Geochemical,and stable and radiogenic isotope records in Devonian and Early Carboniferous carbonates from Valle de Tena,central Pyrenees (Spain): evidence for their diagenetic environments

Mineralogical, textural and geochemical investigations were made to determine the post-depositional evolution of Devonian and Early Carboniferous carbonates from Valle de Tena. The carbonate association is made up of low-Mg calcite, which occurs as micrite, spar cements, neomorphic patches and spar filling veinlets. Non-stoichiometric dolomite and ankerite occur as cements (dolomite also as replacements) in the Middle Devonian, post-dating calcite types. All these phases pre-date tectonic stylolites, indicating compaction after stabilization of the carbonate minerals. Strontium concentrations indicate that Early Devonian and Early Carboniferous micrites initially precipitated as aragonite; Middle and Late Devonian micrites precipitated as high-Mg calcites. Both precursors were diagenetically stabilized to low-Mg calcites through interaction with meteoric waters in phreatic environments. Trace elements in dolomite and ankerite indicate precipitation from Sr-enriched meteoric water. All studied carbonates, except Middle Devonian limestones, precipitated in reducing environments, which favoured incorporation of Fe and Mn. Late calcite generations precipitated from more saline waters than micrites. Light ¹⁸O values in micrites suggest alteration mainly in meteoric-phreatic environments. The dolomites and ankerites precipitated from more ¹⁸O-depleted fluids than the calcites, suggesting a greater contribution from meteoric waters. Variations in ¹³C of micrites represent primary secular trends, according to published ¹³C variations. The ¹³C oscillations within each succession probably relate to sea-level oscillations. Strontium isotopes also point to a meteoric origin of diagenetic fluids. Model calculations suggest that O and Sr isotopes equilibrated between calcites and fluid at relatively low water/rock ratios, whereas C isotopic signatures are inherited from limestones.     

Boron isotope variations in nature: a synthesis

The large relative mass difference between the two stable isotopes of boron, ¹⁰B and ¹¹B, and the high geochemical reactivity of boron lead to significant isotope fractionation by natural processes. Published ¹¹B values (relative to the NBS SRM-951 standard) span a wide range of 90. The lowest ¹¹B values around — 30 are reported for non-marine evaporite minerals and certain tourmalines. The most ¹¹B-enriched reservoir known to date are brines from Australian salt lakes and the Dead Sea of Israel with ¹¹B values up to +59. Dissolved boron in present-day seawater has a constant world-wide ¹¹B value of + 39.5. In this paper, available ¹¹B data of a variety of natural fluid and solid samples from different geological environments are compiled and some of the most relevant aspects, including possible tracer applications of boron-isotope geochemistry, are summarized.

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Résumé La grande différence relative de masse entre les isotopes stables du bore, ¹⁰B et ¹¹B, et la grande réactivité geochimique du bore ont pour conséquence un fractionnement isotopique naturel important. Les valeurs de ¹¹B publiées (par rapport au standard NBS SRM-951) varient de 90. Les valeurs de ¹¹B les plus basses (–30) correspondent aux evaporites non-marines et à certaines tourmalines. Le réservoir le plus enrichi en ¹¹B est représenté par les saumures des lacs salés d' Australie et par la Mer Morte en Israël, qui ont des valuers de ¹¹B allent jusqu'à + 59. L'eau de mer a une valeur de ¹¹B mondialement constante de + 39.5. Des valeurs de ¹¹B des solutions naturelles ainsi que des roches et minéraux de différentes origines, publiées jusqu'à présent, sont présentées ici. En outre quelques aspects importants concernant la géochimie des isotopes du bore y compris quelques applications sont exposés.

     

Stable isotope variations in the Quaternary epithermal calcite-fluorite deposit at Monte delle Fate near Cerveteri (Latium,central Italy)

Carbon, oxygen and hydrogen isotope variations have been measured in samples from the epithermal fluorite vein deposit at Monte delle Fate, Latium. The ranges in ¹³C and ¹⁸O of calcite are –1.3 to 3.4 and 9.5 to 17.3, respectively. D values of water extracted from fluid inclusions are –49 to –39 for calcite and –41 to –34 for fluorite. Fluid inclusion filling temperatures (225°–240°C) and salinites (3.75) are nearly the same for both fluorite and sparry calcite. An elongated form of calcite, of minor abundance, precipitated at lower temperatures. The data indicate that (1) the CO₂ involved in the mineralization was provided by the local marine limestones, (2) the waters were meteoric in origin and underwent an ¹⁸O shift of 10 permil by exchange with marine country rocks, and (3) all geochemical features can be explained by the action of two hydrothermal fluids. Hot brines recently discovered in the Cesano geothermal area, 30 km to the east, have temperatures and some chemical characteristics similar to the hydrothermal fluids at Monte delle Fate.     

