Formation of serpentinite-hosted, Fe-rich arsenide ores at the latest stage of mineralization of the Bou-Azzer mining district (Morocco)

Abstract The Bou Azzer district in the central Anti-Atlas of Morocco is the world’s unique mining district where Co is exploited from serpentinite-hosted hydrothermal Co-Ni-Fe arsenide ores. This paper provides the first-ever mineralogical and geochemical characterization of a suite of Fe-rich arsenide orebodies found during recent mining in the Ait Ahmane area, in central Bou Azzer district. These are lens-like orebodies of massive and disseminated Fe-rich ores fully hosted in serpentinites (hereafter named as serpentinite-hosted ores) that are typically enveloped by arsenide-poor, carbonated serpentinite, evolving irregularly outwards to talc-rich serpentinite. The central part of these orebodies mainly consists of massive ore made up of aggregates of zoned spindle-like crystals of lollingite whereas the outer zone of disseminated ore consists of rosette-like lollingite. These ores often contain accessory grains of chromite that display strong fracturation, zoning and alteration. We propose a general model for the formation of the Bou-Azzer Co-Ni-Fe arsenide ores associated to the infiltration of high-temperature (>400°C) slightly alkaline, oxidized and CaCl2-rich fluids through opening fault and fractures. When channeled through faults separating serpentinite and quartz diorite (or other rocks) the fluids deposited Ni- and Ni-Co-rich ores in these fault-related open spaces (here re-defined contact-type ores). Over the course of mineral deposition, the generated Ni-poor fluids tended to migrate into serpentinite, first through networks of thin entwined veins and progressively by intergranular percolation, promoting dissolution of the infiltrated serpentinite and precipitation of Co-Fe ores (the presence of remnants of chromite included in coexisting arsenides and calcite sustains the role played by dissolution/precipitation, ore-forming process). Further circulation of the evolving fluids through weak zones of massive serpentinite away from the serpentinite-quartz diorite (or other rocks) contact (intra serpentinite faults), formed the serpentinite-hosted, Fe-rich ore type represented by the F55 lens. These fluids most probably evolved at low temperature (∼200°C) towards slightly higher pH values and oxidizing conditions increasing the proportions of CO2 and CO32-, promoting the dissolution of serpentine, the increase in the magnetite component of chromite, the formation of Cr and Fe hydroxides, the mobilization of silica out of the reactive zone and the precipitation of calcite+Fe-rich arsenides.