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Sulfur, oxygen and carbon isotope geochemistry of barite-iron oxide-pyrite deposits from the Apuane Alps (northern Tuscany, Italy)
Chemical Geology, 76 (1989) 249-257 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
249
Sulfur, oxygen and carbon isotope geochemistry of barite-iron oxide-pyrite deposits from the Apuane Alps (northern Tuscany, Italy) G. CORTECCI 1, P. LATTANZI 2 and G. TANELLI 3 1Dipartimento di Scienze della Terra, Universit& di Pisa, 1-56100 Pisa (Italy) 2Dipartimento di Scienze della Terra, Universit~ di Firenze, 1-50121 Firenze (Italy) 3Dipartimento di Scienze della Terra, Universit& di Napoli, 1-80138 Napoli (Italy) (Accepted for publication April 13, 1989)
Abstract Cortecci, G., Lattanzi, P. and Tanelli, G., 1989. Sulfur, oxygen and carbon isotope geochemistry of barite-iron oxidepyrite deposits from the Apuane Alps (northern Tuscany, Italy). In: K. GrSnvold (Guest-Editor), Water-Rock Interaction. Chem. Geol., 76: 249-257. The sulfur, oxygen and carbon isotopic compositions of a total of 64 samples from stratiform and vein barite-iron oxide-pyrite ore bodies of the Apuane Alps are presented. In stratiform ores, sulfide minerals display a wide range of (~34Sfrom -20.9 to + 18.9%o. Similarly, barites are quite variable in •348, with values from +4.3 to +30.8%o, with a clustering around + 19.4%o; on the other hand, their JlSO-values are fairly uniform around + 15.8%o. Calculated isotopic temperatures from barite-pyrite pairs are broadly distributed, even within a single deposit. Gangue carbonates show quite light J13C-values from -29.3 to -20.5%o, and JlSO-values closely around + 18.5%o. In vein ores, sulfide minerals and barite are much more uniform in J34S, with values closely around - 1.1 and + 18.0%o, respectively; barite JlS0-values are nearly equal to + 13.4%o. Moreover, barite-pyrite and barite-galena pairs give concordant isotopic temperatures of ~ 385 ° C. The isotopic data concur with previously acquired lines of evidence to support the following genetic model: (a) Stratiform ores are synsedimentary in origin, the main source of sulfur having been Middle-Upper Triassic seawater sulfate which underwent bacterial reduction. Concurrent oxidation of organic matter supplied carbon in gangue carbonates. Probably, both barite and gangue carbonate suffered post-depositional oxygen isotope equilibration with water under diagenetic conditions. (b) Vein ores formed from remobilization of stratiform ores by metamorphic fluids (Alpine orogeny ) at 380-400 ° C, highly enriched in 'sO probably by interaction with country schists.
1. Introduction In the Apuane Alps region (northwestern Tuscany, Italy), barite, iron oxide + pyrite are presently mined from the deposits of Monte Arsiccio, Pollone and Buca della Vena (Fig. 1 ). These deposits are currently the subject of a comprehensive research program. The main aspects of geological setting, mineralogy and geo0009-2541/89/$03.50
chemistry have recently been described by Cortecci et al. (1984, 1985) and Benvenuti et al. (1986). The data collected so far allowed the aforementioned authors to hypothesize a sedimentary-metamorphic (Middle-Upper Triassic and Alpine) genetic model for the deposits. In this paper, the available S, C and O isotope data on minerals from these deposits are utilized in an attempt to clarify the interaction be-
© 1989 Elsevier Science Publishers B.V.
250
G. CORTECCI ET AL.
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Fig. 1. Simplifiedstructural schemeof the ApuaneAlps (after Ciarapicaand Passeri, 1982), with locationsof the ore deposits studied in the present work (11). Structural units: a=Nucleo MetamorficoApuano; b=Unit~ di Massa; c=Unit~ di Fornovolasco-Panie;d = non-metamo~hic units (Tuscan Nappe and Ligurides). tween the fluids and the mineralizations in the various stages of their sedimentary-metamorphic evolution.
2. Summary of geology and mineralogy of the deposits The complex geological architecture of the Apuane Alps can be schematically described as the result of overthrusting, during the Alpine orogeny, of several metamorphic and non-metamorphic tectonic units corresponding to differ-
ent paleogeographic domains (Carmignani et al., 1978; Ciarapica and Passeri, 1982). The barite-iron oxide-pyrite mineralizations are restricted to or near the contact between the Upper Triassic dolomite formation of "Grezzoni" and the underlying phyllitic complex of "Scisti di Fornovolasco", which, at least in their upper portion, are ascribed to the Ladinian-Carnian (Ciarapica and Zaninetti, 1983) (Fig. 2). Specifically, the mineralizations are mostly associated with calcareous intercalations in the upper part of the phyllitic
251
BARITE IRONOXIDE-PYRITEDEPOSITSFROMTHE APUANEALPS " LINITA D I F O R N O V O I . ~ 4 S C O - P A N I E
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Fig. 2. Time-space relationships between the ore deposits under study ( • ) and the metamorphic tectonic units of Apuane Alps (after Cortecci et al., 1985). S o l i d w a v y l i n e s = tectonic contacts; s o l i d s t r a i g h t l i n e s = conformable stratigraphic contacts; d a s h e d l i n e = disconformity, s F = Scisti di Fornovolasco (Ladinian-Carnian); T~ = Middle Triassic; T~ = Upper Triassic; C a r = Carnian; E o c = Eocene; Ol = Oligocene.
TABLE I Summary of texture and mineralogy of the barite-iron oxide-pyrite deposits of the Apuane Alps Stratiform bodies (Monte Arsiccio, Pollone, Buca della Vena )
Vein bodies (Pollone }
Texture
typically banded, with alternation of millimeter massive barite with minor pyrite, locally crustified and sized layers of barite and iron minerals, often brecciated folded and faulted; massive non-banded bodies may also occur
Main mineralogy
barite, hematite, magnetite, pyrite (arsenopyrite, barite, pyrite (sphalerite, galena, sulfosalts); sulfosalts, sphalerite, galena); calcite, dolomite, quartz, fluorite, sericite, calcite quartz, (albite, mica, chlorite)
complex. The metamorphic grade of the latter corresponds to the greenschist facies (380465°C; 2-4 kbar; Di Pisa et al., 1985; Orberger and Saupd, 1985 ). The mineralization typically consists of lens-shaped or stratiform near-concordant bodies, with subordinate related discordant masses. Table I summarizes the texture and mineralogy of the stratiform and vein ores. The marked stratigraphic control, the mostly stratiform near-concordant attitude of the ore, and the macro-, mesa- and micro-scale evidence of metamorphic deformation and recrys-
tallization led Cortecci et al. (1985) to interpret stratiform bodies as original sedimentarydiagenetic concentrations, subsequently metamorphosed, tectonized and partly mobilized during the Alpine event, during which the vein bodies were emplaced.
