On the Lateritic Textures Retained in the "Emery" at Shinkiura Mine, Kyushu

Abstract

Almost perfect correspondence in textures and chemical compositions of the fine grained "emery" of stratiform deposit in the upper Palaeozoic-lower Mesozoic formation, at Shinkiura mine, Oita Prefecture, Kyushu, Japan, to some lateritic red shales or bauxites were recently found by the present author.
The textures retained in the "emery" ores are either pisolitic or concretional. Those of deformed or reworked fragmental pisolites are also preserved. The "emery" ores are heterogeneous in composition and comprise corundum, andalusite, cordierite, hercynite, ironoxides, biotite, amphibole and tourmaline, assuming apparently complicated mineral assemblages. But, in the basis, the mineral paragenesis is represented by either of the two tetrahedra; andalusite-cordierite-spinel-magnetite-silica rich domain, and corundum-andalusite-spinel-magnetite-silica poor. domain on SiO₂-(FeO+MgO)-Fe₂O₃, -AL₂O₃, diagram. In chemical composition, compared with the lateritic shales, the trend of dehydration and reduction is evident. All these petrographic features of the "emery" ores of Shinkiura mine might be interpreted as the result of thermal metamorphism of some lateritic shales in the sedimentary formation mentioned above. The cause of this metamorphism is attributed to the Tertiary granitic intrution to the south.
A new subject on palaeogeography of the upper Palaeozoic-lower Mesozoic age in this region with related to the presumed occurrence of the lateritic shales in the formation is open to future research.