慶南 東南部地域 鐵鑛床의 成因에 關한 硏究

Genesis of iron ore deposits in the south-eastern part of Gyeongnam Province, Korea

Many hydrothermal skarn-type iron ore deposits including Mulgeum, Yangseong, Maeri and Kimhae mines are distributed in the south-eastern Gyeongnam Province, Korea. The deposits are magnetite veins which occurred in propylitized andesitic rock near the contact with late Cretaceous Masanite. Symmetrical zoned skarns are commonly developed around the magnetite veins. The order of the skarn zones from the vein is garnet-quartz skarn, epidote skarn, and epidote-orthoclase skarn. The garnets include isotropic or anisotropic andradite(Ad100~70), and the epidotes are composed of pistacite(Ps21~31) Fe contents of the epidotes generally increase toward the magnetite veins. Epidotes and garnets often show compositional variations from grain to grain, that is, their Fe and Al contents vary inversely. This suggests that the variations depend mainly upon fo₂ during the skarnization. Oxygen and carbon isotope analyses of minerals from andesitic rock, micrographic granite, major skarn zones and post-mineralization zones were conducted to provide the information on the formation temperature, the origin and the evolution of the hydrothermal solution forming the iron ore deposits. Becoming more distant from the ore vein, temperatures of skarn zones represent the decreasing tendency, but most δO¹⁸ and δO¹⁸H₂O values of skarn minerals represent no variation trend, and also the values are relatively low. Judging from all the isotopic data from the ore deposits, the major source of hydrothemal solution altering the skarn zones and precipitating the ore bodies was magmatic water derived from the more deeply seated micrographic granite. This high temperature hydrothermal solution rising through the fissures of propylitized andesitic rock was mixed with some meteoric water, and the extensive isotopic exchange occurred with the propylitized andesitic rock. During this process, the temperature and δO¹⁸H₂O value of hydrothermal solution were lowered gradually. At the stage of iron ore precipitation, because after all the alteration was already finished, the oxygen isotopic exchange with the wall rock was nearly not taken. The relatively high δO¹⁸ and δ O¹⁸H₂O, and relatively low δC¹³ values of calcites of post mineralization stage, are the results of leaching of the high δO¹⁸ chert xenolith in the andesitic rock and low δC¹³ andesitic rock.