Metallogenesis at paleo-spreading centers: Lead isotopes in sulfides, rocks and sediments from the Troodos ophiolite (Cyprus)

Chemical Geology, 68 (1988) 229-238 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

229

[3]

METALLOGENESIS AT PALEO-SPREADING CENTERS: LEAD ISOTOPES IN SULFIDES, ROCKS AND SEDIMENTS FROM TH'E TROODOS OPHIOLITE (CYPRUS)* 1 BRUNO HAMELIN .2, BERNARD DUPRE, OLIVIER BRI~VART *~ and CLAUDE J. ALLt~GRE Laboratoire de Gdochimie et Cosmochimie, I.P.G., F-75230 Paris Cddex 05 (France) ( Revised and accepted for publication January 15, 1988 )

Abstract Hamelin, B., Duprd, B., Brdvart, 0. and All~gre, C.J., 1988. Metallogenesis at paleo-spreading centers: Lead isotopes in sulfides, rocks and sediments from the Troodos ophiolite (Cyprus). Chem. Geol., 68: 229-238. New Pb isotopic data for metalliferous deposits and pelagic sediments from the Troodos ophiolite complex (Cyprus) are presented. We use these data to re-evaluate the genetic models for these deposits in the light of recent results obtained for different types of hydrothermal sites presently active on spreading ridges. In contrast to previous conclusions of earlier studies, we find that the Troodos hydrothermal deposits (sulfides as well as umbers and Mn crusts) have isotopic compositions very similar to that of most of the rocks from this ophiolite complex: they display a 2°Tpb/2°4pb ratio higher than present-day typical ridges. This result has been interpreted as evidence for a subduction-related origin of the Troodos ophiolite. This different geodynamic setting of the ophiolite is also the reason for the isotopic difference between Troodos and Semail ( Oman ) sulfides, the latter being typical of a normal oceanic crust. When examined in more detail, the Troodos sulfide results are very homogeneous within one site, but show significant differences from one site to another. At least three different isotopic end-members are required to interpret these variations. One of these end-members is found in the pelagic sediments data analyzed here, which have a ~°Tpbff°4pb typical of continental crust signature, higher than that of the rocks and sulfides. On present ridges, the involvement of such a component has been similarly described for the Guaymas Basin and for the Gorda Rise deposits. However, some of the isotopic variations observed between sulfides cannot be interpreted only by this end-member. One possibility is that a second sedimentary component, as yet unidentified, has been involved in the hydrothermal circulation, as recently demonstrated for Red Sea brines. Finally, the determination of the exact proportion of Pb leached out of the ophiolite basaltic crust itself would require a more complete knowledge of the initial isotopic heterogeneities within the massif. Indeed, several rock results are significantly less radiogenic than those of the sulfides, which however show themselves initial compositions. This could result either from a sediment contribution to the sulfide Pb, 6r from a biased sampling of the massif heterogeneities. This question remains open in the present state of the data.

*~I.P.G.P. Contribution No. 991. Present addresses: *~Lamont-Doherty Geological Observatory, Palisades, NY 10964, U.S.A. *:~Elf-Aquitaine,Centre Micouleau, av. Prst. Angot, F-64018 Pau Cddex, France.

0009-2541/88/$03.50

1. Introduction Since their discovery on the East Pacific Rise (EPR) (CYAMEX, Scientific Team, 1978-1979; Rise Project Group, 1980), the ac-

© 1988 Elsevier Science Publishers B.V.

