GEOL.363 MINERAL DEPOSITS
Hand out No. 1
(Semester 1/2001)
DEFINITION
Ore : a mineral or minerals that can be mined and extracted from a rock at a profit.
Gangue: Those minerals which occur with the ore minerals but which have no value, such as
quartz, calcite or pyrite.
Mineralization: A general term which usually refers to ore minerals but which often may refer
to other metallic minerals such as pyrite.
Waste: Rock which is not ore. Usually referred to that rock which has to be removed during the
normal course of mining in order to get at the ore.
Grade: The concentration of each ore metal in a rock sample, usually given as weight percent. If
concentrations are extremely low, as with Au, Ag, Pt and others, the concentration may be given
in grams per tonne (g/t). The average grade of an ore deposit is calculated, often employing
very sophisticated statistical procedures, as an average of the grades of a very large number of
samples collected from throughout the deposit.
Cut-off: The cut-off grade is the arbitrarily defined lowest grade which will be mined from an
ore ore deposit, and usually defines the boundary of the deposit. For example, if the average
grade of a porphyry deposit is 0.5% Cu, the cut-off might be 0.2% Cu. any rock with a grade
below 0.2% Cu would be waste.
Reserves: The amount of ore in a given deposit, usually quoted as the number of tonnes
available at a specific average grade.
Host rock: The rock within which the ore deposit occurs.
Country rock: The rock which surrounds the ore deposit. Also referred to as wall rock , in
particular that rock on either side of a vein.
Hydrothermal: Hot fluids, usually mainly water, sometimes acidic which may carry metals and
other compounds in solution to the site of ore deposition or wall rock alteration.
Alteration: A change in the mineralogy of the country rock as a result of a chemical reaction
with hydrothermal solutions. For example, mafic minerals such as hornblende or biotite may
alter to chlorite and feldspars may alter to clay. An alteration zone describes rocks which have
been altered to a specific group of secondary or alteration minerals, usually around the perimeter
of a mineral deposit.
Vein: A tabular deposit usually formed by deposition of ore and gangue minerals in open spaces
within a fault or other structural environment.
Replacement: A chemical process whereby hydrothermal fluids, passing through permeable
rocks, react with the rocks to dissolve original minerals and relace them with ore and/or other
gangue minerals.
Massive sulphide: A stratiform (see below), usually lens-shaped mineral deposit consisting of at
least 60% sulphide minerals.
Skarn: A replacement of limestone (calcium carbonate) or other carbonate-rich rocks adjacent to
an intrusive contact by calc-silicate minerals usually through the addition of Si and other
elements.
Epigenetic: Mineralization which has been deposited later than its immediate host rocks, for
example a vein. The ore is younger than the host rocks.
Syngenetic: Mineralization which has been deposited simultaneously with its host rocks, for
example placer deposits. The ore is the same age as the host rocks.
Gossan: A rusty, surficial weathering zone which is caused by the oxidation of pyrite to produce
secondary iron oxide minerals. Since pyrite is often associated with ore deposits, gossans can be
a guide to ore.
Reserves : Quantity of a resource that has been found and can be recovered economically
using existing technology. Reserves increase as geological knowledge increases, making it
possible to find more mineral deposits. Reserves also increase due to improved technology
that allows minerals to be produced from previously uneconomic ores. The volume of
reserves will always be less than the volume of resources. For example:

world resources of silver total approximately 2 million million tons in the upper 1
km of the earth's crust

Silver occurs with an average concentration of 0.07 ppm (ppm = parts per million;
0.07 ppm = 1 part silver for 14,285,714 parts of the crust)

Worldwide, we use about 10,000 tons of silver annually.
At first glance it might seem that we have more silver than we can ever use but the big
number is resources, not reserves.

Silver reserves are estimated as 200,000 tons

Consequently, we currently have about a 20 year supply of silver on hand.
However, there is little chance that we will run out of silver after 20 years. Mining companies
will find more silver deposits and prices will inevitably rise (limiting demand) as stocks dwindle.
It is unlikely that we will exhaust the reserves of any key mineral for another 100 years.
Concentration Factor (CF) :The increase in the concentration of an economic mineral required
to generate a metallic ore. The average concentration of minerals in the crust is insufficient to
form an ore. Various geological processes concentrate minerals within the crust. The
concentration factor necessary to generate an economic mineral deposit can be readily
determined by dividing the economic concentration by the average concentration of the mineral
in the crust. For example:

Aluminum is one of the most common elements in crustal rocks. On average, it
makes up 8% of rock

An economic ore of Aluminum must have a concentration of ~35%

Concentration factor is 35/8 = 4.4.
Rare minerals have large CF values, e.g. gold, CF =2,500, whereas common elements
have low CF values, e.g. silicon, CF = 2.
MORPHOLOGY OF MINERAL DEPOSITS
Concordant: Any geologic body, such as an ore deposit, which lies within or parallel to volcanic
or sedimentary bedding and does not cut across the bedding structures. (Also conformable ).
Discordant: A geologic body, such as a dike or vein, which cuts across primary rock structures
such as bedding.
Stratiform: A mineral deposit which occurs as a specific stratigraphic (or sedimentary) bed.
Stratabound: A mineral deposit which occurs within a specific stratigraphic bed or horizon, but
which does not comprise the entire bed.
Footwall : The lower contact of an inclined vein, or the wall rock which lies on the lower side of
a dipping vein.
Hangingwall: The upper contact of an inclined vein.
Fault: A planar feature or fracture zone along which displacement has occurred.
Shear zone: A planar zone of weakness, similar to a fault, but consisting of several parallel
displacement zones usually over a greater width than a single fault.
Lode, shoot: All refer to mineralized zones within a fault or shear zone or a vein fissure, stringer
structure.
Breccia: Angular fragments of rock produced by movement along a fault or explosive igneous
activity. The material which surrounds the fragments and cements them together is called matrix
and might be vein minerals, igneous material or very fine rock fragments.
