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     

Oxygen isotopes in mantle related and geothermally altered magmatites of the Transhimalayan (Gangdese) ranges

In closed magma systems SiO₂ approximately measures differentiation progress and oxygen isotopes can seem to obey Rayleigh fractionation only as a consequence of the behaviour of SiO₂. The main role of ¹⁸O is as a sensitive indicator of contamination, either at the start of differentiation ( ¹⁸O_init) or as a proportion of fractionation in AFC. Plots of ¹⁸O vs SiO₂-allow to determine initial ¹⁸O values for different sequences for source comparison. For NBS-28=9.60, the ¹⁸O at 48% SiO₂-varies between a high 6.4 for Kiglapait (Kalamarides 1984), 5.9 for Transhimalaya, 5.8 for Hachijo-Jima (Matsuhisa 1979), 5.6 for Koloula (Chivas et al. 1982) and a low 5.3 for the Darran Complex, New Zealand. The Transhimalayan batholiths (Gangdese belt) were emplaced in the Ladakh-Lhasa terrane, between the present-day Banggong-Nujiang, and Indus-Yarlung Tsangbo suture zones, after its accretion to Eurasia. The gradient of the least contaminated continuous ( ¹⁸O vs SiO₂-igneous trend line is similar to that of Koloula, and AFC calculations suggest a low secondary assimilation rate of less than 0.05 times the rate of crystallisation. Outliers enriched in ¹⁸O are frequent in the Lhasa, and apparently rare in the Ladakh transsect. Low- ¹⁸O (5.0–0) granitoids and andesites on the Lhasa-Yangbajain axis are the result of present day or recent near-surface geothermal activity; their quartzes still trace the granitoids to the Transhimalaya ¹⁸O trend line, but the distribution of low total rock or feldspar ¹⁸O values could be a guide to more recent heat flow and thermally marked tectonic lineaments. Two ignimbrites from Maqiang show hardly any ¹⁸O-contamination by crustal material.     

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.     

Oxygen and carbon isotopic study on the genesis of the Steirischer Erzberg siderite deposit (Austria)

The carbon and oxygen isotopic composition of Fe-carbonate ore and its calcitic to dolomitic Devonian host rocks at the Steirischer Erzberg siderite deposit (Greywacke zone, Upper Austroalpine Unit) were determined in order to constrain the source and nature of the Fe-rich mineralizing fluid. The ¹⁸O-values obtained for various Fe-carbonate generations and the carbonate host lie within a similar range between + 14.6 and + 21.6 (V-SMOW). No good correlation exists between the relative ages of the carbonate phases and their O isotopic composition. The variation in ¹⁸O-values is due to metamorphic recrystallization with locally variable fluid/rock ratios. The average ¹³C-value of the carbonate host is +0.5 ± 1.2 (PDB) which corresponds well to worldwide Phanerozoic marine carbonate values. The first Fecarbonate generation has slightly lower ¹³C-values, on average -1.4 ± 0.8 (PDB). Recrystallization of both the carbonate host minerals and the ankerite/siderite led to significantly lower ¹³C-values of -4.2 ± 0.6 and-4.7 ± 0.7, respectively. Within the basal breccia of the post-Hercynian transgression series matrix calcite/ dolomite shows an average ¹³C-value of -2.9 ± 0.7, and matrix siderite/ankerite an average value of-4.1 ± 0.4. These data, together with Sr isotope data published previously, strongly support a late-diagenetic or epigenetic first Fe-mineralization from convecting formation waters. They ascended along extension faults and were driven by an increased heat flow caused by crustal thinning during a Devonian rifting phase that initiated the separation of the Noric terrane from Africa. A potential source of the Fe could have been the underlying Ordovician acid volcanics. Regional metamorphism related to collision tectonics in the Late Carboniferous (Hercynian) and later during the Alpine orogeny, caused intensive recrystallization and partial mobilization of the various carbonate phases.     

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.     

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.

     

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.     

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.