230 tive hydrothermal sites at ridge crest have been extensively studied. They indeed constitute invaluable natural laboratories to study the hightemperature interaction between basalts and percolating seawater. These studies have also important metallogenic implications, since hydrothermal processes concentrate numerous metals in the sulfide deposits which precipitate at the vent exits. Huge metalliferous deposits are also well known within ophiolite massifs, where they are often exploited for mining. These are considered as ancient counterparts of the present-day active sites. Therefore, comparison between the present submarine hydrotherreal activity and the past ophiolite deposits exposed on continents allow for a better understanding of the genesis of a metalliferous ore deposit. Among the metals strongly concentrated by hydrothermal processes, Pb is particularly interesting because its sources may be identified isotopically. Studies at 21 °N on the EPR (Br~vart et al., 1981; Vidal and Clauer, 1981 ) showed that the isotopic compositions of Pb in sulfides were identical to that of the basalts of the oceanic crust at that location. This was the first demonstration that Pb in the hydrothermal fluids came from the basaltic crust, and was precipitated as sulfides when these fluids were exiting out of the crust. In contrast, Pb isotopic composition of the Mn crusts sampled in the same area show a continental signature (Br~vart et al., 1981 ), as do Pb in Mn nodules ( Chow and Patterson, 1959). Recent studies, including results on both hydrothermal fluids (Chen et al., 1986; Chen, 1987; Hinkley and Tatsumoto, 1987) and sulfides ( Duprg et al., 1987; LeHuray et al., 1988), have shown that different cases can be found, depending on the type of ridge studied. Several sites on the Juan de Fuca and Gal~ipagos ridges have been shown to contain Pb derived from basalts, as at 21 ° N on EPR. However, on ridges covered by sediments, either because of weak

activity ( Gorda Rise), or because of proximity of continental masses (Guaymas Basin, Red Sea), a large proportion of Pb in the sulfides is also leached out of the sediments. In the case of sulfides associated with Red Sea brines, the sediment contribution is dominant, and moreover two different sedimentary end-members can be identified. The first component is related to recent biogenic and detrital sediments, whereas the other comes from Miocene evaporites in contact with the basalts (Dupr~ et al., 1988b). These studies of a variety of ridge crest sulfides illustrate the sensitivity of the Pb isotope tracing method if data are simultaneously available in the same area for sulfide deposits, basaltic rocks and various associated sediment types. We apply the same method in this paper to the Troodos ophiolite (Cyprus). The origin of this Upper Cretaceous section of oceanic crust has been debated. Most workers now agree that it did not form at a typical spreading center similar to those of the present-day large oceans, but rather in a subduction-related marginal sea, a back-arc or a fore-arc basin environment (A1l~gre et al., 1973; Miyashiro, 1973; Hamelin et al., 1984b, and references therein; Rautenschlein et al., 1985). The characteristics found for the hydrothermal products must be discussed in light of this geodynamic question. The results for sediments, sulfides and Mn crust reported here complement data obtained previously for rocks from Troodos in the course of a general Pb isotope survey of various ophiolites over the world (Hamelin et al., 1984b). We have also compiled other data available in the literature, both for rocks (Spooner and Gale, 1982; Rautenschlein et al., 1985) and for metalliferous deposits (Doe and Zartman, 1979; Gale et al., 1981).

2. Geologic setting The Troodos deposits have been extensively

231

described in relation to the historically important Cyprus Cu-pyrite ores (Constantinou and Govett, 1973). The massive sulfides and the stockworks which represent the feeding conduits for the deposits lie within or just above the main sequence of pillow lava, often referred to as "lower" pillow lava ( Fig. 1 ). The sulfides are overlain by Mn-poor, Fe-rich sediments, the "ochre" group, which are alteration products of the sulfides. Most of the ore bodies are capped by the "upper" pillow lava sequence, which have often been considered as later products of a more differentiated magma (Pearce, 1975; Parrot, 1977; Sun and Nesbitt, 1978), and the geodynamic significance (Gass and Smewing, 1973 ) of which is still debated (Robinson et al., 1983). The "umber" sequences (Robertson and Hudson, 1973; Robertson, 1975) are found interstratified with, or overlying the upper pillow lavas. The ochres and umbers are metalliferous sediments similar to those found on modern spreading ridges. The volcanic pile is overlain by abyssal clays at the base of a thick carbonate sediment pile. Sheeted dyke samples have been dated at 79 Ma by K-Ar (Desmet et al., 1978). K-Ar and Rb-Sr dating of celadonite veins also gave ages between 92 and 85 Ma (Staudigel et al., 1986). We analyzed pyrite, chalcopyrite and sphal-

