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The Dumont disseminated nickel-cobalt-PGE deposit is located ~25 km WNW of Amos, 60 km NE of Rouyn-Noranda and 70 km NW of Val D'Or in the province of Quebec, Canada (#Location: 48° 38' 53"N, 78° 26' 30"W).
The Dumont deposit is hosted within the thick supracrustal succession of Archaean volcanic and sedimentary rocks that occupies ~65% of the Abitibi belt, unconformably overlying an inferred TTG sialic basement complex. The volcanic component of the succession is predominantly of mafic composition, although ultramafic, intermediate and felsic types are also represented. The pillowed and non-vesicular nature of the volcanic rocks and flyschoid character of the sedimentary rocks indicates submarine deposition, although some fluvial sedimentary facies and airfall tuffs demonstrate local non-marine conditions. The succession is cut by numerous small to medium sized synvolcanic intrusions of similar composition to the lavas.
This succession was deformed and intruded by granitoid stocks and batholiths during the 2.68 to 2.70 Ga Kenoran event, producing east-west trending, commonly isoclinal fold axes and regional greenschist and prehnite-pumpellyite facies metamorphism. Amphibolite facies occurs in the contact aureoles surround the Kenoran granites and in the sedimentary rocks of the Pontiac Group.
Two main sets of dolerite dykes cut the rocks of the Abitibi belt, the north-south trending 2.690 Ga Matachewan and NE-SW trending 2.147 Ga Abitibi swarm. The latter are prominent near the Dumont intrusion, although none are known to have cut the body.
The Dumont sill is hosted by Archaean lavas and volcaniclastic rocks of the Amos Group, part of the Barraute volcanic complex, which comprises three cycles of mafic to felsic volcanism. The sill is one of at least five ultramafic-mafic complexes in the Amos district that occur at the same stratigraphic level within the mafic lavas of the middle cycle, mainly iron-rich tholeiitic basalts. Whilst the Amos group wall rocks have been folded and dip steeply, a penetrative deformational fabric is only locally developed, and primary textures persist, even though many of the rocks are strongly altered, containing significant levels of CO2.
The age of the Dumont sill has not been directly determined due the lack of dateable minerals. However, the conformable nature of the body (see below), together with the character of its differentiation, suggests that it was emplaced as a virtually horizontal sill that was folded and faulted during the Kenoran event. It has been concluded that the sill is of Neoarchaean age, only slightly younger than the enclosing lavas, i.e., ~2700 Ma (Duke, 1986).
Three main fault sets are evident in the Amos district, the earliest of which trend east-west to ENE-WSW, following bedding, interpreted to have accompanied the main period of folding. The second, steeply dipping, north to NW set, occurred during the intrusion of the granitoids, while the third, and most prominent, which strike NE-SW, post date granitic intrusion. The Dumont sill is cut and displaced by all three sets.
The deposit and its wall rock contacts are concealed by ~30 m of glacial overburden and swamp, although drilling suggests the sill is conformable with the layering of the enclosing volcanic rocks. Pillowed basalt intersections toward the eastern end of the sill indicate a northeast facing direction. Offsets in the magnetic data at a high angle to the sill axis are consistent with fault sets detailed above, while zones of weakness parallel to the long axis of the intrusion have also been indicated.
The sill is interpreted to be a layered mafic-ultramafic intrusion, comprising a lower ultramafic, and an upper mafic zone (Duke, 1985), and has been divided into a number of structural blocks/domains separated by prominent faults, mainly the NW and ENE trending sets. Although the northwestern extremity of the sill has not been precisely determined, the lenticular ultramafic zone is at least 6.6 km long, with an average true thickness of 450 m, a maximum of ~600 m in the central block, tapering to a minimum of ~150 m to the SE. The true dip of the ultramafic zone varies along the length of the sill from 60 to 70° to the NE. The overlying mafic zone is ~200 m thick, based on limited drilling and outcrop.
The ultramafic zone is subdivided into a central dunite, sandwiched between lower and upper peridotite subzones. The dunite subzone is an extreme olivine adcumulate containing very small amounts of intercumulus chromite and clinopyroxene, while the lower and upper peridotite subzones are olivine-chromite cumulates with variable amounts of intercumulus clinopyroxene. Cumulus sulphide occurs in certain parts of the dunite subzone and also locally in the lower peridotite.
