Source:Targeted Geoscience Initiative 4: Canadian Nickel-Copper-Platinum Group Elements-Chromium Ore Systems — Fertility, Pathfinders, New and Revised Models, Geological Survey of Canada, Volume Open File 7856, p.257–268 (2015)
One challenge in the exploration for Cu-Ni-PGE mineralization in the footwall of the Sudbury Igneous Complex (SIC) is the uncertainty of its origin. The relative proximity of mineralization to the SIC is consistent with models of magmatic fractionation, but the common association of ore in the SIC footwall with amphibole and epidote alteration is consistent with a hydrothermal origin. Although these processes are not mutually exclusive (e.g. ores of magmatic origin could have been later remobilized by hydrothermal fluids), better constraints on which processes operated would greatly assist exploration. This project is a pilot study to assess whether chemical fingerprints can be established for four distinct mineralization types in the Levack-Morrison ore system: (a) contact; (b) a transition zone between contact and footwall ore; (c) sharp-walled veins; and (d) disseminated, S-poor, PGE-rich ores.<br/>Sulphide assemblages consisting primarily of pyrrhotite, chalcopyrite, and pentlandite were characterized in detail (petrography, SEM, EPMA, LA-ICP-MS). The results indicate that (a) Se content increases with depth; and (b) some trace elements (e.g. Cd vs. Se in chalcopyrite, Co vs. Se in pentlandite and pyrrhotite) can discriminate among different ore types. Calculated partition coefficients (± 2σ) for Se inchalcopyrite and pentlandite (1.2 ± 0.1 for contact and transition ores, 0.5 ± 0.2 for sharp-walled veins) are significantly different, which is consistent with different mineralization processes for those ore types. In addition to trace element content calculation in major sulphides, element distribution maps were created from LA-ICP-MS spectra of sulphide assemblages. Some contact-style samples contained abundant euhedral pyrite but pyrite was also present in samples of other ore types. The maps showed complex trace element zonation (e.g. Se, Co, and As) in pyrite in contact ore, as well as some PGE minerals (notably Ir and Os). In contrast, no PGEs were detected in any of the other sulphides or any compositional zoning. Because Ir has very low solubility under most hydrothermal conditions, Co-rich, Ir-bearing pyrite was interpreted to have formed from the cooling of a sulphur-rich sulphide liquid. Such pyrite (when present) could be used as an indicator of a magmatic signature.<br/>To further refine these results, future work would need to focus on three areas: (1) analyses of additional samples from the Morrison-Levack ore system to validate the discrimination diagrams for different ore types; (2) similar work would need to be undertaken elsewhere in the Sudbury mining district, to establish if the proposed discrimination plots are applicable basin-wide; (3) better constraints would need to be established for the origin of the Co-rich, PGE-bearing pyrite to enable it to be used as a marker of ore type.