Chemical Processes of Kuroko Formation
H. Ohmoto, M. Mizukami, S. E. Drummond, C. S. Eldridge, V. Pisutha-Arnond, T. C. Lenagh, 1983. "Chemical Processes of Kuroko Formation", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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This paper attempts to evaluate the causes of the regularities and variations in the mineralogic and chemical constituents of volcanogenic massive sulfide deposits and to put constraints on the origin and the hydrology of the ore-forming systems. This attempt has been carried out mostly through examination of the chemistry of sulfur and metals in the Kuroko ore-forming fluids and of depositional processes.
Sulfur and oxygen isotopic characteristics of sulfides and sulfates from the Kuroko deposits suggest that the precipitation of these minerals was caused by rapid and disequilibrium mixing of hot (T ≥ 200°C) hydrothermal fluids with cold seawater. The δ 34 S and δ 18 O values place constraints on the total sulfate contents of the ore-forming fluids. Concentrations of aqueous sulfur species were also estimated from thermodynamic calculations by employing a simple assumption that the fluids were saturated with pyrite before interaction with cold seawater but were undersaturated with other sulfides and sulfates. The calculations indicate a continuous increase in the H 2 S content from ~10 –4.3m for the early stage fluids (T ≃ 200°C) to ~10 ~15 m for the later fluids (T ≃ 350°C), but a constant ∑SO content of ~10 -3m. The isotopic and chemical characteristics of the fluids suggest that these fluids obtained most of their sulfur from disseminated gypsum-anhydrite which had been formed through interaction of the pore fluids (seawater) and volcanic rocks during the diagenetic and the early hydrothermal stages at T ≤ 150°C. Some of these sulfate ions derived from anhydrite were reduced to form H 2 S through reactions with Fe +2 and organic C in the country rocks at elevated temperatures. The fluids of T ≥ 300°C also acquired some sulfur through dissolution of pyrite in the country rocks.
From this sulfur chemistry of the Kuroko fluids and solubility data of minerals, we have put constraints on the metal contents in the ore-forming fluids and on the amounts of minerals precipitating from the mixtures with normal seawater. The amounts and proportions of minerals in the ores depend on the oxygen content of cold seawater, relative rates of reactions among various oxidized and reduced aqueous species, and rates of chemical reactions vs. rates of fluid mixing, as well as on the concentrations of reduced sulfur, oxidized sulfur, and individual metals in the ore-forming fluids. Rapid mixing above the sea floor of discharging ore-forming fluids with normal seawater causes quantitative precipitation of Ba in the fluids as barite but no precipitation of quartz and native gold, which explains low Si/Ba and Au/Ag ratios in the massive black ores. The calculated metal ratios for the precipitates from the mixtures of the Kuroko fluids of T tnitial = 200° to 300°C with normal seawater are Zn/Pb ≃ 1, Zn/Fe &same 10, Zn/Cu ≥ 10, and Zn/Ba ≃ 1 (wt ratio), which agree well with the observed metal ratios for the massive black ores. The amounts of metals precipitated from a metric ton of the Kuroko ore-forming fluids were on the order of ~1 to ~10 g (~1 to ~10 ppm) each for Zn, Pb, Fe, and Ba, and ~0.1 to ~1 g for Cu.
Continuous reactions of the black ore with the later, hotter hydrothermal fluids lead to conversion of the black ore to the yellow ore and then to the pyrite-rich ore. The key to the process was that the hydrothermal fluids before interactions with the black ore were under-saturated with respect to major sulfides (e.g., sphalerite, chalcopyrite) except pyrite. The principal factors that determine the amounts and proportions of economic metals in massive sulfide deposits are suggested to be the thermal history of an ore-forming system, the dominant rock type in a region, and the composition of seawater (particularly its oxygen and sulfate contents). The differences in these factors can explain both the contrasts between Archean and Phanerozoic deposits and between those associated with basaltic rocks and those with felsic rocks.
The results of examination of the regional metal zoning patterns and of the location of volcanic centers in the Hokuroku district, together with computations of the mass of fluids and rocks based on material balance of Zn, Ba, Fe, S, Mg, Ca, and oxygen isotopes, of the size of heat sources based on heat balance, and of the duration of ore-forming activity based on the δ 34 S values of diagenetic pyrite, suggest that an average Kuroko ore-forming system discharged ~10 11 metric tons of fluids at T ≥ 200°C in an area of ~1.5 × ~3 km and deposited 0.5 to 1 million metric tons of Zn (3 to 20 million metric tons of Ore) in 200 to 50,000 years. The thermal anomalies responsible for these ore-forming systems appear to have been established by a relatively small (~10 km 3) intrusion. It is also, suggested that the rate of accumulation of clastic (and volcanic) rocks on the sea floor is an important factor in determining the formation and preservation of massive sulfide deposits.
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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.