Stable isotope (C,O, S) systematics of the mercury mineralization at Idrija,Slovenia: constraints on fluid source and alteration processes

The world-class Idrija mercury deposit (western Slovenia) is hosted by highly deformed Permocarboniferous to Middle Triassic sedimentary rocks within a complex tectonic structure at the transition between the External Dinarides and the Southern Alps. Concordant and discordant mineralization formed concomitant with Middle Triassic bimodal volcanism in an aborted rift. A multiple isotopic (C, O, S) investigation of host rocks and ore minerals was performed to put constraints on the source and composition of the fluid, and the hydrothermal alteration. The distributions of the ¹³C and ¹⁸O values of host and gangue carbonates are indicative of a fracture-controlled hydrothermal system, with locally high fluid-rock ratios. Quantitative modeling of the ¹³C and ¹⁸O covariation for host carbonates during temperature dependent fluid-rock interaction, and concomitant precipitation of void-filling dolomites points to a slightly acidic hydrothermal fluid (¹³C–4 and ¹⁸O+10), which most likely evolved during isotopic exchange with carbonates under low fluid/rock ratios. The ³⁴S values of hydrothermal and sedimentary sulfur minerals were used to re-evaluate the previously proposed magmatic and evaporitic sulfur sources for the mineralization, and to assess the importance of other possible sulfur sources such as the contemporaneous seawater sulfate, sedimentary pyrite, and organic sulfur compounds. The ³⁴S values of the sulfides show a large variation at deposit down to hand-specimen scale. They range for cinnabar and pyrite from –19.1 to +22.8, and from –22.4 to +59.6, respectively, suggesting mixing of sulfur from different sources. The peak of ³⁴S values of cinnabar and pyrite close to 0 is compatible with ore sulfur derived dominantly from a magmatic fluid and/or from hydrothermal leaching of basement rocks. The similar stratigraphic trends of the ³⁴S values of both cinnabar and pyrite suggest a minor contribution of sedimentary sulfur (pyrite and organic sulfur) to the ore formation. Some of the positive ³⁴S values are probably derived from thermochemical reduction of evaporitic and contemporaneous seawater sulfates.Editorial handling: P. Lattanzi     

Contrasting fluid flow patterns at the Bufa del Diente contact metamorphic aureole,north-east Mexico: evidence from stable isotopes

Impure limestones with interstratified metachert layers were contact metamorphosed and metasomatized by the Bufa del Diente alkali syenite. Massive marbles exhibit mineralogical and stable isotope evidence for limited fluid infiltration, confined to a 17 m wide zone at the contact. Influx of magmatic brines along most metacherts produced up to 4 cm thick wollastonite rims, according to calcite (Cc)+quartz (Qz)= wollastonite (Wo)+CO₂, and were observed at distances of up to 400 m from the contact. The produced CO₂ exsolved as an immiscible low density CO₂-rich fluid. Chert protolith isotope compositions were ¹⁸O (Qz)=27–30%. and ¹⁸O (Cc)=24–27%.. Many wollastonites in infiltrated metacherts have low ¹⁸O ranging from 11–17 and confirm that decarbonation occurred in presence of a magmatic-signatured fluid. Large gradients in ¹⁸O (Wo) across the rims may reach 6 The ¹⁸O of remaining quartz is often lowered to 15–20 whereas caleites largely retained their original compositions. The isotopic reversals of up to 10 between quartz and calcite along with reaction textures demonstrate non-equilibrium between infiltrating fluid in the aquifer and the assemblage calcite+quartz+wollastonite. This is compatible with the assumption of a down-temperature flow of magmatic fluids that occurred exclusively in the remaining quarzite layer. The ¹³C (Cc) and ¹⁸O (Cc) of marble calcites measured perpendicular to two metachert bands reveal significant isotopic alterations along distances of 4.5 cm and 7.5 cm from the wollastonite-marble boundary only into the hanging wall marble, suggesting an advection process caused by a fluid phase which movel upwards. Covariation trends of ¹³C (Cc) and ¹⁸O (Cc) across the alteration front indicate that this fluid was CO₂-rich. Mass balance calculations show that all CO₂-rich fluid produced by the decarbonation reaction was lost into overlying marble. The metachert aquifers did not leak with respect to water-rich fluids.     

