The origin of fluids involved in the formation of gold-bearing skarns of the Andorra granite (Central Pyrenees,Spain): Sulphur isotope data

Summary Arsenopyrite-gold mineralization in the lower Paleozoic series of the eastern Pyrenees is of two types: disseminated in Hercynian metasediments, and veins cross-cutting the main foliation. In the central Pyrenees, gold-bearing arsenopyrite mineralizations have recently been discovered in skarns along the SW contact of the Andorra granite. The ore minerals are similar to those of the veins, with gold also being associated with arsenopyrite, bismuth and sulphides.The late Hercynian Andorra granite intruded Paleozoic metasediments of pre-Caradocian to Carboniferous age. Six types of skarns have developed at or close to the contact between the granite and Devonian limestones. Only three of these, the hedenbergite, pyrrhotite and arsenopyrite skarns, are sulphide-bearing. Gold grades in these skarns range from less than 0.8 g/t in pyrrhotite skarns to 7 g/t in arsenopyrite skarns.The ³⁴S of arsenopyrite from disseminated and vein mineralizations show values from 11 to 15, indicating a sulphur source of crustal origin. Vein sulphides are considered to have formed after a metamorphic remobilization of the sulphide rich disseminations in the Paleozoic series. Sulphur isotopic composition of skarn sulphides is variable: pyrrhotite from pyrrhotite skarns has a ³⁴S +, suggesting a sulphur of magmatic origin. Their gold grades are very low (>0.8 g/t). The heavier values of arsenopyrite from arsenopyrite skarns (³⁴S + 10) and of galena and sphalerite from hedenbergite skarns (³⁴S + 7) correlate with the higher gold grades (7 and 5 g/t, respectively). This is interpreted to reflect a contribution of wall-rock sulphur-rich fluids generated from the thermal metamorphism of arsenopyrite-bearing wall-rock metasediments (³⁴S + 12%o) during skarn formation or by assimilation of country rock by the granitic intrusion during its emplacement. The economic importance of these skarns appears to be related to the amount of crustal fluid involved in skarn formation.

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Lagerstättenbildende Fluide in den goldführenden Skarnen des Andorra-Granites, Zentralpyrenäen, Spanien: Schwefelisotopen-Daten

Zusammenfassung Arsenkies-Goldmineralisation in altpaläozoischen Serien der ostlichen Pyrenäen tritt in zwei Typen auf: Disseminiert in Hercynischen Metasedimenten, und in Gängen, die die Haupt-Foliation durchschlagen. In den Zentralpyrenäen wurden goldführende Arsenkies-Mineralisationen vor kurzem in Skarnen längs des SW-Kontaktes des Andorra-Granites entdeckt. Die Erzminerale sind ähnlich denen in den Gängen, wobei Gold mit Arsenkies, Wismut und Sulfiden assoziiert ist. Der spät-hercynische Andorra-Granit intrudierte paläozoische Metasedimente von Prä-Caradoc bis Karbon-Alter. Im Kontaktbereich Granit/Devonische Kalke kommen sechs Typen von Skarnen vor. Nur drei davon führen Sulflde, nämlich die Hedenbergit-, Magnetkies- und Arsenkiesskarne. Goldgehalte in diesen Skarnen schwanken von weniger als 0,8 g/t in Magnetkiesskarnen bis zu 7 g/t in Arsenkiesskarnen. Das ö34 S von Arsenopyrit in disseminierten und Gang-Mineralisationen schwankt von 11-15%o, und dies weist auf eine Schwefelquelle in der Kruste hin. Gangsulflde haben sich auch nach einer metamorphen Remobilisierung der Sulfid-reichen Disseminationen in den paläozoischen Serien gebildet. Die Schwefelisotopenzusammensetzung der Skarnsulfide schwankt: Magnetkies aus Magnetkiesskarnen hat ö34 S Werte von +4%o, dies weist auf magmatischen Ursprung hin. Ihre Goldgehalte sind sehr niedrig (<0,8 0,8 g/t). Die schwereren Isotopen von Arsenkies in Arsenkies-Skarnen ( ³⁴S+ 10%o) und von Bleiglanz und Zinkblende aus den Hedenbergit-Skarnen ( ³⁴S+ 7%o) lassen sich mit höheren Goldgehalten (7 und 5 g/t) korrelieren. Dies wird Fluiden zugeschrieben, die reich an Schwefel aus den Nebengesteinen sind und durch die thermale Metamorphose der Arsenkies-führenden Metasedimente (S + 12%o) während der Skarnbildung, oder durch Assimilation von Nebengestein durch den Granit wahrend seiner Platznahme entstanden sind. Die wirtschaftliche Bedeutung dieser Skarne scheint in einer direkten Beziehung zur Menge krustaler Fluide zu stehen.

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This paper was presented at the IGCP 291 Project Symposium Metamorphic Fluids und Mineral Deposits, ETH Zürich, March 21–23, 1991.     

