The Bou Madine Polymetallic Ore Deposit, Eastern Anti-Atlas, Morocco: Evolution from Massive Fe–As–Sn to Epithermal Au–Ag–Pb–Zn ± Cu Mineralization in a Neoproterozoic Resurgent Caldera Environment

Abstract

The Bou Madine Au–Ag–Pb–Zn ± Cu epithermal deposit in the Ougnat Pan-African inlier of the eastern Anti-Atlas occurs within the lower unit of the late Neoproterozoic Tamerzaga-Timrachine Formation. Host rocks are a thick succession of alternating rhyolitic to dacitic ignimbrites locally interlayered with andesitic to dacitic lava flows. Mineralization is structurally controlled, consisting of zones of vuggy quartz and veins, veinlets, and tectonic-hydrothermal breccias. Alteration produced a predominantly strong phyllic-argillic assemblage (quartz, sericite, pyrite) that overprints earlier pervasively propylitized rocks. Overall, five mineralized vein systems, referred to as Imariren, Tizi, North zone, Central zone, and South zone are recognized. Higher grade orebodies tend to occur where NNE- and NNW-oriented veins intersect. Three successive and overlapping stages of mineralization are distinguished. Among these, stage II was coeval with dextral reactivation of the N160° E veins and is economically the most important for Au and Ag. Gold is foremost distributed in pyrite and arsenopyrite, and to a lesser extent in galena and sphalerite. Gangue minerals include quartz and minor calcite. Structural and textural data indicate that mineralization took place during the late Neoproterozoic as a result of open-space filling. These data further suggest that the Bou Madine deposit represents a relatively shallow mineralizing system that was open to the surface, and is similar to volcanic-hosted epithermal veins developed in subaerial resurgent caldera environments. Fluid inclusion measurements indicate mean homogenization temperatures of 150 °C and salinities in the range of 1–5 wt% NaCl equiv. These data are interpreted as reflecting fluid mixing and fluid-rock interaction between two contrasting fluids: (1) an ascending, acidic, deep-seated fluid that equilibrated with Neoproterozoic crystalline basement rocks; and (2) formation and/or meteoric waters. Mixing between these two fluid end members would have triggered precipitation of precious- and associated base-metal mineralization.