Geological Survey of Denmark and Greenland Bulletin 28, 2013, 49-52 49 Geochemistry and petrology of gold-bearing hydrothermal alteration zones on Qilanngaarsuit, southern West Greenland Martin Koppelberg, Annika Dziggel, Denis Martin Schlatter, Jochen Kolb and Franz Michael Meyer During field work in 2008, the Geological Survey of Den- mark and Greenland investigated the gold potential of hy- drothermal vein systems in the Nuuk region of the Archaean North Atlantic craton. A new gold occurrence was discov- ered on the island of Qilanngaarsuit, 35 km south of Nuuk. Two cross sections through hydrothermal alteration zones, that locally contain up to 672 ppb Au, were mapped and sampled in detail. In this study, we present petrological and geochemical data in order to constrain the conditions for ore formation and transport of elements during f luid–rock in- teraction. Geological setting Qilanngaarsuit in southern West Greenland (Fig. 1) is situat- ed in the Godthåbsfjord gold province, a c. 20 km by 150 km wide, NE-trending sector along the Ivinnguit fault system. Several gold occurrences such as Storø, Qussuk, Bjørneøen and SW Isua have been described from this sector (Garde et al. 2012; Scherstén et al. 2012; Kolb et al. 2013). The Ivinnguit fault, situated north of the island, is a deep-crustal shear zone, which formed during terrane amalgamation and it represents the border between the Færingehavn and Akia terranes (Nutman & Friend 2007). Qilanngaarsuit island is dominated by Eoarchaean tonalite-trondhjemite-granodi- orite (TTG) gneisses of the Færingehavn terrane, which are overlain by amphibolites and aluminous cordierite-orthoam- phibole gneisses that originated from c. 2840 Ma old proto- liths (Nutman & Friend 2007). Four deformation events can be distinguished, involving north-vergent thrusting, isocli- nal folding and the formation of late, upright, open to tight folds (e.g. Kolb et al. 2013). The gold-bearing quartz veins are hosted by amphibolites in the central part of the island and surrounded by approx. 8 m wide hydrothermal altera- tion zones. The foliation-parallel, c. 10–20 cm wide, quartz veins can be followed over several hundred metres along strike. Structural data indicate that they formed in response to f lexural, slip folding during the late-tectonic evolution of the region (Kolb et al. 2013). One sample from a hydrother- mal alteration zone surrounding the veins contains up to 672 ppb Au and several other vein and alteration zone samples have elevated Au contents (> 20 ppb Au; Kolb et al. 2009). © 2013 GEUS. Geological Survey of Denmark and Greenland Bulletin 28, 49–52 . Open access: www.geus.dk/publications/bull 50°W 64°N 63°N S Q B 50 km Eoarchaean gneiss Anorthosite-gabbro complexes Orthogneiss and granitic rocks (Meso- to Neoarchaean) Tonalitic to granodioritic plutons (Meso- to Neoarchaean) Granite (Meso- to Neoarchaean) Quaternary cover Ice cover Granulite facies Qôrqut granite complex Fault Qilanngaarsuit Fisk efjo rd F ault Go dt hå bs fjo rd Fisk efjo rd Amer alik Amer alik Ser mili k BJØRNESUND BLOCK Færingehavn terrane Tasiusarsuaq terrane Akia terrane Tre Brødre terrane Tre Brødre terrane Isukasia terrane Bu kse fjord Bjør nes un d Grædefjord GODTHÅBSFJORD– AMERALIK BELT Ivinnguit fault SERMILIK BLOCK Nuuk Færingehavn Kapisilik terrane Terrane or block boundary Supracrustal belts (undifferentiated) Greenland Fig. 1. Geological map of the Nuuk region (modified from Allaart 1984). B: Bjørneøen, Q: Qussuk, S: Storø. 5050 Petrology and geochemistry Two profiles from the unaltered wall rocks through the hy- drothermal alteration zones were investigated (Fig. 2). Two types of amphibolite can be distinguished: homogeneous amphibolite in the footwall and compositionally layered am- phibolite in the hanging wall of the hydrothermal vein sys- tem. Their protoliths were low-K tholeiites depleted in light rare-earth elements (LREE), geochemically similar to other amphibolites in the Buksefjord region (Chadwick 1981). Generally, the amphibolites are fine- to medium-grained and consist of hornblende (40 vol.%), plagioclase (30  vol.%), clinopyroxene (20   vol.