The Surface Texture and Morphology of Magnetite from the Izok Lake

Journal of Geochemical Exploration 150 (2015) 84–103

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Journal of Geochemical Exploration

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The surface texture and morphology of magnetite from the Izok Lake volcanogenic massive sulﬁde deposit and local glacial sediments, Nunavut, Canada: Application to mineral exploration

Sheida Makvandi a,GeorgesBeaudoina,⁎,BethM.McClenaghanb, Daniel Layton-Matthews c a Département de géologie et de génie géologique, Université Laval, Quebec, QC, G1V0A6, Canada b Geological Survey of Canada, Ottawa, ON, K1A0E8, Canada c Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, ON, K7L3N6, Canada article info abstract

Article history: Magnetite is a common mineral found in a wide range of mineral deposits and in different geological Received 1 August 2014 environments. The study of surface textures and morphology of magnetite can provide information that is useful Accepted 20 December 2014 to 1) discriminate different types of magnetite such as that attributed to magmatic, metamorphic and supergene Available online 26 December 2014 environments, 2) identify host bedrocks, 3) sediment provenance, and 4) recognize chemical and mechanical processes affecting grains during erosion, transport, and after deposition in sedimentary environments. In this Keywords: study, magnetite grains from the Izok Lake volcanogenic massive sulfide deposit (Nunavut, Canada) and from Magnetite VMS deposit till covering the area have been investigated using scanning electron microscopy, mineral liberation analysis, Izok Lake and optical microscopy to document their mineral associations, surface textures, grain shape and size Surface texture distribution. Evidences such as 1) contact relations between magnetite and sphalerite, 2) sphalerite and Grain-size distribution chalcopyrite inclusions in magnetite, and 3) intergrowths of magnetite with actinolite and gahnite suggest that Shape in Izok Lake deposit and related gahnite-rich stringer zone, magnetite formed by replacement of sulfide minerals during regional, upper greenschist to amphibolite facies metamorphism. Magnetite from iron formation also formed as a result of oxidation–dissolution of almandine, or breakdown of Fe-bearing minerals during metamorphism. Euhedral, fine-grained magmatic magnetite in association with ilmenite, plagioclase and hornblende was identified in bedrock gabbro. Magnetite overgrowths on the surface of existing magnetite and other metamorphic minerals fingerprinted the supergene processes affecting bedrocks and sediments after metamorphism. Magnetite in till around the Izok Lake deposit is mostly imprinted by mechanical microtextures such as crescentic gouges, deep grooves, arc-shaped steps, and troughs that are diagnostic of transportation by thick continental ice sheets. A small proportion of magnetite grains characterized by V-shaped percussion cracks also indicate transportation by fluvial and/or glaciofluvial environments. Shape, grain-size distribution, and mineral association of magnetite in till suggest that in vicinity of the Izok Lake deposit, till has mainly been fed by the deposit and related alteration zones, though, a high proportion of grains have been derived from iron formations, bedrock gabbro, and Mackenzie dikes. © 2014 Elsevier B.V. All rights reserved.

1. Introduction abrasion and chemical alteration during transport from bedrock to depositional environments. During transport, minerals crack and/or The surface texture and shape of detrital mineral grains can be used fracture because of grain-to-grain impact (Mahaney, 2002). Before, to document their geological history from hypogene formation through during, or after deposition detrital grains can be modified by dissolution erosion, transportation and deposition (e.g. McClenaghan, 2005; of chemically unstable interlocked minerals (e.g. sulfides), and precipi- Townley et al., 2003). These eroded minerals may undergo mechanical tation of supergene minerals. Each chemical or mechanical process may impart specific microtextures on mineral surfaces (Mahaney, 2002). The study of ⁎ Corresponding author at: Département de géologie et de génie géologique, Université microtextures and shape of detrital grains may help to 1) distinguish Laval, Pavillon Adrien-Pouliot, 1065 avenue de la Medecine, Québec, QC, G1V0A6, Canada. chemical and/or mechanical processes responsible for the formation Tel.: +1 4186563141. of these textures on mineral surfaces, 2) identify mode of transport E-mail addresses: [email protected] (S. Makvandi), [email protected] (G. Beaudoin), [email protected] (Kleesment, 2009), and 3) provide an estimation of transport distance (B.M. McClenaghan), [email protected] (D. Layton-Matthews). from the source (e.g. DiLabio, 1991; Golubev, 1995; McClenaghan

http://dx.doi.org/10.1016/j.gexplo.2014.12.013 0375-6742/© 2014 Elsevier B.V. All rights reserved. S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103 85 et al., 1997; Pokhilenko et al., 2010). Golubev (1995) studied surface Dare et al., 2011, 2012; Dupuis and Beaudoin, 2011; Nadoll et al., 2012, textures of kimberlite indicator minerals (e.g. pyrope and chromite) in 2014; Sappin et al., 2014). glaciated terrains and showed that these features reflect the distance and mode of glacial transport. However, Pokhilenko et al. (2010) 1.1. Geological setting concluded that these minerals are not useful indicators for short-term glacial transport, since their grain shape and surface textures were not The Izok Lake deposit is an Archean bimodal-felsic Zn–Pb–Cu–Ag changed after a few tens of kilometers of ice transport. Dill (2007) sug- volcanogenic massive sulfide deposit located at 65° 38′ 00″ N, 112° 47′ gested that physically resistant minerals, such as corundum, kyanite 45″ W in the northern part of the Slave Structural Province, Nunavut and zircon, are reliable minerals for morphology-based environmental Territory, Canada (Fig. 1; Morrison, 2004; Hicken, 2012; Paulen et al., analysis in short-distance placer deposits, whereas Fe- and Ti-rich 2013). The deposit is situated under Izok Lake, which is 20 km west of minerals, such as ilmenite and Ti-bearing magnetite, are good measures Itchen Lake and 30 km north of Point Lake. The Slave Structural Province of long distance of transport. is located on the northwest margin of the Canadian Precambrian Shield, The shape of gold particles has been used to estimate the distance and consists of Archean granite, granodiorite and granite gneiss, and of transport from the source (e.g. Averill and Zimmerman, 1986; lesser supercrustal sedimentary and volcanic rocks of the Yellowknife Fisher, 1945; Grant et al., 1991; Knight and McTaggart, 1986; Supergroup. The Izok Lake area is mainly underlain by Archean rocks, Knight et al., 1999; Stendal and Theobald, 1994; Utter, 1979; which are overlain in the northwest by Paleoprotozoic rocks of the Youngson and Craw, 1999). Some researchers have suggested that Interior Plains in the northwest (Bostock, 1980; Thomas, 1978). the morphological evolution of gold particles is a function of the The Izok Lake deposit is hosted in 2.70–2.67 Ga metasedimentary transport media as well as the distance of transport (e.g DiLabio, and metavolcanic rocks of the Point Lake Formation, a subdivision of 1991; Giusti, 1987; Hallbauer and Utter, 1977; Herail et al., 1989; the Yellowknife Supergroup (Bleeker and Hall, 2007; Morrison, 2004; Townley et al., 2003). DiLabio (1991) proposed a simple classifica- Padgham and Fyson, 1992; Paulen et al., 2013). In the Izok Lake tion for characterization of the shape and surface textures of gold area, the Point Lake Formation comprises mostly felsic volcanic particles in till. He stated that the proportion of “pristine”, “modi- rocks with intermediate, mafic metavolcanic and intrusive rocks, and fied” and “reshaped” gold grains along the down-ice path shows metasedimentary rocks (Fig. 1). The Point Lake Formation has been the relative proximity to the ore zone. In vicinity of the ore zone, intruded extensively by syn-volcanic to post-volcanic granitic plutons there are higher proportions of pristine gold particles. DiLabio between 2.58 and 2.68 Ga (Bleeker et al., 1999). The hanging wall stra- (1991) concluded that the abundance of reshaped gold grains in a tigraphy of the Izok Lake deposit includes felsic volcaniclastic rocks, till sample indicates the remoteness to the source area. dacite, andesitic and basaltic flows, thin sulfide-rich iron formations, Most previous textural studies of glacial sediments have primarily and turbiditic sedimentary rocks (Fig. 1). Irregular granitic pegmatite, focused on quartz grains (e.g. Alekseeva, 2005; Mahaney, 1991; and diabase dikes subsequently cut all these lithologies (Hicken, 2012; Mahaney and Andres, 1991; Mahaney and Kalm, 2000; Mahaney et al., Morrison, 2004). The Izok Lake deposit consists of 5 massive sulfide 2001; Mathur et al., 2008; Narayana et al., 2010; Sweet and Soreghan, lenses (Northwest, North, Central West, Central East, and Inukshuk) 2010). These studies reveal that quartz grains in till display characteris- with total resources of 14.8 Mt grading 12.8% Zn, 2.5% Cu, 1.3% Pb, and tic microtextures (e.g. deep directional grooves, arc-shaped steps) that 71 g/t Ag (Costello et al., 2012). The ore is composed of 25% pyrite, are specific to glacial dispersal trains. Evidence of these quartz textures 24% sphalerite, 9% chalcopyrite, 8% pyrrhotite, 3% galena, and 3% has not been reported from other sedimentary environments. Mahaney magnetite (Morrison, 2004). and Andres (1991) and Mahaney (1995) used fractures and abrasion Several diabase dikes of the Mackenzie Swarm crosscut the Point features imprinted on quartz grains to identify local ice transport, over- Lake Formation. These 1.2 Ga diabase dikes trend north to northwest lying ice thickness, and depositional environments. Crescentic gouges and crosscut the Central, East and Inukshuk massive sulfide lenses of are typically formed on the surface of quartz grains that are transported the Izok Lake deposit (Buchan and Ernst, 2004). The Mackenzie dikes by continental glaciers with a thickness of N800 m. These textures have are composed of plagioclase, pyroxenes, and 1–15% fine-grained Fe–Ti never been observed on grains transported by cirque glaciers with oxides (Baragar et al., 1996). Magnetite is dominant in Mackenzie thicknesses of less than ~200 m. dikes, whereas ilmenite is subordinate or rare. In this study, we examine the micro-scale surface textures and mor- In the Izok Lake area, the felsic and intermediate volcanic rocks are ex- phological features of magnetite grains from the ferromagnetic fractions tensively intruded by gabbroic sills and dikes. These intrusive bodies are of bedrock and till at the Izok Lake volcanogenic massive sulfide deposit considered to have been volcanic feeders of the overlying flows. The gab- (Nunavut, Canada) to evaluate if they may be useful for mineral explo- broic complex is typically medium to coarse grained, with equigranular ration. Specifically, the objectives are to determine if the surface texture, and is massive to weakly foliated (Morrison, 2004). To the east textures and morphological features in till can be used to 1) identify and north of Izok Lake, dacite and basalt are overlain by carbonate and the nature of host bedrock, 2) determine the transport mechanisms, sulfide facies iron formations (Hicken et al., 2013; Thomas, 1978). 3) estimate the distance of transport from the source, and 4) identify Carbonate iron formation is well-bedded and comprises carbonate layers supergene processes that affected grains after deposition in sedimentary interbedded with equal-thickness cherty beds. Southward, the carbonate basins. Surface textures and morphology of magnetite grains from facies iron formation changes laterally into sulfide facies, which locally massive sulfides and host rocks are compared to that of magnetite grains contains pods of zinc-rich massive sulfides composed of sphalerite, in up- and down-ice till collected from various distances from the Izok pyrite, chalcopyrite, and pyrrhotite (Hicken et al., 2013). Lake deposit to document the textures formed during erosion, transport East of Izok Lake, a series of dacitic flows overlay rhyolite, marking and/or deposition. Magnetite is the focus of this study because: 1) it is a the end of felsic volcanism (Morrison, 2004; Fig. 1). The felsic flows common iron oxide mineral found in different geological environments, are fine to medium grained, and massive to well-foliated. The footwall 2) it is easily separated from sediments/disaggregated samples because to the Izok Lake deposit is comprised of felsic rocks. These rocks show it has the highest magnetic susceptibility among all naturally occurring the same geochemical signatures as those forming the immediate hang- minerals, 3) its physical properties (e.g. hardness, lack of cleavage) and ing wall. Hydrothermal alteration zones, which are regionally distribut- mechanical behavior (conchoidal fractures) are similar to quartz ed in the area, occur in both hanging wall and foot wall rocks to the (Mandolla and Brook, 2010), and therefore surface textures of magnetite deposit (Morrison, 2004). The felsic and intermediate volcanic bedrocks can be studied as indicators of transport media as well as depositional are strongly altered by widespread and pervasive development of environments, and 4) its chemical composition allows discrimination sericite. In immediate footwall and hanging wall rocks to the Izok lake of different types of mineral deposits (e.g. Beaudoin et al., 2007, 2009; deposit, the sericitization zone consists of sericite, biotite and sillimanite 86 S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103

