Abstract

In the upper Proterozoic marginal basin of Bleida, Morocco, several horizons of copper mineralization are present. In the main orebody, a hydrothermal synsedimentary system has been reconstructed with evidence from thickness and facies variations: it includes two sulfide-rich basins on both sides of a half-horst along a synsedimentary normal fault trending orthogonally to the continental margin. Three successions of ferruginous jasper, sulfide-bearing felsic tuffs, and turbidites occur in adjacent basins, whereas only chloritite and albitite beds are present on the axial half-horst. Toward the fault zone the ferruginous jasper becomes apatite and chlorite rich, then grades laterally to a chlorite- and K feldspar-rich albitite, whereas in the same direction, sulfide-bearing layers show a mineral zoning (pyrite-chalcopyrite-bornite), then grade to a bornite- and chlorite-rich albitite. Chlorite is more magnesian toward the axial half-horst; it crystallized around 250 degrees C. The axial zone is characterized by high B, Ba, and Zr contents; zircon crystals and tourmaline needles are present. Toward the axial zone there is also a strong REE enrichment, up to a hundred times, and a marked light REE fractionation. The REE patterns correspond to hydrothermal fluids linked to felsic volcanism.The synsedimentary faults channeled first K-Na-Mg-rich fluids which deposited chemical sediments on the sea floor, then Cu-Fe-bearing fluids migrated through the more permeable laminations of the overlying felsic tuffs resulting in syndiagenetic crystallization of sulfides.The lateral mineral zoning results mostly from decreasing temperature and increasing f (sub O 2 ) away from the axial zone. These long-lived hydrothermal flows were muffled by the deposition of turbidite material, although recurrent volcanic and hydrothermal activity occurred during stages of stagnating turbidite sedimentation.These complex relationships between synsedimentary faulting, felsic volcanism, turbidite deposition, and fluid expulsion from compressed sediments were controlled by the rapid and episodic subsidence of the marginal basin of Bleida.

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