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87Sr enrichment of ophiolitic sulphide deposits in Cyprus confirms ore formation by circulating seawater
Earth and Planetary Science Letters, 35 (1977) 71-78 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
71
[3]
STSr ENRICHMENT OF OPHIOLITIC SULPHIDE DEPOSITS IN CYPRUS CONFIRMS ORE FORMATION BY CIRCULATING SEAWATER H.J. CHAPMAN and E.T.C. SPOONER
Department o f Geology and Mineralogy, University of Oxford, Oxford OX1 3PR (Great Britain) Received January 10, 1977 Revised version received February 5,1977
The hypothesis that seawater was the source of the hydrothermal fluid which formed the Upper Cretaceous ophiolitic cupriferous pyrite ore desposits of the Troodos Massif (Cyprus) has been tested by analysing the strontium isotopic composition of thirteen mineralized samples from four mines. Initial 87Sr/86Sr ratios range from 0.7052 +0.0001 to 0.7075 -+0.0002, the latter value being indistinguishable from that of Upper Cretaceous seawater at 0.7076 -+ 0.0006 (2a). Hence, the mineralized metabasalt samples have been contaminated with 87Sr, relative to initial magmatic strontium isotope ratios of the Troodos ophiolitic complex (0.70338 -+0.00010 to 0.70365 -+ 0.00005). Since seawater was the only source of strontium available during formation of the Troodos Complex which was isotopically relatively enriched in 87Sr, the data confirm that seawater was the source of the hydrothermal oreforming fluid.
1. Introduction Strontium isotopic ratios (87Sr/a6Sr) have previously been used as geochemical tracers to confirm seawater interaction during downward fluid flow into the Upper Cretaceous ophiolitic rocks of the Troodos Massif, Cyprus [ 1 - 3 ] . This evidence, combined with other information, has led to the development of a model for hydrothermal metamorphism, metasomatism and mineralization of the upper ~ 2 km of the ophiolitic sequence which involves hot seawater circulation during hydrothermal convection (Fig. 1) [ 1 - 6 ] . The permeable layer in which heat and mass transfer occurred consists of pillow lavas underlain by a dyke complex containing screens of gabbroic and trondhjemitic rocks at the base. It has been the object of the work discussed here to test one aspect of this model; namely, that the cupriferous pyrite ore deposits, which occur within the pillow laves/dyke complex sequence, were formed in the discharge zones of the geothermal systems (Fig. 1). The basis of the method is that since Upper Cretaceous seawater had a higher aTSr/a6Sr ratio (0.7076 -+ 0.0006 (2a) [7]) than the initial magmatic 87Sr/
~
SEA
.
WATER .
Fig. 1. Schematic diagram of model of hydrothermal convection, metamorphism and mass transfer within the ophiolitic rocks of the Troodos Massif, Cyprus. General metamorphism [28 ] occurred in zones of recharge flow, whereas formation of the eupriferous pyrite ore deposits [ 14,15,39-42 ] occurred in the discharge zones. Precipitation of Fe/Mn-cnriched sediments ("umbers") [17,43,44] was also probably a consequence of discharge of hot metalliferous solutions [4,43 ]. a = gabbro, metagabbro and variably altered trondhjemite; b = sheeted dyke complex; c = piLlowlavas; d = umbers (position schematic); e = massive sulphide ore; f = mineralized stockwork. Dashed lines approximate to metamorphic boundaries between zeolite fades (z), greenschist facies (g) and amphibolite facies (am). Continuous lines are schematic flow lines for hydrothermal convection; dotted line is approximate form of an isotherm produced by convection.
72 86Sr ratios of the basic igneous rocks of the Troodos Massif (0.70338 -+ 0.00010 to 0.70365 -+ 0.00005 [2]), significant seawater interaction is indicated by marked 87Sr enrichment of altered or mineralized rock. Seawater was the only source of isotopically heavier strontium available during formation of the ophiolitic complex in the submarine environment, since there is no evidence to suggest close proximity of continental crustal material to the Troodos complex when it formed. In this work the initial 87Sr/86Sr ratios of thirteen samples from four mines have been determined, using methods [8,9] previously described (see footnote to Table 1). Hence, this article reports usage of strontium isotopes in economic geology - a field in which they have had relatively limited application [101.
