Geological Survey of Denmark and Greenland Bulletin 11, 53-60 53© GEUS, 2006. Geological Survey of Denmark and Greenland Bulletin 11, 53–60. Available at: www.geus.dk/publications/bull A lead isotope study of an Archaean gold prospect in the Attu region, Nagssugtoqidian orogen, West Greenland Henrik Stendal, Robert Frei and Bo Møller Stensgaard This paper presents a lead isotope investigation of a gold prospect south of the village Attu in the northern part of the Nagssugtoqidian orogen in central West Greenland. The Attu gold prospect is a replacement gold occurrence, related to a shear/mylonite zone along a contact between orthogneiss and amphibolite within the Nagssugtoqidian orogenic belt. The mineral occurrence is small, less than 0.5 m wide, and can be followed along strike for several hundred metres. The mineral assemblage is pyrite, chalcopyrite, magnetite and gold. The host rocks to the gold prospect are granulite facies ‘brown gneisses’ and amphibolites. Pb-isotopic data on magnetite from the host rocks yield an iso- chron in a 207Pb/204Pb vs. 206Pb/204Pb diagram, giving a date of 3162 ± 43 Ma (MSWD = 0.5). This date is interpreted to represent the age of the rocks in question, and is older than dates obtained from rocks elsewhere within the Nagssugtoqidian orogen. Pb-isotopic data on cataclastic magnetite from the shear zone lie close to this isochron, indicating a similar origin. The Pb-isotopic compositions of the ore minerals are similar to those previously obtained from the close-by ~2650 Ma Rifkol granite, and suggest a genetic link between the emplacement of this granite and the formation of the ore minerals in the shear/mylonite zone. Consequently, the age of the gold mineralisation is interpreted to be late Archaean. Keywords: Archaean, gold, magnetite, Pb isotopes, geochronology, West Greenland ________________________________________________________________________________________________________________________________________________________________________________________ H.S. & B.M.S., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: hst@geus.dk R.F., Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. Discovery of the gold prospect described in this study was due to the find of a mineralised sample, which Karl Markussen from Attu submitted to the Bureau of Mine- rals and Petroleum in Greenland. The Geological Survey of Denmark and Greenland (GEUS) visited the locality in 2001 and in 2002 (Stendal et al. 2002, 2004), and the present paper reports Pb-isotopic data for minerals from the prospect and its surroundings. The Attu gold prospect lies within the Nagssugtoqidian orogen of West Greenland (Fig. 1), where geological map- ping and exploration has been carried out for decades by the Geological Survey, the Danish Lithosphere Centre, university research groups and exploration companies (e.g. Kalsbeek et al. 1987; Connelly et al. 2000; van Gool et al. 2002). In addition to the general investigations, Steenfelt (2001) has summarised geochemical signatures from stream sediments, Rasmussen & van Gool (2000) have described geophysical aspects, and Steenfelt et al. (2002), Stendal & Schønwandt (2003) and Stendal et al. (2004) have described mineral occurrences and their economic potential. An overview of the mineral occurrences in the entire region has been presented by Stendal et al. (2004). Detailed, mainly zircon U-Pb geochronological data from the Nagssugtoqidian orogen have been presented by Kalsbeek & Nutman (1996), Connelly & Mengel (2000) and Connelly et al. (2000), and Pb-Pb, Rb-Sr and Sm- 54 Nd whole-rock isotope data from the region have been reported by Kalsbeek et al. (1984, 1987), Taylor & Kals- beek (1990) and Whitehouse et al. (1998). In addition, some Pb-isotopic work has been carried out on sulphide separates, mainly pyrite, from mineral occurrences in the Disko Bugt region (Stendal 1998). Geological setting The Palaeoproterozoic Nagssugtoqidian orogen of West Greenland (van Gool et al. 2002) is located between the Archaean North Atlantic craton to the south and a lesser- known continental mass to the north that includes the Palaeoproterozoic Rinkian fold belt. Most of the orogen consists of variably reworked Archaean orthogneisses. Several