Fluid regimes in the deformation of the Helvetic nappes,Switzerland, as inferred from stable isotope data

The stable isotope composition of veins, pressure shadows, mylonites and fault breccias in allochthonous Mesozoic carbonate cover units of the Helvetic zone show evidence for concurrent closed and open system of fluid advection at different scales in the tectonic development of the Swiss Alps. Marine carbonates are isotopically uniform, independent of metamorphic grade, where ¹³C=1.5±1.5 (1 ) and ¹⁸O=25.4±2.2 (1 ). Total variations of up to 2 in ¹³C and 1.5 in ¹⁸O occur over a cm scale. Calcite in pre- (Type I) and syntectonic (Type II) vein arrays and pressure shadows are mostly in close isotopic compliance with the matrix calcite, to within ±0.5, signifying isotopic buffering of pore fluids by host rocks during deformation, and closed system redistribution of carbonate over a cm to m scale. This is consistent with microstructural evidence for pressure solution — precipitation deformation.Type III post-tectonic veins occur throughout 5 km of structural section, extend several km to the basement, and accommodate up to 15% extension. Whereas the main population of Type III veins is isotopically undistinguishable from matrix carbonates, calcite in the largest of these veins is depleted in ¹⁸O by up to 23 but acquired comparable ¹³C values. This generation of veins involved geopressurized hydrothermal fluids at 200 to 350° C where ¹⁸O H₂O=–8 to +20, representing variable mixtures of ¹⁸O enriched pore and metamorphic fluids, with ¹⁸O depleted meteoric water. Calc-mylonites ( ¹⁸O=25 to 11) at the base of the Helvetic units, and syntectonic veins from the frontal Pennine thrust are characterized by a trend of ¹⁸O depletion relative to carbonate protoliths, due to exchange with an isotopically variable reservoir ( ¹⁸O H₂O=20 to 4). The upper limiting value corresponds to carbonate-buffered pore fluid, whereas the lower value is interpreted as ¹⁸O-depleted formation brines tectonically expelled at lithostatic pressure from the crystalline basement. Carbonate breccias in one of the large scale late normal faults exchanged with infiltrating ¹⁸O-depleted meteoric surface waters ( ¹⁸O=–8 to –10).During the main ductile Alpine deformation, individual lithological units and associated tectonic vein arrays behaved as closed systems, whereas mylonites along thrust faults acted as conduits for tectonically expelled lithostatically pressured reservoirs driven over tens of km. At the latest stages, marked by 5 to 15 km uplift and brittle deformation, low ¹⁸O meteoric surface waters penetrated to depths of several km under hydrostatic gradients.     

Stable isotope and fluid inclusion studies of carbonate deposits from the Tolfa Mountains mining district (Latium,central Italy)

Carbon and oxygen isotope analyses were made of representative samples of calcite and quartz from the carbonate deposits in the Tolfa Mountains mining district. Measurements were also made of hydrogen isotope compositions, filling temperatures and salinities of fluid inclusions in these minerals. There are three stages of mineralization at Tolfa. In stage I, characterized by calc-silicate hornfels, the carbonates have relatively high ¹⁸O values of 14.5 to 21.6 suggesting a rather low water/rock ratio. ¹³C values of –0.3 to 2.1 indicate that appreciable decarbonation or introduction of deep-seated carbon did not occur. Stage II is marked by phanerocrystalline carbonates; ¹⁸O values of 13.1 to 20.0 and ¹³C values of 0.7 to 5.0 identify them as hydrothermal veins rather than marbles. D values of –56 to –50 for inclusion fluids suggest a possible magmatic component to the hydrothermal fluid. Filling temperatures of coarse-grained samples of Calcite II are 309° to 362° C with a salinity range of 5.3 to 7.1 weight percent NaCl. Calculated ¹⁸O values of 11–12 for these fluids are again indicative of low water/rock ratios. The sparry calcites of stage III have ¹⁸O and ¹³C values of 8.1 to 12.9 and –1.7 to 3.2, respectively. D values of inclusion fluids are –40 to –33, clearly heavier than in earlier stages and similar to values of modern local ground waters. A salinity measurement of <0.1 weight percent NaCl in a sample of Calcite III is compatible with a relatively unaltered ground water origin for this fluid. Precipitation of the sparry calcite took place at much lower temperatures, around 160° C. For quartz, ¹⁸O values of 9.3 to 12.4 and D values for inclusions of –53 to –28 are consistent with its late occurrence and paragenetic link with associated carbonates.     

