Multiple Sulphur Isotopic Compositions of Archean Records Determined by SHRIMP-SI

Abstract

Sulphur is intimately involved in the environment, life, and mineral deposits. Although an increasing number of studies on the multiple sulphur isotopes of Archean records have been conducted since the discovery of sulphur mass independent fractionation (S-MIF) in the Archean records, many problems still remain. Two important shortcomings of previous studies are the analytical methods and the lack of investigating pyrite generation. As a result, the obtained data are essentially the compositions of mixed sulphur components. This thesis measures the multiple sulphur isotopic compositions of pyrite in situ using the upgraded SHRIMP-SI (Sensitive High Resolution Ion Micro Probe-Stable Isotopes) based on well-establishes pyrite genesis and generation by the combination of BSE (Back Scattered Electrons) imaging and NaOCl solution etching. The first study on the multiple/quadruple sulphur isotopic compositions of pyrite in metasedimentary rocks of 3.2 Ga-2.72 Ga from the Pilbara Craton, Western Australia shows that the highest S-MIF signal of this Archean interval (-2.40 per mil) is larger than previously reported. Additionally, larger D33S is possible and promising in the sulphides with no or poor preservation. The pyrite with more negative D36S/D33S than that of the Archean Reference Array (ARA, -1) display biological or hydrothermal modifications, indicated by deviations from ARA in a trend approximately parallel to the mass dependent biological fractionation or hydrothermal overprinting and/or modifications on the primary diagenetic pyrite, respectively. The second study shows two generations of pyrite in the pyritic laminae/clusters and barite of the 3.49 Ga Dresser Formation. The pyrite grains that are associated with barite and characterized by considerably negative D34S display negative D33S, suggesting sulfate reduction rather than elemental sulphur disproportionation as the earliest sulphur metabolism. However, such correspondences between D33S and sulphur metabolism require confirmation. Additionally, the D36S/D33S of the pyrite grains depleted in 34S exhibits no biological deviation trends from the associated barite, indicating that a biological origin for such pyrite is still equivocal. The third study reveals two generations of pyrite in two typical Archean shale samples, and each generation has two morphologies (disseminated grains and nodules). The results show that the multiple sulphur isotopic compositions of each generation are similar and the signs of D33S are the same, which are independent of pyrite morphology. In the fourth study, the products of desulphidized pyrite and sulphidized metal iron by the released sulphur are both pyrrhotite. The results of quadruple sulphur isotopic compositions show only slight fractionations in D33S and D36S. Due to the sensitivity of D34S on redox states, D33S is a more appropriate tool of tracing the sulphur sources of Archean ore deposits. The fifth study on a typical Neoarchean BIF-hosted gold deposit from the Rio das Velhas Greenstone Belt combines multiple sulphur isotopes with gold and trace elements. The revealed zonings indicate four generations of pyrite. Generation one shows distinct multiple sulphur isotopic compositions (larger D33S) and concentrations of redox-sensitive trace elements (higher contents of Se and Mo) from generation two to four. Generation two is the main gold precipitation stage with the highest gold contents, and is also characterized by oscillatory zoning. The widespread S-MIF in the four generations of pyrite suggests that the hydrothermal fluids can be metamorphic fluids, metamorphic fluids reacted with surrounding rocks, and magmatic fluids mixed with metamorphic fluids and/or interacted with wall rocks. Reduction is the most likely trigger of gold precipitation.