3. Analytical procedures In this paper, we utilize recently acquired S, C and O isotope data, as well as previously pub-
252
lished S isotope data (Cortecci et al., 1985; Orberger et al., 1985). Barite, pyrite, arsenopyrite, sphalerite and galena were separated by heavy liquids, magnetic and hand-picking methods; barite was further purified by repeated treatments with HF-HC1 mixture. The 34S/32S and lsO/160 ratios of the minerals were determined by means of procedures described in Corsini et al. (1980) and Cortecci et al. (1981). Based on X-ray diffraction, the analyzed gangue carbonates appear to constitute essentially calcite _+minor dolomite. In the hand specimens, they occur intimately associated with barite + pyrite + quartz _+muscovite. After removal of pyrite by hand-picking, the residual material was reacted with 100% H3P04 at 25 ° C (McCrea, 1950) for ~ 72 hr., and the obtained C02 analyzed for 13C/12C and lso/160 ratios. The isotopic ratios are given in 5-notation, in !'i~, relative to CDT for sulfur, PDB for carbon and V-SMOW for oxygen. In calculating the ~SO-values, the value of 1.0412 for the ~sO fractionation factor between COe and H20 at 25°C was used (O'Neil et al., 1975). The analytical uncertainty (difference between complete duplicate analyses) was mostly better than+0.1~i~ for all different kinds of measurements.
4. Isotopic composition of barite and sulfide minerals T h e 534S - and ~aSO-values so far produced for barite and sulfide minerals from the deposits under question are shown in Fig. 3. We mention here that the oxygen isotope enrichment factor between barite and aqueous sulfate is likely near zero (Savin, 1980, p. 304); the same is expected for the sulfur isotope enrichment factor, on the basis of the discussion by Chiba et al. (1981, p. 60) and the sulfur isotope enrichment factor measured for the gypsumaqueous sulfate system (1.65%~; Thode and Monster, 1965).
G. CORTECCI ET AL.
4.1. S t r a t i / o r m ore bodies
Barite and pyrite display highly variable 534Svalues from +4.3 to +30.8%o and from - 2 0 . 9 to + 18.9%~, respectively. Internal variations for each deposit are from 7 to 21%c for barite and from 10 to 34%~ for pyrite. Isotopic compositions of galena and arsenopyrite also appear to be fairly variable. These isotopic features are in agreement with the proposed genetic model and in particular strongly support a mostly bacteriogenic origin of sulfur in the sulfide minerals from non-equilibrium reduction of sulfate in semi-closed shallow-water environment (Ohmoto and Rye, 1979; Chambers, 1982), possibly in brackish water according to the paleogeographic reconstruction of Ciarapica and Passeri (1982). This interpretation basically agrees with that of Orberger et al. (1985, 1986). The main source of sulfur was likely Middle-Upper Triassic seawater sulfate, the 5"~4S of which should have been close to +15%c (Claypool et al., 1980; Cortecci et al., 1981). Barites with 534S > + 15%o can account for sulfate variously enriched in 34S by bacterial reduction, whereas barites with 534S < +15%c probably reflect variable contributions of sulfate from partial reoxidation of isotopically light bacteriogenic sulfide (near-zero fractionation effects; Kaplan and Rittenberg, 1964; Nakai and Jensen, 1964; Mizutani and Rafter, 1969). Bacterial reduction of pore-water sulfate may have continued during early diagenesis, favoured by the presence of organic compounds in the sediments as presently testified by the widespread occurrence of carbonaceous matter in the country schists, thus producing some pyrite with 534S close to that of seawater sulfate (see Ohmoto and Rye, 1979). When coexisting minerals are considered, barite is always enriched in 34S with respect to pyrite (and galena); however, calculated isotopic temperatures are broadly distributed, even within a single deposit, with values from 122 ° to 530°C (Fig. 4). These features most likely indicate disequilibrium conditions between
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Fig. 3. Frequency distributions of 6-values (our data) and ranges of 634S-values from Orberger et al. (1985). The data of these authors refer to both stratiform and vein ore bodies: they did not observe significant differences of isotopic composition between the different types of ores. _Note added in proof, see also Orberger et al. (1986).
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Fig. 4. Relationship between 6:~4S-values of associated barite and sulfide minerals from stratiform ores. Dashed lines connect coexisting minerals. Calculated isotopic temperatures from barite-sulfide pairs in brackets (fractionation factors from Ohmoto and Rye, 1979). Isotopic temperatures from barite-pyrite and sphalerite-galena pairs in vein ore at Pollone are shown up on the right.
barite and pyrite, as expected for minerals formed in a surface, low-temperature environment, and suggest that the original disequili-
bria basically survived the Alpine metamorphic event (Rickard et al., 1979; Willan and Coleman, 1983).
254
G. C O R T E C C I E T A L .
As stated before, reduced sulfur was likely oxidized to sulfate, possibly in a brackish-water environment, i.e. in water of ~isO between ~ 0c& (non-evaporated seawater; note that no sign of sulfate evaporites was found in the deposits and in the host rocks) and -6%c (meteoric water in a temperate climate ). Oxidation may have occurred biologically or abiologically; in the first case, sulfate of 51sO close to that of environmental water could have been produced (Mizutani and Rafter, 1969), whereas in the second case both water-derived and dissolved oxygen should have been incorporated in the product sulfate (Lloyd, 1967, 1968). According to experiments of Lloyd (1967), possible ~sOvalues for sulfate from inorganic oxidation are calculated to be between + 0.8 and + 9.4%~, depending on ~ s O of water ( - 6 to 0%c ) and ~ s O of dissolved oxygen ( +24 to +38%c, as a function of the biological activity in the water; Kroopnick and Craig, 1976). The sulfate from oxidation may have mixed with Middle-Upper Triassic seawater sulfate probably with ~ 8 0 close to + 10 %~ (Claypool et al., 1980 ), or higher depending on activity of sulfate-reducing bacteria. As shown in Fig. 5, barites from Pollone and Monte Arsiccio deposits have quite uniform 5~SO-values from +14.9 to +16.4%~, (mean + 15.7 + 0.4~;i~ ) and quite variable ~34S-values; samples from Buca della Vena deposit display similar, but more variable, ~lSO-values from
18
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Fig. 5. Relationship between 8:~4S- and ~ 180-values of barite from stratiform and vein ore bodies.