Fig. 1. Simplified bedrock geology map of the Izok Lake deposit showing lithologies and massive sulfide lenses (modified from Morrison, 2004), and Location map of the Izok Lake area in Canada. that encloses a zone of chloritization containing chlorite, biotite and 2010 (Hicken et al., 2013; McClenaghan et al., 2012). Heavy mineral cordierite (Morrison, 2004). The hydrothermal alteration zones are concentrates (HMC) of GSC bedrock and till were prepared by the Over- also locally characterzed by intense silicification and sodium depletion. burden Drilling Management Limited (ODM; Ottawa, Canada) using In the northern and central parts of the Izok Lake area, volcanic and methods described in McClenaghan et al. (2012). Massive sulfide and sedimentary rocks are metamorphosed to the upper amphibolite– host rock samples were disaggregated using Electric Pulse Disaggrega- sillimanite facies, whereas in the southern part, metamorphism reached tion (EPD). Electric Pulse Disaggregation liberates minerals from rock the greenschist facies (Bostock, 1980; Morrison and Balint, 1993, 1999; by applying a high-voltage electric current, which breaks the rock Thomas, 1978). The upper amphibolite–sillimanite facies metamor- along grain boundaries. The resulting mineral morphologies after phism formed a gahnite-rich zone at the margins of the massive sulfide EDP reflect the original shape and grain size (Cabri et al., 2008; and the underlying stringer zone. The gahnite-rich dacite hosting the Rudashevsky et al., 2002). A heavy mineral preconcentrate was then stringer zone consists of gahnite, sillimanite, magnetite, illmenite, produced for each disaggregated rock and for till samples using a shak- pyrite, pyrrhotite, chalcopyrite, sphalerite, quartz, biotite, plagioclase, ing table (McClenaghan, 2011). The table preconcentrate was refined and hornblende (Bostock, 1980; Hicken, 2012; Morrison, 2004; using methylene iodide diluted to a specific gravity of 3.2. The heavy Morrison and Balint, 1993; Thomas, 1978). mineral concentrate was then further separated into ferromagnetic The Izok Lake area is a glaciated terrain affected by four phases of ice and non-ferromagnetic fractions using a hand magnet. The ferromag- flow (Dredge et al., 1996; Kerr et al., 1995; Stea et al., 2009). Fig. 2 shows netic fraction contains mainly mineral grains and aggregates in which the ice flow patterns in the Izok Lake region. Paulen et al. (2013) dem- magnetite and/or pyrrhotite are principle components (Hicken et al., onstrated that the oldest ice flow was directed towards the southwest 2013). The b 0.25 mm ferromagnetic fraction was archived, whereas (210° to 254°) followed by a flow directed towards the northwest the 2 to 0.25 mm ferromagnetic fractions of selected GSC till and bed- (300° to 325°). The third ice flow from east to west-northwest (279° rock samples were shipped to Université Laval (ULaval) for further to 296°) was the dominant phase that shaped the present-day land- preparation and examination of grains. scape of the area. Evidence of the youngest glacial flow direction Each ferromagnetic fraction was homogenized by hand shaking the towards the northwest (296° to 318°) is faint and rare, and are only container. Each sample was poured onto a sheet of paper and divided found on outcrops east of Iznogoudh Lake (Paulen et al., 2013). Undulat- into four sections, and one of the four sections was selected as a subsam- ing topography, glacially streamlined bedrock hills surrounded by ple, according to the method described by Gerlach and Nocerino (2003). surficial sediments and abundant lakes are characteristic of the area. The minimum number of grains for a representative subsample, based Till cover in the Izok area is generally thin, varying between b 0.5 m to on previous microtexture investigations, has been determined to be 3m(Dredge et al., 1996; Hicken et al, 2013). 25 (Baker, 1976; Krinsley and Doornkamp, 1973; Madhavaraju et al., 2009; Mahaney, 2002; Townley et al., 2003). Townley et al. (2003) indi- 2. Methodology cated that examination of at least 25 grains is required to document the range in shape of gold grains in stream sediments. Krinsley and 2.1. Sampling methods Doornkamp (1973), Baker (1976),andMahaney (2002), also stated that study of at least 25 quartz grains was sufficient to record variations A total of 22 bedrock samples and 85 till samples were collected by in surface textures and morphology of quartz in a sample. The splitting Geological Survey of Canada (GSC) in the Izok Lake region in 2009 and procedure was repeated until the subsample contained between 25 and S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103 87

Fig. 2. Locations of bedrock and till samples in the Izok Lake area (modiﬁed from Hicken, 2012). In the map, the preﬁx 09-MPB has been removed from the sample names, as sample 09-MPB-R64 is shown as R64.

