**1. Introduction**

Indicator mineral methods applied to sediment samples are important exploration tools in glaciated terrain for diamonds [1] and gold [2–5]. More recently, their potential to aid porphyry Cu [6–8], magmatic Ni–Cu-PGE [5,9], carbonate-hosted Pb–Zn [10] and volcanogenic massive sulfide (VMS) exploration [11] has been demonstrated.

Using current sampling protocols, a large till or stream sediment sample (10–20 kg) is necessary to recover detectable and meaningful numbers of indicator mineral grains in a single sample [12]. Indicator minerals are recovered from these large samples at specialized commercial laboratories using a combination of sizing, magnetic and density concentration methods to reduce the volume of material into a non-ferromagnetic heavy mineral concentrate (HMC). The coarse fraction (>250 μm, medium to coarse sand size) of the HMC is subsequently visually examined to identify and count indicator minerals using a binocular microscope [13]. These methods focus on the recovery of the coarser (>250 μm, medium to very coarse sand) mineral fraction of sediments because it is cost e ffective and relatively easy to recover and visually examine. The resulting HMC is composed of dense mineral grains (specific gravity (SG) >3.2) displaying varying levels of physical and chemical weathering, and the degree of abrasion and wear can be used to infer the transport distance of certain minerals [3]. Mineral associations can be observed in composite grains, while the degree of liberation of interlocked minerals can also serve as an indicator of mechanical weathering during transport.

Developments in rapid scanning electron microscopy (SEM) such as mineral liberation analysis (MLA, Hillsboro, OR, USA) or Quantitative Evaluation of Minerals by Scanning Electron Microscope (QEMSCAN ™, Hillsboro, OR, USA) over the past 10 years make it possible to also examine and analyze the finer (<250 μm) heavy mineral fraction of sediment samples using automated technologies [14]. Automated mineralogy provides the potential for the identification and utilization of additional indicator minerals that traditional visual examination of the >250 μm mineral fraction does not allow.

Volcanogenic massive sulfide (VMS) deposits are an important source of Cu, Pb and Zn in Canada [15,16]. Thus, indicator mineral methods can be an important exploration method in Canada because much of the landscape is covered by glacial deposits. Metamorphism of massive or disseminated sulfide ore bodies results in coarsening of sulfide and alteration mineral grains as a result of recrystallization and grain boundary reduction. Indicator minerals commonly found in glacial dispersal trains extending from metamorphosed VMS deposits can include chalcopyrite, barite, gahnite, spinel-staurolite-sapphirine, kyanite-sillimanite, anthophyllite-orthopyroxene, spessartine, red epidote (Mn), red rutile (Cr), and loellingite [3,17]. Some of these minerals are soft and/or brittle, thus they do not survive glacial transport and deposition or subsequent post-glacial weathering as readily as more resistant phases. Chalcopyrite in glacial sediments is more resistant to weathering than other sulfide minerals and commonly forms dispersal patterns larger than those of other sulfides present in a deposit in greater abundance [3]. Oxide and silicate minerals are typically more physically and chemically robust than sulfide minerals and thus better survive glacial transport, deposition and subsequent postglacial weathering, and their dispersal fans can extend to hundreds of kilometers in length.

To investigate the potential application of MLA to indicator minerals of highly metamorphosed VMS deposits, the heavy mineral concentrates from till samples previously collected around the Izok Lake Zn–Cu–Pb–Ag VMS deposit by the Geological Survey of Canada (GSC), Ottawa, ON, Canada [11,18] were examined using MLA. Four till samples were selected for this study and all had been previously processed at the commercial laboratory Overburden Drilling Management Limited, Ottawa, ON, Canada, to prepare heavy HMC of the <0.25 mm, 0.25–0.5 mm, 0.5–1.0 mm and 1.0–2.0 mm fractions. Hicken et al. [18] and McClenaghan et al. [11] reported on indicator minerals that were visually identified in the 0.25–2.0 mm HMC of the till samples. This study tested and evaluated the e fficacy and e fficiency of MLA to determine all minerals present in the <250 μm HMC fraction, as well as the key indicator minerals of VMS mineralization.

#### *1.1. Bedrock Geology of the Izok Lake Area*

The Izok Lake VMS deposit is located in the central Slave Province, Canada, a granitic-greenstone terrane comprising deformed and metamorphosed Archean rocks hosting the Yellowknife Supergroup, a package of 2.67–2.70 Ga metasedimentary and metavolcanic rocks [19]. The Yellowknife Supergroup is divided into the lower Point Lake Formation (a suite of mafic tholeiitic to felsic calc-alkaline metavolcanic rocks and derived metasedimentary rocks, part of the Banting Group) and the upper Contwoyto

Formation (a series of iron formation-bearing greywacke turbidites) (Figure 1). These formations host numerous granitic plutons along with N–NW trending regional diabase dykes of the Helikian Mackenzie Swarm [20,21].

