Economic Geology-4350
A Brief Guide to the Geology Of Ore Deposits
By
Ron Morton
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Contents
Introduction
3-4
History of Economic Geology
5-10
Mineral Exploration and the Making of a Mine
Some Terms to Know
16-17
Ore Forming Processes and Fluids
Classification of Ore Deposits
Porphyry Copper Deposits
18-25
26-28
29-34
Porphyry Molybdenum Deposits
Porphyry Gold Deposits
34-35
35-37
Epithermal Gold/Silver Deposits
Volcanogenic Massive Sulfides
38-44
45-56
Orogenic (Mesozonal) Gold Deposits
Mississippi Valley Lead Zinc Deposits
57-63
64-68
Sedimentary Exhalative Lead Zinc Deposits
Iron oxide copper-Gold
11-15
69-72
73-77
Banded Iron Formations and Iron Ore Deposits
78-83
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Introduction
Science has, for 100’s of years, had two main functions:
To enable us to do things.
To enable us to know things.
This nicely sums up what earth scientists call the field of economic geology.
To know: How, why, and where minerals are concentrated in specific rocks, facies of
those rocks, structures, and tectonic settings within the earth’s crust. To know how to search for
and find these concentrations
To do: Extract the concentrations in a safe, environmental way for societies benefit and
for the maintenance and expansion of our civilization.
It’s often been said that the most basic of human activities is agriculture. But, in the 21st
century, it would be impossible to feed the world’s billions of people, to improve living
standards in 3rd world countries and do so with environmental safeguards, as well as have
continued expansion and advance of the technology, agriculture, etc. of industrial nations
without mining and the use of minerals and metals. These are necessary for such basic needs as
plowing, fertilization, harvesting, transportation, preservation of food as well as cars, trains,
planes, skyscrapers, microwave ovens, DVD’s, blackberries and iPods, delivery of hot pizza, and
so on.
As one example it takes 38 elements and minerals-copper, zinc, lead, mercury, tin, gold,
tellurium, silver and so on-to make a computer go beep.
A concentration of minerals, what geologists call an ore deposit, is considered economic
if it can be mined and sold at a profit. This is an economic, a business definition and not a
geological one. This definition says nothing about geological processes of concentration and
formation. Under this definition ore is defined as a solid, natural occurring mineral concentration
useable as mined or from which one or more valuable constituents may be economically
recovered. Thus includes not only metals but industrial minerals.
The geological processes of formation of that concentration, say a 10 million ton massive
sulfide deposit of 10% zinc, 1% copper, and 1% lead may be identical to an occurrence of
200,000 tons of 5% zinc, 0.5% copper and no lead. Based on tonnage and grade one is an ore
deposit and the other is not. Yet both formed in the same environment and in the same way. The
differences are in the source of the metals, temperature of the hydrothermal fluid, time the
mineralizing process was operative, nature of the host rocks, and so on. So, in this course, we are
not going to be true economic geologists, we are economic geologists with the emphasis on
geologist and geology-we will be concerned with processes of formation and concentration of
minerals with emphasis on environments of formation, host rocks and facies of host rocks,
tectonic setting, structures, and how a mineral concentration can be defined and classified. This
all leads to exploration criteria for finding these concentrations.
The search for and finding of ore deposits is technical, and because of all the different
earth processes that lead to mineral concentrations economic geology is an interdisciplinary
field. It can involve many different aspects of geology depending on the kind of ore deposit
being sought. Geological fields applicable to exploration include:
a) Volcanology
b) Structural geology
c) Igneous-sed-met petrology
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d) Geochemistry
e) Geophysics
f) Hydrogeology
g) Geomorphology
h) Environmental Geology
From all the above then we will approach economic geology and this course by:
a) Looking at and learning what the most important ore, gangue, and alteration minerals are
for a given ore deposit types-Lab
b) Looking at and learning about the geological and geochemical characteristics of a wide
variety of ore deposits as well as something about their environment of formation and
genesis-lecture and lab
c) Exploration criteria for searching for these concentrations.
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History of Economic Geology
It has been argued that humans have always been scientists. It could also be said they have
always been economic geologists; economic geologists in the sense of prospectors, miners,
explorationists, and entpreneaurs. One example of this was the mining of copper by Native
Americans on Isle Royale from about 7,500 to 3,000 years before the present. Here Native
American prospectors discovered every native copper deposit eventually mined by Americans in
the mid-late 1800’s. Native American miners followed copper veins down dip, sunk shafts up to
60 feet deep in hard basalt, and extracted the copper by what can be called the fire and water
technique. They also had stamping mills for pounding the irregular copper pieces flat, most
likely making it easier to transport off the island. There are more than 1500 Native American
mining excavations on Isle Royale from which it is estimated the miners extracted more than 1.5
million pounds of copper. On top of all this they were also entrepreneurs for they traded the
copper across most of North America!
Another example is the making of “superglue” over 70,000 years ago by people who
lived in what is now South Africa. The glue was made out of red hematite and acacia gum. But
making the glue wasn't easy for the ancient Africans. It was mentally taxing work that would
have required humans to account for differences in the chemistry of gum harvested from
different trees and in the iron content of the powdery hematite from different sites.
Other commodities mined by early humans included salt, flint, (mining for these dates
back at least 100,000 years) silver, gold, cinnabar, lead, malachite and hematite. These
commodities were used for nutrition, hunting, tools, jewelry, paint, fishing weights, and who
knows what else.
There was underground mining for flint in France and Britain between 4000 and 10,000
years ago. These ancient miners sunk shafts over 100 meter deep into the soft chalk with stone
hammers and picks. Over 7000 years ago Egypt sent out expeditions in search of gold, turquoise,
silver, tin, and lead.
In all likelihood, if our ancient ancestors were anything like us, they wondered why and
how particular metals became concentrated in such localized areas. If so that is all lost to oral
history. The first written record on the formation of ore deposits comes from ancient Greece.
Aristotle, who had no direct knowledge of ores in the ground, somehow came to believe it was
the rays of the sun that caused the concentration of metals. He wrote that when the sun’s rays
penetrate into the earth’s crust and interact with water, in various proportions, metal or metallic
ores form. His reasoning for this was that “a metal is a combination of the elements of earth and
water with the presence of water told because metals are malleable and, when heated, will melt
and flow.” In the case of gemstones Aristotle believed it was star light whose “pure, serene, and
heavenly rays give birth to these bright and precious stones.”
Plato, on the other hand, thought there was a great fire at the center of the earth, which
gave off dense clouds of metal making vapors. The existence of this great fire was substantiated
by the flows and floods of molten rock that flowed over the ground, by clouds of ash, steam, and
vapor ejected periodically from Mt. Etna, Mt. Vesuvius, and other active volcanoes in the
Mediterainium region. The fire was kept going by the deep, penetrating rays of the sun.
That was essentially the state of ideas on the origin of ore deposits until the 15th century.
At this time (Middle Ages) the chief prospecting tool for concentrations of metals was the
wooden divining rod! Interest in the origin of ore deposits increased dramatically in the mid5
1500’s thanks to the invention of the printing press. The printing press allowed books to be
duplicated and distributed at a rate much faster than ever before. This along with the rise and
increased influence of the alchemists allowed ideas, principally theirs, to spread across Europe.
The alchemists essentially took Aristotle’s and Plato’s ideas and modified them. One
addition was the idea that not only the suns and stars rays led to mineral concentrations, but so
did the rays of other planets and the moon. Thus different celestial bodies were responsible for
different metals. The yellow rays of the sun gave us gold, the planet mercury quicksilver, iron
came from red Mars, silver from the light of the silvery moon, copper from Venus, tin from
Jupiter, and lead from Saturn.
Since the sun’s rays were responsible for gold deposits it was naturally concluded that
were the sun’s rays were most intense is where you would find the most gold. This idea was so
widely held it ended up having political consequences. Spain signed a 1790 treaty with England
giving up all claims to lands north of the Gulf of California. This included what would become
Canada. Spain’s willingness to cede these lands to the British was in large part due to the fact
these were rainy/snowy, cold, dismal places and thus not much gold to be found there.
A smaller school of alchemists believed it was only the sun that was responsible for ore
deposits. Like Aristotle they believed the sun’s rays penetrated deep into the earth collecting
moisture and vapors which condensed and hardened into “unripe” metals. These then filled veins
and cavities in the earth’s crust. Over time the “unripe” metals were changed or “matured” into
the different mineable metals due to what they called “the alchemy of nature.” This idea led to
the belief that metals could be changed or transmuted and thus the long, futile effort began to
change lead, tin, etc. into gold.
In the 1700’s another idea that was popular amongst both miners and some alchemists
was that of the “Golden Tree” of the earth. In 1753 Johann Lehmann, an alchemist, summed up
this idea writing “the mineral veins are nothing but off shoots from an immense tree trunk which
presumably goes down into the very depths of the earth and for this reason cannot be reached by
mining operations.”
The largest mineral veins were supposed to represent the boughs of the “Golden Tree”
with the smaller ones being its branches and twigs. Nuggets of gold, tin, silver, and copper found
in placer or alluvial deposits were thought to be fruit from the tree that had been washed into
streams from places where the branches outcropped.
Modern economic geology got its start in Germany at a time when the alchemist’s ideas
were prevalent. In 1556 Georgius Agricola, considered the first economic geologist, published a
handbook on mining called De Re Metallica. In the book, which dealt mostly with mining
practices, Agricola summed up his observations on the sulfide ores in the Engeberg (ore
mountains) region of Saxony, Germany. From his field observations he proposed and outlined
the first classification of ore deposits. His classification scheme was simple and field based. He
divided ore deposits into two large groups- alluvial or in place. He then subdivided these two
groups on the form or shape of the deposit. Thus “in place” deposits would be veins, stringers,
stockworks, and bedded ores. In addition to these he also made two fundamental observations: 1)
ore channels are secondary features in rocks, and 2) ores were deposited from solutions
migrating through those channels.
From Agricola the next advance came in 1669 when Steno (of Steno’s Law) made the
observation that ores may represent the condensation of vapors traveling through fissures, not all
ores had to be deposited by fluids. He got this idea from looking at volcanoes and precipitates
around hot springs.
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After Steno the next major contribution to economic geology was that of Carpentier, a
professor at the mining academy of Freiberg, who, in the late 1700’s, studied the lead-zinc mines
around Freiberg, Germany. From his observations he concluded that metals and minerals in ore
veins were the result of reactions between the wall rock hosting the ore and heated water passing
through them. His main evidence for this was the gradation of some mineralized veins into the
wallrock. These observations represent the start of the hydrothermal ore theory, and the
beginning of what has been called lateral secretion. This is the idea that metals are leached out of
the wallrock by heated water and then deposited in openings in the same rock.
From here we come to the late 1700’s and to the Plutonists and Neptunists. The founder
of the plutonist idea of ore formation was James Hutton. Hutton, the father of modern geology,
proposed, in 1788, that not only igneous rocks but also all ore deposits were derived from molten
magmas, and were thus transported in a liquid state to where they are found. Hutton got the idea
from what he saw as the similarity between metallic ores and products observed in smelters. He
argued against Steno and Carpentiers ideas by saying metals were injected into fissures in a
molten state and cooled and crystallized there. Mixed ores were the result of the same processes
that formed the different igneous rocks.
Opposed to Hutton and the Plutonists were the Neptunists lead by Abraham Gottlieb
Werner. Werner, a professor at the Freiberg Mining Acadamy, proposed that all rocks and ore
deposits were formed as sediment in a primordial ocean. Veins were cracks formed on the sea
floor by earthquakes and slumps and then filled in by minerals precipitating out of the water. As
evidence he cited the colliform banding seen in some veins.
The two sides argued back and forth for more than 20 years. Both sides were adamant
that the other was wrong. Plutonists because lava flows were clearly not sediment and Neptunists
because it had been shown metals could be transported in, and precipitate out of water. The
argument between the two camps eventually led to ore deposits being subdivided into two
separate groups–those formed by igneous processes (such as tin and copper) and those formed by
sedimentary processes (like bedded deposits of iron ore)
In the mid 1800’s there was a return to Carpentiers ideas with the writings of Eli de
Beaumont, a French scholar and scientist. He became the first geologist to organize (classify)
hydrothermal (hot water) deposits around and related to igneous centers. He also recognized
replacement type deposits (skarns) and magmatic segregations (chromite).
So, in summary, after 1500 years the state of knowledge about ore deposits in the mid1800’s was: 1) they were either sedimentary or igneous, 2) if igneous they then could be broken
down into those formed by the circulation of hot waters, those due to cooling from a magma,
amd those that were replacements of the wallrocks. Overall, no general classification scheme for
ore deposits had been developed.
At the end of the 1800’s and in the early 1900’s geologists came to the conclusion that 1)
ore deposits formed from a great many geological processes, 2) structures in the rocks were
important in localizing ores and in the movement if fluids, and, 3) that a unifying classification
scheme was desperately needed; deposits needed to be systematically arranged so similarities
and differences between them could be seen and used to not only understand their origin but also
to find other, similar deposits.
This led to a number of geologists proposing different kinds of classification schemes.
The one that turned out to be the most practical and insightful (and still partly in use today) was a
classification system proposed in 1915 by Waldemar Lindgren. Lindgren, who worked 21 years
at the USGS and then taught for 24 years at MIT, based his classification system on 1) whether
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or not an ore deposit was a product of chemical or mechanical concentration and, 2) if chemical,
whether the deposit formed from hydrothermal water, magma, or within bodies of rock (meaning
sedimentary processes). Ores formed by hydrothermal waters were further sub-divided based on
inferred temperature and pressure of formation.
This was pretty much state of the art until the 1970’s and 80’s when geologists began to
recognize that many ore deposits were cogenetic with their host rocks and not exotic to them.
The start of the study of fluid inclusions also demonstrated that Lindgren’s pressure-temperature
categories were not nearly as distinct as had been believed.
Other important discoveries in the 70’s and 80’s that led to increased knowledge of ore
formation and to further revision of Lindgren’s classification were:
a) The recognition that metalliferous hydrothermal fluids are not pure water but some
kind of saline brine,
b) That metals are transported in these brines as complexes; experimental work of
Helgeson and others showed these to be dominantly chloride and bi-sulfide
complexes.
c) Increased understanding of plate tectonics, which led to an improved understanding
of the litho-tectonic setting of metals and the rocks they occur with. Because of this
classification schemes began to take into account tectonic regimes.
d) Studies of ancient volcanoes demonstrated the connection between volcanism and
certain kinds of ore deposits. The study of these deposits in modern settings led to
changes in Lindgren’s pressure-temperature scheme.
e) Recognition that porphyry copper, molybdenum, tungsten, tin deposits have different
geological characteristics and form in different tectonic settings.
f) Discovery of hydrothermal vents on the sea-floor and recognition of different
temperature fluids which had dramatically different metal contents. The start of
investigations into sub-seafloor geothermal systems and the connection of these and
venting fluids to VMS on land.
g) The recognition that meteorite impacts can play a role in ore formation (Sudbury)
h) The recognition that MVT deposits are linked to deep, upwelling brines that have a
relationship to oilfield brines and most likely represent connate waters.
All of this led to a modified Lindgren classification scheme which was presented by
Guilbert and Parks in their 1986 Geology of Ore Deposits book. As part of their classification
they placed deposits in process related groups, and subdivided ore as either syngenetic or
epigenetic. They then went on to try to use environment of formation and origin of the
hydrothermal fluid as further subdivisions.
Since 1986 there has been a great deal of change in the exploration for ore deposits and in
our understanding of them. This has led to new ore deposit classification schemes. Some of these
advances or changes are:
1) The first is the sheer volume of material published on ore deposits from 1986 to the
present; more than 18,000 papers, field trip guides, special symposia volumes,
reports, and special issues of journals. This essentially means that no one geologist
can be current or even close to current in ore deposit geology-the time of the “jack-ofall-trades” geologist is gone. We now live in a geological world of the specialist, the
geochemist, volcanologist, sedimentary petrologist, structural geologist, and so on.
2) Discovery of an ore deposit is partly luck and may always be so-why is this fault
mineralized while the other 10 in the area are not. The answer may be that that’s the
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random or fractal path the fluid decided to take on its way to the surface-very difficult
to reconstruct ancient fluid pathways. Thus geology can put you in right area, can
define and narrow targets, but a discovery hole is definitely partly luck. This is shown
by the successes ratio in exploration which is much less than 1%. This is why
persistence pays off!
3) Experimental work by Seyfried, Bischoff and others on heated seawater and
submarine geothermal systems, which led the way to understanding of sub-seafloor
alteration and the movement of seawater through the ocean crust. This work also
demonstrated the temperature dependency for metals to be transported in seawater.
The downside of this work turned out to be how fast it became the “in way” for
forming all volcanogenic massive sulfide deposits. Based on this magmas were
discounted as ore sources, they were but the heat that drove the geothermal system. It
took 20 years to re-establish the importance of magmatic-hydrothermal fluids and
their impact on seawater systems-which is where we are today.
4) Recognition that porphyry deposits not only have distinct lateral zoning patterns in
terms of alteration and mineralization but also vertical ones. This led to the direct
connection between porphyry, epithermal gold, and hotspring deposits.
5) Recognition of komatiites and their potential for hosting ni-cu-pge ores.
6) The recognition that certain ores and ore forming episodes are unique-Sudbury, Lake
Superior Fe ores
7) The use of cad and GIS systems to integrate, plot, and model data. Ability to see in 3d using computer technology,
8) Increased accuracy and precision of age dates which help sort out stratigraphic
successions (ore and non-ore) as well as help in defining syngenetic vs epigenetic
deposits.
9) Mantle plumes and their roll in ore formation
10) Source of ore fluids based on fluid inclusion studies as well as isotopic studiesdifficulty here is to make sure ore fluid sampled-difficult in long lived geothermalmagmatic systems,
11) Increased understanding of plate tectonics and environments of formation for ore
deposits.
12) Geological time and ore deposits-the uneven distribution of ore. Also ore forming
processes through time and relationship of certain ore deposits to changes in the
atmosphere and climate.
13) Role of mud volcanoes in sedimentary exhalative deposits
So there we are-we have come a long way in understanding ore forming processes,
environments of ore formation, sources of ore fluids, and role of plate tectonics and mantle
plumes in ore formation. But for all of this increased knowledge and more complex and
sophisticated geochemical and geophysical techniques, ore deposits have become harder to target
and locate. We have yet to crack why they occur where they do-ie., why this caldera mineralized
and not adjacent 3, or why this fault and not next 10 mineralized even though all are the same
age, and so on.
This is partially due to the fact that most exploration is targeting hidden or buried ore
bodies, and what we can’t see clearly adds numerous levels of difficulty to interpretations and
targeting. Another reason is the apparent lack of persistence on the part of explorationists and
their under use of the science of geology and ore petrology in exploration programs. Again
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companies want quick results and thus tend to target and drill geophysical and geochemical
anomalies. Geological targets are harder to define and require longer term commitments. What
company will drill a stratigraphic hole, have their geologists routinely look at thin sections for
alteration, rock types, textures, or drill orientated holes for structure, etc. in the targeting part of
the program. I think the road to success is slow, old fashion, and geological, with geophysics and
geochemistry supporting the geological program.
As far as the current classification of ore deposits this will be discussed in a later section.
References
Adams, F.D., 1954, The Birth and Development of the Geological Sciences, New York, Dover
Publications Inc.
Agricola, G., 1556, De Re Metallica, English Translation by H.C. Hoover and L.H. Hoover, New
York, Dover, 1950.
Crook, Thomas, 1933, History of the Theory of Ore Deposits, London, Thomas Murby & Sons.
Guilbert, J.M. and Park, C.E., 1986, The Geology of Ore Deposits, Long Grove, IL., reissued by
Waveland Press, 2007
Hutton, J., 1795, Theory of the Earth, Eddinburgh.
Lindgren, W., 1933, Mineral Deposits, 4th. Ed., New York, McGraw-Hill.
Peters, W.C., 1987, Exploration and Mining Geology, New York, John Wiley & Sons.
Sagan, Carl, 1996, The Demon-Haunted World: Science as a Candle in the Dark, New York,
Ballantine Books.
Stanton, R.L., 1972, Ore Petrology, New York, McGraw-Hill Inc.
.
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Mineral Exploration and the Making of a Mine
The making of a mineable ore deposit entails much more than just finding it, though that, by far,
is the hardest part of the process. So, in the search for, discovery, and development of an ore
deposit the following are of importance:
1) Mineral exploration program, which is the search for, and finding of, an ore deposit. This
involves a study phase to select areas to explore, decide how the exploration will be
carried out, and quick field checks to make certain the geology is what has been reported.
This is followed by the reconnaissance phase which involves field mapping, geochemical
sampling, geophysical surveys, economic petrological studies, channel sampling,
relogging of any core, and data compilation. If the reconnaissance phase is successful it is
then followed by the detailed phase, which is the drill testing of select targets or favorable
stratigraphy. The success ratio at the exploration stage is less than 1/10th of 1 %. With
such slim odds investors keep putting money into exploration because success gives you
a much higher profitability than other industrial ventures. On that note today many large
companies no longer undertake mineral exploration programs or do detailed explorationthey buy deposits from junior companies which are financed by individual investors..
2) Feasibility studies: to see if the mineralization discovered is economically viable. This
depends on many factors including size, tonnage, grade, price, and demand. This stage
takes detailed drilling, assaying, and pilot mill tests. For industrial minerals this stage
involves such things as homogeneity, size, chemical purity, and color. At this stage also
need to consider the effect on the environment (flora, fauna, air, water) and cost of
cleanup and environmental protection
3) Mine development: the establishment of infrastructure, which includes site clearing,
roads, power, mill, disposal areas, mine buildings, and shaft sinking (if underground) or
pit design.
4) Mining: extraction of the ore from the ground-open pit or underground, disposal and/or
storage of waste rock; the ore goes to the mill
5) Processing of the ore: this involves milling (crushing, separation of ore minerals from
gangue, separation of ore minerals into separate concentrates-ie. sphalerite and
chalcopyrite from pyrite-quartz-chlorite gangue, followed by separation of zinc from
copper. Gangue to tailings disposal and concentrate shipped to smelter.
6) Smelting to get metal(s) out of concentrate
7) Marketing
8) Closure and clean-up-waste rock, tailings, mine site, etc.
Since this is economic geology we will focus mainly on the geology part-ie exploration
for and discovery of ore deposits.
The search for mineral deposits (ore deposits) is costly (a few million to more than 100
million dollars), competitive, technical, and time consuming (average time from discovery to
first ore from ground is 10 years). However, it can also be very exciting for the geologist
involved in it for it is much like being on a grand adventure, a treasure hunt and a very
challenging treasure hunt; like solving a mystery from a series of very different clues. In the
search for mineral deposits companies can use many and varied techniques. Again, because of
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the time involved and the uncertainty of long term financing, many companies try to use short
cuts and, as said before, this usually leads to failure and wasting money.
