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.     

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.     

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.     

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     

Isotope studies on the Rosebery,Mount Farrell and Mount Lyell ores,Tasmania

Preliminary studies have been made on the distributions of oxygen and sulphur isotopes in the Rosebery, Mount Farrell, and Mount Lyell ores. These ores lie in Cambrian geosynclinal volcanic rocks in West Tasmania. At each locality the sulphur of the sulphide minerals has a distinctive degree of enrichment in ³⁴S in relation to sulphur in meteorites and a narrow range of ³⁴S values. The dominant ore at Mount Lyell (mainly pyrite-chalcopyrite) has an average ³⁴S value of +7.0, the main lode at Rosebery (pyrite-sphalerite-galenachalcopyrite) averages +10.9, and the Mount Farrell ore (galena-sphalerite) averages +14.1. The degree of enrichment does not appear to be related to local, near-surface geological factors. Other ores of geosynclinal volcanic type with similar mineralogy also show narrow ranges in ³⁴S and varying enrichments in ³⁴S. Barite from a concordant sulphide-barite-carbonate lode at Rosebery has an average ³⁴S of +38.1 and an average ¹⁸O of +10.7. Barite from veins at Mount Lyell has an average ³⁴S of +25.3 and an average ¹⁸O of +10.6.

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Die Verteilung von Sauerstoff- und Schwefel-Isotopen in den Erzkörpern von Rosebery, Mount Farrell und Mount Lyell wurde untersucht. Die Erzkörper sind in kambrische, geosynklinale vulkanische Gesteine Westtasmaniens eingebettet. An jeder dieser Lagerstätten zeigt der Schwefel der Sulfiderze einen charakteristischen Anreicherungsgrad an ³⁴S im Verhältnis zum Meteoritenschwefel und einen eng begrenzten Bereich der ³⁴S-Werte. Die Erze des Mount Lyell-Lagers (hauptsächlich Pyrit-Chalkopyrit) zeigen überwiegend einen ³⁴S-Durchschnittswert von +7.0, das Hauptlager von Rosebery (Pyrit-Sphalerit-Galenit-Chalkopyrit) +10.9, und des Mount Farrell-Erz (Galenit-Sphalerit) +14.1. Der Anreicherungsgrad scheint nicht mit den lokalen geologischen Faktoren verbunden zu sein. Auch andere Erzkörper geosynklinaler vulkanischer Art von ähnlicher mineralogischer Struktur zeigen eng begrenzte ³⁴S-Werte und ³⁴S-Anreicherungsvariationen. Der Baryt des konkordant aufgebauten Sulfid-Baryt-Carbonat-Lagers bei Rosebery hat einen ³⁴S-Durchschnitt von +38.1 und einen ¹⁸O-Durchschnitt von +10.7. Der Baryt aus den Erzgängen von Mount Lyell ist durch einen ³⁴S-Durchschnitt von +25.3 und einen ¹⁸O-Durchschnitt von +10.6 charakterisiert.

     

Fossil sulphate-reducing bacteria in the Bleiberg lead-zinc deposit, Austria

Polished ore samples from the world-class Bleiberg lead-zinc deposit in Austria were studied by Field Emission Scanning Electron Microscopy (FESEM) at magnifications in the range of 10⁵–10⁶. The zinc ore shows nano-sized sphalerite filaments and spherules which are morphologically similar to recent biofilms of sulphate-reducing bacteria. The activity of sulphate-reducing bacteria is suggested by a large ³⁴S difference of ~40 between coeval seawater sulphate (+16 ) and sulphide sulphur (< –25 ) in the Bleiberg ores, and by variable sulphur valences. Peloids of sphalerite, Zn-bearing calcite and pyrite have features typical of bacterial colonies. Combined with geological and mineralogical evidence, a significant role of bacteria during ore deposition at Bleiberg is likely.Eugen F. Stumpfl deceasedEditorial handling: B. Lehmann     

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     

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.     

Sulphur isotope studies of the Mt Molloy,Dianne and O.K. stratiform sulphide deposits,Hodgkinson Province,North Queensland,Australia

Stratiform sulphide deposits which have been metamorphosed to lower greenschist facies occur within the Paleozoic strata of the Hodgkinson Province, northeast Queensland. Massive cupreous pyrite is ubiquitous and Mt Molloy and Dianne also have layered chalcopyrite-rich and sphalerite-rich lenses. Sulphide ³⁴S values for the mineralisation show a narrow spread, around 02030; at the Dianne and O.K. deposits, but a wider spread and an average several per mil higher at the Mt Molloy area. The minerals can not be used for geothermometry due to isotopic disequilibrium. However, metamorphic effects on the isotopic compositions appear not to have been significant. A decrease in temperature and contact of the ore fluid with sea water probably caused the precipitation of the ore minerals. A magmatic ore fluid with ³⁴S_S around 02030; predominated at the Dianne and OK deposits whereas the fluid at Mt Molloy mixed with sea water to acquire a higher ³⁴S_S value.     

