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Geochemistry of chromitites and host rocks from the Pindos ophiolite complex, northwestern Greece
CHEMICAL GEOLOGY t#CCl~C#cG
ELSEVIER
ISOTOPE GEOSCIENCE Chemical Geology 122 (1995) 99-108
Geochemistry of chromitites and host rocks from the Pindos ophiolite complex, northwestern Greece Maria Economou-Eliopoulos a.., Iossif Vacondios b a University of Athens, Department of Geology, Section of Economic Geology and Geochemistry, Panepistimiopolis, 15784 Athens, Greece b l~titute of Geology and Mineral Exploration (IGME), 70 Messoghion Str., 11527 Athens, Greece Received 30 April 1993; accepted after revision 1 December, 1994
Abstract Representative chromite samples of schlieren and massive ore types from various localities of the Pindos ophiolite complex were analyzed for plal~inum-groupelements (PGE), S, Ni, Co and Cu, and mineral chemistry. Host dunites and harzburgite were also analyzed for the chalcophile elements. The chromite ores show a wide compositional variation. The Cr/(Cr+AI) ratio ranges from 0.47 to 0.83 while Mg/ (Mg + Fez ÷ ) ranges between 0.42 and 0.73, but two populations can be distinguished (the high-Cr and high-Al types), which show a more restricted variation. Ranges of PGE contents (in ppb) are: Os = 8-150, Ir = 4-320, Ru = 15-550, Rh = 2-82, Pt-- 3150 and Pd = 1.5-20. Gold ranges between 1 and 13 ppb. The PGE and less chalcophile elements (Ni, Co and Cu), the incompatible/compatible element ratios, the PGE/S ratio and PGE patterns provide 'valuable information for the discrimination of chromite ores with a similar major-element composition. A fractionation trend in both chromitites and associated dunites can be used for stratigraphic orientation in the mantle sequences of ophiolite complexes and evaluation of their chromite potential.
1. Introduction The Pindos ophiolite complex of NW Greece, contains lavas of the complete spectrum from mid-ocean ridge basalts (MORB) through island arc tholeiites (I.AT) to boninite series volcanics (BSV) (Capedri et al., 1980). The degree of depletion in the mantle peridotites ranges from relatively low to extreme depletion (Pearce et al., 1984.). Although the Pindos complex has supra-subduction zone (SSZ) characteristics, which are considered promising for significant chromite potential (Peltrce et al., 1984; Roberts, 1988), only small chromite bodies are known (Higoumenakis * Correspondingauthor. [MBI 0009-2541/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10009 -2541 ( 94 ) 0 0 1 5 4 - 5
et al., 1977; Rassios, 1990). Assuming that variation in chromite composition is related with the composition of parent magma (Augt, 1985: Talkington and Watkinson, 1986; Prichard et al., 1986: Bacuta et al., 1990: Tarkian et al., 1992; Hattori et al., 1992) the platinumgroup element (PGE) content in the Pindos chromitites may provide valuable evidence for the chromite mineralization. This study presents the chemistry (major elements and PGE) of some chromite occurrences from the Pindos complex and discusses the implications with respect to the origin of, and exploration for chromitites.
2. Geotectonie setting The Pindos ophiolite complex, NW Greece, is part of a nappe which is tectonically overthrust on to the
M. Economou-Eliopoulos, I. Vacondios / Chemical Geology 122 (1995) 99-108
100
\
Vourbiani f,
Vourinos
~,'Koziokos le
Krania
(r I
iii!ii!i!i!iiii!!!iii
"4"" ~ ~
I
o
101
D
Main
Thrust
M.t'sovo 0
2
I
I
~:!:3:!:i:!:~:i:~:!:!:!:T!:!:~
"
/. K m I
Fig. 1. Simplified geological map of the Pindos ophiolite complex (alter Migims et ai., 1986) showing the sampling localities. Symbols: D -- Kampos Despoti; K = Korydallos; M = Milia; N = Dramala; T= Trygona; X = Kyra Kali; 1 = Tertiary and Quaternary; 2 = limestones, shales, cherts, sandstones and conglomerates (Triassic-Jurassic); 3=limestones with rudists (Cretaceous); 4=cumulates (mainly gabbros and wehrlites); 5 = isotropic gabbros and extrusives; 6 = tectonic ultramalics.
Eocene flysch of the Pindos zone (Fig. 1). The Pindos, Othrys, Koziakas and Vourinos complexes are considered to represent different parts of a single slab of oceanic crust developed above an SSZ, during the Mid-Jurassic ( ~ 170 Ma) (Smith, 1977; Jones, 1990). The structure of the Pindos ophiolite is very complex. It constitutes a complete complex, although it is mainly comprised of large harzburgite-dunite masses ( > 1000 km2). The degree of depletion in the mantle peridotites (harzburgite and plagioclase-harzburgite range from relatively low to extreme depletion as recorded in the composition of orthopyroxene and coexisting Cr-spinel (Jaques and Green, 1980; Dick and Bullen, 1984) (Table 1; Fig. 2). The magmatic sequence of the complex includes cumulate rocks ranging from ultramafic to gabbros, diabase and a complete spectrum from MORB through IAT to BSV (Parrot, 1967; Paraske-
vopoulos, 1975; Capedri et al., 1980, 1985; Pearce et al., 1984). Thus, in the Pindos complex more than one mantle source and eruptive setting seems to be represented, all having a place in an SSZ environment (Pearce et al., 1984).
3. Location and description of chromite ores
The mantle sequence of the Pindos ophiolite complex resembles to that of Vourinos in the presence of extensive and highly depleted harzburgite. However, in contrast to Vourinos there are only a limited number of small chromite occurrences, and thus there is a low potential for exploitation in the Pindos complex (Higoumenakis et al., 1977; Rassios, 1990). The lmown c~omite occurrences show a umform distribution in
0.98
0.00
0.00
0.00
7.96
50.44
0.15
0.12
0.50
99.52
0.00
7.58
51.07
0.00
0.23
0.47
100.13
FeO
MsO
CaO MnO
NiO
Total
99.17
0.19
1.36
0.25
10.45
17.11
0.00
49.70
99.21
0.00
0.00
0.85
35.20
5.81
0.00
0.49
99.08
0.00
0.26
0.20
10.89
17.77
0.00
52.46
61.11
0.46
0.06
11.79
15.55
0.00
0.00
99.29
60.31
1.62
0.00
11.72
15.05
0.23
0.21
99,32
0.80
0.58
AI2Os Cr203
Fe203
TiO~
MgO
FeO
IdnO
NiO
Total
Cr/(Cr+AI)
M g / ( M g + Fe 2+ )
99.95
0.59
0.81
98.65
0.00
0.00
14.98
11.91
0.08
0.81
60.94
9.82
0.11
0.00
0.61
0.80
99.17
0.25
0.00
14.29
12.31
0.08
1.24
60.76
10.24
0.13
0.58
0,83
99.26
0.00
0.00
15.26
12.07
1.10
1.02
59.50
11.18
0.29
0.58
0.78
98.85
0.11
0.00
16.20
11.31
0.00
0.31
59.66
10.97
6 0.18
0.56
0.79
99.26
0.11
0.00
15.32
11.83
0.00
0.45
61.19
10.18
7
0.58
0.80
98.86
0.00
0.00
14.99
11.86
O.ff/
0.96
61.17
9.81
0.00
8
0.36
0.(30
0.00 0.17 0.20 99.08
0.00 0.51 99.32
8.23
0.19
0.59
0.81
99.16
0.00
0.00
15.35
11.70
0.06
2.21
61.42
9
0.58
0.83
99.31
0.00
0.00
15.47
11.75
0.08
1.41
61.91
8.42
0.27
lO
0.60
0.84
99.50
0.(30
0.00
14.69
12.16
0.11
2.57
61.71
8.18
0.08
11
0.68
0.00
Dramala (D1A)
35.14
50.64
6.46
12
0.00
0.00 7.61
44.13
0.39 0.00
0.00
0.45
0.90
0.68
0.52
99.33
0.36
0.26
12.78
15.19
0.47
2.10
41.75
26.20
0.20
99.22
0.00
0.99
0.15
11.59
18.39
23.52
55.14
0.00
sp
40.56
opx
0.19 0.37 100.10
0.13 0.33 99.67
0.(30
0.57
0.48
99.42
0.21
0.00
16.71
12.70
0.31
4.02
37.97
27.50
0.60
0.49
99.25
0.(30
0.(30
15.67
13.65
0.38
2.72
39.47
27.18
0.18
14
0.70
0.48
99.21
0.29
0.00
12.00
15.79
0.23
2.50
39.88
28.52
0.00
15
0.10
0.73
0.48
98.89
0.34
0.29
10.87
16.26
0.17
3.26
39.24
28.36
0.29
0.64
0.52
99.97
0.28
0.30
14.30
14.48
0.37
2.50
41.41
26.06
0.66
0.51
100.92
0.20
0.29
13.57
15.57
0.30
2.46
41.66
26.61
0.26
18
0.89
17
35.14
0.13
5.51
50.43
7.61
13
16
0.78
0.12 0.00
0.29
0.00
56.93
0.00
opx
40.92
ol
Vourbiani
99.02
0.31
0.78
0.09
11.63
17.30
0.20
48.34
19.89
0A8
sp
Ko~dallos ( K3B )
99.03
0.20
0.35
0.79
35.30
5.28
0.00
0.51
0.61
55.99
opx
Kyra Kali
Data from present study, and Migiros et al. (1991) (No. 12), Economou (1986) (Nos. 17 and 18), and Paraskevopoulos and Economou (1986) (No. 19).
