Archean sulfur reservoirs of the Kaapvaal Craton

Abstract

Archean sulfur is recognized by anomalous isotopic signatures where D36S/D33S* = ~0.90. These signatures have been attributed to photolysis of SO2 in a low oxygen atmosphere and disappear from the rock concurrent with the timing of the Great Oxidation Event and the apparent rise in atmospheric oxygen. The main objective of this thesis is to examine the isotopic composition of sulfide minerals from different depositional environments and explore the mechanisms which preserve the anomalous signatures. The South African Witwatersrand and Pongola Supergroups are the primary study areas. Both are Mesoarchean in age and contain well-preserved sedimentary sequences. They were chosen as they contain a wide variety of lithostratigraphic occurrences of pyrite; here samples were collected from diamictites, paleosols and fluvial and marine clastic sedimentary rocks. The texture and morphology of sulfide grains were documented via reflected light microscopy, etching and backscattered electron imaging. This was combined with LA-ICP-MS trace element measurements to determine paragenesis. The sulfur isotope composition was defined through in-situ four isotope analysis using the SHRIMP-SI. Pyrite from diamictites returned mostly negative D33S and D36S values, in the ranges of -0.43 - 0.09 per mil (pm) and -1.95 - 0.15 pm, respectively. The d34S** values showed a range of -6.09 - 11.7 pm. Deviation in D36S from the atmospheric D36S/D33S array and variation in d34S is consistent with microbial sulphate reduction of atmospheric sulphate. The restricted d34S values observed in one sample set may represent input of mass-dependent crustal sulfur. Paleosol-hosted sulfide minerals had negative or near-zero D33S values, with a range of -0.50 - 0.17 pm, and near-zero D36S values, with a range of -0.99 - 0.52 pm. The d34S values showed a range of -8.88 - 6.87 pm. In samples where both D33S and D36S were ~0 pm, the sulfur is likely magmatic sulfur inherited from the igneous parent rocks. Samples with small negative D33S values likely preserve a mixture of atmospheric sulfate and inherited magmatic sulfur. Pyrite from fluvial rocks typically returned negative D33S and D36S values, with the majority of data falling in the ranges of -0.72 - 0.10 pm for D33S and -0.98 - 0.34 pm for D36S. The d34S values typically fell within the range of -3.21 - 9.65 pm. The muted negative D33S values and small d34S values are consistent with a mixture of atmospheric sulfate and mass-dependent crustal sulfur. The consist deviation in D36S from the atmospheric D36S/D33S array suggests microbial sulfate reduction. Shallow marine samples returned variable isotopic signatures. Several samples showed D36S/D33S = ~6.9, consistent with microbial sulfate reduction. These samples showed D33S values of -0.18 - 0.51 pm, D36S values of -2.05 - 0.02 pm and d34S values of -15.8 - 8.53 pm. Other samples showed D36S/D33S = ~-2, with D33S values of 0.11 - 0.80 pm and D36S values of -2.04 - -0.25 pm. The d34S values showed a range of -1.17 - 7.74 pm. Data from individual samples were observed to show consistent D33S but variable D36S, forming steep arrays extending downwards from the atmospheric D36S/D33S array. This was interpreted to represent microbial fractionation of atmospheric elemental sulfur. These results indicate that terrestrial and marine depositional environments preserve different isotopic signatures. Terrestrial environments favour preservation of atmospheric sulfate and marine environments favour preservation of atmospheric elemental sulfur. Within both environments, microbial processes were involved in preservation of the atmospheric sulfur species. Atmospheric signatures in terrestrial environments are muted, which is attributed to mixing of atmospheric and crustal sulfur. Signatures are also muted in marine samples, which may also be due to mixing, or possibly some other localized environmental effect. *D = Capital delta **d = lowercase delta