Textural and geochemical constraints on the formation of the magmatic Fe–Ti oxide deposit at Boreum Island, South Korea

Abstract

This paper investigates the complex magmatic processes of the Boreum magmatic Fe–Ti oxide deposit through comprehensive field observation, petrography, and geochemistry. Located in the marginal zone of the Gyeonggi Massif, central-western Korean Peninsula, this deposit represents one of South Korea's most important Fe–Ti oxide provinces, providing an ideal natural laboratory for understanding ore formation mechanisms. The sill-like Boreum ultramafic rocks (30 m wide, 300 m long) intrude Paleoproterozoic Boreumdo schist and comprise three distinct phases: actinolite-, diopside-, and olivine-dominant. Cross-cut relationships indicate formation through multiple melt injections rather than single-melt fractionation processes. Iron–Ti oxide mineralization concentrates unevenly in the highly fractured olivine-dominant phase as sill-dyke networks. Magnetite and ilmenite are dominant, with subordinate pleonaste and perovskite (restricted to diopside-dominant phase). Magnetite shows complex exsolution textures indicating subsolidus re-equilibration during slow cooling. It is significantly enriched in compatible elements (Cr: 25,622–140,526 ppm; Ti: 18,160–87,919 ppm; V: 2421–18,433 ppm), plotting within magmatic Fe–Ti, V deposit compositional fields. Mineral chemistry reveals high-temperature magnetite crystallization (T_Mg-Mag = 885–607 °C, T_Ilm-Mag = 754–594 °C) under low oxygen fugacity conditions (ΔFMQ −2.60 to +0.69). Magnetite exhibits restricted δ⁵⁶Fe (+0.12 to +0.40 ‰) and δ¹⁸O (−0.75 to +2.89 ‰) values, consistent with magmatic origin. Fe–Ti oxide mineralization resulted from high-temperature magmatic melt migration into already solidified ultramafic rocks, not in situ crystallization or crystal settling. These findings provide new insights into magmatic processes forming Fe–Ti oxide deposits in mafic–ultramafic intrusions.