Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences

Manitou Islands


Occurrence number: 
Longitude: -79.58, Latitude: 46.25

This is an elliptical complex of 3.2x2.7 km much of which lies beneath Lake Nipissing, the outer fenite ring of which crops out on five islands. Extensive drilling and tunnelling have allowed some definition of the extent of the complex. Ordovician sedimentary rocks overlie the complex unconformably on all five islands. Outer quartz and inner syenitic fenite zones can be distinguished. The syenitic fenite consists essentially of alkali feldspar and sodic pyroxene with pegmatitic veins having cockscomb pyroxenes; other minerals present include sodic amphibole, calcite, occasional quartz, brown biotite, apatite and minor pyrite, magnetite, monazite and uranium pyrochlore. The fenites are cut by carbonatite veins and drilling indicates that the central area contains thick screens of carbonatite. The carbonatites are variably coarse to fine grained calcite rocks with streaks of sodic pyroxene together with biotite, magnetite, apatite and locally uranium pyrochlore and pyrite. Between the syenitic fenites and central carbonatite area are alkali pyroxenites and syenites, presumably fenite variants, in which some altered nepheline occurs. Rocks described as 'altered feldspathic rocks' occur in this zone to the south, and could be potassic fenites. Lamprophyric and syenitic dykes cut the fenites.

Five uranium pyrochlore deposits have been defined and these were the first large pyrochlore deposits to be discovered in North America. Extensive drilling, the sinking of a shaft to over 100 m, and the driving of drifts and cross-cuts were carried out in the early 1950s. The deposits are complex but concentrated in syenitic fenites. One reported deposit contained 1 893 000 tons of mineralized rock averaging 0.049% U3O8 and 0.86% Nb2O5, and another contained 2 961 000 tons averaging 0.041% U3O8 and 0.69% Nb2O5. Rock with up to 60% of apatite and traces of uraninite occur. Details of other deposits are given in Lumbers (1971, p. 81) and Rowe (1958, p. 52) and references cited therein.
Biotite separated from carbonate-rich syenitic fenite gave 560 Ma by K-Ar (Lowdon et al., 1963, p. 93), and from a specimen from Newman Island 570 Ma (Gittins et al., 1967).

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.
LOWDON, J.A., STOCKWELL, C.H., TIPPER, H.W. and WANLESS, R.K. 1963 Age determinations and geological studies. Paper, Geological Survey of Canada, 62-17: 1-140.
LUMBERS, S.B. 1971. Geology of the North Bay area Districts of Nipissing and Perry Sound. Geological Report, Ontario Department of Mines, 94: 1-104.
ROWE, R.B. 1958. Niobium (columbium) deposits of Canada. Economic Geology, Geological Survey of Canada, 18: 1-108.

Fig. 1_43 Occurrences in the vicinity of Lake Nipissing. and Fig. 1_45 Manitou Islands (based on Rowe, 1958, Fig. 14).
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith