Geology and Mineralogy of the Hercynian Koudiat Aïcha Polymetallic (Zn-Pb-Cu) Massive Sulfide Deposit, Central Jebilet, Morocco

Abstract

Koudiat Aicha is a small Zn-Pb-Cu deposit, enclosed in the Visean Sarhlef volcano-sedimentary series of the Moroccan Hercynian Jebilet massif. Base metal mineralization is located between a basal unit consisting of black argillite with arenite intercalations, and an upper unit composed of black argillite with locally fossiliferous calcareous units towards the top. Paraconcordant gabbro sills are present in both the upper and basal units, and the enclosing strata. Three successive phases of deformation linked to regional deformation overprint the volcano-sedimentary rocks and gabbros, as well as the sulfide mineralization. The mineral deposit includes several lenses of massive to semimassive pyrrhotite, 1 to 20 m thick, with a large halo of disseminated sulfide veinlets and sulfide nodules within a zone of intense chlorite alteration in the footwall. The ore mineralogy consists of massive to semimassive pyrrhotite with lesser amounts of sphalerite, chalcopyrite, arsenopyrite, galena, pyrite, and stannite. Lead isotope results ( 206 Pb/ 204 Pb averaging 18.27) suggest that the metals of the Koudiat Aicha deposit are derived from the volcano-sedimentary host rocks. Sulfur isotopes also indicate a volcano-sedimentary origin, with bacterial reduction of sulfate (δ 34 S CDT = −7.5‰ to −10.5‰). The conditions for sulfide metamorphic equilibration range from 250° to 330°C (sphalerite and chlorite geothermometers). The gabbroic sills could have been a local heat source for hydrothermal circulation. Based on these geological and mineralogical features, a Besshi-type model seems appropriate for the genesis of the Koudiat Aicha mineralization.