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TitleReview of reductants potentially involved in the formation of "basin-related" uranium deposits and their relevance in the Athabasca Basin
AuthorYeo, G M; Potter, E G
SourceSaskatchewan Geological Survey open house 2010, Abstract Volume; 2010 p. 16
Alt SeriesEarth Sciences Sector, Contribution Series 20100264
Meeting41st Annual Saskatchewan Open House; CA; November 29 - December 1, 2010
NTS64E; 64L; 64M; 74F; 74G; 74H; 74I; 74J; 74K; 74N; 74O; 74P
Lat/Long WENS-110.0000 -102.0000 60.0000 57.0000
Subjectseconomic geology; uranium; uranium deposits; uranium metallogenesis; mineral deposits; mineralization; Athabasca Basin
ProgramGEM: Geo-mapping for Energy and Minerals, Uranium
AbstractThe essential ingredients required to form any 'basin-related' uranium deposit are a source of oxidizing, U(VI)-bearing fluids and a mechanism to reduce mobile U(VI) to immobile U(IV) for inclusion in oxide or silicate minerals. In the case of Athabasca Basin, both of these remain controversial. Of these two problems, the more important is the reducing mechanism, since it is a constraint on exploration targeting (e.g., if basement graphitic pelites are an essential factor then exploration should be focused on them; whereas if Fe(II) is the reductant, a broader range of basement rocks are favourable).
Potential reducing systems in sedimentary basins fall into two broad categories: carbon-based reductants (e.g., particulate organic material, fluid hydrocarbons, graphite, or compounds derived from graphite) and inorganic reductants (e.g., mineral surfaces, sulphur derived from oxidation of basement sulphides or Fe(II) released during chloritization of ferromagnesian minerals). Microbial activity may be involved in both categories. It is likely that more than one U(VI) reduction mechanism was important in formation of the Athabasca ores.
Plant fragments are the reductants in many Phanerozoic sandstone-hosted uranium deposits (e.g., classic roll-front deposits). The presence of oncoids and biolaminites in the basal Athabasca sandstones suggests that particulate organic material may have been widespread. However, early diagenetic hematization of the sandstones would have oxidized such material prior to the formation of the uranium deposits ca. 1590 Ma.
Fluid hydrocarbons or humates (closely related) are suggested to have been the reductants in some stratiform sandstone-hosted uranium deposits. In Athabasca Basin there is good evidence of hydrocarbons derived from both the 1.54 Ga Douglas Point black shales, and Phanerozoic sedimentary units, but textures indicate that they post-date primary mineralization, except at Dufferin Lake.
Methane and carbon dioxide derived from basement graphitic pelites, or the graphite itself, have been suggested as the reductants for U(VI) in Athabasca Basin. Although graphite is chemically inert at diagenetic temperatures, it has been argued that radiolysis of graphite could precipitate uraninite and generate CO2. Most of the organic material from which the graphite was derived would have been driven off under upper amphibolite faces metamorphic temperatures (ca 750oC), although methane can survive above 800oC if structurally trapped, and hence not readily available to reduce U(VI). Methane has also been generated by experimental interaction of graphite and tritium, but it is doubtful that this could produce the volume of methane required to precipitate a uranium deposit.
U(VI) can be incorporated in Fe-oxides or reduced on the surfaces of Fe-bearing mineral grains. Alternatively, U(VI) can be reduced by Fe(II), or H2S released during oxidation of pyrite, which is ubiquitous in the Wollaston graphitic pelites and other metasedimentary rocks, or by Fe(II) released by chloritization of ferromagnesian minerals. One unresolved issue with sulphide oxidation is the production sulphate since sulphate minerals associated with the deposits are genetically late. Aluminum phosphate sulphate (APS) minerals are one potential sink for sulphate released during oxidation of sulphides. The Fe(III) would precipitate as a hydroxide and ultimately dehydrate to 'hydrothermal hematite', a distinctive alteration feature at most Athabasca deposits.
Although frequently mentioned in literature, mechanisms of reduction and precipitation of aqueous uranium in unconformity-related deposits remains contentious. This review suggests that the most likely reduction mechanism for primary uranium in Athabasca Basin is by Fe(II) released during chloritization of ferromagnesian minerals as suggested by Kyser, Alexandre, and others. Hence, the basement lithology most involved geochemically in formation of unconformity uranium deposits is pelite; not graphitic pelite. Graphitic pelites are more important physically as controls on reactivated faults.