Canadian Mineralogist Vol. 26, pp. 979-990 (1988)

PLATINUM AND PALLADIUM 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 -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 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 in the center of ladinite in ophiolites in northwestern China. Recent- 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 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 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 edges but has been encountered at Harold's Grave only as meeting at 120°. commonly show 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 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. PGM, SHETLAND OPHIOLITE COMPLEX 981

FIG. 1. Reflected-light photomicrographs of Pt- and Pd-bearing minerals from Cliff. (a and c) Sperrylite (pale grey) in a silicate matrix (dark grey). (b) Sperrylite (white) in a silicate vein (dark grey) between two chromite grains (pale grey). (d) Pt-Pd- Cu-Au alloys (white) associated with native (pale grey) surrounded by chro- mite (dark grey) (see Fig. 4). (e) Mertieite or stibiopalladinite (white) i,~ the rim of a chromite grain (pale grey) and associated with (white). Silicate matrix appears dark grey. (f) Mertieite or stibiopalladinite (white) in the silicate matrix (dark grey) near a chromite grain (pale grey). Scale bar represents 10 I'm.

Hongshiite, PtCu given in Table 3. Hongshiite previously had been described only from Hung, China (Peng et at, 1978). A Pt-Cu phase that occurs adjacent to genkinite (Fig. 2) and has an irregular form is also present as Pt-rich alloys a network of branches that enclose a Ni-Cu alloy adjacent to a sperrylite crystal (Fig. 3). The compo- An irregular grain of a Pt-Pd-Cu alloy lies adja- sition of the Pt-Cu phase is given by scans only, as cent to genkinite (Fig. 2) and in contact with irarsite it is too small (6 x 2 JLm)for quantitative analysis, and hollingworthite. The optical properties are given but no other elements were detected. The mineral is in Table 3. The mineral has a rough, chequered sur- likely to be hongshiite. Its optical properties, in face, possibly suggesting a fine intergrowth. The agreement with those reported by Cabri (1981), are grain has an approximate diameter of 5 JLm,and the 982 THE CANADIAN MINERALOGIST

TABLE 2 MICROPROBE COMPOSITIONS OF SPERRY LITE elements present are given qualitatively by scans (Fig.

4 2). An analysis gave Pt 63.00, Pd 16.86, Cu 3.21, Sb 0.56, total 83.63 wt.OJo. Pt 55.79 55.69 55.69 55.60 As 43.79 41.79 42.42 41.82 Figures Id and 4 show four small (less than 5 porn) Fe 0.27 0.12 0.46 0.11 Sb 0.91 1.99 1.96 2.92 irregular grains of a Pt-Pd-Au-Cu alloy clustered total 100.7S 99.59 100.53 100.45 around an undefined arsenide and associated with native copper. Optical properties of the alloys 1. (PtO.9SFeO.01) (As2.00Sb<0.01) r-0.99 r-2.00 are given in Table 3. No occurrence of such alloys ) 2. (PtO.99FeO.01 r-1.00 (As1.94SbO.OS) r-2.00 is known other than in Shetland (Prichard et al. 1986). 3. (PtO.99FeO.03) r-1.02 (As1.94SbO.OS) r-2.00 ) 4. (PtO.98FoO.01 r-0.99 (As1.93SbO.08) r-2.01 Mertieite or stibiopalladinite These crystals are subhedral, irregular (Fig. Ie),

FIG. 2. (a) Back-scattered electron image. (b) Mineral-location map. (c-k) Single- element scans for Rh, Pd, Pt, Ir, Ni, Cu, S, As and Sb, respectively. The PGM in this composite grain from Harold's Grave are irarsite (IrAsS), hollingworthite (RhAsS), hongshiite (PtCu), genkinite [(Pt, Rh, Pd, Ni)4Sb3], and a Pt-Pd-Cu alloy.

