Residual Ore Deposits
Or “Mafic Igneous Intrusion Associated Deposits”
Syngenetic : The Ores are formed after their host rocks.
Primary : Nickle.
Secondary : Chromite, Copper, Molybdenum, (PEG).
Host: The most significant magmatic deposits are related to mafic (gabbro, norite) and ultramafic (peridotite, dunite) rocks originated from the crystallization of basaltic and ultramafic magma.
Process : Magmatic intrusions result in partitioning of elements and contamination of the melt by assimilation of the host rock. Gravitation segregation of Sulphur result in sulfide ore to form at the bottom of the melt. It is also possible to produce this type of ore through the meteorite impacts, rift/continental flood basalt-associated sills and dykes, volcanic flows and troctolite intrusions.
Examples : There are several largest magmatic deposits: they are Cr-PGE deposits at Bushveld Igneous Complex, South Africa, Ni-Cu-PGE deposits at The Great Dykes, Zimbabwe, Ni-PGE-Cr deposits at Sudbury “(meteorite impact-unusual), Canada, Ni-Cu-PGE deposits at Stillwater Igneous Complex, Montana, US.
|Magmatic Ore Deposits|
The sedimentary deposits are concordant and may be integral part of stratigraphic sequence. It is formed due to seasonal concentration of heavy minerals like hematite on the seafloor. The structures consist of repeated thin layers of iron oxides, hematite or magnetite, alternating with bands of iron-poor shale and chert. The examples are Pilbara BIF, Northwestern Australia, Bailadila and Goa iron ore, India.
Similarly economic limestone deposits are formed by chemical sedimentation of calcium magnesium carbonate on the seafloor. Coal and lignite are formed under sedimentary depositional condition. Evaporite deposits form through the evaporation of saline water in lakes and sea, 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.
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|Sedimentary Ore Deposits|
Metamorphic deposits are formed in different types of metamorphic conditions, ranging from low to high temperature and low to high pressure, due to regional prograde or retrograde metamorphic process and hosted by metamorphic rocks. Minerals like garnet, kyanite, sillimanite, wollastonite, graphite and andalusite are end product of metamorphic process. Three general types of metamorphic deposits are known: 1) copper-rich, 2) gold-rich, and 3) lead-zinc-silver-rich.
1- Copper-Rich Types: These metamorphic deposits are characteristically associated with very low grade to low grade metamorphism. Most often they form in terrains where mafic or ultramafic basement rocks are overlain upsection by organic-rich sedimentary rocks. Examples Kennicott, Alaska, Ore = Chalcocite + Bornite. White Pine, Michigan, Ore = chalcocite + bornite + chalcopyrite + minor sphalerite.
2- Gold-Rich Types: are of two general types: 1) Archean iron formation types, and 2) quartz-carbonate veins. Archean Iron Formation Types: Occur mostly in Precambrian shield areas. Vein morphology but most veins apparently concentrated in peculiar iron-rich shales and sandstones which are upgraded by at least one and usually several metamorphic events. Gold occurs in quartz veins in the silicate or sulfide facies host rocks. Examples Precambrian of Wyoming Jardine, Montana.
Often associated with greenstone belts in shield areas. Ore formed in structural zones/shear zones which are regional in scale. The districts usually contain large scale folding as well. Examples Valdez Creek District, Alaska Conn Mine, Eastern Canada AJ Mine, southeast Alaska.
3- Lead-Silver-Rich: deposits typically contain
galena, sphalerite, and locally tetrahedrite and
chalcopyrite as ore minerals.
Examples Coer de Lane district, Idaho.
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|Metamorphic Ore Deposits|
Volcanogenic Massive Sulfide
VMS and VHMS type of ore deposits contribute significant source of Cu-Zn-Pb sulfide +- Au, Ag, formed as a result of volcanic associated hydrothermal events under submarine environments at or near the seafloor. It forms 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 volcano-sedimentary rocks) and often stratiform in nature. The ore formation system is synonymous to black smoker type of deposit.
