Fluid evolution in the Jales Au district, Northern Portugal

The Jales gold district is located in Vila Pouca de Aguiar County, northern Portugal, approximately 100 km ENE of the city of Porto. The district encompasses three areas with epigenetic mineralization of Variscan age. Mineralization is hosted in syn- to late-tectonic Variscan granites and in greenschist-facies metasedimentary rocks, ranging in age from Cambrian to Lower Devonian. Sulfide paragenesis proceeds from early Fe-As sulfides to late base-metal sulfides. The deposits proximal to the intrusion (Campo and Gralheira) are relatively sulfide rich and contain paragenetically late electrum associated with significant amounts of base-metal sulfides, whereas the deposit distal to the intrusion (Três Minas) is sulfide poor and contains high-fineness gold that is paragenetically early. Petrographic studies, coupled with arsenopyrite geothermometry and chlorite solid solution modeling indicate that fluid evolution was different at Três Minas than in the two other areas, explaining the distinct mineral assemblages and metal associations. At Três Minas, gold was precipitated early from sulfide complexes due to oxidation. During the early stage at high temperatures, silver and base metals were transported as chloride complexes and therefore were not affected by oxidation. At Campo and Gralheira, early oxidation was not as extensive as at Três Minas, and gold precipitated later due to decrease in H₂S activity and cooling. During the lower-temperature late stage, silver was transported as a sulfide complex, so that decrease in H₂S activity and cooling resulted in the precipitation of electrum, rather than high-fineness gold. Additionally, cooling destabilized chloride complexes carrying base metals, explaining deposition of significant amounts of base-metal sulfides at Campo and Gralheira.