Summary:
Sulphide deposits sit on broadly defined trends of mineralisation along basal brecciated rocks of the Sudbury Igneous Complex as pentlandite-pyrrhotite chalcopyrite rich concentrations as well as within the underlying footwall in fractured pathways as chalcopyrite dominated polymetallic (Cu, Ni, Au, Ag, Pt, Pd) vein-style sulphides. The total Mineral Reserve tonnage increased from 2016 due to the addition of the Onaping Depth deposit.
Fraser Deposit
Contact-style Ni–Cu–PGE and footwall-style Cu–Ni–PGE deposits at the Fraser mine occur along the northwestern edge of the Sudbury Igneous Complex (SIC). In the vicinity of the Fraser mine, the SIC forms a tabular sheet dipping to the southeast, comprising noritic to gabbroic cumulates overlain by a granophyric residuum. Along the basal contact of the SIC, partially melted target rocks comprise an igneous-textured footwall breccia (locally called “late granite breccia”). At the Fraser mine, the footwall breccia overlies Archean gneiss, granite, migmatite, and diabase dykes of the Superior Province. The cooling of the SIC resulted in a zoned contact-metamorphic aureole (albite–epidote hornfels to pyroxene hornfels in grade) extending about 1 km be low the base of the SIC into the Archean host-rocks (Coats & Snajdr 1984, Dressler 1984, Farrow 1995).
During impact, the excavation, brecciation, and melting of the footwall rocks prepared structural traps along the basal contact of the SIC, in which contact-style Ni– Cu–PGE sulfide mineralization was introduced during the differentiation of the SIC. Magmatic differentiation of sulfide liquid along the lower contact of the SIC resulted in the formation of residual sulfide liquid enriched in Cu, Pt, Pd and Au. This Cu-rich sulfide liquid then settled into zones of brecciated gneissic rock (Sudbury Breccia) in the footwall to the SIC (i.e., Li 1992, Li & Naldrett 1993, Morrison et al. 1994). The zones of Sudbury Breccia that host footwall-style Cu–Ni–PGE mineralization occur below the Strathcona embayment, subparallel to the embayment axis, which plunges approximately 21° SSW (Fedorowich et al. 1999).
Nickel Rim South Deposit
The Nickel Rim South Mine is located along the eastern flank of the Sudbury Igneous Complex. Sulphide deposits sit on broadly defined trends of mineralisation along basal brecciated rocks of the Sudbury Igneous Complex as pentlandite-pyrrhotite chalcopyrite rich concentrations as well as within the underlying footwall in fractured pathways as chalcopyrite dominated polymetallic (Cu, Ni, Au, Ag, Pt, Pd) vein-style sulphides.
The Nickel Rim South deposit is made up of a Contact zone and Footwall mineralized zones that span a region measuring roughly 720 meters along a northwest to southeast axis.
Contact mineralisation consists of massive and semi massive nickel bearing sulphides.
The Nickel Rim South footwall mineralisation compares closely with other Sudbury footwall deposits (copper zones). This copper zone is a mixture of erratically distributed copper veins in a granitic host rock - felsic gneiss and Sudbury breccia, with occasional brecciated dyke fragments. Both are fine grained, hard granitic rocks, although very competent, can be prone to rockbursting at depth. The copper sulphides occur in an erratic assemblage of veins ranging from a few centimeters to several metres thick. The two types of mineralisation are often separated by a few hundred meters, but at Nickel Rim South they merge together at around 1,320 m depth and are only separated by a few tens of meters below that horizon. Overall they lie within a steeply dipping (70°) mineralised envelop which is also mined using blasthole stoping. A large strength contrast exists between the complex copper veins and the host rock, with the veins fracturing and deforming preferentially.
Onaping Depth Deposit
The deposit is located in the Onaping-Levack area of the North Range of the Sudbury Igneous Complex (SIC), in Ontario. The Sudbury Igneous Complex (SIC) occurs as a 60 km x 27 km elliptical bowl-shaped body that formed from a meteorite impact melt sheet. The deposit consists of a basal xenolithic norite breccia (contact sublayer) overlain by norite, quartz-gabbro and granophyre. It represents one of a cluster of nickel-copper sulfide orebodies that have been located at the base of the complex over an approximately 10 km-long zone.
Copper–nickel–PGE mineralized zones at Sudbury are spatially and genetically related to relatively small bodies of inclusion-rich material localized either at the contact between the main mass of the SIC and footwall rocks (contact sublayer) or within radiating and concentric dykes cutting footwall rocks (offset dykes). Three major variants of sublayer are recognized: igneoustextured gabbronoritic material; igneous-textured quartz diorite; and a wide variety of metamorphic-textured rocks collectively known as “footwall breccia”. Gabbronoritic sublayer typically occurs in contact deposits and in the proximal (0–2 km from SIC contact) portions of the North Range offset dykes. Quartz diorite is the main component of South Range offsets and of distal (>2 km from SIC contact) portions of North Range offsets. Footwall breccia occurs as sheets and discontinuous lenses concentrated along the lower contact of the SIC and as a major component of some of the offset dykes. Igneous- and metamorphic-textured sublayer are characterized by disseminated to massive sulphide and by the presence of a variety of xenoliths of both local and unknown or “exotic” derivation.
Ore deposits of the North Range of the Sudbury Igneous Complex can be classified into 5 generic ore types (Coats and Snadjr, 1984):
• Disseminated sulphides in the mafic norite of the Complex
• Disseminated sulphides, stringer sulphides, massive sulphide lenses and interstitial sulphides in breccias in the sublayer norites
• Massive sulphides and interstitial sulphides in breccia at the contact of the sublayer norite and the Footwall rocks
• Sulphide lenses associated with massive sulphide stringers in Footwall rocks
• Cu-rich massive sulphide stringers and dykes in Footwall rocks
Ores of the Craig / Onaping occur as two of the above ore types. The Main or ‘contact’ ore zones are shallower deposits comprising massive, disseminated and breccia sulphides occupying a deep embayment in the mineralized zone of the Footwall breccia of the SIC. The Main ore zone sulphides grade into Footwall zone ores comprising Cu-rich massive sulphide stringers varying in width between 0 – 6m. It is thought that these ores originate from copper remobilization in the contact ore zone. Sulphide stringers in the SIC are typically oriented parallel to the Footwall contact, and are thought to occupy stress-related fractures in the footwall normal to the compressive stress generated by the igneous intrusion. The brecciation at the contact between the sublayer norites and the footwall rocks is related to shock generated by the rate of the igneous intrusion. Principal North Range sulphides are chalcopyrite, pentlandite, pyrrhotite, and pyrite. PGM’s occur as sperrylite or as a solid solution in the chalcopyrite or pentlandite.
Narrow vein orebodies are generally classified as seams of between 0.1 and 3m with an intermediate or steep dip of 20 - 55o (Miller-Tate et al, 1995). The Onaping Depth orebody, like the Strathcona Copper Zone and the McCreedy East footwall ores, comprises multiple stringers of varying thickness within the thickness of the orezone, but can be considered a narrow-vein orebody due to the narrowness and steep inclination of the individual veins.