COPPER ISOTOPE CHARACTERISTICS OF COPPER-RICH MINERALS FROM BESSHI-TYPE VOLCANOGENIC MASSIVE SULFIDE DEPOSITS, JAPAN, DETERMINED USING A FEMTOSECOND LA-MC-ICP-MS

Abstract

The copper isotope ratios of chalcopyrite, secondary malachite, and native copper of metamorphosed Besshi-type volcanogenic massive sulfide (VMS) deposits from two different mines, Japan, were measured by femtosecond-pulsed laser ablation-multiple collector-inductively coupled plasma-mass spectrometry (fs-LA-MC-ICP-MS). The δ 65Cu (where δ 65Cu = [{(65Cu/63Cu)sample/(65Cu/63Cu)NIST-SRM976} − 1] × 1,000) values of chalcopyrite samples of both mines are relatively narrow (−0.34 to +0.29‰) compared to those of previously reported chalcopyrite from modern submarine hydrothermal deposits at mid-oceanic ridge settings (−0.98 to +3.14‰). However, the δ 65Cu values of chalcopyrite are slightly different between the two mines. This suggests that, although metamorphic reequilibration may have reduced the original range of copper isotopes, premetamorphic δ 65Cu characteristics of both mines are generally preserved. The chalcopyrite samples of one deposit might be significantly affected by dissolution during reworking processes after precipitation on the sea floor. Secondary malachite ( δ 65Cu = 2.63–2.97‰) and native copper ( δ 65Cu = 1.43–1.71‰) have higher δ 65Cu values than chalcopyrite from the same deposit. This variation is most reasonably explained in terms of a redox-controlled isotope fractionation at low-temperature conditions during secondary (weathering) processes involving the preferential incorporation of 65Cu in secondary Cu(II) solutions. It is also suggested that significant negative isotopic fractionations accompanying the reduction of Cu (II) solutions to native copper [Cu (0)] occurred.