0.57
0.80
0.17
10.15
0.10
10.09
SiO2
4
5
3
l
2
Trygona (T10D)
Kampos Despoil (DIA)
8.21
0.00
0.14
50.30
Table 2 Representative electron microprobe analyses of chromifites from the Pindos complex
99.37
0.00
35.68
5.44
0.14
0.59
0.00
40.94
0.00
17.25
0.25
Cr203
0.36
56.50
TiO2
19.53
0.56
0.00
0.00
AI203
0.51
56.03
40.35
40.78
SiO,
sp
ol
opx
opx
ol
sp
Milia
Trygona
Table 1 Electron microprobe analyses of olivine (oi) and coexisting orthopyroxene (opx)--spinel (sp) from the mantle harzburgite of the Pindos ophiolite complex
0.71
0.50
100.31
0.26
0.45
11.50
15.50
0.14
4.74
40.78
26.80
0.14
19
Kalambaka
99.03
0.00
0,85
0.20
12.01
17.29
0.00
52.74
15,56
0.38
sp
0.30
12.78 14.22
8.25 20.20
0.80 0.62
0.42
100.97
0.00
0.82
99.32
0.19
0.74
0.16
0.47
0.84 0.06
61.73
10.20
21
KyraKali(Xl)
~.~
0.~
0.~
0.~
11.42
17.~
0.~
50.89
18.~
0.56
sp
1.73
59.55
8.64
0.23
20
Milia(M1)
99.54
0.00
0.15
0.69
34.70
5.33
0.26
0.47
0.59
57.35
opx
~oo
~:~
"~
~-J
-~ 3"
"~.
¢%
t-
102
M. Economou-Eliopoulos, 1. Vacondios / Chemical Geology 122 (1995) 99-108
the mantle sequence and a simple area of concentration cannot be delineated. Small chromite occurrences of all textural types (massive, schlieren, banded, disseminated and nodular) are found within elongate dunite bodies in harzburgite. The maximum length of chromite lenses is a few tens of meters while the maximum width is a few tens of centimeters (Higoumenakis et al., 1977). Due to mantle and emplacement tectonics, a strong plastic and brittle deformation was superimposed on primary magmatic textures and had an important influence on the present form and distribution of chromitites (Roberts et al., 1988; Rassios, 1990). Representative samples of chromite ores and associated dunites and harzburgites from the areas of Dramala, Kampos Despoti, Trygona, Korydallos, Milia, Kyra Kali and Vourbiani, covering much of the Pindos ophiolite complex (Fig. 1) were analyzed for major elements, PGE, Au, S, Ni, Co and Cu.
5.0 + +
4.0
•
Trygona
x
Kyro Kati
+
Koziokos
÷
3.0
0
< 2.0
1.0
"oo~
4. Analytical methods PGE were determined by neutron activation analysis after preconcentration from large (30 g) samples, using the nickel fire-assay technique, following the method of Hoffman et al. (1978) with minor modifications. Sodium borate, sodium carbonate and lithium tetraborate were used as a flux. The nickel sulphide button was dissolved in 12 M HC1. The residue was collected on filter paper and irradiated (together with standards) for 3 min and then allowed to decay for 3 min, and Rh and Pd were then determined. Pt, Os, Ru and Ir were determined following a second irradiation. This was carried out over 7 hr, and samples then were allowed to decay for 10 days. Information on detection limits, precision and accuracy is given by Hoffman et al. (1978). Pt and Pd in samples with a content lower than the detection limits of the Hoffman et al. (1978) method were determined by atomic absorption spectroscopy (AAS) using a heated Graphite Atomizer (Perkin Elmer* Model 2100). Ni, Co and Cu were also determined by AAS. Sulphur was obtained by using a Leco* CS-244, HF100 analyzer. Electron microprobe analyses were carried out at the Institute of Geology and Mineral Exploration (IGME), using a Cam~ca® Superprobe wavelength-dispersive system.
Mitio
o
x
o.o
~o
'
4'0
'
6o'
'
Crx100 - Spinel Or+ AI
Fig. 2. Plot of % AI203 in orthopyroxene (opx) vs. Cr/(Cr+A1) ratio in coexisting spinel from the mantle harzburgite of the Pindos ophiolite complex. Data from Table I and Capedri et al. (1985).
5. Chemical composition of chromite ores
Chromite composition of massive chromite from the area of Dramala shows a Cr/(Cr + A1) ratio of < 0.52 (Migiros et al., 1991). Results indicate that in the area of Dramala, high-Cr chromitites are also found (Table 2). Generally, the C r / ( C r + A I ) ratio ranges between 0.47 and 0.83, while the Mg/(Mg + Fe 2+ ) ratio ranges from 0.42 and 0.73. Two distinctive populations are seen, the metallurgical (high-Cr) and refractory (highAI) types, each of which shows a relatively restricted variation (Fig. 3). Ti contents in chromitites with high C r / ( C r + A I ) ratios (average 0.80) are low (<0.16 wt% TiO2), whereas ehromitites from the areas of Dramala, Korydallos and Vourbiani with low Cr/ (Cr+A1) ratios (average 0.50) have higher Ti contents (up to 0.47 wt% TiO2). Ferric iron is also higher in the high-A1 chromitites and in the Dramala high-Cr chromitites.The chromititcsfrom the area of Milia are
M. Economou-Eliopoulos, L Vacondios / Chemical Geology 122 (1995) 99-108
103
Table 3 Platinum-group element data of chromite ores from the Pindos ophiolite complex Location
Sample No.