Some of the scientific and not so scientific things that go into finding an ore deposit are:
a) Serendipity-luck
b) Understanding the kind of deposit you are looking for in a geological sense-ie. host
rocks, alteration, environments of formation, ore genesis, structure, etc.
c) Geological mapping-from recon (large scale-looking for right rocks, stratigraphy,
alteration) to detailed facies-alteration mapping.
d) Item c should be coupled with economic petrology studies: thin sections, x-ray, SEM,
microprobe, polished sections, etc.
e) C and d also involve lithogeochemistry and the extensive use of GIS, Cad, and
computer modeling
f) In reconnaissance stage will use geochemistry to try and find anomalies-this involves
soil and rock sampling as well as water and lake bottom sediment sampling; may also
involve special surveys like bio-geochemical sampling.
g) Geophysical surveys to support c-f and to pick out geophysical anomalous areasespecially important once favorable geological areas defined-this may entail EM
surveys (V-Tem, IP, etc), magnetometer surveys, gravity surveys, etc..
h) Diamond and rotary drilling (drilling holes only way to finally identify, quantify and
outline an ore deposit).
All or a combination of the above is used to a) delineate areas of potential for the kind of
ore deposit being explored for, b) delineate areas where more detailed follow up work
should/will occur, and 3) outline and test anomalous areas (geologic, geophysical, geochemical).
These techniques will vary depending on the deposit type explored for and the kind of rocks the
potential deposit occurs in. In other words, before you can use the above techniques properly and
effectively you have to know what you are looking for.
Most important to whole program is geological mapping-mapping of outcrops and
logging of drill core to identify and interpret:
1) Rock types, facies of those rocks, stratigraphic successions, environment of
formation, method of emplacement, etc.
2) Alteration minerals, assemblages, and intensity
3) Structures which may control ore deposition
If you do not know or understand the rocks you are working with-stratigraphy, facies,
structure; it is difficult to do any other kind of studies or to interpret geochemical and
geophysical results.
Scale of mapping will vary depending on purpose-why are you mapping in the first
place? Broad reconnaissance scale down to mapping individual outcrops in detail. The object of
this is to figure out what the rocks are, kind of deposits they represent, facies and their
distribution, structures, alteration, relationship of alteration and any mineralization to specific
stratigraphic units, facies, or depositional environments.
Examples:
1) Mississippi Valley Lead Zinc- carbonate rocks, dolomitization and silicification in the
proper facies (reefs), at facies changes, along major faults
2) VMS-right volcanic rocks, nearness to volcanic centers, subaqueous formation,
synvolcanic structures, high temperature alteration, volcanic facies,
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3) Porphyry Copper-multiple intrusions, porphyritic, apalitic phases, breccia pipes,
alteration zoning, distribution of pyrite
4) Orogenic gold-major shear zones, secondary dilational structures along those zones,
alteration
Coupled with stratigraphic mapping for geology/alteration is lithogeochemistry-used to
help define stratigraphic units, alteration assemblages, and alteration changes (elements) in
similar rocks. Detailed mapping plus whole rock geochemistry and thin sections can be a great
aid in defining areas where detailed exploration should be done. This is also the cheapest part of
the exploration program as well as the most important.
GIS can be a valuable tool here-Arcview, ArcGIS, MapInfo (Australia)-company’s own
programs. GIS allows you to integrate all your data plus output it in a visual format. Also need
good system for storage of data and a good drill log plotting program.
GIS works on layers-after entering data can make individual layers to show specific
features or combine layers, ie.-geological units and contacts, faults, copper values-numerous
separate layers here-can turn layers off and on. Also can query data and instantly see the result,
ie. all copper values in basalt that are greater than 100 ppm.
Diamond Drilling and Core Logging
Diamond drilling is the most important phase of exploration. More than any other
exploration technique it provides the exploration geologist with the most concrete and accurate
material upon which an economic evaluation of a property/area can be made. It also provides a
detailed, continuous, look at the subsurface geology. However, drilling can be expensive and
because of this it has become the final and most critical phase of exploration though it should not
be. Drill costs vary depending on hole depth, rock types, core size, etc., and range from about
$50-80 per meter for short holes to $70-150 per meter for deep holes (1000 meters).
Diamond drilling is done to test geological and geophysical anomalies; also zones of
intense alteration. Because of the expense diamond drilling must be carried out accurately,
carefully, and intelligently. Incorrectly placed holes, poorly engineered holes (direction and dip),
lost core, improperly logged core, poorly stored core have been and continue to be common in
the exploration business and cost a lot of lost time, duplicate work, unnecessary expense. Storage
is very important-permanent rock record, can always go back and relog or sample.
The purpose of diamond drilling is to remove a core of representative rock from the
ground. A diamond impregnated bit is rotated at high speed at the end of a core barrel (3-4m’s
long) attached to metal drill rods and rotated by diesel power. As the bit turns it cuts a ring of
rock which is forced up into the core barrel. Once 3 or 4 meters have been drilled the core barrel
is pulled out of the hole and emptied into wooden core boxes and the diamond bit and core barrel
are dropped back down hole (wire line-do not have to take all the drill rods out of hole to retrieve
the core barrel). Core drilled and retrieved can be of various sizes-A (small), BQ, NQ (large) etc.
The drill rods are threaded to each other as drilling proceeds. The diamond bit is turned at very
high rpms and is cooled by water, which is circulated by a pump down the hole, around the bit
and back up again. Purpose of the water is to cool the bit, lubrication for the cutting action of the
bit, and prevent plugging of the bit by cuttings. Bentonite (an expansive clay) is often mixed with
water to drill through less competent rock and fault zones, this prevents loss of water pressure
and lost core. Presently a system called MWD, which means measurements while drilling, tells a
driller the temperature of the bit, what the rate of water flow is, and the pressure on the bit. Also
gyroscope mounted cameras are used down the hole to determine hole angle and/or dip. In
13
diamond drilling the average rate is between 3 and 12 meters per hour. The upper part of a drill
hole (in overburden) is permanently collared by steel casing to prevent collapse.
In drilling core recovery is important because if it is less than 80% it decreases the value
of the hole-ie alteration and mineralized zones are softer and more friable and are the first, along
with faults, to be ground away. In hole with poor recovery the core may not be representative of
the rocks drilled.
Core Logging
Logging diamond drill core is often a long, sometime tedious job but one of the utmost
importance. At 50-150 dollars a meter for the core you want to get all the information out of it
you can and this information has to be as accurate as possible. Also this is a continuous record of
a small part of the subsurface above which you have been mapping and interpreting outcropsthus very important for stratigraphy, facies, alteration, etc.
Typically, a company will have a drill hole plotting program so the holes will be easily
placed on a geological map. From there a drill hole plotting program will make vertical sections
of the holes and these can be projected onto the surface of the geology map. Thus you not only
gain a greater understanding of the geology and fill in part of your map, but you also end up with
a cross-section of the subsurface stratigraphy; a continuous rock record down to the depth
drilled. The only drawback is that the core represents only a small sampling of the subsurface
and thus confidence limits must be placed on the down-hole geology.
The core is also split in half for chemical sampling and assaying. The split core can be
photographed and these digital pictures can be imported into the drill log program and placed
alongside the geological description of the core.
When you log core always wet it down first (spray doc, paint brush)-wet core shows
many more features than dry core and textures are much easier to see. Exploration companies
generally have a set way to log core, but in most cases it will entail 1) graphical logs, 2) detailed
rock descriptions, 3) alteration minerals, 4) observed structures, 5) mineralization. Consistency is
a key factor in core logging
All relevant features are added to the graphical logs in symbol form (breccia, change in
grain size, primary textures, etc. Rock types should be colored in and alteration minerals,
intensity and structures (fractures, joints, foliation, etc.) noted in the descriptions and, where
applicable, shown in symbol form on the graphical log. In the rock descriptions note all primary
textures, crystals, fragments, etc.). Economic minerals are identified (if possible) and their form,
relative %, and association are shown on the log; ie. chalcopyrite 0.5%, disseminated in mafic
lava.
The other kind of drilling is rotary or percussion and is common in exploration because it
is much cheaper than diamond drilling and faster. However, there is no core-the bit simple grinds
the rock up and circulating water pumps the cuttings to the surface where they can be dried and
sampled. End up with a series of small chips of rock which typically represent 3-5 meters of
drilling. Samples of this dried material are pasted onto a piece of cardboard and examined under
a binocular microscope. From this can get gross rock types (if identifiable), gross mineralization,
large scale textures. Fast and cheap way to test for mineralization-used primarily for porphyry
deposits and epithermal gold deposits.
14
References
Australian Drilling Industrial Training Committee, 1997, Drilling: The Manuel of Methods,
Applications, and Management, Danvers, Maryland, CRC Press.
Barnes, J. and Lisle, R., 2004, Basic Geological Mapping, New York, John Wiley & Sons.
Compton, R.R., 1985, Geology in the Field, New York, John Wiley & Sons
Evans, A.M., 1992, Introduction to Mineral Exploration, Blackwell Science
Moon, C., Evens, A.M., Wherteley, M., eds., 2005, Introduction to Mineral Exploration, New
York, John Wiley & Sons.
Spencer, E., 2006, Geological Maps: A Practical Guide to the Presentation and Interpretation of
Geological Maps, Waveland Press
15
Some Terms To Know
Polymetallic: Refers to an ore deposit that is the source of more than one metal suitable for
recovery.
Ore: a solid, natural occurring mineral concentration useable as mined, or from which one or
more valuable constituents may be economically recovered
Ore Deposit: A concentration of useable minerals
Gangue: the commercially worthless minerals associated with economically valuable metallic
minerals in an ore deposit
Footwall: The rocks stratigraphical and/or structurally below an ore deposit
Hanging Wall: The rocks stratigraphically and/or structurally above an ore deposit
Wallrock: Non-mineralized rocks adjacent to an ore deposit.
Syngenetic: Ore deposits that form at the same time, or close to it, as their host rocks.
Epigenetic: Ore deposits that form later than their host rocks.
Hypogene: Primary minerals in an ore deposit that formed from ascending hydrothermal
solutions.
Supergene: Refers to mineralization caused by descending surface waters. Common useage
signifies the enrichment process accompanying the weathering and oxidation of a sulfide and/ore
oxide ore deposit at or near the surface.
Mesozonal: Orogenic, hydrothermal ore deposits formed at depths of 5 to 10 km and
temperatures of 300-4750C.
Metallogeny:The study of the genesis of ore deposit.
Hypozonal: Orogenic, hydrothermal ore deposits formed at depths >10km and temperatures
between 400 and 6000C.
Epizonal: Orogenic, hydrothermal ore deposits that formed at depths of less than 6km and
temperatures of 150 to 3000C.
Metallogenic Province:An area of the earth’s crust characterized by a particular assemblage of
ore deposit types.
Metallogenic Epoch: A period of geological time favorablre for the deposition of ore depositds
characterized by specific deposit types.
Reserves: In metal mining reserves quals ore and is defined as that part of a mineral deposit that
can be economically and legally extracted or produced at the time of the reservedetermination.
Under reserves there are two subclasses- proven-(measured-what mining will actually
16
producegiven current scientific knowledge and technical methods); probable-lessor degress of
certainty of recovery but not to such an extent as to be classifies as a resource.
Resource: measured and indicated, some mining companies also use inferred but this is so
speculative cannot be inclused in a mine feasibility study. Indicated reasourdce is a A resource
sampled by drill holes, underground openings, or other sampling procedures at locations too
widely spaced to ensure continuity but close enough to give a reasonable indication of continuity
and where geo-scientific data are known with a reasonable level of reliability. Inferred An
estimate, inferred from geo-scientific evidence, drill holes, underground openings or other
sampling procedures, and before testing and sampling information is sufficient to allow a more
reliable and systematic estimation.
Hydrothermal Mineral Deposits: Ore deposits that form from a hot, aqueous solution
(dominantly composed of hot water) that flows through permeable units in a restricted portion of
the earth’s crust and precipitates a localized mass of minerals from its dissolved load.
17
Ore Forming Processes and Fluids
Ore deposits form in a great variety of ways and all of these geologically different
processes work toward taking diffuse elements and minerals and concentrating them 10-1000 or
more times thus turning them into viable mineral deposits.
Some of the geological processes for forming ore deposits are:
1) Evaporation of seawater: salt, potash, borax
2) Melting glaciers: sand and gravel
3) Alluvial: concentration of dense, durable minerals like gold, tin, platinum, and
diamonds.
4) Weathering: nickel laterites, bauxite (aluminum)
5) Sedimentary precipitation: iron ore
6) Digenesis and extraction of connate brines: MVT deposits and sedimentary exhalative
deposits
7) Metamorphism-Sapphires, Rubies and metamorphic fluids: orogenic gold
8) Magmatic differentiation plus magma mixing: chromium, magnetite, lithium, REEs
9) Liquid Immiscibility: nickel, copper, platinum, palladium
10) Hydrothermal: copper, zinc, lead, mercury, arsenic, gold, silver, bismuth, antimony.
There are thus a great variety of deposits and lots of processes which lead to mineral
concentrations. Not possible to look at them all thus will concentrate on the most common
processes and the ones that form the most kinds of deposits-these are hydrothermal and
magmatic along with precipitation from seawater.
Ore forming fluids can be subdivided into:
1)
2)
3)
4)
5)
Magmatic
Magmatic Hydrothermal
Seawater-meteoric water-connate water
Mixing of 2 and 3
Metamorphic fluids
Magmatic Fluids and Ore Forming Processes
Ore deposits formed by magmatic processes are typically referred to as orthomagmatic,
which means minerals are selectively crystallized and concentrated in a magma chamber.
Elements and minerals that are commonly concentrated by such a process are nickel, chromium,
copper, titanium, vanadium, PGEs, and iron Common minerals are pentlandite, chromite,
chalcopyrite, ilmenite, platinum, palladium, magnetite, apatite, rutile,. Types of ore deposits are:
a)
b)
c)
d)
chromite
PGEs
copper-nickel-iron sulfides
iron-titanium-vanadium oxides
Processes that lead to concentration of these elements and minerals are:
18
a) Fractional crystallization: Layered igneous rocks result from the gravitational settling or
rising of minerals that precipitate out of a cooling silicate (mafic-ultrmafic) magma. The
composition of the minerals and layers will thus be different than that of the starting
magma. Settling occurs due to density differences between crystals and melt. For
example, early formed olivine and orthopyroxene crystals will be denser than the silicate
magma they crystallize from and will settle to the bottom of the magma chamber.
Plagioclase, however, typically has a density less than that of the melt and will rise.
Oxide minerals will be denser and settle to the bottom and end up forming layers rich in
chromite and/or magnetite. However, experimental evidence suggests that in order to get
chromite or magnetite only layers in mafic-ultramafic intrusions you also need a process
to drive the cooling melt off its normal crystallization path. From experimental work and
detailed mapping this would appear to be 1) mixing of a new magma with the cooling and
crystallizing intrusion, 2) changes in the partial pressure of oxygen in the magma
chamber, or 3) increase in the total pressure in part of the chamber. Increase in fugacity
(partial pressure) of oxygen at high temperatures and pressures promotes chromite
stability and may allow only chromite to precipitate out of the melt. The pressure of
oxygen can be altered by devolitizing the magma. As well, a sudden increase in the total
pressure, especially at the top of the chamber due to vesiculation, can shift phase
boundaries and allow chromite and/or magnetite to precipitate until normal pressure is
restored.
b) Liquid Immiscibility: This is the segregation of 2 coexisting liquid fractions from an
originally homogeneous magma. The 2 fractions may be mineralogically similar (silicatesilicate) or be very different (silicate-oxide, silicate-sulfide). Immiscibility is best seen in
extrusive rocks where rapid quenching prevents the segregated products from being
rehomoginized. For example, in basalts 2 distinctly different glasses interstitial to the
crystalline minerals, have been found. These glasses have the composition of rhyolite and
a magnetite-ilmenite-apatite-pyroxene mix. In an August 1963 eruption of Kilauea it was
directly observed that a sulfide melt separated from a cooling basaltic magma. Closer to
home the Duluth Complex contains massive copper-nickel sulfide mineralization as well
as rare, discordant bodies of ilmenite-magnetite-apatite. These occurrences are inferred to
provide further evidence that both sulfide and iron-titanium-phosphorus immiscible
fractions separated from the Duluth Complex magmas.
Oxygen immiscibility: It is well known that deposits of magnetite-apatite or ilmeniterutile-apatite are preferentially associated with anorthosites and some alkaline rocks.
Experimental work has shown it is possible to create two immiscible liquids, one magnetiteapatite and the other having the general composition of synodiorite. Further studies showed that
a fairly wide range of rock compositions under conditions of high oxygen fugacity can lead to
the formation of an immiscible iron oxide melt. See fig 1.19
Sulfide immiscibility: This has been accepted as a common feature of magma
crystallization. Experimental work shows that silicate and sulfide liquids can coexist over a large
volume of the FeS-FeO-SiO2 system. Sulfide solubility, or the amount of sulfide dissolved in the
magma at saturation, will vary as the magma crystallizes, but at any point where saturation does
occur sulfide droplets form. Sulfide saturation can be achieved through falling temperatures,
increase in the fugacity of oxygen, decrease in ferrous iron, addition of external sulfur, or magma
mixing.
19
There are many important and large ore deposits where sulfide immiscibility is assumed to be the
mode of formation. Central to all of these deposits are 3 fundamental steps:
a) Appearance of a substantial sulfide melt
b) Conditions where the sulfide droplets are in equilibrium with a large volume of silicate
magma
c) Effective accumulation of the sulfide droplets into a single layer or single spatial entity.
Factors that promote sulfide immiscibility are:
a) Addition of externally derived sulfur: komatiite lavas flowing over shale or
iron formation; Duluth complex intruding and assimilating sulfur rich rocks in
the Virginia Formation, Sudbury and melting of crustal rocks due to a meteor
impact.
b) Injection of new magma and magma mixingExamples:
Duluth, Kambalda, and Sudbury-external sulfur
Bushveld and the Merensky Reef-injection of new magma
Bushveld and chromite-magma mixing plus devolitization
Adirondacks titanium-oxide-rich melt.
Hydrothermal Fluids
There is little doubt that the vast majority of ore deposits are either a direct result of
precipitation from a hot aqueous fluid, or have been modified by such fluids. A wide variety of
ore-forming processes are associated with such fluids and these can be found in both igneous,
sedimentary, and metamorphic environments. They occur at temps and pressures that range from
those of shallow crustal levels to those deep in the crust.
These fluids are referred to as hydrothermal and the deposits they form as hydrothermal
mineral deposits. As a family hydrothermal mineral deposits form when a hot, aqueous solution
(dominantly composed of hot water) flows through permeable units in a restricted portion of the
earth’s crust and precipite a localized mass of minerals from its dissolved load. Just how
hydrothermal solutions form, how they react with the rocks through which they pass, and how
they deposit their carried constituents are topics of interest to economic geology.
Hydrothermal fluids are common, and particularly so in active or recently active volcanic
terrains. This conclusion comes from:
1)
2)
3)
4)
5)
Presence of hot springs and numerous subaerial geothermal systems
Fluid inclusions in minerals
Alteration of rocks
Hot mine waters
Geophysical and heat studies that show the oceanic crust at spreading ridges is cooled
by large volumes of seawater moving through it. It is estimated heat loss from the
20
crust implies that seawater circulates through young oceanic crust at a rate of 1017th
to 1018th g/year. This heated seawater, as shown by direct sampling and experimental
work, reacts with rocks of the oceanic crust recrystallizing them and modifying their
chemistry. This heated water eventually exits as warm or hot springs.
6) Number of hydrothermal ore deposits (circular reasoning).
Hydrothermal fluids are often, both subaerially and subaqueosly, parts of geothermal
systems. A geothermal system is one in which fluids circulate and these form where a
combination of favorable structures and rocks occur together in areas of high heat flow. In
general the favorable conditions include:
1)
2)
3)
4)
5)
An aquifer zone to contain the heated water
A cap-rock which is a unit of low permeability overlying the aquifer
Recharge channels
Discharge channels
Heat source
Aquifers maybe a naturally permeable unit, a group of permeable units, or a zone of
fracture-fault permeability or both. The cap rock is a unit of low permeability overlying the
aquifer which insulates the system. It keeps the water hot and prevents leakage. The cap rocks
may be naturally impermeable or become impermeable by a process called self sealing, This is
the result of the precipitation of minerals (silica, carbonates) within permeable units due to
temperature decrease, pressure decrease, and/or oversaturation. Warm water is buoyant and will
rise thus cooling slightly and this, coupled with a slight pressure decrease, causes some
precipitation, particularly of quartz and carbonate.
Recharge channels represent areas where fresh water enters an aquifer and discharge channels
cut up through the cap rock and send heated water to the surface. The heat source is typically a
cooling, near surface magma body, often a subvolcanic intrusion.
A hydrothermal system with recharge and discharge channels is referred to as open
system, ones with only discharge channels are called closed systems. Water to rock ratios in
hydrothermal systems vary from very low (0.1) to very high (150/1). This leads to two kinds of
hydrothermal systems-rock dominated (water-rock ratio <40/1) and water dominated (>40 or
50/1). The distinction is based on the reactivity of seawater magnesium during reactions with
basalt-ie. all the magnesium is extracted (removed from seawater) during the reactions (low
ratios), or not (high ratios).
In hydrothermal systems there are 4 sources for the water:
1) Seawater
2) Connate water (water trapped in sediments and breccias at time of formation)
3) Metamorphic water-especially common at transition from greenschist to amphibolite
grade due to dehydration reactions
4) Magmatic
Source of metals in hydrothermal fluids have 3 origins:
1) Rocks or sediments through which fluids pass and interact
21
2) Magmas
3) Combination of 2-mixing in geothermal systems
Magmatic Hydrothermal Fluids:
Magmatic-hydrothermal fluids originate from magmas as they cool and crystallize at
various levels of the earth’s crust, and are responsible for a wide-range of porphyry type deposits
(copper, molybdenum, tin, gold, tungsten, etc.), epithermal gold deposits, volcanogenic massive
sulfide deposits , skarns, and deposits of such elements as mercury, arsenic, bismuth, and so on.
At some stage, either early or late in the crystallization history of a felsic magma, it will
become water saturated resulting in the exsolution of an aqueous fluid which forms a chemically
distinct phase in the silicate melt. This process is called water-saturation but is also referred to as
either “boiling” or “vapor-saturation.”
This aqueous phase will be in chemical, isotopic, and thermal equilibrium with the igneous melt.
At depths up to a few km’s water is the major constituent although fluids may contain significant
CO2, SO2, H2S, NaCl, KCl, FeCl, CaCl, HCl, HF, and of course a wide variety of metals.
This phase can exist as a liquid or vapor though the term gets blurred in useage and the
word fluid is most commonly used for magmatic-hydrothermal phases. The process of water
saturation can be achieved in two ways: either by progressive crystallization of the magma or by
decreasing pressure of the system.
The term “first boiling” is used when water saturation occurs by virtue of decreasing
pressure (usually due to upward emplacement of the magma or mechanical failure of the
chamber walls).
The term “second boiling” refers to the achievement of water saturation by progressive
crystallization of dominantly anhydrous minerals under isobaric conditions. This pertains more
to deep seated magmas and occurs late in their crystallization history.
Immiscibility in these systems can occur at low pressures where the aqueous fluid
actually separates into two phases-1) a dense, very saline brine and 2) a low salinity aqueous
solution.
Magmas, especially felsic ones, can exsolve a lot of water either in liquid, vapor, or
mulitiphase form. Once a separate hydrous phase is exsolved from a magma, a consideration of
typical magma viscosities, bubble sizes, and relative density of the bubbles compared to the
silicate melt leads to the prediction that exsolved water will rise very rapidly to collect in the
highest parts of the magma chamber. Observation strongly supports this:
1) Ores commonly occur around cuplas within or adjacent to intrusive bodies
2) Initial volcanic eruptions after a period of quiescence are dominantly gas rich and
explosive with exsplosivity decreasing as the eruption progresses.
