The Bou Azzer Co–Ni–Fe–As(±Au ± Ag) District of Central Anti-Atlas (Morocco): A Long-Lived Late Hercynian to Triassic Magmatic-Hydrothermal to Low-Sulphidation Epithermal System

Abstract

The Neoproterozoic Bou Azzer Co–Ni–Fe–As(±Au ± Ag) district in the Central Anti-Atlas Mountains of southern Morocco hosts the only mine in the world where Co is produced as a primary commodity directly from Co- and As-bearing arsenide minerals. The district has a mean annual production of 100,000 t of ore at an average grade of ~1 % Co, 1 % Ni, 3–42 g/t Ag, and 3–4 g/t Au. Major orebodies are developed either within serpentinite or more importantly along the contact between serpentinite and quartz-diorite rocks. Field and textural relationships record three hydrothermal stages characterized by mineral assemblages of pre-arsenide auriferous stage I, (Co, Ni, Fe)-arsenide and sulpharsenide stage II, and sulphide-sulphosalt ± Au ± Ag epithermal stage III; one postore supergene stage IV is characterized by secondary Co minerals (erythrine, roselite, talmessite). Ore mineralogy is dominated by diarsenide, triarsenide, and sulpharsenide minerals, accompanied by various generations of intergrown calcite, dolomite, and quartz. Gold- and silver-bearing assemblages mainly comprise native gold and electrum, polybasite, proustite, xanthoconite, argyrodite, stromeyerite, and freibergite. Similarity of structural orientations of the veins that formed during the three stages suggests a common mechanism of ore formation. Available radiometric dating shows highly discordant ages for the Bou Azzer Co–Ni–Fe–As(±Au ± Ag) deposits that range from Neoproterozoic to Late Triassic. A maximum age constraint indicates that all of the mineralized structures are younger than emplacement of the trachytic sills dated at ~530 Ma, thus precluding a Precambrian age for the mineralization. Fluid inclusion data indicate consistent trend towards lower temperatures and salinities with advancing paragenetic sequence. In this regard, calculated P-T trapping conditions decrease from 1.6 to >2 kbars and 320 to 400 °C during the early pre-arsenide auriferous stage I, to 880–1400 bars and <200 °C during the (Co, Ni, Fe)-arsenide and sulpharsenide stage II, and finally to pressures as low as ~650 bars and temperatures in the range of 150–200 °C during the sulphide-sulphosalt ± Au ± Ag epithermal stage III. Stable isotope compositions (i.e., O, H, S) are consistent with derivation of the mineralizing fluids either from magmatic, metamorphic, or meteoric sources or a mixture of all of these fluid sources. Magmatic and/or magmatic-hydrothermal models propose that crystallizing magmas provided the fluids, ore and gangue components, and thermal energy to drive fluid flow that carried and deposited the metals including gold. In both metamorphic and meteoric models, metals were derived from the serpentinites and enclosing metasedimentary rocks, transported to shallower levels, and then deposited along dilatant faults.