HINKLER WELL - CENTIPEDE URANIUM DEPOSITS

D. CRABB

Carpentaria Exploration Co.

Brisbane, Australia R. DUDLEY Esso Minerals Ltd Nedlands, Australia A.W. MANN

Division of Mineralogy, CSIRO Wembley, Australia

ABSTRACT

HINKLER WELL - CENTIPEDE URANIUM DEPOSITS

The H inkier Well — Centipede deposits are near the northeastern margin of the Archean Yilgarn Block on a drainage system entering Lake Way. Basement rocks are granitoids and greenstones. The rocks are deeply weathered and overlain by alluvism. Granitoids, the probable uranium source, currently contain up to 25 ppm uranium, in spite of the weathering. The host calcrete body is 33 km long and 2 km wide. Uranium up to 1000 ppm occurs in carnotite over a 15 km by 2.5 km area.

1. LOCATION

The Hinkler Well - Centipede drainage system is located near the northeastern margin of the Yilgarn Block, latitude 20°50' N, and extending from longitude 120 °E to longitude 120°20 °E. The drainage system enters the southwestern side of Lake Way.

2. GEOLOGY

Basement geology comprises granitoid and greenstone of the Archaean Yilgarn Block, with Proterozoic sandstone forming part of the catchment divide to the north. Outcrop is scarce, as the rocks are mostly deeply weathered, and often overlain by alluvium and colluvium.

Within the Hinkler Well — Centipede catchment, four major hydrogeological zones have been recognized [1].

These are granitoid, including alluvium and colluvium overlying granitoid basement greenstone, calcrete, and playa, including Lake Way and its margin. These zones are shown on the map (Figure 1).

Knowledge of the detailed mineralogy of the granitoid zone is limited. Information from neighbouring drainages and the Yilgarn Block in general suggests that the unweathered granitoid rocks would comprise quartz, microcline, oligoclase, orthoclase, albite, muscovite and biotite, and that the weathered profile in such areas would be principally kaolinite and quartz with minor amounts of montmorillonite, illite, hematite and goethite.

The granitoids of the Yilgarn Block are the probable source of uranium for carnotite formation, and granite samples from within the catchment show a large variation in uranium content, from less than 2 to 25 ppm.

However, it is worth stressing that the measured concentration of uranium in a granite is perhaps not an accurate guide to its past or future potential as a uranium source, because the weathering rate of uranium from a particular granite is probably a more important determinant of the amount of uranium mobilized.

The mineralogy of the greenstones and associated weathered material in this catchment area is not known in detail. By analogy with other greenstones in the vicinity, the unweathered ultramafics would comprise serpentine, olivine, talc, tremolite-chlorite, magnetite and chromite, and the unweathered amphibolites would contain plagioclase, amphiboles, chlorite, ilmenite and quartz, with some magnetite. Weathered material from these rocks would include hematite, goethite, magnesite, calcite, montmorillonite, kaolinite, saponite, quartz and chalcedony in varying proportions, dependent on the primary composition of the source rock.

3. GEOMORPHOLOGY

The calcrete hosting the mineralization is a body some 33 km long in the axis of a catchment of approximately 1 200 km2. It consists of an elongate calcrete body over 2 km wide in the west that narrows before broadening into a chemical delta where it enters Lake Way. Calcrete within the Hinkler Well - Centipede channel forms a positive relief feature of the order of 1 to 3 m in relation to the surrounding land surface. This topographic feature of the calcrete is mainly due to erosion and incision of the channel margins by the present-day surface runoff. The The original calcrete "profile top" is restricted to a few remnants of limited areal extent. The original calcrete surface before erosion is usually identifiable on the basis of its massive to blocky, very fine granular texture and dirty white colour, and by the presence of a sparse veneer of rounded quartz and other bedrock pebbles.

„...illgool -Outcamp Weif*-4 6

Hmkler Well ^TuMock, Well

Tony Borei..4.?---:?-8»',

, To

**\ Leonora [ [ Alluvium, colluvium

[ j Calcrete

^l Weathered granite Greenstone

Mineralization (carnotite) Defined drainage channel -^>-Drainage divide —•- — Road track ---Well, bore drill hole • Solubility index -3 4

Figure J

Hydrogeology, water sampling locations and solubility index data for the Hinkler Well— Centipede catchment system.

PROCESSES 1 Precipitation

2 Infiltration, COa equilibrium 3 Weathering

4 Lateral groundwater movement 5 Evaporation evapo-transpiration 6 Carbonate silicate precipitation 7 Playa-zone evaporation, precipitation

y**\ Granite [""'j Greenstone [ Atluvium/colluvium [' ] Calcrete

F^D Mineralization (carnotite)

10km

Figure 2

Idealized section along the drainage mid-line of the Hinkler Well — Centipede catchment system.

4. HOST ROCK

The mineralogy of the elongate calcrete body and the calcrete of the chemical delta differ significantly. The elongate calcrete is principally calcite with chalcedony and minor amounts of dolomite, gypsum and sepiolite. It is underlain and sometimes interspersed with kaolinitic and/or smectite-rich clay and in some cases by latérite (hematite and goethite). There is a small area of carbonated weathered granite outcropping and immediately beneath, mineralized calcrete near H inkier Well. The calcrete of the chemical delta is characterized, in general, by increased proportions of dolomite and sepiolite and by the presence of some aragonite [2].

5. MINERALIZATION

Uranium mineralization, both in an elongate calcrete unit and in the chemical delta section of the drainage system, occurs exclusively as the mineral carnotite.