3) Analysis of lava lakes shows that gas content decreases with depth.
4) Hydrous minerals in felsic intrusions (biotite, hornblende) tells us something about
water exsolution-ie to crystallize these minerals the magma needs to have 2-5%
22
water. Yet analysis of such intrusions finds only 0.5-1% water in the crystallized
rock-so a lot has been lost.
The appearance of an exsolved aqueous fluid within a magma is accompanied by the
release of mechanical energy since the volume per unit mass of silicate melt plus low density
aqueous fluid is much greater than the equivalent mass of water saturated magma. At shallow
levels of the crust the volume change accompanying an aqueous fluid separating from a melt
may be as much as 30%. This results in over pressuring of the chamber interior and adjacent
walls and can lead to brittle fracturing. This hydro-fracturing leads to the formation of cracks
with steep dips. These fractures will then propagate into the country rocks and become conduits
for the exsolving fluid phase.
Magmatic-hydrothermal fluids, once exsolved and rising can move directly into the near
surface environment with little interaction with geothermal waters, or they may become
thoroughly mixed with geothermal waters. In the first case the fluid remains saline and acidic, in
the second case the fluid typically loses most of its characteristics and takes on those of the
geothermal system adding its dissolved component to the circulating waters. These two different
paths lead to different ore deposits with different geological characteristics.
Seawater:
Seawater is extensively circulated through the oceanic crust as part of large scale
geothermal systems. It is responsible for widespread alteration and metal redistribution with
seawater losing Mg and Ca and gaining Fe and metals. The drawdown of seawater into faults and
fractures in the oceanic crust, particularly along mid-ocean ridges, its heating up, and chemical
interaction with basalts, gabbros, and diabase as it moves downward and then its subsequent reemergence from exhalative vents was a major oceanographic discovery that helped, and in some
ways hindered, our understanding of VMS deposits. However, these discoveries confirmed the
viability of seawater as a hydrothermal, ore-forming fluid source.
The degree to which metals become concentrated and transported in seawater is very
much dependent on 1) temperature which determines iron oxide or iron sulfide accumulations
versus accumulations of zinc, copper, lead, silver, and/or gold, 2) the degree of mixing with
magmatic hydrothermal fluids.
Meteoric Water:
This is rain, river, and/or lake water (groundwater) that has been able to penetrate to
relatively deep levels in the crust and to become involved in widespread circulation through the
crustal regime. Meteoric water is responsible for hydrothermal ore deposits, especially those with
low temperatures of transport and precipitation (uranium ores, native copper). The heated waters
of Thermopolis in Wyoming represent deep seated circulating meteoric water.
Connate water
This is water included within interstitial pore spaces of sediment as it is deposited, or
within breccia deposits (debris flows). Originally this water is either meteoric or seawater but it
undergoes substantial modification as the sediment or breccia is buried, compacted, and lithified.
The stage of formation from water-rich sediment to lithified rock produces aqueous solutions
23
that evolve with time and depth. Such fluids move upwards through the stratigraphic sequence
and can be involved in the formation of ore deposits.
The progressive burial of sediment to depths of 300 meters or more results in rapid
reduction of pore space and initial formation of a substantial volume of water. The temperature
of these connate fluids increases with depth in the sedimentary sequence with the exact rate of
increase being a function of the geothermal gradient; pressure also increases with depth. Connate
fluids also increase in salinity and density with depth. The increase in salinity may be due to
interaction with evaporate horizons, density increase is due to increase in pressure. Connate
brines are responsible for MVT and sedimentary-exhalative deposits. It is now believed that mud
volcanoes are their surface expression.
Metamorphic water:
As rocks are progressively buried and temperatures exceed 3000C the process of
digenesis evolves into one of metamorphism. The importance of metamorphism to ore forming
processes is where hydrous silicate and carbonate minerals break down to form anhydrous
minerals exsolving water as they do so; this is at the transition from greenscist to amphipolite
grade. This begins at about 3000C (kaolinite to phyropyllite). Kaolinite contains a lot more water
than phyropyllite); so at 4000C chlorite changes to biotite and at even higher grades hornblende
to pyroxene. Carbonates also breakdown and it is not unusually for heated metamorphic waters
to be rich in CO2. Metamorphic reactions and waters are important in orogenic gold deposits.
References
Arndt, N.T., 2005, Mantle-derived Magmas and magmatic Ni-Cu-(PGE) deposits, in:
Hedenquist, J.W., Thompson, F.H., Goldfarb, R.J., and Richards, J.P., eds., 2005, One
Hundredth nniversary Volume, Littleton, Colorado, Economic Geology Press.
Barnes, H. L., ed., 1997, Geochemistry of Hydrothermal Ore Deposits, 3rd edition, New York,
John Wiley & Sons.
Candela, P., and Piccoli, P., 2005, Magmatic Processes in the Development of Porphyry-Type
Ore Systems, in: Hedenquist, J.W., Thompson, F.H., Goldfarb, R.J., and Richards, J.P., eds.,
2005, One Hundredth Anniversary Volume, Littleton, Colorado, Economic Geology Press.
Ernst, W.G., 2000, Earth Systems: Processes and Issues, Oxford, Cambridge University Press.
Guilbert, J.M. and Park, C.F., 1986, The Geology of Ore Deposits, Waveland Press, reissued in
2008.
Helgeson, H., 1964, Complexing and Hydrothermal Ore Deposition, Pergamon Press.
Kirkham, R.V., 1997, Mineral Deposit Modeling, Geological Association of Canada, special
paper 40.
Misra, K.C., 2000, Understanding Mineral Deposits, Dordrecht, Kluwer Academic Publishers.
24
Pirajno, F., 1992, Hydrothermal Mineral Deposits, New York, Spencer-Verlag.
Richards, J.P. and Larson, P.B., 1999, Techniques in Hydrothermal Ore Deposits Geology, El
Paso, Texas, Reviews in Economic Geology, V. 13, Society of Economic Geologists.
Robb, L., 2005, Introduction to Ore Forming Processes, Malden, Maryland, Blackwell
Publishing.
Seltman, R., et. Al., 1997, High-Level Silicic Magmatism and Related Hydrothermal Systems,
Special Issue Journal of Petrology, V. 38, pp. 1617-1807
25
Classification of Ore Deposits
The purpose of classifying ore deposits into similar type’s aids in the description
of them, permits generalizations regarding environments of formation and genesis, and improves
the scientific ability to explore for, and discover them. To be most useful to a student,
explorationist, and/or academic classifying something as complex and variable as ore deposits
must be based on correct field and economic petrologic observations, as well as being as simple
as possible.
Many attempts have been made to classify ore deposits since Agricola’s first efforts, but
most attempts have been discarded because a) the field observations were not correct or were
incomplete, b) the classification was too cumbersome, complex, and/or restrictive to have any
practical value, and c) being based on insignificant or nonuseful variations between similar
deposits. Therefore to be practical and useful an ore deposit classification needs to be based on
common descriptive factors followed by environments of formation; ideas on genesis can then
follow these. The descriptive factors need to be true and correct with environments of formation
as complete and correct as the state of geological knowledge allows; this is also true for theories
on the genesis of the deposit type. In other words the simpler and more field based the better and
the hope is this is good enough to be an aid in the exploration and discovery of similar deposits.
Of all the early classifications the one outlined by Lindgren in 1913 and periodically
modified by him for 20 years was the most useful of all the early classification attempts, and was
especially attractive to North American geologists because it was relevant to vein type ores
which were, at the time, the dominated ore deposit type. The temperature and depth of formation
criteria presented by Lindgren were approximations only, and Lindgren said little about genesis
or environments of formation. Guilbert and Park, in 1986, amended Lindgren’s classification to
reflect current knowledge and ideas; they also added deposit types not recognized in the early
1900’s. Their major modification was to add process-related groups and to more accurately
present temperature and pressure criteria based partially on fluid inclusion data. I might add here
there have been several attempts to classify ore deposits based on plate tectonic settings.
However, without geological accurate descriptions and the fact similar kinds of deposits occur in
very different plate tectonic environments, makes these classification at this time not very useful.
They also offer little in the way of a guide to finding ore deposits. However, plate tectonic
setting is useful when added to other, more field based classification schemes.
The classification used here is a modification of Guilbert and Parks 1986 classification
and is based on the field and geochemical description of an ore deposit along with the
environment of formation. In classifying ore deposits in this manner it quickly becomes obvious
that even though no two ore deposits are exactly alike, most of them fall into a relatively small
number of categories. We also see that each of these categories coincides nicely with generally
accepted hypothesis on how deposits in a particular group form. In other words, although we
began with a geological, petrological, and geochemical descriptive classification, we actually end
up with what are currently perceived to be the genetic processes that form those deposits.
26
Classification of Ore Deposits-2009
Orthomagmatic
1. Cu-Ni Deposits
a) Komatiite (ultramafic lava-hosted)
b) Impact Melt
c) Mafic intrusive rocks
2. PGE’s
a) Stratiform in mafic intrusions
b) Mafic magmatic breccias
3. Diamond Deposits
a) Kimberlites
b) Lamprolites and lamprophyres
4. Chromite Deposits
a) Layered intrusion-hosted
b) Ophiolite-hosted
4a) Magnetite-apatite-vanadium deposits
5.
Deposits Related to granites
• Pegmatites
• Uranium
Hydrothermal Ore Deposits
6. Porphyry Cu-type deposits
a) Porphyry copper in quartz diorite-granitic host rocks
b) Porphyry copper in tonolite to granodiorite host rocks
c) Porphyry copper in monzonite-syenodiorite host rocks
7. Porphyry Molybdenum Deposits
a) Porphyry Molybdenum in quartz-monzonite-granite host rocks
8. Porphyry Gold Deposits
9. Porphyry Tin and Tungsten Deposits
10. Epithermal Gold Deposits
a) Low sulfidation in felsic volcanic rocks
b) High sulfidation in felsic volcanic rocks
c) Sediment hosted deposits
11. Orogenic Gold Deposits
a) Mesothermal (mesozonal)-brittle-ductile)
b) Hypothermal (hypozonal)-(ductile)
12. Volcanogenic Massive Sulfide Deposits
a) Mafic volcanic rocks dominant
 Ore hosted by felsic volcanic rocks
 Ore hosted by mafic volcanic rocks
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 Ore hosted by sedimentary rocks
b) Felsic volcanic rocks dominant
 Ore hosted by volcaniclastic rocks
• low sulfidation deposits
• high sulfidation deposits
1) Ore hosted by felsic lava floews
2) high sulfidation deposits
3) low sulficdation deposits
13. Sedimentary Hosted Deposits
1. Mississippi Valley lead–zinc
2. Sedimentary-Exhalative
3. Evaporite Deposits-Salt and Potash
4. Sedimentary hosted copper
5. Sedimentary hosted uranium deposits
14) Iron Ore Deposits
a. Volcanic hosted
b. Sedimentary hosted (BIF)
15. Skarn Deposits
16. Iron Oxide-Copper-Gold Deposits
a. Calc Alkalic type
b. Alkalic type
17. Deposits Related to Surface Weathering
a) Mechanical Weathering-Placers (including Witswaterrand)
b) Chemical Weathering
1) Bauxite
2) Nickel laterites
3) Manganese
4) Phosphates
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Porphyry Copper Deposits
Definition:
Porphyry copper deposits are large tonnage, low-to medium-grade deposits in which
primary (hypogene) ore minerals are spatially and genetically related to felsic to intermediate
porphyritic intrusions; the ore is dominantly structurally controlled. The deposits are magmatic
hydrothermal with the sulfide and oxide minerals precipitated from saline aqueous solutions at
elevated temperatures. They get their name from the porphyritic texture of the plug-like
intrusions and dikes spatially associated with the mineralization. Multiphase intrusions are
typical and one of these usually has an aplitic texture (sugary groundmass). The ore occurs in
narrow, closely spaced veins (hydrofractures) within hydrothermally altered rock. This produces
stockwork ore and what has been called disseminated ore, which is actually mineralization along
numerous, closely spaced microfractures.. To be mineable these deposits must be amenable to
bulk mining methods.
Geographical Distribution:
Porphyry copper deposits occur throughout the world in a series of extensive, relatively narrow,
and linear metallogenic provinces (Fig. 1). They are predominantly associated with Mesozoic to
Cenozoic orogenic belts in western North and South America and around the western margin of
the Pacific Ocean. They also occur, to a lesser extent, in the Tethyan orogenic belt in eastern
Europe and southern Asia as well as in Paleozoic orogens in Central Asia and eastern North
America. There district location may be controlled by major regional faults such as the Rio
Grande Rift in the southwestern United States.
This global distribution of porphyry copper deposits is a function of the uneven distribution of
magmatism through geological time, which is directly related to plate tectonics and plate
configurations. The dominance of phanerozoic (Cenozoic and Mesozoic) deposits over those in
older terrains is mostly due to preservation and exposure. Porphyry deposits form relatively near
the surface, typically one to four kilometers depth and are subject to subsequent tectonism,
erosion, and/or burial.
Tectonic Setting:
Porphyry copper deposits occur in a variety of tectonic settings and typically represent root zones
or bottoms of stratovolcanoes and/or calderas in subduction-related, continental and
island-arc settings. In the southwestern United States porphyry copper deposits are associated
with granitic rocks emplaced in a continental setting, within or along the margins of calderas that
are now largely eroded. In modern islands arcs that border the Pacific Ocean and along the
continental arc that is associated with the Andes Mountains the deposits are related to both
andesitic stratovolcanoes and calderas. Porphyry copper-gold deposits, such
as those associated with Triassic and Lower Jurassic alkali intrusive and volcanic rocks in British
Columbia, formed in an island-arc setting, although possibly during periods of extension.
Porphyry copper deposits in the Yulong belt of Tibet are related to pull-apart basins in a large,
post-subduction strike-slip fault.
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Size and Grade:
Porphyry copper deposits are large and typically contain hundreds of millions of tons of ore,
although they do range in size from tens of millions to billions of tons. Copper grades are highly
variable ranging from 0.2 to more than 1.6%; they average about 1%. Other common economic
metals associated with the deposits include molybdenum (0.005 to about 0.03%), gold (0.004 to
0.35 g/t), and silver (0.2 to 5 g/t). Average deposit size is 750 million tons at a copper grade of
0.75%.
With low grades (0.2-greater than 1%) what makes these deposits profitable is their very large
size and their shape. Most are relatively near-surface and approximately cylindrical so they lend
themselves to large mining schemes. It was actually at the Bingham Canyon porphyry deposit
where the first large-scale bulk mining method took place. It started in 1899 and by 1907 there
was an open pit mine extracting 6,000 metric tons a day. Today some porphyry copper mines
extract more than 100,000 metric tons of ore per day. The maximum production for a 24 hour
period is from Morenci in Arizona when 1.3 million tons of ore were removed. Chuquicamata in
Chili produces more than 700,000 tons per day from an open pit that is 4.3 km long, 3km wide
and 850 m deep.
Ore Minerals:
Chalcopyrite is the main ore mined often with minor bornite and chalocite. Molybdenite is
commonly present and other, minor minerals are enargite, tetrahedrite, tenentite, electrum, and
cassiterite. The main gangue sulfide mineral is pyrite and magnetite is the main oxide mineral.
Secondary or supergene minerals associated with porphyry copper deposits (see end of this
section) are chalcocite, covellite, cuprite, chrysocolla, azurite, and malachite. It was supergene
enrichment of many porphyry coppers that made them economically viable in the first place.
Also economically mined at most porphyry copper deposits are gold and silver; most of the gold
is micron size. For example Bingham Canyon has produced 23 million ounces of gold since 1900
making it the largest gold producer in the US. Bingham has also produced 190 million ounces of
silver.
Prices:
The price of copper has gone up and down with swings in the economy. In 2004 the price was
1.10 per pound, and with increased demand, especially from China and India, it rose to $4.00 in
2008. With the downturn in the economy in 2008 and 2009 the price dropped dramatically. It
was to $1.20 in Feb 09 then rose to $2.00 in April. Molybdenite was $18 lb in 2004, $46 in 2005,
$33 in 2008 and then fell to $9 April 2009.
Hydrothermal Alteration and Zoning:
Hydrothermal alteration is defined as the chemical and mineralogical changes in rocks brought
about by warm to hot aqueous fluids circulating or passing through those rocks. The fluid/rock
interaction causes chemical and mineralogical changes in the rock as well as changes in the fluid
composition. Obviously such changes depend on 1) the time and duration of the interaction, 2)
30
the permeability of the rocks (alteration will be more intense along more permeable zones), 3)
the composition of the fluid (acidic, basic, or neutral), and 4) the temperature of the fluids.
Alteration is studied by a) economic petrology-looking at alteration minerals and assemblages
present in the rocks and comparing them to unaltered rocks. This is done via hand samples, thin
sections, and mineral chemical studies and 2) lithogeochemical studies looking at chemical
changes that have occurred in the altered rock relative to an unaltered equivalent rock.
In porphyry deposits hydrothermal alteration is widespread (km’s) and is typically zoned
on a deposit scale, as well as around individual veins and fractures (Fig. 15). Alteration zoning
can be defined by distinct mineral assemblages. Typically the upper portions of the host intrusion
become extensively fractured due to a) adjustment of the adjacent country rocks as the intrusion
cools, and b) high vapor pressure of the fluid (first boiling and volume increase). These fractures
are permeable zones along which the ore fluids migrate both laterally and vertically.
Alteration zoning in porphyry copper deposits is based on 1) lateral and vertical movement of the
hydrothermal fluid, 2) cooling of the hydrothermal fluid as it moves upwards and outwards
accompanied by a possible decrease in pressure, and 3) degree of interaction with groundwater.
This leads to distinct horizontal and vertical zonal sequences of alteration minerals in the
intrusion and adjacent rock. These zones may be grouped into different mineral assemblages
based on modal abundances and types of minerals present.
Lateral Zonation:
1) Propylitic Alteration Zone: This is the outermost alteration zone and is widespread (300m
up to 10km’s). It is always present and is composed of chlorite, epidote, carbonate, and
pyrite. Chlorite and epidote are the most common and abundant minerals with pyrite
varying from trace to as much as 10%. This alteration type forms by conversion of the
mafic minerals (hornblende, biotite, and pyroxene) to chlorite-epidote, and plagioclase
going to epidote-carbonate and/or recrystallizing to albite. This zone grades outwards into
unaltered rocks over a few hundred meters.
2) Argillic Alteration Zone: This is an area of clay alteration and is most extensive in rocks
rich in plagioclase feldspar; it is not always present. Prominent minerals are kaolinite and
montmorillonite both of which replace plagioclase. Pyrite is always present. When it
occurs this alteration zone is typically narrow and lense-like.
3) Sericitic or Phyllic Alteration Zone: Sericite, quartz, and pyrite are the dominant minerals
in this zone accompanied by minor amounts of rutile, chlorite, and ilmenite, Significant
amounts of ore are associated with this alteration type with the ore minerals dominated by
chalcopyrite with or without molybdenite and gold.
4) Potassic Alteration Zone: This is an inner or core zone characterized by potassium
feldspar and/or biotite with or without amphibole, magnetite, anhydrite, and fluorite.
Associated ore minerals may be chalcopyrite, bornite, chalcocite, molybdenite, and gold;
pyrite is usually a minor constituent.
At some deposits there is an early alteration composed of barren quartz veins, these are then
overprinted by the potasssic alteration.
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Alteration mineralogy is controlled in part by the composition of the host rocks. In mafic host
rocks with significant iron and magnesium, biotite (and possibly minor hornblende) is the
dominant alteration mineral in the potassic zone, whereas potassium feldspar dominates in more
felsic host rocks. In carbonate-bearing host rocks, calc-silicate minerals, such as garnet
and diopside, may be abundant.
Alteration mineralogy is also controlled by the composition of the mineralizing system. In more
oxidized environments minerals such as pyrite, magnetite, hematite, and anhydrite are common,
whereas pyrrhotite is present in more reduced environments.
Vertical Alteration:
Porphyry copper deposits also exhibit a vertical zonation of alteration minerals that is
distiunct from the lateral zonation assemblages. Vertical alteration zones are not common at
porphyry copper deposits because 1) most of the overlying rocks have been eroded away and 2)
they will only form where there has been little interaction between the hydrothermal fluid and
ground water.. The alteration assemblages and zonation associated with this alteration were
determined by deep drilling in recent volcanic terrains and partial preservation at more recent
discoveries, particularly those in active island arc environments. The vertical alteration has been
subdivided into 3 zones which are referred to as alsic A, P, and K.
Overall alsic alteration represents extreme base leaching from all aluminum phases in the
rocks except for aluminum itself. This alteration type forms from acidic fluids.
Alsic A: This is a high temperature (>4250C) alteration represented by extreme leaching of the
rock, minerals present are andalusite, corundum, and quartz with or without pyrite and
chalcocite. This alteration occurs immediately above the host intrusion.
Alsic P: This alteration type forms at moderate temperatures (300-4250C), with the minerals
present being pryophyllite, diaspore, topaz, and quartz.
Alsic K: This is a low temperature (100-3000C) alteration with the representative minerals being
kaolinite and quartz. This zone may extend all the way to the surface where it is represented by
hot springs and steaming ground. In the near surface environment other minerals commonly
present are alunite, hematite, and diaspore. This zone may not be present.
Host Rocks: Common Characteristics:
a) Associated igneous intrusive rocks range in composition from diorite and quartz diorite to
granodiorite and quartz monzonite. Some are alkalic and include monzonite and syenite
porphyries. The intrusions are typically cylindrical stocks or broad, domal-shaped nearsurvace intrusions.
b) Multi phase intrusive events or phases are common and range from 2-3 to more than 14
(Henderson); there are 7 at Bingham. Host to ore is often the most differentiated and
youngest phase present.
c) One or more intrusive phases is porphyritic (sometimes all phases are porphyries).
Phenocrysts range from 30-55%. Another phase present is an aplitic rock; one with a
fine-grained “sugary” texture (caused by pressure quenching of the porphyry due to rapid
ascent and volatile loss). At depth these porphyritic intrusions merge into the same
underlying magma chamber.
d) Plagioclase is always present with hornblende common in rocks of intermediate
composition and biotite, potassium feldspar, and quartz common in more silicic rocks.
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e)
f)
g)
h)
Rocks containing plagioclase, potassium feldspar, quartz, biotite and/or hornblende
formed at 675-7000 C with more than >4% water in the magma. In alkalic intrusions the
minerals typically present are plagioclase, sodic-rich clinopyroxene, leucite, and garnet.
Accessory minerals may include apatite, zircon, magnetite, titanite, fluorite, and monazite
Breccia pipes (diatremes) are common and may represent hydrothermal explosions
caused by rapid fluid rise and, in the near surface, rapid expansion. This cause rocks to
fracture and break, and may also lead to the flashing of water to steam leading to phreatic
explosions.
All rocks have been extensively altered
Veins in the rocks are ubiqueoutous, They form throughout the life of the porphyry
magmatic-hydrothermal system (as do breccias). Veins contain a large % of the ore
minerals and represent the locus of greatest fluid transport.