Lead-isotope evidence for a Hercynian origin of the Salsigne gold deposit (Southern Massif Central,France)

The Salsigne gold deposit contains a complex association of sulphide layers, gold-rich disseminations, quartz-bearing veins and flat reefs, which are hosted by folded and slightly metamorphosed Paleozoic sediments on both sides of a major thrust zone. It is demonstrated that these various Au-As ore types have similar lead-isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.22–18.56), which are characteristic of Hercynian ore deposits in the southern Massif Central. The Lower Cambrian to Devonian host rocks and associated Pb-Zn-Ba occurrences display distinctly less radiogenic corrected isotopic signatures (²⁰⁶Pb/²⁰⁴Pb = 17.83–17.98), which are characteristic of Cambrian lead in the Montagne-Noire. Concerning the controversial origin of the Salsigne gold mineralization, these results disagree with the former syngenetic hypothesis and support a new model of Hercynian syntectonic gold concentration.This work was supported by BRGM'S scientific program: Le gisement de Salsigne: caractérisation du modèle et évaluation du potentiel aurifère du district     

Origin and regional significance of late Precambrian and early Palaeozoic granitoids in the Pan-African belt of Somalia

Granitoids within the Precambrian basement of north-eastern and southern Somalia are subdivided on the basis of geology, geochronology and petrology into three different assemblages. The post-kinematic assemblage in north-eastern Somalia ( 630 Ma) comprises granodiorites and granites which belong to a medium-K calc-alkaline suite. Average initial Sr, Nd and Pb isotopic ratios [Sr_i = 0.7048, _Nd = –1.8,²⁰⁶Pb/²⁰⁴Pb(i) = 17.704 and²⁰⁷Pb/²⁰⁴Pb(i) = 15.611] indicate that these melts were derived from a mantle or juvenile crustal source with only slight involvement of pre-existing crust as a contaminant. Two different assemblages are found in southern Somalia. The older assemblage is composed of crustal anatectic, synkinematic, parautochthonous granites ( 600 Ma) related to amphibolite facies retrogression of an intensively reworked pre-Pan-African crust [Sr_i = 0.7100, _Nd = –8.4,²⁰⁶Pb/²⁰⁴Pb(i) = 15.403 and²⁰⁷Pb/²⁰⁴Pb(i) = 15.259]. These monzo- and syenogranites are moderately potassic and peraluminous. The younger assemblage ( 470 Ma) consists of post-kinematic monzonites to syenogranites with A-type affinities. Initial Sr, Nd and Pb isotopic data for this metaluminous assemblage [Sr_i = 0.7114, _Nd = –13.1,²⁰⁷Pb/²⁰⁴Pb(i) = 16.913 and²⁰⁷Pb/²⁰⁴Pb(i) = 15.512] indicate a significant lower crustal component but, however, also a mantle signature. The late Proterozoic to early Palaeozoic granitoids in Somalia thus express contrasting regimes, characterized by strong juvenile input in the north, close to the Arabian-Nubian Shield, whereas intense crustal reworking with little addition of juvenile material prevailed in the south. Somalia was definitively not a cratonic area during the Pan-African, but a zone of high crustal mobility.     

Lead isotope study of Zn-Pb ore deposits associated with the Basque-Cantabrian basin and Paleozoic basement, Northern Spain

A total of forty-three galena samples from syngenetic and epigenetic Pb-Zn mineralizations emplaced in the Lower Cretaceous Basque-Cantabrian basin and Paleozoic basement of the Cinco Villas massif in the western Pyrenees, have been analyzed for Pb-isotopic composition. Galena from sedex mineralizations hosted in Carboniferous clastic rocks in the Cinco Villas massif display an homogeneous lead isotopic signature (²⁰⁶Pb/²⁰⁴4Pb 18.43, ²⁰⁷Pb/²⁰⁴Pb 15.66, ²⁰⁸Pb/ ²⁰⁴Pb 38.69) suggesting a single lead reservoir. These values are slightly more radiogenic than lead from other European Hercynian deposits, possibly reflecting the influence of a more evolved upper crustal source. Underlying Paleozoic sediments are proposed as lead source for the Cinco Villas massif ores. Analyses from twenty-six galena samples from the four strata-bound ore districts hosted in Mesozoic rocks reveal the existence of two populations regarding their lead isotopic composition. Galena from the western Santander districts (e.g., Reocin) is characterized by more radiogenic isotope values (²⁰⁶Pb/²⁰⁴Pb 18.74, ²⁰⁷Pb/²⁰⁴Pb 15.67, ²⁰⁸Pb/ ²⁰⁴Pb 38.73) than those from the central and eastern districts (Troya-Legorreta, Central and Western Vizcaya, ²⁰⁶Pb/²⁰⁴Pb 18.59, ²⁰⁷Pb/²⁰⁴Pb 15.66, ²⁰⁸Pb/ ²⁰⁴Pb 38.73). In all districts, the most likely source for these mineralizations was the thick sequence of Lower Cretaceous clastic sediments. The existence of two separate lead isotopic populations could be the result of regional difference in the composition of the basement rocks and the clastic sediments derived of it or different evolution histories. In both sub-basins, isotopic ratios indicate an increase in crustal influence as the age of the ores decreases.     

