platinum-group elements in the palaeogene north atlantic igneous … · 2002-09-19 · they have...

Introduction

Over the last decade, a large number of platinum-group ele-ment (PGE) deposits have been discovered in Palaeogene intru-sions throughout the North Atlantic Igneous Province (NAIP).The deposits occur in some of the best exposed and describedintrusions on Earth, notably the Skaergaard Intrusion on EastGreenland and the Rum Complex in Scotland, and althoughthey are mostly sub-economic they provide excellent opportuni-ties to explore PGE mineralization processes in detail. In partic-ular, the discovery in 1989 of a gold and PGE-rich layer withpotentially economic concentrations in the Skaergaard Intru-sion attracted international attention. Since then, small sub-eco-nomic deposits have been documented throughout the provinceboth on East Greenland and the Inner Hebrides indicating thatthe early stages of the hot-spot now associated with Iceland hada high PGE potential.

The Palaeocene layered intrusions in the North Atlanticregion have been studied in detail for more than a century, andthey have greatly influenced the development of modernigneous petrology. Classical studies include the works of Geikie(1888) and Harker (1909) on the British intrusive centres, andWager (1960), Wager and Brown (1968), Wager and Deer(1939) on East Greenland. Many of the intrusions have beendocumented in far more detail than intrusions elsewhere, andtheir magmatic and post-magmatic histories are extremely wellknown. They thereby provide excellent study areas for PGE frac-tionation processes.

This paper presents the first regional compilation ofPGE occurrences in mafic volcanic and intrusive rocks in theNAIP. The authors provide an account of the different occur-rences and their petrological and geochemical characteristicsand discuss the effect and influence of the Icelandic hot spotfor the PGE mineralization. In this paper, standard cumulatenomenclature are followed as defined by Irvine (1982),except for the widely used and accepted terms “allivalite”(bytownite troctolite) and “eucrite” (bytownite gabbro)(Brown, 1956). The platinum-group elements (PGE) aretaken to include gold, and the platinum-group minerals(PGM) are mineral compounds with PGE as major con-stituents. The Mg# denotes the atomic Mg/(Mg+Fetotal) andis expressed in percent.

Geology of the Icelandic Hot Spot

A very large topographic ridge that extends from the coastof East Greenland via Iceland to the Faeroe Islands dominatesthe North Atlantic Ocean. It is evident from geophysical meas-urements that this ridge represents an anomalously large accu-mulation of mafic magmatic rocks (White, 1988). Onshorevolcanic and intrusive rocks of roughly similar ages (Palaeoceneand early Eocene) are exposed on both sides of the Atlantic; atCape Dyer on Baffin Island; in West Greenland on Qeqer-tarssuaq, the Nuussuaq Peninsula, and the Svartenhuk Penin-sula; in East Greenland from Kap Gustav Holm in the south,along the Blosseville Kyst, around Hold with Hope, to Shan-non Ø in the north; on the Faeroe Islands; the submergedVøring Plateau off the coast of Norway; and on the British Islesin western Scotland and northern Ireland (Fig. 1). Plate tec-tonics link these occurrences to an extensive plateau-basaltprovince, described in this paper as the North Atlantic IgneousProvince (NAIP). Measured stratigraphic sections exceed 6.5km of lavas in the Kangerlussuaq area (Pedersen et al., 1997)and numerous layered intrusions are exposed near the base ofthe succession. When the North Atlantic Ocean opened, thetholeiitic magmatic activity moved offshore, and at the presentday continues on Iceland after more than 60 Ma. On EastGreenland, alkaline and syenitic magmatism continued wellafter the tholeiitic activity ceased.

The anomalously high production of magma in the NAIPhas been linked to the manifestation of a mantle “hot-spot” thatis now situated under Iceland. This link is supported by theoccurrence of primitive Mg-rich basalts and picrites with esti-mated primary compositions of 18 to 21 wt% MgO (Gill et al.,1992; Larsen and Pedersen, 2000), liquidus temperatures up tosome 1560°C (Kent and Fitton, 2000; Larsen and Pedersen,2000), and trace element signatures indicating deep mantle melt-ing (17.5–37 kbar, Scarrow et al., 2000) for the majority of lavas.

Mantle Source Material

Geochemical and geophysical investigations demonstratethat the Iceland hot spot is the surface representation of a narrowzone of upwelling of deep mantle material (a mantle plume). Theplume trail has been traced by geophysical methods to a depth of

Platinum-Group Elements in the Palaeogene NorthAtlantic Igneous Province

JENS C.Ø. ANDERSEN, MATTHEW R. POWERCamborne School of Mines, University of Exeter, Redruth, Cornwall, United Kingdom

PETER MOMMENordic Volcanological Institute

Reykjavík, Iceland

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Fig. 1. (a) The NAIP displays plateau basalts (dark shaded areas) extending from Baffin Island in the west, across West and East Greenland, Iceland,and the Faeroe Islands to the British Isles in the East. Submerged plateau basalts (medium shading) are known from geophysical studies and the OceanDrilling Program to form extensive stratigraphic successions in the seaward-dipping reflector sequence offshore from East Greenland, the Rockall-Faeroe Plateau, and the Vøring Plateau. Magmatic central complexes (circles) are exposed along the coast of East Greenland and on the InnerHebrides on the British Isles, and further are known from geophysical measurements to be located offshore from NW Scotland. At present day, thehot spot is situated underneath the southern part of Iceland (circled dot). Modified from Emeleus, 1991; Larsen et al., 1989, 1998, 1999); (b) Pre-drift reconstruction of the North Atlantic area showing the relative position of the continents at the Palaeocene to Early Eocene stage (cf., Larsen etal., 1989).

Platinum-Group Elements in the Palaeogene North Atlantic Igneous Province

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Fig. 2. Fractionation of Cu (and other incompatible chalcophile elements)in residual liquids during magmatic differentiation. The compositions ofthe primary West Greenland picrite magma (P) (from Larsen and Peder-sen, 2000) and N-MORB (N) (from Rehkämper et al., 1999) are plottedfor comparison.

J.C.Ø. ANDERSEN et al.

at least 1500 km (Pritchard et al., 2000; Shen et al., 1998), andit is likely that it originated in the “D” layer at the core-mantleboundary (e.g., Bijwaard and Spakman, 1999; Christensen andHofmann, 1994; Kempton et al., 2000). The plume head isbelieved to have impacted underneath the lithosphere around 62Ma and spread laterally to a radius of possibly up to 1200 km(indicated by the synchronous onset of magmatic activity on Eastand West Greenland and the British Isles, Tegner and Duncan,1999; Tegner et al., 1998a; White and McKenzie, 1989). At pres-ent, the hot spot is confined to a narrow plume trail of maximum100 km in diameter that is well constrained in both its geophys-ical (Allen et al., 1999) and geochemical (Breddam et al., 2000)signatures. Since the onset of rifting, the magmatic activity iscomplicated by interaction between the hot spot and the tectonicspreading along the Mid-Atlantic Ridge.

Trace element and isotope studies show that hot spots arediverse and may have widely different mantle source material(Zindler and Hart, 1986). The characterization of these materi-als has been the focus of many recent isotope and trace elementstudies (e.g., Hart et al., 1986, 1992; Hauri and Hart, 1993;Zindler et al., 1982). On that basis, the systematics observed inone province cannot be assumed to apply in other types of hotspots. Large-scale PGE studies of plateau basalt provinces mustconsequently be one of the major targets for PGE research in thefuture. Only by understanding the regional PGE fractionationand relating it to the source material can we fully understand theglobal distribution and fractionation of PGE.

Geochemical evidence suggests that the Iceland mantleplume has tapped distinctly different sources in an inhomoge-neous mantle during its lifetime (Hanan and Schilling, 1997;Kerr, 1995b). A recent study into the compositional variationsin volcanic rocks from the NAIP indicates at least four distinctend-member compositions (Kempton et al., 2000). Of these,two are believed to reside in the lower mantle; the third is aplume sheath possibly picked up at the 670 km discontinuity;and the fourth, a normal mid-ocean ridge basalt (N-MORB)component.

The diverse source components involved in the Iceland hotspot indicate that a large diversity of magma compositions canbe expected. However, in terms of PGE fractionation, the singlemost critical factor is silicate-sulphide liquid immiscibility. Dur-ing sulphur-undersaturated magmatic differentiation, Pd, Pt,and Au can be expected to act as perfectly incompatible traceelements where the concentrations are governed by the degree ofpartial melting and fractional crystallization. Ir and Os (andpossibly, Ru) on the other hand, may be weakly compatible inspinel and olivine, which can influence their fractionationbehaviour. During sulphur-saturated fractionation, on the otherhand, all the PGE will be strongly partitioned into the sulphidemelt, and a strong depletion can consequently be expected inthe co-existing silicate magma.

Monitoring Sulphur Saturation in Basaltic Magmas

Platinum-group element concentration and fractionation inmafic-ultramafic magmatic systems is closely linked to sulphur

saturation (with respect to an immiscible sulphide liquid).Therefore, in a regional study it is important to evaluate the sul-phur saturation in the primary magmas. In reduced (around theFMQ buffer), primitive, iron-rich, mafic magmas(FeO>10 wt%) the solubility of sulphur is largely a function ofthe FeO content and temperature (Poulson and Ohmoto,1990). However, because sulphur is easily lost or gained throughhydrothermal processes, even mild hydrothermal alteration islikely to have a large effect in rocks with only trace concentra-tions of sulphides. The sulphur concentration therefore providesa poor monitor for sulphur saturation in ancient plateau basaltprovinces. A better monitor can be found among the chal-cophile elements, where Cu is incompatible during silicate andoxide crystallization, and relatively immobile during slighthydrothermal alteration (cf., Cruse and Seewald, 2001). Post-magmatic oxidation reduces Cu-sulphides to bornite, digenite,covellite, and eventually its native metallic state (all of which areimmobile), but rarely produces Cu-oxides (that are moremobile). Primary magmas are progressively enriched in Cu dur-ing magmatic differentiation, and consequently, when plottedagainst a monitor of differentiation (in this paper MgO wasused), the copper concentrations will vary along a semi-hyper-bolic trend (Fig. 2). Sulphur saturation (with consequent sul-phide-silicate liquid immiscibility) on the other hand, causes afractionation and depletion of Cu, Ni, Co, and PGE in theresidual silicate magma, and consequently a drop in the Cu con-centration can be expected. In the following sections, the Cuconcentration is used to monitor sulphur saturation in theNAIP. The variation of North Atlantic mid-ocean ridge basaltshas been included for comparison.

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Platinum-Group Elements in Basalts of the NorthAtlantic Igneous Province

The North Atlantic Igneous Province is one of the largest,and most long-lived magmatic provinces on the Earth. Mag-matic activity has lasted since the Palaeocene, from around 63Ma, and is continuing at the present day. PGE are known fromearlier studies to be associated with hot-spot magmatism else-where, yet only few regional studies have been carried out. Theassociation is particularly notable in the Siberian plateau-basaltprovince, where economic concentrations are recovered fromthe large Ni-Cu sulphide deposits at Noril’sk (Komarova et al.,2002; Kozyrev et al., 2002). Hot-spot magmas have also beensuggested to be involved in the formation of the Bushveld Com-plex (Hatton, 1995; Hatton and Schweitzer, 1995) that hoststhe world’s major reserves of PGE. On that account, hot-spotmagmas have potentially been involved in the formation ofsome of the richest PGE provinces on earth. Both places, how-ever, involved rather special circumstances in their PGE frac-tionation, and consequently, it is difficult to draw out thesignificance of potential mantle plumes. At Noril’sk, the Siber-ian plateau basalts were contaminated with sulphur from localgypsum- and anhydrite-bearing evaporite beds and yielded sul-phur saturation as a consequence of the contamination (cf., Nal-drett et al., 1995). This affected the PGE concentrations in largevolumes of the associated plateau basalts. Magmas associatedwith the Bushveld display extensive signs of interaction withcontinental crust (Hatton and Schweitzer, 1995).

The NAIP hosts widespread and abundant PGE occur-rences, and it is obvious that particularly favourable conditionsfor PGE concentration were available during its formation.Scattered studies (on Greenland mainly geochemical, on theBritish Isles mainly mineralogical) have been carried out aroundthe province to assess the potential for economic occurrences ofPGE. Despite the widely different approaches, the studies givethe picture of a very high potential for PGE explorationthroughout the province.

The East Greenland Plateau Basalts

The east coast of Greenland displays the greatest volume ofonshore lavas in the NAIP (Fig. 1). Uninterrupted plateaubasalts occur along 400 km of the coast — from the Kangerlus-suaq Fjord in the south to Kangertittivaq (Scoresby Sund) in thenorth — covering a total area of some 65 000 km2 with a strati-graphic thickness up to 6.5 km (Pedersen et al., 1997). Mafic-ultramafic central complexes, dykes, and smaller outcrops ofbasaltic lavas are exposed along a stretch of more than 1000 kmof the coastline from just south of Kap Gustav Holm to Shan-non Ø in the north.

Basaltic magmatism on East Greenland was initiated by aseries of picrites and crustally contaminated tholeiitic basalts (theLower Basalts, Fram and Lesher, 1997; Hansen and Nielsen,1999) overlain by the voluminous uncontaminated plateaubasalts. Tholeiitic basalts dominate the province with syenitecomplexes and minor alkaline lavas formed mainly toward the

end of the magmatic activity (Brooks and Nielsen, 1982; Larsenet al., 1989). The East Greenland plateau basalt successionappears to have formed during three discrete volcanic stages(40Ar-39Ar, Tegner et al., 1998a): Stage 1 at 62 Ma to 59 Ma (theLower Basalts, possibly triggered by plume impact); stage 2 at 57Ma to 54 Ma (the main plateau basalts, possibly triggered bycontinental breakup); and stage 3 at 50 Ma to 47 Ma (the latebasalts and the majority of layered intrusions, possibly triggeredby the passage of the plume axis under the coast) (Bernstein etal., 1998; Tegner et al., 1998a). The deepest stratigraphic levelsof the lava pile are exposed around the mouth of the Kangerlus-suaq Fjord, where a domal uplift of around 6 km has occurredsince the early Palaeogene (Brooks, 1973). The KangerlussuaqFjord and the coastline resemble a continental rift triple junc-tion. Intense dyke swarms follow the coastline and the fjord,locally almost completely replacing the pre-existing strata. Dur-ing the Palaeogene, the coast collapsed during intense faultingcausing most of the exposed strata near the coastline to be rotated20° to 30° (locally up to 60°) southward (Nielsen, 1978).

The Lower Basalts form a succession of picrites with thininterbedded units of contaminated tholeiitic basalts. The picritemagmas have been estimated as having up to 17 wt% MgO andare believed to have differentiated ultramafic cumulates at8 kbar before eruption (Fram and Lesher, 1997). Their osmiumisotopic compositions are near chondritic indicating that theywere unaffected by contamination (Brooks et al., 1999). Thecontaminated tholeiites, on the other hand, have isotopic ratiosconsistent with assimilation of 15% to 20% Archean crust(Fram and Lesher, 1997).

