Gold mineralization in the Ourika Gneiss (High Atlas of Marrakech, Morocco): mineral paragenesis and fluid P-T-X evoluti
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ORIGINAL PAPER
Gold mineralization in the Ourika Gneiss (High Atlas of Marrakech, Morocco): mineral paragenesis and fluid P-T-X evolution Ahmed Barakat
Received: 4 February 2014 / Accepted: 21 April 2014 # Saudi Society for Geosciences 2014
Abstract The Ourika gold showing lies in the High Atlas of Morocco, near Marrakech. It is mainly hosted by Precambrian gneiss and basic dykes, and is controlled by shear zones. Gold mineralization occurs mainly within quartz structures. Fracturing and mineralization are interpreted to have occurred in three stages. Early and intermediate stages involved the deposition of an arsenopyrite–pyrite–quartz–chlorite assemblage, and a chalcopyrite–arsenopyrite–pyrite–quartz–chlorite assemblage, respectively. Early intermediate stage fluid inclusions belong to the C-H-O-N system typical of the retrograde metamorphism that affects the Pan-African terranes at P-T conditions of 140–300 MPa and 240–450 °C, and moderate ductile–brittle deformation. Deposition of native gold, bismuth, bismuthinite and galena during the ore stage from low-salinity aqueous fluids was related to downward penetration of meteoric waters, trapped at P-T conditions of 20– 110 MPa and 170–210 °C. Evolving physical conditions and fluid compositions from early to late fluid stages played important roles in ore-forming processes. Owing to structural observations, ore-forming fluids could be linked to regional Variscan hydrothermal event. Gold crystallised as native particles in microfissures crosscutting earlier sulphides. Thus, the gold deposition was late, and appears as a consequence of deeply penetration of oxidising fluids favoured by microfracturing of the already ore (As–Fe–Cu) structures.
Keywords High Atlas of Marrakech . Precambrian . Ourika gneissic massif . Quartz veins . Gold . Sulphides . Fluid inclusions
A. Barakat (*) Geo-resources and Environment Laboratory, Faculty of Sciences and Techniques, University of Sultan My Slimane, P.O. Box 523, 23000 Beni-Mellal, Morocco e-mail: [email protected]
Introduction In Morocco, the axial zone of the Western High Atlas consists of Precambrian to Palaeozoic basement rocks (Fig. 1). These host several types of ore deposits and showings (barite, Pb–Zn, Cu, Au), such as the Azegour W–Mo deposit (Permingeat 1957), the Erdouz Zn–Pb deposit (Badra et al. 1991), the Tichka gold deposit (Bastoul 1992; Jouhari et al. 1999), the Oukila gold deposit (Barakat et al. 2002b), the Wirgane Pb–Zn deposit (Bensallam et al. 2008; Alansari et al. 2009) and the Ifri Cu–(Ni–Bi–U–Au–Ag) deposit (Barbanson et al. 2003). The importance of gold deposits in the axial zone was only recently recognised, and the Omnium Nord Africain (ONA) Group (Reminex) is currently exploring for this metal in this region. Preliminary exploration led to the discovery of a gold mineralization in the Ourika district (Fig. 2) hosted by Paleoproterozoic gneisses. Several showings are present including the Asggaour prospect, which is, by far, the most promising. Gold mineralization in this region is hosted by qu
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