Bottoms of porphyry deposits (areas below the ore deposit) are regions of abundant
quartz veins, porphyry dikes, and widespread alteration that may include calcic (garnet,
pyroxene, epidote) skarn on the flanks of some systems, and greisens (coarse-grained
aggregates) of muscovite-quartz, which occurs directly beneath the ore (SW USA).
Ore Geometry:
•
•
•
•
Igneous host rocks constitute the central core of the orebody (or all of it). Ore may then
extend outwards for a variety of distances into the country or wall rocks. Distribution of
the ore is based on a) composition of wall rocks, b) degree of fracturing and faulting.
Because most intrusions are steep-walled cylinders there is a strong tendency for the
deposits to exhibit a concentric or shell-like pattern of sulfide and alteration minerals.
At first inspection the mineralization appears to be widely disseminated (gives the rock a
“salt and pepper” look; however on a microscopic scale it can be seen that most sulfide
minerals are actually located along netweorks of microfractures (stockworks). This small
scale fracturing only extends outwards as far as the sulfide-alteration assemblages and is
believed to be due to first boiling of the hydrothermal fluid.
The mineralization tends to exhibit an inverted cup shape with:
a) A low grade core composed of minor chalcopyrite often accompanied by
molybdenite and gold. Bornite and chalcocite may be present in minor
mounts. This zone has low amounts of pyrite.
b) The main ore zone composed of chalcopyrite (1-4%) and gold with or without
silver. Bornite and chalcocite are absent and there are trace amounts of
arsenopyrite, and cassiterite.
c) A pyrite rich shell (10-20%) with minor chalcopyrite (0.1-1%),.
d) C grades outwards into a low Pyrite shell (2-5%) with traces of chalcopyrite.
May get veins of galena-sphalerite-and silver minerals in this zone.
Types of Porphry Copper Deposits:
a) Quartz Diorite-Diorite Type:
1) Host rocks are diorite, quartz diorite, syenite, and monzonite
2) The rocks have low silica/alkali ratios
3) There is little or no free quartz
4) Magnetite is a common accessory mineral
33
5) Hydrothermal alteration: potassic (biotite rich) > propylitic, or potassic >argillic >
to propylitic
6) These deposits have high gold and low molybdenum
b) Quartz Monzonite Type (classic southwest United States)
1) Host rocks are quartz monzonite, granodiorite, and quartz diorite
2) The rocks have high silica/alkali ratios
3) There is lots of free quartz
4) Little or no magnetite, low gold contents, but they have more silver and are enriched
in molybdenum
5) Hydrothermal alteration is potassic to sericitic to argillic to propylitic
Tectonic Setting:
• Type a is associated with Island Arcs; deposits common in Australia, and around the
pacific
• Type b is associated with continental margins, and represented by deposits in the Andes
Mountains, southwest USA, and British Columbia.
Formation:
1. Porphyry copper deposits form in active volcanic environments with the intrusions
feeders for volcanic activity at the surface
2. They are associated with hot springs and epithermal gold deposits which occur in the
upper zones of the vertical alteration. If subaqueous may be hot springs with associated
volcanogenic massive sulfide deposits.
3. Evidence for their genesis comes from isotopes, fluid inclusions, and economic
petrological studies. These show that the hydrothermal fluid was in equilibrium with the
potassic alteration assemblage, which formed at 500-7500 C and is indistinct from
magmatic water.
4. The hydrothermal fluid, from fluid inclusion studies, was up to 60% NaCl, it underwent
boiling, and was high in copper, iron, and sulfur.
Thus:
5. Potassium silicate alteration assemblages and associated ore minerals formed from high
temperature, acidic magmatic brines.
6. Isotopes and fluid inclusions show that sericitic alteration assemblages formed from
mixing of magmatic fluid with ground water. The temperature of formation was 300-6000
C), the ph of the fluid has increased and the salinity decreased. Also K/H ratios had
decreased.
7. Propylitic alteration formed from a highly diluted, low temperature (<2000 C), neutral Ph
fluid that was < 5% NaCl. It had low K/H ratios
8. Vertical alteration, potassic > advanced argillic > qtz is indicative of a boiling magmatic
fluid cooling as it rises toward the surface. Temperatures range from 700 to 3000C, ph is
low and K/H ratios are high; there is little mixing with ground water.
Porphry Molybdenum Deposits
•
•
Similar to porphyry copper deposits in style, alteration, grade, and size
Differences:
34
– The ore is dominantly molybdenum-MoS2 (0.10 – 0.45%), with associated tin,
tungston, and REE’s
– The host rocks are granites, granite porphyries, granodiorite;. They are more felsic
than the intrusions that host porphyry copper deposits and thus are believed to
have been derived from more evolved magmas.
– To get more evolved magmas thick continental crust is needed. Need long
residence time which infers a stable upper crust and also long lived magma
chambers.
– The deposits are associated with continental rifts.
There is one major and one minor type of porphyry molybdenum deposit:
– Major- Climax-type and includes Henderson, Questa, Mt. Emmons, etc.
– Minor-Quartz Hill-type, includes Endako
Climax-Henderson Type
• Multiple Intrusions of aplite, aplite porphyry, granite porphyry
• Intrusions associated with overlapping shells of alteration and mineralization
• Intrusions are alkali-rich, meta-aluminous, high silica rhyolites
• Top part of the intrusions is characterized by intense silicification with associated topaz,
wolframite, fluorine-rich mica, fluorine-rich garnet.
•
Quartz Hill Type
• Similar to Climax-Henderson but less molybdenite and lower enrichment of niobium,
zirconium, rubidium, fluorine, tin, and tungsten.
• Host rocks are granite porphyries with biotite; granodiorite intrusions also in the area
• Niobium is 150-200ppm and rhubidium is 500-1000ppm; this compares to 20-100ppm
niobium and 150-400ppm rhubidium for porphyry coppers, which represent less evolved
magma, and shorter crustal residence times.
Porphyry Gold
• Many porphyry copper deposits (especially in the diorite-quartz and diorite class) are
major producers of by-product gold and silver
• The gold (with or without silver) may occur:
– In centrally located stockworks with the copper +/- molybenum
– Proximal in calcic skarns with copper
• About 20 porphyry deposits in western world contain > 0.4 g/t of gold (0.12 oz/t).
• Bingham (which has produced 23.4 million ounces) is a major US gold producer
• Ok Tedi has 0.07 oz/t
• In alkali porphyry copper-gold deposits:
– The gold is in stockworks
– The deposits have potassic alteration with anomalous Mt (up to 8%)
– Copper minerals are chalcopyrite and chalcocite
35
References
Burnham, C.W., 1997, Magma and hydrothermal fluids, in: Barnes, H.L., ed., Geochemistry of
Hydrothermal Ore Deposits, 3rd edition: Wiley Interscience, New York, p. 63-116.
Candela, P.A., and Piccoli, P., 2005, Magmatic processes in the development of porphyry-type
ore systems, in: Hedenquist, J.W., Thompson J.F.H., Goldfarb, R.J., and Richards, J.R., eds.,
Economic Geology 100th Anniversary Volume: Society of Economic Geologists, Littleton,
Colorado, p. 25-37.
Goldfarb, R.J., and Nielsen, R.L., eds., 2002, Integrated Methods for Discovery: Global
Exploration in the Twenty-First Century: Society of Economic Geologists, Special Publication 9
Lipman, P.W., and Sawyer, D.A., 1985, Mesozoic ash-flow caldera fragments in southeastern
Arizona and their relation to porphyry copper deposits: Geology, v. 13, p. 652-656.
Lowell, J.D., and Guilbert, J.M., 1970, Lateral and vertical alteration-mineralization zoning in
porphyry ore deposits: Economic Geology, v. 65, p. 373-408
Schroeter, T.G., ed., 1995, Porphyry Deposits of the Northwestern Cordillera of North America:
The Canadian Institute of Mining and Metallurgy, Special Volume 46
Seedorf, E., Dilles, J.D., Proffett, J.M., Jr., Einaudi, M.T., Zurcher, L., Stavast, W.J.A., Johnson,
D.A., and Barton, M.D., 2005, Porphyry deposits: characteristics and origin of hypogene
features, in: Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., and Richards, J.R.,
eds., Economic Geology 100th Anniversary Volume: Society of Economic Geologists, Littleton,
Colorado
Sillitoe, R.H., and Hedenquist, J.W., 2003, Linkages between volcanotectonic
settings, ore-fluid compositions, and epithermal precious metal
deposits, in: Simmons, S.F., and Graham, I., eds., Volcanic, Geothermal,
and Ore-Forming Fluids; Rulers and Witnesses of Processes within the
Earth: Economic Geology, Special Publication 10, p. 315-343
Sillitoe, R.H., and Bonham, H.F., Jr., 1984, Volcanic landforms and ore
deposits: Economic Geology, v. 79, p. 1286-1298.
Sillitoe, R.H., 1993a, Epithermal models: genetic types, geometrical controls and
shallow features, in: Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., and
Duke, J.M., eds., Mineral Deposit Modeling: Geological Association of
Canada, Special Paper 40, p. 403-417.
——— 1993b, Gold-rich porphyry copper deposits: geological model and
exploration implications, in: Kirkham, R.V., Sinclair, W.D., Thorpe,
36
R.I., and Duke, J.M., eds., Mineral Deposit Modeling: Geological
Association of Canada, Special Paper 40, p. 465-478.
Sillitoe, R.H., 1973, The tops and bottoms of porphyry copper deposits:
Economic Geology, v. 68, p. 700-815.
Westra, G., and Keith, S.B., 1981, Classification and genesis of stockwork
molybdenum deposits: Economic Geology, v. 76, p. 844-873.
37
Epithermal Gold/Silver Deposits
Definition:
The term epithermal was used by Lindgren in 1913 to refer to those hydrothermal ore deposits
that formed in a relatively near surface environment. Today we know this is from about 50
meters down to 1,500 meters below the surface with temperatures between 100 and 4000C and
low pressures. Surface expressions of these near surface deposits are hot springs. Most geologists
think of epithermal deposits as being mainly continental and associated with subaerial
geothermal systems, however volcanogenic massive sulfide and sedimentary exhalative deposits
are certainly in the epithermal group.
Ore Minerals, Production, Size.
Ore elements associated with epithermal gold/silver deposits include gold, silver, antimony,
arsenic, and mercury. The correspondent minerals are native gold and silver, argentite, tennatite,
enargite, gold tellurides, electrum, stibnite, and cinnabar. Minor elements are tellurium, copper,
zinc, lead, and bismuth represented by chalcopyrite, covellite, sphalerite, galena, and
bismuthinite.
Most epithermal gold/silver deposits are obviously mined for gold and/or silver with individual
deposits varying from gold-rich (silver/gold ratios less than 10) to silver-rich (silver/gold ratios
from 20-200). Open pit methods are used for large, low grade deposits (1-2 g/t Au, 70-90 g/t Ag)
with gold in oxidized (supergene) ore being amenable to low cost, heap-leach treatment.
Underground mining is generally used for smaller, higher grade deposits (10-100 g/t Au, or
greater than 500 g/t Ag). In these deposits grade control is essential and difficult because gold
and silver values can vary dramatically over just a few meters. Tonnages mined often depend on
grade with low grade deposits needing more gold-bearing rock; generally tonnages will be in the
100’s of millions. Higher grade deposits could have as little as 400,000 tons of ore. Large mining
companies typically will not mine a deposit unless it has at least 1 million ozs of gold.
Distribution and Age:
Most known epithermal deposits are Tertiary in age, which goes along with their near surface
level of formation and thus low level of preservation. However, older deposits are being
recognized, including those that are metamorphosed and extensively recrystallized. Most
deposits are closely associated with convergent plate boundaries (continental and island arcs).
Epithermal deposits thus form in areas of active volcanism where there is high heat flow,
tectonic activity, and active geothermal systems. Mined deposits represent parts of fossil
geothermal systems.
Classification and Deposit Types:
Based on the nature of the ore forming fluids as well as the type of volcanic host rocks, alteration
assemblages, and ore minerals, epithermal gold/silver deposits have been divided into 2 groups:
1) Low sulfidation deposits, also called the adularia-sericite class
2) High sulfidation deposits, these are also called the acid sulfate or alunite-kaolinite
pyrophyllite class.
38
The term sulfidation is used here to describe stabilities of sulfur-bearing minerals in terms of
sulfur fugacities (partial pressure).
Low Sulfidation Epithermal Gold/Silver Deposits:
Historically most gold production from volcanic rock-hosted epithermal deposits have come
from this type.
Regional Setting:
This deposit type, in terms of tectonic setting, is related to back-arc basins, continental
margins, and island arcs. The deposits form at shallow depths and are associated with regional
faults that control volcanism and are relatively long-lived. For example the Comstock, Oatman,
Bullfrog, and Tonapah epithermal districts line up along what is called the Walker Lane in
Nevada and the deposits in the San Juan Mountains are related to the Rio Grande Rift.
Local Setting:
Low sulfidation deposits are commonly associated with calderas and, to a lessor extent,
stratovolcanoes. However, in the case of calderas, it needs to be realized that just because you
have identified a caldera does not mean you will necessarily find a gold deposit. In fact most
calderas contain no gold mineralization. In the San Juan Mountains only 1/3 of the known
calderas are mineralized. Within calderas the ore deposits occur:
a) Within permeable rock units like ash flow tuffs, dome breccias, debris flows and
especially where these units are in proximity to synvolcanic faults. Thus margins of
calderas and dome-flow complexes are common environments.
b) In the case of stratovolcanoes the gold mineralization is typically associated with faults
along the flanks of the volcano and permeable rock units such as air fall deposits, debris
flows, and ash flow tuffs.
Host rocks are dominantly Tertiary to Quaternary in age and rhyodacite, rhyolite, and
andesite in composition. Rhyolitic host rocks tend to be enriched in molybdenum, tungston,
fluorine, niobium, and tin. It has been proposed that these epithermal deposits represent the
upper parts of porphyry molybdenum deposits but that claim has not been proven. In some rare
cases volcanic sediments (moat-type) host ore.
Mineralogy:
The mineralogy of the ore is characterized by veins of adularia and sericite with quartz.
Other common associate minerals are chlorite, rhodochrosite, rhodenite, barite, and fluorite.
Pyrite is ubiquitous and may be accompanied by cinnabar, arsenopyrite, and/or stibnite. The ore
minerals are electrum (gold with 20% silver), native gold, native silver, gold tellurides, argentite,
tennatite, chalcopyrite, and tetrahedrite. These occur as replacements of the host rocks and in
single veins or vein stockworks,
In general these deposits contain more silver than gold though there are exceptions like
Round Mountain, a 178 million ton deposit with 0.035 oz gold (300,000 ozs) and no silver. Gold
grades vary from <1 to >100 g/ton; typically the gold is fine-grained and mostly invisible. Silver,
when present, ranges from 1 to more than 5 oz/ton. Minor sphalerite and galena are often
associated with these deposits.
39
Alteration:
At and above the water table the rocks tend to be silicified and alunite often is present with the
quartz. The silicification is due to boiling of the hydrothermal fluid, and precipitation of quartz
as silica sinter, Below the water table clay minerals (kaolinite), sericite, and manganese
carbonate are the common alteration products. This alteration gives way outward to a zone of
epidote, albite, and carbonates with or without pyrite and chlorite. This alteration type is similar
to propolytic alteration in porphyry copper deposits, is quite extensive, and may have formed
prior to ore deposition. Quartz is variable in appearance ranging from banded to crustaform and
cockscomb; varieties include chalcedony, amethyst, and chert.
Zonation of Ore minerals:
•
Above the water table to the surface there may be native gold and/or silver, with minor
stibnite and cinnabar. This will be a low grade zone with the ore minerals occurring as
disseminations and fracture fillings.
•
At and just below the water table (boiling zone) can find native gold and silver, gold
telluride's, stibnite, and arsenopyrite. These minerals occur dominantly as fracture fillings
with some replacement ore. This will be the high grade ore zone.
•
Below water table can find electrum, argentite, tetrahedrite and arsenopyrite as fracture
fillings and replacement ore.,
•
Deeper zones (>600m) may have galena, sphalerite, chalcopyrite, and/or molybdenite
Paleodepth of ore fluid:
•
From geological reconstructions and fluid inclusion studies the low sulfidation deposits
form between 100 to 1000m below the surface.
•
The ore fluids have low salinity (1-5% NaCl), a near-neutral Ph, and contain CO2, H2S,
and NaCl as dominant species.
•
The temperatures of the fluid was between 200 and 3000 C
•
The composition of the fluid is similar to many active geothermal systems dominated by
meteoric water and magmatic gas such as the Broadlands in New Zealand.
•
The amount of magmatic gas/fluid is between 10-30% and thus the fluid represents a
mixture of meteoric water and magmatic hydrothermal fluid.
40
High Sulfidation Epithermal Gold/Silver Deposits:
Tectonic Setting:
These deposits occur in continental margins, island arcs, and back-arc basins. They form
in active volcanic areas where intrusions are near the surface; thus, like the low sulfidation
gold/silver deposits, they are associated with stratovolcanoes and calderas.
Depositional Environment and Geological Setting:
Within calderas the deposits commonly occur along the margins or near synvolcanic
structures within host rocks that range from dome-flow complexes and ash flow tuffs to
hyalotuffs associated with tuff rings and maars. These low sulfidation deposits are postulated to
represent the near surface expression of porphyry copper systems.
Mineralogy:
These deposits are characterized by alunite (KAlSO4) [white, powdery, fine-grained
earthy variety], enargite, and pyrite with or without covellite, quartz, and barite. The ore and
gangue occurs as disseminations, fracture fillings, and replacements. Vein ore is subordinate to
these other types.
Ore Minerals are native gold, enargite, tennatite, argentite, tetrahedrite, bismuthinite, and
gold tellurides
41
Alteration:
Characterized by extensive leaching of the volcanic rocks with ore most often associated
with the more intense alteration.
From the surface down to the water table will find alunite, kaolinite, and pyrophylite with native
sulfur and some cinnabar as well as quartz (chalcedony). There may be extensive zones of
silification at and just below water table. The ore minerals occur beneath this silicified zone and
the alteration here is sericite-alunite-kalinite-quartz (opal, chert), which gives way laterally to
kaolinite and montmorillonite that grades out into a propolytic alteration zone. With depth
pyrophylite and diaspore take the place of kaolinite, The ore minerals found here are enargite,
native gold, electrum, stibnite, with minor native silver. This gives way downward to
tetrahedrite, tennatite, bismuthinite, and chalcopyrite.
Paleodepth of Formation:
•
100-600 m below the surface
•
Hydrothermal fluids have a low Ph (<1-3), are saline (10-30%), and dominated by H2S,
HCl, H2SO4, and SO2
•
These are characteristic of magmatic-hydrothermal fluids that enter the near surface
environment with little or no interaction with meteoric water
Surface expression of these low ph waters are:
•
Solftara’s, acid lakes
•
Abundanrt native sulfur, realgar, and orpiment
•
Examples include White Island, New Zealand, and Osorezan, Japan
Hot Spring Deposits:
Hot spring gold deposits, which include siliceous (chert) sinters and brecciated lower zones, cap
many modern geothermal systems and may be associated with either low or high sulfidation
deposits. Their surface formation makes them very vulnerable to surface weathering and erosion
and thus their preservation is poor. .
In these deposits ore was precipitated within 100 meters of the surface due to the direct
interaction of the hydrothermal fluid with surface waters. The surface manifestations are mud
pots, solftaras, thermal pools, and siliceous sinters. Deposits tend to occur as disseminations and
veins and may be associated with hydrothermal breccias.
Gold mineralization is native gold and electrum with associated arsenic, antimony, mercury,
thallium, silver, and sometimes tungsten.
Today some active geothermal fields are precipitating gold at well heads of geothermal power
stations (Wairki), and at the surface in hotsprings (Osorezan, Japan).
42
Osorezan is a stratovolcanoe with a crater lake at its top. The lake is occupied by a lava dome,
which has been dated at 0.17 million years. Hydrothermal venting occurs along synvolcanic
faults. There is a young intrusion at depth which supplies both heat and fluids,
At Osorezan two kinds of hydrothermal water have been identified:
a) Saline poor-fluid associated with fractures and faults around the edges of the lava dome.
There is no gold in these fluids but you do have minor antimony and arsenic,. The fluid is
low in sulfur and it has been postulated any gold would have precipitated at depth.
b) Saline rich fluids. Again these are discharged along faults and fractures at the bottom of
lake. Water is high in arsenic, lots of native S, and gold which precipitates with arsenic.
This fluid is magmatic-hydrothermal.
References
Bethke, P.M., Rye, R.O., Stoffregen, R.E., and Vikre, P.G., 2005, Evolution of the magmatichydrothermal acid-sulfate system at Summitville, Colorado: Integration of geological, stable
isotope, and fluid inclusion evidence: Chemical Geology, v. 215, p. 281-315.
Bonham, J.F., Jr., 1988, Models for volcanic-hosted precious metal deposits; a review, in:
Schafer, R.W., Cooper, J.J., and Vikre, P.G., eds., Bulk Mineable Precious Metal Deposits of the
Western United States: Geological Society of Nevada, p. 259-271.
Cooke, D.R., and Simmons, S.F., 2000, Characteristics and genesis of epithermal gold deposits:
Society of Economic Geologists Reviews, v. 13, p. 221-244.
Giggenbach, W., 1992, Magma degassing and mineral deposition in hydrothermal systems along
convergent plate boundaries: Economic Geology, v. 87, p. 1927-1944.
Hagemann, S.G., and Brown, P.E.eds., Gold in 2000, Reviews in Economic Geology v. 13,
Society of Economic Geologists
Hedenquist, J.W., and Roberts, P.J., eds., Guide to Active Epithermal (Geothermal) Systems and
Precious Metal Deposits: Mineral Deposits, Monograph 26, 211 p
Sillitoe, R.H., and Bonham, H.F., Jr., 1984, Volcanic landforms and ore deposits: Economic
Geology, v. 79, p. 1286-1298
Sillitoe, R.H.,1993, Epithermal models: genetic types, geometrical controls and shallow
features, in: Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., and Duke, J.M. eds., Mineral Deposit
Modeling: Geological Association of Canada, Special Paper 40, p. 403-417.
Simmons, S.F., White, N.C., and David, J., 2005, Geological Characteristics of Epithrmal
Precious and Base Metal Deposits, in: Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., and
43
Richards, J.P., eds., Economi Geology 100th Anniversary Volume: The Economic Geology
Publishing Company, p. 485-522
Taylor, B.E., 2007, Epithermal gold deposits, in: Goodfellow, W.D., ed., Mineral Deposits of
Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of Geological
Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits
Division, Special Publication No. 5, p. 113-139.
White, N.C., and Hedenquist, J.W., 1990, Epithermal environments and styles of mineralization:
variations and their causes, and guidelines for exploration, in: Hedenquist, J., White, N.C., and
Siddeley, G. eds., Epithermal Gold Mineralization of the Circum-Pacific, Geology,
Geochemistry, Origin and Exploration, II: Journal of Geochemical Exploration, v. 36, p. 445474.
Wood, D.G., Porter, R.G., and White, N.C., 1990, Geological features of some Paleozoic
epithermal gold occurrences in northeastern Queensland, Australia, in: Hedenquist, J., White,
N.C., and Siddeley, G., eds., Epithermal Gold Mineralization of the Circum-Pacific, Geology,
Geochemistry, Origin and Exploration, II: Journal of Geochemical Exploration, v. 36, p. 413443.