     

The sulfur and carbon isotope composition of scapolite-rich granulites from southern Tanzania

Sulfur and carbon isotope data are presented of 15 granulite samples from the Furua Complex, southern Tanzania, in which scapolite is a primary and major rock-forming constituent (up to 30 vol%). From these data, the isotopic composition is deduced of the sulfate and carbonate group in the scapolite structure. Subsequently, the composition and origin is discussed of the volatile species that are present in the deep crustal environment in which these scapolites formed.The ³⁴S-values show a narrow range from 0.3 to 3.6, consistent with a deep-seated (mantle) origin of the sulfur; the mean value of 1.9 is slightly higher than usually found in rocks of assumed mantle provenance. The results of the carbon isotope analyses are more difficult to interpret; they suggest that the granulites contain two different carbon components with different isotopic compositions. Firstly, one component, liberated by phosphoric acid at room temperature, has ¹³Cvalues between –3.8 and –11.2 and a mean value of –6.7. This carbon component is assumed to occur as finely dispersed, submicroscopic carbonate inclusions. The second carbon fraction is liberated by phosphoric acid treatment at temperatures between 200 and 400° C and has considerably lower ¹³Cvalues with a mean value of –14.1 This seems to represent the carbon isotope composition in the scapolite structure. Such low ¹³C-values do not agree with the generally accepted value of –7 for juvenile carbon, but they are comparable to those found in early, primary carbonic inclusions from various granulite regions. It is argued that these low ¹³C-values are typical for granulite-facies metamorphism and that they may characterize an important fluid phase of the lower crust.     

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.     

Formation of Mud-Volcanic Fluids in Taman (Russia) and Kakhetia (Georgia): Evidence from Boron Isotopes

Temperatures of the formation of mud-volcanic waters are determined based on concentrations of some temperature-dependent components (Na–Li, Mg–Li). Estimates obtained for the Taman and Kakhetia regions are similar and range from 45 to 170°, which correspond to depths of 1–4.5 km. The calculated temperatures correlate with the chemical (Li, Rb, Cs, Sr, Ba, B, I, and HCO₃) composition of water and ¹³ (₂) and ¹³ (CH₄) values in spontaneous gases. The isotope values indicate that mechanisms of the formation of ¹³-rich gases, i.e., gases with high ¹³ values (up to +16.0 in ₂ and –23.4 in CH₄) in mud-volcanic systems of Taman and Kakhetia are governed by fluid-generation temperatures rather than the supply of abyssal gases. The ¹¹ value was determined for the first time in mud-volcanic products of the Caucasus region. This value ranges from +22.5 to +39.4 in the volcanic water of Georgia, from –1.2 to +7.4 in the clayey pulp of Georgia, and from –7.6 to +13.2 in the clayey pulp of Taman. It is shown that the ¹¹ value in clay correlates with the fluid-generation temperature and ¹¹ correlates with ¹³ in carbon-bearing gases. These correlations probably testify to the formation of different phases of mud-volcanic emanations in a single geochemical system and suggest the crucial role of temperature in the development of isotope-geochemical features.     

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.

     

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.     

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.     

Das Chausib-Turbiditbecken am Südrand des Damara-Orogens,Südwest-Afrika

Zusammenfassung Am Südrand des Damara-Orogens ist innerhalb der Hakos Subgroup eine Turbiditserie von 1500 m Mächtigkeit entwickelt. Sie stellt als neudefiniertes Chausib Member eine Faziesvertretung der Hakos-Quarzite (Chaibis Member) dar.Die Zusammensetzung der Turbitbänke ist quarzitisch. Infolge metamorpher und tektonischer Überprägung ist eine Gradierung nach der Korngröße nicht mehr vorhanden; sie kann aber aus einer typischen Änderung des Stoffbestandes vom Liegenden zum Hangenden einer Bank abgeleitet werden.Die an sich hochfaltbare Serie liegt in einer einfach gebauten Großstruktur von 10 km Wellenlänge vor; die Faltung wurde durch eine mächtige unterlagernde Quarzitfolge gesteuert. Einige für eine Wechselfolge kompetenter und inkompetenter Lagen charakteristische kleintektonische Phänomene werden beschrieben.Für die paläogeographische Situation wird ein Modell entwickelt, nach dem die Chausib-Turbidite als Beckenablagerungen den Chaibis-Quarziten als gleichalte Schwellensedimente gegenüberstehen.

