Isotopic composition of two Precambrian stratiform barite deposits from the Indian shield

Abstract

Stratiform barite beds, interlayered with (fuchsite) quartzites and mafic metavolcanics and containing minor disseminations of sulfides, occur in the Archaeagn (>3.3 Ga) Sargur Group of rocks at Ghattihosahalli, in the state of Karnataka, south India and near Jagat, within quartzites and metamorphosed flood basalts underlying the middle Proterozoic (∼2.0 Ga) Aravalli Supergroup in the state of Rajasthan, northwest India (Fig. 1). The Sargur barites have δ 34 S in the range of 4.02 to 7.45‰ ( x ̄ = 5.40‰ ). Disseminations of detrital pyrite, occurring in layers in the barite have δ 34 S between −3.62 and−5.87‰. These barites are also characterized by low 87Sr 86Sr ratios (∼0.7018 ± 7). The basal Aravalli barites, in contrast, have exceptionally high 87Sr 86Sr ratios (0.72284 to 0.72382) and δ 34 S between 17.11 and 21.24‰ ( o ̄ vbar|x = 18.75‰ ). δ 34 S in disseminated chalcopyrite within banded barite-amphibolite samples from the same occurrence varies between 6.21 and 8.52‰ ( o ̄ vbar|x = 6.88‰ ). The isotopic data of the Sargur barites most probably reflect Archaean coeval seawater composition. The mantle-like values indicate buffering of the seawater by mantle-derived basaltic crust. In contrast, the Aravalli barites bear the signature of a Proterozoic sea whose sulfur isotope composition is closer to that of present-day oceans. The unusually high 87Sr 86Sr ratios point, however, to a local, highly radiogenic source, which had affected the Sr isotopic composition of coeval, ‘normal’ seawater.