4.5.4. Staurolite

Staurolite is a common metamorphic mineral in upper amphibolite facies terranes [60]. Its presence in all four till samples is expected as the deposit is in a high metamorphic grade terrane [3]. Staurolite with a significant Zn content (>5 wt. %) was identified as a useful marker when exploring for metamorphosed massive sulfide deposits, specifically as an intermediate mineral to form gahnite through the reaction of Zn-bearing biotite and staurolite during metamorphism [61–64]. However, EMPA analysis of several hundred staurolite grains collected from the >250 μm fraction of till HMC at Izok Lake by Hicken [17] identified very few grains containing >5 wt. % Zn. Spectra collected from these grains were added to our study's mineral reference library and no significant abundance of Zn-rich staurolite was observed in our samples.

## 4.5.5. Fe-Oxide Minerals

Fe-oxide minerals hematite, goethite, and magnetite have been used as indicator minerals of porphyry Cu (e.g., Kelley et al. [7]) and VMS mineralization (e.g., Makvandi et al. [65] and McClenaghan et al. [66]). These minerals can be derived from fresh bedrock, sulfide gossans that were glacially eroded and incorporated into glacial sediments, or from postglacial weathering of sulfide grains in till. Automated mineralogical platforms such as MLA cannot distinguish the valence state of individual elements and also has di fficulty discerning grain boundaries between minerals of similar atomic (Z) number due to similarities in the grey level in BSE. This means that Fe-oxide minerals (goethite, limonite, hematite, magnetite) are di fficult to separate and distinguish using EDS or BSE [45]. The MLA mineral reference library for this project has one entry for Fe-oxide and while it is possible to further di fferentiate minerals using the data gathered by the MLA based on the amplitude of individual elemental peaks this would have required more time than was permitted during our routine MLA analysis.

The increase in Fe-oxide abundance down ice of the Izok Lake deposit likely represents the incorporation of iron formation, weathered gossanous material from the Izok Lake deposit, the weathering of sulfide grains within till following deposition, or some combination of all three. Mineralized iron formation makes up a portion of the hanging wall immediately east of the deposit [22]. Fe-oxide abundance in the 185–250 μm fraction increases in the till progressively farther down ice, with the highest value (72.16 grains per 1000 grains) observed in the 185–250 μm fraction of till sample 12-MPB-902, 8 km down ice of mineralization. It is normal for indicator mineral abundance in till dispersal trains to peak down ice of the weathered source as source material is incorporated relative to the reduction in material transported from up ice [67] and likely accounts for the lower abundance of Fe-oxide identified in till sample 09-MPB-058 (0.5 km down ice) compared to till sample 09-MPB-075 (3 km down ice). The further increase in Fe-oxide abundance in distal till sample 12-MPB-902 may reflect the influx of debris from the WIZ showing, 2.5 km up ice of till sample 12-MPB-902.

## 4.5.6. Sulfide Minerals

Sulfide mineral instability in oxidizing surface environments combined with their low resistance to physical abrasion and crushing result in poor preservation in till [3]. Sulfide grains (chalcopyrite, pyrite, sphalerite) were only detected in the coarse (250 μm) fraction of till HMC by Hicken et al. [3] and only in sample 09-MPB-058, immediately down ice of mineralization. Galena was not identified in the >250 μm fraction of any till samples.

This study identifies several sulfide minerals in the <250 μm till HMC, including chalcopyrite, galena, pyrite, sphalerite, and pyrrhotite. The highest abundances of all sulfides are in sample 09-MPB-058, immediately down ice of mineralization, and this establishes values for metal-rich surface till proximal to a VMS deposit in permafrost terrain. In fact, this till sample contains the only significant abundance of sphalerite identified in this study. The relative abundance of sulfide minerals in this proximal metal-rich till sample is sphalerite > pyrite > pyrrhotite > chalcopyrite > galena.

Coarse sphalerite was only visually observed in two of 53 till samples in the GSC study, 09-MPB-058 and 09-MPB-052, both down ice of mineralization between Izok Lake and Iznogoudh Lake. Our study detected sphalerite as discrete grains that occasionally contain inclusions of chalcopyrite or pyrite. The low sphalerite abundance in till in all four size fractions is likely the result of some combination of the following: rapid physical abrasion during glacial transport due to its low hardness (Hardness 3.5–4), chemical weathering of sphalerite postglacially, and perhaps few sphalerite-bearing zones of the deposit being directly exposed to glacial erosion. The low preservation of sphalerite grains in till in all size fractions make it a poor candidate for a till HMC indicator mineral.

Pyrite is a common mineral in mineralized and unmineralized rocks, so its usefulness as an indicator mineral of sulfide mineralization is less obvious. McClenaghan et al. [56] recovered coarse (>250 μm) pyrite in five till samples: 09-MPB-058, -016, -081, -052 and -030, with the highest abundance in samples 09-MPB-081 (56 grains) just up ice of mineralization, and 09-MPB-058 (338 grains) and -016 (82 grains) 500 m down ice of mineralization. Lower abundances were recovered from samples 09-MPB-052 (24 grains) and -030 (13 grains) 1 km and 5 km down ice of mineralization, respectively. No pyrite grains were recovered from till sample 09-MPB-060 up ice of the deposit.

