Sydorchuk V. Uranium mineralization in pegmatites and granites of the Tasiast-Tijirit terrane, the Reguibat Shield (West Africa)

The Reguibat Shield is an important geological province that shows great potential in various mineral deposits, including uranium. The main factor is the presence of numerous geological structures that are favorable to ore mineralization, such as granite intrusions, faults and greenstone belts. The Tasiast-Tijirit terrane represents a North-Western part of Archean domain of the Reguibat Shield. The rocks of the terrane are of Mesoarchean age. The main types of rocks are migmatites, gneisses and amphibolites, greenstone associations, granitic intrusions and pegmatites. Samples that showed an increased radiation level after conducting a ground-based radiometric survey were selected for laboratory studies from the host rocks of the Tasiast-Tijirit terrane. Among the Tasiast-Tijirit terrane rocks the increased uranium content was found in the following rock associations: pegmatites and granites; ferruginous quartzites within the banded iron formation; calcretes in sedimentary rocks. The following types of laboratory analyses were used for the study: 1) spectral analysis by laser-luminescence method to determine the uranium content in samples; 2) atomic emission spectroscopy for the general estimation of concentrations of chemical elements in samples; 3) X-ray fluorescence analysis to determine the chemical composition of samples and its trace elements; 4) electron microprobe analysis to determine the morphology of mineralization and the chemical composition of uranium minerals. As a result of the study, three uranium minerals in granites and pegmatites of the Tasiast-Tijirit terrane were observed: uranophane, uranothorite and brannerite. The first one is the most common mineral and occurs in the veinlets along with uranothorite, and also in fine grains along with galena. In some cases it has a significant admixture of both REE: cerium and yttrium groups. Uranothorite is commonly associated with uranophane. Brannerite is relatively rare and was observed at the edges of biotite grains. The increased content of U in ferruginous quartzites within the banded iron formation is associated with its concentration on the geochemical barriers, provided by various fault structures and dyke complexes crossing the Lebtheinia greenstone belt. The mineral form of U-bearing mineral in ferruginous quartzites was not found. The composition of uranium-containing calcretes was described. According to the results of laboratory studies, it was found that the main mineral concentrator of uranium in calcretes is tuyamunite. The main geological, physical and chemical conditions essential for the formation of uranium mineralization of the calcrete type of the d’Oum-Dherua area were defined: 1) The presence of host rocks that provided a source of uranium, such as pegmatites and granites of the Tasiast-Tijirit terrane. 2) Close location to the banded iron formation of Lebtheinia, as a source of vanadium, essential for the mineralization of tuyamunite type. 3) The presence of paleovalleys in the d’Oum-Dherua area where uraniferous calcretes are concentrated. The genetic relationship between uranium mineralization in granites of the Tasiast-Tijirit terrane and calcretes of the d’Oum-Dherua area was described. The main criteria for finding uranium mineralization within the study area are identified and the possibility of finding new similar uranium deposits in the surrounding areas is considered.