Regional exploration for channel and playa uranium deposits in Western Australia using groundwater
Highlights
► Carnotite saturation in groundwater successfully targets secondary U mineralisation. ► Regional 5 km sample spacing is effective. ► Separation of channel and upland groundwater types improved targeting and background characterisation. ► Surface flow and erosion is important for determining prospective terrane. ► Primary sources of secondary deposits can also be inferred from groundwater data.
Introduction
Uranium is important for energy supplies, and further discoveries are critical for the predicted increase in future energy demand (Macfarlane and Miller, 2007). Previously, regional radiometric surveys have been successful at targeting surficial deposits, but a few metres of sediment cover (>2 m) may block this signal. Groundwater chemistry can be successfully used as a regional targeting tool in these terrains (Cameron et al., 1980, Gamble, 1984, Mann and Deutscher, 1978) and was widely employed by exploration companies in the 1970s and 1980s. More systematic regional exploration will be necessary, however, if more near-surface U ore deposits are to be found, particularly in areas of covered terrain such as in Australia.
Surficial U deposits are hosted in calcrete and associated sediments in palaeodrainage channels and playas in the northern Yilgarn Craton in Western Australia (Butt et al., 1977, Cameron, 1990). The largest known deposit is Yeelirrie (52,500 t U3O8 at 0.15% grade), which has higher reserves than all other known deposits in the region combined, at higher grades and higher cut-off values (Table 1). These deposits are shallow and potentially relatively easy to mine, hence there has been renewed interest in exploration for this type of deposit.
Groundwater interacts with mineralised rocks and may display a geochemical signature with a much greater size than other sampling media (Cameron, 1978, Giblin and Dickson, 1992, Gray and Noble, 2006a, Leybourne and Cameron, 2006, Taufen, 1997). These groundwater geochemical anomalies may reduce the required drilling density needed to explore regionally, an important consideration for cost-effective exploration in covered terrains. Uranium is generally soluble in groundwater and surface water and has shown geochemical haloes around U mineralisation (Mann and Deutscher, 1978, Cameron, 1980, Langmuir and Chatham, 1980, Rose and Wright, 1980, Peuraniemi and Aario, 1991).
Other hydrogeochemical investigations for U exploration have looked at various styles of deposits including sandstone-hosted deposits in the USA. (Langmuir and Chatham, 1980, Rose and Wright, 1980), glacial till/peat in Finland (Peuraniemi and Aario, 1991) and unconformity deposits in Canada (Cameron, 1980). Australian secondary deposits in calcrete have also been subject to hydrogeochemical studies. Mann and Deutscher (1978) provided a comprehensive study of a single catchment hosting the Dawson Well–Hinkler Well–Centipede deposits, adjacent to the Yeelirrie catchment, Heath et al. (1984) studied the Lake Austin deposit, Gamble (1984) studied groundwater near the Lake Raeside deposits and, Cameron et al. (1980) and Cameron (1984) published a summary of water chemistry from a large region sampled around Yeelirrie as part of exploration by Western Mining Corporation. Most showed some degree of success, particularly with oxidised systems. Sample spacing in these studies varied from tens to hundreds of metres (Langmuir and Chatham, 1980, Peuraniemi and Aario, 1991, Rose and Wright, 1980) to a few kilometres (Cameron, 1980, Heath et al., 1984, Langmuir and Chatham, 1980) up to 10 km (Cameron et al., 1980, Mann and Deutscher, 1978, Rose and Wright, 1980).
This paper reports a regional groundwater survey with ∼1400 samples covering ∼92,000 km2 in the northern Yilgarn Craton. This study expands on previous studies, with improved detection limits, more consistent sample spacing, data filtering and the use of palaeodrainage modelling to determine the effectiveness of hydrogeochemistry to detect the major channel and playa U deposits.
Section snippets
Location and climate
The northern Yilgarn region is semi-arid to arid with hot, dry summers and cool winters, with low, irregular rainfall between 200 and 300 mm per annum (Bureau of Meteorology, 2010). The sparse vegetation consists largely of mulga (Acacia aneura), and drought-resistant shrubs and grasses. Halophytic shrubs are located on the fringe of the playas. The study area, including, towns, water catchments, playas and sample locations is shown in Fig. 1. The northern Yilgarn is separated from the south by
Sampling
Groundwater samples were collected from 1419 wells and bores at a spacing of 5 km or greater (Fig. 1). Samples were collected directly from flowing windmills or bailed from 5 to 10 m below the water table in open bores or wells. The proportion of bailed to flowing samples was approximately equal. Full field collection details are reported in Noble and Gray (2010).
Analysis
Electrical conductivity (EC), pH and Eh and were measured in the field. Water table depth and bore/well type were recorded. Samples for
Water chemistry
Uranium concentrations in the northern Yilgarn range from 1 to 700 μg/L with a mean concentration of 14 μg/L (Table 3). Dissolved U alone is an effective targeting element, producing coherent anomalies near known deposits using the ∼5 km sample spacing (Fig. 5). These results confirm the earlier findings of Cameron et al. (1980) and Mann and Deutscher (1978) that indicated hydrogeochemistry can be used effectively to target U ore bodies in the Yilgarn Craton.
In particular, the greatest U
Conclusions
This study has demonstrated that hydrogeochemistry and particularly the calculation of carnotite saturation in groundwater, using farm bores and windmills at the ∼5 km scale is effective for exploration for palaeochannel U deposits in the northern Yilgarn Craton. The research expanded earlier single catchment studies to show that the method can be applied successfully on a very large scale over multiple catchments and varied groundwater conditions; the study extends over nearly 1/4 of the
Acknowledgements
The authors thank their colleagues C. Butt, M. Lintern and J. Walshe for early draft review and comments, two anonymous manuscript reviewers, and T. Naughton and A. Vartesi for figure drafting. This project was supported by CSIRO Minerals Down Under National Research Flagship and by MERIWA. The authors would like to thank many people who contributed to the collection of these data: M. Pirlo, T. Ainsworth, S. Corbel, M. Lintern, T. Naughton, W. Gorczyk, P. Golodoniuc, S. van der Wielen, A.
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