Geological Survey of Denmark and Greenland Bulletin 7, 2004, p 61-64 61 During the past decade the Geological Survey of Denmark and Greenland (GEUS) has carried out two major resource evaluations in the Precambrian basement terranes of South and West Greenland in order to locate potential areas of mine- ral deposits (Steenfelt et al. 2000, 2004; Stendal & Schøn- wandt 2003; Stendal et al. 2004). Based on geological field work and geochemical and geophysical data, these evalua- tions have assessed the interplay between the magmatic, tec- tonic and metamorphic evolution in the study areas and their mineralising events. As a result of the second of these evaluations it is now pos- sible to outline a succession of mineralising events in the northern part of the Nagssugtoqidian orogen and in the Disko Bugt area of central West Greenland (Fig. 1), and relate them to the general Archaean and Palaeoproterozoic geological evolution of this region. However, uncertainties still exist concerning the age and detailed setting of many epi- genetic mineralisations. Geological background The Precambrian rocks of central West Greenland north of the North Atlantic craton, consist of Archaean orthogneisses and supracrustal rocks together with localised belts of juve- nile Palaeoproterozoic intrusive complexes and supracrustal rocks. The whole region was reworked to varying degrees during a major Palaeoproterozoic orogenic event. In West Greenland the Palaeoproterozoic has traditionally been di- vided into the Nagssugtoqidian orogen between Kangerlus- suaq and Disko Bugt (Fig. 1) and the Rinkian fold belt farther north. However, more recent studies suggest that these two belts are largely contemporaneous and probably Precambrian mineralising events in central West Greenland (66°–70°15´N) Henrik Stendal and Adam A. Garde Geological Survey of Denmark and Greenland Bulletin 7, 61–64 (2005) © GEUS, 2005 Ataneq Ilulissat Kan ge rlu ssu aq 70°N 68°N Attu Rifkol Kangaatsiaq Aasiaat Qeqertarsuaq Disko Disko Bugt Itilliarsuk Nuussuaq Saqqaq Eqi Ataa Arveprinsen Ejland Qasigiannguit Naternaq Nordre Isortoq Sisimiut Sukkertoppen Iskappe Sarfartoq 53°W 51°W Inland Ice Nordre S trøm fjor d Nassuttooq Arfers iorfik Greenland Fig. 1. Simplified geological map of central West Greenland with mineral occurrences (modified from van Gool et al. 2002b). Precious metals Base metals Iron and iron alloys Speciality metals Light metals Industrial minerals Gemstones Town Kangerlussuaq Airport Quaternary Ice Palaeogene basalt Cambrian Sarfartoq carbonatite Palaeoproterozoic Sisimiut charnockitte Palaeoproterozoic Arfersiorfik quartz diorite Palaeoproterozoic supracrustal rocks Archaean granite Archaean undifferentiated rocks Archaean supracrustal rocks Cretaceous–Palaeogene sediments represent the southern and northern parts of a single, E–W- trending, c. 1850 Ma collisional orogen. Geological descrip- tions and additional references may be found in Garde & Steenfelt (1999), Connelly et al. (2000, in press), Garde et al. (2002, 2004) and van Gool et al. (2002a). The Nagssugtoqidian orogen is dominated by reworked Archaean orthogneisses in the south (between c. 66°30´N and 69°N; Fig. 1) with original emplacement ages of around 2870–2700 Ma (Connelly et al. 2000). It also incorporates two juvenile, calc-alkaline Palaeoproterozoic plutonic mag- matic complexes, namely the 1920–1870 Ma Arfersiorfik quartz diorite and the more or less contemporaneous Sisimiut charnockite (van Gool et al. 2002a). These must have been emplaced during subduction and prior to the con- tinent–continent collision, and the presence of a suture in the central part of the Nagssugtoqidian orogen has therefore been suggested. Based on new zircon and titanite U-Pb age determinations of the Archaean basement and mylonites in the Disko Bugt area, Connelly et al. (in press) have recently proposed that the main suture between the Rinkian and Nagssugtoqidian components lies in the Disko Bugt area; in the new model these components would represent the north- ern and southern colliding continents. The southern part of the Nagssugtoqidian orogen hosts a rift-related mafic dyke swarm (the Kangâmiut dykes) dated at c. 2040 Ma (van Gool et al. 2002a); the central and northern parts of the orogen contain Palaeoproterozoic metasedimen- tary and metavolcanic belts of limited geographical extent, including the