+14.2 to +18.1%o (mean +16.0_+1.6%~) and nearly equal ~34S-values. Mixing of sulfate from oxidation with more or less bacterially reduced sulfate is not evident from these data. Moreover, the ~lSO-values of barites with ~'~4S between + 16 and + 22%o cannot be explained by isotopic effects in the bacterial reduction of seawater sulfate: during this reaction, the ratio of 180 enrichment factor to 34S enrichment factor in the residual sulfate is commonly between 0.25 and 0.40, with possible variations from - 0.15 to 0.67 (Mizutani and Rafter, 1969, 1973; Zak et al., 1980; Pierre, 1985 ). We conclude that original ~lSO-values of barites suffered postdepositional modifications by equilibrium (or near equilibrium) isotopic exchanges with water of uniform oxygen isotopic composition over a restricted range of temperatures. The possibility that barite formed during a late-diagenetic stage from interstitial sulfate homogenized in 1sO by diffusion, mixing and isotope exchange processes seems to be unlikely. In this case, in fact, much more uniform ~34S-values should also be observed, especially for barites from Monte Arsiccio and Pollone deposits. As barite should have been lithified at time of metamorphism, characterized by low water/ rock ratios in the "Unith di FornovolascoPanie" (Preite Martinez et al., 1978), isotopic equilibration conceivably took place during diagenesis to lithification. Table II gives the calculated isotopic equilibrium temperatures for pore water with initial 3180 of either 0 or - 6%o. Interpolation of the data from 1 m NaC1 experiments by Kusakabe and Robinson (1977) allows 280 yr. to be calculated for barite to attain 99.9% equilibrium with water at a recrystallization temperature of 110 ° C. Extrapolation to 80°C leads to -~ 18,000 yr.; to lower temperatures, extrapolation may be misleading. The equilibration times should be in any case much less than the span of time between ore deposition (Middle-Upper Triassic) and metamorphic overprint (26-11 Myr. ago; Carmignani et al., 1978).
BARITE IRON OXIDE PYRITE DEPOSITS FROM THE APUANE ALPS TABLE II Calculated oxygen isotope temperatures of barites and gangue carbonates from stratiform ore bodies Ore deposit
Mineral
Temperature* ( : C )
Monte Arsiccio
barite
88-97 (mean 93 )
48-55 (mean 52 )
carbonate (calcite)
81-94 (mean +89)
41-50 (mean 46)
Buca della Vena
barite
76-107 (mean 92)
40-61 (mean 51 )
Pollone
barite
91-100 (mean 95)
51-57 (mean 53)
All deposits
barite
76-107 (mean 93+6)
40-61 (mean 52 + 4)
*Barite-water and calcite-water isotope fractionations used in the calculation are from Kusakabe and Robinson (1977) and O'Neil et al. ( 1969 ), respectively.
4.2. Vein ore bodies
The analyzed barites from vein ore bodies at Pollone have fairly uniform c~34S-values ( + 15.8 to + 20.8%0 ), and are similar to samples from stratiform bodies either at Pollone or in the other deposits. Similarly, analyzed pyrite and base-metal sulfides are quite uniform in ~34S from - 3.7 to + 0.5%~. These values are considerably higher than those of stratiform pyrite from the same deposit, and fall in the range observed for the stratiform sulfide ores at Monte Arsiccio and Buca della Vena. Finally, baritepyrite and sphalerite-galena pairs give concordant sulfur isotopic temperatures close to 385 ° C (Fig. 4), in agreement with peak metamorphic temperatures estimated by Di Pisa et al. ( 1985 ) for the area of interest. These isotopic and thermometric data concur to suggest equilibrium deposition of barite and sulfide minerals from metamorphic fluids which remobilized pre-existing stratiform ores. According to the work of Kusakabe and Robinson ( 1977 ), a few weeks should be sufficient for 99.9 % oxygen isotope equilibration between barite and water at 385 ° C, and a ~180-- + 13%o
255
is calculated for water in equilibrium with barite under question by means of the equation: 1000 In O~BaSO4-H~O= 3.01 ( 106/T 2) _ 6.79 where ~ = oxygen isotope fractionation factor. It is suggested that the water has become enriched in 180 by exchange with the host schistose rocks during metamorphism (Taylor, 1979).
5. Isotopic composition of gangue carbonates Preliminary results of gangue carbonates from the stratiform ore deposit of Monte Arsiccio, constituted essentially by calcite, are: 513C = - 29.3 to - 20.5%c (mean - 26.3%0 ) and 5180 = + 17.9 to + 19.4%o (mean + 18.5%o ). The ~13C-values match well with those of carbonates associated with bacterial sulfate reduction (usually between - 3 0 and -20%0; Hudson, 1977), i.e. with an origin of carbon mainly from anaerobic oxidation of organic matter (average ( ~ 1 3 C ~--- - - 25%0; Deines, 1980). These data further support a biogenic origin of sulfur in the sulfide minerals in the deposit. The c~180-values are 2-4%0 higher than those of coexisting barites, as expected at equilibrium [see mineral-water isotope fractionation curves in O'Neil et al. (1969) and in Kusakabe and Robinson (1977)]. Calcite-barite oxygen isotope fractionation depends very little on ternperature (slope is close to zero), and the slope varies from negative to positive values depending on the barite-water curve used (1 m NaC1 or pure water solution, in Kusakabe and Robinson, 1977). Therefore, thermometric and ~8OH20 calculations based on calcite-barite pairs can result in highly uncertain values. In Table II we report the calculated isotopic temperatures for the analyzed carbonates (considered as calcites), assuming that they experienced the same environmental conditions as barites. The obtained values are in good agreement with those calculated for coexisting barites, thus supporting isotopic equilibrium be-
256
tween the two minerals, as well as contemporaneous deposition and a common post-depositional history.
