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(Alexandre et al., 2009). Positive chondrite normalized Ce anomaly in U 6 uraninite may
indicate destruction of monazite and incorporation of Ce into the mineralizing brine
(Kyser et al., 2003), but the similarity of the isotopic composition, temperature and timing
of fluid events, all suggest that U 6 mineralizing brines originated in the Athabasca Basin
and descended downward along fracture systems into the basement rocks (Fig. 3.11F).
Syn-ore Chl 9 chlorite and U 6 uraninite crystal chemistry indicate that F - , Chl - , CO 2- 3 , and
PO 4
3-
were likely present in the ore assemblage and could be effective ligands for
complexing U 6 (e.g. UO 2 F 2 , UO 2 Cl 2 , UO 2 (CO 3 ) 2- , and UO 2 (H 2 PO 4 ) 2 ). In oxidized aqueous
basinal brines with pH between 4 and 7.5, uranyl-phosphate complexes are the important
species (Cuney and Kyser, 2008), uranyl-carbonate complexes are dominant under
relatively oxidizing and near-neutral pH conditions and the chlorine- and fluorinecomplexes
are dominant in fluids under acidic conditions at temperatures up to 200°C
(Romberger, 1984; Kojima et al., 1994). Therefore, the key process controlling U 6
uranium deposition was likely reduction of the mineralizing solution through interaction
between the oxidizing basinal brines and the reducing metamorphic basement lithologies
(Hoeve and Quirt, 1984; Polito et al., 2005, 2006; Cuney and Kyser, 2008).
Destabilization of the aqueous uranyl complexes is indicated by syn-ore Cal 10 carbonate
alteration, high phosphate contents in U 6 uraninite and elevated values of fluorine and
chlorine in Chl 9 chlorite.
3.5.4. Late alteration events
Electron microprobe analyses indicate that Chl 5 , Chl 7 , and Chl 8
chlorites were
subsequently altered to low-temperature Mg-rich chlorite (Table 3.4) recording the effect
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