The geology, petrography, geochemistry and uranium ore mineralogy of the Namibplaas sheeted leucogranites, Damaran orogen, Namibia
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Date
2017
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University of Namibia
Abstract
This research study focused on the previously unknown uranium mineralisation hosted in garnet and magnetite bearing sheeted leucogranites (SLG's) I alaskites at Namibplaas. The various SLG's were distinguished and characterised by means of field, petrographic and geochemical studies into B, C and D-types based on the classification scheme ofNex and Kinnaird (1995) in the Goanikontes area. In addition, the stratigraphic positions of the SLG' s were established. The research techniques applied in the study included review of historical data, ground mapping, diamond drill core logging and sample preparation, thin section preparation, geochemical
analyses and uranium ore mineralogical investigation. At Namibplaas, the Damaran sequence is most complete; the B-types are least abundant and are localised to the Kuiseb Formation, the C-types are most abundant and intrude the whole
stratigraphic sequence. The D-types are emplaced within the Karibib, Chuos, Rossing and Khan Formations and at the Khan-Rossing-Chuos-Karibib Formation boundaries. The B and C- types are post D1- D2 but pre D3 deformation and are barren in uranium. D-types are post D1- D2- D3 deformation, these are uranium enriched and cross-cut the B and C-types making them the youngest of the intrusives. The sheeted leucogranites at both Goanik:ontes and at N arnibplaas have undergone similar
structural events. But there is however a difference in accessory mineralogy and U & Th geochemistry. The D-types at Namibplaas bear garnet and magnetite. The occurrence of these accessory minerals can be explained as a result of host stratigraphic - assimilation. Individual sheeted leucogranite types are discemable from each other by a number of major, trace and rare earth element discrimination plots. Uranium and thorium and U/Th ratio distinguishes and characterises the Namibplaas sheeted leucogranites very well. The D-type contain the highest concentrations of uranium (22.5ppm to 1020ppm) with a U/Th ratio > 1. The C-type sheeted
leucogranites show the highest thorium concentrations (10ppm to 301ppm) with a U/Th ratio < 1. The B-type sheeted leucogranites have lower concentrations of uranium (3ppm to 9ppm) and thorium (Sppm to 17.5ppm) The late- to post- orogenic conformable and transgressive sheeted leucogranitic bodies range in composition from tonalite to syenogranite. The Namibplaas leucogranites are peraluminous to
metaluminous indicating a highly evolved crustal source most probably the Abbabis basement gneisses. Partial melting of the Abbabis basement gneisses which was induced by M2 resulted in the B and C-type leucogranites. Continued partial melting of the Abbabis basement gneisses during M3 resulted into the more uranium enriched D-type leucogranites. QEMSCAN investigation shows that uranium phases present in the D-type sheeted leucogranites are U-oxides as uraninite, U-Ti-oxides as betafite and Th(U)-silicates as uranothorite. The Namibplaas uranium propect is typically a low grade high tonnage deposit with a resource in the measured category; 25 Mt at an average grade of 206 ppm U30s at a cut-off grade of 160 ppm. The project may not be economically viable under the current global uranium price; however, the project may become viable if the global uranium price doubles in the near future. Further work which can be carried out at Namibplaas may include age dating of the
leucogranites in order to establish the sequence of formation of the various leucogranites as well as fluid inclusion studies to determine the pressure and temperature regime, types and compositions of the ore forming fluids, as well as more detailed structural studies to better constrain the various deformation phases of the Damaran Orogen in relation to uranium mineralisation.
Description
A thesis submitted in fulfilment of the requirements for the Degree of Master of Science in Applied Geology
Keywords
Uranium ore, Damaran oregen