Preface: thematic issue on uranium deposits

Abstract

This thematic issue comprises five papers covering various aspects of the genesis of different types of uranium deposits from Australia, Canada, India, and central Europe. Three papers are on Mesoproterozoic uranium deposits: Valhalla from Queensland, Australia, related to sodium metasomatism, by Polito et al., the unconformity related deposits of the Athabasca Basin, Saskatchewan, by Alexandre et al., and an iron oxide–copper–gold (IOCG) type of uranium mineralization from the Singhbhum Shear Zone, India, by Pal et al. Two other papers concern hydrothermal uranium veins hosted by Early Paleozoic metamorphic rocks of the Variscan Bohemian Massif (Dolnicek et al.; Kribek at al.). There are numerous uranium deposits related to Na metasomatism, but the two largest and presently mined ones are those of the Central Ukrainian Province (Belevtsev and Koval 1968) and the Lagoa Real district in Brazil. The geotectonic setting of this type of mineralization and the origin of the mineralizing fluids is debated (Lobato et al. 1983; Turpin et al. 1988). The paper by Polito et al., besides a detailed mineralogical description of the alteration and ore minerals, provides U–Pb isotopic ages on the uranium minerals and Ar/Ar ages on alteration minerals, allowing to link these processes to a very-low-grade metamorphic event at about 1,550 Ma, the Isan orogeny. The temperature of alteration and mineralization (340–380°C) at Valhalla is, however, significantly lower than in the Ukrainian and Brazilian occurrences (>500°C to 300°C). Fluids associated with the Valhalla deposit would be either basinal brines expulsed during the metamorphic event or magmatic fluids, whereas the fluids associated with the Lagao Real deposits have a seawater composition (Lobato et al. 1983). The age of the primary uranium deposition in unconformity-related uranium deposits has been continuously revised since the first determinations during the 1970s. With improvement of the dating techniques, older and older ages have been obtained from about 900 Ma to nearly 1,600 Ma for the latest ages obtained by Kurt Kyser’s group at Queen’s University and reviewed by Alexandre et al. in the present volume. Uranium oxides are easily remobilized by highto low-temperature fluid percolations and tend to lose their radiogenic lead because of the much larger diameter of Pb compared to U. The use of in situ dating techniques by secondary ion mass spectroscopy or laser ablation inductively coupled plasma mass spectroscopy allows analyzing the less disturbed domains of uranium oxide minerals. Alexandre et al. also associate, the first time for these deposits, in situ laser step heating Ar/Ar dating of metamorphic host rock minerals and syn-ore alteration clay minerals. The events younger than 1.6 to 1.5 Ga are interpreted as remobilization of the primary uranium mineralization in response to far-field tectonic or magmatic events. The possibility that some of these events may represent new uranium deposition events, upgrading the uranium deposits and thus explaining their extremely high grade, is excluded. With a 1.6 Ga deposition age, the timing of the formation of the Athabasca deposits becomes very close to the age of the uranium deposits of the Kombolgie Basin, Northern Territory, Australia, which formed in the same P-T-X conditions (Derome et al. 2003, 2005). Miner Deposita (2009) 44:1–2 DOI 10.1007/s00126-008-0218-y