%) and clinozoisite/zoisite/epidote (10  vol.%). Metamorphic garnet is locally present in the lay- ered amphibolite. Retrogression is indicated by the transfor- mation of plagioclase to a fine-grained assemblage of zoisite and quartz, and by the replacement of amphibolite and clinopyroxene by epidote and clinozoisite. Within the alteration zone, the amphibolite-facies min- eral assemblages are replaced by a high temperature altera- tion assemblage of garnet, quartz, plagioclase, biotite, and sillimanite (Figs 2, 3). In contact with the veins, the hy- drothermal alteration zone consists of up to 50   vol.% gar- net, 15  vol.% plagioclase, 15  vol.% quartz, 10  vol.% biotite and 10   vol.% sillimanite (Fig. 2). Relict amphibole facies minerals such as hornblende and clinopyroxene are locally preserved, indicating that these minerals formed during re- gional metamorphism prior to the mineralisation. Ore min- erals make up ≤1 vol. % of the rocks; they include pyrite, pyr- rhotite and chalcopyrite. Mass-balance calculations based on whole-rock, major- and trace- element data (Kolb et al. 2009; Koppelberg 2011) and using the method of Gresens (1967) indicate that the ore f luid was enriched in Si, K, LREE, Au, Cr, Cu, Zn, Mo and As (Fig. 4). In Si-rich, vein-dominated samples, the hydrothermal overprint was associated with a volume increase of 14–62%. In contrast, the sillimanite- bearing samples record a significant volume loss of 15–50%, and are depleted in Si (Fig. 4). This suggests that at least some of the Si in the quartz veins was leached from the surround- ing wall rocks. Garnet in the alteration zones is rich in almandine (Kop- pelberg 2011). Most grains are essentially unzoned and have Fig. 2. Lithological logs of profiles A and B (modified from Schlatter 2009). Amphibolite with weak Grt-Bt alteration (Pl (XAb0.22-0.45), Hbl, Qtz, Cpx, Bt, Grt (Alm46-62, Prp20-39,Grs80-12) Unaltered amphibolite (Hbl, Pl, Cpx, Qtz, Ttn) Retrogressed amphibolite (Pl (XAb0.49-0.61), Hbl, Zo, Cpx ± Qtz, Ttn, Ep, Bt, Cal Unaltered amphibolite with Grt (Alm53-59,Prp13-20, Grs20-22), Hbl, Pl (XAb0.19-0.29), Cpx, Qtz Grt-Bt alteration, vein-dominated, without Sil: Pl (XAb0.59-0.65), Qtz, Grt (Alm53-64-Prp10-33-Grs3-28), Bt, Hbl, Cpx, Chl ± Sil Grt-Bt alteration with Sil: Grt (Alm54-69-Prp13-31-Grs3-23), Pl (mainly andesine, locally An), Qtz, Bt, Sil Grt-Bt alteration without Sil: (Pl (XAb0.56–0.57), Grt (Alm53-75-Prp13-31-Grs4-13), Qtz, Bt, Chl) Unaltered amphibolite, slightly retrogressed (Hbl, Pl (XAb0.31-0.35), Cpx, Czo) 508405 508406 <2 ppb 23 ppb 508407 508408 508409 508410 508411 <2 ppb 12 ppb 4 ppb <2 ppb <2 ppb 508412 508413 508414 508415<2ppb <2ppb 508381 508382 508383 508384 508385 508386 508387 508388 508389 508390 508391 <2 ppb <2 ppb 3 ppb <2 ppb 46 ppb 34 ppb 20 ppb 38 ppb 672 ppb <2 ppb <2 ppb 508392 Grt-Bt-Pl thermobarometry: 540°–620°C 4.5 ± 1 kbar Profile AProfile B(cliff profile) 4 ppb <2 ppb 3 ppb 20 ppb <2 ppb 5 m Petrographical sample (polished thin section) Geochemical sample (FA gold)38 ppb 67 ppb Sillimanite Garnet Sulphides Qtz vein / Qtz-blebs Pegmatite Silicified (schist) Grt-rich schist Bt-rich schist Layered amphibolite (type 1) Homogeneous amphibolite (type 2) Lithogeochemical sample (Whole-rock + trace elements + INAA gold) Legend 51 a composition of Alm53–69, Prp21–31, Grs3–12, depending on bulk composition. In some of the larger grains, the rims have slightly higher Fe concentrations (Alm62–72, Prp20–25, Grs3–10). Metamorphic garnet in the unaltered wall rocks is unzoned and enriched in grossular (Alm53–59, Prp13–20, Grs17–21). Both types of garnet have very low REE contents (<2 ppm), and are depleted in LREE. Biotite in the alteration zone has AlIV contents between 2.4 and 2.65 atoms per for- mula unit and Mg/(Fe+Mg) ratios between 0.25 and 0.45. The composition of plagioclase from the amphibolites varies from andesine to anorthite; most grains can be classified as labradorite and bytownite. Plagioclase in the hydrothermal alteration zone is depleted in Ca and is mostly andesine. In order to achieve reliable pressure–temperature (P–T) estimates, only mineral cores of neighbouring minerals were used for geothermobarometry. Due to the presence of retro- grade reaction rims in some of the garnet grains, it was as- sumed that the mineral cores ref lect the equilibrium mineral composition and were not altered by retrograde processes. P–T estimates on the alteration assemblage using the garnet- biotite-plagioclase-quartz geothermobarometer of Wu et al. (2004) give conditions of c. 540–620°C and 4.5 ± 1 kbar (Fig. 5). P–T pseudosection models using the computer pro- gram PeRpLeX developed by Connolly (1990) confirm these conditions (Koppelberg 2011). 