Stockwork: A large number of small, closely spaced veins, often with many different
orientations, is referred to as a stockwork and sometimes as a stringer zone.
Chimney : Also referred to as a pipe , this is a vertically oriented, cylindrical body, often a
breccia, of vein or replacement mineralization.
Manto: This is a horizontally oriented chimney-like structure, usually of replacement
mineralization.
Reference: http://www.bc-mining-house.com/prospecting_school/dp_intr1.htm
GEOL.363 MINERAL DEPOSITS
Hand out No. 2
(Semester 1/2001)
MODES OF MINERAL DEPOSITION
Syngenetic - A deposit formed at the same time as the rocks in which it occurs.
Ex. Banded Iron Formation
Epigenetic - A deposit introduced into the host rocks at some time after they were deposited.
Ex. Mississippi Valley-type Deposits
Current theories of ore deposit genesis can be divided into internal and external or surface
processes. Keep in mind more than one mechanism may be responsible for the formation of an
ore body. Example stockwork porphyry copper deposit at depth (epigenetic) with a syngenetic
massive sulfide deposit at the surface. The Table at the end of the document summarizes the
principal theories of ore genesis
Magmatic Segregation - Those deposits, not including pegmatites that have formed by direct
crystallization from a magma. Two types:
 Fractional crystallization - Any process whereby early formed crystals can not reequilibrate with the melt. Includes 1) gravitative settling; 2) flowage differentiation; 3)
filter pressing and 4) dilation. Number 1 is the most important and results from the
settling of early formed crystals to the bottom of the magma chamber. Rocks formed in
this manner are termed cumulates and are often characterized by rhythmic layering. In
ore deposits the alternating layers are often magnetite and/or chromite between layers of
silicate. Ex. Bushveld igneous complex.
 Immiscible liquid - Typical example is oil and water. In ore deposits we deal with silicate
and sulfide magmas. As a magma cools sulfides coalesce as droplets and due to higher
density settle out. Most common sulfides are iron sulfides, but nickel, copper and
platinum also occur. Ex. Sudbury, Canada. The settling out of the heavier sulfides results
in the peculiar net-textures ores often found in many of these deposits.
The billiard ball model is often used to explain the ore texture found in immiscible liquid ores
(Figure). Imagine a beaker partly filled with billiard balls and water (A). These represent olivine
grains and interstitial liquid. Mercury is added to represent the immiscible sulfide liquid. This
will sink to the bottom and the balls will tend to float on it. However, the lower balls will be
forced down into the mercury by the weight of the overlying ones. If the contents of the beaker
are frozen before all the mercury percolates to the bottom then a situation such as (B) results.
This is exactly analogous to the massive ore (mercury), net textured ore (mercury and billiard
balls) and disseminated ore (some mercury, billiard balls and water) zones of these deposits.
Pegmatitic Deposition - Very coarse grained igneous rocks. Commonly form dike-like masses a
few meters to occasionally 1-2 km in length. Economic ore deposits are associated with granitic
pegmatites since felsic magmas carry more water. Residual elements such as Li, Be Nb, Ta, Sn
and U that are not readily accommodated in crystallizing silicate phases end up in the volatile
fraction. When this fraction is injected into the country rock a pegmatite is formed. Temperatures
of deposition vary from 250-750 ◦C. Pegmatites are divided into simple and complex. Simple
consist of plagioclase, quartz and mica and are not zoned. Complex have a more varied
mineralogy and are strongly zoned. Crystals in pegmatites can be large, exceeding several
meters. Three hypotheses to explain their formation:
a. fractional crystallization
b. deposition along open channels from fluids of changing composition
c. crystallization of a simple pegmatite and partial to complete hydrothermal replacement
Hydrothermal Processes - Hot aqueous solutions are responsible for the formation of many ore
deposits. Fluid inclusion research indicates most ore forming fluids range in temperature from
50◦C to 450◦C. Analysis of the fluid in inclusions has shown that water is the most important
phase and salinities are often much greater than those of seawater. The chemistry of ore fluids
and the mechanism of deposition of ore minerals remains a subject of hot debate. Arguments boil
down to a) source and nature of the solutions b) means of transport of the metals and c)
mechanism of deposition.
a. Sources of the solutions and their contents - Earliest thought was that hydrothermal fluids
represented the low temperature residual fluids left over from the crystallization of a
magma. These fluids would contain the metallic elements which are not accommodated
in silicate minerals. The solution were assumed to move upward along available
channelways to sites of deposition. The main problem with this theory was that many ore
deposits had no close spatial relationship to igneous rocks of appropriate age to supply
the ore fluids. To overcome this problem we relied on ore fluids often traveling
impossibly long distances or emanating from igneous intrusives beneath the level of
subsurface mapping. The high salinities of fluid inclusions and studies of oil field brines
has led to a more recent theory which favors connate brines trapped in sedimentary rocks
as the source for the fluids. The fluids are driven out of the rocks during compaction and
migrate up-dip in response to fluid/rock density differences. Research has shown that a
typical sedimentary basin with 9 km of sediments will reach temperatures in excess of
300◦C. The hot brines expelled from theses basins then leach metals and perhaps sulfur
from the rocks through which they pass ultimately precipitating their metallic component
i n
n e a r
s u r f a c e
s e t t i n g s .
b. Means of transport - Sulfides have very low solubilities in water. Example amount of
zinc in equilibrium with ZnS in a saturated solution at pH 5 and 100◦C (reasonable
conditions) is about 10-5 g/l. To form an ore body of one million tons zinc would require
1017 liters of solution passing through the ore body. This requires a volume of water
10,000 km2 X 10km high (clearly an impossible situation). Figure further illustrates the
problem. In order to have enough Pb and H2S to form an ore body requires a pH of 3 or
less. This is highly unlikely in any rocks. Fluid inclusions and theoretical geochemistry
suggest the pH of ore solutions is probably near neutral. Thus ionic solubility does not
appear to be a viable mechanism of metal transport.
Complexes
1. S2- - would form complex sulfide ions like HgS22-. However, for to be a dominant anion
requires a pH of 11 or greater. This is not possible under normal geologic conditions.
