Platinum-group mineralization at the margin of the Skaergaard intrusion, East Greenland

Platinum-group element occurrences

The northernmost occurrence (sample WW08, Fig. 2a, Table 1) is hosted by undeformed gabbro that displays weak subhorizontal lamination and includes 2-m-long units of comb-layered crescumulate (Fig. 2a) along with scattered, up to 0.5-m-wide leucogabbro blocks (Fig. 2a, b). The gabbro consists of olivine + clinopyroxene orthocumulates with interstitial plagioclase and orthopyroxene (Fig. 4a). The crescumulate units each evolve from a leucogabbroic base to a mesogabbroic top. Sulfides are distributed throughout the cumulate, including these crescumulate units. The collected sample carries 4.4 vol% (5.8 wt%) disseminated Fe-Cu-Ni sulfide (Fig. 4b) as well as minor ilmenite and biotite. Cumulus olivine is relatively variable at Fo_76–70. The most calcic interstitial plagioclase has An₆₇. The gabbro carries 990 ppb PGE (Electronic Supplement Table 1).

Table 1

Mineral abundances and platinum-group mineral (PGM) associations

Sample		Mineral abundances, vol%							PGM associations
		Watkins Fjord wehrlite			PGE-bearing		Skaergaard picrite xenoliths		PGE-bearing samples
					Cumulate	Schist
		WW 02	WW 05	WW 10	WW 08	WW09	UM1	UM2	WW08	WW09
	X-ray analysis points	6.29 × 106	6.83 × 106	6.55 × 106	6.28 × 106	2.24 × 107	6.28 × 106	6.28 × 106
	X-ray pixel spacing, μm	10	10	10	10	10	10	10
Silicates	Olivine	82.1	88.2	79.0	48.6	4.0	53.6	59.2	1.5	3
	Clinopyroxene	1.8	4.4	7.1	10.9	23.6	13.1	13.0	2.9	5.4
	Orthopyroxene	0.1	0.1	0.7	6.9	n.a.	7.5	3.8	5.9	0
	Plagioclase	6.2	2.7	8.4	25.5	15.9	22.0	21.9	0	1.3
	Actinolite	n.a.	n.a.	n.a.	n.a.	21.5	n.a.	n.a.	0	17.2
	Biotite	0.2	0.2	0.4	1.4	8.6	0.8	0.6	2.1	6.4
	Serpentine	6.8	2.5	3.5	0.2	0.5	1.7	0.2	0	2.5
	Chlorite	0.3	0.1	0.1	0.1	2.6	0.1	0.0	0	0.3
	K-feldspar	0.1	0.1	0.1	0.1	0.3	0.1	0.0	0	0.9
	Quartz	0.0	0.0	0.0	0.0	0.1	0.0	0.0	2.9	1.5
Oxides	Ilmenite	0.0	0.0	0.3	1.4	3.6	0.6	0.7	2.6	0.4
	Ti-magnetite	0.1	0.1	0.1	0.2	2.0	0.1	0.0	3.5	10
	Chromite	2.2	1.6	0.1	0.2	0.0	0.2	0.4	0.6	0
Sulfides	Chalcopyrite	0.0	0.0	0.0	2.4	11.7	0.0	0.0	43.7	17.7
	Pyrite	0.0	0.0	0.0	0.1	0.9	0.0	0.0	6.5	7.3
	Pyrrhotite	0.0	0.0	0.0	1.2	2.4	0.0	0.0	14.4	6.1
	Pentlandite	0.0	0.0	0.0	0.7	1.1	0.0	0.0	10.3	15.2
	Galena	0.0	0.0	0.0	0.0	0.0	0.0	0.0	2.1	1.6
	Gold	0.0	0.0	0.0	0.0	0.0	0.0	0.0	1.2	0.9
Others	Apatite	0.0	0.0	0.0	0.1	0.9	0.1	0.1	0	1.1
	Others	0.1	0.0	0.0	0.0	0.1	0.0	0.0	0	1.1

