HR: 0800h
AN: V51E-0841    [Abstracts]
TI: Fluid speciation controls of low-temperature copper isotope fractionation
AU: * Asael, D
EM: dan.asael@mail.huji.ac.il
AF: Institute of Earth Sciences, Hebrew University of Jerusalem, Edmond Safra campus, Givat Ram, Jerusalem, 91904, Israel
AU: Matthews, A
AF: Institute of Earth Sciences, Hebrew University of Jerusalem, Edmond Safra campus, Givat Ram, Jerusalem, 91904, Israel
AU: Oszczepalski, S
AF: Polish Geological Institute, Rakowiecka 4, Warsaw, 00-975, Poland
AU: Bar-Matthews, M
AF: Geological Survey of Israel, 30 Malchey Israel, Jerusalem, 95501, Israel
AU: Halicz, L
AF: Geological Survey of Israel, 30 Malchey Israel, Jerusalem, 95501, Israel
AB: Since the classic study of Ohmoto (1972) on the sulfur isotope system, it has been clear that mass-balance among reduced and oxidized solution species can exert strong control on the isotopic composition of sulphide ore minerals. Similarly, experimental studies on copper isotope fractionation suggest that fluid speciation among reduced and oxidized copper species could control the Cu-isotopic composition of copper sulfides. We explore this issue for sedimentary copper sulfide deposits through speciation calculations based on thermodynamic stability of reduced and oxidized copper aqueous solutions species. The calculations are made as a function of Eh, pH and Cu-isotopic fractionation factors at sedimentary temperatures. Speciation was calculated using The Geochemists Workbench v4.03, Cu isotopic fractionations were taken from experimental calibrations and diagrams were calculated by Matlab programming. The calculations show that isotopically-light Cu-sulfide minerals (with respect to the bulk δ⁶⁵Cu value) will only form in Eh-pH conditions where oxidized species dominate the mass balance, whereas more reducing conditions will give Cu-sulfide minerals with bulk δ⁶⁵Cu values similar to the bulk solution value. Applying these speciation diagrams to stratiform sedimentary hosted copper deposits of the Timna Valley in Israel and the Kupferschiefer deposits of SW Poland, points to notable differences between the two deposits in terms of the ore formation conditions. Whereas the Timna valley copper sulfides have light Cu-isotopic compositions with bulk δ⁶⁵Cu = -2.04 ± 0.4 ‰, the Kupferschiefer Cu-sulfides show significantly higher δ⁶⁵Cu values = -0.42 ± 0.38 ‰. Assuming that the source solution had a bulk δ⁶⁵Cu = 0 at both locations (i.e., an igneous copper porphyry source), the speciation diagrams indicate that the Kupferschiefer deposits precipitated at more reducing conditions and higher Cu(I)aq/Cu(II)aq values than the Timna deposits. These observations are in accord with field relations showing that Cu(II) minerals dominate the Timna system but that Cu-sulfides are the major minerals of the Kupferschiefer deposits. * Ohmoto, H. 1972. Systematics of sulfur and carbon isotopes in hydrothermal ore deposits. Econ. Geol. 67: 551- 578.
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 1030 Geochemical cycles (0330)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1051 Sedimentary geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting