Alteration Mineralogy and Stable Isotope Geochemistry of Paleoproterozoic Basement-Hosted Unconformity-Type Uranium Deposits in the Athabasca Basin, Canada

Abstract

Unconformity-type uranium deposits are characterized by mineralization developed along the contact between younger sandstone cover and underlying crystalline basement rocks. Mineralization may extend up to 400 m into the underlying basement rocks. Whereas sandstone-hosted unconformity-type deposits have been well studied, deposits hosted primarily in the basement have not. This study examines the deposits at Rabbit Lake, Dawn Lake, and McArthur River, in the Athabasca basin of Canada, which are hosted by the metamorphic Archean and Early Paleoproterozoic rocks forming the basement to younger Late Paleoproterozoic sandstones. Alteration is similar in the three deposits and is characterized by three distinct paragenetic stages: (1) preore alteration involving illitization of plagioclase and amphibole, followed by chloritization of biotite and illite, which formed at ca. 230°C; (2) ore-stage alteration, characterized by uraninite and coarse-grained illite, which formed at ca. 240°C; (3) postore alteration comprising spherulitic dravite, vein chlorite, quartz, calcite, and Fe, Cu, Co, and Pb sulfides, which formed at ca. 135°C. Fluid circulation associated with emplacement of later Mackenzie dikes initiated partial recrystallization of uraninite. A later stage of alteration includes kaolinite and iron hydroxide precipitation formed at much lower temperatures of ca. 50°C.Stable isotope compositions of the alteration minerals in conjunction with their paragenesis indicate that oxidized basinal fluids (δD = -43 to -21‰ . , δ 18 O = 3-8‰) were derived primarily from evolved seawater and leached uranium from the overlying sandstones of the Athabasca Formation and transported it into the basement via infiltration along fracture zones associated with reverse faults. Graphitic units in the basement and preore alteration served as both physical (fractured zones) and chemical (reductants) traps for the uranium mineralization. The basinal fluids were responsible for the preore illite-chlorite, synore uraninite-illite, and the early postore alteration events; this differs from many other sandstone-hosted deposits, where both oxidized basinal and reduced basement-derived fluids were responsible for uranium precipitation.

Advances in understanding the Kombolgie Subgroup and unconformity-related uranium deposits in the Alligator Rivers Uranium Field and how to explore for them using lithogeochemical principles

“…Several studies have now demonstrated that the unconformity-related U deposits are associated with long-lived basement-rooted structures that were likely active during basin-fill, sandstone diagenesis and U mineralisation Alexandre et al 2005Alexandre et al , 2009Polito et al 2005Polito et al , 2006bSouthgate et al 2006;Kyser 2007). In most cases, these structures have been reactivated several times since initial uraninite precipitation as testified by the typically young ages determined for most U deposits in Australia and Canada (Hills & Richards 1976;Gulson & Mizon 1980;Fayek & Kyser 1997;Fayek et al 2000Fayek et al , 2002Kyser et al 2000;Polito et al 2004Polito et al , 2005Alexandre et al 2005Alexandre et al , 2009. Therefore, the conclusion is that elements known to be associated with post-ore alteration may be useful in unconformity-related U exploration because they too indicate formation in a long-lived structure that was originally active when U mineralisation formed.…”

“…Petrographic analysis demonstrates that sericite alteration of feldspar occurs first and furthest from the ore zone (Figure 5a), followed by sericite replacement of amphibole and sillimanite and then by increasing amounts of chlorite toward mineralisation ( Figure 5b; Polito et al 2004Polito et al , 2005. Alexandre et al (2005) demonstrated that the formation of crystalline illite/sericite from amphibole and feldspar around the U deposits in the Athabasca Basin created void space due to the minerals' contrasting molar volume. These authors proposed that void space creation promoted additional fluid flow into the basement, with the release of Mg and Fe from minerals such as amphibole, resulting in the precipitation of void-consuming chlorite.…”

An evaporated seawater origin for the ore-forming brines in unconformity-related uranium deposits (Athabasca Basin, Canada): Cl/Br and δ37Cl analysis of fluid inclusions

“…Similarly, Kotzer and Kyser (1995) concluded that the brines have a significant meteoric water component and that the high chlorinities were achieved by evaporite dissolution and reaction with feldspars. However, based on comparable isotopic analyses, Alexandre et al (2005) stated that the brines result from mixing between evaporated seawater and low-latitude meteoric waters.…”

Formation of the enigmatic Matoush uranium deposit in the Paleoprotozoic Otish Basin, Quebec, Canada

“…This difference underlines the variability of possible reductants for the formation of U deposits in sandstone basins (e.g., Kyser 2006, 2012;Yeo and Potter 2010). Ferrous iron has also been suggested as a reductant for unconformity-related U deposits in the Athabasca Basin, but its source is dominantly chlorite, a product of pre-ore clay alteration (Alexandre et al 2005). Other reductants that have been documented elsewhere include, but are not limited to, S (Beverley, South Australia; Wülser et al 2011), graphite or CO (Athabasca Basin deposits; e.g., Cuney and Kyser 2008), organic matter and H 2 S (tabular deposits, Grants U region in USA; Galloway 1980;Saucier 1980), and hydrocarbons in many types of deposits (e.g., Alexandre and Kyser 2006).…”