Schwefelisotopenuntersuchungen an einigen Sulfid- und Sulfatmineralen der Blei-Zink-Erzlagerstätte Bleiberg/Kreuth,Kärnten

Zusmmenfassung Die Ergebnisse der Schwefelisotopenanalysen von sechs Sulfid- und vier Sulfatmineralproben von Bleiberg/Kreuth (Österreich) variieren von –6,9 bis –25,9 ³⁴S in den Sulfiden und von +14,8 bis +18,9 ³⁴S in den Sulfaten. Die große Variationsbreite der Schwefelisotopen und die Bevorzugung des leichten Schwefels deutet vermutlich auf bakterielle Prozesse der Sulfidfällung. Die Sulfatschwefel fallen in den Bereich der Schwefelisotopenzusammensetzung des mesozoischen (postskytischen) Meerwassers.

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Determination of the sulfur isotopic composition in some sulfide and sulfate minerals of the lead zinc deposit, Bleiberg/Kreuth, Carinthia

Summary Results of sulfur isotope analyses on 6 sulfides and 4 sulfates from Bleiberg/Kreuth (Austria) range from –6.9 to –25.9 ³⁴S (in sulfides) and from +14.8 to +18.9 ³⁴S (in sulfates). A large range of sulfide sulfur isotope fractionation with appreciable light sulfur probably indicates a bacterial sulfur source in sulfide precipiation. The sulfate sulfur plots in the range of Mesozoic (post-Skytian) seawater sulfur isotopic composition.

     

Zur Schwefelisotopenzusammensetzung der Pb-Zn-Vererzung des Grazer Paläozoikums (Ostalpen)

Zusammenfassung Die Schwefelisotopenzusammensetzung von sulfidischen Erzmineralen und Baryten der unterdevonischen schichtgebundenen Bleiglanz-Zinkblende-Baryt-Erzvorkommen des Grazer Paläozoikums (Ostalpen, Österreich) wurde untersucht. Zur Analyse gelangten 64 Proben von etwa 15 Lokalitäten. Die Schwefelisotopenverteilung zeigt Ähnlichkeiten mit anderen an das Devon gebundenen schwerspatführenden Lagerstätten. Die Baryte ergaben ³⁴S-Werte von +23, 1 bis +27, 8, die dem Meerwassersulfat devonischen Alters entsprechen. Die untersuchten Galenite variieren von +2, 4 bis +6, 9 und die Sphalerite von +3, 3 bis +9, 1. Die Werte der Pyrite zeigen einen großen Streubereich (–27, 4 bis +37, 6). Eine massive buntmetallarme Pyritvererzung fällt durch sehr schweren Schwefel auf. Späte Mineralisationsphasen der Sulfide tendieren gleichfalls zur Anreicherung des schweren Schwefels.

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The sulphur isotope compositions of 64 samples of sulphide minerals and barites from the strata-bound Lower Devonian ore deposits of the Paleozoic Complex of Graz in the Eastern Alps (Austria) have been investigated. The ³⁴S-values of galenites and sphalerites show only small variations in the range of +2, 4 up to +6, 9 for PbS and +3, 3 up to +9, 1 for ZnS indicating abiogenic origin. Later remobilized sulphide minerals show a tendency to heavier sulphur. The deposition of pyrites is influenced partly by bacteriogenic processes (–27, 4 up to +37, 6). The enrichment of heavier sulphur characterizes massive pyrite mineralisations being poor on base metals. The sulphur isotope compositions of barites (+23, 1 up to 27, 8) correspond to Devonian sea water sulphate. An ore free barite characterized by increased strontium concentration shows heavier sulphur (+29, 6) than barites from ore beds.

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Derzeit Oberste Bergbehörde, Bundesministerium für Handel, Gewerbe und Industrie (Wien)     

A stable isotope study of the Palaeoproterozoic Tallberg porphyry-type deposit,northern Sweden

The Tallberg deposit is situated in the Skellefte District in northern Sweden. It is a Palaeoproterozoic equivalent of Phanerozoic poryphyry-type deposits. The mineralization is situated within the Jörn granitoid complex and is associated with intrusive quartz-feldspar porphyries. The granitoids are coeval with mainly felsic volcanic rocks hosting several massive sulphide deposits. The alteration is generally of a mixed phyllic-propylitic type, but areas or zones associated with high gold grades exhibit phyllic alteration. Ore minerals are pyrite, chalcopyrite, sphalerite, magnetite, and trace amounts of molybdenite. In this stable isotope study, quartz, sericite, and chlorite from the alteration zones were sampled. The magmatic quartz has a ¹⁸O composition of + 6.2 to +6.7 whereas the quartz in the hydrothermal alteration zones have values ranging from +7.5 to +10.6. The calculated temperatures for this fractionation range from 430° to 520°C. The sericites have ¹⁸O ranging from +4.6 to +8.2 (average +6.6) and D -31 to -54 (average -41). Chlorites range from ¹⁸O +4.2 to +7.7 and D from –34 to –44. The range of ³⁴S of 11 pyrite samples is +3.8 to +5.5 with an average of +4.6 ± 0.5, suggesting a relatively homogeneous sulphur source, probably of magmatic origin. Modelling waters in equilibrium with the minerals indicates early magmatic fluids with ¹⁸O of 6.5. This fluid mixed with a low ¹⁸O and high D fluid, which is tentatively identified as seawater. The ¹⁸O signature of sericite and chlorite also indicates significant water-rock exchange, explaining the positive ¹⁸O values for the waters in equilibrium with the hydrated minerals.     

