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exsolved from the mafic dikes during their emplacement (Fig. 5.1). Faulting and fracturing
of the host rock during extrusion of the mafic dikes provided pathways for exsolved
magmatic-hydrothermal mineralizing solutions. The key geochemical process leading to U
deposition was changes in the pH or Eh state of the mineralizing solution through
interaction between the magmatic hydrothermal brine and fluids resident in metamorphic
host rocks that destabilized the uranyl-phosphate complexes.
5.2.2.2. Unconformity-related U mineralization
Minor U-veins associated with fluids generated in the 1750 Ma Athabasca Basin
(Kyser et al., 2000) formed at ca. 1620 Ma in the Beaverlodge area and were triggered by
tectonic reactivation of fault zones during the Mesoproterozoic Mazatzal Orogen (Eisele
and Isachsen, 2001). This deformation caused minor faulting and fracturing of the basement
and basin rocks followed by deposition of Athabasca-type U-mineralization observed in the
Ace-Fay and Martin Lake deposits (Dieng et al., 2011). δ 18 O and δ 2 H values and chlorites
crystal chemistry of syn-ore chlorite indicates that U mineralization formed oxidizing
basinal brines near 235°C, identical to those of the unconformity-related U-deposits in the
Athabasca Basin (e.g. Fayek and Kyser, 1997). Syn-ore mineral crystal chemistry indicates
that F - , Chl - , CO 2- 3 , and PO 3- 4 were likely present in the ore assemblage and could be
effective ligands for complexing U. The key process controlling U deposition in vein
systems was likely reduction of the oxidizing U-bearing basinal brines through interaction
with reducing basement lithologies (e.g. Hoeve and Quirt, 1984; Cuney and Kyser, 2008).
U mineralization in the El Sherana Basin in the South Alligator River area coincided
with a hydrothermal alteration event during later stages of the Nimbuwah tectonic event at
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