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Insights into B-Mg-Metasomatism at the Ranger U Deposit (NT, Australia) and Comparison with Canadian Unconformity-Related U Deposits

Université de Lorraine, CNRS, GeoRessources Lab, F-54500 Vandœuvre-lès-Nancy, France
Geoscience Australia, Cnr Jerrabomberra Avenue and Hindmarsh Drive Symonston ACT, GPO Box 378, Canberra, ACT 2601, Australia
Department of Earth Sciences, University of Geneva, Rue des Maraîchers 13, CH-1205 Geneva, Switzerland
CRPG, UMR 7358 CNRS-UL, 15 rue Notre Dame des Pauvres, F-54501 Vandœuvre-lès-Nancy, France
Author to whom correspondence should be addressed.
Minerals 2019, 9(7), 432;
Received: 13 June 2019 / Revised: 10 July 2019 / Accepted: 11 July 2019 / Published: 14 July 2019
(This article belongs to the Special Issue Geology and Mineralogy of Uranium Deposits)
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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. View Full-Text
Keywords: tourmaline; chlorite; boron isotopes; metasomatism; unconformity-related uranium deposits; Ranger tourmaline; chlorite; boron isotopes; metasomatism; unconformity-related uranium deposits; Ranger

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Gigon, J.; Skirrow, R.G.; Harlaux, M.; Richard, A.; Mercadier, J.; Annesley, I.R.; Villeneuve, J. Insights into B-Mg-Metasomatism at the Ranger U Deposit (NT, Australia) and Comparison with Canadian Unconformity-Related U Deposits. Minerals 2019, 9, 432.

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