Ore-forming processes in the Khetri Copper Belt, western India: constraints from trace element chemistry of pyrite and C-O isotope composition of carbonates

The Khetri Copper Belt of the Aravalli-Delhi Fold Belt in western India hosts Cu (± Au ± Ag ± Co ± Fe ± REE ± U) mineralization that is likely of iron oxide-copper-gold (IOCG) type. The study on the Madan-Kudan deposit in this belt documents four vein types: Type-1 (pyrite ± chalcopyrite ± magnetite ± biotite ± scapolite ± amphibole ± chlorite), Type-2 (chalcopyrite-pyrrhotite-pyrite-magnetite-amphibole-chlorite), Type-3 (chalcopyrite-pyrrhotite-pyrite-dolomite-quartz), and Type-4 (chalcopyrite-pyrrhotite-biotite). Pyrite is grouped on texture and major and trace element chemistry into Pyrite-1A, Pyrite-1B, Pyrite-1C (Type-1 veins), Pyrite-2 (Type-2 veins), Pyrite-3A, and Pyrite-3B (Type-3 veins). This sequence documents changing fluid composition and suggests that sulfide mineralization was associated with Na-Ca-K alteration (Type-1 and Type-2 veins), carbonate alteration (Type-3 veins), and K-Fe-Mg alteration (Type-4 veins). The C and O isotope composition of dolomite from Type-3 veins suggests that the ore fluid contained mantle-derived carbon (possibly carbonatite-related) and mixed with an isotopically heavier fluid or exchanged isotopes with crustal rocks. A strong positive correlation between Au and Cu is interpreted to reflect their “coupling” in the pyrite structure. In contrast, Pb, Zn, Bi, and Ag are present in mineral inclusions. Intragrain Fe, Co, As, and Ni variability in pyrite suggests that replacement by coupled dissolution-precipitation and formation of overgrowths were important. Pyrite-1A has high Co (up to 3.3 wt%) and Co/Ni ratios (500 to 16,000) that have not been reported elsewhere. The Co/Ni ratios of KCB pyrite are similar to those from iron oxide-apatite and other IOCG deposits, although the latter do not have a characteristic Co/Ni ratio but consistently have high Co concentrations (up to 1 wt% or more).