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TitleOre fluids recorded in the compositions of magnesiofoitite and alumino-phosphate-sulfate (APS) minerals in the basement along the P2 structure and the McArthur River deposit, Athabasca Basin
AuthorAdlakha, E E; Hattori, K; Zaluski, G; Kotzer, T; Potter, EORCID logo
SourceGeological Association of Canada-Mineralogical Association of Canada, Joint Annual Meeting, Abstracts Volume vol. 37, 2014 p. 4-5 Open Access logo Open Access
LinksOnline - En ligne
Alt SeriesEarth Sciences Sector, Contribution Series 20140044
PublisherGeological Assoc. Canada - Mineralogical Assoc. Canada
Meeting Gerological Association of Canada - Mineralogical Association of Canada annual meeting; Fredericton; CA; May 21-23, 2014
File formatpdf
NTS74H/15; 64L
AreaMcArthur River
Lat/Long WENS-105.0000 -104.5000 58.0000 57.7500
Subjectseconomic geology; deformation; faults; structural features; uranium; uranium deposits; alteration; diagenesis; hydrothermal alteration; McArthur River deposit; Athabasca Basin
ProgramTargeted Geoscience Initiative (TGI-4) Uranium Ore Systems
AbstractThe P2 reverse fault is a reactivated structure in the southeastern Athabasca Basin that hosts the McArthur River uranium deposit, the largest high-grade unconformity-type uranium deposit yet discovered. This study is to document alteration below the unconformity, specifically within and near the P2 deformation zone and the basement-hosted Zone 2 ore body, in order to evaluate the role of the P2 fault as the conduit for basinal and basement fluids and identify fertile alteration. The results of this study show that the P2 structure is the site of multiple stages of diagenetic-hydrothermal alteration, which produced illite, sudoite, Fe-Mg chlorite, clinochlore, kaolinite, aluminum phosphate-sulfate minerals (APS) and tourmaline. Below the unconformity, the assemblage of illite plus sudoite is common in pelite and pegmatite regardless of proximity to the P2 fault, and kaolinite is distributed along the unconformity; however, the assemblage of magnesiofoitite (alkali-deficient Mg-tourmaline) plus LREE-rich APS has only been found in close proximity to the P2 structure. Along the P2 fault, magnesiofoitite forms fine-grained (<0.2 mm) matrix, aggregates, overgrowths on metamorphic/magmatic dravite (< 2 mm), and veinlets (< 2 mm) that cross-cut sudoite and illite. APS form zoned pseudo-cubes (1 - 20 um), disseminated and clustered within fine-grained matrix in altered metapelite and pegmatite and vary compositionally from LREE-rich, to Ca- or Sr-rich. Magnesiofoitite contains low contents of LREE ([LREE]N/[HREE]N) ~ 0.2), yet significant amounts of U (0.2 - 3.7 ppm), Cr (2.9 - 110 ppm), V (65 - 260 ppm) and W (0.03 - 0.347 ppm). The low LREE in magnesiofoitite is consistent with its close proximity to LREE-rich APS, implying that the two minerals are contemporaneous. Some crystals of magnesiofoitite are essentially free of alkalis. Overall, low alkalis (<0.3 in apfu) and high U plus W in magnesiofoitite suggests that the fluids were acidic and oxidized in order to transport soluble complexes of U6+ and W6+. This fluid character is further supported by the coprecipitation of APS: significant Al solubility requires acidic fluids and the presence of SO4 2- confirms the oxidized nature of the fluids. The presence of these co-genetic minerals along the P2 structure suggests that the P2 fault was a conduit for uraniferous fluids; however, the fluids did not form uranium deposits all along the P2. The evidence further substantiates models in which the localization of large deposits required the focusing of an ascending reduced fluid to precipitate uraninite from the descending oxidized uraniferous fluid.
Summary(Plain Language Summary, not published)
The Targeted Geoscience Initiative (TGI-4) is a collaborative federal geoscience program that provides industry with the next generation of geoscience knowledge and innovative techniques to better detect buried mineral deposits, thereby reducing some of the risks of exploration. This conference presentation summarizes the chemistry of alteration minerals precipitated during ore-forming processes along the P2 fault, which hosts the world's highest grade uranium deposit (McArthur River). The results reinforce existing deposit models in that oxidized, uranium-bearing fluids permeated down the fault into the basement rocks, with uranium precipitation controlled by the presence of reducing media in the basement.

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