Cobalt–nickel–copper arsenide, sulfarsenide and sulfide mineralization in the Bou Azzer window, Anti-Atlas, Morocco: one century of multi-disciplinary and geological investigations, mineral exploration and mining

Abstract

AbstractThe 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 (As1−) to the sulfarsenide stage (As3+); (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.