ELSEVIER Ore Geology Reviews 11 (1996) 363 -381

ORE GEOLOGY REVIEWS

Platinum-group element distribution in chromite ores from ophiolite complexes: implications for their exploration

Maria Economou-Eliopoulos Department of Geology, Section of Economic Geology and Geochemistry, Athens University, Panepistimiopolis, 15714 Athens, Greece

Received 19 July 1995; accepted 12 July 1996

Abstract

Compilation of some new data on ophiolites for Greece and Yugoslavia, and published data from previous studies, indicate that platinum-group element (PGE) and gold concentrations in chromite ores are generally low, ranging from less than 100 ppb to a few hundred ppb. However, samples from several ophiolite complexes exhibit an enrichment (of a few ppm) (a) only in Os, Ir and Ru,(b) only in Pt and/or Pd or (c) in all PGE. This enrichment (up to 10s ppm) is mainly related with chromitites hosted in supra-Moho dunites and dunites of the uppermost stratigraphic levels of the mantle sequence and it seems to be local, independent of the chromitite major element composition and the chromite potential of the ophiolite complexes. The contents of PGE combined with less chalcophile elements (Ni, Co, Cu), the ratios of incompatible/compati- ble elements, and PGE-pattems provide evidence for discrimination between chromitites derived from primitive magmas and those derived from partially fractionated magmas, although they have a similar major element composition. Thus, they can be used for a stratigraphic orientation in the mantle sequence, and therefore for exploration targets. Moreover, PGE data offer valuable information for the evaluation of the chromite potential in ophiolite complexes. The most promising ophiolites seem to be those which apart from the petrological and geochemical characteristics indicating extensive degree of partial melting in the mantle source contain only one chromite ore type (the other type being only in small proportion) of limited compositional variation, in both major elements and PGE, low ratios of Pd/Ir, while PGE-enriched chromitites in the mantle sequence are only occasionally present. In contrast, ophiolites which contain both high-Cr and -A1 chromitites, and where their chalcophile element data implies relatively extensive fractionation trend are not good exploration targets for chromite ores, although they are related with a SSZ environment.

Keywords: ophiolite; chromite; platinum-group elements; chromite exploration; geochemistry; Greece

1. Introduction

The geotectonic environment is considered to be important for chromite exploration, since chromite deposits of economic interest are restricted to ophio- lites with supra-subduction zone features (Pearce et

al., 1984; Roberts, 1988), However, some ophiolite complexes with petrological and geochemical char- acteristics of SSZ ophiolites, like the Pindos complex and Rhodope massif ophiolites (Greece), Bay of Islands (Newfoundland) ophiolite complexes and elsewhere are known only for small chromite bodies

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364 M. Economou-Eliopoulos / Ore Geology Reciews 11 (1996) 363-381

(Rassios, 1990; Paktunc, 1990; Zhelyaskova-Pan- ayotova and Economou-El iopoulos , 1994; Economou-Eliopoulos and Vacondios, 1995).

Assuming that crystal fractionation or partial melting processes rather than alteration and meta- morphism is a major factor controlling PGE content (Auge, 1985; Prichard et al., 1986; Cina et al., 1986; Bacuta et al., 1990; Tarkian et al., 1992; Hattori et al., 1992; Pilchard and Lord, 1993; Bridges et al., 1994) changes in the whole-rock PGE content may provide evidence for chromite mineralization.

In the present study, some new PGE data on chromitites and host rocks from Greece and Yu- goslavia are given. These data are compared to those from Albania and Bulgaria and other ophiolite com- plexes and their implication on the chromite genesis and exploration is discussed.

2. Geotectonic setting

The majority of ophiolites contained in the Hel- lenides occur as a series of thrust sheets, elongated

NW-SE, and restricted in their central and northern parts. They crop out (a) in two discontinuous and narrow belts to the west of the Pelagonian massif (Othrys and Pindos complexes), in the Pelagonian massif (Vourinos complex) and to the east of the Pelagonian massif (Vermio, Veria and E. Thessaly), (b) in W. Chalkidiki peninsula and (c) in the Serbo- macedonian and Rhodope massifs (Fig. 1). The geol- ogy, petrology and geochemistry of these ophiolite complexes, including chromite deposits, have been studied in depth. Most of the related data are re- viewed by Bebien et al. (1980); Economou et al. (1986); Economou-Eliopoulos (1993), and are sum- marized in Table 1 and Fig. 2.

Petrological and geochemical characteristics indi- cate that the ophiolites of Greece have been formed in marginal basins (Capedri et al., 1980; Pearce et al., 1984; Economou et al., 1986). The Vourinos complex is now considered among the supra-subduc- tion zone (SSZ) ophiolites and to have boninitic affinities. The Pindos complex contains a spectrum of lavas from MOR basalts through island arc tholei- ites (IAT) to boninite series volcanics (BSV), and the

Table 1 Characteristics of some ophiolite complexes of Greece and Yugoslavia

Mantle Magmatic Chromitite Range of C r /

peridotite sequence type potential (Cr + A1)

Greece Vourinos H IAT, BSV M ~ 10 million tons 0.77-0.81 Kissavos H IAT R&M thousands tons 0.46-0.81 Rodiani H IAT R thousands tons 0.55-0.66 Pindos H, L IAT, MOR, BSV M, R thousands tons 0.48-0.82 Othrys L, H MOR, IAT, BSV R 3 million tons 0.49-0.52 Edessa-Vermio-Veria H IAT M thousands tons 0.70-0.79 E. Thessaly-Olympos H IAT M thousands tons 0.76-0.83 W. Chalkidiki H IAT M thousands tons 0.70-0.73 Skyros island H MOR, IAT R, (M) thousands tons 0.56-0.75 Euboea H, L MOR, IAT R, M thousands tons 0.51-0.82 Rhodos island H, L MOR, IAT R, M thousands tons 0.54-0.82 Serbomacedonian massif H R, M thousands tons 0.49-0.75 Rhodope massif (Greek and Bulgarian) H. (L) MOR, IAT R, M thousands tons 0.57-0.76

Yugoslavia Radusa (Liuboten) H M ~ 4 million tons 0.75-0.82 Brezovica H M 200.000 t 0.78-0.80 Veluce H, (L) M thousands tons 0.72-0.74

Symbols: H = harzburgite; L = lherzolite; IAT = island-arc tholeiites; MOR = mid-oceanic ridge basalts; BSV = boninite series volcanics; ( ) = less frequent; M = high-Cr (metallurgical type); R = high-Al (refractory type).

