Summary:
The Bucko Lake Deposit and its satellite deposits are magmatic sulphide deposits formed as a product of komatiitic magmatism during formation of the Thompson Nickel Belt (“TNB”), a segment of the Circum-Superior Craton Belt (Ciborowsky et al., 2017). Since formation, the magmatic sulphide deposits have been variably modified and remobilized during post-depositional tectonism and high-grade metamorphism of the TNB.
The Bucko Lake area of the Property is underlain by Archean gneisses and Paleoproterozoic Ospwagan Group metasedimentary and ultramafic intrusive rocks. The Archean gneisses are intruded by Paleoproterozoic ultramafic sills, including the Bucko Lake Ultramafic, which hosts the Bucko Lake nickel sulphide deposit. The Bucko Lake Ultramafic sill is on the northeast flank of the Resting Lake Pluton. The footwall contact of the nickel sulphide deposit occurs in close contact with granodiorite gneiss associated with this intrusion.
The Bucko Lake Ultramafic sill is mainly composed of metamorphosed peridotite and dunite with smaller amounts of olivine orthopyroxenite, poikilitic harzburgite, orthopyroxenite and amphibole-bearing peridotite. This sill has been interpreted to be a hook-shaped body 800 m long and dips 75° to 80° east. It is approximately 20 m wide at the south end and gradually increases to >150 m wide at the north end, where it wraps around the nose of a synformal fold structure that plunges steeply to the south.
Contacts of the ultramafic bodies with the surrounding country rocks are generally obscured by alteration, shearing or late-stage pegmatite dykes. Blocks of plagioclase amphibolite gneiss occur in the northern part of the ultramafic sill. The larger xenoliths occur within a distinct bulge or keel in the footwall of the ultramafics adjacent to the Hinge Zone. These blocks appear to be xenoliths of country rock incorporated into the sill during emplacement.
The Bucko Lake Ultramafic Sill has undergone two stages of metasomatic alteration (Good and Naldrett, 1993). The first phase was serpentinization of the olivine, with concurrent alteration of orthopyroxene to anthophyllite, tremolite and phlogopite. The second stage of alteration was superimposed on the serpentinized ultramafics and occurs as envelopes around pegmatite dykes and fractures. The envelopes range from cm to m wide and consist of an outer zone of talc and tremolite, a central zone of fibrous tremolite, and an inner zone of phlogopite and minor anthophyllite.
DEPOSIT GEOLOGY AND MINERALIZATION
Within the Bucko Lake Deposit, three main zones of nickel sulphide mineralization have been recognized:
1. The West Limb Zone or western limb of the fold structure. The Lower, Middle, and Upper Zones follow interpreted continuity in elevated mineralization between drill hole intercepts through corresponding portions of the Bucko Lake Intrusion. Two corridors of elevated nickel within this area are referred to as the North and South trends;
2. The Hinge Zone occupies the “hinge” area between the western and eastern fold limbs and represents the northernmost portion of the Deposit and consists of three zones of mineralization interpreted to be folded extensions to the Lower, Middle, and Upper Zones observed on the West Limb; and
3. The Footwall Zone represents a recently discovered mineralized horizon that was intersected during the course of infill drilling and driving footwall development on the 1,000 ft (304.8 m) level in 2008. This Zone is interpreted to ties within mineralization intersected by historical exploration drill holes near the southern limit of drilling on the 1,400 ft (426.7 m) level.
Wide zones of lower-grade disseminated mineralization (generally >1.0% Ni) typically envelope higher grade net-textured to semi-massive sulphide layers or shoots (>3.0% Ni) within the host ultramafic intrusion. The overall appearance is one of a brecciated mass with sub-angular breccia fragments of mineralization rimmed with a mass of altered tremolite. This ‘breccia’ creates unequal breakage and subsequent weakness in unsupported faces.
A network of remobilized sulphide veinlets range in size from mm to m and are associated with a fracture-controlled talc/tremolite/phlogopite/anthophyllite alteration network that overprints the intrusion. Sulphides are found along altered contacts with pegmatite dykes that cross cut the intrusion.
Mineralization consists of disseminated to net-textured sulphides, mainly pentlandite, pyrrhotite, pyrite and chalcopyrite with minor mackinawite, violarite and cubanite. The massive sulphides from Bucko Lake, like those from the Thompson 1D Deposit, have the highest Ni/Co ratios among the TNB deposits (>100) (CAMIRO, 2004). Given that the Ni contents in the pentlandites from the Bucko Lake and Thompson 1D Deposits are in the same range as the other TNB deposits, the high Ni/Co ratios can be attributed to relatively lower Co abundances in the two Deposits. The lower Co abundances could reflect low magmatic Co contents. However, given the extent of deformation in the Thompson 1D Deposit, it is possible that the 1D pentlandites lost Co due to deformation. The Bucko pentlandites are also deformed and may have lost Co as a result.
At Bucko Lake, primary disseminated nickel sulphide mineralization is typical of that in komatiitic dunite-associated deposits (Good and Naldrett, 1993). Mobilized sulphides occur in amphibolite xenoliths, sheared granitic pegmatite dykes and sheared peridotite, and are subdivided into two types: 1) early xenolith-hosted sulphides and 2) later stringer sulphides. The abundance of PGM, Au, Cu and Ni in primary sulphides is apparently unaffected by serpentinization and amphibolitegrade metamorphism, however, Cu and Au were lost during the metasomatic alteration adjacent to granitic pegmatite dykes. The composition of the xenolith-hosted mobilized sulphides is similar to that of the primary sulphides. However, the stringer sulphides are relatively enriched in Cu and depleted in Ni and Ir.