A sulphur isotopic investigation of the potential sulphur sources for Lower Palaeozoic-hosted vein mineralization in the English Lake District

Abstract

Current models for Cu–Fe–As (Devonian), Pb–Zn (Carboniferous) and barite (Upper Carhoniferous/Permian) vein mineralization in the Lake District invoke a variety of sources for fluids and metals including the Skiddaw Group sediments, the Borrowdale Volcanic Group, the Lake District batholith, meteoric water and Carboniferous seawater. This study isotopically characterizes the sulphur reservoirs within the Lower Palaeozoic sedimentary and igneous sequences and Late Caledonian intrusives, and assesses their importance as sources of sulphur for the vein mineralization. The Ordovician Skiddaw Group is a reservoir of isotopically heavy sulphur with a range of δ 34 S from +11‰ to +29‰ [& x~ (1 σ ) = 18.53 ±6.7‰ ( n = 10)]. Four analyses of Borrowdale Volcanic Group rocks yielded values of +1‰ to +15‰. Such positive values are thought to be the result of closed system bacteriogenic reduction of Ordovician seawater sulphate in the case of the Skiddaw Group, and mixing of mantle-derived magmatic sulphur with Ordovician seawater sulphate for the Borrowdale Volcanic rocks. The sulphides in the vein systems hosted by the Skiddaw Group in the Vale of Newlands formed from fluids with δ 34 S H 2 S signatures ranging from +16.7‰, to +22.5‰: partially homogenized Skiddaw Group sulphur is considered to be the source. Mineralization at Coniston, hosted by the Borrowdale Volcanic Group, shows similar sulphur isotopic characteristics, and Skiddaw Group at depth is considered to be the predominant source of sulphur with a possible minor input from the wallrock volcanic rocks at the site of deposition. Although the Lake District batholith provided a heat source for the Cu–Fe–As vein mineralization, the granite did not supply any sulphur to those systems. However, magmatic sulphur was an important constituent in mineralization associated with the Shap and Skiddaw granites. The δ 34 S values for barite mineralization at Force Crag, Greenside, Skiddaw and Shap ranges from +13‰ to +18‰, and Carboniferous seawater provides the most reasonable source for sulphate.