The Occurrence of Malayaite in Pyrometasomatic Deposits of the Southwestern Japan

Abstract

The occurrence of malayaite (CaSnO⋅SiO₄) has been studied at several mines such as the Mitate mine, Miyazaki Prefecture, the Hoei mine, Oita Prefecture and the Kuga mine, Yamaguchi Prefecture. In each ore deposit, this mineral has been found from the periphery of pyrometasomatic deposists, or from the silicified zones of the country rocks near skarn bodies. From the results of the macroscopic and microscopic studies of the occurrence of malayaite, it is inferred that this mineral would have been formed by the following three processes.
1) Sn has been concentrated at the front of skarnization or at the silicified zone, and it has reacted with Ca and Si to form malayaite.
2) Ca- and Si-rich hydrothermal solutions have reacted with tin-bearing minerals such as cassiterite and tin-bearing andradite in the skarn, and malayaite has been formed using Sn supplied from these tin-bearing minerals.
3) Sn, Ca and Si have been transported into veinlets by the hydrothermal solutions and malayaite has been formed there with quartz and calcite.
Under the Fe-rich condition such as the skarnization, Ca and Si would have reacted with Fe, forming garnet or clinopyroxene, and Sn would have crystallized as cassiterite without forming malayaite. According to the electron probe microanalysis, malayaite from the Mitate mine contains 2.0%-4.6% Ti, while the one from the Kuga mine contains 0.3% Ti. The contents of Sn and Ti within a single crystal grain of malayaite vary distinctively and the negative correlation between the two elements is conspicuous.