Sargipali galenas: Unusual lead-isotope data from eastern India

The Sargipali sulphide deposit, hosted by a sequence of metasediments of the Precambrian Gangpur Series, represents the stratiform and stratabound Pb-Cu mineralization. Lead-isotope analyses obtained from six galena samples of different forms and stratigraphical levels, yielded a model Pb age of 1665 Ma, which predates the episodes of deformation and metamorphism ( 850 Ma) as well as syn-to late-tectonic magmatism. The model Pb age, on the contrary, shows a relationship to the closing phase of the Gangpur sedimentation (1700 Ma) in a shallow-water marine environment. The notable paucity of the interlayered volcanic rock in the deposit and its Pb-rich nature possibly point to its being the sedimentary exhalative (SEDEX) type. Documentary evidence for the Pb component from the geological environment of the upper continental crust is revealed by an exceptionally high (= ²³⁸U/²⁰⁴Pb) and the slightly above average W(= ²³²Th/²³⁸U) values. These indicate that the source rock was initially rich in U-Th-Pb concentration. For the Sargipali ore Pb, the parent is assumed to be late Archean-lower Proterozoic soda granite.     

Geological,fluid inclusion and stable isotope studies of Mo mineralization,Galway Granite,Ireland

Mo mineralization within the Galway Granite at Mace Head and Murvey, Connemara, western Ireland, has many features of classic porphyry Mo deposits including a chemically evolved I-type granite host, associated K- and Si-rich alteration, quartz vein(Mace Head) and granite-hosted (Murvey) molybdenite, chalcopyrite, pyrite and magnetite mineralization and a gangue assemblage which includes quartz, muscovite and K-feldspar. Most fluid inclusions in quartz veins homogenize in the range 100–350°C and have a salinity of 1–13 eq. wt.% NaCl. They display Th-salinity covariation consistent with a hypothesis of dilution of magmatic water by influx of meteoric water. CO₂-bearing inclusions in an intensely mineralized vein at Mace Head provide an estimated minimum trapping temperature and pressure for the mineralizing fluid of 355°C and 1.2 kb and are interpreted to represent a H₂O-CO₂ fluid, weakly enriched in Mo, produced in a magma chamber by decompression-activated unmixing from a dense Mo-bearing NaCl-H₂O-CO₂ fluid. ³⁴S values of most sulphides range from c. 0 at Murvey to 3–4 at Mace Head and are consistent with a magmatic origin. Most quartz vein samples have ¹⁸O of 9–10.3 and were precipitated from a hydrothermal fluid with ¹⁸O of 4.6–6.7. Some have ¹⁸O of 6–7 and reflect introduction of meteoric water along vein margins. Quartz-muscovite oxygen isotope geothermometry combined with fluid inclusion data indicate precipitation of mineralized veins in the temperature range 360–450°C and between 1 and 2 kb. Whole rock granite samples display a clear ¹⁸O-D trend towards the composition of Connemara meteoric waters. The mineralization is interpreted as having been produced by highlyfractionated granite magma; meteoric water interaction postdates the main mineralizing event. The differences between the Mace Head and Murvey mineralizations reflect trapping of migrating mineralizing fluid in structural traps at Mace Head and precipitation of mineralization in the granite itself at Murvey.     

Sulfur isotope study of source and deposits of stibnite in the Turhal Area,Turkey

The Turhal antimony sulfide ore deposits are hosted by a Permian-Jurassic sequence which consists of black phyllites at the base followed by interbedded phyllites and calcareous quartzites with metabasite interlayers and then by brown-gray phyllites with marble blocks. Four different styles and three distinct episodes of mineralization were distinguished according to deposition features of the ores and kinkbands in the stibnite crystals. Stibnite from stratiform, disseminated and vein occurrences as well as pyrite from black phyllites showed the following sulfur isotope composition (³⁴S): +2.8 and +3.0 for stratiform stibnite (n = 2), +3.6 and +5.5 for disseminated stibnite (n = 2), +2.5 to +7.8 for vein stibnite (n = 11) and -6.1 to +0.1 for pyrite (n = 3). The ³⁴S compositions of stibnite are interpreted as suggesting an ultimately single source for sulfur in the various styles of mineralization, i.e. synsedimentary volcanic exhalations for the stratiform and disseminated together with ores and hydrothermal mobilisation of these as well as leaching of volcanic rocks to form the vein ores. Deep basinal fluids probably under normal geothermal gradient conditions caused the leaching of the primary sulfides as suggested by the oxygen isotope composition of vein quartz associated with the ores. By contrast sulfur in pyrite is essentially a derivation of seawater sulfate through bacterial and/or chemical reduction.     

Pre-orogenic volcanic assemblages and structure in the Western Cordillera of Ecuador between l°40'S and 2°20'S

A major reverse fault with a separation of at least a few thousand meters is exposed in the western Cordillera of Ecuador near 2°S.Ree and other incompatible elements patterns show (i) that the rocks exposed in the upthrust block to the east of the fault areMorb that had not yet been identified and (ii) that a volcanic island-arc assemblage extensively outcrops in the downthrust block west of the fault.The fault therefore bounds a steeply east-dipping thrustsheet comprising the arc and its basement. Thrustsheets of the same type are known farther north in Ecuador. They were stacked in a presently inactive subduction-zone during the early Tertiary when the island-arc, drifting from the west, finally collided with continental south America.The large dextral-slip Guayaquil-Dolores fault system, traditionally but possibly erroneously thought to be the suture-zone between continental south America and the oceanic allochthone blocks collided during the Cretaceous and Tertiary, subsequently disrupted most of that structural arrangement.

