Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway)

Abstract

Graphite formation in the deep crust during granulite facies metamorphism is documented in the Proterozoic gneisses of the Lofoten–Vesterålen Complex, northern Norway. Graphite schist is hosted in banded gneisses dominated by orthopyroxene-bearing quartzofeldspathic gneiss, including marble, calcsilicate rocks and amphibolite. The schist has major graphite (<modality 39%), quartz, plagioclase, pyroxenes, biotite (Mg# = 0.67–0.91; Ti < 0.66 a.p.f.u.) and K-feldspar/perthite. Pyroxene is orthopyroxene (En69–74) and/or clinopyroxene (En33–53Fs1–14Wo44–53); graphite occurs in assemblage with metamorphic orthopyroxene. Phase diagram modelling (plagioclase + orthopyroxene (Mg#-ratio = 0.74) + biotite + quartz + rutile + ilmenite + graphite-assemblage) constrains pressure-temperature conditions of 810–835 °C and 0.73–0.77 GPa; Zr-in-rutile thermometry 726–854 °C. COH fluids stabilise graphite and orthopyroxene; the high Mg#-ratio of biotite and pyroxenes, and apatite Cl < 2 a.p.f.u., indicate the importance of fluids during metamorphism. Stable isotopic δ13Cgraphite in the graphite schist is −38 to −17‰; δ13Ccalcite of marbles +3‰ to +10‰. Samples with both graphite and calcite present give lighter values for δ13Ccalcite = −8.7‰ to −9.5‰ and heavier values for δ13Cgraphite = −11.5‰ to −8.9‰. δ18Ocalcite for marble shows lighter values, ranging from −15.4‰ to −7.5‰. We interpret the graphite origin as organic carbon accumulated in sediments, while isotopic exchange between graphite and calcite reflects metamorphic and hydrothermal re-equilibration.

A Case History of Graphite Exploration in North Norway Integrating Various Geophysical Surveys

“…The graphite-bearing rocks in Vesterålen occur in sequences belonging to a precambrian domain comprising Lofoten, Vesterålen, and the western islands of Troms County (Figure 2). [5]).…”

“…However, only a few deposits are exposed to the surface as flake graphite deposits, which are not of economic importance [4]. All Norwegian graphite is of sedimentary origin; however, hydrothermal processes during subsequent stages of high-degree metamorphism have also contributed to its formation [5].…”

“…These geophysical surveys and follow-up work led to the discovery of many new graphite mineralization sites. Part of the work was published in [5,6,13,14]. In the early fifties, all the known graphite occurrences in Northern Norway were investigated as co-occurrences with uranium [8].…”

“…These geophysical surveys and follow-up work led to the discovery of many new graphite mineralization sites. Part of the work was published in [5,6,13,14].…”