Tom V. Segalst ad
Universit y of Oslo
Mineralogical-Geological Museum
Sars' Gat e 1, N-0562 Oslo, Norw ay
May 1997
Short Course:
Environmental Geochemistry of Ore Deposits and Mining Activities.
ORE MATERIALS:
PRIMARY AND SECONDARY MINERALOGY OF ORE
DEPOSITS
Introduction
Our societ y and civilizat ion is built upon resources of all kinds, also eart h
resources. For example, and ordinary t elephone receiver is made from more than
40 dif f erent chemical element s, most of them being dif f erent met als.
The element s may occur in their nat ive stat e in t he Eart h, but most oft en w e find
t hat the chemical element s make a substance t ogether. The nat ive element s and
t heir compounds, formed by and occurring in nat ure, are called minerals. Minerals
should be solids charact erized by a unique chemical compositions, w hich can be
expressed in a chemical formula, and charact erized by a unique at omic
arrangement (st ruct ure), w hich can be expressed in a unique X-ray dif f ract ion
pat t ern.
In order t o exploit the dif f erent element s, it is best to produce t he element s from
places w here dif f erent processes have lead to their enrichment . In nat ure, such
a place is called a mineral deposit or an ore deposit. An ore deposit usually
includes nat urally occurring mat erials w hich can be subject ed t o mining w it h
economical prof it. " Ore" is in the English language also used about such mat erials
even if they cannot be exploit ed w ith a prof it.
In Norw egian law t he w ord " malm" for ore is def ined as a mineral cont aining
met als subject ed t o exploit at ion w ith a density $ 5 g/cm 3 . In Norw egian t he w ord
" ert s" is derived from the German w ord " Erz" meaning the ore mineral w ithout the
demand it being economical.
The English w ord " ore" is derived from Greek " aurum" for gold (chemical symbol
Au) derived f rom Sanskrit aw es and aus meaning " t o shine" . The German w ord
" Erz" is also derived f rom t he same Sanskrit w ords. Sanskrit as means to burn,
w hich has led to Greek " eos" for the sunrise colors and it s English derivat ion
" east " for t he direct ion to the daw n. The Scandinavian w ord " malm" is derived
f rom Sanskrit mai w hich means cut and soil, i.e. gravel.
2
Metals
Most ore deposits are mined f or t he exploitat ion of metals. A met al is any
chemical element (or element s in an alloy) having a charact erist ic lust er, is
opaque, and is a good conduct or for heat and elect ricity. Approximat ely 80 of our
chemical element s are met als. Heavy metals have a density $ 5 g/cm 3 . Light
metals have a densit y < 5 g/cm 3 .
We may divide met als int o noble metals (rare met als w ith high value due to high
lust er; gold, silver, plat inum-group metals), semi-noble metal (mercury), base
metal (more common met als, of t en major met al in alloys; copper, lead, zinc), alloy
metals (minor met als in alloys; molybdenum, chromium, tungst en, vanadium,
et c.), semi-metals (t in, arsenic), and fissionable metals (uranium, thorium).
Metallogeny or metallogenesis are w ords used about the st udy of how mineral
deposits formed, included their place in time and space, and t heir relat ion to the
regional pet rographic and tect onic charact erist ics of the Eart h' s crust . We may
also t alk about met allogenet ic maps, met allogenet ic provinces, and met allogenet ic
epochs.
