Interpretation of the lead isotopic composition from sulfide mineralizations in the Proterozoic Sjangeli area, northern Sweden.

Abstract

Cu-, Fe- and Zn-mineralizations occur in the Lower Proterozoic supracrustal rocks of the Sjangeli area, a tectonic basement window in the Caledonides of northern Sweden and Norway. The supracrustal belt consists of basic and ultrabasic metvolcanites which are intercalated with quartz-mica schists and banded carbonate-silicate rocks. The metavolcanite consists of tuffs, lava flows, and pillow lavas. Cu-mineralizations, which form three different types, occur within basic metatuffs. Two types represent epigenetic remobilizations of Cu-mineralizations, while the third type is characterized by stratiform bornite-chalcocite-quartz-magnetite layers of possible syngenetic origin. The Fe- and Zn-mineralizations occur in quartz-mica schists and form stratiform lenses.At about 430 My a disturbance of the lead isotope system occurred, which changed the 238U\/204Pb, 232Th\/204Pb, and 232Th\/238U ratios. Radiogenic lead was added to the mineralizations during the 430 My event, U was mobile, which is illustrated by the significantly increased 238U\/204Pb ratios of the massive pyrrhotite mineralizations.Lead lines from the recalculated lead composition (430 My) indicate mixing model ages of 1546 My to 212 My. Since the mixed lead components were non-cogenetic, the model age variation of the mixing lines indicates differences of the mixing lines indicates differences of the mixing component, rather than real age differences. However, a lead source of Middle Proterozoic age can be deduced.The Middle Proterozoic disturbance resulted in the formation of vein mineralizations which have predominantly high-uranogenic, low-thorogenic lead. The in-situ lead growth indicates high 238U\/204Pb ratios and implies that U had also been mobile during the Proterozoic metamorphism.Model ages for the least radiogenic lead (Kopparåsen) yield a minimum age 2050 My.