Abstract

Sulphur isotopic compositions were determined for sulphide mineral phases in Meguma-Group-hosted gold deposits, Nova Scotia, in order to resolve (i) potential source regions for sulphur in these deposits, (ii) prevailing ambient physiochemical conditions during sulphide (and gold?) deposition, and (iii) possible implications of δ34S values on genetic models. The Beaver Dam deposit was selected as a test case, and results (± 1σ) from 32 sulphides analyzed for δ34S are as follows (‰): pyrite, 9.9 ± 1.1 (n = 16); pyrrhotite, 9.9 ± 0.2 (n = 4); arsenopyrite, 10.5 ± 0.4 (n = 11); and 9.4 for a single galena. The strikingly narrow range for sulphur isotope data and the sulphide mineral assemblage together are interpreted as implying that the prevailing chemical conditions during sulphide deposition remained at or below the H2S-SO4 buffer and constant relative to it, thus δ34Smineral is a good approximation of δ34Sfluid. Considering that the sulphide minerals analyzed represent a wide range in paragenesis and mode of occurrence, the uniform values suggest a homogeneous sulphur composition in the fluid, with little if any influence of local wall rocks during sulphide deposition. This is consistent with extensive alteration zones (i.e., silicification) throughout the deposit, indicating both high fluid/rock ratios and disequilibrium conditions. Data for an additional seven Meguma Group gold deposits (arsenopyrite only) indicate for δ34S a total range of about 9–25‰; however, the intradeposit variation is consistently small (2–3‰). These results are confirmed by previously published data for a variety of sulphide phases from three other deposits.Comparison of the results for Meguma-hosted deposits with a wide range of both similar and dissimilar gold deposits of variable age indicates that the large positive values (i.e., >9–10‰) are unique to the deposits examined. Two possible explanations are offered: first, the δ34S values originally may have been similar to those of other deposits (i.e., around 0‰ or slightly enriched) and then modified during transport to the site of deposition, or second, the source region of the fluids contained sulphur in an oxidized form (e.g., evaporites). The former hypothesis is currently favoured.Potential reservoirs for sulphur in the Meguma Zone include the Meguma Group metasedimentary rocks and younger peraluminous granitoid batholiths. The sulphur isotopic data are inconsistent with an unmodified magmatic source, and if the Meguma Group is favoured, then the dominant contribution was from the greywacke component. In the latter case, results for the Beaver Dam deposit may represent relatively unmodified fluid, whereas deposits characterized by enriched sulphur may reflect increasing amounts of greywacke-derived sulphur.The data somewhat constrain the possible genetic models for Meguma Group gold deposits. Syngenetic and magmatic models are considered unlikely; instead, a metamorphogenic origin is favoured. However, the ultimate source for mineralizing fluids is considered as lying beneath the Meguma Group, with variable amounts of contamination of this fluid during passage through Meguma Group strata. The relationship between gold mineralization and sulphur isotopic signatures of the associated sulphides is unclear, although a metamorphic protolith is probable.

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