CALCRETE AND GYPCRETE URANIUM MINERALIZATION IN SOUTHERN AFRICA IN RELATION TO SOURCE TERRANE GEOMORPHOLOGY AND CLIMATE

Three calcrete-gypcrete uranium areas have been extensively explored in southern Africa: 1 ) the Namib Desert in Namibia, 2) a region centered on Upington in South Africa and, 3) a small area near Beaufort West. All have arid to very arid climates very comparable to Western Australia, although in the Namib Desert this is complicated by marine mist.

In the Namib Desert, carnotite-bearing groundwater calcrete and lesser gypcrete occur within palaeochannels on the mid-Tertiary African Surface from the Atlantic coast to the foot of the Great Escarpment 100 km inland (Figure 2b). The calcretes, and probably the uranium mineralization as well, predate Quaternary gorge cutting.

The orebody at Langer Heinrich, in fact, is exposed only because of headward erosion of the rejuvenated Gawib River. Most occurrences are not thus exposed and the mineralization itself has developed on extremely broad low-gradient surfaces that remained stable for large portions of the Late Tertiary. A significant geomorphic difference from Western Australia is the Great Escarpment, rising steeply to the Khomas Highland immediately east of the calcreted Namib platform. The calcretes differ from Australian calcretes, in their Late Tertiary age, the abundance of coarse detrital fragments, the fact that carbonate is predominantly sparry calcite but with some dolomite [24] the presence of abundant gypsum in the upper parts of occurrences within 60 km of the Atlantic, and in the superposition of unmineralized pedogenic sheet calcrete and gypcrete. In the Langer Heinrich orebody, carnotite occurs in gravelly, sandy and silty faciès and is not uncommonly richest where calcification is least.

Gypcrete, with the same kind of aggregate as the calcite, occurs directly above groundwater calcrete within the belt of habitual fog from the Atlantic Ocean. Thicknesses up to 25 m have been drilled in palaeochannels on the Tumas River where a potential orebody occurs beneath Holocene surficial gypcrete. The sulphate is derived largely from marine mist [19]. The uranium is probably derived in part at least from uraniferous calcrete updrainage and is reconcentrated as carnotite in a sparsely gypcreted sand faciès adjacent to a palaeo-water table [10]. A ubiquitous overlying layer of surficial gypcrete or, inland from the fog limit, surficial calcrete, covers almost the entire Namib Desert. Carnotite is found only very rarely in this superficial gypcrete or calcrete, a consequence mainly of capillary rise from subjacent mineralization.

In spite of the differences and much greater difficulty in delineating valley calcretes and gypcretes in Namibia, numerous examples support the conclusions that lateral transport of uranium in groundwater is essential to ore deposition and that bedrock barriers or constrictions which narrow the channel of subsurface flow, and thus force the water closer to the evaporative surface, greatly favour formation of uraniferous calcretes. Source rocks are not hard to find: Proterozoic migmatites, pegmatites, alaskites, and granites abound, many appreciably anomalous in uranium. The unusually uraniferous alaskitic pegmatites and migmatic granites of the central metamorphic zone of the Damara Belt (e.g. Rossing, Valencia) are not directly updrainage from any well-known calcrete occurrences, however, and may not be uniquely significant as source rocks. As in Australia, vanadium is probably sufficiently abundant in the granitic source terrane to permit carnotite deposition, but certain of the schists in the Damara Belt are enriched in vanadium [19]. Within a given drainage, the ratio of catchment or source terrane to mineralized calcrete is of the same general order of magnitude as in Western Australia. Salt lakes are absent.

Do the climatic parameters separating uraniferous groundwater calcrete from non-uraniferous pedogenic calcrete in Westerna Australia apply in southern Africa? Is summer-only episodic rainfall and extreme aridity essential for calcrete uranium? If one compares the distribution of calcretes in southern Africa as recorded by Netterberg [20] with present-day climates and inferred soil moisture regimes [10] two points become clean

1. The overall distribution of calcretes in general, but predominantly the pedogenic type, coincides with present-day aridic and subaridic soil moisture regimes.

2. Although the best developed pedogenic calcretes, including great stretches of "Kalahari Limestone", are in an area with a present climate almost identical with the valley calcrete region of Western Australia, they are in fact of Tertiary to Pleistocene age, recording a wetter climate than that of today. With rare exceptions attributable only to especially favourable circumstances, pedogenic calcretes are not forming in that climatic region today. Instead, throughout the Upington region, for example, nonpedogenic calcretes of Pleistocene (?) to Holocene age are found in shallow valleys eroded into the older calcrete. In some of these, in constricted parts of the valleys, there are subeconomic concentrations of carnotite.

Although these nonpedogenic calcretes consist very largely of eroded and recemented fragments of older pedogenic calcrete, the climatic parameters for pedogenesis vs. nonpedogenesis would seem to apply.

