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Mineral deposits that are
essentially as originally formed are called primary or hypogene. The term
hypogene also indicates formation by upward movement of material. Deposits that
have been altered by weathering or other superficial processes are secondary or
supergene deposits. Mineral deposits that formed at the same time as the
enclosing rock are called syngenetic, and those that were introduced into
preexisting rocks are called epigenetic.
The distinction between metallic and
nonmetallic deposits is at times an arbitrary one since some substances
classified as nonmetals, such as lepidolite, spodumene, beryl, and
rhodochrosite, are the source of metals. The principal reasons for
distinguishing nonmetallic from metallic deposits are practical ones, and
include such economic factors as recovery methods and uses.
Fig. 1 Typical forms of deposits. (a) Vein
developed in fissured or sheeted zone. (b) Brecciated vein in
granite.
Concentration
The Earth's crust consists of
igneous, sedimentary, and metamorphic rocks. Table 1 gives the essential
composition of the crust and shows that 10 elements make up more than 99% of the
total. Of these, aluminum, iron, and magnesium are industrial metals. The other
metals are present in small quantities, mostly in igneous rocks (Table 2).
Most mineral deposits are natural
enrichments and concentrations of original material produced by different
geologic processes. To be of commercial grade, the metals must be present in
much higher concentrations than the averages shown in Table 2. For example, the
following metals must be concentrated in the amounts indicated to be considered
ores: aluminum, about 30%; copper, 0.7–10%; lead, 2–4%; zinc, 3–8%; and gold,
silver, and uranium, only a small fraction of a percent of metal. Therefore,
natural processes of concentration have increased the aluminum content of
aluminum ore 3 or 4 times, and even a low-grade gold ore may represent a
concentration of 20,000 times. Economic considerations, such as the amount and
concentration of metal, the cost of mining and refining, and the market value of
the metal, determine whether the ore is of commercial grade.
Forms of
deposits
Mineral deposits occur in many forms
depending upon their origin, later deformation, and changes caused by
weathering. Syngenetic deposits are generally sheetlike, tabular, or lenticular,
but may on occasion be irregular or roughly spherical.
Epigenetic deposits exhibit a
variety of forms. Veins or lodes are tabular or sheetlike bodies that originate
by filling fissures or replacing the country rock along a fissure (Fig. 1a).
Replacement bodies in limestone may be very irregular. Veins are usually
inclined steeply and may either cut across or conform with the bedding or
foliation of the enclosing rocks. The inclination is called the dip, and is the
angle between the vein and the horizontal. The horizontal trend of the vein is
its strike, and the vertical angle between a horizontal plane and the line of
maximum elongation of the vein is the plunge. The veins of a mining district
commonly occur as systems which have a general strike, and one or more systems
may be present at some angle to the main series. In places the mineralization is
a network of small, irregular, discontinuous veins called a stockwork.
Mineral deposits are seldom equally
rich throughout. The pay ore may occur in streaks, spots, bunches, or bands
separated by low-grade material or by gangue. These concentrations of valuable
ore are called ore shoots; if roughly horizontal they are called ore horizons,
and if steeply inclined they are called chimneys. After formation, mineral
deposits may be deformed by folding, faulting, or brecciation (Fig. 1b ).
Metasomatism, or
replacement
Metasomatism, or replacement, is the
process of essentially simultaneous removal and deposition of chemical matter. A
striking manifestation of this process in mineral deposits is the replacement of
one mineral by another mineral or mineral aggregate of partly or wholly
different composition. A large volume of rock may be transformed in this manner,
and the resulting deposit is generally of equal volume. Commonly the original
structure and texture of the replaced rock is preserved by the replacing
material.
Replacement, evidence for which is
found in many mineral deposits, operates at all depths under a wide range of
temperature. The evidence indicates that the new minerals formed in response to
conditions that were unstable for the preexisting ones.
Fig. 2 Replacement of limestone by ore along a
fissure. Disseminated ore, indicated by the dots, is forming in advance of the
main body.
Usually the replacing material moves
to the site of metasomatism along relatively large openings such as faults,
fractures, bedding planes, and shear zones. It then penetrates the rock along
smaller cracks and finally enters individual mineral grains along cleavage
planes, minute fractures, and grain boundaries where substitution may take place
on an atomic scale until the entire mass has been transformed (Fig. 2). After
gaining access to individual grains, the replacement may proceed by diffusion of
ions through the solid, controlled in large part by imperfections in the crystal
structure. In many deposits repeated movement has opened and reopened
channelways, which would otherwise have become clogged, to permit continued and
widespread replacement. The process may take place through the action of gases
or solutions or by reactions in the solid state. See also: Metasomatism
Fig. 3 Association of contact metasomatic and
vein deposits with intrusive magmas.
Classification
Mineral deposits are generally
classified on the basis of the geologic processes responsible for their
formation as magmatic, contact metasomatic, pegmatitic, hydrothermal,
sedimentary, residual, and regional metamorphic deposits.
Magmatic
deposits
Some mineral deposits originated by
cooling and crystallization of magma, and the concentrated minerals form part of
the body of the igneous rock. If the magma solidified by simple crystallization,
the economically valuable mineral is distributed through the resulting rock;
diamond deposits found in peridotite are believed by some geologists to be of
this type. However, if the magma has differentiated during crystallization,
early formed minerals may settle to the bottom of the magma chamber and form
segregations such as the chromite deposits of the Bushveld in
Contact
metasomatic deposits
During the crystallization of
certain magmas a considerable amount of fluid escapes. This fluid may produce
widespread changes near the contacts of magma with the surrounding rocks (Fig.
3). Where such changes are caused by heat effects, without addition of material
from the magma, the resulting deposits are called contact metamorphic. If
appreciable material is contributed by the magma, the deposits are termed
contact metasomatic. The term skarn is applied to the lime-bearing silicates
formed by the introduction of Si, Al, Fe, and Mg into a carbonate rock; some
skarns contain ore bodies. The magmas that produce these effects are largely
silicic in composition and resulting mineral deposits are often irregular in
form. See also: Skarn
A complicating case exists where
little fluid escaped from the magma but the heat of the intrusion was great
enough to cause dissolution and movement of certain metals from the surrounding
rocks. It is believed by some investigators that solutions formed in this manner
may become concentrated in metal content and subsequently deposit these metals
near the contact of the intrusion and the surrounding rocks. In this case, the
ore minerals were deposited by replacing preexisting rocks but the source of the
ore is the surrounding rocks, not the magma. To further complicate matters, the
ore in some deposits appears to consist of material derived from both the
intrusion and the surrounding rocks. In such deposits the source of the ore is
generally controversial, and the size, amount, and composition of the
mineralization would depend upon the relative contributions from the intrusion
and the associated rocks.
