Alkaline Rocks and Carbonatites of the World

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

Nemag Lake


Occurrence number: 
Longitude: -81.23, Latitude: 46.37

The Nemag Lake fenite occurrence covers about 800x600 m and lies 5 km northeast of the Kusk Lake occurrence (031-00-078). As at Kusk Lake Mississagi Quartzite, consisting essentially of feldspathic sandstones and greywackes, has been fenitized with the development of veining and brecciation, and growth of secondary aegirine, alkali amphibole, alkali feldspar and some late biotite. Several stages of increasing fenitization have been mapped from shattering of the rocks with little mineralogical change through stages of veining and intensive metasomatism, often with the spectacular marginal replacement of angular quartzite blocks (Siemiatkowska and Martin, 1975, Fig. 4). There are late quartz veins and a single 1.5x0.6 m ankerite vein is the only carbonate to have been found in the fenites. Igneous rocks are not present. Rock analyses are available, as are microprobe analyses of aegirine, alkali amphibole, alkali feldspar, Nb-rutile and opaque phases.

A K-Ar determination on biotite from late porphyroblasts in the fenite gave 1196±11 Ma (Siemiatkowska and Martin, 1975, Table 6), but the fenites are considered to have formed between 1700 and 2150 Ma ago (op. cit., p. 1117).

SIEMIATKOWSKA, K.M. and MARTIN, R.F. 1975. Fenitization of Mississagi Quartzite, Sudbury area, Ontario. Bulletin of the Geological Society of America, 86: 1109-22.

Fig. 1_41 Kusk Lake and Nemag Lake fenites (after Siemiatkowska and Martin, 1975, Fig. 2).
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