Geological Survey of Denmark and Greenland Bulletin 33, 2015, 49-52 49 A quartz-wolframite-molybdenite vein and scheelite in amphibolite horizons from Thrudvang peninsula, Skjoldungen, SE Greenland Diogo Rosa and Thomas Ulrich During the South-East Greenland Mineral Endowment Task (SEGMENT) expedition in 2012, the possible min- eral potential of the Skjoldungen region was investigated. Th e region is part of the Archaean North Atlantic Craton, and includes the Skjoldungen Alkaline Province (Nielsen & Rosing 1990; Blichert-Toft et al. 1995; Kolb et al. 2013). A quartz-wolframite-molybdenite vein with phyllic alteration was identifi ed during the reconnaissance work in the north- western part of the peninsula of Th rudvang, close to the Kangertikajik fj ord (GGU 446946; Figs 1, 2). Th e c. 30 cm wide, subvertical vein is hosted in mafi c granulite. However, the deformed nature of the vein and steep terrain did not al- low us to establish its extent or general trend. Two rusty amphibolite horizons, in the mafi c granulite hosting the vein, have anomalous concentrations of tung- sten, but not of molybdenum (GGU 446934 and 446948; Table 1). Ultraviolet light examination conducted during the study revealed that tungsten is present as pale blue luminesc- ing scheelite grains, rather than as wolframite, as in the vein. As such, these anomalous amphibolite samples are similar to scheelite-rich stratabound horizons documented in suprac- rustal sequences in the Godthåbsfj ord region in southern West Greenland, which were interpreted by Appel & Garde (1987) to be of exhalative origin. Fluid inclusion study A sample from the quartz vein (GGU 446946) was investi- gated for fl uid inclusions in a thick section. Th e quartz is very transparent and shows only very few fl uid inclusions which are generally aligned in secondary trails that are typically parallel and rarely crosscut each other (Fig. 3A). Th e inclusions vary in size from 5 to 20 mm and are euhe- dral in shape. Only two inclusion trails showed irregular or deformed inclusion shapes. Overall, only one type of inclu- sion was found, characterised by a dark single phase that fi lls the entire inclusion (Fig. 3B). Selected trails of 24 inclusions from three fl uid inclusion assemblages were used for microthermometry, to determine the phase transition during their cooling. Th e dark phase in the inclusions contracted at around –95° to –110°C when a small vapour bubble appeared (Fig. 3C). Th e inclusions © 2015 GEUS. Geological Survey of Denmark and Greenland Bulletin 33, 49–52 . Open access: www.geus.dk/publications/bull Greenland Mafic granulite, minor paragneiss, meta-peridotite, amphibolite Grey tonalitic to granodioritic gneiss (2781 ± 6 Ma in Skjoldungen area) Tonalitic to granodioritic gneiss, locally agmatitic 1 km 63°30´N 41°W 446934 446946, 446948 T h r u d v a n g Dragsfjord Kangertikajik Jættefjorden Fig. 1. Geological map of the peninsula of Thrudvang and the surrounding areas (after Kolb et al. 2013), showing the locations of the samples discussed in this paper. Contour line spacing: 100 m. 5050 were then slowly heated at a rate of 3–5°C/min. Th e vapour (liquid-gas) bubble homogenised between –59° and –50.4°C forming a single phase. Th e range of the diff erent fl uid inclu- sion assemblages in individual trails is small (Fig. 4). Th ere was no melting of ice or clathrate observed. Th e behaviour of the fl uid inclusions during the micro- thermometric experiments is typical for high-density liquid Fig. 2. Top: Sampling the vein described in this paper. Mid and bottom: Field work in the Kangertikajik fjord region, South-East Greenland in 2012. Pho- tographs (mid and bottom): Lars Lund Sørensen. 