Rock type
Cr Cr + AI
(ppb) Os
Ir
Ru
Rh
Pt
Pd
(ppm)
Pd/Ir
Au
S
1.5 7 3 5
12 8 8 8
26 52
135 105
0.82
60 3,250 60 2,500 70 70 1,350
35
110
0.38 1.16 1.00 1.25
14 1
0.80
40 2,300 60 1,450
20 105 67 290
2.00 0.22
0.80
40 4,200 50 2,480 40 1,340
20 115 18 110 34 285
0.30 4.50 0.24
0.48
50 2,920 12 160 16.70 40 1,630 310 190 30.00 50 40.00 70 1,750 120 260 1.00
0.82
40 2,160 70 3,640 60 2,000
5 140 45.00 24 156 3.50 30 750 0.02
0.80
60 3,400 60 2,400 80 2,080
10 125 0.33 22 105 12.00 34 650 0.03
0.52
40
50 285
Ni
Cu
Co
Dramala:
Dunite (ore hosting) Harzburgite Pyroxenite Chromitite (massive)
NIB NIC NID NIA
<5 <5 <5 26
4 6 3 4
<4 <4 6 51
1.5 2 4 2
2 6 26 6
DIB DIA
<5 30
4 18
4 50
3 3
9 7
8 4
<5 <5 90
5 2 34
<4 <4 120
2 2 6
3 8 3
1.5 1 9 18 8 12
K3C K3Z K3D K3B
<5 <5 <5 40
0.3 1 <0.3 20
4 <4 <4 33
1 5 3 5
3 5 20 30 8 12 25 20
12 12 10 11
M3 M4 M1
<5 0.4 <5 2 150 320
<4 <4 550
3 2 82
10 18 10 7 150 7
13 3 1
X3 X4 X1
<5 <5 40
3 1 51
<4 <4 190
1 2 11
V1
8
5
15
2
Kambos Despoti:
Harzburgite Chromitite (massive) T~. gona:
Dunite (ore hosting) TIOB Harzburgite TIOC Chromitite (schlieren) TIOA Ko~. dallos:
Dunite (ore hosting) Dunite (barren) Gabbros Chromitite (massive) Milia:
Dunite (ore hosting) Harzburgite Chromitite (schlieren) Kyra Kali:
Dunite (ore hosting) Harzburgite Chromitite (schlieren)
2 1 1 10 12 9 1 1.5 4
Vourbiani:
Chromitite (massive)
characterized by higher total iron, although ferric iron is relatively low (Table 2). Ni, Cu and Co concentrations are similar in chromitites from the areas of Dramala, Kampos Despoti and Trygona, while the concentrations o f these elements are higher in chromitites from Korydallos, Milia, Kyra Kali and Vourbiani (Table 3). The P GE concenlxations for chromitites show a wide variation, ranging between 8 and 150 ppb Os, 4 and 320 ppb Ir, 15 and 550 ppb Ru, 2 and 82 ppb Rh, 3 and
3
10
4
1,600
2.00
150 ppb Pt, and 1.5 and 20 ppb Pd. The m i n i m u m P G E content found in chromitites from the Vourbiani area and the m ax i m u m in those from Milia (Table 3). P G E content is lower in high-A1 chromitites (Vourbiani and Korydallos) than in high-Cr ones. However, the highCr chromitites from Dramala are PGE poor too. The P d / I r ratio ranges from 0.02 to 2.00, and the highest values are observed in both high-A1 and high-Cr chromitites (Table 3).
104
M. Economou-Eliopoulos, 1. Vacondios / Chemical Geology 122 (1995) 99-108
The sulphur contents in the chromitites range between 40 and 80 ppm and there appears to be no correlation between PGE and S contents. It is noticeable that the Milia chromitites with the highest PGE content (1260 ppb) also have low S contents, but, at the same time, show the highest PGE/S ratios and an increase of Pt over Pd (Table 3). All chromitites from the Pindos complex show an enrichment in Os, Ir and Ru compared to Pt and Pd, resulting in chondrite-normalized PGE patterns with a negative slope (Fig. 4). They are similar to published data from other ophiolites (Page et al., 1982, 1984; Economou, 1986; Konstantopoulou and EconomouEliopoulos, 1991). Harzburgite which is thought to be a residual mantle peridotite, is characterized by homogeneous PGE and less chalcophile element (Ni, Co and Cu) contents throughout the Pindos complex. In contrast, some
o.1! /
K(~mbos
=o
'\,\
o
\\
~// /
\\
/1'~
'\\,~jll, '~ V
.:.2 ao,
g 5 100
/ \
\
/
/
\X / 90 •)oro
8c
ds
4
~ x~''
°1
5C
# N
",
~0
~u
N
o
Trygona
• Nilia N Kor ydallos
A
Karnpos Despoti
• Vourbiani
x
Kyra
• Kalambaka
•X- D r a m a l a
30
R'.
dunite and chromitite bodies show significant variability in concentrations of the above elements and relatively high Pd/Ir ratios, especially at the area of KorydaUos (Fig. 5), where the presence of gabbros is common (Table 3).
6C
40
~'o
Fig. 4. Chondrite(C2) -normalizedPGE patternsof high-Crchromititesfromthe Pindosophiolitecomplex.
7¢
x ÷ ¢. ¢. u u
,'r
100
Kali ~ ,
,
i 50
I
Mg = 100 M g + Fez *
Fig. 3. Plotof Cr/(Cr + AI) vs. Mg/(Mg + Fez+) ratioforchromite ores from the Pindos ophiolite complex. Data from Table 2 and Paraskevopoulosand Economou(1986) and Migiroset al. (1991).
6. Discussion Assuming that chromitites with dunite lenses within the tectonite harzburgite of ophiolite complexes, have formed by crystallization from ascending magmas at palaeo-spreading center (Brown, 1979 ), then a number of different factors can account for the compositional variation of the chromite ores. The documented presence of laurite and Os--Ir-Ru alloys as inclusions in chromite grains, the lack of any correlation between
M. Economou-Eliopoulos, L Vacondios / Chemical Geology 122 (1995) 99-108
"7
.E
o"""
O.C
E o .c u
Fig. 5. Chondrite (C2) -normalized PGE patterns of high-Al chromitites from the Pindos ophioUte complex.
PGE content and degree of serpentinization and Os isotopic data on pliatinum-group metal inclusions in chromite (Aug6, 1985; Prichard et al., 1986; Talkington and Watkinson,, 1986; Bacuta et al., 1990; Hattori et al., 1992; Tarkian et al., 1992) suggest that the composition of parent magma is among the most important factors. This may Ix,'a function of the degree of melting of the mantle source or subsequent modification due to fractional crystallization. Variations of physico-chemical conditions of crystallization (foe, P, T) are also considered to influe,nce the composition of chromitites (Tindle and Pearce, 1983). The high-Al chromitites from the areas of Vourbiani, Korydallos and Dr~anala, which are characterized by a relatively high Mg'O, TiO2 and Fe203 contents, may have been produced by a combination of increasing foe and decreasing T, or from a parental magma that was rich in A1 and Ti. Sometimes it is difficult to distinguish between the degree of partial melting and fractional crystallization. Due to the incompatible behavior ofAl, Ti, Pt and Pd, in contrast to the compatible nature of Cr, Os, Ir and Ru (Tindle and Pearce, 1983; Barnes et al., 1985, 1988:1 Cocherie et al., 1989), high-Cr chromitites could be interpreted as resulting from a
105
higher degree of partial melting in the upper mantle compared to high-A1 chromitites (Jaques and Green, 1980; Dick and Bullen, 1984: Economou, 1986; Bacuta et al., 1990; Economou-Eliopoulos and Vacondios, 1990). Also, since the compatible elements are more strongly partitioned into initial melts they are depleted in relatively evolved magma (Brown, 1987; Economou-Eliopoulos and Zhelyaskova-Panayotova, 1989; Bacuta et al., 1990; Konstantopoulou and EconomouEliopoulos, 1991). Therefore high-Al chromitites in a spatial association with high-Cr chromitites may suggest generation of the former from more evolved parent magmas. Alternatively, their composition may be related to a lower degree of partial melting of the mantle source, and they represent juxtaposed slices of different ophiolitic fragments during the evolution of a marginal basin. The relatively high Pd/Ir ratios and smooth-shaped PGE patterns of high-Al chromitites from the Pindos ophiolite complex, reflecting a relatively high degree of fractionation (Barnes et al., 1985, 1988), suggest that parent magma of high-Al chromitites was more evolved than that of high-Cr ones. Furthermore, the presence of high-Cr chromitites from the area of Dramala with Pd/Ir ratios as high as 1.25, in combination with of the presence of exclusively high-Al chromitites with a very low TiO2 and Fe203 content and Pd/Ir ratios, within defined ophiolite complexes like the Othrys ophiolite complex (Economou-Eliopoulos, 1993), suggests that these chromitites were derived from separate parent magmas. Oxygen fugacity and sulphur saturation are also important factors controlling the PGE content in chromitites (Buchanan, 1988; Amosse et al., 1990). Naldrett et al. (1990) concluded that once plagioclase has appeared on the liquidus of the resident magma, a fresh input can result in a mixture forming either a sulphide-bearing and PGE-enriched chromitite or, if the magma mixture is not particularly enriched in Cr, a PGE-rich sulphide layer such as the Merensky Reef (Bushveld complex, South Africa). The PGE content in the analyzed chromitites and dunites from the Pindos complex, in particular from the area of Korydallos, exhibit a fractionation trend, as it is shown by the Pd/Ir ratios. At Milia, the PGEenriched chromitites exhibit a preference of Pt over Pd, and a high PGE/S ratio (Table 3) may reflect a relatively high R factor (silicate/sulphide) (von Griine-
106
M. Economou-Eliopoulos, 1. Vacondios / Chemical Geology 122 (1995) 99-108
waldt et al., 1990). Moreover, a process of mixing with a more evolved magma is also suggested by the association of the Milia chromitites with a dunite body showing a relatively high Pd/Ir ratio (45) (Table 3). The compositional variation characterizing the Pindos chromitites and host dunites can be interpreted assuming that chromitites in ophiolite complexes were formed from magmas derived by varying degree of melting over a large region of the upper mantle, during the evolution of a marginal basin (Pearce et al., 1984; Ribe, 1988; Nicolas, 1989). Thus, the presence of both high-Cr and high-A1 chromitites and their geochemical features in the Pindos complex are consistent with the variability in the degree of depletion in an SSZ environment and the presence of a wide spectrum of lavas (Fig. 2; Capedri et al., 1980), suggesting more than one mantle source and eruptive event. In addition, the wide variation in the content of chalcophile elements in chromitites with a similar major-element composition in the Pindos complex, in contrast to that in the Vourinos complex which is the main source of chromite in Greece, suggests that the studied chromitites and host dunites do belong neither to deep mantle chromitites nor to the dunites of the cumulate sequence which hosts PGE-enriched chromitites (Ohnenstetter et al., 1991 ). Thus, the petrological data and the present geochemical data on chromitites and host dunites of the Pindos complex provide evidence for a formation mainly at higher stratigraphic levels of the mantle sequence.