Local enrichment makes a deposit
For an ore deposit to form, the chemical element in quest ion must be enriched
considerably versus it s average crust al abundance. Hence it s concentration factor
in nat ure may be large, and put const rains on the ore-processes concent rat ing
element s in nat ure. The follow ing table is modif ied from Fergusson (1985):
Element
Mercury
Lead
Chromium
Tin
Gold
Zinc
Uranium
Manganese
Nickel
Cobalt
Copper
Tit anium
Iron
Aluminum
Crust al abundance
ppm
0.089
12
110
1.7
0.004
94
1.7
1,300
89
25
63
6,400
58,000
83,000
Cut -of f grade
ppm
1,000
40,000
230,000
3,500
~ 5
35,000
700
250,000
9,000
2,000
3,500
100,000
200,000
185,000
Concent rat ion fact or
(rat io)
11,200
3,300
2,100
2,000
~ 1,250
370
350
190
100
80
56
16
3.4
2.2
3
Study of enrichment processes
The big research challenge is t o f ind out how nat ure managed to concent rat e the
element s int o ore deposit s. If w e understand how t hese processes w ork, w e may
be able to find more ore deposit s.
A common type of ore deposit is the one formed by act ions of hot w at er, the
hydrot hermal t ype. For this type of deposit w e may ask the follow ing types of
questions:
-
Where did the w at er come from?
Where did the heat come from?
Where did the dif f erent chemical element s come from?
How w ere the dif f erent chemical element s transport ed?
How w ere the minerals precipit at ed (in open or closed syst em, by cooling,
heat ing, change of pressure, boiling, w all-rock react ions by changing pH and/or
redox pot ent ial, et c.)?
- What geological conditions led t o t he format ion of the deposit?
In order t o address such quest ions w e need field descriptions and samples of the
mineralizat ion, w e need t o est ablish t he cryst allizat ion sequence of the minerals
(paragenet ic sequence) including the w all-rock alterat ions. We st udy the inclusions
of fluids and gases in the minerals in order to est ablish temperat ure, pressure,
salinit y, gas cont ent s, et c. w hen the mineralizat ion occurred. We also analyze
isot opes from the deposit; the light st able isot opes of H, O, C, and S can give us
inf ormat ion about sources of w at er, carbon, and sulf ur. Finally w e use comput er
modelling of thermodynamic relat ions bet w een minerals, w at er, and gas, to
calculat e st abilit ies of minerals and concent rat ions of chemical element s in the
f luid, trying t o quant itat ively simulat e the format ion of the observed deposit.
From such ext ensive st udies w e learn more about how such deposits are formed,
and can then ut ilize t he inf ormat ion to bet t er search for new ore and mineral
deposits. Our st udent s learning t hese techniques get jobs in ore geology and
prospect ing, pet roleum geology, geochemical and chemical analysis, and
environmental chemist ry.
The ore materials
The chemical element s can be classified in dif f erent w ays. The fat her of
geochemistry w as the Norw egian prof essor Vict or Morit z Goldschmidt (18881947) of the Universit y of Oslo. For geochemical purposes he divided the chemical
element s int o atmophile (forming the at mosphere), chalcophile (sulfide-f orming,
concent rat ing in the Eart h' s crust ), siderophile (occurring as noble met als,
concent rat ing in t he Eart h' s core), and lithophile (associat ing w ith oxygen in the
Eart h' s crust and mant le).
4
The accompanying table from Fergusson (1985) show s the most important ore
minerals, classif ied bet w een the abundant met als (iron, aluminum, tit anium,
manganese, magnesium) and the scarce met als (subdivided bet w een chalcophile,
siderophile, and lit hophile chemical element s).
The abundant ore met als mainly occur as met al oxide minerals, except for
magnesium ore occurring as met al carbonat e minerals. Among the scarce met als,
w e see t hat t he base met als and some of t he alloy met als are chalcophile,
occurring as met al sulfide minerals. The noble met als are siderophile (except
silver), mainly occurring in nat ive f orm (or alloys), but may occur as rare minerals
w hen t hese met als combine w ith t ellurium, arsenic, and sulf ur. Ot her alloy met als
and the fissionable met als are lit hophile, commonly occurring as met al oxide
minerals, but also as met al-oxyanion minerals (e.g. tungst enat e, phosphat e).
The next t able, also f rom Fergusson (1985), show s t he met als classified slight ly
dif f erent than bef ore, list ing know n reserves and their approximat e project ed
lif et ime, t he countries w ith main reserves, and the principal ore mat erial (except
magnesium, w here seaw at er is an import ant " commodit y" ).