elongate and commonly lath-shaped. Some crystal growth are approximately 1J.lmwide, but larger areas edges meet at 1200 (Fig. If). Two grains of an inter- of each member of the intergrowth are developed growth of Pd antimonide and a nickel telluride have locally (Fig. 5). Some typical compositions of these been found in the interstitial silicate matrix of the minerals are given in Table 4. same polished section of material from Cliff. One Stibiopalladinite and mertieite I and II can be dis- grain is rhomb-shaped and the other, surrounded by tinguished from one another only by X-ray-dif- millerite, is tabular. Although apparently optically fraction data (Cabri 1981), as there are no significant homogeneous, higher-magnification scans show a optical differences among them. No special reflect- fine intergrowth of crystals. The lamellae of the inter- ance data have yet been given for mertieite I, and PGM, SIIETLAND OPHIOLITE COMPLEX 983 the reflectance curves of mertieite II and stibiopal- TABE & OPnCA! MOPENTIEA TOA N|E TOBE UXUAUAI PI. ND Pd.BEARIXS pod ladinite are very similar, as is the microhardness.All FNOU SHETLAIID three minerals are anisotropic, and there are no dif- AMmy ll,tt@ Gdddtdo lPlBh,P4NDa€63fsb ffi dolqly llardqM ferencesinthe extinction color of the anisotropism. W ad3otpb br€8hnoud glatbbwt srttdlhanfp Mertieite I is not fully characterized by crystal- R-ro{u anot structure analysisbut may be pseudohexagonal.Mer- ffi€ffio POU @qffi aroropc miuo ye g€yb qoffio tieite II is rhombohedral, and stibiopalladinite is hex- g@ffi gr€t agonal. According to Cabri (1981),mertieite I may dan(

Ftc. 3. (a) Back-scattered electron image. O) Mineral-location map. (c-f) Single- elementscans for Cu, Ni, Pt and As, respectively.The minerals in this composite grain from Cliff are sperrylite (PtA&),.honeshiite (PtCu), and a Ni-Cu alloy. The single-elementscans define this compositenineral grain, cited and shown in plate 9 in Prichard et ol, (1986). 984 THE CANADIAN MINERALOGIST

FIG. 4. (a) Back-scattered electron image. (b) Mineral-location map. (c-i) Single- element scans for Au, Ni, Pd, Pt, Cu, S, and As, respectively. The Pt-Pd-Au- Cu alloys from Cliff are associated with native copper and a nickel arsenide. The single-element scans define this composite mineral grain, cited and shown in plate 12 in Prichard et al. (1986).

not be distinguished by the differences in chemical Most grains are located in chromite rims, but some composition that were found. are in the interstitial silicate or are associated with pentlandite. Potarite, PdHg

One grain of potarite from Cliff has been qualita- TEXTURAL RELATIONSHIPS OF THE PGM tively analyzed (Prichard et al. 1986). PGM in relation to their host minerals Pd-Au alloys All the PGM studied in this paper are from Small (3 x 3 J.'m) anhedral grains of Pd-bearing chromite-rich samples, containing 50 to 900/0chrome gold have been qualitatively identified from Cliff. spinel. The interstitial silicates are secondary serpen- PGM, SHETLAND OPHIOLITE COMPLEX 985

FIG. 5. (a) Back-scattered electron-microscope image. (b) Location-map showing areas A, Band C. (e-h) Single-element scans for Pd, Ni, Sb, and Te, respectively, in area C. This grain from Cliff consists of an intergrowth of a palladium antimonide and a nickel telluride, marked (i) and (ii), respectively.

tine, chlorite associated with , and nickel PGM is shown in Figure 6. The only PGM found carbonate (Prichard et al. 1987). The PGM occur in entirely enclosed within the center of chromite grains the center and 'spongy' rim of chromite grains and is an Os-rich laurite (RuSz). Os-free laurite, with in the silicate matrix interstitial to the chromite. A native osmium inclusions and irarsite, occurs in the schematic diagram of the textural settings of the rim of chromite grains. Euhedral native osmium also 986 THE CANADIAN MINERALOGIST

TABLE 4. MICROPROBE ANALYSES OF MERTIEITE OR STIBIOPALLADINITE FROM CLIFF

2 3 4 5 6

Pd 66.31 67.68 66.57 67.55 67.02 65.61 Cu 2.79 1.71 2.74 1.75 2.84 3.26 Pt 0.48 0.78 0.61 0.60 0.42 0.49 Ni Sb 28.34 29.27 29.05 27.24 30.47 31.21 As 0.60 0.32 0.36 1.72 total 98.52 99.76 99.33 98.86 100.75 100.57

