U redox fronts and kaolinisation in basement-hosted unconformity-related U ores of the Athabasca Basin (Canada): late U remobilisation by meteoric fluids |
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| Authors: | Julien Mercadier Michel Cuney Michel Cathelineau Mathieu Lacorde |
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| Institution: | 1.G2R, Nancy-Université, CNRS, CREGU,Vand?uvre lès Nancy,France |
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| Abstract: | Proterozoic basement-hosted unconformity-related uranium deposits of the Athabasca Basin (Saskatchewan, Canada) were affected
by significant uranium redistribution along oxidation–reduction redox fronts related to cold and late meteoric fluid infiltration.
These redox fronts exhibit the same mineralogical and geochemical features as the well-studied uranium roll-front deposits
in siliclastic rocks. The primary hydrothermal uranium mineralisation (1.6–1.3 Ga) of basement-hosted deposits is strongly
reworked to new disseminated ores comprising three distinctly coloured zones: a white-green zone corresponding to the previous
clay-rich alteration halo contemporaneous with hydrothermal ores, a uranium front corresponding to the uranium deposition
zone of the redox front (brownish zone, rich in goethite) and a hematite-rich red zone marking the front progression. The
three zones directly reflect the mineralogical zonation related to uranium oxides (pitchblende), sulphides, iron minerals
(hematite and goethite) and alumino-phosphate-sulphate (APS) minerals. The zoning can be explained by processes of dissolution–precipitation
along a redox interface and was produced by the infiltration of cold (<50°C) meteoric fluids to the hydrothermally altered
areas. U, Fe, Ca, Pb, S, REE, V, Y, W, Mo and Se were the main mobile elements in this process, and their distribution within
the three zones was, for most of them, directly dependent on their redox potential. The elements concentrated in the redox
fronts were sourced by the alteration of previously crystallised hydrothermal minerals, such as uranium oxides and light rare
earth element (LREE)-rich APS. The uranium oxides from the redox front are characterised by LREE-enriched patterns, which
differ from those of unconformity-related ores and clearly demonstrate their distinct conditions of formation. Uranium redox
front formation is thought to be linked to fluid circulation episodes initiated during the 400–300 Ma period during uplift
and erosion of the Athabasca Basin when it was near the Equator and to have been still active during the last million years.
A major kaolinisation event was caused by changes in the fluid circulation regime, reworking the primary uranium redox fronts
and causing the redistribution of elements originally concentrated in the uranium-enriched meteoric-related redox fronts. |
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