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Review of Reducing Mechanisms Potentially Involved in the Formation of Unconformity-type Uranium Deposits and their Relevance to Exploration G.M. Yeo 1 and E.G. Potter 2 Yeo, G.M. and Potter, E.G. (2010): Review of reducing mechanisms potentially involved in the formation of unconformity-type uranium deposits and their relevance to exploration; in Summary of Investigations 2010, Volume 2, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2010-4.2, Paper A-12, 13p. Abstract The essential ingredients for ‘basin-related’ uranium deposits are oxidizing, U 6+-bearing fluids and focussed reduction of mobile U 6+ to immobile U 4+. The more critical problem is the reducing mechanism, since it constrains exploration targeting (i.e., is graphitic metapelite essential in the basement of the Athabasca Basin, or is a broader range of basement rocks favourable?). Potential reducing systems in sedimentary basins may be carbon based (e.g., particulate organic material, fluid 2+ hydrocarbons, graphite or graphite-derived compounds) or inorganic (e.g., mineral surfaces, Fe or H2S from oxidation of sulphides, or Fe2+ from chloritization or illitization of ferromagnesian minerals). Microbial activity may be involved in both and more than one U 6+ reduction mechanism may have been responsible for development of the various Athabasca deposits. Plant fragments reduced many Phanerozoic sandstone-hosted uranium deposits. Oncoids and biolaminites in the basal Athabasca sandstones suggest widespread primary particulate organic material. Early diagenetic hematitization of the sandstone, however, would have oxidized this prior to formation of the uranium deposits >1590 Ma. Fluid hydrocarbons or related humates are also thought to have reduced some stratiform sandstone-hosted uranium deposits elsewhere. Athabasca Basin hydrocarbons derived from both the 1.54 Ga Douglas Point black shale and Devono-Mississippian strata are common, but texturally post-date primary uranium phases, except at Dufferin Lake. 4+ Graphite or CH4 and CO2 derived from basement graphitic metapelite have been proposed as reductants for U in the Athabasca Basin. Although graphite is chemically inert at diagenetic temperatures, it has been argued that radiolysis of graphite generated CO2. Most hydrocarbons from which the graphite was derived would have been lost during upper amphibolite facies metamorphism (ca. 750°C). If structurally trapped, CH4 can survive above 800°C, but it would not then be readily available to reduce U 6+. Experimental interaction of graphite and tritium has generated CH4, but it is doubtful that this could produce sufficient CH4 or CO2 to form an economic deposit. Uranyl ions can be incorporated in Fe-oxide or reduced on Fe-bearing mineral surfaces. Alternatively, U 6+ can be 2+ 2+ reduced by Fe or H2S generated during oxidation of pyrite, common in metapelite, or by Fe released by chloritization of ferromagnesian minerals. In either case, Fe3+ would precipitate as a hydroxide and dehydrate to “hydrothermal hematite”, a distinctive alteration at most Athabasca deposits. Aluminium phosphate-sulphate minerals are a potential synchronous sink for sulphate released during sulphide oxidation. Although commonly mentioned in the literature, mechanisms for reduction/precipitation of uranium in unconformity-related deposits remain contentious. This review supports arguments by Kyser, Alexandre, and others that the most likely reduction mechanism for primary uranium in Athabasca Basin is by Fe2+ released during 2+ chloritization of ferromagnesian minerals, and possibly also by Fe or H2S from oxidation of pyrite. Variation in paragenetic sequences at individual deposits suggests local variation in the dominant reduction mechanism. The basement rock type most important geochemically to formation of unconformity uranium deposits, however, is broadly metapelite – not just graphitic metapelite. Graphitic metapelite remains important for controlling reactivated faults which were conduits for fluids. 1 Denison Mines Corp., 230 - 22nd Street East, Suite 200, Saskatoon, SK S7K 0E9. 2 Natural Resources Canada, Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8. Saskatchewan Geological Survey 1 Summary of Investigations 2010, Volume 2 Keywords: Athabasca, unconformity, uranium, graphite, reductant. 1. Introduction The essential requirements for any uranium-mineralizing system are suitable uranium source rocks, suitable fluids and fluid pathways, and effective reducing mechanisms (Skirrow et al., 2009). The reducing mechanisms are particularly important, since they are generally tightly focussed, resulting in small exploration targets compared to the volume of potential host rock. In the Athabasca Basin, the source of uranium remains somewhat contentious with a weak consensus suggestive of derivation from the basin-fill sandstones (Hoeve and Sibbald, 1978; Fayek and Kyser, 1997; Alexandre et al., 2009a), whereas a minority view holds that uranium was derived from underlying crystalline rocks (Hecht and Cuney, 2000; Madore et al., 2000; Richard et al., 2010). Regardless of the source, there is a consensus that dissolved uranium was likely transported in oxidizing basin fluids in response to hydrodynamic gradients. Mineralogy and fluid inclusion analyses show that the diagenetic basin fluids were slightly acidic (kaolinite-illite equilibrium), hot (160° to 220°C), oxidized (f O2 in the hematite stability field) Na-Ca chloride brines (Derome et al., 2005; Cuney, 2009a). Fluid movement was constrained broadly by the unconformity between relatively permeable sandstones and impermeable crystalline rocks and more narrowly by the topography of the unconformity surface and by fault zones, particularly reverse faults. Although a range of reducing mechanisms have been proposed, the spatial association of many Athabasca deposits with basement graphitic pelites led Hoeve and Sibbald (1978) to suggest a diagenetic-hydrothermal reduction mechanism involving graphite for the formation of the deposits. According to this model, “... at elevated temperatures and under a thick sedimentary cover, oxidizing diagenetic solutions of the Athabasca Formation penetrated the metamorphic basement along breccia and fault zones and reacted with graphitic rocks to yield reducing solutions containing carbon dioxide and methane. The newly generated reducing solutions, which formed part of a percolating system, may have flowed upwards along other portions of the same fault or breccia zone, eventually reaching the unconformity. Mineralization resulted from interaction of flows of methane-bearing reducing solutions and of oxidizing diagenetic solutions carrying ore constituents and hence was subject to hydrodynamic controls.” Alternative mechanisms suggested include reduction by: fluid hydrocarbons (Alexandre and Kyser, 2006); direct 6+ reduction of U by radiolysis of graphite (Alexandre et al., 2005); H2S from the breakdown of pyrite (Cheney, 1985; Ruzicka, 1993); Fe2+ from pyrite oxidation, chloritization of biotite or illitization of hornblende (Wallis et al., 1985; Alexandre et al., 2005); or mixing of geochemically distinct brines (Richard et al., 2010). Recent discussions of unconformity-type uranium deposits are commonly not specific about potential reduction mechanisms (e.g., Jefferson et al., 2007; Kyser and Cuney, 2008a; Skirrow et al., 2009; Burrows, 2010), typically referring to them only as basement-derived reducing fluids. Graphite, or CH4- and CO2-bearing fluids generated from oxidation of graphite, however, are still widely considered the reductants for U6+ in the oxidizing basinal fluids (e.g., Cuney, 2009a, 2009b; Belyck, 2010; International Atomic Energy Agency, 2010). If, as inferred by Hoeve and Sibbald’s (1978) diagenetic-hydrothermal model, graphitic pelites are essential to the formation of Athabasca Basin unconformity deposits, then exploration should be focussed on areas underlain by graphitic pelites, such as the western Wollaston Domain, and electromagnetic conductors should continue to be the exploration targets of choice. On the other hand, if graphitic pelites are not essential to the formation of Athabasca Basin deposits, then a much broader area of the basin is relatively prospective and other methods may be effective in defining exploration targets. To resolve the question of whether graphitic pelites are essential to the formation of Athabasca unconformity uranium deposits, various reducing mechanisms for U6+ and the evidence for them in the Athabasca Basin are reviewed below. 2. Reducing Mechanisms Reducing mechanisms for U6+ in oxidizing solutions can be grouped into two broad categories, those involving carbon-based reductants and those involving inorganic reductions. In addition, U6+ may be adsorbed directly onto Saskatchewan Geological Survey 2 Summary of Investigations 2010, Volume 2 metal surfaces, and anaerobic bacteria may be involved in the reducing process. Discussion of the latter, however, is outside the scope of this review. a) Carbon-based Reducing Mechanisms In addition to fluid hydrocarbons, basement graphite or graphite-derived hydrocarbons, another carbon-based reducing mechanism to consider is early diagenetic reduction by particulate organic matter. Detrital Organic Material Fossil plant fragments (or diagenetic sulphides associated with them) are widely considered the principal reductants for dissolved U6+ in Silurian and younger sandstone basins due to the development of vascular plants (Kyser and Cuney, 2008b).

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