The results of a detailed fluid inclusion study of a zone of auriferous quartz vein in the Clogau-St Davids goldmine, North Wales are presented. These show that both aqueous and an immiscible pair of H2O-CH4 fluids were present during the mineralizing event. Compositions of the immiscible fluid pair are estimated from Laser Raman and bulk inclusion volatile analysis data. Thermometry and barometry using these fluid compositions shows that the ores were formed at c. 300–320°C and c. 1.8 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 proceeds to react with graphite in the wall-rocks to produce H2O–CH4 fluids. As H2O is consumed to produce CH4, an increase in salinity is observed in the residual aqueous inclusion fluids. Deposition of gold is ascribed to destabilization of either AuCl−2 or Au(HS)−2 complexes in the incoming fluid as a result of interaction with graphite. Auriferous zones within the veins are defined by high CH4 values and low CO2/CH4 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.