erite from the Mathiati, Kambia, Peristerka (New Kambia), Pitharokoma and Limni mines (Fig. 1 ). Two different samples have been analyzed from each site. Constantinou and Govett (1973) proposed that these sulfides are secondary products resulting from dissolution and later reprecipitation of primary deposits. However, more recent work (Guillemot, 1979) has claimed that the sulfides are primary, and that vent-chimney fragments are well preserved. The deposits are now believed to have formed before the emplacement of the upper pillow, and may have suffered only slight secondary leaching, possibly yielding the heavy-metal enrichment of the umbers. We analyzed two umber samples from Aredhiou, complementing the extensive study of these metalliferous sediments by Gale et al. (1981). We also report measurements of two Mn crusts capping basalt samples from Aredhiou and Margi. Mn crusts are a third type of metal-rich deposit commonly found associated with ridge tholeiites, and result from still another precipitation process that of sulfides and umbers. Pb isotopic data for similar modern Mn crusts have been published by Br~vart et al. (1981) and Boul~gue et al. (1984). Samples from the different units of the ophiolite sequence have been analyzed, includ-

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Fig. 1. Location map of Troodos samples.

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232 TABLE l Lead isotopic ratios and lead contents of pelagic sediments and metalliferous deposits from the Troodos ophiolite complex Sample

z°6pb/2°4pb

2°7pb/zO4pb

2°sPbff°4pb

Pb (ppm)

18.21 17.95 18.40 18.61 18.27 18.23 18.82 18.73 23.52

15.56 15.57 15.58 15.57 15.59 15.58 15.60 15.57 15.86

38.05 37.81 38.18 38.44 38.17 38.18 38.43 38.33 43.21

0.013 0.018 -

18.65 18.67 18.59 18.55 18.57 18.58 18.56 18.59 18.52 18.53

15.59 15.61 15.58 15.55 15.57 15.58 15.56 15.58 15.60 15.61

38.63 38.68 38.57 38.48 38.52 38.56 38.49 38.57 38.49 38.51

57 -

18.55 18,55

15.57 15.55

38.50 38.42

133 84

18.72 18.74

15.57 15.55

38.42 38.33

2.6

19.06 18.82 18.92

15.65 15.67 15.69

38.61 38.87 39.03

5.1 4 5

Rocks: 7022907, ultramafic 7161003, ultramafic CY4, gabbro CHY 59, gabbro CH 62, whole-rock gabbro CH 62, plagioclase K 7, gabbro 7160304, sheeted dyke 7160303, sheeted dyke

0.229 0.726

Sulfides: Mathiati Peristerka Kambia I Kambia 3 Pitharokoma Limni

a b a b a b a a b

-

14

Metalliferous sediments. C9-11, Umber Aredhiou C9-12, Umber Aredhiou

MnO~ crust." C9-15, Aredhiou C9-25, Margi Other Sediments: C9-13, detritic, Aredhiou C9-24, pink shale, Margi CHY 50, green shale

ing the upper and lower pillow lava, sheeted dykes, gabbros and ultramafic cumulates. Raut e n s c h l e i n e t al. ( 1 9 8 5 ) a n a l y z e d w e l l - p r e served basaltic glasses. In order to identify possible sources of Pb in the hydrothermal deposits, we analyzed three sediment samples. Two are shales from Margi

a n d K a m b i a , t h e t h i r d is a d e t r i t a l l a y e r i n terbedded in one of the umbers from Aredhiou. The analytical technique used has been rep o r t e d p r e v i o u s l y ( B r ~ v a r t e t al., 1 9 8 1 ) . T h e r e s u l t s a r e g i v e n i n T a b l e I. T h e r o c k r e s u l t s o f H a m e l i n e t al. ( 1 9 8 4 b ) a r e a l s o r e p o r t e d f o r comparison.

