Fluid evolution in base-metal sulphide mineral deposits in the metamorphic basement rocks of southwest Scotland and Northern Ireland

The Dalradian and Ordovician-Silurian metamorphic basement rocks of southwest Scotland and Northern Ireland host a number of base-metal sulphide-bearing vein deposits associated with kilometre-scale fracture systems. Fluid inclusion microthermometric analysis reveals two distinct fluid types are present at more than half of these deposits. The first is an H2O-CO2-salt fluid, which was probably derived from devolatilization reactions during Caledonian metamorphism. This stage of mineralization in Dalradian rocks was associated with base-metal deposition and occurred at temperatures between 220 and 360degreesC and pressures of between 1.6 and 1.9 kbar. Caledonian mineralization in Ordovician-Silurian metamorphic rocks occurred at temperatures between 300 and 360degreesC and pressures between 0.6 and 1.9 kbar. A later, probably Carboniferous, stage of mineralization was associated with base-metal sulphide deposition and involved a low to moderate temperature (T-h 70 to 240degreesC), low to moderate salinity (0 to 20 wt% NaCl eq.), H2O-salt fluid. The presence of both fluids at many of the deposits shows that the fractures hosting the deposits acted as long-term controls for fluid migration and the location of Caledonian metalliferous fluids as well as Carboniferous metalliferous fluids. Copyright (C) 2004 John Wiley Sons, Ltd.