The overlying mafic zone has been divided into three subzones, comprising from the base upwards: i). clinopyroxenite - an extreme clinopyroxene adcumulate at its base, grading upward into ii). gabbro, characterised by clinopyroxene + plagioclase cumulate rocks, that passes upward, in turn, into iii). quartz-gabbro which includes both plagioclase + clinopyroxene cumulates and noncumulate gabbros that contain modal and normative quartz. Olivine and chromite are restricted to the ultramafic zone whereas plagioclase occurs only in the mafic zone.
The Mg/Mg-Fe ratios of the ferromagnesian cumulus phases have been shown to increase gradually from the base of the sill upwards through the lower peridotite, with an abrupt increase at or just above the base of the dunite. It remains essentially constant through the upper part of the dunite and into the base of the upper peridotite, before following a normal iron enrichment trend upward through to the overlying part of the intrusion.
The ultramafic rocks have been variably serpentinised from partial to complete, while along the basal contact of the sill, beyond the resource envelope, serpentinisation is frequently overprinted by variable talc-carbonate alteration. The dominant secondary assemblage is lizardite + magnetite + brucite + chlorite + diopside ±chrysotile ±pentlandite ±awaruite ±heazlewoodite, with local antigorite, particularly in the uppermost ultramafic zone. Native copper accumulates in and along major fault systems and beside intercumulus nickel sulphide and awaruite mineralisation, frequently in partially serpentinised zones. Traces of millerite occur in steatitised rocks of the basal contact zone and more rarely in large fault zones. The mafic zone is ubiquitously altered to an assemblage of actinolite + epidote + chlorite ±quartz. Primary textures are pseudomorphed throughout most of the intrusion.
Although the serpentinisation is isovolumetric and isochemical on a microscopic scale, on a macroscale, as the major elements are re-partitioned, with the addition of hydrogen, oxygen (water) and chlorine to the system, some phase can be dissolved and transported, resulting in losses of calcium, iron and sulphur. The textures and secondary mineral assemblages are indicative of nonequilibrium, retrograde, low temperature (<350°C) alteration that may well have occurred as a result of an influx of water during the initial cooling of the intrusion.
Magmatic sulphides and a nickel-iron alloy are restricted to the dunite subzone and the lower peridotite, mainly representing a post-cumulus phase in the latter. In the former they
show a strong affiliation with the magnesium-rich upper dunite. Four distinct olivine-sulphide cumulate layers, with enriched grades of >0.35% Ni, are recognised within the dunite subzone, although these do not extend over the entire strike length of the sill. Fewer than four enriched layers may be present at any one point. In addition, the same sulphides and alloy are broadly disseminated throughout the dunite and lower peridotite subzones and carry lower grades between the enriched layers.
Mineralisation occurs as: i). primary low- to medium-grade disseminated nickel (Duke, 1986); ii). contact type nickel-copper-platinum group elements (PGE) occurrence (Oswald, 1987); iii). discontinuous PGE mineralisation associated with disseminated sulphides has also been encountered at lithological contacts elsewhere in the layered intrusion and within the dunite.
Disseminated Nickel Mineralisation - characteristically occurring as disseminated blebs of pentlandite ((Ni,Fe)9S8), heazlewoodite (Ni3S2), and the ferro-nickel alloy, awaruite (Ni2.5Fe), which are present in various proportions throughout the sill. These minerals may occur together as coarse agglomerates of up to 10 000 µm (10 mm), predominantly associated with magnetite, or as individual disseminated grains ranging from 2 to 1000 µm (0.002 to 1 mm). Nickel also occurs in the crystal lattice of several silicate minerals including olivine and serpentine.
The mineralogy of the Dumont deposit has been influenced by the serpentinisation of the original dunite protolith, an extreme olivine adcumulate, that contained a primary, disseminated (intercumulus) magmatic sulphide assemblage (dominantly pentlandite) with nickel contained entirely within olivine and the pentlandite (± other primary trace sulphides; Duke 1986). This assemblage was subjected to metamorphism, involving the addition of heat and water, with the olivine in low-silica mafic and ultramafic rocks being oxidised and hydrolysed to form an assemblage of serpentine, magnetite and brucite, creating a strongly reducing environment where the nickel released from the decomposition of olivine was partitioned into low-sulphur sulphides and newly formed awaruite. The resultant secondary silicates, hydroxides and magnetite were nickel-poor while the intercumulus metallic phase blebs had an increased nickel tenor (Duke, 1986), from primary 26 to 34 wt. % Ni in pentlandite to 70 to 75% Ni in awaruite and heazlewoodite.