7. C o n c l u s i o n s
Although, further detailed geochemical analysis is required, the present data lead to the following conclusions: (1) On the basis of chalcophile element (PGE, Ni, Cu and Co) concentrations in chromitites with a similar major-element composition, these can be divided into those derived from primitive magmas and those derived from partially fractionated magmas. (2) The relatively high Pd/Ir ratios of some chromitite occurrences, reflecting a relatively extensive fractionation trend, in combination with the high variability of the PGE distribution in chromitites throughout the Pindos complex (in contrast to the Vourinos complex), suggest that they were derived by mixing of
relatively small influxes of magmas, at higher stratigraphic levels. (3) The spatial association of high-Cr and high-A1 chromitites, coupled with the petrological and geochemical features of the ophiolitic rocks and chromitites, suggest more than one mantle source and eruptive event during the formation of the Pindos complex. (4) The Pindos ophiolite complex is not considered to contain a significant potential for chromite, although its formation is related with a supra-subduction environment.
Acknowledgements
The authorities of the Economic European Community (contract MAIN 0068-GR T r ) are thanked for financial support for this work. Mr. J. Mitsis, Athens University, is also thanked for his assistance with atomic absorption analysis, and Dr. G. Economou, of IGME for performing electron microprobe analyses. Finally, many thanks are expressed to the reviewers of this journal for their constructive criticism and suggestions.
References
Amosse,J., Allihert,M., Fischer,W. and Piboule, M., 1990. Experimental study of the solubilityof platinumand iridium in basic silicate melts- - implicationsfor the differentiationof platinumgroup elements during rnagmatic processes. Chem. Geol., 81: 45-53. Aug& T., 1985. Platinum-group mineral inclusions in ophiolitic chromitite from the Vourinos complex.Greece.Can. Mineral., 23(2): 163-171. Bacuta, G.C., Kay, R.W., Gibbs,A.K. and Lipin, B.R., 1990. Platinum-group element abundance and distribution in chromite deposits of the Acoje Block,Zambalesophiolitecomplex,Philippines. J. Geochem.Explor., 37:113-145. Barnes, S.-J., Naldrett, A.J. and Gorton, M.P., 1985. The origin of the fractionationof platinum-groupelementsin terrestrial magmas. Chem.Geol., 53: 303-323. Barnes, S.-J., Boyd, R., Korneliussen,A., Nilsson, L.P., Often, M., Pedersen, R.-B. and Robins, B., 1988. The use of mantle normalizationand metalratiosin discriminatingbetweeneffectsof partial melting,crystalfractionationand sulphidesegregationon platinum-group elements, gold, nickel and Cu: Examples from
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M. Economou-Eliopoulos, 1. Vacondios / Chemical Geology 122 (1995) 99-108 Brown, M.A., 1979. Textural and geochemical evidence for the origin of some chromite deposits in the Oman ophiolites. In: A. Panayotoy (Editor), Int. Ophiolite Symp., Nicosia, 1979, pp. 714-721. Brown, M.A., 1987. Textural and geochemical evidence for the origin of some chromite deposits in the Oman ophiolites. In: A. Panayoton (Editor), Int. Ophiolite Symp., Nicosia, 1987, pp. 714-721. Buchanan, D.L., 1988. Platinum-group Element Deposits. Elsevier, Amsterdam, 185 pp. Capedri, S., Venturelli, G. Bocchi, G., Dostal, J., Garuti, G. and Rossi, A., 1980. The geochemistry and petrogenesis of an ophiolitic sequence from Pindos, Greece. Contrib. Mineral. Petrol., 74: 189-200. Capedri, S.M., Lekkas, E., Papanikolaou, D., Scarpolis, N., Venturelli, G. and Gallo, F., 1985. The ophiolite of the Koziakas range, Western Thessaly (Greece). Neues Jahrb. Geol. Pal[iontol. Abh., 152( 1): 45-64. Cooherie, A., Aug6, T. aad Meyer, G., 1989. Geochemistry of the platinum-group elen,~nts in various types of spinels from the Vourinos ophiolite complex, Greece. Chem. Geol., 77: 27-39. Dick, H.J.B. and Bullen, T.H., 1984. Chromian spinel as petrogenetic indicator in abyssal ;rod Alpine-type peridotites and spatially assooiated lavas. Con~yib. Mineral. Petrol., 86: 54-76. Economou, M., 1986. Platinum group elements (PGE) in chromite and sulfide ores related with ophiolites. In: M.J. Gallagher, R.A. Ixer, C.R. Neary and H.M. Prichard (Editors), Metallogeny of Basic and Ultrabasic Rocks. Inst. Min. Metall., London, pp. 441453. Economou-Eiiopoulos, IVI., 1993. Platinum-group element (PGE) distribution in chromile ores from ophiolite complexes of Greece: Implications for chromite exploration. Ofioliti, 18( 1): 83-97. Economou-Eliopoulos, 1~[.and Vacondios, I., 1990. Distribution of platinum group elements (PGE) in chromite ores and host rocks from the Pindos opkiolite complex. In: Tectonic Controls on Chrome Ore Localization in Ophiolites, Greece. IGME (Inst. Geol. Miner. Explor. ), Athens, Eur. Econ. Comm. Rep., pp. 207228. Economou-Eliopoulos, M. and Zhelyaskova-Panayotova, M., 1989. Platinum-group elements and gold in chromite ores within Rhodope massif ultramafic rocks (Balkan Peninsula). 28th Int. Geol. Congr., Washington, D.C., Proc., 1:433 (abstract). Hattori, K., Burgath, K-P. and Hart, S., 1992. Os-isotope study of platinum-group minerals in chromitites. In: Alpine-type Ultramarie Intrusions and Associated Placers in Borneo. Mineral. Mag., 58: 157-164. Higoumenakis, A., Kyritopoulos, P. and Maltzaris, F., 1977. Exploration for chromite deposits in the ophiolite mass south of Trikala. IGME (Inst. Geol. Miner. Explor.), Athens, Intern. Rep., 30 pp. Hoffman, E., Naldrett. A., Van Loon, J., Hancock, R.G. and Manson, A., 1978. The determination of all the platinum group elements and gold in rocks anti ore by neutron activation analysis after preconcentration by nickel sulfide fire assay technique on large samples. Anal. Chim. Acta, 102: 157-166. Jaques, A.L. and Green, D.H., 1980. Anhydrous melting ofperidotite at 0-15 kb pressure and the genesis of tholeiitic overlying harzburgite tectonite. Geol. Soo. Am. Bull., 86: 390-398.