Preliminary investigations of 18O/16O and D/H compositions in rhyo-ignimbrites in the In Hihaou (In Zize) Magmatic Center,central Ahaggar,Algeria

The 620 M.y.-old in Hihaou (In Zize) magmatic complex located at the north-western boundary of the Archaean In Ouzzal block (western Ahaggar), is composed of massive alkaline rhyo-ignimbrites and rhyolitic domes, which are intruded by a granophyric and granitic body. The whole is preserved in a cauldron structure. Extrusive rocks are strongly ¹⁸O-depleted, with -values as low as –1.5/SMOW, while granophyres are less depleted (minimum -¹⁸O value=+2.0/SMOW. The granite has values around + 6/SMOW. D/H compositions are rather low, with D–90 to –110/SMOW. Isotopic zoning of quartz phenocrysts, ¹⁸O/¹⁶O fractionation among coexisting phases, and heterogeneity of the whole-rock -¹⁸O values, suggest that the volcanic rocks have interacted with meteoric water after the eruption. Several mechanisms of isotopic alteration are discussed. The hydrothermal alteration does not seem to have been controlled by the granitic intrusion, but rather seems to have followed the deposition of thick pyroclastic deposits on permeable arkosic sandstones and fluvio-glacial conglomerates. Pervasive circulation of water through the cooling volcanic deposits could have produced the observed ¹⁸O depletion.     

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)     

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.     

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     

Schwefel-Isotopenuntersuchungen an der Blei-Zink-Erzlagerstätte Grund (Westharz,Bundesrepublik Deutschland)

Zusammenfassung Die S-Isotopenverteilung wurde an 67 Sulfid- und 17 Barytproben aus der Blei-Zink-Erzlagerstätte Grund untersucht. Die ³⁴S-Werte der Zinkblende der Mineralisationsphase II liegen im Westfeld-Erzmittel I und in den östlich anschließenden Erzmitteln zwischen +4 und +6, in dem am weitesten westlich liegenden Westfeld-Erzmittel II zwischen +6 und +10. Die Werte für Bleiglanz der Mineralisationsphase II sind +2 bis +4 bzw. +4 bis +7. Die Sulfide der Mineralisationsphase III haben allgemein niedrigere -Werte. Koexistierende Sulfide zeigen eine deutliche Fraktionierung, wobei stets _ZnS > _PbS ist; die Differenz beträgt in der Mineralisationsphase II im Mittel 1,8, in der Phase III 3. Dies deutet auf niedrigere Bildungstemperatur der Minerale der Phase III hin. Zur genetischen Deutung der beobachteten -Abnahme beim Übergang zur Mineralisationsphase II werden vier Modelle diskutiert. Baryte zeigen innerhalb der Lagerstätte recht einheitliche ³⁴S-Werte zwischen +11 und +14,5%. Diese Einheitlichkeit wird durch den Einfluß deszendenter Zechstein-Lösungen erklärt.

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³⁴S-values are given for 67 sulfide and 16 barite specimens from the Pb-Zn-deposit Grund (Harz mountains, W-Germany). In the central part of the deposit the sulfide 's of the first major mineralization (phase II) range from: ZnS +4 to +6 and PbS +2 to +4. The sulfides of the second major mineralization (phase III) are depleted in ³⁴S and range from: ZnS +2 to +4, PbS –1,4 to 3. The sulfides at the western end of the vein system are heavier; the phase II minerals ranging from: ZnS +6 to 10 and PbS +4 to 7. The mean -difference between co-existing ZnS and PbS in phase II is 1,8, in phase III 3. This indicates lowering of temperature of formation for the phase III ore. Four models have been set up in order to explain the observed -variation. Barites with rather uniform 's from +11 to +14,5 are probably affected by descendent solutions from overlying sulfate sediments of Permian age.

     

Oxygen isotope geochemistry of hydrothermally-altered synmetamorphic granitic rocks from South-Central Maine,USA