Naternaq supracrustal belt (Østergaard et al. 2002) which has been dated at around 1950 Ma. These belts are thought to have been deposited along the margin of the rifted Archaean continent. The northern part of the Nagssugtoqidian orogen, the area east of Disko Bugt, and the Precambrian rocks of eastern Nuussuaq are all characterised by very variable Palaeopro- terozoic reworking. The Archaean gneisses in southern Nuussuaq are about 3000 Ma old and include the Itilli dior- ite dated at 3030 Ma (Connelly et al. in press). The northern part of the area east of Disko Bugt hosts two Archaean, E–W-trending, amphibolite facies metavolcanic and metasedimentary belts, including a large sill complex and a low-grade Palaeoproterozoic sedimentary succession (Fig. 1; Garde & Steenfelt 1999; Marshall & Schønwandt 1999; Stendal et al. 1999). The northern of the two Archaean belts was probably deposited unconformably on the gneisses of southern Nuussuaq and has been dated at c. 2950 Ma (Garde & Steenfelt 1999; Connelly et al. in press). Following the Palaeoproterozoic orogeny, a suite of E–W- trending ultramafic lamprophyres and small plugs was em- placed in the Ataa area about 1750 Ma ago (Larsen & Rex 1992). Much later, at c. 600 Ma, the Sarfartoq carbonatite complex and numerous lamprophyric and carbonatitic dykes were intruded at the southern margin of the Nagssugtoqidian orogen (see Jensen et al. 2004), an event that has been associ- ated with the opening of the Iapetus ocean (Larsen & Rex 1992). 62 Age (Ma) ~ 600 1775–1600 1870–1775 1920–1870 2000–1920 ~ 2040 ~ 2650 ~ 2800 ~ 3000 Geological setting Rifting – carbonatite Rifting – kimberlite Cooling of the crust from c. 600–400ºC Main Nagssugtoqidian metamorphism and de- formation. Pegmatite formation ~ 1800 Ma Subduction – calc- alkaline magmatism. Peak metamorphism in Disko Bugt region Drifting and sedimentation Rifting Granite intrusion into gneiss Continental rift or active continental margin Continental rift or active continental margin Table 1. Proterozoic mineralising events and commodities in West Greenland Locality and type of mineralising event Sarfartoq – igneous Kangerlussuaq – igneous Hydrothermal activity in the whole region from 66º to 70º15´N. Albitisation in Disko Bugt area Remobilisation of metals during metamorphism and deformation in the whole region. Monazite and allanite in pegmatite Arfersiorfik diorite and Sisimiut charnockite. Hydrothermal activity n the Disko Bugt region Naternaq, Nordre Strømfjord supracrustal rocks (Ataneq). Exhalative volcanic massive sulphides Kangâmiut dykes Rifkol, at Attu intrusive related mineralisation Eqi and Arveprinsen Ejland. Exhalative volcanic massive sulphides and hydrothermal activity Southern part of Nuussuaq (Saqqaq) and Itilliarsuk. Exhalative volcanic massive sulphides and hydrother- mal activity Commodity Niobium, tantalum, phosphor Diamond Mainly minor sulphide showings with minor base metal contents and low in gold Copper-zinc, graphite and formation of magnetite during metamorphism in mafic rocks. Cesium in pegmatite Minor oxide occurrences (Fe-Ti-V) in Arfersiorfik. In Disko Bugt region minor copper, zinc and gold occurrences Banded iron formation, Cu-Zn and gold Copper Gold Copper, zinc, lead, gold and banded iron formation Copper, zinc, gold and banded iron formation Timing of mineralising events The timing of mineralising events in relation to the geologi- cal evolution of the study area is summarised below and in Table 1. > 3000 Ma The oldest known mineral occurrence of the region, at Itilliarsuk in southern Nuussuaq, comprises syngenetic ban- ded iron formation (BIF) and associated semi-massive sul- phide occurrences, whereas the age of epigenetic gold oc- currences in the same area is not known (Stendal et al. 2004). 2800–2000 Ma A Pb isotope study of pyrite in the Disko Bugt area has yielded an age of c. 2800 Ma (Stendal 1998). Gold occur- rences related to syngenetic, semi-massive sulphide occur- rences at Eqi are presumed also to be of this age. A gold mineralisation at Attu is interpreted to be of Middle Archaean age. The age of the Pb source in the associ- ated ore minerals is compatible with that of the nearby Rifkol granite (Kalsbeek et al. 1984), suggesting gold mineralisation at around 2650 Ma. The pre-Nagssugtoqidian continental break-up is repre- sented by the well-known c. 2040 Ma Kangâmiut dyke swarm. The dykes themselves carry copper mineralisation with no economic significance. 