ABSTRACT

Mineral chemistry of biotite, chlorite and epidote associated with the granitoid ore body of the Malanjkhand copper-molybdenum deposit have been studied to constrain temperature, oxidation and sulfidation states as well as fugacity ratios of HF, HCl and H₂O of the hydrothermal mineralizing fluid. Calculated mineralizing temperatures from biotite, chlorite and epidote are mutually consistent and agree well with earlier estimates derived from fluid inclusion studies. Major element chemistry of biotite indicates low oxidation state (FMQ-NNO) and re-equilibrated nature (∼300°C). However, exchange of Cl-F-OH between biotite and the fluid continued to lower temperatures. Subtle differences between the Malanjkhand deposit and known porphyry copper deposits have been evidenced by the fugacity ratios of halogens and water. Chlorite registers equilibrium with fluid down to 200°C and, together with biotite composition, indicates that copper was transported as chloride complex in the fluid. Sulfide deposition occurred during a prompt rise in fugacity of H₂S at 300°C, possibly resulting from a coupled oxidation of Fe²⁺ and reduction of SO₄^2-. Interaction of ore fluid with epidote in the wall-rock favored the deposition of chalcopyrite, enhanced the Ca²⁺ activity in the fluid and inhibited a fall in pH and a subsequent acid alteration of K-feldspar in the wall-rock. These physicochemical characteristics of the mineralized fluid suggest that the Malanjkhand deposit represents an ancient granitoid-associated geothermal system rather than a classic porphyry copper ore deposit.