Field procedures

A total of thirteen 5–12 kg till samples were collected from twelve sites in the vicinity of the NICO deposit in 2007 (Figs 2 and 3). Samples were collected in the upper C horizon soils developed on till from hand dug pits, at an average depth of 50 cm, to obtain relatively unaltered material. Permafrost or sometimes bedrock was encountered at the bottom of most holes. Most minerals are typically weakly oxidized in C horizon soils of northern Canada except for the most labile minerals which may be partly or completely destroyed by postglacial weathering (e.g. McMartin & Campbell 2009). Three till samples were collected immediately down-ice (<150 m) from exposed mineralized zones (Fig. 3; 07MOB005, 90 m west of #25 Zone; 07MOB006, 150 m west of #3 Zone; 07MOB007, 20 m west of Discovery Zone). On the NW side of the Bowl Zone (Fig. 3), one 6 m thick section exposing a glacial diamicton in a borrow pit was sampled above (07MOB009: 180 cm depth) and below (07MOB010: 350 cm depth) the zone of oxidation to evaluate the effects of post-glacial surface weathering on till composition. Four till samples were collected at different sites away from mineralization: two samples along Lou Lake (Fig. 2) over potassic-altered porphyritic volcanic rocks of the Faber Group (07MOB012) and monzonite of the Great Bear batholith (07MOB013); and two samples over magnetite-altered metasedimentary rocks and porphyry dyke (07MOB008 and 07MOB011) (Fig. 3). Four till samples (07MOB001 to 004) were collected approximately 10 km NE from the NICO deposit area over altered metasedimentary and weakly altered porphyritic volcanic rocks of the Treasure Lake and Faber groups (Fig. 2). Although these samples were collected up-ice from NICO, they can not be considered as representing non altered background sites since additional mineral occurrences and alteration zones are located in their vicinity, including one mineral occurrence 120 m up-ice from sample 07MOB001 (Tan U-Fe showing).

Twenty-seven representative 0.5–2 kg bedrock samples were collected for indicator mineral recovery at most drift sampling sites, in the vicinity of and at nearby surface occurrences, and from the sub-surface “Portal” area at various bulk sample stock piles (see Table 1 and Figs 2 and 3). To determine what the regional and local rocks may contribute to till in terms of heavy minerals, bedrock samples included mineralized rocks from the NICO deposit (n=15), barren host rocks in the immediate NICO area (n=5), mineralized metasedimentary rocks from the Tan showing (n=1), and unmineralized and least-altered regional rocks (n=6) (Table 1). Detailed till and bedrock sample locations and descriptions are provided in McMartin et al. (in press).

Analytical procedures

Unmineralized or weakly mineralized bedrock samples were milled to <2 mm at a commercial laboratory and the mineralized rocks were milled to <2 mm at Overburden Drilling Management Ltd (ODM). All crushed bedrock samples were then processed at ODM for heavy mineral separation, panning and indicator mineral picking (Fig. 4). Samples were crushed and processed in order of increasing content of visible sulphides to minimize the potential for indicator mineral carryover between samples. Crushed vein quartz (i.e. mineral blank) was processed between bedrock samples in order to reduce and monitor contamination from laboratory equipment. The crushed bedrock fraction was pre-concentrated with respect to density using a shaking table (McClenaghan 2011). Each sample was tabled twice to increase gold grain recovery. Visible gold and sulphide grains recovered from this table concentrate and/or by subsequent panning were counted and described, then returned to the table concentrate after examination. Heavy liquid separation using methylene iodide (SG 3.2) was used to produce a final heavy mineral concentrate (HMC) from the table concentrate. After a ferromagnetic separation, the non-ferromagnetic HMC (NFM-HMC) was sieved to obtain the sand fraction (0.25–2 mm) for picking. The ferromagnetic fraction (FM-HMC) was sieved (0.25–2 mm) for further picking.

