Ferguson Lake

Nunavut, Canada

Main commodities: Ni Cu Co Pd Pt PGE PGM
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The Ferguson Lake Ni-Cu-Co-platinum-group element (PGE) deposit is located in Nunavut, Canada, ~250 km west of Rankin Inlet, which is on the western coast of Hudson Bay.

The Ferguson Lake deposit lies within the Hearne domain of the Archaean Churchill Province in the Canadian Shield, east of the Snowbird tectonic zone separating the Rae domain to the NW from the Hearne domain (Hanmer et al., 2004). The NE-trending and NW-dipping Tyrrell shear zone divides the Hearne domain into the northwestern and central Hearne subdomains, both composed of tholeiitic-dominated greenstone belts and associated sedimentary and plutonic rocks, enclosed by extensive Archaean tonalite-granodiorite rocks, overlain by Palaeoproterozoic metasedimentary rocks of the Hurwitz Group, and intruded by the 1.83 Ga Hudson granite (MacLachlan et al., 2000). In the structural hanging wall of the Tyrrell shear zone, the NW Hearne subdomain contains the NE-SW striking, ~2.71 to 2.66 Ga Neoarchaean Yathkyed greenstone belt (Hanmer et al., 2004, 2006).

The Ferguson Lake deposit is located on the northeastern tip of the Yathkyed greenstone belt, which has been divided into two distinct lithologic panels: i). a lower overturned sequence of greenschist to lower amphibolite facies volcanic and metasedimentary rocks, and ii). an overlying upper panel composed of amphibolite facies gneisses (after volcanic and siliciclastic protoliths) and amphibolite facies metavolcanic rocks, both intruded by multiple granitic sheets (MacLachlan et al., 2005).

Syntectonic granitic sheets within the Tyrrell shear zone, and cutting the base of the Yathkyed greenstone belt, have been dated at 2.655 to 2.629 Ga (U-Pb; MacLachlan et al., 2005), overlap several deformational episodes postulated at ~2.66 to 2.62, 2.62 to 2.60 and 2.56 to 2.49 Ga within the NW Hearne subdomain (MacLachlan et al., 2005). The ~2.66 to 2.62, 2.62 to 2.60 Ga ages are interpreted to represent the amalgamation of the NW Hearne subdomain and the Rae Domain along the Snowbird tectonic zone, while the 2.56 to 2.49 Ga thermal event is believed to be the result of the uplift and emplacement of the upper panel of the Yathkyed greenstone over the lower panel (MacLachlan et al., 2005).

The Ferguson Lake area is located near the NW periphery of the Yathkyed greenstone belt, at its NE tip (MacLachlan et al., 2000), and is underlain by deformed Archaean gneissic, metavolcanic and metasedimentary rocks, and by tonalitic, granitic and gabbroic intrusive rocks (Martel and Sandeman 2004), all of which are intruded by Proterozoic gabbroic and syenitic plutons and associated lamprophyre dykes (Martel and Sandeman 2004). The dominant regional metamorphic assemblage in the rocks of the area, hornblende-plagioclase±biotite±garnet, indicates that peak metamorphism and deformation occurred under amphibolite facies conditions, with at least three phases of deformation having been recognised in the Ferguson Lake area (Martel and Sandeman 2004; Carter 2006). Campos-Alvarez et al., 2012 report there are no published geochronological data for the rocks immediately hosting mineralisation or for the subsequent metamorphic and deformation events at Ferguson Lake, nor to determine the stratigraphic and structural relationships between the upper and lower panels of the Yathkyed greenstone belt.

Massive, disseminated and vein sulphide mineralisation at Ferguson Lake is hosted toward the structural hanging wall of a variable 10 to 600 m thick, north-dipping gabbroic intrusion, that is concordant with the enclosing gneisses and amphibolites (Nicholson, 2007). The massive to semi-massive sulphide is near the structural hanging wall of the metagabbro, while a low-sulphide, high-PGE zone of sulphide veins and disseminations is locally found ~30 to 50 m below the main massive sulphide.

The host sequence is inferred to be on the north-dipping overturned limb of a regional, NE-striking, F2 fold, in which the main foliation is interpreted to represent deformation associated with an F2 folding event (Martel and Sandeman 2004), placing the massive sulphide mineralisation at the base of the gabbroic sill (Miller 2007; Martel and Sandeman 2004). The F1 and associated foliation, and the F2 folds, are inferred to represent deformation events at ~2.62 and 2.55 Ga, described to the south of Ferguson Lake (Martel and Sandeman 2004). F3 folding and ductile-brittle deformation overprinted the earlier events (Martel and Sandeman 2004). Amphibole ±biotite schists, hornblende-plagioclase gneisses, and hornblendite represent the metamorphosed gabbro sill, which whole-rock geochemistry suggests is of tholeiitic affinity (Carter 2006; Miller 2007). Martel and Sandeman (2004) can see no distinctive lithological breaks within this intrusive, with only gradational contacts evident. As such, it is believed to represent a multiphase or differentiated mafic intrusion and has been informally named the Ferguson Lake Intrusive Complex (Carter 2006; Miller 2007). The Ferguson Lake Ni-Cu-Co-PGE ores are interpreted to have originated through segregation and subsequent crystallisation of an immiscible sulphide liquid, derived from a tholeiitic parental magma (i.e., the metagabbro) that was emplaced into a sequence of supracrustal rocks (Carter 2006; Miller 2007; Martel and Sandeman 2004).