thin belts of supracrustal and intrusive igneous rocks occur within this gneiss terrain. Granitoid rocks and numerous pegmatites intrude the gneisses. Formations of Palaeoproterozoic age are limited to the Arfersiorfik and Sisimiut igneous suites and minor supracrustal sequences (Connelly et al. 2000). The Attu area itself is located in the southern part of the northern Nagssugtoqidian orogen (NNO; Fig. 1). The metamorphic grade is granulite facies; metamorphism and deformation of the Archaean granitoid rocks in the NNO gradually decrease northwards, from granulite to amphi- bolite facies, and from high strain to lower strain with more open structures. Steeply and shallowly dipping shear and fault zones are common in contact zones between 68° Inland Ice 54°54°54° 67°30' AttuAttu gold prospect Ar NSSZ NNO CNO Rifkol Undifferentiated Orthogneiss Attu gold prospect Amphibolite and meta- sedimentary rocks Archaean Mainly meta- sedimentary rocks Palaeoproterozoic A 18 60 74 18 64 74 55 58 60 60 56 70 62 70 48 70 63 70 62 70 62 82 50 60 15 40 75 1 km Attu gold prospect 53 °3 0' 67°51' 67°48' 53 °2 1' Biotite gneiss, mainly granodioritic-tonalitic Orthopyroxene gneiss, mainly tonalitic Granite/charnockite Amphibolite Metasedimentary and metavolcanic rocks Strike and dip of dominant lithological layering and foliation Direction and plunge of fold axis, measured, constructed Rock samples with elevated gold values Fault, shear zone, mylonite zone Quaternary deposits Kangiussaq Tateraat 50 km50 km50 km 50°52° B 500 km Inland Ice Greenland Inland Ice Greenland Iceland Ketilidian orogen North Atlantic craton Nagssug- toqidian orogen Rinkian fold belt Canada Fig. 1. A: Geological map of the Attu region with index map of Greenland. CNO, central Nagssugtoqidian orogen; NNO, northern Nagssugto- qidian orogen; NSSZ, Nordre Strømfjord shear zone. B: Geological map of the Attu gold prospect area (modified from Olesen 1984). 55 different lithologies. Major fault zones generally strike NNE to NE. The major Nordre Strømfjord shear zone (van Gool 2002) is located c. 20 km south of the study area. The shear zone is traceable from the coast to the Inland Ice and forms the southern boundary of the NNO. The gneisses of the NNO are late Archaean, with ages between 2870 and 2700 Ma (Kalsbeek & Nutman 1996; Connelly & Mengel 2000; Hollis et al. 2006, this volume; Thrane & Connelly 2006, this volume). Discordant sheets of granitoid rocks of Archaean age occur in the centre of the NNO and large charnockite/granite bodies including the Rifkol granite are situated 20 km to the northwest and just south of the study area (Fig. 1; Hansen 1979; Kalsbeek et al. 1984). Only a few younger Palaeoproter- ozoic ages have been obtained from the NNO: Thrane & Connelly (2006, this volume) have obtained an approxi- mate depositional age of the Naternaq supracrustal belt some 80 km north-east of Attu of c. 1950–1900 Ma, and an undeformed pegmatite between Attu and Aasiaat has yielded an age of c. 1790 Ma (Connelly & Mengel 2000). The Attu gold prospect The Attu gold prospect is located south of the village At- tu within a 100–330 m wide, complex tract hosting sev- eral parallel shear/mylonite zones and faults that strike NNE to NE and dip 60–70°W (Figs 1, 2). The fault zone can be followed along strike in a north-easterly direction for several kilometres. The host rocks are layered, brown 2.24 ppm Au 0.34% Cu 124 ppb Au 0.11% Cu 12 ppb Au 17 ppb Au 2–6 ppm Au < 0.5% Cu Fig. 2. The site of the Attu gold prospect. Gold values are given for rock samples. Red circles are sample sites with gold values obtained from fine-grained stream sediments (see Fig. 1 for geographical location). The red line shows the approximate position of the gold-bearing zone in the shear/mylonite zone. The black line shows the approximate position of a stream. Fig. 3. Layered brown gneiss with black bands of amphibolite. The hammer shaft is 50 cm long. 56 weathering gneiss and amphibolite (Fig. 3). At the west- ern border of the tract a gold-bearing shear/mylonite zone follows the contact between brown gneisses and amphib- olites. The gold-bearing shear/mylonite zone (Fig. 4) is invaded by pegmatite sheets as well as centimetre-wide veins consisting of