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Fig. 4

Generalized flow sheet showing steps in bedrock and till sample processing to recover indicator minerals.

Till samples were processed at ODM to recover heavy minerals, using a method similar to that used for the bedrock samples (Fig. 4). Samples were disaggregated and sieved to obtain the <2 mm (matrix) fraction prior to processing using a double-run across the shaking table. Blank samples were not inserted. After heavy liquid and ferromagnetic separations, the NFM-HMCs of till samples were sieved at >0.25 mm to obtain the 0.25–2 mm fraction. Only half of this fraction was picked for indicator minerals (the other half was used for geochemistry); therefore, all grain counts for till in the 0.25–2 mm NFM-HMC fraction were normalized by a factor of 2.

Prior to indicator mineral examination and selection, the NFM-HMCs recovered from till and bedrock samples were sieved to 0.25–0.5 mm, 0.5–1 mm and 1–2 mm. For the bedrock samples, only the 0.25–0.5 mm fraction was examined as it consisted of individual minerals grains; the 0.5–1 and 1–2 mm fractions consisted of impure lithic fragments containing the same heavy minerals. The 0.25–0.5 mm fraction was further sorted with an electromagnetic separator into fractions with different paramagnetic characteristics to help reduce the volume of concentrate to be visually examined (Averill & Huneault 2006). All fractions were examined under a stereoscopic microscope at ODM to determine the abundance of potential IOCG indicator minerals, metamorphosed or magmatic massive sulphide indicator minerals (MMSIMs) and kimberlite indicator minerals (KIMs). Checks were performed on selected grains using SEM-energy dispersive x-ray spectrometer (EDS) to confirm mineral identity. Grains considered having possible IOCG affinities (mainly sulphides and silicates) were mounted for further study. Because of their significant abundance in some bedrock samples, no more than 20–40 representative grains of the same mineral species were selected per sample for further study.

A total of 94 grains from till samples and 532 grains from bedrock samples from the NFM-HMCs were mounted on 25-mm epoxy-impregnated stubs and analyzed to confirm their identity and quantify their chemical composition. Analyses were conducted at the Geological Survey of Canada Microbeam Laboratory, Ottawa, using a CAMECA SX50 electron microprobe (EMP) equipped with four wavelength-dispersive spectrometers. Operating conditions were 20 kV accelerating voltage, and 10 nA beam current using a focused spot. Count times on peak were 10 seconds, with 5 seconds off-peak. The raw data were processed with the ZAF matrix correction. Standards comprise a range of natural and synthetic pure metals, simple oxides and simple compounds. The analysed grains were classified (or re-classified when necessary) on the basis of their chemical composition. Theoretical chemical compositions of mineral end-members (Le Maitre 1982) were used to calculate cut-off values (at approximately 50:50 mol %) for members of binary solid solution series. For minerals that contain substantial amounts of more than two end-members, the threshold values were lowered accordingly.

Grains from the 0.25–2 mm ferromagnetic fraction (FM-HMC) were visually picked randomly for each sample. As a first pass to test the usefulness of using iron oxide composition to fingerprint IOCG mineralization, five grains per till sample for a total of 65 grains and ten grains per bedrock sample for a total of 265 grains were mounted on 25-mm epoxy-impregnated stubs at SGS Mineral Services, Lakefield. The grains were analyzed at Université Laval using a CAMECA SX-100 5-WDS electron microprobe under a beam of 15 kV at 100 nA, using a range of natural and synthetic standards. After counting over the peak for 20–30 seconds, background was measured on both sides for 10 seconds at offset from the peak empirically determined to improve sensitivity. Matrix corrections were computed using CAMECA's implementation of the PAP method. These settings yield minimum detection limits as low as 20 ppm for elements such as K, Ca, Al, Si, Ti and Mg, 50 ppm for Mn, Cr and V, 200 ppm for Cu and Ni and 400 ppm for Zn. The optimized analytical routine allows analysis of one spot in c. 20 min. Of the 65 till and 265 rock sample grains, only 46 and 211, respectively, were analyzed for minor and trace elements because some of the ferromagnetic fraction grains consisted of other minerals with ferromagnetic mineral inclusions too small for accurate analysis. The complete tabling, picking and mineral chemistry results for both the 0.25–2 mm NFM-HMCs and FM-HMCs are provided in McMartin et al. (in press).