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Zusammenfassung Die Gesteine einer bei 2° Südlicher Breite in der Westkordillere Ecuadors zu Tage tretenden und um mehrere tausend Meter versetzten Verwerfung wurden in geochemischer Hinsicht untersucht. Die Verteilung der seltenen Erden und der anderen inkompatiblen Elemente in den Proben zeigte östlich dieser Verwerfung das Vorkommen von Tholeiiten desMorb-Typs auf, deren Auftreten hier bis jetzt unbekannt war. Außerdem konnten die westlich der Verwerfung auftretenden Diabase, Andesite und Rhyolite dem Typ eines Inselbogens zugeordnet werden.Diese Verwerfung begrenzt infolgedessen eine stark nach Osten einfallende Schuppe des Sockels. Andere Schuppen dieser Art treten weiter im Norden Ecuadors auf, wo sie zu Beginn des Tertiärs während des Zusammentreffens des von Westen ankommenden, aus der Kreidezeit stammenden Inselbogens mit dem südamerikanischen Kontinent in eine Paläosubduktionszone eingebettet worden sind.Dieses Strukturgefüge ist anschließend zum größten Teil durch das System der rechtsseitigen Verwerfungen von Guayaquil — Dolores verdeckt worden. Dieses System wird für gewöhnlich — aber wahrscheinlich zu Unrecht — für die Nahtstelle des kontinentalen Südamerikas und der während der Kreidezeit und des Tertiärs gegen den Kontinent sto– Benden und/oder obduzierten allochtonen Blöcke ozeanischen Substrats gehalten.

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Résumé Une faille inverse exposée dans la cordillère occidentale de l'Equateur vers 2°S a un rejet de plusieurs milliers de mètres. L'étude géochimique des terres rares et autres éléments incompatibles permet de reconnaître à l'est de la faille des tholéiites de ride médio-océanique jusqu'alors non identifiées, alors que les diabases, andésites et rhyolites présentes à l'ouest de la faille appartiennent à une formation d'arc insulaire.Cette faille délimite donc une écaille de socle à fort pendage vers l'est. D'autres écailles de ce type sont connues plus au nord en Equateur. Elles se sont imbriquées au début du Tertiaire dans une paléozone de subduction, lors de la collision de l'arc insulaire, venu de l'ouest, avec le continent sud-américain.Ce dispositif structural a été ensuite oblitéré en majeure partie par le systême faillé dextre de Guayaquil-Dolores, qui passe classiquement, mais probablement à tort, pour être la suture entre l'Amérique du sud continentale et les blocs allochtones à substratum océanique collés au continent et/ou obductés durant le Crétacé et le Tertiaire.

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A gravity-derived model for the steinheim impact crater

The gravity survey of the Steinheim impact crater comprises about 500 gravity stations resulting in aBouguer anomaly map of the crater and its surroundings. From this map aBouguer residual anomaly of the impact structure was deduced which shows considerable character. A central negative anomaly with an amplitude of about -2 mgal and a halfwidth of 3 km is surrounded by ring-like relative positive and negative anomalies which extend to a radial distance of about 5–6 km. The interpretation is based on nine radial profiles with close station spacing. Model calculations were performed suggesting the Steinheim crater to be much larger and of different formation than has been assumed. Based on the gravity interpretation, on a morphological analysis, and on additional geological and geophysical observations a model of the Steinheim crater and its development is proposed. The main characteristics are a 500–600 m deep primary excavation and a final, apparent, diameter of roughly 7 km, contrasting to an up to now favoured shallow excavation and a 3.5–4 km final diameter. The model considerations include mass and energy calculations and references to the Ries impact crater.

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Zusammenfassung Im Gebiet der Impakt-Struktur Steinheimer Becken wurden in Ergänzung zu früheren Untersuchungen neue Schweremessungen durchgeführt. Mit den nunmehr etwa 500 vorliegenden, nachBouguer reduzierten Schwerewerten wurde eine Karte derBouguer-Anomalien konstruiert, aus der nach Abzug eines Regionalfeldes eine Restfeld-Anomalie für den Impakt-Krater gewonnen wurde. Sie ist grob rotationssymmetrisch und zeigt außerhalb eines zentralen Schwereminimums von –2 mgal ringförmig angeordnete, relative positive und negative Schwereanomalien. Die Interpretation stützt sich auf neun radiale Profile erhöhter Stationsdichte, von denen vier für Modellrechnungen ausgewählt wurden. Auf Grund der berechneten Dichtemodelle, einer ergänzenden morphologischen Analyse der Kraterstruktur sowie zusätzlicher geologischer und geophysikalischer Befunde wird ein Modell für den Aufbau und die Entstehung des Steinheimer Beckens vorgeschlagen. Danach ist das heutige Becken mit einem Durchmesser von rund 3,5 km der Ausdruck einer primären Kraterstruktur, die in einer Exkavationsphase entstand und eine Tiefe von 500–600 m besaß. Ausgleichsbewegungen führten zu einer Massenkonvergenz und Anhebung in der Kratermitte und einer Absenkung außerhalb des primären Kraters, die die endgültige Struktur auf grob 7 km Durchmesser vergrößerte. Die Modell-Betrachtungen schließen Massen- und Energieabschätzungen sowie Vergleiche zum Ries-Krater ein.

