2.3 CLASSIFICATION OF MINERAL DEPOSITS
2.3.7 Genetic Model .1 Magmatic
Magmatic deposits are genetically linked with the evolu- tion of magma that was emplaced into the continental or ocean crust (Fig. 2.16). Mineralization is located within the rock types derived from differential crystallization of parent magma. The significant magmatic deposits are related to mafic (gabbro, norite) and ultramafic (peridotite, dunite, pyroxenite [Fig. 2.17]) rocks originated from crystallization of mafic and ultramafic intrusive magma. Ore minerals are formed by separation of metal sulfides and oxides in molten form within an igneous melt. The deposit types include chromite-nickel-copper and PGE. There are several large magmatic deposits: CrePGE deposits at Bushveld Igneous Complex, South Africa, NieCuePGE deposits at Great
FIGURE 2.15 Layers of chromite (black) and PtePd-bearing gabbro (white), Sittampundi Igneous Complex, Tamil Nadu, India.
FIGURE 2.16 Field photograph of mantle peridotite tectonite from Manipur Ophiolite, Eastern India. Rocks are phanerocrystalline, coarse grained, and melanocratic. Color varies between dark (less altered) and pale green (more altered), intensely serpentinized, layered, and foliated.
Credit: Aparajita Banerjee.
Dyke, Zimbabwe, NiePGEeCr deposits at Sudbury, Canada, NieCuePGE deposits at Stillwater Igneous Complex, Montana, USA, and CreNiPGE deposits at Sukinda and Nausahi Intrusive, India.
2.3.7.2 Sedimentary
Sedimentaryrocks are formed by the process of deposition and consolidation of loose materials under aqueous con- ditions. The sedimentary deposits are a concordant and integral part of the stratigraphic sequence. They are formed due to seasonal concentration of heavy minerals like he- matite on the sea floor. The structures consist of repeated thin layers of iron oxides, hematite, or magnetite, alter- nating with bands of iron-poor shale and chert. Examples are Pilbara BIF, Northwestern Australia, and Bailadila and Goa iron ore, India. The limestone deposits are formed by chemical sedimentation of calcium-magnesium carbonate on the sea floor. Coal and lignite are formed under sedi- mentary depositional conditions.
The evaporite deposits form through evaporation of saline water in lakes and seas, in regions of low rainfall and high temperature. The common evaporite deposits are salts (halite and sylvite), gypsum, borax, and nitrates. The original character of most evaporite deposits is destroyed by replacement through circulating fluids. Examples are sodium chloride and potassium salt deposits at Death Valley (Fig. 2.18), occupying an interface zone between the arid Great Basin and Mojave deserts of California and Nevada, USA, and gypsum deposit at Bikaner, India.
FIGURE 2.17 Pyroxenite is an ultramafic-layered igneous rock con- sisting essentially of minerals of the pyroxene group, such as augite and diopside, hypersthene, bronzite, or enstatite.
FIGURE 2.18 Death Valley formed on Manly Lake during the Pleistocene period in Eastern California. Surface elevation at the feet of the children is e279 ft (e85m) below the mean Sea Level (mSL). It is one of the hottest places on Earth, and known for salt mining for a variety of food-processing applications, including baking, cheese manufacturing, meat processing, seasonings, and prepared mixes.
2.3.7.3 Metamorphic
Metamorphic deposits are transformed alteration prod- ucts of preexisting igneous or sedimentary materials.
Reconstruction is formed under increasing pressure and temperature caused by igneous intrusive body or tectonic events. Metamorphic mineral deposits are formed due to regional prograde or retrograde metamorphic processes and are hosted by metamorphic rocks. Minerals like garnet, kyanite, sillimanite, wollastonite, graphite, and andalusite are end products of the metamorphic process. The copper deposits of Kennicott, Alaska, and White Pine, Michigan, are formed by low-grade metamorphism of organic-rich sediments resting over mafic or ultramafic rocks. The low copper values of underlying source rocks liberate during a leaching process caused by the passing of low-temperature hydrothermalfluids. Thefluids migrate upward along the fractures and faults and precipitate high-grade copper in the rocks containing organic matter.
2.3.7.4 Volcanogenic Massive Sulfide and Volcanic-Hosted Massive Sulfide
VMS and VHMS-type ore deposits contribute significant sources of CueZnePb sulfideAu and Ag, formed as a result of volcanic-associated hydrothermal events under submarine environments at or near the seafloor. They form in close time and space association between submarine volcanism, hydrothermal circulation, and exhalation of sulfides, independent of sedimentary process. The deposits are predominantly stratabound (volcanic derived or volca- nosedimentary rocks) and often stratiform in nature. The ore formation system is synonymous with black smoker- type deposit. Kidd Creek, Timmins, Canada, is the largest volcanogenic massive sulfide deposit in the world. Kidd is also the deepest (þ1000 m) base metal mine. The other notable VMS/VHMS deposits are Iberian Pyrite Belt of Spain and Portugal, Wolverine ZneCuePbeAgeAu de- posit, Canada, and Khnaiguiyah ZnePbeCu, Saudi Arabia.
2.3.7.5 Black Smokers Pipe Type
Black smokers pipe-type deposits are formed on the tectonically and volcanically active modern oceanfloor by superheated hydrothermal water ejected from below the crust. Water with high concentrations of dissolved metal
sulfides (Cu, Zn, Pb) from the crust precipitates to form black chimney-like massive sulfide ore deposits around each vent andfissure when it comes in contact with cold ocean water over time. The formation of black smokers by sulfurous plumes is synonymous with VMS or VHMS deposits of Kidd Creak, Canada, formed 2.4 billion years ago on the ancient seafloor.
