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

Abstract

The Mociços copper deposit is located in the Ossa-Morena Zone, at the Sousel-Barrancos metallogenic belt (SBMB) and belongs to a cluster of ancient Cu mines and occurrences, such as Miguel Vacas, Ferrarias and Bugalho. This deposit is characterised by a main sulphide assemblage of chalcopyrite + pyrite in a quartz + carbonate decametric lode, with an NNW-SSE general trend, which cross-cuts metasedimentary Paleozoic successions with circa 1 km length. The model proposal for SBMB deposits (Mateus et al., 2003) states that the Cu contents were remobilised from metasedimentary host rocks by low to moderate salinity and temperature fluids. The study of more than 200 fluid inclusions (FI´s) allowed identifying four main FI types. LVS1-3 where L and V are H2O, S1 is halite, S2 is a carbonate and S3 is an opaque. Most of these inclusions contain a halite crystal. L1-2VS1-2 each phase representing L1 is H2O, L2 and V are CO2, S1 is halite and S2 is a carbonate or opaque. L1-2V where L1 is H2O, L2 and V are CO2. LV consists of two-phase inclusions where L and V are H2O. In addition, CO2-rich biphasic FI´s (L and V are CO2) were studied in milky deformed quartz. These inclusions suggest a metamorphic fluid circulation with possible CO2 devolatilisation from the metasedimentary host rocks.