In the elongate calcrete unit, carnotite, generally as coatings on vugs and fructure surfaces, is distributed in discontinuous sheets or lenses at, above and below the present water table. This uranium mineralization, which at many localities averages less than 10 ppm uranium in grade, occurs over a 15 x 1.5 to 2.5 km area. Some very isolated lenses of mineralization up to 1 000 ppm uranium have been located between Hinkler and Dawson Well (Figure 2).

When mineralized the massive, dark-coloured, dolomitic calcrete of the chemical delta area often contains fine-grained, disseminated carnotite throughout the carbonate matrix. Carnotite also occurs as coatings on fracture surfaces and on the walls of small cavities. Three distinct pods of ore-grade mineralization have been outlined in the chemical delta. (Economic parameters have been used to define edges of pods and these edges are not natural limits of mineralization, which is erratically distributed between all three pods.) The mineralized pods are hosted by soft grey to black manganiferous calcrete, covered by varying amounts of overburden — from near zero to up to 6 m of windblown sands and silts. The thickness of the mineralized zones varies from one to five metres, with the bulk of the mineralization concentrated near or just below the present-day water table; grades are variable, with concentrations of up to 2 000 ppm uranium. The soft grey-black calcrete is transitional into comparatively harder calcrete, typical of the elongate calcrete unit, on the western edge or upstream side of the mineralized pods. Grade and thickness of mineralization fall off rapidly in this transition zone.

6. RADIOMETRICS

Surface traverses with hand-held scintillometers revealed several radiometric spot anomalies, as well as more widely distributed anomalies, generally elongated parallel to the drainage system. The majority of radiometric highs occur over exposed, weakly mineralized calcrete, or in areas of less than one metre of overburden over strongly mineralized calcrete.

Maximum readings with down-hole gamma-ray logging instruments always correlated well with the peak concentration of uranium in drill holes, which varied from 2 to 6 m below surface, depending on the overburden thickness. Similarly, correlation between surface and drill hole radiometric anomalies was observed to be fairly good in locations with shallow (less than 1 m) overburden; but as overburden thickness increased, strong radiometric anomalies observed in drill holes were not detectable on the surface. Very few detectable radiometric anomalies were recorded below an average depth of 10 m throughout both the elongate calcrete zone and chemical delta zone. In a study of disequilibrium in the mineralized Chemical Delta area, Dickson [5, 6] found that the mean disequilibrium ratio (UgOg/eUaOs) for 311 mineralized samples to be 1.04. The only significant variation in the disequilibrium ratio occurs with depth, when samples from above the water table tended to have a ratio greater than 1, while those from below the water table tended to have a ratio of less than 1.

A large number of isolated radon anomalies of low magnitude were observed in addition to several elongate anomalies coincident with the calcrete. These radon anomalies, together with the anomalous radiometrics, extend for nearly the full length of the calcrete from west of Dawson Well to Lake Way. Radon content of the groundwater was generally less than 10emans.

7. HYDROGEOCHEMISTRY

Potassium concentrations range from 10 to 20 ppm on the western edge of the catchment to values of over 3 000 ppm near Lake Way; the potassium concentration increases rapidly near the lake margin. Uranium concentrations range from 10 to 30 ppb near the granitoid breakaways to 440 ppb at Justit bore at the western end of the calcrete. On the calcrete, from Justit bore to Lake Way, uranium concentrations are in the range of 50 to 200 ppb and there is no increase in uranium concentration concomitant with increasing salinity near Lake Way [2]. The uranium concentrations are highest in groundwaters on the southwestern side of the catchment, but there appears to be no large uranium contribution from the Proterozoic rocks along its northern boundary.

Vanadium concentration in groundwater does not exceed 20 ppb, except in the calcrete. In the upstream sections of the calcrete, the vanadium concentrations are between 20 and 60 ppb, increasing to higher values (up to 180 ppb) near the lake margin.

From the groundwater analysis results for potassium, uranium and vanadium, a simple solubility index reflecting the saturation status of the groundwater with respect to carnotite has been calculated for each sampling point (Figure 1).

Present-day groundwaters in most parts of the representative drainage are seen to be undersaturated with respect to carnotite (index negative), even near some areas of the calcrete known to be mineralized, for example between the Dawson a,pd Hinkler wells. This implies that the carnotite in this area, if it is in contact with groundwater, should dissolve. High vanadium concentrations in the groundwater are a feature of this area.

Toward the margin of Lake Way, the solubility indices increase, and one of the holes on the edge of Lake Way (DH 3) has a positive value implying supersaturation of the groundwater with respect to carnotite.

This is in contradiction to the age of >106 years suggested by the apparent equilibrium of the Centipede deposit from radiometric data [5]. Dickson [6], however, suggests the likelihood of differential mobility of uranium and its daughter products and has proposed a model to accommodate this. It should also be pointed out that a unique

"average age for carnotite deposits may be misleading, since mineralization may have occurred over an extended period of time.

REFERENCES

[1] MANN, A.W., DEUTSCHER, R.L., Hydrogeochemistry of a calcrete containing aquifer near Lake Way, Western Australia, J. Hydrol. 38 (1978) 357-377.

[2] 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.

[3] CARPENTARIA EXPLORATION COMPANY (PTY) LTD., Final Report Hinkler Well Prospect (1972).

[4] ESSO AUSTRALIA, LTD. Minerals Department, Company Reports and maps of the Centipede Uranium Deposit (1982).

[5] DICKSON, B.L, CSIRO, Institute of Earth Resources, Division of Mineral Physics, Restricted Report 1048R(1980).

[6] DICKSON, B.L., Uranium series disequilibrium in the carnotite deposits of Western Australia, this Volume.