Under contact metasomatic
conditions, the (ore-forming) fluids extensively replace the country rock to
produce a variety of complex minerals. Contact metasomatic deposits include a
number of important deposits, whereas contact metamorphic deposits are rarely of
economic value. Many garnet, emery, and graphite deposits are classed as contact
metasomatic, as are such metalliferous deposits as the iron ores of
Pegmatite
deposits
Pegmatites are relatively
coarse-grained rocks found in igneous and metamorphic regions. The great
majority of them consist of feldspar and quartz, often accompanied by mica, but
complex pegmatites contain unusual minerals and rare elements. Many pegmatites
are regular tabular bodies; others are highly irregular and grade into the
surrounding rocks. In size, pegmatites range from a few inches in length to
bodies over 1000 ft (3000 m) long and scores of feet across. Some pegmatites are
zoned, commonly with a core of quartz surrounded by zones in which one or two
minerals predominate. See also: Feldspar; Quartz
Pegmatites may originate by various
igneous and metamorphic processes. Fractional crystallization of a magma results
in residual solutions that are generally rich in alkalies, alumina, water, and
other volatiles. The volatiles lower the temperature of this liquid and make it
unusually fluid; the low viscosity promotes the formation of coarse-grained
minerals. The rare elements that were unable by substitution to enter into the
crystal structure of earlier-formed minerals, principally because of differences
in size of their atomic radii, are concentrated in the residual pegmatite
solutions. Late hydrothermal fluids may alter some of the previously formed
pegmatite minerals.
Some pegmatites develop by
replacement of the country rock and commonly these are isolated bodies with no
feeders or channels in depth. They occur in metamorphic regions usually devoid
of igneous rocks and contain essentially the same minerals as those in the
country rocks. In some regions small pegmatites have grown by forcing apart the
surrounding metamorphic rock, and others have formed by filling a fissure or
crack from the walls inward. In both cases growth is believed to have taken
place by diffusion and consolidation of material in the solid state. See also:
Pegmatite
Hydrothermal
deposits
Most vein and replacement deposits
are believed to be the result of precipitation of mineral matter from dilute,
hot ascending fluids. As the temperature and pressure decrease, deposition of
dissolved material takes place. It is not altogether certain how important the
gaseous state is in the transport of ore material. It may be that at relatively
shallow depth and high temperature gaseous solutions transport significant
amounts of ore-forming material.
W. Lindgren, who developed the
hydrothermal theory, divided these deposits into three groups on the basis of
temperature and pressure conditions supposed to exist at the time of formation.
Deposits thought to form at temperatures of 50–200°C (120–390°F) at slight depth
beneath the surface are called epithermal. Many ores of mercury, antimony, gold,
and silver are of this type. Deposits formed at 200–300°C (390–570°F) at
moderate depths are known as mesothermal and include ores of gold-quartz,
silver-lead, copper, and numerous other types. Hypothermal deposits are those
formed at 300–500°C (570–930°F) at high pressures; certain tin, tungsten, and
gold-quartz ores belong to this type.
The nature of hydrothermal fluids is
inferred by analogy with laboratory experiments, and by investigation of
deposits forming around volcanoes and hot springs at the present time. Studies
of liquid inclusions in minerals, of mineral textures, and of inversion
temperatures of minerals indicate that mineralization takes place at elevated
temperatures. Layers of minerals on the walls of open fissures with crystal
faces developed toward the openings suggest deposition from solution. In some of
these cavities later crystals were deposited on earlier ones in a manner that
suggests growth in moving solutions. Certain secondary replacement phenomena,
such as weathering and oxidation of mineral deposits, also indicate deposition
from liquid solutions. Studies of wall rock alteration where hydrothermal
solutions have attacked and replaced rock minerals indicate that these solutions
change in character from place to place. Sulfur in such solutions may react with
or leach metals from the surrounding rocks or partly solidified magma to form
certain kinds of mineral deposits. On the basis of geochemical data it has been
estimated that most hydrothermal ore-forming solutions had a temperature in the
range 50–600°C (120–1100°F), formed under pressures ranging from atmospheric to
several thousand atmospheres, commonly contained high concentrations of NaCl and
were saturated with silica but were not highly concentrated in ore metals, were
neutral or within about 2 pH units of neutrality; and that the metals probably
were transported as complexes.
The principal objections to the
hydrothermal theory are the low solubility of sulfides in water and the enormous
quantities of water required. W. Lindgren realized this and, for some deposits,
favored colloidal solutions as carriers of metals. Laboratory synthesis of
sulfide minerals by G. Kullerud shows that some ore-bearing solutions must have
been considerably more concentrated than is generally believed.
Two common features of hydrothermal
deposits are the zonal arrangement of minerals and alteration of wall rock.
1. Zoning of mineralization. Many
ore deposits change in composition with depth, lateral distance, or both,
resulting in a zonal arrangement of minerals or elements. This arrangement is
generally interpreted as being due to deposition from solution with decreasing
temperature and pressure, the solution precipitating minerals in reverse order
of their solubilities. Other factors are also involved such as concentration,
relative abundance, decrease in electrode potentials, and reactions within the
solutions and with the wall rocks as precipitation progresses.
Zonal distribution of minerals was
first noted in mineral deposits associated in space with large igneous bodies,
and has since been extended to include zoning related to sedimentary and
metamorphic processes in places where no igneous bodies are in evidence.
Although many geologists interpret zoning as a result of precipitation from a
single ascending solution, others believe deposition is achieved from solutions
of different ages and of different compositions.
The distribution of mineral zones is
clearly shown at Cornwall, England, and at Butte, Montana. At Cornwall, tin
veins in depth pass upward and outward into copper veins, followed by veins of
lead-silver, then antimony, and finally iron and manganese carbonates. Such
zoning is by no means a universal phenomenon, and, in addition to mines and
districts where it is lacking, there are places where reversals of zones occur.
Some of these reversals have been explained more or less satisfactorily by
telescoping of minerals near the surface, by the effects of structural control
or of composition of the host rock in precipitating certain minerals, and by the
effects of supergene enrichment on the original zoning, but many discrepancies
are not adequately explained.
2. Wall rock alteration. The wall
rocks of hydrothermal deposits are generally altered, the most common change
being a bleaching and softening. Where alteration has been intense, as in many
mesothermal deposits, primary textures may be obliterated by the alteration
products. Chemical and mineralogical changes occur as a result of the
introduction of some elements and the removal of others; rarely a rearrangement
of minerals occurs with no replacement.
Common alteration products of
epithermal and mesothermal deposits are quartz, sericite, clay minerals,
chlorite, carbonates, and pyrite. Under high-temperature hypogene conditions
pyroxene, amphibole, biotite, garnet, topaz, and tourmaline form. In many mines
sericite has been developed nearest the vein and gives way outward to clay
minerals or chlorite. The nature and intensity of alteration vary with size of
the vein, character of the wall rock, and temperature and pressure of
hydrothermal fluids. In the large, low-grade porphyry copper and molybdenum
deposits associated with stocklike intrusives, alteration is intense and
widespread, and two or more stages of alteration may be superimposed.
Under low-intensity conditions, the
nature of the wall rock to a large extent determines the alteration product.
High-intensity conditions, however, may result in similar alteration products
regardless of the nature of the original rock. Exceptions to this are
monomineralic rocks such as sandstones and limestones. Wall rock alteration may
develop during more than one period by fluids of differing compositions, or it
may form during one period of mineralization as the result of the action of
hydrothermal fluids that did not change markedly in composition. Alteration
zones have been used as guides to ore and tend to be most useful where they are
neither too extensive nor too narrow. Mapping of these zones outlines the
mineralized area and may indicate favorable places for exploration.
Sedimentary and
residual deposits
At the Earth's surface, action of
the atmosphere and hydrosphere alters minerals and forms new ones that are more
stable under the existing conditions. Sedimentary deposits are bedded deposits
derived from preexisting material by weathering, erosion, transportation,
deposition, and consolidation. Different source materials and variations in the
processes of formation yield different deposits. Changes that take place in a
sediment after it has formed and before the succeeding material is laid down are
termed diagenetic. They include compaction, solution, recrystallization, and
replacement. In general, the sediment is consolidated by compaction and by
precipitation of material as a cement between mineral grains. Sedimentation as a
process may itself involve the concentration of materials into mineral deposits.