Authigenic Carbonate and Barite Mineralization in Sediments of the Deryugin Basin (Sea of Okhotsk)

Results of the analysis of mineralogy, geochemistry, and isotope parameters (¹³C, ³⁴S, ¹⁸O, and ⁸⁷Sr/⁸⁶Sr) of carbonates and barites from sediments of the Deryugin Basin in the Sea of Okhotsk are presented. The diagenetic nature of carbonates and barites, which are formed as a result of the prolonged activity of cold seeps acting along a fracture zone and supplying methane–and barium-saturated fluids, is determined. Any signs for hydrothermal activity were not observed.     

The Enåsen gold deposit,central Sweden

Stable isotope analyses of quartz, sulphides, and magnetite were conducted to provide information on thermal history and source of hydrothermal fluids in the Palaeoproterozoic Enåsen gold deposit. Reequilibration and homogenization of oxygen isotopes throughout the rock have apparently not occurred despite the upper amphibolite to granulite facies regional metamorphism that has affected the rocks. However, oxygen isotope geothermometry on a coexisting quartz-magnetite pair gave a minimum temperature for peak metamorphism of around 650 °C which agrees with Fe-Mg geothermometry. This suggests that grain-scale equilibrium is achieved. The variation in oxygen isotope ratios (¹⁸O = 7.3 – 10.5) on quartz from the metamorphosed acid sulphate alteration zone is suggested to represent a cooling trend in the fossil hydrothermal system with higher ¹⁸O-values in more superficial parts. Temperatures of alteration and silicification and isotopic composition of hydrothermal fluids could not be defined from the present data but it was recognized that the data is compatible with a epithermal genesis for the deposit. It is suggested that alteration, silicification, and mineralization at the Enåsen gold deposit took place in a high sulphidation epithermal environment at temperatures of around 200–250 °C and that the hydrothermal fluids consisted of meteoric and magmatic water. A tentative reconstruction of the fossil hydrothermal system is presented. Sulphur isotope ratios of sulphides from the fold-bearing quartz-sillimanite gneiss gave ³⁴S-values close to zero indicating a magmatic source of the sulphur.     

Bacteriogenic sulfur isotopic ratios in geology

Stable sulfur isotopes may aid in distinguishing sulfides of a magmatic hydrothermal origin from sulfides containing biogenic sulfur. For those sulfide ore depositis that are intimately associated with the intrusive body from which it is inferred their ore solutions were derived, the variation in S³⁴ values is generally less than ± 5 permil. Biogenic sulfides, on the other hand, exhibit a broad spread in S³⁴ values that is rately less than a few permil. The reason for this is that the sulfur produced, as hydrogen sulfide by anaerobic bacteria, is isotopically fractionated by variable amounts resulting in a relatively broad spread in S³⁴ values.Raw culture experiments have illustrated the isotopic effects resulting from reduction of sulfate to hydrogen sulfate by sulfate reducing bacteria. The characteristics of enrichment of S³² to a highly variable extent in hydrogen sulfide is verified by these experiments.In addition, a series of closed system raw culture experiments resulted in fractionation factors between 1.043 to 1.062 which are similar to variations in S³⁴ between juxtaposed sulfides in nature produced by bacteriogenic processes.Speech delivered on October 1, 1965, on the occasion of the colloquy concerning Sulphurisotopes organized by the Deutsche Forschungsgemeinschaft at Bad Sooden-Allendorf.     

Carbon and oxygen isotopic covariations in hydrothermal calcites

Isotopic covariations of carbon and oxygen in hydrothermal calcites are quantitatively modeled in terms of the following three mixing processes: (1) mixing between two different fluids which leads to the precipitation of calcite; (2) mixing between fluid and rock: (a) calcite precipitation due to fluid/rock interaction, (b) secondary alteration of primary calcite by interaction with a subsequent fluid. The models are derived from mass balance equations. A distinction among the three mixing processes can be made on a ¹³C vs ¹⁸O diagram, which places important constraints on the genesis of hydrothermal mineralization. The variables which control the ultimate isotopic composition of hydrothermal calcites include the composition of the initial fluid and the wallrock, temperature, and dissolved carbon species. Owing to significant temperature-dependent fractionation effects during equilibrium precipitation of calcite from a hydrothermal fluid, the mixing processes may be distinguished by telltale patterns of isotopic data in ¹³C vs ¹⁸O space. In particular, caution must be exercised in postulating the fluid mixing as the cause for mineral deposition. This is demonstrated for hydrothermal Pb-Zn deposits in the western Harz Mountains, Germany. A positive correlation between ¹³C and ¹⁸O values is observed for calcites from the Bad Grund deposit in the Upper Harz. Two sample profiles through calcite veins show similar correlations with the lowest -values at the center of the veins and the highest -values at the vein margins. Because the correlation array has a greater slope than for calcite precipitation at equilibrium in a closed system and because fluid mixing may not proceed perpendicular to the vein strike, it is assumed that a fluid/rock interaction is responsible for the observed correlation and thus for the precipitation of calcite. A deep-seated fluid is inferred with a ¹³C value of — 7% and a ¹⁸O value of +10%., as well as H₂CO₃ as the dominant dissolved carbon species; precipitation temperatures of the calcites are estimated to be about 280 170°C. Quite different isotopic distributions are observed for calcites from the St. Andreasberg deposit in the Middle Harz. An alteration model is suggested based mainly on the isotopic distribution through a calcite vein. In addition to a primary fluid which has the same isotopic composition as that in the Bad Grund deposit and thus seems to be responsible for the precipitation of calcite associated with sulfides, an evolved, HCO ₃ ^- -dominant subsurface fluid with ¹³C about -20 — 15% and ¹⁸O 0% is deduced to alter the primary calcite at low temperatures of 70 40°C.     

