Compositional zoning in ore minerals at the Craig mine, Sudbury, Ontario, Canada

The Craig mine, located on the North Range of the Sudbury Igneous Complex, in Ontario, is a typical Sudbury-district nickel sulfide deposit with announced reserves of 14.7 million tonnes of ore, grading 2.01% Ni and 0.74% Cu. The ores consist of massive to disseminated pyrrhotite with randomly dispersed veinlets and flames of pentlandite and scattered grains of chalcopyrite and magnetite. Very small quantities of pyrite occur as scattered subhedral to anhedral grains. The physical textures are typical of those of magmatic nickel sulfide deposits and appear to have formed through diffusion of nickel from a pre-existing monosulfide solid-solution during cooling. Chemical textures have been examined using the electron microprobe; element mapping used analysis times up to 12 hours and beam currents up to 1000 nA. The pyrrhotite has been found to contain nickel-depletion zones around pentlandite veinlets and flames; depletion zones also exist around non-nickel-bearing phases. The depletion zones appear to have formed as a result of nickel diffusion, which was arrested at low temperatures. Individual grains of pentlandite within veinlets contain significant inhomogeneities. Representative examples of the scattered grains of pyrite, although showing no physical variation, contain well-defined compositional zoning in terms of cobalt contents; nickel zoning is much less well defined. This zoning presumably resulted from selective loss of cobalt (and, to a lesser extent, nickel) from the monosulfide solid-solution as its compositional range was reduced at low temperatures.