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A 1500 m thick turbidite series, developed within the Hakos Subgroup at the southern margin of the Damara Orogen, is defined as the Chausib Member. It represents a timeequivalent of the Hakos-Quartzite (Chaibis Member) in a different facies.The turbidites are quartzitic. As a result of the metamorphic and tectonic overprint, they no longer show a grading of grain size. However, a former graded bedding can be inferred from a typical compositional change from the base to the top within each bed.The turbidite series is potentially very susceptible to folding, but occurs in a simple structure with a wave-length of about 10 km. The style of folding was determined by an underlying, thick, quartzite unit. Several small-scale tectonic phenomena are described, which are characteristic for such an alternation of competent and incompetent layers.A palaeogeographic model is envisaged in which the Chausib turbidites are basinal deposits bordering the time-equivalent sediments of a submarine rise, represented by the Chaibis quartzites.

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Résumé A la bordure sud de l'orogène de Damara, on trouve une série de turbidites d'une épaisseur de 1500 m intercalée dans le Hakos Subgroup. En tant que Chausib Member, nouvellement défini, elle représente un faciès équivalent des quartzites de Hakos (Chaibis Member).Les bancs de turbidite ont une composition quartzitique. Mais la superposition métamorphique et tectonique ne permet plus de discerner le granoclassement qui peut cependant être déduit du changement typique dans la composition chimique et minéralogique du matériau d'un banc, du bas vers le haut.La série dont la capacité de plissement est certaine, se présente avec une structure majeure simple, d'une longueur d'onde de 10 km, le plissement ayant été handicapé par une puissante série sous-jacente de quartzite. Quelques phénomènes microtectoniques caractéristiques d'une série interstratifiée de couches compétentes et incompétentes, sont décrits.Un modèle de situation paléogéographique est développé, d'après lequel les turbidites de Chausib s'opposent comme dépôts sédimentaires de bassin contemporains, aux quartzites de Chaibis, considérés comme sédiments de seuil de même âge.

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Hakos Turbidite 1500 . , Chausib, « Hakos». Turbit . . . 10 . , . ., Chausib-Turbidite , Chaibis . .

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Diese Arbeit ist im Sonderforschungsbereich 48 der Universität Göttingen (Entwicklung, Bestand und Eigenschaften der Erdkruste, insbesondere der Geosynklinalräume) entstanden.Die Förderung erfolgte aus Mitteln, die dem Sonderforschungsbereich 48 durch die Deutsche Forschungsgemeinschaft (DFG) zur Verfügung gestellt wurden.     

Oxygen,sulfur, and strontium isotope and fluid inclusion studies of barite deposits from the Iglesiente-Sulcis mining district,Southwestern Sardinia,Italy

This paper deals with barite from stratiform, karst, and vein deposits hosted within Lower Paleozoic rocks of the Iglesiente-Sulcis mining district in southwestern Sardinia. For comparison sulfates from mine waters are studied. Stratiform barite displays ³⁴S=28.8–32.1, ¹⁸O=12.7–15.6, and ⁸⁷Sr/⁸⁶Sr=0.7087, in keeping with an essentially Cambrian marine origin of both sulfate and strontium. Epigenetic barite from post-Hercynian karst and vein deposits is indistinguishable for both sulfur and oxygen isotopes with ³⁴S=15.3–26.4 and ¹⁸O=6.6–12.5; ⁸⁷Sr/⁸⁶Sr ratios vary 0.7094–0.7140. These results and the microthermometric and salinity data from fluid inclusions concur in suggesting that barite formed at the site of mineralization by oxidation of reduced sulfur from Cambrian-Ordovician sulfide ores in warm, sometimes hot solutions consisting of dilute water and saline brine with different ¹⁸O values. The relative proportion of the two types of water may have largely varied within a given deposit during the mineralization. In the karst barite Sr was essentially provided by carbonate host rocks, whereas both carbonate and Lower Paleozoic shale host rocks should have been important sources for Sr of the vein barite. Finally, ³⁴S data of dissolved sulfate provide further support for the mixed seawater-meteoric water composition of mine waters from the Iglesiente area.     