2. HS- - bisulfide complex is an important constituent of near neutral solutions which
contain abundant H2S. It can form complexes such as PbS(HS)-. This can increase the
solubility of galena by 10 to lOOX. Unfortunately, it also requires a high concentration of
H2S and HS- to remain stable, much higher than that found in fluid inclusions.
3. Chloride - favored due to the high salinities of most ore fluids. Typical chloride complex
would be AgCl2-.
4. Metal-organic - have not been extensively studied, but may be important for some ore
deposits.
The transport of sulfur is also a problem. If we assume chloride complexes are the important
transporting agent we must also assume S(r) must be less than 105M to transport sufficient metal.
Thus S must be supplied at the site of deposition or abundant sulfate must be reduced at the site
of deposition.
Two possible mechanisms of sulfide deposition
1. sulfate reduction
2. carrying reduced sulfur with the metals and precipitating ores in response to cooling
and/or pH changes
Anderson Figure (see diagram) summarizes many of the problems:
1. only solutions above and to the left of the 10-5 Pb contour contain enough lead to make
ore
2. 10-5 sulfur contour is the boundary below which sufficient reduced sulfur is present to
make ore
3. ore forming area on the left side of the diagram is not realistic. Diagram appears to show
that reduced sulfur must be added at the site of deposition.
Lateral Secretion - Formation of ores by leaching of metals and SiO2 from host rocks adjacent
to open fractures. Difficult to distinguish from basement-derived hydrothermal fluids. However,
geochemical patterns should be different (Figure). In Case A quartz is added both to the vein and
the surrounding host rock and as such must be derived from a distant source (hydrothermal). In
Case B silica has obviously been leached from the wall rocks adjacent to the vein and lateral
secretion is a viable genetic model.
Case history Yellowknife Goldfields - Deposits occur in quartz-carbonate lenses in chloritic
shear zones cutting amphibolites. Principle minerals are quartz, carbonates, sericite, pyrite,
arsenopyrite, stibnite, chalcopyrite, pyrrhotite, sphalerite, galena and native gold. Alteration
haloes of carbonate-sericite and chlorite-carbonate occur in the host rocks adjacent to the
deposits. Quartz is the most important mineral. Figure shows that substantial quartz has been
subtracted from the alteration haloes. Some subtraction of Fe, Mg, Ca, Ti and Mn has also
occurred and this is doubtless the source for the ore minerals py-cp-po. Alumina is depleted in
the outer halo and concentrated in the inner halo in the form of sericite. C02 and H20 seem to
have been added from outside sources. Calculations indicate that the sheared amphibolites have
20X the concentration of metals necessary to produce all the ore in the deposit. Thus, in this case
lateral secretion is a viable model.
Volcanic Exhalative Processes
Ores often, show spatial relationships to volcanic rocks. They are conformable with the host and
frequently banded suggesting sedimentary processes. Principal constituent is pyrite with lesser
cp, sp, gn, barite and Ag-Au. Thought until the late 60’s to be epigenetic, but now realized they
are syngenetic. Show a progression of types with three distinct end members:
1. Cyprus type - Associated with mafic volcanics and ophiolite sequences. Found in
spreading centers and back arc basins. Consist predominantly of pyrite with lesser
chalcopyrite. Typified by the Cyprus pyrite-cu ores.
2. Besshi type - Associated with basaltic to dacitic volcanism. Thought to form during the
initial stages of island arc formation. Many Besshi type deposits occur in Precambrian
rocks and these may have been generated in entirely different tectonic settings. Pyrite
dominant, but chalcopyrite and sphalerite very common. Typified by many of the
volcanogenic deposits of Canada.
3. Kuroko type - Associated with dacitic to rhyolitic volcanics. Form during the waning
stages of island arc volcanism. Pyrite occurs, but is not dominant Usually galena or
sphalerite are predominate with lesser chalcopyrite and tetrahedrite. Also significant
silver in this type. Typified by the Kuroko deposits.
Although it is agreed ores are associated with volcanism the source of the ore bearing solutions
continues to be debated. Many feel ore fluids are of magmatic origin, but others feel they are
merely convecting seawater. Stable isotopes have provided the best evidence to support the
convecting seawater hypothesis. As can be seen in the Figure the Cyprus deposits have O/H
isotopic values which plot exactly on the seawater field. Kuroko ores show some deuterium
depletion relative to seawater, but still appear to have had a dominant seawater component.
Figure shows the model suggested from the isotopic studies.
Metamorphic Processes
Pyrometasomatic deposits (skarns) developed at the contact of plutons and host rock. Generally,
host rock is a carbonate and new minerals formed are the calc-silicates diopside, andradite and
wollastonite. T’s involved thought to be 300-500◦C, but pressure probably quite low. Three stage
process:
1. Recrystallization
2. Introduction of Si, Al, Fe, Mg
3. Hydration and introduction of elements associated with volatile fraction
Other metamorphic processes are relatively unimportant, but hydration/dehydration during
regional metamorphism may concentrate metals at the metamorphic front. Sodic metasomatism
of K-spar is thought to have been important in the concentration of gold at Kalgoorlie.
Conversion of feldspar from K-spar (1.33A) to Na plag (.97A) resulted in the expulsion of gold
(1.37A) which could no longer be accommodated in the feldspar lattice. Skip over Mechanicalchemical sedimentary processes since they are covered in the other course.
References: http://geology.csupomona.edu/drjessey/class/GSC433/Genesis.htm
Magmatic
Segregation
Pegmatitic
Deposition
Hydrothermal
Lateral Secretion
Metamorphic
Processes
THEORIES OF ORE DEPOSIT GENESIS
A: Origin Due to Internal Processes
Separation of ore minerals by fractional
crystallization during magmatic differentiation.
Pt—Cr deposits
Bushveld, S.A.
Titanium deposit
Tahawas, N.Y.