6. Conclusions T h e sulfur isotopic features s u p p o r t the syns e d i m e n t a r y origin in M i d d l e - U p p e r Triassic t i m e s of the s t r a t i f o r m deposits. T h e m o s t imp o r t a n t source of sulfur was s e a w a t e r sulfate, which u n d e r w e n t b a c t e r i a l r e d u c t i o n p r o b a b l y in a n e a r - s h o r e e n v i r o n m e n t . C a r b o n isotopic c o m p o s i t i o n of gangue c a r b o n a t e s is in keeping with such a genetic model. B o t h b a r i t e a n d gangue c a r b o n a t e p r o b a b l y were p r e c i p i t a t e d during s e d i m e n t a t i o n a n d / o r early diagenesis, a n d suffered p o s t - d e p o s i t i o n a l oxygen isotopic equilibration with e n v i r o n m e n t a l water, probably in a later diagenetic stage at t e m p e r a t u r e s possibly of 4 0 - 1 1 0 ° C . Vein ore bodies at P o l l o n e f o r m e d d u r i n g Alpine orogeny, f r o m m e t a m o r p h i c fluids which remobilized s t r a t i f o r m ores. D e p o s i t i o n of barite a n d sulfide m i n e r a l s should have o c c u r r e d u n d e r isotopic equilibrium at t e m p e r a t u r e s of 3 8 0 - 4 0 0 ° C from w a t e r e n r i c h e d in 1sO b y int e r a c t i o n with c o u n t r y rocks, p r o b a b l y schists.
Acknowledgements T h e r e s e a r c h was s u p p o r t e d b y t h e M i n i s t e r o della P u b b l i c a I s t r u z i o n e t h r o u g h grants 60%1984 (G.C.) a n d 40%-1985 ( G . T . ) , a n d b y the C e n t r o N a z i o n a l e delle R i c e r c h e t h r o u g h a N.A.T.O. S e n i o r Fellowship (G.C.) at the Lab o r a t o i r e d ' H y d r o l o g i e et de G~ochimie Isotopique, Universit~ de P a r i s - S u d ( O r s a y ) , where h o s p i t a l i t y a n d assistance p r o v e d v e r y good. A d d i t i o n a l s u p p o r t f r o m t h e C e n t r o C.N.R. di Mineralogia e G e o c h i m i c a dei S e d i m e n t i (Fir e n z e ) is also acknowledged. We wish to express our a p p r e c i a t i o n to M. B e n v e n u t i for valuable c o o p e r a t i o n a n d helpful discussions. Finally, m a n y t h a n k s to R. Tosi for d r a f t i n g t h e figures.
G. CORTECCI ET AL.
References Benvenuti, M., Lattanzi, P., Tanelli, G. and Cortecci, G., 1986. The Ba-Fe-pyrite deposit of Buca della Vena, Apuane Alps, Italy. Rend. Soc. Ital. Mineral. Petrol., 41: 347-358. Carmignani, L., Giglia, G. and Kligfield, R., 1978. Structural evolution of the Apuane Alps: an example of continental margin deformation in the northern Apennines, Italy. J. Geol. (Chicago), 86: 487-504. Chambers, L.A., 1982. Sulfur isotope study of a modern intertidal environment, and the interpretation of ancient sulfides. Geochim. Cosmochim. Acta, 46: 721-728. Chiba, H., Kusakabe, M., Hirano, S., Matsuo, S. and Somiya, S., 1981. Oxygen isotope fractionation factors between anhydrite and water from 100 to 550°C. Earth Planet. Sci. Lett., 53: 55-62. Ciarapica, G. and Passeri, L., 1982. Panoramica sulla geologia delle Alpi Apuane alla luce delle pi5 recenti ricerche. Mem. Soc. Geol. Ital., 24: 198-208. Ciarapica, G. and Zaninetti, L., 1983. Faune ~ Foraminif~res ladino-carniens dans les schistes de Fornovolasco, Unit~ delle Panie (Alpes Apuanes, Italie). Rev. Pal~obiol., 2: 47-59. Claypool, G.E., Holser, W.T., Kaplan, I.R., Sakai, H. and Zak, I., 1980. The age curves of sulfur and oxygen isotopes in marine sulfate and their mutual interpretation. Chem. Geol., 29: 199-260. Corsini, F., Cortecci, G., Leone, G. and Tanelli, G., 1980. Sulfur isotope study of the skarn-(Cu-Pb-Zn)-sulfide deposit of Valle del Temperino, Campiglia Marittima, Tuscany, Italy. Econ. Geol., 75: 83-96. Cortecci, G., Reyes, E., Berti, G. and Casati, P., 1981. Sulfur and oxygen isotopes in Italian marine sulfates of Permian and Triassic ages. Chem. Geol., 34: 65-79. Cortecci, G., Lattanzi, P. and Tanelli, G., 1984.Barite-iron oxide-pyrite ( + Pb, Zn, Ag) ore deposits ofApuane Alps, Tuscany, Italy. Abstr. 27th Int. Geol. Congr., Aug. 4-14, Moscow, Vol. VI, Sect. 12, pp. 69-70. Cortecci, G., Lattanzi, P. and Tanelli, G., 1985. Barite-iron oxide-pyrite deposits from Apuane Alps (northern Tuscany, Italy). Geol. Zb., Geol. Carpathica, 36: 347357. Deines, P., 1980. The isotopic composition of reduced organic carbon. In: P. Fritz and J.Ch. Fontes (Editors), Handbook of Environmental Isotope Geochemistry, Vol. i, The Terrestrial Environment, A. Elsevier, Amsterdam, pp. 329-406.