44
Volcanogenic Massive Sulfide Deposits
Definition:
These ore deposits are defined on the basis of 4 major characteristics:
•
Their spatial and temporal relationship to volcanism and volcanic rocks
•
The “massive” ore is stratabound or strataform (or nearly so) and greater than 60%
sulfide minerals
•
The ores are composed of the elements zinc, copper, and lead with minor but significant
amounts of gold and silver
•
The ore deposits form at or near the seawater-rock/sediment interface with deposition
from a hot hydrothermal fluid. Near the seafloor because there are VMS deposits where
the ore and alteration minerals replace volcanic deposits that have high porosity.
The term massive, when applied to the ore, means it consists of 60% or greater by weight of
sulfide minerals.
Importance: From both a practical and a purely scientific perspective VMS deposits occupy a
unique position of importance among mineral deposit types.
Practical:
•
They are a major source of copper, zinc, lead, silver and gold, a significant source of
cobalt, tin, cadium, tellurium, bismuth, germanium, gallium, and barium. Some also
contain mineable mercury, antimony, and arsenic. For this reason these deposits are
called polymetallic ore deposits because they are mined for a variety of elements. Thus
they remain a very desirable kind of deposit to explore for because they give a company
some protection from the fluctuating price of different metals.
•
They are a significant source of gold
Scientific:
•
They occur with volcanic rocks dated at 3.4 by old and are currently forming on the
modern sea floor. In that regard they have probably been forming as long as volcanoes
have been erupting
•
They gives us evidence for and of early plate tectonic processes, start and evolution of
life, evolution of the atmosphere, and ore-forming processes through time
•
They are natural laboratories for we can see them forming today
Grade and Tonnage:
Tonnage: There are over 850 deposits worldwide and they range from lenses of 200,000 tons to
giant deposits containing more than 150 million tons. Among the largest are Rio Tinto with 1.5
45
billion tons, Kidd Creek with 140mt, Geco at 66mt, Horne with 52mt, Crandon with 60mt, and
Bathurst at 320 mt.
Grades: The grades of the deposits vary considerably and this appears to be a function of
temperature of the hydrothermal fluid, amount of magmatic input, environment of formation and
kind of host rocks. In general the deposits contain greater than 1% copper, 3% zinc, 0.5% lead,
1g/t gold and 20 g/t silver. Some deposit grades are (see table)
Tectonic Setting:
VMS deposits form anywhere there is volcanism in a subaqueous environment; this
includes ocean floor divergent zones, island arcs, and back arcs. Many VMS deposits have
probably been destroyed by subduction driven tectonic activity. There may be some deposits,
such as Kidd Creek, that are associated with mantle plumes. Continental back arc settings
contain the world’s most economically important VMS districts. The most common local
environments of formation are along grabens, and within and along the edges of both cauldrons
and calderas. Most large deposits are associated with calderas. Because of such local settings
VMS deposits tend to occur in clusters.
Ore types:
VMS deposits contain two kinds of ore, each of which is distinguished on the bases of
sulfide content and the relationship of the ore to the volcanic stratigraphy. The two types are
massive and stringer or vein ore. Massive ore consists of greater than 60% sulfide minerals and
is stratabound or strataform. Stringer ore contains less than 20% sulfides by weight and crosscuts stratigraphy.
Massive Ore:
Occurs parallel to the attitude of the enclosing rock types with sharp (almost knife-like)
contacts with the hanging wall rocks and gradational contacts with the footwall rocks.
Stratigraphic thickness of the massive ore varies from less than 0.3 meters to more than 100
meters; with the strike length of the ore greater than the width and much greater than the
thickness (10’s of meters wide and 100’s of m in strike length. For example, at Kidd Creek the
massive ore is 40 m thick, 160 m wide, and 800 m long.
Much of the ore is truly massive, ie. featureless, but parts of the ore body may contain
banded or layered ore. This banding or layering can be due to a) different sulfide minerals, 2)
sulfide-silicate minerals; 3) grain size differences, or 4) replacement. The layering is due to three
different causes:
1) Replacement, which occurs due to the flow of hydrothermal fluid through the forming
massive ore resulting in remobilization of previously deposited metals along temperature and
chemical gradients; these tend to be perpendicular to the seawater interface. This process
results in what has been called “ore refining” and results in a deposit with a chalcopyritephyrotite-pyrite core and a sphalerite-galena-pyrite outer zone. In extreme cases much of the
base metal and precious metal content can be remobilized out of the sulfide deposit and
carried by the fluids up into the overlying seawater. In this case you end up with a barren
pyrite body with maybe a thin base metal enriched outer margin.
46
2) Recrystallization during metamorphism
3) Sedimentation, which is associated with a cloud-like brine in a depression on the sea floor or
is due to “plume” fall out.
Stringer Ore:
The stringer ore always occurs on the footwall side of the massive ore and has a total
sulfide content that seldom exceeds 20% by weight. The ore zone consists of stock work-like
veins as well as pod-like to cloud-like replacements of the host rock. The stringer zone is roughly
perpendicular to the massive ore and thus x-cuts stratigraphy. It narrows with depth and grades
upwards into the massive ore.
There are VMS deposits that contain only massive ore and others that contain only
stringer ore. Stringer ore only could be a result of erosion or the water depth was too shallow and
the fluid simply boiled away. Massive ore only is more problematic, but could be due to “plume”
fallout, slumping and sliding away from areas of formation, or precipitation from a brine pool.
Mineralogy of the Ore:
Ore mineralogy is similar in both massive and stringer ore though zinc and lead contents
are much higher, and copper lower in the massive ore. Iron sulfides (pyrite and phyrrhotite)
compose more than 50% of the massive ore and can make up 90 to 100%. Sphalerite, galena, and
chalcopyrite occur in widely varying amounts and these three minerals make up most of the rest
of the sulfides. Minor minerals, some of which are economic to recover, include tetrahedrite,
native gold, argentite, cassiterite, cinnabar, stibnite, arsenopyrite, gold tellurides, barite,
bismuthinite, and bornite. The iron content of the sphalerite is highly variable so it varies from
honey-yellow to blackjack in color; often the sphalerite is cadium enriched.
Classification of VMS deposits:
VMS deposits have been classified on the basis of host rocks, metal content, and
alteration assemblages. All three ways of classifying them pretty much agree. For our purpose
we will use a classification scheme based on host rocks, with each class named for a VMS
deposit or camp in that class. This is done because it is easier to associate host rocks, metals, etc.
with a place name than with a generic name like bimodal mafic. In a host rock classification
scheme there are five major deposit types:
1) Noranda-type: This type is named for deposits found in the Noranda area of Quebec. In
this deposit type mafic lava flows (dominantly basalt) are the dominant volcanic deposit
type throughout the stratigraphy, but the VMS deposits are hosted by felsic (rhyolitedacite) lava flows and/or domes. These are copper-zinc deposits and are also referred to
as the bimodal mafic class.
2) Mattabi-type: This type is named for VMS deposits in the Sturgeon Lake area of
northwestern Ontario and for the largest deposit found in that camp. Felsic volcanic rocks
(dominantly rhyolites) comprise most of the volcanic stratigraphy, and the ore is hosted
by volcaniclastic rocks. These are zinc-copper-lead deposits and are also called
siliciclastic felsic. The Mattabi-type can be subdivided into two sub-types which are
similar to epithermal gold deposits: low and high sulfidation deposits.
47
3) Bathurst-Rio Tinto-type: This deposit type is named for the large VMS deposits in the
Bathurst camp in New Brunswick and also for the giant Rio Tinto deposit in Spain. The
volcanic stratigraphy is similar to Mattabi-type deposits but they also contain a lot more
volcanically derived sedimentary rocks including iron formations. The ore is hosted by
either felsic volcaniclastic rock or felsic lava flows and domes. These are zinc-copperlead deposits which are also referred to as siliciclastic felsic-sedimentary.
4) Kuroko-Kidd Creek-type: These deposits are named for the Kidd Creek Mine in
Timmons, Ontario and the numerous VMS deposits found in the “green tuff” region of
Japan. The ore deposits are hosted by felsic lavas and domes as well as block and ash
flows, and dome breccias. In the case of the Kidd Creek deposit ultramafic lavas occur in
the footwall stratigraphy. These are zinc-lead-copper-silver deposits and are also called
bimodal felsic. They, like the Mattabi-type, may be subdivided into two subtypes: low
and high sulfidation.
5) Cyprus-type: This kind of VMS deposit is associated with ophiolite assemblages and is
similar to what is forming today on the sea-floor at divergent zones. The VMS
mineralization is hosted by basalts that vary from pillow lavas and hyaloclastites to sheet
flows. Mafic dykes are ubiquitous and there may be small cryptodomes composed of
plagiogranites. These are copper-zinc deposits.
All of the volcanic host rocks formed subaqueously, which is a necessary condition for
the formation of VMS deposits. The location of a VMS deposit within a volcanic stratigraphic
sequence appears to be related to a change in either magma composition or eruptive style; ie.
basalt to andesite, pyroclastic flows to lava domes.
The felsic volcanic rocks tend to be high in silica and have low Zr/Y ratios (<12). Overall they
are high in Zr (>300ppm) and y (>30ppm). La/Lb ratios are low(<6) and the rhyolites have
elevated LREEs and HREEs, which are typical of high temperature magmas.
Deposit Types, Volcanic Rocks, and Ore Grades:
Cyprus-Type
The volcanic stratigraphy is dominated by mafic lava flows (basalts) with abundant
pillow lavas, hyaloclastites, and sheet flows. Sheeted diabase dykes are everywhere and marine
sediments, including chert (Mn-rich), are common. This stratigraphy is a “classic” ophiolite
sequence though the VMS deposits seem to occur where there is a change from basalt to
andesitic volcanism. Plagiogranitic intrusions may occur near the top of the stratigrahic section.
There are no felsic volcanic rocks. The sulfide minerals in these deposits are pyrite, chalcopyrite,
sphalerite, and marcasite.
Cyprus-type VMS deposits are typically small (less than 2 million tons). Examples
include deposits in Oman, Cyprus, and on the modern sea floor along divergent zones (black
smokers).
Noranda-Type
48
The volcanic stratigraphy is > than 80% mafic volcanic (basalt and andesite) and these
lavas vary from pillowed flows and hyaloclastite to sheet and massive flows; rare “cinder cone”
and “tuff ring” deposits (hyalotuffs) are present and mark a distinct shallowing up of the
sequence. Though the stratigraphy is dominated by mafic volcanics there is up to 20% felsic
lavas found throughout the succession and these vary from lobe-hyaloclastite complexes and
cryptodomes to blocky domes and dome breccias. These felsic rocks (rhyolites and dacites) are
of great importance because they host the VMS deposits. They thus represent a dramatic change
in magma type and style of volcanism.
Over all the deposits are small though Geco and Flin Flon are exceptions. Grades average
3.5% copper, 5% zinc, 1.1 oz/t silver, and 0.02 gr/t gold; there is no lead.
Examples include Noranda, Flin Flon, Winston Lake, Geco, and in Minnesota the 5-mile
Lake occurrence.
Mattabi-Type
In Matabi-type VMS deposits felsic volcanic rocks make up more than 70% of the
footwall and hanging wall stratigraphy; these rocks are dominantly fragmental. The most
abundant rock types are pyroclastic flows and debris flows and the ore deposits are hosted by
these units. These deposits, on average, are larger than Noranda-type deposits and they average
1% copper, 8% zinc, 1% lead, 3.5oz/t of Ag, and 0.07oz/t of Au.
Examples incluse the Sturgeon Lake mining camp which includes the Mattabi deposit,
Que River, Selbaie, and the Onaman River.
Bathurst-Rio Tinto-Type
These VMS deposits are similar to Mattabi in that they have abundant felsic fragmental
rocks in the stratigraphy but they differ by the presence of volcanically derived sedimentary
rocks including wackes, siltstones, iron formation, and argillites. These sedimentary rocks
represent either caldera fill (in the hanging wall) or basin fill (in the footwall). Like Matabi
sediments also occur intermixed with the felsic volcanic and represent material derived from
basin or caldera walls.
Fragmental felsic rocks (pyroclastic flows or debris flows) may host the ore but so do
domes and lavas that mark the end of the felsic volcanic succession. Similar to Mattabi in grade
they tend to be much larger deposits.
Kuroko-Kidd-Type
In these kinds of VMS deposits the hanging wall and footwall rocks are dominantly felsic
(>70%) but they differ from the previous two types in that the felsic volcanic rocks are domes
and lava flows with associated block and ash flows. Volcanic sedimentary rocks occur
throughout the succession and in places (Kidd Creek) ultramafic lava flows occur in the footwall.
These deposits vary from small (< 1 million tons) to giant (>100 million tons) with
grades that average 1.5% copper, 1% lead, variable zinc (3-7%), 3.5 oz/t silver, 12% barium for
49
non Archean deposits. Examples include the Kuroko deposits in Japan, Kidd Creek, and Myojin
Knoll.
Hydrothermal Alteration Zones:
Zones of hydrothermally altered rock are associated with all types of VMS
deposits. These zones are petrologically distinct and they either cross-cut or are semiconformable with the volcanic stratigraphy. The alteration is produced by reaction of a hot
(metal-rich) fluid with the volcanic or volcanically derived wall rocks.
Hydrothermal Alteration and Zoning Patterns
The alteration zones around and below VMS deposits are defined by the alteration
minerals present, their chemistry, and their distribution. These vary considerably but of
importance is that they can be classified according to 1) the VMS deposit type they are
associated with, and 2) the volatile fragmentation depth. In other words the style of volcanism
and the associated depth of water are important controls on the alteration minerals and their
distribution.
The volatile fragmentation depth (VFD) is defined as the water depth at which volatiles
will exsolve from a magma rapidly enough to tear the magma apart (pyroclastic eruption), or
rapidly enough to allow external water to “trigger” explosive disruption (hydromagmatic). The
VFD differs for the different kinds ov VMS deposits.
VFD and VMS:
Noranda and Cyprus-types:
These kinds of VMS deposits form below the VFD and thus there is little or no products
of explosive volcanism in the stratigraphy; ie they form in relatively deep water. The stratigraphy
is thus dominated by lava flows and domes.
Thus porosity and permeability of the rocks is low leading to alteration zones that are
narrow, straight, and well confined. These deposit types are found in dominantly mafic volcanic
stratigraphy, and this creates a distinct mineralogical and chemical alteration signatures.
Mattabi-Bathurst-types:
These VMS deposits are hosted by rocks that formed above the VFD and thus the
stratigraphy is dominated by both explosively formed fragmental rocks and clast-bearing
sedimentary rocks formed by erosion and mass wasting of the explosive volcanic products; ie
shallow water and lots of explosive volcanism.
Thus permeability and porosity are high, and this creates alteration zones that are much
more wide-spread, irregular, and poorly defined. These deposit types occur in dominantly felsic
volcanic stratigraphy leading to distinctive alteration assemblages.
50
Kuroko-Kidd-type:
These kind of VMS deposits for both above and below the VFD, ie in deep and shallow
water. Overall the volcanic stratigraphy is composed largely of felsic lavas and domes and thus
porosity and permeability are relatively low. Thus alteration zones are narrow, confined, and
straight.
VMS and Hydrothermal Alteration
As mentioned above, VMS deposits have extensive zones of hydrothermally altered
volcanic and sedimentary rocks (derived from erosion of the volcanic material). These zones
vary in the kinds of minerals present, their geometric shape, and their size. These variables are a
direct reflection of the environment of formation of the volcanic rocks as well as the source and
nature of the hydrothermal fluids. Overall alteration is a great exploration tool.
Noranda-type (Bimodal Mafic):
The alteration associated with Noranda-type deposits can be subdivided into pipe-like
and semi-conformable.
Pipe-like Alteration:
•
Well defined and confined pipe-shaped alteration zone, which is mineralogically and
chemically zoned. This alteration type cross-cuts stratigraphy, varies from 100-400 m
wide and 300-1500m deep; it narrows with depth and at the top is slightly wider that the
VMS deposit. The alteration is controlled by synvolcanic fault structures.
•
Within the alteration pipe there is a inner iron chlorite-quartz +/-clay zone that gives way
outwards to a middle zone of magnesium chlorite-quartz-sericite-clay minerals, which
grades into an outer zone of sericite-quartz-clays (kaolinite-montmorillonite). Pods of
talc may be associated with the middle zone. The stringer ore occurs in the inner zone
and pyrite can be found throughout all the alteration zones.
Semi-Conformable Alteration:
This alteration type occurs as regionally extensive areas of altered rock up to 10’s of km’s long
and 1-3 kms deep, it may extend down to the sub-volcanic intrusion and up to the paleo-seafloor.
Semi-conformable alteration may be difficult to recognize because:
1) It is mineralogically similar to zeolite, greenschist, and possibly amphibolite grade
metamorphic assemblages. It may be recognizable or distinguished based on the
abundance of the minerals present and their distribution. That is a greenschist grade
basalt should not contain 70% actinolite or epidote and have this concentrated only in
permeable horizons such as amygdaloidal flow tops and hyaloclastites.
2) The alteration is patchy-it comes and goes and is most intense along the most permeable
rocks and horizons.
Mineral assemblages include:
A) Zeolite Alteration-rare in older rocks, common on modern seafloor
51
B) Spilitization- similar to a greenschist grade mineral assemblage of chlorite, albite,
actinolite/tremolite, quartz, epidote
C) Silicification- quartz-albite-most prominent in mafic rocks
D) Epidote - epidite-quartz-actinolite
E) Carbonate-ankerite, ferrigious dolomite
Silicification- This is a common alteration type in the semi-conformable zone and represents an
increase in the SiO2 content of the rocks. It is most pronounced along permeable rock horizons
like fractures, faults, pillow selvages, etc. It forms by either:
a. Addition of SiO2 to the rock-that is the hydrothermal fluid was saturated in silica.
b. Leaching of Ca-Na-K-Mg out of the volcanic roks leaving excess Si and Al
Spillitization- this commonly occurs below the silicified zone and consists of epidote,
tremolite/actinolite, and albite. This alteration is patchy.
Epidote-Quartz (may form rocks called epidosites)- this is also similar to regional greenschist
metamorphism. Composed of epidote and quartz (pale green rocks) the intensity of this alteration
assemblage increases adjacent to synvolcanic faults and synvolcanic intrusions. This alteration
type is believed to define higher temperature reaction zones in mafic volcanic rocks. It represents
an increase in SiO2, CaO, and Na2O, and a decrease of K and some Fe, Cu, and Zn
Kuroko-Kidd Type (bimodal felsic)
Alteration assemblages associated with these deposit types are the mirror image of the
Noranda-Cyprus type with the difference due to the dominance of felsic over mafic rocks (more
K, Si, Na; less Fe and Mg). Thus there is a well defined and constrained alteration pipe with an
inner zone composed of sericite-quartz-clays-iron chlorite which grades out into a zone of ironmagnesium chlorite-quartz-clays. This then gives way outwards to albite-clay alteration. The
stringer ore is associated with the sericite zone and the size of the pipe is similar to the NorandaCyprus type.
The semi-conformable alteration is less studied than that of the Noranda-type but appears to be
similar.
Mattabi-type:
The alteration associated with Mattabi-type deposits consists of a poorly defined and
irregular (Christmas tree shape) alteration pipe that forms along synvolcanic faults and then
grades upwards into “cloud-like” zones of alteration, which tend to host the VMS deposit. The
alteration minerals are dominantly aluminum silicates-quartz-and iron-rich carbonates and/or
iron clays, chlorite and/or chloritoid. This alteration assemblage is similar to those associated
with high-sulfidation” epithermal gold deposits.
The pipe-like alteration is composed of an inner zone of andalusite-quartz-+/- iron
carbonate (siderite) and iron-rich clay minerals, and this grades out into a chloritoid (iron clays)phyrophillite-ankerite zone. The cloud-like alteration zones develop in permeable units
(pyroclastic flows, debris flows) and consist of zones of intense aluminum silicate alteration
52
(andalusite-quartz-kyanite-pyrite) which are then surrounded by zones of iron carbonatephyrophyllite-chloritoid alteration. The chloritoid forms in rocks that have undergone extreme
leaching of sodium and calcium and then have been subjected to greenschist grade
metamorphism; thus in older deposits iron-rich clay minerals will be absent.
Semi-conformable alteration zones consist of:
1) An upper silicified zone which is widespread and well developed.
2) This is underlain by a an iron-chlorite-chloritoid-iron-carbonate-sericite zone which may
extend down to the synvolcanic intrusion..
Bathhurst-Rio Tinto-type:
This is similar to a low sulfidation epithermal gold deposit. There is a poorly defined
alteration pipe that can be up to 1.5km wide that is subdivided into:
1) an upper zone of quartz-fe chlorie
2) an lower zone of sericite-fe chlorite which hosts the stringer ore.
3) marginal to these is a zone of iron-magnesium chlorite and sericite, which gives way to a
sericite-magnesium chlorite zone.
Hanging wall alteration:
In long-lived hydrothermal systems alteration assemblages may form in the hanging wall rocks
to the ore deposit. This results in alteration that simply ends in the hanging wall rocks or in
stacked VMS ore bodies. If the alteration system is waning (ie after ore formation is cooling
down) then the alteration assemblages will differ from those in the footwall rocks. In basalts can
get increases in the rock of Ni, Cr., V, Ag, As, Mo, CaO, Na2 O, Mgo. This hanging wall
alteration is not well documented and is really only noticeable in well constrained alteration
pipes; would be hard to pick out with the more-replacement type VMS deposits.
Origins:
Noranda-Cyprus-Type
Associated with large geothermal systems with the current model having the metals
leached out of basalts-diabase-gabbros of the oceanic crust. The geothermsl system is driven by
sub-volcanic intrusions and the heated fluid rises to the surface along syn-volcanic faults. This
leads to many small deposits. Thus the hydrothermal fluid is dominantly modified sea-water with
little magmatic input. However with the Noranda-type deposits hosted by felsic volcanic rocks
the possibility of a magmatic-hydrothermal fluid mixing with seawater is quite possible.
Origins-Kuroko-Kidd-Bathurst
Associated with a large, circulating geothermal system. The hydrothermal fluid is both
magmatic and sea water with the magmatic component thoroughly mixing with the seawater in
the geothermal system; thus the “low sulfidation” element and alteration assemblages..
53
Origins-Mattabi-type:
Associated with a large, circulating geothermal system. The ore fluid is magmatic and
there is little or no mixing with the sea-water system; thus the “high sulfidation” alteration
and element assemblages. It should be noted here that either the Mattabi or Bathurst-types
could show similar alteration assemblages depending on the amount of reaction of the
magmatic-hydrothermal fluid with seawater.
Summary
54
References
Barrie, C.T., and Hannington, M.D., eds., 1999, Volcanic-Associated Massive Sulfide Deposits:
Processes and Examples in Modern and Ancient Settings: Reviews in Economic Geology, v. 8,
Society of Economic Geologists, p. 297-324.
Franklin, J.M., Gibson, H.L., Jonasson, I.R., and Galley, A.G., 2005, Volcanogenic Massive
Sulfide Deposits, in: Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., and Richards, J.P.,
eds., Economi Geology 100th Anniversary Volume: The Economic Geology Publishing
Company, p. 523-560.
Galley, A.G., Hannington, M.D., and Jonasson, I.R., 2007, Volcanogenic massive sulphide
deposits, in: Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of Major DepositTypes, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods:
Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, p. 141161.
