Isotopic composition of lead in Proterozoic anoxic metasedimentary and volcanogenic rocks from the Bohemian Massif (Czech Republic) with metallogenetic implications

Abstract

The isotopic composition of lead in normal and metal-rich black shales and associated volcanogenic rocks has been studied in the Barrandian Upper Proterozoic of the Bohemian Massif, which is a part of the Central European Variscan Belt. These rocks played an important role in the metallogeny of the Bohemian Massif. The Late Proterozoic age of metamorphism is evidenced by the slope of the secondary isochron of the uranogenic lead corresponding to the age of 635 ± 45 Ma, and by the model age of 540–590 Ma of thorogenic lead in the samples with low Th/U ratios. In most of the samples, the leached Pb is less radiogenic than the residual Pb. No correlation was established between the presence of more radiogenic samples and the distance towards Variscan granitoids. The samples are mostly associated with the Upper Proterozoic volcano-sedimentary sequences far from Variscan intrusions and only some occur in the exocontacts of Variscan plutons. This suggest that most of the uranium was introduced into rocks during Cadomian metamorphism rather than Variscan tectonomagmatic phases. However, a possible contribution of radiogenic lead in connection with the latter events as well as some heterogeneity in U/Pb and Th/Pb ratios from the time between sedimentation and metamorphism cannot be excluded. The comparatively low 208 Pb 204 Pb ratios indicate that thorium was not mobilized by the metamorphic fluids, and testifies that Th is less mobile under metamorphic conditions. The 207 Pb 204 Pb values (15.57–15.63) of less radiogenic lead in the rocks are close to those of galenas from different Variscan polymetallic mineralizations in the Bohemian Massif, which suggests a common source from a well-mixed reservoir as observed in ore leads in mature island arcs or at active boundaries between continental and oceanic plates.