LAKE AUSTIN URANIUM DEPOSIT, WESTERN AUSTRALIA

A.G. HEATH

Formerly, Broken Hill (Pty) Ltd Perth, Australia

R.L DEUTSCHER

CSIRO Division of Minerals Chemistry Melbourne, Australia

C.R.M. BUTT

CSIRO Division of Mineralogy Wembley, Australia

ABSTRACT

LAKE AUSTIN URANIUM DEPOSIT, WESTERN AUSTRALIA

The Lake Austin uranium deposit is a calcrete type deposit in the Yilgarn Block, near Cue, in a catchment area of granitoids and greenstones. The uranium is in valley fill and the sediments of the Lake Austin playa. The mineralization occurs over 1 to 6 meter thickness close to the water table in calcrete overlying clays and/or weathered bedrock. The principal uranium mineral is carnotite. Waters in nearby channels have an uranium content of over 30 ppb. The chloride content of the water increases downstream in the nearby drainages, as does the uranium and vanadium content.

1. LOCATION

The Lake Austin uranium deposit is 20 km west-southwest of Cue, Western Australia, at latitude 27°27'S, and longitude 117°57'E.

2. GEOLOGY

The basement rocks of the whole catchment are the granitoids and greenstones of the Archaean Yilgarn Block [1 ].

The granitoids are of a wide range of compositions, from granite to tonalité, and are the predominant basement rocks of the catchment. The "greenstones" are the metamorphosed acid volcanics and sedimentary rocks of the Cuddingwarra Formation and metamorphosed basic and ultrabasic intrusive and extrusive rocks of the Moyagee Formation. The granitoids are the probable source rocks for the uranium; some adamellites contain up to 70 ppm uranium but the majority of the granites, which do not outcrop, probably contain less than 10 ppm uranium.

Outcrop is generally poor and rocks are mostly deeply weathered to 30 m or more, commonly overlain by Cainozoic alluvium and colluvium. Calcretes have been deposited in the axes of the larger valleys, and a playa.

Lake Austin, has formed in the lowest part of the landscape. The calcrete and its enclosing sediments, and the upper sediments of the playa, are the hosts to uranium mineralization.

3. GEOMORPHOLOGY

The mineralization at Lake Austin is mostly in a narrow arm of the playa at the termination of an extensive, calcrete drainage system entering from the northeast (Figure 1 ). The main tributaries commence some 80 km to the east and 70 km to the north. Relief throughout the drainage is very subdued; the plateau surfaces which form the drainage divide are generally between 455 and 500 m and the playa is at 408 m [1 ]. The gradient of the valley floor is about 1:3 200. Calcretes are present in the axes of the tributary drainages for much of their length, tending to form mounds 1 to 3 m above the surrounding alluvial-colluvial plains but with local karstic depressions. The calcrete in the main tributary is more or less continuous, 50 km long, up to 5 km wide and 2 to 1 5 m thick. It widens into a nearly flat platform of over 50 km2 adjacent to the playa. The drainages have surface flow only after exceptional rains; however, the calcrete is an important and prolific aquifer, with the water table at 3 to 5 m.

Vegetation on the calcrete consists of low shrubs and grasses, with a few open stands of trees (Acacia and Eucalyptus spp). The playa surface is mostly bare, with fringing halophytic shrubs. The area is semi-arid, with erratic rainfall averaging about 180 mm per annum, falling mainly between January and June. Summers are hot (average daily range in January is 22.7 to 38.8 °C) and winters mild (average daily range in June is 7.4 to 21.3°C).

4. HOST ROCKS

The host sequence generally consists of calcrete overlying red-brown clays and/or weathered bedrock. A surface cover of sand or alluvium up to 2 m thick may be present locally. The calcrete is mostly white to grey, occasionally brown, ranging in texture from earthy and friable to porcellaneous and hard. In the channel, the calcrete

117°45E 118°00 E 118<>15 E 118830 E 27°OOS

- 27°15 S

- 27°30 S

UPLANDS

Fresh and weathered rock sandplam on plateau

VALLEYS Calcrete

Alluvium and colluvium Saline alluvium in playa

— — Drainage axis

0 L Austin uranium occurrence

• Minor uranium occurrences

\Q-~~~ Groundwater uranium content in jug/I

Figure 1

Regional geological setting. Lake Austin uranium occurrence, showing distribution of uranium m groundwaters.

commonly has a lower siliceous horizon, in places up to 8 m thick On the platform, this silicified zone is present mthe north but is absent from the south, toward the playa; the calcrete also thins from 10 to 14 m, to 4 to 5 m near the playa The calcrete consists of calcite and dolomite, with chalcedony in silicified zones, some gypsum and minor detntal quartz, feldspar, goethite and kaolmite. The underlying orange-brown sandy clays, 3 to 5 m thick, are the usual host to mineralization. They are carbonate-rich and contain quartz and minor magnetite in a kaolmite-smectite matrix On the platform area, these in turn overlie up to 30 m of red-brown clay and either bedrock or green-grey clays of unknown thickness.