The Candelaria silver deposit,Nevada — preliminary sulphur,oxygen and hydrogen isotope geochemistry

The pre-Cenozoic geology at Candelaria, Nevada comprises four main lithologic units: the basement consists of Ordovician cherts of the Palmetto complex; this is overlain unconformably by Permo-Triassic marine clastic sediments (Diablo and Candelaria Formations); these are structurally overlain by a serpentinitehosted tectonic mélange (Pickhandle/Golconda allochthon); all these units are cut by three Mesozoic felsic dike systems. Bulk-mineable silver-base metal ores occur as stratabound sheets of vein stockwork/disseminated sulphide mineralisation within structurally favourable zones along the base of the Pickhandle allochthon (i.e. Pickhandle thrust and overlying ultramafics/mafics) and within the fissile, calcareous and phosphatic black shales at the base of the Candelaria Formation (lower Candelaria shear). The most prominent felsic dike system — a suite of Early Jurassic granodiorite porphyries — exhibits close spatial, alteration and geochemical associations with the silver mineralisation. Disseminated pyrites from the bulk-mineable ores exhibit a ³⁴S range from — 0.3 to + 12.1 (mean ³⁴S = +6.4 ± 3.5, 1, n = 17) and two sphalerites have ³⁴S of + 5.9 and + 8.7 These data support a felsic magmatic source for sulphur in the ores, consistent with their proximal position in relation to the porphyries. However, a minor contribution of sulphur from diagenetic pyrite in the host Candelaria sediments (mean ³⁴S = — 14.0) cannot be ruled out. Sulphur in late, localised barite veins ( ³⁴S = + 17.3 and + 17.7) probably originated from a sedimentary/seawater source, in the form of bedded barite within the Palmetto basement ( ³⁴S = + 18.9). Quartz veins from the ores have mean ¹⁸O = + 15.9 ± 0.8 (1, n = 10), which is consistent, over the best estimate temperature range of the mineralisation (360°–460°C), with deposition from ¹⁸O-enriched magmatic-hydrothermal fluids (calculated ¹⁸O fluid = + 9.4 to + 13.9). Such enrichment probably occurred through isotopic exchange with the basement cherts during fluid ascent from a source pluton. Whole rock data for a propylitised porphyry ( ¹⁸O = + 14.2, D = — 65) support a magmatic fluid source. However, D results for fluid inclusions from several vein samples (mean = — 108 ± 14, 1, n = 6) and for other dike and sediment whole rocks (mean = — 110 ± 13, 1, n = 5) reveal the influence of meteoric waters. The timing of meteoric fluid incursion is unresolved, but possibilities include late-mineralisation groundwater flooding during cooling of the Early Jurassic progenitor porphyry system and/or meteoric fluid circulation driven by Late Cretaceous plutonism.     

Sulfur isotope ratios of Icelandic rocks

Sulfur isotope ratios have been determined in 27 selected volcanic rocks from Iceland together with their whole rock chemistry. The ³⁴S of analyzed basalts ranges from –2.0 to +0.4 with an average value of –0.8 Tholeiitic and alkaline rocks exhibit little difference in ³⁴S values but the intermediate and acid rocks analyzed have higher ³⁴S values up to +4.2 It is suggested that the overall variation in sulfur isotope composition of the basalts is caused by degassing. The small range of the ³⁴S values and its similarity to other oceanic and continental basalts, suggest that the depleted mantle is homogeneous in its sulfur isotope composition. The ³⁴S of the depleted mantle is estimated to be within the range for undegassed oceanic basalts, –0.5 to +1.0     

Sulfur isotopes and origin of some sulfide deposits,New England,Australia

The S-isotopic compositions of sulfide deposits from Steinmann, granitoid and felsic volcanic associations have been examined. Ores of Steinmann association have ³⁴S values close to zero per mil (³⁴S=+0.3±3.1) it appears they are of mantle origin. Isotopically, ores of granitoid association regularly show a variable enrichment in ³²S relative to meteoritic (³⁴S=–2.7±3.3). The composition is in accord with an upper mantle/lower crustal source. Two stratiform accumulations of felsic volcanic association show a narrow spread of ³⁴S values (+0.2 to 2.4); a mantle origin for the sulfur in these deposits is favored. In contrast, vein, stockwork and cement ores are moderately enriched in ³²S relative to meteoritic (³⁴S=–4.0±6.4). These ores are polygenetic; sulfur and metals appear to have been leached from local country rocks where volcanogenic and biogenic sulfur predominate.     