50 ferromagnetic particles. A ferromagnetic particle is a mineral aggre- 2.2.2. Mineral liberation analysis (MLA) gate in which magnetite may be a component. In this study, 150 magne- The subsamples for the MLA were mounted in epoxy, and the surface tite grains from 6 bedrock samples, and 225 magnetite grains identified of the mounts was polished. The analyses were performed using the in ferromagnetic fractions of 9 till samples were examined for their sur- MLA 650 Field Emission Gun Environmental SEM at Queen's University, face textures using Scanning Electron Microscopy (SEM). The 6 rock using an accelerating voltage of 25 kV and beam currents of 10–15 nA. samples include 1) sample 09-MPB-R60 from massive sulfides, 2) sam- The MLA software collects BSE images over the sample frame to auto- ple 09-MPB-R64 from massive sulfides adjacent to gahnite-rich zone, matically discriminate mineral grain boundaries and map distribution. 3) sample 09-MPB-R61 from gahnite-rich dacite within the deposit Each particle is subdivided into grains, and grain boundaries are distin- stringer zone, 4) samples 09-MPB-R42 and 5) 09-MPB-R90 from iron guished based the greyscale variations in the BSE images. Each grain is formations located 6 km up ice to the east of the deposit, and 6) sample further analyzed by X-ray, and the collected spectra are compared to 09-MPB-R92 from the bedrock gabbro. The location of till samples, and the library of reference spectra for mineral identification (Fandrich directions of ice flowsasidentified by Paulen et al. (2013) are illustrated et al., 2007; Sylvester, 2012). For each sample, MLA produces different in Fig. 2. Other subsamples were also prepared from the bedrock and data sets including modal mineralogy, mineral association, particle till samples to determine modal mineralogy, mineral association, grain properties, grain properties (including angularity, area, perimeter, size distribution, and angularity of magnetite grains using Mineral weight proportion, length, breadth, angle length, aspect ratio, and Liberation Analysis (MLA). form factor), particle size distribution, and grain size distribution (Fandrich et al., 2007). Quantitative mineralogical data (e.g. modal min- 2.2. Analytical methods eralogy, and grain properties and size distribution) generated by the MLA measurement software for each sample are presented by DataView 2.2.1. Scanning electron microscopy (SEM) software and can be exported by the operator (Fandrich et al., 2007). The SEM was used to examine and document the surface micro- Table 1A and B summarize the modal mineralogy and weight propor- textures, morphological features, and shape of magnetite grains in the tion of minerals in the ferromagnetic fractions of the Izok Lake bedrock ferromagnetic fraction. Subsamples were cleaned using acetone in an and till samples, respectively. ultrasonic bath for 5 min. Longer immersion in ultrasonic baths may alter existing surface textures and even create new microtextures 3. Results (Porter, 1962; Vos et al., 2014). Grains were mounted on 3 mm thick carbon discs taped to 12.5 mm aluminum stubs. Grains were coated 3.1. Surface textures of magnetite in bedrock samples with gold and palladium. A JEOL JSM-840A SEM at ULaval equipped with Back-Scattered Electron (BSE) and Secondary Electron (SE) Table 2A summarizes the proportion of different microtextures modes were used to examine magnetite grains. The accelerating voltage found on the surface of magnetite grains in bedrock samples. The pro- was 15 keV and the beam current was 60 μA, at a working distance of portion of a surface texture in bedrock samples indicates the percentage 20 mm. Magnetite grains in each subsample were identified during of grains that are characterized by the given texture. The occurrence of Scanning Electron Microscopy using Energy Dispersive X-ray. each surface texture has been assessed independently to that of other 88 S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103

Table 2 Proportion of microtextures found on the surface of magnetite in A) bedrocks, and B) till at the Izok Lake area.

Type Microtextures Massive Dacite Iron Gabbro sulﬁde (%) formation (%) (%) (%)

C Precipitation 20 43 30 15

rope, almandine, spessartine, and C Dissolution 50 14 17 15 M Parting planes 10 36 17 0 C Crystallographic defects 0 21 0 14 M Conchoidal fractures 3 14 4 7

Type Microtextures % Type Microtextures %

M Adhering particles 72 M Conchoidal fractures 19 C Dissolution 70 M Sub-parallel linear fractures 17 M Abrasion 62 M Linear steps 16 M-C Overprinted surfaces 50 M Arc-shaped steps 14

rt: Garnet group minerals such as py M-C Medium relief 47 M Parting planes 8 C Precipitation 42 M Straigth grooves 8 M Fracture faces 35 M V-shaped percussion cracks 8 M-C Low relief 30 M Troughs 7 M-C High Relief 22 M Crescentic gouges 5 M Uneven fractures 21 M Curved grooves 2

Microtextures are listed based on their frequency of observation on magnetite in till. M: Mechanical texture; C: Chemical texture; M-C: Mechanical–Chemical texture. etic fractions of each sample are presented.

e, hornblende, and grunerite. Mic: Mica group minerals such as biotite and muscovite. Fds: Feldspare microtextures, as even if one texture is overprinted by another both

taurolite. Px: Pyroxene. Crn: Corundum. textures have been counted. A variety of grain morphologies and surface microtextures were observed on magnetite from the Izok Lake massive sulfides (Fig. 3). Grains from massive sulfide samples are dominated by mineral aggregates in which magnetite is associated with sulfide and silicate minerals (Fig. 3A). Magnetite from massive sulfides (MS-Mag) is subhedral to anhedral, medium to coarse grained (0.05–0.8 mm), and commonly hosts sphalerite and chalcopyrite inclusions (Fig. 3B). Pyrite, sphalerite, roportions of minerals in ferromagn and chalcopyrite are the abundant sulfide minerals in association with MS-Mag. Pyrite is typically cubic shaped, whereas MS-Mag is intergrown

y: Pyrite. Gn: Galena. Po: Pyrrhotite. Ccp: Chalcopyrite. Ghn: Gahnite. G with sphalerite and contains inclusions of muscovite and sphalerite. Muscovite, biotite, and amphibole (actinolite with lesser amounts of hornblende) are either associated with sulfides and MS-Mag, or are inclusions in these minerals. Mineral aggregates from the central parts of massive sulfide lenses (sample 09-MPB-R60) tend to be coarse-grained, whereas samples from margins of the massive sulfides (sample 09-MPB-R64) are characterized by fine-grained magnetite with sulfide anite. Qz: Quartz. Amp: Amphibole group minerals such as actinolit ed till samples description. Weight p inclusions (Fig. 3A & B). Fig. 3C shows irregular grains of magnetite in sphalerite. Sphalerite contains inclusions of chalcopyrite, whereas magne- : Zircon. Mnz: Monazite. Ax: Axinite. Sil: Sillimanite. Spl: Spinel. St: S tite contains inclusions of both sphalerite and chalcopyrite. Despite the central parts of massive sulfides where magnetite is intergrown with actinolite (Fig. 3B), towards the margins, near the contact between massive sulfides and gahnite-rich host rocks, banded actinolite and magnetite are common (Fig. 3D). In massive sulfides, parting planes are a characteristic texture in 10% of magnetite grains (Fig. 3E). Parting planes form as a

enite, titanomagnetite, and tit result of external stress in minerals, without formation of cleavage planes (Chang et al., 1998). External stress breaks minerals along their planes of structural weakness. Parallel micro-intergrowths of ﬁne layers of magnetite in sphalerite can cause octahedral parting planes (Fig. 3E). Dissolution textures are the most widespread surface texture characterizing 50% of des. IF: iron formations. Mag: Magnetite. Hem: Hematite. Sp: Sphalerite. P ﬁ MS-Mag (Fig. 3F). Dissolution textures are commonly overprinted by precipitation textures. Porous magnetite (Mag2) is a precipitation product observed on the surface of 20% of the grains (Fig. 3G). In Fig. 3H, the surface of an actinolite aggregate is covered by a thin layer of porous Mag2. Mag2 precipitated on the surface of early-formed grains locally forms layers of non-porous crystals as shown in Fig. 3I. Based on previous petrographic studies (Hicken, 2012), gahnite-rich dacite contains higher contents of magnetite relative to other bedrock A. SamplesMS (09-MPB-60)MS (09-MPB-R64)Dacite (09-MPB-R61)IF (09-MPB-R42) 49.80 111.00IF 52.00 (09-MPB-R90) Sampling depth (m)Gabbro (09-MPB-R92) UTM 140.00 East 11.00B. 141.00 UTM NorthSamples Mag 417,291 417,314 417,291Till (09-MPB-060) HemTill 7,279,944 (09-MPB-016) 7,279,934 7,279,944 SpTill 417,291 (09-MPB-066) 418,784 418,684 16 23Till 418,032 (09-MPB-052) 29.88 416,525 7,279,944Till Py UTM (09-MPB-075) East 7,279,626 416,204 3.08 7,281,312Till 0.50 7,280,202 (09-MPB-077) 2.20 415,547 2.2 0.48Till 5.2 7,280,183 (09-MPB-020) 414,413 2.6 58.06 UTM 32 Gn NorthTill 7,279,260 (09-MPB-032) 414,887 3.00 10.92 30Till 0.07 2.50 (09-MPB-002) 7,280,635 Po 411,773 7.69 18.00 15 0.006 Mag 7,281,103 0.02 413,154 0 0.00 22 0 7,280,812 Ccp 410,547 0.70 0.004 4.75 3.32 12 7,281,594 6.50 Hem Gnh 0.00 0.00 5.34 19 0.03 7,285,377 6.00 0.01 Grt 12 7.74 0.00 0.00 0 7,282,107 20.00 12 0.00 16.08 0.6 Po 5.29 0.00 0.00 Ti-m 0.00 0.00 22.5 0.85 11.00 0.72 0.73 0 0.02 Qz 0.9 0.03 23 0 11.00 13.00 1 0 0.02 0.00 Gnh 0.00 0 0 0.00 25.12 12.5 0 0 Amp 0 0.00 0.00 16.37 0.02 0.01 7.00 0.00 Mic 0 Grt 4 0.00 0.03 0.00 17.00 0.17 25 0.005 0.5 Fds 0.8 3.67 7 1.02 11.80 52.00 3.48 6 0.00 0.005 0.01 Ti-m 0.02 3 13 Ap 29.10 1.5 21 0.009 1 11 14 3 0.00 0.60 0 3.00 0.8 Zrn 0.1 0 Qz 0.06 16 0.05 60.00 39.59 15.6 Mnz 2.50 0.02 10.00 17.4 0 0.06 16 2.50 15 0.00 13 11 Ax 0.00 6 0.09 Amp 4 5.00 0.00 3.00 0.002 8.40 8.80 0.00 Sil 0.10 0.02 0.00 8.00 0.01 4.70 0.00 5.00 2.40 0.00 Mic 0.00 0.00 17.00 12.00 0.05 17 0.00 9.5 7 0.00 0.00 10.5 0.01 16.00 0.00 4.50 13.00 Fds 0.00 9 5.5 6.00 1.50 5 0.04 3.5 0.00 12 4 5 Ap 0.46 8 2 4.2 18.00 0.40 0.50 1.2 0.20 Zrn 5 2.5 0.20 0.20 10 0.02 0.10 0.10 0.00 Spl 0.02 0.00 1.50 0.01 2.00 1.84 0.00 0.00 0 St 1.80 0.00 0.02 0.03 0.00 0.65 0.20 0 0.09 0.00 0.00 0.03 16 0.1 Px 0.01 33.5 18 0.00 38 16 0.01 0.00 0.00 0.00 2.00 Crn 20 17 0.50 0.00 0.00 0.00 33 0.00 17 0.02 0.00 0.00 Abbreviations. MS: massive sul andradite. Ti-m: Ti-bearing minerals such asgroup ilm minerals such as albite, anorthite, and orthoclase. Ap: Apatite. Zrn Table 1 A) The Izok Lake studied bedrock samples description; B) The Izok Lake studi lithologies in the Izok Lake area. Fig. 4A illustrates a mineral aggregate S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103 89