233

3. R e s u l t s 15.64

3.1. Ophiolite rocks As emphasized above, the isotopic study of the genesis of metalliferous deposits requires a precise knowledge of the composition of the rocks in which the deposits are found. This is especially important for ophiolites because these massifs often have, in addition to heterogeneities known for variations related to "atypical" original settings, such as marginal seas, back-arc basins or early stages of rifting (Hamelin et al., 1984b). The identification of initial isotopic signature in ophiolite sequences, requires age correction. In all cases studied so far [Semail (Chen and Pallister, 1981 ), Tibetan ophiolites ( GSpel et al., 1984) and the different massifs studied by Hamelin et al. (1984b)], measured intramassif Pb isotopic variations show the influence of U and Th decay since the formation of the massifs. Precise determination of initial compositions, and therefore also of the initial intra-massif heterogeneities, can be severely hampered by perturbations of the U - T h - P b system by alteration. Hamelin et al. (1984b) and Rautenschlein et al. (1985) agree that the Troodos massif has an isotopic composition different from that of the present-day normal ridges and from their ancient analogs such as the Semail ophiolite. The main characteristic of the Troodos rocks is their high 2°7pb/2°4pb ratio relative to that of typical ridge tholeiites. This conclusion is independent of any age correction, since the 2°TpbF°4Pb ratio (in contrast to 2°6pb/2°4pb and 2°8pb/2°4pb) is virtually insensitive to decay correction for the elapse of hundred million years since ophiolite formation. The high 2°Tpb/2°4pb has been interpreted as reflecting the involvement of continental components in the Troodos magma genesis and therefore to an origin relating to a subduction zone for this massif, which is in good agreement with other geochemical evidence (Miyashiro, 1973; Richard and A1-

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19.0

206Pb/204Pb

Fig. 2.2°Tpb/2°+Pbvs. e°6pbF°4Pbdiagramfor the Troodos hydrothermal sulfides ([] =this work;B= Spooner and Gale, 1982 ).

l~gre, 1980; McCulloch and Cameron, 1983; Rautenschlein et al., 1985). However, the exact range of initial heterogeneity of the Troodos ophiolite is still poorly constrained. The range of variation measured by Hamelin et al. (1984b) for cumulate samples with low U / P b ratios is 17.95 <2°6pbF°4Pb<18.61. However, one initial 2°6pbF°4pb for a dolerite sample is much more radiogenic: 2°6pbF°4Pb -~22.1. Moreover, Spooner and Gale (1982) reported much more scattered results, some of them with very unradiogenic compositions as low as e°6Pb/2°4Pb---16.62. U contents of the glasses studied by Rautenschlein et al. (1985) are not available for decay correction.

3.2. Sulfide results Although much less scattered than the rock results, the sulfides contain significantly different Pb isotopic variations: 18.40 <2°6pb/2°4Pb<18.89. The compositions of samples from a single mine show an excellent internal consistency, with good inter-laboratory agreement between our data and those of Gale et al. (1981). Data from these two studies allow a clear isotopic fingerprinting of t h e Troodos deposits (Fig. 2). The Mathiati and Limni deposits have higher 2°7pb/2°4pb than the others (mean values of 15.60 vs. 15.58). How-

234 SEDIMENTS~

15.65

n a. o 15.80 ~ Q. oo4 15.55 "---UMBERS ,

18.2

I

I

18.4

Mn02 I

18.6

,

I

18.8

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19.0

,

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206Pb/204Pb

Fig. 3. Comparison between sulfides, umbers and sediments. Data fromthis work,and Gale et al. (1981). ever, these two mines differ from each other in 2°~Pb/2°4pb (Mathiati, 18.66; Limni, 18.53). Among the mines with lower 2°Tpb/2°4pb, there are also some 2°6pb/2°4pb distinctions. Especially, the samples from Moussoulos (southeast part of the massif) analyzed by Gale et al. (1981) have distinctly more radiogenic '~°6pb/2°4pb (18.89). As a whole the data define a triangular array, suggesting that at least three end-members contributed Pb to Troodos sultides. The same structure is observed in the 2°Spb/2°4pb vs. 2°6pbff°npb diagram (not shown).

