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Type: Thesis
Title: Genesis of the Blackbush Uranium Deposit, South Australia
Author: Domnick, Urs
Issue Date: 2019
School/Discipline: School of Chemical Engineering
Abstract: The Blackbush deposit, northern Eyre Peninsula, South Australia is a recently discovered sediment-hosted U deposit. The resource is mainly hosted at the unconformity between Eocene sandstone (Kanaka Beds) and a massive saprolite derived from subjacent ~1585 Ma granites, affiliated with the Samphire Pluton. Three distinct granitoids are recognised in basement underlying the deposit. The southern part of the pluton features a characteristic green-coloured granite (B) whereas the northern part (granite C), immediately underlying the Blackbush deposit, is characterised by reddened feldspars. These two granites are separated by an arcuate domain of a yellowish granite (A), which is significantly less evolved than the other two, as indicated by higher Ca, and lower U. All three granites show complex alteration overprints and textures, as well as crosscutting veins. Alkali feldspar is replaced by porous K-feldspar and albite, and plagioclase is overprinted by an assemblage of porous albite + sericite ± calc-silicates. In granites A and B, igneous biotite is replaced by calc-silicate minerals, the products of Ca-metasomatism, sourced from the anorthite component of altered plagioclase. Vein assemblages include quartz, hematite, coffinite, fluorite and clay minerals. The geological evolution of the Blackbush deposit is constrained by new SHRIMP U-Pb zircon data from the three granites and microprobe U-Pb ages for coffinite and uraninite in veins within granite, saprolite and overlying sandstone. The new geochronological data for the granites: 1585 ± 9 Ma (Granite B), 1579 ± 9 Ma (Granite C), and 1588 ± 9 Ma (Granite A) show statistical overlap. Their distinct appearance and geochemistry may be attributed, in part, to fractionation within a single magmatic event. Granite C, immediately beneath the deposit, is significantly more altered than the other granites. Anomalously high U contents (10-81 ppm), as well as highly variable Th/U ratios, and the presence of hydrothermal uranothorite and coffinite are clear evidence for U mobility facilitated by porosity created during feldspar alteration, and strongly indicate that the granite is the most probable source rock for the uranium. Pervasive alteration is also expressed in the modified chemistry and textures displayed by contained zircons from Granite C, which have been examined and chemically mapped down to the nanoscale. In the deposit, uranium is present as coffinite and is hosted mainly in the sandstone, along the unconformity, as well as within a roll front downstream, and to a minor degree also in saprolite. Coffinite in the sandstone and saprolite occurs intergrown with framboidal Fe-sulphides and lignite grains, as well as coatings around grains of quartz. It is poor in trace elements such as REE or Th. In contrast, coffinite within the granite-hosted veins contains a significant Y (11 wt.%) and HREE. Minor U also occurs absorbed in lignite. Chemical U-Pb dating (608 spot analyses) of coffinite in sandstone, saprolite and veins gives a normal distribution with a mean age of 17 ± 1.6 Ma. Uraninite in the granite-hosted vein yielded a significantly older age (41 ± 2 Ma). Coffinite ages are interpreted as evidence for a single ore-forming event during the Miocene. Such an event likely coincided with tectonic movement, indicated by horst and graben structures in the early Miocene Melton Limestone. The latter unit overlies the Eocene sandstone, and is, in turn, overlain by Pliocene Gibbon Beds that show no evidence of tectonism. The significantly older age of granite-vein uraninite indicates this predates Eocene sedimentation, and likely indicates transport and redeposition of uranium by oxidising fluids within the exposed granite. Combined, the new ages for hydrothermal minerals indicate that uraninite formed in granite veins and was subsequently dissolved and reprecipitated as coffinite in younger sediments during Tertiary tectonic events.
Advisor: Cook, Nigel J.
Ciobanu, Cristiana
Bluck, Russel
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering and Advanced Materials, 2020
Keywords: Uranium
Blackbush prospect
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