A detailed fluid‐inclusion study of post‐Variscan quartz ± carbonate ± sulphide ± anhydrite ± fluorite veins hosted by Palaeozoic basement (Porthleven, Menheniot, Cornwall) and Permo‐Triassic sediments (Western Approaches Basin) has identified two broad fluid types. A high salinity (20–27 wt% NaCl), low temperature (80–150°C) brine is present in base metal mineralized 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 confirms that such basins are likely sources for the mineralizing fluids. A second, hotter (200°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 affected by the local high heat producing Cornubian granite batholith. There is also limited evidence for fluid mixing of these two fluids which may have been responsible for base metal precipitation.