The Roc Blanc orogenic Pb-Zn-Ag-Au deposit (Morocco): a product of metamorphic dehydration and CO2 devolatilization during exhumation of the Variscan Jebilet massif

What is it about?

The Roc Blanc Pb-Zn-Ag ± Au vein system in the Variscan Central Jebilet massif, Morocco, is confined within the contact metamorphic aureole of S- and I-type calc-alkaline granitic stocks (327 ± 4 to 295 ± 15 Ma) along the Marrakech Shear Zone. Host rocks consist of a succession of greenschist- to amphibolite-facies metasedimentary and metavolcaniclastic rocks of Carboniferous age. The ore mineralogy is predominantly base metal and Ag-bearing sulfides and sulfosalts, intergrown with quartz and carbonates. Late-stage gold mineralization is commonly present as electrum in intimate association with all major generations of sulfide and sulfosalt minerals. Ore-related hydrothermal alteration includes silicification, sericitization, chloritization, and carbonatization. Chlorite and arsenopyrite geothermometry suggest mean temperatures of 364 and 350 °C; respectively. Calculated δ18Ofluid values of 15 to 18‰ are consistent with metamorphic fluid sources, by which the mineralizing fluids were produced during the emplacement of granitic intrusions and subsequent devolatilization of graphitic black shale and interlayered carbonate beds. Lead and strontium isotope data constrain the source of the ore-forming components (i.e., metals and sulfur) to the enclosing host rocks. A decrease in temperature during fluid ascent and subsequent alteration-associated fluid-rock interaction resulted in the deposition of the Pb-Zn-Ag ± Au mineralization in a post-collisional extensional setting.

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