LEAD ISOTOPE RATIOS FROM THE COBALT–NORANDA AREA, CANADA

Abstract

Lead isotope ratios indicate that very old galenas in Archean volcanics and sediments in the vicinity of Timmins, Ontario; Noranda and Val d'Or, Quebec, have had additions of younger lead of the type found near Cobalt, Ontario. The mixture of leads of two distinct ages has produced isotopic ratios which are linearly related. The greatest degree of mixing has occurred in areas such as Kirkland Lake which are close to the boundary between Archean rocks and the Bruce–Cobalt Series. Most of the lead appears to have been derived from a subsialic source region. From the relationship of the anomalous lead line and the single-stage growth curve commonly accepted for ordinary leads it is found that lead sulfide mineralization occurred 3 250 ± 150 million years (m.y.) ago in Archean rocks of Ontario and Quebec. The leads associated with mines along the northeastern edge of the Cobalt basin were separated from their uranium and thorium source region 2 300 ± 150 m.y. ago. Leads from Chibougamau, Quebec, appear to be related in age to the Cobalt leads. This study of 50 lead isotope ratios is a clear illustration of the claim that single-stage lead models must not be applied indiscriminately to isolated analyses of galena. It also produces evidence for the existence of crustal rocks in North America prior to 3 200 m.y. ago.