See also: Diagenesis
The mineral deposits that form as a
result of sedimentary and weathering processes are commonly grouped as follows:
(1) sedimentary deposits, not including products of evaporation; (2)
sedimentary-exhalative deposits; (3) chemical evaporites; (4) placer deposits;
(5) residual deposits; and (6) organic deposits. See also: Volcano
1. Sedimentary deposits. Included in
this group are the extensive coal beds of the world, the great petroleum
resources, clay deposits, limestone and dolomite beds, sulfur deposits such as
those near Kuibyshev, Russia, and the deposits of the Gulf Coast region, and the
phosphate of North Africa and Florida. Metalliferous deposits such as the
minette iron ores of Lorraine and Luxembourg, and Clinton iron ores of the
United States, and the manganese of Tchiaturi, Georgia, and Nikopol in the
Ukraine also belong here. There are other deposits of metals in sedimentary
rocks whose origin remains an enigma, such as the uranium of the Colorado
Plateau, the Witwatersrand in South Africa, and Blind River in Ontario; and the
copper deposits of Mansfeld, Germany, and of the Copperbelt of Zambia and the
Democratic Republic of the Congo. These deposits have characteristics of both
syngenetic and epigenetic types. A controversy centers around the genesis of
these and similar deposits of the world. See also: Heavy minerals; Sedimentology
2. Sedimentary-exhalative deposits.
Many large stratiform deposits are found in marine sedimentary rocks associated
with volcanic rocks. It is well known that volcanoes and fumaroles carry in
their gases a number of metals. On land these gases escape into the atmosphere.
Under water the gases, if they carry matter which is insoluble under the
existing conditions, will precipitate their metals as oxides, sulfides, or
carbonates in the vicinity of the gas emission. If the gases contain matter that
is soluble, the metal content of the seawater will increase, and upon reaching
saturation level will precipitate an extensive disseminated ore deposit. Where
submarine emissions take place in a large ocean basin, they may be deposited
over the floor of the basin as part of the sedimentation process. See also:
Volcano
Deposits exemplified by
lead-zinc-barite-fluorite mineralization, most commonly found in carbonate
rocks, occur in the Mississippi Valley region of North America and also on other
continents. These ores are included with the sedimentary-exhalative type, but
could also be discussed under several other classes of deposits since they are
very difficult to categorize. They have been considered by various geologists to
be true sediments, diagenetic deposits, lateral secretion deposits, deposits
formed by downward leaching of overlying lean ores, deposits formed by solutions
that descended and subsequently ascended, deposits resulting from
magmatic-hydrothermal processes, and sea-floor deposits from thermal springs.
Most geologists favor either a syngenetic-sedimentary hypothesis or an
epigenetic-hypogene one. Some studies hypothesize a source of metal-bearing
waters similar to those in deep brines which rise and move through fissures in
overlying rocks or are poured out on the sea floor and are added to accumulating
sediments. A single generally acceptable hypothesis of origin, if such
eventually emerges, must await the accumulation and interpretation of additional
geological and geochemical data.
3. Chemical evaporites. These
consist of soluble salts formed by evaporation in closed or partly closed
shallow basins. Deposits of salt or gypsum that are several hundred feet thick
are difficult to explain satisfactorily. Oschsenius suggested that they formed
in basins which were separated from the ocean by submerged bars except for a
narrow channel (inlet); such barriers are common along coastal areas.
Intermittently, seawater flowed over the barrier and was concentrated into
saline deposits by evaporation. Modifications of this theory have been proposed
to account for the omissions of certain minerals and the interruptions in the
succession.
Deposits of gypsum and common salt
(halite) are found in many countries, whereas the larger concentrations of
potash salts, borates, and nitrates are much more restricted in occurrence. See
also: Saline evaporites
4. Placer deposits. Placers are the
result of mechanical concentration whereby heavy, chemically resistant, tough
minerals are separated by gravity from light, friable minerals. Separation and
concentration may be accomplished by streams, waves and currents, and air, or by
soil and hill creep. The most important economic placer deposits are those
formed by stream action (Fig. 4).
Fig. 4 Deposition of placer by stream action on
the inside of meander bends.
Stream and beach placers are
widespread in occurence and include the famous gold placers of the world, as
well as deposits of magnetite, ilmenite, chromite, wolframite, scheelite,
cassiterite, rutile, zircon, monazite, and garnet. Placer deposits of diamond,
platinum, and gemstones are less common.
5. Residual deposits. Complete
weathering results in distribution of the rock as a unit and the segregation of
its mineral constituents. This is accomplished by oxidation, hydration, and
solution, and may be accelerated by the presence of sulfuric acid. Some iron and
manganese deposits form by accumulation without change, but certain clay and
bauxite deposits are created during the weathering of aluminous rocks. Residual
concentrations form where relief is not great and where the crust is stable;
this permits the accumulation of material in place without erosion. See also:
Weathering processes
Large residual deposits of clay,
bauxite, phosphate, iron, and manganese have been worked in many parts of the
world, as have smaller deposits of nickel, ocher, and other minerals.
6. Organic deposits. Plants and
animals collect and use various inorganic substances in their life processes,
and concentration of certain of these substances upon the death of the organisms
may result in the formation of a mineral deposit. Coal and peat form from
terrestrial plant remains and represent concentration by plants of carbon from
the carbon dioxide of the atmosphere. Petroleum originates by the accumulation
of plant and animal remains. Many limestone, phosphate, and silica deposits also
form by plant and animal activity. Hydrated ferric oxide and manganese dioxide
are precipitated by microorganisms; anaerobic bacteria can reduce sulfates to
sulfur and hydrogen sulfide. There is considerable controversy, however, as to
whether microorganisms are responsible for the formation of certain iron,
manganese, and sulfide deposits. Some uranium, vanadium, copper, and other
metalliferous deposits are considered to have formed, in part at least, by the
activity of organisms.
Deposits formed
by regional metamorphism
Regional metamorphism includes the
reconstruction that takes place in rocks within orogenic or mountain belts as a
result of changes in temperature, pressure, and chemical environment. In these
orogenic belts, rocks are intensely folded, faulted, and subjected to increases
in temperature. The changes that occur in this environment affect the chemical
and physical stability of minerals, and new minerals, textures, and structures
are produced, generally accompanied by the introduction of considerable material
and the removal of other material.
Some geologists believe that the
water and metals released during regional metamorphism can give rise to
hydrothermal mineral deposits. Along faults and shear zones movement of fluids
could take place by mechanical flow, though elsewhere movement might be by
diffusion. The elements released from the minerals would migrate to low-pressure
zones such as brecciated or fissured areas and concentrate into mineral
deposits. It has been suggested that the subtraction of certain elements during
metamorphism also can result in a relative enrichment in the remaining elements;
if this process is sufficiently effective, a mineral deposit may result. Certain
minerals also may be concentrated during deformation by flow of material to
areas of low pressure such as along the crests of folds.
Deposits of magnetite, titaniferous
iron, and various sulfides may form in metamorphic rocks, as well as deposits of
nonmetallic minerals such as kyanite, corundum, talc, graphite, and garnet.