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Jones, G., 1990. Tectono-stratigruphy and evolution of the Mesozoic Pindos ophiolite and associated units, northwest Greece. Ph.D. Thesis, University of Edinburgh, Edinburgh, 394 pp. l(onstantopoulou, G. and Economou-Eliopoulos, M., 1991. Distribution of platinum-elements and gold within the Vourinos chromitite ores, Greece. Econ. Geol., 86: 1672-1682. Migiros, G., Karantasi, S., Kanaki-Mavridis, F. and KoroneouSkourtsi, V., 1986. Geological structure of N. Pindos. IGME (Inst. Geol. Miner. Explor.), Athens, Intern. Rep. (unpublished). Migiros, G., Gartzos, E. and Economou, G., 1991. Chromitites from the ultramafic sequence of Dramala area, North Pindos, Greece, and their geotectonic significance. EUG VI (6th Symp. Eur. Union Geosci.), Strasbourg. Naldrett, A.J., Brugmann, G.E. and Wilson, A.H., 1990. Models for the concentration of PGE in layered intrusions. Can. Mineral., 28: 389-408. Nicolas, A., 1989. Structures of Ophiolites and Dynamics of Oceanic Lithosphere. Kluwer, London, 367 pp. Ohnenstetter, M., Karaj, N., Neziraj, A., Johan, Z. and Cina, A., 1991. Le potential platinif'ere des ophiolites: min6ralizations en 616mentsdu groupe du platine (PGE) dans les massifs de Tropoja et Bulqiza, Albanie. C.R. Acad. Sci. Paris, 313, S6r. II: 201-208. Page, N., Cassard, D. and Haffty, J., 1982. Platinum, rhodium, ruthenium and iridium in chromitites from the Massif du Sud and Tiebaghi Massif, New Caledonia. Econ. Geol., 77: 1571-1577. Page, N., Engin, T., Singer, D.A. and Haffty, J., 1984. Distribution of platinum-group elements in the Bali Kef chromite deposit, Guleman-Elazig area, eastern Turkey. Econ. Geol., 79:177-184. Paraskevopoulos, G., 1975. Cycles in cumulates of layered gabbroic sequences of southern Pindos complex. Pract. Akad. Athens, pp. 429--442. Paraskevopoulos, G. and Economou, M., 1986. On the origin of chromite ores of the Vourinos ophiolite complex, Greece. Neues Jahrb. Mineral. Abh., 154: 179-192. Parrot, J.F., 1967. Le cortege ophiolitique du Pinde septentrional (Grace). O.R.S.T.O.M. (Off. Rech. Sci. Tech. Outre-mer), Paris, pp. 1-114. Pearce, J.A., Lippard, S.J. and Roberts, S., 1984. Characteristics and tectonic significance of suprasubduction zone ophiolites. Geol. Soc. London, Spec. Publ., 16: 77-94. Prichard, H., Neary, C. and Potts, P.J., 1986. Platinum group minerals in the Shetland ophiolite. In: M.J. Gallagher, R.A. Ixer, C.R. Neary and H.M. Prichard (Editors), Metallogeny of Basic and Ultrabasic Rooks. Inst. Min. Metall., London, pp. 395-414. Rassios, A., 1990. Internal structure and pseudostratigraphy of the Dramala peridotite. Geol. Soo. Greece Bull., 25( 1 ): 293-305. Ribe, N.M., 1988. On the dynamics of mid-ocean ridges. J. Geophys. Res., 93: 429-436. Roberts, S., 1988. Ophiolitic chromitite formation: A marginal basin phenomenon. Econ. Geol., 83: 1034-1036. Roberts, S., Vacondios, I., Wright, L., Rassios, A., Vrachatis, G., Grivas, E., Nesbitt, R., Neary, C., Moat, T. and Konstantopoulou, G., 1988. Structural controls on the location and form of the Vourinos chromite deposits. In: J. Boissonas and P. Omenetto (Editors), Mineral Deposits within the European Community. Springer, Berlin, pp. 249-266.
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M. Economou-Eliopoulos, L Vacondios / Chemical Geology 122 (1995) 99-108
Smith, A.G., 1977. Othrys, Pindos and Vourinos ophiolites and the Pelagonian zone. Proc. Colloq. on Aegean Region, Proc., 3: 1369-1374. Talkington, R.W. and Watldnson, D.H., 1986. Whole rock platinumgroup elements trends in chromite rich rocks in ophiolitic and stratiform igneous complexes. In: M.J. Gallagher, R.A. Ixer, C. bleary and H.M. Prichard (Editors), Metallogeny of Basic and Ultrabasic Rocks. Inst. Min. Metall., London, pp. 427--440.
Tarkian, M., Economou-Eliopoulos, M. and Eliopoulos, D.G., 1992. Platinum-group minerals and tetraauficuprite in ophiolitic rocks of the Skyros island, Greece. Mineral. Petrol., 47: 55--66. Tindle, A.G. and Pearce, J.A., 1983. Assimilation and partial melting of continental crest: Evidence from the mineralogy and geochemistry of autoliths and xenoliths. Lithos, 16:185-202. yon Griinewaldt, G., Dicks, D., de Wet, J. and Horsch, H., 1990. PGE mineralization in the western sector of the Eastern Bushveld Complex. Mineral. Petrol., 42: 71-95.
ELSEVIER
ISOTOPE GEOSCIENCE Chemical Geology 122 (1995) 99-108
Geochemistry of chromitites and host rocks from the Pindos ophiolite complex, northwestern Greece Maria Economou-Eliopoulos a.., Iossif Vacondios b a University of Athens, Department of Geology, Section of Economic Geology and Geochemistry, Panepistimiopolis, 15784 Athens, Greece b l~titute of Geology and Mineral Exploration (IGME), 70 Messoghion Str., 11527 Athens, Greece Received 30 April 1993; accepted after revision 1 December, 1994
Abstract Representative chromite samples of schlieren and massive ore types from various localities of the Pindos ophiolite complex were analyzed for plal~inum-groupelements (PGE), S, Ni, Co and Cu, and mineral chemistry. Host dunites and harzburgite were also analyzed for the chalcophile elements. The chromite ores show a wide compositional variation. The Cr/(Cr+AI) ratio ranges from 0.47 to 0.83 while Mg/ (Mg + Fez ÷ ) ranges between 0.42 and 0.73, but two populations can be distinguished (the high-Cr and high-Al types), which show a more restricted variation. Ranges of PGE contents (in ppb) are: Os = 8-150, Ir = 4-320, Ru = 15-550, Rh = 2-82, Pt-- 3150 and Pd = 1.5-20. Gold ranges between 1 and 13 ppb. The PGE and less chalcophile elements (Ni, Co and Cu), the incompatible/compatible element ratios, the PGE/S ratio and PGE patterns provide 'valuable information for the discrimination of chromite ores with a similar major-element composition. A fractionation trend in both chromitites and associated dunites can be used for stratigraphic orientation in the mantle sequences of ophiolite complexes and evaluation of their chromite potential.
1. Introduction The Pindos ophiolite complex of NW Greece, contains lavas of the complete spectrum from mid-ocean ridge basalts (MORB) through island arc tholeiites (I.AT) to boninite series volcanics (BSV) (Capedri et al., 1980). The degree of depletion in the mantle peridotites ranges from relatively low to extreme depletion (Pearce et al., 1984.). Although the Pindos complex has supra-subduction zone (SSZ) characteristics, which are considered promising for significant chromite potential (Peltrce et al., 1984; Roberts, 1988), only small chromite bodies are known (Higoumenakis * Correspondingauthor. [MBI 0009-2541/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10009 -2541 ( 94 ) 0 0 1 5 4 - 5
et al., 1977; Rassios, 1990). Assuming that variation in chromite composition is related with the composition of parent magma (Augt, 1985: Talkington and Watkinson, 1986; Prichard et al., 1986: Bacuta et al., 1990: Tarkian et al., 1992; Hattori et al., 1992) the platinumgroup element (PGE) content in the Pindos chromitites may provide valuable evidence for the chromite mineralization. This study presents the chemistry (major elements and PGE) of some chromite occurrences from the Pindos complex and discusses the implications with respect to the origin of, and exploration for chromitites.
