Metasomatic history and origin of uranium mineralization at Mary Kathleen, northwest Queensland
Authors / CoAuthors
The Mary Kathleen uranium deposit in northwest Queensland is contained in west-dipping middle Proterozoic calcareous, dolomitic and alkali-rich metasediments of the Corella Formation, which are metamorphosed to hornblende hornfels grade by the Burstall Granite, and are also intruded by a swarm of rhyolite and microgranite dykes associated with this granite. An early, higher temperature metasomatic event has formed K feldspar-rich and scapolite-pyroxene bands from the original metasediments; a later lower temperature metasomatic event has resulted in extensive garnetization and skarn replacement of earlier formed rocks, and uraninite-allanite mineralization. Fluids associated with this second event appear to be enriched in Na, Cl, H2O, O, U and rare earths, and are spatially and probably genetically related to the rhyolite dyke swarm. This is supported by the high uranium content of the dykes (12 ppm average) compared to that in Burstall Granite (7 ppm), metasediments (3.5 ppm), quartzite (1 ppm) and basic rocks (0.7 ppm). Uranium-rich fluids moved westwards from the dykes, following decreasing temperature gradients. They were impeded in most places by a chemically inert layer of quartzite, but breached this barrier in at least two places and intersected a sequence containing permeable lenses of conglomerate, within which the broadly stratabound orebody was deposited. Similar orebodies could be expected in the vicinity of other permeable zones stratigraphically above quartzite and adjacent to fractures and acid dykes, but such sites appear to have been removed by faulting and erosion. However, fractured areas of the quartzite hanging wall (i.e. the eastern side of the quartzite), in the vicinity of rhyolite dykes, could also be sites of economic uranium mineralization at depth.