Pyrite abundance in <250 till HMC at Izok Lake is di fferent. Abundance is highest immediately down ice of mineralization but reaches background levels (0.2–0.4 grains per 1000 total grains) in most size fractions at sample site 09-MPB-075, 3 km down ice of mineralization. In the <64 μm fraction, pyrite values remain elevated above background (0.06 grains per 1000 total grains) up to sample site 12-MPB-902, 10 km down ice of mineralization. This pattern suggests that till sample site 09-MPB-060 (1 km up ice) may not represent the background for most size fractions, or that pyrite in the coarse fraction is rapidly comminuted to the finer fraction by abrasion and crushing during glacial transport. The high concentrations of pyrite overlying and just down ice of base metal mineralization indicates that it can be a useful indicator in combination with other sulfide minerals, and the dispersal distance may be greater when examining the finest (<64 μm) fraction. Pyrite occurs in both discrete grains and as inclusions in other grains (Table 5), and some grains containing pyrite inclusions also contain inclusions of chalcopyrite (Figure 9) and pyrrhotite. Rapidly identifying these fine pyrite inclusions can generate targets for more precise analytical tools (EMPA, LA-ICP-MS) as pyrite geochemistry has been used in exploration for lode Au [68] and Sedimentary Exhalative (SEDEX) base metal [69] deposits.

Chalcopyrite has been noted as more resistant to weathering in oxidized till by Averill [3]. Chalcopyrite was reported in disaggregated bedrock HMC and bedrock PTS by Hicken [17] and Hicken et al. [31]. Chalcopyrite in bedrock HMC ranged in size between 0.015 and 1.0 mm, and in bedrock PTS ranged in size between 0.1 and 5 mm. McClenaghan et al. [56] reported

chalcopyrite in the 250–500 μm fraction of till HMC from Izok Lake, primarily in samples located within 1 km down ice of the deposit.

In our study, chalcopyrite is present in the <250 μm fraction of the three till samples overlying and down ice of mineralization, along with a small number of grains in the finest fraction of sample 09-MPB-060, directly up ice of the deposit. The presence of chalcopyrite in the fine fraction of till is significant because it represents an increase in the dispersal distance down ice from 1 km for the >250 μm HMC fraction to at least 8 km for the <250 μm HMC fraction. Chalcopyrite is predominantly present as inclusions in other, more robust grains (Figure 9) such as epidote, hornblende, garnet, and Fe-oxide.

Chalcopyrite and pyrite are commonly present in small amounts in mineralized and barren metamorphosed mafic rocks, and thus the presence of chalcopyrite or pyrite inclusions in other minerals alone, although most likely related to mineralization at this specific site, can be an unreliable indication of mineralization in a regional exploration context [47]. The preservation of chalcopyrite and pyrite as inclusions in more robust grains, combined with the common occurrence of both minerals in mineralized and unmineralized bedrock, means that establishing accurate background values for both minerals is important when using automated mineralogical methods. The presence of whole grains of chalcopyrite and pyrite in till HMC (Table 5) remains a valuable indicator of proximity to mineralization given the relatively rapid physical and chemical weathering of sulfide grains following glacial transport and deposition. The identification of decreasing abundance in dispersal trains at a more detailed survey level can still be useful when vectoring to targets, and the ability of automated mineralogical systems to detect small inclusions of sulfide minerals on polished surfaces preserved as inclusions in other minerals makes them a better tool for this task than optical identification methods.

Hicken [17] reported the presence of galena in the 250–500 μm HMC fraction of only one till sample, collected 1 km down ice of mineralization. The abundance of galena (area percentage) in the samples down ice of the deposit is minor, but the grain abundance is anomalously elevated in the coarsest (180–250 μm) of the four fractions of each sample (Tables 3 and 4), highlighting the utility of reporting both area percentage and grain abundance data for indicator minerals. Area percentage values alone would not qualify this sample as anomalous but combined with grain abundance information indicates that galena is present as multiple very small grains.

The anomalous grain counts are due to the presence of small (10–15 μm) galena inclusions in other minerals (garnet, hornblende, chlorite) with most inclusions in the coarsest fraction, extending up to 8 km down ice of mineralization. Galena is not identified in the finest (<64 μm) HMC fraction of the four till samples. This distribution represents a significant increase of galena dispersal distance from a single sample 1 km down ice to 8 km down ice. This suggests that very fine galena inclusions in coarser grains, previously not identifiable with optical methods, are an important indicator of this style of mineralization, significantly extending the dispersal train of an ore mineral that is poorly preserved as whole grains in till.

No pyrrhotite was recovered from the coarse non-ferromagnetic HMC of till samples in the previous GSC study because pyrrhotite would have been removed during ferromagnetic separation. Pyrrhotite detected in the <250 μm fraction of non-ferromagnetic till HMC by this study is present as inclusions in composite grains where the pyrrhotite content is so minor that the magnetic forces of attraction could not overcome the mass of the grain. The abundance of pyrrhotite in the <250 μm HMC fraction of sample 09-MPB-060, 1 km up ice of mineralization, is comparable to the amount identified in till sample 09-MPB-075, located 3 km down ice (west) of mineralization. This distribution suggests that either (1) the abundance of pyrrhotite in till has diminished to regional background levels within 3 km of down ice, or (2) that up ice sample 09-MPB-060 (1 km up ice) does not represent the regional background for pyrrhotite.
