Sulphur isotope geochemistry of black shale-hosted antimony mineralization, Arnsberg, northern Rhenish Massif, Germany

Abstract

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.