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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

By
Visut Pisutha-Arnond
Visut Pisutha-Arnond
Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802
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Hiroshi Ohmoto
Hiroshi Ohmoto
Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802
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Published:
January 01, 1983

Abstract

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 forumla 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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Contents

Economic Geology Monograph Series

The Kuroko and Related Volcanogenic Massive Sulfide Deposits

Hiroshi Ohmoto
Hiroshi Ohmoto
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Brian J. Skinner
Brian J. Skinner
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Society of Economic Geologists
Volume
5
ISBN electronic:
9781629490007
Publication date:
January 01, 1983

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