Multi-stage fluid system responsible for ore deposition in the Ossa-Morena Zone (Portugal): constraints in Cu-ore deposits formation

Abstract

The Mociços Cu-deposit is part of a cluster of ancient copper mines in the Sousel-Barrancos metallogenic belt in the Ossa-Morena Zone at the SW Iberia. The orebodies develop along NNW-SSE quartz-carbonate-sulfides veins with pyrite and chalcopyrite as the main sulfide phases, and ore emplacement has been attributed to copper remobilization from the metasedimentary host-rocks, though no detailed studies were conducted. A novel multi-stage fluid circulation model is hereby proposed, supported by petrography and fluid inclusion data evidencing the P–T–V-x evolution of the deposit. Stage (i) is an early metamorphic stage with a predominance of carbonic fluids, identified in highly deformed milky quartz (QzI), with estimated pressures between 338 and 486 MPa compatible to the regional metamorphic events (greenschist facies). Stage (ii) corresponds to a late-metamorphic manifestation of H2O–NaCl–CO2 fluids, with low salinity (eq. w(NaCl) from 0.4 to 5.0%) and CO2 dominated. Stage (iii) in which ore emplacement took place and is characterized by dominant multisolid H2O–NaCl hypersaline, halite-bearing fluid inclusions (eq. w(NaCl) from 29.3 to 44.3%) with an H2O–NaCl–CO2 endmember and features indicative of magmatic-hydrothermal brines. Many of these inclusions homogenize by halite dissolution, with pressures as high as 320 MPa, and the coexistence of both fluids in the same fluid inclusion assemblages (FIA) could indicate phase separation caused by fluid pressure variations. Although there is no direct evidence of the magmatism responsible for these fluids, the geodynamic settings could favor deep-seated magmatism. Stage (iv) is characterized by low-salinity (eq. w(NaCl) from 0.18 to 15.57%) and low-temperature (68 to 160°C) primary two-phase fluid inclusions hosted by late-stage quartz (QzIV), suggesting a late-meteoric fluid circulation phase responsible for the leaching, oxidation and supergene enrichment observed at surficial levels. Throughout the P–T evolution of the system a decrease in pressure and temperature was registered, especially in fluid inclusions hosted in quartz from the sulfide bearing veins, suggesting that the transition from ductile to brittle regimes might have favored ore deposition.