A HIGH-GRADE METAMORPHIC, GRENVILLE-AGE, GRAPHITE DEPOSIT, LAC-DES-ILES, QUEBEC, CANADA

Within the Grenville metasedimentary belt, the Stratmin (TimCal) graphite mine at Lac-des-Iles, Quebec is currently the only active producer of high-quality graphite in Canada. The major graphite-producing lithologies contain up to 15% graphite along with various percentages of diopside. Also common, but not everywhere present, are calcite and phlogopite, and lessor quantities of quartz, plagioclase, K-feldspar, sphene and apatite; individuals of the latter group are commonly absent from a given assemblage. Associated and enclosing the graphite-rich rocks are dolomitic marbles and gneisses. The marbles commonly contain in addition to calcite and dolomite, forsteritic olivine and phlogopite with or without chondrodite and spinel. The gneisses are either biotite-orthopyroxene-bearing or highly pelitic containing biotite + quartz + plagioclase + K-feldspar + garnet ± sillimanite and accessory minerals. The large number of variable assemblages have permitted a number of geothermobarometers and calculation schemes to be tested where they delimit metamorphic conditions to between 4.5 and 6 kb pressure and temperatures that vary from 725 ̊C to 675 ̊C. Preliminary carbon isotope data suggest that the graphite had a premetamorphic sedimentary origin.