Abstract: Veins developed during contact metamorphism associated with the emplacement of the Cornubian granite batholith contain both H₂O-rich and CO₂-rich fluid inclusions. Microthermometric data indicate that unmixing of a low-CO₂, low-salinity fluid occurred at 400–200°C and 1000–500 bars to produce low-density CO₂-rich vapour and saline aqueous fluids (8–42 wt. % NaCl equivalent). Decrepitation-linked ICP analyses show that the cation composition of the brines is dominated by Na, K and Ca, but that significant amounts of Li, Sr, Ba, Fe, Mn, Zn and B are also present. Bulk volatile analyses confirm the dominance of CO₂ over N₂ and CH₄ in the vapour phase, with CO₂/N₂ molar ratios of 15.3–28.7 and CO₂/CH₄ molar ratios of 66.9–292. The relative abundance of nitrogen suggests an aureole-derived ‘organic’ component is present.

The source of the fluids is ambiguous as they are intermediate in composition between ideal ‘magmatic’ and ‘metamorphic’ end-members. It is proposed that this is due to mixing of the two types of fluid in the contact aureole during granite intrusion. A model is suggested in which magmatic-metamorphic circulation occurred synchronously with granite emplacement and subsequently evolved to a meteoric-dominated system with the bulk of the ore deposits forming in response to the influx of meteoric fluids.