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 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.  

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).

 (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

 (2) 

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) 

(3), where the equilibrium constant has the form shown in Eq.

 (4)  

(4). Likewise, the quantitatively small but nevertheless important partial pressures of sulfur and oxygen may be governed by such reactions as

 (5) 

(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: Geologic thermometry; Lead isotopes (geochemistry); Lithosphere

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• Alifazeli = egeology.blogfa.com
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• Alifazeli = egeology.blogfa.com
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• Alifazeli = egeology.blogfa.com
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• Alifazeli = egeology.blogfa.com
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• Alifazeli = egeology.blogfa.com
• F. Pirajano, Hydrothermal Mineral Deposits: Principles and Fundamental Concepts for the Exploration Geologist, 1992
• Alifazeli = egeology.blogfa.com

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