2. Initial 87Sr/S6Sr ratios of mineralized rocks The mines (Limni, Alestos, Mathiati and Mousoulos) were chosen for their wide distribution over the Troodos Massif (for locations see Fig. 2). Limni, which
is currently the largest producer in Cyprus (~450,000 tonnes ore per annum at ~0.85 wt.% Cu [11,12] consists of a medium-level cupriferous pyrite stockwork' ore deposit developed in reconstituted pillow lava (Fig. 3). In the southeast corner of the Limni opencast pit is an unusual exposure of a deepqevel stockwork developed in Sheeted Dyke Complex (Fig. 3) [ 12,13]. This material is strongly pyritised, and contains a metamorphic mineral assemblage characteristic of the greenschist facies (quartz-albite-epidote-chloriteactinolite-sphene-calcite-pyrite). Alestos is another deep-level stockword, in this case developed in the transition zone between pillow lavas and dykes (the "Basal Group"). Mathiati [14] and Mousoulos [15] are high-level cupriferous pyrite stockworks which formed just below the original rock/seawater interface. The analytical results are given in Table 1, and are compared graphically in Fig. 4 with the 87Sr/a6Sr ratio of Upper Cretaceous seawater [7], and with initial 87Sr/S6Sr ratios of hydrothermally metamorphosed pillow lavas and dykes and unaltered gabbroic material from the Troodos Complex [2]. Relative to the rubidium content of the ocean-floor basalt (~1.5 ppm [ 1 8 -
|
S
I0
IS
II
Kiiimetros
Alsstos ) i)ji
(:.i )i i • i
,f
~)// i~
ii~~ii~ii~i~~iIi~i~i i~I~)iii~i~ii~~ii~i~i~~i~i~i ii~ "-]Sedimentary ~ rocks
~ Pi|lowLaves
Bassi group Sheeted Dyke co.,,.x Gabbro end Trondhjemite Ultremefic recks and Serpentinit4s
i~iil
Fig. 2. Simplifiedgeologicalmap of the Troodos Massif, Cyprus (modified after the Cyprus GeologicalSurvey map) showing the locations of the mines from which samples were obtained for strontium isotopic studies.
73
Fig. 3. Limni opencast copper mine from the south showing positions of specimens obtained for strontium isotopic analysis. Samples from the main stockwork were from point a (CY/75/55 and 56). Samples from the pyritised Sheeted Dyke Complex (b) were from a location just to the right of the photograph.
20]) the rubidium contents of these mineralized whole-rock metabasalt samples are very variable ( < 0 . 1 11.8 ppm). Strontium, on the other hand, was clearly leached from the stockworks, since, relative to a deepsea basalt value of ~105 ppm [ 18-20], mineralized whole-rock samples show marked depletions. In fact, several samples from the cores of ore deposits (data not given in Table 1) contained no analytically detectable strontium. The initial strontium isotope ratios of all thirteen samples are significantly higher than the small range of primary magmatic initial 87Sr/86Sr values (0.70338 -+ 0.00010 to 0.70365 -+ 0.00005 [1,2]). The measurements range from 0.7052 -+ 0.0001 (Alestos) to ratios as high as tha t of Upper Cretaceous sea water (Limni main ore deposit; CY/75/55 = 0.7075 -+ 0.0002). No higher 87Sr/86Sr ratios were obtained. Most values are between 0.7060 and 0.7067 and are therefore higher than most of the initial 87Sr/86Sr ratios of the altered pillow lavas and dykes [1,2] (see Fig. 4). At Limni Mine, it was hoped to obtain an 87Sr/ S6Sr ratio close to that of the ore-forming fluid by analysing material from the core of the deposit. However, this material contained insufficient strontium for
analytical purposes. Nevertheless, it is considered significant that sample CY/75/55, which was from closest to the centre of the stockwork, has an initial strontium isotope ratio indistinguishable from that of Upper Cretaceous seawater. An 875r/86Sr ratio of the hydrothermal fluid was obtained from a vein sample (CY/ 75/67A) from the pyritic stockwork in the southeast corner of Limni mine. The ~1 cm thick vein consisted of quartz-epidote-actinolite-chlorite-pyrite-calcite, and, because of strontium ionic substitution for calcium in epidote, contained a relatively large amount of strontium (196 ppm). Studies of fluid inclusions in this same vein [21 ] have given homogenization temperatures of 309 -+ 19°C (21 measurements) and have shown that the hydrothermal fluid was a relatively dilute solution (~3.8 equivalent wt.% NaCI) with a freezing point of -2.3 -+ 0.4°C (22 measurements) close to that of seawater (~-1.9°C) [21 ]. The initial strontium isotopic composition of the vein is high at 0.7061 -+ 0.0001. This value is statistically indistinguishable from that of the altered dykes in which the vein occurred (samples CY/75/62, 64, 67B and 67D = 0.7060/1 -+ 0.0001). This indicates complete homogenization of the aTsr/a6Sr ratios of the hydrothermal
