Pyrite chemistry and S isotope systematics associated with paragenesis of epithermal-style gold mineralization in the Eastern Cobequid Highlands, Nova Scotia, Canada

Abstract

Within the northeastern Cobequid Highlands (CH), Nova Scotia, Canada recent bedrock mapping and bulk rock geochemistry have identified a potential epithermal Au system. The Warwick Mountain area shows the most potential for Au mineralization with two zones of intensely silicified and sulphidized basalt identified. Assays show anomalous As, Sb, Cd, W, Hg, with Au concentrations up to ~660 ppb. Trace element chemistry and S isotope systematics of sulphide minerals are used to track deposit evolution and epithermal processes at a variety of scales, and develops exploration criteria using these data. The host rock setting, enrichments in bulk rock Au, As, Sb, and Hg, and sulphide mineralogy are consistent with a low sulphidation epithermal Au style of mineralization. The bulk S (as mainly pyrite) correlates to bulk δ³⁴S_VCDT, possibly as a function of sulphate content enriched during sample weathering in core. The As content of pyrite correlates weakly to in-situ δ³⁴S_VCDT (by SIMS) in pyrite with As-rich zones often showing high δ³⁴S_VCDT, suggesting that ³⁴S enrichment and As enrichment were coupled. Bulk δ³⁴S_VCDT correlates to in-situ δ³⁴S_VCDTbut bulk δ³⁴S_VCDT values are consistently higher by several ‰, suggesting partial oxidation of sulphides to sulphates and preferential loss of ³²S in core during long-term storage (degassing, removal by rain water?). Pyrite grains with rare, late overgrowths showing very high δ³⁴S_VCDT values are attributed to sulphide derived from the reduction of seawater sulphate. There are systematic variations in As and δ³⁴S_VCDT in surface samples and a few core samples. The biggest variation between core and rim δ³⁴S_VCDT to more negative values occurs at the Nuttby Mountain occurrence, where late enrichment in Au, Ag, As, and Sb occurs in the rims of pyrite grains. Similar rim enrichment in Au was observed in some pyrite from drill core.