RECONSTRUCTING THE PROVENANCE, TECTONIC SETTING AND PALEOWEATHERING OF LOWER PALEOZOIC BLACK SHALES FROM NORTHERN EUROPE

Metalliferous Lower Paleozoic black shales are widespread in Northern Europe. They are commonly highly enriched in uranium, vanadium, and molybdenum. This significant metal enrichment makes the lower Paleozoic black shale a potential future source of raw material for Europe.

However, the source and distribution of the metals in the black shale is still up for debate. Recent studies have shown that the geochemical composition of the black shale is heterogenous. This pronounced geochemical variation could be a consequence of several factors, including variation in primary clastic input, and depositional environmental conditions. In this study, we reconstruct the provenance, tectonic setting and paleoweathering of the black shales from Sweden, Estonia, and Russia.

Major and trace elements concentration from 10 drill cores from Estonia and 34 drill cores from western Russia, and outcrops from eight locality in Sweden were studied using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) and X-ray fluorescency (XRF) respectively.

The provenance signatures (Th/Sc versus Zr/Sc, Al₂O₃ versus TiO₂, Zr versus TiO₂ plots, and Zr/Sc ratio) of the black shales suggest that they were derived from rocks of intermediate to felsic composition and from recycled sediments. The likely provenance region was the Paleoproterozoic igneous and metamorphic basement of southern central and southern Finland, which consists predominantly of felsic to intermediate metamorphic (acidic to intermediate gneisses, felsic volcanics, microcline granites and migmatites) and igneous rocks (small granitic intrusions and large rapakivi granite intrusions), and reworked older Ediacaran and Lower Cambrian sediments; however, the proportion of clastic input from these sources is not uniform in the three regions studied. The discrimination of the tectonic settings of source materials of the black shale using the SiO₂ versus K₂O/Na₂O plot and a new discriminant method (APMdisc) favors a passive margin setting. The Chemical Index of Weathering (CIW) indicates that the clastic material in the black shale of the studied regions has experienced an intense degree of chemical weathering. Weathering indices (Chemical Index of Alteration CIA and CIW) also show that the black shale has experienced significant secondary potassium enrichment.