M. Econoraou-Eliopoulos / Ore Geology Reviews 11 (1996) 363-381 365

9 . .29 . ~ n

Fig. 1. Sketch map showing the geotectonic zones (after Jacobsha- gen et al., 1978), the distribution of ophiolites in the Balkan peninsula and location of the studied ophiolites. Symbols: (A) Rbodope massif; (B) Serbomacedonian massif; (C) Circum- Rbodope massif; (D) Axios zone; (E) Pelagonian zone; (F) Pin- dos-Olonos zone; (1) Othrys; (2) Vourinos; (3)= Pindos; (4) Edessa-Vermio-Veria; (5) Gomati; (6) Nigrita; (7) Soufli- Dadia-Tsoutoura; (8) Exochi; (9) Brousevci; (10) Dobromirci; (11) Skyros; (12) Euboea; (13) Rhodos; (14) W. Chalkidild Radusa; (Vavdos, Ormilia, Gerakini); (15) Radusa; (16) Brecov- itsa; (17) Veluce; (18) Tropoja; (19) Bulqiza.

Othrys ophiolite besides the evidence for its develop- ment at MOR environment, has subduction related features, suggesting changes in the geotectonic set- ring of the complex (Economou-Eliopoulos, 1993). The Vourinos, Othrys and Pindos complexes are considered to be parts of a single slab of oceanic crust; MOR- and SSZ-type ophiolites, e.g. fore-arc and back-arc ridges, were obducted, tectonically fragmented and separated by overlapping sediments of the Meso Hellenic trough (Smith, 1977; Pearce et al., 1984; Jones, 1990). In addition, the main part of the Vourinos complex is considered to represent a spreading centre (Jackson et al., 1975; Harkins et al.,

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1980; Ross et al., 1980), while the Pindos (Dramala) complex represents off-axis mantle (Jones et al., 1990).

The Pindos ocean extends further north in Albania and Yugoslavia. Ophiolites in Yugoslavia, can be distinguished into the western and central ophiolites, both of which are characterized by the dominance of lherzolite and very low chromite potential. The southern part of the ophiolite belt consisting mainly of harzburgite-dunite exhibits economically signifi- cant chromite potential (Jankovic and Karamata, 1986; Table 1).

3. Ore geology

Chromite ores in the Vourinos complex occur in the tectonite and cumulate sequences, although only those in the tectonites are under exploitation. While

366 M. Eeonomou-Eliopoulos / Ore Geology Reviews 11 (1996) 363-381

all chromite ores are found within dunite bodies or dunite envelopes in harzburgite, there is not a sys- tematic relation between size of the dunite body and that of the ore body. The sizes of the ore bodies vary widely and contain all textural types (massive, schlieren, banded, disseminated and nodular), but usually a single type dominates. High-temperature deformation, superimposed on primary magmatic textures is very common (Burgath, 1983; Economou et al., 1986). Schlieren ores form the bulk of the economic deposits of metallurgical type. Massive ore bodies are rare.

The Othrys complex includes two tectonically separated chromite deposits, namely Eretria and Domokos, in addition to several other occurrences. Massive chromite ores occur as spherical, lenticular or irregular bodies of varying size. The host rock is moderately depleted harzburgite and no systematic relation exists between the distribution or size of the ore bodies to the size of host masses (Economou et al., 1986; Economou-Eliopoulos, 1993).

Ophiolite masses, mostly serpentinized dunite- harzburgite, in the western margin of the Axios zone, including the areas of Edessa, Vermio, Veria, E. Thessaly (Fig. 1) contain mainly massive chromite bodies of small size (Economou and Economou, 1986; Migiros and Economou, 1988).

The chromite deposits of W. Chalkidiki peninsula occur in altered dunite and mostly are of schlieren type, though occurrences of massive and nodular ore have been reported (Burgath, 1983).

Ophiolites associated with the Serbomacedonian massif (Gomati and Nigrita) and Rhodope massif (Soufli, Dadia, Tsoutoura and Exochi) are com- pletely serpentinized and locally sheared and meta- morphosed to antigorite-tremolite and /or talc schists. A number of chromite occurrences are known, from which a few thousands tons of ore have been exploited. The schlieren and massive ore types occur in elongate lenses following the host rocks

trend (Scarpelis and Economou, 1978; Magganas and Economou, 1988).

Aside from the Vourinos and Othrys complexes with an estimated ore potential of about 10 and 3 million tons respectively, the ores recovered from the remaining complexes is in the order of several thou- sand tons, but their resources are still unknown (Economou et al., 1986; Konstantopoulou, 1990).

4. Analytical methods

Platinum-group elements were determined by neu- tron activation analysis after preconcentration by nickel sulphide fire-assay technique on large samples lollowing the method of Hoffman et al. (1978) with minor modifications. Information on detection limits, precision and accuracy is given by Hoffman et al. (1978). Platinum and palladium samples with a con- tent lower than the detection limits of the Hoffman et al. (1978) method, were determined by atomic ab- sorption spectrometry (AAS) using heated graphite atomizer (Perkin Elmer 2100 Model). Nickel, cobalt and copper were determined by AAS too.

Sulfur was determined using a LECO CS-244, HF-100 device. Electron microprobe analyses of chromite ores were carried out at the Institute of Geology and Mineral Exploration (IGME), Athens, using the Cameca Superprobe wavelength-dispersive automated system.

5. Chemical composition of chromite ores

5.1. Major elements"

Analyses of chromite concentrates from ophiolites of Greece have been published by Panagos (1965), while representative microprobe analyses, are given by Economou et al. (1986). Additional analyses were

Fig. 3. Plot of Cr/(Cr + AI) versus Mg/(Mg + Fe z+ ) for chromite ores from (a) the Vourinos ophiolite complex; (b) east of the Pelagonian massif; (c) the ophiolite complexes of Othrys and Pindos; (d) the Serbomacedonian and Rhodope massifs; (e) the Radusa, Brecovitsa and Veluce areas of Yugoslavia. Data from Zhelyaskova-Panayotova and Ivchinova (1971); Economou (1978); Scarpelis and Economou (1978); Mussallam et al. (1981); Economou (1984); Economou and Economou (1986); Magganas and Economou (1988); Eliadis and Papadopoulos (1988); Konstantopoulou and Economou-Eliopoulos (1991); Economou-Eliopoulos and Vacondios (1990); Economou- Eliopoulos and Eliopoulos (1996). The number of the analyses used for the plotted compositional fields ranges between 30 and 100.