Kidd Creek, Timmins, Canada, is the largest VMS deposit (35 Mt @ 2.2% Cu, 5.3% Zn, 0.22% Pb and 60 g/t At at 2005) in the world. Kidd is also the deepest (þ1000 m) base metal mine. The other notable VMS/VHMS deposits are IPB of Spain and Portugal, Wolverine Zn-Cu-Pb-Ag-Au deposit, Canada and Khnaiguiyah Zn-Pb-Cu, Saudi Arabia.
Deposits of this class have been classified by numerous workers in different ways (e.g., metal sources, type examples, geodynamic setting - see Franklin et al. (1981) and Lydon (1984)). Recently, VMS deposits have been classified according to their setting and rock associations into five subclasses, including (after Barrie and Hannington (1999) and Franklin et al. (2005)).
VMS deposits associated with geological environments dominated by mafic rocks, commonly ophiolite sequences. The Cyprus and Oman ophiolites host examples and ophiolite-hosted deposits are found in the Newfoundland Appalachians represent classic districts of this subclass.
VMS deposits associated with environments dominated by mafic volcanic rocks, but with up to 25% felsic volcanic rocks, the latter often hosting the deposits. The Noranda, Flin Flon-Snow Lake and Kidd Creek camps would be classic districts of this group.
VMS deposits associated with sub-equal proportions of mafic volcanic and siliciclastic rocks; felsic rocks can be a minor component; and mafic (and ultramafic) intrusive rocks are common. In metamorphic terranes may be known as or pelitic-mafic associated VMS deposits. The Besshi deposits in Japan and Windy Craggy, BC represent classic districts of this group.
VMS deposits associated with siliciclastic sedimentary rock dominated settings with abundant felsic rocks and less than 10% mafic material. These settings are often shale-rich siliciclastic-felsic or bimodal siliciclastic. The Bathurst camp, New Brunswick, Canada; Iberian Pyrite Belt, Spain and Portugal; and Finlayson Lake areas, Yukon, Canada are classic districts of this group.
VMS deposits associated with bimodal sequences where felsic rocks are in greater abundance than mafic rocks with only minor sedimentary rocks. The Kuroko deposits, Japan; Buchans deposits, Canada; and Skellefte deposits, Sweden are classic districts of this group.
|Volcanogenic Massive Sulfide|
Mississippian Valley Type
Carbonate-hosted lead-zinc ore deposits are important and highly valuable concentrations of lead and zinc sulfide ores hosted within carbonate (limestone, marl, dolomite) formations and which are epigenetic, stratabound, rhythmically banded ore with replacement of primary sedimentary features. MVT and Irish type deposits are commonly associated with a 'dolomite front' alteration.
Process The formation of ore minerals occurs later when tectonic processes and mountain building events channel metal-bearing fluids through the carbonate host rock. MVT deposits are closely associated with orogenic forelands – large valleys running parallel to mountain belts. The deposits formed by diagenetic recrystallization of carbonates creating low-temperature hydrothermal solution that migrates to suitable stratigraphic traps like fold hinge and faults at the continental margin and intra-cratonic basin setting.
The OFM are predominantly sphalerite, galena and barite. Calcite is the most common gangue mineral. Low pyrite content supports clean concentrate with high metal recovery of +95%. Some deposits are surrounded by pyrite/ marcasite halo. Exploring for MVT deposits involves targeting the locations where MVT mineralization is most likely to occur: carbonate platforms in foreland basins with the necessary structural channels. Aside from outright mineral occurrences, geologists will use existing maps to identify the target host rocks and important geologic structures. Field methods such as stream sediment or soil geochemistry may be used to target lead or zinc mineralisation. Geophysical methods including gravity and magnetic surveys may be employed. Gravity may identify mineralized areas (high density rock) or buried open structures which may have been fluid channels (low density rock). Magnetic surveys may be useful since the magnetic iron sulfide mineral pyrrhotite is known to occur in MVT deposits.Examples There are numerous Zn-Pb-Ag sulfide deposits along the Mississippi river in US, Pine Point, Canada, San Vicente, Central Peru, Silesia, Southern Poland, Polaris, British Columbia, Lennard Shelf and Admiral Bay, Western Australia.
