Skaergaardite, PdCu, a new platinum-group intermetallic mineral from the Skaergaard intrusion, Greenland

Abstract

Skaergaardite, PdCu, is a new mineral discovered in the Skaergaard intrusion, Kangerdlugssuaq area, East Greenland. It occurs in a tholeitiic gabbro associated with plagioclase, clinopyroxene, orthopyroxene, ilmenite, titanian magnetite, fayalite and accessory chlorite-group minerals, ferrosaponite, a member of the annite-phlogopite series, hornblende, actinolite, epidote, calcite, ankerite, apatite and baddeleyite. The mineral is found in composite microglobules composed of bornite, chalcocite, digenite, chalcopyrite, with rare cobalt pentlandite, cobaltoan pentlandite, sphalerite, keithconnite, vasilite, zvyagintsevite, (Cu,Pd,Au) and Pt-Fe-Cu-Pd alloys, unnamed PdCu 3 , (Pd,Cu,Sn), Au 3 Cu and PdAuCu. Skaergaardite occurs as droplets, equant grains with rounded outlines, subhedral to euhedral crystals and as irregular grains that vary in size from 2 to 75 mm, averaging 22 mm. It is steel grey with a bronze tint, has a black streak, a metallic lustre and is sectile. Neither cleavage nor fracture was observed. The mineral has a micro-indentation hardness of VHN 25 = 257. It is isotropic, non-pleochroic and exhibits neither discernible internal reflections nor evidence of twinning. Skaergaardite varies from bright creamy white (associated with bornite and chalcopyrite) to bright white (associated with digenite and chalcocite). Reflectance values in air (and in oil) are: 58.65 (47.4) at 470 nm, 62.6 (51.1) at 546 nm, 64.1 (52.8) at 589 nm and 65.25 (53.95) at 650 nm. The average of 311 electron-microprobe analyses gives: Pd 58.94, Pt 1.12, Au 2.23, Cu 29.84, Fe 3.85, Zn 1.46, Sn 1.08, Te 0.28 and Pb 0.39, t o t a l 9 9 . 1 9 w t . % , c o r r e s p o n d i n g to ( P d 0 . 9 6 7 Au 0 . 0 2 0 Pt 0 . 0 1 0 ) S 0 . 9 9 7 ( C u 0 . 8 2 0 Fe 0 . 1 2 0 Zn 0.039 Sn 0.016 Te 0.004 Pb 0.003 ) S1.002 . The mineral is cubic, space group Pm3m, a = 3.0014(2) Å , V = 27.0378 Å 3 , Z = 1. D calc is 10.64 g/cm 3 . The six strongest lines in the X-ray powder-diffraction pattern [d in Å (I)(hkl)] are: 2.122(100)(110), 1.5000(20)(200), 1.2254(50)(211), 0.9491(20)(310), 0.8666(10)(222), 0.8021 (70)(321). The mineral has the CsCl-type structure. It is believed to be isostructural with wairauite (CoFe), synthetic CuZn (b-brass) and is structurally related to hongshiite (PtCu). Skaergaardite developed from a disordered Pd-Cu-rich metal alloy melt that had exsolved from an earlier Cu-(Fe) sulphide melt. Ordering of Pd and Cu (beginning at T & 600ºC) results in development of the CsCl structure from a disordered face-centred cubic structure.

Platinum-Group Element Deposits in Layered Intrusions: Recent Advances in the Understanding of the Ore Forming Processes

“…In the JM-Reef of the Stillwater Complex, detailed mineralogical analysis (Godel and Barnes 2008a;Godel and Barnes 2008b) of altered samples revealed the presence of Pd-Cu alloys (with composition similar to Skaergaardite (Rudashevsky et al 2004)) and to a lesser extent Pd sulfides observed either as interstitial phase between magnetite crystal or as small veinlets crosscutting the base-metal sulfide minerals ( Fig. 9.22).…”

“…f AP and PV Reefs of the Penikat intrusion. The data are extracted from and McDonald 2007; Locmelis et al 2010;Oberthür et al 2003;Osbahr 2012;Osbahr et al 2013;Rudashevsky et al 2009;Rudashevsky et al 2004;van der Merwe et al 2012;Yudovskaya et al 2012). These studies complemented the extensive literature available (e.g.…”

Extreme enrichment of Se, Te, PGE and Au in Cu sulfide microdroplets: evidence from LA-ICP-MS analysis of sulfides in the Skaergaard Intrusion, east Greenland

“…Detailed descriptions of the mineralogy of the Platinova Reef can be found in Bird et al (1991), Andersen et al (1998), Rudashevsky et al (2004), Cabri et al (2005), Nielsen et al (2005), Godel et al (2014), Rudashevsky et al (2014;2015) and Nielsen et al (2015). The latter compiles and summarizes a series of Geological Survey of Greenland and Denmark reports that record details of more than 4000 PGM (e.g., Nielsen et al 2003;Rudashevsky et al 2009).…”

“…2c, f, j) and PGM. Previous studies by Bird et al (1991), Andersen et al (1998) Rudashevsky et al (2004), Cabri et al (2005), Nielsen et al (2005;2015 and references therein) and McDonald et al (2009) have shown that for the main part of the Platinova Reef, the dominant PGM are the Pd-Cu alloys skaergaardite (PdCu) and nielsenite (PdCu 3 ) along with tetra-auricupride (AuCu) in the Au zone. The Au content of the Pd-Au-Cu alloys increases with height in line with the upward bulk rock Pd/Au ratio decrease (Fig.…”

“…Sulfide mineralization is present as tiny (<100 µm) grains of bornite-digenite/chalcocite intergrowths with associated precious metal-bearing minerals, which are dominantly Pd-Pt-Au-Cu-Fe alloys (Bird et al 1991;Andersen et al 1998;Rudashevsky et al 2004;Cabri et al 2005;Nielsen et al 2005;McDonald et al 2009;Godel et al 2014;Rudashevsky et al 2014Rudashevsky et al , 2015Nielsen et al 2015). Figure 1d shows the typical geochemical variations and offsets of base and precious metals through a generalized section of the Triple Group.…”