ARCHEAN VOLCANIC STRATIGRAPHY, AND PETROLOGY AND CHEMISTRY OF MAFIC AND ULTRAMAFIC ROCKS, CHROMITE, AND THE SHEBANDOWAN NICKEL-COPPER MINE, SHEBANDOWAN, NORTHWESTERN ONTARIO

Shebandowan Ni-Cu Mine, owned by Inco Metals Ltd., is located within the Shebandowan Greenstone Belt, 70 km west of Thunder Bay, northwestern Ontario. Two successions of Archean tholeiitic basalt underlie the Shebandowan mine area. The older is characterized by pillowed basalt, mafic pyroclastic rock, plagioclase-phyric flows and co-eval volcaniclastic rock; the younger is divided into two distinct basalt types: (1) high Mg-basalt with intercalated tuff and (2) subaqueous pillowed basalt, pillow breccia, hyaloclastite with minor fine-grained ash and lapilli tuff, and iron formation. All rocks are steeply dipping and have been isoclinally folded. Faulting has occurred both during and after this folding. All rocks have been metamorphosed to greenschist facies.;Chromite is ubiquitous throughout the ultramafic bodies, making up 1-2% of each body. It is zoned with fresh cores, ferritchromit rims and irregular magnetite overgrowths. Ferritchromit is always lower in MgO, usually lower in Al(,2)O(,3) and Cr(,2)O(,3) and higher in Fe(,2)O(,3) and FeO relative to fresh cores. Stratigraphic compositional changes in chromite have been documented; there is an overall increase in Fe, decrease in Cr/R('3) and increase in Fe('3)/Fe('3)+Cr up section.;On the basis of TiO(,2), Al(,2)O(,3), and Fe(,2)O(,3) contents of chromite, the ultramafic bodies are divided into groups: one with TiO(,2)-Fe(,2)O(,3)-rich, Al(,2)O(,3)-poor chromite, and one with TiO(,2)-Fe(,2)O(,3)-poor, Al(,2)O(,3)-rich chromite. These divisions correspond respectively to the extrusive and intrusive ultramafic rocks. . . . (Author's abstract discontinued here because it exceeds stipulated maximum length.) UMI.;Two distinct types of ultramafic rock are recognized within the Shebandowan mine area: they are ultramafic flows, associated with iron formation and basaltic hyaloclastite, and ultramafic sills with no particular rock association. The former are found only in the younger succession, the latter predominantly in the older. Petrographically they are similar; pseudomorphs after primary minerals indicate that olivine, and some orthopyroxene are cumulate phases, clinopyroxene is intercumulus. No spinifex textures are recognized in either group. Thin bodies (15-20m), whether flows or sills, consist of harzburgite and lherzolite, whereas larger flows and sills (> 80 m) are layered from harzburgite to olivine gabbro. These bodies are now serpentinite, talc-carbonate or tremolite schist.