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Skaergaard Intrusion - Platinova Reef |
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Greenland |
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Au PGE PGM Pt Pd
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Super Porphyry Cu and Au
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The Skaergaard layered, ferrobasaltic/gabbroic intrusion is of Early Eocene age and is located in Eastern Greenland. It is related to the major rifting event that culminated in the opening of the North Atlantic Ocean. The intrusion hosts the Pd-Au bearing Platinova Reef (#Location: 68° 10' 40"N, 31° 40' 36"W).
The east coast of Greenland is a volcanic rifted margin characterised by voluminous igneous activity. The progressively younger magmatism has been divided into:
i). Lower Basalts, up to 2 km thick of tholeiites, olivine tholeiites, picrites and ankaramites;
ii). Plateau Basalts - >5 km of olivine tholeiites and tholeiites representing perhaps a volume of 10 M km3;
iii). Layered gabbros and related dykes, which includes the Skaergaard Intrusion; and
iv). Wehrlites, occurring as plugs of limited extent, and other related rocks.
The Skaergaard Intrusion was emplaced between 55.4 and 56.6 Ma, whilst the associated basalts have been dated at 55 to 60 Ma. The basement to the basalts and which underlies the intrusion, is composed of strongly folded and foliated orthogneisses, which range from diorite and granodiorite to tonalite-trondhjemite-granite, and are locally migmatitic. These orthogneisses also contain bands and lenses of ultramafics, amphibolites, quartzites and sillimanite-bearing garnet-biotite schists.
The Skaergaard intrusion is a relatively small, ~300 km3, melt-dominated pluton that crystallised in response to cooling from the roof and margins upwards and inward, forming upper, marginal and bottom series, the latter referred to as the Layered Series. It was intruded during continuing continental rifting of the East Greenland continental margin, and was emplaced at a high crustal level, along the unconformity that separates the Archaean gneiss basement from a thin succession of Cretaceous to Paleocene sediments and the Paleocene to Eocene East Greenland flood basalts. The intrusion, at least in part, post-dates the Lower Basalts and was contemporaneous with the extrusion of the main Plateau Basalts and the transition from rift to drift tectonics in the North Atlantic. The initial cooling produced a marginal gabbro (including a chilled margin) at the contact with the country rocks. The subsequent crystallisation proceeded upward and inwards from the margins of the chamber producing the Marginal Border Series and Upper Border Series formed by crystallisation from the walls and roof of the chamber, respectively, whilst the Layered Series formed concurrently on the bottom of the chamber (Thy et al., 2023).
The phase layering in the Layered Series is interpreted to suggest an evolved, olivine-normative tholeiitic melt saturated in plagioclase and olivine, followed by augite, and then, simultaneously, by ilmenite and magnetite forming primocrysts. Pigeonite appears in the lower parts and continues until the centre of the series. Apatite appears in the upper part concurrently with liquid immiscibility. Cryptic variations of the individual primocrysts record a systematic upward increase in iron and decrease in magnesium for the mafic minerals and a systematic increase in sodium and decrease in calcium for plagioclase (Thy et al., 2023).
The stratigraphy of the Layered Series may be summarised as follows (after Thy et al., 2023):
• The Lower Zone, which is 876 m thick within the composite profile that predominantly comprises gabbronorite, characterised by plagioclase, olivine, clinopyroxene, orthopyroxene and Fe-Ti oxides. The Lower Zone has been subdivided into three subzones. These are the ~350 m thick Lower Subzone a gabbronorite, with 56 wt.% plagioclase, 9 wt.% olivine, 13 wt.% clinopyroxene and 18 wt.% orthopyroxene, with up to 4% interstitial ilmenite, magnetite and apatite; the ~542 m thick Lower Subzone b gabbronorite, with 51 wt.% plagioclase, 5 wt.% olivine, 28 wt.% clinopyroxene and 13 wt.% orthopyroxene and ~5% Ilmenite, magnetite and apatite; and the ~160 m thick Lower Subzone c Fe-Ti oxide gabbronorites that average 34 wt.% plagioclase, 6 wt.% olivine, 30 wt.% clinopyroxene, 7 wt.% orthopyroxene, 17 wt.% ilmenite, 7 wt.% magnetite and trace apatite.
