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Coldwell Complex - Marathon

Ontario, Canada

Main commodities: PGE PGM Pd Pt Cu
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The Marathon PGM-Cu deposit is located ~10 km north of the town of Marathon, on the NE shore of Lake Superior, ~250 km ENE of Thunder Bay and 300 km NNW of Sault Ste. Marie, in Ontario, Canada.

  The deposit is hosted within the Eastern Gabbro Series of the Proterozoic Coldwell Complex, which intrudes and bisects the much older Archaean Schreiber-Hemlo Greenstone Belt. The ovoid Coldwell Complex has a diameter of ~25 km and a surface area of 580 km2, and is the largest alkaline intrusive complex in North America.

  The Coldwell Complex occurs as three nested intrusive centres interpreted to have been emplaced into restricted dilatant zones within a ring dyke complex, possibly associated with ongoing caldera collapse on the north shore of Lake Superior (Walker et al., 1993; Shaw, 1997). It is localised near where the northern end of the Thiel Fault intersects Archaean rocks. This north-south structure is a major transform that punctuated the Late Mesoproterozoic Mid Continental Rift. The three Coldwell Complex intrusive centres are roughly contemporaneous but do young to the west. Of the three, Centre I is the oldest and consists of syenodiorite, layered ferroaugite-amphibole syenite, syenite, and the host Eastern Gabbro Suite (Walker et al., 1993). Centre II is composed of nepheline syenites with alkaline gabbro whilst Centre III consists of metaluminous hypersolvus syenites, which in order of intrusion were magnesio-hornblende syenite, contaminated ferro-edenite syenite, ferro-edenite syenite and quartz syenite (Mitchell et al., 1993). The intrusive rocks of the Coldwell Alkalic Complex were emplaced into a sequence of related mafic volcanic and subvolcanic rocks. The intrusion of gabbroic and syenite magma into the mafic volcanics has caused large and small scale brecciation and assimilation of this unit, leaving mafic volcanic xenoliths from <1 m to >1 km in size. The larger xenoliths form a roof pendant overlying the central part of the complex (Walker et al., 1993). Age determinations for various phases of the Coldwell Complex range from 1108 ±1 Ma (Heaman and Machado, 1992) to 1105.3 ±0.6 Ma (Good and Dunning, 2018). The Complex is considered to be related to other intrusive complexes associated with the Mid Continental Rift System e.g., the Duluth Complex, Logan Sills and Crystal Lake Gabbro which were also emplaced at ~1108 Ma (Heaman and Machado, 1992). The complex is interpreted to have undergone emplacement and rapid cooling at the onset of rifting and Keweenawan tholeiitic magmatism which are probably equivalents of the mafic volcanics into which it is intruded.

  The Eastern Gabbro is up to 2 km thick, has a strike length of 33 km and exposed width of 0.5 to 2 km, and forms the crescent shaped eastern fringe of the Coldwell Complex. The intruded metabasalts, which are slightly older to coeval, occur throughout the Eastern Gabbro, and are composed of basalt flows and sills that were metamorphosed to hornfels by the later injection of large volumes of gabbroic magma (Good et al., 2015). Unlike the metabasalts, which lack Cu-PGE mineralisation, the Eastern Gabbro hosts numerous Cu-PGM occurrences along its extent and is interpreted to be the oldest intrusive phase of the complex. Mineralisation at Marathon is part of this very large magmatic system where at least two major intrusive events, each predominantly composed of olivine gabbroic units, make up the Eastern Gabbro. The first of these is known as the Layered Gabbro Series, made up of alternating layers and intrusions of gabbro, olivine gabbro, gabbroic anorthosite and troctolite. The second intrusive pulse is the Marathon Series which intrudes the Layered Gabbro Series and occurs as multiple horizons within the layered package that makes up the latter. The Marathon Series is composed of coarse grained to pegmatitic, relatively homogeneous gabbro bodies and layered troctolite sills. The most significant phase of the Marathon Series is the Two Duck Lake Gabbro which is the host rock for Cu-PGM mineralisation at Marathon. Late quartz syenite and augite syenite dykes cut all of the gabbros but only account for a minor amount of the intrusive assemblage.