Fig. 3. Typical morphology and microtextures of magnetite from Izok Lake massive sulfide lenses. A) A mineral aggregate including euhedral crystals of pyrite (Py), and magnetite (Mag) intergrown with sphalerite (Sp). B) Mineral aggregate derived from massive sulfides close to gahnite-rich zone showing intergrowths of small crystals of magnetite and sphalerite. Actin- olite (Act) filled the inter-minerals fractures. C) Magnetite replacing sphalerite and chalcopyrite (Ccp). Inclusions of these minerals are seen in magnetite. D) An aggregate of banded actinolite and magnetite. E) Parting planes in an octahedral crystal of magnetite. F) Dissolution texture on the surface of magnetite. G) Rims of porous magnetite (Mag2) precipitated on the surface of metamorphic magnetite (Mag). The arrow shows a shrinkage crack. H) Porous Mag2 precipitated on the surface of actinolite. I) Precipitationoflayersofnon-porouscrystalsof Mag2 on the surface of metamorphic magnetite grains. composed of magnetite, pyrrhotite and pyrite with quartz, gahnite, sil- The ferromagnetic fractions of iron formation samples contain limanite and sphalerite. In gahnite-rich dacite, magnetite grains are anhedral, very fine-grained (N0.1 mm) magnetite in aggregates with bi- generally larger (0.1–1.3 mm) than MS-Mag (0.05–0.8 mm). Precipita- otite, hornblende, almandine, hematite, and pyrite. Magnetite from iron tion textures characterize the surface of 43% of magnetite from formations (IF-Mag) is commonly intergrown with almandine (Fig. 4E), gahnite-rich dacite (GRD-Mag). A few GRD-Mag are partially covered and less commonly with biotite (Fig. 4F). Rims of porous Mag2 are by porous Mag2 (Fig. 4B). Typically, porous Mag2 forms complex, found on the surface of 30% of IF-Mag (Fig. 5F). Hematite has a platy tex- circular-shaped precipitation textures (Fig. 4C). Octahedral parting ture in association with magnetite (Fig. 4G). Octahedral parting planes planes characterize 36% of GRD-Mag, whereas 21% of grains show trian- are observed on the surface of 17% of IF-Mag. Crystallographic defects gular crystallographic defects (Fig. 4D). These crystallographic defects and dissolution textures are not found on IF-Mag. Magnetite from a bed- are triangular voids formed as a result of lattice irregularities, extrinsic rock gabbro sample is euhedral to subhedral, and very fine grained substitutions or interstitial impurities introduced during or after miner- (≥0.1 mm; Fig. 4H & I). In this sample, hornblende and plagioclase are al growth (Miller et al., 2009). major constituents, whereas magnetite and ilmenite are accessories 90 S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103

Fig. 4. Typical surface textures of magnetite from gahnite-rich dacite (A to D), iron formations (E to G), and gabbro (H & I). A) Mineral aggregate including magnetite, pyrite, pyrrhotite (Po), sillimanite (Sil), gahnite (Ghn), and quartz (Qz). B) Overgrowth rims of non-porous Mag2 on the surface of metamorphic magnetite. C) Rims of porous Mag2 on the surface of metamorphic magnetite. D) Triangular crystallographic defects (shown by arrows). E) Intergrowths of magnetite and almandine garnet (Alm). Enlargement shows rims of porous Mag2 precipitated on the surface of almandine. F) Intergrowths of magnetite and biotite (Bt). G) Hematite in association with magnetite in iron formations. H) A mineral aggregate from gabbro sample including euhedral to subhedral magnetite grains in association with ilmenite (Ilm), hornblende (Hbl) and plagioclase (Pl). I) Aneuhedralﬁne-grained magnetite. The surface of magnetite is characterized with precipitation texture (arrows 1 & 2).

(Fig. 4H). Ilmenite hosts hornblende microinclusions, and magnetite is grained euhedral grains disseminated in mica, amphibole or pyrox- partly replaced by hornblende-plagioclase intergrowths. Fifteen percent ene aggregates (Fig. 5D & E). In some detrital particles, magnetite is of magnetites from gabbro (G-Mag) are characterized by precipitation intergrown with quartz (Fig. 5F), or associated with Ti-bearing min- and dissolution textures (Fig. 4I). Fourteen percent of the grains also erals such as ilmenite, titanomagnetite, titanite, and/or associated show crystallographic defects. with almandine garnet (Table 1B). In contrast to bedrock samples, sulﬁdes and gahnite are found in trace amounts in the ferromagnetic 3.2. Surface textures of magnetite in till heavy mineral fractions of till samples (Table 1B). A variety of mechanical and chemical microtextures has imprinted the surface Magnetite in till in the Izok Lake area forms individual grains of magnetite in till samples (Figs. 5 to 8). The proportions of the (Fig. 5A) sporadically with layered textures (Fig. 5B). It locally con- surface textures are presented in Table 2B. Since magnetite grains tains small inclusions of apatite (Fig. 5C), muscovite, and/or rarely from different till samples were characterized by the same types zircon. Magnetite also occurs as ﬁne-grained anhedral to medium- and distribution of surface textures, the proportions in Table 2 S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103 91

Fig. 5. Typical morphology of till magnetite. A) Individual till magnetite grain with semi-rounded edges and dissolution textures. B) Platy magnetite. C) Apatite (Ap) inclusion in magnetite characterized with dissolution texture. D) Small magnetite grains disseminated in amphibole (Amp). E) Euhedral magnetite with fractured edges in association with quartz. F) Magnetite and quartz intergrowths. indicate what percentage of magnetite grains in all till samples is Strand et al. (2003), Van Hoesen and Orndorff (2004),andVos et al. characterized by each surface texture. (2014). Arc-shaped steps are deep accurate tears, similar to conchoidal fractures, but steps are deeper and more widely spaced (N5 μm). Arc- 3.2.1. Mechanical microtextures shaped steps are seen on the surface of 14% of magnetite grains in till Abrasion is defined as mechanical scraping of grain surfaces by friction (Fig. 6C), whereas, 16% of grains show linear steps that are caused by between grains during transport, producing rubbed or worn down impacts (Mahaney and Andres, 1996; Mahaney and Kalm, 2000; surfaces (Mahaney, 1995, 2002; Mahaney and Andres, 1996; Mahaney Mahaney et al., 2001). Seventeen percent of detrital magnetites display and Kalm, 2000; Mahaney et al., 2001; Strand et al., 2003). It is the most sub-parallel linear fractures (Fig. 6D). Twenty one percent of magnetite common mechanical texture characterizing the surface of 62% of magne- grains also show uneven fractures, which are characterized by a rough tite grains in till samples. Fig. 6A shows an octahedral magnetite grain that surface with random irregularities. Fig. 6E shows a magnetite grain has been abraded into a subrounded shape. More intense abrasion may with an uneven surface that was subsequently partly abraded. erase primary microtextures and smooth sharp angular edges. Grains Curved and straight grooves are shallowly indented tears on the may be abraded in various modes of transport, such as glacial or fluvial surface of till grains produced with grain-to-grain collisions during transport (Van Hoesen and Orndorff, 2004; Vos et al., 2014). transport (Fig. 6F; Krinsley and Donahue, 1968; Campbell and Fracture faces are large, features with smooth fresh surfaces, mostly Thompson, 1991; Helland and Diffendal, 1993; Helland et al., 1997; found on grains transported by alpine glaciers (Mahaney and Andres, Mahaney et al., 1988a, 1988b; Mahaney and Andres, 1996; Mahaney 1991, 1996; Mahaney and Kalm, 2000; Mahaney et al., 2001). These tex- et al., 2001, 2004; Mahaney, 1990b, 1995; Vos et al., 2014). These textures are fragile and can be overprinted by other microtextures such as tures are restricted to coarse grains (≥0.4 mm) with surfaces large sub-parallel linear fractures, conchoidal fracture, arc-shaped steps, and/ enough to be scratched by other grains during transport (Margolis and or deep grooves (Mahaney and Andres, 1991; Singh et al., 2011). Thirty Krinsley, 1974). The low percentage of groove features in magnetite five percent of magnetite grains in till show fracture faces (Fig. 6B). Con- grains in till at the Izok Lake area (Table 2B) is consistent with magnetite choidal fractures defined by Campbell and Thompson (1991), Mahaney grain size distribution in both bedrock and till samples (discussed later (1991, 1995), Mahaney et al. (1988a, 1988b, 1996, 2001), Mahaney and in this text). Throughs (Fig. 6F) and crescentic gouges (Fig. 6G) charac- Kalm (2000), Mazzullo and Ritter (1991),andStrand et al. (2003) also terize small proportions of detrital magnetite in Izok Lake till samples characterize the surface of 19% of magnetite grains in till samples (Table 2B). Throughs are deeper and wider than grooves, and crescentic (Fig. 6B). gouges are different from conchoidal fractures by their depth and cres- Steps are divided into arc-shaped and linear steps such as those that cent shapes (Campbell and Thompson, 1991; Mahaney, 1995; Mahaney have been described by Campbell and Thompson (1991), Helland and and Andres, 1996; Mahaney et al., 1988a,1988b, 2004; Watts, 1985). Diffendal (1993), Helland et al. (1997), Mahaney and Andres (1996), Parting planes on the surface of magnetite in bedrock samples Mahaney et al. (2001), Mahaney and Kalm (2000), Mahaney (1995), are also observed on the surface of detrital grains (Table 2B; Fig. 6H). 92 S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103

