Fluid inclusion studies of the Drammen Granite, Oslo Paleorift, Norway

Abstract

The Drammen Granite is a subvolcanic complex, intruded during the Permian igneous activity in the Oslo Paleorift. Molybdenite deposits occur within the complex in large discrete quartz veins, accompanied by moderate alteration of the granite. Microscopic and microthermometric studies of fluids trapped in quartz from miarolytic cavities allow characterization of the late-magmatic fluids in the granites. The fluids from which the cavity quartz precipitated were present during and shortly after the magmatic stage, from 750–800° C to ca. 500° C. The salinity decreased from 20–25 eq.w.% NaCl at the earlier stages, to ca. 5 eq.w.% NaCl in the latest fluids, while the CO₂-content increased from 0–2 mol.% to 6–8 mol.%. The presence of devitrified glass in some samples is direct evidence for the coexistence of a magma and a fluid phase. The fluids were under lithostatic pressure (1,300–1,500 Kb) within most of the granite, but were under hydrostatic conditions locally near the contacts, where boiling occurred. The early separation of a fluid phase implies that only moderate concentration and extraction of metals took place. The early saline fluids became diluted, probably by introduction of ground water, after the first aliquots separated.