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Hydrothermal flake graphite mineralisation in Paleoproterozoic rocks of south-east Greenland

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Abstract

Flake graphite mineralisation is hosted in the Kuummiut Terrane of the Paleoproterozoic Nagssugtoqidian Orogen, south-east Greenland. Eclogite-facies peak-metamorphic assemblages record temperatures of 640–830 °C and pressures of 22–25 kbar, and are retrogressed in the high-pressure amphibolite-facies during ca. 1870–1820 Ma. Graphite occurs as lenses along cleavage planes in breccia and as garnet-quartz-graphite veins in various metamorphic host rocks in the Tasiilaq area at Auppaluttoq, Kangikajik, and Nuuk-Ilinnera. Graphite contents reach >30 vol% in 0.2–4 × 20 m wide semi-massive mineralisation (Auppaluttoq, Kangikajik). Supergene alteration formed 1- to 2-m-thick and up to a 2.5 × 2.5 km wide loose limonitic gravel containing graphite flakes in places. The flake size ranges from 1 to 6 mm in diameter with an average of ~3 mm. Liberation efficiency is at minimum 60%. Hydrothermal fluids at ~600 °C, transporting carbon as CO2 and CH4, formed the mineralisation commonly hosted by shear zones, which acted as pathways for the mineralising fluids. The hydrothermal alteration assemblage is quartz-biotite-grunerite-edenite-pargasite-K-feldspar-titanite. The δ13C values of graphite, varying from −30 to −18‰ PDB, indicate that the carbon was derived from organic matter most likely from metasedimentary sources. Devolatilisation of marble may have contributed a minor amount of carbon by fluid mixing. Precipitation of graphite involved retrograde hydration reactions, depleting the fluid in H2O and causing graphite saturation. Although the high-grade mineralisation is small, it represents an excellent example of hydrothermal mineralisation in an eclogite-facies terrane during retrograde exhumation.

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Acknowledgements

The Ministry of Mineral Resources (MMR) and GEUS are gratefully acknowledged for financial support of the field and analytical work. This contribution is a product of collaboration between GEUS, the Centre for Exploration Targeting (CET) at the University of Western Australia and the ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS). This is contribution 846 from the CCFS (www.CCFS.mq.edu.au). Marco Fiorentini also acknowledges support from the Australian Research Council through Linkage Project LP120100668 and the Future Fellowship Scheme (FT110100241). Nanna acknowledges Selskabet for Arktisk Forskning og Teknologi (SAFT). We thank Troels F.D. Nielsen for unpublished information about the Nuuk-Ilinnera locality, Kristine Thrane for unpublished dates on diorite dikes in the area, Trygvi Bech Árting for unpublished information on the geochronology of the AIC and discussions and Helene Almind for the technical help during the XRD analyses. This paper is the result of N. Rosing-Schow’s MSc project. We thank the editor Bernd Lehmann and the reviewers including Gilpin Robinson and Karen Kelley for useful comments, which helped improve the manuscript.

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Correspondence to Nanna Rosing-Schow.

Additional information

Editorial handling: K. Kelley

Electronic supplementary material

ESM 1

Sample list with graphitic carbon content; list of carbon isotope values measured in the graphite and marble samples; and grain size distribution curve of two samples on which liberation analysis were done. (DOCX 71 kb)

ESM 2

Photographs of: a garnet-quartz vein (coarse light coloured feature cutting gneiss) in hand specimen and transmitted light photomicrograph. The graphite is growing on the garnet in spherulitic patterns. Grt = garnet; gr = graphite; gru = grunerite; hbl = hornblende; b semi-massive graphite mineralisation at locality B; and c a shear zone within an amphibolite, where graphite is forming anastomosing structures draped around fragments of amphibolite. (GIF 393 kb)

High Resolution Image (TIFF 10316 kb)

ESM 3

Diagram showing carbon isotope variations through time in carbonates (black) and organic matter (grey shaded area) (modified after Schidlowski 2001). Two marble samples from Charcot Bjerge and one from Nuuk-Ilinnera have been plotted in the diagram. The Nuuk-Ilinnera marble plots within the carbonate isotope spread, whereas the Charcot Bjerge marble is more depleted in δ13C. (GIF 27 kb)

High Resolution Image (TIFF 3546 kb)

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Rosing-Schow, N., Bagas, L., Kolb, J. et al. Hydrothermal flake graphite mineralisation in Paleoproterozoic rocks of south-east Greenland. Miner Deposita 52, 769–789 (2017). https://doi.org/10.1007/s00126-016-0701-9

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Keywords

  • Hydrothermal graphite
  • Flake graphite
  • Carbon isotopes
  • Raman spectroscopy
  • Greenland