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Résumé La recherche gravimétrique dans le cratère d'impact de Steinheim et ses environs (Allemagne du Sud) comprend environ 500 stations de mesure qui ont été utilisées pour construire une carte des anomalies deBouguer. Un champ régional a été tracé pour obtenir l'anomalie résiduelle du cratère. On observe une anomalie négative (–2 mgal) au centre de la structure cernée d'anomalies positives et négatives relatives de faible amplitude, jusqu'à une distance radiale de 5–6 km. Pour l'interprétation neuf profils radiaux, où les stations sont à faible distance, ont été utilisés. Les modèles calculés pour quatre profils radiaux suggèrent que la structure de Steinheim est plus grande qu'on ne l'a supposé jusqu'à présent. En partant de l'interprétation des mesures gravimétriques, d'une analyse topographique du cratère et d'observations géologiques et géophysiques complémentaires un modèle de la structure Steinheim et de son développement est proposé. On en conclut que le bassin actuel avec un diamètre de 3,5 km est l'expression d'une structureprimaire du cratère qui s'est formée au cours d'une phase d'excavation et possédait une profondeur de 500–600 mètres. Des mouvements d'égalisation conduisirent à une convergence de masses avec soulèvement dans le centre du cratère et un affaissement à l'extérieur du cratère primaire, lequel s'accrut jusqu'à la structure finale de quelque 7 km. Le modèle inclut le calcul des masses déplacées et des énergies d'impact ainsi qu'une comparaison avec le cratère d'impact du Ries.

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Geology and geochemistry of the Changba SEDEX Pb-Zn deposit,Qinling orogenic belt,China

The Changba Pb-Zn SEDEX deposit occurs in the Middle Devonian sequence of the Anjiaca Formation of the Western Qinling Hercynian Orogen in the Gansu Province, China. The Changba-II orebody is hosted in biotite quartz schist and is the largest of 143 stratiform orebodies that are hosted either in biotite quartz schist or marble. The Changba-II comprises two types of mineralization: a bedded facies and an underlying breccia lens. The bedded section exhibits three sulfide sub-facies zoned from bottom to top: 1) banded sphalerite intercalated with quartz albitite; 2) interbedded massive pyrite and sphalerite ore; and 3) banded sphalerite ore intercalated with banded baritite. Major metallic minerals are sphalerite, pyrite, galena, with minor arsenopyrite, pyrrhotite, boulangerite, and rare chalcopyrite. The bedded sulfides are underlain by a lens of brecciated and albitized biotite-quartz schists cemented by sulfides and tourmaline.Massive and bedded sulfide ³⁴S values range from 8.1 to 29.3, whereas barite ³⁴S values range from 20.8 to 31.5. Disseminated pyrite in footwall schists has ³⁴S values ranging from 8.1 to 10.6, and increase to values ranging from 11.1 to 14.7 in the hangingwall. The lower ³⁴S values for massive and bedded sulfides are interpreted to be derived from progressive bacterial sulfate reduction (BSR) of Devonian seawater in a sulfate-restricted sub-basin. The higher ³⁴S values for massive and bedded sulfides could be a product of quantitative BSR but this is incompatible with barite being more abundant above the bedded sulfides. Instead, it is more likely that thermochemical sulfate reduction of seawater sulfate or of evaporite was the source of heavy hydrothermal sulfur. Heavy hydrothermal sulfur was injected into a sulfate-restricted sub-basin where it mixed with low ³⁴S BSR sulfide to form the massive and bedded sulfides. The REE patterns of sulfide layers and associated quartz albitite and baritite are similar to those of the host biotite quartz schists, suggesting that the hydrothermal fluids leached REE from the underlying rocks. Pb isotope ratios in galena form an array between the Upper Crust and the Mantle reservoir curves, which indicates that the lead is derived from upper crustal rocks comprising mafic igneous units. The Sr⁸⁷/Sr⁸⁶ ratio of 0.7101 for carbonate within the sulfide layers also suggests that Sr is derived from the mixing of Sr leached from upper crustal rocks with Middle Devonian seawater Sr. A Rb-Sr isochron age of 389.4 ± 6.4 Ma for sulfide layers and the interbedded hydrothermal sediments is consistent with the age of host Mid-Devonian strata. Ar³⁹/Ar⁴⁰ plateau age at 352.8 ± 3.5 Ma and Ar³⁹-Ar⁴⁰ isochron age of 346.6 ± 6.4 Ma for albite in the quartz albitite intercalated with sulfide layers indicate either albite formation after the sulfides or thermal resetting of the Rb-Sr system at about 350 Ma, the age of collision between the North China and Yangtze cratons.Editorial handling: E. Frimmel     

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 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.     