Data from QEMSCAN. Mineral abundances based on field scans; PGM associations based on trace mineral searches

n.a. not analyzed

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The second occurrence, around 50 m further to the south (sample WW09, Fig. 2c, d, Table 1), is a clinopyroxene-actinolite-plagioclase-biotite-ilmenite schist (Fig. 4c) that carries significant apatite (0.9 vol%) and titanomagnetite (1.4 vol%) and up to 16 vol% (20 wt%) Fe-Cu-Ni sulfide (Fig. 4d). The abundant clinopyroxene combined with a lack of quartz, K-feldspar, and garnet suggests that the schist formed from a mafic igneous protolith. Orthopyroxene has Mg/(Mg + Fe) of 33–42; plagioclase has An₂₅, and apatite has F/(F + Cl) of 93–97 at%. The rock carries 1760 ppb PGE (Electronic Supplement Table 1).

The Watkins Fjord wehrlite

The Watkins Fjord wehrlite plug consists of mesocumulates of olivine (79–88 vol%) ± chromite (<2 vol%) with interstitial plagioclase (2–8 vol%), poikilitic clinopyroxene (1–7 vol%), and locally also with poikilitic orthopyroxene (<1 vol%) and/or ilmenite (<0.5 vol%) (Fig. 5a, b, Table 1). The cumulates are unaltered and show no evidence for tectonic deformation. Only very minor serpentinization occurred along fractures in the rocks. The wehrlite shows little mineralogical variation with only minor differences in the relative abundances of interstitial minerals across the plug. The whole-rock Mg/(Mg + Fe) is 76–78 at%, and the total PGE varies from 27 to 40 ppb (Electronic Supplement Table 1).

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The northern margin of the Skaergaard intrusion

The northern margin represents the structurally lowest exposures and most primitive rocks of the Skaergaard intrusion. The contact is visible in outcrop around 1.5 km to the west on Uttental Plateau (Fig. 2e). Here, the margin dips around 20° to the south consistent with a subhorizontal orientation at the base of the intrusion prior to rift-related collapse of the East Greenland continental margin (coastal flexure, Nielsen and Brooks 1981). The Skaergaard chilled margin is followed inward by the zone with abundant picrite xenoliths which is succeeded by the cross-bedded belt that defines the transition to the Layered Series. The chilled margin is a two-pyroxene dolerite that is locally olivine-phyric (Fig. 5c). Olivine has compositions of Fo_57–60. At the northern margin of the intrusion, the characteristic lithologies of the outermost parts of the MBS (the Tranquil Division) along the eastern and western margins of the intrusion are largely missing. The abundant picrite blocks and other xenoliths found along this margin are distributed within plagioclase-olivine orthocumulates with poikilitic clinopyroxene that locally display phase segregation and crude layering as well as very rare pegmatitic domains. Olivine has Fo_70–71 and plagioclase An_72–71. Inward from the zone with blocks, the cross-bedded belt marks the transition to the Layered Series. The lowermost plagioclase-olivine cumulates (Fig. 5d) display modal banding in an orientation that would originally have been subhorizontal. Augite appears as a cumulus mineral after around 220 m of stratigraphy (Fig. 5e), and iron-titanium oxides become abundant at around 740 m (Fig. 5f). Cumulus olivine at the base of the Layered Series has Fo₆₇ and plagioclase An₆₆.

Picrite blocks in the Marginal Border Series are olivine-plagioclase cumulates with interstitial clinopyroxene and orthopyroxene (Fig. 5g–h). Olivine forms large euhedral crystals, while cumulus plagioclase crystals are much smaller and interspersed with the interstitial pyroxenes. Although Kays and McBirney (1982) suggested that the blocks were derived from the Watkins Fjord wehrlite, the blocks are distinctly different in their cumulus mineral assemblages and compositions. Their assemblage of olivine and plagioclase is more evolved than the olivine (+chromite) assemblage of the wehrlite plug, but the composition of olivine (Fo_81–79) in the blocks is slightly more primitive than the wehrlite (Fo_79–78).

Relationship to the Watkins Fjord wehrlite

Although the occurrences are in closest proximity to the wehrlite, the PGE-bearing cumulate is compositionally and structurally very different. The wehrlite is compositionally very uniform across the plug, and there is no field evidence to suggest an evolution in mineral assemblages or compositions toward this outcrop, as would be expected if they formed part of the same magmatic succession. The difference in olivine compositions between the plug and the PGE-bearing cumulate would require extensive magmatic differentiation, and this is at variance with the uniform olivine compositions within the plug. In contrast, the much more variable olivine compositions in the PGE-bearing cumulate suggest that it formed from a relatively large, differentiating body of magma. The included leucogabbro blocks further support that the parental magma was in contact with earlier formed differentiated products. These blocks are very different to the primitive olivine + chromite cumulates of the Watkins Fjord wehrlite, and they must have been derived from a magma that crystallized a very different mineral assemblage. As above, this would be expected to involve extensive differentiation, which would be more consistent with a formation in the much larger Skaergaard intrusion

The strongly fractionated PGE in the cumulate also greatly contrasts to the unfractionated PGE within the wehrlite. Although sulfide liquids are generally expected to carry Pt, Pd, and Au in preference to Ir, Os, and Ru, the extreme fractionation required to generate these different trends is difficult to reconcile with sulfide-silicate partitioning from a common parental magma (e.g., Fleet et al. 1996; Fleet et al. 1999; Sattari et al. 2002; Mungall and Brenan 2014).

Relationship to the Skaergaard intrusion

The PGE-bearing cumulate is structurally and mineralogically similar to lithologies within the Skaergaard intrusion. Although the cumulus mineral assemblage (cumulus olivine and intercumulus plagioclase) is at variance with the earliest formed Skaergaard cumulates (cumulus plagioclase and interstitial olivine, Maaløe 1976; Hoover 1989a), it is fair to say that our knowledge of the MBS at the base of the intrusion is limited. The chilled margin is variable in texture and composition (Hoover 1989a), and it is likely that heterogeneity could lead to local variations in the cumulus mineral assemblages. Localized fluctuations in the SiO₂ activity, caused, for example, by contamination, could explain the anomalous occurrence of orthopyroxene. Recent research by Holness et al. (2015) demonstrated that the Skaergaard magma was emplaced in multiple injections during the evolution of the lowermost cumulates, and it would be reasonable to assume that several of these magmas could also have contributed to the compositional variability of the chilled margin.

We initially entertained the idea that the cumulate might represent a previously unknown source region for the picrite blocks in the Marginal Border Series. However, although the modal proportions of minerals are very similar to these blocks, the cumulus assemblages are different and the composition of cumulus olivine is significantly more evolved and variable in the PGE-bearing cumulate. We found no evidence for sulfide equilibration in olivine within the picrite blocks to suggest a connection with the PGE-bearing cumulate.

Olivine compositions within the PGE-bearing cumulate closely match the most primitive olivine in the Skaergaard cumulates (Fo₇₄, Hoover 1989b). As expected from the interstitial habit, plagioclase is more evolved than the equivalent cumulus plagioclase in the Layered Series. Olivine in parts of the MBS are clearly Ni depleted, suggesting that sulfide saturation at least locally affected the marginal parts of the intrusion.

The crescumulates and leucogabbroic blocks are further indications that the cumulate formed from evolved magma in a relatively dynamic magma chamber. The crescumulates evolve from feldspathic bases and appear to be controlled by the upward growth of plagioclase. In this respect, they are similar to the perpendicular feldspar rock in the Marginal Border Series (Wager and Deer 1939). Similar crescumulates have also been found among the picrite blocks (M. Holness, personal communication, 2012).