(ldeBI) Pd:Sb PdSSb3(ldeBI) Pd:Sb 32:12 PdsSb2 (ldeBI) Pd:Sb =30:12 Pd"Sb. =33:12 = 1. (Pd4.79CUa.34Pta.a2)I=5.1S(Sb,,79Asa.a6h:.=,.S5 (Pd, o.27Cua, 72Pta,04):t=1' ,03(Sb3,84ASa, 13):&=3,97 (Pd7.53Cua,s3Pta.03):t=a,09(Sb2,S1 Asa, 1a):t=2.9' 2, (Pd.,s9Cua.2,Pta.a3)I=5.,3(Sb,.s.Asa.a3):t=,.s7 (Pd, O,.7CUa,..Pta.a7):t=, a.9s(Sb3,95ASa.a7):t=4,a2 (Pd7,6sCua.32Pta.05):t=a,05(Sb2,90Asa,a5):t=2.95 (Pd 3. (Pd4.79Cua,33PtD,D2)I=5,1.(Sb1.82Asa.04):t=,.S6 1a.2SCUD,7' PID.OS):t=" ,a1 (Sb3,9' ASD,DS):t=3,99 (Pd7,S2Cua.S2Pta.03):t=a,a7(Sb2,S7Asa,D6):t=2.93 (Pd, 4. (Pd"s7Cua,2' Pta.a2)I=5.1a(Sb,.7~Sa,1S):t=,.9a a...CUa,45Pta,aS):t=' a,94(Sb3.SsASa,3S):t=4,a6 (Pd7.66Cua.33Pta,04):t=a,03(Sb2.7aAsa.27):t=2,97 5. (Pd4.75CUa,3.Pta,a1)I=5.1DSb1.9a (Pd, a, (Pd7..7Cua,S3Pta.a3):t=a,03Sb2,97 '9Cua, 72Pta.04):t=, D.9SSb "as 6, (Pd4.65Cua,39Pta,a2)I=5,06Sb',94 (Pd9.9sCua...Pto,04):t=1a,S6Sb..1S (Pd7,32CUD,6' Pta.a3):t=7, 96Sb3,a.

of chromite grains. Nearly all the sperrylite is sur- rounded by chlorite, and some is intergrown with chlorite to form zig-zag crystals. Some sperrylite is ::P(jAu::: ::::::::::::::,,:: associated with nickel carbonate, and one sperrylite ~llandl18 grain is enclosed by mesh-textured serpentine. ::;:::::::::::::::::;:::::::::::::::::::::::::::::;:;:::::;:::. ~US2 Genkinite and hongshiite also occur in the intersti- ?1'0"...::~:::.. Os tial silicate matrix. ~::~ PICu : Textures of the composite PGM grains :: PIAs2 .,iill~Ii~li PGM commonly form composite grains, examples of which are illustrated in Figures 2 and 3. In Tar- ...£3..:::::::::::' RhAsS~lrASS kian & Prichard (1987), the composite Ru-, Ir-, Os- and Rh-bearing PGM are discussed in detail, but a general summary is included here to enable the FIG, 6. Schematic diagram of the PGM and their textural genetic sequence of formation of the Pt- and Pd- positions in relation to chromite grains. bearing PGM to be discussed. Figure 7 is a diagram- matic representation of the textural relationships of the PGM at Cliff and Harold's Grave. is present, enclosed by ruthenian pentlandite, which In general, PGM and antimonides are is located in the silicate matrix in association with enclosed by arsenides where they are found in associ- nickel carbonate. Pure laurite and irarsite also occur ation. For example, laurite (RuSz), commonly con- in the interstitial silicate matrix; the irarsite is com- taining abundant small (2-3 pm) inclusions of native monly rimmed by hollingworthite (RhAsS). No Rh- osmium, is commonly intergrown and rimmed by bearing minerals have been observed in the center irarsite (IrAsS) (Fig. 8). Irarsite also contains a num- or rim of chromite grains (Tarkian & Prichard 1987). ber of antimony-rich phases including Rh-Sb-S, Ir- Most Pd-bearing PGM occur in the altered sili- Sb-S, Rh-Ni-Sb, small grains of a Pd antimonide cate matrix, but some are located in chromite rims. and breithauptite (NiSb). Another example of an Sb- Generally, the mertieite or stibiopalladinite is situ- bearing phase enclosed by an As-rich phase is a Rh- ated in chlorite or serpentine between chromite grain, Sb-S inclusion, which is surrounded by sperrylite. and only rarely is it in chromite rims, where it is The majority of the Pd forms Pd antimonide, which associated with silicate inclusions and occasionally is mostly isolated from the other phases but is found pentlandite (Figs. Ie, f, 6). Potarite also is found within and adjacent to irarsite and partly enclosed within these rims, as are the majority of the Pd- by sperrylite. In one case the Pd antimonide encloses bearing gold grains. hollingworthite, which has a pure end-member com- All sperrylite is associated with the interstitial sili- position devoid of Pt. In the reverse case, where hol- cates between chromite grains. Only rarely is sper- lingworthite encloses Pd antimonide, the hollingwor- rylite attached to the edge of the chromite grains, thite is Pt-bearing (Tarkian & Prichard 1987). and none has been found within the center or rim Hollingworthite commonly rims irarsite (Fig. 2) and PGM. SHETLAND OPHIOLITE COMPLEX 987

