Host Rocks

The deposit is situated on a low-lying peninsula on the northern shore of the lake It is possible that this is a delta which has been broached by the moderately sized stream which enters the lake just to the west. Alternatively, the peninsula may be a chemical delta associated with the groundwater, and the stream conducts the occasional surface flow.

* ~™• Drainage Axis Breakaway

*•• • • • — " • • > Radiometrie Anomaly

® Sample Point With Uranium Content in P P B

28°45 S

Figure 1

Lake Raeside area regional water sampling — water soluble uranium contours.

120 45 E

Legend

—— Divide

*^~ Drainage Axis Breakaway Radiometrie Anomaly 0 -2 35 Sample Point With Carnotite S I

Figure 2

Lake Raeside area regional water sampling - carnotite solubility index (SI) contours (log).

TABLE 1

XRF Analyses of Samples from Lake Raeside Peninsula (Surficial Host Rock)

A = accessory (roughly between 5 and 20 %) Tr = trace (less than 5 %)

The host rocks are predominantly red or brown calcareous clays and clayey grits, overlying indurated ferruginous clays. Gypsum is common, and in places has formed thick "kopi" dunes, which are impervious to gamma radiation.

Some mineralogical and chemical analyses are shown in Table 1.

2.3 Mineralization

The mineralization is confined to the peninsula, in a zone approximately 5.6 km long by 100 to 800 m wide and 1 to 2 m thick. The mineralization is shallow, between 1 to 5 m below surface, and is generally slightly above the water table. Using a cut-off grade of 200 g/tonne, the resource potential of the area is estimated at 1 700 tonnes of contained U308 with an average grade of 0.025 % U308. This figure is based principally on radiometric probing of drill holes on a 200 x 800 m grid, and is only a very crude estimate. A typical cross section is shown in Figure 3.

The mineralization is strongly associated with the more calcareous clays. Carnotite is the only uranium mineral observed.

2.4 Radiometrics

Airborne radiometric surveys were carried out by both the Bureau of Mineral Resources (BMR) [2] and Consolidated Goldfields Australia ( Pty) Ltd (CGFA) [3]. The BMR survey, published at 1:250 000 scale, indicates a strong total-count anomaly coincident with Lake Raeside. The CGFA survey covered this area in more detail, and showed a strong uranium anomaly over the lake. Values over the peninsula were not regarded as anomalous.

However, later ground traverses over the peninsula using a Geometries DISA 400 gamma-ray spectrometer and a chart recorder detected the mineralization with an anomaly to background ratio of 5:1. A limited regional alpha track survey using "TRACK ETCH" cups indicated a strong anomaly on the lake margins, and it is concluded that the lake anomaly is probably due primarily to radium leaching from the peninsula mineralization. Subsequent breakdown to 214Bi would produce anomalous radiometric readings.

2.5 Hydrogeochemistry

Regional water sampling of wells and boreholes in 1974 had indicated that waters to the north had high uranium contents (greater than 100 ppb), but were undersaturated with respect to carnotite. During the 1976 drilling program, water samples were collected from the holes for analysis (Figure 4).

Eh, pH, salinity, conductivity and temperature were measured on site, and filtered samples were sent to Perth to be analysed for potassium, uranium and vanadium. Bicarbonate was determined both in the field and in Perth with very similar results. Eh and pH were determined using aTitron meter; salinity, conductivity and temperature were measured with a YSI salinity meter with the probe in the hole. Measurements downhole indicated little stratification, although measurements in other areas had shown strong stratification associated with uranium mineralization. Bicarbonate was determined by standard alkalinity titration. Potassium was determined by atomic absorption spectrometry with no preconcentration. Uranium and vanadium were determined down to 1 ppb levels using a proprietary XRF method by Sheen Laboratories. Other techniques, including delayed neutron activation, were tried, but XRF proved the most satisfactory. Typical results are shown in Table 2.

From these samples, and a large number of others collected elsewhere, it is possible to make the following generalizations:

1. Potassium and calcium contents increase linearly with salinity, from less than 20 ppm to more than 100 ppm.

2. pH is relatively constant in the range 6.5 to 7.5.

3. Waters are generally well oxidized compared to a standard calomel electrode (SCE). Hole A42 (Table 2) is a rare exception. This is significant in that under reduced conditions carnotite precipitation is unlikely to occur.

The calculated solubility index does not make allowance for oxidation state; however, Mann [4] has suggested that some carnotite may precipitate due to changes in oxidation state.

4. There is a sharp decrease in bicarbonate associated with saline areas from 300 to 500 ppm to about 150 ppm This is probably due to the precipitation of calcium carbonate as calcrete.

5. Vanadium does not conform to any particular pattern, ranging from trace amounts to greater than 200 ppb.

There does not appear to be any association with particular rock types.

Table 2

Hole A51 is about 400 m up-drainage from the mineralization, holes A42 and A35 are both at the northern end (updip) of the mineralization, and hole A1 5 is in the middle. Central Well is located in granitic rocks 8 km north of the mineralization.

Table 2 illustrates these points. The data are arranged roughly in order of location along the drainage. The first sample from Central Well (Figure 1) has quite high uranium and bicarbonate contents but low salinity and potassium contents. The carnotite solubility index (SI) indicates that the sample is very undersaturated The second sample A51 (Figure 4) is just outside the mineralized area. The uranium content is quite low, but potassium and vanadium contents are higher and, together with the lower pH, give a carnotite SI value much higher than at Central Well, but still undersaturated. Sample A35 is actually in carnotite mineralization Potassium, vanadium and uranium contents have all increased, and together with the lower bicarbonate, yield a SI very close to zero. This indicates that the solution is saturated and that carnotite should be precipitating, as is observed. Sample A15 continues this trend, and the SI indicates that the water is slightly supersaturated.

From these observations, the following mode of formation is postulated:

1. Weathering of the granites to the north of Lake Raeside provides a source of soluble uranium, potassium, and calcium. The source of the vanadium is not well understood, but may be leached from the latentes 2. The uranium is strongly complexed by the formation of uranyl dicarbonate complex (UDC) and uranyl

tricarbonate complex (UTC), due to the high bicarbonate concentration This effectively precludes a reaction involving uranium ions in this environment

3. Groundwaters carry the complexed ions slowly down-gradient. Calcium and potassium ion concentrations increase due to evaporative concentration until calcite and/or dolomite precipitate as calcrete, removing calcium and/or magnesium and bicarbonate from the groundwater.

4. Due to the reduction in bicarbonate concentration, the UDC and UTC complexes break down, releasing uranium ions. These combine with the abundant potassium and smaller quantities of vanadium to form potassium uranyl vanadate — i.e carnotite.

ACKNOWLEDGEMENTS

This work was carried out by the author whilst employed by Derry, Michener and Booth (Pty) Ltd, consultants acting on behalf of BP Minerals Australia (Pty) Ltd. Their assistance is gratefully acknowledged. Numerous discussions with Dr A.W. Mann and Dr R.L Deutscher were of mutual benefit, and the work of Professor

P.B. Hosteller was invaluable.

I »Water Table

so-Legend

[ | Strong Acid Reaction j Weak Acid Reaction

• lsoradsmC.PS

1 m-r Vertical I Scales

Om-l———I 100m Horizontal

Figure 3

Lake Raeside Deposit typical cross section (see Figure 4 for orientation).

Figure 4

Lake Raeside Deposit detailed water sampling - carnotite solubility index (SI) contours (log).

APPENDIX

Calculation of Solubility Indices for Calcite and Carnotite

The solubility index (SI) of a particular mineral is a figure which, if greater than unity, indicates that a solution is supersaturated and precipitation can proceed. It is calculated from the product of the molar concentration of the active ionic species, divided by the solubility product.

In the case of calcite.

Sl = (Ca2+) (C02~) 10~85

Normally, C0§~ is present in very small quantities compared to HC03, and must be calculated from the equation (CO§-)(H+) =10io.3

(HCC-a)

The calculation for carnotite is similar, using the equation CS.=

(H¹⁺²¹ 10'^I-6.8

Several complications arise, however, in that the active concentrations of UO|+ are very much reduced by the formation of the complex ion UO2(C03)2. 2H202~ (UDC) and U02(C03)^- (UTC). Similarly, the active concen-tration of K"¹" is altered by the very high salinities which occur in the saline areas. The formation of the H²V04 i°ⁿ '^s dependent on Eh.

The formation of UDC and UTC can be estimated using the total dissolved uranium concentration, and dissocia-tion constants given by Hostetler and Garrels [5].

A short program for an HP25 programmable calculator is given in Table 3. This program estimates the carnotite solubility index from pH, bicarbonate (alkalinity), uranium, potassium and vanadium. It gives good results in the calcrete areas of Western Australia, but does not allow for reducing conditions which will affect the concentration of available vanadium. Nor does the program allow for the ionic strength of the solutions.

Table 3 5. Repeat step 4 any number

of times

Table 4

[1] THOM, R., BARNES, R.G., Leonora, Western Australia, West. Aust. Geol. Survey 1:250 000 Geol. Series Explan. Notes (1977).

[2] LEONORA - SW, Total Magnetic Intensity and Radioactivity, Aust. Bur. Min. Res. Geol. Geophys. Sheet H51/B1-91-3.

[3] CONSOLIDATED GOLDFIELDS AUSTRALIA "Leonora" 1972 West. Australia Geol. Survey, Open File Report, roll 237 M1187.

[4] MANN, A.W., Calculated solubilities of some uranium and vanadium compounds in pure and carbonated waters as a function of pH, Aust. CSIRO Div. of Mineral. Report FP 6 (1974) 39.

[5] HOSTETLER, P.B., GARRELS, R.M., Transportation and precipitation of uranium and vanadium at low temperature, with special reference to sandstone-type uranium deposits, Econ. Geol., 57 (1962) 137-167.