3.3. Umber results Metalliferous sediments have a narrower range of variation than sulfides: 18.42 < 2°6Pb/2°4pb < 18.63. In contrast to the sulfides, umbers do not have any clear geographical variation (Fig. 3 ) .The umbers sampled close to the Mathiati mine (Kataliondas) and from Limni (Kinousa) do not have the same high 2°Tpbff°4pb as sulfides from these mines, nor do the samples from Drapia, close to Moussoulos, have the high 2°~Pb/2°4pb of the sulfides. Finally, our results confirm the conclusion of Gale et al. (1981) of high Pb content in umbers (84 and 133 ppm) relative to stockwork sulfides.

3.4. Mn crusts The two Mn crusts that we analyzed have a similar isotopic composition, although they

come from different sites. Their 2°Tpb/2°4pb is identical to that of umbers and of most of the sulfides. Their 2°6pb/2°tpb (18.73) is slightly more radiogenic than that of the other hydrothermal products, except for the sulfides from Moussoulos (Fig. 3). Analyses of modern analogs have shown that Pb isotopes in Mn crusts vary according to geologic setting. Mn crusts sampled at 21 ° N on the EPR have 2°Tpbff°4pb compositions that reflect mixing with continental-derived Pb (Brdvart et al., 1981 ). However, samples from 7 ° N (Boul~gue et al., 1984) have compositions close to that of neighbouring basalts. The Troodos samples apparently belong to the latter type. Indeed, the 2°6Pbff°4Pb difference with the urnbers (A2°~Pbff°4pb=0.18) can be interpreted as due to radioactive decay since the deposition of the crusts, if their U content is 0.5 ppm. This value is in the range found by Boul~gue et al. (1984).

3.5. Comparison between results of rocks and hydrothermal deposits Despite the internal differences described just above, the Troodos hydrothermal products nonetheless cluster in a restricted range of isotopic compositions. This range is distinguished by 2°Tpb/2°4Pb values higher than those of present-day mid-ocean ridge basalts (MORB) (Fig. 4). The isotopic compositions of Troodos metalliferous deposits plot "above" the MORB trend on a 2°Tpbff°4pb vs. 2°6pbff°4Pb diagram, shifted toward the continental crust values. On the basis of this result, Gale et al. (1981) suggested that a large proportion of "seawater Pb" was involved in the genesis of the Troodos deposits. However, more recent data have shown that this high 2°Tpb/2°4pb is in fact also a characteristic of most Troodos rocks themselves. Indeed, only sulfides from Limni and Mathiati have 2°TPb/2°4pb (slightly) higher than those measured in the rocks (Fig. 4). As discussed below, this does not rule out that a continental Pb component might be present in some sul-

235

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SEDIMENTS" - - ~ f ~ 15.7 -IYDROTHERMAL ~ ~ OPHl: EPOSITS "-~,~')~.~ 15.6 L ~

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15.5 17.5

18.5

19.5

206Pb/204Pb

Fig. 4. Comparison between hydrothermal deposits and rocks from the Troodos ophiolite. Data for rocks: Hamelin et al. (1984b); Rautenschein et al. (1985). Referencefields for present ridges and for abyssal sediments are also shown. Isotopic variations between sulfides may be due either to oceanic crust heterogeneities, or to additions of Pb from sediments (I), or from another unidentified radiogenic source (2). See text for explanations. tides. On the other hand, this result puts stronger constraints on the evaluation of the mixing proportions of Pb from different sources in the deposits. We suggest that the similarity between the 2°TPb/2°4pb in the rocks and sulfides must be considered chiefly as an indepen-

dent confirmation for a different geodynamic environment than that by M O R B for the origin of Troodos and Semail ophiolites, as proposed from the rock results by Hamelin et al. (1984b). 4. D i s c u s s i o n

