Geological Society of America

T190 Tectonics and Metallogeny (Posters)

Tuesday, 11 October 2011
9:00 AM - 6:00 PM
Hall C

IN SITU COPPER ISOTOPIC ANALYSIS OF SULFIDE MINERALS IN HYDROTHERMAL ORE DEPOSITS AND IGNEOUS ROCKS BY FEMTOSECOND LA-MC-ICP-MS

Paper number: T190-9
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Kei Ikehata Presenting Author
Faculty of Life and Environmental Sciences, University of Tsukuba, Tsukuba
Junichiro Ishibashi
Fukuoka
Ryohei Suzuki
Fukuoka, N/A
Hiroshi Shimizu
Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shimabara, N/A
Takeshi Matsushima
Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shimabara, N/A
Satoshi Matsumoto
Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shimabara, N/A
Kenji Uehira
Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shimabara, N/A
Takafumi Hirata
Kyoto, N/A

Abstract

We investigated copper isotopic variations of sulfide chimney samples from two seafloor hydrothermal sites in the southern Mariana Trough by femtosecond-pulsed laser ablation multiple collector inductively coupled plasma mass spectrometry (fs-LA-MC-ICP-MS). The δ⁶⁵Cu (where δ⁶⁵Cu = [(⁶⁵Cu/⁶³Cu)_sample/ (⁶⁵Cu/⁶³Cu)_NIST-SRM976-1] × 1000) values of Cu-rich sulfide minerals of both sites are significantly large (-0.70 to 4.0‰) compared to those of Cu-rich minerals in ancient submarine hydrothermal deposits at mid-oceanic ridge settings (e.g., Besshi-type volcanogenic massive sulfide deposit, Japan, -0.31 to 0.26‰; Ikehata et al., 2011) and in mantle peridotite (Horoman, Japan) and island-arc volcanic rocks (e.g., Shinmoedake volcano, Japan)(-0.10 to 0.40‰).

These large copper isotopic variations in the chimney samples are most likely explained in terms of a redox-controlled isotope fractionation during hydrothermal reworking of copper sulfides below sea floor or alteration of primary hydrothermal copper sulfides by seawater (possibly by bacteria), involving the preferential incorporation of heavy copper isotopes in secondary Cu(II) solutions. These results also suggest that sub-seafloor recrystallization and metamorphic reequilibration may have reduced the original range of copper isotopes.

Therefore, copper isotopic geochemistry might be a useful tool for understanding details of ore-forming processes. However, further improvements in precision and accuracy of in situ copper isotopic analysis are needed to fully understand the copper isotopic behavior during high-temperature processes.