Amitsoq Graphite deposit, South Greenland: petrological and geochemical study of competing carbon and hydrogen storage processes

The cycling of carbon and hydrogen in the Earth’s lithosphere, including their geological storage, is complex. The Amitsoq graphite deposit provides an outstanding example of such processes, showing possible evidence of the interaction between hydrogen and graphite, a process that generates methane, an aspect that has to be taken into account while considering carbon and hydrogen storage processes. The deposit is located in the Nanortalik region, South Greenland. It is one of the richest graphite deposits worldwide, with an estimate of more than 23Mt at ∼20% graphitic carbon. The deposit was mined in the early 20th century, yet its origin and formation conditions are still unknown. The deposit formed in the context of the Ketilidian orogeny, during the Paleoproterozoic, a geological period when the majority of the world's richest graphite deposits formed. The ore is hosted in amphibolite-facies psammite, which represents part of the fore-arc of the Ketilidian subduction zone and was formed by the erosion of the nearby Julianehåb batholith. Shortly after the orogeny, the area was intruded by granitoid plutons. These factors led to high-temperature, low-pressure metamorphism with local anatexis. Folding observed in thin sections from the ore allows to infer that graphite is either pre- or syn-kinematic with respect to the D1-D2 metamorphic events described in literature. The ore is a graphite-sulphide schist in which graphite is present as millimetre scale, elongate flakes, present in stacks, and cryptocrystalline agglomerates. Graphite flakes are variably folded and interleaved with biotite and pyrrhotite. δ13C analysis of graphite yields an average of -32‰, compatible with biotic sources. δ34S analyses yielded values of ∼3‰, overlapping with different sources. Thermometry studies yield temperatures of ∼500 °C and ∼575 °C ± 50 °C. Several models are proposed to explain the deposition of graphite under assumption derived from literature and thermometry.