The minerals are ident if ied mainly by t heir physical propert ies (color, lust er, cryst al
f orm, hardness, et c.) by t he naked eye or w ith a hand-lens. Furt her ident ificat ion
can be made by the help of opt ical propert ies in the ref lect ed light (ore minerals)
or transmit t ed light microscope of polished and t hin sect ions, respect ively. Mineral
ident if icat ion can also be done by X-ray dif f ractomet ry (XRD) and chemical
analysis in the elect ron microprobe.
Ore deposit types
A lot of w ork has been spent over t he years t rying to classify ore deposits int o a
f ew major t ypes. The hope w as that if a few charact erist ics of an ore deposit
under invest igat ion could be classified as belonging t o one of t he f ew " t ypes" , the
rest of it s charact erist ics, including the genesis of that deposit, w ould be
est ablished from t he " t ype" . Furt her research has show n that this " classificat ion
game" does not w ork in all inst ances. There is a tendency in nat ure not to " put"
t he dif f erent chemical elements in diff erent " boxes" (t ypes), but there are
gradat ions bet w een the dif f erent " t ypes" . We also find cases w here nat ure has
obt ained the same " result " from complet ely dif f erent processes.
As an example of early classificat ion of hydrot hermal ore deposit s, here is
enclosed Lindgren' s (Evans, 1993) classif icat ion based on approximat e
t emperat ures of format ion: Hypothermal (300-600EC), mesothermal (200-300EC),
epithermal (50-200EC), and telethermal (~ 100EC). Based on this simple
classificat ion criterion (t emperat ure) Lindgren' s " t ypes" included generalizat ions
on depth of format ion, occurrence, nature of ore zones, met al af f iliat ions, ore
minerals, gangue minerals, w all-rock alterat ion, text ures and st ruct ures, and
zoning.
5
In some examples this outline seems t o fit beaut ifully, but in ot her cases not. For
inst ance for mesot hermal deposit s the silver deposit of Cobalt (Ont ario, Canada)
is included. Going int o det ails, w e don' t find t hese silver deposit s " in or near
int rusive rocks" that can have had anything to do w ith t heir genesis, and f luid
inclusion and geochemical dat a show a much more complex genesis than
ant icipat ed from Lindgren' s " t ype" (Kissin, 1993).
A very general genet ic classificat ion is using the rock type classificat ion:
Magmatic, sedimentary, and metamorphic. Adding the action of volat iles, w e get
metasomatic (replacement t hrough volat ile act ion), hydrothermal (act ion of hot
w at er) and pneumatolytic (act ion of gases). We may talk about supergene (act ion
of surf ace processes). And w e may dist inguish bet w een syngenetic (f ormed
simult aneously w ith the rocks) and epigenetic (f ormed af t er the rocks) ore
deposit s.
Of t en w e choose to use a classificat ion based on w ay of occurrence rat her on
some physical-chemical paramet er (like t emperat ure) or genesis. We may talk
about a stratiform deposit w hen occurring in or as a st rat igraphic bedding, but a
stratabound deposit w hen just rest rict ed t o a part icular part of the st rat igraphic
column.
Other times w e ref er to ore deposit s typical of cert ain places, like Bushveld Type
(st rat iform) and Alpine Type (podif orm) ort homagmat ic chromit e deposit s;
Sudbury Type ort homagmat ic nickel (plus copper and iron) deposit; Cyprus Type
st rat iform sulfide deposits in ophiolit e environment ; Besshi Type st rat iform sulfide
deposits in t errigenous sediment s in Japan; Mississippi Valley Type (MVT)
epigenet ic carbonat e-host ed lead-zinc deposits; and Kiruna Type iron-oxide/apat ite
ores of Sw eden.
