A fluid inclusion model for the genesis of the ores of the Dolgellau Gold Belt, North Wales

The results of a detailed study of fluid inclusions in a zone of auriferous quartz veins in the Clogau-St Davids gold mine are presented. Both aqueous and an immiscible pair of H (sub 2) O-CH (sub 4) fluids were present during the mineralizing event. Calculations based on laser Raman and bulk inclusion volatile analysis data indicate that the ores were formed at approx 300-320 degrees C and approx 1.6 kbar. A model is developed which is consistent with both the fluid inclusion data and other considerations. This involves the introduction of aqueous fluid into the graphitic shales hosting the mineralized veins, which reacts with graphite to produce H (sub 2) O-CH (sub 4) fluids. As H (sub 2) O is consumed to produce CH (sub 4) , an increase in salinity is observed in the residual aqueous inclusion fluids. Deposition of gold is ascribed to destabilization of either 042AuCl (super -) (sub 2) or 059Au(HS) (sub 2) (super -) complexes in the incoming fluid as a result of interaction with graphite. Auriferous zones within the veins are defined by high CH (sub 4) values and low CO (sub 2) /CH (sub 4) ratios in bulk inclusion analyses. These effects are thought to be the result of increased fluid-rock interaction in these zones, related to structural features in the wall-rocks.