is itself enclosed by sperrylite, the latter being the most arsenic-rich phase. Genkinite, hongshiite, and PtCu the Pt-Pd-Cu alloy are all found in association with other Pt-bearing PGM. AhAoS + .PI GENESIS OF THE PGM These textural relationships suggest constraints on BB the order of PGM formation and changes in availa-

bility of PGE and arsenic. If chromite crystalIIiza- Rh-Sb-S IrAsS tion preceded that of the primary igneous intersti- Pd-Sb tial silicates, then the PGM inclusions in chromite grains formed prior to those in the silicate matrix. Ir-Sb-S AuS, PGM in the spongy rim of chromite grains, and com- Pd-Sb monly associated with silicate inclusions, probably formed at an intermediate stage. On this basis the Pt-Pd-Cu earliest mineral to form was Os-rich laurite, which occurs as isolated crystals in chromite grains (Tar- ..~ 8 B kian & Prichard 1987). Irarsite inclusions associated '0'11,.'" with silicates in chromite rims represent increasing e; availability of arsenic. Antimony-rich inclusions that (Fe,NI,Ru)S PtCu occur within As-rich phases such as irarsite and sper- rylite may indicate increasing availability of As in place of Sb. The presence of Pd-rich PGM in chromite rims and interstitial silicate, and the restriction of PGM containing Pt as a major PGE, to the silicate matrix, FIG. 7. Schematic diagram showing the textural relation- suggest that Pd preceded Pt availability for PGM ships of PGM within composite grains. Only minerals formation. Similarly, Rh-bearing PGM are found found in mutual contact are considered, the size of the only in the silicate matrix. HoIIingworthite com- area representing a particular PGM giving an indica- monly rims irarsite grains in the silicate matrix, but tion of its relative abundance; e.g., the square represent- ing Os is enclosed by no such rims are found around irarsite within chro- RuS2' indicating that inclusions of native osmium occur within laurite. The minerals mite rims. As for Pt, this relationship is interpreted represented are: native osmium, laurite, nickel to reflect increasing availability of Rh after the for- antimonide, rhodium antimony , mation of chromite, Ru-, Os- and Ir-bearing PGM, antimony sulfide, breithauptite, irarsite, Pt-free holling- and early Pd-bearing PGM within chromite rims. worthite, Pt-rich hollingworthite, mertieite or stibiopal- There is also evidence from the hoIIingworthite com- ladinite, sperrylite, Pt-Pd-Cu alloys, genkinite, hong- positions that Pt became available later than Rh. shiite and ruthenian pentlandite. HoIIingworthite is Pt-free where surrounded by Pd antimonide, suggesting formation prior to the avail- ability of Pt. However, the hoIIingworthite is Pt- laurite to a Ru-rich alloy. Shetland Ru-bearing pent- bearing where it surrounds Pd antimonide and also landite has been found to surround laurite and where it is surrounded by sperrylite. euhedral native osmium. This pentlandite is The last POM to form are the Pt- and Pd-rich associated with nickel carbonate and contains Ru alloys. For example, hong shiite occurs adjacent to that may have been released from the laurite during sperrylite and appears to be an alteration product low-temperature alteration. This process may also of the sperrylite, which breaks down along cleavages have recrystallized native osmium into euhedral crys- (Fig 3). This hongshiite is adjacent to a late cross- tals within the Ru-bearing pentlandite. cutting silicate vein that may be associated with a late phase of hydrothermal alteration involving the DISCUSSION removal of As. Hongshiite has been described as an alteration product by Cabri (1981), who suggested Primary igneous silicate minerals are absent in the that it replaced (PtS). The Pt-Pd-Au-Cu chromite-rich Shetland samples examined in this alloy (Fig. 4) also may be a late phase, as it is adja- study. It is not possible, therefore, to compare cent to native copper, probably a product of low- directly the POM in primary igneous assemblages temperature alteration. Stockman & Hlava (1984) with those in altered silicate lithologies in order to suggested that the POM may be reduced in the determine whether the present PGM are a modified natural environment, and they cited alteration of assemblage of primary minerals or the result of POE 988

