The genesis of the west shropshire orefield: Evidence from fluid inclusions, sphalerite chemistry, and sulphur isotopic ratios

Abstract

AbstractThe Pb + Zn + Ba veins of West Shropshire, England, occupy fractures in Ordovician and Precambrian rocks of the Shelve Inlier. Precipitation of sphaleritic ores was succeeded by galena + baryte mineralization, with chalcopyrite also occurring late in the mineralizing episode. Three generations of sphalerite are recognized, and second being chemically zoned with distinct Fe + Cd‐rich growth zones. Associated with these iron‐rich bands are smaller (∼ 10 μm) zones rich in indium (>1.0 wt%) and copper, electron electron probe microanalyses suggesting coupled substitution of Cu+ + In+3 in the ZnS. Based on the sequence of growth zones and their chemistry a sphalerite stratigraphy can be recognized within the orefield.Fluid inclusions studies reveal the mineralizing fluids to be highly saline (18–30 wt% CaCl2 equivalent) Na + Ca‐richbrines, with mineralizing temperatures in the range 200–120°C. A trend from higher temperature‐low salinity to low temperature‐high salinity fluids with time is recognized. Sulphide sulphur isotopic ratios are consistent and suggest a ΔS34SH2S of the mineralizing fluid of 10%, while 34Sbaryte values are in the range + 14 to + 19%, indicating separate sulphate and sulphide sulphur sources. The mineralogical, fluid inclusion, and isotopic data suggest the saline fluids rose into an open plumbing system where mineral precipitation was mainly controlled by fluid cooling. The baryte however, formed due to mixing with oan overlying sulphate‐bearing reservioir.Theree possible fluid sources are considered, namely: Lower Carboniferous seawater, basinal brines, and metamorphic fluids. However, the information available does not allow the source to be positively identified.