The Montcalm nickel/copper deposit, one of Canada’s largest undeveloped nickel resources, is worthy of serious reappraisal in light of the current strength in nickel prices. Located 35 km northwest of Timmins, Ont., the deposit was discovered by Teck Corp. (in partnership with Domik Exploration of Japan and Metallgesellschaft of West Germany) in September, 1976. It was then that the first drill hole testing an airborne electromagnetic anomaly intersected 74.4 m of sulphide mineralization. Another 17,000 m of drilling outlined 4.2 million tonnes of mineralization, with average grades of 1.4% nickel and 0.7% copper. This deposit is unusual in three respects. First, most nickel/copper deposits occur at the base of mafic- ultramafic intrusions or flows, a feature generally ascribed to gravity settling of a dense sulphide liquid through a less dense silicate magma. However, the Montcalm deposit occupies an intermediate level within a large mafic intrusion. Second, the deposit has a platinum group metal (PGM) tenor nearly two orders of magnitude lower than most other magmatic sulphide deposits hosted in gabbroic rocks, such as those found in the Sudbury district, whereas its nickel and copper contents are considered normal for gabbro-hosted intrusions. And third, pyroxenitic rocks in the stratigraphic footwall contain abundant pegmatitic gabbroic apophyses (a branch from a dike) and dikes, similar to parts of the PGM-enriched Robie zone of the Lac-des-Iles mafic-ultramafic intrusion, in northwestern Ontario.
The magmatic/hydrothermal processes involved in the PGM depletion in the nickel/copper deposit, and local PGM enrichments in the footwall gabbro pegmatite dikes, have obvious significance in the exploration for nickel/ copper/PGM deposits. These processes are being studied as part of a doctoral thesis at the University of Toronto, under the supervision of Prof A. J. Naldrett, a well-known authority on magmatic sulphide deposits. A brief review of the geology of the Montcalm gabbroic complex and the Montcalm nickel/copper deposit are presented here, based on new drill- core and field o bservations and geochemistry. Four Zones The Montcalm gabbroic complex is one of two large late Archean gabbroic complexes in the western Abitibi Subprovince, including the Kamiskotia gabbroic complex, 20 km to the east. It is a large (about 85 sq km) sub-vertical, crudely layered intrusion with a crescent shape. The Montcalm gabbroic complex apparently interdigitates with iron-rich tholeiitic basalts, and calc-alkalic andesites and dacites, suggesting parts of the complex extended laterally into the volcanic stratigraphy. To the east, the complex is bordered by a large composite tonalite-granodiorite-granite batholithic complex. A smaller gneissic tonalite cuts the Montcalm complex along its southern margin, and ultramafic sills and dikes cut the intrusion and neighboring volcanic rocks. A preliminary uranium/lead age of 2,702 +/– 2 million years has been established for the Montcalm complex, whereas a felsic dike that cuts the complex and the nickel/copper deposit has a preliminary age of 2,700 +/– 2 million years.
The Montcalm is divided into four zones: the Pyroxenite, Gabbro, Anorthositic gabbro and Ferroan gabbro zones, which broadly indicate a northwest-southeast iron enrichment fractonation trend. The pyroxenite zone occurs 300 m northwest of the nickel/ copper deposit as a lens-shaped mass and is principally composed of pyroxenite and pegmatitic gabbroic rocks. The pyroxenites are massive, medium grained clinopyroxenites with interstitial hornblende, orthopyroxene and plagioclase. Pegmatitic gabbroic rocks have subequal proportions of plagioclase, clinopyroxene and hornblende and iron/titanium oxides. Both rock types have up to 2% sulphides, predominantly pyrite and pyrrhotite with lesser chalcopyrite, and pentlandite locally.
The Gabbro zone is a 400-m-thick zone of predominantly medium-to- coarse-grained clinopyroxene plagioclase mesocumulate and adcumulate; it is host to the nickel/copper deposit. Poorly developed modal layering is present in about 5% of the Gabbro zone. The Anorthositic gabbro zone is greater than 200 m thick in the vicinity of the nickel/copper deposit. It is principally composed of coarse- grained plagioclase mesocumulate with greater than 65% plagioclase, together with lesser clinopyroxene plagioclase mesocumulate and adcumulate units. The southern two-thirds and the western “tail” of the Montcalm complex make up the Ferroan gabbro zone, which is characterized by the presence of interstitial or cumulus magnetite, along with medium-to- coarse-grained plagioclase, clinopyroxene and calcic hornblende.