Gibson, H.L., Jonasson, I.R., and Galley, A.G., 2005, Volcanogenic Massive Sulfide Deposits, in
Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., and Richards, J.P., eds., Economi Geology
100th Anniversary Volume: The Economic Geology Publishing Company, p. 523-560.
Gibson, H.L., Morton, R.L., and Hudak, G.J., 1999, Submarine Volcanic Processes, Deposits,
and Environments Favorable for the Location of Volcanic-associated massive sulfide deposits,
Reviews in Economic Geology, V.8, pp 13-51.
Goodfellow,W.D., McCutcheon, S.R., and Peter, J.M., eds., 2003, Massive Sulfide
Deposits of the Bathurst Mining Camp, New Brunswick, and northern Maine: Economic
Geology Monograph 11, Society of Economic Geologists
Hannington, M.D., and Barrie, C.T., eds., 1999, The Giant Kidd Creek Volcanogenic Massive
Sulfide Deposit, Western Abitibi Subprovince, Canada: Economic Geology Monograph 10,
Large, R.R., 1992, Australian volcanic-hosted massive sulphide deposits:features, styles and
genetic models: Economic Geology, v. 87, p. 471-510.
Santaguida, F., Gibson, H.L., Watkinson, D.H., and Hannington, M.D.,1998, Semi-conformable
epidote-quartz hydrothermal alteration in the Central Noranda Volcanic Complex: Relationship
to volcanic activityand VMS mineralization: Canadian Minerals Research Organization Project
94E07, Annual Report, The Use of Regional-Scale Alteration and Subvolcanic Intrusions in the
Exploration for Volcanic-Associated Massive Sulphide Deposits, p. 139-180 Franklin, J.M.,
Sillitoe, R.H., Hannington, M.D., and Thompson, J.F.H., 1996, High sulfidation
deposits in the volcanogenic massive sulfide environment: Economic Geology, v. 91, p. 204-212.
Spry, P.G., Marshall, B., and Vokes, F.M., eds., 2000, Metamorphosed and Metamorphogenic
Ore Deposits: Reviews in Economic Geology, v. 11,
55
Stix, J., et. Al., 2003, Caldera-Forming Processes and the origin of Submarine Volcanogenic
Massive Sulfide Deposits, Geology, V. 31, pp. 375-378
Tucker, B.C., and Hannington, M.D., eds., Volcanic-Associated Massive Sulfide Deposits:
Processes and Examples in Modern and Ancient Settings: Reviews in Economic Geology, v. 8
56
Orogenic (Mesozonal) Gold Deposits
Definition:
Epigenetic, structurally hosted lode gold deposits in metamorphic rocks that formed
predominantly at depths between 5 and 12 km’s, and temperatures of 300 to 4250 C (called
mesozonal). However, there are some orogenic lode gold deposits that formed at higher
temperatures (>4250C) and depths more than 12 kms; these deposits are refered to as hypozonal.
The majority of mesozonal lode gold deposits are Archean in age and hosted in greenstone belts
by metamorphosed and deformed rock sequences. Deposits are therefore structurally controlled
and consist dominantly of veins, but disseminated ores do occur. The vein deposits consist of
simple to complex networks of gold bearing, laminated, quartz-carbonate fault fill veins in
moderately to steeply dipping, compressional, brittle to ductle shear zones and faults. There are
locally associated extensional veins and hydrothermal breccias. The ores are dominantly hosted
by mafic metamorphic rocks of greenschist to lower amphibolite grade. The relative timing of
the mineralization is syn-late deformation, and typically post peak greenschist or syn-peak
amphibolite facies metamorphism.
Some orogenic lode gold deposits are related to intrusions that formed late in the orogenic
history of the greenstone belt, typically post deformational. The gold is hosted by quartzcarbonate veins that cut the intrusion, gold may also be disseminated in the adjacent country
rocks.
Grade and Tonnage
There are about 100 world class mesozonal gold deposits (> 2.5 million ounces or 70 tons
of gold). Some of the better known deposits are the Golden Mile in Australia, the Dome,
Hollinger, Cambell-Red Lake, and Kirkland Lake in Canada, and Homestake in the United
States. The Superior Province contains the most mesozonal deposits followed by the Yilgarn
Craton in Australia. The concentration of these deposits in Archean rocks is thought to be due to:
1) Continental growth and a large number of collisions between continental fragments or
arc complexes.
2) The associated development of major faults and large scale fluid flow along these
structures.
Greenstone-hosted mesozonal gold deposits are second only to Witwatersrand
paleoplacer deposits of South Africa in total gold tonnage. The largest greenstone-hosted deposit
in terms of total gold content is the Golden Mile in Kalgoorlie, Australia, which contains more
than 60 million ounces (1,800 tonnes). The Hollinger-McIntyre deposit in Timmins, Ontario is
the second largest with more than 30 million ounces (1000 tonnes). Gold grades in these deposits
range from 5-15 g/t; tonnage is highly variable ranging from a few 1000 to over 100 million
tons; the Canadian average is 20 million tonnes.
Distribution:
Mesozonal gold deposits have a direct association with Precambrian shield areas of the
world. More than 50% of the entire Precambrian gold production and reserves is concentrated in
Late Archean greenstone belts of the Superior and Yilgarn cratons. The best future potential is in
57
Africa, which has a very large, exposed and relatively unexplored Archean craton. China also
has numerous Archean terrains but these are mostly at high metamorphic grades so gold potential
is uncertain. Most of the Archean deposits occur in greenschist facies rocks and/or at the
greenschist/amphibolite transition. As well most of the known deposits formed during two
distinct time intervals:
1) 2.8 to 2.6 BY
2) 2.1 to 1.8 by
The prevalence for these two time periods is believed to be due to continental and/or
island arc collisions along with the breakup of the first super continent. Younger deposits (650 to
50 my) are due to the breakup of Rodinia and active collisional events on the margins of
Gondwana and Laurentia. The oldest known mesozonal gold deposits occur 1) in the Barberton
Land Greenstone belt in Africa (3.1 by) and 2) in the Pilbara Craton in Australia (3.4 by).
Ore Minerals:
The ore minerals are native gold (typically micron size), gold tellurides and native silver
with gold to silver ratios of 5 to 10 to 1. Sulfide minerals are variable ranging from 3-10% with
pyrite and/or phyrrhotite the most common ones in greenschist grade volcanic hosted deposits.
Arsenopyrite is the dominant sulfide mineral in deposits hosted by metasediments and in
amphibolite grade rocks. Small amounts of molybdenite occur in some deposits and scheelite
(W) is present in intrusion and some metasediment hosted deposits.
Typical minerals are native gold, native silver, electrum, arsenopyrite, pyrite, pyrrhotite,
tourmaline, molybdenite, scheelite; base metal contents are generally low.
Gangue and/or alteration minerals in the deposits are dominantly quartz and carbonate (ankerite,
siderite, calcite, and dolomite) with varying amounts of sericite, muscovite, chlorite, biotite,
fuchsite, talc, tourmaline, magnetite, and hematite; mineral assemblages depend on the
composition of the host rocks and metamorphic grade.
The gold is commonly associated with either 1) pyrite and/or pyrrhotite, and 2) quartz-carbonate
veins. More rarely it occurs as disseminations in alteration halos surrounding barren to low grade
gold quartz and carbonate veins. Veins often occur as stockworks and quartz-carbonate with or
without gold can be a cement in hydrothermal breccias.
Host Rocks:
Most orogenic, mesozonal and hypozonal gold deposits occur in greenstone belts as veins or
disseminations and can be hosted by a wide variety of rock types. Host rocks can include mafic
to ultramafic lava flows, felsic volcanic rocks, iron-rich gabbroic sills and dykes, granitic
intrusions, iron formations, and clastic sedimentary rocks. Numerous deposits are hosted by
more than one lithology.
Structural Environment:
Virtually all of the mesozonal gold deposits are linked to large scale fault zones though the ores
themselves are not directly hosted by these structures. These large-scale faults (such as the
58
Lardner Lake-Cadillac break or the Boulder-Lefore Shear [Golden Mile], and Porcupine –
Destor) are typically several 100’s of km’s long and a few meters to a few hundred meters in
width. Many are not single faults but segmented structures that exhibit multiple deformation
events. They tend to be parallel or subparallel to the volcanic-sedimentary stratigraphy in the
greenstone belts, and to accreted terrain margins in Phanerozoic settings where they mark
collision zones.
The faults are near vertical and believed to represent major crustal dewatering features. They
appear to have a relatively complex, long lived history, which commonly begins as crustal
shortening and high angle reverse motion (basin forming). This, with time and continued
tectonism, changes to major strike-slip motion.
Stratigraphic rotation and truncation are prominent features at the regional scale and “false
bedding” may be created along the plane of the shear. Strain intensity is not uniform across the
deformation zone so can end up with discrete zones of more intense shearing (often focused
along less competent units) which results in a pattern of anastamosing sears enclosing relatively
undeformed blocks.
Fluid migration accompanies shearing so there is most likely hydrothermal water along the entire
deformation zone. However, to form a gold deposit, what is needed is focused upflow and such
focused regimes are associated with specific structures within the major shear zone.
Within the major shear, lithologic differences allow the development of numerous flexures. That
is shearing and accompanying faulting are not straight. Faults curve or bend and it’s at the bends,
curves or flexures where local extensional or dilation zones (pull apart features) form; these are
referred to as 2nd and 3rd order faults and are where the majority of the gold deposits occur.
Good sites for gold mineralization associated with these structures would be 1) jogs in the shear,
2)changes in strike or bifurcation of the 1st order systems, 3)marked competency contrasts in the
rocks, and 4)regional fault intersections.
On a more local scale most of the gold deposits are composed of one or more ore zones whose
shape and orientations are related to small scale structures within the deformation zone and/or
within the more local flexure (2nd and 3rd order faults). Ore zones are typically rod-like or
tabular, with the maximum dimension being parallel to the shear plane. The deposits are
composed of either:
1) Large quartz-carbonate veins with gold and associated breccia zones
2) Stockworks of gold and quartz-carbonate
3) Disseminated in sulfide-rich rocks with or without quartz-carbonate veining.
The shape and style of the gold-bearing zones, as well as the morphology of the dilation zones, is
dependent on the following variables:
1)
2)
3)
4)
Strain rate
Fluid pressure
Temperature plus lithostatic pressure
Host rock lithology
59
Ambient pressure and temperature along with lithology appear to be the most important as this
determines whether deformation in response to shearing will be brittle, ductile or both. This is
important because it means there will be a relationship between style of mineralization and
metamorphic grade of the wall rocks.
In lower greenschist grade rocks brittle structures (veins, breccias) host gold mineralization. In
middle to upper greenschst grade where there is both brittle and ductile deformation, the gold is
in stockeworks of veins. In amphibolite grade, where the deformation is ductile, the
mineralization tends to be parallel to the foliation with little or no veining-here can get
disseminated and replacement gold.
Host rock lithology-ie. competence of the rock-is also an important variable because maximum
dilation occurs in rocks of the highest competency such as felsic intrusions.
The gold deposits are also commonly associate with Timiskaming-like regional unconformities
which form during the tectonic event.
Alteration:
All mesozonal gold deposits are accompanied by intense and extensive wall-rock alteration. The
most common types of alteration are silicification, carbonitization, potash enrichment, and
sulfidation. In the larger deposits the alteration can extend away from the ore for more than 2km.
Silicification: This consists dominantly of vein quartz and, much less obvious, fine-grained
quartz in the wall rocks. This is a more local or close to deposit alteration.
Carbonitization: This includes such minerals as siderite, ankerite, and ferruginous dolomite.
These carbonate minerals occur as veins with quartz and as disseminations in the wall rocks. The
carbonate alteration occurs on a regional scale but as the ore deposit is approached the carbonate
minerals tend to become more iron-rich.
Sulfidation: This occurs in the wall rocks immediately adjacent to the main ore zone and/or
forms narrow zones which host gold mineralization. The minerals present are pyrite and/or
pyrrhotite with base metals contents very low.
Potassium Enrichment: This is dominantly sericite that forms within the deformation zones.
Sericite replaces feldspar, which is destroyed during shearing. In amphibolites grade rocks biotite
and muscovite take the place of sericite.
Overall the extent and intensity of the alteration is a function of the degree of interaction between
the hydrothermal fluid and the wall rocks, as well as the temperature and pressure. Also the
development of abundant sericite in the wallrocks can result in strain softening, which promotes
ductile responses; the formation of carbonate, on the other hand, increases rock competency and
can lead to brittle deformation in originally low competence units such as chlorite-rich basalts.
Proximal alteration at greenschist grade is characterized by iron carbonate, quartz, sericite, and
pyrite. Further away from the ore veins and/or disseminations the carbonate is less iron rich and
chlorite may be present as well as magnetite. In ultramafic rocks green mica and talc may also
be common.
60
In amphibolite grade rocks biotite, amphiboles, and arsenopyrite are common and at even high
grades biotite, diopside, garnet, actinolite, staurolite, and andalusite may be present.
Minor Elements:
Arsenic, as arsenopyrite, can be directly associated with the ore. In some deposits as the % of
arsenopyrite increases so does the gold grade (Porcupine)
Stibnite, tourmaline, molybdenite, and scheelite are anomalous close to many gold deposits.
However this enrichment is irregular and not always present. These particular minerals are most
commonly found where the host rocks are quartz-rich (felsic intrusions and volcanics).
Chromium, vanadium, and barium green micas are associated with a number of deposits,
especially those hosted by ultramafic rocks; barium occurs as barite and is enriched only at
Hemlo.
Metamorphic Grade and Orogenic Gold Deposits
The spatial association of mesozonal gold deposits with greenschist grade rocks within
greenstone belts has been well known for decades. Such a setting is an indication of the ambient
temperature-pressure conditions within mid-crustal environments where gold deposition is taking
place. The specific reasons for this are uncertain, but ideas include 1) the fact that large fluid
volumes are created during the change from greenschist to amphibolite grade metamorphism and
this fluid is released upwards into the greenschist zone, 2) the structurally favorable brittleductile zone lies just above this transition zone, 3) fluid focusing is most likely to occur as fluids
ascend into the greenschist pressure regimes and 4) gold solubility shows a dramatic drop under
greenschist temperatures. Also brittle fractures form more easily in greenschist grade rocks.
Depth
Metgrade
Def
Ore Morph
FeSul Carbonate
Shallow
Sub-greenschist
Brittle
Breccia
Py
Mod
Greenschist
Brittle/duct
Veins, bx
Py/po Ank
Deep
Amphibolite
Ductle
Foliation parallel
Po
Ank, Fe-Dol
None
Deposit Types
1) Quartz-carbonate-gold veins with the host rocks ranging from basalt and komatiite to
quartz-feldspar porphyries; lesser graywacke and siltstone; greenschist grade, regional
carbonitization-examples are Dome, Kerr Addison, and Hollinger.
2) Intrusion hosted-quartz-carbonate-tourmaline veins in granitic rocks, ore controlled by
fractures and faults in competent units, Kirkland lake
3) Iron-formation hosted-quartz-pyrite-arsenopyrite-gold. The BIF is highly folded and
transposed, ore typically in fold hinges, Examples are Homestake, Musselwhite, Lupin,
Beardmore
4) Disseminated gold in wall rocks, tabular orebodies, amphibolite grade, Hemlo
Ore fluids
61
Based on fluid inclusions in the ore and alteration minerals the ore forming fluids are
believed to be rich in CO2 and have NaCl contents of 3-12 %; this low salinity may be why there
are so few base metals with these deposits.
Genesis:
The gold is deposited in structurally prepared sites from hydrothermal fluids raising along
the shear system. There is no consensus as to the source of the gold. The hydrothermal fluids are
dominantly metamorphic, though they could have a magmatic component to them. Fluids are
believed to be derived from dewatering at the greenschist-amphibolite grade boundary
(hornblende to pyroxene, etc.) The fluids then rise along the shear structures. As pressures and
temperatures decrease so does gold solubility and precipitation occurs in dilatent structures.
Relationships to magmas is not certain though intrusive rocks ranging from lamprophyres to
porpyritic intrusions are common in the vicinity of the deposits and do host some deposits. The
magmas are late tectonic, often coeval with gold formation, produced from melting deep on the
crust and therefore have high variability in composition. Thus gold and melts seem to be
controlled by the same deep crustal thermal event.
It is believed there is a connection between the fluids in mesozonal zones and epizonal mercuryantimony deposits common in the less eroded parts of orogenic belts. Thus there could be a
continuum in the upper crust from mercury-antimony to gold-rich zones with increasing depth.
This possible zoning reflects temperature controls on the mineralization with gold no longer
soluble below 2500C and antimony at about 1750C; mercury is a volatile and carried upward to
the shallowest depths. Fluid inclusions show the fluids responsible for mercury and antimony
deposits in orogenic environments are enriched in CO2 and O18 making them much different than
magmatic epithermal gold fluids.
References
Bateman, R.J., Ayer, J.A., Dubé, B., and Hamilton, M.A., 2005, The Timmins-Porcupine Gold
Camp, Northern Ontario: The Anatomy of an Archaean Greenstone and Its Gold Mineralization,
Discover Abitibi Initiative: Ontario Geological Survey, Open File Report 6158, 90 p.
Dubé, B., and Gosselin, P., 2007, Greenstone-hosted quartz-carbonate vein deposits, in:
Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of Major Deposit-Types,
District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods:
Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, p. 4973.
Goldfarb, R.J., Baker, T., Dubé, B., Groves, D.I., Hart, C.J.R., Robert, F., and Gosselin, P., 2005,
World distribution, productivity, character, and genesis of gold deposits in metamorphic
terranes, in: Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., and Richards, J.P., eds.,
Economic Geology One Hundredth Anniversary Volume: 1905 - 2005: Society of Economic
Geologists, p. 407-450
62
Groves, D.J., Goldfarb, R.J., Gebre-Mariam, M., Hagemann, S.G., and Robert, F., 1998,
Orogenic gold deposits: A proposed classification in the context of their crustal distribution and
relationships to other gold deposit types: Ore Geology Reviews, v. 13, p. 7-27.
Groves, D.I., Goldfarb, R.J., Robert, F., and Hart, C.J.R., 2003, Gold deposits in metamorphic
belts: Overview of current understanding, outstanding problems, future research, and
exploration significance: Economic Geology, v. 98, p. 1-29.
Hagemann, S.G., and Cassidy, K.F., 2000, Archean orogenic lode gold deposits, in: Hagemann,
S.G., and Brown, P.E., eds., Gold in 2000: Society of Economic Geologists, Reviews in
Economic Geology, v. 13, p. 9-68.
Hodgson, C.J.,1993, Mesothermal lode-gold deposits, in: Kirkham, R.V., Sinclair, W.D., Thorpe,
R.I., and Duke, J.M., eds., Mineral Deposits Modelling: Geological Association of Canada,
Special Paper 40, p. 635-678.
Poulsen, 1995, Disseminated and replacement gold, in: Eckstrand, O.R., Sinclair, W.D.T., and
Thorpe, R.I., eds., Geology of Canadian Mineral Deposit Types: Geological Survey of Canada,
Geology of Canada No. 8, p. 383-392.
Poulsen, K.H., Robert, F., and Dubé, B., 2000, Geological Classification of Canadian Gold
Deposits: Geological Survey of Canada, Bulletin 540, 106 p.
Sibson, R.H., 1990, Faulting and fluid flow, in Nesbitt, B.E., ed., Short Course on Fluids in
Tectonically Active Regimes of the Continental Crust: Mineralogical Association of Canada,
Short Course Handbook, v. 18, p. 93-132.
Sibson, R.H., Robert, F., and Poulsen, K.H., 1988, High-angle reverse faults, fluid-pressure
cycling, and mesothermal gold-quartz deposits: Geology, v. 16, p. 551-555.
Solomon, M., Groves, D.I., and Jaques, A.L., 2000, The lode gold deposits of the Western
Australian Shield, Chapter 5, in: Solomon, M., and Groves, D.I., eds., The Geology and Origin
of Australia's Mineral Deposits: Hobart: Centre for Ore Deposit Research, University of
Tasmania, Oxford Monographs on Geology and Geophysics, No. 24, p. 54-84.
63
Mississippi Valley-Type Deposits
Definition:
Mississippi Valley Type (MVT) ore deposits are part of a spectrum of deposits that form during
the evolution of a sedimentary basin. They are defined as ores of the minerals galena and
sphalerite with associated barite and fluorite. The ores are stratabound and largely confined to
carbonate beds. MVT deposits are found throughout the world but are most abundant in Europe
and North America. Their name comes from several “classic” districts that are located in the
drainage basin of the Mississippi River. Some of the better known and studied districts are:
a)
b)
c)
d)
e)
Tri-state district-southwest Missouri-Oklahoma-Kansas
Central and eastern Tennessee
Wisconsin area of the Upper Mississippi
Pine Point and Polaris-NWT
English Pennies, Ireland, and the Eastern Alps.
Deposits within a district have similar geological and geochemical attributes and ore controls;
whereas individual districts can be quite distinct from one another. This diversity is illustrated by
deposits in the central and southern USA: those in Tennessee (Vibernam Trend) are lead-rich,
those in the Tri-state are zinc-rich, and that in southeast Missouri are barium-rich. The diversity
between ore districts is not only expressed in the dominant ore mineral, but also in overall ore
assemblages, alteration minerals, host rocks, and ore controls. The diversity is believed to be due
to a range of depositional processes, paleo-hydrological controls on the ore fluids, and
hydrothermal fluid/rock interactions.
Minerals:
Most MVT deposits have a simple mineralogy consisting primarily of sulfide minerals that are
dominated by sphalerite (commonly yellow to light brown due to low iron content), galena (low
in silver), pyrite, and marcasite. Barite may be present in minor amounts though in a few
deposits it is the dominant mineral (Central Missouri). Fluorite is also a common but minor
constituent. Some deposits also contain a complex assemblage of carbonate, sulfide and sulfosalt
copper, zinc, lead, cobalt, silver, and antimony minerals; cadmium is also a common constituent
and is associated with sphalerite. The ore minerals and gangue occur in several different ways:
1) Fine-grained banded, strataform ore (banding on the cm-micron scale) that occurs as beds
that extend from inches to several 10’s of feet, banding can also occur within individual
crystals.
2) Replacement and open space filling: replacement of carbonate beds can produce massive
sulfides over short distances. Most common is open space fillings which vary from the
lining to the filling of cavities. Here the ore minerals are commonly colloform and
dendritic. These form spheroidal aggregates of colloform sphalerite, fiberous aggregates
and crystals of galena and sphalerite, dendritic galena, and bands as well as boytroidal
masses of iron sulfides. In places where the cavities are not completely filled (karst
features), crystals of large size and great perfection can form. For example calcite 2/3 of a
meter long, galena cubes that are 1/3 of a meter on a side, dodecahedral sphalerite that is
64
1/3 m across, and 2 meter long gypsum crystals. Indeed some VMT deposits are more
valuable as a source of crystals than as a resource of zinc and lead.
3) Cementing material of breccias, which include landslide, talus, dissolution, and collapse
(Karsts).
4) Special forms such as “zebra texture ore” which is the partial replacement of galena
and/or sphalerite by dolomite, snow on the roof, where sulfide minerals coat the tops of
crystals or breccia clasts in open spaces;, and finally “sulfide speleotherms” such as
dripstones, stalagmites, stalactites, and curtains.