The deposit s may be named aft er their rock t ype, like pegmatite (huge cryst als);
porphyry copper (disseminat ed st ockw ork associat ed w ith plut onic int rusives w it h
copper ± molybdenum ± t ungst en, or tin); skarn (calc-silicat e rock); massive
sulfides; Kuroko (= black ore), Oko (= yellow ore), Keiko (= silicious ore),
Sekkoko (= sulf at e ore), and Tetsusekiei (f erruginous chert ), all descript ively
named in Japanese and charact erist ic of st rat if orm sulf ide deposit s in volcanic
environment s in Japan; and Kupferschiefer (European copper shale).
Other types of deposit s may be know n by t heir form, like Roll-front uranium
sandst one deposit s.
Some deposit types are bet t er know n from their acronyms: BIF = Banded Iron
Format ion iron-oxide/quart z ores; MVT = Mississippi Valley Type epigenet ic
carbonat e-host ed lead-zinc deposit s; SedEx =
Sediment ary Exhalat ive
(sediment ary hosted) lead-zinc deposits; and VMS = Volcanic-associat ed Massive
Sulf ide deposit s.
6
The ore morphology
From all the dif f erent t ypes of classif icat ions show n above, it must be quit e clear
t hat ore bodies and mineral deposit s show enormous variat ion in all respect s:
Chemist ry, mineralogy, text ures, st ruct ures, w all-rock int eract ion, geologic
set t ing, et c. It is simply not possible to generalize on t he morphology of ore
deposits. Some ores are massive, others are disseminat ed.
The room for the deposits is given in diff erent w ays. Sediment ary accumulat ion
on t he sea-f loor is easier t o give room than a skarn replacement deposit ; in t he
f irst case the seaw at er have to give w ay t o the sediment arily deposit ed ore
minerals, in the second case there has to be a dissolut ion of w allrocks (e.g.
carbonat es) bef ore the ore minerals have space to form.
For hydrot hermal ore deposits w e need permeability and room f or t he hot f luid to
f low and to let it s ore minerals deposit. This permeabilit y can be supplied by
ordinary pore spaces in t he rocks, but t he most ef f icient flow w ill be had through
f ract ures and fault s. Fract ures can be formed in dif f erent w ays: tect onic fract uring
and folding, by shrinkage of solidifying intrusives, by hydraulic fract uring by the
high-pressured fluid it self, and by high-pressured gas vent s, pipes, chimneys or
breccias. Hence the shapes and st ruct ures of hydrot hermal ore deposits w ill be
highly variable.
The ore texture, i.e. cryst al size, shape, and w ay of int ergrow t h, is dependent on
t he cryst al nucleat ion, cryst al grow t h, and the physical-chemist ry of the
mineralizing syst em. This also det ermines t he sequence of mineralization, i.e.
paragenetic sequence.
Open-space mineral deposit ion from hydrot hermal solut ions can result in comb
st ruct ures and in symmet rically and rhytmically crust ified veins. In such veins it
is quit e easy t o ident ify the dif f erent minerals and t heir position in t he veins in
order to est ablish the paragenet ic sequence.
Zoning
In some types of ore deposit s zoning of minerals, lat erally and/or vertically, occur.
Such zoning is w ell know n f rom t he granite-associat ed hydrot hermal vein deposits
in Cornw all (England), w here the met als are zoned in t he sequence tin - copper lead/zinc - iron out w ard from t he granit e cont act . In submarine hydrot hermal
deposit s there is a common met al zoning out w ard like iron - (t in) - copper - zinc lead - silver - barium. In syngenet ic sediment ary ore deposits there is commonly
a met al zoning proceeding basinw ise like copper/silver - lead - zinc.
This means that there w ill of t en be a change of mineralogy across a mineral
deposit or across a mining dist rict due to zoning. We can t heref ore not assume the
mineral composition to be const ant .
