Mineral deposits are natural accumulations of minerals formed by geological processes. These formations develop through different methods, such as magmatic crystallization, metamorphism, weathering, and cavity filling. They are classified according to their formation process and economic importance, which is essential for mining and resource management.
Mineral Deposits:
It is an aggregate of a single mineral or several minerals irrespective of shape, size, and origin but of some value. It represents a local accumulation and concentration of minerals that remain diffused in the earth’s crust.
Classification of Mineral Deposit:
Mineral Deposits can be classified based on the mechanism responsible for concentrating the valuable substance.
- Magnetic Mineral Deposits
- Hydrothermal Mineral Deposits
- Metamorphic Mineral Deposits
- Sedimentary Mineral Deposits
- Placer Mineral Deposits
- Residual Mineral Deposits
Role of Temperature and Pressure:
A temperature and pressure decrease promotes precipitation from aqueous solution (or magma). The more soluble salts will stay in solution and be precipitated later than the less soluble salts. This phenomenon explains the formation of the sequences of minerals in mineral deposits and mineral zoning. Changes in pressure are more important in promoting precipitation.
Gases in solution are very sensitive to a change in pressure. For example, the release of dissolved CO2 by lessened pressure causes precipitation of CaCO3 from solution in a tropical environment. Precipitation of solutes in solution can be affected isothermally by the decrease in pressure alone.
Likewise, escaping fluids from magmatic solutions under decreasing pressure promotes the crystallization of the magma. The reduction of metals in solution also promotes the precipitation of minerals. For example, complex metal chlorides react with H2S to form Sulfides.
The modes of formation of mineral deposits:
To know the formation of mineral deposits, a knowledge of the constituents of the mineral deposits is
necessary. The temperature, pressure, and chemical character (pH and Eh) of the mineralizing agencies help to understand the origin of the mineral deposits. The formation of a mineral generally indicates a change from a mobile(fluid) to a solid state.
A brief description of the modes of formation of mineral deposits:
Crystallization from magmas:
When magma cools and the saturation point of the solution is exceeded for any given mineral, the mineral will crystallize, provided that the temperature at the existing pressure is below the mineral’s fusion(melting) point. This process gives rise to all types of mafic and felsic minerals. Diamond, Chromite, and Magnetite deposits are related to the cooling and crystallization of magma.
Metasomatism or Metasomatic replacement or simply replacement:
It is a simultaneous capillary solution and deposition process by which new minerals replace earlier minerals or rocks. The metasomes(replacing minerals) are carried in the solution, and the replaced substances are carried away in the solution.
Replacement is essential in forming epigenetic mineral deposits (those formed later than the enclosing rocks) and supergene mineral deposits (those formed by ascending fluids from magma). Metallic sulfides such as sphalerite, Chalcopyrite, bornite, galena, etc, are formed by this process.
The relative solubility of solid(mineral) and solute(solution):
The relative solubility determines the precipitation of many minerals from the solution. For example, if a CuSO4 solution meets ZnS (sphalerite), CuS(Copper Sulphide) will be deposited at the expense of the
ZnS. On the other hand, if the CuSO4 solution meets HgS (Cinnabar), no mineral deposition will occur.
Oxidation and Reduction:
Hematite(Fe2O3) and Bauxite (Al2O3.2H2O) form in an oxidizing environment, whereas pyrite (FeS2) forms in a reducing environment.
Direct deposition in open space:
By decreasing temperature and pressure, the mineralizing solution may favor vein or cavity-filling deposits in the host rock without involving replacement.
Catalytic action:
Certain substances cause the precipitation of minerals without entering into a solution, which act as catalysts.
Adsorption:
Adsorption is the taking up of one substance at the surface of another. For example, Kaolin adsorbs copper from solution to form chrysocolla (copper ore used as a gemstone).
Base exchange (cation exchange):
It occurs between solids and liquids, whereby cations are exchanged, and new mineral deposits are formed.
Precipitation by bacteria:
This is exemplified by bacteria’s well-known precipitation of iron(hematite). Also, sulfate-reducing bacteria are known to produce native sulfur deposits.
Unmixing of solid solutions (Homogeneous mixture of substances- alloys):
At higher temperatures, two metals or minerals may form solid solutions, which, at decreasing temperatures, release one of the metals or minerals. This is known as unmixing or exsolution. Examples:
- Gold contains Silver in a solid solution
- Gold unites with Mercury to form an amalgam.
- Solid solutions of magnetite, ilmenite, chalcocite, and covellite are standard.
Minerals formed by exsolution remain as inclusion.
Colloidal deposition:
Colloids are precipitated from natural solutions as flocculent masses, but they do not contribute much to the formation of mineral deposits.
Weathering processes:
Three types of mineral deposits are formed which are:
- Mechanical concentration (placer deposit),
- Residual deposit(bauxite deposit) and
- Supergene sulphide enrichment(copper sulphide deposit).
Through this process, low-grade mineral deposits undergo enrichment to form economically valuable
mineral deposits.
Metamorphism:
By this process, the high pressure, temperature, and chemically active fluid act upon pre-existing rocks and minerals to form new minerals through recombination and reconstitution—examples, Garnet, Graphite, Talc, Sillimanite, Marble, Quartzite, Schists etc.