Di Pisa, A., Franceschelli, M., Leoni, L. and Meccheri, M., 1985. Regional variation of the metamorphic temperature across the Tuscanidi I Unit and its implications on the Alpine metamorphism (Apuane Alps, N. Tuscany). Neues Jahrb., Mineral. Abh., 151: 197-211. Hudson, J.D., 1977. Stable isotopes and limestone lithification. J. Geol. (London), 133: 637-660. Kaplan, I.R. and Rittenberg, S.C., 1964. Microbiological
BARITE-IRON OXIDE-PYRITE DEPOSITS FROM THE APUANE ALPS
fractionation of sulfur isotopes. J. Gen. Microbiol., 34: 195-212. Kroopnick, P. and Craig, H., 1976. Oxygen isotope fractionation in dissolved oxygen in the deep sea. Earth Planet. Sci. Lett., 32: 375-388. Kusakabe, M. and Robinson, B.W., 1977. Oxygen and sulfur isotope equilibria in the BaSO4-HSO4-H20 system from 110 to 350 ° C and applications. Geochim. Cosmochim. Acta, 41: 1033-1040. Lloyd, R.M., 1967. Oxygen-18 composition of oceanic sulfate. Science, 156: 1228-1231. Lloyd, R.M., 1968. Oxygen isotope behavior in the sulfatewater system. J. Geophys. Res., 73: 6099-6110. McCrea, J.M., 1950. On the isotopic chemistry of carbonates and a paleotemperature scale. J. Chem. Phys., 18: 849-857. Mizutani, Y. and Rafter, T.A., 1969. Oxygen isotopic composition of sulphates, Part 4. Bacterial fractionation of oxygen isotopes in the reduction of sulphate and in the oxidation of sulphur. N.Z.J. Sci., 12: 60-68. Mizutani, Y. and Rafter, T.A., 1973. Isotopic behaviour of sulphate oxygen in the bacterial reduction of sulphate. Geochem. J., 6: 183-191. Nakai, N. and Jensen, M.L., 1964. The kinetic isotope effect in the bacterial reduction and oxidation of sulphur. Geochim. Cosmochim. Acta, 28: 1893-1912. Ohmoto, H. and Rye, R.O., 1979. Isotopes of sulfur and carbon. In: H.L. Barnes (Editor), Geochemistry of Hydrothermal Ore Deposits. Wiley, New York, N.Y., 2nd ed., pp. 509-567. O'Neil, J.R., Clayton, R.N. and Mayeda, T.K., 1969. Oxygen isotope fractionation in divalent metal carbonates. J. Chem. Phys., 51: 5547-5558. O'Neil, J.R., Adami, L.H. and Epstein, S., 1975. Revised value for the 1sO fractionation between CO2 and water at 25°C. U.S. Geol. Surv. J. Res., 3: 623-624. Orberger, B. and Saup~, F., 1985. Baryte, pyrite and haematite mineralizations of the SW Apuane Alps {Tuscany, Italy). Abstr. Dtsch. Mineral. Ges.-Soc. G~ol. Appl.-Ges. Dtsch. Metallhtitten Bergleute, Joint Meet., Aachen.
257 Orberger, B., Arnold, M. and Saupd, F., 1985. Sulfur isotope geochemistry of baryte, pyrite and galena in baryte deposits from the Apuane Alps (Tuscany, Italy). Terra Cognita, 5:155 (abstract). Orberger, B., Arnold, M. and Saup~, F., 1986. Sulfur isotopic studies of the minerals from the Pollone and Monte Arsiccio deposits (SW Apuane Alps, Tuscany, Italy). Fortschr. Mineral., 64: 215-226. Pierre, C., 1985. Isotopic evidence for the dynamic redox cycle of dissolved sulphur compounds between free and interstitial solutions in marine salt pans. Chem. Geol., 53: 191-196. Preite Martinez, M., Turi, B., Di Sabatino, B. and Negretti, G., 1978. Considerazioni geotermometriche su alcune formazioni metamorfiche di eta Alpina della Toscana. Period. Mineral., 47: 63-71. Rickard, D.T., Zweifel, H. and Donnelly, T.H., 1979. Sulfur isotope systematics in the Asen pyrite-barite deposits, Skellefte district, Sweden. Econ. Geol., 74: 1060-1068. Savin, S.M., 1980. Oxygen and hydrogen isotope effects in low-temperature mineral-water interactions. In: P. Fritz and J. Ch. Fontes (Editors), Handbook of Environmental Isotope Geochemistry, Vol. 1. The Terrestrial Environment, A. Elsevier, Amsterdam, pp. 283-327. Taylor, H.P., 1979. Oxygen and hydrogen isotope relationships in hydrothermal mineral deposits. In: H.L. Barnes (Editor), Geochemistry of Hydrothermal Ore Deposits. Wiley, New York, N.Y., 2nd ed., pp. 236-277. Thode, H.G. and Monster, J., 1965. Sulfur isotope geochemistry of petroleum, evaporites and ancient seas. In: Fluids in Subsurface Environments. Am. Assoc. Pet. Geol. Mere., 4: 367-377. Willan, R.C.R. and Coleman, M.L., 1983. Sulfur isotope study of the Aberfeldy barite, zinc, lead deposit and minor sulfide mineralization in the Dalradian metamorphic terrains, Scotland. Econ. Geol., 78: 1619-1656. Zak, I., Sakai, H. and Kaplan, I.R., 1980. Factors controlling the 1~O/1~O and :~4S/:~2Sisotope ratios of ocean sulfates, evaporites and interstitial sulfates from modern deep-sea sediments. In: Isotope Marine Chemistry. Uchida Rokakuho, Tokyo, pp. 339-373.
249
Sulfur, oxygen and carbon isotope geochemistry of barite-iron oxide-pyrite deposits from the Apuane Alps (northern Tuscany, Italy) G. CORTECCI 1, P. LATTANZI 2 and G. TANELLI 3 1Dipartimento di Scienze della Terra, Universit& di Pisa, 1-56100 Pisa (Italy) 2Dipartimento di Scienze della Terra, Universit~ di Firenze, 1-50121 Firenze (Italy) 3Dipartimento di Scienze della Terra, Universit& di Napoli, 1-80138 Napoli (Italy) (Accepted for publication April 13, 1989)
Abstract Cortecci, G., Lattanzi, P. and Tanelli, G., 1989. Sulfur, oxygen and carbon isotope geochemistry of barite-iron oxidepyrite deposits from the Apuane Alps (northern Tuscany, Italy). In: K. GrSnvold (Guest-Editor), Water-Rock Interaction. Chem. Geol., 76: 249-257. The sulfur, oxygen and carbon isotopic compositions of a total of 64 samples from stratiform and vein barite-iron oxide-pyrite ore bodies of the Apuane Alps are presented. In stratiform ores, sulfide minerals display a wide range of (~34Sfrom -20.9 to + 18.9%o. Similarly, barites are quite variable in •348, with values from +4.3 to +30.8%o, with a clustering around + 19.4%o; on the other hand, their JlSO-values are fairly uniform around + 15.8%o. Calculated isotopic temperatures from barite-pyrite pairs are broadly distributed, even within a single deposit. Gangue carbonates show quite light J13C-values from -29.3 to -20.5%o, and JlSO-values closely around + 18.5%o. In vein ores, sulfide minerals and barite are much more uniform in J34S, with values closely around - 1.1 and + 18.0%o, respectively; barite JlS0-values are nearly equal to + 13.4%o. Moreover, barite-pyrite and barite-galena pairs give concordant isotopic temperatures of ~ 385 ° C. The isotopic data concur with previously acquired lines of evidence to support the following genetic model: (a) Stratiform ores are synsedimentary in origin, the main source of sulfur having been Middle-Upper Triassic seawater sulfate which underwent bacterial reduction. Concurrent oxidation of organic matter supplied carbon in gangue carbonates. Probably, both barite and gangue carbonate suffered post-depositional oxygen isotope equilibration with water under diagenetic conditions. (b) Vein ores formed from remobilization of stratiform ores by metamorphic fluids (Alpine orogeny ) at 380-400 ° C, highly enriched in 'sO probably by interaction with country schists.