51 CO2 inclusions that homogenise close to the CO2 triple point at –56.6°C. Th e density of the inclusions is calculated to c. 1.1 g/cm3. Th e range of the homogenisation tempera- tures indicates that a small amount of other elements such as nitrogen is probably present in the fl uid. Th e occurrence of one-phase liquid CO2 inclusions in metamorphic rocks was interpreted by Hollister (1990) and Johnson & Hollister (1995) to be related to grain boundary migration during re-crystallisation of quartz. Th ey proposed that selective removal of H2O from H2O–CO2 fl uid mix- tures leads to enigmatic pure CO2 inclusions. Such processes cannot be excluded for GGU sample 446946, but are dif- fi cult to recognise. Th erefore, it cannot conclusively be ar- gued that the CO2-rich fl uid is directly related to the W-Mo mineralisation observed in these rocks, because it could well be that the fl uid inclusions were modifi ed during the meta- morphic history of the sample. Geochronology A molybdenite concentrate from the sampled vein was Re– Os dated and provided a Neoarchaean age of 2749 ± 11 Ma (Table 2). Th is age is similar to the Laser Ablation Induc- tively Coupled Plasma Mass Spectrometry (LA-ICP-MS) date of c. 2740 Ma for a porphyritic monzogranite and a LA- ICP-MS date of 2753 ± 5 Ma for the Skirner Bjerge Syenite in the Skjoldungen Alkaline Province, which are interpreted to have been emplaced during the fi rst (DS1) stage of regional transpression of the Skjoldungen Orogeny (Kolb et al. 2013). Th e vein documents a previously unknown tungsten- molybdenum mineralisation occurrence in Greenland. Pre- viously known molybdenite occurrences in East Greenland are related to Palaeogene intrusions. Furthermore, the age determination fi ts into the earliest of fi ve Mo mineralising pulses, which, according to Golden et al. (2013), correspond to supercontinent assembly events. In the case of the Neo- archaean Mo mineralising pulse, it can be linked to the as- sembly of Kenorland during the Neoarchaean, also known as Superia (Golden et al. 2013). Mineral potential Th e analysed molybdenite has a relatively low Re concentra- tion (Table 2), which is typical of Archaean molybdenite. Th is low Re concentration probably refl ects the limited mobility of Re in the reducing environment that prevailed prior to oxidation of the atmosphere (Golden et al. 2013). Notwith- standing the reported secular variation of Re concentrations 446934 Rusty zone in 49.75 14.61 11.18 0.14 7.39 10.50 3.14 0.65 0.84 0.02 0.89 99.12 3 0.37 amphibolite 446946 W-Mo quartz 94.95 1.57 1.10 0.01 0.11 0.36 0.44 0.11 0.05 0.02 0.49 99.2 263 1.60 vein 446948 Rusty zone in 49.03 13.99 13.05 0.16 4.77 12.15 2.81 0.30 0.74 0.23 2.18 99.4 5 0.55 amphibolite Table 1. Whole rock geochemistry of W–Mo mineralised samples from Thrudvang, SE Greenland * GGU Description SiO 2 Al 2 O 3 Fe 2 O 3 MnO MgO CaO Na 2 O K 2 O TiO 2 P 2 O 5 LOI Total Mo W no. % % % % % % % % % % % % ppm % * The samples were analysed at Actlabs (Canada), using fusion ICP-OES (majors) and fusion ICP-MS (Mo and W). CB A 50 μm 50 μm 200 μm Fig. 3. Photomicrographs. A: Parallel f luid inclusion trails. B: Fluid inclu- sion trails with one-phase liquid CO2 inclusions at room temperature. C: The same f luid inclusion trails at –120°C. 5252 in molybdenite, according to Stein (2006), the Re concentra- tion can be used to establish the type of mineral occurrence and the economic potential. However, the Re concentration in the analysed sample is intermediate between that of likely subeconomic molybdenite occurrences formed by local dehy- dration melting of biotite gneiss (with <20 ppm Re or even sub-ppm Re), and that of molybdenite of possible