The main plateau basalts can be subdivided into geochem-ically distinct groups that are most clearly expressed by the dif-ferences in their TiO2 content (Larsen et al., 1989; Nielsen,1978; Tegner et al., 1998a, Hald and Tegner, 2000). The low-Tibasalts have 0.8 to 2 wt% TiO2; the high-Ti basalts 1.5 to4 wt% TiO2; and very high-Ti basalts 4 to 5.5 wt% TiO2 (Teg-ner et al., 1998a; Hald and Tegner, 2000). The offshore basaltsdrilled during the ODP legs 152 and 163 almost exclusivelybelong to the low-Ti series (Larsen et al., 1999). The correla-tions between the basalts have not yet been fully worked out.

In all of the analyzed East Greenland plateau basalts, Cudisplays an almost perfect incompatible trace element behaviourindicating that sulphur saturation did not occur (Fig. 3). As aconsequence, magmatic differentiation could enrich Cu andPGE to very high concentrations, and any sulphide-bearingmafic-ultramafic cumulates in the region are likely to host sig-nificant concentrations of PGE. On the other hand, the lack ofextensive volumes of sulphide-saturated melts suggests that, ifpresent, Noril’sk type PGE-bearing sulphide deposits in the areaare likely to be relatively small.

Nielsen and Brooks (1995), Momme (2000), and Mommeet al. (2002) have studied PGE in the East Greenland plateaubasalt province. Nielsen and Brooks (1995) investigated dykesand the Lower Basalts in the region around the KangerlussuaqFjord, and presented a model for PGE fractionation in theSkaergaard Intrusion and the Kap Edvard Holm Complex.Momme (2000) carried out a detailed study of the Lower

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Fig. 3. MgO-Cu in East Greenland basalts (data from Larsen et al., 1989;Momme, 2000; and Hald and Tegner, 2000). Basalts from Kangertittivaq(157 analyses, Larsen et al., 1989), the Blosseville coast (58 analyses;Momme, 2000; L.M. Larsen, pers. comm., 2001), and dykes on Jame-son Land (43 analyses, Hald and Tegner, 2000) evolve along narrowlydefined sulphur-undersaturated trends in the diagram. The most MgO-richBlosseville coast data can be explained in terms of olivine accumulation.A plagioclase ultraphyric basalt sill (PUB, 5 analyses, Hald and Tegner,2000) on Jameson Land appears to have anomalously low Cu concen-trations consistent with sulphur-saturated fractionation. Data for NorthAtlantic N-MORB for comparison were collected from DSDP Leg 37 (situ-ated well outside the influence of the Icelandic hot spot). The N-MORBevolves along a sulphur-saturated trend below 8 to 10 wt% MgO, and thevariations above ~10 % MgO can be explained by olivine accumulation.Three anomalous analyses have been excluded.


Basalts, plateau basalts, dykes, and sills in the area and recog-nized distinct differences between the different suites of basalts.Philipp et al. (2001) presented a study of PGE in the basalt suc-cession off the coast of SE Greenland (ODP legs 152 and 163)that is treated separately below.

The Lower Basalts

Investigations on the Lower Basalts by Nielsen and Brooks(1995), Momme (2000), and Momme et al. (2002) show thatthey have low PGE concentrations and experienced only lowdegrees of PGE fractionation. Pt, Pd, and Ir concentrations varybetween 1 to 8, 2 to 9 and 0.2 to 1.1 ppb, respectively. Thepicrites have the highest concentrations but, apart from that, theLower Basalts show no correlation of the PGE concentrationswith Mg# (Momme, 2000). Brooks et al. (1999) demonstratedthat the picrites represent sulphur-undersaturated, uncontami-nated, primitive magmas (based on their osmium isotopic com-

position and Cu-Pd ratio), and the PGE variations are likely tobe related directly to their source material. The evolved tholei-ites display negative correlations between the PGE concentra-tions and Zr and poor correlations between the PGE and Mg#interpreted by Momme (2000) to reflect sulphide saturation andfractionation due to crustal contamination. Cu concentrationsin the tholeiites show a similar scatter with concentrations gen-erally below the incompatible fractionation trend (Fig. 3), whichcan be explained in a similar way.

Low-Ti Plateau Basalts

The low-Ti series basalts have compositions from whole-rockMg# of 75% to 50%, where the most Mg-rich carry abundantolivine phenocrysts. The basalts display a wide range of PGE-con-centrations that correlate with Mg# of these samples. Ir and Ptdecrease in concentration with decreasing Mg#: Ir from 0.4 ppbto 1 ppb to below detection limit (0.05 ppb) and Pt from around10 ppb to 2.5 ppb. Cu and Pd, on the other hand, increase from100-130 to 245 ppm Cu and around 10-24 ppb Pd.

The close correlation between the PGE and major elementvariations suggests that primary processes of magma differentia-tion without crustal contamination govern the distribution ofPGE. The PGE trends can be explained with compatible behav-iour of Ir and Pt and incompatible behaviour of Pd (and Cu)during fractionation. Although a well-established hydrothermalsystem resulted in a slight alteration to greenschist facies miner-als, no evidence is seen for a redistribution of any of the PGE(Momme, 2000; Momme et al., 2002). The observed incom-patible behaviour of Cu and Pd strongly suggest sulphur-under-saturated differentiation (cf. Momme, 2000).

The High- and Very High-Ti Plateau Basalts

Both the high-Ti and very high-Ti series basalts are rich inincompatible elements, e.g. (La/Sm)N 1.2-1.6 (N denotes man-tle normalization). This has been interpreted to reflect lowdegrees of partial melting (3% to 9%) and segregation from anasthenospheric mantle source at depth (Tegner et al., 1998b). Pt(3 ppb to 11 ppb) and Ir (<0.05 ppb to 0.22 ppb) correlate pos-itively with compatible elements, e.g., Cr and Ni, whereas, Pd(6 ppb to 25 ppb) and Cu (120 ppb to 450 ppm) clearlyincrease with differentiation (Mg# = 60-43%) (cf. Momme,2000; Momme et al., 2002). In addition to the incompatiblebehaviour of Pd and Cu, the relatively constant Cu/Pd ratios(high-Ti series: 12 3 103 –33 3 103; very high-Ti series: 18 3 103 –35 3 103) of these basalts clearly indicate that theydid not fractionate any sulphide during differentiation (Barneset al., 1993; Momme, 2000; Momme et al., 2002).

Dykes and Sills

The East Greenland plateau basalt province includes anumber of prominent dyke and sill complexes, notably thecoast-parallel (THOL-1, Nielsen, 1978) and the Skaergaard-likedyke swarms (THOL-2, Brooks and Nielsen, 1978; Nielsen,

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Fig. 4. MgO-Cu in the East Greenland offshore basalts (data from ODPLegs 152 and 163). The syn-breakup picrites (15 analyses) and tholeiitesfrom site 918 and 989 (18 analyses) display sulphur-undersaturated frac-tionation, whereas, the pre-breakup basalts (28 analyses) and syn-breakup basalts from site 990 (29 analyses) display sulphur-saturatedfractionation trends. The sulphur-undersaturated trend falls within thetrend of the Blosseville coast basalts, whereas, the sulphur-saturated fol-low that of North Atlantic N-MORB. Three anomalous analyses have beenexcluded.


1978); the Sorgenfri Gletcher (Gisselø et al., 1999), the Jame-son Land (Larsen and Marcussen, 1992), and the Traill Ø sillcomplexes (Price et al., 1997).

Significant differences are observed between the PGE con-centration in the THOL-1 and THOL-2 dyke swarms and theLower Basalts (Nielsen and Brooks, 1995). When compared withthe Lower Basalts, the THOL-1 dykes display slightly higherdegrees of fractionation and slightly higher concentrations of Pt,Pd, and Au (Nielsen and Brooks, 1995). The THOL-2 dykeshave similar major, trace and PGE distributions and are identicalto the similarly evolved high-Ti series basalts. They have highPGE concentrations between 15 to 27 ppb Pd, 1 to 20 ppb Ptand <0.05 ppb to 0.4 ppb Ir in samples with Mg# 60-46%.Chondrite-normalized PGE diagrams presented by Nielsen andBrooks (1995) show a slightly higher fractionation in the THOL-2 dykes compared to the Lower Basalts and the low-Ti basalts.

Chilled margin samples from the Sorgenfri Gletscher andTraill Ø sill complexes are in many respects similar to the high-and very high-Ti series of the plateau basalts (Gisselø et al.,1999). However, despite the samples have evolved compositions(Mg# = 58-43%), their PGE concentrations are significantlylower than similarly evolved plateau basalts (Pd: 5 ppb to 9 ppb,Pt: 3 ppb to 7 ppb, Ir: 0.08 ppb to 0.5 ppb). This could indi-cate that the sill complexes have fractionated PGE at depth. Cuconcentrations have not been found in the literature.

Of particular interest are the Jameson Land and Traill Ø sillcomplexes (Larsen and Marcussen, 1992; Price et al., 1997;Hald and Tegner, 2000). Both complexes are situated in a riftbasin just to the north of Kangertittivaq and intrude a sedimen-tary succession that include gypsum-bearing Permo-Triassicstrata that locally include copper-lead-zinc mineralization(Clemmensen, 1980; Larsen and Marcussen, 1992; Thomassenet al., 1982). As such, the complexes form interesting analogiesto intrusions in the Noril’sk region. A plagioclase-ultraphyricbasalt sill (Hald and Tegner, 2000) in the Jameson Land sillcomplex has low Cu concentrations indicating that it has frac-tionated sulphides (Fig. 3).

The East Greenland Offshore Basalts

The ODP Legs 152 and 163 drilled a substantial successionof plateau basalts off the Southeast Greenland coast (Fig. 1). Thelegs are located some 400 km to 500 km south of the axis of theIcelandic hot spot and transect mostly pre- and syn-breakuplavas comparable in age with the East Greenland basalts (Bern-stein et al., 1998). Post-breakup basalts are only reported fromsite 988 (Tegner and Duncan, 1999). Cu concentrations evolvealong two distinctly different trends; picrites and uncontami-nated tholeiites evolve along a similar sulphur-undersaturatedevolution trend as the East Greenland basalts, but contaminatedbasalts evolve along a sulphur-saturated trend (Fig. 4).

Pre-breakup Basalts

The pre-breakup (continental) basalts are represented bythe lower and middle series from site 917 (Sinton and Duncan,

1998; Tegner and Duncan, 1999). These basalts range in com-position from picrite to evolved tholeiite and display signs ofhaving undergone fractionation and crustal contamination dur-ing their evolution (Larsen et al., 1998). The pre-and early syn-breakup basalts are distinctly continental in contrast to the latesyn- and post-breakup basalts that display oceanic trace-elementsignatures (Larsen et al., 1999).

The pre-breakup basalts display scattered PGE concentra-tions that correlate poorly with the MgO concentration of thebasalts (Philipp et al., 2001). Concentrations vary from 0.10 to0.60 ppb Ir; 0.39 to 5.55 ppb Pt; and 0.52 to 5.91 ppb Pd(Philipp et al., 2001). This indicates that sulphur-saturationtook place to a variable extent during contamination and frac-tionation — similar to the evolved tholeiites in the LowerBasalts in East Greenland and the Vaîgat formation in WestGreenland (see below).

Syn-breakup Basalts

The syn-breakup basalts are represented by the upper seriesfrom site 917 (Sinton and Duncan, 1998; Tegner and Duncan,1999) and sites 915, 918, 989, and 990. The basalts displaytwo distinctly different suites: Site 917 transects a succession of

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high-Mg basalts and picrites with up to 24 wt% MgO; theother sites display successions of slightly younger evolvedoceanic tholeiites.

Major and trace element analyses suggest that the picritesand high-Mg basalts have undergone little or no fractionation orcontamination during their formation (Larsen et al., 1998). Thelavas evolve along a similar fractionation trend as the high-Tibasalts in East Greenland (Larsen et al., 1999). They displaystrong correlations between the PGE and the MgO concentra-tions. Concentrations vary from 2.5 ppb Ir and 10 ppb Pt andPd in the most magnesian rocks to 0.5 ppb Ir, 14 ppb Pt, and16 ppb Pd in the least magnesian rocks (Philipp et al., 2001).The decreasing trends of Ir, Ru, and Rh with decreasing MgOindicate compatible behaviour, and increasing trends of Pd andPt incompatible behaviour during magmatic differentiation.These fractionation trends are consistent with sulphur-under-saturated fractionation (Philipp et al., 2001) during the accu-mulation or removal of olivine and chromite.

The tholeiite suite is more evolved and shows much moreuniform compositions than the pre- and early syn-breakupbasalts (Mg# = 62.3-49.3%, 8.23-6.27 wt% MgO). They havein previous papers been described as post-breakup basalts (Fit-ton et al., 2000; Larsen et al., 1999), but although they are dis-tinctly oceanic in their trace-element signatures, they comparein age to the late syn-breakup basalts of the East Greenlandprovince (cf. Tegner and Duncan, 1999; Tegner et al., 1998a)and display isotopic evidence of crustal contamination (Larsenet al., 1999; Saunders et al., 1999). The basalts evolve along alow-Ti fractionation trend.

In the evolved tholeiites, all of the PGE evolve alongdecreasing trends with decreasing MgO: Ir from 0.4 ppb to 0.1ppb, Pt from 14 ppb to 3 ppb, and Pd from 13 ppb to 2 ppb(Philipp et al., 2001). This was interpreted by Philipp et al.(2001) to signify compatible behaviour during sulphur-satu-rated fractionation. Cu concentrations confirm sulphur-satu-rated fractionation trends in basalts from site 990, but basaltsfrom site 918 and 989 display clear sulphur-undersaturated frac-tionation similar to the fractionation observed in the EastGreenland basalts. They display a consistent trend of decreasingPd with increasing Cu incompatible with sulphur-saturatedfractionation. Instead, this indicates (confer with our discussionbelow) that they represent mixing between two distinctly differ-ent magma sources — one rich in PGE and poor in Cu, theother rich in Cu and poor in PGE. This could be, for example,a hot spot and a N-MORB component.

