Variability of sulphur isotope ratios in pyrite and dissolved sulphate in granitoid fractures down to 1 km depth - evidence for widespread activity of sulphur reducing bacteria

Drake, H., Åström, M.E., Tullborg, E.L., Whitehouse, M. and Fallick, A.E. (2012) Variability of sulphur isotope ratios in pyrite and dissolved sulphate in granitoid fractures down to 1 km depth - evidence for widespread activity of sulphur reducing bacteria. Geochimica et Cosmochimica Acta, 102, pp. 143-161. (doi: 10.1016/j.gca.2012.10.036)

Abstract

Euhedral pyrite crystals in 46 open bedrock (granitoid) fractures at depths down to nearly 1 km were analysed for sulphur isotope ratios (δ34S) by the in situ secondary ion mass spectrometry (SIMS) technique and by conventional bulk-grain analysis, and were compared with groundwater data. Twenty nine of the fractures sampled for pyrite had corresponding data for groundwater, including chemistry and isotopic ratios of sulphate, which provided a unique opportunity to compare the sulphur-isotopic ratios of pyrite and dissolved sulphate both at site and fracture-specific scales. Assessment of pyrite age and formation conditions were based on the geological evolution of the area (Laxemar, SE Sweden), and on data on co-genetic calcite as follows: (1) the isotopic ratios of the calcite crystals (δ18O, δ13C, 87Sr/86Sr) were compared with previously defined isotopic features of fracture mineral assemblages precipitated during various geological periods, and (2) the δ18O of the calcites were compared with the δ18O of groundwater in fractures corresponding to those where the calcite/pyrite assemblages were sampled. Taken together, the data show that all the sampled fractures carried pyrite/calcite that are low-temperature and precipitated from the current groundwater or similar pre-existing groundwater, except at depths of −300 to −600 m where water with a glacial component dominates and the crystals are from pre-modern fluids. An age of <10 Ma are anticipated for the pre-modern fluids. The δ34Spyr showed huge variations across individual crystals (such as −32 to +73‰) and extreme minimum (−50‰) and maximum (+91‰) values. For this kind of extreme S-isotopic variation at earth-surface conditions there is no other explanation than activity of sulphur reducing bacteria coupled with sulphate-limited conditions. Indeed, the most common subgrain feature was an increase in δ34Spyr values from interior to rim of the crystal, which we interpret are related to successively higher δ34S values of the dissolved source SO42− caused by ongoing bacterial sulphate reduction in fractures with low-flow or stagnant waters. The measured groundwater had δ34SSO4 values of +9‰ to +37‰, with the highest values associated with low sulphate concentrations. These values are overall, and especially in the sulphate-poor waters down to −400 m, somewhat higher than the anticipated initial values, and can thus, like for the 34S-enriched pyrites, be explained by a Rayleigh distillation process driven by microbial sulphate reduction. An intriguing feature was that the δ34SSO4 values of the groundwater were in no case reaching up to the values required to produce biogenic pyrite with δ34S values of +40‰ to +91‰. To explain this feature, we suggest that groundwater in low-flow fractures with near-stagnant water (carrying sulphate and pyrite with high δ34S) is masked by high-flow parts of the fracture system carrying groundwater that often contains sulphate in abundance and considerably less fractionated with respect to 34S and 32S. In order to gain detailed knowledge of chemical processes and patterns in groundwater in fractured rock, fracture-mineral investigations are a powerful tool, as we have shown here for the sulphur system.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Fallick, Professor Anthony
• Authors: Drake, H., Åström, M.E., Tullborg, E.L., Whitehouse, M., and Fallick, A.E.
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Geochimica et Cosmochimica Acta
• ISSN: 0016-7037