The mineralisation strikes ~east-west over an interval of >10 km, and is evident at the surface as an up to 30 m wide gossan. Drilling and geophysics suggest the mineralised zones, which extend to depths of ~450 m below surface, and comprise the West Zone Extension, West, Central and East zones along strike. The west zone hosts the bulk of the indicated resources (as listed below). Geophysical data have established that massive sulphide mineralisation extends further along strike and plunges deeper in the western part of the deposit area (Visser 2002).

Two styles of mineralisation are recognised at the deposit:
    i). The main Cu-Ni-Co-Pt-Pd mineralisation, comprising massive to semi-massive pyrrhotite-pentlandite-chalcopyrite-pyrite, occurring as laterally discontinuous and irregular pods and lenses toward the base/structural hanging wall of the host metagabbro. Magnetite is an important constituent, and Visser (2002) suggested PGE grades increase in the presence of magnetite. Minor galena is also present (Beirnes 2004). Massive sulphide mineralisation (40 to 50% total sulphides) is dominated by pyrrhotite, and occurs as a series of irregular, north dipping, stacked to coalescing lenses, individually 1.5 to >8.0, to as much as 21 m thick, and from tens to hundreds of metres long. Semi-massive sulphides (a combination of disseminated, vein, interstitial and replacement sulphides) are found at the transition from massive sulphide to the enclosing country rocks. Semi-massive sulphide lenses locally overlie (structurally underlie) the massive sulphides at deeper levels, and have the same mineralogy. In some cases, the sulphide of these deep lenses are interstitial to coarse garnet aggregates. These semi-massive sulphide intervals contain ~0.19 to 0.50 g/t Pt and 2.18 to 4.30 g/t Pd and are enriched in PGE by a factor of about 2 to 3 compared to the main massive sulphides which carry 0.01 to 1.00 g/t Pt and 0.42 to 2.78 g/t Pd.
    ii). A low-sulphide (3 to 5% total sulphides) style that comprises mm- to cm-thick vein, disseminated, interstitial and varitextured sulphides hosted by the metagabbro. This style commonly occurs about 50 to 70 m structurally below the main massive sulphide mineralisation, and contains higher Pt (0.27 to 17.01 g/t) and Pd (0.35 to 23.85 g/t) compared to the massive sulphide (Nicholson 2007). Drill core assay data indicate intervals of low-sulphide high-PGE mineralisation occurring immediately below the massive sulphide and, in some cases, extending continuously throughout the metagabbro, toward the contact with the enclosing country rocks (Campos-Alvarez et al., 2012).

Pentlandite and minor violarite are the dominant nickel bearing sulphides and are finely intergrown with pyrrhotite. Chalcopyrite is the copper-bearing sulphide, occurring around pyrrhotite grains, as coarse grained intergrowths with pyrrhotite and as flame structures. Cobalt mineralisation occurs as the cobalt-iron arsenide glaucodot, which is typically fine-grained and locked in the pyrrhotite. PGE mineralisation within the massive sulphide lenses is characterised by very fine grained palladium dominant Bi and Te bearing minerals that are intergrown with the pyrrhotite. The PGE mineralisation within the lower sulphide zone often contains sperrylite, which is locally free and commonly coarse grained.

For the deposit as a whole, Pd/Pt ratios are variable, mostly >1, overall decoupled from the Pd+Pt grades, and show no systematic variation with increasing distance from the main massive sulphide mineralisation, although in places, high Pd/Pt ratios of up to 150 are found close to the structural footwall contact between the host metagabbro and the enclosing gneisses and amphibolites. These high Pd/Pt ratios are associated with low-grade mineralisation (Campos-Alvarez et al., 2012).

Platinum-group minerals (PGM) occur as i). inclusions in magmatic pyrrhotite and chalcopyrite in both the massive sulphide and high-PGE zones, ii). at the contact between sulphides and hornblende or magnetite inclusions in the massive sulphide, iii). in undeformed sulphide veins and adjacent chlorite and/or epidote halos, iv). in hornblende adjacent to hydrothermal veins, and v). in plagioclase-chlorite aggregates replacing garnet cemented by sulphide (Campos-Alvarez et al., 2012).

The PGM are mostly represented by the kotulskite (PdTe)-sobolevskite (PdBi) solid solution but also include michenerite (PdBiTe), froodite (PdBi
2), merenskyite (PdTe2), mertieite II (Pd8[Sb,As]3), and sperrylite (PtAs2) and occur in variety of textural settings. The PGM that occur in massive and interstitial sulphides are interpreted to be of magmatic origin, formed through exsolution from base metal sulphides at temperatures <600°C, and are dominantly Bi rich (i.e., Te-bearing sobolevskite), whereas those that occur in late-stage hydrothermal sulphide/silicate veins and their epidote-chlorite alteration halos tend to be more Te rich (i.e., Bi-bearing kotulskite) (Campos-Alvarez et al., 2012).

The deposit area also contains widespread, post-metamorphic, epidote-chlorite-calcite veins, and replacement assemblages that contain variable amounts of sulphides and platinum-group minerals (PGM).

Ore reserve and mineral resource estimates at the end of 2007 (Starfield Resources, Inc, 2012) were:
    Total indicated resource - 15.3 Mt @ 0.71% Ni, 1.04% Cu, 0.08% Co, 0.28 g/t Pt, 1.64 g/t Pd;
    Total inferred resource - 28.9 Mt @ 0.67% Ni, 1.01% Cu, 0.08% Co, 0.28 g/t Pt, 1.75 g/t Pd.

The most recent source geological information used to prepare this summary was dated: 2012.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Campos-Alvarez N O, Samson I M and Fryer B J  2012 - The roles of magmatic and hydrothermal processes in PGE mineralization, Ferguson Lake deposit, Nunavut, Canada: in    Mineralium Deposita   v.47 pp. 441-465

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