Magnetite and Spinel Series Minerals from the Magnesian Skarn-type Iron Deposit at the Shinyemi Mine, Korea
Spinel, magnesioferrite and magnetite are found in the Shinyemi magnesian skarn-type iron deposit. There are close relationships between the mode of occurrence and the chemical composition of these minerals . Spinel formed during the metamorphic stage has a chemical composition close to MgAl2O4. Metasomatic spinel, nearly simultaneously formed with "magnetite I" during the progressive skarn formation, can be expressed in the spinel-hercynite binary system with ranging from 24.1 to 34.5 mole% FeAl2O4 component. "Magnetite I", commonly associated with olivine, ranges in composition from 62.4 to 98.3 mole% FeFe2O4, from 0.0 to 30.3 mole% MgFe2 O4, from 0.0 to 7.3 mole% FeAl2O4, and from 0.6 to 3.1 mole% MnFe2O4 components. It generally contains exsolution bodies of spinel with the composition of 68.7-83.1 mole% MgAl2O4, 6.2-21.3 mole% FeAl2O4 and 5.5-9.7 mole% MgFe2O4 components. "Magnetite I" is frequently replaced by the late formed magnetite ("magnetite II") associated with Mg-rich chlorite, serpentine and carbonate minerals during the retrogressive process. Compositions of "magnetite II" are represented in the MgFe2O4-FeFe2O4-MnFe2O4 ternary system. It extends to about 28.2 mole% MnFe2O4. During the almost same stage to that of "magnetite I", "magnesioferrite I" was crystallized in dolostone near the magnetite ore bodies, it contains 59.1-70.7 mole% MgFe2O4, 5.7-11.7 mole% MgAl2O4, 2.627.3 mole% FeFe2O4 and small amounts of FeAl2O4 and MnFe2O4 components. The late formed magnesioferrite ("magnesioferrite II") has considerable amounts of FeFe2O4 and MnFe2O4 components. Introduction Spinels are common accessory phases in igneous and metamorphic rocks and have been used as an indicator of physical and chemical conditions such as temperature and oxygen fugacity. On the other hand, Mg-rich spinel group minerals frequently occur in magnesian skarn-type deposits (SOKOLOV and GRIGOR'EV, 1977; ZHARIKOV, 1970). There are, however, few detailed mineralogical studies on the spinel group minerals from these deposits. In the course of the mineralogical investigation on the magnetite ores from the Shinyemi deposit, the authors found the close relationship between the occurrence and the chemical compositions of spinel and magnetite series minerals of the group. The present paper describes the mode of occurrence, optical properties, X-ray diffraction data and chemical compositions of these minerals, and discusses the phase relation for some spinel group minerals at lower temperatures of 400-650•Ž.