院洞 Fe-Pb-Zn 스카른광상의 物理化學的 特徵

Physicochemical study of the Wondong Fe-Pb-Zn skarn deposit, Korea

The Wondong Fe-Pb-Zn deposit is located in endo and exoskarns formed along the contact between the Makkol limestone interbedding pelitic limestone of Ordovician age and quartz porphyry of Cretaceous age. At the Wondong mine, the endoskarn shows a discontinuous zonal arrangement from quartz porphyry to pelitic limestone as follows: unaltered quartz porphyry, weakly altered quartz porphyry zone, intensively altered pinkish quartz porphyry zone, garnet zone, and greyish white and fine-grained clinopyroxene zone developed at pelitic limestone side. In terms of chemical mass balance, intensively altered pinkish quartz porphyry relative to unaltered quartz porphyry shows substantial enrichments in K₂O, Na₂O, and HREE and depletions in MgO, CaO, total Fe₂O₃, and LREE. On the other hand, garnet zone of endoskarn is enriched in CaO, MnO, total Fe₂O₃, MgO and depleted in K₂O, Na₂O, Al₂O₃ seems to be determining inert component. Thus the behavior of elements indicates that the mobility of elements depends on the equilibration of hydrothermal fluid and minerals and affects on enrichments by fractionation from and depletions by partition to hydrothermal fluid, respectively. Traversing toward pelitic limestone from a central zone of exoskarn, the exoskarn also shows a zonal arrangement as follows: garnet zone, clinopyroxene zone, and decolored pelitic limestone. The arrangement of mineral assemblages in skarns of the Wondong mine is the result of an increase in CaO and K₂O toward the pelitic limestone. Skarn and ore minerals were formed in the following sequence: early skarn, late skarn and magnetite, pyrite, sphalerite, galena, and molybdenite. On the basis of stabilities of mineral assemblages, physicochemical conditions of the late skarn and magnetite mineralization are estimated to be 350℃≤T≤400℃ at 1 Kb, -23≤log fO₂≤-18, and 0.005≤XCO₂≤0.01, while those of the early skarn to be 420℃≤T≤550℃ at 1 Kb.