M. Economou-Eliopoulos / Ore Geology Reviews 11 (1996) 363-381 367

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368 M. Economou-Eliopoulos / Ore Geology Reviews 11 (1996) 363-381

Table 2 Electron microprobe analyses of chromite ores from Greece and Yugoslavia

Exochi Tsoutoura Euboea Skyroe Radusa Brezovica Veluce

Ex2 Ex3 Ex5 Tsou- 1 N.Art. Ag.l. Rad3 Rad4 Rad5 Rad6 Br I Br2 Yu5

SiO 2 0.17 0.19 0.16 0.00 0.21 0.67 0.42 0.28 0.52 0.24 0.41 0.56 0.41 AI203 11.72 12.37 12.99 19.60 9.48 12.33 10.88 9.64 11.77 9.85 10.66 10.64 14.46 Cr203 56.02 55.66 56.16 49.05 60.94 58.30 58.43 59.56 58.67 59.22 58.05 56.13 56.02 Fe~O 3 4.04 1.50 1.60 2.00 2.14 2.07 1.89 2.30 1.08 3.96 3.52 3.68 0.23 FeO 14.19 21.27 14.76 13.87 12.113 11.26 13.13 14.76 12.32 13.97 13.40 16.53 15.24 TiO 2 0.17 0.20 0.16 0.20 0.16 0.23 0.36 0.10 0.18 0.10 0.10 0.10 0.26 MgO 12.73 8.38 12.30 14.47 13.86 14.75 13.40 12.00 13.78 12.97 13.18 11.20 0.48 MnO 0.18 0.16 0.27 0.15 0.20 0.16 0.81 0.45 0.94 0.28 0.46 0.63 12.57 NiO 0.21 0.15 0.14 0.20 0.50 0.14 0.20 0.23 0.26 0.10 0.34 0.11 0.10

99.43 99.88 98.54 99.54 99.52 99.91 99.53 99.32 99.52 100.71 100.12 99.58 99,77

C r / 0.76 0.75 0.74 0.63 0.81 (I.76 0.78 0.81 0.77 0.80 0.78 0.78 0,72 (Cr + A1) M g / 0.62 0.41 0.60 0.65 (I.67 0.7(I 0.64 0.59 0.67 0.62 (I.63 0.55 0.60 (Mg + Fe z+ )

completed for the present study. Representative anal- yses of chromite from other chromite-enriched areas of Greece and Yugoslavia are given in the Table 2. The analytical data of chromite ores, by electron microprobe, indicate a small compositional variation for chromite ores throughout the Vourinos complex, the Edessa-Vermio-Veria of E. Thessaly ophiolites and those of the W. Chalkidiki peninsula, all of which are high-Cr and also in the Othrys complex (Eretria and Domokos) and the area of Rodiani, which are high-A1 (Fig. 3). In contrast, the chromite ores from Skyros island, Euboea, the Kissavos area (central Vourinos), Pindos complex, Serbomacedo- nian and Rhodope massifs are classified as both high-Cr and A1 (Fig. 3).

5.2. PGE in chromitites

The platinum group element (PGE) and gold con- centrations in concentrated chromite ores are gener- ally low, less than 100 ppb to a few hundred ppb. In

particular, the Vourinos and Othrys ophiolite com- plexes, hosted the largest chromite deposits in Greece, with exception the central part of the former, are characterized by low PGE concentrations and Pd/ I r ratios. However, some chromitite samples ex- hibit an enrichment (up to 3-5 ppm): (a) only in Os, Ir and Ru; (b) only in Pt and/or Pd; or (c) in all PGE, which seems to be local and independent of their major element composition, e.g. Skyros island, Central Greece (Agiorgitis and Wolf, 1978; Economou, 1984; Economou, 1986), Pindos com- plex (Economou-Eliopoulos and Vacondios, 1995; Tarkian et al., 1996). In the Table 3 average and representative PGE analyses of chromitites mainly from Greek and Yugoslavian ophiolites are given.

The majority of chromite samples show a rela- tively high content in Ir, Os and Ru, while Pt and Pd are low resulting in chondrite(C2)-normalized PGE patterns with a negative slope (Fig. 4) and a low Pd/ I r ratio. In contrast, some chromitite samples from the Pindos complex, which are Rh, Pt and Pd

Fig. 4. Chondrite-normalized platinum-group element patterns for chromitites (a) high-A1, showing a difference in the degree of fractionation; (b) high-Cr from the Pindos complex; (c) average composition of the Vourinos complex (the PGE-enriched samples are not included) and Kissavos-Rodiani ores; (d) high-Cr, from Vermio-Veria-Edessa ophiolites; (e) from the Serbomacedonian massif; (f) and (g) from the Rhodope massif (Greek and Bulgarian); (h) from the Skyros island and (i) from the Chalkidiki peninsula; ~) from Radusa, Brecovitsa and Veluce areas of Yugoslavia. Data from Table 3.

M. Economou-Eliopoulos / Ore Geology Reviews 11 (1996) 363-381 369

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M. Economou-Eliopoulos / Ore Geology Reviews 11 (1996) 363-381 371

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enriched, show smooth sloped to positive slope PGE-patterns (Fig. 4a and b). Moreover, chromitites with a similar major-element composition may ex- hibit a variation in the PGE-patterns and the values of the Pd / I r ratios (Table 3, Fig. 4). In particular, high-A1 ores, like those from the areas of Vourbiani and Korydallos of Pindos complex, Rodiani eastern of Vourinos, and Kissavos area (Central Vourinos), show a depletion of noble metals, both of absolute values and relative to Ni and Cu (Table 3). Also, although high-Cr chromite ores have a higher PGE content compared to high-A1 ores (Economou, 1986; Bacuta et al., 1990; Zhou et al., 1994) there are high-A1 chromite ores from the Othrys complex, Skyros island and the area of Korydallos, of the Pindos complex, which have a total PGE content comparable or /and higher to that of high-Cr ones (Table 3). Such a PGE content may suggest that those high-Cr ores were crystallized from magmas derived from mantle sources which have experienced

prior depletion and have not attained sulphur satura- tion during their ascent.

