Sulphur isotope geochemistry of pyrite from the Upper Cretaceous Marshybank Formation, Western Interior Basin

Variations in the texture and sulphur isotopic composition of pyrite in the Upper Cretaceous Marshybank Formation are linked to depositional environment. Abundant, framboidal (and lesser euhedral) pyrite precipitated in offshore marine rocks. Moderate quantities of pyrite also crystallized in brackish, coastal plain rocks. However, in contrast to marine pyrite, coastal plain pyrite is dominantly euhedral in texture, reflecting direct precipitation from a porewater with a relatively low dissolved sulphide concentration. In the marine rocks, pyrite 634 S values range from -35.7 parts per thousand to +27.4 parts per thousand (avg. -4.8 parts per thousand Canyon Diablo Troilite, CDT). Pyrite within carbonate concretions hosted in these marine rocks has a similar isotopic composition (-49.8 parts per thousand to +10.6 parts per thousand CDT). However, isotopic values ate often highly variable within individual concretions as a result of the heterogeneous nature of sulphate reduction and pyrite formation within marine sediments. Pyrite in coastal plain rocks is characterized by relatively high delta(34)S values (-4.2 parts per thousand to +35.5 parts per thousand I avg. +13.2 parts per thousand CDT), while the overlying sideritized conglomerates have the lowest delta(34)S values reported for Cretaceous rocks from the Western Interior Basin of North America (-49.8 parts per thousand to -41.7 parts per thousand CDT). Very low delta(34)S values, which are only observed in the marine rocks, are indicative of microbial sulphate reduction and pyrite formation in a sulphate-replete (i.e., open) system. Higher delta(34)S values (up to +18 parts per thousand), which were obtained for both the marine and coastal plain rocks, are indicative of progressive pyrite crystallization in a sulphate-limited (i.e., closed) system. Such conditions are expected in marine sediments as burial occurs, and in brackish (i.e., low sulphate) sediments. Pyrite with very high delta(34)S values (>+18 parts per thousand) is common in the coastal plain rocks. These high values are the result of influx of S-34-enriched, residual sulphide derived from overlying marine units. A minor amount of S-34-enriched pyrite is also present within, and proximal to marine sedimentary Unit H, which was deposited at the same time as the coastal plain rocks. We hypothesize that porewater containing S-34-enriched, residual sulphide flowed from the coastal plain and along Unit H, into the marine sediments, thus producing anomalously high delta(34)S pyrite. (C) 2002 Elsevier Science B.V. All rights reserved.