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The Khetri Copper Belt, Rajasthan: Iron Oxide Copper-Gold Terrane in the Proterozoic of NW India
by
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.

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ABSTRACT

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 8478 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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