The origin and evolution of base metal mineralising brines and hydrothermal fluids, South Cornwall, UK

Abstract

A fluid inclusion geochemical study has been carried out on quartz from post-Variscan quartz ± carbonate ± base metal sulphide ± anhydrite ± fluorite veins hosted by Palaeozoic basement (Porthleven, Menheniot, Cornwall) and Permo-Triassic sediments (Western Approaches). Data indicate that the base metal mineralising fluids have a similar bulk chemical composition to the saline fluids found in the Permo-Triassic basinal sequence and support the hypothesis that these basins are the source of the mineralising fluids. Cl and Br systematics suggest that the brines were formed either by the evaporation of seawater or a seawater–meteoric water mixture past the point of halite precipitation. The major cation composition (Na, Ca, K, Mg) of the brines is not consistent solely with evaporation processes but may be explained by dolomitisation processes, albitisation processes, or both, which are recognised in the basinal sequences. The presence of seawater in the base metal mineralised veins suggests that the first marine incursions (Late Triassic) into the region must act as a lower age limit for the mineralisation. The halogen chemistry of a second, hotter (200°C), more dilute (0–5 wt.%) fluid identified in fault-hosted E-W trending veins in the Porthleven area, suggests that the chlorinity of these fluids has a magmatic origin. Circulation of these fluids in post-Variscan extensional structures was driven by the local high-heat-producing Cornubian batholith. The local high-heat-producing granites provided fracture permeability and a heat source that heated the base metal mineralising fluids as they entered the horst block and the dilute fluids circulating around the granites. Petrographic evidence suggests that both palaeohydrologic systems were active contemporaneously. However, each flow system was isolated in differently orientated structures, and there is little evidence for fluid mixing.