The search for Kimberlite in the James Bay Lowlands of Ontario

CIM Bulletin, Vol. 84, No. 947, 1991

L.E. Reed, Mining Division, BP Resources Canada Ltd., and I.G.L.Sinclair, Min Scan Consultants Ltd.

An exploration program seeking kimberlite pipes, the common primary hosts for diamonds, was conducted during the years 1979 through 1982 in the James Bay Lowlands of Ontario by Selco Mining Corporation (now the Mining Division ofBP Resources Canada) andEsso Minerals Canada. The area to be explored was selected on the basis of its regional geological setting, and because of the known presence of kimberlite indicator minerals in the region. The primary exploration technique was low level airborne magnetic surveying, followed by detailed ground geophysics. This work located many pipe-like bodies, with diameters of a few hundred metres, as well as several dykes. Targets located by these geophysical methods were drilled, intersecting a total of 45 intrusions, consisting of 34 alkaline diatremes containing tuffisitic breccias, 7 carbonatites, and 4 massive alnoites. K-Ar determinations indicated ages of 152 Ma and 180 Ma for two of the massive alnoites. The texture and general mineralogy of the tuffisitic breccias are similar to those of diatreme fades kimberlites, but detailed examination of their mineralogy and chemistry demonstrated that they lie within the alnoitic rather than the kimberlitic compositional field. Although the exploration program did not lead to the discovery of true kimberlites, it did demonstrate the presence of a large, previously unknown province of alkaline volcanic activity, which has no surface expression.

Keywords: Exploration, James Bay Lowlands, Magnetic surveying, Kimberlite, Geophysics, Geochemistry