The West Greenland Plateau Basalts

Palaeogene plateau basalts are exposed along the west coastof Greenland on Disko Island, the Nuussuaq Peninsula, Ubek-endt Ejland, and the Svartenhuk Peninsula. The basalts formedbetween 64 and 59 Ma ago (Larsen et al., 1992) covering an areaof some 55 000 km2 with a maximum stratigraphy of around5 km (Clarke and Pedersen, 1976). Unlike East Greenland, nomajor intrusive centres are exposed, the only known centralcomplex being the small (15 km2) Sarqâtâ qátâ gabbro-

granophyre complex on Ubekendt Ejland. In composition thelavas vary from picrites to olivine and quartz tholeiites withminor occurrences of trachybasalt, trachyte, nepheline norma-tive olivine basalt, and lamprophyre. In contrast to East Green-land, no major occurrences of syenite are exposed. The volcanicsuccession is divided into a lower Vaîgat formation dominatedby picrites and olivine tholeiites, and an upper Malîgat forma-tion dominated by feldspar-phyric quartz tholeiites (Clarke andPedersen, 1976).

The Vaîgat formation consists of an approximately 3 kmstratigraphic succession of picrites interrupted by minor unitsof evolved tholeiites and thin beds of marine mudstones(Larsen et al., 1992; Lightfoot et al., 1997). Isotopic evidencesuggests that the Vaîgat picrites represent the primitive, uncon-taminated products of the early mantle plume (e.g., Holm etal., 1988; Holm et al., 1993; Schaefer et al., 2000). The tholei-ites, on the other hand, carry abundant xenoliths of shale aswell as up to metre-sized blocks of troilite and metallic iron(Ulff-Møller, 1985) and display strong compositional evidenceof contamination. The formation consists of two magmaticcycles, each initiated by submarine picritic pillow breccias andhyaloclastites that gradually make the way for successions ofthin (3 m to 5 m thick) subaerial lava flows toward the top(Larsen et al., 1992). Thick, massive plagioclase-phyric lavas oftholeiitic affinity, in contrast, dominate the Maligat formation.The formation displays both submarine and subaerial lavas nearits base but evolves to exclusively subaerial conditions upward(Larsen et al., 1992).

Copper concentrations in West Greenland volcanic rocksare considerably higher than mid-ocean ridge basalts and basaltson East Greenland (Fig. 5). The Vaîgat picrites and most of thebasalts of the Malîgat formation evolve along the incompatibletrace-element trend. In contrast, the contaminated basalts fromthe Vaîgat formation appear to have fractionated large amountsof sulphides (Lightfoot et al., 1997). The higher copper con-centrations must reflect directly on the mantle source material,and it is likely that it represents a similar component as in theEast Greenland offshore basalts. Unlike East Greenland, largeamounts of magmas have low concentrations of copper indicat-ing that sulphur-saturation occurred at an early stage. The dom-inant sulphur-saturated lavas, however, appear to be some of theearliest contaminated tholeiites of the Vaîgat formation. There-fore, in contrast to the Noril’sk region, they have not had thepossibility of mixing with (and collecting PGE from) evolvedsulphur-undersaturated magmas in basal intrusions below thelava pile. Consequently, the hope of finding large Noril’sk-typeNi-Cu-PGE sulphide deposits in West Greenland is limited(Lightfoot et al., 1997). Given the favourable conditions, how-ever, possibilities for gabbro-hosted PGE mineralization similarto East Greenland still exist.

Pearson et al. (1999) and Schaefer et al. (2000) report a fewPGE and Os-isotopic analyses. Their studies are consistent withearlier conclusions that the Vaîgat picrites represent primaryuncontaminated magmas from the plume head, and that theyformed by large degrees of partial melting (>25%) under sul-phur-undersaturated conditions with residual garnet in the

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mantle (Schaefer et al., 2000). The picrites display unfraction-ated PGE patterns (Pd-Ir ratios of 5.8-8.0) and unusually highIr and Os concentrations (Pearson et al., 1999; Schaefer et al.,2000). 187Os/188Os are close to chondritic values, but186Os/188Os reach supra-chondritic values suggesting involve-ment of a source with high Re/Os and Pt/Os values (Pearson etal., 1999). Similar observations were made on Hawaii by Bran-don et al. (1998), where they have been interpreted to representa slight entrainment of material from the earth’s core in themantle plume.

The Palaeogene British Volcanic Province

The Palaeogene British Volcanic Province (previouslyknown as the British Tertiary Volcanic Province) is an elongateregion of predominantly basic volcanism that stretches fromnorthwest Scotland to northeast Ireland together with sporadicigneous bodies that are present across northern England and asfar south as Lundy in the Bristol Channel. In western Scotland,crustal extension and fracturing along the present-day NorthAtlantic margin has been occurring since the late Palaeozoic andhas resulted in basins filled with a thick Mesozoic successionseparated by ridges of Precambrian basement. Increased crustalthinning, coupled with enhanced heat flow associated with the

proximity of the Iceland hot spot, resulted in the generation oflarge volumes of basic magma (e.g., Emeleus, 1997) giving riseto the plateau basalts, dyke swarms and central intrusive com-plexes. The lavas cover an area of some 7200 km2 and reach athickness of up to 1.8 km (Emeleus, 1991). The magmaticactivity initiated at around 63 Ma (Pearson et al., 1996) with theextrusion of plateau basalts on Muck and Eigg and extendeduntil about 52 Ma with the extrusion of the Sgurr of Eigg pitch-stone (Dickin and Jones, 1983) by which time magmatism waslargely restricted to the zone of active rifting. Age determina-tions of the main gabbroic components of some of the centralcomplexes such as Rum (60.53 ± 0.08 Ma) and Skye (58.91 ±0.07 Ma) (Hamilton et al., 1998) indicate that the basic mag-matism was typically short lived. Indeed, the Rum intrusive cen-tre had been unroofed and was actively eroded down to similarelevations as today (Emeleus, 1985) by the time basalts of theCanna lava formation were extruded at 60.1 Ma (Mussett et al.,1988) suggesting locally very rapid uplift and erosion.

No regional study of PGE has been carried out in the lavasof the province, but copper concentrations from Kent and Fit-ton (2000) indicate that both sulphur-undersaturated and sul-phur-saturated fractionation took place (Fig. 6). The basalts plotfrom an upper limit on the trend for the East Greenland basaltsto lower values, the lower limit being that of mid-ocean ridgebasalts. Consequently more diverse possibilities exist for PGEfractionation than on East Greenland.

The Icelandic Plateau Basalts

Basalts on Iceland record the most recent activity (16-0 Ma,Saemundsson, 1980) of the hot spot. The Mid-Atlantic spread-ing ridge transects the island and the oldest rocks are foundtoward the northwest and the southeast. Tholeiitic basalts dom-inate the magmatic activity with minor alkaline activity in vol-canoes off the central rift axis. Basalts on Iceland (data fromHardarson et al., 1997) display both sulphur-undersaturatedand sulphur-saturated trends (Fig. 7). Cu concentrations varybetween the incompatible trend characteristic for East Green-land and the Cu-poor N-MORB trends. This gives the possibil-ity for a large diversity of PGE fractionation mechanisms.

Relatively few PGE analyses have been published from Ice-landic basalts. Rehkämper et al. (1999) present analyses of sixMORB samples from Kolbeinsey Ridge and four olivine tholei-ites from Iceland. The analyses display positive correlationsbetween Ni, Pd (0.09 ppb to 6 ppb), Pt (0.04 ppb to 2.5 ppb),and Ir (0.02 ppb to 0.09 ppb) concentrations in the six MORBsamples, which Rehkämper et al. (1999) interpreted to reflectsulphur-saturated differentiation during the fractionation of sil-icate phases. One Kolbeinsey Ridge MORB sample, with 6.1ppb Pd, 2.5 ppb Pt and 0.09 ppb Ir, has PGE concentrationssignificantly higher than other MORB samples, and probablyreflects the elevated degree of partial melting beneath Kolbein-sey Ridge compared to other oceanic spreading ridges. The fourIcelandic olivine tholeiite samples have 0.8-16.7 ppb Pd, 1.21-10.9 ppb Pt, and 0.22-0.03 ppb Ir, and display sulphur-saturated trends similar to the Kolbeinsey Ridge MORB sam-

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Fig. 5. MgO-Cu in the West Greenland plateau basalt province (data fromLightfoot et al., 1997). The Vaîgat picrites (26 analyses) evolve along asulphur-undersaturated trend; whereas, the contaminated Vaîgat tholei-ites (18 analyses) display sulphur-saturation below around 13 wt% MgO.The Malîgat tholeiites (12 analyses) display both sulphur-undersaturatedand -saturated fractionation. Four anomalous analyses have beenexcluded. Trends for the Blosseville coast basalts and North Atlantic N-MORB are included for comparison.


ples. An off-rift alkaline basalt (7.8 wt% MgO) analyzed fromthe Snaefellsnaes Peninsula has extremely high Pd concentra-tions (66 ppb) and has probably accumulated sulphide that orig-inally exsolved from the silicate magma.

Platinum-group elements in Icelandic picrites, olivinetholeiites, and Mg-poor evolved tholeiitic basalts are the subjectof an ongoing project by P. Momme. Preliminary results indi-cate that Early Holocene picrites (having 14 to 10 wt% MgO)from Theystareykir (northern Iceland) and the ReykjanesPeninsula (southern Iceland) have variable Pd (4 ppb to 17ppb), Pt (4 ppb to 7 ppb), and Ir (0.1 ppb to 0.3 ppb) concen-trations. The PGE display positive inter-element correlations,but show negative correlations with Cu and Au (similar to theEast Greenland offshore basalts). The most PGE-rich and Cu-poor sample is a picrite from Theystareykir (Mg# 75%, 74 ppmCu, 17 ppb Pd, 7 ppb Pt, and 0.3 ppb Ir), whereas, the low-PGE and high-Cu end-member is a phenocryst-poor primitiveolivine tholeiite sample (Mg# = 70%, 145 ppm Cu, 6 ppb Pd,4 ppb Pt, and 0.1 ppb Ir), i.e., Cu and PGE vary systematicallybetween primitive olivine tholeiite and the most depletedpicrite. The observed variations between PGE and Cu are incontrast to any expected magmatic differentiation trend. Theycan, however, be generated by mixing of magmas from the twodistinctly different sources (a PGE-rich Cu-poor, and a PGE-poor Cu-rich source) suggested for the East Greenland offshorebasalts above.

The olivine tholeiites (7 to 10 wt% MgO) display decreas-ing Ir (from 0.15 ppb to 0.05 ppb) and increasing Pd (from 4ppb to 18 ppb) with decreasing MgO, indicating sulphur-undersaturated differentiation. The highest Pd concentration inthis study is found in an evolved olivine tholeiite (Mg# = 53%,18 ppb Pd, 0.13 ppb Ir) from central Iceland, which must havedifferentiated sulphur-undersaturated. The high PGE-concen-trations found in the Icelandic picrites and olivine tholeiitescontrasts with the overall low concentrations in PGE in theevolved (7 to 4 wt% MgO) samples reflecting the sulphur-satu-rated status of Icelandic magmas with less than 7 wt% MgO.

PGE Occurrences in East Greenland Intrusions

At least 10 major (and several minor) layered intrusions areexposed in the Kangerlussuaq area of East Greenland and alongthe coast (Fig. 8). The intrusions appear to form two distinctmagmatic suites contemporaneous with the later parts of theplateau basalt volcanism; the older being 57 Ma to 54 Ma(including the Skaergaard Intrusion, the Imilik I Intrusion, andthe Sorgenfri Gletcher Sill Complex) and the younger 50 Ma to47 Ma (including the Kap Edvard Holm Complex, the KruuseFjord Complex, the Imilik II and III intrusions, the LilloiseIntrusion, and the Nordre Aputitêq Complex) (cf. Tegner et al.,1998a). PGM have been reported from the Skaergaard Intru-sion (Andersen et al., 1998; Bird et al., 1991; Nielsen, 2001),

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Fig. 6. MgO-Cu in the British Volcanic Province (106 analyses from Kentand Fitton, 2000, and R. Kent, pers. comm., 2001). The basalts mostlyfall below the sulphur-undersaturated evolution trend indicating that theyhave fractionated significant amounts of sulphides. Trends for the Blos-seville coast basalts and North Atlantic N-MORB are included for com-parison.

Fig. 7. MgO-Cu in Icelandic plateau basalts (133 analyses from Hardar-son et al., 1997). The Icelandic plateau basalts display a similar sulphur-undersaturated fractionation as the Early Palaeogene plateau basalts.Trends for the Blosseville coast basalts and North Atlantic N-MORB areincluded for comparison.

Fig. 9. Geological map of the Skaergaard Intrusion (modified after McBir-ney, 1989a). The Skaergaard Intrusion is situated across the unconfor-mity between the Archean basement, a thin succession of sediments(horizontal ruling) and the plateau basalts (inverted v). The intrusion issubdivided internally into a Marginal Border (MBS), an Upper Border(UBS), and a Layered Series. The Layered Series consists of a Lower(LZ), a Middle (MZ) and an Upper Zone (UZ). The intrusion has later beencrosscut by younger intrusions (the Vandfaldsdalen Macrodyke and theBasistoppen Sheet, crosshatched). The Platinova reefs (crosses) occur inthe uppermost 100 m of the MZ cumulates and can be traced in outcropacross the entire Layered Series. Glaciers and glacial deposits are white,the sea is marked by a light grey shading.

Fig. 8. The East Greenland plateau basalt province. The plateau basaltsare exposed along the Blosseville coast from the Kangerlussuaq Fjord toKangertittivaq. The base of the succession is exposed around the mouthof the Kangerlussuaq Fjord, where it overlies conformably a successionof Cretaceous to Palaeocene sediments. The sediments were depositedunconformably on the Archean basement. Intrusions mentioned in thetext are: 1. The Skaergaard Intrusion; 2. The Kap Edvard Holm Complex;3. The Kruuse Fjord Complex; and 4. The Nordre Aputitêq Intrusion. Lightgrey areas represent the sea, the inland ice, white.


the Kap Edvard Holm Complex (Arnason and Bird, 2000; Birdet al., 1995), the Kruuse Fjord Complex (Arnason et al., 1997a),the Miki Fjord Macrodyke (Arnason and Bird, 1994), and theNordre Aputitêq Intrusion (Arnason and Bird, 1994). In addi-tion, PGE have been reported from the Kraemer Ø Macrodyke(Momme, 2000).

The Skaergaard Intrusion

The Skaergaard Intrusion (Fig. 9) hosts a succession ofstratiform layers rich in Au and Pd described by Bird et al.(1991), Andersen et al. (1998), and Nielsen (2001). The layerswere discovered during an exploration program in 1987 byPlatinova Resources Ltd. (now Platinova A/S) in a zone of dis-tinct rhythmic layering known as the Triple Group (Wager andDeer, 1939). Further sampling and drilling carried out in 1988-1990 confirmed the existence of a stratabound PGE-rich zone,here referred to as the Platinova reefs that can be traced in out-crop and followed underground across 2/3 of the intrusion. TheAu resource was estimated by Platinova to 1.8 ppm Au in 91million tons of ore (cf., Andersen et al., 1998), and a recent esti-mate of the Pd yielded 1.87 ppm total Pd in 280 million tonsof ore over a 4.7 m mining width at a 1 ppm cut-off (Nielsen,2001). As such, the Platinova reefs constitute by far the largestproven resource in the province.

