Origin of Magnetite- and Ilmenite-series Granitic Rocks in Sulawesi, Indonesia: Magma Genesis and Regional Metallogenic Constraint

Abstract

Lateral and spatial variations magnetic susceptibilities of the granitic rocks from Sulawesi, Indonesia were studied to constrain its implication for their magma genesis and regional metallogenic. The magnetic susceptibility of the granitic rocks varies between 0.08 x 10-3 SI to 18.58 x 10-3 SI, corresponding respectively to ilmenite series (< 3x 10-3 SI; oxidized type) and magnetite series (> 3x 10-3 SI; reduced type) granite. This wide range result is due to large variation of rock types within one suite which cause a variety of magnetic minerals content in the rocks. Although regionally, Sulawesi granitic rocks are dominated by ilmenite series granites, the ratio of ilmenite/magnetite series granites decreases substantially from the southern part to the northern part of the island. The occurrence of the ilmenite-series with I-type characteristic granitic rocks in Sulawesi Island may be explained by assimilation process between magma and crustal material that containing a various quantity of reduced C- and S-bearing sediments. The ilmenite series granitic rocks are characterised by the occurrences of ilmenite and hematite, higher Sn – W, U – Th, V and total REE+Y, whereas the magnetite series are characterized by abundance of hematite and sulphide minerals but lower Sn – W, U – Th, V and total REE+Y. The absence of Sn and W mineralization in ilmenite-series granitic rocks could be explained by the high oxidation state of the granitic magma.