Abstract

Twenty-seven lead-isotope analyses for galena specimens from 14 localities in the Cobalt district represent sulphide-rich interflow sedimentary units in the Archean volcanic sequence, mineralized zones, fracture fillings, and local sulphide concentrations in the Cobalt Group (Proterozoic) sedimentary rocks, and vein-filling and breccia-cementing sulphides from within and near silver–arsenide veins. The analyses define a very homogeneous composition, averaging 206Pb/204Pb = 14.732, 2O7Pb/204Pb = 15.114, and 208Pb/204Pb = 34.301, for lead from these different geological settings. This result is in agreement with textural observations indicating a late paragenetic position for galena in the different settings mentioned. The model age for this lead is in general accord with the U–Pb age obtained in a study by Andrews et al. for a Nipissing diabase sheet at Gowganda.About one third of the galena specimens are interpreted by a two-stage evolution model to have formed or been remobilized during an event in the time range 1650–1945 Ma. The timing and character of this event are not clearly defined. The radiogenic composition of a specimen from the Pan Silver property is comparable to radiogenic values reported by Thorpe and suggests a young galena-forming event in the area.The lead-isotope data are permissive evidence for a genetic link between the Nipissing diabase sheets and mineralization of a variety of types in the Cobalt district. The metal source is not clearly indicated, but one analysis for Archean interflow sulphides suggests than an Archean source is unlikely for the lead in the silver–arsenide veins and other mineralized zones.

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