Liquid immiscibility. Settling out from magmas
Cu-Ni ores of
of sulfide, sulfide-oxide or oxide melts which
Sudbury, Canada and
accumulate beneath the silicates or are injected
the nickel extrusives
into country rocks or extruded on the surface.
of Kambalda, West
Australia.
Crystallization as disseminated grains or
Li-bearing pegmatites
segregations in pegmatites.
of Kings Mtn. N.C.
Deposition from hot aqueous solutions of various Porphyry Cu-Mo
sources.
deposits of the
W. Cordillera.
Diffusion of ore and gangue forming materials
Gold deposits of
from the country rocks into faults and other
Yellowknife, B.C. and
structures.
the Mother Lode, CA.
Pyrometasomatic (skarn) deposits formed by
W deposits at Bishop,
replacement of wall rocks adjacent to an
CA. Fe deposits
intrusive.
Iron Mtn UT.
Initial or further concentration of ore elements by Homestake Au Mine,
metamorphic processes.
Lead, South Dakota.
Mechanical
Accumulation
Sedimentary
Precipitation
B: Origin Due to Surface Processes
Concentration of heavy minerals into placer
Precipitation of certain elements in sedimentary
environments.
Residual Processes
Leaching of soluble elements leaving
concentrations of insoluble elements.
Secondary or
Supergene
Enrichment
Volcanic Exhalative
Process
Leaching of certain elements from the upper part
of a mineral deposit and their reprecipitation at
depth to produce higher concentrations.
Exhalations of sulfide-rich magmas at the
surface, usually under marine conditions.
Placer Au deposits of
Alaska and California.
Banded Iron Fm.
of the Canadian
Shield.
Nickel laterites
of New Caledonia
and Arkansas bauxite.
The upper portion of
many porphyry copper
deposits.
Mt. Isa, Aust.,
Sullivan and Kidd
Creek,Canada,
Kuroko,Japan.
GEOL.363 MINERAL DEPOSITS
Hand out No. 3
(Semester 1/2001)
CLASSIFICATION OF MINERAL DEPOSITS
In order to more readily study mineral deposits and explore for them more effectively, it is
helpful to first subdivide them into categories. This subdivision, or classification, can be based
on a number of criteria, such as minerals or metals contained, the shape or size of the deposit,
host rocks (the rocks which enclose or contain the deposit) or the genesis of the deposit (the
geological processes which combined to form the deposit).
Since there is considerable debate among geologists as to the exact mode of formation (genesis)
of most mineral deposits, this is not a good classification criterion. It is best to stick to features
we can all agree on, namely, the physical description of the deposit. We soon see that, even
though no two mineral deposits are exactly alike, most of them fall into one or another of a small
number of categories. We also see that each of these categories coincides with a generally
accepted hypothesis as to how the mineral deposits formed. In other words, although we started
out with a physically descriptive classification, we end up with a classification which also
coincides with what we perceive to be unique genetic processes. It is therefore useful to define a
small number of terms used in the classification which have a genetic connotation:
Hydrothermal
Hot water or hydrothermal solutions have actually been observed forming mineral deposits, for
example, the "black smokers" on the sea floor. The ore constituents, such as Cu, Pb, Au or other
metals are dissolved in a hot aqueous solution along with other deposit constituents such as Si, S
and Fe. These elements are deposited to form the ore and gangue minerals in response to a
change in the solution, very often a sharp decrease in temperature. an example of this process
would be if you dissolved as much table salt as possible in boiling water. If you then cool the
solution in the fridge, much of the salt will precipitate or come out of solution.
Magmatic
Some mineral deposits, particularly those containing Ni, Cr and Pt, form by the separation of the
metal sulphide or oxides in the molten form, within an igneous melt before it crystallizes. These
are known as magmatic deposits. They occur within the igneous rock from which they were
derived, such as a gabbro. The ore metals concentrated as liquid in much the same manner as
metals are purified in a smelter or blast furnace. The heavier metal-rich liquids sink and
concentrate at the base of the intrusive body, while lighter silicate liquid and crystals tend to rise,
the same as the slag in a blast furnace.
Syngenetic
A syngenetic mineral deposit is a deposit which formed at the same time as the rocks that
enclose it. Magmatic deposits are syngenetic in that the ore minerals crystallize from the same
liquid that produces the silicate minerals which form the bulk of the intrusive - they crystallize
more or less simultaneously as the melt cools. Deposits which form on the earth's surface in the
form of a sedimentary layer are also syngenetic. The rocks which they lie upon were deposited
just prior to the mineralizing event, while the overlying rocks were deposited just after - all three
layers being deposited at essentially the same time in terms of the geological time frame.
Epigenetic
If a mineral deposit formed much later than the rocks which enclose it, it is said to be epigenetic.
An example is a vein. The first step in the formation of a vein is the fracturing or breaking of
rock along a fault zone, at a depth ranging from surface to several kilometers below surface. The
rock must be solid (lithified) and brittle, creating open spaces when it breaks. Hydrothermal
solutions pass along the fault zone and deposit or precipitate the ore and gangue minerals within
the open spaces. Thus, the vein is necessarily younger than the rocks that contain it.
Since we are fairly certain which deposits are syngenetic and which are epigenetic (although
there will always be some degree of uncertainty and overlap), it is convenient to begin the
classification with this discrimination. Beyond this, the various categories are based on their
physical description, including size and shape. A third level of subdivision is usually based on
the metals contained. Here, then, is the classification:
1. Epigenetic
1. Porphyry
Large, low grade deposits usually associated with a porphyritic intrusive body.
A. Cu-Mo
B. Cu (-Au)
C. Mo (-W)
2. Skarn
Mineral deposits formed by replacement of limestone by ore and calc-silicate minerals, usually
adjacent to a felsic or granitic intrusive body.
A. W-Cu (-Zn, -Mo)
B. Zn-Pb-Ag (-Cu, -W)
C. Cu (-Fe, -Au, -Ag, - Mo)
D. Fe (-Cu, - Au)
E. Sn (-Cu, -W, -Zn)
F. Au (-As, -Cu)
3. Vein
Fracture filling deposits which often have great lateral and/or depth extent but which are usually
very narrow.