As serpentinisation proceeded, olivine was consumed, and the resultant iron-rich serpentine and brucite broke down to produce additional magnetite, while the remaining serpentine and brucite became progressively more Mg-rich. As this process continued, the Mg-rich serpentine and brucite began to consume the previously formed awaruite. Under these conditions, pentlandite and awaruite continued to break down to produce heazlewoodite (Klein and Bach, 2009; Sciortino et al., 2013).
Therefore, where serpentinisation is incomplete, the nickel content present within the silicate crystal structure or as microscopic inclusions, is generally higher, and corresponds to a population of lower tenor pentlandite and high Fe-serpentine. But where serpentinisation is complete, intercumulus blebs contain abundant magnetite±pentlandite±heazlewoodite with little to no awaruite. Therefore, where the heazlewoodite content is high, awaruite is low, and the nickel tenor of pentlandite is higher, while the nickel tenor in silicates is low (~15% Ni). Where sulphides are absent, awaruite occurs as finely disseminated grains associated with magnetite or brucite mesh rims (Sciortino et al., 2013).
Millerite (NiS) is rare, but is locally present in minor amounts near host rock contact zones and in major fault zones, typically occurring as fine secondary overgrowths, overprinting pentlandite and heazlewoodite in intercumulus blebs.
Disseminated sulphide zones containing pentlandite-heazlewoodite-awaruite assemblages may been divided into the following types:
Sulphide dominant - predominantly composed of heazlewoodite and/or pentlandite, with or without lesser awaruite. This style occurs as higher-grade bands (>0.35% Ni), subparallel to the dip of, and principally within the centre of, the sill. Pentlandite and heazlewoodite occur as medium- to coarse-grained blebs filling intercumulus spaces in a primary magmatic texture, and sometimes as secondary overgrowths within magnetic blebs. These blebs are often intimately associated with magnetite ±brucite ±chromite ±awaruite in intercumulus spaces. Where awaruite occurs with sulphides, it is frequently a secondary overgrowth on pentlandite, within the primarily intercumulus magnetite blebs. Sulphide bands are present in all three mineralised layers within the dunite where it is the thickest in the central SE part of the sill.
Alloy dominant - predominantly awaruite ±lesser heazlewoodite ±lesser pentlandite. Awaruite occurs as fine grains (generally <1 mm) associated with small intercumulus magnetite or chromite blebs, as well as secondary overgrowths on serpentine within the pseudomorphed grain. Alloy mineralisation zones predominantly occur where primary sulphides are absent and serpentinisation is near complete.
Mixed - which consists of sulphides and alloy in comparable proportions, representing a transition from sulphide to alloy or sulphide (pentlandite) to sulphide (heazlewoodite) mineralisation types. Mineralisation can occur as coarse sulphide-magnetite blebs associated with awaruite or as finely disseminated discrete grains.
Nickel in silicates - in parts of the deposit, a very low percentage of the nickel in the ultramafic rock is contained in sulphide or alloy minerals, but rather in silicate minerals such as serpentine or olivine. These zones are generally low grade (<0.25% Ni), and are where serpentinisation is incomplete and nickel is held within the crystal structure of olivine ((Mg,Fe,Ni)2SiO4) and/or serpentine (Mg,Fe,Ni)3(Si)2O5(OH)4. In some cases, the nickel is not contained within the crystal structure of the serpentine, but as very fine (<1 µm) sulphide or awaruite inclusions within the serpentine matrix. The proportion of nickel in silicates varies throughout the sill and is dependent on the degree of serpentinisation, being highest in zones of weak or no serpentinisation, and inversely proportional to the sulphide content. This form of nickel is not commercially recoverable.
Contact-type Nickel-Copper-PGE Mineralisation - this style of mineralisation is not fully delineated, and appears to occupy an interval of ~1 m in thickness at the contact between the lower peridotite of the Dumont sill and the footwall mafic volcanic rocks. Intersections include semi-massive to massive (>90%) sulphides, mainly pyrrhotite with pentlandite and trace chalcopyrite, with grades of 0.6 to 1% Ni, 0.1 to 0.2% Cu, 0.3 g/t Pt, 1 g/t Pd and 0.07 g/t Au.