1. Introduction In the Apuane Alps region (northwestern Tuscany, Italy), barite, iron oxide + pyrite are presently mined from the deposits of Monte Arsiccio, Pollone and Buca della Vena (Fig. 1 ). These deposits are currently the subject of a comprehensive research program. The main aspects of geological setting, mineralogy and geo0009-2541/89/$03.50
chemistry have recently been described by Cortecci et al. (1984, 1985) and Benvenuti et al. (1986). The data collected so far allowed the aforementioned authors to hypothesize a sedimentary-metamorphic (Middle-Upper Triassic and Alpine) genetic model for the deposits. In this paper, the available S, C and O isotope data on minerals from these deposits are utilized in an attempt to clarify the interaction be-
© 1989 Elsevier Science Publishers B.V.
250
G. CORTECCI ET AL.
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Fig. 1. Simplifiedstructural schemeof the ApuaneAlps (after Ciarapicaand Passeri, 1982), with locationsof the ore deposits studied in the present work (11). Structural units: a=Nucleo MetamorficoApuano; b=Unit~ di Massa; c=Unit~ di Fornovolasco-Panie;d = non-metamo~hic units (Tuscan Nappe and Ligurides). tween the fluids and the mineralizations in the various stages of their sedimentary-metamorphic evolution.
2. Summary of geology and mineralogy of the deposits The complex geological architecture of the Apuane Alps can be schematically described as the result of overthrusting, during the Alpine orogeny, of several metamorphic and non-metamorphic tectonic units corresponding to differ-
ent paleogeographic domains (Carmignani et al., 1978; Ciarapica and Passeri, 1982). The barite-iron oxide-pyrite mineralizations are restricted to or near the contact between the Upper Triassic dolomite formation of "Grezzoni" and the underlying phyllitic complex of "Scisti di Fornovolasco", which, at least in their upper portion, are ascribed to the Ladinian-Carnian (Ciarapica and Zaninetti, 1983) (Fig. 2). Specifically, the mineralizations are mostly associated with calcareous intercalations in the upper part of the phyllitic
251
BARITE IRONOXIDE-PYRITEDEPOSITSFROMTHE APUANEALPS " LINITA D I F O R N O V O I . ~ 4 S C O - P A N I E
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Fig. 2. Time-space relationships between the ore deposits under study ( • ) and the metamorphic tectonic units of Apuane Alps (after Cortecci et al., 1985). S o l i d w a v y l i n e s = tectonic contacts; s o l i d s t r a i g h t l i n e s = conformable stratigraphic contacts; d a s h e d l i n e = disconformity, s F = Scisti di Fornovolasco (Ladinian-Carnian); T~ = Middle Triassic; T~ = Upper Triassic; C a r = Carnian; E o c = Eocene; Ol = Oligocene.
TABLE I Summary of texture and mineralogy of the barite-iron oxide-pyrite deposits of the Apuane Alps Stratiform bodies (Monte Arsiccio, Pollone, Buca della Vena )
Vein bodies (Pollone }
Texture
typically banded, with alternation of millimeter massive barite with minor pyrite, locally crustified and sized layers of barite and iron minerals, often brecciated folded and faulted; massive non-banded bodies may also occur
Main mineralogy
barite, hematite, magnetite, pyrite (arsenopyrite, barite, pyrite (sphalerite, galena, sulfosalts); sulfosalts, sphalerite, galena); calcite, dolomite, quartz, fluorite, sericite, calcite quartz, (albite, mica, chlorite)
complex. The metamorphic grade of the latter corresponds to the greenschist facies (380465°C; 2-4 kbar; Di Pisa et al., 1985; Orberger and Saupd, 1985 ). The mineralization typically consists of lens-shaped or stratiform near-concordant bodies, with subordinate related discordant masses. Table I summarizes the texture and mineralogy of the stratiform and vein ores. The marked stratigraphic control, the mostly stratiform near-concordant attitude of the ore, and the macro-, mesa- and micro-scale evidence of metamorphic deformation and recrys-
tallization led Cortecci et al. (1985) to interpret stratiform bodies as original sedimentarydiagenetic concentrations, subsequently metamorphosed, tectonized and partly mobilized during the Alpine event, during which the vein bodies were emplaced.
3. Analytical procedures In this paper, we utilize recently acquired S, C and O isotope data, as well as previously pub-
252
lished S isotope data (Cortecci et al., 1985; Orberger et al., 1985). Barite, pyrite, arsenopyrite, sphalerite and galena were separated by heavy liquids, magnetic and hand-picking methods; barite was further purified by repeated treatments with HF-HC1 mixture. The 34S/32S and lsO/160 ratios of the minerals were determined by means of procedures described in Corsini et al. (1980) and Cortecci et al. (1981). Based on X-ray diffraction, the analyzed gangue carbonates appear to constitute essentially calcite _+minor dolomite. In the hand specimens, they occur intimately associated with barite + pyrite + quartz _+muscovite. After removal of pyrite by hand-picking, the residual material was reacted with 100% H3P04 at 25 ° C (McCrea, 1950) for ~ 72 hr., and the obtained C02 analyzed for 13C/12C and lso/160 ratios. The isotopic ratios are given in 5-notation, in !'i~, relative to CDT for sulfur, PDB for carbon and V-SMOW for oxygen. In calculating the ~SO-values, the value of 1.0412 for the ~sO fractionation factor between COe and H20 at 25°C was used (O'Neil et al., 1975). The analytical uncertainty (difference between complete duplicate analyses) was mostly better than+0.1~i~ for all different kinds of measurements.
4. Isotopic composition of barite and sulfide minerals T h e 534S - and ~aSO-values so far produced for barite and sulfide minerals from the deposits under question are shown in Fig. 3. We mention here that the oxygen isotope enrichment factor between barite and aqueous sulfate is likely near zero (Savin, 1980, p. 304); the same is expected for the sulfur isotope enrichment factor, on the basis of the discussion by Chiba et al. (1981, p. 60) and the sulfur isotope enrichment factor measured for the gypsumaqueous sulfate system (1.65%~; Thode and Monster, 1965).