A. Hypothermal - Cu (-Au)
B. Mesothermal - Cu-Pb-Zn-Ag-Au
C. Epithermal - Au-Ag (-Hg)
4. Mississippi Valley
Named for the region where they were first described, these deposits formed within porous
carbonate rocks (limestone reefs or caves). They are Pb-Zn deposits with low Ag values.
2. Syngenetic
1. Volcanic Massive Sulphide (VMS)
These deposits formed as massive (over 60% sulphide) lens-like accumulations on or near the
sea floor in association with volcanic activity.
A. Felsic volcanic hosted - Cu-Pb-Zn-Ag-Au
B. Mafic volcanic hosted - Cu (-Zn, -Au)
C. Mixed volcanic/sedimentary - Cu-Zn (-Au)
2. Sedimentary Massive Sulphide (Sedex)
These are formed by hydrothermal emanations on or near the sea floor in association with the
deposition of sedimentary rocks.
A. Pb-Zn-Ag
B. Ba
3. Magmatic
During the crystallization of a magma, usually mafic or ultramafic, heavy, metal-rich liquids
settle and accumulate at specific sites, often at the base, within the intrusion.
A. PGM (Platinum group metals)
B. Chromite
C. Ni-Cu (-PGM)
4. Placer
Formed within sediments by the concentration of heavy resistant minerals (Au diamond,
cassiterite) by stream or wave action.
REFERENCES
McKinstry, H.E., 1948,Mining Geology: Englewood Cliffs, NJ, Prentice Hall, 680 p.
Peters, William C., 1978, Exploration and Mining Geology: John Wiley & Sons Inc., 696 p.
Guilbert, John M. and Park Jr., Charles F., 1986, The Geology of Ore Deposits: W.H. Freeman
and Company, 985 p.
Evan, Anthony M., 1993, Third Edition, Ore Geology and Industrial Minerals-An Introduction:
Blackwell Scientific Publications Inc., 389 p.
Eckstrand, O.R., 1984, Canadian Mineral Deposit Types: A Geological Synopsis, Geological
Survey of Canada, Economic Geology Report 36: Canadian Government Publishing
Centre, 86 p.
Roberts, R.G. and Sheahan, P.A., 1988, Ore Deposit Models, Geoscience Canada, Reprint Series
3: Geological Association of Canada, 194 p.
Reference: http://www.bc-mining-house.com/prospecting_school/dp_intr2.htm
GEOL.363 MINERAL DEPOSITS
Hand out No. 4
(Semester 1/2001)
PLATE TECTONICS AND MINERALIZATION
Many of the processes for mineral concentration that were described in the previous section
depend upon the presence of magma bodies underground. In Unit 3, Plate Tectonics, you learned
that magma often results from tectonic activity near plate boundaries, and so it should seem
reasonable that there are links between the occurrence of mineral deposits and the plate tectonic
history of a region. In particular, magma generation is associated with oceanic spreading ridges
and with subduction zones. Let us examine the deposits that are likely to result from each of
these settings.
1. Mineral Deposits at the Oceanic Ridges
We have already discussed the process by which hydrothermal fluids can concentrate metal
sulfide deposits at the oceanic ridges. Ocean water circulating through cracks in the new basaltic
sea floor created at spreading ridges is heated by contact with hot rocks. Metals are dissolved in
the saline water and precipitate near the discharge points of the hot springs when they combine
with hydrogen sulfide to form metal sulfides. In this manner copper, iron, lead, and zinc sulfide
minerals precipitate from solution. When the mineralized water encounters and mixes with cold
ocean water, oxides of iron and manganese may precipitate.
At the sites along the East Pacific Rise where the submersible Alvin was used to investigate the
spectacular hot spring vents described in Unit 6, Dynamics of the Oceans, the black smoker
chimneys were found to consist largely of metal sulfides. Chimneys have been discovered 15
meters (50 ft) tall resting on a metal sulfide mound of possibly equal height and up to 30 meters
(100 ft) across. Such chimney-mound structures may weigh several thousand tons and one that
was sampled was found to contain 14% iron, 0.7% copper, and 31% zinc, along with some
cobalt, silver, and gold.
Only a small portion of the oceanic ridge system has been investigated in detail to date, but on
the basis of present evidence, it seems that the extremely active chimney-vent type of activity
may be restricted to fast-spreading ridges such as the East Pacific Rise, where new sea floor is
being added at a rate of about 18 cm (8 in) per year. This type of vigorous sea floor spreading
activity is reflected in the hydrothermal regime present at the ridge crests.
Sea-floor spreading rates are much slower in other parts of the world, and this seems to influence
the style of hydrothermal activity as well. For instance, at sites investigated on the Mid-Atlantic
Ridge, where the spreading rate is on the order of two to three centimeters (one inch) per year,
surface deposits consist of nearly pure manganese oxide encrusting the ocean floor. Where
deeper layers of the sea floor are exposed, metallic sulfide deposits are found, indicating that the
hydrothermal fluids have had a chance to mix thoroughly with ocean water before emerging
from cracks in the walls of the central rift valley at the crest of the ridge.
Still another type of spreading boundary environment is encountered in the Red Sea mineral
deposits. The Red Sea is a new ocean, formed by the rifting of the Arabian peninsula away from
Africa. Its deposits were discovered in the 1960s during an international oceanographic
expedition when echo sounders on the research vessels recorded an unusual reflection within the
ocean water some distance above the seafloor. When the water was sampled at that depth, very
high salinities were found, indicating that the sonar reflection marked a boundary between
normal seawater and denser salt brine that was warm and rich in metals. The brine was collected
in a series of pools located along the axis of the Red Sea. When cores were taken of the sea floor
sediments, it was found that they contained layers of metal-rich sediment ten meters or more
thick.
The Red Sea deposits are rich in iron, copper, zinc, and small amounts of silver and gold. It is
estimated that the largest of these pools, the Atlantis II Deep, contains three million tons of
metals, not counting the iron minerals. The two countries flanking that part of the Red Sea, Saudi
Arabia and Sudan, have formed a joint commission to study the feasibility of mining this deposit.
