Zn-Pb-Cu sulfide-bearing sandstone glacial erratics near Raahe on the Western Coast of Finland and a discussion on their origin

In the 1970’s and 1980’s, a series of sandstone eratics were collected from the Raahe area and analyzed using XRF. The results suggest that these samples are highly enriched in Cu, Zn, and Pb. The aim of this study is to make observations on these samples in order to place the samples in a regional geological context. The petrographic analysis in this study shows galena, sphalerite, marcasite, chalchopyrite, and pyrite as the ore-bearing minerals located as pore infilling along with calcite and quartz. No metamorphism is evident in samples in either grain deformation or mineralogic changes which contrasts with the amphibolite facies metamorphism in the underlying Svecofennian bedrock. The observations from grain shapes favor a shallow marine depositional environment where the detrital grains likely get those well-rounded shapes. Fine-scale laminations in sediments can further support a marine conditions. Electron microprobe analyses (EPMA) conducted on samples provided the quantitative data showing the chemistry of the ore minerals. Data collected show low Fe in sphalerite and low concentrations of harmful trace elements, such as Cd and As. Indium was also measured for its potential economic interests however contents proved low. The lead isotope data provided by Pyhäsalmi Mine Oy show very radiogenic Pb isotope compositions, with 206 Pb/ 204 Pb ratios falling in the range of 20.55−21.06 and 207 Pb/ 204 Pb in the range of 15.90−15.94. These compositions are similar those measured for the Laisvall Pb-Zn deposit in the Swedish Caledonides, being consistent with a similar Ordovician age of ore formation. In contrast, sulfur isotope analyses of pyrite yielded mostly negative δ 34 S values from -15.6‰to -7.6‰and only one positive value (+6.7‰), which are distinctly different from the generally heavy sulfur isotope compositions (ave. +24‰) reported for the Laisvall deposit or other Laisvall-type mineralization in Sweden. The conclusion based on the mineral assemblages is that the samples are Mississippi Valley Type (MVT) style mineralized. Although the samples show MVT type mineralization they are not easily fit into a regional perspective. Local mineralizations near Raahe in the Pyhäsalmi area are well documented however the closest MVT type mineralizations are located 400 km west of the discovery site in Laisvall. The lack of previously discovered MVT type mineralizations causes debate on the understanding of the late regional hydrothermal activity. The samples may be from the Hailuoto Formation under the Bothnia Bay or younger Paleozoic sedimentary rocks correlative with those occurring further south in the Bothnian Sea