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.
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
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 Cornwall, Pennsylvania, Iron Springs, Utah, and Banat, Hungary; many copper ores of Utah, Arizona, New Mexico, and Mexico; the zinc ores of Hanover, New Mexico; and various tungsten ores of California and Nevada.
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
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
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
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
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.
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 .
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 .
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.