If the venture appears to be profitable, the Red Sea may be the site for the first commercial
application of deep-sea mining. The technique most likely to be used is a dredge combined with
a powerful suction device controlled from a sea-surface vessel. The device would scoop up loose
sediment from the ocean floor and pump it up as a slurry to a ship on the surface.
In a few places, bits of oceanic crust have been scraped off the lithosphere at a subduction zone
and added to continental crust, making them easily accessible. These hunks of displaced ocean
floor are called ophiolites and have been mined for their stores of copper and other metals since
antiquity. The best-known example is the Troodos Massif on the island of Cyprus in the
Mediterranean Sea.
2. Mineral Deposits Related to Subduction Zones
Important occurrences of copper ore are found in what are called porphyry copper deposits.
These form by magmatic concentration with hydrothermal alteration and replacement in magma
intrusions into continental crust. Figure 11-4 shows the worldwide distribution of regions
containing major porphyry copper deposits as shaded zones. Note that many occur in close
association with subduction zones, such as those in South America, the Philippines, and the
Middle East. In other cases, such as in western North America, eastern Australia, and in the Ural
Mountains of Russia, the deposits possibly were associated with subduction that took place in the
past.
Recall that in a subduction zone, oceanic crust on the downgoing slab is heated, melted, and
chemically differentiated, producing relatively felsic magma that is supplied to the volcanic arc
above the subduction zone. The subduction process not only supplies the magma intrusions to
fuel the hydrothermal process, but because the subducted ocean floor was already enriched in
metal deposits by the processes described previously, it taps a source that has an abundant supply
of metals.
Porphyry copper deposits tend to be large and relatively low grade, containing from 0.2% to 2%
copper. Fortunately, the refining process is relatively easy, and because of the size and grade of
the deposits, they are often profitable to mine. A single porphyry deposit may contain up to
several million tons of copper, though most are considerably smaller.
Porphyry Copper Deposits in Relation to Plate Boundaries
In South America, the subduction zone associated with the Peru-Chile Trench is hard against the
continental coastline, while in many places along the northwest margin of the Pacific Ocean,
Back-Arc Spreading Shown in Relation to a Subduction Zone
subduction takes place some distance offshore from the continent of Asia, forming a volcanic
island arc separated from the main continental landmass by an oceanic basin in which slow seafloor spreading may be taking place. The Japan Sea is an example in which spreading has
widened the separation of the Japanese islands from China. Figure 11-5 shows the relation
between this spreading basin and the subduction zone.
A number of different mineral deposits are associated with back-arc spreading, resulting from
similar concentrating mechanisms to those active on the other oceanic spreading ridges. Chief
among these are the Kuroko-type massive sulfide deposits found in northern Japan and
elsewhere that contain copper, zinc, lead, gold, and silver concentrations. Although smaller than
the porphyry copper deposits, these are often significantly higher-grade ores and are of economic
value.
In some places, the extensional or pulling-apart forces that generate back-arc basins can operate
on a continent as well, so long as active subduction takes place nearby. This forms continental
rift systems much like the oceanic rifts, but within the continent. Where these rifts are flooded,
either with inflow of seawater or as lakes, like the great lake system of the East African Rift,
subaqueous hydrothermal activity may form other types of lead-zinc-silver-rich deposits in a
manner like that described for the Red Sea.
The western part of the United States was the scene of continuous subduction of Pacific Ocean
floor along the west coast until about 26 million years ago. Continental rifting occurred inland
from the subduction, similar to back-arc spreading and, where this was accompanied by certain
types of igneous intrusions, resulted in hydrothermal emplacement of large molybdenum
deposits in the front ranges of Colorado. These give the United States its dominant position in
reserves of this important steel-making element, as shown in Figure 11-3.
F. SOME NOTABLE MINERAL DEPOSITS
This is a good time for us to put together the various concepts discussed so far in terms of the
origin and setting for different kinds of deposits.
1. The Bushveld Igneous Complex, South Africa
The Bushveld igneous complex in South Africa is magmatic in origin and consists of a series of
layered igneous rocks that contain vast reserves of chromium along with platinum, nickel, and
iron. It is the largest single deposit of chromium ore found anywhere in the world and has been
estimated to contain reserves in the range of 6,000 million tons of chromite.
The Bushveld complex is a good example of magmatic concentration as described earlier, in
which chromite (chromium ore) accumulates toward the bottom of a layered magma intrusion
due to its high density and tendency to crystallize before the rest of the magma. The metals are
concentrated in a series of relatively thin layers ranging in thickness from centimeters to as much
as one meter. They extend over an area of many thousands of square kilometers, however,
accounting for large reserves.
Radioisotope dating of the Bushveld complex yields ages on the order of 2 billion years, placing
its origin nearly half way between the origin of Earth and the present. The complex occurs as a
series of nearly circular igneous intrusions that are relatively isolated in that the surrounding rock
is much older and essentially undisturbed. It does not appear to be associated with either
subduction or sea-floor spreading and so its origin is still a bit of a mystery, although an ancient
form of continental rifting may have been involved. Two interesting hypotheses have been
advanced -- one is that the intrusion is due to hot spot activity, and the other is that the circular
igneous complexes resulted from the impacts of fragments of an asteroid.
2. The Troodos Massif in Cyprus
The Troodos Massif in the western part of the Mediterranean island of Cyprus contains the
Troodos ophiolite segment of ocean floor produced by sea-floor spreading in Cretaceous time. It
contains massive sulfide deposits of iron, copper, zinc, and cobalt that were concentrated by seafloor hydrothermal activity at an ocean ridge, as well as magmatic nickel sulfide and chromium
deposits that were formed in deeper intrusive rocks at the ridge. The copper deposits on Cyprus
have been worked since antiquity, and indeed the metal derives its name from this locality,
having been called the "Cyprian metal."