Hydrothermally-altered mesozonal synmetamorphic granitic rocks from Maine have whole-rock ¹⁸O (SMOW) values 10.7 to 13.8. Constituent quartz, feldspar, and muscovite have ¹⁸O in the range 12.4 to 15.2, 10.0 to 13.2, and 11.1 to 12.0, respectively. Mean values of _Q–F ( ¹⁸O_quartz– ¹⁸O_feldspar)=2.4 and _Q–M ( ¹⁸O_quartz– ¹⁸O_muscovite)=3.3 are remarkably uniform (standard deviations of both are 0.2). Measured _Q–F and _Q–M values demonstrate that the isotopic compositions of the minerals are altered from primary magmatic ¹⁸O values but that the minerals closely approached oxygen isotope exchange equilibrium at subsolidus temperatures. Analyzed muscovites have D (SMOW) values in the range –65 to –82.Feldspars in the granitic rocks are mineralogically altered to either (a) muscovite+calcite, (b) muscovite+calcite+epidote, (c) muscovite+epidote, or (d) muscovite only. A consistent relation exists between the assemblage of secondary minerals and the oxygen isotope composition of whole rocks, quartz, and feldspar. Rocks with assemblage (a) have whole-rock ¹⁸O>12.1 and contain quartz and feldspar with ¹⁸O>13.8 and >11.4, respectively. Rocks with assemblages (b), (c), and (d) have whole-rock ¹⁸O<11.4 and contain quartz and feldspar with ¹⁸O< 13.1 and <11.0, respectively. The correlation suggests that the mineralogical alteration of the rocks was closely coupled to their isotopic alteration.Three mineral thermometers in altered granite suggest that the hydrothermal event occurred in the temperature range 400°–150° C, 100°–150° C below the peak metamorphic temperature inferred for country rocks immediately adjacent to the plutons. Calculations of mineral-fluid equilibria indicate that samples with assemblage (a) coexisted during the event with CO₂-H₂O fluids of and ¹⁸O=10.8 to 12.2 while samples with assemblages (b), (c), or (d) coexisted with fluids of and ¹⁸O=9.4 to 10.1. Compositional variations of the hydrothermal fluids were highly correlated: fluids enriched in CO₂ were also enriched in ¹⁸O. Because CO₂ was added to the granites during hydrothermal alteration and because fluids enriched in CO₂ were enriched in ¹⁸O, some or all of the variation in ¹⁸O of altered granites may have been caused by addition of ¹⁸O to the rocks during the hydrothermal event. The source of both the CO₂ and ¹⁸O could have been high-¹⁸O metasedimentary country rocks. The inferred change in isotopic composition of the granites is consistent with depletion of the metacarbonate rocks in ¹⁸O close to the plutons and with large volumes of fluid that were inferred from petrologic data to have infiltrated the metacarbonate rocks during metamorphism.A close approach of minerals to oxygen isotope exchange equilibrium in altered mesozonal rocks from Maine is in marked contrast to hydrothermally-altered epizonal granites whose mineral commonly show large departures from oxygen isotope exchange equilibrium. The difference in oxygen isotope systematics between altered epizonal granites and altered mesozonal granites closely parallels a differences between their mineralogical systematics. Both differences demonstrate the important control that depth exerts on the products of hydrothermal alteration. Deeper hydrothermal events occur at higher temperature and are longer-lived. Minerals and fluid have sufficient time to closely approach both isotope exchange and heterogeneous chemical equilibrium. Shallower hydrothermal events occur at lower temperatures and are shorter-lived. Generally there is insufficient time for fluid to closely approach equilibrium with all minerals.     

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.     

Hydrothermal alteration and REE-Th mineralization at the Rodeo de Los Molles deposit,Las Chacras batholith,central Argentina

REE (rare-earth-element) and Th mineralization at the Rodeo de Los Molles deposit occurs within an elliptical body of hydrothermally altered rocks (fenite) located in a biotite monzogranite of the Las Chacras batholith. Ore assemblages are found as isolated patches of intergrown britholite, allanite, apatite, bastnaesite, fluorite, sphene, quartz, and aegirine-augite, as well as nodules of uranothorite and late-stage veins of calcite, fluorite, and bastnaesite. Composition-volume computations suggest that the fenite was produced by alteration of the biotite monzogranite by addition of K and Na, and loss of Ca and Sr. Petrographic evaluations indicate that microcline and plagioclase have been replaced by perthite, and biotite was converted to aggregates of clinochlore, anatase, kaolinite, and hematite. Relict biotite is characterized by lower Fe/(Fe+Mg) and Ti values with progressive alteration. Fluorine-rich phlogopite is present in mineralized areas, but textural evidence suggests that it was not produced via biotite alteration. Mass-balance constraints also show that Ca and Mg in ore zones may result from redistribution, rather than their being a result of external derivation. The ¹⁸O values of quartz (8.6–11.1) and feldspar (7.8–10.6) suggest that feldspar continued to exchange oxygen isotopes with a fluid to lower temperatures than did quartz. Feldspars equilibrated with a fluid of ¹⁸O8 at a fluid/rock ratio less than 1. The ¹⁸O values of quartz and aegirine-augite that crystallized during REE mineralization also suggest equilibration with a fluid of ¹⁸O8. The D values of biotite (-83 to-120) are relatively low for igneous rocks and are thought to have resulted from exsolution of a D-enriched magmatic vapor. The D values of both mineralized and barren fenites are consistent with equilibration with fluid of magmatic origin. Meteoric water was involved in the production of calcite and clinochlore alteration, and late-stage calcite-fluorite-bastnaesite veins. The ¹³C values of calcite and bastnaesite (-7.8 to-13.5%) suggest that carbon was derived by leaching of carbon from igneous and/or enclosing metamorphic rock types, and that a majority of carbon ultimately was derived from sedimentary organic meterial.     