Textural and isotopic variations in graphite from plutonic rocks,South-Central New Hampshire

Graphite occurs in two distinct textural varieties in syntectonic granitoids of the New Hampshire Plutonic Series and in associated metasedimentary wall rocks. Textural characteristics indicate that coarse graphite flakes were present at an early stage of crystallization of the igneous rocks and thus may represent xenocrystic material assimilated from the wall rocks. The range of ¹³C values determined for flake graphite in the igneous rocks (–26.5 to –13.8) overlaps the range for flake graphite in the wall rocks (–26.0 to –16.7), and spatial correlation of some ¹³C values in the plutons and wall rocks supports the assimilation mechanism. The textures of fine-grained irregular aggregates or spherulites of graphite, on the other hand, indicate that they formed along with secondary hydrous silicates and carbonates during retrograde reactions between the primary silicates and a carbon-bearing aqueous fluid phase. Relative to coexisting flake graphite, spherulitic graphite shows isotopic shifts ranging from 1.9 higher to 1.4 lower in both igneous and metasedimentary samples.The observed isotopic shifts and the association of spherulitic graphite with hydrous silicates are explained by dehydration of C-O-H fluids initially on or near the graphite saturation boundary. Hydration of silicates causes dehydration of the fluid and drives the fluid composition to the graphite saturation surface. Continued dehydration of the fluid then requires coprecipitation of secondary graphite and hydrous silicates and drives the fluid toward either higher or lower CO₂/CH₄ depending upon the inital bulk composition. Isotopic shifts in graphite formed at successive reaction stages are explained by fractionation of ¹³C between secondary graphite and the evolving fluid because ¹³C is preferentially concentrated into CO₂ relative to CH₄.Epigenetic graphite in two vein deposits assiciated with the contacts of these igneous rocks is generally enriched in ¹³C (–15.7 to –11.6) relative to both the igneous and wall-rock ¹³C values. Values of ¹³C vary by up to 3.4 within veins, with samples taken only 3 cm apart differing by 2.0 These variations in ¹³C correlate with textural evidence showing sequential deposition of different generations of graphite in the veins from fluids which differed in proportions of carbon species or isotopic composition (or both).     

The genesis of emeralds and their host rocks from Swat,northwestern Pakistan: a stable-isotope investigation

Emerald deposits in Swat, northwestern Pakistan, occurring in talc-magnesite and quartz-magnesite assemblages, have been investigated through stable isotope studies. Isotopic analyses were performed on a total of seven emeralds, associated quartz (seven samples), fuchsite (three samples) and tourmaline (two samples) from the Mingora emerald mines. The oxygen isotopic composition ( ¹⁸O SMOW) of emeralds shows a strong enrichment in¹⁸O and is remarkably uniform at + 15.6 ± 0.4 (1,n = 7). Each of the two components of water in emerald (channel and inclusion) has a different range of hydrogen isotopic composition: the channel waters being distinctly isotopically heavier (D = –51 to –32 SMOW) than the other inclusion waters (D = –96 to –70 SMOW). Similarly the oxygen isotopic compositions of tourmaline and fuchsite are relatively constant ( ¹⁸O = + 13 to + 14 SMOW) and show enrichment in¹⁸O. The ¹⁸O values of quartz, ranging from + 15.1 to + 19.1 SMOW, are also high (+ 16.9 ± 1.4 1, n = 7). The meanD of channel waters measured from emerald (–42 ± 6.6 SMOW) and that of fluid calculated from hydrous mineralsD_calculated (–47 ± 7.1 SMOW) are consistent with both metamorphic and magmatic origin. However, the close similarity between the measuredD values of the hydroxyl hydrogen in fuchsite (–74 to –6 SMOW) and tourmaline (–84 and –69 SMOW) with pegmatitic muscovite and tourmaline suggests that the mineralization was probably caused by modified (¹⁸O-enriched) hydrothermal solutions derived from an S-type granitic magma. The variation in the carbon and oxygen isotopic composition of magnesite, locally associated with emerald mineralization, is also very restricted ( ¹³ –3.2 ± 0.7%, PDB; ¹⁸O + 17.9 ± 1.27 SMOW). On the basis of the isotopic composition of fluid ( ¹³C –1.8 ± 0.7 PDB; ¹⁸O + 13.6 ± 1.2 SMOW calculated for the 250-550 °C temperature), it is proposed that the Swat magnesites formed due to the carbonation of previously serpentinized ultramafic rocks by a CO₂-bearing fluid of metamorphic origin.     