2. Geotectonie setting The Pindos ophiolite complex, NW Greece, is part of a nappe which is tectonically overthrust on to the
M. Economou-Eliopoulos, I. Vacondios / Chemical Geology 122 (1995) 99-108
100
\
Vourbiani f,
Vourinos
~,'Koziokos le
Krania
(r I
iii!ii!i!i!iiii!!!iii
"4"" ~ ~
I
o
101
D
Main
Thrust
M.t'sovo 0
2
I
I
~:!:3:!:i:!:~:i:~:!:!:!:T!:!:~
"
/. K m I
Fig. 1. Simplified geological map of the Pindos ophiolite complex (alter Migims et ai., 1986) showing the sampling localities. Symbols: D -- Kampos Despoti; K = Korydallos; M = Milia; N = Dramala; T= Trygona; X = Kyra Kali; 1 = Tertiary and Quaternary; 2 = limestones, shales, cherts, sandstones and conglomerates (Triassic-Jurassic); 3=limestones with rudists (Cretaceous); 4=cumulates (mainly gabbros and wehrlites); 5 = isotropic gabbros and extrusives; 6 = tectonic ultramalics.
Eocene flysch of the Pindos zone (Fig. 1). The Pindos, Othrys, Koziakas and Vourinos complexes are considered to represent different parts of a single slab of oceanic crust developed above an SSZ, during the Mid-Jurassic ( ~ 170 Ma) (Smith, 1977; Jones, 1990). The structure of the Pindos ophiolite is very complex. It constitutes a complete complex, although it is mainly comprised of large harzburgite-dunite masses ( > 1000 km2). The degree of depletion in the mantle peridotites (harzburgite and plagioclase-harzburgite range from relatively low to extreme depletion as recorded in the composition of orthopyroxene and coexisting Cr-spinel (Jaques and Green, 1980; Dick and Bullen, 1984) (Table 1; Fig. 2). The magmatic sequence of the complex includes cumulate rocks ranging from ultramafic to gabbros, diabase and a complete spectrum from MORB through IAT to BSV (Parrot, 1967; Paraske-
vopoulos, 1975; Capedri et al., 1980, 1985; Pearce et al., 1984). Thus, in the Pindos complex more than one mantle source and eruptive setting seems to be represented, all having a place in an SSZ environment (Pearce et al., 1984).
3. Location and description of chromite ores
The mantle sequence of the Pindos ophiolite complex resembles to that of Vourinos in the presence of extensive and highly depleted harzburgite. However, in contrast to Vourinos there are only a limited number of small chromite occurrences, and thus there is a low potential for exploitation in the Pindos complex (Higoumenakis et al., 1977; Rassios, 1990). The lmown c~omite occurrences show a umform distribution in
0.98
0.00
0.00
0.00
7.96
50.44
0.15
0.12
0.50
99.52
0.00
7.58
51.07
0.00
0.23
0.47
100.13
FeO
MsO
CaO MnO
NiO
Total
99.17
0.19
1.36
0.25
10.45
17.11
0.00
49.70
99.21
0.00
0.00
0.85
35.20
5.81
0.00
0.49
99.08
0.00
0.26
0.20
10.89
17.77
0.00
52.46
61.11
0.46
0.06
11.79
15.55
0.00
0.00
99.29
60.31
1.62
0.00
11.72
15.05
0.23
0.21
99,32
0.80
0.58
AI2Os Cr203
Fe203
TiO~
MgO
FeO
IdnO
NiO
Total
Cr/(Cr+AI)
M g / ( M g + Fe 2+ )
99.95
0.59
0.81
98.65
0.00
0.00
14.98
11.91
0.08
0.81
60.94
9.82
0.11
0.00
0.61
0.80
99.17
0.25
0.00
14.29
12.31
0.08
1.24
60.76
10.24
0.13
0.58
0,83
99.26
0.00
0.00
15.26
12.07
1.10
1.02
59.50
11.18
0.29
0.58
0.78
98.85
0.11
0.00
16.20
11.31
0.00
0.31
59.66
10.97
6 0.18
0.56
0.79
99.26
0.11
0.00
15.32
11.83
0.00
0.45
61.19
10.18
7
0.58
0.80
98.86
0.00
0.00
14.99
11.86
O.ff/
0.96
61.17
9.81
0.00
8
0.36
0.(30
0.00 0.17 0.20 99.08
0.00 0.51 99.32
8.23
0.19
0.59
0.81
99.16
0.00
0.00
15.35
11.70
0.06
2.21
61.42
9
0.58
0.83
99.31
0.00
0.00
15.47
11.75
0.08
1.41
61.91
8.42
0.27
lO
0.60
0.84
99.50
0.(30
0.00
14.69
12.16
0.11
2.57
61.71
8.18
0.08
11
0.68
0.00
Dramala (D1A)
35.14
50.64
6.46
12
0.00
0.00 7.61
44.13
0.39 0.00
0.00
0.45
0.90
0.68
0.52
99.33
0.36
0.26
12.78
15.19
0.47
2.10
41.75
26.20
0.20
99.22
0.00
0.99
0.15
11.59
18.39
23.52
55.14
0.00
sp
40.56
opx
0.19 0.37 100.10
0.13 0.33 99.67
0.(30
0.57
0.48
99.42
0.21
0.00
16.71
12.70
0.31
4.02
37.97
27.50
0.60
0.49
99.25
0.(30
0.(30
15.67
13.65
0.38
2.72
39.47
27.18
0.18
14
0.70
0.48
99.21
0.29
0.00
12.00
15.79
0.23
2.50
39.88
28.52
0.00
15
0.10
0.73
0.48
98.89
0.34
0.29
10.87
16.26
0.17
3.26
39.24
28.36
0.29
0.64
0.52
99.97
0.28
0.30
14.30
14.48
0.37
2.50
41.41
26.06
0.66
0.51
100.92
0.20
0.29
13.57
15.57
0.30
2.46
41.66
26.61
0.26
18
0.89
17
35.14
0.13
5.51
50.43
7.61
13
16
0.78
0.12 0.00
0.29
0.00
56.93
0.00
opx
40.92
ol
Vourbiani
99.02
0.31
0.78
0.09
11.63
17.30
0.20
48.34
19.89
0A8
sp
Ko~dallos ( K3B )
99.03
0.20
0.35
0.79
35.30
5.28
0.00
0.51
0.61
55.99
opx
Kyra Kali
Data from present study, and Migiros et al. (1991) (No. 12), Economou (1986) (Nos. 17 and 18), and Paraskevopoulos and Economou (1986) (No. 19).
0.57
0.80
0.17
10.15
0.10
10.09
SiO2
4
5
3
l
2
Trygona (T10D)
Kampos Despoil (DIA)
8.21
0.00
0.14
50.30
Table 2 Representative electron microprobe analyses of chromifites from the Pindos complex
99.37
0.00
35.68
5.44
0.14
0.59
0.00
40.94
0.00
17.25
0.25
Cr203
0.36
56.50
TiO2
19.53
0.56
0.00
0.00
AI203
0.51
56.03
40.35
40.78
SiO,
sp
ol
opx
opx
ol
sp
Milia
Trygona
Table 1 Electron microprobe analyses of olivine (oi) and coexisting orthopyroxene (opx)--spinel (sp) from the mantle harzburgite of the Pindos ophiolite complex
0.71
0.50
100.31
0.26
0.45
11.50
15.50
0.14
4.74
40.78
26.80
0.14
19
Kalambaka
99.03
0.00
0,85
0.20
12.01
17.29
0.00
52.74
15,56
0.38
sp
0.30
12.78 14.22
8.25 20.20
0.80 0.62
0.42
100.97
0.00
0.82
99.32
0.19
0.74
0.16
0.47
0.84 0.06
61.73
10.20
21
KyraKali(Xl)
~.~
0.~
0.~
0.~
11.42
17.~
0.~
50.89
18.~
0.56
sp
1.73
59.55
8.64
0.23
20
Milia(M1)
99.54
0.00
0.15
0.69
34.70
5.33
0.26
0.47
0.59
57.35
opx
~oo
~:~
"~
~-J
-~ 3"
"~.
