COPPER ISOTOPE CHARACTERISTICS OF COPPER-RICH MINERALS IN SEAFLOOR HYDROTHERMAL DEPOSITS AND IGNEOUS ROCKS, ANALYZED USING A FEMTOSECOND LA-MC-ICP-MS

The copper isotope variations of primary and secondary copper-rich minerals from modern (Mariana Trough) and ancient (Besshi-type and Kuroko-type volcanogenic massive sulfide deposits, Japan) seafloor hydrothermal deposits have been measured by a 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 copper-rich sulfide minerals of active seafloor hydrothermal deposits are significantly large (δ⁶⁵Cu = -0.7 to 4.0‰) compared to those of copper-rich minerals in ancient submarine hydrothermal deposits (δ⁶⁵Cu = -0.3 to 0.4‰; e.g., Ikehata et al., 2011) and in igneous rocks (δ⁶⁵Cu = -0.3 to 0.3‰; e.g., Ikehata et al. in press). These large copper isotope variations in the modern active seafloor hydrothermal deposits are most likely explained in terms of a redox-controlled isotope fractionation during hydrothermal reworking or alteration of precipitated copper-rich minerals. These results also suggest that sub-seafloor and metamorphic recrystallization effects probably have reduced the original range of copper isotopes.

Secondary malachite (δ⁶⁵Cu = 2.6 to 3.0‰) and native copper (δ⁶⁵Cu = 1.4 to 1.7‰) in the Besshi-type deposits have heavier copper isotopic values compared to precursor copper-rich minerals. These variations are mainly due to isotope fractionations during redox reactions (weathering) at low temperatures involving the preferential incorporation of heavy copper isotopes in secondary Cu(II) solutions.

Therefore, copper isotope geochemistry might be a useful tool for understanding details of ore-forming processes.