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Mineralogy, geochemistry and petrology of a pyrochlore-bearing carbonatite at Seabrook Lake, Ontario Osatenko, Myron John

Abstract

The Seabrook Lake carbonatite complex is one of the smallest of nine known carbonatite complexes in central Ontario. The complex, which is one-half square mile in area and pear-shaped in plan, consists of fenitized granite and breccia, mafic breccia, ijolite and related breccia, and carbonatite. The bulbous northern part of the complex consists of a plug-like core of carbonatite surrounded by mafic breccia and carbonatite dykes. The narrow southern part consists of ijolite and related breccia. Enveloping all of these rocks is a fenitized aureole which grades outward to unaltered granite that underlies much of the surrounding area. The carbonatite is composed of calcite with the following minor mineral, in decreasing order of abundance: goethite, microcline, magnesioriebeckite-riebeckite, magnetite-ulvospinel, apatite, hematite, pyrite, albite, biotite, chlorite, pyrochlore, brookite, sphene, ferroan dolomite (ankerite?), aegirine, chalcopyrite, wollastonite and quartz. The chemical constituents are as follows: Major CaO + CO₂ Minor Fe₂O₃, SiO₂, MgO, Nb₂O₅, SrO, BaO, Na₂O, K₂O, MnO, Al₂O₃, P₂O₅, S and H₂O. Trace Cu, Pb, Zn, As, Ce, Y, Li, Cr, Co, Ni, V, In, Zr, and Ti. The complex is believed to have formed by desilication and metasomatism of fractured and brecciated granite by a soda-iron-rich carbonatite magma of unknown origin.

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