Furthermore, to evaluate the potential for finer-grained and/or lighter-weight indicator minerals in till, a 1 kg split from each bulk till sample was wet sieved to recover the 0.063–0.25 mm fraction (Fig. 4). The resulting finer fraction, weighing between 57–470 g, was separated at ODM using methylene iodide (SG 3.2). After a ferromagnetic separation, a portion of the liquid light fraction (SG<3.2), the heavy mineral fraction (SG>3.2) and the ferromagnetic fraction were mounted in epoxy on glass slides at the Geological Survey of Canada. The slides were examined using stereoscopic and petrographic microscopes at Consorminex Inc., and the grains counted (NFM grains only). Grain count results are also included in McMartin et al. (in press).

Quality Assurance / Quality Control

One arsenopyrite and two pyrite grains were found in the vein quartz blank processed prior to the mineralized bedrock concentrates at ODM. Seven of the thirteen quartz blank samples crushed and processed with the batch at ODM yielded no heavy mineral grains >0.25 mm. Traces (1–11 grains) of 0.25–0.5 mm arsenopyrite and/or pyrite were found in four of the other six blank concentrates indicating some carryover from the bedrock crusher or during the processing at ODM. Thirty hematite grains were found in the initial vein quartz blank crushed at the commercial laboratory and are presumed to represent crusher carryover from a previous client's samples. One to four pyrite or arsenopyrite grains were found in five of the other twelve vein quartz blanks processed at this commercial laboratory.

Sulphides

Six sulphide species were observed in the 0.25–0.5 mm NFM-HMCs of the bedrock samples in decreasing order of abundance: arsenopyrite >> pyrite > chalcopyrite > bismuthinite = molybdenite = cobaltite ( Table 2). Arsenopyrite is the most abundant As-bearing mineral observed in this fraction. Although arsenide minerals such as loellingite and cobaltite have been observed in mineralized bedrock at NICO (e.g. Gandhi & Lentz 1990; Goad et al. 2000a), only one grain of loellingite (FeAs₂) and three grains of cobaltite (CoAsS) were recovered from mineralized bedrock samples in this study. Arsenopyrite was found in the 0.25–1 mm size range of the NFM-HMCs from 15 of the 27 bedrock samples, in trace amounts in one barren host rock sample (CQA-07 445C) and up to about 129,674 grains/kg in mineralized and hydrothermally altered metasedimentary rocks collected at the #25 Zone (CQA-07 443A) and from the sub-surface Portal area (CQA-07 480Q). The grains are fresh, prismatic to massive, and have a silver colour and metallic luster. A few grains are euhedral and some grains contain trace amounts of Co, as confirmed by the SEM. Arsenopyrite sometimes contains inclusions of amphibole and bismuthinite (Fig. 5A). Arsenopyrite is also abundant (up to 1000 grains/sample) in the 0.025–0.150 mm size range of the pan concentrate of several mineralized bedrock samples. Arsenopyrite is entirely absent in the till concentrates, even in those samples located directly down-ice from mineralized zones enriched in arsenopyrite (e.g. #2 and #25 zones).