Sulfur isotopes and the origin of stibnite mineralisation in New England,Australia

Stibnite mineralisation in the antimony province of New England can be divided into Central type ores (veins of stibnite + quartz ± berthierite) and Peripheral type ores of stibnite + quartz + native antimony ± berthierite. The Central stibnites have ³⁴S_CDT values of –5±2 (1) which may represent equilibrium precipitation from mantle sulfur at about 200°C. Peripheral stibnites have ³⁴S values between 0 and –25, with a large group at 0±2. They represent precipitation from a limited supply of mantle sulfur and the acquisition of sedimentary sulfur. We consider that the different ore types were produced from distinct ore solutions derived from two immiscible melts. These originated in the deep mantle, were mobilised by tectonic activity and supplied the antimony and most of the sulfur to the ores.     

The Rosita Hills epithermal Ag-Base metal deposits, Colorado, USA A stable isotope and fluid inclusion study

The Rosita Hills volcanic centre is an alkalicalcic, mid-Tertiary complex overlying orthoand paragneissic basement, on the eastern margin of the Rio Grande Rift in south central Colorado, USA. The centre contains vein-hosted, adularia-sericite type, epithermal Ag and base-metal mineralisation with minor Au. Stable isotope studies (O and H) of whole rock and mineral separate (quartz and sericite) samples from veins and hydrothermal eruption breccias show that the hydrothermal fluid had both magmatic and meteoric components. The D and ¹⁸O values of the hydrothermal fluid, calculated from mineral values, range from -22 to -103 and 0.5 to 5.9 respectively. Fluid inclusion data from vein minerals (quartz, baryte and sphalerite) and from an advanced argillic lithocap overlying the veins again show that the hydrothermal system had more than one component fluid. Fluid inclusions have salinities which range from 1.7 to 25.1 wt% NaCl equivalent and show evidence of boiling in the advanced argillic lithocap. Homogenisation temperatures range from 135°C to 298°C. Liquid CO₂ is present in some inclusions. These data indicate that a saline, isotopically heavy fluid mixed with a dilute, isotopically light fluid to precipitate the ore. We argue that the saline, isotopically heavy fluid is magmatic and derived from a resurgent rhyolitic magma below the mineralisation.     

Boron isotopic variations in hydrous rhyolitic melts: a case study from Long Valley,California

In this paper, we present boron isotope analyses of variably degassed rhyolitic glasses from Long Valley, California. The following results indicate that pre-eruptive boron isotopic signatures were preserved in degassed glasses: (1) averaged secondary ionization mass spectrometry (SIMS) measurements of H₂O-rich (~3 wt%) melt inclusions from late erupted Bishop Tuff pumice are indistinguishable from positive thermal ionization mass spectrometry (PTIMS) analysis of vesiculated groundmass glass (¹¹B=+5.0±0.9 and +5.4±5, respectively); (2) SIMS spot-analyses on H₂O-poor obsidian (~0.15 wt% H₂O) from younger Glass Mountain Dome YA (average ¹¹B=+5.2±1.0) overlap with compositionally similar late Bishop Tuff melt inclusions; and (3) four variably degassed obsidian samples from the 0.6 ka Mono Craters (H₂O between 0.74 and 0.10 wt%) are homogeneous with regard to boron (average ¹¹B=+3.2±0.8, MSWD=0.4). Insignificant variations in ¹¹B between early and late Bishop Tuff melt inclusion glasses agree with published experimental data that predict minor ¹¹B depletion in hydrous melts undergoing gas-saturated fractional crystallization. Melt inclusions from two crystal-rich post-caldera lavas (Deer Mountain and South Deadman Dome) are comparatively boron-rich (max. 90 ppm B) and have lower ¹¹B values (average ¹¹B=+2.2±0.8 and –0.4±1.0 ) that are in strong contrast to the boron isotopic composition of post-caldera crystal-poor rhyolites (27 ppm B; ¹¹B=+5.7±0.8). These variations in ¹¹B are too large to be caused by pre-eruptive degassing. Instead, we favor assimilation of ¹¹B depleted low-temperature hydrothermally altered intrusive rocks subsequent to fresh rhyolite recharge.Editorial responsibility: J. HoefsAn erratum to this article can be found at