Opponents of the concept of mineral
formation by regional metamorphism believe that a dispersal of minerals, rather
than a concentration, would result from the operative processes. However, if
movement of material were confined to specific channelways, this objection would
not necessarily hold. See also: Metamorphism
Fig. 5 Vein deposit of sulfide ore, showing
changes due to oxidation and supergene
enrichment.
Oxidation and
supergene enrichment
Many sulfide minerals form at depth
under conditions differing markedly from those existing at the surface. When
such minerals are exposed by erosion or deformation to surface or near-surface
conditions, they become unstable and break down to form new minerals.
Essentially all minerals are affected.
The oxidation of mineral deposits is
a complex process. Some minerals are dissolved completely or in part, whereas
elements of others recombine and form new minerals. The principal chemical
processes that take place are oxidation, hydration, and carbonation. The
oxidation of pyrite and other sulfides produces sulfuric acid, a strong solvent.
Much of the iron in the sulfides is dissolved and reprecipitated as hydroxide to
form iron-stained outcrops called gossans. Metal and sulfate ions are leached
from sulfides and carried downward to be precipitated by the oxidizing waters as
concentrations of oxidized ores above the water table. Oxides and carbonates of
copper, lead, and zinc form, as do native copper, silver, and gold. The nature
of the ore depends upon the composition of the primary minerals and the extent
of oxidation. If the sulfates are carried below the water table, where oxygen is
excluded, upon contact with sulfides or other reducing agents they are
precipitated as secondary sulfides. The oxidized zone may thus pass downward
into the supergene sulfide zone. Where this process has operated extensively, a
thick secondary or supergene-enriched sulfide zone is formed. Enrichment may
take place by removal of valueless material or by solution of valuable metals
which are then transported and reprecipitated. This enrichment process has
converted many low-grade ore bodies into workable deposits. Supergene enrichment
is characteristic of copper deposits but may also take place in deposits of
other metals. Beneath the enriched zone is the primary sulfide ore (Fig. 5).
The textures of the gossan minerals
may give a clue to the identity of the minerals that existed before oxidation
and enrichment took place. These have been used as guides in prospecting for
ore.
Sequence of
deposition
Studies of the relationships of
minerals in time and space have shown that a fairly constant sequence of
deposition, or paragenesis, is characteristic of many mineral deposits. This
sequence has been established largely by microscopic observations of the
boundary relationships of the minerals in scores of deposits. Subsequent
experimental studies of mineral phases have contributed to the knowledge of
paragenesis. In magmatic and contact metasomatic ores, silicates form first,
followed by oxides and then sulfides. W. Lindgren presented the paragenesis for
hypogene mineral associations, and others have discussed the problems involved.
The sequence of common minerals starts with quartz, followed by iron sulfide or
arsenide, chalcopyrite, sphalerite, bornite, tetrahedrite, galena, and complex
lead and silver sulfo salts. It indicates the existence of some fundamental
control but attempts to explain the variations in it have been largely
unsuccessful, or are applicable to only part of the series or to specific
mineralized areas. Local variations are to be expected since many factors such
as replacement, unmixing, superimposed periods of mineralization, structural and
stratigraphic factors, and telescoping of minerals may complicate the order of
deposition.
Paragenesis is generally thought to
be the result of decreasing solubility of minerals with decreasing temperature
and pressure. It has also been explained in terms of relative solubilities, pH
of the solutions, metal volatilities, decreasing order of potentials of
elements, free energies, and changing crystal structure of the minerals as they
are deposited. R. L. Stanton has reevaluated paragenetic criteria as applied to
certain stratiform sulfide ores in sedimentary and metamorphic rocks. He
proposes that the textures of such ores do not represent sequences of deposition
but are the result of surface energy requirements during grain growth, or
annealing of deformed minerals. To explain mineral paragenesis more
satisfactorily, many additional experiments must be made to determine phase
relations at different temperatures and pressures. See also: Depositional
systems and environments; Mineral
Mineralogenetic
provinces and epochs
Mineral deposits are not uniformly
distributed in the Earth's crust nor did they all form at the same time. In
certain regions conditions were favorable for the concentration of useful
minerals. These regions are termed mineralogenetic provinces and they contain
broadly similar types of deposits, or deposits with different mineral
assemblages that appear to be genetically related. The time during which these
deposits formed constitutes a mineralogenetic epoch; such epochs differ in
duration, but in general they cover a long time interval that is not sharply
defined. Certain provinces contain mineral deposits of more than one epoch.
During diastrophic periods in the
Earth's history mountain formation was accompanied by plutonic and volcanic
activity and by mineralization of magmatic, pegmatitic, hydrothermal and
metamorphic types. During the quieter periods, and in regions where diastrophism
was milder, deposits formed by processes of sedimentation, weathering,
evaporation, supergene enrichment, and mechanical action.
During the 1960s numerous studies
were made of the regional distribution of mineral deposits associated with long
subsiding belts of sediments, or geosynclines, and with platform areas of
relatively thin sediments adjoining the thick geosynclinal wedge. Geosynclinal
areas commonly suffer folding and later uplift and become the sites of complex
mountain ranges. It has been proposed that the outer troughs and bordering
deep-seated faults contain ore deposits of subcrustal origin, the inner uplifts
contain deposits of crustal origin, and the platforms contain ores derived from
subcrustal and nonmagmatic platform mantle rocks. V. I. Smirnov and others have
summarized available information on types of mineral deposits characteristic of
the processes most active during the evolutionary stages of geosynclinal and
platform regions. In the early prefolding stage of subsidence, subcrustal
juvenile basaltic sources of ore fluids prevail, and the characteristic metals
are Cr, titanomagnetite, Pt metals, skarn Fe and Cu, and deposits of pyritic Cu
and Fe and Mn. In the folding episode, rocks of the geosyncline are melted to
produce magma from which ore components are extracted or leached by postmagmatic
fluids. The most typical ores of this stage are Sn, W, Be, Ni, Ta, and various
polymetallic deposits. The late stage is characterized by ore deposits
associated with igneous rocks and other deposits with no apparent relationship
to igneous rocks. Smirnov believes these ores originated by the combined effect
of subcrustal, crustal, and nonmagmatic sources of ore material. Typical metals
of this stage include Pb, Zn, Cu, Mo, Sn, Bi, Au, Ag, Sb, and Hg. In the
tectonically activated platform areas, deposits of Cu-Ni sulfides, diamonds,
various magmatic and pegmatitic deposits, and hydrothermal ores of nonferrous,
precious, and rare metals are found. In addition, there are nonmagmatic deposits
of Pb and Zn. Some ore material is believed to be both subcrustal and
nonmagmatic in origin. Relative proportions of types of mineralization differ
from one region to another.
Fig. 6 Strong vein in granite dividing into
stringers upon entering schist.
Fig. 7 Ore in limestone beneath impervious
shale.
The relationship between mineral
deposition and large-scale crustal movements permits a grouping of
mineralogenetic provinces by major tectonic features of the continents such as
mountain belts, stable regions, and Precambrian shields. The Precambrian shield
areas of the world contain the Lake Superior, Kiruna, and Venezuelan iron
provinces, the gold provinces of Kirkland Lake and Porcupine in Canada, the
gold-uranium ores of South Africa, the gold deposits of western Australia, and
the base metals of central Australia. In the more stable regions are the
metalliferous lead-zinc province of the Mississippi Valley and provinces of salt
and gypsum, iron, coal, and petroleum in different parts of the world. The
mountain belts are the location of many diverse kinds of mineral provinces such
as the gold-quartz provinces of the Coast Range and the Sierra Nevadas, various
silver-lead-zinc provinces of the western United States, the Andes, and
elsewhere, and numerous base-metal provinces in the Americas, Africa, Australia,
and Europe.
