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. 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, is marked by a contact of metamorphism, which responsible for the replacement phenomena observed essentially in the limestones of the region which are transformed into graphitic Sn-W exoskarns. Syn-tectonic, skarnification is polyphase : (i) prograde stage (T= 500- 750°C) : with graphite, garnets, idocrase, pyroxene, wollastonite, quartz, scheelite and molybdenite, crossed by a network of successive veins (garnet, idocrase, wollastonite, quartz and calcite; (ii) retrograde stage (T= 400-500°C): with garnet, idocrase, pyroxene wollastonite, quartz, calcite, epidote scheelite and cassiterite; (iii) hydrothermal alteration stage (T ≤400°C) : with sericite and clay accompanied by abundant scheelite and cassiterite, favoured by the high oxidation rate. Therefore, they would have formed in a reduced environment, in relation to large batholiths or stocks of quartz diorites and/or granites in syn, late- to post-orogenic to anorogenic contexts, in the absence of any cogenetic volcanism. The economic mineralisation of graphite was formed mainly during prograde stage, under reducing conditions. It was formed by devolatilization of a bituminous origin linked to the reef limestones and organic matter, trapped in the shales according to the following reaction: