AN ASSESSMENT OF THE USE OF HYDROGEOCHEMISTRY IN EXPLORATION FOR CALCRETE URANIUM IN AUSTRALIA

W.G. MIDDLETON

Formerly, Uranerz Australia (Pty) Ltd Subiaco, Australia*

ABSTRACT

AN ASSESSMENT OF THE USE OF HYDROGEOCHEMISTRY IN EXPLORATION FOR CALCRETE URANIUM IN AUSTRALIA

The role of hydrogeochemistry in exploration for calcrete uranium deposits in Australia is reviewed and the sampling and analytical procedures used are described. The concept of carnotite solubility index (CSI) is introduced and a simplified derivation,

CSI = log (U»V"K+) 1.13 x 10⁴(HC0³)²

is given for field use. The various interpretation schemes are reviewed and compared. On the basis of experience in Australia, the uranium content of the aquifer was found to provide a guide to the fertility of the system, and anomalous vanadium concentrations in the groundwater could be related to carnotite mineralization. Using the simplified CSI function, values of—3 to zero and upwards were found to be indicative of prospective drainages. It is concluded that water sampling surveys carried out in conjunction with shallow drilling programs make for the most efficient use of hydrogeochemistry in calcrete exploration.

1. INTRODUCTION

A review of the descriptions of calcrete uranium occurrences identified in Western Australia indicates that the majority give either an enhanced radiometric signature [1, 2] or a regional hydrogeochemical anomaly [3]. Thus, in the early years of calcrete uranium exploration, airborne and ground radiometric methods used in conjunction with regional hydrogeochemical surveys were the standard approach. However, as the more obvious outcropping carnotite mineralizations were discovered, reconnaissance radiometric prospecting has been replaced by hydrogeochemical surveys [3] and follow-up drilling in areas selected, for example, by Landsat image palaeodrainage interpretation [4].

Research by government agencies and industry on the genesis of calcrete uranium deposits was undertaken concurrently with exploration, and this resulted in the development of more refined hydrogeochemical evaluation techniques. These innovative approaches were used increasingly as the exploration target changed to buried calcrete uranium deposits.

An example of the more sophisticated hydrogeochemical approach is the method based on calculation of the Carnotite Solubility Index (CSI). The derivation and use of the CSI function was described first by Mann [5] and later again by Mann and Deutscher [6] and is discussed further, elsewhere in this volume [2, 7, 8].

2. SAMPLING AND ANALYTICAL PROCEDURE 2.1 Sampling

For regional hydrogeochemical surveys, any stock wells, stock bores or abandoned drillholes are sampled. Stock watering points are usually equipped with windmills and it is important that water samples be taken directly from the pump outlet pipe only after the line has been flushed when the pump is operating. For unequipped stock wells and standing water drillholes, water samples are obtained by a down-hole sampler. Sufficient water is collected to give two 250 ml samples which are filtered on site. The temperature, pH and conductivity of the sample are measured. One sample is acidified to pH 2 using 10 ml of 10 % nitric acid. Polyethylene screw-top bottles are used for sample storage.

There has been considerable discussion in the literature about the advantages and disadvantages of acidifying water samples for uranium analyses. Waters with high iron or aluminium ion contents will tend to flocculate on standing, even after filtering, so that uranium in the waters will be adsorbed by the flocculant. Acidification prevents this effect and also increases the reliability and precision of any repeat analyses required after a period of storage.

Both samples are then despatched to the laboratory for analysis. The acidified water sample is analysed for uranium, vanadium and potassium and the unacidified sample for bicarbonate, sulphate, and total dissolved solids (TDS). Every tenth sample, or at least one sample per batch, is duplicated to maintain quality control on the analyses.

*New address 74, Kenwick Rd. Kenwick, Western Australia

2.2 Analyses

Water samples are analysed for vanadium and uranium by XRF using a proprietary method (limit of detection for both elements is 5 ppb). For potassium, a standard atomic absorption spectrometry analysis (AAS) is used and classical methods are used for the determination of sulphate, bicarbonate and TDS.

3. DATA EVALUATION

The data are best presented by preparing regional distribution maps for each element, contoured according to sample population boundaries, and by calculation of carnotite solubility indices. Sample populations are identified from cumulative frequency graphs having ten equal log-normal classes [9] either by calculation [1 0] or estimation.

The Carnotite Solubility Index (CSI) is calculated for each sample point, using a simplified function derived as follows [11]:

The solubility index for carnotite is given by CSI = log [UOJ⁺] [H²V04] [Kl

[H+]2 1.41 x 1CT7

The total concentration of uranium in solution [U] is given by:

[U] = [UOi+] + [UDC] + [UTC]

By rearranging and substituting for the various U, V and carbonate terms, CS. = log _____________ M M ^ __________

(1 + KUDC [C0§-]2 + KUTC [C0§-]3) [H+]2 x 1.41 x 1CT7

where KUDC and KUTC are the equilibria constants for the uranyl dicarbonate complex (UDC) and the uranyl tricarbonate complex (UTC) respectively. For waters with apH below 8, the predominant complex present is UDC, so that KUOC[CO§~]2» KUTC [C0§~]2» 1 and the UTC equilibria reactions can be neglected. By substituting the HCOJ equilibria reactions, rearranging and converting molarities to ppm and ppb,

CSI = log

1.13 x 104[HC03]2

where uranium and vanadium concentrations are in ppb or jug/I and potassium and bicarbonate concentrations are in ppm or mg/l.

The CSI is a function of the uranium, vanadium and potassium interrelationships in carbonated groundwaters, and is a measure of the state of equilibrium between the groundwaters and carnotite mineralization, which may be present in the channel. When CSI is zero, the channel waters and carnotite within the same system are in equilibrium, and when CSI is negative, there will be a tendency for the carnotite to go into solution. When the CSI is positive, the solution is supersaturated with carnotite.

4. DISCUSSION

4.1 Interpretation of Hydrogeochemical Data

Interpretation of regional hydrogeochemical surveys is limited by the diversity of aquifers sampled, but can lead to a broad regional estimate of the amount of uranium in the hydrological system, i.e. the fertility of the system.

Most stock wells and bores are restricted to areas of low to medium salinity that are nonprospective for this type of mineralization, although some abandoned bores may be in suitable areas.

To be efficient, detailed hydrogeochemical surveys need to be undertaken in conjunction with reconnaissance drilling. This allows control both of the siting and spacing of samples of the aquifer sampled. In calcreteterrane this can be significant, as the calcretes are themselves often good aquifers and the composition of their waters will differ from those below the calcrete.

Although the fertility of an aquifer can be gauged by the uranium concentration in the water, the CSI is a better overall guide to its prospectivity [1 2]. The discovery of the Lake Raeside deposit [7] exemplifies the use of this technique and highlights the advantages to be gained from this approach. Nonetheless, the CSI cannot be used uncritically. The calculations assume ideal behaviour of ions in solution, but this is not the case for brines, and experiments by the CSIRO have shown that the measured CSI differs from the theoretical CSI by as much as— 2 (A.W. Mann, personal communication).

In hypersaline brines, carnotite tends to dissolve incongruently rather than form equilibrium concentrations of uranium, vanadium and potassium, furthermore, waters with a salinity of above 20 000 ppm inhibit the solution of uranium because of their low bicarbonate content.

Experimental errors resulting from field procedures such as on-site sample filtering also contribute to CSI variation from ideal behaviour through removal of bicarbonate from solution [8].

All the conditions mentioned above tend towards a lowering of the bicarbonate concentration in solution and therefore to more negative CSI values at equilibrium.

A fact seldom considered is that CSI calculations assume the vanadium content of a particular water to be in the pentavalent state. If this is not true and vanadium is present as V (IV), then the vanadium is not available for carnotite formation and the CSI calculation is meaningless.

Thus, calculated CSI values need to be used with caution in relation to previous experience and relative to CSI values in the same drainage. From experience, the lower threshold for uranium hydrogeochemical anomalies is 40 to 50 ppb U30g. It has been found that using the simplified CSI, prospective areas for calcrete uranium deposits were indicated by a CSI of —3 and upwards (towards positive values).

Generally, when the CSI tends to approach zero (from below the threshold of—3) downstream along the channel, it is indicative of an approaching site of carnotite deposition and a prospective area for calcrete uranium [5].

Within a prospective channel, significant variations in the CSI of the water usually take place within 1 km of the mineralization. Nonetheless, regional CSI variations indicative of mineralization within the channel waters may be defined by sample spacings up to 4 km apart along the channel axis. However, it must be emphasized that the main purpose of the CSI is to indicate prospective drainages rather than to pinpoint areas of carnotite mineralization. This is exemplified by the case study described below.

Research by Hostetler [in 12] on the carbonate-carnotite-gypsum precipitation sequence in maturing bicarbonate-rich waters, has documented the changes in concentration levels of the major components (U, V, K, Ca, bicarbonate and carbonate) that occur as precipitation proceeds within the hydrological system. Variations are compared against the potassium content of the water. Most carbonate precipitation occurs at 300 to 400 ppm potassium and, where sufficient uranium and vanadium are available, carnotite precipitation takes place in the range 700 to 800 ppm potassium. Carnotite precipitation tapers off until the potassium content reaches 1 000 ppm, when gypsum is precipitated.

As there is an apparent broad linear relationship between the potassium content, the salinity and the conductivity of a water, potassium content can be readily estimated in the field. The relationship varies regionally but can be established for a given area with a few orientation samples. Because waters of low to medium salinity are not considered prospective, conductivity field measurements can lead to reductions in analytical costs.

The potassium concentration equivalent to carnotite precipitation also may apply to the dissociation of carnotite.

However, as carnotite may dissolve incongruently, the channel waters can have enhanced vanadium contents.

Therefore, the presence of anomalous vanadium concentrations may be an important criterion forthe recognition of prospective calcrete channels [6].

When CSI function values forming a local data set are evaluated individually with reference to the potassium concentrations derived from Hostetler's precipitation sequence, the efficiency of the CSI approach is increased.

An example of this is described below.

4.2 Hydrogeochemical case study from Central Australia

From interpretation of Landsat imagery, it was noticed that an extensive valley system formed the source to an area where patchy carnotite mineralization within calcretes was previously known. It was considered that the valley may contain valley-fill uranium deposits.

The valley area was auger drilled on 4 x 1 km centres and all holes geologically and radiometrically logged. Where the water table was intersected, water samples were taken and the temperature, pH and conductivity of the water noted. A selection of water samples was analysed for uranium, vanadium, potassium, bicarbonate and total dissolved solids (TDS).

The data in Table 1 represent a traverse of drillholes within the calcreted valley, extending for 32 km downstream from the headwaters to the confluence with the mineralized channel. Samples WS777 and WS799 are taken from within the area containing calcrete uranium mineralization.

Although the uranium and vanadium concentrations are moderately enhanced within the valley, the potassium content is consistently less than 100 ppm and reflects the low salinity of the water. Even the sample point furthest downstream (MH24), which contains 95 ppb U3O8, has a potassium content far less than the carnotite precipitation value. The two samples from within the mineralized area are also low in potassium but others in the vicinity contain several thousand ppm potassium. Sample WS65, taken during an earlier survey within the mineralized channel, is an example. The influence of salinity both on the concentration of uranium in solution and the potassium concentration is exemplified by WS65. Although the uranium content is only marginally anomalous, the high vanadium concentration and the —1.90 CSI mark this sample as being derived from a mineralized channel. For further illustration, the results from WS56, sampled at the same time and in the same area as WS65, are also given. Although these samples from within the mineralized channel show a range of values for uranium, vanadium, and potassium, all CSI's were in the —3 to zero range.

It is suggested that the carnotite mineralization is the result of the mixing of uraniferous waters of low salinity and hypersaline ponded waters. Because carnotite equilibrium is approached only after the valley confluence with the

Table 1

mineralized area, it is considered that the valley area holds no potential for calcrete uranium deposits. This conclusion is reinforced by the geological characteristics of the valley-fill sediments.

The variation in results for the mineralized channel highlights the difficulties experienced when comparing hydrogeochemical results from different sampling programmes. The greatest variation is shown by potassium and this may be the result of one or several factors:

1. areal and/or vertical inhomogeneity within the aquifer, giving rise to compositional differences;

2. partial adsorption of potassium by clays;

3. variation in solution equilibria within the mineralized channel over the period between the two sampling programs;

4. variation in sampling practice such as difference in time elapsed between sampling and drilling of boreholes.

5. CONCLUSION

The most effective method of exploration in Australia is shallow auger drilling combined with geological and hydrogeochemical sampling and downhole radiometry within the calcreted channel. The fertility of a channel system can be gauged by the uranium content of the water, but a better overall guide to prospectivity is the Carnotite Solubility Index (CSI).

ACKNOWLEDGEMENT

This article is published by permission of Uranerz (Australia) (Pty) Ltd.

REFERENCES

[1 ] 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 FP16 (1977) 67.

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

[3] CAMERON, E., MAZZUCCHELLI, R.H., ROBINS, T.W., Yeelirrie calcrete uranium deposit, Murchison Region, W.A., In: Conceptual Models in Exploration Geochemistry Butt, C.R.M., Smith, R.E., (Ed.), 4, J.

Geochem. Explor. 2 (1 980) 350-353.

[4] KRISCHE, E.U., QUIEL, F., The application of digital analysis to Landsat imagery for locating surficial uranium deposits in Western Australia, this Volume.

[5] MANN, A.W., Nature and origin of uranium deposits in calcrete. In: Recent Geological Research Relating Low Temperature Aqueous Geochemistry to Weathering, Ore Genesis and Exploration, Glover, J.E., Groves, D.E.,(Eds.). Western Aust. Geol. Dept. and Extension Service, Publ. 1 (1980) 14-27.

[6] 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.

[7] GAMBLE, D.S., The Lake Raeside uranium deposit, this Volume.

[8] HAMBLETON-JONES, B.B., SMIT, M.C.B., Calculation of the carnotite solubility index, this Volume.