ISOTOPIC COMPOSITION OF 2 PRECAMBRIAN STRATIFORM BARITE DEPOSITS FROM THE INDIAN SHIELD

Stratiform barite beds, interlayered with (fuchsite) quartzites and mafic metavolcanics and containing minor disseminations of sulfides, occur in the Archean (> 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 (approximately 2.0 Ga) Aravalli Supergroup in the state of Rajasthan, northwest India (Fig. 1). The Sargur barites have delta-S-34 in the range of 4.02 to 7.45 part per thousand (xBAR = 5.40 part per thousand). Disseminations of detrital pyrite, occurring in layers in the barite have delta-S-34 between-3.62 and 5.87 part per thousand. These barites are also characterized by low Sr-87/Sr-86 ratios (approximately 0.7018 +/- 7). The basal Aravalli barites, in contrast, have exceptionally high Sr-87/Sr-86 ratios (0.72284 to 0.72382) and delta-S-34 between 17.11 and 21.24 parts per thousand (xBAR = 18.75 parts per thousand). delta-S-34 in disseminated chalcopyrite within banded barite-amphibolite samples from the same occurrence varies between 6.21 and 8.52 parts per thousand (xBAR = 6.88 parts per thousand). 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 Sr-87/Sr-86 ratios point, however, to a local, highly radiogenic source, which had affected the Sr isotopic composition of coeval, 'normal' seawater.