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3.5.3. Later basin-related U mineralization
Formation of the Martin Lake Basin (Langford, 1981; Mazimhaka and Hendry,
1984) and associated 1818±4 Ma alkaline mafic dikes (Morelli et al., 2009) is coincident
with periods of active tectonism related to back-arc extension following peak Trans-
Hudson Orogen (Hoffman, 1989; Corrigan, 2009). Extrusion of mafic dikes is associated
with the ca. 1820 Ma volcanic-type U 5 -mineralization (Dieng et al., 2011) identified at the
Cinch Lake, Ace-Fay, and Gunnar deposits (Fig. 3.11E). Syn-ore Chl 8 chlorite and Cal 9
calcite have δ 18 O, δ 13 C and δ 2 H values typical of magmatic fluids (Fig. 3.8) that are likely
exsolved from mafic dikes during their emplacement. Chl 8
chlorite crystal chemistry
indicates that the U 5 -bearing hydrothermal fluid formed at ca. 320°C from an Mg- and Ferich
fluid. REE patterns of U 5 uraninite are similar to those in the mafic dikes and there is
a close spatial field relationship between mafic dikes and fractures hosting U 5 uraninite.
Therefore, during the mafic dikes intrusion and subsequent magmatic-hydrothermal
degassing, pre-existing fractures would have provided pathways for U 5
uranium
mineralizing solutions. Chondrite normalized REE patterns of U 5 uraninite indicate that
Eu is relatively more depleted in U 5 uraninite than in mafic dikes, suggesting that the
exsolved fluid changed from oxidized to less so during U 5 uranium deposition. The
shallow structural levels of emplacement and the presence of Mn-oxides and Hem 7
hematite alteration associated with U 5
uraninite indicate that during early stages, the
hydrothermal fluids were oxidized and capable of transporting U. In such an oxidizing
environment with intermediate pH conditions (4-8) and temperatures near 320°C, uranylphosphate
complexes were likely the most stable (Langmuir, 1978). This is corroborated
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