The representativeness of in-situ sulfur isotopic results for sedimentary pyrite: Implications for tracing ore-forming fluids

Abstract

The sulfur isotopic composition of sedimentary pyrite is frequently measured and compared with that of ore stage sulfides to assess the contribution of sulfur from sedimentary pyrite. In-situ analysis methods, notably secondary ion mass spectrometry (SIMS) and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), are both efficient and convenient for determining the sulfur isotopic composition of sedimentary pyrite and have been widely used in recent studies.

However, the representativeness of in-situ analysis results has rarely been investigated. In this study, we investigate this topic by analyzing sedimentary pyrite using SIMS. The samples were collected from the Meishan section in South China. The results show that framboidal and euhedral pyrite tend to have low and elevated δ³⁴S values, respectively. The average δ³⁴S value obtained by SIMS analysis is generally higher (up to ∼15 ‰) than that derived using bulk analysis in which pyrite was extracted using chromium reduction method. Since LA-MC-ICP-MS employs a larger spot size (25–50 μm), and only euhedral pyrite typically has a relatively large diameter, LA-MC-ICP-MS analysis would like yield even higher δ³⁴S values than SIMS. This suggests that in-situ sulfur isotopic results may not be fully representative of the sulfur isotopic composition of sedimentary pyrite.

A compilation of reported in-situ sulfur isotopic compositions of pyrite in the sedimentary host rocks in the “Golden Triangle” area shows that the average δ³⁴S value (∼+9‰) is significantly higher than those obtained through bulk analysis from South China and globally (<−10 ‰), which is consistent with observation at Meishan.

Consequently, we suggest that combining bulk analysis, in which all the sulfur was extracted, with in-situ method, which provides sulfur isotopic compositions with detailed textural and temporal contexts, offers the most robust interpretation when tracing the source of sulfur in the ore-forming fluids.