Whole-Rock Oxygen Isotope Distribution in the Fukazawa-Kosaka Area, Hokuroku District, Japan, and Its Potential Application to Mineral Exploration
Geoffrey R. Green, Hiroshi Ohmoto, Jiro Date, Toshio Takahashi, 1983. "Whole-Rock Oxygen Isotope Distribution in the Fukazawa-Kosaka Area, Hokuroku District, Japan, and Its Potential Application to Mineral Exploration", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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Oxygen isotope analyses of 151 samples of Miocene volcanic and sedimentary rocks from a 20 × 15-km area of the Hokuroku district, Japan, show a consistent relationship between alteration assemblages and δ 18 O values of whole rocks. Excluding quartz-rich sandstones, which are resistant to hydrothermal alteration, the δ 18 O values of all types of igneous rocks (dacite, andesite, basalt) and sedimentary rocks (tuff and mudstone) lie within the ranges of 16.9 ± 2.7 (1 σ value) per mil in the zeolite zone, 11.1 ± 2.5 per mil in the montmorillonite zone, and 6.7 ± 1.3 per mil in the sericite-chlorite zone. Essentially, all the Miocene rocks in the Hokuroku district are altered to zeolite or higher grade zones.
Concentric zoning patterns of the whole-rock δ 18 O vaines and of alteration minerals are particularly well developed in the footwall volcanics around the Fukazawa Kuroko deposit. The δ 18 O values of the whole rocks gradually increase from less than 8 per mil in the sericite-chlorite zone that extends about 0.5 km outside of the ore zone, to 8 to 14 per mil in the montmorillonite zone, a 1- to 3-km-wide zone occurring outside the sericite-chlorite zone, to mostly greater than 14 per mil in the peripheral zeolite zone.
In the hanging-wall rocks above the Fukazawa deposit, δ 18 O and mineralogical zonations extend at least 400 m above the ore horizon (i.e., to the present-day surface), but these zones gradually contract upward. Such features together with the radiometric ages of the igneous rocks suggest that the Fukazawa mine was the site of discharge of high-temperature (T > 200 º C) hydrothermal fluids for a period of over 3 million years, although" the hydrothermal activity may have been episodic and decreased in intensity with time.
Comparison of the spatial variation of δ 18 O values of the footwall rocks in the Fukazawa area with that of Na, K, Mg, Ca, Sr, Cu, Pb, Zn, and S contents and of the magnetic susceptibility shows that the areal extent of the δ 18 O anomaly is mnch larger than that of other geochemical anomalies, and that the variability of δ 18 O values within a site is much less than that of the elemental compositions. This suggests that the analysis of whole-rock δ 18 O values is a useful method of exploration for volcanogenic massive sulfide deposits (and also for vein deposits occurring in submarine volcanic sequences) during the reconnaissance stage whereas other geochemical methods are more suitable in the developments, tage. In fact, 18 of 21 drill sites where low δ 18 O (<8‰) rocks were discovered in this study are located within 1 km of known Kuroko-type or vein-type deposits. Analyses of whole-rock δ 18 O values are also useful in the exploration for these types of deposits in metamorphosed and deformed terrains, because the oxygen isotope zoning is less likely to be destroyed than the alteration mineral zoning during regional metamorphism.
The δ 18 O values of the altered volcanic rocks around the Knroko deposits and their correlation with alteration assemblages can be interpreted as a result of interaction between the rocks and seawater at different temperatures (25°-200°C for the zeolite zone, 150°-300°C for the mont-morillonite zone, and 200 o -400°C for the sericite-chlorite zone) under water-dominated (i.e., water/rock > 1) conditions. This interpretation also explains most of the hydrogen isotope data of the whole-rock samples (δD = —34 to — 80‰) from the Hokuroku district.
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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.