DISTRIBUTION OF LITHIUM IN THE GRANITIC ROCKS

Abstract

Three batholiths with associated lithium-bearing pegmatites are located in the Preissac - Lacorne area of north-western Quebec. Results of previous workers have indicated that the rocks of the batholiths represent a differentiation sequence. These rocks and their constituent minerals were analyzed spectrographically for their lithium content, for the purpose of studying the distribution of lithium during magmatic crystallization. Two working-curves were constructed for the analyses, one for minerals and the other for rocks. The curve for mineral analysis covered a concentration range of 3 to 3160 p.p.m. Li, whereas the curve for rock analysis covered the concentration range of 25 to 500 p.p.m. Li. Sodium was used as internal standard for mineral analyses, the spectral line Na 6160 being used with the spectral lines Li 6707 and Li 6103. For rocks, indium was the internal standard, the spectral line In 3258 being used with the analysis line Li 3232. Analyses of the rocks show that the lithium content increases with the acidity of the rocks, that is most probably with increasing differentiation. Averages for the various rocks are as follows: amphibolite (lithium not detected); hornblende monzonite, 7 p.p.m.; granodiorite, 53 p.p.m.; granite, 159 p.p.m. Analyses of the minerals show that quartz and feldspar usually contain much less than 100 p.p.m. lithium. Hornblende, on the other hand, averages about 208 p.p.m. and micas are even richer, some exceeding 3200 p.p.m. Strock’s (1936) theory for the increase in the Li x 100 ratio Mg in rocks with increasing differentiation is found to hold true for the batholithic rocks of the Preissac - Lacorne area. Furthermore, the results suggest that as a result of the increase of lithium with increasing differentiation, the last major differentiate of the batholiths (biotite-muscovite granite) was parental to the lithium-bearing pegmatites found in the area.