Page 81 - Introduction to Mineral Exploration

64 C.J. MOON & M.K.G. WHATELEY

100 Paardefontein Rietkuil

80
Counts

60

40
0 10 20 30 40 50
Distance (km)
FIG. 4.8 Trace of a carborne radiometric survey similar to that which discovered the first major radiometric
anomaly in the Karoo Basin. (After Moon 1973.)

discovered to the west of the small town of that coated sand grains and filled pore spaces.
Beaufort West (Fig. 4.7). Once the initial dis- Diagenesis rendered the deposits impermeable
covery was made the higher cost reconnais- to further remobilization. The outcrop charac-
sance technique of fixed wing airborne teristics of the deposits are either calcareous,
radiometrics was used as this provided much manganese-rich sandstone, or bleached altera-
fuller coverage of the area, about 30% at 30 m tion stained with limonite and hematite, that
flying height. This immediately indicated to locally impart yellow, red, and brown colora-
Union Carbide a number of areas for follow up tion. Typical sizes of deposits, which occur as a
work and drilling. number of pods, are 500 by 100 m. Average
It became rapidly apparent that the deposits grades are about 0.1% U 3 O 8 and thicknesses
are similar to those of the Uravan area. In both vary from 0.1 to 6 m but are typically 1 m.
the uranium is in the fluviatile sandstones in From the airborne radiometrics and follow-
an alternating sequence of sandstone, siltstone, up drilling Union Carbide defined one area of
and mudstone; in the South African case, in the potential economic interest and inevitably
Upper Permian Beaufort Group of the Karoo word of the discovery leaked out. In addition
Supergroup (Turner 1985). These sediments the price of uranium rose following the 1973
were deposited by braided to meandering Arab–Israeli war leading to interest on the part
streams which resulted in interfingering and of a large number of other companies. The
overlapping of sandstone–mudstone transi- reconnaissance approach of competitor com-
tions and made stratigraphical correlation diffi- panies was however markedly different in that
cult. Significant mineralisation is confined to they knew that the basin hosted significant
thicker multistorey sandstones, usually >14 m mineralisation and that Union Carbide had
thick, and is epigenetic (Gableman & Conel important areas under option to purchase. In
1985). The source of the uranium is unknown, general these companies selected at least one
possibly it was leached from intrabasinal vol- area around the known Union Carbide dis-
canic detritus but certainly mobilized as uranyl covery and a number of areas elsewhere within
carbonate complexes by oxygen-rich, alkaline the basin. These were chosen on the basis of
ground waters which then migrated into the comparison with deposits in the western USA
porous sandstone (Turner 1985, Sabins 1987). where the deposits are preferentially hosted
The migrating ground water then encountered at specific horizons and within thicker fluvial
reducing conditions in the presence of organic or lacustrine sandstones. Mosaics of the, then
material, bacterially generated hydrogen (1973) newly available, Landsat MSS images,
sulfide, and pyrite. This change from oxic to as well as government-flown air photography,
anoxic conditions caused the uranium to pre- were used to derive a regional stratigraphy
cipitate as minerals (coffinite and uraninite) and to highlight thicker sandstone packages.