Graphite from Palaeoproterozoic enhanced carbon burial, and its metallogenic legacy

John Parnell,Adrian J Boyce,Connor Brolly

doi:10.1017/s0016756821000583

John Parnell, Adrian J Boyce + Show 1 more

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https://doi.org/10.1017/s0016756821000583

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Abstract

AbstractThe episode of widespread organic carbon deposition marked by peak black shale sedimentation during the Palaeoproterozoic is also reflected in exceptionally abundant graphite deposits of this age. Worldwide anoxic/euxinic sediments were preserved as a deep crustal reservoir of both organic carbon, and sulphur in accompanying pyrite, both commonly >1 wt %. The carbon- and sulphur-rich Palaeoproterozoic crust interacted with mafic magma to cause Ni–Co–Cu–PGE mineralization over the next billion years, and much uranium currently produced is from Mesoproterozoic deposits nucleated upon older Palaeoproterozoic graphite. Palaeoproterozoic carbon deposition has thus left a unique legacy of both graphite deposits and long-term ore deposition.

Highlights

The Palaeoproterozoic Lomagundi–Jatuli Event was a major anomaly in the global cycling of carbon which records a positive excursion in δ13C composition in carbonates worldwide (Melezhik et al 2007)
The data will confirm if the Palaeoproterozoic was a period of anomalous carbon burial, if the carbon was derived from sedimentary organic matter rather than carbonic fluids during metamorphism, and if the carbon caused burial of sulphur as in younger sediments
The graphitic beds have a bedding-parallel form that suggests that they represent layers in the original succession, for example in Palaeoproterozoic successions in South Australia (Keeling, 2017) and North Norway (Gautneb et al 2020)

Summary

Introduction

The Palaeoproterozoic Lomagundi–Jatuli Event was a major anomaly in the global cycling of carbon which records a positive excursion in δ13C composition in carbonates worldwide (Melezhik et al 2007) This event was closely followed by the deposition of extensive black (carbon-rich) shales, the Shunga Event2.0 Ga, on several continents (Condie et al 2001; Strauss et al 2013; Martin et al 2015). The abundance of carbon may reflect intense weathering of the continents following the Great Oxidation Event, and high productivity in the nutrient-rich oceans (Melezhik et al 2013) The identification of this episode as the peak of black shale sedimentation in the Precambrian, from 2.0 to 1.85 Ga (Condie et al 2001), is based on data from Australia (57 %), North America (37 %) and Russia (6 %). The data will confirm if the Palaeoproterozoic was a period of anomalous carbon burial, if the carbon was derived from sedimentary organic matter rather than carbonic fluids during metamorphism, and if the carbon caused burial of sulphur as in younger sediments

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