The situation in the Namib Desert is complicated by the sulphate-bearing fogs which occur during three quarters of the year, and which at places account for about two-thirds of the total precipitation. Gypsum is at present crystallizing on the moist surfaces. Climates during the Late Tertiary, however, when uraniferous groundwater calcretes were being formed, were apparently arid to extremely arid and even then probably characterized by summer rains and dry winters [21]. I n spite of a favourable climate, calcrete would not form and still does not form on the escarpment and the highlands because of the relief and rejuvenation. Instead, the groundwaters migrating into alluviated valleys on the gently sloping Namib platform retained abundant carbonate and have progressively calcified almost the entire valley-fill. Locally, carnotite has been precipitated apparently in essentially the same way as in Western Australia. Presumably, this process may have continued even as pedogenic calcrete may have been accumulating on the overlying desert surface, though this is by no means clear at this time. Reworking and the evaporative succession characteristic of Western Australia are apparently operative on the Namib Desert but distorted by topography and marine mist.

REFERENCES

[I] CULBERT, R.R., BOYLE, D.R., LEVINSON, A.A., Surficial uranium deposits in Canada, this Volume.

[2] OTTON, J.K., Surficial uranium deposits in the United States of America, this Volume.

[3] GILE, L.H., PETERSON, F.F., GROSSMAN, R.B., Morphological and genetic sequences of carbonate accumulation in desert soils. Soil Sei., 101 (1966) 347-360.

[4] NETTERBERG, F., The interpretation of some basic calcrete types, S.Afr. Archaeol. Bull., 24 (1969) 117-122.

[5] GOUDIE, A., Duricrusts in tropical and subtropical landscapes, Oxford, Claredon Press (1973) 174.

[6] CARLISLE, D., MERIFIELD, P.M., ORME, AR.R., KOHL, M.S., KOLKER, 0., The distribution of calcretes and gypcretes in southwestern United States and their uranium favourability based on a study of deposits in Western Australia and South West Africa (Namibia), US Dept. of Energy, Open-File Rpt. GJBX-29(78) (1978)274.

[7] MANN, A.W., HORWITZ, R.C., Groundwater calcrete deposits in Australia. Some observations from Western Australia, J. Geol. Soc. Australia, 26 (1979) 293-303.

[8] SOFOULIS, J., The occurrence and hydrological significance of calcrete deposits in Western Australia, Western Australia Geol. Surv. Ann. Report (1963) 38-42.

[9] SANDERS, C.C., Hydrology of a calcrete deposit on Paroo Station, Wiluna, and surrounding areas.

Western Australia Geol. Surv. Ann Report (1973) 15-26.

[10] CARLISLE, D., Possible variations on the calcrete-gypcrete uranium model, US Dept. Energy, Open-file Report, GJBX 53(80), (1980) 38.

[II] CAVANEY, R.J., Lake Maitland uranium deposit, this Volume.

[12] LIVELY, R.S., HARMON, R.S., LEVINSON, A.A., BLAND, C.J., Disequilibrium in the uranium series in samples from Yeelirrie, Western Australia, J. Geochem. Expl. 12 (1979) 57—65.

[13] DICKSON, B.L, FISHER, N.L, Radiometrie disequilibrium analysis of the carnotite uranium deposit at Yeelirrie, Western Australia, Proc. Aust. Inst. Min. Metall., No. 273 (1980) 13-19.

[14] AIREY, P.L., ROMAN, D., Uranium series disequilibria in the sedimentary uranium deposit at Yeelirrie, Western Australia, J. Geol. Soc. Aust, 28 (1981) 357-363.

[15] DICKSON, B. L., Uranium series disequilibrium in the carnotite deposits of Western Australia, this Volume.

[16] MANN, A.W., DEUTSCHER, R.L, Genesis principles for the precipitation of carnotite in calcrete drainages in Western Australia, Econ. Geol. 73 (1978) 1724-1737.

[17] BUTT, C.R.M., MANN, A.W., HORWITZ, B.C., Regional setting, distribution and genesis of surficial uranium deposits in calcretes and associated sediments in Australia, this Volume.

[18] BOWLER, J.N., Aridity in Australia. Age, origins and expression in aeolian landforms and sediments. Earth Sei. Rev. 12 (1976) 279-310.

[19] HAMBLETON-JONES, B.B., The geology and geochemistry of some epigenetic uranium deposits near the Swakop River, South West Africa, University of Pretoria, D.Sc. Thesis Unpubl. (1976) 306.

[20] NETTERBERG, F., Calcrete in road construction, Nat. Inst. for Road Research, S. Afr., CSIR Bull. 10, Report 286 (1971)73.

[21 ] KORN H., MARTIN, H., The Pleistocene in South West Africa, In: Clark, J.D., (Ed.), Proc. Third Pan-African Cong. On Prehistory, London (1975).

[22] CARLISLE, D., Concentration of uranium and vanadium in calcretes and gypcretes. In, Wilson, R.C.L, (Ed.), Residual deposits: surface related weathering processes and materials. Geol. Soc. London (1983) 258.

[23] BETTENEY, E., CHURCHWARD, H.M., Morphology and stratigrahic relationships of the Wiluna Hardpan in arid Western Australia, J. Geol. Soc. Aust. 21 (1974) 73-80.

[24] HAMBLETON-JONES, B.B., Surficial uranium deposits in Namibia, this Volume.

PETROLOGY, MINERALOGY AND GEOCHEMISTRY OF SURFICIAL URANIUM DEPOSITS