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

The Khetri, Alwar and Lalsot-Khankhera Copper Belts contain widespread Cii±Au±Ag±Co±Fe±REE±U mineralisation over a 150x 150 km area of Rajasthan and Haryana, NW India. Mineralisation 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, defonned 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 characterised 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 mineralisation 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. Mineralisation 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 mineralisation, massive magnetite-haematite vein-deposits, fluorite mineralisation 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 the Madhan-Kudhan Cu mine at Khetri (897+125 Ma).

There is a variation in the sulphide-oxide mineralogy of Cu deposits across the >100 km wide Khetri Copper Belt, with four dominant types recognised: (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 sulphide Cu-Au deposits, whereas types (3) and (4) are iron oxide Cu-Au deposits hosted by oxidised rocks. Copper mineralisation in the Khetri Copper Belt is epigenetic, broadly synchronous with late (0.75-0.85 Ga) A-type granitoid emplacement, and has a mineralisation and structural style, and regional- and deposit-scale alteration assemblages comparable to known IOCG and iron-sulphide Cu-Au mineralised districts.