Multi-sulfur isotopes of Neoarchean pyrites from the Yilgarn Craton, Western Australia

Abstract

This thesis brings together four individual studies of the geochemistry of the pyrites from the ~2.7 Ga Eastern Goldfields Superterrane Greenstone Belt (EGST), Yilgarn Craton, Western Australia. The data are used to provide a synthesis of the sulfur cycles in the Neoarchean, with a particular focus on the nature and origin of sulfur species in the (near) surface environment before the pervasive rise in oxygen. Multi-sulfur isotopes were analyzed for pyrites from relatively deep-water marine cherts and shales that are interbedded with the ~2.7 Ga EGST greenstone lava flows. The observed D33S anomalies for the EGST pyrites at or above the mantle value, implying a component of atmospheric elemental sulfur, and little or no contribution from negative D33S seawater sulfate. Variations in D33S and d34S are interpreted to be due to changes in the relative contributions of photolytic S0 and hydrothermal H2S to the growing pyrite crystals. Detailed time-series S isotopic trend show that D33S changes are dominated by local variations in the hydrothermal flux through seafloor vents rather than global changes in the atmospheric compositions. The marked global increase in D33S in Neoarchean sedimentary pyrites is attributed to the emergence of several cratons above sea level, accompanied by synchronous subaerial felsic volcanism, at ~2,650 Ma. The ~2.7 Ga Kapai Slate contains pyrite corona nodules, with distinctive cores and mantles. The cores are enriched in highly compatible trace elements (Ni, Ag, Bi, etc.), but depleted in incompatible elements (Mo and Tl), relative to the mantles. The striking feature of the data is the remarkable linear correlations between the compatible elements (R > 0.95) and their inverse correlation with incompatible Mo. A two-stage model is proposed for the formation of nodules. The cores crystallized slowly from stagnant pore fluid prior to the eruption of the overlying Paringa Basalt, whereas the mantles crystallized rapidly in conjunction with the eruption of the basalt. Deposition of the basalt compacted the unconsolidated Kapai clays, driving out the pore fluid, which had a profound impact on the geochemistry of nodule mantles. The relative compatibilities of the analyzed trace elements are also estimated, which provide a new constraint on the partition coefficients of the trace elements between Archean ocean water and sedimentary pyrite. A detailed quadruple S isotopic study were conducted for the Kapai Slate pyrite nodules. Statistical methods are applied to show that the best-fit lines for the S isotopic compositions of these pyrites have D36S/D33S gradients varying between -0.7 and -1.1, in agreement with most other studies of Archean sedimentary pyrites. Two of these best-fit lines, however, have positive intercepts on both the D36S and D33S axes. The observed linear relationship between D36S and D33S preludes the mixing between different atmospheric S reservoirs. It can, however, be modelled using a combination of at least two reactions, which simultaneously fractionate SO2 in a single atmospheric reservoir by utilizing different light bandwidths. A mathematical model is provided to constrain the S-MIF contribution from each of the likely photochemical reactions. It is suggested that this process can be called simultaneous fractionation. Finally, multi-sulfur isotopes were used to trace the sources of sulfur in the ~2.7 Ga Teutonic Bore Volcanic Complex VMS deposits to evaluate the ore formation model, and to quantitatively estimate the relative contribution of different sulfur reservoirs. The magmatic input, other than wallrock leaching, dominates the production of ore fluids. Most important, the quantity of seawater sulfur in the whole sulfur budget may be related to the size of the VMS deposits. The multi-sulfur isotope is shown to have great potential for investigating the genesis of metal sulfides mineral deposits in future economic geology studies. Show more