An Analysis of the Hydrothermal System Responsible for Massive Sulfide Deposition in the Hokuroku Basin of Japan
L. M. Cathles, 1983. "An Analysis of the Hydrothermal System Responsible for Massive Sulfide Deposition in the Hokuroku Basin of Japan", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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The hydrothermal system responsible for Miocene massive sulfide mineralization in the Hokuroku basin of northern Japan is analyzed by combining simple heuristic calculations with critical field data, and by incorporating redistribution of anhydrite, silica, and δ18Or into a finite difference model of the hydrothermal system and comparing model chemical predictions to field observations. Kinetic constraints are taken into account. The exercise suggests that the Kuroko massive sulfide deposits were formed as the result of multiple small (1 × 3 km in cross section) intrusive pulses that cooled by convection and formed individual sulfide and sulfate lenses in less than 5,000 years and probably ~100 years. The general formation permeability was at least a few millidarcies and probably a few hundred millidarcies. Sulfate and silica deposition preceded sulfide deposition, performing the necessary function of isolating hot deeply circulating solutions from cool shallowly circulating solutions so they could vent uncooled into the sea. Surface discharge was controlled by major fractures. Calculated δ18Or anomalies are in excellent agreement with those observed near the Fukazawa massive sulfide deposit. The model deduced for the Knroko hydrothermal system is compatible with and supported by observations that have been made on ophiolite suites, and on hot and Warm springs at mid-ocean ridges. The correspondence between δ18Or profiles calculated here and those measured in the complete composite section through the Semail Ophiolite, Oman, is particularly striking. Insights are gained into the nature of deep (greater than 2 km) equilibrium oxygen isotope alteration. Exploration implications are indicated and a confirmatory test suggested.
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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.