Sample description
hydrothermaUy mineralized and metamorphosed pillow lava (quart z-chlorite-illitepyrite-chaleo-pyrite sphalerite-haematite)
CY/75/67A
quar tz-epido te-chloriteactinolite-pyrite-calcite vein
pyritised and hydrothermally metamorphosed sheeted dykes with interstitial finergrained pillow lava screens (quartz-albitechlorite-epidote-actinolite-sphene-pyrite)
Limni; stockwork in Sheeted Dyke Complex
CY/75/55A CY/75/55B CY/75/56A
Limni; main ore deposit
Sample locality and number
0.4
10.2 11.8 0.3
Rb (ppm) -+ 10%
196
12 14 16
Sr (ppm) -+ 10%
0.002
0.853 0.842 0.018
Rb/Sr (-+1% for Rb/Sr > 0.01)
Initial strontium isotope ratios of samples from ophiolitic sulphide ore deposits, Cyprus
TABLE 1
0.70633 -+ 0.00005
0.71027 -+ 0.00012 0.71027 ± 0.00010 0.70672 ± 0.00014
87Sr/a6Sr (measured)
0.7061 -+ 0.0001
0.7075 -+ 0.0002 0.7075 -+ 0.0002 0.7066 -+ 0.0001
87Sr/86Sr (initial; adjusted relative to an Eimer and Amend standard value of 0.7080)
hydrothermally mineralized and metamorphosed pillow lava
hydrothermally mineralized and metamorphosed pillow lava
hydrothermaUy mineralized and metamorphosed Basal Group
altered dyke altered dyke altered dyke altered dyke altered pillow lava altered pillow lava
<0.1
0.5
0.3
2.7 0.3 4.5 4.3 0.6 3.7
2
2
12
27 20 35 73 23 18
0.006
0.276
0.025
0.099 0.016 0.130 0.059 0.026 0.205
± 0.00005 ± 0.00011 ± 0.00007 ± 0.00007 ± 0.00007 ± 0.00007
0.70687 -+ 0.00013
0.70679 ± 0.00014
0.70547 ± 0.00013
0.70640 0.70629 0.70664 0.70651 0.70657 0.70723
± 0.0001 ± 0.0001 -+ 0.0001 +- 0.0001 ± 0.0001 ± 0.0001
0.7067 ± 0.0001
0.7058 -+ 0.0002
0.7052 ± 0.0001
0.7060 0.7061 0.7060 0.7061 0.7063 0.7064
Rb, Sr and Rb/Sr ratios were determined by X-ray fluorescence following the methods of Pankhurst and O'Nions [9], but with extended counting times due to the low elemental concentrations encountered [45]. 87Sr/86Sr ratios were measured on single Ta filaments in a solid-source 12-inch mass-spectrometer [ 8 ]. Analytical errors are quoted at the 2o level. Initial 87Sr/86Sr ratios were obtained by correcting to an age of 75 Myr B.P., since the Troodos ophiolitic rocks were formed in Campanian-Maestrichtian (Upper Cretaceous) time [16,17]. These initial strontium isotopic ratios are given to ±0.0001, which is the maximum error caused by the combined uncertainties in Rb/Sr ratios, measured 87Sr/86Sr ratios and in the age of the complex. The 87Rb decay constant of 1.39 × 10 - t l yr -1 was used for age correction, and the initial 87Sr/86Sr ratios have been normalized to an Eimer and Amend SrCO 3 standard isotopic value of 0.7080.
CY/75/100A
Mousoulos
CY/75/104
Mathiati
CY/75/980
Alestos
CY/75/62 CY/75/64 CY/75/67B CY/75/67D CY/75/65A CY/75/65B
76
I
I
t
|
I
i
[]
"t
CAMPANIAN- MAESTRICHTIAN SEA WATER
LIMNI;MAIN ORE DEPOSIT
LIMNI;PYRITISEO SHEETED OYKE COMPLEX ALESTOS, MATHIATI MOUSQULOS
!
I
I
z
AND
HYOROTHERMALLY METAMOIIPHOSEO PILLOW LAVAS ANn DYKES FRESH GABBROIC MATERIAL
0-7030
I 0.7040
i
I
l
I
0.70§0
0"7060
0.7070
0.7000
INITIAL
07Sr/USr RATIOS
Fig. 4. Initial strontium isotope ratios of samples from ophiolitic sulphide ore deposits, Cyprus (diamond symbols), compared with those of Campanian-Maestrichtian (Upper Cretaceous) seawater [7 ], fresh gabbroic material (Troodos Massif) [1,2] and hydrotbermaily metamorphosed pillow lavas and dykes (Troodos Massif) [1,2 ]. Open diamond denotes the aTsr/a6Sr ratio of the epidotic vein from the pyritised dyke complex, Limni (CY/75/67A; Fig. 3). Numbers refer to numbers of specimens analysed, and Z denotes the 87Sr/S6Sr ratio of an interstitial zeolite sample from altered pillow lavas [ 1,2 ].
fluid and the rock. The values obtained from altered pillow lava screens between dykes tend to be slightly higher at 0.7063/4 -+ 0.0001 (samples CY/75/65A and 65B). The three samples from the other three mines which contained sufficient strontium for isotopic analysis show high initial STSr/S6Sr ratios (0.7052 + 0.0001 to 0.7067 + 0.0001).
