The Mary Kathleen uranium deposit is located in the Eastern Fold Belt of the Mount Isa Block of North West Queensland, Australia, approximately 60 km east of Mt Isa and 50 km west of Cloncurry. The Mary Kathleen deposit lies within the fault bounded, 5 to 20 km wide and 150 km long Mary Kathleen Zone, which adjoins the Kalkadoon-Leichhardt Belt to the west, which forms the old core of the Mt Isa Block, and is followed to the east by the Quamby-Malbon Zone.
The main sequence within the Mary Kathleen Zone comprises the Mary Kathleen Group, overlain by the Mount Albert Group, both of which belong to Cover Sequence 2, which are intruded by the regional 1740 to 1720 Ma Wonga Granite. The >3000 m thick Mary Kathleen Group is composed of sandstone, siltstone and shale, which are commonly calcareous and/or evaporitic and/or carbonaceous, with greywacke, limestone, chert, calcareous breccia, minor felsic and mafic volcanics, banded iron formation and conglomerates. It includes the Corella Formation in the lower section of the group, which is up to 1200 m thick and composed of mainly thinly bedded calcareous and commonly evaporitic sediments. This unit overlies the 1250 m thick Ballara Quartzite sandstone, conglomerate, minor carbonates and felsic volcanics. The Mount Albert Group which overlies the Mary Kathleen Group is composed of >3000 m of sandstone, siltstone, dolomite, shale limestone and conglomerate.
Intrusive rocks, skarn formation and ore were generated in a number of discrete events spanning a period of 500 Myrs Uranium ore was hosted by garnetiferous calc-silicates (garnetites) within the micaceous schist, quartzite, scapolite-diopside granulites and marbles of the Corella Formation, representing an altered metasedimentary sequence which is intruded by felsic and mafic bodies with 1730-40 Ma ages. The orebody has been interpretted as an epigenetic skarn related to the intrusion of the nearby granite and rhyolite dykes of the 1740-1720 Ma Wonga Granite.
In more detail, the sequence in the deposit area comprises, from east to west: fine grained pyritic quartzites with interbedded calc-silicate granulites; thinly bedded feldspathic and siliceous granulites; scapolite granulite with patches of garnet; eastern "garnetite" zone which forms a nose to the south and broadens substantially on an apparent western limb where it hosts the orebody, and is composed mainly of garnet replacement of scapolite granulite; scapolite-diopside-hornblende granulite; scapolite-bearing diorite or meta-gabbro; a 6 m thick bed of cobble breccia-conglomerate; partly brecciated scapolite-diopside granulite; cobble to breccia-conglomerate; scapolite-diopside granulite; banded amphibolite; impure marble with scapolite-calc silicate granulite with discrete bands of mica schist.
The orebody comprised a series of elongate lensoid ore shoots which roughly paralleled the margins of the broader garnetite envelope. In section the ore dipped at 30-50° in its upper section to vertical down dip with a total dip extent of 450 m. Lenses of ore were developed over a "stratigraphic" interval of around 100 m and outcropped over a strike extent of around 200 m with a northerly plunge. The uranium mineralisation occurs as uraninite which is present as fine disseminations enclosed within allanite, which usually replaces garnet.
The total mined reserve amounted to 9.483 Mt @ 0.131% U3O8, for 12 425 t of U3O8.
The rare earth oxides occur in allanite (orthite) and stillwellite (La boro-silicate), the main uranium bearing component of the orebody. Yttrium occurs within the garnet lattice and is essentially non-recoverable. Allanite comprised approximately 35% of the ore in outcrop. The REE is broadly distributed with the uranium, but is not a straight line correlation. Ore with 0.05% U3O8 generally contains around 1.8% total REE, while 0.5% U3O8 carries 9% REE.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 1990)