U-bearing syenitic rocks found at the Mbanga Massif (South Region, Cameroon):  Geochemistry, mineralogy, radiometry and ore processing

Abstract

U-bearing syenitic rocks cropping at the Mbanga Massif in the South Region of Cameroon were geochemically and mineralogially charactarized, before determination of their radiometric data, and processing of their uranium ores by H2SO3 acid leaching. The syenitic rocks are alkaline syenite, high-K syenite, and alkaline quartz syenite, with the first two rocks being metaluminous and the third, peraluminous. Both rocks show a compositional difference in uranium abundances (133- 447 ppm) and those of other elements in wt.% (e.g., SiO2 : 61.6-66.6, Al2O3: 16.9-17.9, Fe2O3: 1.9-3.9, MgO: 0.4-1.3, CaO: 1.5-3.4, Na2O: 4.7-9.7, and K2O: 0.1-6.4) and in ppm ( e.g., Zr: 111-599.7, Rb: 4.5- 287, Ba : 100.1-399, Sr: 157.1-999, V: 16.1-131, Pb:30.9-121.5, and Th: 12.0-27.6). The ƩLREE (37.3- 168 ppm), ƩHREE (8.7- 22.2 ppm), ƩLREE/ ƩHREE (1.6-13.8), and Eu/Eu* < 0.4 values are also variable. U-ore minerals (uraninite, uranophane, autunite, coffinite, carnotite, torbernite, and/or coffinite) are associated with other ore minerals (U-rich titanium oxide, zircon, magnetite, and/or ilmenite) in a gangue made up of chlorite, calcite, quartz, alkaline feldspar, albite, plagioclase, and/or biotite. The uranium contents (< 465 ppm) in the studied U-bearing syenites classified them within low grade ores; suggested to have formed with the aid of circulating hydrothermal fluids source of the precipitated OH, H2O, and OH-H2O bearing uranium ore minerals. The measured raw radiometric values range from 3542-6600 c/s and U3O8, from 0.33-0.59 wt.%. Samples with U3O8 ≥ 0.54 wt.% were found interesting for uranium industries. The presence of some minerals in the studied ores and the use of oxidant such as manganese dioxide during the H2SO3 acid leaching, provided elements which positively impact the process by increasing the U extraction with a total recovery of 80-90 %. The maximum leaching temperatures 40-60 oC were less compared to those used to process some other low grade uranium ores.