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Fig. 5

Secondary electron microscope images of non-ferromagnetic mineral grains from bedrock and till samples collected in the NICO deposit area: (A) arsenopyrite grain with ferroactinolite and bismuthinite inclusions, from Discovery Zone metasedimentary rocks; (B) chalcopyrite grain partly altered to goethite, from till collected in the Bowl Zone; (C) tourmaline grain intergrown with quartz, from porphyry breccia; (D) tourmaline grain with Ti-magnetite inclusions from metasedimentary rocks at the Tan showing; (E) silt-sized prismatic crystals of ferroactinolite grain, from Discovery Zone metasedimentary rocks; (F) sub-rounded ferroactinolite grain from till sample collected near the Discovery outcrop.

Bismuthinite was identified by its metallic steel-grey color. It was found in the NFM-HMCs of two mineralized bedrock samples, both collected from the sub-surface Portal area (Table 2). The grains mainly occur as small inclusions in 0.25–0.5 mm sized arsenopyrite grains (Fig. 5A). They are so fine-grained (mostly silt-size) that no individual grains are present in this fraction. Smaller bismuthinite grains were also recovered in abundance (up to 1% of sample) in the same two mineralized bedrock samples from the pan concentrates (<0.1 mm in size). Bismuthinite was recovered only in one till sample collected at the south end of Lou Lake (07MOB013), where a single bismuthinite grain was found in the 0.25–0.5 mm fraction ( Table 3).

Chalcopyrite was found in the NFM-HMCs of eight bedrock samples collected from various mineralized zones of the NICO deposit (Table 2). It occurs generally in trace amounts (a few grains) but up to 310 grains/kg at #3 Zone (CQA-07 487A2) and 3,093 grains/kg in one of the sub-surface Portal samples (CQA-07 465A). The grains are fresh, massive, and have a metallic brass-yellow colour in bedrock. About 20 grains of chalcopyrite were also observed in the pan concentrate (0.1 mm in size) of the bedrock sample collected at the #3 Zone. In till, chalcopyrite occurs in only one sample (07MOB010) but in minor amounts (18 grains/10 kg), mainly in the 0.25–0.5 mm fraction (Table 3). Some grains are fresh but most are partly altered to goethite (Fig. 5B).

Molybdenite was identified by its metallic lead-grey color. It was recovered in the 0.25–0.5 mm NFM-HMCs of only one bedrock sample collected in the sub-surface Portal area (CQA-07 482F: 61 grains/kg). Molybdenite is completely absent from the till sample concentrates.

Pyrite was found in trace to minor amounts in the 0.25–0.5 mm NFM-HMCs of several bedrock samples collected as part of this study. However it does occur in abundance (up to 27 670 grains/kg) in three bedrock samples collected at the#3 and #25 zones and from the Portal area (Table 2). The grains are sub-angular with a brassy colour and a metallic luster, but they are commonly mantled by goethite and some contain very fine, <0.1 mm magnetite inclusions. Seven representative pyrite grains with minor Co from two mineralized bedrock samples were selected for further study. Pyrite is also present generally in trace amounts in the 0.05–0.1 mm size range of eight bedrock samples as identified by panning. In till, pyrite occurs in trace amounts in a few samples, mainly in the 0.25–0.5 mm size range but also in the pan concentrate (0.025–0.2 mm) and in the 0.063–0.25 mm heavy mineral fraction (Table 3). Goethite, an alteration product of iron-bearing minerals, was found in trace amounts in a few samples and in minor amounts (32 grains/10 kg) in till sample 07MOB013. Goethite grains were also identified in both the heavy and the light 0.063–0.25 mm mineral fractions. Being non-distinctive and non-diagnostic of IOCG deposits, goethite grains and the remaining pyrite grains were not submitted for EMP analysis.

