Alkaline Rocks and Carbonatites of the World

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St Lawrence Granite

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Occurrence number: 
031-00-164
Country: 
Canada
Region: 
Newfoundland
Location: 
Longitude: -55.5, Latitude: 47

A more or less continuously exposed batholith of 30x10 km with a number of outliers, the St Lawrence granite cuts sharply across Proterozoic lavas, pyroclastics and volcanogenic sediments that are conformably overlain by Cambrian sediments. There is a well developed contact aureole containing andalusite, cordierite etc. The granite is composed essentially of quartz and K-feldspar with albite in some specimens together with small and variable amounts of alkali amphibole, aegirine, biotite, fluorite, magnetite and hematite. In the northern parts of the batholith there are riebeckite-bearing volcanics which are interpreted as being extrusive equivalents of the granite. Fluorite veins are widespread but do not occur in the western lobe, and rhyolite porphyry dykes of essentially the same mineralogy as the granite occur. The alkali amphiboles of the granite have been investigated by Strong and Taylor (1984). Colman et al. (1984), in an oxygen isotope study, investigated the nature and extent of fluid involvement in metasomatism at the contacts of the granite.

Economic: 
This is the only pluton in Newfoundland known at present to contain economic mineral deposits. Fluorite is found in veins within the granite, and they vary from a few hundred m to 7 km in length, and three mines are currently in production. Associated with the fluorite is quartz, calcite, and rare baryte, galena, pyrite and sphalerite.
Age: 
Rb-Sr on four rocks gave 315+-5 Ma (Bell et al., 1977). A zircon age of 355 Ma was obtained by B.J. Fryer (K.L. Currie, personal communication, 1985).
References: 
BELL, K., BLENKINSOP, J. and STRONG, D.F. 1977. The geochronology of some granitic bodies from eastern Newfoundland and its bearing on Appalachian evolution. Canadian Journal of Earth Sciences, 14: 456-76. COLEMAN, M., COX, M. and TAYLOR, R. 1984. Isotopic evidence for the nature and extent of fluid involvement in metasomatism of the St. Lawrence granite (Newfoundland, Canada). Chemical Geology, 45: 289-98. Currie, K.L. personal communication, 1985 STRONG, D.F. and TAYLOR, R.P. 1984. Magmatic-subsolidus and oxidation trends in composition of amphiboles from silica-saturated peralkaline igneous rocks. Tschermaks Mineralogische und Petrographische Mitteilungen, 32: 211-22. TENG, H.C. and STRONG, D.F. 1976. Geology and geochemistry of the St. Lawrence peralkaline granite and associated fluorite deposits, southeast Newfoundland. Canadian Journal of Earth Sciences, 13: 1374-85
Fig. 82 The St Lawrence granite (after Teng and Strong, 1976, Fig. 2).
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith