Biogenic and thermogenic sulfate reduction in the Sullivan Pb–Zn–Ag deposit, British Columbia (Canada): Evidence from micro-isotopic analysis of carbonate and sulfide in bedded ores

Abstract

The layered, clastic-hosted (Sedex-type) Proterozoic (ca. 1475 Ma) Sullivan Pb–Zn–Ag deposit was metamorphosed by several episodes of metamorphic recrystallization that have extensively modified or erased primary grain-to-grain textural relationships. Yet, genetically significant variations of sulfur and carbon isotope ratios have been preserved at the millimeter and greater scale. Whereas sulfur isotope fractionations on a grain-to-grain scale cannot distinguish between likely formation temperatures and post-peak metamorphic cooling, the preserved fine-scale and deposit-scale stratigraphic variations of δ34S and δ13C bear on the origin and formational history of this deposit.High-spatial resolution documentation of accurate grain-to-grain and stratigraphic sulfur isotope variations in selected portions of bedded ore from the Sullivan deposit was made possible by use of a cold stage during in situ laser-assisted isotopic analysis. Pyrrhotite formed from biogenic sulfate reduction in the water column can be distinguished from sulfides (e.g., sphalerite) with higher δ34S values that formed in response to hydrothermal fluids in a brine pool.Isotopic analysis of carbonates in the presence of abundant sulfides was facilitated by development of a gas chromatographic carbonate extraction system with a high-efficiency cryogenic trap. Calcite, present in interlayered massive sulfide and siliciclastic layers in the bedded ore zone in trace amounts to a few weight percent, records a carbon isotopic stratigraphy that suggests variable thermogenic sulfate reduction.Stratigraphic curves illustrating the variation in carbon and sulfur isotopes from the base of the Main Band to above the “HU” bands (mine stratigraphic names) in the bedded ore zone are similar in shape. Values of δ13C decrease abruptly at the base of the Main Band, decrease slightly to the C band, then increase to pre-ore values. The values of δ34S decrease from the base of the Main Band to the C band, and then increase to pre-ore values. This similarity suggests some linkage between carbon and sulfur isotopic systematics, and a sulfur budget dominated in the early stages of massive sulfide precipitation by hydrothermal processes involving thermogenic sulfate reduction, giving way to bacteriogenic sulfate reduction in a closed, then finally open marine basin.