Skarn units in the Leveäniemi and Gruvberget mines An overview of the skarns in Svappavaara, northern Sweden

Abstract

Skarns have been investigated a lot worldwide and are used as a common term for calc-silicate-rich rock with associated minerals, usually connected to some sort of mineralization. In Sweden, the term usually is used a bit more freely and as a term for waste rock or side rock around mineralization. In this thesis, the definition of skarns by Varela and Mannelqvist (2023) and Meinert (1992) will be used. LKAB is one of Sweden's biggest mining companies, owning three big mines in northern Sweden, mining iron from magnetite and skarns of different kinds can be found in these mines. However, even if LKAB knows about the different skarn types, there has been very little to no deeper investigation of these rock types in Gruvberget and Leveäniemi in the Svappavaara mining area. With the use of LA-ICP-MS and SEM, information about the Sr-isotopes in the minerals found in the skarn samples and trace elements in magnetite located in the different skarns has been gathered. With this information, together with logging data from LKAB, this thesis has tried to find connections and relations between the different skarns and if they also have any connection with the main mineralization. 5 different skarn types are being investigated in this thesis. The skarns are Actinolite skarn (Ask), Limestone skarn (Lsk), Albite altered skarn (Alb), Skarn (SKA) and Anthophyllite skarn (Ant). The results from the discrimination diagram from Ovalle, J. T. (2018) shows that the Svappavaara main mineralization (Sample 19.2 Mag) was formed during higher temperatures than the Kiruna field and that the magnetite samples were all (except sample 13 Ask) formed during moderate temperatures of 300 – 500 °C. However, discrimination diagrams should be seen as a pointer and not solid facts. The samples nearest to the main mineralization has 87Sr/86Sr ratio values similar to 87Sr/86Sr ratio values found when magnetite is forming. Samples that are further away has an increase in 87Sr/86Sr ratio values. The theory is that the Sr in the surrounding rocks of the deposit has been leeched and transported further away, increasingly precipitating as the temperatures, pH and chemistry of the fluid changes. If this trend and theory is correct, then finding skarns with a low 87Sr/86Sr ration could mean that an iron mineralization are close by, where the ratio would determine the distance to the ore. A new skarn type may have been discovered. The Diopside skarn (Dsk) has a clearer matrix which represents a carbonate-rich parent rock, compared to the Ask, where the protolith usually is completely altered beyond recognition. Also, the Dsk usually has another appearance than the Ask, lighter in colour and more carbonates visible on the core. Sample 1, 2, 3, 9, 10, 12, 13, 15, 18.1, 18.2, are, with the data collected in this project, skarns. Sample 19.1 Ant could be one, but there is too little information to be able to decide that in this thesis.