In situ sulfur isotopes in texturally characterized pyrite, pyrrhotite and chalcopyrite by LA-ICPMS/MS for application in sulfide bearing systems

The demand for in situ sulfur isotopes analysis has increased in recent years. Yet, no rapid method for collecting high-resolution texturally-constrained data with minimal sample preparation time has been available. In this study, we develop an analytical technique using LA-ICP-MS/MS to rapidly acquire in situ δ³⁴S measurements in thin section-hosted sulfides by removing interferences on the ³²S and ³⁴S mass via mass shifting. This new method required the development of matrix-matched reference material for pyrite, pyrrhotite, and chalcopyrite. New reference materials have been petrographically and chemically characterized, prior to being analysed by both high-precision bulk SF₆ isotope ratio mass spectrometry (IRMS) and in situ secondary ion mass spectrometry (SIMS) S isotope methods. The new reference materials include Iberia pyrite (δ³⁴S = 8.9 ± 1.1 ‰; Δ³³S = −0.02 ± 0.06 ‰), Sullivan pyrite (δ³⁴S = 6.0 ± 0.6 ‰; Δ³³S = −0.05 ± 0.05 ‰), 1869 chalcopyrite (δ³⁴S = 9.4 ± 0.2 ‰; Δ³³S = −0.03 ± 0.04 ‰), Pierre pyrrhotite (δ³⁴S = 4.7 ± 0.2 ‰; Δ³³S = −0.01 ± 0.06 ‰), and Montpellier pyrrhotite (δ³⁴S = 3.3 ± 0.2 ‰; Δ³³S = 0.18 ± 0.07 ‰). Using the developed method, the reproducibility of the primary reference material during each analytical run approximates ±1.5–2.0 ‰. The method was tested on sulfides previously measured by SIMS, with δ³⁴S values yielded being within-error of these, confirming the robustness of the technique. We apply the method to case study pyrite-enargite chalcopyrite veins from Cerro de Pasco deposit, Peru, a porphyry related epithermal polymetallic deposit. The results yield heavier sulfur isotope signatures in trace element (TE)-enriched pyrite domains (δ³⁴S = 2.2 ± 1.1 ‰) and lighter values (δ³⁴S = −1.2 ± 1.7 ‰) in TE-depleted domains. The grain-scale S isotope variation is coupled to microstructure and TE internal zonation, and is interpreted to indicate successive pulses of condensed magmatic vapour. This study opens new analytical opportunities for rapid in situ sulfur isotope analysis in petrographically-characterized thin sections in various sulfide-bearing geological systems.