Marine hdrothermal alteration at a Kuroko ore deposit,Kosaka, Japan

Isotopic compositions were determined for quartz, sericite and bulk rock samples surrounding the Uwamuki no. 4 Kuroko ore body, Kosaka, Japan. ¹⁸O values of quartz from Siliceous Ore (S.O.), main body of Black Ore B.O.) and the upper layer of B.O. are fairly uniform, +8.7 to +10.5. Formation temperatures calculated from fractionation of ¹⁸O between sericite and quartz from B.O. and upper S.O. are 250° to 300° C. The ore-forming fluids had ¹⁸O values of +1 and D values of –10, from isotope compositions of quartz and sericite.Tertiary volcanic rocks surrounding the ore deposits at Kosaka have uniform ¹⁸O values, +8.1±1.0 (n=50), although their bulk chemical compositions are widely varied because of different degrees of alteration. White Rhyolite, which is an intensely altered rhyolite occurring in close association with the Kuroko ore bodies, has also uniform ¹⁸O values, +7.9±0.9 (n=19). Temperatures of alteration are estimated to be around 300° C from the oxygen isotope fractionation between quartz and sericite. Paleozoic basement rocks phyllite and chert, have high ¹⁸O values, +18 and +19. The Sasahata formation of unknown age, which lies between Tertiary and Paleozoic formations, has highly variable ¹⁸O, +8 to +16 (n=4). High ¹⁸O values of the basement rocks and the sharp difference in ¹⁸O at their boundary suggest that the hydrothermal system causing Kuroko mineralization was mainly confined within permeable Tertiary rocks. D values of altered Tertiary volcanic rocks are highly variable ranging from –34 to –64% (n=12). The variation of D does not correlate with change of chemical composition, ¹⁸O values, nor distance from the ore deposits. The relatively high D values of the altered rocks indicate that the major constituent of the hydrothermal fluid was sea water. However, another fluid having lower D must have also participated. The fluid could be evolved sea water modified by interaction with rocks and the admixture of magmatic fluid. The variation in D may suggest that sea water mixed dispersively with the fluid.     

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.

     

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.     

Lower Triassic alkaline granites of Central Chile (30‡S) in the high-Andean Cordillera

The Na₂O + K₂O/Al₂O₃ molar ratio (agpaitic index) of Lower Triassic leucogranites samples from the Rios Elqui-Lamarí batholith is close to one which is typical of alkaline granites. Loss of alkalis due to groundwater interaction should have occur causing a depletion of the original peralkalinity.It is assumed that this alkaline magmatism has been generated, during crustal extension period, by partial melting of water undersaturated deep crustal rocks.

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Zusammenfassung Das Mol-VerhÄltnis von Na₂O + K₂/Al₂O₃ (agpaitischer Index) der untertriassischen Leukogranit-Proben des Batholithen vom Rios Elqui-Limari liegt in der NÄhe von 1. Dieses ist typisch für Alkaligranite. Der Verlust von Alkali könnte durch Grundwassereinwirkung zustandegekommen sein, was eine Verringerung der ursprünglichen HyperalkalinitÄt verursacht hat.Es wird angenommen, da\ dieser alkalische Magmatismus in einer Phase der Krustenausdehnung entstanden ist durch partielles Aufschmelzen von wasseruntersÄttigten Gesteinen der Tiefenkruste.

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Resumen Leucogranitos de edad triásica inferior pernecientes al batolito compuesto Ríos Elqui-Limarí están caracterizados por presentar una razón molecular Na₂O + K₂O/Al₂O₃ (índice agpaítico) cercano a uno, propio de los granitos alcalinos. Una probable pérdida de álcalis de estos granitos, provocada por el agua infiltrada, sería causante de una disminución de la peralcalinidad original.Se infiere un mecanismo de fusión parcial de rocas corticales profundas subsaturadas en agua, para la génesis de este magmatismo alcalino en un ambiente tectónico de extensión cortical.

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Na₂O + K₂/l₂₃ - - -- 1. . , . , .