The role of metamorphic fluids in the development of the Cornubian orefield: fluid inclusion evidence from south Cornwall

Abstract

AbstractVeins 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.