Textural variations in uranium-bearing sheeted leucogranites, with specific attention to “oxidation halos” from the Rössing mine, Namibia
Date
2022
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Abstract
The Rössing Uranium Mine is ~60 km northeast of Swakopmund, Namibia, and it is one of the largest sheeted leucogranite-hosted primary uranium deposits in the world. The sheeted leucogranites (SLGs) are divided into six different types (A-F) based on field characteristics of colour, grain size, structural setting and mineralogy. Three pre-D3 A-C types are barren, while post-D3 D-F alaskites are undeformed and have potential for uranium mineralisation, especially types D and E. The E-type SLGs have notable diagnostic features; oxidation halos which occur in almost all uranium deposits in the southern Central Zone of the Damara Orogen, and have been mined for uranium since the establishment of the Rössing Uranium Mine in 1976. This project aims to document the textural variations of oxidation halos in relation to uranium mineralisation and distribution.
The Rössing uranium deposit is found within the southern Central Zone of the Damara Orogen which is composed of the Abbabis Metamorphic Complex (AMC) a Damaran-aged metasedimentary sequence and various granitic intrusions. The geology of the Rössing area comprises the Etusis, Khan, Rössing Formations and granitic intrusions. The primary uranium ore minerals at the Rössing Mine are uraninite and U-silicates. The mineralised uranium SLGs are typically found within the stratigraphic intervals of the Khan and Rössing Formations. Four techniques were used to analyse the rock samples: hand specimen observations; thin section petrographic analysis; Micro-XRF analysis to determine major element distribution of the whole E-type SLG rock sample and TIMA studies to determine mineral phases, elemental distribution, textural and alteration variations of the oxidation halos and their host rocks.
The results show that the grain size of the oxidation halos and their host rocks varies from fine to pegmatitic. In the oxidation halo sample, the D-type section and the oxidation halo core zone are pegmatitic; the E-type section has a coarse texture with centrimetric crystal sizes and the oxidation halo transition zones have a fine-grained texture. In the D-type SLG rock sample, TS1 sections 1, 3 and 6 have a fine texture, TS1 sections 2 and 5 exhibit pegmatitic textures and TS1 section 4 has a coarse-grained texture. Rock texture controlled uranium mineralisation and distribution, alteration rates and colour of the oxidation halo and their hosting rocks. The pegmatitic sections are less altered and have less abundant uranium (U)-bearing minerals. Whereas, the fine-grained textured sections are highly altered and they have abundant U-bearing minerals such as uranophane, uraninite and zircon. The pegmatitic-fine-grained textured interface acted as fluid traps, retaining more fluids for a long period causing intensive alteration of mineral phases characterising the finelytextured sections. In the pegmatitic sections, hydrothermal fluids were not retained for a long period, hence their low alteration. High abundance of U-bearing minerals in the fine-textured sections resulted in the presence of smoky quartz, which is responsible for light-dark variation in the D-type SLG rock sample. In the oxidation halo sample, red colouration is due to unequal distribution of iron oxides such as haematite and magnetite. The various alteration processes both rock samples were subjected to had three major effects: iron rich and primary U-bearing minerals were oxidised to form iron oxides and secondary U-bearing minerals, respectively; this promoted the rock permeability which in turn resulted in an uneven redistribution of U-bearing minerals across the rock samples; and controlled the rock colour variations. High abundance of uranium-bearing minerals in the oxidation halo transitional zones is a result of abundant biotite phases, whose alteration helped to provide Fe2+ to the REDOX system to reduce and precipitate more uranium-bearing minerals, and forming iron oxide phases such as haematite, magnetite, rutile and titanite as by-products.
Description
A dissertation submitted in fulfilment of the requirements for the degree of Master of Science to the Faculty of Science, University of the Witwatersrand, Johannesburg, 2022
Keywords
Rössing Uranium Mine, Oxidation Halos, Sheeted leucogranites (SLGs)