Fig. 6. A selection of mechanical textures on the surface of Izok Lake till magnetite. A) An octahedral magnetite with abraded edges. B) Magnetite with fracture faces (arrow 1), conchoidal fractures (arrows 2 & 3), and adhering particles in different sizes (arrows 4, 5 & 6). C) Arc-shaped steps on the surface of magnetite. D) Sub-parallel linear fractures shown by the arrow. E) Magnetite with a low relief, uneven surface subsequently affected by abrasion during transport. F) Magnetite with an accurate groove (arrow 1), a short and deep (arrow 2) and a long and shallow (arrow 3) troughs. G) Magnetite grain with overprinted surfaces including crescentic gouges (arrow 1), fracture faces (arrow 2), a shallow trough (arrow 3), and a abraded surface overprinted by adhering particles and sub-parallel linear fractures (arrow 4). H) Magnetite parting planes shown by arrow 1. The grain also has dissolution textures (arrow 2). I) V- shaped percussion cracks are shown by an arrow.

V-shaped percussion cracks are triangular-shaped fractures with or irregular (Fig. 7B) channels on the surface of magnetite. Fig. 8C variable size and depth, formed by mechanical impacts between grains shows small cavities on the surface of till magnetite produced by disso- collision (Krinsley and Donahue, 1968; Krinsley and Doornkamp, 1973; lution of magnetite (arrow 1) or by dissolution of mineral inclusions Krinsley and Marshall, 1987; Campbell and Thompson, 1991; Helland (arrow 2). Numerous, small pits on the surface of ﬁne-grained magne- and Diffendal, 1993; Helland et al., 1997; Mahaney, 1991; Mahaney tite in an amphibole aggregate (Fig. 7D), dissolved ilmenite lamellae and Andres, 1996; Mahaney et al., 2001; Mahaney and Kalm, 2000; (Fig. 7E), and also dissolved muscovite ﬂakes leave tabular cavities Mahaney et al., 2010;). A small proportion of magnetite grains in (Fig. 7F) are other textures produced by dissolution on the surface of Izok Lake till samples are imprinted by V-shaped percussion cracks magnetite in Izok Lake till samples. (Table 2B; Fig. 6I). Precipitation textures characterize 42% of magnetite grains in till (Fig. 8). In Fig. 8A to C, dissolution features are partly covered by precip- 3.2.2. Chemical mixrotextures itation of supergene magnetite (Mag2). Fig. 8D shows rims of Mag2 Dissolution features imprint the surface of 70% of detrital magnetite crystals on the broken surface of magnetite grains in till. The surface of grains (Fig. 7). Dissolution features form either linear shallow (Fig. 7A) some grains is almost entirely covered by Mag2 masking the earlier S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103 93

Fig. 7. Different forms of dissolution characterizing the surface of till magnetite grains: linear shallow channels (A), irregular channels (B), and small cavities (C). D) Dissolved surface of fine- grained magnetite aggregate in amphibole. E) Dissolution ilmenite lamellae exsolved from magnetite. F) Tabular cavities remained after dissolution of flakes of muscovite (Ms) inclusions in magnetite. mechanical microtextures (Fig. 8E). In rare occurrence, dissolution pits magnetite grains in Izok Lake till samples show overprinted surfaces are completely filledbyprecipitationofMag2(Fig. 8F). (Table 2B). Although overprinted surfaces mask or destroy other surface textures, their frequency indicates the proportion of grains 3.2.3. Mechanical and chemical microtextures that have been affected by chemical and mechanical processes. Fig. 7F Some microtextures form as a result of combination of mechanical illustrates overprinted surfaces where primary dissolution textures on and chemical processes (Vos et al., 2014). Relief refers to a mechanical the surface of magnetite have been later partly erased by abrasion. and chemical texture that reflects the variation in elevation of an area Overprinted surfaces in Fig. 8F have also formed by filling the dissolu- on the grain surface. Relief describes the roughness or smoothness of a tion pit by precipitation. grain surface, and can be classified into three different categories: low, medium, and high (Campbell and Thompson, 1991; Helland et al., 3.3. Magnetite grain shape 1997; Van Hoesen and Orndorff, 2004; Vos et al., 2014). The large difference between high and low points on the surface of a grain gives high 3.3.1. Grain shape in bedrock samples relief (Fig. 6B). Grains with low relief have smooth surfaces without Euhedral, subhedral, and anhedral are common terms used to de- topographic irregularity (Fig. 6E). The irregularities can be caused scribe the shape of minerals. Euhedral grains are well-formed, and by grain-to-grain collision during transport, or chemical weathering bounded with their easily recognized crystal faces (Best, 2002). In con- (Vos et al., 2014). In the Izok Lake area, almost half of the magnetite trary, anhedral grains have irregular shapes with undeveloped crystal grains in till have medium relief (Table 2). faces. Grains with partially developed crystal faces are classified as Adhering particles are those that are attached to the surface of subhedral. Fig. 9 displays proportions of euhedral, subhedral and grains; 72% of magnetite grains in till samples display this microtexture. anhedral crystals of magnetite in different Izok Lake bedrock samples. Adhering particles have variable grain sizes and composition (Fig. 6B; In the sample from gabbro, G-Mag occurs as euhedral to anhedral Higgs, 1969; Vos et al., 2014). Vos et al. (2014) categorized adhering grains. In this sample, 59% of magnetite grains are anhedral, and the particles as mechanical and chemical microtextures. These particles rest are either subhedral (29%) or euhedral (12%). Iron formation sam- are commonly remnants of source rocks or diagenetic environments ples contain only anhedral grains of magnetite (Fig. 9). In samples from (Mahaney, 2002). However, a low proportion of adhering particles are massive sulfides and gahnite-rich dacite, a high proportion of magnetite produced during glacial grinding, and/or transport in eolian environ- grains are anhedral (65% and 71% respectively), and the rest are ments (Krinsley and Doornkamp, 1973; Mahaney, 2002; Mahaney subhedral. and Kalm, 2000; Smalley, 1966). Adhering particles can be useful in provenance studies (Mahaney, 2002; Vos et al., 2014). 3.3.2. Grain shape in till samples Overprinted surfaces feature chemical and/or mechanical mi- In sediments, shape is commonly described by roundness or angu- crotextures overprinted by younger microtextures. Fifty percent of larity that indicate the morphology of grain corners (Wadell, 1932; 94 S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103

Fig. 8. A to C) Precipitation texture overprinting dissolution texture (shown by arrows). Arrow 2 in 9A shows triangular crystallographic defects on the surface of till magnetite. D) Overgrowth rims of Mag2 precipitated on broken surfaces of till magnetite. E) Widespread precipitation of Mag2 masked mechanical microtextures. F) A dissolution pit ﬁlled completely by precipitation of Mag2.

Janoo, 1998). Grain roundness is a function of the history of transport these grains (Fig. 10). The subangular and subrounded classes show and abrasion (Folk, 1978; Kasper-Zubillaga, 2009; Kasper-Zubillaga grains with sharp and rounded edges. In subangular grains sharp cor- et al., 2005; Pettijohn et al., 1973; Sagga, 1993). Russell and Taylor ners are dominant. Rounded edges are more common in subrounded (1937), Pettijohn (1957),andPowers (1953) developed a classiﬁcation grains. Rounded grains have very few sharp edges and corners (Fig. 10). to assess the roundness of grains. Fig. 10 illustrates established exam- Fig. 11 shows histograms illustrating the distribution of grain round- ples of magnetite grains from the Izok Lake till samples for 5 of the 6 ness of magnetite from different till samples as a function of distance classes of the roundness qualitative scale developed by Powers from the Izok Lake deposit. Only 14% of the up-ice sample (09-MPB- (1953). Very rounded grains were not identiﬁed. Very angular grains 060) is made up of angular grains, and 86% are subangular or subrounded. have sharp and jagged corners, whereas angular grains have fewer Down ice from the Izok Lake deposit, the highest proportions of till grains angles and sharp corners, though there are no signs of abrasion on are subangular, while the rest of grains are almost equally distributed

Fig. 9. Histograms show the proportions of euhedral, subhedral, and anhedral crystals of magnetite from different Izok Lake bedrock samples. S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103 95

