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

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 Fe tot /(Fe tot + 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 + Fe tot ) = 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 δ 11 B 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 (δ 11 B~40% ), and (ii) boron from the crystalline basement (δ 11 B~−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.

Petrogenetic Constraints of Early Cenozoic Mafic Rocks in the Southwest Songliao Basin, NE China: Implications for the Genesis of Sandstone-Hosted Qianjiadian Uranium Deposits

“…Uranium is a typical redox sensitive element. It is 'fluid-mobile' when present as oxidized state (U 6+ ) but largely immobile when present as reduced species (U 4+ ) [88][89][90][91][92][93]. Sandstone-hosted U deposits are epigenetic deposits in which U minerals are present as disseminations and mineral replacements primarily in fluvial, lacustrine, and deltaic sandstones [94].…”

Tourmaline occurrence and gold mineralization at a granitoid-metasediment contact in the Upper Lom Basin, east Cameroon

“…Tourmaline chemical composition can provide exploitable information on the source and evolution of ore fluid [2,[7][8][9], including possible fingerprints on the origin of associated gold mineralization [10]. The elemental composition of tourmaline, including boron isotopic data, have been widely employed in studies aimed at unraveling: (1) The source of the boron, (2) Fluid-rock interaction processes and transport, (3) The evolution of the magmatic fluid, (4) Temperature of the fluids related to B-metasomatism, and (5) The influence of magmatic-hydrothermal process on formation of orogenic gold deposits and the timing of magmatic, metamorphic and hydrothermal processes [1,5,6,[11][12][13][14]. The relative abundance of δ 11 B in nature [~ 60%; 5] makes the boron isotope system a valuable tracer of boron source in magmatic-hydrothermal and metamorphic systems [14].…”