4.5.1. Gahnite

Hicken [17] reported that gahnite in polished thin-section (PTS) ranged between 0.2 and 3.0 mm in size, and disaggregated bedrock HMC of mineralized bedrock ranged between 0.25 and 1.0 mm in size. This range reflects the original size of the gahnite presented to the overriding glacier during erosion of the deposit and generation of the till. As these grains were transported down ice they would have been crushed to smaller sizes with increasing transport distance, producing the pattern observed in our samples, i.e decreasing abundance and size of gahnite in till with increasing distance down ice.

The pattern of gahnite abundance in the <250 μm HMC is similar to that observed by McClenaghan et al. [11] and Hicken et al. [18] for the 250 to 500 μm HMC. Abundance is highest in sample 09-MPB-058, immediately down ice of the deposit, and decreases with increasing distance down ice. Of the four size fractions of the most proximal sample, gahnite is most abundant in the coarsest (185–250 μm) fraction, whereas in the sample farthest down ice, gahnite is most abundant in the finest size fraction. This is consistent with a model of grain comminution by abrasion and/or crushing during transport down ice, where grains are gradually reduced to a terminal grain size from their original size in bedrock [48]. No gahnite was recovered from the coarse (>250 μm) HMC fraction of sample 09-MPB-060 up ice of the deposit [18], however, in our study, a few gahnite grains are found in the sample. Because sample 09-MPB-060 is located only 1 km up ice (east) of massive sulfide mineralization, it is possible that an alteration halo containing fine-grained gahnite extends this far east. Note minor amounts of 0.25–0.5 mm gahnite grains (few grains/10 kg) were previously reported in regional till samples farther up ice of Izok Lake by McClenaghan et al. [56].