G. CORTECCI ET AL.
4.1. S t r a t i / o r m ore bodies
Barite and pyrite display highly variable 534Svalues from +4.3 to +30.8%o and from - 2 0 . 9 to + 18.9%~, respectively. Internal variations for each deposit are from 7 to 21%c for barite and from 10 to 34%~ for pyrite. Isotopic compositions of galena and arsenopyrite also appear to be fairly variable. These isotopic features are in agreement with the proposed genetic model and in particular strongly support a mostly bacteriogenic origin of sulfur in the sulfide minerals from non-equilibrium reduction of sulfate in semi-closed shallow-water environment (Ohmoto and Rye, 1979; Chambers, 1982), possibly in brackish water according to the paleogeographic reconstruction of Ciarapica and Passeri (1982). This interpretation basically agrees with that of Orberger et al. (1985, 1986). The main source of sulfur was likely Middle-Upper Triassic seawater sulfate, the 5"~4S of which should have been close to +15%c (Claypool et al., 1980; Cortecci et al., 1981). Barites with 534S > + 15%o can account for sulfate variously enriched in 34S by bacterial reduction, whereas barites with 534S < +15%c probably reflect variable contributions of sulfate from partial reoxidation of isotopically light bacteriogenic sulfide (near-zero fractionation effects; Kaplan and Rittenberg, 1964; Nakai and Jensen, 1964; Mizutani and Rafter, 1969). Bacterial reduction of pore-water sulfate may have continued during early diagenesis, favoured by the presence of organic compounds in the sediments as presently testified by the widespread occurrence of carbonaceous matter in the country schists, thus producing some pyrite with 534S close to that of seawater sulfate (see Ohmoto and Rye, 1979). When coexisting minerals are considered, barite is always enriched in 34S with respect to pyrite (and galena); however, calculated isotopic temperatures are broadly distributed, even within a single deposit, with values from 122 ° to 530°C (Fig. 4). These features most likely indicate disequilibrium conditions between
BARITE-IRON OXIDE-PYRITE DEPOSITSFROMTHE APUANEALPS
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Fig. 3. Frequency distributions of 6-values (our data) and ranges of 634S-values from Orberger et al. (1985). The data of these authors refer to both stratiform and vein ore bodies: they did not observe significant differences of isotopic composition between the different types of ores. _Note added in proof, see also Orberger et al. (1986).
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Fig. 4. Relationship between 6:~4S-values of associated barite and sulfide minerals from stratiform ores. Dashed lines connect coexisting minerals. Calculated isotopic temperatures from barite-sulfide pairs in brackets (fractionation factors from Ohmoto and Rye, 1979). Isotopic temperatures from barite-pyrite and sphalerite-galena pairs in vein ore at Pollone are shown up on the right.
barite and pyrite, as expected for minerals formed in a surface, low-temperature environment, and suggest that the original disequili-
bria basically survived the Alpine metamorphic event (Rickard et al., 1979; Willan and Coleman, 1983).
254
G. C O R T E C C I E T A L .
As stated before, reduced sulfur was likely oxidized to sulfate, possibly in a brackish-water environment, i.e. in water of ~isO between ~ 0c& (non-evaporated seawater; note that no sign of sulfate evaporites was found in the deposits and in the host rocks) and -6%c (meteoric water in a temperate climate ). Oxidation may have occurred biologically or abiologically; in the first case, sulfate of 51sO close to that of environmental water could have been produced (Mizutani and Rafter, 1969), whereas in the second case both water-derived and dissolved oxygen should have been incorporated in the product sulfate (Lloyd, 1967, 1968). According to experiments of Lloyd (1967), possible ~sOvalues for sulfate from inorganic oxidation are calculated to be between + 0.8 and + 9.4%~, depending on ~ s O of water ( - 6 to 0%c ) and ~ s O of dissolved oxygen ( +24 to +38%c, as a function of the biological activity in the water; Kroopnick and Craig, 1976). The sulfate from oxidation may have mixed with Middle-Upper Triassic seawater sulfate probably with ~ 8 0 close to + 10 %~ (Claypool et al., 1980 ), or higher depending on activity of sulfate-reducing bacteria. As shown in Fig. 5, barites from Pollone and Monte Arsiccio deposits have quite uniform 5~SO-values from +14.9 to +16.4%~, (mean + 15.7 + 0.4~;i~ ) and quite variable ~34S-values; samples from Buca della Vena deposit display similar, but more variable, ~lSO-values from
18
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+14.2 to +18.1%o (mean +16.0_+1.6%~) and nearly equal ~34S-values. Mixing of sulfate from oxidation with more or less bacterially reduced sulfate is not evident from these data. Moreover, the ~lSO-values of barites with ~'~4S between + 16 and + 22%o cannot be explained by isotopic effects in the bacterial reduction of seawater sulfate: during this reaction, the ratio of 180 enrichment factor to 34S enrichment factor in the residual sulfate is commonly between 0.25 and 0.40, with possible variations from - 0.15 to 0.67 (Mizutani and Rafter, 1969, 1973; Zak et al., 1980; Pierre, 1985 ). We conclude that original ~lSO-values of barites suffered postdepositional modifications by equilibrium (or near equilibrium) isotopic exchanges with water of uniform oxygen isotopic composition over a restricted range of temperatures. The possibility that barite formed during a late-diagenetic stage from interstitial sulfate homogenized in 1sO by diffusion, mixing and isotope exchange processes seems to be unlikely. In this case, in fact, much more uniform ~34S-values should also be observed, especially for barites from Monte Arsiccio and Pollone deposits. As barite should have been lithified at time of metamorphism, characterized by low water/ rock ratios in the "Unith di FornovolascoPanie" (Preite Martinez et al., 1978), isotopic equilibration conceivably took place during diagenesis to lithification. Table II gives the calculated isotopic equilibrium temperatures for pore water with initial 3180 of either 0 or - 6%o. Interpolation of the data from 1 m NaC1 experiments by Kusakabe and Robinson (1977) allows 280 yr. to be calculated for barite to attain 99.9% equilibrium with water at a recrystallization temperature of 110 ° C. Extrapolation to 80°C leads to -~ 18,000 yr.; to lower temperatures, extrapolation may be misleading. The equilibration times should be in any case much less than the span of time between ore deposition (Middle-Upper Triassic) and metamorphic overprint (26-11 Myr. ago; Carmignani et al., 1978).