Grade and Tonnages:
MVT deposits are distributed over hundreds of square km’s that define individual ore districts.
Large districts include southeast Missouri (3,000km2), Tri-State (1,800km2), Irish Midlands
(8,000 km2), and the Upper Mississippi Valley (8000km2). Pine Point contains more than 80
individual deposits, and the Upper Mississippi Valley more than 400.
In the Pine Point District most deposits contain between 0.2 to 2 million tons, the largest has 18
million tons. The average deposit size in the Upper Mississippi Valley was between 0.1 and
0.5mt. Production data are commonly presented only on a district scale so grade and tonnage of
individual deposits is hard to find. At Pine Point the ore averages 2.9% lead and 6.5 % zinc.
Ages:
The age of MVT deposits varies from Cambrian to Cretaceous though Cambrian-Carboniferous
and Traissic to Cretaceous are where more than 95% of the deposits occur. There are few in the
Silurian-Devonian and even though carbonate rocks are abundant in Precambrian terrains there
are few MVT deposits.
Host Rocks and Regional Setting:
Host rocks are limestone and dolstone with most deposits found in dolstone. It is believed this
preference for dolstone is due to the fact it is more permeable then limestone. In general the
carbonate rocks form parts of reef complexes, with individual ore deposits occurring either along
belts conforming to ancient elongated reef complexes or as local groupings that conform to
irregular reefs or banks. The distribution of the reefs and associated carbonate rocks is related to:
A) Old shore lines
B) Bottom topography
C) Climate.
So given a climate suitable for reef formation, the distribution of the reefs is then controlled by
paleogegraphy. Imposed on this is the observation that most deposits are located in reefs not far
from major geological faults; the faults are due to compressional tectonic events, especially those
associated with foreland basins related to collisional and or subduction tectonics. For example
Ordovician normal faults related to continental collision (Taconic) are associated with the
deposits in Newfoundland.
65
Local Setting:
1) Above unconformities in carbonate environments such as reefs and facies changes-ie. a
carbonate reef to shale.
2) Straigraphic pinchouts
3) Talus or landslide breccias (carbonate)
4) In solution collapse breccias (karst)
5) Drape structures
6) At a facies change
Alteration:
Hydrothermal alteration associated with the carbonate rocks that host the MVT deposits is
typically represented by:
1) Dissolution of carbonate rocks which included hydrothermal brecciation and dissolving
of the rocks by acidic fluids.
2) Dolotimization-hydrothermal dolomite, which replaces the host limestone or occurs as
cement in open space areas such as breccia zones or karst areas. Dolotimization increases
the rocks porosity.
3) Silicification-this is a minor alteration type and its intensity is dependent on the
temperature of the hydrothermal fluids along with the amount of cooling that occurred
during ore formation. Silicification is greater the hotter the system (>2000C)
Tectonic setting:
The most important period of MVT formation was from the Devonian to the Permian, which
corresponds to a series of tectonic events that occurred during the formation of Pangaea. The
second most common time period for these deposits was the Cretaceous to the Tertiary and is
associated with plate tectonic activity along North Americas east coast. There is thus a direct
correlation between MVT deposits and orogenic forelands; the type of foreland does not appear
to be important as they are found in collsional, Andean, and transpressional orogens. Some
deposits are also associated with fold and thrust belts –ie. formed in flat-laying rocks later caught
up in folding and thrusting.
Origin of Fluids and Ores:
The following features are pertinent to the origin of MVT deposits:
1) The ore minerals are epigenetic.
2) Fluid inclusion data from the ore zones show a temperature range of 80 to 2500C. The
salinities of the fluids range from 10-30% NaCl and are similar to oil field brines. The
high salinities can be explained by dissolution of evaporates, incorporation of connate
brines during diagenesis, or the incorporation of evaporated surface waters.
3) For dome deposits fluid inclusions show distinct mixing trends between 2 different
fluids-one saline, one not so saline.
4) Organic material, such as bitumen, kerogen, or hydrocarbons are commonly observed in
the ore deposits.
66
5) Sulfur isotopes show a variety of sources-evaporite deposits, connate seawater, diagenetic
sulfides, sulfide-bearing organic material, and H2S reservoir gas. Most common source
appears to be seawater sulfate that occurs in connate waters trapped in sediments or
minerals.
6) Association with foreland basins and, within those basins, the deposits tend to occur
along major faults that formed or were reactivated during compressional tectonic events.
7) In any one area sulfide minerals tend to favor particular facies and horizons (collapse
breccias, paleokarst, facies changes, etc.) and are absent from other large masses or beds
of carbonate. The ore deposits are particularly associated with reef features.
8) Lead, zinc, and iron, as sulfides, are the principle metals; copper is almost absent.
Origin:
Putting all the above together it turns out the formation of MVT deposits is not much different
than that of the movement and localization of oil; that is the source material for oil formation
occurs in off-shore shale and sand beds and, with compaction of these, oil, gas and water migrate
up dip and into a variety of traps important among which are permeable reef complexes.
The idea for the origin of MVT deposits is similar. It is thought that the base metal ions are
initially fixed in off shore sediments in foreland basins by absorbtion onto clays and organic
matter, and also by precipitation directly as sulfides in black shales. Upon burial and compaction
the metals are leached into, or dissolved by, the pore space brines (connate sea water) and start to
migrate up dip and/or along suitable structures (normal compressive faults). Brines of the oilfield type are known to develop high salinities similar to those found in the MVT fluid
inclusions. Oil field brines can also be rich is Zn and Pb; their ph is between 4 and 6 and metal
content can reach several hundred parts per million.
During diagenesis and tectonism the large volume of connate pore fluid is heated up and
migrates up dip along major faults . Where the hydrothermal fluid encounters local traps (reefs,
karst, facies changes, etc.) it precipitates out metals as sulfides. The precipitation may occur in 1
of 2 ways:
a) Biological-sulfate reducing bacteria, living on organic matter in the reef complex,
reduces SO2 in the brines to S, which combines with the metal anions.
b) Abiological-methane is common in organic sediments and in the fluid inclusions of the
ores. Reactions like the following can occur:
CH4 + ZnCl2 + SO4 +Mg + CaCO3 = ZnS + CaMg9(CO3) + Ca +Cl + HCO3 + H2O
This type of reaction would account for the production of metallic sulfide and also, at the same
time, dolotimization of the limestone. This kind of process would explain a lot of the alteration,
mineralization, isotopic, and fluid inclusion data for these deposits.
67
References
Anderson, G.M., and Macqueen, R.W., 1982, Ore deposit models- 6. Mississippi Valley-type
lead-zinc deposits: Geoscience Canada, v. 9, p. 108-117.
Arnold, B.W., Bahr, J.M., and Fantucci, R., 1996, Paleohydrogeology of the Upper Mississippi
Valley Zinc-Lead District: Society of Economic Geologists, Special Publication No. 4, p. 378389.
Bradley, D.C., and Leach, D.L., 2003, Tectonic controls of Mississippi Valley-Type lead-zinc
mineralization in orogenic forelands: Mineralium Deposita, v. 38, p. 652-667.
Hannigan, P., 2007, Metallogeny of the Pine Point Mississippi Valley-Type zinc-lead district,
Southern Northwest Territories, in: Goodfellow, W.D., ed., Mineral Deposits of Canada: A
Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of Geological Provinces,
and Exploration Methods: Geological Association of Canada, Mineral Deposits Division, Special
Publication 5, p. 609-632.
Hitzman, M.W., and Beaty, D.W., 1996, The Irish Zn-Pb(-Ba) orefield, in Sangster, D. F., ed.,
Carbonate-Hosted Lead-Zinc Deposits: Society of Economic Geologists, Special Publication
Number 4, p. 112-143.
Leach, D., et. Al., 2005, Sediment–hosted Lead-Zinc Deposits, in: Hedenquist, J.W., Thompson,
J.F.H., Goldfarb, R.J., and Richards, J.P., eds., Economi Geology 100th Anniversary Volume:
The Economic Geology Publishing Company, p. 561-609.
Paradis, S., and Nelson, J.L., 2007, Metallogeny of the Robb Lake carbonate- hosted zinc-lead
district, northeastern British Columbia, in: Goodfellow, W.D., ed., Mineral Deposits of Canada:
A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of Geological
Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits
Division, Special Publication 5, p. 633-654.
Paradis, S., Hannigan, P., and Dewing, K, 2007, Mississippi Valley-type Lead-Zinc deposits
(MVT), in: Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of Major DepositTypes, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods:
Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5
Sangster, D.F., 1988, Breccia-hosted lead-zinc deposits in carbonate rocks, in:James, N. P., and
Choquette, P. W., eds., Paleokarst: Springer-Verlag, New York, NY, p. 102-116.
Sharp, R.J., Ste-Marie, C.P., and Lorenzini, C., 1995a, Field study of Polaris Mine area,
Northwest Territories, Canada, in: Misra, K.C., ed., Carbonate-hosted lead-zinc-fluorite-barite
deposits of North America. Guidebook Series: Geological Association of Canada, 22, p. 38-41.
Sverjensky, D.A., 1984, Oil field brines as ore-forming solutions: Economic Geology, v. 79, p.
23-27.
——— 1986, Genesis of Mississippi Valley-type lead-zinc deposits: Annual Review of Earth
and Planetary Sciences, v. 14, p. 177-199.
68
Sedimentary Exhalative Lead-Zinc Deposits
Definition:
Laminated sulfide deposits of zinc and lead that form in a sedimentary basin from the submarine
venting of hydrothermal fluids and/or as replacement deposits of sediments by hydrothermal
fluids in the shallow subsurface.
Grade and tonnage:
Globally these deposits comprise 50% of the world’s lead-zinc reserves and 25% of the
world’s production of lead and zinc. Out of 130 known deposits there are 25 with a total tonnage
greater than 50mt, 12 of these contain greater than 10 million tons of combined lead and zinc.
The average tonnage and grade for all deposits is: 41.3 mt at 6.8% Zn, 3.5% Pb and 50 g/t Ag.
The largest deposits are Broken Hill in Australia (205 mt), Macarthur River in Tasmania (237
mt), Sullivan in British Columbia (162 million tons), Red Dog (165 mt) and Mount Isa in
Australia(124 mt).
Distribution and Tectonic Setting:
Sedimentary exhalative deposits are widely distributed but are most common in North America,
Australia, and Asia. They are found in intra-cratonic and epicratonic sedimentary basins. The
tectonic settings include intra-cratonic rifts (Belt-Purcell Basin), reactivated rifted margins
(Howards Pass), back arc rifts (McMillan Pass and Mt. Isa).
Regardless of tectonic setting most of the sedimentary basins that host these deposits have
similar stratigraphic elements. These consist of a clastic and or volcanic dominated succession
overlain by a thick package of shales and carbonates. The lead-zinc deposits occur within the
upper succession and are linked by 2 common factors:
1) They are hosted by reduced, fine-grained siltstones, shales, and/or mudstones and/or carbonate
units. They thus reflect deposition in a relatively deep water setting
2) They occur in areas affected to some degree by syn-sedimentary faulting. This can be the most
important feature of the local geology and is reflected in abrupt lateral facies and thickness
changes, and interfingering of coarse-grained fault scarp debris flow deposits with the finegrained host rocks.
Host Rocks:
Host rocks range from organic-rich shales and siltstones to carbonates. Organic-rich
shale hosted deposits are the largest in terms of tonnage. Otherwise there appears to be no
difference in lead and zinc grade and host rock lithology. Locally derived fragmental rocks are
common to all of the ore-hosting lithologies and these represent fault breccias or debris flows.
Ore Mineralogy, Morphology, and Textures:
The main economic elements are zinc, lead, and silver, which occur primarily as sphalerite and
galena. Pyrite is the dominant iron sulfide mineral and can vary from rare to minor. Tetrahedrite
may be present and, if so, will contain most of the silver; otherwise the silver occurs in galena.
69
Other constituents are carbonates (calcite, dolomite, ankerite, and/or siderite), barite (peripheral
to or above the ore), apatite, and quartz as chert and siliceous shale.
The ore minerals commonly occur parallel to bedding and they alternate with beds of the host
rock. Individual sulfide layers vary from mms to several tens of cms thick with bedding contacts
typically sharp. The sulfides are typically fine-grained with coarser grained ores having formed
due to recrystallization during metamorphism. Many deposits contain zones of disseminated
and/or breccia filling ores and these appear to be related to the upflow zones for the
hydrothermal fluid.
Ores are thus both syn-sedimentary and replacement. Textures found in the syn-sedimentary ores
are: 1) sulfide-rich beds that are intricately interlayered with host sediments and which have
sharp boundaries between sulfides and host rock, 2) compaction load cast features, 3)
microbreccia layers and round to subround sulfide clasts in graded sedimentary rocks. These are
interpreted to represent reworked sulfides layers, 4) beds that are composed of multiple sulfide
layers which exhibit grading. This suggests a sedimentary origin and deposition from a density
current.
Evidence for replacement ore is 1) carbonate or barite nodules and layers that have been partly
replaced by pyrite, galena, or sphalerite, 2) base metal sulfides that replace early pyrite, and 3)
base metal sulfides that replace fossils. These features could be due to hydrothermal replacement
below an exhalative ore body.
Some deposits have a well defined stockwork-like feeder zone beneath the stratiform ore and
these zones are referred to as “vent-proximal.” Typically such zones narrow with depth and
terminate in a syn-sedimentary fault. These zones consist of breccia and altered sedimentary
rocks overprinted by quartz, iron-manganese-rich carbonate, and sulfide-rich veins and vein
networks.
Overall the deposit morphology reflects the depositional environment. Mound or lens-like
deposits (vent proximal) have low aspect ratios, deposits with high aspect ratios (long and thin)
are thought to have formed in stratified brine pools or by replacement of carbonate layers within
shale or mudstone units. Many deposits have both morphologies.
Deposits with both stratiform and lense-like ore exhibit an increase in the zn/pb ratio away from
the vent complex. Also, in and near the vent complex sphalerite is iron poor, it becomes more
iron-rich away from the “vent” zone.
Those deposits with stringer ore zones are characterized by three distinct facies-bedded sulfides
above and lateral to the vent complex, a stringer zone, and distal hydrothermal sediments. Distal
hydrothermal sediments either represent plume fallout that has been dispersed by bottom
currents, or clastic sediment shed from sulfide mounds or chimneys. The distal sediments consist
of :
1)
2)
3)
4)
Laminated to disseminated barite, and/or carbonate with or without pyrite
Manganese and iron-rich carbonates
Chert and pyrite, the chert may be phosphorous rich
Galena and pyrite.
70
Hydrothermal Alteration:
Hydrothermal alteration associated with sedimentary exhalative deposits is commonly
widespread extending for 100’s of meters into both post and pre-ore sedimentary rocks, and up to
km’s away from the deposits. Common alteration minerals are quartz, carbonate, muscovite
(sericite), chlorite, tourmaline (which may be abundant and form a distinct alteration
assemblage) as well as sulfides such as arsenopyrite, tetrahedrite, and pyrrhotite)
The style of alteration, its distribution, and the minerals present depend on the composition of the
sedimentary rocks, their permeability, and porosity.
Ore Fluids:
Fluid inclusion data and oxygen isotopes indicate that the ore fluids were hot, metalliferous,
basinal brines. The deposits are thought to have formed from oxidized and H2S poor fluids
generated in a hydrothermal reservoir within syn-rift clastic sediments, which were capped by
fine-grained marine sediments or carbonates. During basin formation, which may be long lived
(200 million years for the Selwyn Basin) zinc and lead are believed to have been leached from
the detrital sediments. The apparent variability in the temperature (175-4000C), salinity (4-18%),
and metal content of the fluids was controlled by the local thermal regime, composition of the
source rocks, presence or absence of evaporates, and time.
Release of these fluids into the near surface environment is thought to be due to tectonic events
leading to faults that breach the cap rocks. Since fluid inclusions show low S one of the
necessary parameters for the formation of sedex deposits is a source of sulfur. This is believed to
be biogenic H2S which is enriched in anoxic water columns. The presence or absence of barite
depends on sulphate in the ore fluid and activity of sulphate in seawater, which controls barite
precipitation.
Sedex and MVT:
Both of these deposit types form from basinal brines with similar temperatures and salinities.
They also have similar ore and gangue minerals with the ore minerals occurring in about the
same proportions. MVT deposits formed mostly in platform carbonate sequences localized
within extensional zones associated inboard of orogenic belts. Sedimentary exhalative deposits
formed in intra continental or failed rifts and rifted continental margins. Sedex deposits are
hosted by reduced, fine-grained siltstones, shales, and mudstones and/or carbonate units within
clastic sedimentary rocks. MVT deposits are located in carbonate dominated sequences. Both are
fault controlled. Thus deposits are similar but they formed in different tectonic settings leading to
different host rocks. This may account for the other minor differences between them (sulfur
isotopes, deposit morphology, and distribution).
Final note:
Large submarine mud volcanoes in the abyssal part of the Black Sea south of the Crimean
Peninsula are similar in many respects to syn-sedimentary mud volcanoes outlined in the
Mesoproterozoic Belt-Purcell basin. One of the Belt-Purcell mud volcanoes directly underlies the
71
giant Sullivan Pb-Zn-Ag deposit in southeastern British Columbia. Footwall rocks to the Sullivan
deposit comprise tourmaline-rich siltstone, conglomerate, and related fragmental rocks; local thin
pyrrhotite-rich and garnet-quartz beds are interpreted as iron and iron-manganese exhalites,
respectively. Analogous iron and manganese-rich sediments occur near the abyssal Black Sea
mud volcanoes. Massive pyrite crusts and associated carbonate chimneys discovered in relatively
shallow waters (about 200 m depth) west of the Crimean Peninsula indicate an active sea-floor–
hydrothermal system. Subaerial mud volcanoes on the Kerch and Taman Peninsulas (about 100
km north of the abyssal mud volcanoes) contain saline thermal waters that locally have very high
boron contents (up to 915 mg/L). These data suggest that tourmalinites (tourmaline-rich
sediments) might be forming in or near submarine Black Sea mud volcanoes. If so this area then
has the potential to host Sullivan-type Pb-Zn mineralization.
References
Goodfellow, W.D., and Lydon, J.W., 2007, Sedimentary exhalative (SEDEX) deposits, in:
Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of Major Deposit Types,
District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods:
Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, p. 163183.
Betts, P.G., Giles, D., and Lister, G.S., 2003, Tectonic environment of shale-hosted
massive sulfide Pb-Zn-Ag deposits of Proterozoic northeastern Australia: Economic Geology, v.
98, p. 557–576.
Lydon, J.W., Höy, T., Knapp, M., and Slack, J.,eds., 2000, The geological environment of the
Sullivan deposit, British Columbia: Geological Association of Canada, Mineral Deposits
Division, Special Publication 1,
Cooke, D.R., Bull, S.W., Large, R.R., and McGoldrick, P.J., 2000, The importance
of oxidized brines for the formation of Australian Proterozoic stratiform sediment-hosted Pb-Zn
(SEDEX) deposits: Economic Geology, v. 95, p. 1–18.
Goodfellow,W.D., 2004, Geology, genesis, and exploration of SEDEX deposits, with
emphasis on the Selwyn Basin, Canada, in Deb, M., and Goodfellow, W.D., eds., Sedimenthosted lead-zinc sulphide deposits: Attributes and models of some major deposits of India,
Australia, and Canada: Delhi, India, Narosa Publishing House, p. 24–99.
Leach, D.L., Sangster, D.L., Kelly, K.D., Large, R.R., Garven, G., Allen, C.R., Gutzmer, J., and
Walter, S., 2005, Sediment-hosted lead-zinc deposits: A global perspective: Society of Economic
Geologists, One Hundredth Anniversary Volume 1905–2005, p. 561–607.
72
Iron Oxide Copper –Gold Deposits
Definition:
Iron oxide gold copper deposits (IOGC) encompass a wide spectrum of sulfide-deficient, low Ti
magnetite and/or hematite ore bodies of hydrothermal origin, which contain more than 20% iron
oxides. Within the deposits the ore is hosted in breccias and/or veins and also occurs as
disseminations. The deposits are polymetallic being composed of two or more of the elements
copper, gold, silver, uranium, bismuth, cobalt, niobium, phosphorus, vanadium, REEs and iron.
These are associated with continental A to I type alkali to granitic intrusions, and large fault
zones. Alteration zones associated with the deposits consist of an early calcic-sodic enrichment
of the rocks upon which is superimposed potassic and/or iron-rich alteration assemblages. The
deposits form at shallow crustal levels in extensional continental settings such as intracratonic
rifts, continental magmatic arcs, and back arc basins. Margins of Archean cratons appear to be
particularly favorable for the formation of these deposits.
From this long, involved definition it is obvious these relatively new deposit types are rather
enigmatic and as yet not really understood.
Distribution and Age:
Currently economic IOGC deposits are relatively uncommon, however subeconomic deposits are
abundant and widely distributed in both space and geological time; they are found on all
continents and range in age from the recent back to the late Archean.
Classification:
A consequence of the diversity of IOCG deposits along with the lack of understanding is the
divergent opinions on their genesis and the way to classify them. With all the on-going debate
probably the most realistic and simplest classification is to divide then into 2 main types based
on the composition of the host rocks: calc alkaline and alkaline. The deposits can then be further
subdivided based on ore mineralogy, alteration, and the development of breccias. Each of the
subtypes is named after a particular deposit or district.
Using this scheme there are three subdivisions in the calc-alkaline group and two in the alkalinecarbonatite group.
Calc Alkaline Types:
a) Olympic Dam sub-type:
These are granite associated, breccia hosted deposits with the polymetallic ores spatially and
temporally associated with either iron oxide or potassic alteration. This deposit sub-type is
named after the Olympic Dam Cu-Au-U-Ag-REE deposit on the eastern margin of the Gawler
Craton of South Australia. These type deposits are hosted by hydrothermal breccias (at the
Olympic Dam Deposit the breccia is funnel shaped, hematite-rich, and is 7 by 5 km in size).
Typically the breccias are composed of a hematite-quartz core, a peripheral hematite granitic
breccia, and a halo of weakly altered and brecciated granitic and/or country rocks. The breccia’s
73
were formed close to the surface through progressive hydrothermal activity and alteration of Atype granites (Roxby Downs Granite at Olympic Dam).
Potassic alteration with associated hematite with or without chlorite, and carbonate is
superimposed on an early magnetite-biotite alteration. The potassic alteration hosts the ore,
which consist of copper sulfides, and REE-bearing minerals with or without urannite and
pitchblende. These deposits occur close to Archean cratons and form mostly in intracontinental
back arcs.
Cloncurry subtype:
This subtype is named after the Cloncurry district at Mt Isa in NW Queensland, Australia. The
deposits are characterized by copper-gold mineralization that overprints preexisting iron
formations or distinctly earlier iron oxide mineralization and alteration. Mineralization is fault
controlled. At the Ernest Henry deposit (1.5 by old) mineralization postdates peak
metamorphism by 24 my and is synchronronus with granite intrusions. Early magnetite-apatite
alteration and associated sodic-calcic alteration were followed by brecciation in a zone between
2 major shears; this was then over printed by Cu-Au mineralization.
Kiruna subtype:
This subtype comprises low titanium and vanadium-bearing magnetite with associated apatite
and actinolite along with related copper-iron oxide deposits; there is little or no gold. These
deposits are named after the classic Kiruna District in northern Sweden; over 355 similar
deposits are known worldwide. The deposits are coeval with, and genetically related to, their
host volcanic and intrusive rocks. Alteration is calcic-sodic with the formation of abundant
albitie. The deposits occur in intracratonic and continental arc settings.
