Post-magmatic hydrothermal circulation and the origin of base metal mineralization, Cornwall, UK

A detailed fluid-inclusion study of post-Variscan quartz+/-carbonate +/- su lphide +/- anhydrite +/- fluorite veins hosted by Palaeozoic basement (Port hleven, Menheniot, Cornwall) and Permo-Triassic sediments (Western Approach es Basin) has identified two broad fluid types. A high salinity (20-27 wt% NaCl). low temperature (80-150 degrees C) brine is present in base metal mi neralized veins hosted by Palaeozoic rocks onshore and in sulphate-bearing veins within the Permo-Triassic offshore succession. Microthermometric data indicate that saline fluids found in the Permo-Triassic basinal sequences have similar compositions to the base metal mineralizing fluids and confirm s that such basins are likely sources for the mineralizing fluids. A second , hotter (200 degrees C) more dilute (0-5 wt%) fluid has been identified in fault-hosted east-west-trending veins in the Porthleven area. It is likely that the temperatures of both the dilute and saline fluids have been affec ted by the local high heat producing Cornubian granite batholith. There is also limited evidence for fluid mixing of these two fluids which may have b een responsible for base metal precipitation.