Simultaneous determination of sulfur isotopes and trace elements in pyrite with a NanoSIMS 50L

Abstract

Pyrite is common sulfide mineral involved in the formation of various ores and hydrothermal and biogenetic activities, and its S isotopic ratios and trace element contents and their spatial distribution have been recorded in the processes of these events. We established simultaneous analyses of ³⁴S/³²S ratios and trace element contents of pyrite using nanometer-scale secondary ion mass spectrometry (NanoSIMS). Firstly, the images of S (³⁴S⁻), As (⁷⁵As⁻), Se (⁸⁰Se⁻), Cu (⁶³Cu³²S⁻), Au (¹⁹⁷Au⁻) and Pb (²⁰⁸Pb³²S⁻) of pyrite were acquired by rastering areas ranging from 20 × 20 μm² to 40 × 40 μm² using a Cs⁺ beam of 7–10 pA with a diameter of ∼250 nm. Then, the ³⁴S/³²S ratios (³²S⁻ measured by using a Faraday cup and ³⁴S⁻ measured by using an electron multiplier) and the concentrations of these trace elements of the individual layers of the zoned pyrite grains were simultaneously measured in spot analysis mode, by rastering the same current over an area of 2 × 2 μm². The ³⁴S/³²S ratios were calibrated for matrix effects with pyrite standard Balmat or Py1117, and the external reproducibility (1SD) is <0.5‰. The concentrations of trace elements were calibrated using the relative sensitivity factors (RSFs) of As, Se, Cu, Au and Pb, which were determined from pyrite grains with zoning layers. The determined RSFs of As, CuS, Au and PbS are 4.43 ± 0.28, 0.36 ± 0.04, 0.18 ± 0.03 and 38.0 ± 15.1, respectively. Pyrite from Lannigou Carlin type gold deposits was analyzed as an example, and the results revealed three main episodes of its formation, with each superposed by micron-width oscillation zonings of trace elements. This method has important potential applications in isotopic and elemental investigation of thin layers of pyrite and other sulfides.