The Ryonchon podiform chromitite from the Chongjin ophiolite, northern Korea Peninsula

Abstract

Ryonchon chromitite is located structurally in the Chongjin area, the northern part of the Korean Peninsula, which crops out at Chongjin terrane, the eastern end of the Jilin-Yanbian-Hambuk Fold Belt. We made on-site geological survey of this deposit and conducted the analyses on the investigated chromitite by means of microscopes, single crystal X-ray diffraction (SXRD) and electron probe micro-analysis (EPMA) to elucidate the phase composition, the chemical composition and the texture and structure of the chromitite. The chromian spinels from the study area are separated into four groups according to chemical composition of spinel as ordinary spinel-group minerals are. The studied chromites have different compositions according to their occurrences, which is not owing to the precipitation of different pararental melts but the subsolidus re-equilibration during the hydrous alternation.The massive/semi-massive unaltered chromite in chromitite from dunite have consistently low Cr (Cr# < 0.6) and low Al (Al2O3 23–26 wt%), whereas Al2O3melt and (FeO/MgO)melt values range 14.58–15.43, 0.40–0.52, respectively, which have been contrasted with that of the typical podiform chromitite. By comparing present analyses with previous works, the parental melt of the Ryonchon chromitite from the Chongjin ophiolite is similar in chemical compostion to Island arc tholeiitic melts rather than boninitic melt, whose tectonic setting corresponds to a forearc environment of a suprasubduction zone. In addition, both cumulus texture and euhedral crystal shapes of the chromite grains in massive/semimassive chromitite from dunite support their magmatic origin. We found that the chromitite from this area have undergone the hydrothermal alteration as appearance of fluidal ferritchromit, Cr# of ferritchromites higher than 0.8, the high MgO content of the ferritchromit, occurrence of typical hydrothermal minerals such as heazlewoodite and the differences in the products formed during several metamorphic processes.