OCCURRENCES AND GENESIS OF ILMENITE DEPOSITS IN ANORTHOSITE COMPLEX IN HADONG AREA, KOREA

Ilmenite deposits are developed along the zonal distribution of Precambrian anorthosite complex in the Hadong-Sancheong area in the southern part of Korea. The massif-type anorthosite complex in the Jirisan gneiss complex comprise elongate body (about 2~3 km x 25 km) in N-S direction. They show gradual zonation of massive, intercumulated and foliated types from margin to inside, which are classified into anorthosite (massive) and leuconorite (intercumulated and foliated) based on the mineral compositions.

Jikjeon-ri orebodies whose outcrops show various extension between 1.9 and 23 m in length is located in southern part of ilmenite deposits. They are related by various intrusive rocks including diorite. Field work and drilling exploration reveal that Ti-rich main orebodies are developed in NW-SE direction with extension of minor vein and veinlets in N-S direction, showing dipping westward below the ground. Ore minerals mainly consist of ilmenite, amphibole, and plagioclase. Accessary minerals are quartz, apatite, chlorite, magnetite, and rutile. Fine grained amphibole is filled in the crack of plagioclase (labradorite) megacrysts. Amphiboles as main constituent of matrix are ferrohornblende (<1mm) or ferropagasite (>1mm) in composition. Ilmenite closely occurs with amphiboles and apatite, filling interstitial spaces of amphiboles and apatite. The chemical compositions of orebodies show SiO₂ 38.54, Al₂O₃ 11.48, Fe₂O₃ 24.44, CaO 10.88, TiO₂ 6.61, MgO 3.21, P₂O₅ 2.72, Na₂O 1.27, and K₂O 0.47 wt%. The chemistry of this type of ores like oxides-bearing leuconorite indicates low in SiO₂ and Al₂O₃, wheres high in Fe₂O₃, MgO, TiO₂, and P₂O₅ compared to anorthosite, implying formation from more differentiated magma than primitive magma for anorthosite. The variations of major elements shows increasing of Fe₂O₃, TiO₂, P₂O₅ contents and decreasing of Al₂O₃ as increasing SiO₂ content. This indicates similar trends with the differentiated rock from ferrodioritic magma. Anorthosite first formed by fractional crystallization of plagioclase from primitive basaltic magma and leuconorite formed later during further diffrentiation. Then more diffrentiated magma evolved into Fe-Ti-P concentrated ferrodioritic magma which assume to be the parental magma for deposition of ilmenite ores in Hadong area.