Localization of
mineral deposits
The foregoing discussion has shown
that mineral deposits are localized by geologic features in various regions and
at different times. Major mineralized districts within the shield areas and
mountain belts are often localized in the upper parts of elongate plutonic
bodies. Specific ores tend to occur in particular kinds of rocks. Thus tin,
tungsten, and molybdenum are found in granitic rocks, and nickel, chromite, and
platinum occur in basic igneous rocks. In certain regions mineral deposits are
concentrated around plateau margins. Tropical climates favor the formation of
residual manganese and bauxite deposits, whereas arid and semiarid climates
favor the development of thick zones of supergene copper ores. Major mineralized
districts are also localized by structural features such as faults, folds,
contacts, and intersections of superimposed orogenic belts. The location of
individual deposits is commonly controlled by unconformities, structural
features, the physical or chemical characteristics of the host rock (Fig. 6),
topographic features, basins of deposition, ground-water action, or by
restriction to certain favorable beds (Fig. 7). See also: Pluton
Source and
nature of ore fluids
Widely divergent views have been
expressed as to the original source and mode of transport of mineral deposits.
Each view has certain advantages when applied to specific types of deposits.
However, the complex nature of some mineralizations and the highly diverse
physicochemical environments in which mineral deposits form make it impossible
to select one theory to account for the source of all ore-forming materials.
According to one view, the source of
the ore material was a juvenile subcrustal basaltic magma from which mineral
deposits crystallized by simple crystallization, or in some cases were
concentrated by differentiation. Most of the ore deposits associated with such
magmas show a close spatial relationship to the enclosing igneous rocks and are
similar in composition from one province to another. The exceptions to this are
certain pyritic copper and skarn ores that apparently were introduced into
sedimentary rocks and are now removed from the postulated source magma.
Another hypothesis holds that many
ore deposits associated with granitic rocks were derived from magmas generated
by remelting of deep-seated sedimentary rocks, followed by movement of the magma
into higher levels of the Earth's crust. As an end product of crystallization
and differentiation, an ore fluid was produced containing concentrations of
metals originally present in the magma. Commonly, such deposits are confined to
the apical portions of granitic plutons that have been altered by postmagmatic
fluids. An increasing number of geologists ascribe to the view that the ore
material was removed from the solidified magma by these late-stage fluids. Such
ore deposits are complex, and their composition, dependent in part on the
composition of the remelted rocks, is diverse and variable from one region to
another. For certain ores associated with major deep-seated faults or with
intersections of extensive fault or fissure systems, an ore source in deeper,
subcrustal, regions has been advocated.
Circulation of surface waters may
have removed metals from the host rocks and deposited them in available
openings; this is the lateral secretion theory. The metals were carried either
by cool surface waters or by such waters that moved downward, became heated by
contact with hot rocks at depth, and then rose and deposited their dissolved
material.
As sediments are compacted and
lithified, huge volumes of water may be expelled. It has been suggested that the
ore-forming fluid in some sedimentary deposits contains metals that were in
solution before the sediment was buried, plus metals acquired during diagenesis
of the sediments. For certain ores with colloform textures, it is believed that
movement took place as a finely divided suspension, and that the ore minerals
initially precipitated as gels. The metals could have been held as adsorbates on
clays and other colloids and then released for concentration during later
crystallization of the colloids. Crystallization would exclude the metals as
finely divided material which could combine with released water and move to
places favorable for precipitation.
A considerable amount of
experimental work has been done on the geochemistry of ore fluids in an attempt
to determine the source, nature, solubility, transport, and deposition of these
fluids. Studies of metal-bearing saline waters and of thermal waters and brines
in igneous, sedimentary, and metamorphic rocks have also contributed to the
knowledge of this complex subject. D. E. White has analyzed and summarized these
studies, and he stresses that four mutually interdependent factors must be
considered: a source of the ore constituents, the dissolving of these
constituents in the hydrous phase, migration of the ore-bearing fluid, and the
selective precipitation of the ore constituents in favorable environments. The
ore-bearing fluids are Na-Ca-Cl brines that may form by magmatic or connate
processes, solution of evaporates by dilute water, or membrane concentration of
dilute meteoric water.
During regional metamorphism large
quantities of hydrothermal fluids may be released from rocks in deep orogenic
zones. These fluids remove metals and other minerals from the country rock and
redeposit them at higher levels along favorable structures. Elements may also
move by diffusion along chemical, thermal, and pressure gradients.
A number of the famous mineralized
districts of the world that have characteristics of both epigenetic and
syngenetic deposits have been modified by later metamorphism, thereby further
obscuring their origin. In some of these districts the fissure and joint systems
in the rocks reflect the pattern in deeper-seated rocks. H. Schneiderhohn has
suggested that repeated rejuvenation of these systems by tectonic movements,
accompanied by the dissolving action of thermal waters on old ore deposits in
depth, would result in upward movement and reprecipitation of metals in higher
formations; Schneiderhohn calls these deposits secondary hydrothermal ores.
Elsewhere old folded rocks and ore deposits have been greatly deformed, and the
ores taken into solution and transported to higher and younger strata; such
deposits Schneiderhohn terms regenerated ores. Controversy centers around
suitable criteria for epigenetic and syngenetic deposits, the problems of
solubility of metals in thermal waters, their transport over long distances, and
whether such rejuvenated and regenerated ores would be dispersed or concentrated
by the processes envisaged by Schneiderhohn.
A. F. Hagner
Mineral and
chemical composition
The common minerals of hydrothermal
deposits (Table 3) are sulfides, sulfo salts, oxides, carbonates, silicates, and
native elements, although sulfates, a fluoride, tungstates, arsenides,
tellurides, selenides, and others are by no means rare. Many minor elements
which seldom occur in sufficient abundance to form discrete minerals of their
own may substitute for the major elements of the minerals and thus be recovered
as by-products. For example (as shown in Table 3), the ore mineral of cadmium,
indium, and gallium is sphalerite; the major ore mineral of silver and thallium
is galena; and pyrite is sometimes an ore of cobalt. See also: Elements,
geochemical distribution of
Ore deposits consist, in essence, of
exceptional concentrations of given elements over that commonly occurring in
rocks. The degree of concentration needed to constitute ore varies widely, as
shown in Table 4, and is a complex function of many economic and sometimes
political variables. The quantity of these elements in the total known or
reasonably expected ore bodies in the world is infinitesimal when compared with
the total amounts in the crust of the Earth. Thus, each and every cubic mile of
ordinary rocks in the crust of the Earth contains enough of each ore element to
make large deposits (Table 4). Although there is a large number of geologic
situations that are apparently favorable, only a few of them contain significant
amounts of ore. Thus, it is evident that the processes leading to concentration
must be the exception and not the rule, and obviously any understanding or
knowledge of these processes should aid in the discovery of further deposits.
Each step in the process of ore
formation must be examined carefully if this sporadic occurrence of ore is to be
placed on a rational basis. In order for ores to form, there must be a source
for the metal, a medium in which it may be transported, a driving force to move
this medium, a “plumbing system” through which it may move, and a cause of
precipitation of the pre elements as an ore body. These interrelated
requirements are discussed below in terms of the origin of the hydrothermal
fluid, its chemical properties, and the mechanisms by which it may carry and
deposit ore elements.
