The Bou Azzer mining district in the central part of the Anti-Atlas belt, Morocco, is renowned worldwide for its serpentinized Neoproterozoic ophiolitic complex that hosts peculiar cobalt-dominated and nickel-rich Co–Ni–Fe arsenide and sulfarsenide deposits with significant credits of gold and copper. The Bou Azzer central mine and its satellite deposits, which are spatially associated with serpentinite bodies and tectonically controlled, generally form breccias at the contact between serpentinites and more competent rock types (e.g. diorite) but may also comprise anastomosing vein and tension gashes networks rooted in the serpentinites and breaching into the competent rocks. A consensus on a genetic model for Bou Azzer has not been reached given the most controversial and still highly debated topics dealing with the absolute age of mineralization, and the identification of the source(s) of metals and arsenic. Here, within the geological framework of the Bou Azzer window and the geodynamic history in the Anti-Atlas, we summarize the key features of ore mineralogy, textures, paragenetic sequence and district-scale metal zonation. Using knowledge based on the current state of research – until high-precision and robust geochronological data are produced and permit a refinement of the genetic model – the key parameters for the origin of the Bou Azzer ores are: (1) arsenides of Ni and Co, which are followed by Co–Ni-sulfarsenides and Cu-sulfides, initiating a sequence of mineralization with precipitation controlled by the parameters mentioned in (2)–(4); (2) hydrothermal high salinity fluids (>30 wt% total salt eq.) fostering leaching of Co and Ni from debated source rocks and their transport as highly soluble chloride complexes; (3) dilution and cooling of those fluids from <320 to <200°C; (4) variable redox conditions that would control the progressive oxidation of As from the arsenide stage (As ¹⁻ ) to the sulfarsenide stage (As ³⁺ ); (5) unlike Neoproterozoic metalliferous and As-rich black shales, serpentinites fail to convincingly stand as a robust source of As despite being a reasonable source rock for Ni and Co; (6) the controversial U–Pb and Sm–Nd dates of brannerite and carbonate, respectively, although arguing in favour of an Hercynian timing of mineralization, do not preclude the possibility of a primary Pan-African mineralization (<615 Ma) and/or at c. 540 Ma, followed by remobilization of a pre-existing mineralization during Hercynian events.

Cobalt arsenide deposits associated with Neoproterozoic serpentinite in Morocco represent the highest-grade cobalt resource worldwide. Yet, genetic models for their origin remain controversial. We report here mineralogical and geochemical evidence for arsenide-calcite mineralization at Bou Azzer to constrain the temporal framework and identify the geodynamic trigger for mineralization mechanisms. To this end, radiometric ages for ore minerals are paramount for understanding the origin of the Bou Azzer cobalt arsenide deposit. New safflorite (CoAs₂) rhenium-osmium (Re-Os) ages are Late Devonian in age: 380.4 ± 2.9 and 373.4 ± 1.2 to 368.1 ± 5.0 million years ago (Ma) for coarse-grained and fine-grained safflorite, respectively. These dates overlap with the timing of break-up of the supercontinent Gondwana, and the building of an arch-and-basin geometry from northern Africa to Arabia. Our findings temporally and spatially contextualize previous knowledge of fluid chemistry and mineralization mechanisms involving a two-fluid mixing. Arsenide mineralization resulted from mixing of a methane-dominated fluid with highly saline basinal brines that leached Os (¹⁸⁷Os/¹⁸⁸Os_initial = 0.120 ± 0.001), and by corollary cobalt, from Neoproterozoic serpentinite. Carbon and sulfur stable isotope data of ore-stage calcite and arsenides, respectively, show that hydrocarbons acted as the main reductant for mineralization. We speculate that the seawater-derived brines sank into the sedimentary basins adjacent to a carbonate platform with the Bou Azzer serpentinite in its basement in the Late Devonian. In the context of an enhanced geothermal gradient, such brines would have been involved in warm hydrothermal alteration of hydrocarbon source rocks of the local expression of the Kellwasser event in the geological record of present-day Morocco. This warm hydrothermal alteration of hydrocarbon source rocks may have taken place for coarse-grained safflorite mineralization (380.4 ± 2.9 Ma) shortly after ca. 382–381 Ma Lower Kellwasser horizons were deposited, or, for fine-grained safflorite mineralization (373.4 ± 1.2 to 368.1 ± 5.0 Ma) while the Upper Kellwasser horizons of present-day Morocco were being deposited.