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The geology of the Pioneer Ultramafite, Bralorne, British Columbia Wright, Robert Leslie


The Pioneer Ultramafite, south of Bralorne, British Columbia, is a fault-bounded lens of alpine-type peridotite enclosed in lower greenschist facies sediments and volcanics of the middle Triassic Fergusson Group and late Triassic Noel, Pioneer and Hurley Formations. The body consists of a core of well layered harzburgite, dunite and orthopyroxenite, rimmed by serpentinite and talc-carbonate alteration zones containing tectonic inclusions of sediments, volcanics and rodingite. Foliated harzburgite forms approximately 80 per cent of the ultramafite; dunite, as dikes, sills and irregular pods in harzburgite, comprises about 17 per cent; and orthopyroxenite as layers (1 to 15 cm thick), parallel to the foliation, in harzburgite, constitutes the remaining 3 per cent. Electron microprobe analyses of the primary minerals indicate olivine composition ranges from Fo 90.2 to 92.5, orthopyroxene from En 89.2 to 90.2 and clinopyroxene averages Ca₄₆․₉Mg₄₉․₇Fe₃․₄. Compositions of coexisting primary minerals indicate a temperature of equilibration of about 950°C at an unknown pressure. Alteration assemblages in the serpentinized contact zone indicate migration of 0, H₂O, C0₂, CaO, MgO and S10₂ resulting in metasomatism of the ultramafite and country rocks, producing rodingite, nephrite (jade) and talc-carbonate. Serpentinization apparently occurred during emplacement of the ultramafite into the surrounding country rocks. Plastic deformation and recrystallization of the peridotite produced the pervasive planar foliation or layering, which has been disrupted by several later episodes of folding and fracturing. Country rocks show evidence of two phases of deformation prior to emplacement of the ultramafite, A strong foliation of serpentinite and country rocks, near the contact, was produced during emplacement. The ultramafite and country rocks are interpreted as a partial, dismembered ophiolite which was emplaced in the late Triassic or early Jurassic by obduction of oceanic crust onto the continental margin.

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