Alkaline Types:
The Palabora subtype consists of magnetite-rich deposits coeval with, and proximal to, alkalinecarbonatite intrusions. Primary characteristics are the presence of apatite, enrichment in REEs, F,
Niobium, and phosphorus; typically there is intense development of fennite.
The Bayan Obo subtype consists of magnetite-rich copper-gold deposits that have low REEs and
niobium. The deposits are associated with carbonatite intrusive complexes with the ore occuring
in the wall rocks adjacent to the intrusions as veins, disseminations, and/or lenses. Ore minerals
are dominated by magnetite, fluorite, alkali amphiboles and pyroxenes. Other minerals present
are apatite, aegerine, phlogopite, alkali amphiboles, and barite.
Ore elements, grades and tonnages:
These deposits, as a whole, are major sources of Fe, Cu, Au, U, REE, F, vermiculite, and
significant sources of Se, Te, and Zr. The Olympic dam deposit contains the world’s largest U
resources (1.4 mt); it also contains 42 mt of copper and 55.1 million ounces of gold. Tonnages of
these kinds of deposits range from 4 by down to 100 million. Overall the largest deposits contain
more than 1 billion tons of iron ore and have significant quantities of P, REE’s, Co, Ag, and U.
Au and iron ore seem to be mutually exclusive. Copper grades range from .4 to 4 % and average
about 1 %. Gold ranges from 0.5 to 2 g/ton, Ag 0.5 to 4 g/t.
74
Mineralogy and Textures:
One of the most striking characteristics of IOCG deposits is their alteration zones. There appear
to be 3 main types of alteration: sodic-calcic, iron-rich, and potassic.
The sodic-calcic alteration zone is a regional one (>1km wide) and consists of formation of albite
(albitites) with or without pyroxene and titanite. In some deposits albite is associated with garnet
and amphibole.
If the host rocks are calcium-rich then iron-garnet, clinopyroxene, scapolite skarn-like
assemblages may form. This type of alteration appears to be early in the mineralizing process.
Iron-rich alteration consists of magnetite plus or minus biotite, iron carbonates, and iron silicates
(chlorite, grunerite, hastingite.) Apatite may also be present.
The ore forming stage is often associated with potassic alteration consisting of k-feldspar and or
sericite with or without chlorite, iron carbonate, fluorite, monazite, titanite, rutile and fe-cu
sulfides.
The latter 2 alteration zones may be restricted to brecciated zones in the host rocks.
Ore minerals are highly variable and not all of the following are associated with each deposit or
district. This adds to their diversity and interpretation of their genesis. The principle minerals are
hematite, low-ti magnetite, bornite, cp, cc, with or without subordinate arsenides, bismuth,
molybdenite, uranite, pitchblende, native copper, gold and silver, vermiculite, tellurides. Gangue
mineraloguy consists of allanite, barite, epidote, fluorite, garnet, tourmaline, scapolite, rutile,
phlogopite, quartz, k-feldspar, sericite, chlorite, carbonate. Amphibles are hastingite, grunerite,
actinolite.
The morphology of the alteration and ore varies significantly and includes breccia zones, veins,
stratifom lenses, irregular bodies, and disseminations. The ore can be hosted in subvertical to
subhorizontal single to polyphase breccia zones, in stockworks, in diatreme breccias, and also
occur as disseminations through the host rock.
Brecciated host rocks are commonly heterolithic and composed of angular to sub angular
fragments some of which may be oxide fragments. Breccias vary in size from an outcrop to
mountain scale (1m to 10km2) with fragment sizes from <1cm to 100’s of meters in length or
thickness making the breccias the most striking feature of many of the deposits. Core zones of
these breccias may be composed almost totally of iron oxides, which grade out into crackle
breccias. The breccias are typically aligned along fault zones or parallel intrusive contacts.
Fragments may be replaced by iron oxides.
Over all large scale faults to local fault zones control the location of many IOCG deposits as well
as the breccia zones. In some deposits (Candelaria) the intersection of permeable units with fault
zones makes an ideal place for ore to accumulate, in others, like Olympic dam, duplexes, shears
(Carajas District) or complex intercalation of high and low permeability units near faults have
determined location of alteration zones, location of breccias, and ore deposition.
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Host Rocks
Host rocks of IOCG deposits are varied and do not appear to constitute a diagnostic feature.
They include coeval and/or pre-existing sedimentary, mafic, and felsic volcanic and plutonic
rocks, as well as schists and gneisses. These serve as lithological (permeability contrasts) or
chemical traps for fluids associated with long lived batholiths, or for very reactive fluids from
short-lived magmatism such as carbonatite stocks. All the deposits formed in oxidized settings
such as pre-existing iron formations, oxidized plutonic-volcanic environments, iron-rich
sediments, or reactive units along major faults.
Geological Setting:
IOCG deposits vary widely but what they all seem to have in common is:
1) Proximity to an Archean Craton
2) Extensional settings-intracratonic to continental arc, rift structures, and regional scale
long lived fault zones
3) Presence of A or I type granites with intermediate to mafic facies indicative of a
sustained magmatic system over a long period of time
4) Structural control of the mineralization
5) Oxidized settings
6) Extensive brecciation both fault induced and hydrothermal.
A-type granites are those with high SiO2, Na2O, K2O, Fe/Mg ratios, Y, Zr, REE’s and F. These
granites have high crystallization temperatures (900-10000C) and are commonly associated with
Archean cratons and extensional settings
I-type granite means derived from an igneous protolith-they have high Na2O
References
Belperio, A., and Freeman, H., 2004, Common geological characteristics of Prominent Hill and
Olympic Dam - Implications for iron oxide copper-gold exploration models. Australian Institute
of Mining Bulletin, Nov-Dec. Issue, p. 67-75.
Chao, E.C.T., et. Al., 1997, The sedimentary carbonate-hosted giant Bayan Obo REE-Fe-Nb ore
deposit of Inner Mongolia, China: A cornerstone example for giant polymetallic ore deposits of
hydrothermal origin: U.S. Geological Survey Bulletin 2143.
Collins, W.J., Beams, S.D., White, A.J.R., and Chappell, B.W., 1982, Nature and origin of Atype granites with particular reference to south-eastern Australia: Contributions to Mineralogy
and Petrology, v. 80, p. 189-200.
Corriveau, L., 2007, Iron Oxide Coppr Gold (+/- Ag, Nb, P, Ree, U) Deposits, in: Hedenquist,
J.W., Thompson, J.F.H., Goldfarb, R.J., and Richards, J.P., eds., Economi Geology 100th
Anniversary Volume: The Economic Geology Publishing Company, p. 523-560.
76
Daliran, F., 2002, Kiruna-type iron oxide-apatite ores and "apatitites" of the Bafq district, Iran,
with an emphasis on the REE geochemistry of their apatites, in: Porter, T.M., ed., Hydrothermal
iron oxide copper-gold and related deposits: A global perspective: PGC Publishing, Adelaide, v.
2, p. 303-320.
Groves, D.I., and Vielreicher, N.M., 2001, The Phalabowra (Palabora) carbonatite-hosted
magnetite-copper sulfide deposit, South Africa: An end-member of the iron-oxide copper-goldrare earth element deposit group: Mineralium Deposita, v. 36, p 189-194.
Haynes, D. W., 2000, Iron oxide copper(-gold) Deposits: Their position in the ore deposit
spectrum and modes of origin, in: Porter, T. M., ed., Hydrothermal Iron Oxide Copper-Gold &
Related Deposits a Global Perspective, 1: Adelaide, Australia, Australian Mineral Foundation, p.
71-90.
Haynes, D. W., Cross, K. C., Bills, R. T., and Reed, M. H., 1995, Olympic Dam ore genesis: a
flui mixing model: Economic Geology, v. 90, p. 281-307.
Hitzman, M.C., 2000, Iron oxide-Cu-Au deposits: What, where, when, and why? in: Porter, T.M.,
ed., Hydrothermal iron oxide copper-gold and related deposits: A global perspective: PGC
Publishing, Adelaide, v. 1, p. 9-25.
Hitzman, M.W., Oreskes, N., and Einaudi, M.T., 1992, Geological characteristics and tectonic
setting of Proterozoic iron oxide (Cu-U-Au-LREE) deposits: Precambrian Research, v. 58, p.
241-287.
Mark, G., and Foster, D.R.W., 2000, Magmatic-hydrothermal albite-actinolite-apatite-rich rocks
from the Cloncurry district, NW Queensland, Australia: Lithos, v. 51, p. 223-245.
Partington, G.A., and Williams, P.J., 2000, Proterozoic lode gold and (iron)-copper-gold
deposits: A comparison of Australian and global examples: Reviews of Society of Economic
Geologists, v. 13, p. 69-101.
Porter, T. M., ed., 2000, Hydrothermal Iron Oxide Copper-Gold & Related Deposits A Global
Perspective, v. 1: Adelaide, Australia, Australian Mineral Foundation, 330 p.
Porter, T.M., 2002, Iron oxide alteration/ mineralizing systems and copper-gold and related
mineralisation, in: Porter, T.M., ed., Hydrothermal iron oxide copper-gold and related deposits:
A global perspective: PGC Publishing, Adelaide, v. 2, p. 3-7.
Reynolds, L.J., 2000, Geology of the Olympic Dam Cu-U-Au-Ag-REE deposit, in: Porter, T.M.,
ed., Hydrothermal iron oxide copper-gold and related deposits: A global perspective: PGC
Publishing, Adelaide, v. 1, p. 93-104.
Sillitoe, R.H., 2002, Some metallogenic features of gold and copper deposits related to alkaline
rocks and consequences for exploration: Mineralium Deposita, v. 37, p. 4-13.
77
Banded Iron Formations and Iron Ore Deposits
Definition:
Banded Iron Formations are bedded, iron-rich (≥ 15% Fe), chemically precipitated sedimentary
rocks that consist of interlayered iron-rich beds and chert or carbonate-rich beds. There are two
main types of banded iron formation: 1) the Lake Superior type, which is sedimentary in origin
and 2) the Algoma-type which is associated with volcanic successions, and is considered to be of
hydrothermal-hot spring origin.
The Lake Superior-type is much larger and supplies most of the world’s iron. Lake Superior
type banded iron formations can be subdivided into two end members: a) granular iron formation
(called upper and lower cherty on the Mesabi Iron Range) which is sand-textured and has thick
(several inches to several feet), discontinuous beds that are composed dominantly of chert and
iron oxides, and b) banded iron formation (upper and lower slaty on the Mesabi Iron Range)
which is mud-textured and consist of thin (< 2 inches) continuous beds composed dominantly of
iron oxides, iron silicates, and iron carbonate with local chert beds. The two are often
interlayered.
In the Lake Superior type, iron-rich beds that are not highly metamorphosed or altered by
weathering and supergene processes are refered to as taconite; the more metamorphosed
equivalents are called metataconite or itabirite. To be mined for taconite the iron-rich beds
usually contain at least 30% iron and a minimum amount of carbonate and silicate minerals.
Distribution and Size:
Algoma-Type: These are dominantly BIF’s and are associated with volcanic successions that
represent divergent and subduction zone volcanism. Common in Archean Greenstone Belts these
are “hot spring” type deposits that are relatively small (<10 feet thick and a mile or two in strike
length at most). There are two notable exceptions to this: the Sudan Iron Formation in
northeastern Minnesota (30 miles along strike) and the Michipicotin Iron Formation in Wawa
Ontario (45-50 miles along strike).
The Lake Superior Type: These were, for the most part, deposited over a 600 my year period
ranging from 2.4 to 1.8 by ago when there formation abruptly ended. They consist of both GIF
and BIF’s and occur mostly in continental margin successions. They are much larger than the
Algoma type and are regionally extensive. For example, the Biwabik Iron Formation on the
Mesabi Iron Range is 200-800 feet thick, 120 miles long, and ¼ to 3 miles wide. All the world
famous iron ore producing areas (Hamersley, Labrador Trough, Mesabi) are Lake Superior Type
deposits.
There are also younger iron formations (Lower Paleozoic to Pliocene), called ironstones, but
these differ from the Proterozoic ones in being distinctly more Al2O3-, P2O5- and Fe2O3-rich and
by having an oolitic or pisolitic texture. They are less cherty, associated with glaciogenic
sediments, and are much smaller. A comparison of the two is given below:
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Characteristic
Age
Minimum age
Major development
Maximum age
Thickness of major units
Original areal extent, max.
dimension
Physical character
Ironstones
Superior Iron Formations
Pliocene
Lower Paleozoic; Jurassic
Proterozoic (~2.0 Ga)
1-50 m
< 150 km
Proterozoic
2.4-1.8Ga
2.4
50-600 m
> 100 km
massive to poorly banded;
silicate and oxide-facies
oolitic
Thinly to thickly bedded;
layers of hematite,
magnetite, siderite, or
silicate alternating with
chert; chert ~50%
dominant
fairly common
relatively rare
dominant primary silicate
minor
common
common
common
relatively abundant
none
rare
common
high iron
none
common
common
absent
absent
common
rare
fairly common
absent
dominant primary silicate
major constituent
common
lower Al, Na, K and minor
elements; much lower P
Mineralogy
goethite
hematite
magnetite
chamosite
glauconite
siderite
calcite
dolomite
pelletal collophane
greenalite
quartz (chert)
pyrite
Chemistry
Tectonic Setting and Depositional Environment:
Lake Superior type iron formations are believed to have formed in platform-foredeep settings on
the margins of Proterozoic cratons. Iron-rich beds were deposited in shallow marine conditions
associated with transgressing seas along the craton margin. Associated with the deposition of the
iron-rich material are sequences of argillic and stromatolitc dolstone, chert, and quartz arenite.
Lateral to these, in deeper water, there was deposition of thick successions of greywacke,
siltstone, black shale and argillite. Due to the transgressive oceanic environment these different
sedimentary successions may be interbedded.
Algoma-type iron formations formed dominantly in Archean to Proterozoic volcanic arcs and
spreading centers (Greenstone Belts). Rocks associated with the volcanic assemblages include,
greywacke, shale, and metalliferous sedimentary rocks. The volcanic and volcanically derived
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sedimentary successions hosting Algoma-type iron-formations are believed to have formed in
island arc/back arc basins and intracratonic rift zones.
Tectonic Setting:
Lake Superior type iron formation was deposited on a stable continental platform within a
relatively shallow-water to transitional deep-basin environment. The Algoma type formed in
tectonically unstable marine environments (“greenstone belts”). Iron formations have a common
spatial association with syndepositional fault and rift zones. The Lake Superior type has an
association with faults on a regional, distal scale and the Algoma type on a more local, proximal
scale.
Facies and Mineralogy:
On the basis of the dominant iron minerals present in unenriched iron formations four
sedimentary facies are recognized within Gif’s and BIF’s. These are Oxide (hematite and
magnetite), Silicate (greenalite, stilpnomeline, minnesotite), carbonate (siderite), and sulfide
(pyrite). The primary minerals are likely to have been amorphous iron hydroxide, siderite, and
pyrite. Typically oxygen content of the waters and sediments, water circulation in the basin of
deposition and organic matter present will be major factors in determining which minerals form.
In general GIF’s are dominantly oxide and silicate faces whereas BIF’s can contain all 4.
Mineralogically the oxide facies consists of magnetite and hematite (taconite); the silicate facies
is composed of cummingtonite, grunerite, greenalite, minnesotite, stilpnomelane, iron-rich
chlorites, and/or iron-rich amphiboles; carbonate facies contains ankerite and siderite; and the
sulphide facies may be pyrite and/or pyrrhotite. The classic facies zonation from shallow to
deeper water deposition is oxide to silicate to carbonate to sulphide. For the Algoma type there
may also be a zonation relative to the volcanic vents from proximal sulphide facies to more
widespread distal oxide and carbonate facies.
Textures and Styles of Mineralization:
Banded iron formations are stratigraphically-controlled, chemical sedimentary units which are
lenticular to tabular in shape and laterally extensive unless deformed. The Lake Superior type is
typically much thicker and more laterally extensive than the Algoma type. There is a common
structural thickening by folding and thrusting. Economic deposits are units with thicknesses of
>30 meters, strike lengths of several kilometers and grades of >30% Fe. They are typically
banded at a millimeter to meter scale and were originally fine-grained, but most have undergone
recrystallization (grain coarsening) during metamorphism.
Alteration:
Post-depositional, protracted supergene alteration can be an important economic factor in
upgrading the primary iron formation. This results in the conversion of primary magnetite to
martite (hematite pseudomorphs), goethite and other hydrous iron oxides, and the leaching of
soluble gangue minerals.
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Characteristics of Iron Formations Mined for Iron Ore:
1) Iron content > than 30%
2) Discrete units of oxide facies iron-formation which are clearly segregated from
carbonate, silicate, and sulphide facies or barren rock. They must also be amenable to
concentration and beneficiation as well as meet required chemical and physical
qualifications for iron ore.
3) Iron minerals are uniformly distributed in discrete grains or grain clusters of hematite,
magnetite, and goethite in a cherty quartz matrix.
4) Iron formations repeated by folds and faults provide thick sections for mining
5) Metamorphic recrystallization improves quality of the crude ore concentration and
processing.
Iron Formation-Hosted Iron Ore Deposits
BlF- and GIF-hosted iron ores can be subdivided into three classes: (1) primary iron formation
(hypogene) with typically 30 to 45 wt percent Fe (taconite); (2) martite-goethite ore formed by
supergene processes, with iron oxides containing 56 to 63 wt percent iron. Martite is a
commonly used textural term to denote hematite pseudomorphs after primary magnetite where
the octagonal outlines of much of the original magnetite are preserved; and (3) high-grade
hematite ores thought to be of supergene or metamorphic origin with 60 to 68 wt percent iron.
The high-grade hematite ores can be further subdivided into hematite residual ore and hematite
replacement ore. Hematite residual ore is oxidized, metamorphosed, and deformed BIF that
formed by supergene leaching of chert, carbonate, and or silicates and residual accumulation of
hematite. Hematite replacement ore is formed by the replacement of silicate and carbonate bands
in the iron formation by hematite. Individual high-grade hematite iron ore deposits range from a
few million tons to over 2 billion tons at >64 wt percent iron, although most fall within the range
of 200 to 500 Mt.
During the formation of types two and three much of the primary layering, on both the meso and
micro-scale, has been preserved. This preservation has resulted from replacement of chert and
carbonate beds by hematite or goethite,or residual accumulation of martite as the result of
leaching out of chert and carbonate from bands which once contained disseminated magnetite.
Magnetite rich bands in the iron formation were oxidized to martite and/or replaced by secondary
hematite in ore, whereas silicate bands have been leached and partly replaced by clays to form
shalelike bands. In martite goethite ores, chert- and carbonate-dominant bands have been leached
out and replaced by goethite so that the overall iron content has been enriched. Disseminated
magnetite in chert and carbonate bands is oxidized to martite in martite-goethite ores.
Origin
Lake Superior-type banded iron formations were chemically precipitated on marine continental
shelves in shallow basins. They are commonly interlayered with other sedimentary rocks such as
stromatolitic dolstone, quartz arenite, argillic dolostone, greywacke, shale and siltstone. Most
Lake Superior-type banded iron formations formed during the Proterozoic, between 2.4 and 1.8
billion years ago. Prior to this, earth's primitive atmosphere and oceans had little or no free
oxygen to react with iron or silica resulting in high iron and silica concentrations in seawater.
81
On the basis of depleted rare earth element (REE) patterns and Nd isotopic signatures, it is now
generally accepted that mid-ocean-ridge- or hotspot- style tectonic settings act as a distal source
of iron. Iron output is pulsed, and is most probably supplemented by normal weathering of ironrich rocks with the iron transported to the sea as water-soluble Fe+2. Upwelling currents or
plumes bring hydrothermal waters onto the outer continental shelf. Silica is believed to have
been derived dominantly from weathering of continental land masses supplemented by discharge
from hydrothermal vents.
Under calm, shallow marine conditions, the iron and silica in seawater combined with oxygen
released during photosynthesis by stromatolites to precipitate magnetite and chert which settled
to the bottom of the sea floor. Stromatolites are cyanobacteria, which is a form of primitive bluegreen algae. It is at this time in earth history that these life forms, which had been around since
the early Archean, began to proliferate in warn, shallow waters. Prior to the development of
abundant stromatolites there was nothing in the seawater to cause the precipitation of the iron or
silica. Silica precipitation could have also been achieved, in part, through evaporation of silica
saturated seawater.
Uniform, regional-scale precipitation of minerals in a shallow water depositional basin gave rise
to the extensive, horizontally continuous iron and or silica-rich mesobands. The alternating of
layers, iron-rich then silica-rich back to iron-rich, etc., and the large, lateral extent of individual
thin layers implies the changes in oxygen or iron/silica content of seawater was regional, and
necessitates calm depositional conditions. To date there is no definitive answer as to how this
change from one to the other might occur. Some proposed theories are: a) temporary failure of
hydrothermal fluids reaching the depositional site due to changes in ocean circulation (termed
‘current reorganization’) thus allowing silica-rich beds to form, b) periods of relative
hydrothermal quiescence, or c) seasonal changes in the temperature of the seawater. Any of the
above, in the final analysis, reflects competing controls through dominance of continental silica
and hydrothermal iron sources respectively.
The iron and silica-rich layers were most likely originally deposited as amorphous gels, and
subsequently lithified to form banded iron formations. The distribution of Lake Superior-type
banded iron formations of the same age range associated with Precambrian cratons worldwide
suggests that they record a period of global change in the oxygen content of the earth's
atmosphere and oceans. Also, the worldwide abundance of large, calm, shallow platforms where
cyanobacterial mats flourished and banded iron formations were deposited may imply a global
rise in sea level.
Banded iron formations are highly anisotropic rocks. When shortened parallel to their layering,
they deformto form angular to rounded folds, kink bands, and box folds. Folds in banded iron
formations are typically doubly plunging and conical. Banded iron formations may interact with
hot fluids channeled along faults and more permeable, interbedded horizons such as dolomite
during deformation. This may remove large volumes of silica, resulting in concentration of iron
(replacement deposits). Iron, in the form of platy hematite, can also crystallize in structurally
controlled sites such as fold hinges and along detachment faults. If there is sufficient enrichment,
82
a “natural” iron ore body is formed. Iron may also be leached, redeposited, and concentrated
during weathering to form supergene iron ore deposits.
References:
Gross, G.A., 1996, Lake Superior-type Iron-Formation, in Geology of Canadian Mineral Deposits, eds.
Eckstrand, O.R., Sinclair, W.D., and Thorpe, R.I., Geological Survey of Canada, Geology of Canada, no.
8, p. 54-68.
Clout, J.M. and Simonson, B.M., 2005, Precambrian Iron Formations and Iron Formation-Hosted Iron
Ore Deposits, in
James, H.L., 1954, Sedimentary Facies of Iron Formations, Econ. Geol., 49, pp. 235-293.
Isley, A.E., 1993, Hydrothermal Plumes and the Delivery of Iron to Banded Iron Formation, Jour. Of
Geology, 103, pp. 169-185.
Gole, M.J., and Klein, C., 1981, Banded Iron Formations Through Much of Precambrian Time, Jour. Of
Geology, 89, pp. 169-183.
83