red alkali-feldspar and quartz with oc- casional pyrite and magnetite. The estimated relative vol- ume of pegmatite in the tract varies from 1 to 10% (Sten- dal et al. 2002, 2004). The most promising gold showings are found in a coast- al profile along the shear/mylonite zone, which can be followed along strike for several hundreds of metres (Figs 1, 2). The studied site is a cliff exposure consisting of mylonite (Fig. 4) and a rusty weathered band (10–20 cm in width) mineralised with pyrite, magnetite and some chalcopyrite (Fig. 5). Pyrite and chalcopyrite replace magnetite. The magnetite is predominantly cataclastic in nature, but recrystallised ore also occurs. The gold is found within pyrite and chalcopyrite. The gangue mineralogy comprises quartz, K-feldspar, muscovite, biotite and car- bonates (calcite, dolomite and/or ankerite). The mylonite zone is silicified at the contact with the mineralised zone, and sulphide-rich parts are weathered. Secondary goethite and malachite are common (Fig. 5). The ore is structurally controlled by and confined to favour- able sites (sulphide-bearing zones) within the mylonite/ shear/fault zone. The Attu gold prospect has returned reproducable gold Fig. 4. The gold bearing mylonite zone. K- feldspar occurs on the right side of the yellow magnet pen (10 cm long). The zone also contains pyrite, chalcopyrite and magnetite. Amphibolite and orthogneiss (host rocks) 446601 magnetite 14.631 0.007 14.642 0.009 44.688 0.033 0.961 0.939 446602 magnetite 15.051 0.014 14.752 0.015 36.702 0.040 0.969 0.942 446610 magnetite 17.540 0.051 15.361 0.046 37.613 0.112 0.977 0.988 446614 magnetite 17.002 0.025 15.225 0.024 38.086 0.061 0.976 0.967 Shear zone and mineralised rock 446616 magnetite 15.423 0.023 14.844 0.023 41.598 0.068 0.979 0.957 2000368 magnetite 15.286 0.009 14.832 0.010 41.201 0.034 0.962 0.936 481093 magnetite 14.247 0.042 14.625 0.044 41.821 0.130 0.982 0.960 446615 pyrite 14.241 0.009 14.587 0.010 42.001 0.035 0.963 0.934 481078 pyrite 14.447 0.011 14.633 0.012 40.805 0.039 0.967 0.925 446616 K-feldspar 15.123 0.008 14.893 0.010 36.451 0.029 0.958 0.932 Table 1. Pb-isotopic ratios of magnetite, pyrite and K-feldspar from the Attu gold prospect and its host rocks Sample number Mineral 206Pb/204Pb ± 2σ* 207Pb/204Pb ± 2σ 208Pb/204Pb ± 2σ r1** r2† * Errors are two standard deviations absolute (Ludwig 1990). ** r1 = 206Pb/204Pb versus 207Pb/204Pb error correlation (Ludwig 1990). † r2 = 206Pb/204Pb versus 208Pb/204Pb error correlation (Ludwig 1990). 57 concentrations in the range 2.3–5.8 ppm. Other localities in the same fault structure yielded 2.24 ppm and 124 ppb Au (Fig. 2). The gold concentrations are positively corre- lated with concentrations of copper, and gold-bearing sam- ples often contain magnetite. Two stream sediment sam- ples yielded anomalous gold concentrations of 12 ppb and 17 ppb Au, respectively (Fig. 2). The host gneisses are brownish in colour and comprise orthopyroxene, amphibole, biotite and feldspar, but little quartz. Magnetite is in equilibrium with the rock form- ing minerals and has the same granular texture. In the amphibolites magnetite forms up to millimetre-thick lay- ers, and also occurs in disseminated form. Within the mylonite zone, magnetite occurs as a primary phase in the host gneiss as cataclastic grains with cracks filled with pyrite and chalcopyrite, and as a residual phase resulting from sulphide replacement. Ten samples were analysed for Pb-isotopic compositions. Fig. 5. The gold bearing zone (10 cm wide) within the mylonite zone, with malachite and rusty weathered sulphides. 14 15 16 17 1918 14.4 14.6 14.8 15.0 15.2 15.4 15.6 15.8 206Pb/204Pb 20 7 P b/ 20 4 P b 2400 2000 1600 1200 800 400 0 Kfsp Mt Mt Mt Mt brown gneiss Py Py Mt amphibolite Mt amphibolite Mt amphibolite Rifkol granite - trend line c. 2650 Ma 60 Ma Lower intercept 3143 Ma Age = 3162 ± 43 Ma MSWD = 0.50 Fig. 6. 206Pb/204Pb–207Pb/204Pb diagram for minerals from the Attu area listed in Table 1. Open squares, mineral separates from the gold-bearing mylonite zone. Red squares, host rock data. Black diamonds, whole-rock samples from the Rifkol granite for comparison (data from Kalsbeek et al. 1984). Mt, magnetite; Py, pyrite; Kfsp, K-feldspar. Blue curve, the Pb-isotopic growth curve from Stacey & Kramers (1975). 