MINERALOGY AND FLUID INCLUSIONS' ANALYSIS IN ZN-PB- BA-SR BOU DAHAR MINING DISTRICT, MOROCCAN HIGH ATLAS

The Bou-Dahar Pb-An-Ba-Sr district is one of the most important metallogenic areas in the Moroccan High Atlas province. This district is integrated into the geodynamic evolution of the High Atlas Mesozoic basin. This basin has undergone, since the Triassic period, an overall tensional activity, during the opening of the Atlantic Ocean, maintained during the Jurassic. This overstretching is followed by a post-Cretaceous compressive activity during the collision between Africa and Europe.

The mineralization, enclosed mainly in Lower and Middle Liassic limestone, is post Jurassic. It occurs mainly along E-W faults. These faults were created during tensional-Jurassic phase then remobilized during compressive post-cretaceous-mineralizing phase. The paragenetic sequence determined by petrographic studies at different ore deposits is listed as follow: quartz-calcite-pyrite-melnicovite-sphalerite-galena-barite-fluorite-celestite.

The analysis of fluid inclusions carried on sphalerite, barite, and celestite reveals a cooling-dilution phenomenon as the cause of the precipitation of the mineralization. This cooling-dilution effect is due to the mixing between a descending cooler and diluted superficial fluid and an ascending, hotter and higher-salinity fluid (brines). This cooling-dilution results in the precipitation of the first stage of mineralization, represented by the sulfides (sphalerite, galena), from fluid F1 (Homogenization Temperature = 165 ^oC, Salinity = 25 eq. Wt% NaCl). With time, the mixing ratio (descending cool fluid/ hot brine) increases and the dilution and cooling of the ascending brines by the superficial fluid continues and becomes more pronounced. This mixing results in the precipitation of barite from a rare fluid F2a (Homogenization Temperature= 125 ^o C, Salinity = 22 eq. Wt% NaCl) and a cooler dominant fluid F2b (Homogenization temperature ≤ 50 ^o C, Salinity = 14 to 19 eq. Wt% NaCl). At the final stage, the descending cooler fluid becomes dominant in the mixing ratio, leading to a very cool and diluter fluid F3 (Homogenization Temperature ≤ 50 ^o C, Salinity = 5.5 eq. Wt% NaCl) from which Celestine precipitates.