Ophiolites around the world have their origins in the ocean floor and in addition to their mineral
deposits are of interest to geologists because they provide dry-land exposures where the structure
of what was once oceanic crust may be studied with relative ease.
3. Kuroko Massive Sulfide Deposits, Japan
We have already mentioned the Kuroko-type deposits in the previous section, relating their
origin to back-arc sea-floor spreading. The Kuroko massive sulfide deposits are found in the
northern portion of the main Japanese island of Honshu and contain zinc, copper, and lead along
with minor amounts of precious metals. The concentrating mechanism was hydrothermal activity
on the sea floor approximately 12 to 15 million years ago, making this a relatively young deposit,
but there are many similar, much older deposits in more ancient rocks in other parts of the world.
4. The Noranda District of Quebec, Canada
The Horne Mine and others at Noranda in northern Quebec is an example of a layered massive
sulfide deposit of considerably greater age, dating back to the early Precambrian. It is a classic
example of a submarine volcanic process forming layered sulfide deposits over the hot springs
on the ocean floor, and is similar to the Kuroko deposits of Japan. It produces copper, zinc, gold,
and silver.
5. Uranium Deposits in Saskatchwan, Canada
Uranium deposits are generally found in sedimentary environments and are of several types, but
their exact origin is still a matter of debate. Some geologists believe that the concentration
mechanism is chemical in nature, due to circulating groundwater that collects dispersed uranium
from the sediments and deposits it in favorable settings.
G. SEARCHING FOR MINERALS
Our greatly improved knowledge of how mineral deposits form has strongly affected the
approach used in locating them. If we return to our grizzled prospector with his pan and mule,
we can gain some perspective on how far we have come. For the most part, these hardy fortuneseekers had limited knowledge of geology or mineralogy. They were simply looking for a
particular type or color of rock. Not that they expected gold or other mineral deposits to appear
so obviously, but they knew that many of the minerals that they sought were to be found in rocks
that had been hydrothermally altered and were rich in sulfides. The prospector, then, was looking
for an environment favorable for mineralization, even though he knew little of the mineral
concentration mechanisms.
Modern prospecting is involved in similar searches, but in a more sophisticated manner. An
important first step is in the production of geologic maps of a region, which identifies the rock
types at the surface (or outcrop) in each area of the map. The geologic structure of an area can
provide important clues as to where mineralization may occur. Faults and fracture zones in the
rock can provide pathways for hydrothermal fluids and such linear features are often found in
association with mineralized zones. But these are all surface features, and many mineral deposits
are covered by unmineralized rocks or surface debris. How may these be found?
Dense or magnetic mineral deposits may be located by using gravity or magnetic surveys, in
which the Earth's gravity and magnetic fields are measured and mapped to high precision. For
example, if the gravity field shows an anomalously high value over a portion of ground, this
probably indicates that dense rock is buried below.
Geochemical surveys provide other techniques for literally "sniffing out" buried minerals. The
concentration mechanisms that we discussed may tend to act in a somewhat diffuse manner,
placing most of the deposit in one restricted location, but spreading smaller amounts of the
mineral throughout the rock layers over a much wider area. Leaching of minerals by
groundwater subsequent to the formation of a deposit may also serve to provide widespread
indications of its presence.
Samples of rock and soil are chemically analyzed and examined for traces of compounds that
indicate the presence of particular minerals. Even vegetation may yield important clues, since
trace minerals in the ground may be taken up by the plant's roots and incorporated into wood and
leaves. These analyses are usually carried out in sophisticated chemical laboratories, but new
technology has provided portable and even airborne instruments that allow analyses to be done
on the spot, with greater efficiency and less cost.
1. Mineral Exploration
The use of genetic models-which relate the geologic processes such as have been discussed to
the specific geologic environment that is most likely to host the particular kind of ore body being
sought-is an important first step in the exploration process. Choosing this geologic environment - "getting in close" -- is the "art" of exploration, the successful practice of which requires a broad
knowledge of the science and of the ore deposits, the real-life ore models.
Once the geologist has "gotten in close," largely by the use of general geologic principles,
augmented by fieldwork, the target commonly can be narrowed by geochemical and geophysical
surveys. Although a geochemical anomaly may be a direct reflection of a buried deposit, a
geophysical anomaly is only an indirect reflection of mineral deposits. Geophysical methods, by
and large, measure electrical responses and magnetic properties and responses that are not
specific to a given mineral or ore.
Some of the metallic mineral deposits have conductive properties that, when excited by an
induced current, generate a magnetic field which can be recorded by sophisticated
instrumentation. A number of electrical-magnetic prospecting methods and instruments are in
wide use, both as ground and airborne systems, and many hidden massive sulfide ore bodies
have been discovered around the world by their use.
The natural radioactivity of rocks can sometimes be more valuable a clue to mineral
concentration than those of the more "classical" physical properties. Apart from uranium and
thorium and the products of their radioactive decay, the only naturally occurring radioactive
element of any importance in prospecting is potassium.
Most rocks contain some uranium, thorium, and potassium, so radioactivity surveys, both ground
and airborne, can be of great use in general geologic mapping. Such surveys, of course, are of
direct use in prospecting for uranium deposits. Radioactivity sensors can also be lowered into
drill holes to detect various elements and properties in such holes not otherwise directly
detectable.
Since none of the geochemical and geophysical tools allow the geologist to see what is below the
surface, his most important exploration tool is the drill. It alone allows him to "see into the
ground" by taking samples from below the surface. Since a drill hole through rock is expensive,
and "sees" only the rock penetrated by the hole, the geologist must place his holes judiciously.
Moreover, since the hole may narrowly miss a deposit, he must study the samples closely and he
must recognize the "alteration halo" around the hidden deposit, in the same -- but more
sophisticated -- way as the prospector recognized the obvious altered rocks.