Isotopic Composition of Oxygen and Carbon and Formation Temperature of Accessory Minerals from Evaporitic Sediments in the Pripyat Trough

The isotopic composition of oxygen and carbon was studied in accessory carbonates and quartz separated from salts in Upper Devonian halogenous formations of the Pripyat Trough (Belorus). It is established that isotopic characteristics vary in a wide range. Values of ¹⁸O vary in the following range (SMOW): from 18.2 to 29.2 in calcites, from 15.7 to 32.5 in dolomites, and from 17.4 to 27.2 in quartz. Values of ¹³C range from –13.4 to 1.4 in calcites and from –11.1 to 1.7 in dolomites (PDB). Results obtained indicate highly variable isotope-geochemical conditions of sedimentation and early diagenesis during the formation of evaporitic sediments. Accessory minerals were repeatedly formed in a wide temperature range and probably at various stages of the lithogenesis.     

Oxygen-isotope geochemistry of metamorphosed,massive sulfide deposits of the Flin Flon — Snow Lake belt,Manitoba

Oxygen-isotope compositions have been measured for whole-rock and mineral samples of host and hydrothermally altered rocks from three massive sulfide deposits, Centennial (CL), Spruce Point (SP), and Anderson Lake (AL), in the Flin Flon — Snow Lake belt, Manitoba. Wholerock ¹⁸O values of felsic metavolcanic, host rocks (+8.5 to +16.1) are higher than those of altered rocks from the three deposits. The ¹⁸O values of altered rocks are lower in the chlorite zone and muscovite zone-I (CL=+ 5.3; SP=+5.4 to +8.3; AL= +3.7 to +5.9) than in the gradational zone (CL= +9.9 to +11.7; SP= +8.4 to +9.8; AL= + 6.6 to +7.7). Muscovite schist (Muscovite Zone-II) enveloping the Anderson Lake ore body has ¹⁸O values of +7.2 to +8.3. Quartz, biotite, muscovite, and chlorite separated from the altered rocks have lower ¹⁸O values compared to the same minerals separated from the host rocks. However, isotopic fractionation between mineral-pairs is generally similar in both host and altered rocks.It is interpreted that differences in the oxygen-isotope compositions of the altered and host rocks were produced prior to metamorphism, during hydrothermal alteration related to ore-deposition. Isotopic homogenization during metamorphism occurred on a grain-to-grain scale, over no more than a few meters. The whole-rock ¹⁸O values did not change significantly during metamorphism. The generally lower ¹⁸O values of altered rocks, the Cu-rich nature of the ore and the occurrence of the muscovite zone-II at Anderson Lake are consistent with the presence of higher temperature hydrothermal fluids at Anderson Lake than at the Centennial and Spruce Point deposits.     

Isotopic Composition (δ13C, δ18O) and Origin of Manganese Carbonate Ores of the Usa Deposit (Kuznetskii Alatau)

Isotopic compositions of carbon and oxygen are studied in different (rhodochrosite, calcareous-rhodochrosite, and chlorite–rhodochrosite) types of manganese carbonate ores from the Usa deposit (Kuznetskii Alatau). The ¹³C value varies from –18.4 to –0.7, while the ¹⁸O value ranges between 18.4 and 23.0. Host rocks are characterized by higher values of ¹³C (–1.9 to 1.0) and ¹⁸O (21.2 to 24.3). The obtained isotope data suggest an active participation of oxidized organic carbon in the formation of manganese carbonates. Manganese carbonate ores of the deposit are probably related to metasomatic processes.     

Changes in stable isotope ratios of metapelites and marbles during regional metamorphism,Mount Lofty Ranges,South Australia: implications for crustal scale fluid flow

