"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, oerom 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 "residual" 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.

The redox state of granitoids relative to tectonic setting and earth history: The magnetite–ilmenite series 30 years later

“…The granitoids of the Sanin Province, in contrast, plot towards the magnetite-haematite side, implying that they contain haematitised magnetite, a feature confirmed by microscopic observation (Tsusue & Ishihara 1974). We now have good quality aeromagnetic data, which are useful in discriminating the two series of granitoids in regions where little or no overlapping of later magnetite-series igneous activity is known.…”

Robust stochastic control modeling of dam discharge to suppress overgrowth of downstream harmful algae

“…In addition, fish catches of P. altivelis have been significantly decreasing since the dam construction.” and that “River water around the green algae smell bad.” Although relationships between the algae and fish are still unclear, suppressing the overgrowth of the algae can be a matter of life and death for inland fisheries of Hii River. Riverbed of the river is rich in sand due to the old traditional iron‐making method from the sand, referred to as Kanna‐nagashi . The riverbed just downstream of Obara Dam is mainly composed of gravels with diameters of 10‐100 (cm).…”

Flooding history of Lake Nakaumi, western Japan, inferred from sediment records spanning the past 700 years

“…Because of this stratification, the bottom water often becomes anoxic, especially during the summer (Sakai et al, 2004). The Paleocene-Eocene granitoids in the San'in region, which have abundant magnetite as the major magnetic mineral (Ishihara and Chappell, 2008), are widely distributed around Lake Nakaumi (Tsusue and Ishihara, 1975) (Fig. 2).…”