Source of
metals
It is not easy to determine the
source for the metals in hydrothermal ore deposits because, as shown above, they
exist everywhere in such quantities that even highly inefficient processes could
be adequate to extract enough material to form large deposits.
Fluids associated with igneous
intrusion
In many deposits there is evidence
that ore formation was related to the intrusion of igneous rocks nearby, but in
many other deposits intensive search has failed to reveal any such association.
Because the crystal structures of the bulk of the minerals (mostly silicates)
crystallizing in igneous rocks are such that the common ore elements, such as
copper, lead, and zinc, do not fit readily, these elements are concentrated into
the residual liquids, along with H2O, CO2, H2S, and other substances. These hot,
water-rich fluids, remaining after the bulk of the magma has crystallized, are
the hydrothermal fluids which move outward and upward to areas of lower pressure
in the surrounding rocks, where part or all of their contained metals are
precipitated as ores. A more detailed discussion of the composition of these
fluids is presented below.
Fluids obtained
from diagenetic and metamorphic
processes
Fluids of composition similar to the
above also could be obtained from diagenetic and metamorphic processes. When
porous, water-saturated sediments containing the usual amounts of hydrous and
carbonate minerals are transformed into essentially nonhydrous, nonporous
metamorphic rocks, great quantities of water and carbon dioxide must be driven
off. Thus, each cubic mile of average shale must lose about 3 × 109 tons of
water (each cubic kilometer, about 6.5 × 108 metric tons) and may lose large
amounts of carbon dioxide on metamorphism to gneiss. The great bulk of the water
presumably comes off as connate water (entrapped at time of rock deposition)
under conditions of fairly low temperature. In many respects this water has the
same seawater composition as it had to start with. However, as metamorphism
proceeds, accompanied by slow thermal buildup from heat flow from the Earth's
interior and from radioactivity, the last fluids are given off at higher
temperatures and are richer in CO2 and other substances. These fluids would have
considerably greater solvent power and can be expected to be similar to those
coming from cooling igneous rocks.
Role of surface
and other circulating waters
It is very likely that the existence
of a mass of hot rock under the surface would result in heating and circulation
of meteoric water (from rain and snow) and connate water. The possible role of
these moving waters in dissolving ore elements from the porous sedimentary
country rocks through which they may pass laterally and in later depositing them
as ore bodies has been much discussed. The waters may actually contribute ore or
gangue minerals in some deposits. The test of this theory of lateral secretion
on the basis of preise analyses of the average country rocks around an ore body
would involve an exceedingly difficult sampling job. It also would require
analytical precision far better than is now feasible for most elements, as each
part per million uncertainty in the concentration of an element in a cubic mile
of rock represents about 10,000 tons of the element or 1 × 106 tons of 1% ore.
Movement of
ore-forming fluids
In addition to the high vapor
pressures of volatile-rich fluids acting as a driving force to push them out
into the surrounding country rocks and to the surface, there may well be
additional pressures from orogenic or mountain-building forces. When a silicate
magma has an appreciable percentage of liquid and is subjected to orogenic
forces, it moves en masse to areas of lower pressure (it is intruded into other
rocks). But if the magma has crystallized 99% or more of its bulk as solid
crystals and has only a very small amount of water-rich fluid present as thin
films between the grains, and then is squeezed, this fluid may be the only part
sufficiently mobile to move toward regions of lower pressure. (If the residual
fluid, containing the ore elements, stays in the rock, it reacts with the early
formed, largely anhydrous minerals of the rock to form new hydrated ones, such
as sericite, epidote, amphibole, and chlorite, and its ore elements precipitate
as minute disseminated specks and films along the silicate grain boundaries.)
The ore-bearing fluid leaves the
source through a system comprising joints, faults, porous volcanic plugs, or
other avenues. As the fluid leaves the source, it moves some appreciable but
generally unknown distance laterally, vertically, or both, and finally reaches
the site of deposition. This system of channels is of utmost importance in the
process of ore formation.
Localization of
mineral deposits
It is stated frequently that ore
deposits are geologic accidents; yet there are reasons, however abstruse, for
the localization of a mineral deposit in a particular spot. One reason for
localization is mere proximity to the source of the ore-forming fluids, as in
rocks adjacent to an area of igneous activity or near a major fracture system
which may provide plumbing for solutions ascending from unknown depths. Zones of
shattering are favored locales for mineralization since these provide plumbing
and offer the best possibility for the ore solution to react with wall rock, mix
with other waters, and expand and cool, all of which may promote precipitation.
Some types of rock, particularly limestone and dolomite, are especially
susceptible to replacement and thus often are mineralized preferentially. The
chemical or physical properties which cause a rock to be favored by the
replacing solutions often are extremely subtle and certainly not understood
fully.
Zoning and
paragenesis
Mineral deposits frequently show
evidence of systematic spatial and temporal changes in metal content and
mineralology that are sufficiently consistent from deposit to deposit to warrant
special mention under the terms zoning and paragenesis. Zoning may be on any
scale, though the range is commonly on the order of a few hundred to a few
thousand feet, and may have either lateral or vertical development. In mining
districts, such as Butte, Montana, or Cornwall, England, where zoning is
unusually well developed, there is a peripheral zone of manganese minerals
grading inward through successive, overlapping silver-lead, zinc, and copper
zones (and in the case of Cornwall, tungsten, and finally tin). The same
sequence of zones appears in many deposits localized about intrusive rocks,
suggesting strongly that the tin and tungsten are deposited first from the
outward-moving hydrothermal solutions and that the copper, zinc, lead, and
silver were deposited successively as the solutions expanded and cooled. In
other districts the occurrences of mercury and antimony deposits suggest that
their zonal position may be peripheral to that of silver or manganese. The
paragenesis, or the sequence of deposition of minerals at a single place, as
interpreted from the textural relations of the minerals, follows the same
general pattern as the zoning, with the tin and tungsten early and the lead and
silver late. With both zoning and paragenesis there are sometimes reversals in
the relative position of adjacent zones, and these are usually explained as
successive generations of mineralization. Some metals, such as iron, arsenic,
and gold, tend to be distributed through all of the zones, whereas others, such
as antimony, tend to be restricted to a single position.
The sequence of sulfide minerals
observed in zoning and paragenesis matches in detail the relative abilities of
the heavy metals to form complex ions in solution. This observation strongly
supports the hypothesis developed later that most ore transport occurs through
the mechanism of complex ions, since no other geologically feasible property of
the ore metals or minerals can explain the zoning relations.
Environment of
ore deposition
Important aspects of the environment
of ore deposition include the temperature, pressure, nature, and composition of
the fluid from which ores were precipitated.
Temperatures
Although there is no geological
thermometer that is completely unambiguous as to the temperatures of deposition
of ores, there is a surprising number of different methods for estimating the
temperatures that prevailed during events long since past that have been applied
to ores with reasonably consistent results. Those ore deposits which had long
been considered to have formed at high temperatures give evidence of formation
in the range of 500–600°C (930–1100°F), or possibly even higher. Those that were
thought to be low-temperature deposits show temperatures of formation that are
in the vicinity of 100°C (212°F) or even less, and the bulk of the deposits lie
between these extremes. See also: Geologic thermometry
Pressures
It would be useful to know the total
hydrostatic pressure of the fluids during ore formation. Most of the phenomena
used for determination of the temperatures of ore deposition are also
pressure-dependent, and so either an estimate of the correction for pressure
must be made, or two independent methods must be used to solve for the two
variables.