3. Conclusions (1) The above data indicate significant 87Sr enrichment of the ore deposit material relative to the initial magmatic STSr/86Sr ratios of the ophiolitic rocks. (2) For the reasons given in the introduction the hydrothermal fluid which formed the cupriferous pyrite ore deposits was, therefore, of s e a w a t e r origin. This confirms previous suggestions based on hydrogen
[6], oxygen [6] and sulphur [3] isotope geochemistry. It has also been shown from fluid inclusion studies that the bulk chemistry of the ore-forming solution was indistinguishable from that of seawater [21 ]. (3) The fact that the initial aTsr/a6sr ratio of the sample from as close as possible to the centre of the Limni stockwork is indistinguishable from that of Upper Cretaceous seawater suggests that the quantity of circulating fluid was sufficiently large to remain close to its initial isotopic composition. This is consistent with a previous prediction of a high discharge fluid STsr/a6sr ratio based on calculations of a large integrated water/rock ratio [2]. (4) The seawater convection model for hydrothermal metamorphism is supported, since the ore deposits may now be confidently identified as the original discharge zones. Since ophiolitic rocks in general, and the Troodos Complex in particular, are considered to
77 be tectonically e m p l a c e d fragments o f oceanic crust and upper mantle [ 2 2 - 3 4 ] the data also support the more general t h e o r y of seawater c o n v e c t i o n within spreading oceanic ridges [ 4 , 5 , 3 5 - 3 8 ] . Mineralization o f layer 2 o f the oceanic crust is an integral part o f this m o d e l [4,21 ].
Acknowledgements We w o u l d particularly like to t h a n k Dr. S. M o o r b a t h w i t h o u t whose support this w o r k w o u l d n o t have been done. E.T.C.S. is very grateful to the Director o f the Cyprus Geological Survey (Mr. Y. Hjistavrinou), Dr. G. Constantinou, Mr. T.O. Trennery and the m a n a g e m e n t o f Limni Mine for ensuring an enjoyable and valuable visit to Cyprus. We also t h a n k Messrs. R. G o o d w i n , M. H u m m , P.J. Jackson and K.A.C. Parish for skilled technical assistance. H.J.C. acknowledges the support o f a N E R C grant.
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8 R.K. O'Nions and R.J. Pankhurst, Secular variations in the Sr-isotope composition of Icelandic volcanic rocks, Earth Planet. Sci. Lett. 21 (1973) 13. 9 R.J. Pankhurst and R.K. O'Nions, Determination of Rb/ Sr and 87Sr/86Sr ratios of some standard rocks and evaluation of X-ray fluorescence spectrometry in Rb-Sr geochemistry, Chem. Geol. 12 (1973) 127. 10 C.E. Hedge, Strontium isotopes in economic geology, Econ. Geol. 69 (1974) 823. 11 P.G. Petropoulos, Annual Report of the Senior Mines Officer for the Year 1974 (Government Printer, Nicosia, Cyprus, 1975). 12 T.O. Trennery and B.G. Pocock, Mining and milling operations at Limni mine, Cyprus, Trans. Inst. Min. Metall., Sect. A: Min. Ind. 81 (1972) A1. 13 N.G. Adamides, Geological history of the Limni concession Cyprus, in the light of the plate tectonics hypothesis, Trans. Inst. Min. Metall., Sect. B: Appl. Earth Sci. 84 (1975) B17. 14 G. Constantinou and G.J.S. Govett, Geology, geochemistry, and genesis of Cyprus sulfide deposits, Econ. Geol. 68 (1973) 843. 15 D.L. Searle, Mode of occurrence of the cupriferous pyrite deposits in Cyprus, Trans. Inst. Min. Metall., Sect. B: Appl. Earth Sci. 81 (1972) B189. 16 M. Mantis, Upper Cretaceous-Tertiary foraminiferal zones in Cyprus, Epitiris, Cyprus Res. Centre 3 (1970) 227. 17 A.H.F. Robertson and J.D. Hudson, Pelagic sediments in the Cretaceous and Tertiary history of the Troodos massif, Cyprus, Spec. Publ. Int. Assoc. Sedimentol. 1 (1974) 403. 18 E.J. Dasch, C.E. Hedge and J. Dymond, Effect of seawater interaction on strontium isotope composition of deep-sea basalts, Earth Planet. Sci. Lett. 19 (1973) 177. 19 R.K. O'Nions and R.J. Pankhurst, Petrogenetic significance of isotope and trace element variations in volcanic rocks from the Mid-Atlantic, J. Petrol. 15 (1974) 603. 20 S.R. Hart, L1L-element geochemistry of Leg 34 basalts, in: R.S. Yeats, S.R. Hart et al., Initial Report of the Deep Sea Drilling Project 34 (1976) 283. 21 E.T.C. Spooner and C.J. Bray, Hydrothermal fluids of seawater salinity in ophiolitic sulphide ore deposits, Cyprus, Nature (1977) in press. 22 I.G. Gass, Is the Troodos Massif, Cyprus, a fragment of Mesozoic ocean floor?, Nature 220 (1968) 39. 23 E.M. Moores, Petrology and structure of the Vourinos ophiolite complex of northern Greece, Geol. Soc. Am., Spec. Paper 118 (1969). 24 R.G. Coleman, Plate tectonic emplacement of upper mantle peridotites along continental edges, J. Geophys. Res. 76 (1971) 1212. 25 J.F. Dewey and J.M. Bird, Origin and emplacement of the ophiolite suite: Appalachian ophiolites in Newfoundland, J. Geophys. Res. 76 (1971) 3179. 26 E.M. Moores and F.J. Vine, The Troodos Massif, Cyprus, and other ophiolites as oceanic crust: evaluation and implications, Philos. Trans. R. Soc. Lond., Ser. A, 268 (1971) 443. 27 J.A. Pearce and J.R. Cann, Ophiolite origin investigated