In the playa, 10 to 1 5 m of saline, gypsiferous orange-brown clays with carbonate nodules overlie up to 20 m of red-brown clay These two clay units are equated with the similar clays beneath the calcrete.

5. URANIUM MINERALIZATION

Uranium mineralization, as carnotite and minor concentrations of uranium in opaline silica, occurs sporadically throughout the mam calcrete channel, centred particularly at Tamcrow (50 km updramage), Nallan (30 km updramage) and on the platform area [2, 3]. The mineralization occurs as minor enrichments, mostly 30 to 70 ppm uranium (maximum 250 ppm uranium) overthicknessesof 1 to 6 m, usually close to the watertable mthe silicified lower horizons of the calcrete On the platform, mineralization is usually m the orange-brown sandy clays below the calcrete Significant mineralization is found only in the two arms of the playa itself, mostly in the top 1 to 5 m The carnotite occurs as patches and coating on the clays, with the maximum concentrations close to the water table (0.5 m). It is also disseminated throughout the clay matrix as fine crystallites (30 ju. m). The amount of visible carnotite varies periodically, implying chemical activity, although the degree of mobility is probably not great. Uranium contents exceeding 450 ppm (maxima over 2 000 ppm) are found over an area of about 1 500 by 50 m (Figure 2).

6. RADIOMETRICS

Over the calcretes, ground radiometric surveys, initially on lines at 1 km spacings, gave low-contrast anomalies, two to three times above background near Tamcrow, Nallan and on parts of the calcrete platform. These anomalies indicate broad areas of uranium enrichment and some, but not all, occurrences of outcropping mineralization Mineralization at the base of the calcrete or m the underlying sediments has no surface

\l , I , I , I , I , I , I , I , I , I , I , I . I , I I I I I I I I I I I I I I I I I

Geological map and uranium distribution, Lake Austin. Sections illustrate distribution of uranium in playa lake sediments, determined by drilling and pitting [3]. (Published by permission of Elsevier Scientific Publishing Co.).

Table 1

Analytical Results and Carnotite Solubility Indices (SI) in Groundwaters, Lake Austin Drainage, W.A.

expression. In the playa, an anomaly with a two to five times contrast over an area of 1 900 by 100 m clearly outlined the mineralized zone. Radiometrie logging of drill holes gives four to thirty times contrasts over the mineralized intervals.

Figure 3

Chloride contents and carnotite solubility indices. Lake Austin [3] (Published by permission of Elsevier Scientific Publishing Co.).

7. HYDROGEOCHEMISTRY

The results of sampling groundwaters in stock wells, exploration drill holes, and in the playa, shallow pits, in the Lake Austin drainage, are summarised m Table 1. On a regional scale (Figure 1), waters in the Tamcrow-Nallan and Coodardy channels have elevated uranium contents, exceeding 30 ppb in the area of the calcrete platform and in one possible source area at Coodardy. Detailed sampling [3] showed that chloride contents of calcrete waters increased downdramage, as a result of evaporation, concentrations ranged from below 1 300 ppm, 50 km from the playa, to nearly 20 000 ppm on the calcrete platform, with a sharp increase to 54 000 to 93 000 ppm close to and in the playa. Potassium increased proportionately with chloride, from below 20 to 1 800 ppm;

although some potassium is precipitated as carnotite, this is negligible in comparison with the effect of evaporation. Bicarbonate concentrations were m the range 185 to 510 ppm m the calcrete, but declined where the chloride content exceeded 50 000 ppm, close to the playa, this implies that the concentrations of carbonate and uranium-carbonate complexes had also fallen. Vanadium and uranium concentrations varied markedly, but tended to increase with chloride content, downdramage The carnotite solubility indices become less negative closer to the playa, approaching zero in the area of mineralization, implying active precipitation (Figure 3).

ACKNOWLEDGEMENTS

Published by permission of Australian Silicates (Pty) Ltd.

REFERENCES

[1] de la HUNTY., Cue, Western Australia. Explanatory notes, Aust. Bur. Mines. Resour. Geol. Geophys.

1:250 000 Geol. Ser. (1 973).

[2] BUTT, C R M., HORWITZ, R.C., MANN, A.W., Uranium occurrences in calcrete and associated sediments in Western Australia, Aust. CSIRO Div. Mineral. Report FP 16 (1977) 67.

[3] HEATH, A.G., DEUTSCHER, R.L, BUTT C.R.M., In: Conceptual Models m Exploration Geochemistry:

Australia, C.R.M. Butt, R.E. Smith (Ed.), J. Geochem. Explor. 12 347-349.

HINKLER WELL- CENTIPEDE URANIUM DEPOSITS