Abstract
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•Gold mineralized sheeted veins hosted in Variscan granites.•Mineral assemblage formation occurred under reduced conditions.•Coarse native gold result from Bi-melt deposition and scavenging of Au.•Mineralized structures have a coincident structural framework of the Penacova-Régua-Verín regional strike-slip fault.•Orogenic gold deposit in Iberian Variscan belt.
The Bigorne deposit is classified as an orogenic gold system that comprises gold-bearing veins cutting through the synorogenic Variscan granites in the northern Iberia. The mineralized structures are subparallel of the regional Penacova-Régua-Verín strike-slip fault system. The mineralized bodies include quartz-sulfide veins, disseminated sulfides with pervasive hydrothermal alteration, and oxidize fractures. The thermal evolution of the mineralization was estimated based on i) mineralogical and thermodynamic stabilities of Au-Bi-Te-(S) phases and ii) arsenopyrite geothermometer. The temperature ranges from 455° to 354°C for arsenopyrite precipitation. For Au-Bi-Te-(S) mineral phases was possible to define a range of deposition temperatures from 371°C to 235°C. The main gold deposition episode is inferred to be the last at lower temperatures. In order to constrain the origin of gold-bearing fluids and metals in the Bigorne deposit, oxygen, hydrogen, and sulfur isotope studies were undertaken. δ18O from quartz in veins shows a narrow range of +13.0 to +13.3‰. δD from quartz ranges between −62.5 and −64.7‰. δ34S in arsenopyrite clustered around −2.8‰. These stable isotope data, interpreted in the context of the regional and local geology and the estimated timing of mineralization, suggest that the sulfur- and gold-bearing fluid was generated from deep-crustal rocks during decompression triggered by crustal uplift, late in the orogenic evolution of the area.