Abundant sulphate in the Neoproterozoic ocean: implications of constant δ34S of barite in the Aberfeldy SEDEX deposits, Scottish Dalradian
Norman R. Moles, Adrian J. Boyce, Anthony E. Fallick, 2015. "Abundant sulphate in the Neoproterozoic ocean: implications of constant δ34S of barite in the Aberfeldy SEDEX deposits, Scottish Dalradian", Ore Deposits in an Evolving Earth, G. R. T. Jenkin, P. A. J. Lusty, I. Mcdonald, M. P. Smith, A. J. Boyce, J. J. Wilkinson
Download citation file:
The timing and extent of ocean oxygenation is controversial. Proterozoic sulphur isotope datasets often show marked fluctuations over small stratigraphic intervals, suggesting that oceanic sulphate concentrations were much lower than modern values. A large accumulation of Neoproterozoic sulphate (>8 million tonnes preserved), as stratiform barite rock, is located in the Grampian Highlands near Aberfeldy. Diagenetic/metamorphic alteration has caused pronounced δ34S variations near bed margins. This aside, barite throughout the deposits shows a narrow range in δ34S, mean 36±1.5‰. We infer that this is representative of contemporaneous seawater sulphate, and that δ34Sseawater was constant during deposition of a stratigraphical thickness >250 m of mostly fine-grained clastic sediments. Uniformity of δ34Sseawater during barite precipitation, even in thick (>10 m) beds and with the co-occurrence of abundant sulphides incorporating bacteriogenically reduced sulphur, implies no limit to availability of seawater sulphate during hydrothermal exhalative events. Our data, combined with previous δ34S research on Dalradian metasediments, suggest a stability, abundance and constancy of ocean sulphate in the Neoproterozoic. This contrasts with isotopic data using trace sulphate in limestones. It appears that, around the time of the Marinoan glaciation (c. 635 Ma), the ocean, although stratified at least locally, comprised a substantial reservoir of sulphate-bearing oxygenated seawater.
Figures & Tables
Ore deposits form by a variety of natural processes that concentrate elements into a volume that can be economically mined. Their type, character and abundance reflect the environment in which they formed and thus they preserve key evidence for the evolution of magmatic and tectonic processes, the state of the atmosphere and hydrosphere, and the evolution of life over geological time. This volume presents 13 papers on topical subjects in ore deposit research viewed in the context of Earth evolution. These diverse, yet interlinked, papers cover topics including: controls on the temporal and spatial distribution of ore deposits; the sources of fluid, gold and other components of orogenic gold deposits; the degree of oxygenation in the Neoproterozoic ocean; bacterial immobilization of gold in the semi-arid near-surface environment; and mineral resources for the future, including issues of resource estimation, sustainability of supply and the criticality of certain elements to society.