BARITE IRON OXIDE PYRITE DEPOSITS FROM THE APUANE ALPS TABLE II Calculated oxygen isotope temperatures of barites and gangue carbonates from stratiform ore bodies Ore deposit
Mineral
Temperature* ( : C )
Monte Arsiccio
barite
88-97 (mean 93 )
48-55 (mean 52 )
carbonate (calcite)
81-94 (mean +89)
41-50 (mean 46)
Buca della Vena
barite
76-107 (mean 92)
40-61 (mean 51 )
Pollone
barite
91-100 (mean 95)
51-57 (mean 53)
All deposits
barite
76-107 (mean 93+6)
40-61 (mean 52 + 4)
*Barite-water and calcite-water isotope fractionations used in the calculation are from Kusakabe and Robinson (1977) and O'Neil et al. ( 1969 ), respectively.
4.2. Vein ore bodies
The analyzed barites from vein ore bodies at Pollone have fairly uniform c~34S-values ( + 15.8 to + 20.8%0 ), and are similar to samples from stratiform bodies either at Pollone or in the other deposits. Similarly, analyzed pyrite and base-metal sulfides are quite uniform in ~34S from - 3.7 to + 0.5%~. These values are considerably higher than those of stratiform pyrite from the same deposit, and fall in the range observed for the stratiform sulfide ores at Monte Arsiccio and Buca della Vena. Finally, baritepyrite and sphalerite-galena pairs give concordant sulfur isotopic temperatures close to 385 ° C (Fig. 4), in agreement with peak metamorphic temperatures estimated by Di Pisa et al. ( 1985 ) for the area of interest. These isotopic and thermometric data concur to suggest equilibrium deposition of barite and sulfide minerals from metamorphic fluids which remobilized pre-existing stratiform ores. According to the work of Kusakabe and Robinson ( 1977 ), a few weeks should be sufficient for 99.9 % oxygen isotope equilibration between barite and water at 385 ° C, and a ~180-- + 13%o
255
is calculated for water in equilibrium with barite under question by means of the equation: 1000 In O~BaSO4-H~O= 3.01 ( 106/T 2) _ 6.79 where ~ = oxygen isotope fractionation factor. It is suggested that the water has become enriched in 180 by exchange with the host schistose rocks during metamorphism (Taylor, 1979).
5. Isotopic composition of gangue carbonates Preliminary results of gangue carbonates from the stratiform ore deposit of Monte Arsiccio, constituted essentially by calcite, are: 513C = - 29.3 to - 20.5%c (mean - 26.3%0 ) and 5180 = + 17.9 to + 19.4%o (mean + 18.5%o ). The ~13C-values match well with those of carbonates associated with bacterial sulfate reduction (usually between - 3 0 and -20%0; Hudson, 1977), i.e. with an origin of carbon mainly from anaerobic oxidation of organic matter (average ( ~ 1 3 C ~--- - - 25%0; Deines, 1980). These data further support a biogenic origin of sulfur in the sulfide minerals in the deposit. The c~180-values are 2-4%0 higher than those of coexisting barites, as expected at equilibrium [see mineral-water isotope fractionation curves in O'Neil et al. (1969) and in Kusakabe and Robinson (1977)]. Calcite-barite oxygen isotope fractionation depends very little on ternperature (slope is close to zero), and the slope varies from negative to positive values depending on the barite-water curve used (1 m NaC1 or pure water solution, in Kusakabe and Robinson, 1977). Therefore, thermometric and ~8OH20 calculations based on calcite-barite pairs can result in highly uncertain values. In Table II we report the calculated isotopic temperatures for the analyzed carbonates (considered as calcites), assuming that they experienced the same environmental conditions as barites. The obtained values are in good agreement with those calculated for coexisting barites, thus supporting isotopic equilibrium be-
256
tween the two minerals, as well as contemporaneous deposition and a common post-depositional history.
6. Conclusions T h e sulfur isotopic features s u p p o r t the syns e d i m e n t a r y origin in M i d d l e - U p p e r Triassic t i m e s of the s t r a t i f o r m deposits. T h e m o s t imp o r t a n t source of sulfur was s e a w a t e r sulfate, which u n d e r w e n t b a c t e r i a l r e d u c t i o n p r o b a b l y in a n e a r - s h o r e e n v i r o n m e n t . C a r b o n isotopic c o m p o s i t i o n of gangue c a r b o n a t e s is in keeping with such a genetic model. B o t h b a r i t e a n d gangue c a r b o n a t e p r o b a b l y were p r e c i p i t a t e d during s e d i m e n t a t i o n a n d / o r early diagenesis, a n d suffered p o s t - d e p o s i t i o n a l oxygen isotopic equilibration with e n v i r o n m e n t a l water, probably in a later diagenetic stage at t e m p e r a t u r e s possibly of 4 0 - 1 1 0 ° C . Vein ore bodies at P o l l o n e f o r m e d d u r i n g Alpine orogeny, f r o m m e t a m o r p h i c fluids which remobilized s t r a t i f o r m ores. D e p o s i t i o n of barite a n d sulfide m i n e r a l s should have o c c u r r e d u n d e r isotopic equilibrium at t e m p e r a t u r e s of 3 8 0 - 4 0 0 ° C from w a t e r e n r i c h e d in 1sO b y int e r a c t i o n with c o u n t r y rocks, p r o b a b l y schists.
Acknowledgements T h e r e s e a r c h was s u p p o r t e d b y t h e M i n i s t e r o della P u b b l i c a I s t r u z i o n e t h r o u g h grants 60%1984 (G.C.) a n d 40%-1985 ( G . T . ) , a n d b y the C e n t r o N a z i o n a l e delle R i c e r c h e t h r o u g h a N.A.T.O. S e n i o r Fellowship (G.C.) at the Lab o r a t o i r e d ' H y d r o l o g i e et de G~ochimie Isotopique, Universit~ de P a r i s - S u d ( O r s a y ) , where h o s p i t a l i t y a n d assistance p r o v e d v e r y good. A d d i t i o n a l s u p p o r t f r o m t h e C e n t r o C.N.R. di Mineralogia e G e o c h i m i c a dei S e d i m e n t i (Fir e n z e ) is also acknowledged. We wish to express our a p p r e c i a t i o n to M. B e n v e n u t i for valuable c o o p e r a t i o n a n d helpful discussions. Finally, m a n y t h a n k s to R. Tosi for d r a f t i n g t h e figures.
G. CORTECCI ET AL.
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