Genesis of Cu-PGE-rich footwall-type mineralization in the Morrison deposit, Sudbury

Abstract

The Morrison deposit, located at the Levack mine in the City of Greater Sudbury, is a footwall-type Cu-Ni-platinum-group-element (PGE) deposit hosted within a zone of Sudbury Breccia in the Archean Levack Gneiss Complex beneath the North Range of the Sudbury Igneous Complex. It consists of sharp-walled, sulfide-rich veins that are enriched in Cu-Pt-Pd-Au relative to contact-type mineralization and can be subdivided based on vein geochemistry, mineralogy, texture, and morphology into a pyrrhotite-rich upper domain, a chalcopyrite-rich lower domain, and a pyrrhotite equal to chalcopyrite middle domain. All domains contain steeply to vertically dipping first-order sulfide veins, irregular and discontinuous second-order sulfide veins, and disseminated sulfides in country rocks. First- and second-order veins can be further subdivided into inclusion-free veins typically within Sudbury breccia matrix or along clast-matrix boundaries, and very irregular and inclusion-rich veins associated with leucosomes in mafic gneiss clasts and granophyric-textured dikes. First-order veins consist of pyrrhotite > chalcopyrite = pentlandite > magnetite in the upper domain, pyrrhotite = chalcopyrite > pentlandite > cubanite > magnetite in the middle domain, and chalcopyrite >> pentlandite > pyrrhotite = cubanite > magnetite in the lower domain. Second-order veins consist of pyrrhotite = chalcopyrite > pentlandite > magnetite and chalcopyrite = millerite = pentlandite in the middle domain, and chalcopyrite >> millerite, millerite > chalcopyrite, bornite >> chalcopyrite, and millerite > bornite > chalcopyrite in the lower domain. Second order veins are adjacent to and in contact with epidote, amphibole, chlorite, carbonate, quartz, and magnetite alteration minerals. Sulfide mineralization in the Morrison deposit is similar to other footwall mineralization associated with the SIC. The veins appear to have been emplaced preferentially into zones of Sudbury Breccia that were within ~400m of the basal contact of the SIC, because that lithology is more permeable and because those zones are within the thermal aureole of the cooling SIC permitting penetration of sulfide melts. The mineralogical, textural, and geochemical zoning in the chalcopyrite-pentlandite-pyrrhotite-rich parts of the Morrison deposit are best explained by partial fractional and/or equilibrium crystallization of MSS and ISS. Bornite ± millerite-rich mineralization are interpreted to have formed by reaction of residual sulfide melts with wall rocks, consuming Fe and S to form actinolitemagnetite- epidote-chlorite-sulfide reaction zones and driving the sulfide melt across the thermal divide in that part of the Fe-Cu-Ni-S system to crystallize borniteSS ± milleriteSS. Gold-Pt-Pd appear to have been more mobile than other metals, forming localized zones of enrichment, although it is not clear yet whether they were mobile as Au-Pt-Pd-Bi-Te-Sb-rich melts or aqueous fluids.