Marine hdrothermal alteration at a Kuroko ore deposit,Kosaka, Japan |
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Authors: | Keiko Hattori Karlis Muehlenbachs |
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Institution: | (1) Department of Geology, University of Alberta, T6G 2E3 Edmonton, Alberta, Canada;(2) Present address: Physics Department, University of Calgary, T2N 1N4 Calgary, Alberta, Canada |
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Abstract: | Isotopic compositions were determined for quartz, sericite and bulk rock samples surrounding the Uwamuki no. 4 Kuroko ore body, Kosaka, Japan.
18O values of quartz from Siliceous Ore (S.O.), main body of Black Ore B.O.) and the upper layer of B.O. are fairly uniform, +8.7 to +10.5 . Formation temperatures calculated from fractionation of 18O between sericite and quartz from B.O. and upper S.O. are 250° to 300° C. The ore-forming fluids had
18O values of +1 and D values of –10 , from isotope compositions of quartz and sericite.Tertiary volcanic rocks surrounding the ore deposits at Kosaka have uniform
18O values, +8.1±1.0 (n=50), although their bulk chemical compositions are widely varied because of different degrees of alteration. White Rhyolite , which is an intensely altered rhyolite occurring in close association with the Kuroko ore bodies, has also uniform
18O values, +7.9±0.9 (n=19). Temperatures of alteration are estimated to be around 300° C from the oxygen isotope fractionation between quartz and sericite. Paleozoic basement rocks phyllite and chert, have high
18O values, +18 and +19 . The Sasahata formation of unknown age, which lies between Tertiary and Paleozoic formations, has highly variable
18O, +8 to +16 (n=4). High
18O values of the basement rocks and the sharp difference in
18O at their boundary suggest that the hydrothermal system causing Kuroko mineralization was mainly confined within permeable Tertiary rocks. D values of altered Tertiary volcanic rocks are highly variable ranging from –34 to –64% (n=12). The variation of D does not correlate with change of chemical composition,
18O values, nor distance from the ore deposits. The relatively high D values of the altered rocks indicate that the major constituent of the hydrothermal fluid was sea water. However, another fluid having lower D must have also participated. The fluid could be evolved sea water modified by interaction with rocks and the admixture of magmatic fluid. The variation in D may suggest that sea water mixed dispersively with the fluid. |
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