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Journal of the Geological Society

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Sulphur isotope geochemistry of black shale-hosted antimony mineralization, Arnsberg, northern Rhenish Massif, Germany: implications for late-stage fluid flow during the Variscan orogeny

^{1 1}Mineralogisches Institut, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
^{2 2}Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montréal H3A 2A7, Quebec, Canada (e-mail: wagner@eps.mcgill.ca)
^{3 3}Scottish Universities Environmental Research Centre (SUERC), East Kilbride, Glasgow G75 0QF, UK

Vein-type and bedding-concordant mesothermal (180–410 °C) stibnite–sulphosalt mineralization at Arnsberg, NE Rhenish Massif, Germany, is hosted by Carboniferous pyrite-rich black shales and siliceous limestones. A detailed sulphur isotope study of the stibnite–sulphosalt mineralization and pyrite from a variety of regional host-rock lithologies has been carried out using an in situ laser combustion technique. The ³⁴S values of stibnite of various textural types are distinctly negative and lie in a narrow range between –23.9 and –17.1 (mean –20.1). In contrast, regional sedimentary–diagenetic pyrites display a large variation of their ³⁴S values between –45.4 and +9.3. There is little evidence for significant modification of the hydrothermal fluid during deposition and the S isotope signatures suggest that the sulphur of the stibnite mineralization was not locally derived. The ³⁴S values of pyrite in Givetian shales display a significantly narrower range of –28.2 to –7.5 and their mean composition (–17.1) is close to the ³⁴S values of the Arnsberg stibnite deposits. Considering the temperature-dependent isotopic fractionation between stibnite and reduced sulfur species, the ³⁴S values of the mineralizing fluid (–16.8; 200 °C) and the Givetian rock source are essentially identical. Therefore, we propose a model of leaching and isotopic homogenization of sulphur from the Middle Devonian shales and a subsequent northward migration of these fluids. The fluids were trapped in permeability-controlled positions within anticlinal zones, where fluid cooling induced deposition of stibnite and sulphosalts.

KEYWORDS: antimony deposits, black shales, sulphur, isotopes, fluid flow