58 Analytical methods The Pb-isotopic study was carried out on magnetite from host gneisses and amphibolites, and on K-feldspar, magne- tite and pyrite from the shear zone-hosted mineralised zone (Table 1). The isotope analyses were carried out at the Danish Centre for Isotope Geology, Geological Institute, University of Copenhagen. Near-pure mineral fractions were separated from dry split aliquots of crushed and sieved (100–200 µm) rock powders using a hand magnet, a Frantz isodynamic separator and heavy liquid techniques. Ore minerals were dissolved in concentrated aqua regia. Total procedural blanks for Pb amounted to < 120 pg which is considered insignificant for the measured Pb-iso- topic results, relative to the amount of sample Pb estimat- ed from the mass spectrometer signal intensities. Isotope analyses were carried out on a VG Sector 54-IT instru- ment in static collection mode. Fractionation for Pb was controlled by repetitive analysis of the NBS 981 standard (values of Todt et al. 1993) and amounted to 0.103 ± 0.007% / amu (2 σ; n = 11). All results are quoted with 2 σ precisions. Results The Pb-isotopic compositions of mineral separates from the gold-bearing mylonite zone and its host rocks are listed in Table 1. In the 207Pb/204Pb vs. 206Pb/204Pb diagram shown in Fig. 6, the Pb-isotopic compositions of magnetite from the four host rock samples of brown gneiss and amphibo- lite (red squares) define a straight line with a slope corre- sponding to 3162 ± 43 Ma (MSWD = 0.50). This line has intercepts with the Stacey & Kramers (1975) Pb-iso- topic growth curve at 3143 and 60 Ma. Based on the good fit of the data points on the isochron, and the agreement of the isochron age with the intercepts of the growth curve, we interpret the 3162 Ma date as the age of the rocks in question. However, farther south, in the central part of the Nagssugtoqidian orogen, Palaeoproterozoic granulite facies metamorphism has led to U loss in Archaean rocks, resulting in Pb-isotopic compositions plotting above and to the left of an 2800 Ma reference isochron (Whitehouse et. al. 1998). If this process had also taken place in the area of the present study, the 3162 Ma date might give a false impression of the age of the rocks. However, the good fit of the data points on the isochron and the agreement of the intercepts with the Stacey & Kramers (1975) growth curve with the isochron age would then be accidental, a coincidence which we regard as very unlikely. Six mineral separates from the gold-bearing mylonite zone (Fig. 6, open squares) lie close to or slightly above the isochron obtained for magnetite from the host rocks. The most primitive 206Pb/204Pb and 207Pb/204Pb ratios have been measured in pyrite and magnetite from the ore-bear- ing zone, whereas the two primary magnetites with cat- aclastic texture from within the shear zone plot very close to the host rock magnetite isochron. This suggests that their crystallisation took place at about the same time as the magnetites from outside the shear zone. Whole-rock Pb-isotopic ratios from the Rifkol granite (Kalsbeek et al. 1984) are also plotted on Fig. 6 for comparison, and the isotopic values are listed in Table 1. The errorchron de- fined by these samples has a slope corresponding to an age of 2653 ± 110 Ma, which has been interpreted as emplace- ment age of the granite (Kalsbeek et al. 1984). This er- rorchron is oblique and discordant to the isochron ob- tained for magnetite from the host rocks, but the three least radiogenic data points from ore minerals associated with native gold from within the shear zone are conform- able with this younger trend. This suggests that the fluids in the shear zone from which the gold mineralisation was deposited were somehow genetically linked to the intru- sion of the Rifkol granite. Alkali feldspar from the shear zone has its own Pb-isotopic signature, which is neither compatible with a ‘Rifkol’ source nor with a source typi- cal of the immediate host rocks. The uranogenic vs. thorogenic isotopic pattern (not shown in a figure) is more disperse than the uranogenic pattern and does not add to the understanding of the urano- genic