Abstract
The Ranger deposit (Northern Territory, Australia) is one of the largest uranium deposits in the world. Uranium mineralisation occurs in crystalline basement rocks and is thought to belong to the unconformity-related category. In order to address the sources of magnesium and boron, and the temperature of the fluids related to boron and magnesium metasomatism that occurred shortly before and during the main uranium stage, in situ analyses of chlorite and tourmaline were carried out. The chemical composition of tourmaline shows an elevated X-site vacancy and a low Fetot/(Fetot + Mg) ratio typical of Mg-foitite. Uranium-related chlorite has relatively low Fe content (0.28–0.83 apfu) and high Mg content (3.08–3.84 apfu), with Si/Al = 1.08−1.22 and Mg/(Mg + Fetot) = 0.80−0.93 indicating a composition lying between the clinochlore and Mg-amesite fields. Chlorite composition indicates crystallisation temperature of 101–163 °C. The boron isotopic composition of tourmaline shows a range of δ11B values of ~1–9‰. A model is proposed involving two boron sources that contribute to a mixed isotopic signature: (i) evaporated seawater, which is typically enriched in magnesium and boron (δ11B ~ 40‰), and (ii) boron from the crystalline basement (δ11B ~ −30 to +10‰), which appears to be the dominant source. Collectively, the data indicate similar tourmaline chemistry but significant differences of tourmaline boron isotopic composition and chlorite chemistry between the Ranger deposit and some of the Canadian unconformity-related uranium deposits. However, lithogeochemical exploration approaches based on identification of boron- and magnesium-enriched zones may be usefully applied to uranium exploration in the Northern Territory.
Highlights
IntroductionThey are located in the vicinity of the intersection between the basin/basement unconformity and multiply-reactivated basement-rooted faults and are currently accounting for ~25% of the world U production [1,2,3]
Unconformity-related uranium (U) deposits occur mainly in or close to the unmetamorphosedProterozoic Athabasca and Thelon basins (Saskatchewan and Nunavut, Canada) and the McArthurBasin (Northern Territory, Australia)
Using the world class Ranger deposit, we present a detailed investigation of the conditions for B-Mg-metasomatism in Australian unconformity-related U deposits [4]
Summary
They are located in the vicinity of the intersection between the basin/basement unconformity and multiply-reactivated basement-rooted faults and are currently accounting for ~25% of the world U production [1,2,3]. Halogen geochemistry of fluid inclusions indicates that the basinal brines were produced by evaporation of seawater [9,21,22,23,24]. In addition to basinal brines, a low-salinity fluid of possible meteoric origin was involved at the time of U deposition in the Australian deposits [9,10]
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