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

Graphite 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.


| INTRODUC TI ON
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). Exceptional demand has driven a revolution in graphite exploration, and the need to understand controls on graphite properties (Jara et al., 2019;Scogings, 2015). 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.
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). In the UK, the Lewisian of north-west Scotland includes several supracrustal successions ( Figure 1), most of which contain graphite. Three of the supracrustal successions have been dated at 1.8-2.1 Ga, like commercial deposits elsewhere. However, the graphite occurrences in the Lewisian are very poorly documented. We characterize graphite from six successions, using carbon isotope composition, Raman spectroscopy and microscopy, and investigate: 1. Is all the graphite derived from organic matter in shales, or is some derived from reduction of carbonate carbon in marbles in the supracrustal successions? 2. Is there evidence for mobilization of carbon from beds of carbonrich sediment, i.e. graphite was deposited from a fluid phase rather than simply metamorphism of kerogen?
3. Is the carbon all fully ordered graphite, as required in graphite of commercial quality (e.g. Palosaari et al., 2020), or is some incompletely ordered?
Graphite occurs in the supracrustal rocks in two distinct forms.
Graphitic pelites and schists represent sedimentary rocks in which carbon was deposited as black shales. Graphite also occurs as laminae and nodules within marbles (Figure 2), deposited as limestones.
In limestones, primary reduced organic matter was less likely, and the graphite may instead represent alteration of the limestone, or post-depositional introduction of carbon from shales elsewhere in the sequence. The graphite shows no relationship with major faults or other structures, but at two localities is associated with albitite veining.
The graphite in the six successions occurs as: South Harris: Beds of graphitic schist, coloured silvery due to large flake size (< 2mm; Fettes et al., 1992). Gairloch-Loch Maree: Beds of graphitic schist, coloured grey to black (Park et al., 2001).

Scanning electron microscopy (SEM) was conducted in the Aberdeen
Centre for Electron Microscopy, Analysis and Characterisation Stable carbon isotope analysis was conducted on graphitic samples digested in 10% HCl overnight to remove trace carbonate.
Samples were analysed by standard closed-tube combustion method by reaction in vacuo with 2 g of wire form CuO at 800℃ overnight.
Samples from the Lewisian Complex were supplemented by samples from elsewhere in the North Atlantic region.

| RE SULTS
The graphite normally occurs as microscopic crystals (less than 0.1 mm crystal size), among quartz, feldspar, mica and other grains in a schistose fabric. In some cases the graphite crystal size is greater, up to 5 mm, conveying a silvery colour to the rock. Graphite accounts for organic carbon contents above 1% (Figure 7). In addition, graphite occurs as partial coatings around phenocrysts, especially allanite, in schists and marbles at Gairloch and Tiree ( Figure 3). The phenocrysts are typical of metamorphism in sedimentary rocks, including albite, anorthite, scapolite, dolomite, apatite, allanite and mica (Cartwright, 1992). Albitization is extensive enough to form albitite rock. The metasediments have experienced retrograde metamorphism from 11 kbar and 800℃ to greenschist facies (Cartwright, 1992;Westbrook, 1972), but this would not affect graphite, whose structural order is irreversible (Palosaari et al., 2020). At Vaul, Tiree, black graphitic material also occurs in quartz veins cutting the black metasediments.
Graphite occurs in marbles at Tiree and Scardroy. At Gott, Tiree, marble exhibits intermittent 'laminae' of graphite (Figure 2), which are associated with phenocrysts that exhibit rotation. Both 'laminae' and rotation reflect shearing focussed along the marble layers and the supracrustal rocks in general (Westbrook, 1972). At Scardroy, pellets of graphite about 0.1 mm size occur in the marble, especially where the marble is partially replaced by albite. Albitite crystals at Gott and Vaul also contain graphite in vuggy cavities up to 1 mm size.
Representative Raman spectra are shown in Figure 4. Some spectra show a single G peak for ordered carbon. Several spectra additionally show a D peak for disordered carbon. The D peak shows variable degrees of development. It is minor in Iona and Scardroy.
Spectra from graphitic beds at two localities in the Loch Maree Group, at Loch Gairloch and Kerrysdale, both exhibit the D peak. A range of samples from Tiree ( Figure 5) exhibit the D peak to different degrees, but in the section at Vaul it is strongly developed, and even shows a secondary D2 disorder peak. The G peak positions are typical of graphite (Wopenka & Pasteris, 1993).
The carbon isotope compositions of the graphite have a wide range, which indicates two distinct compositions. Graphite from marble at Tiree and Scardroy has a composition heavier than −10‰, while graphite samples from other localities are lighter than −20‰ (Table 1).  The graphite in schists very probably represents metamorphosed organic matter in the original sediments. However, several occurrences suggest that some graphite was precipitated from a fluid. Vein-hosted graphite at Vaul, Tiree, was clearly a fluid product.