• The Middle Zone, which comprises 260 m of the composite profile, and is an Fe-Ti oxide gabbronorite, containing, on average 40 wt.% plagioclase, 2 wt.% olivine, 34 wt.% clinopyroxene, 6 wt.% orthopyroxene, 14 wt.% ilmenite and 6 wt.% magnetite, with trace apatite (≤0.1 wt.%). Fractionation in the final stages of crystallisation led to the formation of the PGE and gold deposits in what is commonly referred to as the Triple Group of the upper part of the Middle Zone. The upper 100 m or so of the Middle Zone is occupied by three leucocratic-melanocratic pairs referred to as the "Triple Group" which host the Pd and Au mineralisation, as described below. This interval is ~1600 m above the base of the intrusion.MZ
• The Upper Zone, which represents 987 m of the composite stratigraphic profile, and varies from gabbronorite in the lower section, to an upper gabbro. It is characterised throughout by plagioclase, olivine, orthopyroxene (which disappears at 1600 m elevation), clinopyroxene and Fe-Ti oxides. The Upper Zone has been divided into three subzones, namely: Upper Subzone a, which comprises 432 m of Fe-Ti oxide gabbronorite containing, on average, 44 wt.% plagioclase, 7 wt.% olivine, 34 wt.% clinopyroxene, 4 wt.% orthopyroxene, 8 wt.% ilmenite, 5 wt.% magnetite and ~0.2 wt.% apatite. Throughout this subzone, olivine levels are markedly higher than in the Middle Zone, while orthopyroxene is relatively rare compared that zone. The Upper Subzone b, comprises 424 m of Fe-Ti oxide and apatite gabbro with an average of 40 wt.% plagioclase, 24 wt.% olivine, 22 wt.% clinopyroxene, 7 wt.% ilmenite, 3 wt.% magnetite and 4 wt.% apatite. The most conspicuous features of this zone are the high olivine mode, the near absence of orthopyroxene and the occurrence of abundant apatite. The Upper Subzone c comprises 123 m of chiefly ferrograbbros (or ferrodiorites), with an average of 33 wt.% plagioclase, 20 wt.% olivine, 34 wt.% clinopyroxene, 5 wt.% ilmenite, 2 wt.% apatite, 5 wt.% quartz and 1 wt.% orthoclase. Magnetite is scarce or absent. The ferrogabbro also contains abundant interstitial granophyre or micropegmatite that increases up-section to as much as 14 wt.% of the rock composed of a granophyric intergrowth of quartz, albite and orthoclase.
Gold and PGE mineralisation occurs within a layered succession of leucocratic and melanocratic gabbros in the upper 100 m of the Middle Zone of the intrusion. This interval includes three prominent, 15 to 20 m thick layers known as the "Triple Group", each of which comprises a leucocratic lower and melanocratic upper section. The upper melanocratic section of each contains large euhedral liquidus olivines, although the Middle Zone is otherwise characterised by being generally olivine free. Each of these Triple Group pairs is separated by units of mesogabbro of variable thickness, which show evidence of macrorhythmic layering on a 5 to 15 m scale (Nielsen and Brooks 1995).
The precious metal mineralisation is restricted to a 70 m interval occupied by the Triple Group and contains at least one major Pd and at least ten smaller reef, each a few tens of centimetres to the largest which is several metres thick, containing significant Pd and Pt enriched layered leucocratic or melanocratic gabbro, separated by PGE and Au poor gabbro. The major Pd and Au concentrations are separated in a systematic fashion across the intrusion, from near the centre, where the Au concentration is ~60 m higher in the stratigraphy than the main Pd reef, gradually decreasing towards the margin, where the two are only a few metres apart (Andersen et al., 1998).
These intervals are concordant with the modal layering of the intrusion. Each has a chemical zonation outwards from the centre of the intrusion with decreasing Pt:Pd ratios (due to the increase in Pd), followed by a strong increase in Au, before both Au and PGE's drop off. The mineralised zones take the form of a series of stacked saucers of PGE mineralisation, with a gold outer annulus. The diameter of these "saucers" decrease upwards (Nielsen and Brooks 1995).
The dominant precious metal minerals are small (Cu,Fe)(Au,Pd,Pt) alloys that are typically included in interstitial sulphides which include bornite, digenite and minor chalcopyrite. Other alloys such as electrum, atokite [Pd3Sn] and zvyagintsevite [Pd3Pb] have been reported (Andersen et al., 1998).
The main mineralised zone is referred to as the Platinova Reef, which was discovered in 1986.