Silicates

Tourmaline was recovered from the 0.25–0.5 mm NFM-HMC fraction of three bedrock samples in considerable amounts, varying from 179 to 1088 grains/kg (Table 2). In two of these samples, both from barren brecciated porphyry dyke (CQA-07–445B&C), tourmaline grains are comprised of fractured mineral grains that are <0.05 mm in size and intercalated with small quartz grains (Fig. 5C). The third bedrock sample, from the Tan U-Fe showing, contains 179 grains/kg of dark brown, sand-sized, euhedral to massive tourmaline grains with Ti-magnetite inclusions (Fig. 5D). In till, tourmaline occurs in trace amounts (1–3 grains) in several samples, and the grains observed are black and not visually distinct, except for one till sample collected near the Tan showing (07MOB002) that contains a single distinctive, sand-sized, blue-grey tourmaline. Because the few tourmaline grains in till lack distinct physical characteristics between those collected in the vicinity of the NICO deposit and those from the up-ice samples, the utility of tourmaline as an indicator mineral at NICO is limited. Low quantities of tourmaline (1–6 grains/200 grains counted) were also found in the heavy fine sand fraction (SG>3.2: 0.063–0.25 mm) of six till samples but these have no distinct physical features either.

Ferroactinolite grains were observed, but not systematically counted, in the 0.25–0.5 mm NFM-HMC fraction of seven mineralized bedrock samples, mainly from the sub-surface Portal area, forming the major paramagnetic assemblage mineral in these samples. Ferroactinolite typically forms fragile aggregates of silt-sized prismatic crystals (Fig. 5E) or occurs as small inclusions in arsenopyrite (cf. Fig. 5A). In till, undifferentiated ferroactinolite-actinolite was found in only one sample collected near the Discovery Zone (07MOB007). This sample contains a relatively large amount of rounded grains of prismatic crystals (52 grains/10 kg) in the 0.25–0.5 mm fraction (Fig. 5F). In both the bedrock and till samples, ferroactinolite grains containing inclusions of heavier minerals such as magnetite were recovered with the ferromagnetic fraction.

Other non-ferromagnetic minerals

Other distinctive heavy minerals were observed in minor amounts or only in a few samples (Tables 2 and 3). Allanite, occurring as greenish black columnar crystals, forms the major mineral assemblage with arsenopyrite in one mineralized bedrock sample from the #2 Zone (CQA-07 441D). Scheelite, a glassy colourless mineral, was found in trace to major amounts (3,093 grains/kg) in the 0.25–0.5 mm NFM-HMCs of the two bismuthinite-bearing bedrock samples (CQA-07 458F&465A) but is absent in the till samples. The unoxidized till sample collected at depth near the Bowl Zone (07MOB010) yielded major gedrite (25% of table concentrate: 1,744 grains/10 kg), an alteration mineral that is essentially an aluminous anthophyllite. A significant number of sand-sized grains of bismutite (116 grains/10 kg), an oxidation product of bismuthinite or native bismuth, were also found in the same till sample.

Gold

Only five gold grains, 0.025–0.125 mm in size, were recovered in the pan concentrates of the entire suite of bedrock samples, mainly from the NICO deposit area (Table 2). Gold at the NICO deposit is known to occur in abundance as microscopic grains ranging from <0.001 to >0.1 mm in size, typically as inclusions within sulphides, particularly cobaltian arsenopyrite, or at sulphide and telluride grain boundaries (Goad et al. 2000a, b). Perhaps gold in the processed bedrock samples was too small to be visually seen and/or was encapsulated in sulphide minerals and therefore, was not recovered during the processing. In contrast to bedrock, gold grains (0.025–0.2 mm in size) are relatively abundant in till, averaging 11 grains/10 kg, and up to 39 grains/10 kg in one till sample collected near #25 Zone (07MOB005). Pristine gold grains are abundant in till collected down-ice of the #25 and #3 zones. None of the gold grains from till collected up-ice of NICO are pristine in shape.

