Sn-W Mineralization Associated with the DJILOUET Complex, Eastern Hoggar (Algeria)

Abstract

The Djilouet complex forms a cupola made up of leucocratic granites associated with Sn-W mineralization. It could represent the Hoggar’s easternmost rare-metal granite (GMR). It is located in the Djanet terrane, 12 km NE of the town of the same name in the far east of the Tuareg shield in Algeria. The geology of the area is characterized with the presence of a magmatic complex evolving from quartz-diorites to leucocratic granites. The latter intrudes into the Upper Proterozoic terrigenous sedimentary rocks of the Djanet series. Djilouet is located in a tectonically weakened sublatitudinal area, known as the Tamrit area of major accidents affecting the basement, and considered as the region’s main structural control. These extend, further north, affecting the Paleozoic cover. The granite cupola covers an area of 0.56 km2, with a zoned concentric structure, typical of the Hoggar post-orogenic granites. It is made up of leucocratic granites with progressive mutual transitions. The latter enclose fine-grained garnet bearing granite contains, in addition to almandine-spessartite garnet, other accessory minerals, observed in SEM, such as rutile, apatite, xenotime, zircon, monazite and uranothorite. The Djilouet W-Sn mineralization is located in a network of quartz-greisen veins and veinlets subparallel to each other, but sometimes forming a real stockwork. The ore is mainly made up of wolframite and, to a lesser extent, cassiterite. The iso-content contours of tin and tungsten, as produced from a sampling covering the whole cupola, overlap very little. They would indicate that these metals have different origins and come from different levels, where the parent magmas have crystallized. The Chemical analysis of these granites’ major elements reveals that they are clearly peraluminous and of calc-alkaline affinity, highly potassic and strongly fractionated. In discriminant diagrams, they project into the post-orogenic domain, with a tendency towards the anorogenic domain. As for the facies that outcrop in the Djilouet cupola, they cannot be qualified as rare metal granite as a whole. Petrographyically, a great similarity proves to persist with the post-orogenic granites of the Tamanrasset area. Nevertheless, its mineralization could be related to a more recent event (probably Hercynian), and has a source that evolved rare metal granite, not observed at surface level.