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

A fluid inclusion geochemical study has been carried our on quartz from pos t-Variscan quartz +/- carbonate +/- base metal sulphide +/- anhydrite +/- f luorite veins hosted by Palaeozoic basement (Porthleven, Menheniot, Cornwal l) and Permo-Triassic sediments (Western Approaches). Data indicate that th e base metal mineralising fluids have a similar bulk chemical composition t o the saline fluids found in the Permo-Triassic basinal sequence and suppor t the hypothesis that these basins are the source of the mineralising fluid s. Cl and Br systematics suggest that the brines were formed either by the eva poration 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 expla ined by dolomitisation processes, albitisation processes, or both. which ar e recognised in the basinal sequences. The presence of seawater in the base metal mineralised veins suggests that the first marine incursions (Late Tr iassic) into the region must act as a lower age limit for the mineralisatio n. The halogen chemistry of a second, hotter (200 degreesC), 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 dri ven 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. Pet rographic evidence suggests that both palaeohydrologic systems were active contemporaneously. However, each flow system was isolated in differently or ientated structures, and there is little evidence for fluid mixing. Copyrig ht (C) 2001 Elsevier Science Ltd.