Magnetite and hematite

Magnetite and hematite occur in the 0.25–2.0 mm ferromagnetic fraction of all bedrock samples in varied concentrations. The weight of the ferromagnetic fraction varies between 0.2 and 328.6 g per kg, on average 38.6 g (Table 2). It is abundant in the few bedrock samples collected from the Discovery (241 g/kg) and East (160 g/kg) zones and from two sub-surface samples from the Portal area (96 and 329 g/kg). It consists of individual magnetite (dominant) or hematite grains, bedrock fragments with disseminated magnetite (Fig. 6A), non-ferromagnetic minerals with small magnetite inclusions, and magnetite-hematite samples mixtures. In till, the 0.25–2 mm ferromagnetic fraction is generally smaller than in bedrock and varies from 0.5–87.5 g per kg, on average 8.5 g. It is significant in the till sample collected down-ice from the Discovery Zone (07MOB007: 88 g/10kg), forming close to 9% by weight of the <2 mm table concentrate. Similar to bedrock, the ferromagnetic fraction in till consists of individual magnetite (magnetite) or hematite grains (Fig. 6B), polymetallic grains including magnetite, and some hematite-magnetite mixtures. Examination of the 0.063–0.25 mm ferromagnetic fraction of till shows that many magnetite grains are irregular and have a weathered appearance while some grains have a high luster and a conchoidal fracture but few are euhedral. Several magnetite grains in this fraction appear to be intermixed with hematite. Hematite is also present in the 0.063–0.25 mm non-ferromagnetic fraction.

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Fig. 6

Secondary electron microscope images of ferromagnetic grains from bedrock and till samples collected in the NICO deposit area: (A) aggregate of magnetite and other minerals from a sub-surface bedrock sample; (B) magnetite grain from till collected at the south end of Lou Lake.

Sulphides

One of the most abundant heavy minerals recovered in mineralized bedrock is arsenopyrite. Compositions of 318 grains range from pure FeAsS to up to 17 wt.% Co trending towards cobaltite (CoAsS) endmember composition. The highest Co values are in grains from bedrock collected at the NICO #3 Zone porphyry, the NICO stockpile and the Discovery Zone magnetite-amphibole-biotite altered metasedimentary rocks (Fig. 7). Arsenopyrite compositions reported by Sidor (2000) for the NICO deposit are similar to those reported here. Sidor (2000) reported a high degree of substitution of Co for Fe particularly in gold-rich ironstone (Co range from 1.2–17.5 wt.%), which also contained endmember cobaltite and slightly arsenian pyrite, whereas unmineralized (Au-poor) ironstone contained arsenopyrite with only up to 4.9 wt.% Co and As-poor pyrite but no cobaltite.

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Fig. 7

Fe versus Co concentrations in arsenopyrite grains from bedrock (n=318). Co ± Ni substitution for Fe trends towards the cobaltite endmember composition (at c. 35% Co and 0% Fe, not shown).

Seventy-one chalcopyrite grains were analyzed from bedrock; mainly from hydrothermally altered metasedimentary rocks and 22 grains from porphyry dyke samples. Chalcopyrite is near stoichiometric in composition in most ore assemblages and this is also the case here (see McMartin et al. in press). There is no discernible difference in the composition of chalcopyrite from hydrothermally altered metasedimentary rocks versus those from porphyry samples. Eight grains of chalcopyrite from till sample 07MOB010 were analyzed and, similar to those recovered from bedrock, they are near stoichiometric in composition.

Twenty-one molybdenite grains from mineralized and hydrothermally altered metasedimentary rocks in a NICO bulk sample stockpile were analyzed. Molybdenite, like chalcopyrite, is near stoichiometric in composition with only trace amounts of Se (up to 0.6 wt.%) substituting for S (McMartin et al. in press). Seven pyrite grains were analyzed: five from mineralized and hydrothermally altered metasedimentary rocks and two from a mineralized porphyry dyke near the # 25 zone. The pyrite grains are stoichiometric in composition, and the grains from porphyry dyke contain only slightly higher As and Bi than those from the metasedimentary rocks (McMartin et al. in press).