¢%
t-
102
M. Economou-Eliopoulos, 1. Vacondios / Chemical Geology 122 (1995) 99-108
the mantle sequence and a simple area of concentration cannot be delineated. Small chromite occurrences of all textural types (massive, schlieren, banded, disseminated and nodular) are found within elongate dunite bodies in harzburgite. The maximum length of chromite lenses is a few tens of meters while the maximum width is a few tens of centimeters (Higoumenakis et al., 1977). Due to mantle and emplacement tectonics, a strong plastic and brittle deformation was superimposed on primary magmatic textures and had an important influence on the present form and distribution of chromitites (Roberts et al., 1988; Rassios, 1990). Representative samples of chromite ores and associated dunites and harzburgites from the areas of Dramala, Kampos Despoti, Trygona, Korydallos, Milia, Kyra Kali and Vourbiani, covering much of the Pindos ophiolite complex (Fig. 1) were analyzed for major elements, PGE, Au, S, Ni, Co and Cu.
5.0 + +
4.0
•
Trygona
x
Kyro Kati
+
Koziokos
÷
3.0
0
< 2.0
1.0
"oo~
4. Analytical methods PGE were determined by neutron activation analysis after preconcentration from large (30 g) samples, using the nickel fire-assay technique, following the method of Hoffman et al. (1978) with minor modifications. Sodium borate, sodium carbonate and lithium tetraborate were used as a flux. The nickel sulphide button was dissolved in 12 M HC1. The residue was collected on filter paper and irradiated (together with standards) for 3 min and then allowed to decay for 3 min, and Rh and Pd were then determined. Pt, Os, Ru and Ir were determined following a second irradiation. This was carried out over 7 hr, and samples then were allowed to decay for 10 days. Information on detection limits, precision and accuracy is given by Hoffman et al. (1978). Pt and Pd in samples with a content lower than the detection limits of the Hoffman et al. (1978) method were determined by atomic absorption spectroscopy (AAS) using a heated Graphite Atomizer (Perkin Elmer* Model 2100). Ni, Co and Cu were also determined by AAS. Sulphur was obtained by using a Leco* CS-244, HF100 analyzer. Electron microprobe analyses were carried out at the Institute of Geology and Mineral Exploration (IGME), using a Cam~ca® Superprobe wavelength-dispersive system.
Mitio
o
x
o.o
~o
'
4'0
'
6o'
'
Crx100 - Spinel Or+ AI
Fig. 2. Plot of % AI203 in orthopyroxene (opx) vs. Cr/(Cr+A1) ratio in coexisting spinel from the mantle harzburgite of the Pindos ophiolite complex. Data from Table I and Capedri et al. (1985).
5. Chemical composition of chromite ores
Chromite composition of massive chromite from the area of Dramala shows a Cr/(Cr + A1) ratio of < 0.52 (Migiros et al., 1991). Results indicate that in the area of Dramala, high-Cr chromitites are also found (Table 2). Generally, the C r / ( C r + A I ) ratio ranges between 0.47 and 0.83, while the Mg/(Mg + Fe 2+ ) ratio ranges from 0.42 and 0.73. Two distinctive populations are seen, the metallurgical (high-Cr) and refractory (highAI) types, each of which shows a relatively restricted variation (Fig. 3). Ti contents in chromitites with high C r / ( C r + A I ) ratios (average 0.80) are low (<0.16 wt% TiO2), whereas ehromitites from the areas of Dramala, Korydallos and Vourbiani with low Cr/ (Cr+A1) ratios (average 0.50) have higher Ti contents (up to 0.47 wt% TiO2). Ferric iron is also higher in the high-A1 chromitites and in the Dramala high-Cr chromitites.The chromititcsfrom the area of Milia are
M. Economou-Eliopoulos, L Vacondios / Chemical Geology 122 (1995) 99-108
103
Table 3 Platinum-group element data of chromite ores from the Pindos ophiolite complex Location
Sample No.
Rock type
Cr Cr + AI
(ppb) Os
Ir
Ru
Rh
Pt
Pd
(ppm)
Pd/Ir
Au
S
1.5 7 3 5
12 8 8 8
26 52
135 105
0.82
60 3,250 60 2,500 70 70 1,350
35
110
0.38 1.16 1.00 1.25
14 1
0.80
40 2,300 60 1,450
20 105 67 290
2.00 0.22
0.80
40 4,200 50 2,480 40 1,340
20 115 18 110 34 285
0.30 4.50 0.24
0.48
50 2,920 12 160 16.70 40 1,630 310 190 30.00 50 40.00 70 1,750 120 260 1.00
0.82
40 2,160 70 3,640 60 2,000
5 140 45.00 24 156 3.50 30 750 0.02
0.80
60 3,400 60 2,400 80 2,080
10 125 0.33 22 105 12.00 34 650 0.03
0.52
40
50 285
Ni
Cu
Co
Dramala:
Dunite (ore hosting) Harzburgite Pyroxenite Chromitite (massive)
NIB NIC NID NIA
<5 <5 <5 26
4 6 3 4
<4 <4 6 51
1.5 2 4 2
2 6 26 6
DIB DIA
<5 30
4 18
4 50
3 3
9 7
8 4
<5 <5 90
5 2 34
<4 <4 120
2 2 6
3 8 3
1.5 1 9 18 8 12
K3C K3Z K3D K3B
<5 <5 <5 40
0.3 1 <0.3 20
4 <4 <4 33
1 5 3 5
3 5 20 30 8 12 25 20
12 12 10 11
M3 M4 M1
<5 0.4 <5 2 150 320
<4 <4 550
3 2 82
10 18 10 7 150 7
13 3 1
X3 X4 X1
<5 <5 40
3 1 51
<4 <4 190
1 2 11
V1
8
5
15
2
Kambos Despoti:
Harzburgite Chromitite (massive) T~. gona:
Dunite (ore hosting) TIOB Harzburgite TIOC Chromitite (schlieren) TIOA Ko~. dallos:
Dunite (ore hosting) Dunite (barren) Gabbros Chromitite (massive) Milia:
Dunite (ore hosting) Harzburgite Chromitite (schlieren) Kyra Kali:
Dunite (ore hosting) Harzburgite Chromitite (schlieren)
2 1 1 10 12 9 1 1.5 4
Vourbiani:
Chromitite (massive)
characterized by higher total iron, although ferric iron is relatively low (Table 2). Ni, Cu and Co concentrations are similar in chromitites from the areas of Dramala, Kampos Despoti and Trygona, while the concentrations o f these elements are higher in chromitites from Korydallos, Milia, Kyra Kali and Vourbiani (Table 3). The P GE concenlxations for chromitites show a wide variation, ranging between 8 and 150 ppb Os, 4 and 320 ppb Ir, 15 and 550 ppb Ru, 2 and 82 ppb Rh, 3 and
3
10
4
1,600
2.00
150 ppb Pt, and 1.5 and 20 ppb Pd. The m i n i m u m P G E content found in chromitites from the Vourbiani area and the m ax i m u m in those from Milia (Table 3). P G E content is lower in high-A1 chromitites (Vourbiani and Korydallos) than in high-Cr ones. However, the highCr chromitites from Dramala are PGE poor too. The P d / I r ratio ranges from 0.02 to 2.00, and the highest values are observed in both high-A1 and high-Cr chromitites (Table 3).
104
M. Economou-Eliopoulos, 1. Vacondios / Chemical Geology 122 (1995) 99-108
The sulphur contents in the chromitites range between 40 and 80 ppm and there appears to be no correlation between PGE and S contents. It is noticeable that the Milia chromitites with the highest PGE content (1260 ppb) also have low S contents, but, at the same time, show the highest PGE/S ratios and an increase of Pt over Pd (Table 3). All chromitites from the Pindos complex show an enrichment in Os, Ir and Ru compared to Pt and Pd, resulting in chondrite-normalized PGE patterns with a negative slope (Fig. 4). They are similar to published data from other ophiolites (Page et al., 1982, 1984; Economou, 1986; Konstantopoulou and EconomouEliopoulos, 1991). Harzburgite which is thought to be a residual mantle peridotite, is characterized by homogeneous PGE and less chalcophile element (Ni, Co and Cu) contents throughout the Pindos complex. In contrast, some
o.1! /
K(~mbos
=o
'\,\
o
\\
~// /
\\
/1'~
'\\,~jll, '~ V
.:.2 ao,
g 5 100
/ \
\
/
/
\X / 90 •)oro
8c
ds
4
~ x~''
°1
5C
# N
",
~0
~u
N
o
Trygona
• Nilia N Kor ydallos
A
Karnpos Despoti
• Vourbiani
x
Kyra
• Kalambaka
•X- D r a m a l a
30
R'.
dunite and chromitite bodies show significant variability in concentrations of the above elements and relatively high Pd/Ir ratios, especially at the area of KorydaUos (Fig. 5), where the presence of gabbros is common (Table 3).
