Multiple sulfur isotopes in post-Archean deposits as a potential tracer for fluid mixing processes: An example from an iron oxide–copper–gold (IOCG) deposit in southern Peru

Abstract

Sulfur has multiple stable isotopes (32S, 33S, 34S, and 36S), but little research has been conducted to investigate the information that low abundance sulfur isotope ratios (i.e., 33S/32S and 36S/32S) may carry on ore-forming processes in post-Archean deposits. In this study, the Cretaceous Mina Justa iron oxide–copper–gold deposit, was selected to examine the capability of multiple sulfur isotopes to trace ore-forming processes in post-Archean mineral deposits. In situ secondary ion mass spectrometry was applied to the pyrite of the magnetite–pyrite stage in the deposit to characterize the spatial variation of multiple sulfur isotopes at the microscale. This revealed a clear co-variation trend between δ34S (= 1000 × [(34S/32S)sample/(34S/32S)V-CDT − 1]) and Δ33S (= δ33S − 1000 × [(1 + δ34S/1000)0.515 – 1]). Simple modeling suggests that the triple sulfur isotope data recorded is best explained by mixing magmatic and externally derived (e.g., basinal brine) fluids. The contribution of external fluids is also supported by the trace element distribution patterns in pyrite, as revealed by laser ablation-inductively coupled plasma-mass spectrometry. This study is the first attempt to use multiple sulfur isotopes to trace ore-forming processes in post-Archean deposits and demonstrates that multiple sulfur isotopes are a faithful recorder of ore-forming processes at the microscale, especially when multiple fluids mix.