Granitoid Fertility Assessment as a Potential Source Rock for Granite-Related Hydrothermal Uranium Mineralisation in the Kindi Radiometric Anomalous Area, Central West Region, Burkina Faso (West Africa)

Abstract

National-scale uranium resource potential assessment in Burkina Faso revealed several radiometric anomalies detected by airborne spectrometric geophysical surveys. These anomalies (>8 ppm eU) are mainly associated with granitoids, which account for around 50% of the country’s geological framework. The Koudougou anomalous zone in central-western Burkina Faso accounts for one of the most significant radiometric anomalies, mainly associated with syn- to late- and post-orogenic granitoids of the Kindi area, including medium-grained biotite granite, porphyroid biotite granite, and two-mica leucogranite, along with aplo-pegmatite veins, which are intruded into early granodiorite/trondhjemite and tonalite from the pre-orogenic domain. The pre-orogenic granitoids, defined as poorly fractionated, low-K, calc-alkaline, biotite- and amphibole-bearing facies rich in Na (4.41 - 5.10 wt% oxide), Ca (3.04 - 3.87 wt% oxide), Fe (2.9 - 4.18 wt% oxide), and Mg (1.52 - 2.19 wt% oxide) but with relatively low U and Th concentrations, are defined as non-favourable sources for a granite-related hydrothermal uranium system. In contrast, the syn- to late- and post-orogenic granitoids are characterised by fractionated high-K calc-alkaline to shoshonitic magmatic series with a biotite, sphene, allanite, and (+/− muscovite) mineral assemblage. They display low Na (3.52 - 4.64 wt% oxide), Ca (0.92 - 2.23 wt% oxide), Fe (0.85 - 2.90 wt% oxide), and Mg (0.08 - 1.11 wt% oxide) contents but are relatively enriched in incompatible elements such as U (2.8 - 10.9 ppm) and Th (7.50 - 24 ppm), which could be explained by crustal reworking during partial melting of the source magmas. The metamict textures observed around allanite and sphene crystals of these granitoids suggest that these accessory minerals are likely the major host of uranium, which may have been released from the mineral structure and then leached during late magmatic to later hydrothermal alteration and mobilised along the structures, as demonstrated by Th/U > 10 ( i.e. , U leaching) or Th/U ≤ 1 (U enrichment) in some samples. Therefore, this type of U-rich granite may represent a fertile U source for potential granite-related hydrothermal mineralisation in the region, as a similar typology of late Birrimian granitoids in other parts of the West African Craton (Senegal and Mauritania) have already demonstrated their potential in sourcing hydrothermal uranium mineralisation.