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by discriminant analysis using Ti, Zr and Y, Earth Planet. Sci. Lett. 12 (1971) 339. I.G. Gass and J.D. Smewing, Intrusion, extrusion and metamorphism at constructive margins:evidence from the Troodos Massif, Cyprus, Nature 242 (1973) 26. J.A. Pearce and J.R. Cann, Tectonic setting of basic volcanic rocks determined using trace element analyses, Earth Planet. Sci. Lett. 19 (1973) 290. R.G.W. Kidd and J.R. Cann, Chilling statistics indicate an ocean-floor spreading origin for the Troodos Complex, Cyprus, Earth Planet. Sci. Lett. 24 (1974) 151. A. Hynes, Comments on "The Troodos ophiolitic complex was probably formed in an island arc" by A. Miyashiro, Earth Planet. Sci. Lett. 25 (1975) 213. E.M. Moores, Discussion of "Origin of Troodos and other ophiolites: a reply to Hines" by Akiho Miyashiro, Earth Planet. Sci. Lett. 25 (1975) 223. I.G. Gass, C.R. Neary, J. Plant, A.H.F. Robertson, K.O. Simonian, J.D. Sinewing, E.T.C. Spooner and R.A.M. Wilson, Comments on "The Troodos ophiolitic complex was probably formed in an island arc", by A. Miyashiro and subsequent correspondence by A. Hynes and A. Miyashiro, Earth Planet. Sci. Lett. 25 (1975) 236. R.W. Kay and R.G. Senechal, The rare earth geochemistry of the Troodos ophiolite complex, J. Geophys. Res. 81 (1976) 964. C.R.B. Lister, On the thermal balance of a mid-ocean ridge, Geophys. J.R. Astron. Soc. 26 (1972) 515. D.L. Williams, R.P. Von Herzen, J.G. Sclater and R.N. Anderson, The Galapagos spreading centre; lithospheric
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cooling and hydrothermal circulation, Geophys. J.R. Astron. Soc. 38 (1974) 587. E. Bonatti, J. Honnorez, P. Kirst and F. Radicati, Metagabbros from the Mid-Atlantic Ridge at 06°N: contacthydrothermal-dynamic metamorphism beneath the axial valley, J. Geol. 83 (1975) 61. R.J. Ribando, K.E. Torrance and D.L. Turcotte, Numerical models of hydrothermal circulation in the oceanic crust, J. Geophys. Res. 81 (1976) 3007. L.M. Bear. The mineral resources and mining industry of Cyprus, Bull. Geol. Surv. Dep. Cyprus 1 (1963) 208 pp. R.W. Hutchinson and D.L. Searle, Stratabound pyrite deposits in Cyprus and relations to other sulphide ores, Soc. Min. Geol. Japan, Spec. Issue 3 (1971) 198. G. Constantinou and G.J.S. Govett, Genesis of sulphide deposits, ochre and umber of Cyprus, Trans. Inst. Min. Metall., Sect. B: Appl. Earth Sci. 81 (1972) B32. A.E. Johnson, Origin of Cyprus pyrite deposits, 24th Int. Geol. Congr., Montreal, Sect. 4 (1972) 291. A.H.F. Robertson and J.D. Hudson, Cyprus umbers; chemical precipitates on a Tethyan ocean ridge, Earth Planet. Sci. Lett. 18 (1973) 93. A.H.F. Robertson, Cyprus umbers: basalt-sediment relationships on a Mesozoic ocean ridge, J. Geol. Soc. Lond. 131 (1975)511. B.W. Chappell, W. Compston, P.A. Arriens and M.J. Vernon, Rubidium and strontium determinations by X-ray fluorescence spectrometry and isotope dilution below the part per million level, Geochim. Cosmochim. Acta 33 (1969) 1002.
71
[3]
STSr ENRICHMENT OF OPHIOLITIC SULPHIDE DEPOSITS IN CYPRUS CONFIRMS ORE FORMATION BY CIRCULATING SEAWATER H.J. CHAPMAN and E.T.C. SPOONER
Department o f Geology and Mineralogy, University of Oxford, Oxford OX1 3PR (Great Britain) Received January 10, 1977 Revised version received February 5,1977
The hypothesis that seawater was the source of the hydrothermal fluid which formed the Upper Cretaceous ophiolitic cupriferous pyrite ore desposits of the Troodos Massif (Cyprus) has been tested by analysing the strontium isotopic composition of thirteen mineralized samples from four mines. Initial 87Sr/86Sr ratios range from 0.7052 +0.0001 to 0.7075 -+0.0002, the latter value being indistinguishable from that of Upper Cretaceous seawater at 0.7076 -+ 0.0006 (2a). Hence, the mineralized metabasalt samples have been contaminated with 87Sr, relative to initial magmatic strontium isotope ratios of the Troodos ophiolitic complex (0.70338 -+0.00010 to 0.70365 -+ 0.00005). Since seawater was the only source of strontium available during formation of the Troodos Complex which was isotopically relatively enriched in 87Sr, the data confirm that seawater was the source of the hydrothermal oreforming fluid.
