Uranium ore-forming systems of the - Geoscience Australia

Uranium ore-forming systems of the Lake Frome region2. Uranium sources: lithology and stratigraphyAllison Britt, Roger Skirrow, Simon van der Wielen2.1 LITHOSTRATIGRAPHY OF THE PROTEROZOIC BASEMENTCambrian to Cenozoic basins of the Lake Frome region overlie Proterozoic basement of unusuallyuranium-rich character. These basins are bounded to the south by Paleoproterozoicmetasedimentary and metaigneous rocks of the Willyama Supergroup, deposited up to ~1640 Maand metamorphosed during the Olarian Orogeny at ~1600 Ma. This sequence forms a major partof the Curnamona Province. Whereas the metasedimentary strata of the Willyama Supergroup arenot unusually endowed with uranium, several granitoid suites emplaced between ~1730 and ~1580Ma have elevated uranium contents. These include alkaline I-type granitoid rocks in the CrockersWell area that are associated with a number of small uranium deposits and occurrences. Thesegranitoids intruded metamorphic rocks of the Willyama Supergroup at ~1580 Ma (Ludwig andCooper, 1984) and have isotopic compositions suggesting involvement of possibly anomalous,metasomatised, mantle (Rutherford et al., 2007). The Radium Hill uranium-radium deposit in thesouthern Curnamona Province (McKay and Miezitis, 2001) also appears to be associated with synorpost-orogenic intrusive rocks.The north-south Benagerie Ridge occupies the central part of the Curnamona Province and isentirely concealed by Cambrian and younger sedimentary basins. The geology is poorlyunderstood but is known to include the early Mesoproterozoic Benagerie Volcanics, a flat lyingsequence of unmetamorphosed dominantly felsic volcanic rocks compositionally similar to theGawler Range Volcanics in the Gawler Craton to the west. A number of small but significant ironoxide copper-gold (IOCG) deposits are known from the Benagerie Ridge and southern CurnamonaProvince, including the Portia and Kalkaroo deposits. The North Portia deposit contains uraninite(Teale and Fanning, 2000), whereas deposits further south appear to be less uraniferous (Williamsand Skirrow, 2000).The Mt Painter and Mt Babbage Inliers situated along the northeastern margin of the CurnamonaProvince comprise early Mesoproterozoic metasedimentary and metaigneous rocks with acomplex tectonic history. The effects of the Cambro-Ordovician Delamerian Orogeny werelocally intense, with granite intrusion and metamorphism up to amphibolite facies.The Mt Painter and Babbage Inliers include granites with some of the highest uranium contents ofany igneous rocks in Australia. For example, the early Mesoproterozoic Yerila Granite in theBabbage Inlier contains an average of 116 ppm uranium (Neumann et al., 2000). Numerousuranium prospects are present in the Mt Painter Inlier, containing uraninite, torbernite, autuniteand other uranium minerals (Drexel and Major, 1990; McKay and Miezitis, 2001). At the Mt Geeprospect breccia-hosted uranium mineralisation with anomalous REE contents and raremolybdenite occurs in the hematite-chlorite-rich matrix between granitic clasts. The breccias areunmetamorphosed, and may have formed in the Late Ordovician to Silurian (Drexel and Major,1990; McKay and Miezitis, 2001; Elburg et al., 2003), or Permo-Carboniferous (Idnurm andHeinrich, 1993).The Mt Painter and Babbage Inliers and southern Curnamona Province represent highlyuraniferous crust, in the form of both magmatic and hydrothermal enrichments. These arepotentially excellent sources of uranium for basin-related uranium mineral systems, whetherdirectly via leaching or indirectly via erosion of uranium-rich detritus into flanking basins.However, the relative importance of the various potential sources depends partly upon themineralogy of the uranium-bearing phases and the chemical stability of these minerals (seeSkirrow et al., 2009, and references therein). Minerals that are relatively soluble in oxidisedfluids, such as uraninite, offer a good source, whereas refractory uranium-bearing phases such asbrannerite (e.g., Crocker Well), davidite (e.g., Mt Victoria) or zircon are unlikely to represent aviable source unless the minerals are metamict (Cuney and Kyser, 2008) or partially weathered.Page 12 of 151