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

• Published in: Mineralium deposita 2017-06, Vol.52 (5), p.769-789
• Main Authors: Rosing-Schow, Nanna, Bagas, Leon, Kolb, Jochen, Balić-Žunić, Tonči, Korte, Christoph, Fiorentini, Marco L.
• Format: Article
• Language: English
• Subjects: Amphibolites; Biotite; Breccia; Carbon; Carbon dioxide; Earth and Environmental Science; Earth Sciences; Eclogite; Feldspars; Flakes; Fluids; Garnet; Geology; Graphite; Gravel; Hydrothermal alteration; Isotopes; Magma; Marble; Mineral Resources; Mineralization; Mineralogy; Organic matter; Quartz; Rock; Rocks; Saturation; Titanite
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-016-0701-9

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 CO 2 and CH 4 , 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 δ 13 C 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 H 2 O 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.