Invasion of anthropogenic CO<Subscript>2</Subscript> recorded in planktonic foraminifera from the northern Gulf of Aqaba

The stable carbon isotopic composition of the planktonic foraminifera Globigerinoides sacculifer and G. ruber (white) and sedimentary organic matter from the northern Gulf of Aqaba have been investigated to estimate changes in ¹³C_DIC in surface waters during the last 1,000 years. The high sedimentation rates at the core sites (about 54 cm/Kyear) provide high temporal resolution (~10 years). Recent sediments at the top of the cores reflect conditions younger than 1950. The ¹³C records of the planktonic foraminifera from three multicores display similar trends, showing a uniform and consistent pattern before the 1750s, and a gradual decrease of approximately 0.63 over the last two centuries. This decrease seems to track the decrease of ¹³C_DIC in surface waters, which is mainly caused by the increase of anthropogenic input of ¹³C-depleted CO₂ into the atmosphere. Similarly, a trend towards lighter values of the carbon isotopic composition of sedimentary organic matter (¹³C_org) during the last 200 years supports the interpretation obtained from the planktonic foraminiferal ¹³C. Furthermore, direct measurements of seawater show that ¹³C of the dissolved inorganic carbon (DIC) in the northern Gulf of Aqaba has decreased by about 0.44 during the period 1979–2000. The average annual decrease is 0.021, which is similar to that observed globally. The ¹³C values of planktonic foraminifera combined with organic matter ¹³C from marine sediments are good indicators for reconstructing past changes in atmospheric CO₂ concentrations from the northern Gulf of Aqaba.     

Oxygen and hydrogen isotope studies on the basal complexes of the Canary Islands: implications on the conditions of their genesis

¹⁸O/¹⁶O and D/H ratios have been measured in rocks and mineral separates from the basal complexes of Fuerteventura, Gomera and La Palma. These complexes comprise alkali plutonic rocks ranging from olivine gabbros to syenites and dikes of various mineralogy cutting them, all metamorphosed under greenschist conditions. K-Ar determinations gave ages from 48 to 17 My. The ¹⁸O's and D's of these rocks range from –1,4 to +11 and –113 to –45 respectively, with a majority of rocks depleted in heavy isotopes relative to normal, deep seated values. These values do not correspond to those found in ophiolitic sequences, which, for a comparable temperature range, give minimum ¹⁸O's>3. These values are explained in terms of water-rock interactions involving limited amounts of meteoric waters from high elevations. This implies the existence of a very important volcanic edifice over Fuerteventura in the Miocene Oligocene period with elevations between 2,500 and a maximum of more than 4,000 m.     

Contact metamorphism/hydrothermal alteration of Tertiary basalts from the Isle of Skye,northwest Scotland