|Mississippian Valley Type|
Skarns are calcium-bearing silicate rocks of any age, can form during regional or contact metamorphism and from a variety of metasomatic processes involving fluids of magmatic, metamorphic, meteoric, and/or marine origin. They are found adjacent to plutons, along faults and major shear zones, in shallow geothermal systems, on the bottom of the seafloor, and at lower crustal depths in deeply buried metamorphic terrains. What links these diverse environments, and what defines a rock as skarn, is the mineralogy. This mineralogy includes a wide variety of calc-silicate and associated minerals but usually is dominated by garnet and pyroxene. Skarns can be subdivided according to several criteria. Exoskarn and endoskarn are common terms used to indicate a sedimentary or igneous protolith, respectively. Magnesian and calcic skarn can be used to describe the dominant composition of the protolith and resulting skarn minerals. Not all skarns have economic mineralization; skarns which contain ore are called skarn deposits. In most large skarn deposits, skarn and ore minerals result from the same hydrothermal system even though there may be significant differences in the time/space distribution of these minerals on a local scale.
Major skarn types:
1-Iron (Fe ) Skarns. Is The largest skarn deposits, are mined for their magnetite content and although minor amounts of Cu, Co, Ni, and Au may be present, iron is typically the only commodity recovered (Grigoryev et al., 1990). Many deposits are very large (>500 million tons, >300 million tons contained Fe) and consist dominantly of magnetite with only minor silicate gangue. Ex. Um Nar area, central Eastern Desert, Egypt. Fe Skarn, Iron Oxide Cu-Au, and Manto Cu-(Ag) Deposits in the Andes Cordillera of Southwest Mendoza Province, Argentina.
2-Gold Skarns. Most Gold skarn deposits are associated with relatively mafic diorite, granodiorite plutons and dyke/sill complexes. Most gold produced from skarn deposits came as a byproduct of the mining of other metals, particularly Cu. The term "gold skarn" is used here in the economic sense suggested by Einaudi et al. (1981) and refers to ore deposits that are mined solely or predominantly for gold and which exhibit calc-silicate alteration, usually dominated by garnet and pyroxene, that is related to mineralization. Ex. The Nickel Plate mine in the Hedley district, British Columbia is the largest and highest grade gold skarn in Canada. Fortitude Deposit, Battle Mountain District, Nevada. Gold skarn mineralization at the Crown Jewel.
3-Tungsten Skarns. Tungsten skarns are found on most continents in association with calc-alkaline plutons in major orogenic belts. are associated with coarse-grained, equigranular batholiths (with pegmatite and aplite dikes) surrounded by large, high-temperature, metamorphic aureoles. These features are collectively indicative of a deep environment. Plutons are typically fresh with only minor myrmekite and plagioclase-pyroxene endoskarn zones near contacts.
4-Copper Skarns. are perhaps the worlds most abundant skarn type. They are particularly common in orogenic zones related to subduction, both in oceanic and continental settings. Most copper skarns are associated with I-type, magnetite series, calc-alkaline, porphyritic plutons, many of which have co-genetic volcanic rocks, stockwork veining, brittle fracturing and brecciation, and intense hydrothermal alteration. The largest copper skarns are associated with mineralized porphyry copper plutons. These deposits can exceed 1 billion tons of combined porphyry and skarn ore with more than 5 million tons of copper recoverable from skarn.
5-Zinc Skarns. Most zinc skarns occur in continental settings associated with either subduction or rifting. They are mined for ores of zinc, lead, and silver although zinc is usually dominant. They are also high grade (10-20% Zn+ Pb, 30-300 g/t Ag). Related igneous rocks span a wide range of compositions from diorite through high-silica granite. They also span diverse geological environments from deep-seated batholiths to shallow dike-sill complexes to surface volcanic extrusions. The common thread linking most zinc skarn ores is their occurrence distal to associated igneous rocks. Major reviews of zinc skarn deposits include Einaudi et al. (1981), Megaw et al. (1988), and Megaw (1998).