The Skaergaard Intrusion is the type example of extrememagmatic differentiation by fractional crystallization in a closedsystem (Wager and Brown, 1968; Wager and Deer, 1939). It iswell exposed, extremely well described, and its magmatic andpost-magmatic history is well known. As such it provides anideal place to study processes of PGE fractionation. The intru-sion displays three lithological series (the Layered, Marginal

Border, and Upper Border series) evolving toward the centre ofthe intrusion with roughly parallel fractionation trends (Fig. 9).Irvine et al. (1998) provide an extensive review of the geology,and its structure, and volume; Nielsen (2001) modelled thechemical evolution.

The Skaergaard Intrusion formed during the syn-breakupmagmatic stage and is one of the oldest of the Palaeogene intru-sions on East Greenland. It has been dated at 55.7 ± 0.3 Ma(40Ar-39Ar, Hirschmann et al., 1997), which is significantlyolder than the majority of intrusions that formed at the post-breakup stage (Bernstein et al., 1998; Tegner et al., 1998a). Theinitial magma was evolved from Ti-rich tholeiitic basalt proba-

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Fig. 10a. Structural relations of the Platinova reefs (from Andersen et al.,1998). The subsurface extent of the intrusion has been constructed fromgravimetric data from Blank and Gettings (1973) as referred by McBirney(1975), the internal subdivision is constructed from drill core intercepts.The topography is a graphical representation of the intrusion traced fromthe topographical map, (A) north-south section facing east from UttentalSund, (B) east-west section facing south from Forbindelsesgletcher. Lightshading denotes lithologies within the intrusion, dark shading the hostlithologies.


bly similar to the high- or very high-Ti plateau basalts (Momme,2000, supported by the compositional modelling by Nielsen,2001). The earliest formed cumulates have plagioclase withAn73 and olivine with Fo74 (Hoover, 1989), and are extremelypoor in Ni and Cr (McBirney, 1989b) indicating that the initialmagma had fractionated substantial amounts of olivine andchromite before entering the magma chamber. A suite ofchromite-bearing peridotite blocks included in the MarginalBorder series may represent fragments of this early precipitate(Irvine et al., 1998). The liquid line of decent is disputed, butmost authors favour a trend of iron-enrichment up to the lateststages of the crystallization — perhaps reaching 20 to 22 wt%FeO (McBirney, 1975; Tegner, 1997; Wager, 1960; Wager andBrown, 1968). The magmatic differentiation involves systematictrends in mineral assemblages (having progressively more phaseson the liquidus), mineral compositions (toward progressivelylower temperature compositions), and trace elements (consis-tent with predicted distribution coefficients for the fractionatingmineral assemblage), with little or no change in initial Sr- orNd-isotopic compositions. All these factors are consistent withmagmatic differentiation by fractional crystallization withminor contamination in a closed system (cf., McBirney, 1975;Stewart and DePaolo, 1990; Wager and Brown, 1968; Wagerand Deer, 1939).

The Platinova reefs (Fig. 10) occur in a succession of large-scale modal layering known as the Triple Group (Wager andBrown, 1968; Wager and Deer, 1939). The host rocks consistof plagioclase-augite-titanomagnetite-ilmenite cumulates withsporadic pigeonite or olivine. Trace interstitial minerals includeapatite, baddeleyite, quartz, and Cu-Fe sulphides. The reefsconsist of possibly more than 10 distinct layers rich in PGE

that are perfectly concordant with the modal layering. The lay-ers get successively smaller upward, with the lowermost span-ning the entire width of the Layered series and the uppermostbeing confined to its centre. The reefs are laterally and strati-graphically zoned with the highest concentrations of Pt(although quantitatively minor) occurring at the centre of thelowermost layer, Pd dominating the lower and central parts ofthe succession, and Au being concentrated toward the marginsand in the uppermost layers. The highest concentrations of Pdare found consistently in the lowermost reef and the Au in thelocally uppermost reef. This gives a systematically increasingseparation of the Pd and Au toward the centre of the intrusion— from only a few metres near the margin to 65 m in the cen-tre (Andersen et al., 1998).

The PGE occur in discrete mineral grains associated withthe first occurrence of interstitial Cu-Fe sulphides in the cumu-lates. The sulphides are nowhere abundant and occupy typicallyless than 0.5 modal percent. They consist of an assemblage ofbornite, digenite, and chalcopyrite with minor idaite. Nickel-and Fe-sulphides have not been found, but abundant magnetiteis intimately associated with the sulphides — suggesting exten-sive post-magmatic oxidation (Andersen et al., 1998). Themajority of PGM are completely or partially enclosed within thesulphide minerals, but rare grains are included in the postcu-mulus rims of the primary silicate minerals and a few have beenfound in interstitial secondary silicates (amphibole, biotite).

The dominant PGM are alloys close to the stoichiometriccomposition (Cu,Fe)(Pd,Au,Pt), but a variety of other phasesare locally significant. Grains of electrum, atokite, zvyagintse-vite, vasilite, keithconnite, arsenopalladinite, hongshiite, kotul-skite, and melonite have been reported (Andersen et al., 1998;Arnason and Bird, 1995; Bird et al., 1991; Nielsen, 2001) alongwith an unnamed mineral of the composition (Pd,Cu)2S (Arna-son and Bird, 1995). The PGM assemblage in the sulphides

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Figure 10b. The Platinova reefs occur in the upper part of the MZ in aprominent unit of macro-rhythmic layering, the Triple Group. Drill coreassay values are compiled above the lithological cross-sections, whiterepresents the Pd and black the Au concentrations. The individual leuco-cratic layers (TG-0L, TG-1L, and TG-2L) are traced across the assayedlogs. The uppermost Triple Group unit occurs above the sections.

Fig. 11. Geological map of the northern part of the Kap Edvard HolmComplex (modified after Bernstein et al., 1992; Bird et al., 1995; andArnason and Bird, 2000). The Lower (LLS) and Middle Layered series(MLS) form the lower stratigraphic units of the complex. The Willow RidgeReef occurs in the LLS in the mountains to the south of HutchinsonGletcher. Later intrusions (dark grey) include the Hutchinson GletcherSyenites I and II and a large transgressive breccia pipe. Glaciers and gla-cial deposits are white, the sea is marked by a light grey shading.


appears to be restricted to the (Cu,Fe)(Pd,Au,Pt) alloys (consis-tent with a high temperature of formation (Karup-Møller andMakovicky, 1999), whereas, the assemblage in the silicate min-erals is more varied (H. Rasmussen, pers. comm., 1999).

Brooks et al. (1999) reported that the Platinova reefs haveunusually low Os concentrations and high Re-Os ratios. TheirRe-Os isotopic compositions yield a model age significantlyyounger than the 55 Ma isochron defined by other samples fromthe intrusion. The low Os concentrations are probably a conse-quence of the late formation of the Platinova reefs within themagmatic stratigraphy (Brooks et al., 1999); the high Re-Osratio and the young model age implies that some Re mobiliza-tion occurred after the formation of the reefs — possibly duringthe oxidation and desulphurization of the sulphides.

The Platinova reefs are unusual in many respects, whencompared to the well-investigated resources in the Bushveld,Great Dyke, and Stillwater complexes. The main characteristicsof the reefs are:1. They occur high in the stratigraphic succession well aftermagnetite appeared as a cumulus phase.2. They are extremely fractionated with respect to their PGEhaving only traces of Pt and virtually no Ir, Os, Ru, and Rh.3. They contain high concentrations of Au.4. They display a compositional zoning with offset peak con-centrations of Pt, Pd, and Au.5. They are associated with Ni- and Fe-poor sulphides that areextremely Cu-rich with bornite and digenite as the major sul-phide phases.

The reefs are closely associated with the first appearance ofsulphides in the cumulates, changes in the whole-rock Cu con-centration and the Cu-S ratio (Andersen et al., 1998; Turner,1986) — all features pointing toward a formation by sulphide-silicate liquid immiscibility. However, in contrast to, for exam-ple, the Merensky and UG2 Reefs of the Bushveld, the J-M Reefof Stillwater, and the Main Sulphide zone of the Great Dyke, thereefs display no reversals to more primitive cumulates or higher-temperature mineral compositions. This indicates that the reefsformed by sulphide-silicate liquid immiscibility reachedthrough crystal fractionation. The sulphur-undersaturatedPGE-rich composition of the initial magma, combined with anextreme, closed-system magmatic differentiation, were impor-tant requisites for the formation of the reefs. Since the discoveryof the Platinova reefs, similar PGE reefs have been found else-where (Sonju Lake, Duluth, Miller, 1999; Rincon del Tigre,Brazil, Prendergast, 2000), indicating that “Skaergaard-typePGE reefs” may constitute widespread but previously unrecog-nized resources of PGE.

An interesting and puzzling aspect of the Platinova reefs isthat the ratio between Pt, Pd, and Au is very far from the man-tle ratio. This indicates that a major fractionation of the ele-ments has occurred before the reefs formed. Local fractionationprocesses within the intrusion can probably account for theabundance of Pd and Au as well as the scarcity of Os, Ir, Rh, andRu, but where is the Pt? Likely places include early fractionatedultramafic cumulates (which may be represented by a suite ofultramafic blocks in the Marginal Border series), sulphide-bear-

ing parts of the Marginal Border series, or a succession of verymagnetite-rich layers that are exposed in the Layered Seriesstratigraphy around 800 m below the reefs.

The Kap Edvard Holm Complex

Platinova Resources Ltd. explored the Kap Edvard HolmComplex for PGE from 1986. They located a zone rich in PGEin the layered gabbros of the lower part of the complex. Subse-quent drilling confirmed a stratabound layer (the reef hasremained unnamed in earlier publications, but for convenienceit is in this paper called “the Willow Ridge Reef” after the dis-covery site) extending for around 6 km along strike with anom-alous concentrations of PGE. Studies of the Willow Ridge Reef,published by Bird et al. (1995) and Arnason and Bird (2000),form the basis for this paper.

The Kap Edvard Holm Complex (Fig. 11) evolved as adynamic open-system magma chamber during its crystalliza-tion. The complex covers some 360 km2 and is subdivided intoa Lower, Middle, and an Upper Layered Series building a totalexposed cumulate stratigraphy of around 7500 m (Wager andBrown, 1968). In its present setting, the layering is dippingsouthward by some 30° to 40°, and neither the base nor the roofof the intrusion is exposed. The intrusion has been dated at 47.3 ± 0.3 Ma with the 40Ar-39Ar dating method (Tegner et al.,1998a). Several smaller syenite intrusions and breccia pipes havelater transgressed the complex, one (the Hutchinson Glacier IIsyenite) in the immediate vicinity of the Willow Ridge Reef. Incontrast to the closed system fractionation of the SkaergaardIntrusion, numerous episodes of magma mixing have beenrecorded in the Kap Edvard Holm Complex. These are promi-nently expressed in the cumulate succession as intraplutonic

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Fig. 12. Geological map of the southern part of the Kruuse Fjord Com-plex (after Arnason et al., 1997a, 1997b). Glaciers and glacial depositsare white, the sea is marked by a light grey shading.


quench-zones (Tegner et al., 1993) that also display reversals inthe cumulus mineral assemblages and compositions.

The Willow Ridge Reef occurs near the lowermost exposedlevel of the intrusion, in a succession of plagioclase-augite-olivine orthocumulates. A prominent layer of wherlite occurs25 m below the reef forming a prominent, although distant,marker horizon. The reef appears to be roughly concordantwith this layer throughout its exposed length. The gabbros dis-play gradual compositional reversals in the cumulus and inter-cumulus mineral vicinity of the Willow Ridge Reef.Hydrothermal alteration is extensive near the HutchinsonGlacier II syenite where 10% to 50% of the gabbro has beenreplaced by secondary minerals.

The structural relations of the Willow Ridge Reef are some-what similar to those of the Platinova reefs in the SkaergaardIntrusion. The reef is stratabound but displays a stratification inthe concentrations of Pd, Pt, and Au (Arnason and Bird, 2000),which is similar to that of the Main Sulphide Zone of the GreatDyke of Zimbabwe (Naldrett and Wilson, 1990; Oberthür,2002; Wilson and Tredoux, 1990) and the Munni Munni Com-plex of Australia (Barnes et al., 1990; Hoatson and Keays,1989). Typically, it displays a lower (minor) Pd peak followedupward by a Pt peak and finally an Au peak. The stratigraphicoffset of the peaks are in the order of 0.5 m to 1 m. Average con-centrations were reported by Arnason and Bird (2000) to250 ppb Pt, 40 ppb Pd, and 50 ppb Au over a 3 m stratigraphicinterval; the maximum concentrations in individual samplesreaching 5 ppm Pt and 8 ppm Au. The magnitude of the strati-graphic offset varies along strike, but no systematic relationshave been established.

The Au occurs in minute alloy grains included in post-cumulus rims of silicate minerals, Cu-Fe sulphide minerals(digenite, bornite, and chalcopyrite), or secondary hydrother-mal minerals (Arnason and Bird, 2000). Gold grains have beenreported up to 25 µm (averaging 5 µm) across. They are almostpure Au (>88 wt% Au) with small amounts of Ag, Cu, and Fe.PGM grains are up to 100 µm (averaging 10 µm) across and dis-play a similar association with silicate and sulphide minerals.The dominant minerals are Pt-alloys (probably isoferroplat-inum), sperrylite, cooperite, and moncheite.

The close association between the PGE and sulphide min-erals in the Willow Ridge Reef is similar to that of the Platinovareefs in the Skaergaard Intrusion. The composition of the sul-phides could have formed by a similar oxidation of a primarypyrrhotite-chalcopyrite assemblage leading to the formation ofmagnetite, bornite, and digenite. Arnason and Bird (2000) sug-gested on the basis of the stratigraphic control, the composi-tional reversal in mineral compositions, and the texturalassociation of PGM, primary silicates and sulphides, that thereef formed as a result of sulphide-silicate liquid immiscibilityduring magma mixing.

