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Abstract

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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