Geosynklinalbecken,Tektonik, Granite und junger Vulkanismus in Afghanistan

Zusammenfassung Mit räumlich und zeitlich differenzierten Unterbrechungen ist Afghanistan vom Paläozoikum bis in das Jungtertiär Sedimentationsgebiet. Sedimentationsunterbrechungen zeigen orogene oder epirogene Phasen an. Im Mesozoikum sind die Hindukusch-Schwelle und das Zentrum konsolidiert. Dieses variszische Kernland wird im Norden und im Süden von jungen Geosynklinalen flankiert, deren Sedimentationsgeschichte und Magmatismus geschildert werden.Im Laufe der Erdgeschichte wird Afghanistan wiederholt Schauplatz epirogener und orogener Bewegungen. Die variszischen Phasen sind die Vorläufer der alpidischen Bewegungen und erst in dieser Ära wird das Orogen konsolidiert. Die mit der Tektonik parallel gehende Metamorphose hat lokalen, meist selektiven Charakter.Basischer und saurer Magmatismus ist mit den tektonischen Hauptzügen genetisch verknüpft.

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Afghanistan is a region where sedimentation predominated besides only some interruptions which were restricted regionally and in time, from Paleozoic to Upper Tertiary.Gaps in the sedimentary record indicate orogenetic or epeirogenetic activities. During Mesozoic time the Hindukush-geanticline and the central region were consolidated. This hercynian center is bordered to the Nord and South by young geosynclines. The history of their sedimentary and magmatic evolution is delineated.Afghanistan has been the field of epeirogenetic and orogenetic movements for several times during its geological history. Hercynian movements preced the alpidic and not until this late period consolidation takes place. Metamorphosis, linked to the structural events is bound to local, generally however selected characters. Basaltic and granitic magmatism is connected with the main structural events.

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Résumé L'Afghanistan a été un domaine de sédimentation depuis le Paléozoïque jusqu'au Néogène, avec toutefois des interruptions distinctes dans l'espace et le temps. Les interruptions dans la sédimentation indiquent des phases orogéniques ou épirogéniques. Au Mésozoïque il s'est produit la consolidation du «Bombement de l'Hindou Kouch» et du «Centre». Ce noyau varisque est ensuite flanqué au Nord et au Sud par des géosynclinaux plus récents dont l'auteur décrit la sédimentation et le magmatisme.Au cours de son évolution géologique, l'Afghanistan a été maintes fois le siège de mouvements orogéniques et épirogéniques. Les phases varisques sont les «épisodes précurseurs» des mouvements alpins, et ce n'est qu'à l'occasion de ceux-ci que l'orogène est consolidé. Le métamorphisme lié aux mouvements tectoniques a un caractère local et le plus souvent sélectif. Un magmatisme basique et acide est génétiquement lié aux structures tectoniques principales.

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Herrn Prof. Dr. Dr. h. c.Erich Bederke zum 70. Geburtstage gewidmet.     

Amazonite from the alkali granite of the Avdar Massif,Mongolia

Summary The green feldspar in alkali granite from the Avdar deposit of decoration stone (150 km W of Ulan-Bator) has been identified by means of physical and chemical methods as amazonite. Its structural state corresponds to that of microcline with a high degree of triclinicity and displaying the following anomalous dimensions of the unit cell (nm):a=0.8568,b=1.2967,c=0.7217 nm, =90.732, =115.851, =87.702°;V=0.72115 nm³.The amazonite contains in increased amount of Pb (1950 ppm) as well as of Rb (1.15 mass %). The genesis of this amazonite is associated with younger metasomatic processes in the alkali granite.

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Amazonit aus dem Alkali-Granit des Avdar-Massivs, Mongolei

Zusammenfassung Der grüne Feldspat im Alkaligranit aus der Dekorationsstein-Lagerstätte im Avdar Massiv (150 km W von Ulan-Bator) wurde auf Grund physikalischer und chemischer Methoden als Amazonit bestimmt. Sein Strukturzustand entspricht demjenigen eines Mikroklins, der eine hohe Triklinität und anomale Elementarzelle aufweist (in nm):a=0.8568,b=1.2967,c=0.7217 nm, =90.732, =115.851, =87.702°;V=0.72115 nm³.Der Amazonit hat einen erhöhten Gehalt an Pb (1950 ppm) und an Rb (1.15 Gew.-%). Die Genesis dieses Amazonits ist mit jüngeren metasomatischen Prozessen im Alkaligranit verknüpft.

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Low δ18O silicic volcanic rocks at the Calabozos caldera complex,southern Andes

Sr- and Pb-isotope data from the Calabozos center (⁸⁷Sr/⁸⁶Sr= 0.7043, ²⁰⁶Pb/²⁰⁴Pb=18.64–18.66, ²⁰⁷Pb/²⁰⁴Pb=15.59–15.60, ²⁰⁸Pb/²⁰⁴Pb=38.52–38.55) fall within the range of values reported for the southern volcanic zone (33–42° S) of the Andean arc. The range of ¹⁸O (5.0–6.3), however, includes unusually low values compared to volcanic rocks of similar bulk composition in the region. The Calabozos caldera complex lies at 35 °30 S, where the continental crust under the Andes thins southward from >45 to 30 km. Three voluminous late Pleistocene ashflow tuffs, collectively called the Loma Seca Tuff, constitute the bulk of >1,000 km³ of eruptive products at the Calabozos caldera complex and are evidence for a major, longlived andesitic-to-rhyodacitic magma reservoir at shallow crustal levels. The ¹⁸O values of the most evolved volcanic rocks from the Calabozos center are lower than predicted for rhyodacite produced by crystal fractionation from basalt typical of the region. Variation of ¹⁸O independent of bulk composition and inferred magmatic water contents indicates that the ¹⁸O depletion is a late-stage, upper-crustal phenomenon that cannot simply be attributed to magmatic interaction with meteoric water. The data are interpreted to be the result of assimilation of 5–30% of roof and wall rocks previously depleted in ¹⁸O by isotopic exchange in a meteoric hydrothermal system overlying the magma reservoir. Combined assimilation and fractional crystallization calculations applied to Sr isotope data show that the isotopic contrast between the Calabozos magmas and the assimilated rocks is very small. Hydrothermally-altered volcanic and plutonic rocks from the Tertiary Andean arc complex and Mesozoic-to-Cenozoic volcaniclastic sediments typical of the local basement provide a geologically reasonable contaminant compatible with the Sr- and O-isotope data. Pb-isotope data from the Calabozos system lend no significant insight into upper crustal contamination.     

Die Wirbeltierfauna der Trias der Tessiner Kalkalpen

Zusammenfassung Nach einer Skizzierung der stratigraphischen Verhältnisse der Trias des Monte San Giorgio (Kt. Tessin, Schweiz) wird eine Liste der Wirbeltierfaunen der anisischen und ladinischen Stufe gegeben. Mit Ausnahme des Pseudosuchiers Ticinosuchus handelt es sich um marine oder amphibisch lebende Vertreter von Fischen (Selachier, Actinopterygier und Coelacanthiden) und Reptilien (Ichthyosaurier, Placodontier, Rhynchocephalen, Prolacertilier und Sauropterygier). Der Monte San Giorgio birgt gegenwärtig die reichste Lagerstätte mariner Fische und Reptilien der mittleren Trias der Erde. Die Untersuchung der Sauropterygier und Placodontier zeigt, daß zwischen diesen beiden Ordnungen keine Verwandtschaft besteht. Von embolomeren Labyrinthodonten abstammend, haben sie neben anderen Zweigen selbständig das Übergangsfeld Amphibien/Reptilien überschritten. Die Klasse der Reptilien ist genetisch keine Einheit, sondern eine Entwicklungsstufe der Wirbeltiere. Zum Schluß wird auf die Ziele einer neuen Grabung im Grenzbitumenhorizont der anisischen Stufe der Trias des Monte San Giorgio hingewiesen. Neben einer statistischen Erfassung des Fossilinhaltes jeder Schicht werden die Beziehungen zwischen Sediment und Fossil studiert, um die wiederholten Änderungen der Lebensräume und ihrer Bewohner an der Nordküste der Tethys rekonstruieren zu können.

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An outline of the stratigraphical relationships of the Triassic of Mt. San Giorgio (Kanton Tessin, Switzerland) is presented, followed by a list of the vertebrate fauna of the Anisian and Ladinian (Middle Triassic). The fauna deals with marine fishes (Selachii, Actinopterygii, Coelacanthini) and marine or amphibious living reptiles (Ichthyosauria, Placodontia, Rynchocephalia, Prolacertilia, Sauropterygia), with the exception of the Pseudosuchian Ticinosuchus. Mt. San Giorgio is the richest place in the world where Middle Triassic marine fishes and reptiles are found nowadays. Investigation of the Sauropterygia and Placodontia shows that no affinity between these two orders exists. They are both descended from embolomer labyrinthodonts and have with other branches independently crossed over the amphibian-reptilian line of transition. The class Reptilia is not a genetical unit, but a grade of vertebrate evolution. Finally, reference is made to the aims of a new excavation in the Anisian (Grenzbitumenhorizont) of Mt. San Giorgio. In order to be able to reconstruct the repeated changes of the environment and its inhabitants on the north coast of the Tethyan Sea, the relations between sediments and fossils are studied and the fossil content of each stratum is observed.

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Résumé Après un bref exposé des relations stratigraphiques du Trias du Mt. San Giorgio (Tessin, Suisse), une liste des faunes de vertébrés de l'Anisien et du Ladinien est présentée. A l'exception du Pseudosuchien Ticinosuchus, il s'agit de poissons marins (Sélaciens, Actinoptérygiens et Coelacanthides) et de reptiles également marins ou amphibiens (Ichthyosaures, Placodontes, Rhynchocephales, Prolacertiliens et Sauroptérygiens). Le Monte San Giorgio livre actuellement la faune la plus riche du monde en poissons et reptiles marins du Trias moyen. L'analyse des Sauroptérygiens et des Placodontes montre que ces deux groupes ne sont pas en relations de parenté. Descendant de Labyrinthodontes embolomères, ils ont passé indépendamment à côté d'autres rameaux la zone de transition entre les amphibiens et les reptiles. La classe des reptiles n'est pas une unité génétique, mais un stade évolutif des vertébrés. Finalement les buts de nouvelles fouilles dans l'Anisien (Grenzbitumenhorizont) du Trias du Mt. San Giorgio sont mentionnés, Outre un inventaire statistique du contenu en fossiles de chaque couche, les relations entre les fossiles et le sédiment sont étudiées afin de pouvoir reconstruire les changements répétés de l'habitat et des habitants du bord septentrional de la Tethys.

