A model for the lead isotope evolution of early proterozoic Svecofennian sulphide ores in Sweden and Finland

Abstract

Pb isotope compositions of galena-bearing sulphide ores from different areas in the Early Proterozoic Svecofennian province of the Baltic shield in Sweden and Finland are discussed in the light of the plumbotectonic concept. The individual deposits are thought to have a common syngenetic origin, formed more or less simultaneously with the onset of the early volcanism during the Svecofennian orogeny. The distinctive Pb isotopic signatures of the various areas, which form linear arrays in the conventional PbPb plots, resulted from differences in the mantle-upper crust mixing processes taking place at an initial stage of the orogeny. A three-stage model, comprising a mantle residence stage, followed by a mixing event and a final crustal residence period, is proposed which quantitatively describes the Pb isotope evolution up to the time of ore formation.