CLIMATE AND VEGETATION

The area is arid, with an average annual rainfall of 249 mm and evaporation in excess of 3 000 mm. Most of the rainfall occurs in periods lasting less than 48 hours between January and June. Dry periods may last from 6 to 9 months and droughts lasting several years have been recorded. Monthly mean maximum temperatures range

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Figure 1

Surficial geology of the Lake Way uranium deposit

from 19 °C in June-July to 37 °C in December-February; mean minimum temperatures range from 7 °C in July to 23 °C in January-February. Summer daytime temperatures commonly exceed 40 °C, but winter days are generally mild and frost may occur at night.

Vegetation in and around the mineralized area varies in accord with the underlying rock type, depth and salinity of the groundwater. Low-lying saline areas near the lake margin support the more salt-tolerant halophytic shrubs and are almost exclusively vegetated by samphires. Atriplex, Maireana, and Enchyleana spp. dominate the chenopod flora of the higher flats and, in areas underlain by fresher water, the shrubs Lycium, Cratystylis, Olearia, and Muehlenbeckie are present. Stands of Acacia and Melaleuca spp. form a distinct border to the halophytic communities and mark the change to sandy, less saline soils.

In the higher calcrete areas there is a predominance of Acacia and Eucalyptus spp. forming an overstorey to the chenopod (saltbush) flora. Open spinifex grasslands are the feature of the sand plain and dunal areas.

Soils in the area are predominantly coarse-grained red silt sands; they are variously saline and have low phosphorus, nitrogen and organic levels.

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Figure 2

Geology of the basement rocks in the vicinity of Lake Way

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Figure 3

Ground and airborne radiometric responses over the uranium mineralization and cross-section through the ore body. See Figure 6 for locality of NS

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Figure 4

Cross-section costeans A4-B4-D12 through the uranium mineralization

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OUTLINE OF MINERALIZED A R E A AT 250ppm U C U T - O F F

Figure 5

Overburden isopach map

Analyses of plants and associated soils from the area of major uranium mineralization are shown in Table 2.

Most of the carnotite has been deposited within carbonated reworked fluvial elastics which occupy the southern extremity and peripheral areas of the Uramurdah-Negrara drainage, but significant mineralization also occurs in the southern edge of the "valley-fill" calcreteand intheyounger"chemical delta" carbonates at the edge of Lake Way (Figures 1 and 3). Mineralization is not lithologically controlled, but is directly related to sediment permeability (primary or secondary), water table depth and rate of groundwater evaporation.

Fluvial deltaic sediments consist of an interfingered and transitional sequence of carbonated sand, silt, clay, and calcrete. Quartz sands are subangular, dark brown to grey and vary in grain size from very fine to grit and become more argillaceous towards the delta area. Reworked carbonate fragments and nodules are common and carbonaceous material is found lining fossil root tubes.

Carbonated silts and clays are typically brown to light grey, thinly bedded and very arenaceous. Thin lenses of sand, grit, and reworked calcrete fragments are present, especially in Area 'B'. Former soil surfaces are in evidence, as shown by fossil root tubes which are filled with calcareous and carbonaceous material.

Dominant minerals in the clay-sized fractions in Areas 'A' and 'B' (Figure 1} are very fine quartz and dolomite.

Kaolin is the subordinate clay mineral, but a single horizon in Area 'A' contains smectite and smectite-illite mixed-layer clays. Accessory and trace minerals in Area 'A' are gypsum, kaolin, feldspar, attapulgite, halite, mica, illite and geothite; Area 'B' contains all the above except smectite and attapulgite.

Calcretes are generally light to dark grey, but the younger, more massive and dolomitic calcretes in the "chemical delta" are distinctly brown. Quartz grains are common inclusions and sepiolite is found lining cavities and fractures. Oolitic and spherulitic carbonate forms occur at or near the surface on the lake periphery and are composed of aragonite crystals, often coated with or in a sepiolite matrix.

Some reprecipitation of carbonate is apparent within the calcrete, where clear microcrystalline calcrete is present as growths in voids and on fracture planes. Fluvial and deltaic sediments associated with the calcretes are subject to carbonate replacement, especially those immediately above the water table.

Karst features, dissolution voids, shrinkage cracks and coarse irregular bedding provide for rapid infiltration and movement of groundwater towards local base level.

