Uranium distribution in the promise reefs of the Mesoarchean Westrand Group, Witwatersrand Supergroup, South Africa

The Mesoarchean Westrand Group (2932 – 2923 Ma) constitutes the lower portion of the Witwatersrand Supergroup. Compared to the overlying Central Rand Group which is well known for its world-class conglomerate-hosted gold-uranium deposits, the economic potential of palaeoplacers in the Westrand Group has not been fully explored. This study aims to assess the uranium potential of the Promise Reefs in the Westrand Group. Field exposures of the Promise Reefs and boreholes drilled down-dip from the surface outcrops were investigated in the Westrand area. Three reef zones were recognized consisting of well-mineralized, matrix-to clast-supported, small pebble conglomerates with associated medium- to coarse-grained quartzite.

Radioactive minerals found in the conglomerates comprise leucoxene, brannerite, carbon nodules, monazite, allanite, thorite, and uraninite.

Analyses of U and Th from borehole samples reported mean values of 120 ppm for U and 60 ppm for Th. Maximum U and Th concentrations are 630 ppm and 125 ppm, respectively. The highest U/Th ratio was 17.4 with a mean value of 2. The conglomerates from surface outcrops have maximum U and Th concentrations of 111 ppm and 140 ppm, respectively, with a mean U value of 19.5 ppm and a mean Th value of 26.6 ppm, and a mean U/Th ratio of 0.6. Comparison of the data shows that surface outcrops generally report lower U concentrations than their unweathered equivalents.

Also, U assays by γ-spectrometry and INAA of the same samples from surface outcrops yielded inconsistent results. As U exploration by field γ-spectrometry relies on the existence of ²³⁴U/²³⁸U equilibrium and because of the U depletion in the surface samples, further investigations using α-spectrometry and Pb-isotope analyses were employed.

The results obtained from α-spectrometry point to complex U redistribution processes in the surface conglomerate with several stages of uranium loss and/or gain. The bulk of the U was removed during an early stage of surface exposure and supergene alteration, probably more than 1.4 Ma ago.

The estimation of the original pre-weathering U concentration in the surface conglomerates is based on the measured Pb-isotope ratios ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb and the Pb and Th concentrations. The data were inserted into U-Th-Pb age equations which were reformulated and solved for the U concentrations. To test the method, for a suite of borehole samples the same treatment was applied. The calculated fresh-rock U tenor indicates the occurrence of supergene uranium losses which are of the same magnitude as the U losses caused by experimental leaching of fresh conglomerate samples. The high amount of radiogenic Pb found in the surface samples points to severe uranium losses earlier than 100 Ma ago.

The data show that U redistribution in weathered Mesoarchean conglomerates is not governed by systematic processes which obscure the original pre-weathering U concentration. Application of the Pb isotope technique proves to be the only method yielding usable estimates of the fresh-rock uranium tenor in deeply weathered terranes.