Abstract

A thin carbonatite marble unit, referred to here as the Mount Grace carbonatite, has been traced discontinuously for approximately 25 km along the northwestern margin of Frenchman Cap dome, southeastern British Columbia, Canada. It forms essentially one conformable layer that averages 3 m in thickness. It contains abundant apatite, phlogopite, and sodic amphibole and contains high concentrations of Mn, Sr, Ba, Nb, and the light rare earth elements, La, Ce, and Nd. Based on detailed stratigraphy and structural setting, the Mount Grace carbonatite is correlative and assumed to be continuous, with a similar carbonatite in the Perry River area, 10 to 15 km to the south. Here, nepheline syenite gneiss and discontinuous carbonatite lenses intrude the stratigraphic succession.The Mount Grace carbonatite is interpreted to be a pyroclastic carbonatite that was deposited on shallow marine tidal flats. Evidence includes its great lateral extent, its distinctly layered nature, its occurrence at a specific stratigraphic horizon, and its lack of contact alteration. It is distinguished mineralogically and chemically from the Perry River intrusive carbonatites, which are discontinuous lenses, have fenitized margins, and contain abundant magnetite, ilmenite, and columbite, minerals which are rare or absent in the Mount Grace carbonatite. The intrusive carbonatites are also distinguished by a higher average concentration of Nb but lower average concentrations of the light rare earth elements.Although the analyzed Mount Grace carbonatite samples are generally low grade, with total Nb, La, Ce, and Nd ranging from approximately 0.04 to 0.8 percent, the large lateral extent makes it a tremendous potential source for these elements.

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