The Kruuse Fjord Complex

Arnason et al. (1997a) document a PGE-rich zone from theKruuse Fjord Complex. They reported elevated concentrations

of PGE from a zone along the contact of a transgressive ultra-mafic body within the complex. The Kruuse Fjord Complex(Fig. 12) is a large layered intrusion exposed around 100 km tothe south of the Kangerlussuaq Fjord. It occupies an area ofaround 20 km by 13 km (Arnason et al., 1997b), but because ofits remote location, inaccessible terrain, and poor exposure, verylittle is known about it. The complex is situated inland from thecoast parallel dyke swarms, and bordered on all sides by Pre-cambrian basement gneisses. It has been dated at 48.0 ± 1.2 Mawith the 40Ar-39Ar method (Tegner et al., 1998a). The stratig-raphy of the complex is poorly documented but an extrapola-tion from existing structural data (Arnason et al., 1997b)indicates an exposed succession in excess of 5 km. Structuralmeasurements indicate that it has not been rotated during theregional coastal collapse (unlike the Skaergaard Intrusion andKap Edvard Holm Complex) and consequently probably only asmall part of its stratigraphy is exposed. Arnason et al. (1997b)subdivided the complex into an outer gabbro series and an innertroctolite series, both cut by a transgressive ultramafic sill. Therelations of a narrow zone of trondhjemite are not clear but itspresence suggests that contamination could have played a sig-nificant role at some points during the evolution of the complex.Local alteration has resulted in secondary mineral assemblagesof amphiboles, epidote, albite, serpentine, zeolites, and calcite(Bird et al., 1988).

The PGE-rich zone in the Kruuse Fjord Complex followsthe lower contact of a transgressive ultramafic (melanogabbro-wherlite) intrusion toward the gabbro. The contact is undulat-ing and clearly discordant to layering in the gabbro, but nochilled margin has been documented. In places, the undulating

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Fig. 13. Geological map of the Nordre Aputitêq Intrusion (from Arnason,1995). The Marginal gabbro is exposed along the north coast of theisland along with a small exposure of the host basalts. The central andsouthern part of the island forms the Central layered gabbro. A zone ofpegmatitic poikilitic gabbro with abundant xenoliths of basalt occurs inthe Marginal series, locally having PGE-bearing sulphides. Snow coveredareas are white, the sea is marked by a light grey shading.


behaviour resembles resorption structures, and localized pods ofperidotite at the boundary indicate that chemical reactionoccurred at the contact (cf. Arnason et al., 1997a, Figs. 3 and 4).The PGE-rich zone is spatially associated with the contact andhas been documented within the melanogabbro for some 100 malong strike. PGE concentrations reach 690 ppb Pd, 630 ppbPt, and 162 ppb Au (Arnason et al., 1997a).

The host lithology for the PGM is a coarse-grained unit ofmelanocratic olivine-chromite-(diopside) cumulate with locallayers and lenses of dunite and clinopyroxenite (Arnason et al.,1997a). Sulphide minerals occur as interstitial phases or smallinclusions in (presumably the post-cumulus rims of ) the silicateminerals. The sulphides include pyrrhotite, chalcopyrite, pent-landite, and cubanite, locally in association with magnetite(Arnason et al., 1997a). Local alteration to violarite, millerite,and bornite has been reported in rocks containing secondaryalteration of silicate phases.

The PGM occur as minute (<35 µm) grains included in thesulphide minerals or situated at the grain boundaries betweenmagmatic and secondary silicate minerals. The dominant phasesare sperrylite, kotulskite, moncheite, and native gold (with vari-able content of silver) with minor Pt-Fe alloy (Pt3Fe), a copper-palladium mineral (Cu3Pd), hollingworthite, platarsite, and anunnamed Pd-bismuthotelluride (Arnason et al., 1997a).

The strong lithological control of the PGE suggests a mag-matic origin. The elevated concentrations are found along acontact between two magmatic lithologies that display resorp-tion structures rather than a chilled margin. A magmatic originis consistent with the close association between the PGM andthe magmatic sulphide and silicate minerals. However, the asso-ciation with secondary silicates and the occurrence of sperrylitein small veins demonstrate a local redistribution of PGE duringthe post-magmatic stage (Arnason et al., 1997a).

The Nordre Aputitêq Intrusion

Arnason (1995) describes a minor occurrence of PGMfrom the Nordre Aputitêq Intrusion. The intrusion occupiesan island of some 5 km2 offshore to the south of the KapEdvard Holm Complex (Fig. 13). It has been dated at 48.0 ±0.2 Ma with the 40Ar-39Ar method (Nevle et al., 1994). Themargins of the intrusion are only exposed along the north-western part of the island, where it is in contact with basalticlavas and tuffs. The intrusion is notable for having very calcicplagioclase (Brooks and Nielsen, 1982) — similar to the intru-sions on the British Isles. Compositional reversals in cumulussolid-solution series minerals indicate that several magmaadditions took place during crystallization (Bird et al., 1988),but little is known about their distribution and relations. Rose(1989) recognized two major lithological divisions — a centraland a marginal series.

The central series occupies the major part of the island. Itconsists of layered leucocratic gabbro with interbedded melano-cratic layers and is separated from the marginal series by a tran-sitional zone of non-layered and strongly deformed gabbro.Layering in the central series is locally well developed and dis-

plays an overall dip of some 35° to 40°E (Brooks and Nielsen,1982; Rose, 1989).

The marginal series is exposed along the northwestern coastof the island. It consists of fine- to medium-grained gabbro withabundant metabasaltic xenoliths, patches of poikilitic pyroxenegabbro, and pegmatites with miarolitic cavities filled with epi-dote, prehnite, amphibole, and other hydrous silicate mineralsindicating strong hydrothermal alteration (Arnason, 1995; Birdet al., 1988). The pegmatites occur along lithological bound-aries, at the margins of metabasaltic xenoliths, and as transgres-sive bodies (Arnason, 1995).

Arnason (1995) reports a minor occurrence of PGM frompegmatites in the marginal series. The host lithologies consist ofmagnetite gabbro (with poikilitic augite) and leucogabbro (withcumulus augite). The PGM occur in association with interstitialsulphides (pyrrhotite, chalcopyrite, and pentlandite) and trans-gressive sulphide-bearing hydrothermal veins. PGM occur asanhedral grains of 5 µm to 12 µm; the dominant phases arenative gold and Pd-bismuthotellurides (kotulskite, telluropalla-dinite, and keithconnite) with subordinate sperrylite (Arnason,1995). The close association of the PGM with pegmatites andhydrothermal veins suggest a late- to post-magmatic origin.

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Arnason (1995) suggested that the PGE were originally carriedin the magmatic high-temperature monosulphide and interme-diate solid-solution series, and that the PGM formed by sub-solidus exsolution.

The Miki Fjord Macrodyke

Three very large layered gabbro dykes (macrodykes) areexposed in the immediate area around the Skaergaard Intrusion.The macrodykes vary considerably in width (up to 1200 m) anddisplay well developed modal layering in their central parts. Thedykes belong to the THOL-2 generation developed radially tothe mouth of the Kangerlussuaq Fjord (Nielsen, 1978), and it islikely that they acted as major feeders for the plateau basalts dur-ing the continental breakup (Momme, 2000). The Miki Fjordmacrodyke (Fig. 14) is the largest and can be followed fromMiki Fjord to the northeast for around 14.5 km before disap-pearing underneath a glacier. (The dyke extends possibly forsome 80 km inland but gets thinner away from the coast; T.F.D.Nielsen, pers. comm., 2001.) Its maximum width is around 1km (Deer, 1976). The dyke transects the Archean metamorphicbasement, a succession of Cretaceous sediments, and the LowerBasalts (Arnason, 1995; Deer, 1976).

The Miki Fjord macrodyke consists of two major en-echelon segments, each displaying a zone of isotropic gabbrofollowing the lower parts and the margin of the intrusion, anda zone with well-developed modal layering, on a centimetre tometre scale in its upper parts. The dominant lithology is olivinegabbro, but a variety of hybrid rocks have formed during exten-sive (and complex) contamination by the variable incorpora-tion of anatectic melts from the basement (Blichert-Toft et al.,1992). The upper parts of the dyke carry abundantmetabasaltic and picritic xenoliths.

Arnason (1995) reported platinum-group element bearingsulphides from the parts of the dyke that crosscut the gneissicbasement. The sulphides are confined to the intrusive lithologieswith the host basement rocks being barren. They are found dis-seminated in the gabbro, apparently being most abundant nearto the intrusive margin and decreasing in concentration towardits centre. The sulphides are dominated by chalcopyrite andpyrrhotite, locally with minor bornite, magnetite or pyrite(Arnason, 1995). PGM are up to 40 µm and in most cases com-pletely or partially included within the sulphides. PGM includekotulskite, atokite, and stibiopalladinite (Arnason, 1995).

The abundance of PGE-bearing sulphides along the mar-gins of the macrodyke is comparable to sulphide occurrencesfound in layered intrusions elsewhere. For example, similarPGE-rich sulphides have been reported from the layered intru-sions in Finland (the Portimo Complex, Huhtelin et al., 1989;Iljina et al., 1989; the Koillismaa Complex, Alapieti and Lahti-nen, 1989, 2002) and intrusions in the Duluth Complex(Hauck et al., 1997; Theriault et al., 2000). The obvious modelfor formation relies on footwall contamination and the strongdependence of the sulphur-saturation with the FeO contentand temperature of mafic magmas (Poulson and Ohmoto,1990). Contamination with felsic material locally lowers the

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Fig. 14. Geological map of the Miki Fjord Macrodyke (from Arnason,1995). The dyke consists of a lower and marginal unlayered unit and anupper-layered unit.

Fig. 16. Simplified geological map of Rum. The ELS, WLS, and CS denotethe eastern-layered, western-layered, and central series. Numbers referto the ELS cyclic units (after Emeleus et al., 1996).

Fig. 15. Extent of the BVP (after Bailey et al., 1924). Shaded areas areplateau basalt, open circles predominantly basic intrusive complexes,crossed circles, mostly acidic complexes. Lines represent dyke swarms.


FeO content and temperature thus triggering local sulphuroversaturation.

PGE Occurrences in Intrusions of the British Isles

Several mafic-ultramafic central complexes are exposed inthe Palaeogene British Volcanic Province, where they are foundalong a stretch of more than 700 km (Fig. 15). Perhaps the bestexposed and well documented are those on the Hebrideanislands of Rum, Mull and Skye, all of which represent the rem-nants of dynamic, open system magma chambers. The com-plexes are preferentially situated in zones where intrabasin ridgesare cut by dyke swarms, probably following pre-existing struc-tural weakness zones (e.g., Emeleus, 1997). These complexescomprise several closely spaced (both temporally and spatially)intrusive centres with mixed basic and acidic components; andsignificant for the PGE, all contain unaltered and well-exposedultramafic bodies. Indeed, the ultramafic lithologies on Rum arethe most extensive intrusive ultramafic rocks exposed in theBritish Isles.

In contrast to the East Greenland intrusions, the Britishcentral complexes on Rum, Mull, and Skye have abundant lay-

ers, laminae, and stringers of chrome-spinel rich lithologies(here called chromitite layers) that host abundant and diversePGM (Butcher et al., 1999; Pirrie et al., 2000; Power et al.,2000a). Neither chromitite nor PGM have been found in otherof the central complexes in the region. On Rum, Mull, andSkye, chrome-spinel concentrations within the chromitite layerslocally exceed 60 modal percent and commonly include inter-stitial Ni-Cu-sulphides. Some chromitites are relatively sul-phide-rich, others sulphide-poor, suggesting that a variety ofmineralization processes were active. The research has so farbeen concerned mainly with these chromitites, and thechemostratigraphy of PGE of the complexes is currently beingexplored. The following sections are consequently focussed onthe mineralogical and petrological association of PGM ratherthan the geochemical distribution (as in the East Greenlandcomplexes). Despite the restricted lithological association of thePGE in chromitite layers and laminae, each of the intrusive cen-tres displays a wide variety of platinum-group minerals and min-eral associations.

The Rum Complex

The Rum Complex (formerly Rhum, reverted to its origi-nal spelling in 1992) appears to host the richest PGE occur-rences in the province. It consists of layered mafic andultramafic lithologies that are well exposed over more than30 km2 (Fig. 16). Three subdivisions have been delineated on

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the basis of field relations (McClurg, 1982; Volker, 1983): theapproximately synchronous Eastern and Western Layered Series(ELS and WLS respectively) separated by the younger trans-gressive Central Series (CS). Platinum-group minerals havebeen identified from all of the subdivisions but appear to beparticularly abundant in the ELS (Power and Andersen, 2001;Power et al., 2000a).

The Eastern Layered Series

Platinum-group elements in the ELS are closely associatedwith the magmatic stratigraphy. The ELS displays 16 megacyclicunits with well developed modal layering, each evolving fromolivine cumulates at their base to plagioclase cumulates at theirtop (e.g., Volker and Upton, 1990). The megacyclic units arenumbered sequentially from bottom (unit 1) to top (unit 16).Individual units pinch and swell laterally and are in some casestruncated by overlying megacyclic units (e.g., Bédard et al.,1988). Thin, laterally persistent chromitite laminae occur typi-cally at the interface between the megacyclic units but also, morerarely, within the ultramafic lithologies of individual units.Chromitites are especially well developed at the unit 5-6, 6-7, 7-8 and 11-12 boundaries and within unit 14 (e.g., Power et al.,2000a, 2000b). Small sulphide grains (typically <100 µm) arescattered throughout the chromitites but rarely exceed one modalpercent. Pentlandite and chalcopyrite dominate the sulphideassemblage with only minor pyrrhotite. Local alteration of thesulphides results in the formation of Fe-oxides and hydroxidesafter pentlandite and pyrrhotite; and bornite, chalcocite, covelliteand rare native Cu after chalcopyrite. Sulphides are mostly con-fined to the chromitites, except for the melano-troctolites of unit1 where large (up to 7 mm) intercumulus sulphide grains areabundant (more than 5 modal percent) (Faithfull, 1985). Unit 1displays a primary assemblage of pentlandite, chalcopyrite, andpyrrhotite (Faithfull, 1985) with localized alteration to Fe-oxidesand a variety of Cu-sulphide phases.

Platinum-group minerals are ubiquitously associated withthe chromitite layers. Three grains of electrum (Au, Ag alloy)were identified by Dunham and Wilkinson (1985) within sul-phides of the 11-12 chromitite, and recent research (Butcher etal., 1999; Power et al., 2000a) established widespread occur-rences of PGM throughout the ELS. The occurrences are thinbut laterally extensive, and have concentrations typically in therange 1-2 ppm total PGE. Concentrations up to 2.6 ppm havebeen recorded in the 7-8 chromitite. PGE concentrations in thesulphide-rich unit 1 are low (232 ppb, Hulbert et al., 1992) andno discrete PGM have been identified. The following accountlargely follows that of Power et al. (2000a).

The PGM occur as discrete grains within or immediatelyadjacent to the chromitite laminae. Minerals containing PGEwith S, Cu, As, Ag, Sn, Sb, Hg, Pb, and Bi (among others) arepresent. However, due to the small size of the PGM, relativelyfew phases have been positively identified and include laurite,keithconnite, sperrylite, irarsite, kotulskite, braggite, michener-ite, moncheite, and telargpalite. The PGM are usually inti-mately associated with interstitial sulphides, where they

commonly occur at the boundaries between the sulphides andtheir host silicates. Within the sulphides, the PGM are domi-nantly associated with pentlandite in preference to chalcopyriteand pyrrhotite. Grain sizes usually display lognormal distribu-tions with more than half of the identified grains being less than2 µm. The largest discrete grains are up to 35 µm although rarePGM rims surrounding sulphides may exceed 100 µm. Thesmaller PGM are usually anhedral blebs while the larger grains(>5 µm) are commonly sub- to euhedral. No correlationbetween PGM grain size and mineralogy or stratigraphy hasbeen determined.