6. MINERALIZATION

Uranium mineralization commonly occurs below or near the water table, and in areas with little overburden, high evaporation, and high groundwater salinity (Figures 4 and 7). Distribution of the most significant mineralization recognized to date occurs over an area 3.5 km eastwest (around the periphery of the calcretes and in the fluvial detritus) and extending 2.5 km north from the north shore of Lake Way. Additional mineralization has been found to be present in sediments beneath 15 m of calcrete, but has not yet been fully defined. The ore zones average 1.55 m in thickness, with a maximum thickness in excess of 6 m (using a 250 ppm uranium cut-off). Overburden ranges from 0 to 10m and averages 4 m (Figure 5).

Total reserves drilled on 76 m spacings are 5 200 tonnes U308 at 250 ppm U cut-off and 3 300 tonnes U3O8 at 550 ppm U cut-off. Six additional target areas remain to be drilled.

Most of the visible uranium mineralization occurs in the form of micro- to cryptocrystalline carnotite (less than 0.04 mm) precipitated as coatings along bedding and slippage planes, in interstices between sand grains, in fissures in calcrete and dolomite and in root tubes in buried soil horizons. Some carnotite crystals are present in an argillaceous matrix with micro- to cryptocrystalline calcrete or micro-oolitic and spherulitic carbonate (aragonite). Both the crystalline and the oolitic calcrete are distinctly separate and younger than the massive calcrete valley-fill and are more prevalent in the associated clay/silt horizons. Leaching tests indicate that a small fraction (± 2 96) of the carnotite may be associated with recent silicification.

OUTLINE OF MINERALIZED AREA AT 250ppm U CUT-OFF I CROSS SECTION

Figure 6

Total count ground radiometric survey contour map

Downhole radiometric surveys were conducted in most of the open drill holes and selected reverse circulation drill cuttings were analysed for uranium by XRF. Overall correlation between the data sets is considered to be within assay limits, but data from a few drill holes show significant variation. Field selection of drill cuttings for XRF analyses was based on the radiation level of the 1 m or 0.33 m sample as measured with a hand-held scintillometer. The relative accuracy of the selection method was confirmed when only 50 additional samples required analyses.

A similar technique was successfully used to select bulk samples from costeans for pilot plant processing. During trial mining studies an area of 1 149 m2 was predrilled on 4.6 m centres and mining of the 5 000 tonne block was controlled with a hand-held scintillometer and a beta-gamma analyser coupled to a ratemeter. Each hole was sampled, chemically assayed and radiometrically logged; bulk samples were taken from the excavator bucket, a channel sample was taken from the pit face and each truckload was radiometrically logged. The five crosschecks correlated within chemical assay limits (± 5 %). Agreement between reserve data calculated from drilling and mining is of the order of 0.5 % grade and 1 % tonnage.

7. RADIOMETRICS

The Lake Way uranium deposit was discovered during an aerial spectrometer survey conducted in 1972.

Radiometric data showed anomalies of twice the background to off-scale on the total count, K-U-Th, and U-Th channels, but zero to minor increases on the Th channel. Subsequent drilling and costeaning revealed that the most intense anomalies generally correspond to near-surface accumulations of uranium. Large areas of twice the background radiation are a reflection of a very thin veneer of remobilized uranium that has been widely dispersed by surface water.

Detailed ground radiometric surveys (Figures 3 and 6) confirmed the airborne data and showed anomalies of twice to seven times the background on total count over areas of substantial mineralization. Major anomalies coincide with most of the chemical deltaic deposits and where Uramurdah Creek has eroded most of the overburden. A small anomaly of twice the background coincides with significant mineralization 3 km northwest of the delta area. Two anomalies 2.5 and 3.5 times reflect areas where only minor surface mineralization has been encountered. A large part of the deposit, which is covered by 4 to 10 m of overburden, was not identified by airborne or ground radiometric surveys (Figures 3 and 6).

Disequilibrium studies conducted by the CSIRO [1] showed a range of 1:1.02 to 1:1.16 (average 1:1.09).

Maximum disequilibrium was recognized in the "chemical delta" where the mineralization is considered to be slightly younger and probably derived from the higher, more northerly deposits.

8. HYDROLOGY

Uramurdah Creek and Scorpion Creek (Figure 2) drain more than 60 km2 of uranium-bearing adamellite and granite gneiss and form part of an intermittent drainage system that flows southwesterly to the north shore of