Hydrothermal carbonates in altered wall rocks at the Uwamuki Kuroko deposits, Japan |
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| Authors: | N Shikazono M Hoshino M Utada M Nakata A Ueda |
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| Institution: | (1) Department of Earth Sciences, Keio University, Hiyoshi, Yokohama, Japan, JP;(2) University Museum, University of Tokyo, Bunkyoku, Hongo, Tokyo, Japan, JP;(3) Department of Astronomy and Earth Sciences, Tokyo Gakugei University, Koganei, Tokyo, Japan, JP;(4) Mitsubishi Material Co. Ltd., Ohmiya, Saitama, Japan, JP |
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| Abstract: | Magnesite, siderite and dolomite are characteristic alteration minerals occurring in Miocene hanging wall rocks of dacitic
composition which host the Kuroko orebodies. These carbonates generally occur in a more stratigraphically upper horizon than
chlorite alteration zone surrounding the orebodies. The Mg/(Mg+Fe) ratios of the carbonates decrease from the central alteration
zone to marginal zone. The Mg/(Mg+Fe) ratios of carbonates and chlorite positively correlate. The δ18O and δ13C values of magnesite, siderite and dolomite positively correlate with each other and lie between the igneous and marine carbonate
values. The petrographic, isotopic and fluid inclusion characteristics and thermochemical modelling calculations indicate
that magnesite and dolomite formed in the central zone close to the orebodies due to the interaction of hydrothermal solutions
with the biogenic marine carbonates. Calcite formed further from the orebodies from hydrothermal fluids which did not contain
a biogenic marine carbon component. The compositional and textural relationships indicate that superimposed alterations (chlorite
alteration and carbonate alteration) occurred in hanging wall rocks. The mode of occurrences and the Mg/(Mg+Fe) ratios of
magnesite and dolomite occurring in hanging wallrocks are useful in the exploration for concealed volcanogenic massive sulfide-sulfate
deposits.
Received: 9 September 1997 / Accepted: 23 September 1997 |
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