Genesis of As-Pb-Rich Supergene Mineralization: The Tazalaght and Agoujgal Cu Deposits (Moroccan Anti-Atlas Copperbelt)

Abstract

Abstract In the Moroccan Anti-Atlas, sulfide deposits hosted by Neoproterozoic to Cambrian formations underwent significant weathering, leading to the formation of supergene profiles. In the Tazalaght Cu-As deposit, three mineralogical steps are distinguished: (1) the replacement of hypogene sulfides (chalcopyrite, pyrite, tennantite) by supergene sulfides (bornite, chalcocite) in the large cementation zone; (2) the formation of oxidized minerals (malachite, azurite, olivenite, and chenevixite, mainly) in a more oxidizing and neutral environment; and (3) the precipitation of goethite, hematite, and quartz in the gossan. In the Cu-As-Pb-V deposit of Agoujgal, the mineralogical units are spatially less confined than at Tazalaght. The narrow cementation zone hosts chalcocite, resulting from the weathering of hypogene chalcopyrite, pyrite, tennantite and galena, while the much more extended and diversified oxidized zone is rich in Cu and Pb carbonates, arsenates, sulfates, phosphates, vanadates, and oxides. Goethite, hematite, mottramite, and late calcite occur in the gossan. Both deposits are characterized by As-rich secondary ores that were formed through similar processes, despite some mineralogical and chemical variations highlighting the influence of the host rocks on weathering. The restricted oxidized mineralization at Tazalaght and the Agoujgal cementation zone most likely arise from the contrasting omnipresence of quartzite at Tazalaght that could not enable a fast and effective neutralization of the fluid’s acidity, and the large amounts of dolomitic host rocks that could be dissolved at Agoujgal. At both sites, the weathering of tennantite through a boxwork texture records the transition from the cementation zone (chalcocite), the oxidized zone (arsenates), and the gossan, and reflects the fluids evolution with time.