Alkali olivine basalts from Skye were simultaneously contact metamorphosed by Tertiary gabbro and granite intrusions and altered by the hydrothermal convection system that the plutons induced. Four metamorphic zones were mapped around the plutons. Furthest from the intrusions, in the primary olivine zone, metabasalts are composed of combinations of igneous olivine, augite, plagioclase, titaniferous magnetite, ilmenite, zeolites, gyrolite, sulfides, and chlorite-smectite intergrowths. Closer to the plutons, in the smectite zone, saponite and carbonate appear, primary olivine and gyrolite disappear, and zeolites decrease dramatically in abundance. Still closer to the plutons, in the amphibole zone, actinolite, edenite, chlorite, sphene, epidote, andradite, and quartz appear and saponite and chlorite-smectite intergrowths disappear. Along parts of the contact between gabbro and basalt, in the orthopyroxeneolivine zone, orthopyroxene, metamorphic olivine, and biotite appear and amphibole, chlorite, sphene, epidote, andradite, carbonate, and quartz disappear. Whole-rock chemical data indicate only minor change in the major-element chemical composition of the metabasalts during progressive metamorphism/hydrothermal alteration. Two-pyroxene eothermometry and various mineral-fluid equilibria suggest the range of peak temperatures attained in the metamorphic zones: orthopyroxene-olivine zone, 900°1, 030° C; amphibole zone, 400°–900° C; smectite and primary olivine zones, < 400° C. Mineralogical and oxygen isotopic alteration of the metabasalts were closely coupled: Basalts from the primary olivine zone with nearly unaltered igneous mineralogies have normal or near-normal wholerock ¹⁸O>+5 (SMOW); mineralogically more altered basalts from the smectite zone have whole-rock ¹⁸O=+2 to +5; still more mineralogically altered basalts from the amphibole zone (with one exception) have ¹⁸O<+ 2; completely recrystallized hornfelses from the orthopyroxene-olivine zone have ¹⁸O<0. The principal mechanism of isotope exchange between basalt and metamorphic/ hydrothermal fluid probably was heterogeneous mineralfluid reaction.Metabasalts from the orthopyroxene-olivine zone are mineralogically fresh pyroxene hornfelses that record crystallization temperatures > 1,000° C yet have highly altered whole-rock oxygen isotope compositions, ¹⁸O<0%. The hornfelses chemically interacted with metamorphic/hydrothermal fluids either at very high temperatures or while they were heated to > 1,000° C or both. Their mineralogy, however, rules out significant water-rock interaction after they cooled below 900° C. Hydrothermal convection on Skye was a two-stage process: (a) fluid flow through wall rocks initially was pervasive while they are heated; (b) fluid flow after the thermal peak in the wall rocks was sufficiently channelized that rocks such as those in the orthopyroxeneolivine zone were isolated from further fluid-rock interaction during all or almost all of the cooling history of the hydrothermal system.     

Mineralogical criteria in the origin of marine iron-manganese nodules

In iron-manganese nodules from the floor of Pacific ocean, Baltic, White Sea and Kara Sea, iron bydroxide '-FeOOH was analysed in the laboratory. In buried ooze, reduction processes generate Fe(HCO₃)₂ which migrates into the upper part of the bottom ooze and into near bottom sea water where Fe(OH)₂ is formed. The oxidation process of Fe²⁺ to Fe³⁺, without participation of iron bacteria, leads to the topotactic transformation of Fe(OH)₂ to '-FeOOH. Marine water does not contain Fe²⁺ and cannot be a direct source of iron deposited in the nodules. Discovery of '-FeOOH in marine nodules permits the consideration that both iron and manganese were derived from the buried bottom mud, which during diagenetic processes led to the transfer of these metals in solutions and their upward migration.     

Reconstruction of palaeohydrological conditions in a lagoon during the 2nd Zechstein cycle through simultaneous use of δD values of individual n-alkanes and δ<Superscript>18</Superscript>O and δ<Superscript>13</Superscript>C values of carbonates

For the first time ¹⁸O and ¹³C values from carbonates and D values of individual n-alkanes were used to reconstruct palaeohydrological conditions in a lagoon at the southern margin of the Central European Zechstein Basin (CEZB). A 12-m core covering the complete Ca2 interval and adjacent anhydrites (A1 and A2) was analyzed for ¹⁸O and ¹³C values of dolomitized carbonates and D values of individual n-alkanes. ¹⁸O_carb values (+2 to +5 vs. VPDB) were strongly influenced by evaporation and temporal freshwater input into the lagoon. The ¹³C_carb values (–1 to +4 vs. VPDB) were controlled mainly by changes in primary production. Both isotopic ratios show an inverse relationship throughout most of the core, contradicting diagenetic alteration, since ¹³C_carb values are not altered significantly during dolomitization. Assuming a temperature range of 35–40 °C in the lagoon, ¹⁸O_carb values of +2.5 to +8 (vs. VSMOW) for the lagoonal water can be reconstructed. The lagoon may have desiccated twice during the Ca2 interval, as indicated by very high ¹⁸O_carb and low ¹³C_carb values, coinciding with increasing amount of anhydrite in the analyzed sample. These events seem to reflect not just local but a regional intra-Ca2 cyclicity. Measured D values of the short-chain n-alkanes, namely n-C₁₆ and n-C₁₈ which are widely used as indicators for photosynthetic bacterial and algal input, reflect the isotopic composition of the lagoonal water. Assuming constant fractionation during incorporation of hydrogen into lipids of –160, an average D value of +70 (vs. VSMOW) can be reconstructed for the lagoonal water, accounting for very arid conditions. The long-chain n-alkanes n-C₂₇, n-C₂₈, n-C₂₉ and n-C₃₀, thought to be derived from the leaf waxes of terrestrial higher plants, were shown to be depleted in D relative to the short-chain alkanes, therefore indicating a different hydrogen source. Terrestrial plants in arid areas mainly use water supplied by precipitation. By using a smaller fractionation of –120 due to evaporation processes in the leaves, reconstructed values vary between –74 and –9 (vs. VSMOW). These values are not indicating extremely arid conditions, implying that the long-chain n-alkanes were transported trough wind and/or rivers into the lagoon at the Zechstein Sea coast. D_water values, reconstructed using the n-C₁₆ alkane and ¹⁸O _water values, independently reconstructed on the same sample using the temperature dependant fractionation for dolomites are good agreement and suggest high amounts of evaporation affecting the coastal lagoon. Altogether, our results indicate that hydrogen isotopic ratios of n-alkanes give information on their origin and are a useful proxy for palaeoclimatic reconstruction.     

