Mineral ages and isotopic composition of primary lead at Manitouwadge, Ontario

Abstract

Granitic rocks at Manitouwadge, Ontario, have an age of 2720±50 m.y. based on isotopic lead data from zircon and sphene, and on Rb-Sr measurements from whole rocks. The rocks are stratigraphically and lithologically similar to ones called Algoman elsewhere in the Canadian Shield. Lead from the conformable ore body at Manitouwadge yields a model age of 3000 m.y., using an age of 4550 m.y. for the earth, and using a closed-system evolution model for the lead. Field relations show that the ore is younger than the granites. Lead in potassium feldspars from the granitic rocks appears to be closely related to lead in the ore; in particular, the Pb208/Pb204 ratios are identical in ore and feldspars. Lead from ores along a 1100 km traverse across the Superior Province from Noranda, Quebec, to Kenora, Ontario, have isotopic compositions generally similar to the composition of the ore at Manitouwadge. Radiometric age data do not indicate ages older than 2700 m.y. in the area. Less complete data from the continents of Africa, Australia, Asia, and Europe suggest that ore leads with model ages of 3000 m.y. are associated with rocks having radiometric ages of 2700 m.y. For these reasons the Manitouwadge data do not appear to represent a local anomaly. If this is true, several possibilities can explain the data: (1) the age of the earth is not 4550 m.y., in which case closed-system evolution and terrestrial primordial lead identical in isotopic composition to that in iron meteorites can be retained; (2) terrestrial primordial lead did not have the same isotopic composition as iron meteorite lead, in which case a 4550 m.y. age of the earth and closed system isotopic evolution for lead can be retained; (3) isotopic evolution of lead did not take place in a chemically closed system. None of these possibilities can be eliminated with certainty. It is difficult to reconcile lead data with a closed-system evolution model, a 4550 m.y. age of the earth, and terrestrial primordial lead identical in composition to iron meteorite lead.