Mineralogy and Geochemistry of Tuffaceous Exhalites (Tetsusekiei) of the Fukazawa Mine, Hokuroku District, Japan
Stavros I. Kalogeropoulos, Steven D. Scott, 1983. "Mineralogy and Geochemistry of Tuffaceous Exhalites (Tetsusekiei) of the Fukazawa Mine, Hokuroku District, Japan", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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Tetsusekiei, translated literally from Japanese as iron quartz, occurs primarily in the immediate hanging wall of the massive stratiform Kuroko ore and, in places, well within the overlying tuff, There are two forms: bedded, which is predominant, and networks cementing hanging-wall breccia fragments. Bedded tetsusekiei is discontinuous and usually covers an area about twice that of the ores without exceeding a thickness of 20 to 30 cm. It appears to have formed late in the development of the stratiform ores and consists of two contrasting constituents: a predominantly aluminosilicate (chlorite + sericite) tuffaceous or, simply, clastic constituent, and a predominantly chert, hematite, and pyrite or, simply, chemical constituent. Textures, mineralogy, and compositions of the tetsusekiei and related tuffs are consistent with the formation of tetsusekiei by the mixing in various proportions of the chemical and clastic constituents. The major source of iron, sulfur, and silica in tetsusekiei was hydrothermal.
During intensification of the hydrothermal system at the time of sulfide deposition, tetsusekiei formed at low temperatures (<150°C) around and at some distance from zones of main fluid discharge whereas, during declines in hydrothermal activity, it formed in close spatial association with the ore. High temperatures (~300°G) recorded by fluid inclusions in cavity-filling quartz, chalcopyrite disease in sphalerite, and sulfidation of hematite and chlorite, indicate that tetsusekiei was chemically and texturally modified after deposition. Several stages of tetsusekiei formation are envisaged to have occurred during the lifetime of the ore-forming system: Tetsusekiei can provide exploration guides at several different scales: (1) its recognition in the field identifies a possible ore-forming horizon; (2) the alteration index, R = , increases from about 50 to 90 percent at Fukazawa over a distance of about 3 km with a concomitant decrease in oxygen isotope values approaching the ore; and (3) trace elements of exhalative input are present at elevated concentrations. The areal distributions of trace elements with respect to the Fukazawa orebodies are complicated by multiple input sources from scattered hot springs along linear structures and do not provide a simple panacea for exploration.
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This paper consists of three parts. The first is an overview of the geologic history of the Green Tuff region where all Kuroko deposits occur. The second part presents a description of the stratigraphy and an interpretation of the structural and igneous history of the Hokuroku district, the most important Kuroko mining district. The third part is an analysis of the role of submarine calderas in Kuroko genesis.
The sequence and causes of the major geologic events that have occurred in Japan and its vicinity since the Cretaceous are interpreted as follows: (1) an active but shallow-dipping north-northwestward subduction of the Pacific plate under the Asian continent during a period from approximately 130 to 65 m.y. ago resulted in ilmenite series magmatism in the outer zone of Japan, then still a part of mainland Asia; (2) about 65 to 40 m.y. ago, the direction of the subducted Pacific plate changed to westward and the angle of subduction steepened, initiating back-arc spreading in the Japan basin province and migration of Japan away from the Asian mainland until about 30 m.y. ago; (3) during the period 65 to 30 m.y. ago, the basaltic crust created in the Japan basin province was subducted eastward under the Yamato Ridge province, resulting in calc-alkaline and magnetite series igneous activity in the inner zone of Japan; (4) about 25 m.y. ago, the first sea (proto-Japan Sea) was formed in the Japan basin province as a result of the eustatic rise of the sea following cessation of spreading there about 30 m.y. ago; (5) back-arc spreading was active in the Yamato basin province during the period between 25 and 5 m.y. ago, cansing bimodal volcanism and subsidence in the flanking Inner Honshu and Yamato Ridge provinces [the Hokuroku basin (i.e., a Kuroko-bearing basin), Niigata oil field basin, and Akita oil field basin were all fault-bounded, deep (>2,500 m) marine basins created by rapid subsidence of crustal blocks within a few million years around 17 m.y. ago, although Kuroko mineralization and the accumulation of organic matter were not synchronous]; and (6) the dip of the subducted Pacific plate returned to a shallow angle about 5 m.y. ago, causing the cessation of back-arc spreading and the initiation of subsidence of the Yamato basin province and uplift of the flanking Inner Japan and Yamato Ridge provinces. The Green Tuff activity is, therefore, synonymous with the tectonic and igneous activity that accompanied the formation of the Japan Sea and the Japanese islands during the period from ~65 m.y. ago to the present.