Pressures vary widely from nearly
atmospheric in hot springs to several thousand atmospheres in deposits formed at
great depth. Maximum reasonable pressures are considered to be on the order of
that provided by the overlying rock; conversely, the minimum reasonable
pressures are considered to be about equal to that of a column of fluid open to
the surface. Pressures therefore range from approximately 500 to 1500 lb/in.2
per 1000 ft (10–30 MPa per 1000 m) of depth at the time of mineralization. See
also: High-pressure mineral synthesis
Evidence of
composition
Geologists generally concede that
most ore-forming fluids are essentially hot water or dense supercritical steam
in which are dissolved various substances including the ore elements. There are
three lines of evidence bearing on the composition of this fluid. These are
fluid inclusions in minerals, thermal springs and fumaroles, and the mineral
assemblage of the deposit and its associated alteration halos.
1. Fluid inclusions in minerals.
Very small amounts of fluid are trapped in minute fluid-filled inclusions during
the growth of many ore and gangue minerals in veins, and these inclusions have
been studied intensively for evidence of temperature and composition. Although
the relative amounts may vary widely, these fluids will have 5–25 or even more
weight percent soluble salts, such as chlorides of Na, K, and Ca, plus highly
variable amounts of carbonate, sulfate, and other anions. Some show liquid CO2
or hydrocarbons as separate phases in addition to the aqueous solution. A few
show detectable amounts of H2S and minor amounts of many other substances. After
losing some CO2 and H2S through release of pressure and oxidation when the
inclusions are opened, the solutions are within 2 or 3 pH units of neutral.
There is little evidence of sizable quantities (>1 g/liter or 0.13 oz/gal) of
the ore metals in these solutions, and the evidence indicates that the
concentrations of the ore elements must generally be very low (<0.1 g/liter
or 0.013 oz/gal). Even if the concentrations were in the range of 0.1 g/liter,
there should be analytical evidence in the fluid inclusion studies, but this is
lacking. In addition, if fluids of such composition were trapped in fluid
inclusions in transparent minerals and on cooling precipitated even a fraction
of their metal content as opaque sulfides, these should be visible (under the
microscope) within the inclusions, but none are seen. If the concentrations of
ore elements are much less than 0.001 g/liter (0.00013 oz/gal), the volume of
fluids that must be moved through a vein to form an ore body becomes
geologically improbable.
2. Thermal springs and fumaroles.
These provide the closest approach to a direct look at the processes of ore
deposition as some ore and gangue minerals form within the range of direct
observation. The solutions from these springs give diluted and possibly
contaminated, partly oxidized and partly devolatilized samples of the sort of
fluid that presumably forms ore bodies at greater depths. Isotopic studies show
that the solutions have been diluted by local meteoric water until less than 5%
(if any) of the fluid emitted at the surface is of deep-seated origin. The
compositions of these thermal springs, after correction for such dilution, are
in good agreement with the data from fluid inclusions.
3. Mineral assemblage. The
assemblage of minerals that occurs within a deposit provides a great deal of
information about the chemical nature of the fluid from which the ores were
precipitated. There are a great number of stable inorganic compounds of the
heavy metals known, yet unaltered ore deposits contain only a relatively small
number of minerals. For example, lead fluoride, lead chloride, lead carbonate,
lead sulfate, lead oxide, lead sulfide, and many others are known stable
compounds of lead, yet of these, primary ore deposits contain only the sulfide
(galena). Some elements, such as calcium, which occur in combination with
several types of anions, for example, the carbonate, fluoride, sulfate, and
numerous silicates, are found with the ore minerals. A quantitative approach to
the compositional problem may be made by considering such reactions as shown in
(1).
The equilibrium constant for this
reaction is (CO32−)/(F−)2 = 101.4 at 25°C (77°F). Thus when calcite and fluorite
are in equilibrium, the requirements for the constant are met, and the
(CO32−)/(F−)2 ratio is known. A large number of such equations can be evaluated
and from comparison with the mineral assemblage known to occur in ores, limits
on the possible variation of the composition of the ore-forming fluid may be
estimated. Unfortunately, calculations of this sort involving ionic equilibria
are limited to fairly low temperatures (less than 100–200°C or 212–390°F) since
there are few reliable thermodynamic data on ionic species at high temperature.
At any temperature, reactions such as shown in (2) can be used
to evaluate or place limits on the
possible variation of the chemical potential of some components in the
ore-forming fluid. See also: Ionic equilibrium; Sulfide phase equilibria
The composition of the ore fluid
tends to become adjusted chemically by interaction with the rocks with which it
comes in contact, and these changes may well contribute to the precipitation of
the ore minerals. Thus, the K+/H+ ratio may be controlled by such reactions
as
(3), where the equilibrium constant
has the form shown in Eq.
(4). Likewise, the quantitatively
small but nevertheless important partial pressures of sulfur and oxygen may be
governed by such reactions as
(5). Such changes in the wall rock
come under the general heading of wall-rock alteration and may be of many types,
only a few of the more common of which are mentioned below.
High-temperature alteration of
limestones usually results in the formation of water-poor calcium silicates,
such as garnet, pyroxenes, idocrase, and tremolite, and the resulting rock is
termed skarn. At lower temperatures in the same types of rock, dolomitization
and silicification are the predominant forms of alteration, because the partial
pressure of CO2 is too high to permit calcium silicate to form. See also:
Silicate phase equilibria
At high temperatures in igneous and
metamorphic rocks near granite in composition, the solutions are approximately
in equilibrium with the primary rock-forming minerals, and thus there is little
alteration except development of sericite and occasionally topaz and tourmaline.
At lower temperatures, the characteristic sequence of alteration from fresh rock
toward the vein is first an argillic zone, then a sericitic zone, and finally a
silicified zone bordering the vein.
Summary
Summarizing the environment of ore
deposition, there are various lines of evidence to show that most hydrothermal
ore deposits were formed at temperatures of 100–600°C (212–1100°F) and at
pressures ranging from nearly atmospheric to several thousand atmospheres. The
solutions were dominantly aqueous and were fairly concentrated in sodium
chloride and potassium chloride; however, they were relatively dilute in terms
of the ore metals.
Mechanisms of
ore transport and deposition
The ore minerals, principally the
sulfides, are extremely insoluble in pure water at high temperatures as well as
low; the solubility products are so low, in fact, that literally oceans of water
would be required to transport the metal for even a small ore body. Thus, it is
not easy to explain the mechanism whereby the minerals are solubilized to the
extent necessary for ore transport.
Crystals of ore and gangue minerals
frequently exhibit evidence of repeated partial re-solution (or leaching) and
regrowth. This demonstrates that the process of ore formation may, at least in
some instances, be reversible. In such cases studies of artificial systems at
equilibrium are applicable.
The re-solution of ore minerals is
important in another connection. Some geologists have advocated colloidal
solutions, or sols, as an alternative to true solutions for ore transport. This
was based on the belief, now known to be generally false, that colloform
textures in ore minerals are a result of original deposition as a colloidal gel.
