| Abstract | Morocco has been for the past two centuries one of the top ten Pb-Zn producers with two thirds of base-metal production derived from three major Mississippi Valley-type districts (Touissit-Bou Beker,
Upper Moulouya, and Jbel Bou Dahar). Collectively, these districts have produced more than 100 Mt of ore at an average grade of * 3 wt% Zn and 4 wt% Pb. At the present time, none of the three districts is active. Economic orebodies are hosted by a
succession of Lower to Middle Jurassic unmetamorphosed, platform carbonate rocks. The epigenetic and stratabound sulphide deposits occur as open-space fillings and metasomatic replacements of carbonate. Mineralization that fills open spaces (i.e.,
veins, interconnected cavities, solution-collapse breccias) accounts for most of the higher grade orebodies. Overall, the mineral paragenesis consists principally of variable proportions of sphalerite and galena, accompanied by different generations
of saddle dolomite (Touissit-Bou Beker), calcite (Jbel Bou Dahar), or barite (Mibladen). In all of the districts, paleogeographic reconstructions indicate that the orebodies are located above basement topographic highs against which the Triassic and
Early Jurassic formations pinch out. Regionally, ENE-WSW-and E-W-trending faults appear to have been a critical factor in ore genesis, having provided favorable fluid channels for metal-bearing brines into permeable host rocks and dissolution
structures. The geometry of the orebodies that parallel the major alpine faults, coupled with lead isotopic constraints, suggest that the MVT mineralizing event occurred during midle Tertiary time (i.e., Cretaceous to Miocene) coincident with closing
stages of the Alpine orogeny in the Atlasic orogenic belt. Alpine mineralization is thought to have been promoted by the mixing of older, high-temperature, rock-buffered, dense brines stored within the Paleozoic basement, and a downwelling, cooler
fluid probably of meteoric origin. The resulting mixed brines were centered mainly on the basement high structure and its flanks, then flowed laterally away from the basement high and giving rise to the lower grade mineralization of the distal
prospects. Fluid migration towards the ore districts could have been achieved either by a gravity-driven system (Touissit-Bou Beker, Mibladen, and Jbel Bou Dahar) or sediment compaction in the foredeep (Jbel Bou Dahar), or a combination of both. An
alternative buoyancy-driven fluid convection model is proposed for the Touissit-Bou Beker MVT mineralization. |