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 annou nced 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 dispe rsed veinlets and flames of pentlandite and scattered grains of chalcopyrit e and magnetite. Very small quantities of pyrite occur as scattered subhedr al to anhedral grains. The physical textures are typical of those of magmat ic nickel sulfide deposits and appear to have formed through diffusion of n ickel from a pre-existing monosulfide solid-solution during cooling. Chemic al textures have been examined using the electron microprobe; element mappi ng 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 pentland ite veinlets and flames; depletion zones also exist around non-nickel-beari ng phases. The depletion zones appear to have formed as a result of nickel diffusion, which was arrested at low temperatures. Individual grains of pen tlandite within veinlets contain significant inhomogeneities. Representativ e examples of the scattered grains of pyrite, although showing no physical variation, contain well-defined compositional zoning in terms of cobalt con tents; nickel zoning is much less well defined. This zoning presumably resu lted from selective loss of cobalt (and, to a lesser extent, nickel) from t he monosulfide solid-solution as its compositional range was reduced at low temperatures.