Due to the behavior of Ni, like Os, Ir and Ru, it is compatible with most early crystallizing phases and trend to be enriched in olivine and chromite-bearing rocks. On the other hand, CuO like Pt and Pd, is an incompatible element and, therefore, the Pd / I r and Ni /Cu ratios have been proposed as an evidence of the magma fractionation (Barnes et al., 1985, 1988). The nickel content in chromite ores ranges from 2200 to 750 ppm, copper from 25 to 177 ppm, cobalt from 110 to 750 ppm and sulphur from 20 to 80 ppm (Table 3). There is not any significant correlation between the Ni, Cu, Co and S, and PGE content, but it seems likely that chromitite samples from the Pindos and Skyros ophiolites, with a relatively high Pd / I r ratio exhibit a lower Ni /Cu ratio, ranging from 9 to 23. Also, PGE-enriched chromitites from the Pindos complex (Milia and Korydallos) and Sky- ros island are characterized by a high P G E/S ratio

372 M. Economou-Eliopoulos / Ore Geology Reviews 11 (1996) 363-381

Table 3

Chalcophile element data for chromite ores from ophiolite complexes

Location Sample Concentration (ppb) Concentration (ppm)

Os Ir Ru Rh Pt Pd Au C r / S Ni

(Cr +AI )

Cu Co P d / N i / P G E / Ref

lr Cu S

Othrys

(Eret)

x (n = 12) 41 15 41 3 4 1 3 0.54 60 1550 38 320 0.10 40.8 1.8 (u.d.)

Vourinos south (SV) 30 18 32 9 10 4 3 0.77

Vourinos north (NV) I 1 13 50 12 4 2 2 0.80

Kissavos I -Cr 14 12 34 8 2 5 1 0.81

Kissavos I-AI 6 5 15 3 3 6 2 11.46

Rodiani R 13 8 34 7 6 13 3 11.60

- - 1760 35 240 0.29 50,3 - - (1)

- - 2000 35 240 0.30 57,1 - - (1)

- - 1800 33 280 0.42 54,5 - - (1)

- - 2000 30 230 1.20 66,7 - - (1)

2200 35 205 1.60 62,8 - - (1)

Pindos Vul

Pindos K3B

Pindos AIA

Pindos K 1

Pindos X 1

Pindos K174

Pindos K 193

8 5 15 2 3 10 4 0.52

40 20 33 5 25 20 I1 0.48

26 4 51 2 6 5 8 0.82

150 320 550 82 1511 7 1 1/.82

40 51 190 II 7 1.5 4 0.80

62 47 80 112 14611 337 10 0.69

70 55 1t0 98 2221t 766 10 0.42

40 1600 50 285 2.00 32.0 1.1 (2, 3)

70 1750 120 280 1.00 14.6 2.0 (4)

70 1350 35 110 1 .25 38.6 1.8 (4) 60 2000 30 750 11.02 66.7 21.0 (4)

80 2000 34 650 0.03 60.8 3.9 (4)

30 800 35 200 7.2 22.9 70.0 (5)

50 1580 170 190 13.9 9.3 66.4 (5)

E. ThessalyL2

Vermio V 1

Veria V2

Edessa Edl

25 14 78 5 411 31 12 079

30 211 55 4 4 I 2 11.71i

70 t 5 100 9 3 2 4 0.77

40 15 100 6 3 2 3 0.79

30 750 50 290 2.2 15.0 6.4 (3.6)

30 1080 40 3611 0.05 27.0 3.8 (7)

40 1280 30 300 0.13 42.7 5.0 (7)

70 1500 50 300 0.13 30.0 2.4 (7)

Serbomacedonian massif Gomati Gom 1 14 30 80 10

Gomati Gom2 7 9 23 3

Gomati Gom3 6 20 32 6

Nigrita N.D.Sk 13 20 45 10

Rhodope massif Soufli S 1 24 19 108

Soufli $2 11 18 68 Soufli $3 24 37 124

Dadia DI 16 34 140 Tsoutoura Tsl 25 6 40 2

Erochi Ex.2 30 25 90 5

Erochi Ex.3 25 20 70 4

Erochi Ex.5 15 25 60 10

Dobromirci 243/63 16 32 120 6

(Bulgaria) 13/63 < 3 9 25 3

Brusevci 1356/9 5 13 23 4

Skyros Ax2 island

Skyros Ax2b island Skyros Ag.I

island Euboea N.Art

Euboea Madl Euboea Mad2

1 0 5 5 0 . 4 6

3 3 7 0.56 2 4 7 0.45

5 5 10 11.75

6 15 I t 3 (1.80

4 2 1 6 11.70

9 3 1 2 0.70

3 2 1 2 1/.84 3 6 2 1/.67

5 5 5 11.76

4 6 4 1t.75

5 10 5 1t.74 4 14 2 1t.74

2 12 6 0.57

2 12 6 0.74

382 63(1 1200 190 244 30 17 0.62

382 630 120 190 244 30 17 0.62

13 26 225 10 22 25 5 1/.56

55 29 76 7 4 2 1 0.61 16 22 40 8 39 1 13 0.78 20 18 35 5 44 1 40 0.56

30 1100 45 340 0.17 24.4 4.6 131

40 1000 40 310 0.33 25.0 1.2 (3)

30 1020 40 350 0.20 25.5 2.3 (3) 30 1200 35 350 0.25 34.3 3.3 (3)

40 0.58 - - 4.6 (7)

30 1250 38 320 0.06 32.9 3.5 (3)

40 1180 40 300 0.03 29.5 5.2 (3)

30 1160 55 400 0.03 21.1 6.7 (3)

30 1600 40 320 1.00 40.0 3.0 (7)

10 1050 40 285 0.20 26.2 18.0 (3)

7 1000 45 300 0.30 25.0 19.6 (3)

20 1250 35 290 0.40 35.7 6.2 (3)

- - 0.44 - - - - (8)

- - 1 . 3 6 - - - - ( 8 )

- - 0.92 - - - - (8)

60 1300 125 400 0.05 10.4 51.0 (2)

60 1300 125 400 0.05 10.4 51.0 (2)

60 1900 37 480 0.69 51.3 5.6 (10)

10 1300 30 300 0.07 43.3 8.9 (3) 30 1100 50 280 0.05 22.0 4.2 (3) 35 1290 50 250 0.06 25.6 3.5 (3)

M. Economou-Eliopoulos / Ore Geology Reviews 11 (1996) 363-381 373

Table 3 (continued)

Location Sample Concentration (ppb) Concentration (ppm)

Os Ir Ru Rh Pt Pd Au C r / S Ni Cu Co P d / N i / PGE/ Ref (Cr + AI) Ir Cu S

N. Chalkidiki Vavdos Bal 15 85 210 10 15 10 10 0.73 30 1050 25 350 0.12 42.0 11.5 (2, 3) Vavdos Ba2 70 23 190 8 6 6 10 0.72 20 1090 30 450 0.26 36.3 15.1 (3) Vavdos Ba3 85 54 135 9 14 6 34 0.73 30 1000 30 360 0.15 33.3 0.6 (2) Ormilia Orml 52 46 127 10 18 10 38 0.72 20 1100 25 380 0.21 44.0 12.7 (2, 3) Ormilia Orm2 160 53 230 13 26 13 3 0.73 30 950 35 350 0.25 27.1 16.5 (3) Gerakini Gerl 120 62 230 9 3 7 4 0.70 40 1009 26 202 0.11 94.8 10.8 (1, 3)

