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Location: 71 km SW from Wiluna, Western Australia, Australia
Lvl 3 1060 Hay StPO Box 1395, 6872PerthWestern Australia, Australia6005
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A deeply incised palaeodrainage system traverses the region, including a palaeochannel system traversing the length of the Yeelirrie catchment, which forms part of the Carey Palaeodrainage. The Yeelirrie palaeochannel consists of fractured rock, palaeochannel sand and alluvium that has washed down from the top and sides of the catchment basin, as well as surficial calcrete bodies that have formed since in the central portion of the valley. It is within these calcrete bodies that the uranium mineralisation is localised.The Yeelirrie uranium deposit is the largest known uranium deposit in Western Australia. It occurs in calcrete hosted material in the central drainage channel of a wide, flat and long valley which is flanked by granitic breakaways of low topographic relief with elevations between 490 m AHD and 610 m AHD.The mineralisation extends from the surface to an approximate depth of 10 m, with the main concentration centred about 4 m below the surface, with a thickness ranging from 1 to 7 m. The surface extent of the identified resource is 9 km long and an average of 1 km wide, with a maximum width of about 1.5 km. Lithological and mineralogical studies conducted in the Yeelirrie valley (WMC 1975, and studies conducted for this PER) show that there are four principal lithological units at the proposed development site:1. Overburden: consisting of a combination of sandy loam, siliceous and ferruginous cemented hard- pan and carbonated loam, which is probably a weathered calcrete.2. Calcrete: a calcite and/or dolomite replacement of the clay-quartz, although relics of partially replaced clay-quartz are common throughout the calcrete. The upper portion of the calcrete comprises friable ‘earthy calcrete’, a continuous layer grading upwards into the overlying soils. Nodular porcellanous calcrete represents the lower layer of the calcrete and consists of up to 70% carbonate (McKay and Miezitis 2001).3. Clay-quartz: a kaolinitic clay-quartz alluvial fill material. Bands of quartz grit and arkose are randomly scattered through the clay-quartz as horizontal beds. Upper clays are predominantly montmerillonite, with kaolinite becoming more abundant at depth.4. Archaean granitic basement complex: generally seen in drill holes at depths of around 30 m below the surface near the ore body. Uranium mineralisation occurs as carnotite, a potassium uranyl vanadate (K2(UO2)2(VO4)2.3H20), which is found in the overburden and clay quartz unit. However, mineralisation is richest within the calcrete and transitional calcrete material. It typically fills fractures and voids, occurring as a coating on surfaces and as a very fine-grained dispersion through the mineralised units. Although found throughout the ‘earthy calcrete’ and the nodular porcellanous calcrete, approximately 90% of the ore is in the clay-rich carbonated rocks of the transition zone at the base of the calcrete unit.Uranium mineralisation in the (mainly) calcrete is related to groundwater levels and chemistry. Key processes involved in the precipitation of uranium mineralisation can be summarised as:• oxidation of mildly reducing uranium, potassium and vanadium-bearing waters, either by direct contact with air, or by mixing with more oxidised surface water; and• evaporation concentration of water during drier climate cycles or along the flow path towards salt lakes. The majority of the uranium mineralisation occurs beneath the water table due to the leaching of uranium by carbon dioxide in rainfall infiltrating from the surface.The majority of the uranium mineralisation occurs beneath the water table due to the leaching of uranium by carbon dioxide in rainfall infiltrating from the surface.