Pb-isotopic data; as expected, it mostly reflects the differences in U and Th concentrations in the different analysed minerals. Summary, discussion and conclusions The Attu gold prospect is small. The gold mineralised zone does not exceed 0.5 m in width, and its length is now known to be only a few hundred metres. Gold has also been detected along strike several kilometres away, but the mineralisation does not show a continuous outcrop pattern. However, the fact that gold is present indicates that the NE-striking shear/mylonite zone is mineralised and that hydrothermal activity seems to have occurred in most of the prominent lineaments in the region. The gold- bearing sulphide deposit is of replacement type, where pyrite and chalcopyrite grew at the expense of e.g. magne- tite. It is envisaged that gold was introduced contempora- neously with the replacement processes. Reworked Archaean orthogneisses dominate all seg- ments of the Nagssugtoqidian orogen. Published age deter- 59 minations range from 2870–2700 Ma (e.g. Kalsbeek & Nutman 1996; Connelly & Mengel 2000), but no chrono- logical information has yet been available from the Attu region. The 3162 ± 43 Ma magnetite age obtained from the Attu host rocks suggests that the rocks in this part of the Nagssugtoqidian orogen may be significantly older than similar rocks elsewhere in the orogen. However, Sm- Nd isotope data from Archaean gneisses in the central part of the orogen suggest the involvement of pre-2800 Ma crustal material (possibly 3100 Ma or older) in their source (Whitehouse et al. 1998). Large parts of the Nagssugto- qidian orogen underwent Palaeoproterozoic granulite facies metamorphism around 1850 Ma (e.g. Willigers et al. 2001), which resulted in severe disturbance of the Pb- isotopic evolution of the rocks (Whitehouse et al. 1998). In view of the well-preserved 3162 Ma isochron relation- ships for the Attu gneisses it appears possible that these rocks escaped high-grade Nagssugtoqidian metamorphism and that granulite facies metamorphism here is of Archae- an age, in agreement with the conclusions of Mazur et al. (2006, this volume) and Thrane & Connelly (2006, this volume). The Pb-isotopic data of the gold bearing samples (Fig. 6) suggest a genetic link between the Rifkol granite intru- sion and the fluids percolating through the shear zone, implying an Archaean age of the mineralisation. Without further analytical work we are unable to elaborate and comment on a possible source of the Pb that has been incorporated into the K-feldspar in the shear zone. Acknowledgements The authors acknowledge F. Kalsbeek, P.M. Holm and an anonymous reviewer for thorough criticism and construc- tive suggestions, which greatly improved the manuscript. The authors would also like to thank the participants in the resource assessment programme Mineral resources of the Precambrian shield of central West Greenland (66°– 70°15′N) for valuable discussions concerning mineralis- ing events in the region. Input from other scientists in the region is also gratefully acknowledged. Special thanks go to Karl Markussen, Attu, who submitted the first gold bearing sample, for showing us the exact sample locality. References Connelly, J.N. & Mengel, F.C. 2000: Evolution of Archean compo- nents in the Paleoproterozoic Nagssugtoqidian orogen, West Green- land. Geological Society of America Bulletin 112, 747–763. Connelly, J.N., van Gool, J.A.M. & Mengel, F.C. 2000: Temporal evo- lution of a deeply eroded orogen: the Nagssugtoqidian orogen, West Greenland. Canadian Journal of Earth Sciences 37, 1121–1142. Hansen, B.F. 1979: Some charnockitic rocks in the Nagssugtoqidian of West Greenland. Rapport Grønlands geologiske Undersøgelse 89, 85–96. Hollis, J.A., Keiding, M., Stensgaard, B.M., van Gool, J.A.M. & Garde, A.A. 2006: Evolution of Neoarchaean supracrustal belts at the north- ern margin of the North Atlantic Craton, West Greenland. In: Garde, A.A. & Kalsbeek, F. (eds): Precambrian crustal evolution and Creta- ceous–Palaeogene faulting in West Greenland. Geological Survey of Denmark and Greenland Bulletin 11, 9–31 (this volume). Kalsbeek, F. & Nutman, A.P. 1996: Anatomy of the Early Proterozoic Nagssugtoqidian orogen, West Greenland, explored by