Abstract

The newly discovered silver mineralization of Bou Azzer (Anti-Atlas, Morocco) is located along a major quartz diorite-serpentinite tectonic contact in an upper Proterozoic ophiolite complex (780 m.y.). From the quartz diorite to the serpentinite the following succession is observed: massive and retromorphosed quartz diorite, cataclastic quartz diorite, and K-Na metasomatized cataclasite, rodingite, chloritite, serpentinite. Rodingite resulted from Mg-Ca-(Mn) metasomatic processes related to serpentinization; the rodingite mineral assemblage (grossularite, salite, piemontite, prehnite) overprinted the cataclastic quartz diorite, releasing K-Na-rich fluids, in a temperature range (350 degrees -200 degrees C) and under pressure conditions (2 kbars) which are in agreement with Pan-African greenschist metamorphism (685 m.y.). The Bou Azzer silver mineralization is generally restricted to the K-Na metasomatic zone along the rodingite front in the form of a network of very thin ore veinlets comprising a native silver and Co-Fe arsenides assemblage, with accessory sulfosalts and copper sulfides, in a silicatecalcite gangue. The Co-Fe arsenides crystallized with the earlier silicates (salite, piemontite, prehnite in rodingite; K-feldspar and albite in the K-Na metasomatic zone; chlorite in cataclastic quartz diorite), whereas the native silver is associated with late gangue minerals (prehnite and K-feldspar in rodingite; calcite and quartz in the K-Na zone and in cataclastic quartz diorite). The investigated silver mineralization is clearly connected to the metasomatic processes related to serpentinization (rodingitization and resulting K-Na metasomatism). A serpentinite source rock is suggested for this silver mineralization.

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