1. Introduction Strontium isotopic ratios (87Sr/a6Sr) have previously been used as geochemical tracers to confirm seawater interaction during downward fluid flow into the Upper Cretaceous ophiolitic rocks of the Troodos Massif, Cyprus [ 1 - 3 ] . This evidence, combined with other information, has led to the development of a model for hydrothermal metamorphism, metasomatism and mineralization of the upper ~ 2 km of the ophiolitic sequence which involves hot seawater circulation during hydrothermal convection (Fig. 1) [ 1 - 6 ] . The permeable layer in which heat and mass transfer occurred consists of pillow lavas underlain by a dyke complex containing screens of gabbroic and trondhjemitic rocks at the base. It has been the object of the work discussed here to test one aspect of this model; namely, that the cupriferous pyrite ore deposits, which occur within the pillow laves/dyke complex sequence, were formed in the discharge zones of the geothermal systems (Fig. 1). The basis of the method is that since Upper Cretaceous seawater had a higher aTSr/a6Sr ratio (0.7076 -+ 0.0006 (2a) [7]) than the initial magmatic 87Sr/
~
SEA
.
WATER .
Fig. 1. Schematic diagram of model of hydrothermal convection, metamorphism and mass transfer within the ophiolitic rocks of the Troodos Massif, Cyprus. General metamorphism [28 ] occurred in zones of recharge flow, whereas formation of the eupriferous pyrite ore deposits [ 14,15,39-42 ] occurred in the discharge zones. Precipitation of Fe/Mn-cnriched sediments ("umbers") [17,43,44] was also probably a consequence of discharge of hot metalliferous solutions [4,43 ]. a = gabbro, metagabbro and variably altered trondhjemite; b = sheeted dyke complex; c = piLlowlavas; d = umbers (position schematic); e = massive sulphide ore; f = mineralized stockwork. Dashed lines approximate to metamorphic boundaries between zeolite fades (z), greenschist facies (g) and amphibolite facies (am). Continuous lines are schematic flow lines for hydrothermal convection; dotted line is approximate form of an isotherm produced by convection.
72 86Sr ratios of the basic igneous rocks of the Troodos Massif (0.70338 -+ 0.00010 to 0.70365 -+ 0.00005 [2]), significant seawater interaction is indicated by marked 87Sr enrichment of altered or mineralized rock. Seawater was the only source of isotopically heavier strontium available during formation of the ophiolitic complex in the submarine environment, since there is no evidence to suggest close proximity of continental crustal material to the Troodos complex when it formed. In this work the initial 87Sr/86Sr ratios of thirteen samples from four mines have been determined, using methods [8,9] previously described (see footnote to Table 1). Hence, this article reports usage of strontium isotopes in economic geology - a field in which they have had relatively limited application [101.
2. Initial 87Sr/S6Sr ratios of mineralized rocks The mines (Limni, Alestos, Mathiati and Mousoulos) were chosen for their wide distribution over the Troodos Massif (for locations see Fig. 2). Limni, which
is currently the largest producer in Cyprus (~450,000 tonnes ore per annum at ~0.85 wt.% Cu [11,12] consists of a medium-level cupriferous pyrite stockwork' ore deposit developed in reconstituted pillow lava (Fig. 3). In the southeast corner of the Limni opencast pit is an unusual exposure of a deepqevel stockwork developed in Sheeted Dyke Complex (Fig. 3) [ 12,13]. This material is strongly pyritised, and contains a metamorphic mineral assemblage characteristic of the greenschist facies (quartz-albite-epidote-chloriteactinolite-sphene-calcite-pyrite). Alestos is another deep-level stockword, in this case developed in the transition zone between pillow lavas and dykes (the "Basal Group"). Mathiati [14] and Mousoulos [15] are high-level cupriferous pyrite stockworks which formed just below the original rock/seawater interface. The analytical results are given in Table 1, and are compared graphically in Fig. 4 with the 87Sr/a6Sr ratio of Upper Cretaceous seawater [7], and with initial 87Sr/S6Sr ratios of hydrothermally metamorphosed pillow lavas and dykes and unaltered gabbroic material from the Troodos Complex [2]. Relative to the rubidium content of the ocean-floor basalt (~1.5 ppm [ 1 8 -
|
S
I0
IS
II
Kiiimetros
Alsstos ) i)ji
(:.i )i i • i
,f
~)// i~
ii~~ii~ii~i~~iIi~i~i i~I~)iii~i~ii~~ii~i~i~~i~i~i ii~ "-]Sedimentary ~ rocks
~ Pi|lowLaves
Bassi group Sheeted Dyke co.,,.x Gabbro end Trondhjemite Ultremefic recks and Serpentinit4s
i~iil
Fig. 2. Simplifiedgeologicalmap of the Troodos Massif, Cyprus (modified after the Cyprus GeologicalSurvey map) showing the locations of the mines from which samples were obtained for strontium isotopic studies.
73
Fig. 3. Limni opencast copper mine from the south showing positions of specimens obtained for strontium isotopic analysis. Samples from the main stockwork were from point a (CY/75/55 and 56). Samples from the pyritised Sheeted Dyke Complex (b) were from a location just to the right of the photograph.