reconnais- sance SHRIMP U-Pb zircon dating. Geology 24, 515–518. Kalsbeek, F., Taylor, P.N. & Henriksen, N. 1984: Age of rocks, struc- tures, and metamorphism in the Nagssugtoqidian mobile belt, West Greenland – field and Pb-isotope evidence. Canadian Journal of Earth Sciences 21, 1126–1131. Kalsbeek, F., Pidgeon, R.T. & Taylor, P.N. 1987: Nagssugtoqidian mo- bile belt of West Greenland: a cryptic 1850 Ma suture between two Archaean continents – chemical and isotopic evidence. Earth and Planetary Science Letters 85, 365–385. Mazur, S., Piazolo, S. & Alsop, G.I. 2006: Structural analysis of the northern Nagssugtoqidian orogen, West Greenland: an example of complex tectonic patterns in reworked high-grade metamorphic ter- rains. In: Garde, A.A. & Kalsbeek, F. (eds): Precambrian crustal evo- lution and Cretaceous–Palaeogene faulting in West Greenland. Geo- logical Survey of Denmark and Greenland Bulletin 11, 163–178 (this volume). Olesen, N.Ø. 1984: Geological map of Greenland, 1:100 000, Agto 67 V.1 Nord. Copenhagen: Geological Survey of Greenland. Rasmussen, T.M. & van Gool, J.A.M. 2000: Aeromagnetic survey in southern West Greenland: project Aeromag 1999. Geology of Green- land Survey Bulletin 186, 73–77. Stacey, J.S. & Kramers, J.D. 1975: Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Sci- ence Letters 26, 207–221. Steenfelt, A. 2001: Geochemical atlas of Greenland – West and South Greenland. Danmarks og Grønlands Geologiske Undersøgelse Rap- port 2001/46, 39 pp., 1 CD-ROM. Steenfelt, A., Stendal, H., Nielsen, B.M. & Rasmussen, T.M. 2004: Gold in central West Greenland – known and prospective occur- rences. Geological Survey of Denmark and Greenland Bulletin 4, 65–68. Stendal, H. 1998: Contrasting Pb isotopes of Archaean and Palaeo- proterozoic sulphide mineralisation, Disko Bugt, central West Green- land. Mineralium Deposita 33, 255–265. Stendal, H. & Schønwandt, H.K. 2003: Precambrian supracrustal rocks 60 and mineral occurrences, Northeast Disko Bugt. Danmarks og Grøn- lands Geologiske Undersøgelse Rapport 2003/24, 57 pp. Stendal, H., Blomsterberg, J., Jensen, S.M., Lind, M., Madsen, H.B., Nielsen, B.M., Thorning, L. & Østergaard, C. 2002: The mineral resource potential of the Nordre Strømfjord – Qasigiannguit region, southern central West Greenland. Geology of Greenland Survey Bulletin 191, 39–47. Stendal, H., Nielsen, B.M., Secher, K. & Steenfelt, A. 2004: Mineral resources of the Precambrian shield of central West Greenland (66° to 70°15′). Part 2. Mineral occurrences. Danmarks og Grønlands Geologiske Undersøgelse Rapport 2004/20, 212 pp. Taylor, P.N. & Kalsbeek, F. 1990: Dating the metamorphism of Pre- cambrian marbles: examples from Proterozoic mobile belts in Green- land. Chemical Geology 86, 21–28. Thrane, K. & Connelly, J.N. 2006: Zircon geochronology from the Kangaatsiaq–Qasigiannguit region, the northern part of the 1.9– 1.8 Ga Nagssugtoqidian orogen, West Greenland. In: Garde, A.A. & Kalsbeek, F. (eds): Precambrian crustal evolution and Cretaceous– Palaeogene faulting in West Greenland. Geological Survey of Den- mark and Greenland Bulletin 11, 87–99 (this volume). Todt, W., Cliff, R.A., Hanser, A. & Hofmann, A.W. 1993: Re-calibra- tion of NBS lead standards using a 202Pb + 205Pb double spike. Terra Abstracts 5, Supplement 1, 396 only. van Gool, J.A.M., Connelly, J.N., Marker, M. & Mengel, F. 2002: The Nagssugtoqidian orogen of West Greenland: tectonic evolution and regional correlations from a West Greenland perspective. Canadian Journal of Earth Sciences 39, 665–686. Whitehouse, M.J., Kalsbeek, F. & Nutman, A.P. 1998: Crustal growth and crustal recycling in the Nagssugtoqidian orogen of West Green- land: constraints from radiogenic isotope systematics and U-Pb zir- con geochronology. Precambrian Research 91, 365–381. Willigers, B.J.A., Krogstad, E.J. & Wijbrans, J.R. 2002: Comparison of thermochronometers in a slowly cooled granulite terrain: Nags- sugtoqidian orogen, West Greenland. Journal of Petrology 42, 1729– 1749. __________________________________________________________________________________________________________________________________________________________________________________ Manuscript received 4 October 2004; revision accepted 19 December 2005