Uranium ore-forming systems of the - Geoscience Australia

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Uranium ore-forming systems of the Lake Frome region5. Uranium deposits of the Lake Frome regionSubhash JairethPost-Paleozoic basins of the Lake Frome region host four known uranium deposits and manyoccurrences and prospects including the recently discovered Beverley North (Pepegoona) prospect(Heathgate Resources, announcement September 2009). The Honeymoon and Oban deposits inthe southern part of the region and Four Mile deposit in the northwest are hosted by organic-richsands of the Paleogene Eyre Formation, whereas Beverley is hosted by organic-rich permeablesand lenses (Beverley Sands) of the Neogene Namba Formation. Beverley is the only uraniumdeposit known to be hosted by the Namba Formation. Most deposits in the southern part of theLake Frome region are located within mapped fluviatile paleochannels in the Eyre Formation(such as Yarramba, Lake Namba, Billeroo paleochannels, Fig. 4.11). Paleochannels in theBeverley and Four Mile area have been poorly defined, although available data suggest that atleast the Beverley deposit is associated with a paleochannel in the Namba Formation (McConachyet al., 2006).Both the Four Mile and Beverley deposits are located within 20 kms of uranium-rich rocksoutcropping in the Mount Painter Inlier, whereas the Honeymoon and Oban deposits in the southoccur within 50 km of uranium-rich Proterozoic basement of the Curnamona Province.5.1 BEVERLEY URANIUM DEPOSITThe Beverley uranium deposit is located ~15 km east of the outcropping Mount Painter Inlier. Itwas the first sandstone uranium deposit to be discovered in the area, and contains a total resource(production + remaining resource) of ~6 million tonnes of ore at an average grade of 0.23 % U 3 O 8which is equivalent of 16 300 tonnes of U 3 O 8 (Geoscience Australia, 2009). Since 2000 it hasbeen mined by the in-situ leaching technique. Summary descriptions of the Beverley deposit havebeen published by Brunt (2005), McConachy et al. (2006), Wülser (2005), Marsland-Smith(2005), Curtis et al. (1990), Haynes (1975), and by Heathgate Resources in various on-linereports. Wülser (2009) recently completed a doctoral thesis on the Beverley deposit. Thefollowing descriptions are based on these publications.5.1.1 Host rocksUranium mineralisation occurs in the Beverley Clay, Beverley Sands and Alpha Mudstone units ofthe Namba Formation (late Oligocene to Miocene), although the bulk of mineralisation is locatedin the lower part of the Beverley Sands close to the contact with the Alpha Mudstone (Brunt,2005; McConachy et al., 2006). Mineralised sands are thus sealed from above and below byrelatively impermeable clays units. Provenance studies indicate that both the Beverley Sands andBeverley Clays were derived from the reworking of Early Cretaceous glacio-lacustrine sediments(Wülser, 2009).The Namba Formation is overlain by unconsolidated sands, gravels and clays of the WillwortinaFormation (Pliocene to Quaternary) which is sourced from the Mount Painter Inlier. Within themine area the ore-bearing Namba Formation is unconformably underlain by the CretaceousBulldog Shale (early Aptian to mid Albian) which in turn overlies a fine-grained sandy sequenceof the Cadna-owie Formation (mid-Neocomian to early Aptian). The sediments of Cadna-owieFormation and the equivalent Parabarana Sandstone were deposited in fluviatile conditions.Sands of the Eyre Formation which underlies the ore-bearing Namba Formation is intersected indrill holes outside the Beverley deposit. They are composed dominantly of carbonaceous andPage 57 of 151
Uranium ore-forming systems of the Lake Frome regionpyritic sands, intercalated with clays and gravels interpreted to be deposited in braided riverchannels (Wülser, 2009).5.1.2 Geometry of paleochannelsMineralisation at Beverley is interpreted to be hosted by sands which infill Miocene paleochannelsincised into the underlying lacustrine Alpha Mudstone (Fig. 5.1, Haynes, 1975; Curtis et al.,1990). The paleochannel and the associated NE trending half graben is controlled by the Paralana-Wertaloona fault system (Curtis et al., 1990). According to Curtis et al. (1990) an oxidationinterface with anomalous uranium is traceable within the Namba Formation along a NE trendingzone measuring 25 x 15 km. Interpretation by Heathgate Resources shows a system of branchingpaleochannels (Fig 5.1). Three-dimensional modelling of the underlying Alpha Mudstone surfacebased on drilling reveals several paleo-highs and lows with the depressions interpreted as channels(McConachy et al., 2006). The N-S drainage system is complicated by several cross-cuttingstructures (McConachy et al., 2006). The shape and orientation of paleochannels are importantfactors which determine possible provenance of infill-sediments, fluid-flow direction at the timeof mineralisation and distribution of in-situ organic reductants in the sediments. In Figure 4.1 thearrows show the directions of present day groundwater flow. However, the geometry ofpaleochannel branching suggests that sediment transport was originally from the south.Figure 5.1: Map of Beverley uranium deposit (resources outlined) and inferred paleochannels (yellow)within the Namba Formation. Source: Heathgate Resources 1998).Page 58 of 151
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