Numerous dikes ranging in composition from ultramafic to felsic comprise fully 25% of the drill core in the vicinity of the Montcalm deposit. Pyroxenite-hornblendite and gabbro pegmatite dikes ranging in thickness from 2 cm to 12 m occur principally within the pyroxenite zone. The pyroxenite-hornblendite dikes generally have sharp boundaries with fine grained chilled margins locally, and are composed of clinopyroxene and/ or hornblende, with lesser iron/titanium oxides, and up to 2% sulphides. Gabbro pegmatite dikes are up to 8 m thick and have both sharp and gradational contacts. They range in composition from hornblende-pyroxene metagabbro to hornblende gabbro with minor quartz, chromium ilmenite and up to 2% sulphides locally.
It is noteworthy that the pegmatitic textures found in the pyroxenite zone are similar to mafic pegmatite in parts of the palladium/platinum-rich Robie Zone at Lac-des-Iles. In both locations, a volatile phase has been involved with the formation of pegmatitic textures, either during crystallization of the cumulus mafic phases or shortly after, and may have played a role in local precipitation or redistribution of sulphides.
Using a novel, highly sensitive radiochemical neutron activation technique refined by G. Brugmann at the University of Toronto, a study is underwa y to document the PGM values within the outside pegmatitic zones. Preliminary analyses indicate the pyroxenite rocks have low PGM values, with a slight but significant enrichment in gabbrol pegmatite dikes locally.
A second dike suite ranges in composition from dacitic to siliceous, and is more likely related to the massive felsic plutonism of the granitoid terrain to the east. This dike suite is nearly coeval with the Montcalm complex and may have released the volatiles necessary for the development of pegmatitic textures in the pyroxenite zone.
From structural measurements of bedding, foliation and lineation in outcrops to the south of the nickel/copper deposit and surrounding lithologies, it would appear that the Montcalm complex has been subject to at least two deformation events. The first event produced a regional foliation and may have been responsible for tilting the regional stratigraphy into its present subvertical position. The arcuate shape of the regional foliation, generally parallel to the crescentic shape of the Montcalm complex, suggests that a broad doming to the northwest may have been coincident with, or responsible for, this first deformation event. A second deformation event, probably related to the emplacement of the granitoid rocks to the east and south, imposed a strong flattening and stretching fabric on the eastern and southern parts of the Montcalm. Two Lenses Located at the extreme northern end of the Montcalm complex, the nickel/copper deposit is composed of two subvertical sulphide-rich lenses termed the West zone and the East zone, each with a maximum subsurface strike length of 200 m and a maximum width of 25 m. The West zone has an average grade of 1.51% nickel and 0.74% copper; almost equal in size and grade to the East zone, which has average grades of 1.39% nickel and 0.61% copper. The zones are hosted by medium-to-coarse-grained gabbros of the gabbro zone. The wall rock gabbro contains discrete high strain zones characterized by a well- developed flattening fabric along with a stretching fabric locally.
Sulphide mineralization has a relatively
simple mineralogy of pyrrhotite, pentlandite and chalcopyrite, with minor magnetite, pyrite and violarite. It can be subdivided into three texturally distinct groups. In order of decreasing abundance, these are: disseminated sulphide (10%-50% sulphide by volume), massive and net- textured sulphide (more than 50%) and inclusive breccia sulphide (more than 50%). Significantly, the inclusion breccia ore contains an abundance of pyroxenitic fragments, which suggests at least some of the fragments were derived from an ultramafic part of the intrusion, or from a lower crustal magma chamber. There is no significant zonation of nickel or copper grade along strike or with depth.