Other Types of PGE Mineralisation - this mineralisation, which is associated with disseminated sulphides, has also been encountered at lithological contacts elsewhere in the layered intrusion and within the dunite. These include the pyroxenite layer overlying the upper peridotite, where a zone that varies in thickness from 0.4 to 51 m carries grades ranging from 0.08 to 1.46 g/t Pt and 0.04 to 2.39 g/t Pd. Another zone lies below the main sulphide layer, ranging from 0.4 to 34.5 m thick with grades ranging from 0.1 to 1.4% Ni, trace to 0.75 g/t Pt, and trace to 0.2 g/t Pd. A third PGE horizon is located ~100 m below the lowest sulphide layer, near the contact between the dunite and lower peridotite, ranging in thickness from 1.0 to 140 m thick with grades of from 0.1 to 0.5% Ni, trace to 0.9 g/t Pt, and trace to 2 g/t Pd. These zones generally appear to be continuous along strike and down dip, and all consist of three alloys: Pd/Sn, Pt/Cu and Pt/Ni, which are intimately associated with Ni sulphides.
All minerals analysed show a low variability in nickel tenor throughout the sill except for pentlandite and serpentine. Heazlewoodite is the least variable of the three main nickel bearing minerals of interest. The variability of nickel content is also low in awaruite. Pentlandite has the highest variability of the three, and exhibits a bimodal population. In samples where the nickel tenor in pentlandite is lower, the lower nickel values are mostly associated with an increase in iron, and less so, sulphur. Serpentines show a wide range of nickel tenors.
The final mineral assemblage and texture of the disseminated nickel mineralisation in the Dumont deposit and its variability, has been controlled primarily by the variable degree of serpentinisation that the host dunite has undergone. The nickel tenor of the sulphides (heazlewoodite vs pentlandite) and recoverable (sulphide) vs unrecoverable (silicate) metals strongly influences the concentrate grade and viability of the deposit and domains within it.
Four metallurgical domains have been established that correspond to the degree of serpentinisation, defined mineralogically on the basis of heazlewoodite to pentlandite ratio and iron-rich serpentine (proportional to Ni tenor in silcates) abundance. These domain can be mapped as large coherent blocks within the sill. The domains and their average proportion of Ni in silicates (i.e., [(Nickel Assay - Metallic Nickel)/Nickel Assay]) are Heazlewoodite Dominant - 37.3%, Mixed Sulphide - 34.1%, Pentlandite Dominant - 31.1% and High Iron Serpentine - 55.8%.
The proportion of the proven + probable reserves listed below in each metallurgical domain, is as follows (Staples et al., 2013):
Heazlewoodite Dominant - 348 Mt @ 0.25% Ni;
Mixed Sulphide - 223 Mt @ 0.27% Ni;
Pentlandite Dominant - 358 Mt @ 0.29% Ni;
High Iron Serpentine - 250 Mt @ 0.27% Ni;
TOTAL - 1179 Mt @ 0.27% Ni.
Published NI 43-101 compliant mineral resources and ore reserves at June 17, 2012 (Report to Royal Nickel Corporation, 2013) include:
Measured + indicated resources - 1665.60 Mt @ 0.27% Ni, 107 ppm Co, 0.020 g/t Pd, 0.009 g/t Pt;
Inferred resource - 499.80 Mt @ 0.26% Ni, 101 ppm Co, 0.014 g/t Pd, 0.006 g/t Pt; including,
Proven + probable reserves - 1178.60 Mt @ 0.27% Ni, 107 ppm Co, 0.019 g/t Pd, 0.009 g/t Pt, at a 0.15% Ni cutoff.
Measured + indicated resources - 1114.30 Mt @ 4.27% Magnetite;
Inferred resource - 832.00 Mt @ 4.02% Magnetite.
Projected metallurgical recovery of both Ni and Co is 43% (Staples et al., 2013).
The information in this summary is largely drawn from: Staples et al., July 2013, Technical Report on the Dumont Project, Launay and Trécesson Townships, Quebec, Canada, coordinated by Ausenco Solutions Canada Inc., and submitted to Royal Nickel Corporation.
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.
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