The stratigraphy on the eastern margin of the Coldwell Complex may be summarised as follows:
Footwall - The footwall to the Marathon PGM-Cu deposit hosts comprises Archaean intermediate pyroclastic rocks that have been partially melted by the heat of intrusion of the Eastern Gabbro Series. The Archaean basement unconformably underlies the Mesoproterozoic metabasalt package. The gabbro along the basal contact of the complex is referred to as Rheomorphic Intrusive Breccia (RIB). The contact is not simple, as blocks of RIB are found within the Eastern Gabbro series, whilst intrusions of gabbro extend deep into the footwall sequence. In addition, a few thin, near vertical promontories of RIB extend into the gabbroic rocks. The breccia has been described to be a matrix supported heterogeneous mixture of angular and sub rounded fragments composed of fine to coarse grained gabbroic material, quartzite, pyroxenite and layered quartz pyroxenite. The composition of the breccia matrix is estimated to be close to that of a quartz norite. A distinguishing feature of the RIB is the common occurrence of elongate curved pyroxenite fragments (Abolins, 1967).
Layered Gabbro Series, which is the oldest and most voluminous part of the Eastern Gabbro Series, containing the most diverse range of rock types, although it is predominantly composed of fine grained gabbro. It can be up to 2 km thick, strikes near north-south and dips moderately to the west. Basal fine grained gabbro is interlayered with footwall RIB and Archaean pyroclastic rocks, whilst near the top it is intruded by late syenite of the Coldwell Complex. The abundant fine grained gabbro is composed of subhedral clinopyroxene, olivine and magnetite with interstitial plagioclase. Layering occurs at the metre scale by gradational changes in grain size. Thin layers of massive magnetite up to 20 cm thick occur locally within the fine grained gabbro. Contacts with other gabbro units are sharp. The fine grained gabbro commonly contains 1 to 2 cm sized zoned amoeboid shaped blebs with either a clinopyroxene or olivine core and/or a thin plagioclase rich rim, particularly close to intrusions of Two Duck Lake Gabbro. This texture is interpreted to have formed by heating during intrusion of the latter. Whilst the Layered Gabbro Series has similar textures throughout the Eastern Gabbro, there are subtle but distinct variations in its bulk chemistry (Cao et al., 2016). It is dominated by olivine gabbro with modal layering defined by variations in the proportion of plagioclase and pyroxene and contains oxide augite melatroctolite subunits. Mineralisation in this series comprises chalcopyrite that is spatially associated with albite and actinolite alteration, but it is devoid of Pd, and has no other detectable PGEs (Brzozowski et al., 2020).
Layered Magnetite Olivine Cumulate, which accounts for <5% of the Layered Gabbro Series. It is found to the west of, and has been intruded stratigraphically above, the Two Duck Lake Gabbro, although is locally cut by thin intrusions of the latter. It occurs as irregular and pod-shaped discontinuous units that strike north-south for a few tens of metres to up to 200 m and is interpreted to have intruded the predominantly fine grained Layered Gabbro Series. It is composed of alternating and gradational layers of medium to coarse grained magnetite and olivine cumulates with interstitial plagioclase. Magnetite cumulate layers, comprising up to 95% magnetite, range in thickness from several centimeters to tens of metres and commonly contain associated disseminated chalcopyrite and minor pyrrhotite.
Marathon Series, which is composed of coarse grained to pegmatitic, relatively homogeneous gabbro, olivine gabbro, sill- to pod-like bodies of oxide melatroctolite, apatite clinopyroxenite, apatite olivine clinopyroxenite and layered troctolite sills. It represents the youngest of the mafic intrusive events in the Eastern Gabbro and is geochemically and texturally distinct from the metabasalts and Layered Gabbro Series (Good et al., 2015; Cao et al., 2018).
  At the Marathon deposit, the Marathon Series is dominated by the tubular-shaped Two Duck Lake Gabbro, a coarse-grained to pegmatitic and sub-ophitic olivine gabbro that hosts the Cu-PGE mineralisation. It is a massive, poorly-layered unit, ~50 to 250 m thick, that strikes near north-south for more than 6 km and generally dips at 5 to 45°W. It intrudes the Layered Gabbro Series, Layered Magnetite Olivine Cumulate, the footwall RIB close to the basal contact of the Layered Gabbro Series, and the metabasalt package just above the contact with Archaean country rocks, which at the Marathon deposit, comprises intermediate pyroclastic metavolcanic rocks (Walker et al., 1993; Good et al., 2015). It is, however, intruded by thin dykelets of RIB that are interpreted to be partial melt derivatives and, also, by late NNW trending quartz syenite dykes. It is distinguished from other gabbros in the Eastern Gabbro Series by cross cutting relationships and mineral textures resulting from different crystallisation histories. Within the Two Duck Lake Gabbro, plagioclase was the first major mineral to crystallise, forming elongate laths that are surrounded by ophitic textured clinopyroxene or olivine. In contrast, in other rocks of the Eastern Gabbro, olivine, clinopyroxene and magnetite were the first to crystallise with plagioclase being late, filling voids between the cumulate minerals. Pegmatitic-textured Two Duck Lake Gabbro occurs locally as pods within coarse grained gabbro or as rims on Eastern Gabbro xenoliths. Mineralised pegmatite comprises <5% of all mineralised zones. Relative to other Cu-PGM deposits in the region, e.g., Lac des Isles, the host intrusion at Marathon is unaltered and the nature of primary minerals and textures are preserved. Secondary minerals such as chlorite, serpentine and calcite are only locally developed on a microscopic scale and can only be detected in thin section. Only minor fluctuations in mineral compositions have been observed across the Two Duck Lake Gabbro (Good and Crocket, 1994). Plagioclase crystals are normally zoned with compositions ranging between 65 and 52% anorthite, typically exhibiting replacement of grain margins by a more calcic plagioclase (69 to 79% anorthite). The average olivine composition is 56.9% forsterite with 540 ppm Ni. Clinopyroxene and orthopyroxene respectively lie within the fields of augite and hypersthene with magnesium values of between 0.6 and 0.7% MgO.
  The Two Duck Lake Gabbro stoped its way along fracture sets or geologic contacts, e.g., the Eastern Gabbro/footwall RIB contact, and consequently has an anastomosing shape with numerous offshoots into the surrounding rock, but also resulted in the formation of thick brecciated units.
Breccia Units, which are composed of heterogeneous subangular blocks of Eastern Gabbro and/or footwall RIB. Hanging wall breccia units typically only contain blocks of Eastern Gabbro in a Two Duck Lake Gabbro matrix, whereas those in the footwall comprise both footwall and Eastern Gabbro blocks. Brecciated units are usually associated with copper-PGM mineralisation.

