Petrological and geochemical constraints on the origin of apatite ores from Mesozoic alkaline intrusive complexes, Central High-Atlas, Morocco

Abstract

Five main apatite deposits from the Moroccan Central High-Atlas —including Anemzi, Tirrhist-Inouzane, Tassent, Ait-Daoud-Toumliline and Tasraft— have been investigated through integrated field, petrographic, mineralogical, and geochemical studies. The apatite ore occurs as veins, few mm to several cm thick, spatially associated with syenite-quartz monzonite (dominant) to monzodiorite-gabbrodiorite bodies emplaced during the Atlas Jurassic-Cretaceous alkaline magmatic event. Gem-quality apatite crystals, up to 15 cm in size, are associated with albite, clinopyroxene (hedenbergite–augite–diopside), amphibole (hornblende–edenite–actinolite), K-feldspar, quartz, magnetite, titanite, epidote, prehnite, and calcite. Two types of apatite have been distinguished according to their halogen contents: F-rich and Cl-rich apatites. Both types are enriched in REE (up to 2 wt% ΣREE), and have major and trace element contents consistent with a magmatic origin, corroborated by the chemical similarity with accessory apatite in the host alkaline intrusions, as well as with other reference igneous apatites. However, hydrothermal fluids percolating through country sedimentary rocks were also involved, as indicated by the striking Na enrichment observed in the Cl-rich apatites, likely inherited from Triassic evaporites, showing close spatial relationships with the alkaline intrusions. These fluids would also account for pervasive Na-metasomatism of alkaline magmatic rocks, as observed in the wall rocks of the apatite veins.