GOLD-BEARING As-RICH PYRITE AND ARSENOPYRITE FROM THE EL VALLE GOLD DEPOSIT, ASTURIAS, NORTHWESTERN SPAIN

Abstract

The El Valle deposit is included within the El Valle – Boinas Au–Cu deposit in Asturias, northwestern Spain, an important gold-producer in Europe. Gold mineralization occurs within a Cu–Au skarn overprinted by a younger hydrothermal process accompanying the intrusion of subvolcanic rocks, which produced oxidation, silicification, argillization and sulfidation. The gold produced by this process is “invisible gold”, and the main ore minerals in unoxidized zones are pyrite and arsenopyrite. Their textures and major- and trace-element composition, determined by EPMA and SEM, allowed us to distinguish three generations of arsenopyrite and four of pyrite. Among these, pyrite II is the only one that is rich in trace elements, As being the most abundant (up to 9.5 wt%), together with Au (up to 800 ppm), Sb (5000–18000 ppm), Ni (1000–2000 ppm), Tl (500–9000 ppm), Cu (100–6000 ppm), Hg (600–1200 ppm) and Se (up to 2000 ppm). This As-rich pyrite II displays various textures, sizes and morphologies. Where it occurs as a late colloform overgrowth, it has the highest amount in As and other trace elements except Au, which mainly occurs where this pyrite has a pyritohedral habit and small size. The pyritohedra commonly result from sulfidation of pre-existing Fe-bearing minerals. All types of arsenopyrite contain gold (1250–3000 ppm) and other trace elements such as Cu (200–1300 ppm), Sb (up to 9000 ppm) and Tl (3000–4500 ppm). This study shows that the highest contents of Au are in the As-rich zones of Py II pyritohedra and in arsenopyrite. Moreover, a slightly negative correlation between the Au and Fe is observed in the compositions of Au-bearing pyrite and arsenopyrite crystals, and the Au/As values are below the solubility limit proposed in the literature. We favor a mechanism in which gold is removed from ore fluids by chemisorption at As-rich, Fe-deficient surface sites and incorporated into the sulfides as solid solution. This process took place during sulfidation of the host rocks.