Yugoslavia Radusa Bdl 10 40 109 0 9 1 4 1.79 30 1110 40 285 0.02 37.8 5.9 (3) Brecovitsa Br 1 10 36 70 6 2 1 4 0.78 30 1400 36 200 0.02 40.0 4.2 (3) Veluce Yu5 9 24 52 4 2 1 3 0.72 20 1240 30 300 0.04 41.3 4.1 (3)

Symbols: (I) Konstantopoulou (1990); (2) Economou (1986); (3) Present study; (4) Economou-Eliopoulos and Vacondios (1990); Economou-Eliopoulos and Vacondios (1995); (5) Tarkian et al. (1996); (6) Zervos (1994); (7) Economou-Eliopoulos (1993); (8) Magganas and Economou (1988); (9) Zhelyaskova-Panayotova and Economou-Eliopoulos (1994) and (10) Economou-Eliopoulos and Eliopoulos (1996).

(Table 3), which may reflect a high R factor (silicate magma/sulphide liquid) (Von Gruenewaldt et al., 1989).

Host and barren dunites, and harzburgites, which are strongly depleted (clinopyroxes are absent or less than 1%) show low PGE Cu and S content, while Ni ranges between 4200 and 1630 ppm in dunites, and from 1850 to 2950 ppm in harzburgites (Table 4; Konstantopoulou, 1990; Economou-Eliopoulos and Vacondios (1995). Typically, the variation of all chalcophile elements within harzburgite is more or less homogeneous, whereas dunites show a signifi- cant variability, but a correlation between Ni and Cu and PGE content is not obvious.

6. Comparison of PGE data from Greece with those in other ophiolites

Chromite deposits related with ophiolite com- plexes are known for their low PGE content, up to a few hundreds ppb. After recent investigations how- ever, a PGE enrichment in ophiolites is more com- mon than previously recognized and has been de- scribed within several ophiolite complexes of the Mediterranean region and elsewhere. In the Shetland ophiolite complex extremely anomalous PGE values are very restricted. In particular, PGE enrichment (over 60 ppm Pt + Pd) is known at one site cliff,

within a chromite-rich sulphide-bearing dunite, form- ing part of a dunite lens in mantle harzburgite. Lower values, in the order of 6 ppm Pt + Pd occur in chromite-poor, sulphide-rich dunite also within this dunite lens. Elsewhere in the ophiolite complex, Pt + Pd enrichment (up to 4 ppm) occur in sulphide-bearing dunites associated with chromite layers in the cumulate dunite overlying the harzbur- gite, and concentrations up to 1 ppm Pt + Pd are present in sulphide-bearing pyroxenites and werlhites _ chromite (Prichard et al., 1986; Prichard and Lord, 1993). In Albania, most of the chromite deposits of metallurgical type occur in the Bulqiza, one of the largest chromite deposits (40 million tons) and Tropoja massifs (eastern ophiolite belt). The PGE content in these complexes varies from 260 to 11100 ppb. They are characterized by: (1) a low PGE (Os + Ir + Ru) content in the deep mantle chromi- rites; (2) P t -Os- I r -Ru enrichment in the upper man- tle chromitites; (3) the presence of base-metal sul- phide (BMS)-related Pd mineralization in supra- Moho chromite-dunite of the Bulqiza complex; and (4) a Pt mineralization in the dunite-pyroxenite in- terface of the Tropoja cumulate sequence (Ohnens- tetter et al., 1991; Cina et al., 1995). Also, in Acoje, Zambales ophiolite complex (Philippines) the PGE concentrations and PGE-patterns display a wide range. The Rh, Pt and Pd-rich sulphide-bearing chromitites are found at the top of the cumulate

374 M. Economou-Eliopoulos / Ore Geology Reviews 11 (1996) 363-381

Table 4 Chalcophile element data for harzburgites (H) and dunites (D)

Location n Type Concentrations (ppb) Concentrations (ppm)

Os Ir Ru Rh Pt Pd Au Ni Cu Co

Othrys 2 H < 5 2 -3 9-11 0.5-1 2-14 3-5 2 2150-2200 8-15 85-100 Vourinos 8 H 4 -7 3-5 6-11 1-5 3-10 3-34 1-2 (2700) (20) (130)

(5) (4) (8) (3) (6) (10) (1)

Vourinos 8 D 4 -19 2 - 3 5 - 2 6 (1.3-6 0 .3-37 0 .1-2 1-46 (2690) (10) (130) (12) (3) (12) (2) (6) (1) (8)

Pindos 5 H < 5 1-6 < 4 4 2 6-10 7-12 3-18 2300-2640 18-52 105-156 (3) (4) (9) (9) (10) (2660) (27) (116)

Pindos 6 D < 5 0.3-5 < 4 - 4 1-5 2-20 1-30 1-20 1630-4200 5-310 115-160 (2) (4) (2) (7) (10) (4) (2930) (64) (140)

Vermio 2 H < 5 2-3 4-18 1-3 5-15 3-8 4 -5 (2950) (30) (110) Vermio 2 D < 5 6 -8 15-16 1 5-15 1-6 3-10 (2800) (25) (1051 Rhodope 2 H < 5 3 -4 9-17 2 -3 4 t2 4 -5 1-3 1850-2180 28-25 105-110 Rhodope I D < 5 3 I 0 1 10 1 5 2300 25 100 Skyros 1 H < 5 3 9 2 "~ 5 4 2200 28 105 Skyros 1 D < 5 3 10 1 2 6 17 2900 25 98 Rhodos 2 D < 5 3 8 -9 2 2-90 5 2-3 3100-2280 6-10 93-96 Vavdos 2 D < 5 3-5 10-15 1-3 3 -6 7-11 2 -8 1760-2900 25-30 95-98 Yugoslavia Rudusa 2 D < 5 3 -4 11-13 1 1 2 0 .5-I 2-3 (2500) (25) (1001

Present study, except Vourinos (Konstantopoulou, 1990) and Pindos (Economou-Eliopoulos and Vacondios, 1995).

peridotite unit (dunite-wehrlite cumulates), which are interbanded with barren chromitites and dunites, and show PGE-patterns with a positive slope. In contrast, all chromite deposits occurring within dunite bodies of the mantle harzburgite are depleted in Rh, Pt and Pd and show PGE patterns with negative slope (Bacuta et al., 1990). Chromitites from the mantle sequence of the Braganca ophiolitic massif, Portugal, show a total PGE content from hundred ppb to as high as 11.2 ppm (Bridges et al., 1994). A small enrichment in Os, Ir and Ru is also observed in chromitites from the areas of Pletena and Dobromirci of the Bulgarian Rhodope massif, which are found in chromitites hosted in dunites at a close proximity to the petrological Moho (Tarkian et al., 1991; Zhelyaskova-Panayotova and Economou-Eliopoulos, 1994).

