Kimberlites and other ultramafic alkaline rocks in the Sisimiut–Kangerlussuaq region, southern West Greenland

includes swarms of dykes described as kimberlites and lamproites (Larsen 1991), and these rock types are widely distributed in the Sisimiut–Sarfartoq–Kangerlussuaq region (Figs 1, 2). Kimberlites and lamproites are potential carriers of diamond, and since the description of the Sarfartoq carbonatite complex and the kimberlitic dykes related to this complex (Larsen 1980; Secher & Larsen 1980), the Sisimiut–Sarfartoq–Kangerlussuaq region has seen several campaigns of commercial diamond exploration. The latest and most persistent stage of exploration began in the mid-1990s and has continued to date, with varying intensity. Numerous reports of diamond-favourable indicator minerals from till sampling, finds of kimberlitic dykes, and recovery of actual diamonds from kimberlitic rocks have emerged since 1995 (Olsen et al. 1999). A drilling programme in late 2001 confirmed the unusually great length and width of a magnetic kimberlitic dyke (Ferguson 2001). The alkaline ultramafic dykes within the Sisimiut– Kangerlussuaq and Sarfartoq regions intrude the border zone between the Archaean craton and the Palaeoproterozoic Nagssugtoqidian orogen (van Gool et al. 2002, this volume). This border is defined as the southern boundary of Palaeoproterozoic reworking of the Archaean basement gneisses. The reworking has affected the Palaeoproterozoic Kangâmiut dolerite dykes, which

. Geological map of the region with the project area (Fig. 2) outlined in a red frame.Modified from Mengel et al. (1998).
were intruded into the Archaean gneisses prior to deformation and are now highly strained and boudinaged north of the boundary.The intrusion of alkaline dykes appears to have taken place during two events.Lamproite dykes in the Sisimiut region are around 1.2 Ga old, whereas the Sarfartoq complex (Fig. 2) and a swarm of associated kimberlitic dykes have ages of around 0.6 Ga (Larsen & Rex 1992).A precise spatial relationship between the two intrusive events has not been established.
A project concerned with general scientific aspects of kimberlitic and related rocks in the Sisimiut-Kangerlussuaq region was established in 2000.The aim is to gather knowledge from companies' exploration activities and to incorporate Survey expertise such as petrology, structural geology, isotope geology, geochemistry, geophysics and the geographic information system (GIS).Field work was carried out in 2000 and 2001, with the Kangerlussuaq International Science Support (KISS) centre at Kangerlussuaq airport as logistical base.A combination of daily reconnaissance covering large areas using a helicopter, and fly camps of 2-3 days duration was applied.
The broad term 'kimberlitic' is used here in acknowledgement of the fact that the classification of the ultramafic dyke rocks of West Greenland is not resolved with unanimity.Many of the dyke rocks resemble kimberlites and have previously been described as such (Larsen 1980(Larsen , 1991;;Scott 1981;Larsen & Rex 1992).Mitchell et al. (1999), however, take the view that the dyke rocks are not 'archetypal' kimberlites, but ultramafic lamprophyres that should be termed melnoites or aillikites.In the light of the actual occurrence of diamonds in the West Greenland dyke rocks, Mitchell et al. (1999) consider them to represent one of the few examples of diamond-bearing ultramafic lamprophyres.One of the aims of the present project is to contribute to a generally accepted classification of the West Greenland kimberlitic and related rocks.Larsen's (1991) compilation of Greenland's kimberlite, lamproite and ultramafic lamprophyre occurrences proved to be a valuable initial guide to companies when the ongoing diamond exploration commenced in West Greenland in the mid-1990s (Olsen et al. 1999).The compilation includes data collected by earlier workers (e.g.Scott 1981), who typically operated by boat and thus were often restricted to working near the coast.consequence, the known occurrences had an uneven distribution, and some areas appeared to be without occurrences.

Distribution of kimberlitic and related rocks
Many of the occurrences of in situ dykes and boulders (float) in the Sisimiut-Sarfartoq-Kangerlussuaq region known at present are indicated in Fig. 2. The approximately 600 occurrences include those from Larsen's (1991) compilation and new localities added by exploration companies and recent field work by the Geological Survey of Denmark and Greenland (GEUS).It should be noted that the map does not include a large number of company localities that are at present confidential because they lie within active exclusive exploration licence areas.The map illustrates the concentration of occurrences in the vicinity of the Sarfartoq carbonatite complex, which Larsen (1980) interpreted as a swarm of kimberlitic cone-sheets centred on the carbonatite.
Bedrock exposure is generally poor in the study region, and dykes and sills of kimberlite, lamproite and carbonatite can be difficult to trace because they are thin, easily eroded and often covered by overburden and vegetation.Therefore, both stream geochemical data and aeromagnetic data are investigated with the aim of identifying geochemical and geophysical signatures reflecting occurrences of kimberlitic and related dykes.

