COPPER ISOTOPE VARIATIONS IN CU-RICH MINERALS FROM METAMORPHOSED BESSHI-TYPE VMS DEPOSITS, JAPAN, MEASURED USING A FEMTOSECOND LA-MC-ICP-MS

The copper isotope variations of primary and secondary Cu-rich minerals from metamorphosed Besshi-type volcanogenic massive sulfide deposits of two different mines, Japan, have been measured 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 primary chalcopyrite samples of both mines are relatively narrow (-0.34 to 0.29‰) compared to those of reported chalcopyrite from modern submarine hydrothermal vents at mid-oceanic ridge settings (δ⁶⁵Cu = -0.98 to 3.14‰; Zhu et al., 2000, Rouxel et al., 2004). However, the δ⁶⁵Cu values in primary chalcopyrites are slightly different between the two mines. This suggests that metamorphic reequilibration may have reduced the original range of Cu isotopes, but premetamorphic δ⁶⁵Cu characteristics of both mines are generally preserved. Secondary malachite (δ⁶⁵Cu = 2.63-2.97‰) and native copper (δ⁶⁵Cu = 1.43-1.71‰) have heavier Cu isotopic values compared to primary chalcopyrite from the same deposit. This variation is most likely explained in terms of a redox-controlled isotope fractionation at low temperature conditions during secondary (weathering) processes involving the preferential incorporation of heavy Cu isotopes in secondary Cu(II) solutions. It is also suggested there occurred a significant negative isotopic fractionations accompanying the reduction of Cu (II) solutions to native copper [Cu (0)]. Therefore, Cu isotope geochemistry might be a useful tool for understanding details of ore-forming processes.