2000–1920 Ma Several 1950–1920 Ma old supracrustal suites in the north- ern Nagssugtoqidian orogen (van Gool et al. 2002a), e.g. at Naternaq and Ataneq, contain syngenetic massive sulphide occurrences (Fig. 2), and the latter area also contains promi- nent graphite deposits. Unpublished Pb isotopic composi- tions of magnetite from amphibolite and banded iron formation yield intercept ages around c. 1940 and 2140 Ma, in agreement with the syngenetic nature of the sulphide occurrences. 1920–1870 Ma The 1920–1870 Ma Arfersiorfik and Sisimiut intrusive suites are associated with Nagssugtoqidian convergence and sub- duction of oceanic lithosphere prior to the main collision. The only known mineral occurrences resulting from this major event are minor igneous oxides (magnetite, ilmenite), although remobilisation and recrystallisation of previously formed mineral occurrences may also have taken place. In the Disko Bugt region, epigenetic fault zones and shear zones with small occurrences of base metals, nickel and gold are hosted in Palaeoproterozoic supracrustal rocks, and have yielded a pyrite Pb-Pb model age of c. 1900 Ma (Stendal 1998). This important mineralising event was probably con- temporaneous with the Palaeoproterozoic thermal event recorded by K-Ar and Ar-Ar ages in parts of the Disko Bugt region (Rasmussen & Holm 1999). 1870–1600 Ma The Nagssugtoqidian collisional orogeny is dated by early fabric-forming deformation at 1860–1840 Ma, large-scale folding at c. 1825 Ma, and formation of steep shear belts in the Nassuttooq region at c. 1775 Ma (van Gool et al. 2002a). Unpublished Pb-Pb ratios obtained by stepwise leaching of allanite and monazite indicate emplacement of pegmatites at c. 1800 Ma in both central (Nassuttooq) and northern (Attu and Qasigiannguit) regions of the orogen. Metamorphic conditions in the central part of the Nagssugtoqidian orogen reached temperatures above 650°C at 1800 Ma, declining to 540°C at c. 1740 Ma and 420°C at c. 1670 Ma (Connelly et al. 2000; Willigers et al. 2001). Pb- Pb isochron ages of c. 1740 Ma from magnetite in amphibo- lites probably represent closure of the U-Pb system in magnetite, and post-date the formation of prominent shear belts at around 1775 Ma. The magnetite was probably for- med during several different stages of metamorphism. Signs of hydrothermal activity have been observed along NE- to NNE-striking fault zones throughout the region. The coo- ling history of the orogen indicates that hydrothermal acti- vity in the temperature range 650°–400°C extended from the time of pegmatite emplacement at c. 1800 Ma until c. 1600 Ma. 63 Fig. 2. Outcrop of volcanogenic-exhalative, semi-massive sulphides of Palaeoproterozoic age at Naternaq in the northern Nagssugtoqidian oro- gen, with formation of gossan and sulphide weathering. In the Disko Bugt region, lamproites and ultramafic lam- prophyres intruded the basement gneisses in the Ataa area at around 1750 Ma, and extensive post-kinematic albitisation was reported by Kalsbeek & Taylor (1999). However, miner- alisations of this age are not known. 600 Ma The formation of diamondiferous kimberlites and related carbonatite Nb-Ta mineralisation took place around 600 Ma in the Sarfartoq area (Jensen & Secher 2004; Jensen et al. 2004). Concluding remarks Mineralising events in the Precambrian basement terrain of central West Greenland are confined to certain periods, of which some are more economically promising than others. The most prospective areas are the Archaean greenstone belts in the Disko Bugt region, especially with respect to syn- and epigenetic gold occurrences. Known Palaeoproterozoic metal occurrences are all small, and probably without economic sig- nificance, with the possible exception of graphite deposits in the Palaeoproterozoic supracrustal rocks in the Sisimiut – Nordre Strømfjord region. Niobium-tantalum and diamond occurrences in the Neoproterozoic, c. 600 Ma alkaline intru- sions are currently under investigation and may be economi- cally feasible. References Connelly, J.N., van Gool, J.A.M. & Mengel, F.C. 2000: Temporal evolution of a deeply eroded orogen: the Nagssugtoqidian Orogen, West Greenland. Canadian Journal of Earth Sciences 37, 1121–1142. Connelly, J.N., Thrane K., Krawiec, A.W. & Garde, A.A. in press: Linking