6C
40
~'o
Fig. 4. Chondrite(C2) -normalizedPGE patternsof high-Crchromititesfromthe Pindosophiolitecomplex.
7¢
x ÷ ¢. ¢. u u
,'r
100
Kali ~ ,
,
i 50
I
Mg = 100 M g + Fez *
Fig. 3. Plotof Cr/(Cr + AI) vs. Mg/(Mg + Fez+) ratioforchromite ores from the Pindos ophiolite complex. Data from Table 2 and Paraskevopoulosand Economou(1986) and Migiroset al. (1991).
6. Discussion Assuming that chromitites with dunite lenses within the tectonite harzburgite of ophiolite complexes, have formed by crystallization from ascending magmas at palaeo-spreading center (Brown, 1979 ), then a number of different factors can account for the compositional variation of the chromite ores. The documented presence of laurite and Os--Ir-Ru alloys as inclusions in chromite grains, the lack of any correlation between
M. Economou-Eliopoulos, L Vacondios / Chemical Geology 122 (1995) 99-108
"7
.E
o"""
O.C
E o .c u
Fig. 5. Chondrite (C2) -normalized PGE patterns of high-Al chromitites from the Pindos ophioUte complex.
PGE content and degree of serpentinization and Os isotopic data on pliatinum-group metal inclusions in chromite (Aug6, 1985; Prichard et al., 1986; Talkington and Watkinson,, 1986; Bacuta et al., 1990; Hattori et al., 1992; Tarkian et al., 1992) suggest that the composition of parent magma is among the most important factors. This may Ix,'a function of the degree of melting of the mantle source or subsequent modification due to fractional crystallization. Variations of physico-chemical conditions of crystallization (foe, P, T) are also considered to influe,nce the composition of chromitites (Tindle and Pearce, 1983). The high-Al chromitites from the areas of Vourbiani, Korydallos and Dr~anala, which are characterized by a relatively high Mg'O, TiO2 and Fe203 contents, may have been produced by a combination of increasing foe and decreasing T, or from a parental magma that was rich in A1 and Ti. Sometimes it is difficult to distinguish between the degree of partial melting and fractional crystallization. Due to the incompatible behavior ofAl, Ti, Pt and Pd, in contrast to the compatible nature of Cr, Os, Ir and Ru (Tindle and Pearce, 1983; Barnes et al., 1985, 1988:1 Cocherie et al., 1989), high-Cr chromitites could be interpreted as resulting from a
105
higher degree of partial melting in the upper mantle compared to high-A1 chromitites (Jaques and Green, 1980; Dick and Bullen, 1984: Economou, 1986; Bacuta et al., 1990; Economou-Eliopoulos and Vacondios, 1990). Also, since the compatible elements are more strongly partitioned into initial melts they are depleted in relatively evolved magma (Brown, 1987; Economou-Eliopoulos and Zhelyaskova-Panayotova, 1989; Bacuta et al., 1990; Konstantopoulou and EconomouEliopoulos, 1991). Therefore high-Al chromitites in a spatial association with high-Cr chromitites may suggest generation of the former from more evolved parent magmas. Alternatively, their composition may be related to a lower degree of partial melting of the mantle source, and they represent juxtaposed slices of different ophiolitic fragments during the evolution of a marginal basin. The relatively high Pd/Ir ratios and smooth-shaped PGE patterns of high-Al chromitites from the Pindos ophiolite complex, reflecting a relatively high degree of fractionation (Barnes et al., 1985, 1988), suggest that parent magma of high-Al chromitites was more evolved than that of high-Cr ones. Furthermore, the presence of high-Cr chromitites from the area of Dramala with Pd/Ir ratios as high as 1.25, in combination with of the presence of exclusively high-Al chromitites with a very low TiO2 and Fe203 content and Pd/Ir ratios, within defined ophiolite complexes like the Othrys ophiolite complex (Economou-Eliopoulos, 1993), suggests that these chromitites were derived from separate parent magmas. Oxygen fugacity and sulphur saturation are also important factors controlling the PGE content in chromitites (Buchanan, 1988; Amosse et al., 1990). Naldrett et al. (1990) concluded that once plagioclase has appeared on the liquidus of the resident magma, a fresh input can result in a mixture forming either a sulphide-bearing and PGE-enriched chromitite or, if the magma mixture is not particularly enriched in Cr, a PGE-rich sulphide layer such as the Merensky Reef (Bushveld complex, South Africa). The PGE content in the analyzed chromitites and dunites from the Pindos complex, in particular from the area of Korydallos, exhibit a fractionation trend, as it is shown by the Pd/Ir ratios. At Milia, the PGEenriched chromitites exhibit a preference of Pt over Pd, and a high PGE/S ratio (Table 3) may reflect a relatively high R factor (silicate/sulphide) (von Griine-
106
M. Economou-Eliopoulos, 1. Vacondios / Chemical Geology 122 (1995) 99-108
waldt et al., 1990). Moreover, a process of mixing with a more evolved magma is also suggested by the association of the Milia chromitites with a dunite body showing a relatively high Pd/Ir ratio (45) (Table 3). The compositional variation characterizing the Pindos chromitites and host dunites can be interpreted assuming that chromitites in ophiolite complexes were formed from magmas derived by varying degree of melting over a large region of the upper mantle, during the evolution of a marginal basin (Pearce et al., 1984; Ribe, 1988; Nicolas, 1989). Thus, the presence of both high-Cr and high-A1 chromitites and their geochemical features in the Pindos complex are consistent with the variability in the degree of depletion in an SSZ environment and the presence of a wide spectrum of lavas (Fig. 2; Capedri et al., 1980), suggesting more than one mantle source and eruptive event. In addition, the wide variation in the content of chalcophile elements in chromitites with a similar major-element composition in the Pindos complex, in contrast to that in the Vourinos complex which is the main source of chromite in Greece, suggests that the studied chromitites and host dunites do belong neither to deep mantle chromitites nor to the dunites of the cumulate sequence which hosts PGE-enriched chromitites (Ohnenstetter et al., 1991 ). Thus, the petrological data and the present geochemical data on chromitites and host dunites of the Pindos complex provide evidence for a formation mainly at higher stratigraphic levels of the mantle sequence.
7. C o n c l u s i o n s
Although, further detailed geochemical analysis is required, the present data lead to the following conclusions: (1) On the basis of chalcophile element (PGE, Ni, Cu and Co) concentrations in chromitites with a similar major-element composition, these can be divided into those derived from primitive magmas and those derived from partially fractionated magmas. (2) The relatively high Pd/Ir ratios of some chromitite occurrences, reflecting a relatively extensive fractionation trend, in combination with the high variability of the PGE distribution in chromitites throughout the Pindos complex (in contrast to the Vourinos complex), suggest that they were derived by mixing of
relatively small influxes of magmas, at higher stratigraphic levels. (3) The spatial association of high-Cr and high-A1 chromitites, coupled with the petrological and geochemical features of the ophiolitic rocks and chromitites, suggest more than one mantle source and eruptive event during the formation of the Pindos complex. (4) The Pindos ophiolite complex is not considered to contain a significant potential for chromite, although its formation is related with a supra-subduction environment.
Acknowledgements
The authorities of the Economic European Community (contract MAIN 0068-GR T r ) are thanked for financial support for this work. Mr. J. Mitsis, Athens University, is also thanked for his assistance with atomic absorption analysis, and Dr. G. Economou, of IGME for performing electron microprobe analyses. Finally, many thanks are expressed to the reviewers of this journal for their constructive criticism and suggestions.
References
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