2.3.7.6 Mississippi Valley Type
Mississippi Valley-type deposits are epigenetic, strata- bound, rhythmically banded ore with replacement of pri- mary sedimentary features, predominantly carbonate (limestone, marl, dolomite, and rarely sandstone) host rocks. Mineralization is hosted in open space filling, collapse breccias, faults, and hydrothermal cavities. The deposits are formed by diagenetic recrystallization of car- bonates creating a low-temperature hydrothermal solution that migrates to suitable stratigraphic traps like fold hinge and faults at the continental margin and intracratonic basin setting. The ore-forming minerals are predominantly sphalerite, galena, and barite. Calcite is the most common gangue mineral. Low pyrite content supports clean concentrate with high metal recovery of þ95%. Some deposits are surrounded by a pyrite/marcasite halo. Pros- pects can be defined by regional stream sediment, soil, and gossan sample anomaly supported by aeromagnetic and gravity survey. There are numerous Zne PbeAg sulfide deposits along the Mississippi River in the United States, Pine Point, Canada, San Vicente, Central Peru, Silesia, Southern Poland, Polaris, British Columbia, and Lennard Shelf (Fig. 2.19) and Admiral Bay, Western Australia.
2.3.7.7 SEDEX Type
SEDEX-type ore deposits are formed due to concurrent release of ore-bearing hydrothermal fluids into aqueous reservoirs, mainly oceans, resulting in the precipitation of stratiform zinc-lead sulfide ore in a marine basin environ- ment. The stratification may be obscured due to post- depositional deformation and remobilization. The sources of metals and mineralizing solutions are deep-seated, superheated formational brines migrated through intra- cratonic rift basin faults, which come in contact with the sedimentation process. In contrast the sulfide deposits are more intimately associated with intrusive or metamorphic
FIGURE 2.19 Sphalerite (yellow) and galena (black) mineralization in calcite (white) bands indicating differentfluid phase events, Lennard Shelf MVT deposit, Western Australia.
processes or trapped within a rock matrix and are not exhalative. Formation occurred mainly during the Mid- Proterozoic period. SEDEX deposits are the most impor- tant source of zinc, lead, barite, and copper with associated by-products of silver, gold, bismuth, and tungsten. This type of deposit shows two mutual structures: (1) layered by the sedimentary-exaltation process and (2) veins by accu- mulation/remobilization/localization of hydrothermal fluid in fractures. Examples are zinc-lead-silver deposits of Red Dog, northwest Alaska, MacArthur River, Mt. Isa, HYC, Australia, Sullivan, British Columbia, Rampura-Agucha, Zawar Group, and Rajpura-Dariba (Fig. 2.20), India, and the Zambian copper belt.
2.3.7.8 Skarn Type
Skarn-type deposits are formed in a similar process to porphyry orebodies. Skarn deposits are developed due to replacement, alteration, and contact metasomatism of the surrounding country rocks by ore-bearing hydrothermal solution adjacent to a mafic, ultramafic, felsic, or granitic intrusive body. They most often develop at the contact of intrusive plutons and carbonate country rocks. The latter are converted to marbles, calc-silicate hornfels by contact metamorphic effects. Mineralization can occur in mafic volcanics and ultramafic flows or other intrusive rocks.
There are many significant world-class economic skarn de- posits: Pine Creek tungsten, California, Twin Buttes copper, Arizona, and Bingham Canyon copper, Utah, USA, OK Tedi gold-copper, Papua New Guinea, Avebury nickel, Tasma- nia, and Tosam tin-copper, India (reconnaissance stage).
2.3.7.9 Residual Type
Residual-type deposits are formed by a chemical weath- ering process such as leaching, which removes gangue minerals from protore and enriches valuable metals in situ or at a nearby location. The most important example is formation of bauxite under a tropical climate where abun- dance of high temperature and high rainfall during chemical weathering of granitic rocks produces highly leached cover rich in aluminum. Examples are bauxite deposit of Weipa,
Gove Peninsula, Darling Range, and Mitchel Plateau in Australia, Awaso and Kibi, Ghana, East Coast, India, and Eyre Peninsula kaolin deposit, Australia. Basic and ultra- basic rocks tend to form laterites rich in iron and nickel, respectively. Nickel-bearing laterites may or may not be associated with PGE, are mined at New Caledonia, Norseman-Wiluna greenstone belt of Western Australia, Central Africa, and Ni-bearing limonite overburden at Sukinda, India. The other residual type deposits are aurif- erous laterites in greenstone belts (Western Australia) and NieCo and Cr in laterites on top of peridotites (New Caledonia and Western Australia, respectively), and Ti in soils on top of alkali igneous rocks (Parana Basin, Brazil).
2.3.7.10 Placer Type
Placer-type deposits are formed by surface weathering and ocean, river, or wind action resulting in concentrations of some valuable, heavy resistant minerals of economic quantities. The placer can be an accumulation of valuable minerals formed by gravity separation during sedimentary processes. The type of placer deposits are alluvial (trans- ported by a river), colluvial (transported by gravity action), eluvial (material still at or near its point of formation), beach placers (coarse sand deposited along the edge of large water bodies), and paleoplacers (ancient buried and converted rock from an original loose mass of sediment).
The most common placer deposits are those of gold, plat- inum group minerals, gemstones, pyrite, magnetite, cassit- erite, wolframite, rutile, monazite, and zircon. The California Gold Rush in 1849 began when someone discovered rich placer deposits of gold in streams draining the Sierra Nevada Mountains. Recently formed marine placer deposits of rutile, monazite, ilmenite, and zircon are currently being exploited along the coast of eastern Australia, India, and Indonesia.