  The Marathon PGM-Cu deposit comprises several large, thick and continuous zones of disseminated sulphide mineralisation hosted within the Two Duck Lake Gabbro. These zones occur as shallow-dipping, sub-parallel lenses that follow the basal gabbro contact and are known as the Footwall and Main Zones, and the W Horizon. Individual zones range from 4 m, to as much as 183 m in thickness, averaging 35 m, with a standard deviation is 28 m. The Main Zone is the thickest and most continuous.
  The three principal mineralised lenses at Marathon are each texturally, mineralogically and geochemically distinct, although the character of the host rocks to the various zones is similar. From the base to top, the three lenses are (Good et al., 2015; Brzozowski et al., 2020):
i). Footwall Zone with Cu/Pd ratios (in ppm) of ~6000 to 35 000 and grades of 0.52 wt.% Cu, 0.1 g/t Pt, 0.49 g/t Pd, which occurs at the base of the Two Duck Lake Gabbro, at the contact with the Archaean country rocks; base metal sulphides comprise semi-massive and disseminated pyrrhotite with lesser chalcopyrite;
ii). Main Zone, with Cu/Pd ratios of ~1000 to 20 000 and grades of 0.55 wt.% Cu, 0.31 g/t Pt, 1.5 g/t Pd, which occurs above the Footwall Zone, and is the thickest and most continuous zone of mineralisation; it is thickest in the northern portion of the deposit; base metal sulphides are dominated by disseminated chalcopyrite and pyrrhotite, with lesser cubanite and bornite, all of which are interstitial to primary silicates, oxides and apatite;
iii). W Horizon, with Cu/Pd ratios of ~500 to 1000 and grades of 0.34 wt % Cu, 0.92 g/t Pt, 3.7 g/t Pd, which occurs above the Main Zone, close to the upper margin of the Two Duck Lake Gabbro; this zone only occurs in the southern half of the deposit; although base metal sulphides are also disseminated, this horizon is distinguished from the Main Zone by a lower modal abundance of both sulphides and hydrous silicates, a different suite of platinum-group minerals, a lower proportion of pyrrhotite, and a distinctly higher proportion of both bornite and Pd values.

