The Svecofennian gabbro-hosted Koivusaarenneva magmatic ilmenite deposit, Kälviä, Finland

Abstract

The Svecofennian Koivusaarenneva gabbro at Kalvia, Finland, is a small intrusion emplaced at 1881 Ma into tonalitic bedrock near the boundary of the Mid Finland granitoid complex. The intrusion is mainly composed of even-grained metamorphosed gabbro, which can be subdivided into three zones. The lower, middle and upper zones are recognized on the basis of petrography (ilmenomagnetite, ilmenite and apatite, respectively) and geochemistry (uniform TiO2/Fe2O3TOT, variable TiO2/Fe2O3TOT and high REE and phosphorus, respectively). The Fe–Ti oxide-rich layers are common in lower and middle zones. The TiO2/Fe2O3TOT ratio is about 0.2 and constant for all rock types in the lower zone. The middle zone is characterized by a broad variation of the major elements as compared with the two other zones. The TiO2/Fe2O3TOT ratio is between 0.25 and 0.50. The middle zone hosts an ilmenite deposit consisting of layers of ilmenite ore, which grades between 8 and 48 wt% ilmenite. In the 2-km-long mineralized zone, the inferred resources amount to 44 Mt with 15% ilmenite and 6% vanadium-rich magnetite (0.7 wt% V). Ilmenite is free of haematite lamella and has a low MgO content (0.1–0.4%). The upper zone consists of phosphorus-rich gabbro and leucogabbro. The lower and upper zones are interpreted as resulting from crystal–liquid fractionation of two different batches of magma. The middle zone and ilmenite ore are interpreted as resulting from separate multiple batches of magma, which are products of an open-system magmatic process. The proposed model for ore genesis invokes stepwise or continuous flow of magma through the sill-like middle zone and the extraction of suspended Fe–Ti oxides that sink to form the ilmenite ore. Replacement of titanium- and iron-depleted magma with fresh parent magma resulted in a large mass of ilmenite ore in a relatively small volume intrusion. The Koivusaarenneva gabbro-hosted ilmenite deposit is a new type of Fe–Ti deposit and indicates that small mafic intrusions elsewhere have the potential to host significant titanium resources.