Field activities
Field work in 2000 consisted of one week's reconnaissance and visits to new dyke occurrences discovered by exploration companies.The 2001 field work was focused on the spatial distribution of kimberlitic dyke rocks in areas with little or no previous information, and detailed studies on mantle xenoliths from the kimberlitic dykes.Studies of mantle xenolith-bearing rocks are described in a separate section below.The field work also included testing the ease of locating occurrences described in companies' assessment reports using various Global Positioning System (GPS) equipment and GIS methods.
Locality F (Fig. 2) provides an example of the density of new observations made in an area with no previously reported kimberlitic or lamproitic occurrences.Within the framed area 26 kimberlitic boulders and one new dyke were registered (Fig. 3).Three of the boulders measure about 1 x 1 x 1 m in size, indicating that the dyke or dykes from which they originated must be of substantial dimensions.Several large boulders were broken up into many smaller fragments, resulting in a much higher count of individual pieces of kimberlitic rock.The high number of kimberlitic boulders in this small area suggests they may have been derived from a nearby dyke outcrop.Similar observations were made at localities G and H (Fig. 2).

Stream sediment geochemical anomalies reflecting alkaline rocks
Geochemical reconnaissance-scale stream sediment surveys have been carried out over large parts of West Greenland, including the project region considered here.The chemical analyses have been compiled and calibrated (Steenfelt 1999(Steenfelt , 2001a)), and presented in a geochemical atlas of West and South Greenland (Steenfelt 2001b).The atlas displays element distribution patterns for 43 major and trace elements based on analysis of the < 0.1 mm grain size fraction of stream sediment samples collected systematically with a density of about one sample per 30 km 2 .The atlas also includes maps of high-pressure mineral phases of assumed mantle derivation, so-called kimberlite indicator minerals, identified in the 0.1 to 1 mm fraction of stream sediment samples collected between latitudes 61°N and 67°N.
The Sarfartoq carbonatite complex comprises rocks enriched in P, Ba, Sr and rare-earth elements (REE), and hosts a niobium-rich pyrochlore mineralisation (Secher & Larsen 1980).In the geochemical maps, the Sarfartoq complex is easily recognised as a local but very pronounced anomaly with high concentrations of P, Ba, Sr, Nb and REE, as well as Mo, Ta and Th.
Kimberlites and lamproites have olivine-and carbonate-rich groundmass (high Mg and Ca) and high concentrations of the trace elements Ba, Cr, Ni, Nb, Sr, P, Ti and light REE.In view of the particular mineralogy and chemistry of the alkaline rocks, their presence within the gneiss-dominated Archaean basement should be readily detectable in the stream sediment geochemical survey.However, because of the small volume of alkaline rocks, their geochemical imprint on the stream sediment composition may be obscured by the presence of other igneous rocks that have intruded the tonalitic gneisses in the same region.Gabbroic inclusions within the gneisses, and the Palaeoproterozoic Kangâmiut dolerite dyke swarm, release so much Ni, Cr and Ti to the streams that the contribution of these metals from alkaline ultramafic rocks is easily disguised.Furthermore, Palaeoproterozoic intrusions within the Nagssugtoqidian domain comprise considerable volumes of charnockite and syenite, enriched in Ba, Sr, P and REE (Steenfelt 1997), which give rise to anomalies that are similar to those expected to be caused by kimberlites and lamproites.
However, one element, niobium, has proved convincing as a 'pathfinder' for kimberlites and lamproites.contents of Nb.The regional stream sediment anomaly pattern for Nb in the study region (Fig. 4) does, in fact, coincide with the province of known kimberlites and lamproites.
It was recognised at an early stage that the Sarfartoq complex and surroundings were characterised by a regional stream sediment Nb anomaly (Steenfelt 1991).The anomaly was then attributed partly to Nb mineralisation, and partly to the known ultramafic alkaline dykes.As more stream sediment data became available (Steenfelt et al. 1993) and more kimberlite occurrences were registered, the apparent spatial relationship between the high Nb in stream sediments and occurrences of ultramafic alkaline dykes was strengthened.
Field visits to high-Nb stream sites were carried out in 2000 and 2001 to test the validity of high Nb in stream sediments as an indicator of kimberlite and lamproite.At one site with a very high Nb value a lamproite dyke was seen in outcrop in a stream gorge, and at other sites boulders of kimberlite were observed along the stream course.Outcrops or boulders of kimberlite or lamproite were not found in all the high-Nb sites visited.However, it is worthy of note that thin dykes and float of beforsite have been recorded some distance from the Sarfartoq carbonatite complex; beforsite is a magnesiocarbonatite rock with high concentrations of Nb and LREE, which probably contributes to the Nb-enriched province.
Stream sediment samples with high Nb and highpressure chromite have been collected in areas east and south-east of known kimberlite occurrences suggesting a wider extent of the kimberlite field than recognised hitherto.A small cluster of high Nb values in the north-eastern part of Fig. 4 is at present unexplained.Kimberlites or lamproites have not so far been recorded here.However, the generally poor exposure in the stream surroundings would hinder ready recognition of alkaline dykes or other rocks with high Nb such as granitic pegmatites.
High concentrations of P, REE, Ba and Sr are recorded in stream sediment samples collected in a 15 x 15 km area near the coast south of Itilleq (locality I, Fig. 2).The geochemical signature of these samples is similar to that obtained in streams draining Palaeoproterozoic syenitic intrusives in the Nagssugtoqidian domain (Steenfelt 1998).It is noteworthy that the Nb concentrations are as low as in the Nagssugtoqidian syenites, and hence it was not expected to find kimberlites, but some other kind of alkaline rock.
Two field days were spent in a valley upstream from the most pronounced stream sediment anomaly.The area is dominated by a gneiss complex intruded by a 10 to 15 m wide Kangâmiut dyke and numerous thin, less than 0.5 m wide, dykes and sills of grey lamprophyre and a brown dolerite with star-shaped aggregates of phenocrystic plagioclase.Blocks of a coarse-grained alkaline ultramafic rock were frequently seen in the moraines of the valley glacier and were particularly abundant in the screes covering the southern valley slopes.The alkaline rock is dominated by black, shiny amphibole and has variable to large amounts of brown phlogopite and light green apatite.The finds explain the geochemical anomaly and enlarge the province of known alkaline rocks in this part of Greenland.

