Thermal History, and Chemical and Isotopic Compositions of the Ore-Forming Fluids Responsible for the Kuroko Massive Sulfide Deposits in the Hokuroku District of Japan
Visut Pisutha-Arnond, Hiroshi Ohmoto, 1983. "Thermal History, and Chemical and Isotopic Compositions of the Ore-Forming Fluids Responsible for the Kuroko Massive Sulfide Deposits in the Hokuroku District of Japan", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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Detailed petrographic and fluid inclusion studies of the stockwork siliceous ores from five major Kuroko deposits (Kosaka, Fukazawa, Furutobe, Shakanai, and Matsumine) have revealed an essentially identical sequence of mineralization and thermal history for each deposit: formation of the majority of black ore minerals (sphalerite, galena, pyrite, and minor barite and quartz) during intensifying stages of hydrothermal activity at T = ~200° to 330°C, followed by formation of the majority of yellow ore minerals (chalcopyrite and quartz) during a thermal maximum at T = 330° ± 50°C and minor sphalerite mineralization while the temperatures again decreased to 260° ± 50°C. Despite the change in temperature, salinities of the ore-forming fluids remained fairly constant at about 3.5 to 6 equivalent weight percent NaCl except for occasional values up to around 8 weight percent. Concentrations of elements in the ore-forming fluids, in moles/kg H 2 O, as determined by the chemical analyses of fluids extracted from sulfides and quartz, were: Na = 0.60 ± 0.16, K = 0.08 ± 0.05, Ca = 006 ± 0.05, Mg = 0.013 ± 0.008, Cl = 0.82 ± 0.32, and C (as CO 2) = 0.20 ± 0.15; and less than 6 ppm each for Cu, Pb, Zn, and Fe.
The range of δD values of the Kuroko fluids, estimated from the analyses of fluids extracted from sulfides and quartz, is between —30 and +15 per mil, a range much larger than the range of —26 to —10 per mil reported by previous investigators. The range of δ 18 O values of the Kuroko fluids (—6 to +4‰), esfmated from the δ 18 O analyses of quartz and the temperature data (this study) and from the δ 18 O values of quartz and hematite in the tetsusekiei ores (Tsutsumi and Ohmoto, 1983), is also much larger than that suggested by previous investigators (0 ± 1‰). There is a distinct correlation between the δ 18 O values and the temperatures of the Kuroko fluids: fluids of pre-and post-main-stage sulfide mineralization with temperatures less than about 200°C had much lower δ 18 O values than those of the main sulfide mineralization at T 250°C (-2 to +4‰).
Comparison of the isotopic and chemical compositions of the Kuroko fluids with those of pore fluids in the Deep Sea Drilling Project core samples and thermochemical consideration of the stability relationships among alteration minerals have led to a model of continuous interactions during the diagenetic through hydrothermal stages between pore fluids (seawater) and volcanic rocks within a thermally intensifying system. The model explains the following observed characteristics: (1) the ranges of δD, δ 18 O, and salinity values of the Kuroko. fluids; (2) the high H2O contents and δ 18 O values of country rocks around the Kuroko deposits; (3) the correlation between the δ 18 O values and fluid temperatures; (4) the concentrations of Na, K, Ca, Mg, and Cl in the fluids; (5) the spatial and temporal relationships among the alteration minerals (zeolite, montmorillonite, transition, and sericite + chlorite assemblages) around the Kuroko deposits; and (6) the Na, K, Ca, Mg, and Cl geochemical halos in the rocks around the Kuroko deposits. Some of the important processes that have caused alteration of the isotopic and chemical compositions of fluids and rocks are thought to be: formation of hydrous minerals(zeolites and smectites) in the country rock and cation exchange reactions between fluids and these minerals during diagenetic stages; formation of calcite in the country rock through reactions between fluids and organic matter during the diagenetic and low-temperature hydrothermal stages; dehydration reactions involving diagenetic minerals; and oxygen isotope exchange reactions between fluids and plagioclase, hydrogen isotope exchange reactions between fluids and smectite, dissolution of plagioclase, and cation exchange reactions between fluids and minerals during high-temperature hydrothermal stages.
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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.