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 δ65Cu (where δ65Cu = [{(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 δ65Cu 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 δ65Cu 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 (δ65Cu = 2.63–2.97‰) and native copper (δ65Cu = 1.43–1.71‰) have higher δ65Cu 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.

You do not currently have access to this article.