Oxygen isotope composition of magnetite in iron ores of the Kiruna type in Chile and Sweden

摘要

Magnetite-apatite iron ores of the Kiruna type, unaffected by deformation, have structures and textures similar to those of igneous rocks. The best examples are the El Laco deposits in northern Chile which resemble lava flows, pyroclastic deposits and dikes. El Laco magnetites have delta O-18 values between 2.3 and 4.2 parts per thousand (V-SMOW). Magnetite from ore with a magmatic texture has a mean of 3.7 parts per thousand, and the mean for magnetite intergrown with pyroxene in veins is 2.4 parts per thousand. Oxygen isotope data given here, fluid inclusion results and geological evidence indicate that ore formation took place in a cooling magmatic system. Major orebodies resembling lava flows and near-vent pyroclastic deposits crystallized from magma at ca. 1000 degrees C. Fluids from cooling magma deposited magnetite and pyroxene (+/-apatite) at ca. 800 degrees C in fissures and open spaces, now present as veins cutting major orebodies. There is little evidence for significant magnetite precipitation during hydrothermal conditions. A large province of magnetite-apatite iron ore in central Chile (the Cretaceous iron belt) and the Kiruna district in northern Sweden also contain primary ore of magmatic appearance. Major deposits in the Chilean iron belt and Kiruna contain magmatic-textured magnetites with the following delta O-18 means: Algarrobo = 2.2 parts per thousand, Romeral = 1.2 parts per thousand, Cerro Iman = 1.6 parts per thousand, and Kiirunavaara = 1.5 parts per thousand. We consider all oxygen isotope data for unoxidized, magmatic-textured magnetite as representative of the Fe-rich magmas. Magnetites affected by hydrothermal alteration, recrystallization and subaerial oxidation have modified isotope signatures.