The Tapuli deposit is a skarn iron ore located in the Pajala municipality in Northern Sweden. It is situated at the margin of the Karelian craton, at the stratigraphic contact between Karelian and Svecofennian rocks. The ore forms stratabound lenses concordant with the metasedinnentary sequences and dips 45 - 60 degrees towards NW. Footwall rocks are dolomitic marbles, phyllites and graphitic phyllites; the hanging wall comprises phyllites and quartzites. Mafic dykes and sills crosscut the stratigraphic succession. Magnetite is the only ore mineral. The skarn minerals are serpentine, diopside, tremolite and actinolite. The skarn altered rocks show a zonation with serpentine skarn closest to or as part of the ore, thereafter, tremolite-diopside skarn and, finally, actinolite skarn closest to the phyllites and quartzites in the hanging wall. Sulfides occur in minor amounts, but their content generally increases with proximity to the footwall rocks. The dolomitic marble was the precursor of the serpentine and tremolite-diopside skarn. Immobile element data suggests that mafic dykes and sills were the precursors of the actinolite skarn. Possible controls of the sulfide distribution are the presence of sulfide-bearing source rocks (dolomitic marble, graphitic phyllite) and rennobilization of sulfides during the intrusion of mafic dykes.