Microthermometric characteristics of fluids associated with granite and greisen quartz, and vein quartz and beryl from the Rosses Granite Complex, Donegal, NW Ireland

Abstract

The 404 Ma Rosses Granite Complex is a member of the late-Caledonian Donegal Granites of NW Ireland. The Rosses Complex comprises an annular assemblage of four nested intrusions (G1 to G4). Beryl occurs in greisen, in quartz veins and in pegmatites within the innermost G3 and G4 granites. Fluid inclusions were studied in granite and greisen quartz and in vein quartz and beryl. The predominant fluid inclusion type is two-phase liquid rich aqueous inclusions. Fluid inclusions in granite quartz are low salinity (< 5 eq. wt.% NaCl) inclusions, believed to be meteoric in origin. In contrast fluid inclusions hosted in greisen quartz and in vein quartz and beryl have higher salinities of between 5 and 10 eq. wt.% NaCl. These fluids may represent Be-rich late-magmatic fluids associated with greisenisation and beryl mineralisation. Consideration of isochores indicates that beryl mineralisation occurred at temperatures below 328 °C.