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The Pan-African orogeny along the boundaries of the West African craton |
1 Université Cadi Ayyad, Faculté des Sciences et Techniques de Marrakech, Département de Géologie, B.P. 549, Marrakech 40 000, Morocco (e-mail: abelkabir{at}fstg-marrakech.ac.ma)
2 Université du Québec à Montréal, Département des Sciences de la Terre et de l'Atmosphère, CP 8888 Centre Ville, Montréal, Québec H3C 3P8, Canada
3 Reminex Exploration, 235, Lot. Hamra, Marrakech, Morocco
4 Akka Gold Mining, Tafraout, Morocco
The newly discovered (1998) West Bleida gold mineralization (3 tonnes metal Au) lies west of the main Moroccan Bleida copper deposit (1981–1991) in the central Anti-Atlas (southern Morocco). It is hosted by metamorphosed and deformed mafic to intermediate volcanic rocks that are part of the Neoproterozoic tholeiitic volcanosedimentary series forming the stratigraphically upper part of the Bou Azzer ophiolite sequence. Strong sericitization and local silicification are associated with mineralization. These altered rocks represent a proximal hydrothermal alteration halo around the West Bleida ore zones. Normative chlorite characterizes the metamorphic assemblage away from the ore zones. Gold mineralization primarily occurs as deformed gold-bearing quartz veins and disseminations in Cu-rich chert zones (chalcopyrite–malachite), Fe-rich lithofacies and breccia zones. Gold is accompanied by small amounts of copper sulphides (<1% modal chalcopyrite). Scanning electron microscope–energy dispersive spectrometry analyses of gold grains from veins and disseminations reveal the presence of palladium as inclusions of Pd–As–Sb, Pd–Bi–Se and Pd–Te mineral phases. An electron microprobe study confirms the presence of two types of gold. The first is an alloy of Au–Ag–Pd, typically bordered by small grains of Pd and Bi (Te,Sb) phases and associated with a metamorphic assemblage. Isomertieite, Pd11(Sb2,As2), was identified as one of the phases. The second type of gold is electrum (10% Ag, 90% Au), which is always associated with fractures and occurs with hematite and white mica. Based on its form and habits, West Bleida gold reflects two distinct generations of fluid activity. The primary event precipitated Au–Ag–Pd alloys from Au–Pd-bearing hydrothermal fluids and produced auriferous quartz veins and disseminations within mafic rocks of the Bleida ophiolitic accretionary complex. It was structurally and lithologically controlled. This early event is preserved in the deeper (and thus fresher) zones more than 80 m below the surface. Intense tectonic overprinting obscures the genetic relationship between vein and disseminated styles of mineralization, both of which contain Pd-rich gold, but some of the auriferous quartz veins are observed to crosscut disseminated mineralization. Two possible hypotheses are considered: the pre-tectonic root of a volcanogenic massive sulphide system, or a late tectonic orogenic (mesothermal) deposit. The presence of Pd minerals and anomalous cobalt concentrations suggest a source in ultramafic rocks. The second event, characterized by inclusion-free electrum, occurred much later and represents the alteration and weathering of the primary Pd-rich gold assemblage by oxidizing surface fluids. It affected all mineralized units and structures to a depth of 80 m. This post-tectonic surficial alteration also caused leaching of Cu-sulphides, which may explain their low abundances in the upper parts of the ore zones.
