Sulfur isotopic studies of continental flood basalts in the Noril'sk region: Implications for the association between lavas and ore-bearing intrusions

Previous studies of both ore and non-ore-bearing intrusives in the Permo-Triassic flood basalts of the Siberian platform in the Noril'sk area have shown that high-grade Ni-Cu-platinum group elements (PGE) mineralization is associated with anomalously high delta(34)S values of similar to8 to 12parts per thousand, In addition, several researchers have proposed that observed depletions in the Cu, Ni, and PGE content of basaltic lavas of the Nadezhdinsky (Nd) Formation are related to diffusional exchange with, and upgrading in metal tenor of, sulfides in the volcanic conduit system. Sulfur isotopic studies of the lavas at Noril'sk were initiated to determine if interaction with crustally derived sulfur in the conduit system was evident, and if the Nd lavas in particular were characterized by an anomalous isotopic signature. delta(34)S values of the lavas range from -4.5 to 8.7parts per thousand Vienna Canon Diablo Troilite (VCDT), with S concentrations from <40 to 1373 ppm. The majority of delta(34)S values range from 0 to 4parts per thousand, and are similar to those from S-poor intrusions in the Noril'sk area. Although textural data are not supportive of early sulfide saturation and the presence of immiscible sulfide droplets in the lavas, recrystallization may have erased expected mineralogical and textural evidence. Mineralogical data indicate that hydrothermal alteration of the lavas has occurred, but S redistribution has been restricted to localized areas and delta(34)S values have not been affected. The relatively low S concentrations of the lavas are thought to be due in large part to degassing of the lavas in the shallow conduit system and during eruption. Our calculations are consistent with the premise that degassing of basaltic magmas at temperatures in excess of similar to900degreesC at QFM leads to only minor S-34-depletion of sulfur remaining in the melt, and decreases in delta(34)S values of less than 2parts per thousand at 90% degassing. For this reason all lavas with delta(34)S values in excess of similar to 2parts per thousand require a contribution of S-34-enriched country rock sulfur. Because of the high S Content and delta(34)S value (similar to 16-20parts per thousand) of evaporites in the country rocks at Noril'sk, contamination of less than 0.5% is required to explain the most S-34-enriched lavas. The Nd lavas have an average delta(34)S of 2.9parts per thousand, but show no difference in S isotopic composition relative to the other lavas, suggesting that metal depletion involved only limited S transfer, or that exchange between mantle-derived S and S of crustal origin buffered delta(34)S values to less than similar to5parts per thousand. Anomalously positive delta(34)S values, similar to those of the ore-bearing intrusives in the Noril'sk region, are not consistently found in low-S rocks, either lavas or intrusives. Although the mechanism for the derivation of sulfide in the ore-bearing intrusions remain speculative, it is clear that the formation of sulfide ores characterized by high metal tenors proceeded only in the presence of sulfur of crustal origin. Copyright (C) 2003 Elsevier Ltd.