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A Preliminary Oxygen Isotope Study of Tetsusekiei Ores Associated with the Kuroko Deposits in the Hokuroku District, Japan

By
Makoto Tsutsumi
Makoto Tsutsumi
Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802Present address: Hot Spring Research Institute, Okayama University, Misasa, Tottori, Japan.
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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

Tetsusekiei beds or chert-hematite layers stratigraphically overlie massive sulfide ores in many Kuroko deposits. Because of the small size of individual grains (less than 1 µm) in these samples, fluid inclusion studies or mechanical separation of individual minerals for isotopic or chemical analysis is difficult. A chemical method, which involves treatment of the powdered samples with an HCl-HI-H 3 PO 2 mixture and with HF, was developed to attain quartz- and hematite-enriched fractions. Oxygen isotope analyses of these fractions, together with the determination of Fe and Si contents of each fraction, allow estimates to be made of the oxygen isotope compositions of "pure quartz" and hematite in the samples.

The δ 18 O values of these "pure" minerals were estimated for eight tetsusekiei samples from four Kuroko mines in the Hokuroku district of Japan: a range of 6 to 21 per mil for the quartz and —19 to +2 per mil for the hematite. The application of the oxygen isotope fractionation factors of quartz-water and magnetite-water of Becker and Clayton (1976) to the δ 18 O values of coexisting quartz and hematite in these samples yields the following values for the temperature and δ 18 O values of hydrothermal fluids responsible for the formation of the tetsusekiei ores: Fukazawa mine: T = 110° ± 30°C, δ 18 O water = -9 ± 5 per mil, and Furutobe mine: T = 90° ± 30°C, δ 18 O water = —3 ± 5 per mil. These data, together with the occurrence of the tetsusekiei bed and the temperature and δ 18 O history of hydrothermal fluids during sulfide mineralization obtained by Pisutha-Arnond and Ohmoto (1983), suggest that the tetsusekiei ores formed during the waning stage of a hydrothermal activity and that a positive correlation exists between the temperature and the δ 18 O values of fluids: negative δ 18 O values at T < 200°C and near zero to positive δ 18 O values at higher temperatures.

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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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