The Hiendelaencina epithermal silver-base metal district,Central Spain: Tectonic and mineralizing processes

The northeastern sector of the Spanish Central System hosts important Stephanian-Permian silver-base metal epithermal mineralizations defining the so-called Hiendelaencina District. The overall geotectonic evolution of this region indicates a major late Variscan extensional period involving the unroofing of dome-shaped metamorphic core complexes, which ultimately led to the radial brittle disruption of these bodies allowing the ascent of andesitic magmas and high-level hydrothermal activity. The deposits are hosted by high-grade metaphorphic rocks belonging to these complexes. Mineralogical and fluid inclusion studies reveal that the mineralizations were formed during two to four hydrothermal stages. These are the result of complex interactions between fluids of contrasted temperatures and salinities. Data on sulphur isotopes suggest that two sources of sulphur existed: magmatic (andesitic derived, with ³⁴S + 6) and metasediment-derived (with initial ³⁴S probably + 25).     

Fluid-rock interaction between syenites and marbles at Stephen Cross Quarry,Québec,Canada: petrological and stable isotope data

Grenville dolomitic marbles and calc-silicates at Stephen Cross Quarry, Québec, underwent contact metamorphism and metasomatism associated with the intrusion of the Wakefield syenite at ambient pressures of 0.4GPa at 1090–1070Ma. Fluid infiltration produced exoskarns, calcite+periclase+forsterite±diopside±orthoclase assemblages in the marbles, and quartz±calcite±wollastonite±diopside±anorthite assemblages in the calc-silicates. Phase-equilibria in the CaO–MgO–Al₂O₃–SiO₂–H₂O–CO₂ system suggest that fluid infiltration occurred close to the thermal peak of contact metamorphism (715–815°C) and that the fluids hadXCO₂0.15. In the metasediments, ¹⁸O values of calcite (Cc) are as low as 8.6, suggesting that the fluids were in isotopic equilibrium with the syenites (¹⁸O =8.8–10.2). Marble ¹³C(Cc) values are-0.1 to-3.2; the lack of correlation between ¹³C(Cc) and ¹⁸O(Cc) is consistent with the infiltration of water-rich fluids. The resetting of stable isotopes and the mineralogical changes can be explained by time-integrated fluid fluxes of up to 110 m³/m² (4×10⁶ mol/m²), corresponding to actual fluxes of 3×10^-11 to 3×10^-12 m³/m²-s and intrinsic permeabilities of 10^-18 to 10^-20 m² for fluid flow lasting 0.1-1Ma. Marble ¹⁸O(Cc) values do not correlate well with distance from the syenite, and fluids were probably channelled across lithological layering. The correlation between the degree of resetting of marble ¹⁸O(Cc) values with the abundance of submillimetre-wide veins, suggests that fluid focussing may have resulted from variations in fracture density. Late, lower temperature (<500°C), fluid flow formed serpentine (Serp) and brucite (Br) from periclase and forsterite. ¹⁸O(Br) and ¹⁸O(Scrp) values correlate with ¹⁸O(Cc), suggesting that retrogression involved only limited volumes of fluid. The observation that ¹⁸O(Cc-Br) and ¹⁸O(Cc-Serp) values are higher in marbles that have lower ¹⁸O(Cc) values is interpreted as indicating that fluid flow persisted to lower temperatures in those rocks due to higher intrinsic permeabilities. Calcite in the syenite was also formed by the influx of fluids during cooling. Syenite ¹⁸O(Cc) values are approximately in isotopic equilibrium with the high-temperature silicate minerals, suggesting that again only minor volumes of fluid were involved. In detail fluid flow was prolonged and complex, creating problems for the application of quantitative fluid flow models.This paper is a contribution to IGCP 304, Lower Crustal Processes     