Perhaps the most striking feature of the ELS is the markedstratigraphic variation in PGM assemblages. Each chromititehas its distinctive PGM assemblage although an overall sys-tematic trend is not apparent. For instance, Pt-Pd sulphides(e.g., braggite and cooperite) and laurite are common withinthe 7-8 chromitite while electrum and native Au are rare. Incontrast, electrum and Pt-Pd tellurides are abundant whilePGE sulphides are uncommon within the 6-7 chromitite.Whole rock PGE contents relate well with the observed min-eralogy and also vary with stratigraphy. Although many of thesulphides have undergone alteration to some degree, few of thePGM show any textural evidence of alteration. Indeed, evenadvanced alteration of sulphide grains appears to result inrelict PGM within Fe-oxides.

The strong stratigraphic control on the PGM assemblagessuggests a magmatic origin. This is supported by the close asso-ciation between the PGM and magmatic sulphides and theunique character of PGM assemblages that can be linkeddirectly to the different units. Post-magmatic oxidation of thesulphides had no visual effect on the PGM — even where thesulphides have been completely converted into Fe-oxides and —hydroxides.

The Western Layered Series

The WLS is exposed on the western flanks of the centralcomplex and consists of predominantly ultramafic cumulates(cumulus plagioclase is largely absent). Layering is developed,but in contrast to the ELS, predominantly defined by grain sizerather than modal variations. Cryptic layering is sporadicallypresent. Three main litho-stratigraphic divisions have beendelineated (Wadsworth, 1961; reappraised by Volker, 1983).The lowermost Harris Bay member is dominated by very coarse(harrisitic) olivine gabbro that passes upward into the moremafic and generally finer grained Transitional member. Alter-nating layers of coarse harrisitic and fine granular dunite domi-nate the uppermost Ard Mheall member. Very thin (commonlyless than 1 mm thick) chromitite layers are abundant through-out the Transitional and Ard Mheall members extending later-ally for up to 100 m. In contrast to the ELS, chromitites in theWLS are not preferentially developed at lithological contacts;most occur within individual members. A similar sulphideassemblage to that reported in the ELS is present in associationwith the chromitites. However, the sulphides are less abundantand more altered; chalcocite and native Cu are relatively com-

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mon. No chromitites have been identified within the poorlyexposed Harris Bay member.

Platinum-group minerals have been identified within theTransitional and Ard Mheall members of the WLS (Butcher etal., 1999; Power and Andersen, 2001) In comparison to theELS, the PGM within the WLS are smaller (0.3 µm to 5 µm buttypically <1 µm) and tend to be anhedral. The PGM are presentboth as inclusions within intercumulus plagioclase and clinopy-roxene and partially enclosed by sulphides. No stratigraphicvariation has been identified. A wide variety of PGM have beenrecorded, although the assemblage is less varied than in the ELS.Identified phases include laurite, sperrylite, michenerite, andirarsite.

The close association between the PGM and primary sili-cate phases and sulphides indicates a magmatic origin. However,a more detailed study is required to determine true extent of theoccurrences and the mineralization processes involved.

Other Lithological Units

Platinum-group minerals have also been identified in theCS and a suite of transgressive ultramafic plugs associated withthe Rum Complex (Power and Andersen, 2001). Similarly tothe ELS, the CS displays modal layering that is well developedin the olivine and plagioclase-olivine cumulates. However,numerous broad, near vertical igneous breccia zones crosscut theCS and strike approximately north-south giving rise to the verydisjointed appearance typical of the CS. These breccia zonescontain a variety of locally derived (e.g., Emeleus et al., 1996)igneous clasts of anorthosite, peridotite and troctolite within avariable matrix. Chromitite laminae are sporadically presentthroughout the CS and generally form convoluted bands locallyexceeding 5 cm in thickness. In addition, chromitites are abun-dant within clasts throughout the CS breccias. Sulphides (pre-dominantly pentlandite and chalcopyrite) are common,although locally altered. The PGM are closely associated withsulphides within the chromitite laminae and are dominated bylaurite, PtFe, and Pd-Cu alloys (Power and Andersen, 2001).

The transgressive ultramafic plugs are found in the centralpart of the Rum Complex, but isolated plugs also occur in thehost sedimentary rocks. The plugs are locally rich in sulphides(more than 5 modal percent) and include dykes with up to 40%sulphides. PGM are closely associated with the sulphides andconsist mostly of Pd-bismuthotellurides, with minor sperryliteand electrum (Power and Andersen, 2001).

The Cuillin Complex, Isle of Skye

Pirrie et al. (2000) reported PGM from the Peridotite seriesof the Cuillin Complex. The Peridotite series forms the south-western margin of the Outer Layered series of the Cuillin Com-plex and is dominated by a succession of olivine cumulatesexposed in an arcuate band above the western shores of LochCoruisk (Fig. 17). The series displays 6 discrete lithological units(Claydon and Bell, 1992) with the lower stratigraphic levelsdominated by olivine cumulates (units 1 to 3) and the higher of

olivine-plagioclase cumulates (units 5 and 6). Unit 4 is anigneous breccia dominated by small (<1 m) rotated troctoliteclasts (probably derived from units 1 and 3; Claydon and Bell,1992) within a leucocratic matrix. Layering is poorly developedin the olivine cumulates but is well developed where cumulusplagioclase becomes a significant component. The eastern mar-gin of the Peridotite series has been intruded by the bytownitegabbro of the younger Eucrite series. Numerous large ultramaficxenoliths derived from the Peridotite series (Weedon, 1961;1965) crop out within the gabbro in a broad zone extendingfrom the contact with the Peridotite series down to the shores ofLoch Coruisk (Fig. 17).

Chromitite laminae are well developed throughout thelower three units of the Peridotite series (Bell and Claydon,1992). They are considerably more substantial than on Rumand have variable morphologies from approximately planar lam-inae, 2 mm to 2 cm thick that can be followed along strike formore than 50 m, to discontinuous highly convoluted irregularseams (e.g., Pirrie et al., 2000). Individual laminae pinch andswell and may converge locally to form layers and pods in excessof 10 cm thick. The breccias of unit 4 contain rare clasts ofchromitite, and chromitites are additionally found in many ofthe xenoliths within the surrounding gabbros. Sulphides (pre-dominantly chalcopyrite and pentlandite) are present in associ-ation with the chromitites but are typically small (<100 µm) andwidely disseminated.

Platinum-group minerals are present in most chromititesbut are nowhere as abundant as on Rum; typically less than 5grains occur in a single (2 cm by 4 cm) polished section com-pared with over 30 PGM in a sample of similar size from theELS of Rum. PGM are also present within both the chromititeclasts within the igneous breccias of unit 4 and the xenolith-hosted chromitites within the surrounding gabbro. PGM arefound enclosed within the chrome-spinel and silicate grains; atchrome-spinel — sulphide and silicate — chrome-spinel grainboundaries. Only very few PGM are enclosed within the sul-phides. Grain sizes ranges from <0.2 µm to 7 µm (mode = 1 µm).

By far, the most abundant PGM is laurite (57% of the totalnumber of grains), which mostly forms euhedral crystals in thechrome-spinel grains. In addition, a grain of irarsite occurswithin a chrome-spinel crystal, and a large laurite grain withincumulus olivine. Sperrylite, irarsite, Pt-Rh-Fe alloy, Pd-As andPd-Bi-Te phases (among others) are anhedral and occur pre-dominantly at grain boundaries. No stratigraphic variation inPGM assemblage has been established. The intimate associationof PGM with chrome-spinel, primary silicates, and sulphidesindicates that the PGM crystallized during the magmatic stage,however, further documentation of the stratigraphic control isrequired to fully understand the occurrence.

The Ben Buie Intrusion, Isle of Mull

The Mull central complex comprises a large number ofmafic to acidic intrusive bodies approximately centred over inter-section of the Great Glen Fault and the Moine Thrust (e.g., Kerr

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Fig. 17. Simplified geological map of the peridotite series of Skye. Numbers refer to the stratigraphy of Claydon and Bell (1992).


et al., 1999). It represents a dynamic open-system magma cham-ber that evolved during several stages of magma replenishment,fractional crystallization, and magma tapping. The complex issubdivided into three evolutionary stages, each displaying severalintrusive episodes. Countless cone-sheets, ring dykes, and otherintrusive units crosscut older lithologies and their original tem-poral and geological relations are complex to resolve.

Three large gabbroic bodies (the Ben Buie, Corra Bheinn,and Bheinn Bheag intrusions) are exposed within the Mull cen-tral complex. All are associated with the earliest (Centre 1) evo-lutionary stage of the complex, and they probably representmaterial that accumulated on the magma chamber floor at dif-ferent times. The intrusions are truncated toward the intrusivecentre, and it is likely that they were originally much larger

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Fig. 18. Simplified geological map of the Ben Buie intrusion of Mull (afterLobjoit, 1957).


intrusions that have been faulted and broken up by later mag-matic activity.

In the Mull central complex, only the Ben Buie Intrusion(Fig. 18) contains ultramafic lithologies (e.g., Bailey et al., 1924;Lobjoit, 1959). This intrusion belongs to the earliest stage of theevolution of the complex and hosts some of its most primitiverocks. It covers an area of around 14 km2 in the southern partof the complex. Modal layering is generally weakly exposed, dis-tinct in places and becomes prominent around Loch Fuaran. Atotal cumulate stratigraphy of more than 1000 m has beenmeasured in the eastern part of the intrusion (S.J. Prout, pers.comm., 2001), but the stratigraphic compositional variationsare limited. The intrusion is dominated by bytownite gabbrosand has an overall composition suggesting that it is a product ofthe high-Ca, low alkali tholeiitic magmatism that is distinct forCentre 1 (Skelhorn et al., 1979). Small (< 100 m) bodies of troc-tolite and subordinate peridotite crop out sporadically through-out intrusion (e.g., Lobjoit, 1959).

Chromitites are exposed within peridotites at two discretelocalities (Pirrie et al., 2000), near Craig and on Caigeann Doirenan Cuileann (Fig. 18). At Craig, chromitite layers and laminaeare found in association with ultramafic bodies in cumulates.The contact relations of the bodies are not visible in the field,but their limited lateral extent suggest that they are either xeno-liths from a hidden ultramafic zone or ultramafic pipes (similarto those observed in the Bushveld Complex, Schiffries, 1982;

Scoon and Mitchell, 1994) rather than discrete stratigraphicunits. Similar bodies have not been found elsewhere in theintrusion. Furthermore, several small, rounded anorthosite pods(possibly partially resorbed xenoliths) up to 1 m in diameteroccur within peridotites and are partially rimmed by thin (<1cm) chromitites.

On Caigeann Doire nan Cuileann, numerous approxi-mately planar chromitites are present within layered olivinecumulates. These chromitites are laterally persistent across theexposure (>10 m) and typically have sharp but undulating upperand lower contacts. All of the chromitites exposed are thin,mostly less than 5 mm, although some reach 2 cm in places.Highly convoluted, laterally discontinuous (<2 m) irregularchromitites are also present.

Sulphides are rare at both Craig and on Caigeann Doirenan Cuileann. However, small (<100 µm) pentlandite and chal-copyrite grains are present in very small quantities throughout.The presence of residual chalcocite and secondary silicates sug-gests that sulphides were originally present but have been oxi-dized (Pirrie et al., 2000). Alteration of silicate and sulphidephases appears to be particularly pronounced adjacent to thechromitites, suggesting subsolidus reaction.

Platinum-group minerals are associated with the chromi-tites at both Craig and Caigeann Doire nan Cuileann (Pirrie etal., 2000). However, the two localities host very different PGMassemblages. The chromitites at Craig contain locally very abun-dant PGM (>50 grains within an area of 100 µm2). The major-ity are enclosed within intercumulus plagioclase andclinopyroxene; relatively few occur at grain boundaries orenclosed within chrome-spinel, and none have been found inassociation with the sulphides. Grains are up to 3 µm across.The relatively restricted PGM assemblage is dominated by Pd-phases (Pd-Te-Bi, Pd-Sb-Bi and Pd-Sb-Pb phases). Less com-mon phases include laurite, sperrylite, and minerals of theirarsite-hollingworthite-platarsite series. At Caigeann Doire nanCuileann, PGM are rare and form mostly anhedral blebs up to2.5 µm across (mostly 0.5 µm to 1 µm). The PGM occur dom-inantly at silicate-silicate or chrome-spinel-silicate grain bound-aries but a few have been found in contact with sulphides orenclosed within chrome-spinel. A moderately restricted PGMassemblage is present with the majority of the grains comprisinglaurite or Rh-rich sperrylite. Other PGM qualitatively analyzedinclude Pd-As, Pd-Pb-S, and (Pt,Ir,Rh)-As-S phases. WherePGM (mostly laurite) are enclosed within chrome-spinel grains,they are euhedral.

In terms of mineralogy, the occurrence at Caigeann Doirenan Cuileann is almost identical to that in the Cuillin Complexon Skye, having abundant euhedral laurite inclusions withincumulus chrome-spinel and Pd-Pt minerals occurring withininterstitial silicate minerals. This suggests that the PGM formedduring the primary crystallization of cumulates. Conversely, atCraig the PGE occurs in ultramafic lithologies that are not lat-erally extensive and may represent xenoliths or ultramafic plugssimilar to those of the Bushveld Complex (Schiffries, 1982;Scoon and Mitchell, 1994). The locally abundant PGM arefound within post-cumulus rims on cumulus minerals, within

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discrete intercumulus phases, or at grain boundaries. This sug-gests that the PGM formed during the late magmatic stage, pos-sibly from a PGE-rich interstitial melt.

Models of PGE Concentration in the North AtlanticIgneous Province

The large number of PGE deposits recorded from EastGreenland and the British Isles indicate that particularlyfavourable conditions for PGE concentration existed during theearly stages of the formation of the NAIP. During their initialstudy, Nielsen and Brooks (1995) highlighted the high potentialfor PGE deposits in the layered igneous complexes on EastGreenland and promoted a high degree of exploration interest inthe region. Their model relied on the formation of PGE-rich,sulphur-poor magmas by a two-stage mantle-melting model.Such a model has been used to explain the occurrence of silicichigh-magnesian magmas in large igneous provinces (cf. Craw-ford et al., 1989) and was applied to parental magmas for theBushveld Complex by Hamlyn and Keays (1986). At normaldegrees of partial melting, the first suite of magmas would formwith residual sulphides in the mantle and would consequentlybe sulphur-saturated. During partial melting of sulphides, thePGE would fractionate strongly into the residual solid phaseand thereby be retained in the mantle. Further adiabaticdecompression, perhaps combined with mantle metasomatismwould lead to the formation of a smaller suite of second-stagemelts (Crawford et al., 1989). These melts would form with noresidual sulphides in the mantle and therefore include the PGEand be sulphur-undersaturated (Hamlyn and Keays, 1986). Inthe second-stage melts, significant PGE concentration couldoccur during magmatic differentiation before sulphur-satura-tion was reached. The sulphide liquid formed by subsequent liq-uid immiscibility would scavenge the magma for PGE.