In summary, recent advances in scientific knowledge of how and where mineral deposits form
have given today's exploration geologist the intellectual tools unavailable to both his predecessor
and the prospector. These tools have allowed the geologist to narrow the search area. Theoretical
and technological advances in geochemistry and geophysics have allowed him to focus the
search. However, no methods yet known enable us to remotely detect a hidden ore deposit. Only
the expensive drill hole can make the discovery. The single most important element of
exploration, therefore, has been and remains, the drill hole.
Important as they are to the existence, well-being and improvement of modern society, mineral
deposits occupy much less that 1% of the Earth's surface. Since they occur only where the
vagaries of geologic processes placed them, if society is to continue to depend on an everincreasing supply of minerals for its well-being, it must carefully weigh the benefits to be gained
from exploration for and extraction of minerals against the benefits from other uses or
designations for the same land that would exclude these activities.
2. Remote Sensing
Some of the most intriguing new developments in mineral exploration do not even require the
presence of a geologist at the time a site is being investigated. Remote sensing, the analysis of
the surface of Earth from aircraft or spacecraft, has provided another method of exploration
geology.
The first application of remote sensing was apparently made by Galileo, who turned his
telescope on the Moon as it rose during daytime from behind a stone wall in his garden. He noted
that the wall, which was illuminated by sunlight, appeared brighter than the surface of the Moon.
He concluded that the lunar surface must reflect less light than terrestrial rocks and so must be
made of darker material.
Remote sensing, as we have already seen, was developed to high levels of sophistication during
the investigation of the solar system by spacecraft carrying a wide variety of instruments. That
technology has been turned back on Earth itself in a series of satellites designed to provide a new
look at our own planet from a much larger perspective than the ground-based observer ever
could obtain. In 1972, the first in a series of Earth-resource satellites, called LANDSAT, was
launched. Three additional Landsats have followed, providing unbroken surveillance for more
than a decade.
The first three Landsats carried a multispectral scanner, or MSS. This is a device that scans a
narrow strip of the surface beneath the satellite as it moves along its orbit. The strip is 185 km
(115 mi) wide, and the scanner is able to resolve features that are only 200 meters (660 ft) or less
in size. The unique feature of the MSS, however, is its ability to record the image as seen
through four different filters, each admitting radiation of a particular color or wavelength range.
Two of these spectral ranges are in visible light (green and red) and two are in the infrared
portion of the spectrum (see Figure 7-3). The images are digitized and radioed to Earth, where
they are processed and reconstructed into pictorial images. Modern satellite images can now
record 255 wavelengths of radiation with spatial resolutions of 10 meters.
A more recent version of these satellites contains a more advanced version of the MSS, called a
thematic mapper that uses seven spectral bands instead of four, and is capable of even higher
resolution (LANDSAT 7), approximately 20 meters per pixel side.
When different materials reflect sunlight, they selectively reflect more or less of each wavelength
of light in a pattern that is often unique to that material. This reflectance spectrum may then be
used to identify the material that is illuminated. The four (or seven) spectral bands measured by
the Landsats allows this kind of analysis to be made remotely from space. You have probably
seen the brightly colored images of portions of Earth from Landsat, in which the four spectral
images are superposed and printed in four false colors. The false color requirement arises from
the fact that infrared is invisible to the human eye, and so these images are printed in arbitrary
colors that have been chosen to make them stand out against the visible light images. A variety
of computer enhancements have been applied to these images in order to emphasize the
differences between rock and soil types, vegetative cover, and so on.
Landsat images have been used to study the shape of landscape features, geologic structures such
as folds and faults, and to produce rough geologic maps of broad areas that have not yet been
mapped in detail by traditional methods. The exploration geologist can use Landsat images in a
variety of ways to aid in the search. For instance, mineral concentration zones may reveal
themselves via the distinctive colors and reflectance spectra of the altered rock that often
surrounds them. Uranium deposition in sedimentary rocks is sometimes marked by distinctive
colors in the rocks. Landsat's sensitivity to the reflectance of chlorophyll can also be used to
detect characteristic vegetation patterns over mineralized zones. These kinds of data have
provided geologists with new tools and new perspectives for mineral exploration that can be
applied worldwide and, in some cases, at relatively little cost.
Another new remote sensing technique is provided by Side-Looking Airborne Radar (SLAR).
Radar is microwave radiation, with the unique capability of penetrating clouds. Unlike Landsat,
which is dependent upon clear weather for its observations of Earth's surface, radar can function
through overcast and view the surface without interference from the weather.
SLAR imagery shows the shape of the landscape so clearly that many geological structures may
be clearly seen. Linear alignments of valleys, cliffs, or other structures, called lineaments,
commonly betray the existence of faults and fractures within the rocks that may be sites of
mineralization.
Synthetic Aperature Radar (SAR) remote sensing instruments use a similar microwave radiation
source. However, later additional computer processing builds a "virtual antenna" equivalent to a
kilometers-long space craft. Fully processed images, such as those from the European Space
Agency ERS-1 satellite, have resolutions of approximately 10 meters. Related instruments have
also been flown in the Space Shuttle. These Shuttle Imaging Radars (SIR-A,B,C) have been
useful in studying geological and structures worldwide.
RECOMMENDED READING
For a basic description of resources check Skinner (1976) or Park and MacDiarmid (1976). For
specific deposits refer to Dixon (1979) or Kunz (1967). The geology of these deposits and their
relation to plate tectonic processes is well described in Bates (1969), Brookings (1990), and
Sawkins (1990).
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Robert L. Bates, Geology of the Industrial Rocks and Minerals, Dover, 1969.
D. G. Brookins, Mineral and Energy Deposits, Merrill, 1990.
C. J. Dixon, Atlas of Economic Mineral Deposits, Cornell University Press, 1979.
George F. Kunz, Gems and Precious Stones of North America, Dover, 1967.
C. F. Park Jr., and R. A. MacDiarmid, Ore Deposits, W. H. Freeman, 1975.
F. J. Sawkins, Metal Deposits in Relation to Plate Tectonics, Springer-Verlag, 1990.
Brian J. Skinner, Earth Resources, Prentice Hall, 1976.
Reference: http://mac01.eps.pitt.edu/harbbook/c_xi/chap11.html