The Kemi stratiform chromitite deposit, northern Finland

Abstract

The Kemi chromite deposit is hosted by an early Proterozoic (2.44 Ga) layered intrusion. The lower part of the intrusion is composed of peridotitic and pyroxenitic cumulates and chromitite layers. Olivine and chromite have not been observed in the upper part, where plagioclase is the predominant cumulus phase occurring either alone or together with pyroxenes. The intrusion was metamorphosed during the Svecokarelidic orogeny. Mafic minerals of the lower and upper parts of the complex are completely altered to chlorite, serpentine, talc, amphiboles, and carbonates, whereas the middle part is well preserved. The chromite grains, too, suffered from alteration, but the cores of the grains still exhibit primary compositions.A continuous chromitite layer can be traced for the entire length of 15 km of the intrusion. It varies in thickness from a few millimeters to as much as 90 m in the central part of the intrusion. The mineable thicker part of the main chromitite layer is about 4.5 km long. The main chromitite layer is overlain by a sequence of thin chromite-rich layers which continues upward to the stratigraphic level of 500 m above the basal contact of the intrusion. The upper part of the main chromitite is layered, but the lower part is more massive and brecciated.The ore reserves for open-pit mining are ca. 40 million metric tons with an average grade of 26.6 percent Cr 2 O 3 and a Cr/Fe ratio of 1.53. The whole area has additional reserves of about 110 million metric tons of ore. The ore zone is intensely fractured. The host rocks have undergone alteration, which has degraded the quality of the ore.The present surface section of the Kemi intrusion is interpreted as a cross section typical of a funnel-shaped intrusion and plunging about 70 degrees to the northwest. The cumulate sequence, especially the main chromitite layer, is at its thickest in the middle of the intrusion, and the magmatic conduit which fed the intrusion is also thought to have been located just below this thickening.It is suggested that the chromitite was deposited when the input of magma into the Kemi intrusion was contaminated by salic material from the underlying basement complex. This magma mixed with the fresh input of primitive magma resulting in chromite saturation. The chromite crystals formed during mixing in a plume according to the model of Huppert et al. (1986) and accumulated preferentially around the magmatic conduit. Evidence of contamination is found in chromite grains in the form of small inclusions, rich in alkalies, which are thought to represent trapped droplets of the contaminant salic melt.