In Greece, the chromite ores of the Vourinos complex, hosted the largest chromite deposits of metallurgical type (10 million tons), are character- ized by a small PGE content and variation (in both major and trace elements), with the exception of its

central part and a few samples from the north Vouri- nos (Konstantopoulou, 1990; Konstantopoulou and Economou-Eliopoulos, 1991). The Othrys complex is similar to the Vourinos one in having a relatively high-grade refractory chromite ore of uniform com- position (in both, major and trace elements) and very low values of Pd / I r ratios. In contrast, in the Pindos complex, chromitites and host dunites show a re- markable variation (Tables 3 and 4; Economou- Eliopoulos and Vacondios, 1990; Economou-Elio- poulos and Vacondios, 1995; Tarkian et al., 1996). The highest Pt-Pd enrichment (in the order of 5 ppm) was reported in chromitites hosted in and/or near supra-Moho dunites of the area of Korydallos (Tarkian et al., 1996). The Radusa chromite deposit, which is the main source of high-Cr chromite in Yugoslavia (over 4 million tons) on the basis of the limited compositional variation of chromitites (Ta- bles 2 and 4; unpub, data) and petrological data of the ophiolite complex, seem to be comparable to the Vourinos complex.

Thus, considering the PGE distribution and the

M. Economou-Eliopoulos / Ore Geology Reviews 11 (1996) 363-381 375

PGE-enrichment in chromite ores related with ophio- lite complexes it seems likely that they have some common features: (a) a high Pt-Pd __+ Rh enrichment usually is associated with sulphides in dunites within the uppermost part of the mantle sequence and/or supra-Moho dunites/pyroxenites; (b) the PGE-en- richment is very restricted and independent of the major element composition of chromite; (c) large chromite deposits hosted in the mantle sequences of ophiolites of Greece (Vourinos and Othrys), Albania (Bulqiza), Yugoslavia (Radusa) and Zambales, Philippines (Acoje) contain only one chromite type (metallurgical or refractory), the other being only in small proportion, and a significant PGE (in particular Pt and Pd) enrichment is only occasionally present.

7. Discussion

The PGE content and PGE-patterns of bulk rock analyses are considered to depend on the timing of chromite and sulphide saturation and that the nega- tive slopes are produced by the chromite (main collector of Os, Ir and Ru as laurite and alloys) and positive slopes by sulphides (associated with Pt and Pd) (Barnes et al., 1988; Von Gruenewaldt et al., 1990). The association of the highest Pt-Pd enrich- ment with sulphide-bearing dunites and accompany- ing chromitites in ophiolites (Prichard et al., 1986; Bacuta et al., 1990; Ohnenstetter et al., 1991; Prichard and Lord, 1993; Bridges et al., 1994; Cina et al., 1995; Yang et al., 1995), resembles layered intru- sions. Chromitites like those in the lower ultramafic zone of the Bushveld complex (i.e. LG-6 horizon) could result from the mixing of primitive magma with resident magma before its sulphide saturation and therefore they are not enriched in Pt and Pd, in contrast to sulphide-bearing reefs (Merensky) or PGE enriched chromitite horizons (the UG-2) (Naldrett et al., 1990). Thus, the PGE-pattems and the values of the Pd/ I r ratio seem to be affected by the proportion of the phases of chromite and sulphide in the whole ore. However, chromitite samples from the Pindos ophiolite complex, with a high Pt-Pd-enrichment (5 ppm), high values of the Pd/ I r ratio (14) and PGE- pattern with positive slope are extremely poor in sulphides (Tarkian et al., 1996) and any correlation between Ni, Cu and S, and PGE content is not obvious. A remobilization of Pt-Pd and S, during

serpentinization, is suggested by the texture and mineralogy of PGM, which seems to be restricted in a small scale (Tarkian et al., 1996). If that is the case then, the bulk-rock PGE composition represent the primary PGE distribution, and the sulphide-poor PGE-enriched chromitite samples from Pindos may indicate that base-metal sulphides have not played a major role in that PGE mineralization [Amosse et al., 1990). Nevertheless, more research is required to clarify the mechanism by which the PGE-enrichment takes place, and to define the proportion of PGE, in the studied PGE-enriched chromitites, which is pre- sent as discrete primary PGM inclusions in chromite (Cotherie et al., 1989), for a well documented inter- pretation of the variation of the Pd/I r ratios.

Besides Rh, which appears to act either as an incompatible or compatible element, Ir, Os, Ru and Ni are compatible in chromite and olivine, whereas Pd, Pt and Cu are incompatible (Barnes et al., 1985; Barnes et al., 1988; Cotherie et al., 1989; Capo- bianco and Drake, 1990). The compilation of present and previous published data on chromite ores, indi- cate that certain ophiolites, like Othrys and the Vourinos, with exception of its central part, are remarkably homogeneous in both major, PGE, Ni, Cu content and Pd/Ir, Ni /Cu ratios. Other com- plexes, like Pindos are characterized by a wide range in the PGE, Ni and Cu content, and values of the Pd/ I r (0.02-150) and Ni /Cu (9-67) ratios (Table 3; Tarkian et al., 1996). Moreover, chromitite sam- pies from the Pindos complex, with a relatively high Pd/I r ratio (1 to 150) exhibit a lower Ni /Cu ratio, ranging from 9 to 23 (Table 3; Tarkian et al,, 1996). The Pd/ I r increase and Ni /Cu decrease point to a fractionation trend of PGE and other chalcophile elements, at the area of Korydallos of the Pindos complex and elsewhere. A fractionation of PGE from mantle to magmatic sequence of ophiolite complexes has been suggested also by Prichard et al. (1986); Bacuta et al. (1990); Ohnenstetter et al. (1991); Prichard and Lord (1993); Bridges et al. (1994); Cina et al. (1995); Yang et al. (1995).

