Magnetite and Spinel Series Minerals from the Magnesian Skarn-type Iron Deposit at the Shinyemi Mine, Korea

Abstract

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% MgFe₂O₄, from 0.0 to 7.3 mole% FeAL₂O₄, and from 0.6 to 3.1 mole% MnFe₂O₄ components. It generally contains exsolu-tion 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, 2.6-27.3 mole% FeFe₂O₄ and small amounts of FeAL₂O₄ and MnFe2O4 components. The late formed magnesioferrite ("magnesioferrite II") has considerable amounts of FeFe₂O₄ and MnFe₂O₄ components.