Mantle xenoliths in kimberlitic rocks
Mantle xenoliths are widely used in studies of the mantle lithosphere (Menzies & Hawkesworth 1987;Nixon 1987Nixon , 1995)), and are here used to shed light on how the plate-tectonic history affected the deeper parts of the lithosphere across the Archaean-Proterozoic transition in the Sisimiut-Sarfartoq-Kangerlussuaq region.An extensive collection of xenoliths from several kimberlitic occurrences in the unreworked Archaean terrane south of Søndre Strømfjord was made in 2000, and two kimberlitic dykes containing abundant mantle xenoliths (localities A and B, Fig. 2) were visited and sampled in 2001.At least 90% of the xenoliths encountered have peridotitic or pyroxenitic compositions, and range in size from less than 1 cm to 15 cm across.Some xenoliths contain visible purple pyrope garnet (Fig. 5) and green Cr diopside, minerals that -given the right In the Palaeoproterozoic reworked Archaean terrane north of Søndre Strømfjord another kimberlitic dyke reported by exploration companies was visited (locality C, Fig. 2).The dyke consists of a 5-10 m wide, c. 500 m long train of kimberlitic boulders and outcrops in banded grey gneiss.Peridotitic and pyroxenitic xenoliths are abundant, ranging in size from less than 1 cm to around 10 cm.Dunitic (ol), harzburgitic (ol + opx) and lherzolitic (ol + opx + cpx ± gt) xenoliths were identified in the field.The Survey campaign located two further dykes north of Søndre Strømfjord (localities D and E, Fig. 2).The locality D dyke is vertical, 2-5 m wide and can be traced for c. 1.5 km.Xenoliths are much less abundant here than in the dykes south of Søndre Strømfjord, and their sizes range from less than 1 cm to about 5 cm.Macroscopically, the peridotitic xenolith types are dunite (ol) and lherzolite (ol + cpx + opx).The locality E dyke is a gently dipping (16-20°S), 30-50 cm thick sheet, and contains small xenoliths that are mostly less than 2 cm in diameter.