Studies of sulfides precipitated at presently active hydrothermal fields on active ridges have demonstrated that hydrothermal circulation averages Pb isotopic compositions of the basalts through which the fluids percolate (Brdvart et al., 1981; Vidal and Clauer, 1981). This is confirmed here by the isotopic homogeneity observed within each site of the Troodos massif. However, the differences observed between different sites of this ophiolite constitute a new result, since similar systematic comparison of various vents on a scale as large as the Troodos massif (80 × 30 km) has not been carried out so far on submarine sampling. The fact that these differences are observed for the sulfides

but not for the umbers is again in good agreement with a rapid precipitation of sulfides at the vent exits, whereas metalliferous sediments result from dispersion and re-deposition of hydrothermal products on a larger scale, thus allowing for further mixing of Pb from various vents. The difference between deposits can be interpreted either: (1) as reflecting isotopic heterogeneities within the basalts themselves, or (2) resulting from variations in the mixing proportions of Pb from various sources from one vent to another. In any case, these vent-to-vent differences indicate the scale at which "disconnected" hydrothermal cells operate, demonstrated by lack of Pb isotopic homogenization between deposits.

4. I. Sulfides with high 2°Tpb/2°4pb At modern spreading centers, sulfides with 2°Tpb/2°4Pb values higher than closely related basalts have been reported from the Guaymas Basin and Gorda Rise (Chen et al., 1986; L e H u r a y et al., 1988), and the Red Sea (Duprd et al., 1988). In each case, they have been interpreted as indicating a Pb component coming from sediments overlying the oceanic crust. This Pb component has been demonstrated to come from the sediments and not just from Pb dissolved in seawater, since direct measurements in the fluids (Chen et al., 1986; Chen, 1987) have shown Pb enrichment well above the low Pb contents of "uncontaminated" seawater. This interpretation of mixing with Pb from sediments is also valid in the case of Troodos, at least for the two mines Limni and Mathiati which have 2°Tpb/2°4pb ratios higher than both the other sulfides and host rocks. Measurements of Troodos sediments (Table I ) show that they constitute appropriate end-members for this mixing (Fig. 2). The Pb isotope composition of the sediments is similar to those measured in present-day Atlantic sediments (Sun, 1980), and to those analyzed by Barrett (1980) from sediments above Apennine

236 ophiolites in Italy. When corrected for U decay, the Troodos sediments plot very close to the radiogenic end-member of the sulfide trend (Fig. 3). However, a more extensive study would be required to document the isotopic variability in these sediments. Especially, the 2°6pbff°4pb distinction between Limni and Mathiati samples can result either from heterogeneities in the basaltic crust end-member, or from different compositions of the sediments corresponding to the recharge area of each mine. Solving this question would require detailed study of both basalts and sediments around these sites. 4.2. 2°6pbf°4Pb variations: what was (were) the Troodos initial composition(s) ? In contrast to the 2°Tpb/2°4pb variability, which calls for the straightforward sediment hypothesis, the 2°6pbff°4pb variations raise more difficult questions, which are yet not definitively solved. We must explain: (1) the 2°6Pb/2°4Pb differences observed between sulfide deposits; and (2) why sulfides bear isotopic compositions significantly more radiogenic than for gabbros and cumulates measured by Spooner and Gale (1982) and Hamelin et al. (1984b). This result is in marked contrast to the case of Semail (Chen and Pallister, 1981 ), where both sulfides and low-U/Pb samples have similar isotopic compositions, simultaneously giving the initial composition of the Oman ophiolite. We can first rule out the hypothesis of differences due to radiogenic ingrowth: for instance, U contents in the two Moussoulos samples analyzed by Spooner and Gale (1982) should be 1.4 and 2.2 ppm to account for their more radiogenic 2°6pb/2°4pb than Kambia. These contents are too high, compared to the range expected in this type of sample. Only very few direct U and Pb concentration measurements have been made in volcanogenic sulfides (LeHuray, 1984, 1987 ). The results in all cases indicate very low/1 (23sU/2°4pb) ratios. Moreover, analyses of ridge hydrothermal fluids

