Origin of the Amensif Zn-Cu (Pb-Ag-au) Distal Skarn Deposit (western High Atlas, Morocco): Constraints from C-O-S Isotopes

The Amensif Zn-Cu (Pb-Ag-Au) ore deposit is located in the northern part of the western High Atlas. The deposit hosts 1 million tons (Mt) of Zn-Cu-Pb metal reserves at 3% Zn, 2% Cu, and 1% Pb. The lenticular sulfide ores are structurally controlled open-space fillings (fractures, faults) in the Lower Cambrian carbonates. The mineralogical paragenetic sequence consists of andradite-quartz-epidote-dolomite-calcite-sphalerite-chalcopyrite-pyrite-galena with subordinate amounts of barite. The pre-ore stage and ore stage I dolomite samples have δ13CPDB and δ18OSMOW values in the range of −10.2 to −2.6‰ and 17.4–26.8‰, respectively. The lower δ18O value of these dolomites suggests oxygen isotope exchange between the ore-forming fluids and the carbonate host rock whereas negative δ13CPDB values reveal the interaction between these hydrothermal fluids and black shales. The δ34S values of sulfides ore (sphalerite, chalcopyrite, and pyrite) are high, ranging from 6.4 to 14.4‰. These S isotopic values reveal that sulfur derived from the mixture of magmatic sulfur and lower Cambrian carbonate-derived sulfur in almost equal proportions. Sulfide ores precipitated from the mixing between the magmatic-hydrothermal fluids exsolved from a hidden magma or the nearby Azegour granite of Permian age and an external cooler fluid of unknown origin. Fluid-rock interactions with the carbonate host rock contributed in the precipitation of the ore. Mineralogical and geological features allow us to ascribe the Amensif Zn-Cu (Pb-Ag-Au) polymetallic ore as a distal skarn system which is spatially associated with the lower Permian granite intrusion.