FIG. 8. (a) Back-scattered electron image. (b) Mineral-location map. (c-i) Single- element scans for Ni, Ru, as, Ir, S, As and Sb. The minerals in this composite grain from Harold's Grave are laurite, irarsite, native osmium and breithauptite. addition by secondary processes of enrichment. At cate that laurite crystallization preceded or was con- Cliff, however, PGM have been found in dunite con- temporary with chromite formation, whereas all Pt- taining unaltered olivine (Prichard & Lord 1988). The bearing minerals are in the interstitial silicate matrix PGM-bearing chromite-rich samples from both Cliff and postdate chromite. Assuming a magmatic ori- and Harold's Grave occur as chromite-rich lenses gin, the trend may represent a fractionation surrounded by a dunite envelope within harzburgite. sequence. These lenses are typical of ophiolite complexes and Superimposed on the effects of igneous processes are thought to form by crystallization from diapirs are those of lower-temperature processes of altera- of basaltic magma that rise through the mantle to tion, which may have modified and redistributed the feed the overlying crustal magma-chamber (Gass & PGM. For example, chlorite and sperrylite, which Smewing 1981). The association of PGM with the occur interstitially to the chromite grains, are inter- dunite envelope and with the chromite-rich samples grown. The sperrylite forms an unusual zig-zag- suggests a primary magmatic origin. The textural shaped crystal among the chlorite laths, suggesting relationships between the PGM and their hosts indi- that the sperrylite formed at the same time or post- PGM, SHETLAND OPHIOLITE COMPLEX 989 dates the chlorite. As and Sb could have been ACIO{oWLEDcEIuENTs introduced with late magmatic fluids, or by some later, lower-temperature process; neverthelessthe We thank Barbara Cornelisenfor patient help with chlorite-sperrylite associationsuggests the presence the microprobe analyses,Drs. Chris Neary and Phil of an As-rich environment during or after the low- Potts for constructivecomments, Professor Norman temperature hydrous conditions of chlorite forma- Page and Dr. Thierry Aug6 for helpful refereeing, tion. Work in progress on both the associatedNi- and Professor Robert F. Martin and Dr. John L. and Cu-bearing minerals and the variably altered Jambor for clarification of the manuscript.by care- chromite-rich host lithologies may enable the ful editing. We are very grateful to John Taylor for processesof PGM formation to be determinedmore drafting the figures, to Carol Whale for typing, and exactly. to Graham Ward for preparing the tablesand check- PGM associatedwith serpentinization include the ing the manuscript. This researchwas made possi- Pt-Pd-Au-Cu alloys, which occur adjacent to other ble by funding from the EEC Raw Materials research PGM, but within late serpentiniteveins that cut com- program. posite PGM. These alloys contain no As or Sb (unlike the adjacent PGM), suggesting that this processof PGM formation, associatedwith vein for- REFERENcES mation, led to removal of the As and Sb. This deple- tion is illustrated for As in the example where AucE, T. 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-, Nnany,C.R. & Por-rs,P.J. (1986):Platinum- ReceivedMorch I 1, 1987,revised manuscript accepted group mineralsin the Shetlandophiolite. /r? Metar- February24, 1988.