Mineral deposits formed by cavity filling process:
The cavity-filling process has given rise to a vast number of mineral deposits of diverse sizes and forms. These deposits have yielded a great assemblage of metals and mineral products. The mineral deposits resulting from cavity filling may be grouped as below:
Fissure veins:
A fissure vein is a tabular ore body that occupies one or more fissures. It is the most Critical cavity-filling deposit, yielding various minerals and metals. Fissures may be formed by stresses operating within the earth’s crust and may be enlarged at the time of mineralization by the intrusive force of hydrothermal solution.
Shear-zone deposits:
A shear zone serves as an excellent channel-ways for mineralizing solution and deposition of minerals within the seams and crevices in the form of refined grains or thin plates of minerals. For example, gold with pyrite forms workable deposits at Otago, New Zealand. Ores of copper(Chalcopyrite) form workable deposits at Singhbhum, Jharkhand,India
Stockworks:
A stockwork is an interlacing network of small ore-bearing veinlets traversing a rock mass. The individual veinlets rarely exceed an inch or so in width or a few feet in length. The entire rock mass is mined. In general, the veinlets exhibit comb structure and crustification. Stockworks yield tin, gold, silver, copper, molybdenum, cobalt, lead, zinc, mercury and asbestos.
Saddle Reefs:
When a stack of alternating beds of competent and incompetent rocks (e.g. quartzite and slate) are folded, openings are formed at the crest. If ore bodies fill these openings, they are called saddle reefs. The saddle reefs(gold) of Bendigo, Australia, are the type examples.
Ladder veins:
Ladder vein is the name applied to more or less regularly spaced, short, transverse fractures in dykes. The fractures generally extend roughly parallel to each other, from wall to wall of the dyke. Such openings may be filled with mineral matter to form mineral deposits. The morning star gold-bearing dike in Victoria, Australia, is the type example.
Pitches and flats (Synclinal fold cracks):
Synclinal and anticlinal tension cracks are formed due to brittle sediment rocks’ gentle, open folding. Synclinal tension cracks are called pitches and flats. These openings occur in the upper Mississippi Valley in the dolomite limestone and are filled with zinc and lead ores.
Breccia-filling mineral deposits:
The haphazard arrangement of the angular rock fragments in the breccia gives rise to numerous openings. Breccias may result from volcanism, collapse, or shattering. The breccia openings may permit solutions entry, and mineral deposits may form, giving rise to breccia-filling deposits.
Solution cavity fillings (cave deposits):
Various solution openings in soluble rocks have afforded primary and secondary mineral deposits receptacles. They occur most commonly in limestone at shallow depths.
Pore-space fillings:
Pore spaces in rocks may contain ores, oil, gas, and water. For example, copper ores occur in sandstone pores (2% Cu)
Vesicular fillings:
Permeable vesicular lava tops may serve as channelways for mineralizing solutions. For example, vesicles in basalts may be filled with native copper.
Mineral deposits formed by contact metasomatism:
The mineral deposits that result from contact metasomatism constitute an assemblage of uncommon
ore and gangue minerals. These deposits are difficult to exploit because of their small size. Mining of
The deposits must be undertaken with caution based on the following factors:
Position:
The deposits occur within the contact aureole. The deposits are generally scattered irregularly.
Form and Size:
Contact metasomatic deposits are notably irregular in outline. They may have a shape. In general, contact metasomatic deposits are small in size, with dimensions of 100 to 400 feet, and contain from a few thousand to a few hundred thousand tons of ore. A few deposits may contain several million tons.
Texture:
Commonly, the ores are coarse in texture, containing large crystals or clusters of crystals. Crustification and banding are absent. The individual minerals appear to be closely interlocked. The metallic minerals, except pyrite and arsenopyrite, generally lack crystal outlines.
Mineralogy:
By contact metasomatism, an assemblage of high-temperature gangue minerals such as tremolite, actinolite, wollastonite, epidote, diopside, anorthite, albite, fluorite, chlorite, and micas are formed. The ore minerals consist of oxides (magnetite, ilmenite, corundum), native metals (gold, platinum, graphite), and sulfides (chiefly base-metal sulfides).
Relation of Mineral Deposits to Intrusive:
Contact metasomatism does not give rise to mineral deposits indiscriminately with all magmas. Formation of mineral deposits depends on the composition of the intrusive, size and form of intrusive, depth of intrusive, and alteration of intrusive as described below:
Composition of intrusive:
Magmas that yield mineral deposits are mostly silicic ones of intermediate composition, such as quartz, monzonite, granodiorite, or quartz diorite. Felsic magma has a higher content of fluids, which favors mineral formation of economic importance.
Size and form of intrusive:
Most contact metasomatic mineral deposits are associated with stocks and batholiths. They are rarely associated with laccoliths and large sills.
Depth of intrusive:
Relatively slow cooling at considerable depth (1-2kms) favors the formation of contact metasomatic mineral deposits.
Alteration of intrusive:
The intrusive undergoes little change during contact metamorphism.
