Abstract Oxygenation of the Proterozoic atmosphere caused the progressive build-up of dissolved sulphate on the continents and in marine environments. However, oxygen levels in the Proterozoic were low enough to allow the early burial of biological material into low redox potential environments where permineralization and the authigenic replacement of organic material, including micro-organisms, occurred by a range of minerals. Consequently, microbial sulphate reduction caused the widespread degradation of organic matter and, where iron was available, the precipitation of pyrite. By contrast, where sulphate levels were low, early preservation by other minerals (e.g. phosphate or silica) could be excellent. We show, using two Proterozoic lake sequences with low and high sulphate chemistries, but with otherwise similar characteristics, that microbial sulphate reduction caused a profound loss of morphological detail and diversity within preserved microfossils. The results could imply that there is a significant bias in the Proterozoic fossil record towards low sulphate environments, which were in reality relatively scarce. Gold Open Access : This article is published under the terms of the CC-BY 3.0 license.

Abstract Ore deposits form by a variety of natural processes that concentrate elements into a small 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 in 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.

Abstract 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 δ 34 S variations near bed margins. This aside, barite throughout the deposits shows a narrow range in δ 34 S, mean 36±1.5‰. We infer that this is representative of contemporaneous seawater sulphate, and that δ 34 S seawater was constant during deposition of a stratigraphical thickness >250 m of mostly fine-grained clastic sediments. Uniformity of δ 34 S seawater 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 δ 34 S 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.