JultikaUniversity of Oulu repository

Abstract

The Karelian craton is one of the critical minerals exploration target in Europe as it shares similarities with the most mineralized Archean cratons in the world i.e., the Yilgarn and Superior cratons, and a lot of studies have been carried out in this region to understand the mineral systems and potentials. The wide adoption of the Mineral Systems Approach (MSA) has encouraged the application of isotope systems to metallogeny and regional exploration. The knowledge of the geologic history and evolution of geological terrains and their paleo lithospheric architectures may reveal a component of their mineral systems, including possible metal sources, pathways of fluids, and processes responsible for mineralization. Isotope systems have been used to constrain the paleo lithospheric structure in Archean cratons. They have also been used in generating regional-scale isotopic maps to identify metallogenic blocks for decades. More and more isotopic data are being generated from different studies especially for the Sm-Nd isotope system, and more recently, for the Lu-Hf system, and the availability of more data and relevant software has now made it easier to construct isotopic maps from regional to continental scales. Abundant whole rock Sm-Nd data of Archean granitoids have been published for the Karelian craton but not much data has been generated for the Lu-Hf isotope system of zircon from Archean granitiods. Compared to bulk rock Sm-Nd isotope, zircon Lu-Hf may be less affected by later stage metamorphic events. In this study, in-situ U-Pb and Lu-Hf analyses of zircons of Archean granitoid samples from the Western Karelian province have been obtained, and the data is compared with already existing Sm-Nd isotope data and bulk zircon U-Pb age data to ascertain co-relation or disparities and possible reasons, and to investigate the geochronology, Archean tectonics and lithospheric structure of the different terrains/blocks in western Karelia, and to link the occurrence of metallogenic belts and lithospheric structure within the study region. The U-Pb and Lu-Hf analyses were conducted on ~2.7- 2.8 Ga granitoid samples from the Western Karelian province, and in addition to existing Sm-Nd data, spatial distribution of εHf and εNd isotopic maps were generated for the region. When the Hf data was compared with the Nd data, a positive co-relation was noticed in many areas, but some disparities were noticed in some areas like the Suomassalmi block. More Lu-Hf analyses needs to be carried out to generate more data to match the available Nd data for a more robust interpretation, especially the Iisalmi block. Evidence from Lu-Hf data, Sm-Nd data, inherited zircons and isotope maps suggest that there is a rift zone or continental margin around the northern part of the Pudasjärvi and Koillismaa blocks and that the Kuhmo block is mostly composed of relatively juvenile crust. These areas of the Pudasjärvi and Koillismaa blocks show an association with mafic layered intrusions hosting chromium, PGE, vanadium, while the Kuhmo-Suommusalmi greenstone belt is associated with orogenic Au and komatite related Ni.