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character of the fluid that formed them, early vein- and cataclasite-type mineralization can
be classified as metamorphic-related U mineralization.
5.2.1.1. Granite-related uranium mineralization
Extensional deformation during deposition of Neoarchean to Paleoproterozoic
supracrustal rocks of the Murmac Bay Group may have provided channel ways for granite
intrusions. In the Beaverlodge area, the Gunnar granite (Hartlaub et al., 2004a) was
emplaced along extensional fault systems (Fig. 5.2, Mary’s Channel and Frasier) that likely
formed during the ca. 2.35 Ga Arrowsmith Orogen (Hartlaub et al., 2007; Berman et al.,
2010), concomitant with deposition of the Murmac Bay Group (Ashton et al., 2009b). The
magmatic-related U mineralization in the Gunnar deposit produced about 7,420 t of U
metal from ores grading 0.15% U (Evoy, 1986). Isotopic compositions of syn-ore minerals
indicate that the U mineralization was derived from magmatic fluids that exsolved from the
albitized granitic rock itself during its emplacement (Fig. 5.1). Uraninite formed when
reduced and cooling exsolved residual hydrothermal magmatic fluids near 315 o C were
deposited in voids left after quartz and calcite dissolution. The loss of a large volume of
CO 2 during magmatic-fluid degassing controlled U precipitation. However, the U
associated with the granite-related mineralization is minor in the deposit and is overprinted
later by more substantial breccia-type U mineralization. The metasomatism that affected the
granite produced an albitized rock that is more competent than the surrounding rocks and
therefore capable of focusing brittle reactivation caused by tectonic events. The age of the
magmatic-related U mineralization at Gunnar is not known with certainty because of the
multiple late fluid events that affected the uraninite, but it can be bracketed between the age
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