20]) the rubidium contents of these mineralized whole-rock metabasalt samples are very variable ( < 0 . 1 11.8 ppm). Strontium, on the other hand, was clearly leached from the stockworks, since, relative to a deepsea basalt value of ~105 ppm [ 18-20], mineralized whole-rock samples show marked depletions. In fact, several samples from the cores of ore deposits (data not given in Table 1) contained no analytically detectable strontium. The initial strontium isotope ratios of all thirteen samples are significantly higher than the small range of primary magmatic initial 87Sr/86Sr values (0.70338 -+ 0.00010 to 0.70365 -+ 0.00005 [1,2]). The measurements range from 0.7052 -+ 0.0001 (Alestos) to ratios as high as tha t of Upper Cretaceous sea water (Limni main ore deposit; CY/75/55 = 0.7075 -+ 0.0002). No higher 87Sr/86Sr ratios were obtained. Most values are between 0.7060 and 0.7067 and are therefore higher than most of the initial 87Sr/86Sr ratios of the altered pillow lavas and dykes [1,2] (see Fig. 4). At Limni Mine, it was hoped to obtain an 87Sr/ S6Sr ratio close to that of the ore-forming fluid by analysing material from the core of the deposit. However, this material contained insufficient strontium for
analytical purposes. Nevertheless, it is considered significant that sample CY/75/55, which was from closest to the centre of the stockwork, has an initial strontium isotope ratio indistinguishable from that of Upper Cretaceous seawater. An 875r/86Sr ratio of the hydrothermal fluid was obtained from a vein sample (CY/ 75/67A) from the pyritic stockwork in the southeast corner of Limni mine. The ~1 cm thick vein consisted of quartz-epidote-actinolite-chlorite-pyrite-calcite, and, because of strontium ionic substitution for calcium in epidote, contained a relatively large amount of strontium (196 ppm). Studies of fluid inclusions in this same vein [21 ] have given homogenization temperatures of 309 -+ 19°C (21 measurements) and have shown that the hydrothermal fluid was a relatively dilute solution (~3.8 equivalent wt.% NaCI) with a freezing point of -2.3 -+ 0.4°C (22 measurements) close to that of seawater (~-1.9°C) [21 ]. The initial strontium isotopic composition of the vein is high at 0.7061 -+ 0.0001. This value is statistically indistinguishable from that of the altered dykes in which the vein occurred (samples CY/75/62, 64, 67B and 67D = 0.7060/1 -+ 0.0001). This indicates complete homogenization of the aTsr/a6Sr ratios of the hydrothermal
Sample description
hydrothermaUy mineralized and metamorphosed pillow lava (quart z-chlorite-illitepyrite-chaleo-pyrite sphalerite-haematite)
CY/75/67A
quar tz-epido te-chloriteactinolite-pyrite-calcite vein
pyritised and hydrothermally metamorphosed sheeted dykes with interstitial finergrained pillow lava screens (quartz-albitechlorite-epidote-actinolite-sphene-pyrite)
Limni; stockwork in Sheeted Dyke Complex
CY/75/55A CY/75/55B CY/75/56A
Limni; main ore deposit
Sample locality and number
0.4
10.2 11.8 0.3
Rb (ppm) -+ 10%
196
12 14 16
Sr (ppm) -+ 10%
0.002
0.853 0.842 0.018
Rb/Sr (-+1% for Rb/Sr > 0.01)
Initial strontium isotope ratios of samples from ophiolitic sulphide ore deposits, Cyprus
TABLE 1
0.70633 -+ 0.00005
0.71027 -+ 0.00012 0.71027 ± 0.00010 0.70672 ± 0.00014
87Sr/a6Sr (measured)
0.7061 -+ 0.0001
0.7075 -+ 0.0002 0.7075 -+ 0.0002 0.7066 -+ 0.0001
87Sr/86Sr (initial; adjusted relative to an Eimer and Amend standard value of 0.7080)
hydrothermally mineralized and metamorphosed pillow lava
hydrothermally mineralized and metamorphosed pillow lava
hydrothermaUy mineralized and metamorphosed Basal Group
altered dyke altered dyke altered dyke altered dyke altered pillow lava altered pillow lava
<0.1
0.5
0.3
2.7 0.3 4.5 4.3 0.6 3.7
2
2
12
27 20 35 73 23 18
0.006
0.276
0.025
0.099 0.016 0.130 0.059 0.026 0.205
± 0.00005 ± 0.00011 ± 0.00007 ± 0.00007 ± 0.00007 ± 0.00007
0.70687 -+ 0.00013
0.70679 ± 0.00014
0.70547 ± 0.00013
0.70640 0.70629 0.70664 0.70651 0.70657 0.70723
± 0.0001 ± 0.0001 -+ 0.0001 +- 0.0001 ± 0.0001 ± 0.0001
0.7067 ± 0.0001
0.7058 -+ 0.0002
0.7052 ± 0.0001
0.7060 0.7061 0.7060 0.7061 0.7063 0.7064
Rb, Sr and Rb/Sr ratios were determined by X-ray fluorescence following the methods of Pankhurst and O'Nions [9], but with extended counting times due to the low elemental concentrations encountered [45]. 87Sr/86Sr ratios were measured on single Ta filaments in a solid-source 12-inch mass-spectrometer [ 8 ]. Analytical errors are quoted at the 2o level. Initial 87Sr/86Sr ratios were obtained by correcting to an age of 75 Myr B.P., since the Troodos ophiolitic rocks were formed in Campanian-Maestrichtian (Upper Cretaceous) time [16,17]. These initial strontium isotopic ratios are given to ±0.0001, which is the maximum error caused by the combined uncertainties in Rb/Sr ratios, measured 87Sr/86Sr ratios and in the age of the complex. The 87Rb decay constant of 1.39 × 10 - t l yr -1 was used for age correction, and the initial 87Sr/86Sr ratios have been normalized to an Eimer and Amend SrCO 3 standard isotopic value of 0.7080.