economic interest with a porphyry-style intrusion-related origin (with hundreds to thousands ppm Re). Th erefore, we cannot dis- criminate between the two mineralisation types and we can- not assess the economic potential of this occurrence, using this criterion. As such, the possibility that the studied vein may be linked to an intrusion and could be part of a wider mineralising system with economic potential remains. Finally, bearing in mind the close spatial relations, it is considered that the mineralisation in the rusty amphibolite horizons is contemporaneous with the dated vein, and not of exhalative or syn-genetic origin. In this case, the scheelite in the amphibolite horizons was precipitated in the previ- ously carbonatised mafi c to ultramafi c horizons in the host package, due to their enhanced reactivity to vein-derived mineralising fl uids. Th is type of reaction can yield skarn-like occurrences, but they are probably not of economic interest – in contrast to the vein mineralisation, which could be of economic interest. Acknowledgements Th e work was carried out as part of the SEGMENT project, jointly fi - nanced by the Ministry of Industry and Mineral Resources of the Govern- ment of Greenland and the Geological Survey of Denmark and Greenland. References Appel, P.W.U. & Garde, A.A. 1987: Stratabound scheelite and stratiform tourmalinites in the Archaean Malene supracrustal rocks, southern West Greenland. Bulletin Grønlands Geologiske Undersøgelse 156, 26 pp. Blichert-Toft , J., Rosing, M.T., Lesher, C.E. & Chauvel, C. 1995: Geo- chemical constraints on the origin of the Late Archean Skjoldungen alkaline igneous province, SE Greenland. Journal of Petrolog y 36, 515–561. Golden, J., McMillan, M., Downs, R.T., Hystad, G., Goldstein, I., Stein, H.J., Zimmerman, A., Sverjensky, D.A., Armstrong, J.T. & Hazen, R.M. 2013: R henium variations in molybdenite (MoS2): evidence for progressive subsurface oxidation. Earth and Planetary Science Letters 366, 1–5. Hollister, L.S. 1990: Enrichment of CO2 in fl uid inclusions in quartz by removal of H2O during crystal-plastic deformation. Journal of Struc- tural Geolog y 12, 895–901. Johnson, E.L. & Hollister, L.S. 1995: Syndeformational fl uid trapping in quartz; determining the pressure-temperature conditions of deforma- tion from fl uid inclusions in the formation of pure CO2 fl uid inclusions during grain-boundary migration. Journal of Metamorphic Geolog y 13, 239–249. Kolb, J., Th rane, K. & Bagas, L. 2013: Field relationship of high-grade Neo- to Mesoarchaean rocks of South-East Greenland: tectonometa- morphic and magmatic evolution. Gondwana Research 23, 471–492. Nielsen, T.F.D. & Rosing, M.T. 1990: Th e Archaean Skjoldungen alka- line province, South-East Greenland. Rapport Grønlands Geologiske Undersøgelse 148, 93–100. Stein, H.J. 2006: Low-rhenium molybdenite by metamorphism in north- ern Sweden: recognition, genesis, and global implications. Lithos 87, 300–327. Authors’ addresses D.R., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: dro@geus.dk T.U., Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, DK-8000 Aarhus C, Denmark. 446946 54.02 0.14 33.95 0.09 1591 1 2749 11 Table 2. Re–Os dating of a molybdenite occurrence at Thrudvang, SE Greenland * GGU Re 187Re 187Os Model age ± 2σ no. ppm ± 2σ ppm ± 2σ ppb ± 2σ Ma Ma * Carried out at ALS Minerals (Canada), using isotope dilution mass spectrometry with a Carius-tube, solvent extraction, anion chromatography and negative thermal ionisation mass spectrometry techniques. 5 4 3 2 1 0 F re q u e n c y Temperature (°C) –60 –58 –56 –54 –52 –50 Fig. 4. Histogram of f luid inclusion homogenisation temperatures for three-phase f luid inclusions.