Graphite, Sheelite and Cassiterite Mineralized Skarns of Frag Al Ma, Sidi Bou Othmane District, Jebilet, Morocco

Abstract

Sidi Bou Othmane region in the Central Jebilet is formed by a Devonian schistose series, initially sandstone-pelitic with carbonate intercalations. The polyphase Hercynian deformation in this area is characterised by penetrative, subvertical, synmetamorphic schistosity (S1), accompanied by folding. Syntectonic granitic intrusions, hidden in the Sidi Bou Othmane region, are at the origin of a contact metamorphism, responsible for the replacement phenomena observed essentially in the limestones of the region which are transformed into skarns. These graphitic skarns are essentially formed of garnet (grossular) with pyroxene (ferrosalite), idocrase, wollastonite, more or less feldspar, epidote, quartz, calcite. Thus, skarnification is polyphase and also syn-tectonic. Following the example of skarns known in the literature, three phases have been observed: (i) Prograde stage: Corresponds to the major stage of skarnification underlined by the development of massive skarns and banded skarns (graphitic garnets, +/- idocrase, pyroxene, wollastonite, quartz, scheelite (CaWO4) and molybdenite (MoS2)), crossed by a network of successive veins (garnet, idocrase, wollastonite, quartz and calcite). It is associated with the deposition of graphite in a reducing medium; (T between 750 and 500°C. The economic mineralisation of graphite was formed mainly during this phase. (ii) Retrograde stage: this is a phase of retromorphosis of first phase minerals (garnet, idocrase, pyroxene and wollastonite) into (quartz, calcite and epidote). It is accompanied by the deposition of scheelite (CaWO4) and cassiterite (SnO2) in an oxidising environment, 500 to 400°C). (iii) Hydrothermal alteration stage: a phase marked by phyllite alteration with sericite and clay in massive, banded and veinlet skarn zones. It is accompanied by abundant scheelite and cassiterite, favoured by the high oxidation rate.