Most molybdenum skarns are associated with leucocratic granites and range from high grade, relatively small deposits (Azegour, Morocco, Permingeat, 1957) to low grade, bulk tonnage deposits (Little Boulder Creek, Idaho, Cavanaugh, 1978). Numerous small occurrences are also found in Precambrian stable cratons associated with pegmatite, aplite, and other leucocratic rocks (Vokes, 1963). Most molybdenum skarns contain a variety of metals including W, Cu, Zn, Pb, Bi, Sn, and U and some are truly polymetallic in that several metals need to be recovered together in order for the deposits to be mined economically. Mo-W-Cu is the most common association and some tungsten skarns and copper skarns contain zones of recoverable molybdenum. Most molybdenum skarns occur in silty carbonate or calcareous clastic rocks; Cannivan Gulch, Montana (Darling, 1990) is a notable exception in that it occurs in dolomite. Hedenbergitic pyroxene is the most common calc-silicate mineral reported from molybdenum skarns with lesser grandite garnet (with minor pyralspite component), wollastonite, amphibole, and fluorite. This skarn mineralogy indicates a reducing environment with high fluorine activities. These deposits have not received significant study outside of the Soviet Union and there has not been a modern review since the brief summary by Einaudi et al. (1981).
Tin skarns are almost exclusively associated with high-silica granites generated by partial melting of continental crust, usually caused by rifting events. The skarn destructive stages of alteration are particularly important in tin skarn deposits. As noted by Kwak (1987), the most attractive ore bodies occur in the distal portions of large skarn districts where massive sulfide or oxide replacements occur without significant loss of tin in calc-silicate minerals like garnet.
|Skarn Deposits Type|
Black smokers pipe Type
“Black smokers” pipe-type deposits are formed on the tectonically and volcanically active modern ocean floor by superheated hydrothermal water ejected from below the crust. The 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 and fissure 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 2400 million years ago on ancient seafloor.
Sedimentary Exhalative Deposit
SEDEX ore deposits are formed due to concurrent release of ore-bearing hydrothermal fluids into aqueous reservoir mainly ocean, resulting in the precipitation of stratiform zinc-lead sulfide ore in a marine basin environment. The stratification may be obscured due to post depositional deformation and remobilization. The source of metals and mineralizing solutions are deep-seated superheated formational brines migrated through intra-cratonic rift basin faults which come in contact with sedimentation process. The formation occurred mainly during Mid-Proterozoic period. SEDEX deposits are the most important source of zinc, lead, barite and copper with associated by-products of silver, gold, bismuth and tungsten. Depending on the deposit sub-type they also host variable amounts of valuable by-products including copper, gold and silver. SedEx deposits are high grade, with an average size of approximately 70 Mt, and can host about 12 percent lead and zinc. SedEx deposits are easily distinguished from many other deposit types by the fact that their formation is the result of minerals being deposited through the discharge of metal-bearing fluids into seawater. This is a strong contrast from other deposit types that are formed as a result of some type of intrusive or metamorphic process. The examples are zinc-lead-silver deposits of Red Dog, Northwest Alaska, McArthur River, Mt Isa, HYC, Australia, Sullivan, British Columbia, Rampura-Agucha, Rajpura-Dariba, India, and Zambian copper belt.
|Sedimentary Exhalative Deposit|
“Residual” deposits are formed by chemical weathering process like leaching which removes gangue minerals from protore and enrich valuable metals in situ or nearby location. The most important example is formation of bauxite under tropical climate where abundance 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, Eyre Peninsula Kaolin deposit, Australia. Basic and ultrabasic rocks tend to form laterites rich in iron and nickel respectively. Nickel-bearing laterites, may or may not be associated with platinum group of elements, are mined at New Caledonia, Norseman-Wiluna greenstone belt of Western Australia and Central Africa, Ni-bearing limonite overburden at Sukinda, India. The other residual-type deposits are auriferous laterites in greenstone belts (Western Australia), Ni-Co 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).
|Residual Mineral Deposits|
“Placer” deposits Syngenetic are formed by surface weathering and ocean, river or wind action resulting in concentration 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 namely, alluvial (transported 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 paleo-placers (ancient buried and converted rock from an original loose mass of sediment). The most common placer deposits are those of gold, platinum group minerals, gemstones, pyrite, magnetite, cassiterite, 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.