Fig. 10. Izok Lake till magnetite grains classified into 5 roundness classes of Powers' angularity chart. between the angular and subrounded classes. The highest proportions of samples is almost identical to that of IF-Mag, though, it covers the sub-angular grains (56%) are documented in till samples 09-MPB-066 ranges of angularity of G-Mag and MS-Mag. The median angularity and 09-MPB-002. In addition, the distribution of roundness of magnetite values for G-Mag, MS-Mag, and IF-Mag are 41.5, 54 and 54.5 respectively. grains from sample 09-MPB-016 is almost identical to that of sample 002 The median value ranges from 40 to 51 for magnetite in till samples. (Fig. 11). These three samples are located down ice from outcropping mineralization of the Izok Lake deposit and West Iznogoudh (WIZ) 3.4. Magnetite grain size distribution in bedrock and till samples showing, respectively (Fig. 2). Magnetite grain size was measured using the MLA on grain cross 3.3.3. Quantitative measurement of angularity of magnetite grains sections exposed in epoxy grain mounts. Values for the maximum The quantitative angularity of magnetite grains in both bedrock and length of magnetite grains and grain size distribution are computed till samples was measured using MLA on grain cross sections. Based on for classes based on physical sieve sizes (Sylvester, 2012). The grain G. Figueroa (personal communication, July 17, 2012) angularity (A) in size distribution of a sample is characterized by D10, D50, and D97 MLA is defined as: values where D10 and D97 determine the lower and upper limits respectively, for 10% and 97% of the cumulative grain size distribution. A ¼ Σ ðÞ½Radius −½Radius Equivalent Ellipse =½Radius Equivalent Ellipse The D50 value is the median value of the particle size distribution. ð1Þ 3.4.1. Grain size distribution in bedrock samples with Radius being the distance to the particle boundary, and Radius Fig. 14A displays the grains size distribution of magnetite from Equivalent Ellipse being the minor radius of a hypothetical equivalent different bedrock samples. The grain size distribution of MS-Mag is sim- ellipse enclosed in the particle as it is exemplified in Fig. 12. Box-and- ilar to GRD-Mag, whereas IF-Mag and G-Mag have similar sigmoidal size whisker plots in Fig. 13 display values of MLA angularity for magnetite distribution. The largest magnetite grains belong to GRD-Mag, with 50% grains in bedrock and till samples. Among bedrock samples, IF-Mag of grains (D50) larger than 600 μm, whereas IF-Mag has the smallest and GRD-Mag respectively have the widest and the narrowest range grain size distribution as the value of D97 is 150 μm(Fig. 14A). The of angularity. GRD-Mag also contains the highest proportion of grains grain size ranges determined by petrographic examination support with high values of angularity as its median angularity value is 144. grain size distributions by the MLA calculations. As previously men- The median angularity value of MS-Mag is similar to that of IF-Mag, tioned, the grain size of MS-Mag varies between 0.1 mm to 1.2 mm, thought, IF-Mag covers wider range of angularity values. G-Mag whereas the grain size of If-Mag is ≥ 0.1 mm. contains the smallest median angularity value among the Izok Lake bedrock samples. 3.4.2. Grain size distribution in till samples In till samples, magnetite grains display a wide range of angularity Fig. 14B shows the cumulative grain size distribution of magnetite in with small differences. The range of angularity of magnetite in till till samples. D50 for detrital magnetite from all till samples range from

Fig. 11. Distribution of magnetite grains from Izok Lake till samples among different roundness classes. 96 S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103

et al., 2005; Morris et al., 1997; Spry, 1982, 1987a, 1987b; Spry and Scott, 1986; Spry and Teale, 2009). Gahnite can form during amphibolite facies metamorphism as a result of one of 3 different reaction paths: 1) sphalerite desulﬁdation, 2) decomposition of zincian biotite or Zn-bearing silicates such as staurolite, and 3) reactions of zinc oxide phases (Heimann et al., 2005). Gahnite is stable over a wide range of

oxygen- and sulfur fugacities. At higher ƒO2, gahnite coexists with magnetite, whereas it is associated with Fe-rich silicates and pyrrhotite at

lower ƒO2 (Sandhaus and Craig, 1986). At the Izok Lake deposit, pyrrhotite inclusions in chalcopyrite, and chalcopyrite replacement by magnetite indicate that GRD-Mag and pyrrhotite did not form simultaneously. The coexistence of magnetite and gahnite at the Izok Lake deposit is consistent with the liberation of Fe from sphalerite and chalcopyrite and formation of GRD-Mag during metamorphism. In samples of iron formation, fine-grained IF-Mag is mostly inter- Fig. 12. Equivalent Ellipse enclosed in a particle to calculate the angularity by the MLA. grown with biotite, amphibole, and almandine (Fig. 4E & F). During regional metamorphism, an increase in oxygen fugacity of the fluid 125 to 500 μm. The D10 values range from 27 to 180 μm, whereas the phase causes the breakdown of Fe-bearing minerals such as biotite, and D97 values range from 355 to 710 μm. The smallest grain size distribu- subsequent development of iron formations (Nesbitt, 1986). Brearely tion is displayed in sample 09-MPB-075, located 2.5 km down ice and Champness (1986) studied three metamorphic almandine garnets from the Izok Lake deposit (Fig. 2), in which D97 is 300 μm and D10 is with numerous magnetite inclusions. They concluded that magnetite 27 μm. However, in sample 09-MPB-066, located about 500 m down can exsolve from almandine garnet during cooling. The content of Fe3+ ice from the deposit, the average size of grains are 500 μm or larger. of host mineral and fugacity of oxygen are parameters controlling exsolu- The grain size distribution of the other till samples in both up-ice and tion of magnetite from almandine (Brearely and Champness, 1986). down-ice to the deposit is almost identical (Fig. 14B). In gabbro, textural relationships between G-Mag and ilmenite, and their association with rock forming minerals suggest their simultaneous 4. Discussion formation during crystallization of gabbroic magma (Fig. 4H). Micro- inclusions of hornblende in ilmenite indicate that the Fe–Ti oxides 4.1. Magnetite in bedrock samples formed during the late stages of magmatic crystallization (Tarassova et al., 2013). Plagioclase-hornblende intergrowths replacing G-Mag can In bedrock samples, the morphology and surface textures of a mag- be evidence of alteration of magmatic magnetite during metamorphism netite grain is determined by the conditions under which it initially (Taipale, 2013). As a result, the local gabbro includes two different gener- formed. At the Izok Lake deposit, the irregular grain boundaries be- ations of hornblende: primary magmatic hornblende hosted by ilmenite, tween sphalerite and MS-Mag formed by replacing hypogene sulfide and metamorphic hornblende intergrown with plagioclase (Fig. 4H). minerals (Fig. 3C). Sulfide replacement by magnetite has been docu- Mag2 is commonly porous and precipitated as small irregular rims mented in other VMS deposits including the Ansil deposit in the on the surface of metamorphic magnetites (MS-Mag, GRD-Mag, and Noranda district (Abitibi, Quebec; Galley et al., 1995; Boucher et al., IF-Mag), actinolite, and/or other components of the mineral aggregates 2010; Doyle and Allen, 2003). Actinolite commonly replaces silicates (Figs. 3H&4C). This textural evidence shows that the precipitation of during upper greenschist facies metamorphic conditions (Winter, Mag2 post-dated crystallization of metamorphic magnetites. Erosion 2001). Magnetite intergrown with actinolite in Izok Lake massive of the Izok Lake deposit during successive phases of ice flow exposed sulfides (Fig. 3B, D, & H) suggests formation of MS-Mag during deeper parts of the bedrock to chemical weathering. Infiltration of metamorphism. groundwater into bedrock and/or unconsolidated sediments could In gahnite-rich dacite, GRD-Mag is associated with gahnite, a zincian result in successive dissolution-precipitation processes to form Mag2. spinel (Money and Heslop, 1976), which is an accessory mineral in Dissolution creates microporosity in the labile mineral grain and metamorphosed massive sulfide deposits (Averill, 2001; Heimann increases its permeability, resulting in increased infiltration of fluids

Fig. 13. Box-and-whisker plots showing values of angularity of magnetite grains obtained from Eq. (1) in Izok Lake bedrock and till samples. S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103 97

Fig. 14. Cumulative weight percent passing graphs showing grain size distribution of magnetite in A) bedrock samples and B) till samples. into the mineral grain. Supersaturation of fluids from dissolved min- ilmenite), in association with magnetite in ferromagnetic fractions of erals/materials results in precipitation of new mineral, which may the Izok Lake till samples indicate that a high proportion of detrital par- form porous circular overgrowth deposits. The porous texture facilitates ticles were derived from lithologies other than massive sulfides. In addi- the infiltration of fluids. Development of porosity in unstable minerals is tion to bedrock gabbro, Mackenzie dikes crosscutting the Point Lake a function of: 1) differences in molar volume between parent and Formation in the Izok Lake area (Hicken et al., 2013; Morrison, 2004) product minerals, and 2) the relative solubility of the host mineral in are composed of plagioclase, pyroxene, and Fe–Ti oxides including mag- aqueous fluids (Putnis, 2009; Putnis and John, 2010). The transforma- netite and ilmenite (Baragar et al., 1996). Buddington and Lindsley tion of minerals, such as hematite (a hexagonal structure) to magnetite (1964),andBaragar et al. (1996) suggested that formation of individual (a spinel structure), is accompanied by volume loss, which can increase magnetite and ilmenite grains is the result of oxidation–exsolution of porosity (e.g. Baguley et al., 1983; Deo et al., 1989; Matthews, 1976). spinel at intermediate temperatures, whereas they commonly form a Irregular cracks commonly found near porous textures are “shrink- complete solid solution at high temperatures. In Mackenzie dikes, age cracks”, likely the result of volume loss in mineral replacement magnetite is characterized by pitted and mottled surfaces indicating reactions (Fig. 3G; Larson, 1983; Deo et al., 1989; Boucher et al., incipient alteration (Baragar et al., 1996). The presence of hematite 2010). and almandine garnet in association with magnetite in detrital mineral aggregates (Table 1B) may discriminate the proportion of particles that 4.2. Detrital magnetite have been eroded and transported from iron formations. However, under post-magmatic conditions and in supergene environments