The Mount Lofty Ranges comprises interlayered marbles, metapsammites, and metapelites that underwent regional metamorphism during the Delamarian Orogeny at 470–515 Ma. Peak metamorphic conditions increased from lowermost biotite grade (350–400°C) to migmatite grade (700°C) over 50–55 km parallel to the lithological strike of the rocks. With increasing metamorphic grade, ¹⁸O values of normal metapelites decrease from 14–16 to as low as 9.0, while ¹⁸O values of calcite in normal marbles decrease from 22–24 to as low as 13.2 These isotopic changes are far greater than can be accounted for by devolatilisation, implying widespread fluid-rock interaction. Contact metamorphism appears not to have affected the terrain, suggesting that fluid flow occurred during regional metamorphism. Down-temperature fluid flow from synmetamorphic granite plutons (¹⁸O=8.4–8.6) that occur at the highest metamorphic grades is unlikely to explain the resetting of oxygen isotopes because: (a) there is a paucity of skarns at granite-metasediment contacts; (b) the marbles generally do not contain low-XCO₂ mineral assemblages; (c) there is insufficient granite to provide the required volumes of water; (d) the marbles and metapelites retain a several permil difference in ¹⁸O values, even at high metamorphic grades. The oxygen isotope resetting may be accounted for by along-strike up-temperature fluid flow during regional metamorphism with time-integrated fluid fluxes of up to 5x10⁹ moles/m² (10⁵ m³/m²). If fluid flow occurred over 10⁵–10⁶ years, estimated intrinsic permeabilities are 10^-20 to 10^-16m². Variations in ¹⁸O at individual outcrops suggest that time-integrated fluid fluxes and intrinsic permeabilities may locally have varied by at least an order of magnitude. A general increase in XCO₂ values of marble assemblages with metamorphic grade is also consistent with the up-temperature fluid-flow model. Fluids in the metapelites may have been derived from these rocks by devolatilisation at low metamorphic grades; however, fluids in the marbles were probably derived in part from the surrounding siliceous rocks. The marble-metapelite boundaries preserve steep gradients in both ¹⁸O and XCO₂ values, suggesting that across-strike fluid fluxes were much lower than those parallel to strike. Up-temperature fluid flow may also have formed orthoamphibole rocks and caused melting of the metapelites at high grades.This paper is a contribution to IGCP Project 304 Lower Crustal Processes     

Sulphur and sulphate-oxygen isotopes and the origin of the silvermines deposits,Ireland

New sulphur and sulphate-oxygen isotope measurements for the main discordant and stratiform lead-zinc-barite orebodies at Silvermines Co. Tipperary, allow reappraisal of previously offered differing interpretations (Graham, 1970; Greig et al., 1971) of the bearing of sulphur isotopes on the genesis of this important Irish deposit. The following aspects of the data are confirmed: barite ³⁴ S-values range from 17–21, similar to lower Carboniferous seawater sulphate: stratiform sulphide lens pyrites have ³⁴ S-values ranging from –13 to –36; vein sulphide ³⁴ S-values range from –8 to 4; sulphide ³⁴ S-values increase upwards and outwards respectively in the related discordant and stratiform G orebodies; galena-sphalerite isotope palaeotemperatures are not too consistent, ranging from 40 to 430°C (using the calibration of Czamanske and Rye (1974). New facts are as follows: barite ¹⁸O-values range from –13 to –17, stratiform barites ranging from 13 to 14.5; sulphides separated from a single stratiform ore lens hand specimen usually have ³⁴ S_sl > ³⁴ S_ga > ³⁴ S_py; the outward decrease in ³⁴ S-values in the stratiform G orebody is confined to the first few hundred feet only; pyrite ³⁴ S-values progressively increase downwards through one stratiform sulphide orebody; yet variations of 13 occur within a single colloform pyrite structure from another stratiform orebody. It is concluded that there were at least two sources of sulphur, seawater sulphate and deep-seated sulphur. The former was the dominant source of all sulphate and, via biogenic reduction, of the sulphur in the bulk of the stratiform sulphide. The latter was the source of the sulphur in the vein sulphides. There was minimal isotopic interaction between the cool seawater sulphate and the warm unwelling ore fluid sulphur species, even though the latter precipitated under near isotopic equilibrium conditions when the temperature dropped and/or the pH and Eh increased. The lack of isotopic equilibrium between pyrite and ore sulphides in the stratiform ore lenses may result from the latter having precipitated slightly later than the former because of solubility relationships. Overall the present isotopic evidence supports considerable geological evidence favoring a syngenetic origin for the stratiform Silvermines orebodies.     

Isotopic composition of oxygen and hydrogen in mud-volcanic waters from Taman (Russia) and Kakhetia (Eastern Georgia)

The ¹⁸O and D values in mud-volcanic waters of the Taman Peninsula and Kakhetia vary from +0.7 to +10.0 and from –37 to –13 , respectively. These values increase as the Greater Caucasus is approached. The increase in 18O and D also positively correlates with fluid generation temperatures based on hydrochemical geothermometers. This is accompanied by changes in the chemical composition of waters, in which contents of alkali metals, HCO^– ₃ ion, and boron increase, while the content of halogen ions (Cl, Br, J) decreases. Changes in the isotopic composition of water are also accompanied by the increase of ¹³ in methane and decrease of ¹¹ B in clays. Analysis of formal models of the evolution of isotopic composition of mud-volcanic waters showed that mud volcanoes are recharged by freshened water from the Maikop paleobasin with an inferred isotopic composition of D –40 and ¹⁸ O –6. Based on this assumption, the ¹⁸O and D values observed in mud-volcanic waters can be explained not only by processes of distillation and condensation in a closed system, but also by combined processes of isotopic reequilibration in the water-illite-methane system.Translated from Litologiya i Poleznye Iskopaemye, No. 2, 2005, pp. 143–158.Original Russian Text Copyright © 2005 by Lavrushin, Dubinina, Avdeenko.     