8. Implications of PGE for the exploration of chromite ores

Since large chromite deposits are restricted to ophiolites with supra-subduction zone characteristics

376 M. Economou-Eliopoulos / Ore Geology Reviews 11 (1996) 363-381

(for example, Vourinos and Troodos) the geotectonic environment is considered to be an approach in chromite exploration (Pearce et al., 1984; Roberts, 1988). However, some ophiolite complexes with petrological and geochemical characteristics of SSZ ophiolites, like the Pindos ophiolite complex is known only for a small (thousand of tons) chromite potential, which is of both metallurgical and refrac- tory type (Fig. 3c; Economou-Eliopoulos and Vacon- dios, 1990; Economou-Eliopoulos and Vacondios, 1995; Rassios, 1990). Since the PGE mineralization is genetically linked with the crystallization of chromite (Auge, 1985; Capobianco and Drake, 1990) the implication of PGE, Ni, Cu content, PGE-pat- terns and Pd/Ir, Ni /Cu ratios, in addition to petro- logical and other geochemical characteristics, may provide valuable information for the origin and ex- ploration of chromite.

With respect to the evolution of the geotectonic environment of major ophiolites in Greece, the Pin- dos, Vourinos and Othrys complexes are considered to be parts of an originally continuous oceanic crust, and have probably passed through a number of dif- ferent tectonic settings, related to the formation of MOR-and SSZ-type ophiolites (fore-arc and back-arc ridges) (Smith, 1977; Pearce et al., 1984; Jones et al., 1990). Thus, these complexes are considered to represent different stages in the evolution of a marginal basin; the main part of the Vourinos com- plex is considered to represent a spreading centre, at a SSZ environment (Jackson et al., 1975; Harkins et al., 1980; Ross et al., 1980). The Othrys complex besides the features of MOR-type ophiolite (Pearce et al., 1984), has some characteristics, which suggest an influence of a subduction zone (Fig. 2; Paraskevopoulos and Economou-Eliopoulos, 1994). Finally, the variability in the degree of depletion in the mantle peridotites (Fig. 2) and the presence of a wide spectrum of lavas of the Pindos complex reflect clearly more than one mantle source and eruptive events (Capedri et al., 1980; Jones et al., 1990).

Assuming that variation in chromite composition is best related to the composition of parent magmas, themselves and/or fractionation within the upper mantle, the differences composition of the mantle source and degree of melting and/or multistage melting between various types of ocean ridges (Jaques and Green, 1980; Lago et al., 1982; Ribe,

1988; Nicolas, 1989; Paktunc, 1990) may be related with the differences in the composition of chromite ores. In addition, due to the compatible behavior of Os. It, Ru, Ni in chromite and olivine, in contrast to incompatible behavior of Pt, Pd and Cu, their con- tent, metalpatterns and values of the Pd/Ir and/or Ni /Cu ratios, (which are considered to reflect the fractionation degree (Barnes et al., 1985, 1988) are considered to be of genetic significance.

Sometimes it is difficult to distinguish the degree of partial melting and fractional crystallization. For example, high-A1 chromitites, in a spatial associa- tion with high-Cr ones in the Pindos ophiolite com- plex, which are characterized by a relatively high Fe203 and TiO 2 content, high values of Pd/Ir ratios and smooth-shaped to positive slope PGE-patterns, reflecting a relatively high degree of fractionation (Fig. 4a and b); Economou-Eliopoulos and Vacon- dios, 1995; Tarkian et al., 1996) may suggest that these two types of chromitite were derived from magmas of similar composition, which were more evolved in the case of high-A1 ores. However, the presence of high-Cr chromitites at the area of Dra- mala, with higher Pd/I r ratios, compared to high-A1 chromitites, in combination with the presence of exclusively high-A1 chromite ores with very low Pd/Ir ratios within defined ophiolite complexes, like the Othrys complex (Table 3; Fig. 4; Konstan- topoulou, 1990; Konstantopoulou and Economou- Eliopoulos, 1991; Zhelyaskova-Panayotova and Economou-Eliopoulos, 1994; Tarkian et al., 1996) suggest that these chromitites are derived from sepa- rate parent magmas, which may have derived from mantle source with different composition.

The compositional variation of chromite deposits in the mantle sequence of individual ophiolites, is restricted to a limited range, (Fig. 3) as expected in the case of an open-system fractionation of rising magma in small magma chambers within the mantle (Roberts, 1986). However, sometimes chromite ores with a similar major element composition differ in their PGE or/and Ni, Cu content. For example, in high-A1 chromitites of the Pindos, a wide variation in both absolute values and ratios of Pd/Ir and Ni/Cu is well pronounced. More specifically, chromitites exhibit relatively high (1-14) Pd/I r and low (9-32) Ni/Cu ratios. In contrast, the Othrys ores, which are high-A1 too, are characterized by

M. Economou-Eliopoulos / Ore Geology Reviews 11 (1996) 363-381 377

very low Pd/Ir (0.10) ratios (Table 3; Fig. 3c and Fig. 4a and b; Economou-Eliopoulos and Vacondios, 1995; Tarkian et al., 1996). Such differences may suggest difference in the environment of their crys- tallization, and the effect of combined fractional crystallization and magma mixing on the PGE, Ni and Cu content, which seem to be more sensitive (Haughton et al., 1974; Irvine et al., 1983; Amosse et al., 1990; Buchanan, 1988). Furthermore, due to the geochemical behavior of these elements, the low values of the Pd/Ir ratios may reflect a limited fractionation and therefore more primitive parent magmas for the Othrys chromite ores than the Pindos ones, which may derived from partially fractionated magmas.

Also, small high-Cr chromite bodies hosted in ophiolites to the east of the Pelagonian massif, like E. Thessaly (Fig. 1) exhibits a relatively high (2.2)

Pd/Ir and low (15) Ni/Cu ratios (Table 3), although the compositional variation of major elements is small, suggesting that they have derived from par- tially fractionated magmas. In contrast, the composi- tion of chromite ores from the Vourinos complex (metallurgical type), with exception of its central part, is remarkably homogeneous in both major and chalcophile element compositions. In particular, in South Vourinos, including Xerolivado, one of the world's largest alpine-type chromite deposits (about 6 million tons of ore}, the variation of the Pd/Ir and Ni/Cu ratios is small, while the average values are 0.29 and 50, respectively. Chromite ores in various localities of Northern Vourinos show Ru >> (Os + Ix) content and higher Pd/Ir ratios compared to South Vourinos, while a few PGE-enriched (up to 3 ppm) chromite samples are also present. The chemistry of chromites suggests a crude stratigraphy, which pro-

Table 5 PGE data in large chromite deposits related with ophiolites

Concentration (ppb)

