Fluid constraints in the Miguel Vacas Cu deposit (Ossa-Morena Zone, Portugal)

Abstract

The ancient Miguel Vacas copper mine belongs to a cluster of ancient Cu mines and occurrences in the Ossa-Morena Zone, specifically in the Sousel-Barrancos metallogenic belt (SBMB), such as Mociços, Ferrarias and Bugalho. This deposit is characterised by an anastomosing vein structure mainly composed of quartz + carbonate + sulphides that extends over 2 km in length with an NNW-SSE direction. Chalcopyrite is the main primary Cu mineralisation; strong supergene enrichment is evidenced by the mineral assemblage of libethenite, malachite, bornite and digenite. Prospects in the area also reveal the presence of arsenopyrite. An epigenetic epithermal model for the Cu mineralisation is proposed for Miguel Vacas, as well as for other SBMB deposits (Mateus et al., 2003). This study aims to the characterisation of the fluids related with the mineralising processes. Sampling was done in a borehole at different depths (eight samples from 53 to 188 m). This study allowed identifying three different FI types regarding their composition.