Sulphur isotope patterns from the Bleiberg deposit (Eastern Alps) and their implications for genetically affiliated lead–zinc deposits

Summary A set of 354 sulphur isotope data from the Bleiberg deposit, the type deposit of Alpine low temperature carbonate hosted Pb–Zn deposits (APT deposits), is critically evaluated applying statistical methods. The sulphur isotope patterns vary significantly among the ore horizons. This suggests a long lasting and polyphase mineralisation system. The sulphur isotope composition of barite corresponds to that of Carnian seawater (i.e. 16 ³⁴S). The ³⁴S values of the iron sulphides correspond to data from sedimentary iron sulphides. Pb and Zn sulphides are characterized by three normally distributed ³⁴S populations with mean values of –6 to –8, –13 to –18, and –25 to –29. Heavy sulphur (>–10 ³⁴S) indicates contribution of sulphide sulphur from epigenetic-hydrothermal fluids, whereas light sulphide sulphur (<–21 ³⁴S) was produced by sulphur – reducing bacteria. The intermediate population is explained by mixing of sulphur derived from these two sources. Other sources of local importance, however, can not be excluded. The isotope populations correspond only partly to the paragenetic ore stages. The sulphur isotope patterns in the APT deposits are regionally different. Data from other low-temperature sediment-hosted Pb–Zn deposits support the proposed interpretation. A comparison demonstrates that the sulphur isotope patterns of APT deposits correspond to patterns of the Irish type deposits, but are different to those of Mississippi Valley type deposits.Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1007/s00710-004-0071-3     

Seawater-sediment-basalt interactions: stable isotope (H,O) and elemental fluxes within the Ordovician volcano-sedimentary sequence of Erquy (Brittany,France)

The Ordovician volcano-sedimentary succession of Erquy (northern Brittany) is made of immature sediments thermally metamorphosed at the contact of intruding basic sills. Pillow lavas constitute the upper part of the sequence. The trace element geochemistry of sills and pillow lavas suggests that they were derived from a tholeiitic source located beneath a passive margin. This volcanic sequence was metamorphosed under low to moderate greenschist facies conditions. In this study the direction and amplitude of chemical and isotopic fluxes in the basalt-sediment-water system were established and the oxygen and hydrogen isotope compositions of the aqueous fluid that reacted with the volcanic rocks were characterized. Cationic thermometry on chlorites and isotopic thermometry on plagioclase-chlorite pairs indicate closure metamorphic temperatures in the range 200–250°C for the basaltic sills. Stable isotope compositions of iron-rich chlorites (¹⁸O-5.5; D from-60 to-50) and plagioclases (¹⁸O from +9 to +10) reveal that the source of the fluid was certainly seawater. The ¹⁸O variations within the sills are strongly correlated with the rate of progress of the main metamorphic reaction:clinopyroxene+plagioclase+ilmenite chlorite+albite+epidote+quartz+sphene that produced major element mobility at the scale of the volcanosedimentary sequence. Calculation of elemental fluxes by mass balance combined with oxygen isotopic compositions of basalts shows that the highest water-rock ratios (1) were at sill-sediment boundaries and within pillow lavas at the top of the pile. The volcanosedimentary sequence of Erquy was a net sink for Na and a source for Ca. No Mg uptake could be detected whereas the hydrothermal alteration of the sediments released Fe, Si, and K trapped by the volcanic rocks. The ¹⁸O value of the fluid reacting with sills appears to have shifted no more than +4 after percolation at low temperature through immature sediments (¹⁸O12). The Erquy volcano-sedimentary sequence represents a marine hydrothermal system dominated by low-temperature exchange which allowed a general ¹⁸O-enrichment of the volcanic rocks and a ¹⁸O-depletion of sediments.     

