Geothermometry based on fractionation of Mn and Cd between coexisting sphalerite and galena from some carbonate-hosted sulfide deposits in India

Abstract

Geothermometry based on the fractionation of Mn and Cd between coexisting sphalerite and galena from concordantly banded, strata-bound ores and younger vein ores of Rajpura-Dariba, Zawar, and Bandalamottu has been attempted. Different fractionation trends for the banded and vein ores in the same deposit at Rajpura-Dariba indicate different thermal regimes of final equilibration for the two varieties of ores. In contrast, a single trend incorpo- rating both types of ores is suggestive of isothermal condi- tions of final equilibration for the banded and vein ores at Zawar. Unrealistically high temperatures obtained for the vein ores of Bandalamottu suggest equilibrium in the sphalerite-galena aggregates of this deposit was not at- tained. Thermometric data from Mn and Cd fractionation are compared with other independent geothermometric de- terminations obtained from fluid inclusion homogenization studies and based on phase equilibria. It has been found that, in general, the Cd-fractionation temperature (Tcd) is a more reliable geothermometer than the Mn-fractionation temperature (Tun). This presumably may be due to the sus- ceptibility of aqueous Mn species to subtle changes in fo2 in the ore fluid and consequent heterogeneity in Mn concentra- tion in sphalerite, thus affecting the distribution coefficient. The investigation also suggests that Tca becomes increasing- ly more dependable when the ores are metamorphosed and reequilibrated at higher grades of metamorphism.