Colloidal solutions were attractive also because they permitted ore metal
concentrations—even in the presence of sulfide—many orders of magnitude higher
than true solutions. The re-solution of ore minerals precludes the process of
colloidal ore transport, as colloidal solutions are supersaturated and therefore
cannot redissolve a crystal of the dispersed phase. See also: Colloid
In addition to the fact that the
absolute solubilities, calculated from the solubility products, are extremely
low, the relative solubilities of the sulfides are radically different. For
example, according to the solubility products, FeS is many, many times more
soluble than PbS (about 1010 times at 25°C or 77°F), yet the two minerals occur
together in ore deposits and behave as if galena were slightly more soluble than
pyrrhotite. From this and other lines of evidence, it appears necessary to
conclude that the solubilities of the various contemporaneous minerals in a
given deposit could not have differed among themselves by more than a few orders
of magnitude. See also: Solubility product constant
The only geologically and chemically
feasible mechanism by which these solubilities may be equalized approximately is
the formation of complex ions of the heavy metals. Such complexes can increase
the solubilities of heavy metals tremendously. As an example, the activity
(thermodynamic concentration) of Hg2+ in a solution saturated with HgS
(cinnabar) and H2S at 25°C (77°F), 1 atm (105 Pa) pressure, and pH 8, is only
about 10−47 mole/liter, representing a concentration much less than 1 atom of
mercury in a volume of water equal to the entire volume of the oceans of the
world. However, in the same solution is formed a very stable sulfide complex of
mercury, HgS22−, which increases the total concentration of mercury in solution
by the impressive factor of about 1042, giving a concentration on the order of
0.001 g/liter (0.00013 oz/gal). Not only does complex formation provide a means
to achieve adequate solubility for ore transport, but the relative tendency for
metals to form certain types of complexes matches in detail the commonly
observed zoning and paragenetic sequences mentioned previously. The metals whose
sulfides are the least soluble tend to form the most stable complexes, and
metals whose minerals are comparatively soluble form weaker complexes. See also:
Coordination complexes
There are many kinds of complexing
ions or molecules (ligands) of possible geologic importance; a few of the more
significant are sulfide (S2−), hydrosulfide (HS−), chloride (Cl−), polysulfides
(Sx2−), thiosulfate (S2O32−), sulfate (SO42−), and carbonate (CO32−), with the
first three being most frequently considered. One of the major unsolved problems
concerns the behavior of sulfur: What is its oxidation state and concentration
relative to metals? If solutions were rich in reduced sulfur species, then the
sulfide or hydrosulfide complexes would be dominant. On the other hand,
solutions poor in reduced sulfur may transport the metals as chloride complexes.
The precipitation of minerals from
complexed solutions takes place either by shifts in equilibrium caused by
changing (usually cooling) temperature or by a decrease in the concentration of
the ligand, thereby reducing the ability of the solution to carry the metals.
This latter alternative can take place in several ways, for example, by reaction
with wall rock, by mixing with other solutions, or by formation of a gas phase
through the loss of pressure. See also: Precipitation (chemistry)
Oxidation and
secondary enrichment
When ore deposits are exposed at the
surface, they are placed in an environment quite different from that in which
they were formed, and the character of the deposit is changed through the
processes of oxidation and weathering. The sulfides give way to oxides,
sulfates, carbonates, and other compounds which are more or less soluble and
tend to be leached away, leaving a barren gossan of insoluble siliceous iron and
manganese oxides. Some minerals, such as cassiterite and native gold, may leach
away at a less rapid rate than does the surrounding material; thus they are
concentrated as a surficial residuum.
Where the country rock is relatively
inert to the acid solutions generated by the oxidizing sulfides, as in the case
of quartzites and some hydrothermally altered rocks, copper and especially zinc
are leached away readily; lead and silver may be retained temporarily in the
oxidized zone as the carbonate or sulfate, and the chloride or native metal,
respectively; but eventually these too are dissolved away. The various metallic
ions are carried downward until they reach unoxidized sulfides in the vicinity
of the water table, where the solutions interact with these sulfides to form a
new series of supergene sulfide minerals. Copper sulfide is the least soluble
sulfide of the base and ferrous metals in the solution, and hence the zone of
supergene sulfide enrichment is predominantly a copper sulfide zone with
occasional rich concentrations of silver. Zinc nearly always remains in solution
and is lost in the ground water.
In reactive wall rocks, such as
limestones, reaction with the wall rock prevents the solutions from becoming
acid enough for large amounts of metal to be removed in solution; the base
metals are retained almost in place as carbonates, sulfates, oxides, and
halides, and there is no appreciable sulfide enrichment.
The behavior of some elements is
governed by the availability of other materials. Thus, for example, uranium is
readily leached from the oxidized zone in many deposits; however, when the
oxidizing solutions contain even very small amounts of potassium vanadate, the
extremely insoluble mineral carnotite precipitates and uranium is immobilized.
Highly soluble materials, such as uranium in the absence of chemicals that
precipitate it, may be temporarily fixed in the oxidized zone by adsorption on
colloidal materials such as freshly precipitated ferric oxides.
Trends in
investigation
There has been a great increase in
the degree to which the experimental methods and principles of physical
chemistry have been applied to aid in understanding the processes by which ores
have formed, and this approach can be expected to be even more fruitful in the
future. Several avenues appear promising and are under active investigation in
numerous laboratories. Among these are the following:
1. Phase equilibrium studies of both
natural and synthetic ore and gangue minerals.
2. Distribution coefficients for
trace elements between coexisting phases, and between various forms on the same
crystal.
3. Experimental solubility studies
in dominantly aqueous solutions.
4. Studies of the composition and
origin of thermal spring waters and fluid inclusions in minerals.
5. Thermodynamic properties of
minerals.
6. Isotopic fractionation during
transportation and deposition processes.
7. Rate studies on crystal growth,
habit, diffusion, reaction, and transformation, as well as studies of sluggish
homogeneous reactions, such as the reduction of sulfate.
8. Crystal structure determinations
and crystal chemical studies of ore and gangue minerals.
9. Distribution of elements in the
Earth's crust and in various rock types.
10. Detailed field studies of the
relations between minerals in ore deposits. See also: Heavy minerals; Petroleum
geology
For a discussion of sensitive
chemical analytical techniques used in the search for ore deposits See also:
Geochemical prospecting
For chemical principles involved in
ore deposition See also
- J. W. Barnes, Ores and Minerals: Introducing Economic Geology, 1988
- R. L. Bates, Geology of the Industrial Rocks and Minerals, 1969
- R. L. Bates, Industrial Minerals: How They Are Found and Used, 1988
- D. Derry, Concise World Atlas of Geology and Mineral Deposits: Non-metallic Minerals, Metallic Minerals and Energy Minerals, 1980
- A. R. Dutton, Hydrogeology and Hydrochemical Properties of Salt-Dissolution Zones, 1987
- R. Edwards and K. Atkinson, Ore Deposit Geology and Its Influence on Mineral Exploration, 1986
- R. M. Garrels and C. L. Christ, Solutions, Minerals, and Equilibria, 1965, reprint 1982
- M. L. Jensen and A. M. Bateman, Economic Mineral Deposits, 3d rev. ed., 1981
- K. B. Krauskopf, Introduction to Geochemistry, 3d ed., 1994
- J. Parnell, H. Kucha, and P. Landais, Bitumens in Ore Deposits, 1993
- F. Pirajano, Hydrothermal Mineral Deposits: Principles and Fundamental Concepts for the Exploration Geologist, 1992
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