The leucogabbroic blocks are not unlike those found in the Skaergaard Layered Series (Irvine et al. 1998), and they demonstrate that the parental magma at some point was in contact with earlier formed, differentiated cumulates; that these cumulates were partially disrupted; and that a pathway existed for the blocks to become incorporated into the cumulate. This again suggests a connection to a relatively large magma chamber that was compositionally more evolved than the Watkins Fjord wehrlite plug.

A further indication of a Skaergaard connection comes from the PGE geochemistry, with both occurrences displaying strongly positive mantle-normalized PGE trends that parallel those of Skaergaard chilled margin (Fig. 9). These trends contrast strongly to the unfractionated PGE in the Watkins Fjord wehrlite. The PGEs are systematically enriched by 10–50 times. The strongest enrichments are in Pt and Pd, but the Ir-Pt-Pd ratios remain within the variability of the chilled margin. The deviations in Cu/Pd, Ni/Cu, and Pd/Au can be explained by variability in Au and Cu within the chilled margin (as also reported by Nielsen and Brooks 1995; Momme 2000).

Formation of PGE-bearing schist

The Precambrian basement represents a succession of high-grade metamorphic rocks. Kays et al. (1989) estimated the peak metamorphic conditions to have reached or exceeded 650–700 °C and 3–4 kbar. The coexistence of plagioclase and clinopyroxene (upper amphibolite to granulite facies) with biotite and actinolite (greenschist to lower amphibolite facies) in the schist indicates a fluid-assisted retrograde modification of the high-grade protolith. Apart from differences in the modal abundances, the sulfide and PGM assemblages are identical to the PGE-bearing cumulate, and we consider it unlikely that they formed independently as an integral part of the metamorphic basement.

Magmatic sulfide liquids have been demonstrated elsewhere to migrate into and interact with basement lithologies. A particularly interesting analog is the Kilvenjärvi deposit of the Portimo complex in Finland, where high concentrations of PGE are associated with sulfide more than 50 m below the base of the host intrusion (Iljina 1994; Andersen et al. 2006). This particular deposit appears to have formed by downward migration of sulfide liquid away from the host intrusion and subsequent equilibration with the host rocks (Iljina 1994; Andersen et al. 2006). Other examples include the PGE-bearing skarns beneath the Platreef (Armitage et al. 2002), the Ngala Hill in Malawi (Henckel and Mitchell 2002), and the Talnakh area of Noril’sk (Ryabov et al. 1996).

We suggest that the schist developed by contact metasomatism as a batch of the Skaergaard parental magma came into contact with the basement lithologies. Assimilation of silicic basement could have locally reduced the capacity of the magma to carry sulfide, leading to silicate-sulfide immiscibility. Sulfide liquid could have ponded at the base of the magma and been locally expelled into the host rocks. The abundance of biotite in the schist suggests that the process might have been somewhat fluid assisted. However, as hydrous fluids would be expected to precipitate Cl-rich apatite (e.g., Boudreau et al. 1986), the F-rich composition of the apatite suggests that fluids were not a principal agent of mineralization.

Process of mineralization

Platinum-group element deposits at the margins of layered mafic-ultramafic complexes are of global economic interest. The Platreef alone hosts nearly 17 wt% of the PGE resources in the Bushveld complex, South Africa (Cawthorn 1999), and significant prospects have been documented from the Portimo and Koillismaa complexes, Finland, and the Coldwell complex and the East Bull Lake intrusive suite, Canada (Iljina and Lee 2005).

The sulfide assemblages are typical for orthomagmatic sulfides formed from mafic-ultramafic magmas, and the strong associations between the sulfides and the PGM indicate that the PGEs were collected by magmatic sulfide liquids. Nickel depletion in olivine from the PGE-bearing gabbro suggests that the sulfide formed directly from the local cumulate.