7
Ore mineral stability and transition
On first hand approximat ion w e w ould assume that the minerals observed in a
rock or an ore deposit are the same minerals t hat once deposited. This is not
alw ays so, especially not for a number of sulf ide minerals. Some high-t emperat ure
sulf ide minerals like molybdenit e, arsenopyrit e w it h pyrit e, and sphalerit e seem to
be quit e st able af t er t hey are formed.
On the other hand, other iron sulfides, copper-iron sulfides, copper sulfides and
silver sulfide w ill re-equilibrat e and form other minerals by exsolut ion and
decomposition at low er t emperat ures (see t able enclosed). It is import ant to note
t hat some types of exsolut ion and decomposit ion occur at decreasing volume,
t hereby creat ing pore space available for lat er passage of fluids.
Supergene alteration
The combinat ion of air and surf ace- and ground-w at er (" met eoric w at er" ) in
diff erent proportions is part icularly ef f ective in causing dissolut ion, transport , and
re-deposition of met als f rom previous mineralized ground. It may be considered a
kind of w eathering of ore minerals. Some chemical element s may be leached aw ay
by groundw at er, and some element s, like copper, are re-precipit at ed fart her dow n,
in some cases by a fact or of 2 or more. Wit hout this secondary enrichment , most
copper deposits of t he West ern Hemisphere w ould not be minable (Brimhall,
1991).
The minerals above t he ground w at er table are oxidized, w hile reduct ion occurs
below the ground w at er t able. The leached and oxidized top, of t en brow n in color
f rom iron oxy-hydroxide minerals, is called a " gossan" or " iron hat " (see figure).
Wall-rock alteration
Alterat ion of w all-rocks by circulat ing hydrot hermal solut ions is called hypogene
alt erat ion. The alt erat ion occurs as a response to changes in pressure,
t emperat ure, and composition. It may be considered as a kind of met amorphic
change of t he w all-rocks. But it may be very dif f icult in some areas to dist inguish
w all-rock alt erat ion, such as chlorit ization, from t he ef f ect s of low grade regional
or cont act met amorphism. The w all-rock alterat ion may show zoning around large
ore-bodies, such as porphyry copper and porphyry molybdenum deposit s.
At high t emperat ure w e find advanced argillic alteration charact erized by quartz
and t he aluminum silicat es dickit e, kaolinite, and pyrophyllite. Sericite is usually
present , and frequent ly alunite, pyrite, tourmaline, and topaz. This kind of
alt erat ion commonly displays light colors.
Sericitization is t he most common kind of alt erat ion of aluminum rocks next to ore
bodies. The dominant minerals are sericite, quart z, and pyrite. This kind of
alt erat ion commonly displays yellow ish colors.
Intermediate argillic alteration occurs at low er temperat ures, and the principal
minerals are now kaoline- and mont morillonit e. The rocks w ill be show ing light
colors.
Propylitic alteration is charact erized by chlorit e, epidot e, albit e, and carbonat es.
The alt erat ion color w ill be int ense green.
8
Chloritization is a common product from w all-rock alterat ion The color is green.
Ot her kinds of w all-rock alt erat ion may be charact erized as: Carbonatization,
potassic alt erat ion (pot assium feldspar ± biot it e), silicification, feldspathization
(like albitization), tourmalinization, alunitization, pyritization, hematitization,
bleaching, greisenization, fenitization, serpentinization, zeolitization (Evans, 1993).
References in text:
Brimhall, G. 1991: The genesis of ores. Scient if ic American, May, 48-55.
Evans, A.M. 1 9 9 3 : Ore geology and indust rial minerals. An introduct ion. 3rd
edition. Blackw ell Science, 389 pp.
Fergusson, J.E. 1985: Inorganic chemistry and t he Eart h. Pergamon Press, 400
pp.
Kissin, S.A. 1993: Five-element (Ni-Co-As-Ag-Bi) veins. In Sheahan, P.A. &
Cherry, M.E. (Eds.): Ore deposit models, Vol. II. Geoscience Canada Reprint Series
6, 87-98.