Sulphur-saturation and the Formation of PGE-rich Magmas

The Nielsen and Brooks (1995) model would be compati-ble with the low volumes and late timing of the layered intru-sions (THOL-2, second stage) at the base of the voluminousplateau basalts (THOL-1, first stage). It is also consistent withvariations in Pd and Cu that indicate that the Skaergaard-like(THOL-2) dykes are sulphur-undersaturated (Brooks et al.,1999). However, Cu and PGE concentrations clearly demon-strate that the majority of plateau basalts in the NAIP (in par-ticular on East Greenland) are sulphur-undersaturated and notsaturated as previously assumed (cf., Momme, 2000).

The incompatible fractionation of Cu in many of the basaltsis in disagreement with a second-stage melting model. Most sig-nificantly, a very large volume of sulphur-saturated first-stagemelts would need to be extracted before the mantle could gener-ate the observed volume of sulphur-undersaturated (second-stage) melts. Such lavas are largely missing and cannot beaccommodated within the possible stratigraphic boundaries ofthe plateau basalt successions. Furthermore, the widespread dis-tribution (and predominance) of sulphur-undersaturated lavas

on East Greenland would require a laterally very extensivedepleted mantle source for the production of second-stage melts.

Sulphur-saturation and the presence of residual sulphides inthe mantle have been central for explaining the occurrence ofsulphur-undersaturated PGE-rich magmas (Hamlyn and Keays,1986; Nielsen and Brooks, 1995). However, this model cannotexplain the compositional relations observed in the NAIP whenassuming normal upper mantle compositions. The Vaîgatpicrites have been argued to have formed by very large degreesof partial melting (24% to 30%, Gill et al., 1992) with no resid-ual sulphides in the mantle. However, despite having formed byfar lower degrees of partial melting (10% to 12%, Fram et al.,1998; conditions where residual sulphides would usually still bepresent in the mantle), practically all of the voluminous EastGreenland basalts display sulphur-undersaturated fractionationtrends. Consequently, a widespread mantle source capable ofproducing sulphur-undersaturated magmas during normaldegrees of partial melting must have been available. Studies byMavrogenes and O’Neill (1999) demonstrate that the sulphur-saturation decreases strongly with increasing pressure. Conse-quently, magmas generated at great depth undersulphur-saturated conditions become sulphur-undersaturatedduring adiabatic ascent.

Earlier studies of plateau basalt provinces have assumed thatsulphur-saturation was sustained during low to intermediatedegrees of partial melting in the mantle. Our compiled datademonstrate that the majority of plateau basalts in the NAIPevolved along sulphur-undersaturated trends. In the EarlyPalaeogene basalts, sulphur-saturation is achieved mostly in con-taminated tholeiites and lavas off the axis of the hot-spot trail.The contaminated tholeiites (found in the Lower Basalts on EastGreenland, the Vaîgat Formation on West Greenland, and inODP site 917) incorporated significant amounts of crustalmaterial that would have lowered the sulphur-saturation level.In the ODP legs 152 and 163 (apart from the contaminatedtholeiites in site 917), evidence for sulphur-saturation is seenonly in the late syn-breakup basalts (site 990). Considering themarginal location of the ODP legs in comparison to the hot-spot trail, they are likely to have involved adiabatic melting inthe upper mantle generating magmas with N-MORB trace ele-ment signatures.

When considering the extent and diversity of PGE miner-alization in the NAIP and the predominance of sulphur-under-saturated magmas, it is clear that a primary PGE-rich magmasource region must have been tapped during the formation ofthe province. The variations in Cu and PGE concentrations dis-cussed above cannot be explained purely in terms of sulphur-sat-uration. The compositional variations in the offshore basalts andIceland require two distinct magma sources, one rich in PGEand poor in Cu (a distinct hot-spot component), the other poorin PGE and rich in Cu (perhaps a N-MORB component).

The authors suggest that magmas derived from the Ice-landic hot-spot source were initially sulphur-undersaturated andonly reached saturation after significant fractional crystallizationor when mixed with other material (a crustal contaminant or N-MORB material).

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Significance of a Deep Mantle Plume as a Source of PGE

Geochemical analyses suggest that the upwelling of materialcaused the Icelandic hot spot from the deep parts of the Earth’smantle (a mantle plume). At least four end-member compo-nents are required to explain the observed isotope and trace ele-ment compositional variations (Kempton et al., 2000).Temporal and spatial variations in the geochemical characteris-tics of the magmas could be a combined effect of variableinvolvement of the end-member source compositions (Kerr,1995b; Hanan and Schilling, 1997), changes in the depth andtemperature of melting (Bernstein et al., 1998; Tegner et al.,1998b), and mixing and re-equilibration of magmas at low pres-sures (Larsen et al., 1989). The Early Palaeogene basalts wereprobably largely associated with the impact of the mantle plumehead beneath the subcontinental lithosphere. Recent basalts onIceland, on the other hand formed from a mature, steady stateplume trail under a considerably thinner lithospheric lid. ThePGE potential of the Early Palaeogene basalts do therefore notnecessarily imply a similar potential in the Late Palaeogene torecent igneous rocks on Iceland.

When evaluating the compositional variations of thebasalts, a number of different parameters need to be considered.In this discussion, the authors focus exclusively on Cu and Pd— elements that can be considered perfectly incompatible in thefractionating silicate and oxide minerals. Fractional crystalliza-tion will consequently lead to an indiscriminate enrichment ofthe elements in the residual magma, whereas, if material is accu-mulated, they will be indiscriminately diluted. If partial meltingand fractional crystallization were the governing processes dur-ing the formation of the basalts, Cu and Pd would consequentlydisplay strong positive correlation. In the East Greenland off-shore basalts, this is not observed (Figs. 19-21). On the contrary,the Cu and Pd concentrations define a vaguely negative correla-tion. Such a correlation can only be explained with the existenceof at least two distinctly different mantle source components.

Palladium and Cu concentrations in the syn-breakuppicrites from site 917 strongly correlate with the MgO content(Figs. 20-21), giving systematic variations with the degree ofpartial melting. Picrites with MgO higher than around 15 to16 wt% appear to be diluted in Pd and Cu as would be expectedif they carry accumulated phenocrysts of olivine. Picrites withfrom 15 to 16 to around 8 wt% MgO display a systematicincrease in Pd from around 13 to 17 ppb Pd and decrease in Cufrom around 120 to 80 ppm Cu. The most extreme end-mem-ber displays a Cu-Pd ratio of 4.9 3 103 (Fig. 19). The resultingprimary magmas from partial melting of this distinct source(P1, Figs. 19-21) appear to display significant differences incomposition with the degree of partial melting. Lower Cu-Pdratios correlate with low MgO (P1-l), and higher Cu-Pd ratioswith higher MgO (P1-h). This is consistent with partial meltingof a PGE-rich source with no residual sulphides in the mantle.

The uncontaminated evolved tholeiites from hole 918 shownearly constant MgO of around 7 to 9 wt% but are variable intheir Pd and Cu concentrations. They appear to form a trendtoward the picrite compositions from an extreme end-member

with the lowest Pd (around 2 ppb) correlating with highest Cu(around 280 ppm) concentrations (P2). The most extreme end-member displays a Cu-Pd ratio of 1.4 3 105 (Fig. 19). The nar-row variation in the MgO content suggest that these magmasformed by low (constant) degrees of partial melting with resid-ual sulphides in the mantle. The Cu-Pd of this component isconsistent with partial melting with residual sulphides in thesource. This is surprising because the basalts display sulphur-undersaturated fractionation.

Considering the ODP legs 152 and 163 that were drilledon the flanks of the Greenland-Iceland-Faeroe ridge, they wouldbe likely to involve both hot spot and N-MORB components.Variable mixing between the two sources above combined withdifferences in the degree of partial melting and fractional crys-tallization can adequately explain the Pd and Cu variations inthe East Greenland basalts. However, both end-members displaysulphur-undersaturated fractionation and are notably differentfrom the N-MORB compositions (N in Figs. 19 to 21) given byRehkämper et al. (1999). N-MORB is characterized by nearlyconstant compositions with between 6 and 10 wt% MgO. Pdconcentrations are low, generally around 0 ppb to 2 ppb withscattered values up to 6 ppb, and Cu concentrations vary around80 ppm to 130 ppm (Rehkämper et al., 1999). In addition,most N-MORB compositions would fractionate along sulphur-saturated trends. An N-MORB component would be unrecog-nizable in the Cu-Pd variations because it falls in the middle ofthe observed variations for the contaminated and sulphur-satu-rated basalts.

The Pd-rich (P1) Mantle Plume Source

High Pd and low Cu concentrations are characteristic ofthe picrites (Fig. 19). They display systematic trends of increas-ing Pd and decreasing Cu with decreasing MgO concentrationssuggesting a decoupling between the elements in the sourceregion. Considering the contemporaneous magmatism, theauthors suggest that the picrite source on East and West Green-land is the same.

The Cu-Pd ratio of the picrites is close to that of the man-tle (Barnes et al., 1993) suggesting that no fractionation of theelements has occurred. Similar ratios are found in cumulates inthe parts of layered intrusions that formed before sulphur-satu-ration was reached in the magma (such as the Munni-MunniComplex, Barnes et al., 1993; Hoatson and Keays, 1989). Thisis consistent with partial melting in a mantle source with noresidual sulphides.

Re-Os-Pt isotope systematics in the Vaîgat picrites indicatea small component with high Re/Os and Pt/Os, which Pearsonet al. (1999) suggest to be derived from the Earth’s outer core.This is most prominently expressed in the suprachondritic186Os/188Os values. Brandon et al. (1998) document similarOs-isotope ratios on Hawaii, and Bird et al. (1999) found sim-ilar ratios in detrital Os-Ir-Ru alloy grains from Oregon. Bird etal. (1999) suggest that the suprachondritic 186Os/188Os valuesare consistent with the values expected in the outer core or thecore-mantle transition (the D” layer). However, as highlighted

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Fig. 20. Variations in MgO and Cu. End member compositions as in Fig-ure 19. Horizontal hatching, ODP pre-breakup basalts; dense verticalhatching, ODP syn-breakup picrites; cross-hatching, ODP syn-breakupbasalts from sites 918 and 989; dark shading, East Greenland basalts.Mantle source components are inferred from data from ODP legs 152and 163 and Philipp et al. (2001). East Greenland data from Larsen et al.(1989), Momme (2000), and L.M. Larsen (pers. comm., 2001). Datafrom ODP site 990 has been omitted for clarity.

Fig. 19. Variations in Cu and Pd in ODP Legs 152 and 163 (data fromPhilipp et al., 2001) and East Greenland (from Momme, 2000). Horizon-tal hatching, ODP pre-breakup basalts; dense vertical hatching, ODP syn-breakup picrites; cross-hatching, ODP syn-breakup basalts from sites918 and 989; dark shading, East Greenland basalts including ODP site990. The data define an array between a typical Cu/Pd mantle value of4.9 3 103 and a value of 1.4 3 105 typical of sulphur-saturated frac-tionation. P1 denotes the source of the picrites with l being the end mem-ber composition at low degrees of partial melting, and h the end memberat high degrees of partial melting. (l). P2 denotes the Cu-rich plumesource. Normal MORB (N) plots at low Cu and low Pd values. The Cu/Pdvalue is comparable to the P2 component consistent with residual sul-phides in the source.


by Hattori (2002), further work is required to confirm theiranalytical results. Entrainment of (or mixing with) outer corematerial is likely to occur in a heterogeneous, possibly partiallymolten, dense D” layer, that floats on a liquid outer core (Morse,2000). This could explain the unusual Os-isotopic signature ofthe Vaîgat picrites.

A core involvement might be difficult to trace by othermeans than isotopes, but some effects could appear in the con-centrations of PGE and the major core constituents — in par-ticular, Ni and Fe. Indeed, unfractionated magmatic rocks fromboth East and West Greenland display high concentrations ofOs and unfractionated PGE patterns (Schaefer et al., 2000).Furthermore, elevated concentrations of both Fe (Gill et al.,1992; Scarrow et al., 2000) and Ni (cf. Lightfoot et al., 1997)are observed in the NAIP.

The PGE would be greatly affected by entrained materialfrom the outer core, primarily because the core is likely to haveelevated concentrations of PGE, and also because an addition ofliquid metallic iron would increase the sulphur-saturation con-centration (cf. Poulson and Ohmoto, 1990). A rough mass bal-ance calculation (ignoring the crust and assuming a chondriticinitial bulk Earth, a homogeneous mantle with concentrationsof 1% of chondritic, and no re-fertilization of PGE in the man-tle since core formation) indicates that the core could containup to 300 times the mantle concentrations of PGE. Conse-quently, a core contribution of 0.5% to 1% (as suggested for

Hawaii by Brandon et al., 1998) would result in PGE concen-trations of 2.5 to 3 times mantle concentrations in the plumematerial. Even a small core-mantle mixing in the mantle plumesource would generate significantly elevated PGE concentra-tions. This could perhaps explain the abundance of PGEdeposits associated with the NAIP.

Bennett et al. (2000) discussed in detail the effect of coreinvolvement on the PGE concentrations in Hawaiian picrites.They concluded, that although a core involvement is consistentwith the elevated Re-Yb ratio, the observed variations in PGE inthe picrites more likely reflect variable amounts of residual sul-phides in the mantle during partial melting. The sulphur-under-saturated state of the picrites in the NAIP is consistent withhigher degrees of partial melting with no residual sulphides(Schaefer et al., 2000), and in the pure end-member situation acorrelation can be expected.

The Cu-rich (P2) Mantle Plume Source

The other plume component that can be recognized is char-acterized by high Cu and low Pd (Fig. 19) and melts producedhave nearly constant MgO concentrations. Elevated meltingtemperatures are not required to account for the compositional

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variations, and the source appears to have geochemical similari-ties to the upper mantle MORB-source (Larsen et al., 1998).

In contrast to the picrite component, the Cu-Pd ratio of theCu-rich component is significantly higher than the mantle ratio(1.4 3 105, Fig. 19). Despite being sulphur-undersaturated, theratio is comparable with that of cumulates in the parts of layeredintrusions that formed after sulphur-saturation was reached in themagma (Barnes et al., 1993). This indicates that a fractionation ofthe elements has occurred and suggests that the magmas formedduring partial melting with residual sulphides in the mantle.

