Source of metals and age constraints for epigenetic gold deposits in the Skellefte and Pohjanmaa districts, central part of the Fennoscandian Shield

Abstract

The lead isotopic compositions of galena in Early Proterozoic gold deposits have been determined for three districts in northern Sweden and central Finland. The deposits are hosted by a variety of ≈1870–1890 Ma Svecofennian host rocks including the volcanosedimentary succession within the Skellefte District island arc in Sweden as well as I-type tonalites at Jorn (Sweden) and Pohjanmaa (Finland). The deposits are epigenetic in relation to these Svecofennian rocks and are part of a goldbearing metallogenetic belt, which can be followed for 600 km parallel to the southwestern margin of the Archaean Domain. In spite of these epigenetic relationships, the lead isotopic data indicate that the deposits are not dramatically younger than the ≈1870–1890 Ma Svecofennian host rocks (probably not exceeding 10–20 million years). Two principal lead sources were activated when the gold deposits were formed. The most significant source is represented by the I-type tonalites, which constitute a relatively primitive (μ = 9.3) and widely distributed source in the entire metallogenic belt. In addition, the volcanic components in the westernmost part of the Skellefte District constitute an extremely primitive (μ <9.0) source, which only locally was an important contributor to the epigenetic deposits in this metallogenetic belt. The significantly different lead isotopic composition estimated for these sources indicates that the volcanic rocks in the western part of the Skellefte District were not comagmatic with the I-type tonalites recognized at Jorn and central Finland.