Preliminary results
Seven xenoliths from the locality A dyke have been analysed for major element mineral chemistry by electron microprobe at the Geological Institute, University of Copenhagen, and garnets in three of them have been subjected to reconnaissance laser ablation analyses at the Survey.Garnets in the same three xenoliths were subjected to high-precision analysis of Ni by electron microprobe, and the xenolith rock types (Table 2) have been determined from their mineral assemblages and estimated mineral proportions (e.g.LeBas & Streckeisen 1991).
Generally, the major element compositions of minerals within individual xenoliths are homogeneous (Table 2).The average forsterite (Fo) content of the olivines ranges from Fo87 to Fo92; garnet wehrlites contain the olivines with the lowest Fo, garnet harzburgites and garnet lherzolites the olivines with the highest Fo.Average Ni contents are within the range of 2900 ± 360 ppm reported by Ryan et al. (1996) for garnet peridotites.Orthopyroxenes contain less than 1.6 wt% CaO and have relatively low Al 2 O 3 , the lowest values being from ilmenite-free garnet lherzolites (0.49-0.53 wt%).Clinopyroxene Mg/(Mg + Fe 2+ ) ratios correlate positively with Cr 2 O 3 .Garnet Mg/(Mg + Fe 2+ ) ratios tend to correlate positively with Fo contents of olivine.CaO and Cr 2 O 3 contents of garnets show limited variation within individual xenoliths but large overall variations (Fig. 6), and the garnets from two harzburgite xenoliths clearly 62 Table 2. Calculated temperatures and pressures for xenolith minerals from the locality A dyke T, P and TNi calculated according to: * Nimis & Taylor (2000), † Ryan et al. (1996).ol: olivine, opx: orthopyroxene, cpx: clinopyroxene, gt: garnet, ilm: ilmenite, hzb: harzburgite, lhz: lherzolite, weh: wehrlite.The GGU numbers refer to material from Greenland in the files of the Geological Survey of Denmark and Greenland.Fig. 6.Garnet compositions in xenoliths from the locality A dyke, analysed by electron microprobe.The G9-G10 boundary line is from Gurney (1984) and Gurney & Zweistra (1995), and STD is a reference garnet analysed along with microprobe standards.GGU 464xxx numbers refer to sample material in the files of the Geological Survey of Denmark and Greenland.

Mineral
plot within the G10 field of Gurney (1984) and Gurney & Zweistra (1995).G10-class pyrope garnets are considered to be strongly indicative of conditions favourable for diamond stability.
Equilibration temperatures (T) and pressures (P) for the clinopyroxene-bearing xenoliths were calculated using the Nimis & Taylor (2000) single clinopyroxene thermobarometry equations (Table 2), and these temperature estimates are comparable to T Ni values calculated using the high-precision Ni analyses and the empirical Ni-in-garnet thermometer of Ryan et al. (1996).
Variations of Zr vs. Y in garnets of three xenoliths from the locality A dyke are shown in Fig. 7, and averaged chondrite-normalised REE patterns for garnet and clinopyroxene in each xenolith in Fig. 8.The very low Zr and Y contents in garnets of a garnet harzburgite xenolith (sample GGU 464002.2) are consistent with their low-Ca character (Fig. 6) and indicate that this xenolith represents original lherzolite mantle depleted by partial melting (Griffin et al. 1992).The garnet in sample GGU 464002.2 is more depleted in the middle and heavy REE than the garnets in lherzolites from samples GGU 464003 and 464006.The sinuous pattern resembles that of low-Ca garnet in a harzburgite xenolith from the Sarfartoq area (Garrit 2000).The low REE contents support the idea that the garnet is hosted in a rock depleted by partial melting, whereas the sinuous shape may indicate postdepletion metasomatic enrichment processes (Hoal et al. 1984) or disequilibrium garnet growth (Shimizu & Sobolev 1995).Garnet lherzolites from samples GGU 464003 and 464006 have much higher Zr and Y concentrations in the garnets, and their chondrite-normalised REE patterns are more like those of typical LREE-depleted and MREE-to HREE-enriched garnets of primitive mantle (e.g.Haggerty 1995).Garnets from samples GGU 464003 and 464006 plot within fields indicating phlogopite and melt metasomatism of previously depleted mantle, respectively (Fig. 7).Phlogopite occurs in the xenoliths as part of kelyphitic rims on garnets, and in sample GGU 464003 also as a minor phase away from garnets.Ti contents of garnets, which follow Zr in some metasomatic processes (Griffin et al. 1999), show no significant zoning.This may indicate that if metasomatic processes affected garnet compositions, there was sufficient time for equilibration with the metasomatising agents.