Deep fluid circulation in alpine shear zones,Pyrenees, France: field and oxygen isotope studies

A combined field, stable isotope, and whole-rock chemical study was made on late Cretaceous to Tertiary metasomatic shear zones cutting Hercynian gneisses in the Aston Massif, Pyrenees, France. Mylonitisation occurred during the early stages of Alpine compression under retrograde conditions at 400–450°C and about 10 km depth. Whole-rock ¹⁸O values of (+11 to +12 in the gneisses) was lowered to +5 to +9 in the shear zones, with the quartz-muscovite ¹⁸O/¹⁶O fractionations of about 2 to 4 essentially unchanged. These ¹⁸O/¹⁶O systematics, together with D muscovite=-40 to-50 indicate that large volumes of formation waters or D-rich meteoric waters passed through the shear zones during deformation. The same fluids also redistributed major elements, as shown by the correlation of ¹⁸O shift with muscovitisation and albitisation reactions in granitic wall rocks. However, even though ¹⁸O was universally lowered within the shear zones, the ¹⁸O/¹⁶O ratios were not homogenised, nor do they correlate in detail with the presence or absence of muscovitisation, suggesting that fluid flow was probably fracture-controlled and episodic. Field mapping shows that, along the length of a particular shear zone, muscovitisation of granite gneiss dies out 150m above the contact with underlying sillimanite gneiss. Thus, muscovitisation and albitisation of granite gneiss in shear zones and their wall rocks probably occurred during re-equilibration of acidic, chloride-rich, aqueous fluids that had previously moved upward within the shear zones through underlying sillimanite gneiss. Extremely high material-balance fluid-rock ratios (10³) are required to explain the extent of muscovitisation along this shear zone, implying integrated fluid mass fluxes of about 10⁸ kg/m²; this is probably close to the maximum value for other shear zones in the network. Similar volumes of a more chemically evolved fluid must have passed through the unmuscovitised mylonites, showing that the absence of alteration cannot necessarily be used to infer low values of fluid flux. For reasonable pressure gradients and time scales of fluid movement, effective permeabilities of 10^-15 to 10^-17 m² are required. Such values can be accounted for by short-lived, widely-spaced cracks produced during seismic activity. A model is presented in which formation waters were seismically pumped down an underlying, shallow, southward-dipping decollement and then upward through the steeply-dipping shear zone network.Contribution No 4711: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA     

A sulfur isotope study of volcanogenic massive sulfide deposits of the Eastern Black Sea province,Turkey

Kuroko-type massive sulfide deposits of the Eastern Black Sea province of Turkey are related to the Upper Cretaceous felsic lavas and pyroclastic rocks, and associated with clay and carbonate alteration zones in the footwall and hangingwall lithologies. A complete upward-vertical section of a typical orebody consists of a stringer-disseminated sulfide zone composed mainly of pyrite and chalcopyrite; a massive pyrite zone; a massive yellow ore consisting mainly of chalcopyrite and pyrite; a black ore made up mainly of galena and sphalerite with minor amounts of chalcopyrite, bornite, pyrite and various sulfosalts; and a barite zone. Most of the deposits in the province are associated with gypsum in the footwall or hangingwall. The paragenetic sequence in the massive ore is pyrite, sphalerite, chalcopyrite, bornite, galena and various sulfosalts, with some overlap between the mineral phases. Massive, stringer and disseminated sulfides from eight kuroko-type VMS deposits of the Eastern Black Sea province have a ³⁴S range of 0–7 per mil, consistent with the ³⁴S range of felsic igneous rocks. Sulfides in the massive ore at Madenköy (4.3–6.1 per mil) differ isotopically from sulfides in the stringer zone (6.3–7.2 per mil) suggesting a slightly increased input of H₂S derived from marine sulfate with time. Barite and coarse-grained gypsum have a ³⁴S range of 17.7–21.5 per mil, a few per mil higher than the ³⁴S value of contemporaneous seawater sulfate. The deposits may, therefore, have formed in restricted basins in which bacterial reduction of sulfate was taking place. Fine-grained, disseminated gypsum at Kutlular and Tunca has ³⁴S values (2.6–6.1 per mil) overlapping those of ore sulfides, indicating sulfide oxidation during waning stages of hydrothermal activity.