4.5.2. Corundum

Morrison [22] identified corundum as a component of the chlorite-biotite-cordierite (CBC) alteration zone, thought to represent the metamorphosed equivalent of the hydrothermally altered feeder-zone for the mineralizing fluids at Izok Lake. Both Hicken [17] and Morrison [22] identify corundum in cordierite-bearing bedrock, whereas Nowak [26] identifies it in the "moderately-altered rhyolite" assemblage defined in his work.

Intergrowths of corundum were previously identified in gahnite grains recovered from till samples at Izok Lake and discrete grains were observed in the polished thin section of one bedrock sample from 2 km northeast of the deposit by Hicken [18]. Only blue and red corundum were visually scanned for and counted in the 250–500 μm fraction of Izok Lake till samples, but no grains were found in the till samples selected for use in this study. All other colours of corundum were not scanned for in the 250–500 μm fraction. In this study, corundum is identified by MLA in all four size fractions of the <250 μm HMC of all four samples, with a pattern of abundance similar to that of gahnite.

Gahnite-corundum intergrowth is observed in the finer size fractions of the down ice till samples. Corundum has been previously noted in association with gahnite at the Geco VMS deposit, Canada, by Spry [57] and is considered part of a mineral assemblage formed following high-grade metamorphism of chloritic precursor rocks [58]. These chloritic precursor rocks are characteristic of the footwall hydrothermal alteration pipes of VMS deposits [15]. Backscatter SEM images of corundum grains from the 185–250 μm fraction reveal that it is intergrown with gahnite, but this relationship is not observed in this coarsest (185–250 μm) fraction at any of the other sample sites located down ice (Figure 9). The presence of coarse-grained (185–250 μm) intergrown corundum only in the most proximal till sample suggests that corundum and gahnite are liberated from one another by crushing and/or abrasion in the subglacial environment, and that their presence as coarse composite grains could serve as an indicator of close proximity (<1 km) to a gahnite-rich mineralized source.

**Figure 9.** Mineral liberation analysis (MLA) and backscatter emission (BSE) images of mineral grains from till: (**A,B**) chalcopyrite inclusions (green) in almandine; (**C,D**) intergrown gahnite (pink) and corundum (blue); (**E,F**) galena inclusion (brown) in garnet. Note: scale bars below images vary.