The gold resource has been quoted (Nielsen and Brooks 1995) at 40 Mt @ 2.38 g/t Au and >100 Mt @ 1.9 g/t Au.
PGE grades of up to 3 g/t Pd (Pt:Pd ratio of around 0.1) have been encountered over widths of 10 m.
Two separate zones of mineralisation within the Platinova Reef have been reported by Platina Resources Ltd (2008). These are inferred to contain:
106.8 Mt @ 1.68 g/t Au, and
103.5 Mt @ 1.91 ppm Pd.
In addition to the Skaergaard Intrusion, there are a number of other Palaeogene intrusions in the area related to the same magmatic event, many with associated mineralisation, including the gabbroic Kap Edvard Holm Intrusion, the Kaerven Gabbro Complex, the large syenitic Kangerlussuaq Alkaline Complex, and the dioritic Circe 1320 complex, together with a range of mafic to alkaline dykes (Nielsen 1978). A distinct set of large gabbroic dykes or 'macrodykes', with typical widths of around 100 to 500 m, are exposed in the area around the Skaergaard intrusion, including the Miki Fjord (e.g. Blichert-Toft et al., 1992), Vandfalsdalen (e.g. White et al., 1989), Kraemer Island (Momme and Wilson 2002), Kangerlussuaq (Holm et al., 2006) and the Togeda Macrodykes. Two of these macrodykes, Miki Fjord and Togeda, contain Cu-PGE-Au sulphide mineralisation along their margins. Sulphides occur as disseminated interstitial blebs and rounded globules of chalcopyrite and pyrrhotite with some Fe–Ti oxides and platinum-group minerals, comprising largely Pd bismuthides and tellurides.
The most recent source geological information used to prepare this decription was dated: 2023.
Record last updated: 21/1/2025
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Andersen J C O, 2007 - Postmagmatic sulphur loss in the Skaergaard Intrusion: Implications for the formation of the Platinova Reef: in Lithos v92 pp 198-221
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Andersen, J.C.O. Rollinson, G.K.,McDonald, I., Tegner, C. and Lesher, C.E., 2017 - Platinum-group mineralization at the margin of the Skaergaard intrusion, East Greenland: in Mineralium Deposita v.52, pp. 929-942.
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Andersen, J.C.O., Rasmussen, H., Nielsen, T.F.D. and Ronsbo, J.G., 1998 - The Triple Group and the Platinova Gold and Palladium Reefs in the Skaergaard intrusion: stratigraphic and petrographic relations: in Econ. Geol. v.93, pp. 488-509.
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Ernst R.E., Liikane D.A., Jowitt S.M., Buchan K.L. and Blanchard, J.A., 2019 - A new plumbing system framework for mantle plume-related continental Large Igneous Provinces and their mafic-ultramafic intrusions: in Journal of Volcanology and Geothermal Research, v.384, pp. 75-84. doi.org/10.1016/j.jvolgeores.2019.07.007.
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Holwell D A and Keays R R, 2014 - The Formation of Low-Volume, High-Tenor Magmatic PGE-Au Sulfide Mineralization in Closed Systems: Evidence from Precious and Base Metal Geochemistry of the Platinova Reef, Skaergaard Intrusion, East Greenland : in Econ. Geol. v.109 pp. 387-406
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Mungall, J.E., Jenkins, M.C., Robb, S.J., Yao, Z. and Brenan, J.M., 2020 - Upgrading of magmatic sulfides, revisited: in Econ. Geol. v.115, pp. 1827-1833.
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Nielsen T F D 2004 - The Shape and Volume of the Skaergaard Intrusion, Greenland: Implications for Mass Balance and Bulk Composition: in J. of Petrology v45 pp 507-530
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Nielsen T F D, Brooks C K 1995 - Precious metals in magmas of east Greenland: factors important to the mineralization in the Skaergaard intrusion: in Econ. Geol. v 90 pp 1911-1917
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Thy, P., Tegner, C. and Lesher, C.E., 2023 - Petrology of the Skaergaard Layered Series: in GEUS Geological Survey of Denmark and Greenland Bulletin 56, 139p. doi.org/10.34194/geusb.v56.8327.
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Wernette, B., Li, P. and Boudreau, A., 2020 - Sulfides, native metals, and associated trace minerals of the Skaergaard intrusion, Greenland: evidence for late hydrothermal fluids: in Mineralium Deposita v.55, pp. 1197-1214.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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