4.2.1. Mineral associations where lower temperatures and higher oxygen or H2O fugacity are prev- Abundance of rock forming minerals (e.g. quartz, feldspar, mica, am- alent, hematite can replace magnetite and/or ilmenite (e.g. Cornwall, phibole, pyroxene; Table 1B), and Ti-bearing minerals (most commonly 1951; Hu et al., 2014; René, 2008; Tarassova et al., 2013). Thus, post- 98 S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103 magmatic alteration and/or supergene processes may in part explain (Table 2B), suggest that this proportion of the grains in different till the abundance of hematite in till samples. samples (09-MPB-060, 09-MPB-016, 09-MPB-020, 09-MPB-002, 09- MPB-032) were previously affected by fluvial or glaciofluvial processes. 4.2.2. Mechanical textures High proportions of till grains with overprinted surfaces in the Izok Lake Results show that magnetite from Izok Lake till samples have been area suggest that these grains have likely experienced different cycles of imprinted by a variety of mechanical microtextures (Table 2B). erosion, transport, and deposition. The four different episodes of ice Mechanical textures are mainly diagnostic of the transport media. flow documented in the Izok Lake are likely the causes for reworking Mineral grains transported by glaciers are characterized by conchoidal of grains through time. fractures, subparallel linear fractures, grooves, troughs, steps, and crescentic gouges (Krinsley and Doornkamp, 1973; Mahaney, 1995, 2002; 4.2.3. Chemical textures Mahaney and Andres, 1996; Mahaney and Kalm, 2000; Mahaney et al., Dissolution, and precipitation textures (Mag2) characterize a high 2001; Van Hoesen and Orndorff, 2004). Among these features, crescen- proportion of magnetite in till samples (Table 2B). Dissolution and pre- tic gouges, deep grooves and arc-shaped steps are considered to be typ- cipitation microtextures can pre-date or postdate a glacial episode ical for continental glaciers that are N 800 m in thickness. These textures (Mahaney, 2002). Overgrowth rims of Mag2 on the surface of magnetite have never been observed on grains transported by glaciers b 200 m in till must be a post-depositional product because: 1) this delicate thick (Mahaney, 1990a, 1991). The presence of these textures on till microtexture is not expected to survive during mechanical transport, magnetite grains demonstrates that they have been transported by a and 2) Mag2 commonly overlies mechanical microtextures that formed thick continental glacier, most likely the most recent glacier (Laurentide during fluvial and glacial transport (Fig. 8). During glacial transport, if Ice Sheet) during the Wisconsinan. The grains may also have been some grains sojourn in the ice, they remain unscathed (Mahaney, glacially transported during earlier glaciation of the Izok Lake region 2002). Thus, formation of Mag2 on the surface of magnetite in till can and then re-entrained by the younger Wisconsin ice flow. even pre-date the last glacier affecting the area. Mag2 is most likely a Detrital grains transported by glaciers, especially at the base of thick supergene magnetite that formed during chemical weathering of ice sheets, commonly show high relief because of severe fracturing hypogene minerals in bedrock and till samples. during glacial crushing (Mahaney, 2002). However, in Izok Lake till samples, magnetite grains with low and medium relief are dominant 4.3. Comparison between Izok Lake detrital magnetite and previous studies (Table 2B). During long-distance transport, reworking, and/or transport in mixed environments (e.g. glaciofluvial environments), abrasion is A comparison between the results of the study of surface textures of dominant compared to fracturing, thereby reducing the grains relief magnetite in the Izok Lake till samples with that from other shows the by obliteration of angular edges (Mahaney, 2002). Chemical processes potential of magnetite microtextures as environmental discriminators, such as dissolution of angular edges or filling the dissolution pits with and emphasize the significance of the textural and morphological stud- precipitation (Fig. 8F) can also reduce the grains relief. ies in mineral exploration using magnetite. Fig. 15 compares the surface Previous transportation by glacial meltwater must also be consid- textures of magnetite in Izok Lake till samples and detrital magnetite of ered for some of the magnetite grains recovered from till. Grains modi- the Wahianoa moraines (Ruapehu Mountains, New Zealand; Mandolla fied by fluvial transport are characterized by grain surfaces that are and Brook, 2010) with detrital quartz in Weichselian till in Estonia abraded, have rounded edges, and display V-shaped percussion cracks and Latvia (Mahaney and Kalm, 2000). Among varieties of indicator (Krinsley and Doornkamp, 1973; Mahaney, 2002). V-shaped percussion minerals, the morphology and shape of placer gold particles have been cracks, present on a small proportion of magnetite in till samples well studied so far (e.g. DiLabio, 1991; Grant et al., 1991; Hallbauer