  The W Horizon forms a nearly continuous sheet of mineralisation that strikes north-south for >1 km and persists down dip for >300 m. It ranges in thickness from the minimum sample width of 2, up to 30 m and occurs near the top of the mineralised zones. It is difficult to visually identify in drill core because it commonly contains only trace sulphides but, if sulphide is present, it consists of chalcopyrite and bornite. Continuity of the W Horizon between drill holes is distinguished by minimum PGM abundances of 1 g/t and by Cu/(Pt+Pd) ratios (in ppm) of <3500. Several very high grade lenses, ranging in size from 30 to 200 m across, occur within the W Horizon, with the highest grade intersection to 2012 containing 107 g/t PGM+Au, 1.04 g/t Rh and 0.02% Cu over a thickness of 2 m, whilst the best intersection comprised 45.2 g/t PGM+Au and 0.49% Cu over 10 m. In general, mineralised zones thicken in footwall basins and thin or pinch out over crests where the Two Duck Lake Gabbro unit thins.
  In general, ~60% of PGM grains are less than 5 µm in size. Platinum group minerals (PGM) are predominantly hosted by either sulphides or are associated with other platinum group minerals with ~25% occurring on the boundaries of plagioclase crystals. Around 38% of PGM are associated with the rare hydrous silicates (chlorite and serpentine) in the Main Zone in comparison with only 4.3% in the W Horizon. The suite of platinum group minerals in the Main Zone is very different from that of the W Horizon, with none of the 12 dominant platinum group minerals accounting for >85% of the PGM in the W Horizon, also being found in the Main Zone. The top five of these and their percentage of the total PGMs in the W Horizon are Zvyagintsevite [(Pd,Pt,Au)
3Pb] - 41.8%; Palladinite [(Pd,Cu,Au)O] - 15.5%; Telargpalite [Pd,Ag)3Te] - 5.5%; Skaergaardite [PdCu] - 3.9%; and Pb-rich Kotulskite [Pd(Te,Bi,Pb)] - 3.8%. The five most abundant PGMs in the Main Zone are Kotulskite-Sobolevskite [Pd2Te(Bi,Pb)] - 34.9%; Mertierite [Pd8(Sb,As,Pb)3] - 16.1%; Sobolevskite [PdBi] - 10.1%; Kotulskite [Pd(Te,Bi)] - 9.9%; and Sperrylite [PtAs2] - 6.3%.
  A prominent feature of the Marathon PGM-Cu deposit is the local and extreme enrichment of PGMs relative to that of Cu and Ni (which are also enriched but to a lessor degree), e.g., high grade samples from the W Horizon with between 25 and 50 g/t Pd only contain very low concentrations of Cu and Ni (<0.02%). This proportionately greater enrichment of PGMs compared to Cu is illustrated by the comparison of the composition of the three zones as listed above. It is observed throughout the deposit but is most pronounced near the top of the mineralised zone. In the southern half of the deposit, PGM enrichment is most prominent in the W Horizon.
  Chalcopyrite and pyrrhotite are the dominant sulphides in the Marathon deposit, with minor pentlandite and bornite. Chalcopyrite is the major copper mineral whilst bornite occurs locally, particularly in the W Horizon. In general, sulphides at the base of the Main Zone are pyrrhotite, with the proportion of chalcopyrite increasing up section. The highest concentrations of PGM correspond to zones where chalcopyrite and particularly bornite-bearing sulphides are dominant. Pentlandite is the main nickel-bearing mineral but is only a minor component of the sulphide assemblage. The chalcopyrite:pentlandite ratio for mineralised samples is relatively constant at ~16:1.
  There is a strong and positive correlation between Pd and the other PGM (Pt, Rh and Ir) and Au for all types of mineralisation in the Marathon PGM-Cu deposit.

