Practical Application of Low Na2O Anomalies in Footwall Acid Lava for Delimiting Promising Areas around the Kosaka and Fukazawa Kuroko Deposits, Akita Prefecture, Japan
Hironori Hashiguchi, Ryoichi Yamada, Tsuneshi Inoue, 1983. "Practical Application of Low Na2O Anomalies in Footwall Acid Lava for Delimiting Promising Areas around the Kosaka and Fukazawa Kuroko Deposits, Akita Prefecture, Japan", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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The variation of Na 2 O in the acid lava underlying Kuroko deposits has been examined as a convenient exploration guide. First, the precise spatial distribution of the low Na 2 O anomaly around certain Kuroko deposits was established using a large number of samples collected systematically from the footwall acid lava. Then the applicability of using the anomaly for delimiting promising areas around the Kosaka and the Fukazawa deposits was examined. The following results have been obtained. 1. The most economical threshold can be set at 0.36 percent Na 2 O in the Kosaka area and 0.32 percent Na 2 O in the Fukazawa area. The small difference in the Na 2 O value of the thresholds is compatible with previous results. 2. The areal distribution of the low Na 2 O anomaly, however, is somewhat different in extent and shape from that presented previously. The difference in distribution is probably due to a different interpretation of high Na 2 O masses near the footwall of orebodies; namely, they are. regarded as either unaltered remnants or as intrusions. 3. The general areal extent of the anomaly in this study is two or three times as large as the presently known extent of ore and has a high correlation with small offsets of ore distribution. The variation of Na 2 O in the footwall is particularly useful for detailed exploration, but the extent may be too small for regional explorations.
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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.