Combined igneous and hydrothermal source for the Kiruna-type Bafq magnetite-apatite deposit in Central Iran; trace element and oxygen isotope studies of magnetite

Abstract

The main deposits of the Bafq district (Choghart, Se Chahun and Chadormalou deposits) are magnetite-apatite ore bodies with Early Cambrian age, hosted by Early Cambrian volcano-sedimentary rocks, showing lava flow structure with miarolitic cavities and occasional brecciation. The texture and geochemistry (trace elements and O-isotope) of the studied magnetite crystals show three types of magnetite with different proportions in different parts of the district and the deposits. These are primary ignious magnetite and high-temperature hydrothermal magnetite in the massive parts of the deposits and foam-like magnetite in the veins or in the fractures of the massive part. The lack of zonation, high Ti/Fe ratios and δ18O values (2‰<) indicate that massive magnetites formed at high temperature condition, while foam-like magnetite recrystallized at lower temperature. Textures and Co, V, Ti, Al, and Pb contents are used to distinguish ignious magnetites from the high-temperature hydrothermal magnetites. Foam-like magnetites contain lower REE and has lower δ18O values (2‰>) than the massive magnetites.The magmatic origin for the main part of the ores in the Bafq is more plausible. Contamination of the primary magmas of the Bafq magmatic rocks with crustal phosphorus and evaporate units was the major agent for triggering iron-rich melt immiscibility from the original magma. Concentration of volatiles in the iron-rich melt involving in crystallization of the primary magnetite and nucleation of the aqueous fluid bubbles on magnetite surfaces led to ascending the buoyant fluid bubble-magnetite aggregates. This caused continuation of the magmatic magnetite microlites growth from the iron-rich magmatic-hydrothermal fluid and formed high-temperature hydrothermal magnetite. Formation of hypersaline fluid due to crystallization of iron-rich melt at shallow depth led to crystallization of titanite (and Th-U-REE-bearing minerals) and sodic-calcic alteration related to magnetite mineralization. Microscopic evidence (replacement of magnetite by hematite and filling of fractures of thorite by magnetite) show that degassing continued at shallow depth, which led to re-crystallization of the primary magnetites and formation of the foam-like magnetites.