Deep partial melting with residual sulphides and garnetcould explain the trace element signatures on East Greenland aswell as the high Cu and low PGE. Adiabatic ascent of magmasgenerated at depth could provide a mechanism for forming largevolumes of sulphur-undersaturated Cu-rich, PGE-poor magma.On the basis of the similarities between this component and N-MORB, the authors suggest that it formed from the depletedplume sheath (possibly picked up at the 670 km discontinuity,as suggested by Kempton et al. (2000).

The Effects of Source Mixing for the MgO-Cu Variations

The involvement of different source regions raises theimportant question whether the incompatible trend in theMgO-Cu diagram represents mixing between the plume com-ponents rather than sulphur-undersaturated fractionation. TheCu-rich end-member is close to the most evolved end of theobserved variation.

A simple magma mixing can be excluded on the basis of thevariations of Cu and Pd. The large variation in the Cu-Pd ratioin ODP site 918 is not matched by an evolution of the MgOcontent — as would be expected if they formed as a mixingtrend. Furthermore, if simple magma mixing was the governingparameter for the trend, most of the variation in the plateaubasalts would fall along a straight line between the end mem-bers. They do not, instead they span the Cu/Pd interval betweenthe picrite and the Cu-rich plume component but display sig-nificant fractionation away from the inferred mixing line. Thisimplies that the magmas underwent significant fractionation —possibly in deep-seated magma chambers before eruption. Thisis in accord with the observations by Fram and Lesher (1997).Variable mixing at depth between the picrite source and the Cu-rich plume source (with residual sulphides), combined withfractionation in deep seated magma chambers, could create sim-ilar variations in the PGE as observed on Hawaii (Bennett et al.,2000) and provide an explanation for the diversity of Cu-Pdratios observed in the NAIP. Sulphur-undersaturated fractiona-tion is still required to produce the observed compositionaltrends.

PGE Fractionation Mechanisms

The formation of the PGE deposits in the region relies onthe regional geochemical variations as well as PGE fractionationprocesses during magmatic differentiation. The regional con-straints are determined by the availability of PGE and S in theprimary magmas and their potential for being concentrated dur-ing the formation of the plateau basalt province. The PGE frac-tionation process is largely governed by the magmatic evolutionof the individual layered igneous complexes.

Regional Geochemical Constraints on PGE Fractionation

The regional geochemical studies have implications for thestyles of PGE mineralization that can be expected in the differ-ent regions of the NAIP. The most significant factors for thePGE concentration are: (1) the involvement of the source com-ponents; (2) the degree of partial melting in the source of thepicrites; (3) the sulphur-saturation level; (4) the degree of mag-matic differentiation; (5) magma mixing; and (6) the potentialfor contamination.

The majority of East Greenland basalts analyzed so farevolve along sulphur-undersaturated fractionation trends andare unlikely to have yielded Noril’sk-type Ni-Cu-PGE deposits.However, it is possible that the contaminated Lower Basaltshave fractionated sulphides in undiscovered intrusions. Like-wise, sulphides may have been fractionated from the JamesonLand, Traill Ø, and Sorgenfri Gletcher sill complexes duringinteraction with local sediments. In particular, the JamesonLand sill complex intrudes gypsum-bearing sediments provid-ing an interesting analogy to Noril’sk. Within the layeredigneous complexes, the sulphur-undersaturated fractionationallows the PGE to be concentrated efficiently during magmaticdifferentiation. This implies that the complexes have a large

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Fig. 21. Variations in MgO and Pd. End member compositions as in Fig-ure 19. Horizontal hatching, ODP pre-breakup basalts; dense verticalhatching, ODP syn-breakup picrites; cross-hatching, ODP syn-breakupbasalts from sites 918 and 989; dark shading, East Greenland basalts.Mantle source components are inferred from data from ODP legs 152and 163 and Philipp et al. (2001). East Greenland data from Momme(2000). Data from ODP site 990 has been omitted for clarity.


potential for hosting PGE deposits where magma mixing hasoccurred or during extreme magmatic differentiation (as in theSkaergaard Intrusion).

On West Greenland, the sulphur-saturated fractionation ofthe contaminated tholeiites could potentially lead to the forma-tion of Noril’sk-type deposits. However, as Lightfoot et al. (1997)concluded from their detailed study of the lava succession,because of their early formation in the volcanic succession, thetholeiites had little chance of collecting sulphides from largeamounts of magma, giving little scope for finding large sulphidedeposits in the region. In addition, the lack of significant layeredintrusions in West Greenland implies that the prospects for find-ing PGE deposits formed by magmatic differentiation are limited.

In the British volcanic province, both sulphur-saturated and-undersaturated differentiation is observed. Contamination hasbeen documented from some of the major central complexes andvolcanic areas (Geldmacher et al., 1998; Kerr, 1995a; Kerr et al.,1995, 1999). The extensive possibilities for contamination andthe presence of sulphur-saturated magmas implies that the Britishvolcanic province has a significant potential for undiscoveredNoril’sk-type Ni-Cu-PGE deposits at depth. Large concentrationsof sulphides are found locally in dykes on the Isle of Rum indi-cating that extensive sulphide accumulations may occur else-where. Furthermore, the combination of sulphur-saturated and-undersaturated fractionation gives large possibilities for PGEfractionation and sulphide collection within the central com-plexes. Sulphur-undersaturated magmas could yield Skaergaard-type deposits during extensive magmatic differentiation, andmixing between sulphur-saturated and -undersaturated magmascould yield Merensky-type deposits (as possibly on Rum).

On Iceland, both sulphur-saturated and -undersaturatedtrends are observed. This implies that Noril’sk-type depositscould potentially have formed in the volcanic succession. How-ever, since the potential for contamination is limited consideringthe largely basaltic crust, sulphur-saturation should be attained ina purely magmatic system. Only small mafic layered intrusionsare exposed on Iceland, but examples show significant differenti-ation and evidence for magma mixing that could have inducedsulphur-saturation (Furman et al., 1992a, 1992b). The high con-centration of Pd found in the specimen from the SnaefellsnaesPeninsula indicates that magmatic processes in places have frac-tionated PGE to significant concentrations. The central volcaniccomplexes on Iceland have the potential for mixing of sulphur-saturated magma with differentiated PGE-rich hot-spot magmawhereby they could generate economic concentrations of PGE.

PGE Fractionation in Layered Igneous Complexes

Within the NAIP, the only potentially economic PGEresource discovered so far is the Platinova reefs in the SkaergaardIntrusion. However, the Rum Complex displays the largestdiversity of PGE mineralization styles recorded so far. PGEoccurrences in other intrusions are minor and of only scientificvalue. The wide range of PGE occurrences in the NAIP suggestthat several different mineralization styles were active in the lay-ered mafic-ultramafic complexes of the province.

The layered igneous complexes display stratabound PGEreefs concordant to the modal layering (as in the ELS on Rum, theSkaergaard Intrusion, and the Kap Edvard Holm Complex), PGEin discordant chromitite stringers and seams (as in the CuillinComplex, Isle of Skye), PGE reefs following major discontinuitiesin the intrusions (as in the Kruuse Fjord Complex), PGE occur-ring in sulphides in marginal zones of the intrusions (as in theMiki Fjord macrodyke and the Nordre Aputitêq Intrusion), andPGE associated with discordant ultramafic bodies (as in the BenBuie intrusion, Isle of Mull). The diversity of PGE occurrencessuggests that several concentration and crystallization mecha-nisms operated during the evolution of the complexes.

The stratiform PGE occurrences are either associated withchromite (as in the ELS on Rum) or sulphides (as the PlatinovaReefs in the Skaergaard Intrusion and the Willow Ridge Reef inthe Kap Edvard Holm Complex). Both types display strongstratigraphic controls and are concordant with the modal layer-ing. The PGE occurrence in the Kruuse Fjord Complex is some-what similar to the stratiform occurrences in its setting andcontinuity. It just follows a magmatic unconformity rather thanthe modal layering.

The stratiform PGE occurrences are, with the exception ofthe Platinova reefs, closely associated with reversals in the cumu-lus mineral assemblages and compositions. This is particularlywell expressed in the ELS of the Rum Complex, but similarassociations have been interpreted from the Kap Edvard Holmand Kruuse Fjord Complexes. This indicates that the PGE wereconcentrated in direct response to magma mixing.

The ELS occurrences (although much smaller) are compa-rable to the different occurrences in the Bushveld Complex.Here they have been attributed to magma replenishment andmixing on the basis of variations in the cumulus mineral com-positions and radiogenic isotopes (Kruger and Marsh, 1982,1985; Lee and Butcher, 1990; Naldrett et al., 1986; Von Grue-newaldt et al., 1986). Magma mixing with subsequent oversat-uration in chromite and/or sulphides (as suggested for the UG2Reef of the Bushveld by Von Gruenewaldt et al., 1986) can sat-isfactory account for the occurrences of PGE in the ELS (Poweret al., 2000a). Differences in the PGM assemblage at differentstratigraphic levels in the ELS probably reflect differences in theprimary magmatic composition as indicated by variations in thechrome-spinel compositions (Power et al., 2000b).

The Willow Ridge Reef shows similarities to the Main Sul-phide Zone of the Great Dyke of Zimbabwe (Naldrett and Wil-son, 1990; Oberthür, 2002; Wilson and Tredoux, 1990), mostnotably by its stratigraphic separation of Pt, Pd, and Au (Arna-son and Bird, 2000). This deposit has been attributed to sul-phur-saturation induced by magma replenishment and mixing.

The Platinova Reefs, in contrast, are not associated withcompositional reversals and display larger structural complexitythan any of the other stratiform deposits (Andersen et al.,1998). The reefs are coincident with changes in the Cu concen-trations in the intrusion indicating that they formed in responseto sulphur-saturation in the magma. In this case, however, sul-phur-saturation was reached purely as a result of fractional crys-tallization (Andersen et al., 1998).

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The PGE occurrence in the extensively contaminated MikiFjord macrodyke displays a strong association with sulphides. Itssetting is similar to marginal deposits in the marginal series of lay-ered intrusions in Finland (Alapieti and Lahtinen, 1989, 2002;Huhtelin et al., 1989; Iljina et al., 1989), basal mineralization inintrusions of the Duluth Complex (Hauck et al., 1997; Theriaultet al., 2000), and possibly the Platreef of the Bushveld Complex(Barton et al., 1986; Buchanan and Rouse, 1984). All of theseoccurrences appear to be related to contamination with felsic crust.

The PGE occurrence in the Nordre Aputitêq Intrusion issimilarly associated with the marginal series, but in contrast tothe Miki Fjord macrodyke it shows extensive evidence forvolatile activity. Furthermore, the intrusion is bordered bybasalt, and because of the higher melting point of the wall rocksthe possibility for contamination is smaller. Also, in contrast tothe Miki Fjord Macrodyke no evidence has been presented forpartial melting in the wall rocks. The close association betweenthe PGE and sulphides suggest a magmatic origin of the deposit,but the occurrence is not described well enough to assess anyhydrothermal involvement in the PGE concentration.

Conclusions

This paper has highlighted the high potential for PGEdeposits within the NAIP. This potential appears to be related tothe inherently high PGE concentrations in the mantle sourcematerial. Our most significant conclusions are:1. The relatively high concentrations of PGE in the Icelandichot-spot basalts are a direct reflection of the composition of themantle plume material.2. At least two distinctly different plume sources are required toexplain the observed distribution of PGE in the NAIP. One is ahigh-PGE, low Cu source that produced Mg-rich partial meltswithout residual sulphides; the other produced partial meltswith residual sulphides that became sulphur-undersaturatedduring adiabatic ascent. A third N-MORB component is likelybut cannot be recognized in the MgO-Pd-Cu variations.3. The sulphur-undersaturated fractionation observed in theEast Greenland basalts by Momme (2000) and Momme et al.(2002) is a general feature of the plateau basalts, formed by theIcelandic mantle plume (in contrast to mid-ocean ridge basaltsfrom the Atlantic ocean).4. It is possible that the Icelandic mantle plume involved mate-rial from the Earth’s outer core. The consequent significantlyelevated (2.5 to 3 times) PGE concentrations in the mantleplume magmas compared to N-MORB could explain the largenumber of PGE deposits in the NAIP.5. Osmium-isotopic studies of hot spot related flood basaltprovinces should allow for variations in the 186Os-188Os ratio.6. There is no evidence for the generation of PGE-rich, S-poormagmas on East Greenland by second-stage melting.7. Large Noril’sk type Ni-Cu-PGE deposits are unlikely to haveformed on Greenland, but may potentially be present at depth inthe Palaeogene British Volcanic Province. Local deposits may haveformed in association with the Traill Ø and Jameson Land sill com-plexes during contamination with gypsum-bearing sediments.

8. The layered igneous complexes in the province display a widerange of PGE mineralization styles that are generally associatedwith the magmatic differentiation processes and related silicate-sulphide liquid immiscibility. The PGE occurrences formed pri-marily during processes of magma mixing (the Rum Complex,the Kap Edvard Holm Complex, the Kruuse Fjord Complex),crystal fractionation (the Skaergaard Intrusion, possibly theCuillin Complex), and contamination (the Miki Fjordmacrodyke). Volatile redistribution may have occurred on alocal scale (the Ben Buie intrusion, the Nordre Aputitêq Intru-sion), but appears to have played only a minor role for the pri-mary PGE concentration processes.

Acknowledgments

The authors would like to thank Louis Cabri for invitingthis contribution and encouraging them to synthesize theirideas. This study has benefited from discussions and long-termcooperation with Kent Brooks, Alan Butcher, Neil Irvine,Troels Nielsen, and Duncan Pirrie, and gratitude to John Arna-son for allowing the use of unpublished material from hisPh.D. dissertation. This research used geochemical data pro-vided by the Ocean Drilling Program (ODP). Data were com-piled from the DSDP Leg 37 and ODP Legs 152 and 163.TheU.S. National Science Foundation (NSF) and participatingcountries sponsors ODP under the management of JointOceanographic Institutions (JOI), Inc. Additional data werecompiled from collections of Lotte M. Larsen and Ray Kent.Samples from Rum were collected with the permission of theScottish Natural Heritage.The Danish Lithosphere Centrefinanced collection and PGE analysis of East Greenland basaltsas part of PM’s studies at Aarhus University. Reid Keaysfinanced PGE analyses of Icelandic samples, supplied byNordic Volcanological Institute. The University of ExeterResearch Fund and the Camborne School of Mines Trust pro-vided financial support for the study of the British VolcanicProvince. The authors are grateful for careful and thoroughreviews by Louis Cabri, James D. Miller, and Troels Nielsen.This paper is a contribution to the IGCP Project 427.

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