CY/75/100A
Mousoulos
CY/75/104
Mathiati
CY/75/980
Alestos
CY/75/62 CY/75/64 CY/75/67B CY/75/67D CY/75/65A CY/75/65B
76
I
I
t
|
I
i
[]
"t
CAMPANIAN- MAESTRICHTIAN SEA WATER
LIMNI;MAIN ORE DEPOSIT
LIMNI;PYRITISEO SHEETED OYKE COMPLEX ALESTOS, MATHIATI MOUSQULOS
!
I
I
z
AND
HYOROTHERMALLY METAMOIIPHOSEO PILLOW LAVAS ANn DYKES FRESH GABBROIC MATERIAL
0-7030
I 0.7040
i
I
l
I
0.70§0
0"7060
0.7070
0.7000
INITIAL
07Sr/USr RATIOS
Fig. 4. Initial strontium isotope ratios of samples from ophiolitic sulphide ore deposits, Cyprus (diamond symbols), compared with those of Campanian-Maestrichtian (Upper Cretaceous) seawater [7 ], fresh gabbroic material (Troodos Massif) [1,2] and hydrotbermaily metamorphosed pillow lavas and dykes (Troodos Massif) [1,2 ]. Open diamond denotes the aTsr/a6Sr ratio of the epidotic vein from the pyritised dyke complex, Limni (CY/75/67A; Fig. 3). Numbers refer to numbers of specimens analysed, and Z denotes the 87Sr/S6Sr ratio of an interstitial zeolite sample from altered pillow lavas [ 1,2 ].
fluid and the rock. The values obtained from altered pillow lava screens between dykes tend to be slightly higher at 0.7063/4 -+ 0.0001 (samples CY/75/65A and 65B). The three samples from the other three mines which contained sufficient strontium for isotopic analysis show high initial STSr/S6Sr ratios (0.7052 + 0.0001 to 0.7067 + 0.0001).
3. Conclusions (1) The above data indicate significant 87Sr enrichment of the ore deposit material relative to the initial magmatic STSr/86Sr ratios of the ophiolitic rocks. (2) For the reasons given in the introduction the hydrothermal fluid which formed the cupriferous pyrite ore deposits was, therefore, of s e a w a t e r origin. This confirms previous suggestions based on hydrogen
[6], oxygen [6] and sulphur [3] isotope geochemistry. It has also been shown from fluid inclusion studies that the bulk chemistry of the ore-forming solution was indistinguishable from that of seawater [21 ]. (3) The fact that the initial aTsr/a6sr ratio of the sample from as close as possible to the centre of the Limni stockwork is indistinguishable from that of Upper Cretaceous seawater suggests that the quantity of circulating fluid was sufficiently large to remain close to its initial isotopic composition. This is consistent with a previous prediction of a high discharge fluid STsr/a6sr ratio based on calculations of a large integrated water/rock ratio [2]. (4) The seawater convection model for hydrothermal metamorphism is supported, since the ore deposits may now be confidently identified as the original discharge zones. Since ophiolitic rocks in general, and the Troodos Complex in particular, are considered to
77 be tectonically e m p l a c e d fragments o f oceanic crust and upper mantle [ 2 2 - 3 4 ] the data also support the more general t h e o r y of seawater c o n v e c t i o n within spreading oceanic ridges [ 4 , 5 , 3 5 - 3 8 ] . Mineralization o f layer 2 o f the oceanic crust is an integral part o f this m o d e l [4,21 ].
Acknowledgements We w o u l d particularly like to t h a n k Dr. S. M o o r b a t h w i t h o u t whose support this w o r k w o u l d n o t have been done. E.T.C.S. is very grateful to the Director o f the Cyprus Geological Survey (Mr. Y. Hjistavrinou), Dr. G. Constantinou, Mr. T.O. Trennery and the m a n a g e m e n t o f Limni Mine for ensuring an enjoyable and valuable visit to Cyprus. We also t h a n k Messrs. R. G o o d w i n , M. H u m m , P.J. Jackson and K.A.C. Parish for skilled technical assistance. H.J.C. acknowledges the support o f a N E R C grant.
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