Abstract
The Yanomaki metasomatic anorthosite is developed within the Paleogene Ryukoma granite which is composed mainly of biotite adamellite, and crops out in two separate areas (No.1 and No.2 deposits). The shape of each anorthosite body is significantly different, i.e., the body of the No.1 deposit occurs as a sheet, while the body of the No.2 deposit occurs as a dome. These geometries suggest that the former was formed by fracture-controlled alteration and the latter by pervasive alteration. The upper parts of the metasomatic anorthosite bodies are intensely altered to halloysite due to weathering. In some places the metasomatic anorthosite bodies are associated with greisenized zones. Actinolite-quartz veinlet swarms are observed within the No.1deposit. Transition zones between the metasomatic anorthosite and host granite are very narrow; the zone is usually less than 5cm in width.
The metasomatic anorthosite is composed mainly of plagioclase (oligoclase to andesine) and quartz. Minor amounts of titanite and muscovite also present. Actinolite is an additional accessory phase in the marginal zones to the anorthosite. Original K-feldspar, biotite and Fe-Ti oxides have been completely disappeared. The anorthite contents of plagioclase are distinctly higher than those of the host granite. The characteristics of the metasomatic anorthosite can be summarized as follows:
1) Formation of the metasomatic anorthosite took place at a temperature between about 385°C and 644°C as deduced from the metasomatic mineral assemblage and hornblende-plagioclase geothermometry.
2) Original K-feldspar and mafic mineral phases were replaced by oligoclase or andesine. The sodium-calcium metasomatism would be caused by alteration by a high temperature and Ca-rich hydrothermal fluid.
3) The hydrothermal fluid which formed the metasomatic anorthosite was probably derived from a later granitic intrusive situated beneath the Yanomaki mine.
