Anhydrite in Kuroko Deposits: Mode of Occurrence and Depositional Mechanisms
Naotatsu Shikazono, Heinrich D. Holland, Robert F. Quirk, 1983. "Anhydrite in Kuroko Deposits: Mode of Occurrence and Depositional Mechanisms", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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In the Kuroko deposits of Japan anhydrite is very abundant in sekko ore, which underlies the strata-bound sulfide ores. Anhydrite in sekko ore is usually nodular. The diameters of the anhydrite nodules vary with stratigraphic position and range from a few millimeters to several meters. The pyroclastic rocks in sekko ore horizons have undergone intense hydrothermal alteration. Pyroclastic rocks containing large amounts of anhydrite are altered to Mg chlorite. Pyroclastic rocks containing sericite and sericite-montmorillonite mixed layer clay minerals generally contain small amounts of anhydrite. The strontium content of the sekko anhydrite ranges from about 200 to 2,000 ppm and tends to increase both stratigraphically upward and according to nodule size. The filling temperature of fluid inclusions in the anhydrites ranges from ca. 240° to 340°C.
The mode of occurrence, texture, Sr content, nature of the contained fluid inclusions, and isotopic composition of Sr, S, and O in anhydrite together with the mineralogy of the sekko ore suggest that anhydrite deposition was probably related to subsurface mixing of fluids.
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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.