Sulphur/selenium ratios have been used as indicators of sedimentary sulphide contamination in magmatic deposits. Ideally, a primitive magma should have a chondritic ratio of about 2,700:1; ratios of 2,000:1 to 10,000:1 are considered normal for uncontaminated magmatic sulphide ores. Archaean sedimentary sulphide has variable ratios depending on the reduction state of the depositional environment, but they are generally more than 20,000:1. Sulphur/selenium ratios for sulphide-oxide facies iron formations in the Kamiskotia area average 38,000:1. For Montcalm, the sulphur/selenium ratio is 10,800:1, nearly within the range of normal magmatic sulphide ores.
The most unusual feature of the Montalm deposit is its uniformly low PGM tenor. This presents a problem: how can a gabbro-hosted magmatic sulphide deposit have a nickel content typical of those derived from basaltic magmas, but a low copper content and a very low PGM content? A. J. Naldrett and J. M. Duke addressed this problem in 1980 and found that batch equilibration between a sulphide liquid and the parent basaltic magma from which it has segregated could not account for the metal contents at Montcalm. For their calculations, they used bulk distribution coefficients between sulphide liquid and tholeiitic liquid of 275, 245 and 1,500 for nickel, copper and palladium respectively (this means that nickel in a sulphide liquid, for example, is 250 times more abundant than the nickel in a basaltic magma, given equilibrium conditions); and an initial magma composition similar to an olivine tholeiite hyaloclastite from Munro Twp. of Ontario. Whereas a large batch of sulphide liquid in equilibrium with an olivine tholeiitic magma (a low “R” factor) could account for the low PGM tenor, the nickel and copper contents would be too low. They then modelled the sulphide that would be in equilibrium with an olivine tholeiitic magma after fractional removal of 50% olivine, clinopyroxene, plagioclase, and continuous removal of trace amounts of sulphide. By this method, they achieved a sulphide liquid with 4% nickel, 1.3% copper and 17 parts per billion palladium in 100% sulphide, very similar to the values for the Montcalm deposit.
It would appear that during the waning stages of volcanism in the Abitibi- Wawa subprovinces, the Montcalm gabbroic complex intruded into a volcanic-sedimentary succession as a semi-concordant body, probably in a subhorizontal position. The pyroxenites, hornblendites and pegmatitic gabbros of the pyroxenite zone crystallized at or near the western margin at the northernmost extent of the intrusion. Normal cumulate processes began with the onset of plagiclase fractionation at the base of the gabbro zone. After about one half of the gabbro zone cumulates had formed, an immiscible sulphide liquid was emplaced, along with a slightly more primitive magma that fractionated gabbro-zone cummulates with a slightly higher magnesium number. Either they originated in a slightly more primitive part of the Montcalm gabbroic complex magma chamber and were emplaced laterally into their present location or they were introduced into the chamber from partly fractionated lower crustal chamber that underwent an immiscible sulphide event. During or shortly after crystallization, the pyroxenite zone was apparently subjected to the metasomatic effects of a volatile-rich phase, possibly derived from the nearby felsic dikes, and distinctive pegmatitic textures developed. After crystallization, voluminous granitoid intrusion to the east and south imprinted contact strain zones along the eastern and southern margin of the Montcalm complex, brecciating and flattening parts of the gabbroic wallrock along with the massive sulphide lenses. C. Tucker Barrie is a PhD candidate in economic geology and igneous petrology at the University of Toronto. He would like to thank Dr Matthew Blecha and Teck Corp. for access to the Montcalm deposit drill core and unpublished geochemical and aeromagnetic data; Falconbridge Ltd. for logistical support in the Montcalm and Kamiskotia areas; and Dr D. Davis of the Royal Ontario Museum for guidance with uranium/ lead geochronology. REFERENCES Barrie, C. T., Petrogenesis and Techtonic Evolution of the Kamiskotia-Montcalm Area and Related Ni-Cu Deposits, Western Abitibi Subprovince, Ontario, Canada, PhD thesis in progress, University of Toronto. Naldrett, A. J., Nickel Sulphide Deposits: Classification, Composition, Genesis. Economic Geology 75th Anniv. Vol., 1981, pp. 628-685. Naldrett, A. J., and Duke, J. M., Platinum Metals in Magmatic Sulphide Ores: Science, Vol. 208, 1980, pp. 1417-1424. — 30 —
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