Correlation between alternation textures and varieties of chromite: Case study in Chongjin Ophiolite, NE Korean Peninsula

Chongjin ophiolite, which is an on-land remnant of oceanic lithosphere formed in the upper Paleozoic era, is located tectonically in the suture zone between Sino-Korean massif and Tumangang orogenic zone. To elucidate the varieties of chromite at study area and their alteration texture and formation mechanism, scanning electron microscopy (SEM) and electron microprobe analysis (EPMA) were carried out. The result shows that chromite grains exhibit three major microtextures of: (1) the primary non-porous Al-chromite was partly or fully altered into porous ferritchromite; (2) the zoned alteration texture or partly altered texture in chromite-bearing serpentinite; (3) the primary Al-chromite was fully altered by porous or non-porous ferrian chromite. The result also suggests that the chromite ore at study area are underwent hydrothermal alteration under low-temperature (retrograde) metamorphic conditions before serpentinization. We established an alternative model of chromite based on these results: First, early formed chromite reacts with olivine to form porous ferrous chromite and Cr-chlorite in presence of SiO ₂ -rich fluid. After that, more oxidizing Fe-bearing fluids interact with porous ferrous chromite in chromite-bearing serpentinite to form zoned or porous and non-porous ferrian chromite. Furthermore, measuring susceptibility (per gram) for the purpose of magnetic separation, its range is between 6.81×10 ^-5 and 350×10 ^-5 SI and it varies according to the type of chromite varieties. Thus, we can understand that in magnetic separation of chromite, the aimed varieties can be acquired from the difference of susceptibility.