Anoxic stratified oceans as a source of sulphur in sediment-hosted stratiform ZnPb deposits (Selwyn Basin, Yukon, Canada)

Abstract Three periods of stagnation, viz. Middle Ordovician to Early Silurian, Early Devonian, and Late Devonian, have been recognized in the Selwyn Basin by extremely positive δ 34 S-values in pyrite. These δ 34 S-values in pyrite reflect almost complete bacterial sulphate reduction of sulphate to sulphide within a restricted anoxic layer of a stratified water column. W.C. Shanks and coworkers have reported a fourth anoxic event in the basin during the Early Cambrian. Each period of basin stagnation coincides with times when sedimentary Zn Pb-sulphide and barite deposits formed, while intervening periods correspond with the formation of stratiform barite deposits. The stratigraphic distribution of δ 34 S-values for pyrite are similar across the basin, whether near to or remote from mineralization, indicating a common seawater source of S. The generally parallel tracking of isotopic curves for sphalerite and galena with pyrite indicates that this sulphide also came from the bacterial reduction of sulphate within a stratified water column. This interpretation is supported by data on hydrothermal fluids that produced Zn Pb mineralization, which are near-neutral pH and range in temperature between 100° and 350°C. In such solutions, it is thermodynamically difficult to transport appreciable sulphide in the presence of Pb and Zn. Since these conditions apply to mineralizing fluids of many sedimentary-exhalative Zn Pb deposits, it follows that they too were likely S-poor, requiring an external seawater source of sulphide. This source is most effectively supplied by the sulphidic bottom layers of a stratified water column. This explains the correlation between anoxic events and Zn Pb sedimentary deposits, both within the Selwyn Basin and world-wide during the Phanerozoic. The latter observation also implies that the anoxic events within the Selwyn Basin were not controlled locally, but instead reflect periods of worldwide ocean stagnation.

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