Geophysical properties of kimberlites
Airborne geophysical surveys play an important role in diamond exploration, as kimberlites are often hidden under surficial deposits but also often have magnetic properties that make them distinguishable from the country rocks (e.g.Keating 1995;Macnae 1995).Since 1996 several helicopter-borne surveys have been commissioned by exploration companies to cover their licence areas in the Maniitsoq and Kangerlussuaq regions (Olsen et al. 1999) (1995) and Griffin et al. (1999), and arrows indicate core-to-rim zonation direction in metasomatised garnets of Griffin et al. (1999).

Conclusions and plans for further work
Field work in 2000 and 2001 in poorly known areas has added approximately 50 samples of kimberlitic dykes and more than 300 kimberlitic boulders to the existing databank.
Niobium in the fine fraction (< 0.1 mm) of stream sediments has proved to be a convincing 'pathfinder' element for kimberlites and lamproites in the Sisimiut-Sarfartoq-Kangerlussuaq region.Nb anomalies and the presence of high-pressure chromite in the stream sediment samples in the eastern and south-eastern part of the study area warrant follow-up.
The different xenolith types from the locality A dyke and their distinctive mineral compositions clearly demonstrate the heterogeneous character of the West Greenland lithospheric mantle, with a vertical zonation of depleted and metasomatised zones beneath the Archaean craton.Temperature and pressure calculations suggest that all xenoliths were derived from within the diamond stability field at a depth interval of 49-69 kbar, corresponding to approximately 150-215 km.
Although magnetic data have been used with some success in the search for kimberlitic dykes in the Kangerlussuaq region, there is a need for establishing a database with petrophysical properties of both kimberlitic dykes and host rocks in order to fully utilise the geophysical data available.(Garrit 2000).Laser ablation ICP-MS analyses, same sources as in Fig. 7. level should be conducted at selected kimberlitic dyke occurrences in future field work.Exploration companies have produced a large volume of data relevant to diamond exploration since 1995.The data include mineral analyses of heavy minerals recovered from till samples, dyke and boulder distribution maps, helicopter-borne geophysical surveys, results of diamond testing of mini-bulk sampled dykes, drill logs, etc. Compilation and publication of all non-confidential company data submitted in assessment reports to the Bureau of Minerals and Petroleum, Government of Greenland, is planned for 2002.A compilation of this type covering the North Slave Craton in Nunavut, Canada (Armstrong 2002) has received very positive response from industry.
The ultramafic alkaline rocks observed within the region north-west of the Sarfartoq cone sheet structure (Larsen 1980) have not been proved to be diamondiferous.However, the finds of diamonds in kimberlitic dykes of the Torngat orogen (Derbuch 2001) -the Canadian counterpart of the Nagssugtoqidian orogenis an encouragement for further exploration in West Greenland.
Fig.1.Geological map of the region with the project area (Fig.2) outlined in a red frame.Modified fromMengel et al. (1998).

Fig. 2 .
Fig.2.Distribution of kimberlitic and lamprophyric occurrences within the project area.A-J are localities mentioned in the text.Red triangles are kimberlitic and lamprophyric dykes known prior to the present study: lamproites predominate in the region between Sisimiut and Itilleq, kimberlites in the Sarfartoq region(Larsen & Rex 1992).Green triangles are observations from the present study.Heavy colouring represents in situ finds and light colouring represents boulders (float).
confirmation of a 5 km long and 20 m wide kimberlitic dyke in late 2001 (locality J, Fig. 2; Fig. 9; Ferguson 2001).The petrophysical properties of 22 representative kimberlitic samples collected in 2001 have been determined by the Geological Survey of Finland, and these data will be used in modelling the geophysical response obtained in the airborne surveys.It is planned to collect further samples for petrophysical measurements in 2002.

Fig. 9 .
Fig. 9. Magnetic total field anomaly of locality J (Fig. 2), where a 5 km long and 20 m wide kimberlitic dyke shows up as pronounced linear feature.Data from helicopter-borne DIGHEM survey carried out for Dia Met Minerals Ltd., reproduced with permission.

Table 1 .
Range of Nb concentrations of various rock types in the Sisimiut-Sarfartoq-Kangerlussuaq region . Data from the latest helicopterborne survey in the Kangerlussuaq region in 2000 were used to outline targets for the drilling that led to the Magnetic profiling at groundFig.8. Chondrite-normalised REE patterns of garnets in xenoliths from the locality A dyke (this study) and of garnets in Sarfartoq xenoliths