The Pb-Zn-Ag±Au-W-Sn Vein System in Variscan (Hercynian) Belts in North Africa

The Variscan belts of North Africa are renowned for their diversity and richness in ore deposits of significant economic potential. The Variscan mineral resources, of polymetallic character, are geologically controlled by complex and polyphase tectonic-magmatic events driven by the Variscan orogeny, dating back to the late Paleozoic times. The Variscan (Hercynian) vein-type mineralization, along the northern fringe of the West African Craton, is mainly hosted within amalgamated Precambrian and Paleozoic terranes of the Moroccan Meseta domain and Algerian-Moroccan Atlas system. The development of large ductile to brittle shear zones accompanied or not by plutonic magmatic bodies (mantle, mixed, or crustal) led to the deposition of the main sulfidic and oxidized metallic mineralization as hydrothermal veins during the late Variscan orogeny. Mineralization is predominantly antimony Pb, Sb, Zn, (As, Sn) sulfides and Ag sulfosalts with free or sulfide-trapped gold commonly associated with As quartz structures sheltered along E-W to NE-SW or NW-SE reactivated basement faults. Two mineralization process phases have been identified; the first one with Sn-W-Mo±Au (Be) is associated with the Hercynian calc-alkaline to monzonitic granitoid emplacement, dated at 320 to 280 Ma. The second one with Pb-Zn-Ag accompanied or not by the leucogranites setting resulted from the reactivation of the pre-existing rooted shear zones, concomitant to the uplift and the erosion of the crust during the late Permian–early Triassic. The formation of mineralizing hydrothermal fluids can be linked either to the direct influence of basal-crustal plutonic magma (calc-alkaline granitoids) to purely crustal magma (Leucogranites), or to the circulation of hydrothermal fluids, of metamorphic origin, coming from the base of the upper crust (supracrustal) through active shear zones during the late Permian transpression phase. Hydrothermal fluids can also be originated from the process of dehydration of sediments by compaction, from the initiation of regional metamorphism resulting from the effect of lithostatic and hydrostatic pressures, and the circulations of meteoric and seawater. These hydrothermal fluids infiltrate in-depth, heat up, and create a geothermal circuit which, while going up toward the surface, deposits the mineralization of tin, tungsten, native gold associated with arsenopyrite, gold-bearing pyrite, silver-bearing galena, sphalerite, in zones made permeable by brecciation and opening of the pre-existing fractured or newly created and faulted systems. Undoubtedly, the Tighza-Aouam (Pb, Zn, Ag) deposit is also rich in gold and tungsten, the El Hammam fluorite deposit, not metalliferous, and the Achemach Tin deposit are economically the most prominent Variscan ore deposits, exploited exclusively in the Hercynian Central Massif of Morocco; while those of neighboring Algeria correspond to the polymetallic veins of the Boukais massif, Ougarta belt (Cu-Pb±Zn±Ag and Au) and of the Beni Snouss district (Au, As) in the GharRouban massif.