The source of uranium and vanadium at the Langer Heinrich and Klein Trekkopje uranium deposits-genesis and controlling factors for uranium mineralization, Western Namibia.
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Date
2012
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Abstract
The Langer Heinrich and Klein Trekkopje U deposits are located within the Namib Desert in Central Namibia. Both deposits are hosted by calcretes developed within palaeo-channel systems of Tertiary age. The sources of both U and V in the two deposits and the reasons why the two deposits differ in terms grade and tonnage have remained unclear to many researchers. The study aimed to resolve these uncertainties. During the study, the geology of the area was reviewed and a number of research methods were employed. U and V concentrations and the mineralogy of various rocks in the catchment area were analyzed by ICP-MS, XRF and thin sections. Leachability of various rocks was studied through the analysis of mineralogy, texture, structures, etc. This study found that the Bloedkoppie granite, which is located in the catchment area for Langer Heinrich, contains about 5 – 15 ppm U and 5 – 20 ppm V. The Tinkas schist, which forms most of the bedrock in the catchment area for the Langer Heinrich deposit, contains 3 – 6 ppm U and 120 – 160 ppm V. The Bloedkoppie granite is considered to represent the source of U for the Langer Heinrich deposit. It is deeply weathered due to its relatively high proportion of ferromagnesian minerals, compared to average granites. The fluid which mobilized U and V from the Bloedkoppie granite also mobilized V from the Tinkas schist. In the case of Klein Trekkopje, the main source of U was the Klein Spitzkoppe granite, Gross Spitzkoppe granite and some Damaran leucogranites. V is considered to be derived from the Kuiseb schist, which contains about 100 – 160 ppm V. The Klein Spitzkoppe granite contains about 10 - 20 ppm U and about 1-3 ppm V whereas the Gross Spitzkoppe granite contains about 15 – 25 ppm U and 3 – 5 ppm V. The catchment area for Langer Heinrich had more V than that of Klein Trekkopje, and V was found to be a key factor in the precipitation of carnotite. The geomorphology around the Langer Heinrich deposit (Etusis and Schieferberge mountains) is more favourable for constriction and ponding of subsurface water unlike that of Klein Trekkopje, which lacks resistant rocks which would restrict the fluids. Oxidizing conditions were fully achieved at Langer Heinrich because there were less saline marine fogs and the subsurface water spent longer periods of time in ponds, resulting in lengthy interaction with air and seasonal rains. By contrast, at Klein Trekkopje, oxidizing conditions were regularly offset by the inflow of highly saline marine fogs and by the fact that poor constriction and ponding resulted in subsurface water moving faster downstream and having less time to interact with oxidizing atmospheric air and rain waters. As a result V4+ could not be effectively oxidized to V5+, which is a pre-requisite for the precipitation of carnotite. Controlling factors for U precipitation in calcretes are redefined in the context of Langer Heinrich and Klein Trekkopje U deposits. Multiple sources of V are proposed. Exploration for calcrete – hosted U deposits could therefore start with investigations of the catchment, by gaging the availability of U and V as well as the suitability of the geomorphology and palaeo-climate.
Description
A thesis submitted in fulfilment of the requirements for the Degree of Master of Science.
Keywords
Uranium, Vanadium