Silicates

Fifty-nine tourmaline grains were analyzed: 20 grains from tourmaline-rich breccia (CQA-07 445C), 19 grains from porphyry (CQA-07 445B) and 20 from hydrothermally-altered metasedimentary rocks at the Tan showing (CQA-07 437A-1). Tourmaline in altered metasedimentary rocks at the Tan showing have considerably higher contents of CaO and Mg-# (closer to the dravite endmember) compared to tourmaline from porphyry and tourmaline breccia, which overlap in their compositional range and correspond to schörl (Fig. 8).

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Fig. 8

Mg-# versus wt.% CaO for tourmaline grains from bedrock (n=59). Tourmaline from metasedimentary rocks hosting the Tan showing has considerably higher contents of CaO and Mg-# compared to tourmaline from barren porphyry and breccia.

Twenty-seven ferroactinolite/actinolite grains were analyzed from one till sample collected near the Discovery Zone (07MOB007) and from two mineralized bedrock samples collected at the East Zone and Discovery Zone (CQA-07 447A and CQA-07 228A). The amphibole grains from till sample 07MOB007 are Fe-poor actinolite grading to actinolitic hornblende and ferro-actinolitic hornblende (Fig. 9). Mineralized bedrock contains much more Fe-rich amphibole than the till sample, spanning the range from ferro-actinolite to ferro tschermakitic hornblende (Fig. 9). The amphibole compositions for mineralized bedrock are similar to those reported by Sidor (2000) for mineralized and unmineralized ironstones and hornfels at NICO, which have variable but generally low mg-#(<50) and variable Si/Al ratios with Si(IV) ranging from c. 6.0–8.0.

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Fig. 9

Si(IV) versus Mg/(Mg+Fe²⁺) for amphibole grains from bedrock and till. The mineralized bedrock samples contain much more Fe-rich amphibole than the till sample, from which most amphibole grains were classified as Fe-poor actinolite.

Ferromagnetic fraction

Of the 46 ferromagnetic grains from till and 211 from bedrock analyzed by EMP, 41 and 186 consisted of magnetite whereas 5 and 25 grains were hematite, in till and bedrock respectively. The chemical composition of the NICO magnetite and hematite can be plotted in a preliminary discriminant diagram defined by Beaudoin & Dupuis (2010) to fingerprint a range of mineral deposit-types based on iron oxide composition. In a Ti+V v. Ni/(Mn+Cr) plot (Fig. 10), most magnetite and hematite grains from bedrock collected in altered metasedimentary rocks at the Tan Fe-U showing have higher Ti+V compositions compared to grains from bedrock collected at or near the NICO deposit. There is a reasonable differentiation between magnetite and hematite from NICO altered metasedimentary rocks and magnetite and hematite from regional metasedimentary rocks. In till, magnetite and hematite from samples collected up-ice of NICO have higher Ti+V compositions compared to grains from till collected over or directly down-ice of the NICO deposit and have a similar composition to many grains from bedrock collected in altered metasedimentary rocks at the Tan Fe-U showing. There is a good correspondence between the composition of grains from bedrock collected in altered NICO metasedimentary rocks and grains from till collected over or directly down-ice of NICO. These preliminary results indicate magnetite and hematite compositions have the potential to fingerprint the mineralization at NICO; however the low number of analyses prevents any further interpretation and more grains per sample must be analyzed to properly evaluate the usefulness of the method.

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Fig. 10

Iron oxide composition from till and bedrock samples plotted on a Ti+V v. Ni/(Mn+Cr) diagram to help discriminate magnetite and hematite from the NICO deposit, barren host rocks near NICO and regional rocks (see Table 1 for classification of bedrock samples). Magnetite and hematite from till samples are divided between those samples collected over or <150 m down-ice from NICO, away from NICO (<3 km) and 10 km up-ice of NICO. Till and bedrock samples collected near the TAN showing are also differentiated. For both till and bedrock samples, only Fe-oxide grains having >0.01 Ni/(Mn+Cr) are plotted.