Stable isotope geochemistry of the Archean Val-d‘Or (Canada) orogenic gold vein field

A systematic study of the auriferous quartz veins of the Val-dOr vein field, Abitibi, Quebec, Canada, demonstrates that the C, O, S isotope composition of silicate, carbonate, borate, oxide, tungstate and sulphide minerals have a range in composition comparable to that previously determined for the whole Superior Province. The oxygen isotope composition of quartz from early quartz–carbonate auriferous veins ranges from 9.4 to 14.4 whereas later quartz-tourmaline-carbonate veins have ¹⁸O_quartz values ranging from 9.2 to 13.8 . Quartz-carbonate veins have carbonate (¹⁸O: 6.9–12.5 ; ¹³C: –6.2– –1.9 ) and pyrite (³⁴S: 1.2 and 1.9 ) isotope compositions comparable to those of quartz-tourmaline-carbonate veins (¹⁸O: 7.9–11.7 ; ¹³C: –8.0 – –2.4 ; ³⁴S: 0.6–6.0 ). ¹⁸O_quartz values in quartz-tourmaline-carbonate veins have a variance comparable to analytical uncertainty at the scale of one locality, irrespective of the type of structure, the texture of the quartz or its position along strike, across strike or down-dip a vein. In contrast, the oxygen isotope composition of quartz in quartz-tourmaline-carbonate veins displays a regional distribution with higher ¹⁸O values in the south-central part of the vein field near the Cadillac Tectonic Zone, and which ¹⁸O values decrease regularly towards the north. Another zone of high ¹⁸O values in the northeast corner of the region and along the trace of the Senneville Fault is separated by a valley of lower ¹⁸O values from the higher values near the Cadillac Tectonic Zone. Oxygen isotope isopleths cut across lithological contacts and tectonic structures. This regional pattern in quartz-tourmaline-carbonate veins is interpreted to be a product of reaction with country rocks and mixing between (1) a deep-seated hydrothermal fluid of metamorphic origin with minimum ¹⁸O=8.5 , ¹³C=0.6 and ³⁴S=–0.4 , and (2) a supracrustal fluid, most likely Archean seawater with a long history of water-rock exchange and with maximum ¹⁸O=3.9 , ¹³ C=–5.6 and ³⁴S=5.0 .     

Sea water basalt interaction in spilites from the Iberian Pyrite Belt

Low grade hydrothermally metamorphosed mafic rocks from the Iberian Pyrite Belt are enriched in ¹⁸O relative to the oxygen isotopic ratio of fresh basalt (+6.5±1). The observed ¹⁸O whole rock values range from +0.87 to +15.71 corresponding to positive isotopic shifts of +5 to +10, thus requiring isotopic exchange with fluids under conditions of high water:rock ratios at low temperatures. The lowest ¹⁸O observed corresponds to an albitized dolerite still and is compatible with independent geochemical data suggesting lower water: rock ratios for the alteration of these rocks.The isotope data are consistent with the hypothesis that the spilites from the Pyrite Belt were produced by interaction of basaltic material with sea water.Significant leaching of transition metals from the mafic rocks during alteration coupled with available sulphur isotopic data for the sulphide ores also suggest that sea water may have played an important role in the formation of ore deposits in the Iberian Pyrite Belt.     

The isotopic constitution of sulphur in some stratiform lead-zinc sulphide ores

Sulphur isotope measurements on five Australian stratiform lead-zinc sulphide ores are presented. Each deposit is characterised by a conspicuously narrow spread of values, mean ³⁴S heavier than meteoritic, average crustal, and, where sampled stratigraphically, ³⁴S related to stratigraphy rather than propinquity. When considered with other deposits of the same class from elsewhere, several seemingly general features appear: Mean ³⁴S heavier than meteoritics; ranges for individual deposits narrow and rarely more than 8 with standard deviation approximately 1 to 2; individual values for the whole group spread between, but almost entirely restricted by, the values for meteoritic S and seawater SO₄ ^—; no age effect; no apparent relationship between ³⁴S and lead type; and, where investigated, a stratigraphical affiliation. While no positive interpretation of these features is made, it is suggested that the sulphur concerned has been derived neither from migrant hydro-thermal solutions nor from the H₂S of normal biological sulphate reduction in seawater.

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Zusammenfassung Es werden Meßergebnisse von fünf australischen geschichteten Blei-Zink-Sulfid-Lagerstätten mitgeteilt: Jedes Vorkommen ist durch einen bemerkenswert engen Schwankungsbereich der Werte, Mittelwert ³⁴S schwerer als meteoritischer Schwefel/Durchschnittswert aus der Erdkruste, gekennzeichnet. Wo die Proben in stratigraphischer Abfolge entnommen wurden, zeigt der Wert ³⁴S eine stärkere Affinität zur Stratigraphie als zu absoluter Entfernung der Muster. Vergleicht man diese Werte mit solchen von anderen Vorkommen der nämlichen Lagerstättengruppe, so liegen einige allgemeine Eigenheiten auf der Hand: Für einige Vorkommen ist der Mittelwert ³⁴S schwerer als meteoritischer Schwefel; für einzelne Lagerstätten ist der Bereich eng und überschreitet kaum mehr als 8 mit der Standardabweichung von ungefähr 1 bis 2; Einzelwerte für die gesamte Lagerstättengruppe fallen fast ausschließlich zwischen die Werte für meteoritischen Schwefel und SO₄ ^2–-Ionen im Meerwasser; es herrscht kein dem geologischen Alter zuzuschreibender Effekt; auch gibt es keine augenscheinliche Beziehung zwischen dem ³⁴S-Wert und dem Blei-Typ; in jenen Fällen aber, wo Untersuchungsergebnisse vorliegen, ist eine Beziehung der Werte zur Stratigraphie zu erkennen. Obwohl keine positive Interpretation dieser Phänomena angestellt wurde, wird die Vermutung geäußert, daß der betreffende sulfidische Schwefel weder von wandernden hydrothermalen Lösungen noch vom H₂S einer normalen biologischen Sulfatreduktion im Meerwasser abgeleitet werden kann.