(Michard et al., 1983; Michard and Albar~de, 1985; Chen et al., 1986; Chen 1987) have also demonstrated strong U - P b fractionation during hydrothermal circulation. The exiting fluids are indeed strongly depleted in U and enriched in Pb compared to seawater. Finally, within-site Pb isotope homogeneity of the Troodos deposits, regardless of their Pb concentration, gives a strong argument suggesting that radiogenic ingrowth is of negligible effect in the isotopic differences between sites. We therefore assume that sulfide Pb isotopic compositions represent initial values, frozen since the formation of these deposits. Two different hypotheses can then explain the data: (1) either the sulfides are effectively representative of an heterogeneous initial isotopic composition of the oceanic crust, or (2) the 2°~pb/e°4pb difference between deposits, and between the sulfides and the least radiogenic rocks, is again due to addition of a more radiogenic Pb component mixed with Pb coming from basalts (Fig. 4). (1) The hypothesis of sulfide compositions representing original basalt heterogeneities implies that the Troodos crust was more heterogeneous than other examples of oceanic crust sections, such as the Semail ophiolite (Chen and Pallister, 1981 ) or FAMOUS (Dupr~ et al., 1981 ) area on the Mid Atlantic Ridge. This may be compatible with an "atypical" geodynamic site of origin for the Troodos ophiolite. As an argument favouring geographic variations within the massif, the most extreme values of Moussoulos are found in the southeast part of the massif, separated from other deposits by the Arapakas fault, interpreted as a paleo-transform fault (Simonian and Gass, 1978). On modern ridges, some transform faults are associated with large discontinuities in isotopic compositions of the ridge basalts, such as the Kane FZ (Machado et al., 1982) and Hayes FZ (Hamelin et al., 1984a) on the Atlantic Ridge. As for the difference between sulfides and rocks, one can argue that the relatively Pb-rich Cu-pyrite deposits are a more reliable record of

237

the initial crust composition than the very'Pbpoor gabbros and cumulates. The difference between sulfide and rock Pb may then be explained by postulating that the rocks have been contaminated by late exchanges with unradiogenic continental crust Pb. This explanation is especially appealing for the unusually unradiogenic values measured in several gabbros by Spooner and Gale (1982). (2) The second viable hypothesis is that 2°6pb/2°4pb variability has been induced by mixing basaltic Pb with a more radiogenic Pb component. This is presently our preferred model, because of the close analogy with recent data from the Red Sea sulfide deposits (Duprd et al., 1988). We propose that Pb leached from the oceanic crust, with an initial composition of 18.1 (as recorded in the low-z gabbros), has been mixed with Pb from a source with same 2°TPbff°4pb but higher 2°6pb/2°4pb than the basalts (Fig. 4). This source, as yet unidentified, must be different from the abyssal sediments, the 2°Tpb/2°npb of which is too high to constitute an appropriate end-member. A similar case is found in the Red Sea brines deposits, for which Miocene evaporites and abyssal sediments constitute two distinct sources of Pb in sulfides, the isotopic composition of which is consequently very different from that of the underlying basalts. The resulting isotopic pattern is therefore similar to that of Troodos. However, a further check of this model will require extended isotopic investigation of Cyprus sediments to identify the missing Pb "contaminant".

Acknowledgements We thank Z. Johan, J.F. Parrot and E. Soler who provided the samples for this study. We also thank M. Tatsumoto for his comments on an early version of this paper, and A. LeHuray for providing ridge sulfide data before publication and for very useful comments on the manuscript. This work has been supported by CNRS ATP "Metallogdnie".

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