Geological setting of the Bigorne gold deposit, Iberian Variscan belt (Northern Portugal) and Au-Bi-Te mineral assemblages as indicators of the ore-forming conditions

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-Re ' gua-Verin 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 degrees to 354 degrees C for arsenopyrite precipitation. For Au-Bi-Te-(S) mineral phases was possible to define a range of deposition temperatures from 371 degrees C to 235 degrees 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. 818O from quartz in veins shows a narrow range of +13.0 to +13.3 parts per thousand. 8D from quartz ranges between -62.5 and -64.7 parts per thousand. delta S-34 in arsenopyrite clustered around -2.8 parts per thousand. 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.

Automated summary

The Bigorne deposit is a orogenic gold system located in the northern Iberia, with gold-bearing veins cutting through synorogenic Variscan granites. The mineralization includes quartz-sulfide veins, disseminated sulfides, hydrothermal alteration, and oxidize fractures. The thermal evolution suggests that gold deposition occurred at lower temperatures. Isotope studies indicate that the gold-bearing fluid was generated from deep-crustal rocks during decompression triggered by crustal uplift.