Alkaline Rocks and Carbonatites of the World

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Prairie Lake

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Occurrence number: 
031-00-052
Country: 
Canada
Region: 
Ontario
Location: 
Longitude: -86.72, Latitude: 49.03

Prairie Lake is a circular complex covering 8.8 km2 and with a strong aeromagnetic expression (Ferguson, 1971). Exposure is poor but it has been trenched and drilled. The principal rock types are ijolites into which are intruded arcuate bands of carbonatite, more especially near the margins. The ijolites are very variable and pass into mixed carbonate-silicate rocks. Melteigitic varieties contain magnetite and biotite and these pass into more leucocratic rocks, with wollastonite, melanite, calcite and some alkali feldspar. Pseudoleucites, some with polygonal outlines, occur in urtites and the petrography and mineral analyses of these are given by Watkinson (1973). Pegmatitic segregations may contain up to 50% wollastonite, the rest being predominantly nepheline. A striking orbicular ijolite occurs on the eastern side of the complex with orbs 2-3 cm in diameter. There is minor aegirine syenite. The carbonatites are principally calcitic but there is local dolomite and late ferruginous varieties. Pyrochlore, sometimes uraniferous, apatite, pyrite and brookite are present. The complex is emplaced into granitic Archaean rocks which are fenitized adjacent to the contacts. Mica and pyroxene-bearing lamprophyres and micro-ijolite dykes occur within the intrusion.

Economic: 
Exploratory drilling, trenching and assaying have been conducted, and a zone rich in uranopyrochlore has been defined towards the centre of the complex, and estimated at 109 024 tons grading 0.12% U3O8. Uranium-free pyrochlore is much more extensive (Sage, 1975, pp. 64-6). A geochemical survey of tills and residual soils has been made (Closs and Sado, 1982).
Age: 
1112 Ma by K-Ar on biotite (Gittins et al., 1967), while a whole rock Rb-Sr isochron gave 1023+-74 Ma (Bell and Blenkinsop, 1980, Table 3).
References: 
BELL, K. and BLENKINSOP, J. 1980. Grant 42 ages and initial 87Sr-86Sr ratios from alkalic complexes of Ontario. Miscellaneous Paper, Ontario Geological Survey, 93: 16-23. CLOSS, L.G. and SADO, E.V. 1982. Orientation overburden geochemistry and Quaternary geology investigations of carbonatite-alkalic complexes in the Prairie Lake and Killala Lake areas District of Thunder Bay. Study, Ontario Geological Survey, 23: 1-65. FERGUSON, S.A. 1971. Columbium (niobium) deposits of Ontario. Mineral Resources Circular, Ontario Department of Mines and Northern Affairs, 14: 1-58. GITTINS, J., MACINTYRE, R.M. and YORK, D. 1967. The ages of carbonatite complexes in eastern Canada. Canadian Journal of Earth Sciences, 4: 651-5. SAGE, R.P. 1975. Carbonatite-alkalic complexes. Miscellaneous Paper, Ontario Geological Survey, 63: 58-66. SAGE, R.P., BATHE, D., WRIGHT, W., CHAMOIS, P. and SHEWBRIDGE, K. 1976. Prairie Lake carbonatite district of Thunder Bay. Ontario Division of Mines, Preliminary Map P.1070, Geology Series. WATKINSON, D.H. 1973. Pseudoleucite from plutonic alkalic rock-carbonatite complexes. Canadian Mineralogist, 12: 129-34
Fig. 28 Prairie Lake (after Sage et al., 1976, Map P.1070).
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith