JultikaUniversity of Oulu repository

Abstract

Kylylahti is a Cu-Zn-Ni-(Co)-Au deposit found in year 1984 by Outokumpu Company and exploited by Altona Mining and Boliden in years 2012–2020. It locates in Polvijärvi, Eastern Finland, 20 kilometres northeast of the famous Outokumpu deposit with which it shares its main geological features. One of the main rock types in Kylylahti host rock assemblage is tremolite skarns, but also other types of rocks traditionally reported as skarns are present, such as actinolite skarns and cummingtonite skarns. In older mapping and core logging, actinolite skarn was the category in which all dark, amphibole bearing host/wall rocks were classified. Boliden felt it was necessary to improve the understanding and classification of the skarn types, especially in terms how the actinolite skarns differ from tremolite skarns. Drill cores of 10 holes in one profile that crosscuts the whole Kylylahti deposit were chosen to be completely re-logged. All darker, amphibole bearing rocks, customarily logged as skarns, were classified into four groups based on their mineralogical and textural features, and with the help of pXRF, their geochemical signatures. Three to five representative samples from each group were studied with polarizing microscope, FESEM, EDS and analysed for their whole rock geochemistry. Three of the groups comprised of hydrothermally altered refractory peridotites of the ophiolitic Kylylahti massif. One group was from carbonate±amphibole interlayers in the enveloping metasedimentary Upper Kaleva rocks. As expected, the 4 groups turned out to be both geochemically and mineralogically distinct from each other. Remarkably, in terms of the past naming convention, only one sample contained one crystal of actual actinolite. Dark amphiboles were mostly tremolites that were pigmented dark or completely black by fine sulphides, graphite and some unknown matter that could be graphite or sulphides but in so fine size that it was not recognizable in this study. Therefore, actinolite skarn is not a correct name for any of these rocks and should not be used. They are just tremolite skarns or carbonate-containing rocks that have some tremolite as accessory mineral.