Abstract

Magnetite-apatite-actinolite rocks, similar to those at Kiruna, Sweden, and the St. Francois Mountains, Missouri, are associated with seven epizonal plutons that intrude andesitic stratovolcanoes of a broadly folded, early Proterozoic continental magmatic arc located in the northwestern Canadian Shield. The plutons, mainly medium-grained quartz monzonite-monzodiorite-diorite (IUGS) sheets and laccoliths 5 to 25 km in diameter, are spatially, temporally, and compositionally related to host andesitic stratovolcanoes. They were emplaced at 2 to 3 km depth, are compositionally heterogeneous, metasomatically altered, and had plagioclase as the liquidus phase. In addition they have kilometer-wide alteration halos comprising an inner zone of nearly complete wall-rock albitization, an intermediate zone of magnetite-apatite-actinolite veins, pods, breccias and wall-rock replacement, and an outer zone of disseminated sulfides. One pluton (Balachey) has a halo that is more or less completely outside the body, but another (Rainy Lake) has a halo that is partly superimposed over it. Separation of iron phosphate and silicate melts by liquid immiscibility is not supported by: (1) the gradual replacement of plagioclase by chessboard albite toward the roof of the Rainy Lake pluton, (2) the chemical trends in the Rainy Lake intrusion, (3) the mineralogical zoning, low temperature mineralogy, and replacement textures of the magnetite-apatite-actinolite, and (4) the relationship of the magnetite-apatite-actinolite veins and bodies to the plutons. However, all available data are compatible with a deuteric origin. It is concluded that high temperature, low initial water content, and shallow-level emplacement by intermediate plutons are necessary for the development of Kiruna-type deposits.

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