Abstract
The NS trending sill-like Gonamsan gabbroic intrusion in the central region of the Korean Peninsula intruded Neoproterozoic metasedimentary rocks composed of schists intercalated with gneisses, marbles, and quartzites. The intrusion has an east zone (EZ) composed of diorite–monzodiorite, diorite–gabbro, and oxide gabbro–monzodiorite, and a west zone (WZ) composed of quartz monzodiorite. These silicate rocks have similar primitive-mantle normalized trace elements and chondrite-normalized REE patterns, suggesting that they are co-magmatic. Plagioclase anorthite (An) contents (1.9–54.0), pyroxene MgO/ (MgO + FeO) (Mg#) (35.4–73.7), and olivine forsterite (Fo) values (14.5–20.9) show a wide range through the intrusive silicate suite, but each unit has narrow ranges, and reverse fractionation is absent. The Fe–Ti (–V) oxide mineralization only occurs in the EZ and is characterized by variations in morphology, size, mineral assemblages, and chemical compositions of the Fe–Ti oxide minerals. Stratiform Fe–Ti (–V) oxide mineralization interlayered with diorite–monzodiorite is restricted to metasedimentary host rocks in the central region of Gonamsan and comprises magnetite and ilmenite with lesser amount of olivine, tschermakite, apatite, spinel, pyroxene, and plagioclase. Lenticular mineralization occurs along the western margin of the diorite–gabbro and consists of magnetite and ilmenite with minor hornblende, apatite, chlorite, and spinel. Oxide gabbro interlayered with relatively thin monzodiorite bands forms cyclical units irregularly distributed throughout the EZ and consists of olivine and pyroxene with minor interstitial magnetite, ilmenite, and apatite. Magnetite is the principal Fe–Ti oxide mineral, and varies in chemical composition and microtextures in different ore zones. Overall, the compositions of magnetite from all mineralized zones fall in the compositional fields for magmatic Fe–Ti, V deposits defined by Dupuis and Beaudoin (2011). Magnetite from the stratiform Fe–Ti oxide mineralization is compositionally consistent with those from high-temperature silicate melts and displays complex ilmenite and spinel exsolution textures that resulted from re-equilibration of ulvöspinel–magnetite solid solution during cooling. In contrast, magnetite from the lenticular ore and oxide gabbro are compositionally consistent with being derived from evolved low-temperature melt. Based on the whole-rock geochemistry, mineral composition, and mineralization distribution, the Fe–Ti (–V) oxide mineralization in the Gonamsan intrusion seems to be related to Fe–Ti-rich melt formed from magmatic fractionation and migration along a relatively permeable zone, rather than other magmatic and post-magmatic processes.
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