矿床地球化学国家重点实验室知识库

Nano-scale secondary ion mass spectroscopy (NanoSIMS) is a powerful tool for determining the sulfur isotope composition of micrometer-sized minerals. However, the high spatial resolution of d34S analysis comes at the expense of accuracy, which limits the applicability of NanoSIMS to sulfur isotope analysis.The method proposed here couples high lateral resolution (>1
1 mm2) with high precision. A Faraday cup (FC, for 32S)–electron multiplier (EM, for 34S) detector combination is selected to improve static counting and mitigate the quasi-simultaneous arrival (QSA) effect. The instrument is set and tuned to achieve high transmission (70%) and mass resolution power (MRP  5600 for 32S). A primary beam of 3.5 pA was rastered on a scan area of 1
1 mm2 to acquire the d34S isotope ratio. The d34S ratios of the four analyzed standard samples are consistent with previously reported values, with reproducibility (1 SD)
better than 0.5&. Because EM aging affects the accuracy of the sulfur analysis results, periodically adjusting the maximum value of the peak-height distribution (PHDmax) of the EM during the analysis is necessary. Careful optimization of the height (Z position) at different locations (samples) ensures precise and accurate d34S values. The S isotopic characteristics of framboidal pyrite from the Shuiyingdong Carlin-type Au deposit are determined using this method. The results indicate that the pyrite crystallite comprising framboidal pyrite is rimmed by ore pyrite, supporting the magmatic hydrothermal origin of the deposit. The developed method can be used for analyzing the d34S of pyrite samples with a limited analyzable region (>1
1 mm2) with high precision.