New potential pyrrhotite and pentlandite reference materials for sulfur and iron isotope microanalysis

Abstract

Pyrrhotite and pentlandite are the most common Fe sulfide minerals in magmatic ore deposits and meteorites. Multiple S isotopes pairing with Fe isotopes of bulk Fe sulfides have proven to be useful tracers to constrain the formation and evolution of magmatic ore deposits. However, pyrrhotite coexists with pentlandite and other sulfides as intergrowth textures in most cases. This makes the isotope results obtained from bulk samples represent a mixed signal. High spatial resolution S and Fe isotope analytical techniques using secondary ion mass spectrometry (SIMS) and laser ablation-multiple-collector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) provide powerful tools to resolve isotopic zoning and small scale isotopic heterogeneities of Fe sulfide minerals. Matrix-matched Fe sulfide reference materials (RMs) are critical for accurate micro-beam measurements, but no natural RMs for both S–Fe isotopically homogeneous pyrrhotite and pentlandite are available. In this study, natural pyrrhotite (JC-Po) and pentlandite (JC-Pn) from the Jinchuan Ni–Cu–PGE massive sulfide ores (China) are evaluated as S and Fe isotope RMs by multiple methods. The major elements are homogeneous as verified by electron microprobe analyses (EMPA). The results of SIMS for the sulfur isotope and LA-MC-ICPMS for the iron isotope prove the homogeneity of S and Fe isotopes, respectively. From in situ microanalysis, the standard deviation of sulfur isotope composition is 0.27‰ (2SD, n = 204) for JC-Po and 0.19‰ (2SD, n = 179) for JC-Pn. The standard deviation of iron isotope composition is 0.33‰ (2SD, n = 129) for JC-Po and 0.46‰ (2SD, n = 186) for JC-Pn. Isotope ratio mass spectrometry (IRMS) determined that the best recommended δ³⁴S and δ³³S values are 0.06 ± 0.33‰ and 0.03 ± 0.17‰ (2SD, n = 13) for JC-Po, and −0.09 ± 0.15‰ and −0.04 ± 0.08‰ (2SD, n = 13) for JC-Pn, respectively. Based on solution MC-ICPMS analyses, the δ⁵⁶Fe value of −0.34 ± 0.07‰ (2SD, n = 9) and the δ⁵⁷Fe value of −0.50 ± 0.11‰ (2SD, n = 9) for JC-Po, and the δ⁵⁶Fe value of 1.47 ± 0.04‰ (2SD, n = 4) and the δ⁵⁷Fe value of 2.18 ± 0.05‰ (2SD, n = 4) for JC-Pn are provided as recommended. In summary, JC-Po pyrrhotite and JC-Pn pentlandite reference materials are suitable for in situ S and Fe isotope microanalysis and are expected to be used for tracking the formation processes of Fe sulfide minerals in magmatic ore deposits and meteorites soon.