es breccia

Breccia complex deposits of uranium are exclusively the Mesoproterozoic (1590 Ma) Olympic Dam IOCG deposit, the world's largest individual deposit at about 2 200 000 tU, which occurs in a hematite-rich granite breccia complex in the Gawler craton (Direen and Lyons, 2007). IOCG deposits constitute 15% of the global reserves and 10% of the global production of uranium (along with vein-type deposits). They have the lowest grades and normally would not be considered as uranium deposits were they not polymetallic.

The Olympic Dam IOCG (U–Ag–REE) deposit is located on the Gawler craton, South Australia. Mineralization occurs in a hematite breccia complex hosted by the Roxby Downs Granite of the Hiltaba Granite Suite, dated at 1588 Ma (Creaser, 1996), and covered by a thick Mesoproterozoic to Cambrian sedimentary sequence of the Stuart Shelf. The Roxby Downs Granite represents the most fractionated member of the large Mesoproterozoic Burgoyne batholith of the Hiltaba Suite, a quartz–monzonite to granite suite typical of A-type, high-K calcalkaline granites and comagmatic with Gawler Range Volcanics (Oreskes and Hitzman, 1993). The breccia complex extends approximately over 7 km along a NW–SE direction and is zoned with a central barren quartz–hematite breccia associated with volcaniclastic rocks and a variably mineralized outer zone of hematite–granite breccia (Oreskes and Hitzman, 1993). Clasts of surficial sedimentary and volcaniclastic rocks are found within heterolithic breccias. Volcanic rocks range from lapilli tuffs to felsic porphyritic lavas, all with hematite alteration. Mafic to felsic dikes intrude the breccia complex as the final steps of the magmatic activity.

The typical alteration mineralogy with the uranium is sericite–hematite, minor chlorite, quartz, carbonate, and magnetite, and the intensity of alteration is generally linked to the extent of brecciation. The main uranium-bearing phases are uraninite, coffinite, and brannerite, but their paragenesis, timing, and character are wholly understudied. Uranium ore genesis is proposed to be related to the mixing of a hot, high-salinity fluid from a granitic magma, which subsequently mixed with an oxidizing meteoritic fluid (Oreskes and Einaudi, 1992). However, recent results indicate that there were sediments above the intrusion and prior to brecciation and that the hydrothermal system at Olympic Dam did not vent magmatic fluids (McPhie et al., 2011), so that basinal fluids may have been involved in the uranium mineralization.

Although these IOCG deposits do contain some uranium, it is normally a liability for the Cu and Au being extracted, and most IOCG deposits do not have elevated uranium contents. Olympic Dam is anomalous in this respect and normally would not be considered a uranium deposit because of its low grade, despite the large tonnage. The uranium must be removed so the Cu and Au can be marketed – the uranium is a bonus.