Preliminary Rb–Sr geochronology of the Rice Lake – Beresford Lake area, southeastern Manitoba

A Rb–Sr geochronologic study of rocks of the Rice Lake – Beresford Lake area, southeastern Manitoba, shows that gold-quartz veins containing sericite and fuchsite were emplaced at 2720 ± 185 m.y. ago (λ87Rb = 1.39 × 10−11 yr−1). The mineralization postdates, and thus provides a minimum age for, the Rice Lake Group as well as for the successively younger intrusive basic rocks, quartz diorite plutons, and the San Antonio Formation. A whole-rock isochron for the potassic granite that lies north of the greenstones indicates an age of 2550 ± 80 m.y., with an initial 87Sr/86Sr of 0.7031 ± 0.0021, which we also consider as the age of regional metamorphism. This is in agreement with a metamorphic age of 2490 ± 90 m.y., as registered by phyllites from the Rice Lake Group. The gneisses and granites that lie south of the greenstone belt are older than 2630 m.y., as determined by a cross-cutting pegmatite. Because the northern granite is younger than the gold-quartz veins, extensive prospecting of the northern granit...