French River (Bigwood-Rutter)

The French River occurrence, referred to by Currie (1976a, p. 196) as 'Bigwood-Rutter' and by Duke and Edgar (1977) as 'Bigwood', consists of two lenses of nepheline syenite within a sheath of alkaline syenite. The more northerly mass of nepheline syenite is known as Rutter, the more southerly as French River, and they extend over some 12 km with a maximum width of about 1.5 km. There is a small isolated outcrop on the Pickerel River south of the main French River mass. Both the nepheline syenite and syenite are foliated and lie conformably within granitic gneisses on the west side of a north-south-trending syncline, the equivalent rocks on the eastern limb of the syncline being biotite gneisses and amphibolites. The Rutter nepheline syenite, which can be seen to be intruded and replaced by pink alkaline syenite, is a medium grained rock of nepheline (12-49%), albite (16-40%), microcline (6-22%), perthite (3-56%) and hastingsite (1-24%), with occasional biotite, aegirine-augite, apatite, sphene and opaques. When perthite is abundant the nepheline is altered to zeolites. The wide variation reflects the banded nature of the rock. The French River nepheline syenite is similar but nepheline only reaches 25%, perthite is generally less and biotite rather than hastingsite is the principal mafic mineral; accessories are similar together with corundum, graphite, sodalite and cancrinite. Pegmatite patches occur with albites up to 30 cm in diameter, blue sodalite and orange and yellow cancrinite. The syenite which completely envelopes the French River nepheline syenite and flanks that of Rutter, is a medium grained, gneissose rock of microcline microperthite, albite, hastingstite, aegirine-augite, biotite and magnetite; zircon, apatite, garnet, pyrite, carbonate, fluorite and sphene are accessories. The syenite has been referred to as umptekite, bigwoodite, rutterite and fenite. Rock analyses and modes are available in Hewitt (1961, p.173-7) and analyses of biotite, amphibole and pyroxene in Duke and Edgar (1977).