     

Origins of metamorphic lode gold deposits: Implications of stable isotope data from the central Rocky Mountains,Canada

Summary The stable isotope geochemistry of native gold-bearing quartz veins contained within low-grade metasedimentary strata in the central Canadian Rocky Mountains, British Columbia is examined. The data augment previous geological and geochemical studies.Vein pyrite ³⁴S values cluster between + 14.2 and + 16.3 (CDT). Coeval galenas exhibit ³⁴S values between + 11.4 and 13.3. Pyrite-galena geothermometry reveals a mean temperature of mineralization of 300 ± 43°C. Comparison of ³⁴S values for the vein pyrites, with values for pyrite porphyroblasts in country rocks suggests that vein sulfur was probably derived from the host rocks.¹⁸O(SMOW) values of host quartzites and pelites cluster between + 12.0 and + 13.5, and + 9.5 and + 10.5, respectively. Auriferous vein quartz exhibits ¹⁸O values between + 13.0 and + 15.0. Veins were likely deposited from fluids undergoing post-peak metamorphic cooling.Vein inclusion fluids exhibit values between –105 and –124 (SMOW). Combined O-H-isotope data are most compatible with a source fluid involving chemically- and isotopically-evolved meteoric waters.The critical role of H-isotope data in the evaluation of source fluids for such mesothermal gold lodes is stressed. The paucity of H-isotope data pertaining to the study of lode gold deposits in similar low-grade metasedimentary domains suggests that the involvement of meteoric waters may at times be overlooked.

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Der Ursprung metamorphogener Gold-Ganglagerstätten: Bedeutung stabiler Isotopendaten aus den zentralen Rocky Mountains, Kanada

Zusammenfassung Die vorliegende Arbeit befaßt sich mit der Untersuchung der Geochemie stabiler Isotope goldführender Quarzgänge in schwach metamorphen Sedimenten der zentralen Rocky Mountains in Britisch Kolumbien, Kanada. Die Resultate ergänzen früher publizierte geologische und geochemische Daten.Die ³⁴S-Werte von Gang-Pyrit liegen zwischen + 14.2 und + 16.3 (CDT); gleichzeitig gebildeter Bleiglanz hat ³⁴S-Werte von + 11.4 bis + 13.3. Die Isotopengeothermo metrie des Pyrits und Bleiglanzes ergibt eine mittlere Mineralisationstemperatur von 300°C + 43° für diese beiden Minerale. Vergleiche der 8345-Werte des Gang-Pyrits mit denen von Pyrit-Porphyroblasten des Nebengesteins lassen für die Gang-Pyrite eine Herkunft des Schwefels aus dem Nebengestein als wahrscheinlich erscheinen.Die ¹⁸O-Werte von Quarziten und Peliten, die als Nebengesteine auftreten, streuen von + 12.0 bis + 13.5 (SMOW), beziehungweise von +9.5 bis + 10.5 Quarz goldführender Gänge hat ¹⁸O-Werte, die zwischen + 13.0 und + 15.0 (SMOW) liegen. Er wurde als Gangfüllung wahrscheinlich bei sinkenden Temperaturen aus post metamorphen wäßrigen Lösungen abgesetzt.Flüssigkeitseinschlüsse von Gangmineralien zeigen D-Werte von -105 bis -124 (SMOW). Die H-O-Isotope sind deshalb ein Hinweis dafür, daß als mineralisierende Lösungen isotopisch veränderte meteorische Wässer in Betracht zu ziehen sind. Bei der Deutung der Herkunft der mineralisierenden wäßrigen Lösungen von mesothermalen Goldgängen muß die Kenntnis der H-Isotope als kritisch betrachtet werden. Die Seltenheit mit der H-Isotopendaten dieses Lagerstättentyps in der Literatur diskutiert werden, dürfte ein wesentlicher Grund dafür sein, daß die Rolle meteorischer Wässer bei der Genese mesothermaler, in Metasedimenten liegender Goldgänge, vielfach übersehen wurde.

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With 4 Figures