Fig. 15. The diagram shows comparison between frequency of occurrences of microtextures in Izok Lake till magnetite, Wahianoa magnetite, and Weichselian quartz. S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103 99 and Utter, 1977; Townley et al., 2003). However, comparing the results energy. This process consequently increases the mineral stability of magnetite with gold is not useful to understand transport conditions (Best, 2002; Cornell and Schwertmann, 2006; Higgins, 2006). Magnetite of magnetite, because these minerals differ noticeably in their physical most commonly forms octahedral or rhombo-dodecahedra crystals, in properties. These differences can result in formation of different surface between, rhombo-dodecahedra forms are produced in lower energy textures and morphology in response to mechanical and chemical surfaces (Cornell and Schwertmann, 2006; Heider et al., 1987). processes affecting grains during erosion, transport, and deposition. Intergrown octahedra (twins), cubes, spheres, and bullets are less com- Thus, magnetite from Izok Lake was compared to detrital magnetite of mon crystal habits formed by magnetite. In all Izok Lake bedrock sam- the Wahianoa moraines derived from magmatic rocks (Mandolla and ples, the highest proportion of magnetite grains are characterized by Brook, 2010), which is only other study documenting the surface anhedral crystals (Fig. 9). With the exception of G-Mag, abundance of textures of magnetite in glaciated terrains. anhedral magnetite grains in Izok Lake bedrock samples is consistent Unlike magnetite in Izok Lake till samples, Wahianoa magnetite is with formation of magnetite by replacing previous sulfide and/or not imprinted by various types of mechanical fractures. Microtextures silicate minerals during metamorphism. Under metamorphic condi- such as uneven fractures, curved grooves, straight grooves, troughs, tions, new minerals grow in the solid state and to form their crystal arc-shaped steps, parting planes, crescentic gouges, and V-shaped per- habits where they compete with previous, unstable minerals for space cussion cracks were not reported for Wahianoa magnetite (Fig. 15). (Best, 2002). Thus, the shape of minerals in metamorphic rocks is The most common mechanical microtexture documented among controlled to some extent by interfacial free energy between growing Wahianoa magnetite grains is linear steps, which is observed on the sur- and neighbouring crystals (Buerger, 1947; DeVore, 1956, 1957; Kretz, face of 33% of the grains Conchoidal fractures, fracture faces, sub-parallel 1966). Despite tendency of metamorphic systems towards equilibrium linear fractures, and abrasion are other less common mechanical states, metastable minerals are common in metamorphic rocks because textures among Wahianoa magnetite grains. The low diversity and of 1) slow kinetic rates, and 2) modifications in parameters such as tem- frequency of mechanical microtextures along with lack of overprinted perature, pressure and fluid activities over time that drive the system surfaces suggest that Wahianoa magnetite has not experienced out of equilibrium (Best, 2002). In Izok Lake bedrock gabbro, magmatic reworking. These patterns for the Wahianoa magnetite are consistent magnetite is characterized by a range of euhedral to anhedral crystals with the moraine's formation by small cirque glaciers (Mandolla and (Fig. 9). Unlike crystallization under metamorphic conditions, during Brook, 2010). Adhering particles is the most common mechanical– fractional or equilibrium crystallization of a magma, mineral phases can chemical microtexture documented for Wahianoa magnetite, though nucleate in the melt, grow, and frequently form their characteristic crystal its frequency is almost half of that for Izok Lake magnetite from till sam- habits without restriction from neighbouring minerals (Best, 2002). ples (Fig. 15). This difference may indicate that Wahianoa magnetites The distribution of quantitative angularity of magnetite grains in have not been affected by severe glacial grinding during transport Izok Lake bedrock samples (Fig. 13) is consistent with the shape of mag- (Krinsley and Doornkamp, 1973; Mahaney, 2002; Mahaney and Kalm, netite and its qualitative angularity in these samples (Figs. 3 & 4). 2000; Smalley, 1966). The absence of dissolution and precipitation Euhedral crystals of magnetite form angular grains because of their further documents that Wahianoa magnetite was not affected by post developed crystal faces and consequently sharp corners (Fig. 4I), where- deposition chemical processes. As a result, Wahianoa magnetite has as subhedral crystals form both angular and sub-angular grains as had a less complex history than magnetite in till in the Izok Lake area. shown in Fig. 3I & A respectively. Anhedral crystals of magnetite can The result of magnetite in Izok Lake till samples was also compared form very angular (Fig. 3B), angular (3C & 4C), sub-angular (Fig. 4E& to that of quartz because quartz has been extensively studied, and is a F), sub-rounded (3D, 3H & 4G), and rounded (3H) grains. As a result, standard for comparison of grain shape and surface texture of other although all magnetite grains from the iron formation samples are minerals (Basu, 1985; Basu et al., 1975; Beal and Shepard, 1956; considered as anhedral, their distribution among different classes of Griffiths, 1967; Kasper-Zubillaga, 2009; Kasper-Zubillaga et al., 2005; roundness of Powers (1953) explains their distribution over a wide Mahaney, 2002; Mandolla and Brook, 2010; Pettijohn et al., 1973; Shep- range of quantitative angularity (Fig. 13). ard, 1973; Shepard and Young, 1961). In addition, magnetite and quartz have similar physical properties (e.g. hardness, conchoidal fracture), 4.5. Shape of magnetite grains in Izok Lake till samples and both are ubiquitous minerals found in a variety of igneous, metamorphic and sedimentary rocks. Comparison of magnetite in Izok Lake Results from Izok Lake till samples show that angularity of magnetite till samples with Weichselian quartz described by Mahaney and Kalm in sediments, similar to quartz grains (Costa et al., 2013; Goudie and (2000) shows that the majority of the textures on the surface of quartz Watson, 1981; Kleesment, 2009), is initially determined by the original grains are the same as those on magnetite in Izok Lake till samples grain shape in host rocks, and then is noticeably controlled by the mode (Fig. 15). The diversity of microtextures and their frequency in both of transport and distance. The majority of magnetite grains in Izok Lake the Izok Lake magnetite and the Weichselian quartz support complex till samples are angular and subangular (Fig. 11). Proportions of sub- transport histories. Magnetite in Izok Lake till samples and Weichselian angular grains in Izok Lake till samples may indicate the proximity to quartz, both have experienced transportation in basal layers of conti- the mineralization since there is an increase in proportion of sub- nental ice sheets producing significant mechanical forces during grain- angular magnetite grains down ice towards the Izok Lake deposit, and to-grain collision. The V-shaped percussion cracks, common to both immediately after the WIZ showing (Fig. 11). MS-Mag and GRD-Mag Weichselian quartz and Izok Lake magnetite indicate transport in form subhedral to anhedral crystals (Fig. 9)withlimitedrangeofhigh glaciofluvial environments. This hypothesis is also supported by a high angularity values (Fig. 13). In contrary to sub-angular grains, propor- proportion of abrasion textures for both minerals. The dominance of tions of angular and sub-rounded grains decrease towards the Izok mechanical relative to chemical textures in Weichselian quartz suggests Lake deposit (Fig. 11). High proportions of angular and sub-rounded that the surface of this mineral was mainly damaged during transport, grains in till samples away from the mineralized zone likely fingerprint and less affected by chemical processes causing dissolution and/or IF-Mag and G-Mag, and also indicate the impact of transport media. precipitation of SiO2. The abundance of angular and sub-angular magnetite grains, togeth- er with the absence of very rounded grains and the low frequency of 4.4. Shape of magnetite grains in Izok Lake bedrock samples rounded grains in Izok Lake till samples are consistent with glacial transport. In fluvial and/or marginal marine environments, intense The equilibrium form of a mineral is typically controlled by its crys- abrasive processes will round the grain edges and the sediments will tallographic structure (Higgins, 2006). In equilibrium conditions, be dominated by rounded and very rounded grains (Madhavaraju minerals tend to form euhedral, coarse crystals to minimize the total et al., 2009; Mahaney, 2002; Moral Cardona et al., 1997). The glacial 100 S. Makvandi et al. / Journal of Geochemical Exploration 150 (2015) 84–103 crushing of bedrock, entrainment and transport of grains by four phases 3) distinguish transport media. The investigation of magnetite from of ice flow in the Izok Lake area could in part explain the abundance of 0.25 mm to 2.0 mm HMC fraction size of Izok Lake bedrock and samples angular and subangular grains in the surface till. Angular grains can also reveals that: 1) Subhedral to anhedral, medium-grained magnetite con- be formed by short distance transport in highly energetic fluvial envi- taining sulfide inclusions, and in association with gahnite signatures ronments. However, in such environments, there is not sufficient time metamorphosed massive sulfide deposits and related alteration zones. to abrade the grain edges to form subangular grains (Helland and 2) Euhedral to subhedral, fine-grained magnetite in association with il- Holmes, 1997). The absence of very angular grains can be related to menite, hornblende and plagioclase fingerprints formation of magnetite 1) their scarcity in the source rock, and 2) abrasion of grain edges during during crystallization of a gabbroic magma. 3) Anhedral, fine-grained transport. Dissolution and precipitation can also affect the shape of a magnetite in association with/or as an inclusion in almandine, biotite, grain. Dissolving grains edges and/or overgrowths on the surface of or hornblende is the fingerprint of iron formations. Hematite is also a grains modifies their angularity. common association of IF-Mag. 4) Intergrowths of magnetite and actinolite indicate the metamorphic formation of magnetite. 5) Overgrowths 4.6. Grain size distribution of magnetite in Izok Lake bedrock and till deposits of magnetite on existing magnetite and/or other components samples of mineral aggregates are typical of supergene environments. The investigation of magnetite surface textures in Izok Lake till In the Izok Lake massive sulfides and gahnite-rich dacite, the grain samples also shows that they are diagnostic of transport media, as: size of magnetite is likely controlled by rates of dissolution of previous 1) Mechanical textures such as conchoidal fractures, subparallel linear minerals (e.g. hypogene sphalerite) and precipitation rate of magnetite. fractures, grooves, troughs, steps, and/or crescentic gouges documented The dissolution-precipitation reactions are affected by 1) the composi- on magnetite in Izok Lake till samples indicate that magnetite has been tion of fluid phases, 2) solubility of parent and new mineral phases, transported by glaciers. Crescentic gouges, deep grooves and arc-shaped 3) temperature, 4) pressure, 5) Eh-pH stability fields of magnetite and steps are also signatures of thick continental glaciers with N 800 m its precursors, and 6) oxygen and sulfur fugacity (Putnis, 2002; Putnis thickness. 2) Magnetite grains characterized by V-shaped percussion and Putnis, 2007; Xia et al., 2009; Putnis and John, 2010; Pollok et al., cracks indicate transportation by fluvial or glaciofluvial environments. 2011; Jonas et al., 2013; Borg et al., 2014). The grain size of minerals 3) Overprinted surfaces of magnetite grains in till indicates these grains in magmatic rocks such as the Izok Lake bedrock gabbro is mainly con- have likely experienced different cycles of erosion, transport, and depo- trolled by the nucleation and growth rates in magma, which are, in turn, sition that is consistent with 4 phases of ice flow affecting the Izok Lake functions of the cooling rate of magma (Best, 2002; Frost and Frost, area through time. 2014). The cooling rate of magma is also controlled by the temperature The angularity of magnetite in till samples indicates the origin shape of magma, and the temperature and depth of rocks enclosing magma of the mineral, and mode of transport as: 1) Euhedral to subhedral (Frost and Frost, 2014). At slower cooling rates, fewer but larger grains magnetite forms angular to sub-angular crystals, whereas anhedral form since the nucleation rate is slower than the growth rate. The ease magnetite can form very angular to rounded grains. 2) Glacial transport of nucleation can also determine the grain size distribution of magmatic increases the angularity of magnetite by fracturing, whereas fluvial minerals as nucleation of Fe–Ti oxides in magma results in formation of transport reduces the angularity by abrasion and rounding the grain small grain size populations (Best, 2002) similar to G-Mag. corners. In Izok Lake till samples, the abundance of angular and sub- The grain size distribution of magnetite is almost identical among angular magnetite grains, the absence of very rounded grains, and the Izok Lake till samples, and is similar to that of MS-Mag and GRD-Mag low frequency of rounded grains are consistent with glacial transport. (Fig. 13). Specifically in till sample 09-MPB-066, located close to the de- The grain size distribution and angularity of magnetite grains in till sam- posit, magnetite grains have the average size of 500 μm that is between ple located about 500 m down ice from the deposit, suggest that in close the average sizes of MS-Mag and GRD-Mag (425 μm and 600 μm respec- proximity to the deposit, magnetite grains in till are mainly derived tively). In contrast, the average sizes of IF-Mag and G-Mag (32 μmand from the mineralization. 75 μm, respectively) are significantly smaller than that of till samples (125 to 500 μm; Fig. 13). However, 6% of IF-Mag, and 19% of G-Mag Acknowledgements arelargerthan125μm. These data demonstrate that analyzing the 0.25 mm to 2.0 mm HMC fraction size of Izok Lake till samples may This research was funded by the Geological Survey of Canada under have caused under-representation of magnetite from iron formation its Geo-mapping for Energy and Minerals (GEM-1) program (2008– and gabbro due to their fine-grained nature. This also indicates 2013), Natural Science and Engineering Research Council (NSERC) of that magnetite from other lithologies may not be represented in a Canada, and industry partners Overburden Drilling Management Limited, 0.25 mm to 2.0 mm particle-size HMC if it has a small grain-size and/ MMG Limited, Teck Resources Limited, and AREVA Resources Canada. We or is interlocked with other minerals, if their aggregate size is very would like to thank all who collaborated on this project: A. Hicken small. Brooker et al. (1987) and Koski (2012) showed that hypogene (Queen's University) for the bedrock and till sampling; A. Ferland and minerals in un-metamorphosed massive sulfide deposits commonly M. Choquette for providing technical assistance with the SEM and display a narrower range in grain size distribution than that of meta- EPMA at U-Laval; E. Lalonde (U-Laval) for text editing; G. Figueroa morphosed deposits. Klein (2005) also realized that grain size distri- (FEI Australia) for assisting with the MLA software; and M. Javidani bution of magnetite formed during metamorphism is commonly (U-Laval) for his technical assistance. Karen Kelley and Thomas Angerer controlled by the grade of metamorphism wherein higher grades of are thanked for their insightful reviews of this manuscript in Journal of metamorphism are characterized by coarser magnetite. 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