A new potential natural chalcopyrite reference material for in situ copper isotope microanalysis

Abstract

Copper sulfide ore is one of the most important sources of Cu in nature. In situ Cu isotope analysis of sulfides is critical for providing useful information on the sources of Cu and tracking the process of sulfide precipitation. The accuracy of in situ Cu isotope analysis relies heavily on matrix-matched reference materials to correct the instrumental mass bias. In this study, a natural chalcopyrite (TQ-Ccp) from the Tianqiao Pb–Zn deposit (Guizhou Province, China) was evaluated as a potential reference material for Cu isotope microanalysis. Major element mapping and determination by electron probe microanalysis (EPMA) demonstrated that there was no internal growth or zonation within the TQ-Ccp chalcopyrite grains. The Cu isotope homogeneity of the chalcopyrite sample was tested by micro-drill sampling and solution multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) as well as by laser ablation MC-ICP-MS (LA-MC-ICP-MS). Thirty-two chalcopyrite powders were micro-drilled randomly from the whole hand specimen and from four fragments of chalcopyrite prepared in an epoxy mount, and measured by solution MC-ICP-MS. The dissolved chalcopyrite solutions processed with and without column chromatography yielded a mean δ⁶⁵Cu_NIST976 of 0.05 ± 0.03‰ (2SD, n = 20) and 0.04 ± 0.03‰ (2SD, n = 32), respectively, suggesting Cu isotope homogeneity at the whole specimen scale. The Cu isotope homogeneity at the micro-scale was further verified by LA-MC-ICP-MS analyses of the four chalcopyrite fragments in the epoxy mount. In summary, TQ-Ccp chalcopyrite is potentially a suitable reference material for in situ Cu isotope microanalysis, whose recommended δ⁶⁵Cu_NIST976 value is 0.04 ± 0.03‰ (2SD, n = 52), as determined by solution MC-ICP-MS analyses.