Zn-, Mn- and Co-rich chromian spinels from the Bou-Azzer mining district (Morocco): Constraints on their relationship with the mineralizing process

• We report coexistence of Zn, Mn and Co in huge proportions in chromian spinels related with Co–Ni arsenide mineralizations.
• Zn-, Mn-enrichment of chromian spinels linked with mafic-hosted VMS deposit formation
• Co–Ni arsenide ore-forming fluids as factors of Co contents in chromian spinels.
• Exploration tool for mafic volcanic-hosted massive sulphide deposits.

The Co–Ni arsenides from the Bou-Azzer mining district contain disseminated chromian spinels with the highest Zn, Mn and Co contents ever reported up to date in any geological environment. The rationale behind this study was checking the role of Zn, Mn and Co contents in chromian spinel as possible indicators of mineralized environments. To tackle this issue the chemical compositional variations of chromian spinel disseminated in barren serpentinite, in Co arsenide ores and in Cu sulphide ores from three different deposits (Aghbar, Tamdrost and Aït-Ahmane mines) were studied focusing on the alteration patterns of chromian spinel grains, their fracturing degree and relationship with the precipitation of ore minerals. Results show that chromian spinel crystals are zoned and strongly fractured. They record, at least, two fracturation events: an early one developed before or coeval with the alteration process that gave rise to the zoning, and a second one that disrupted the zoning pattern splitting the altered grains in fragments which became included and partly dissolved in arsenide minerals. The early fracturing and alteration of chromite occurred during the Pan-African orogenesis and became fractured again during the Variscan tectono-metamorphic evolution of the Bou-Azzer ophiolite, just before the formation of arsenide ores. Maximum ZnO contents (up to 19.7 wt.%) occur in cores of chromian spinels associated with Co minerals from Aghbar, MnO reaches its maximum (21.4 wt.%) in rims of crystals included in chalcopyrite and CoO (up to 2.3 wt.%) concentrates in cores of grains hosted by skutterudite (CoAs3), all them from Aghbar mine. Chromian spinels from Tamdrost and Aït-Ahmane ores have much lower contents in these elements. Zn and Mn concentration in chromian spinel are neither related with the ore type nor with the mineralization degree of the host suggesting that these elements became enriched in chromian spinel during its early, ocean-floor alteration in a metal-rich environment characterized by the nearby presence of hydrothermal vent fields and forming volcano-sedimentary massive sulphide deposits (e.g. the Bleida deposit). In contrast, Co cannot be upgraded up to the levels measured in these chromian spinel grains in this ocean floor environment but its high contents seem to be related with the formation of the arsenide ores.