Copper isotope variations of copper-rich minerals in seafloor hydrothermal deposits and igneous rocks, measured by a femtosecond LA-MC-ICP-MS

In recent years, the copper isotope systematics has seen an increased interest as a potential tool for understanding copper sources and geochemical processes of copper transport and deposition in ore-forming systems. 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 analyzed 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 chimney samples from 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., 2012). 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, 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. 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.