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HAZEL M. PRICHARD Department of Earth Sciences, Open University, Milton Keynes, Buekinghamshire, U.K

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HAZEL M. PRICHARD Department of Earth Sciences, Open University, Milton Keynes, Buekinghamshire, U.K Canadian Mineralogist Vol. 26, pp. 979-990 (1988) PLATINUM AND PALLADIUM MINERALS FROM TWO PGE-RICH LOCALITIES IN THE SHETLAND OPHIOLITE COMPLEX HAZEL M. PRICHARD Department of Earth Sciences, Open University, Milton Keynes, Buekinghamshire, U.K. MK7 6AA MAHMUD TARKIAN Mineralogiseh-Petrographisehes Institut, Grindelallee48, 2000 Hamburg 13, West Germany ABSTRACT INTRODUCTION The mineralogy of Pt- and Pd-bearing platinum-group Traditionally, the PGM considered typical of minerals (POM), which form a varied assemblage previ- ously un described from other ophiolite complexes, is ophiolite complexes are Os-, Ir- and Ru-bearing recorded from two localities in the Shetland ophiolite, Scot- (Constantinides et al. 1979, Talkington 1981, land. In order of abundance the minerals include sperry- Prichard et al. 1981, Legendre 1982, Auge 1985) and lite, mertieite or stibiopalladinite, Pt-Pd-Cu-Au alloys, are associated with chromitites. Pt and Pd com- hongshiite, genkinite and potarite. All six platinum-group pounds may be more common; for example, sper- elements (POE) form major components of minerals in the rylite has been described from northern Tibet (Yu Shetland ophiolite complex. The different POM occur in & Chou 1979), and one sperrylite grain has been ten- association with "each other. Textural evidence indicates that tatively identified from the Josephine peridotite Ru- and Os-bearing POM formed early and were followed by Ir, Pd-, Rh- and, finally, Pt-bearing POM. The con- (Stockman & Hlava 1984). Chang et al. (1973) have reported the presence of sperrylite and stibiopal- centration of all six POE could be magmatic, but much of the POE mineralogy, except for laurite in the center of ladinite in ophiolites in northwestern China. Recent- chromite grains, has been modified by subsequent processes. ly, a much more diverse assemblage of PGM, includ- During POM formation, the environment became more ing Pt- and Pd-bearing minerals (Neary et al. 1984, antimony- and arsenic-rich, which finally led to the for- Prichard et al. 1984, 1986, Gunn et at. 1985) has been mation of low-temperature alteration minerals. discovered in the Shetland ophiolite complex. These PGM are reminiscent of those found in stratiform Keywords: platinum-group minerals, Shetland, ophiolite, complexes such as the Bushve1d and Stillwater. sperrylite, mertieite, stibiopalladinite, hongshiite, The Shetland mafic and ultramafic complex, genkinite, potarite, Scotland. which lies in the northeast of Scotland, on the is- lands of Unst and Fetlar, was first described as an SOMMAIRE ophiolite complex by Garson & Plant (1973) and sub- Les mineraux du groupe du platine enrichis en Pt et Pd, sequently by Flinn et al. (1979), Gass et al. (1962) decouverts a deux endroits dans Ie massif ophiolitique de and Prichard (1985). The complex is of Caledonian Shetland, en Ecosse, definissent un assemblage varie jamais origin and corresponds to the lower part of a typi- decrit auparavant dans de tels massifs. Dans I'ordre de leur cal ophiolite suite. The PGE are concentrated within abondance, ce sont sperrylite, mertieite ou stibiopalladi- the mantle sequence. They occur in chromite-rich nite, alii ages de Pt-Pd-Cu-Au, hongshiite, genkinite et samples located in two separate lenses of dunite potarite. Les six elements du groupe du platine constituent within harzburgite at Cliff and Harold's Grave (Gass des composants majeurs de ces mineraux dans Ie massif de et at. 1982). The first aim of this paper is to describe Shetland. lis sont associes. Les textures indiquent que les mineraux porteurs de Ru et de as ont ete formes a un stade these Pt- and Pd-bearing minerals, all of which are precoce, et ont ete suivis par des mineraux porteurs de Ir, unusual in an ophiolite complex. The mineralogy of Pd et Rh, et, ala toute fin, Pt. La concentration des six the Os-, Ir-, Ru- and Rh-bearing PGM from Cliff elements pourrait etre magmatique, mais la grande partie and Harold's Grave are described elsewhere (Tarki- des assemblages de mineraux observes aurait ete modifiee an & Prichard 1987). Secondly, the relative sequence par des processus tardifs, sauf dans Ie cas de la laurite pie- of formation of the PGM is examined, and their gen- gee au centre des cristaux de chromite. Au cours de Ia for- esis discussed. mation de ces mineraux, Ie milieu s'est enrichi en antimoine et en arsenic, ce qui a eventuellement mene ala formation THE PGM ASSEMBLAGE AT CLIFF de mineraux d'alteration stables a basses temperatures. AND HAROLD'S GRAVE (Traduit par Ia Redaction) Mots-eMs: mineraux du groupe du platine, Shetland, ophio- The PGM species and relative abundance at Cliff lite, sperrylite, mertieite, stibiopalladinite, hongshiite, and Harold's Grave are summarized in Table 1. Both genkinite, potarite, Ecosse. localities contain PGM in which all six PGE occur 979 980 THE CANADIAN MINERALOGIST TABLE 1. FREQUENCY AND SIZE OF PGM FROM CLIFF AND HAROLD'S GRAVE as well as synthetic PbSb and PbBiTe, were used as standards. Corrections were applied using the com- puter program PAP. It was possible to analyze quan- Cliff Harold's No.ofgrains Av..SI18 Standard Grav. meaaxlld ~ devllltionof titatively minerals with a diameter of greater than avaragaalz8 5 JLm. Many of the PGM are less than 5 JLmin diameter f1 Sperrylite Most 37 11 x4 t8.4x:l:2.9 and commonly form composite grains with abundant abundant inclusions of different PGM, so that it was not Genkinite 2 grains 8x2and 12x4 always possible to obtain quantitative analytical data. Hongshiile 1 grain 1 grain 6x2and 5,. In these cases, single-element scans have been used Pt.Pd-Cu alloy 1 grain 6,5 Pt-Pd.Au-Cu 4 grains 2,2 t 1.8xt 1.0 to demonstrate the complexity of the composite fJI PGM and to provide qualitative data. A back- Mertieiteor Slibiopalladinile Abundant 2 grains 26 9,. t7.8x:t 1.4 scattered electron image defines the surface outline Poterita 1 grain 1 3,2 Au Pd alloy 5 grains 5 3,3 :to.5x:tO.5 of the different PGM in such composite grains. The Bh outline of the single-element scan does not always Holllngworth;t" Moderately Moderately common common 11 x7 :1:8.2x:l:2.7 exactly match the outline of the mineral of which Rh, Sb, S 1 grain 2 grains 4x3, 4x2and it forms a part: extension of the PGM below the sur- 3'2 Rh,NI,Sb 1 grain face of the section, beneath surrounding minerals, 6'2 or the presence of hidden PGM also at a shallow h Irersile Moderately depth within the composite grain may contribute an common Common 27.10 :t:33x:t 11.5 Ir,Sb, S 1 grain 6,6 additional signal to the scan. !!II In the following section we describe the Pt- and Laurfte Rare Common 33 13x 10 :t:14.1 x:!: 15.5 Ruthenieo Pd-bearing PGM and give compositions. For the rare penllandite Common Common 30x 10 :I:15.6x:t:4.5 PGM, their characteristics are described using Qi Native 1 grain Common 50 3,2 :i:1.5 x:t: 0.5 reflected-light microscopy; where no quantitative analysis was possible, records of single-element scans are used. MINERALOGY as major components. However, sperrylite is the Sperrylite, PtAs2 most abundant PGM at Cliff; it has not been found at Harold's Grave. Similarly, mertieite or stibiopal- The Shetland sperrylite is anhedral (Fig. Ic), but ladinite is the second most abundant PGM.at Cliff some grains have adjacent euhedral crystal edges but has been encountered at Harold's Grave only as meeting at 120°. Crystals commonly show cleavage two grains measuring 2 by 3 JLm.The average maxi- in two directions at 120° (Fig. la). Sperrylite inter- mum and minimum diameters and standard devia- grown with chlorite has a zig-zag shape (Fig. lb). tions, of the PGM, as exposed on the polished thin The composition of the sperrylite is rather constant; sections, are given in Table I. The range of sizes is typical compositions are given in Table 2. quite variable. For example, sperrylite ranges from 2 x I JLmto 85 x 45 JLm,and laurite from I x I Genkinite, (Pt,Pd)4Sb3 JLm to 250 x 200 JLm. In contrast, the abundant grains of native osmium have been observed not to Genkinite has been located in two chromite-rich have dimensions greater than 5 JLm.It is not always rocks from Harold's Grave. In one it forms part of possible to accurately measure all PGM, as many are a composite cluster of minerals (Fig. 2), whereas in present in irregular shapes and contain inclusions. the other it is situated in a cluster of PGM but is iso- Hongshiite (Fig. 3) may be present as a single rela- lated from them by silicate matrix. In both cases the tively large crystal that encloses a Ni-Cu alloy. genkinite forms irregular crystals with dimensions of approximately 12 x 4 JLmand 8 x 2 JLm. The optical properties of the genkinite are given in Table TECHNIQUES 3. A typical composition is Pt 41.86, Rh 8.23, Pd 7.01, Ni 3.41, Sb 38.96, total 99.47 wt.OJo,giving a The PGM were studied using both reflected- and formula of (Pt2.o4Rho.76Pdo.62Nio.ssh3.97Sb3.o3' transmitted-light microscopy. Analyses of the PGM Genkinite has been described only three times previ- were obtained using a Cameca Camebax Microbeam ously (Tarkian & Stumpfll975, Cabri et al. 1977, CD wavelength-dispersion electron microprobe at the 1981); this paper reports the fourth occurrence, in University of Hamburg. The analytical conditions this case from a chromite-rich lithology from an were 15 kV and 15 nA. Pure metals, FeS2,FeAsS, ophiolitic environment.
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