Carbon in Mineralised Plutons

The Paleoproterozoic schists of the Leverburgh Belt, South Harris and the Neoproterozoic carbonaceous metasediments of the Dalradian Supergroup were deposited during the two most significant periods of black shale deposition globally. Hosted within these metasedimentary rocks are graphite-bearing mineralised plutons, formed during orogenic events. The assimilation of carbonaceous lithologies during magmatic pluton emplacement is a commonly recognised mechanism in the formation of many metal and semi-metal-enriched deposits. Graphite mineralisation as a result of carbon assimilation is a feature often associated with these mineral deposits, though the source of the carbon and any associated metal deposits is not always understood. In this study, carbon and sulphur isotope analyses demonstrate that the crustal assimilation of the Paleoproterozoic host rocks took place during magmatic emplacement and provided the source of carbon and sulphur during mineralisation of the plutons. Minor enrichments of trace elements are present in the South Harris plutonic lithologies, indicating that mobilisation and enrichment occurred during assimilation of the schists. Petrographic and elemental analysis of a Dalradian-hosted Ordovician pluton indicates a similar but more substantial enrichment of these trace elements during crustal assimilation. The timing and depth of assimilation appear to play key roles in the extent of graphite and associated trace element enrichments.