Chemical variation of magnetite in the Tapuli skarn iron ore deposit, northern Sweden

• Main Author: Hasa, J. (Jenni)
• Format: Master Thesis
• Language: English
• Published: University of Oulu 2015
• Subjects: Geotieteiden koulutusohjelma; Lainio; Pajala; Savukoski; Wall Rocks; karelian; Northern Sweden
• Online Access: http://jultika.oulu.fi/Record/nbnfioulu-201605131727

Abstract. The Tapuli skarn iron ore deposit is located approximately 25 km north of the village of Pajala in northern Sweden. It occurs within the Pajala shear zone and is hosted by Svecofennian and Karelian supracrustal rocks. Magnetite is the only economic ore mineral. The ore body consists of several lenses, which dip 45–60 degrees towards NE. The hanging wall rocks consist of Lainio Group quartzites, phyllites and mica schists, whereas the skarn interval itself belongs to the Savukoski Group metavolcanic rocks. The footwall is composed of dolomites, graphitic schists and phyllites of the Savukoski Group. Ca-Mg-rich skarns (clinopyroxene, actinolite, tremolite) are mainly found in the hanging wall and in short intervals within the ore, while Mg-rich-skarns (serpentine) are the main host of the ore. Tapuli is a skarn iron deposit with Ca-poor and Mg-rich magnetite. The aim of this study was to investigate the amount of Mg and other elements in magnetite and the effects of the surrounding skarn types on the magnetite composition. Dings Davis Tube tests were employed to divide magnetite ore into magnetic and non-magnetic fractions and XRF to analyse both fractions. MLA (Mineral Liberation Analysis) was used to define the modal mineralogy of the separated fractions and the particle size and liberation of magnetite, and the composition of the magnetite was determined using electron probe microscope (EPMA). The surrounding skarn material has a significant impact on the composition of the magnetite in skarn rocks. Different skarn types at Tapuli can be distinguished based on the MgO content of the magnetite. Based on the results obtained from the Dings Davis Tube tests of the magnetic fraction, it can be concluded that when the Fe content falls below the average level, SiO₂ and MgO are in turn getting higher. The MgO and SiO₂ levels show a good correlation with each other. This can be explained by Mg-rich silicates, usually serpentine included in the magnetic fraction.