  The Cu-PGE mineralisation within the Two Duck Lake Gabbro has been described by Good et al. (2015, 2017) as 'conduit type'. This involves injection of multiple pulses of 'mineralised' magma from a large sulphide settling pool located at depth that formed through the addition of sulphur from surrounding assimilated Archaean sedimentary rocks. The sulphide pool contains iron, copper and nickel sulphides and non-sulphide PGMs. As pulses of uncontaminated silicate melt derived from a mantle source flow through the system and interact with the sulphide pool, the R factor (i.e., ratio of silicate melt to sulphide melt) of the sulphide liquid will increase, causing a progressive decrease in the Cu/Pd ratio (Brzozowski et al., 2020). This dilution of the sulphide pool reduces the available S that dampens the formation of Fe-Cu-Ni sulphides, while enhancing the PGM content.
  The sulphide mush that interacted with, and was mobilised by, the initial pulses of silicate melt will have a relatively low R factor, retaining much of the contaminated sulphur signature from the Archaean country rocks, but will have mantle-like Cu/Pd values. The Footwall Zone mineralisation is interpreted to have formed by the emplacement of these early sulphides. However, interaction with subsequent pulses of uncontaminated silicate melt which increased the R factor, diluted the sulphur signature and decreased the Cu/Pd ratio, which was still within the mantle range. Emplacement of these sulphides formed the Main zone. The W Horizon was the result of emplacement of sulphides from the pool that had interacted with significantly larger volumes of uncontaminated silicate melt to further increase the R factor and completely dilute the contaminated signature, and generate the lower-than-mantle Cu/Pd and S signature (Brzozowski et al., 2020). The dilution of sulphur in the sulphide pool is also reflected by the increase in bornite within the W Horizon.

NI 43-101 compliant Mineral Resources at 31 December, 2019 were (Sibanye-Stillwater Mineral Resources and Mineral Reserves report, 2019):
  Measured Resources - 57.4 Mt @ 0.8 g/t 2E PGM, 0.20% Cu;
  Indicated Resources - 65.6 Mt @ 0.6 g/t 2E PGM, 0.21% Cu;
  Inferred Resources - 15.3 Mt @ 0.5 g/t 2E PGM, 0.23% Cu;
  TOTAL Resources - 138.3 Mt @ 0.7 g/t 2E PGM, 0.21% Cu containing 94.5 t of Pd+Pt.
NOTE: 2E PGM = Two Elements PGM, namely Pd+Pt.
Puritch et al. (2009) quoted by Brzozowski et al. (2020) gives grades as 0.27% Cu, 0.75 g/t Pd, 0.23 g/t Pt and 0.09 g/t Au.

  No mining had been undertaken on this resource to 2020.

This information in this summary has been drawn from: Fraser, B., 2012 - Geological conditions at the Marathon PGM-Cu project site; An NI 43-101 Technical Report prepared by EcoMetrix Incorporated for Stillwater Canada Inc., 49p. and from (Brzozowski et al., 2020).

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
   Selected References:
Brzozowski, M.J., Samson, I.M., Gagnon, J.E., Good, D.J. and Linnen, R.L.,  2020 - On the Mechanisms for Low-Sulfide, High-Platinum Group Element and High-Sulfide, Low-Platinum Group Element Mineralization in the Eastern Gabbro, Coldwell Complex, Canada: Evidence from Textural Associations, S/Se Values, and Platinum Group Element C: in    Econ. Geol.   v.115, pp. 355-384.


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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