Geneses of Two Types of Mafic Rocks to Carry Placer‐Magnetite Ores in the Sanin Granitic Belt, SW Japan

Abstract

Two types of mafic rocks from the central Sanin district, and their mafic minerals, were studied chemically and microscopically. They are classified into pyroxene‐containing gabbroid and hornblende–biotite quartz diorite. The gabbroid had higher color index but lower magnetite content; while the quartz diorite had lower color index, but higher magnetite content. The magnetite contents are also related to the amounts of hydrous mafic silicates. The gabbroic magma having pyroxene–amphibole assemblage, originated in the upper mantle, was considered essentially anhydrous, but became partly hydrous on the way to the site of solidification in the continental crust, and crystallized some magnetites with hypersthene and amphibole. The quartz dioritic magma was formed by partial melting of possibly subducting ocean‐floor basalts, once exposed to the sea‐floor then altered; thus the magmas became hydrous and oxidized originally, and precipitated abundant magnetite and hydrous mafic silicates from the early crystallization stage onward. Their weathered parts provided the most placer magnetite ores in the history.

A Tribute to Shunso Ishihara

“…Remarkably, Professor Ishihara's prolific academic productivity did not wane during his increasingly more significant administrative roles and, despite retirement from formal positions, he has continued to be active in petrochemical and metallogenic research at the GSJ, with an extensive publication record of research results, typically in collaboration with younger colleagues. This more recent work has focused on critical metals, particularly In and rare‐earth elements (REE) required by Japanese industry (Ishihara et al, , ; Ishihara & Endo, ; Ishihara & Matsueda, ; Murakami & Ishihara, ), as well as metallogenic syntheses (Ishihara & Qing, ; Ishihara, ) and other topics (Ishihara & Hirano, ).…”