"Residual" iron-sand deposits of Southwest Japan

Abstract

Four petrographic provinces have been recognized in southwest Japan on the basis of chemical and mineralogical composition of granitic rocks. The provinces trend parallel to the island arc and are referred to, from south to north, as the Outer Zone, the Ryoke, Sanyo-Naegi, and Sanin-Shirakawa belts.Iron-titanium oxides in the granitic rocks of southwest Japan have been examined under the microscope and analyzed with the electron probe microanalyzer. Ilmenite containing up to 4 mole percent hematite molecule is essentially the sole iron-titanium oxide present as an early accessory constituent of the granitic rocks in the Outer Zone, Ryoke belt, and Sanyo-Naegi belt. Magnetite, magnetite intergrown with ilmenite, and hemo-ilmenite are present as early accessory constituents in the Sanin-Shirakawa belt. The assemblage of iron-titanium oxides in the gabbroic rocks of each belt is similar to those in the granitic rocks.The granitic rocks of the Sanin-Shirakawa belt are characterized by a higher ferric to ferrous iron ratio than those of the other belts.Residual iron-sand deposits are essentially weathered granitic and mafic plutons containing small amounts of iron-titanium oxides, and are classified into iron and titanium deposits based on their mineralogical characters. The iron deposits are confined to the Sanin-Shirakawa belt and the titanium deposits to the Ryoke belt. The distribution of iron-sand deposits corresponds to the mineralogy of the source rocks.Deposits in the Yokota area of the Sanin-Shirakawa belt occur in irregular discontinuous blanketlike masses scattered over the uneven weathered surface of the plutons. Most of the iron deposits are found in the granodiorite group, but some are in coarse-grained adamellite. The iron deposits derived from mafic plutons are confined to small bodies of diorite and gabbro. The distribution of the deposits is determined by several controls: magnetite content of the surface rocks, degree of weathering of the source rocks, water supply, local relief for ore dressing, and others.Titanium deposits are confined to the weathered gabbroic rocks of the Ryoke belt. Most of the deposits occur in either clay beds derived from residual debris or talus deposits originated from weathered gabbroic plutons.