|The Khetri Copper Belt, Rajasthan: Iron Oxide Copper-Gold Terrane in the Proterozoic of NW India|
Joe Knight, Jon Lowe, BHP Billiton Exploration, Brisbane, QLD, Australia, Sojen Joy, John Cameron, James Merrillees, Sudipta Nag, Nalin Shah, Gaurav Dua & Khamalendra Jhala, BHP Billiton Minerals India, New Delhi, India.
in - Porter, T.M. (Ed), 2002 - Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing, Adelaide, v. 2, pp 321-341.
The Khetri, Alwar and Lalsot-Khankhera Copper Belts contain widespread Cu±Au±Ag±Co±Fe±REE±U mineralization over a 150 km by 150 km area of Rajasthan and Haryana, NW India. Mineralization is hosted by the mid-Proterozoic Delhi Supergroup, which comprises shallow-water, locally evaporitic, sedimentary rocks, with lesser mafic and felsic volcanic rocks. These rocks have been metamorphosed to the low- to mid-amphibolite facies, deformed into NE-SW striking, doubly-plunging folds, and intruded by numerous 1.5-1.7 Ga syntectonic granitoids and 0.75-0.85 Ga post-tectonic granitoids. Post-tectonic granitoids range from tonalite to syenite, contain hornblende and biotite as the dominant mafic minerals and magnetite, titanite, allanite, apatite, fluorite as accessory phases, and are geochemically characterized by A/CNK ratios <1.1, low Al and Ca, high Th and HFSE, and enrichment in LREE, indicating A-type affinities.
The largest deposits in the Khetri Copper Belt are at Khetri (140 Mt @ 1.1-1.7% Cu, 0.5 g/t Au), where mineralization extends over a >10 km strike-length, is hosted by garnet-chlorite schists, andalusite- and graphite-bearing biotite schists, and feldspathic quartzites, and is sited in sub-vertical NE- and NW-striking shear zones. Mineralization forms sub-vertical lens, comprising stockworks of massive to vein-hosted chalcopyrite - pyrite - pyrrhotite, which are broadly foliation-parallel but also cross-cut bedding and peak-metamorphic fabrics. Gold, Ag, Co, LREE, Mo, S, U and W are variably co-enriched with Cu. Alteration at Khetri comprises amphibole (hornblende, actinolite, cummingtonite, anthophyllite) - albite - quartz - biotite - scapolite - chlorite - carbonate, with magnetite and haematite as dominant oxide phases.
Directly to the east of Khetri, a 50 km wide by > 100 km zone of calc silicate and albite-haematite alteration overprints and cross-cuts metamorphic fabrics. Calc silicate alteration comprises coarse-grained clinopyroxene - hornblende - epidote - apatite - scapolite - titanite - magnetite, whereas albite - haematite alteration comprises assemblages of albite - amphibole - haematite - magnetite - calcite, with variable K-feldspar, biotite, epidote, scapolite, titanite, apatite and fluorite, and locally abundant pyrite and chalcopyrite. Albite-haematite alteration is spatially related to vein systems and breccias, which commonly contain Cu-Au mineralization, massive magnetite-haematite vein-deposits, fluorite mineralization and rare uraninite deposits. Calc silicate alteration occurs on the margins of the Khetri Copper Belt, whereas albite-haematite alteration forms a central core to the Belt and locally overprints calc-silicate assemblages. A SHRIMP U-Pb titanite age in the assemblage albite - haematite - amphibole - calcite - titanite constrains the timing of regional alteration to 847±8 Ma. This overlaps the fission-track ages of garnet from ore assemblages at Madhan-Kudhan Cu mine at Khetri (897±125 Ma).
There is a variation in the sulfide-oxide mineralogy of Cu deposits across the >100 km wide Khetri Copper Belt, with four dominant types recognized: (1) chalcopyrite-pyrite-pyrrhotite ores hosted by graphitic schists at Akwali, in the west, (2) chalcopyrite - pyrite - pyrrhotite - magnetite - haematite ores at Khetri and Kho Dariba, in the east, (3) magnetite - haematite - chalcopyrite - pyrite ores hosted by albite-haematite alteration, in the central part of the Khetri Copper Belt, and (4) haematite - chalcopyrite - baryte ores in the eastern part of the Belt. Types (1) and (2) are hosted by mainly reduced rock types and can be classified as iron sulfide Cu-Au deposits, whereas types (3) and (4) are iron oxide Cu-Au deposits hosted by oxidized rocks. Copper mineralization in the Khetri Copper Belt is epigenetic, broadly synchronous with late (0.75-0.85 Ga) A-type granitoid emplacement, and has a mineralization and structural style, and regional- and deposit-scale alteration assemblages comparable to known IOCG and iron-sulfide Cu-Au mineralized districts.
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