|Abstract||The 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.