The vanadiferous magnetite deposits of the Oursi region, Upper-Volta

Abstract

Geologically, Upper-Volta belongs to the West African Shield, which is composed of metamorphic and crystalline rocks of the Liberian and Birrimian orogenic cycles (geochronologic limits: 2,700-1,600 m.y.).The Oursi area comprises a dome of syntectonic granite partly encircled by a complex syncline of porphyrites and related rocks, ophiolitelike greenstones, and volcano-sedimentary rocks. Within these are intrusions of magnetite-bearing gabbros which are subconcordant with the folded structures and may be Birrimian tarditectonic intrusions. In all the masses, the gabbros are layered with the magnetite concentrations forming an integral part of them.Ten significant magnetite deposits have been determined. They are characterized by bundles of magnetite veins with thicknesses ranging from 0.5 to 40 m and lengths of a few hundred meters. At the surface the magnetite concentrations are always martitized. In some places, maghemite may also occur. The final stage of alteration consists of a general transformation into goethite.The magnetite veins are associated with different types of gabbro: augite-gabbros, augite- and hypersthene-gabbros (gabbronorites), and hypersthene-gabbros (norites). Quartz is generally absent but may appear in large quantities in some rocks, e.g., quartz-bearing gabbronorites. Most of the gabbros are igneous cumulates. The cumulus crystals comprise plagioclase, hypersthene, and augite, whereas the intercumulus material is hypersthene, augite, and ilmenite. The veins are essentially an association of magnetite and ilmenite grains with accessory grains of sulfides, such as pyrrhotite, pentlandite, and arsenopyrite. The magnetite is characterized by the development of two types of microtexture: cloth microtexture, determined by the superposition of two series of exsolution lamellae in the (100) plane of the magnetite, and trellis microtexture, determined by the development of ilmenite lamellae in the (111) planes of homogeneous magnetite.Trellis cloth zonation occurs where the two microtextural types are found together in the magnetite. Primary ilmenite is associated with the magnetite grains and reaction rim zonation occurs at the contact between magnetite and ilmenite.The results for some magnetite samples, analyzed by conventional methods and by electron and laser microprobes, show that the V 2 O 5 concentration never rises above 1.4 percent. Goethite samples contain less than 100 ppm vanadium; magnetite, maghemite, and hematite ore have the same vanadium content. The V 2 O 5 concentration and the relatively low Cr 2 O 3 content (<0.11 wt %) of the Oursi ore are similar to those found in Bushveld ores formed during the last magmatic differentiation phases.Microprobe scanning shows that the titanium content decreases along the border of the magnetite grains and that the vanadium content increases at the same time. These results confirm, on the mineral scale, the same negative correlation between the titanium and vanadium content as was observed in similar ores in the Bushveld, Skaergaard, and Bjerkrem-Sogndal masses.