500 µm HbHb GrtGrt BtBt BtBt BtBt SilSil SilSil SilSil Cr Cu Zn Mo As Au Ni Co Sc La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu –100 –50 0 50 100 150 200 –100 –50 0 50 100 150 200 –100 –50 0 50 100 150 200 A B C Quartz-vein dominated alteration zone (VF: 1.14–1.62) Silimanite-bearing alteration zone (VF: 0.5–0.85) G ai n/ lo ss (% ) G ai n/ lo ss (% ) G ai n/ lo ss (% ) SiO2 Al2O3 TiO2 Fe2O3 MgO MnO CaO Na2O K2O Fig. 3. Photomicrograph illustrating the replacement of the regional meta- morphic amphibolite facies mineral assemblages by hydrothermal garnet, biotite and sillimanite in sample GGU 508405. Fig. 4. Results of mass-balance calculations for quartz-vein dominated and sillimanite-bearing alteration zones. VF: volume factor – the change of volume of altered rock relative to unaltered rock. A: major elements, B: trace elements, C: rare-earth elements (R EE). Fig 5. Thermobarometry results (see main text). The Al2SiO5 diagram is from Holdaway & Mukhopadhyay (1993). Red: sample GGU 508384, green: 508385, blue: 508386, yellow: 508405. The Al2SiO5 triple point is at 500°C and 3.75 kbar. Kyanite Silimanite Andalusite KY KY SIL SI L AN D AN D 200 300 500400 600 700 800 900 2 1 3 4 5 6 7 8 9 Temperature (°C) Pr es su re (k ba r) 5252 Discussion and conclusion The majority of the world’s gold deposits formed in the Ar- chaean (c. 2.7 Ga) as a result of crust-forming processes dur- ing collision events of converging continental plates (Groves et al. 2005). These epigenetic deposits are called orogenic gold deposits, and occur in metamorphic terranes that mainly show greenschist facies metamorphism (Groves et al. 1998). Other orogenic deposits are known to have formed at amphibolite-facies metamorphic grades, and these are termed hypozonal deposits (Groves et al. 1998). The replace- ment of regional, metamorphic, amphibolite-facies mineral assemblages by hydothermal minerals surrounding the gold- bearing quartz veins as well as the late timing of quartz-vein formation by ductile, f lexural slip folding (Kolb et al. 2009), indicate that the gold mineralisation and associated hydro- thermal alteration formed late in the metamorphic evolution on Qilanngaarsuit. The low-pressure amphibolite-facies met- amorphism in the surrounding amphibolites has been dated to c. 2715 Ma (Nutman & Friend 2007), while the minerali- sation probably occurred between 2660 and 2600 Ma (Kolb et al. 2013). The Qilanngaarsuit mineralisation is, therefore, interpreted to represent a new example of hypozonal oro- genic gold mineralisation in the Godthåbsfjord gold prov- ince. The origin of other gold prospects (Storø, Qussuk) is still a matter of debate, and both metamorphosed epithermal and orogenic models have been proposed (Garde et al. 2012; Scherstén et al. 2012; Kolb et al. 2013). The alteration sys- tematics, timing and conditions of the gold mineralisation on Qilanngaarsuit are, however, similar to those of other gold occurrences, in particular Storø, in the Godthåbsfjord gold province. The deposits are spatially closely associated with a major terrane boundary, the Ivinnguit fault, suggest- ing that this shear zone may have acted as a major pathway for the gold-bearing f luids between c. 2660–2600 Ma. Acknowledgements The Professor Dr. Karl-Heinrich Heitfeld-Stiftung is thanked for finan- cial support. The work benefited from valuable comments and discussions with Susan Giffin and Nicolas Stoltz. References Allaart, J.H. 1982: Geological maps of Greenland 1:500 000. Map sheet no. 2, Frederikshåb Isblink – Søndre Strømfjord. 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