Invisible gold at the Arcos deposit (Lugo, NW of Spain)

The low-temperature mineralization of the Arcos deposit (Lugo, NW Spain) is spatially related to the intrusion of porphyritic rhyolites that produced host-rock alteration, mainly decarbonation-dolomitization, silicification, argillization and sulfidation. The study of non-oxidized samples has shown an early gold mineralization stage composed of pyrite and arsenopyrite, with gold mostly occurring as "invisible gold". Several types of pyrite and arsenopyrite were defined considering their chemical and textural characteristics. Among these, the Py-II type, which is As-rich (up to 5 wt.%), is the main gold-bearing mineral (up to 0,2 wt.% of Au), whilst the gold content in arsenopyrite is erratic and low. The Py-II occurs as fine-grained pyritohedra and overgrowth rims of previous pyrite crystals, showing oscillatory As-zoning, the highest Au-contents being in the As-richer zones. This pyrite is related to silicification of the host-rocks and sometimes occurs as pseudomorphic replacements of previous Fe-minerals. The EMPA indicate that gold was both deposited as free gold (nanoparticles) and as solid-solution in As-rich pyrite. Removal of gold from the hydrothermal solutions was aided by a sulfidation process. The slightly negative correlation observed between the Au and Fe suggests that the mechanism for Au incorporation into pyrite and arsenopyrite is substitution for Fe.