| D ISCUSS I ON
The graphite that defines a laminar fabric in marble at Gott, Tiree, formed during shearing and fluid movement during metamorphism.
The albitite at Tiree is a replacive and vein-forming rock attributed to metamorphism (Cartwright, 1992), and graphite in cavities in the albitite must have been deposited from a fluid phase. Similarly, the pellets of graphite in albitized marble at Scardroy must also be a fluid product. However, all of the graphite, including that deposited from a fluid, occurs within the supracrustal packages, so have a common origin in the metasediments rather than including a mantle component. It would be Graphite I in the petrographic terminology of Dill et al. (2019).
The Raman spectra for supracrustal rocks show that the carbonaceous material is graphitic rather than kerogenous, based on sharply defined peaks and the position of the order peak (Wopenka & Pasteris, 1993). The spectra for some samples, lacking a D peak, indicate graphite that is fully ordered. However, most samples show at least some degree of disorder. The greatest disorder is exhibited by the samples from Gairloch and most samples from Tiree. Both localities are in amphibolite facies rocks, whereas the other localities are in granulite facies rocks (Table 1). The most fully ordered samples are from South Harris, Glenelg, Scardroy and the albitized rocks of Tiree.
The well-ordered graphite from amphibolite facies albitized rocks at Tiree shows that metamorphic grade is not the sole control on ordering. Previous research concluded that graphite from decarbonation is likely to be fully ordered, while graphite derived from organic matter is less ordered (Pasteris, 1999;Wintsch et al., 1981). Nevertheless, the graphite from marble at Gott shows disorder ( Figure 5). Although there is not a simple relationship between ordering and evidence for deposition from fluid in the Lewisian Complex, most of the fluid-derived samples are well-ordered. F I G U R E 6 Composition of isotopic carbon in samples of graphite in schists and marbles in Lewisian supracrustal inliers, and in selected localities in North Atlantic region (localities recorded in Table 1) [Colour figure can be viewed at wileyonlinelibrary.com]

F I G U R E 7
Cross-plot of carbon and sulphur contents (wt%) in Lewisian schists, showing sulphur enrichments relative to modern mean ratio due to pyrite formation. Plot after Berner (1982) [Colour figure can be viewed at wileyonlinelibrary.com] Graphite in sediments from the 2-1-1.8 Ga interval elsewhere, from Greenland, Norway, India and Argentina, similarly exhibits disorder despite metamorphism to granulite and amphibolite facies (Lajoinie et al., 2015;Mishra & Bernhardt, 2009;Palosaari et al., 2016;Papineau et al., 2009;Rosing-Schow et al., 2017). In contrast, graphite veins, concentrated from Palaeoproterozoic sediments so they can be exploited as graphite ore deposits, typically show complete ordering. High ordering is evident in the mined and prospective deposits in Labrador, Greenland, Norway and Sweden  Baffin Island, Canada (Belley et al., 2017) to West Greenland (Garde, 1978), Finland (Lehtinen, 2015), Tajikistan (Sorokina et al., 2015) and China (Yang et al., 2019). We have measured the carbon isotope composition of graphite in marble from Kimmirut, Baffin Island, and Pargas, Finland, at −8.1 and −2.6‰ respectively ( Figure 6). These heavy compositions indicate, like the graphite in marble from Scotland, an origin in decarbonation of the marble.
We note that the Palaeoproterozoic graphitic marbles in Baffin Island, Tajikistan and China all host gem quality corundum (ruby and sapphire).
A further aspect of Palaeoproterozoic graphite is a consistent occurrence in albitite veins, as in Scotland. Examples include deposits in Brazil (Sirqueira et al., 2018), Russia (Sorokhtina et al., 2010) and India (Mukherjee et al., 2016), which emphasize graphite precipitation from fluids was widespread.

| CON CLUS IONS
Graphite occurs in numerous supracrustal successions within the Lewisian Complex. Petrographic, isotopic and spectroscopic studies show that: (i) The graphite in schists is derived from sedimentary organic matter, while graphite in marbles is derived from decarbonation of limestone.
(ii) In addition, there is evidence of graphite deposition from migrated (i.e. fluid) carbon, in cross-cutting veins and cavities.
(iii) Some of the graphite is fully ordered, especially where it was deposited from a fluid. However, much of the graphite is not completely ordered. Graphite examined from Gairloch and Vaul, Tiree, shows disorder, despite metamorphism to amphibolite facies.
(iv) Comparison of the data from Scottish graphite with that of exploitable deposits in the North Atlantic region shows that ordered graphite that may be commercially valuable is more likely to occur in veins, which should guide exploration.

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.