Stromeyerite, mckinstryite and jalpaite from the Tagmout Ag–Cu deposit, Eastern Saghro, Morocco: Mineralogical insights and possible origin

Characterized by its copper-silver sulfide mineralization, the Tagmout deposit (Eastern Saghro, Anti-Atlas, Morocco) is hosted in plutonic magmatic rocks (quartz-monzodiorite and granodiorite). The sulfide mineralization occurs in the form of disseminations, veinlets/stockworks crosscutting the granitoids, and is associated with quartz and carbonate gangue and adularia, sericitic and argillic alteration of the host rocks. Some silver occurs in the form of the silver-copper sulfides stromeyerite, mckinstryite and jalpaite. These minerals occur sporadically in the copper sulfide ores in association with other copper-bearing phases (mainly chalcopyrite II, bornite II and chalcocite II). Textural relationships show that these minerals are derived from pre-existing argentiferous copper sulfides (bornite I, chalcocite I and chalcopyrite I) as reaction products. Electron probe microanalyses indicate that the structural formula of the Ag–Cu sulfides from Tagmout deposit is slightly different from the theoretical composition and from that of Ag–Cu sulfides elsewhere. Stromeyerite, mckinstryite and jalpaite from Tagmout have average compositions Cu1.02Ag0.89Fe0.05S1.02, Cu3.24Ag4.35 Fe0.34 S4.06 and Cu1.17Ag2.70 Fe0.10 S2.01, respectively. These compositions show a deficiency in Ag and a slight excess in Cu and S, compared to these minerals reported elsewhere. This Ag deficiency reflects the significant Fe contents of these sulfides. Iron content reach values of up to 7.61 wt%, Fe likely substituting for Ag by Ag++Cu+ ≤> Fe2+ in the crystal structure. The Tagmout deposit is to our knowledge the fifth occurrence in the world where all three Ag–Cu sulfides occurs close to each other. Silver at Tagmout deposit was not incorporated in the early Pb and Zn sulfides, but was transported in the ore-forming fluid, where it formed solid solutions in the early copper sulfides. It was later remobilized by lower temperature-high salinity fluids resulting in the deposition of primary silver-copper minerals associated with secondary silver-copper sulfides.