Reconstruction of Physicochemical Environment of Hydrothermal Mineralization at Malanjkhand Copper Deposit, Central India: Constraints from Sulfur Isotope Ratios in Pyrite, Molybdenite and Chalcopyrite

摘要

δ~(34)S values of pyrite, molybdenite and chalcopyrite were determined from the Malanjkhand copper deposit. These minerals constitute the primary sulfide phases that were deposited after the initial magnetite deposition in the main orebody and host granitoid. Pyrite exhibits a depleted range of values (-2.63 to -0.56‰), chalcopyrite, a very narrow range of values around zero (-0.039 to 0.201 ‰) and molybdenite furnishes a range of enriched values (0.68 to 1.98‰). On back calculation of the δ~(34)S values of H_2S in the fluid from which the minerals were likely to have precipitated, using standard expressions for equilibrium fractionation at the temperature range obtained from fluid inclusion and mineral fluid equilibria, it is observed that H_2S in the fluid at pyrite deposition was depleted and gradually became enriched towards molybdenite and chalcopyrite deposition. This trend is best explained as being due to inorganic reduction of SO_4~(2-) in the fluid and is very much in agreement with the paragenetic sequence indicating increasing activity of H_2S in the fluid. The very restricted range in the δ~(34)S values of sulfide minerals in the fluid does indicate a single, possibly magmatic, source of sulfur that also agrees well with the earlier deduced model of genesis of the deposit as an ancient geothermal system associated with granitic magmatism.