Mixed metamorphic and fluid graphite deposition in Palaeoproterozoic supracrustal rocks of the Lewisian Complex, NW Scotland

John Parnell,Connor Brolly,Adrian J Boyce

doi:10.1111/ter.12546

Abstract

AbstractGraphite deposits may form alternatively by metamorphism of sedimentary rocks and from fluids. Both types occur in supracrustal successions within the Lewisian Complex of Northwest Scotland, and similarly in Palaeoproterozoic supracrustal rocks across the North Atlantic region in Canada, Greenland and Scandinavia. Carbon isotope compositions show that the graphite in Scotland had a mixed origin from metamorphism of sedimentary organic matter (schists) and the decarbonation of limestones (marbles). Raman spectroscopy shows that most of the graphite in Scotland exhibits some structural disorder, unlike the complete order in graphite vein ore deposits across the region. Exceptionally, where graphite was precipitated from fluid, in albitized rock in Tiree and Scardroy, it is fully ordered. While organic matter may survive granulite facies metamorphism without being transformed to fully ordered graphite, it can yield commercially more valuable ordered graphite when mobilized in a fluid.

Highlights

Graphite is a critical commodity because of the very high potential of graphite and graphene in future technologies, including its use in electric vehicles (Gautneb et al, 2019; Helmers, 2015; Wang et al, 2018)
Most graphite resources occur in Precambrian rocks, reflecting the high incidence of black shales within the Precambrian (Condie et al, 2001) and the metamorphism of organic matter to graphite in older rocks
Graphite occurs in numerous supracrustal successions within the Lewisian Complex

Summary

Introduction

Graphite is a critical commodity because of the very high potential of graphite and graphene in future technologies, including its use in electric vehicles (Gautneb et al, 2019; Helmers, 2015; Wang et al, 2018). Prospective graphite deposits have been explored in several parts of the North Atlantic region, including Labrador, Canada (Saglek Bay), Greenland (Amitsoq, Akuliaruseq), Norway (Skaland) and Sweden (Woxna). Each of these deposits was deposited during the period 1.8–2.1 Ga (Bergman, 2018; Meyer & Dean, 1988; Palosaari et al, 2016; Thrane & Kalvig, 2019).

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Discussion

Conclusion