An overview of the Co-Ni-Fe arsenide ore zoning in the Bou Azzer mining district (Central Anti-Atlas, Morocco)

Abstract

Co-Ni-Fe-bearing arsenide ores in the Bou Azzer district (Anti-Atlas, Morocco) occur alongside serpentinite and are mainly found as contact-type ores or serpentinite-hosted ores. There is a clear zoning pattern in the contact-type ores, evolving from Ni-Co ores and Co-Ni ores filling fault-related open spaces at the serpentinite contact to Fe-Co types within the serpentinite itself. This trend is supported by drill core geochemistry. In serpentinite-hosted orebodies, the arsenide minerals are primarily Fe-Co ores, which preserve cores of Co-Fe ores. Our key contribution is to identify the chemical evolution of the ore-forming fluid. Co and Fe enrichment facilitated the progressive transition from Ni-Co to Co-Fe ores, while subsequent Ni depletion and Fe enrichment generated the disseminated Co-Fe ores (i.e., contact-type ores). Further circulation of these ore-forming fluids through intra-serpentinite weak fault zones tends to precipitate Fe-Co ores surrounding Co-Fe ores (i.e., serpentinite-hosted ores). Critically, we show that the established west-to-east district zoning (decreasing Co, increasing Ni) is specific to contact-type ores and may be influenced by the composition of the adjacent country rock (high Ni content is related to mafic layer in the contact. Serpentinite-hosted ores in the Ait Ahmane area appear to follow this west-east zoning process.