Hydrothermal Fluid Evolution and Polymetallic Mineralization of the Roc Blanc Pb-Zn-Ag-Au Vein Deposit, Jebilet Massif, Variscan Belt, Morocco

The Roc Blanc polymetallic vein deposit is located in the NW of Marrakech, in the Variscan Central Jebilet massif, and is closely associated with the contact metamorphic aureoles produced by S- and I-type calc-alkaline granitic intrusions (ca. 330–295 Ma) along the Marrakech Shear Zone. The mineralized veins are hosted by Carboniferous black shales and metavolcaniclastic formations metamorphosed from greenschist to amphibolite facies. Ore assemblages are dominated by Pb-Zn-Ag sulfides and sulfosalts, with late gold occurring as electrum intimately intergrown with multiple sulfide generations. Hydrothermal alteration is characterized by silicification, sericitization, chloritization, and carbonatization. Chlorite and arsenopyrite geothermometry yield temperatures of ~350-364 °C, consistent with a metamorphic origin of the mineralizing fluids. Oxygen, lead, and strontium isotopic signatures further indicate that ore-forming metals and sulfur were largely sourced from the enclosing metamorphic host rocks during granite emplacement and devolatilization of carbonaceous sediments.

Two major stages of ore deposition are distinguished. The pre-ore stage (Stage I) includes two quartz-rich vein generations carrying Fe-As-Zn-Cu sulfides. The main ore stage (Stage II) is enriched in Ag-Au-Pb-Zn-Cu-Sb and is hosted within carbonaceous veins and late quartz generations. Fluid-inclusion studies identify three fluid types: (i) liquid-rich H₂O-N₂-CH₄±CO₂, (ii) vapor-rich H₂O-CO₂-CH₄-N₂, and (iii) aqueous H₂O-salt inclusions. These data reflect the mixing of deep metamorphic fluids with surface to subsurface aqueous fluids, locally trapped under boiling conditions. Stage I mineralization formed at 350 ± 20 °C with salinity around 13.7 wt% NaCl eq., whereas the economically significant argentiferous stage precipitated at ~150 °C with salinity of 12.1 wt% NaCl eq. All ore deposition occurred at relatively low pressure (<1–1.1 kbar). Overall, cooling and dilution of the hybrid fluid system represent the key mechanisms driving Ag-rich polymetallic mineralization at Roc Blanc.