the Palaeoproterozoic Nagssugtoqidian and Rinkian orogens through the Disko Bugt region of West Greenland. Journal of the Geological Society (London). Garde, A.A. & Steenfelt, A. 1999: Precambrian geology of Nuussuaq and the area north-east of Disko Bugt, West Greenland. Geology of Green- land Survey Bulletin 181, 6–40. Garde, A.A., Connelly, J.N., Krawiec, A.W., Piazolo, S. & Thrane, K. 2002: A coastal survey in the southern part of the Palaeoproterozoic Rinkian fold belt, central West Greenland. Geology of Greenland Survey Bul- letin 191, 33–38. Garde, A.A., Connelly, J.N., Grocott, J., Hand, M., McCaffrey, K.J.W. & Thrane, K. 2004: Crustal shortening and granite emplacement in the Rinkian fold belt, West Greenland, and implications for Palaeopro- terozoic Laurentian evolution. Danmarks og Grønlands Geologiske Undersøgelse Rapport 2004/17, 16–18. Jensen, S.M. & Secher, K. 2004: Investigating the diamond potential of southern West Greenland. Geological Survey of Denmark and Green- land Bulletin 4, 69–72. Jensen, S.M., Secher, K., Rasmussen, T.M. & Schjøth, F. 2004: Diamond exploration data from West Greenland: 2004 update and revision. Dan- marks og Grønlands Geologiske Undersøgelse Rapport 2004/117, 90 pp. Kalsbeek, F. & Taylor, P.N. 1999: Review of isotope data for Precambrian rocks from the Disko Bugt region, West Greenland. Geology of Green- land Survey Bulletin 181, 41–47. Kalsbeek, F., Taylor, P.N. & Henriksen, N. 1984: Ages of rocks, structures, and metamorphism in the Nagssugtoqidian mobile belt in West Green- land – field and Pb-isotopic evidence. Canadian Journal of Earth Sciences 21, 1126–1131. Larsen, L.M. & Rex, D.C. 1992: A review of the 2500 Ma span of alkaline- ultramafic, potassic and carbonatitic magmatism in West Greenland. Lithos 28, 367–402. Marshall, B. & Schønwandt, H.K. 1999: An Archaean sill complex and associated supracrustal rocks, Arveprinsen Ejland, north-east Disko Bugt, West Greenland. Geology of Greenland Survey Bulletin 181, 87–102. Østergaard, C., Garde, A.A., Nygaard, J., Blomsterberg, J., Nielsen, B.M., Stendal, H. & Thomas, C.W. 2002: The Precambrian supracrustal rocks in the Naternaq (Lersletten) and Ikamiut areas, central West Green- land. Geology of Greenland Survey Bulletin 191, 24–32. Rasmussen, H. & Holm, P.M. 1999: Proterozoic thermal activity in the Archaean basement of the Disko Bugt region and eastern Nuussuaq, West Greenland: evidence from K-Ar and 40Ar-39Ar mineral age inves- tigations. Geology of Greenland Survey Bulletin 181, 55–64. Steenfelt, A., Nielsen, T.F.D. & Stendal, H. 2000: Mineral resource poten- tial of South Greenland: review of new digital data sets. Danmarks og Grønlands Geologiske Undersøgelse Rapport 2000/50, 47 pp. Steenfelt, A., Stendal, H., Nielsen, B.M. & Rasmussen, T.M. 2004: Gold in central West Greenland – known and prospective occurrences. Geological Survey of Denmark and Greenland Bulletin 4, 65–68. Stendal, H. 1998: Contrasting Pb isotopes of Archaean and Palaeopro- terozoic sulphide mineralisation, Disko Bugt, central West Greenland. Mineralium Deposita 33, 255–265. Stendal, H. & Schønwandt, H.K. 2003: Precambrian supracrustal rocks and mineral occurrences, northeast Disko Bugt. Danmarks og Grøn- lands Geologiske Undersøgelse Rapport 2003/24, 57 pp. Stendal, H., Knudsen, C., Marker, M. & Thomassen, B. 1999: Gold mine- ralisation at Eqi, north-east Disko Bugt, West Greenland. Geology of Greenland Survey Bulletin 181, 129–140. Stendal, H., Nielsen, B.M., Secher, K. & Steenfelt, A. 2004: Mineral resources of the Precambrian shield of central West Greenland (66° to 70°15´N). Part 2. Mineral occurrences. Danmarks og Grønlands Geologiske Undersøgelse Rapport 2004/20, 212 pp. van Gool, J.A.M., Connelly, J.N., Marker, M. & Mengel, F. 2002a: The Nagssugtoqidian Orogen of West Greenland: tectonic evolution and regional correlations from a West Greenland perspective. Canadian Journal of Earth Sciences 39, 665–686. van Gool, J.A.M. et al. 2002b: Precambrian geology of the northern Nagssugtoqidian orogen, West Greenland. Geology of Greenland Survey Bulletin 191, 13–23. Willigers, B.J.A., Krogstad, E.J. & Wijbrans, J.R. 2001: Comparison of ther- mochronometers in a slowly cooled granulite terrain: Nagssugtoqidian Orogen, West Greenland. Journal of Petrology 42, 1729–1749. 64 Authors’ address Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: hst@geus.dk