Uranium ore-forming systems of the - Geoscience Australia

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Uranium ore-forming systems of the Lake Frome regionfrom silt to gravel with small cobbles in places, and was deposited by braided streams (Alley andBenbow, 1995). In the Callabonna Sub-basin, the Eyre Formation incorporates the MurnpeowieFormation, which contains clay, lignite and basal gravels.A network of paleochannels filled by Eyre Formation sediments is recognised in the Lake Fromeregion, incising the underlying Cretaceous units of the Eromanga Basin or directly intoProterozoic basement in places. These paleochannels are important as hosts to many of theuranium deposits in the region, including the Honeymoon, Oban and Goulds Dam deposits (Curtiset al., 1990). In the southern part of the Callabonna Sub-basin the paleochannels have a generalnortherly trend and indicate sediment transport from south to north. Although the Eyre Formationhosts the Four Mile deposit (Alliance Resources, 2009, www.allianceresources.com.au) it is notclear whether a paleochannel is present in this area.There is little evidence for further sedimentation in the Lake Eyre Basin until the late Oligocenewhen widespread gentle folding and uplift rejuvenated the Birdsville Track Ridge some 200 km tothe north of the Callabonna Sub-basin. This activity split the Lake Eyre Basin into the Tirari andCallabonna Sub-basins and revived sedimentation which continued to the Pliocene. The newsediments comprise the Namba Formation which disconformably overlies the Eyre Formation.The Namba Formation is equivalent to the Etadunna Formation and this name is commonly usedin the historical drill hole logs from the Lake Frome region, though the Etadunna Formation isnow restricted to the Tirari Sub-basin.The Namba Formation sediments are described by Alley and Benbow (1995) as grey, green andwhite clays, fine-grained sand and carbonate, and minor conglomerate. The formation wasdeposited in a fluvial lacustrine environment with meandering streams and billabongs. Relief inthe region is believed to have been low during deposition of the Namba Formation. The NambaFormation averages 90m thickness, thinning over the Benagerie Ridge and thickening up to 170mtowards the Barrier and Flinders Ranges. However, in detail some areas show the oppositethickening trends, for example in the EL5/6 area to the east of Lake Frome (see Chapter 4 and Fig.4.3). Drill hole logs from the Lake Frome region typically record unconsolidated, very fine- tofine-grained angular sands, black carbonaceous clay, green and grey clays, dolomitic mud and, inplaces, lignite. At the Beverley deposit the Namba Formation comprises three members: a lowerinterval known as the Alpha Mudstone which contains plant fragments and carbonised wood; theoverlying Beverley Sands (grey silt with sandstone lenses) that host uranium mineralisation, andthe overlying Beverley Clay (Curtis et al., 1990). Sinuous sand-filled depressions in the uppersurface of the Alpha Mudstone host much of the uranium mineralisation, and are interpreted torepresent parts of a broadly north-trending paleochannel system (McConachy et al., 2006).2.2.4 QuaternaryQuaternary processes have created the present-day fans, claypans, playas, dunes and ephemeralstreams of the Callabonna Sub-basin. The oldest Quaternary unit, the Willawortina Formation,formed as piedmont fans around the Flinders and Olary Ranges, with sedimentation commencingin the late Cenozoic and continuing until the Plio-Pleistocene. The Willawortina Formationcomprises fine and coarse facies (Callen et al., 1995). The fine facies is an upwards fining,cyclical sequence of sandy mud and silty dolomite, with distal fan, mudflow and playa-lacustrinecomplexes represented by outcrops of green clay and thick, calcareous paleosols (Callen et al.,1995). The coarse facies, represented in drill holes around the Beverley uranium deposit,comprises braided fans of coarse, framework-supported gravels as well as poorly sorted, matrixsupporteddebris flows (Callen et al., 1995).The Coomb Spring and Millyera Formations were deposited from the mid to late Pleistocene(Callen and Benbow, 1995). The beach deposits of the Coomb Spring Formation occur aroundLake Frome and consist of white to yellow, often coarse-grained, well-sorted sand (Callen andBenbow, 1995). Present-day topographic contours suggest that it was deposited when lake levelswere higher and Lakes Frome and Eyre were possibly joined (Callen and Benbow, 1995). ThesePage 17 of 151
Uranium ore-forming systems of the Lake Frome regionCoomb Spring Formation intertongues with the flood-plain and lacustrine deposits of the MillyeraFormation, which consists of greenish clay, thin algal limestone and fine-grained white to greenishsand (Callen and Benbow, 1995).Late Pleistocene sedimentation also resulted in the deposition of the Eurinilla Formation, found inthe northern Callabonna Sub-basin and near Lake Frome. It also intertongues with the MillyeraFormation and was party deposited in paleochannels formed within the Willawortina Formation(Callen and Benbow, 1995). The Eurinilla Formation consists of bright red-brown to yellowbrownsand and gravel of intermittent streams and their overbank deposits (Callen and Benbow,1995).The youngest unit in the Callabonna Sub-basin is the late Pleistocene to Holocene CoonarbineFormation, consisting of aeolian sands of red-coated, frosted quartz grains, yellow to orangegrains, clay pellets and gypsum flakes with aeolian cross-bedding (Callen and Benbow, 1995).The sands form a striking pattern of longitudinal dunes, with wind-eroded claypans alignedobliquely. The dunes are typically up to 15m high (Callen and Benbow, 1995) and trendnorthwards in the northern part of the study area. With Lake Frome as an axis, their orientationprogressively rotates to an east-north-easterly trend in the southern part of the sub-basin (Randell,1973; Callen and Benbow, 1995). Older dunes contain calcareous paleosols and a greaterproportion of clay compared to the younger phases of dune formation (Callen and Benbow, 1995).Large transverse dunes also formed along the downwind margins of playas and claypans and onthe bed of Lake Frome. Gypsum dunes are built upon clay dunes indicating deflation of the lakefloor clays followed by exposure and deflation of the groundwater gypsum horizon (Callen andBenbow, 1995).Table 2.1: Quaternary units in the Callabonna Sub-basin. After Callen and Benbow (1995).Formation Age Maximum Thickness (m) Depositional EnvironmentCoonarbine Formation Pleistocene-Holocene 50 DunefieldEurinilla Formation Late Pleistocene 10 AlluvialCoomb Spring Formation Mid-Late Pleistocene 8 Lacustrine ShorelineMillyera Formation Mid-Late Pleistocene 9 Playa-lacustrineWillawortina Formation Plio-Pleistocene 100 Alluvial and lacustrinePage 18 of 151
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