The sulfide and PGM assemblages of both occurrences are very similar, despite the very different host lithologies. Although the PGMs in the schist have a stronger association with hydrous silicates (Fig. 8), the sulfide association still dominates. The lack of variation between the PGM assemblages suggests that this association reflects the greater modal abundance of these minerals rather than a hydrothermal component to the PGE mineralization. The high F/(F + Cl) indicates that associated apatite crystallized under relatively anhydrous conditions, which could be either directly from the parental magma or as part of the high-grade metamorphic host (cf., Spear and Pyle 2002). Magmatic-derived hydrous fluids would be expected to precipitate Cl-rich apatite instead (Boudreau and McCallum 1992). The near constant Ir-Pt-Pd-Au ratios of the two occurrences confirm that the PGEs were largely unaffected by hydrothermal redistribution.

Differences within the relative abundances of pentlandite and chalcopyrite and in the PGE tenor between the occurrences imply that the sulfide melts formed as separate entities or that they were related through a process of fractionation. The abundance and distribution of sulfide in the gabbro suggest that it did not collect as a single liquid body but that at least some remained dispersed within the silicate magma or mush. Furthermore, the constant Pd/Ir with variations in Ni/Cu is inconsistent with fractionation of monosulfide solid solution (e.g., Maier et al. 1998) from a relatively uniform sulfide body. Local variations in the silicate-sulfide mass ratios (R factors) during immiscibility can explain the variations in PGE tenor between the two occurrences. If the Skaergaard chilled margin is assumed to represent the primary magma, the total PGE concentrations indicate that the PGE-bearing cumulate formed with an R factor around 210 and the schist 110 (Eq. 1).

The setting, mineralogy, and geochemistry of the occurrences are adequately explained by immiscibility in response to contamination (equivalent to the process documented for the Platreef by Holwell and McDonald 2006). Collection of the PGE by magmatic sulfide liquids in an environment with variable host rock assimilation can fully explain the mineralogical and geochemical variations in the sulfide. The retrograde assemblage of the schist is likely to have evolved through the interaction with the Skaergaard magma or associated magmatic fluids. However, the PGEs appear to have been passively carried in the sulfide liquid rather than directly interacted with contaminants or hydrous fluids.

Comparison to the Platinova Reef

Despite the geochemical similarities to the Skaergaard margin, it is notable that the sulfide minerals observed here are very different to sulfides that appear in the Platinova Reef and the upper parts of the Skaergaard Layered Series. The pyrrhotite + pentlandite + chalcopyrite assemblage is similar to traditional orthomagmatic sulfide deposits. The Skaergaard Layered Series, in contrast, is dominated by chalcopyrite, bornite, and digenite and has marcasite in the most fractionated parts. The pentlandite can be explained by early silicate-sulfide immiscibility during the crystallization of the marginal gabbros. During the formation of the Layered Series, in contrast, the Skaergaard magma had been almost completely depleted in Ni through olivine crystallization in the Hidden and Lower Zones prior to sulfide formation and therefore did not produce pentlandite. The differences between the chalcopyrite-pyrrhotite and chalcopyrite-bornite-digenite-marcasite assemblages can be explained by oxidation of the primary orthomagmatic sulfide assemblage in the Layered Series (Andersen 2006) or by iron or oxygen exchange between sulfide and Fe-rich silicate melts (Nielsen et al. 2015).

Implications for exploration

The occurrence of PGE-bearing rocks along the margin of the Skaergaard intrusion indicates that contact-style mineralization should be considered as viable exploration targets for evolved tholeiitic intrusions. Significant potential could exist elsewhere along the margin of the Skaergaard intrusion. The discovery also raises the question whether other intrusions in East Greenland may have significant PGE-bearing sulfide concentrations along their margins, notably the much larger Kap Edvard Holm complex to the west where PGEs have been reported in the past (Bird et al. 1995; Arnason et al. 1997).