Controlling Mechanisms for the Major Element Chemistry of Aqueous Solutions in Tuff-Rich Environments
Masakatsu Mizukami, Hiroshi Ohmoto, 1983. "Controlling Mechanisms for the Major Element Chemistry of Aqueous Solutions in Tuff-Rich Environments", The Kuroko and Related Volcanogenic Massive Sulfide Deposits, Hiroshi Ohmoto, Brian J. Skinner
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Smectite is the most common diagenetic and hydrothermal mineral in the Miocene Green Tuff Formation that hosts the Kuroko deposits. We have conducted a series of laboratory experiments to determine the stability of smectites and the partition coefficients for the cation exchange reactions between smectite and solutions of NaCl-CaCl 3, KCl-CaCl 2, MgCl 2 -CaCl 2, and SrCl 2 -CaCl 2 with a concentration range of up to 1 N in a temperature range of 25° to 300°C. K-rich and Mg-rich smectites were found to be unstable in KCl-CaCl 2 and MgCl 2 -CaCl 2 solutions, respectively, at temperatures above about 150° to 200°C. However, Na-, Ca-, and Sr-rich smectites are stable in Na, Ca, and Sr solutions, respectively, to temperatures of at least 300°C. Cation exchange reactions between smectites and solutions are very fast and readily reversible even at 25°C.
Using the partition coefficients obtained in this study, we have computed the changes in the major element chemistry of solutions during interactions with smectite-rich tuffs as a function of the ratios of meteoric water to seawater and of rock to water, and we have compared our results with the observed compositions of two types of modern waters in the Green Tuff region: formation waters (T ≃ 25°C) from the Seikan undersea tunnel and thermal waters (T = 150°-230 0 C) from several areas. The major element (Na, K, Ca, Mg, Sr, Cl, and SO 4) chemistry of these waters appears to have been controlled by the following five parameters: (1) the ratio of meteoric water to seawater, (2) the cation exchange reactions between smectite and solutions, (3) the solubilities of gypsum-anhydrite and calcite which are also common diagenetic minerals in the Green Tuff Formation, (4) the ratio of rock to water, and (5) the cation exchange reactions between feldspars and solutions (only at T ≥ 150°C). The calculated rock/water mass ratios range from 0 to >5 for the Seikan formation waters but are mostly greater than 5 (i.e., rock dominated) for the thermal waters. It is reasonable to assume that the major element chemistry of the hydrothermal fluids responsible for the Kuroko mineralization was also controlled by the same five parameters under rock-dominated conditions.
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The Kuroko and Related Volcanogenic Massive Sulfide Deposits
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.