The deposit is hosted within the Izok Lake volcanic belt of the Point Lake Formation [22,23] that forms an arcuate belt approximately 18 km long and 1 to 5 km wide [24]. The deposit is located near the stratigraphic top of this belt. To the south of Izok Lake, the belt strikes southwest and dips steeply to the southeast, whereas to the north it makes an abrupt shift to striking northwest and dipping steeply northeast. This abrupt shift in strike is the result of regional deformation, referred to as the Izok Lake antiform.

Rocks to the south of the deposit have been interpreted to be greenschist-facies conditions with a mineral assemblage of albite–epidote–chlorite, whereas rocks to the north are interpreted as upper amphibolite–sillimanite grade with a mineral assemblage of hornblende–cordierite–sillimanite [25]. The difference in metamorphic grade to the north and south was interpreted by Thomas [25] to be the result of two regional metamorphic facies, but this idea was challenged first by Morrison [22] and later by Nowak [26]. They suggested that the rocks in the region were affected by a single craton-wide, high-temperature, low-pressure event. This conclusion is supported by similarities between P–T estimates from mineral assemblages at Izok Lake and other rocks from the Slave Craton [22], whereas the local variation in metamorphic mineral assemblages identified is attributed to variations in pre-metamorphic bulk rock composition brought about by varying degrees of hydrothermal alteration [26].

Nowak [26] related the variations in pre-metamorphic bulk rock composition to the different types and intensities of hydrothermal alteration associated with mineralization. His work calculated bulk rock compositions using detailed geochemical whole-rock analysis, and also by combining detailed modal mineral abundance data gathered by automated mineralogy (QEMSCAN) with compositional data gathered by electron microprobe analysis (EMPA). Nowak [26] used discrimination plots to compare data from Izok Lake to several unmetamorphosed to low-grade metamorphosed VMS deposits, and the bulk rock compositions of the mineral assemblages identified are consistent with those of the associated alteration haloes from the other sites.

Mineralized boulders containing upwards of 30% Zn were discovered along the west and south shores of Izok Lake by Money and Heslop [27], and this initiated more detailed local exploration and the discovery of the Izok Lake deposit. The deposit was explored by several companies, including Minnova Corp., Inmet Mining, Wolfden Resources Corp., and the current property owner MMG [28]. The deposit consists mainly of galena, sphalerite, and chalcopyrite, with a variety of other less abundant ore minerals including covellite, digenite, electrum, and native silver [22,23,27].

Izok Lake is a significant mineral resource, with total indicated and inferred resources of 14.8 Mt grading 2.5% Cu, 12.8% Zn, 1.3% Pb, and 71 g/<sup>t</sup> Ag within a group of five near-surface sulfide lenses (North, Northwest, Central West, Central East, and Inukshuk) [29], making it one of the largest undeveloped Zn–Cu resources in North America [22]. The three westernmost of these lenses subcrop under Izok Lake. The five lenses are arranged with two northern lenses, two central lenses, and one peripheral lens (The Inukshuk lens). The two north lenses are lower grade than the two central lenses and both subcrop under Izok Lake. The North lens is small and located near the surface and is considered to be the remnant trough of a sulfide lens that was eroded by glacial activity. The two central lenses represent the majority of the deposit's metal endowment, being larger and richer in Cu than the two northern lenses, and of the two the central west lens subcrops under Izok Lake. The Inukshuk lens is the least defined of the five sulfide lenses, and does not appear to subcrop in the region [22].

**Figure 1.** Geological map of the Point Lake belt, northwest Slave Province, Nunavut, Canada. The location of the Izok Lake deposit within the belt is indicated by the yellow star. Heavy black and white dashed line indicates the NWT-Nunavut border. Compiled using map data from Stubley and Irwin [30].

Within the region surrounding the Izok Lake deposit, rocks in the footwall of the deposit and to the north have been interpreted as a rhyolite protolith metamorphosed to upper amphibolite–sillimanite grade with characteristic mineral assemblages of hornblende, cordierite, and sillimanite. Hanging wall rocks are interpreted as meta-andesite (cordierite–biotite–epidote–garnet), aphyric meta-rhyolite (quartz–muscovite–sillimanite–garnet), and metagabbro (amphibole–plagioclase–biotite–garnet–magnetite). Gahnite (Zn-spinel)is ametamorphicmineral occurringin stringer-sulfidemineralized zones proximal to one of the deposit's five sulfide lenses. Gahnite was identified by Hicken et al. [18] and McClenaghan et al. [11] as a useful indicator mineral of mineralization at Izok Lake, present in a dispersal fan down ice of the deposit. In bedrock, gahnite is found in close association with quartz, feldspar, biotite, muscovite, sphalerite, chalcopyrite, and pyrrhotite [22,26,31].
