Gold mineralization in the Ourika Gneiss (High Atlas of Marrakech, Morocco): mineral paragenesis and fluid P-T-X evolution

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.