Location Os Ir Ru Rh Pt Pd Au Cr/(Cr + A1) Pd / I r potential

Greece Vourinos a 11-30 13-18 32-50 9-12 4-10 2-4 2-3 0.77-0.80 0.29-0.30 lOMt Orthrys 13-180 8-21 14-110 3-5 2-24 1-15 2-19 0.49-0.62 0.05-1.87 3Mt Pindos 8-150 5-320 15-550 2-112 3-3460 1-1660 1-15 0.42-0.82 0.02-151 th.t

Philippines (Acoje, Zambales) Mantle - - < 20-72 < 100-250 3-11 3-17 2-29 - - > 0.60 0.23 sequence Cumulates - - 30-550 190-1100 3-760 30-5960 74-8350 - - > 0.68 9.50

Albania (Bulqiza) Mantle (Os + Ir + Ru): 150-320 - - < 10-25 < 3 - - 0.78-0.82 sequence Cumulates ( O s + l r + R u ) : 45-110 - - 550-2800 1600-5900 - - 0.51

Cyprus Troodos 18-48 7-23 21-160 2-6 O-15 1-25 0-7 0.63-0.74 0.22-1.50 6 Mt

Turkey Guleman - - 30-43 80 6-9 9-10 3-27 - - 0.76-0.83 0.06-0.91 1.5 Mt

New Caledonia Tiebaghi - - 31-410 < 100-1130 < 5-31 < 10-45 < 4-9 - - 0.53-0.65 0.02-0.07 2.7 Mt

PGE and chromite tonnage data: Johan et al. (1982); Konstantopoulou (1990); Prichard and Lord (1990); Page et al. (1982); Page et al. (1984); Page et al. (1986); Bacuta et al. (1990); LeBlanc and Violette (1983); Nicolas and AI Azd (1990); unpubl, data. a A few PGE--enriched samples are not included; th.t = thousands tons.

378 M. Economou-Eliopoulos / Ore Geology Reviews 11 (1996) 363-381

ceeds from southwest to northeast of the complex (Konstantopoulou, 1990; Konstantopoulou and Economou-Eliopoulos, 1991).

Therefore, assuming that high-A1 and high-Cr chromite ores are derived from separate magmas, chalcophile elements may provide evidence for a discrimination between chromite ores derived from primitive magmas and those derived from partially fractionated magmas. Also, even in the Vourinos chromite ores with a remarkably constant composi- tion, the geochemical trend throughout the complex seem to provide PGE information for a stratigraphic orientation, which is consistent with the structural data suggesting a tectonic separation (Roberts et al., 1988).

Furthermore, considering the Pindos complex, central part of the Vourinos complex (Kissavos), Euboea and Skyros island to the southeast, as well as Serbo-Macedonian and Rhodope massifs, they have some common features; they host chromitites of both high-Cr and high-A1 type, in a comparable propor- tion, have a low potential for chromite and a frac- tionation trend in the chromite composition, as ex- pressed by the PGE, Ni, Cu content, Pd/Ir or/and Ni /Cu ratios (Table 3; Economou et al., 1986; Konstantopoulou and Economou-Eliopoulos, 1991 ; Zhelyaskova-Panayotova and Economou-Eliopoulos, 1994). It seems likely that the composition and variation of parent magmas in SSZ ridges in which the petrological and geochemical data of rocks and chromite ores suggest that magmas have derived by two (or more) stage melting events, may represent ophiolitic portions where the conditions were not favourable for chromite mineralization.

In contrast, large chromite deposits of Greece (Vourinos 10 Mt, Othrys 3 Mr), Cyprus (Troodos 6 Mt), Albania (Bulqiza 40 Mt) and Philippines, Zam- bales (Acoge Block 4 Mt), (Table 5), Kazakstan (Kempirsai 90 Mt of metallurgical type, average 56 wt% Cr203) are characterized by a limited variation of the chemical composition of ores (in both major and chalcophile elements), and they contain only one chromite type (metallurgical or refractory), the other being only in small proportion (Kravchenko, 1986; Stumpfl et al., 1993; Cina et al., 1986; Michaelides, 1983). Although PGE-enriched chromitites may be present in both chromite-poor and chromite-rich ophiolite complexes, the former exhibit a well frac-

tionated trend in many chromite occurrences and host dunites, whereas in the latter PGE-enriched samples are only occasionally present and they are usually associated with sulphide-bearing dunites in a close proximity to the petrological Moho.

9. Conclusions

The platinum-group element (PGE) data in com- bination with petrological and other geochemical data lead to the following conclusions:

(1) Assuming that high-Cr and high-A1 chromite ores are derived from separate magmas, chalcophile element data (PGE, Ni, Cu, content, PGE-patterns, Pd/Ir, Ni /Cu ratios) may provide valuable evidence for discrimination between chromite ores derived from primitive magmas (low Pd/Ir, high Ni/Cu ratios) and those derived from partially fractionated magmas (high Pd/Ir, low Ni/Cu ratios).

(2) Large chromite deposits are characterized by a limited compositional variation in both major and trace (PGE, Ni, Cu) elements and low Pd/I r ratios, while PGE-enriched chromitites are only occasion- ally present.

(3) Chromitites hosted in supra-Moho dunites and dunites of uppermost stratigraphic levels of the man- tle sequence may exhibit a significant Pt-Pd enrich- ment (up to 10s of ppm), which is very restricted, independent of the chromite composition and chromite potential of ophiolite complexes.

(4) Present geochemical data suggest that ophio- lite complexes with a low potential for chromite ore, besides a well fractionated area trend (as expressed by high Pd/Ir and low Ni /Cu ratios) in the majority of chromite occurrences and host dunites, high-Cr and high-A1 chromitites are found in comparable proportions, whereas ophiolites hosting significant potential for chromite, contain only one chromite type, the other being only in small proportion.

Acknowledgements

The authorities of the Ministry of Research and Technology, Greece (program 87 PS 49), and Eco- nomic European Community (contract MAIN 0068- GR TT) are thanked for the financial support of this

M. Economou-Eliopoulos / Ore Geology Reviews 11 (1996) 363-381 379

work. Mr. J. Mitsis, Athens University, is also thanked for his assistance on the atomic absorption analysis, and Dr. G. Economou, of IGME for per- forming the electron microprobe analysis. Finally, many thanks are expressed to Dr. Hazel Prichard, University of Wales, Cardiff, Professor Dr. M. Tarldan, University of Hamburg, Germany and an anonymous reviewer of this journal for their con- structive criticism and suggestions.

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