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Dem Schweizerischen Schulratspräsidenten, Prof. Dr.Hans Pallmann, zu seinem 60. Geburtstag gewidmet.     

Crustal structure along a traverse across the Middle and High Atlas mountains derived from seismic refraction studies

In 1986 explosion seismic investigations have been carried out along a traverse of about 350 km length running from NNW to SSE crossing the High and Middle Atlas. Two further profiles run E/W through the Middle and NW-SE through the High Atlas.Neither the High nor the Middle Atlas mountains have a significant root. The maximum thickness of crust with 38–39 km is found under the northern border of the High Atlas. South and north of the High Atlas the crustal thickness amounts to 35 km. The upper as well as the lower crust are stongly structurized in the vertical direction. Significant for the profiles observed is the change between high and low velocities in the whole crust. The velocities at the uppermost mantle are relatively low with values of 7.7–7.9 km/s. The average velocities for the whole crust are 6.1–6.2 km/s.

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Zusammenfassung Sprengseismische Untersuchungen wurden 1986 längs einer etwa 350 km langen Traverse durchgeführt, die NNW-SSO verläuft und den Mittleren und den Hohen Atlas quert. Zwei weitere Profile verlaufen E-W durch den Mittleren und NW-SO durch den Hohen Atlas.Der Hohe und der Mittlere Atlas besitzen keine ausgeprägte Gebirgswurzel. Die maximale Krustendicke wird mit 38–39 km unter dem nördlichen Rand des Hohen Atlas angetroffen. Südlich und nördlich vom Hohen Atlas beträgt die Krustenmächtigkeit 35 km. Sowohl die obere als auch die untere Kruste sind in vertikaler Richtung stark strukturiert. Auf allen beobachteten Profilen ist ein Wechsel zwischen hohen und niedrigen Geschwindigkeiten für die gesamte Kruste charakteristisch. Die Geschwindigkeiten im obersten Mantel sind mit Werten von 7.7–7.9 km/s relativ niedrig. Die Durchschnittsgeschwindigkeiten für die gesamte Kruste liegen bei 6.1–6.2 km/s.

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Résumé Durant l'année 1986 des sondages sismiques de tirs ont été entrepris le long d'une traverse d'environ 350 km de longueur se dirigeant du nord nordouest au sud sud-est et parcourant le Haut et Moyen Atlas. Deux autres profils parcourent le Moyen Atlas de l'est à l'ouest et le Haut Atlas du nord-ouest au sud-est.Le Haut et Moyen Atlas ne possèdent pas de racine montagneuse. L'épaisseur maximale de 38–39 km de la croûte a été localisée sous le bord nord du Haut Atlas. Au sud et au nord du Haut Atlas l'épaisseur de la croûte s'élève à 35 km. La croûte supérieure ainsi que la croûte inférieure est très structurée en direction verticale.Ce qui est caractéristique pour tous les profils observés c'est un changement rapide entre de grandes et petites vitesses et concernant le total de la croûte. Les vitesses dans le manteau le plus élevé haut sont relativement basses avec 7,7–7,9 km/s. Les vitesses moyennes pour la croûte entière sont de 6,1 à 6,2 km/s.

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1986 350 , NNW SSE . : E-W , NWSE . . , 38/39 , 35 . , . . : 7,7–7,9 /. 6,1–6,2 /.

     

La theorie des deux magmas fondamentaux dans le volcanisme intracontinental et l'evolution geochimique des laves du Mont-Dore (France)

Résumé Les auteurs utilisent toutes les données chimiques, volcanologiques, chronologiques et tectonophysiques fournies pour un certain nombre de massif continentaux (Mont-Dore et autres). La synthèse de toutes ces études montre qu'il existe deux magmas continentaux indépendants: le magma basaltique venant du manteau (sima) et le magma rhyolitique fourni par la croûte supérieure (sial). Le mélange de ces deux magmas et l'action secondaire de la' gravité et de la pneumatolyse expliquent logiquement la totalité des phénomènes géochimiques des volcans continentaux.

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The authors used all the chemical, volcanological, chronological and tectonophysical data supplied for a certain number of continental mountain massifs (Mont-Dore and others). The synthesis of these studies shows that there are two independent continental magmas: the basaltic magma coming from the mantle (sima) and the rhyolitic magma supplied by the upper crust (sial). The mixing of these two magmas, with the secondary action of gravity and pneumatolysis logically explains the totality of the geochemical phenomena in continental volcanoes.

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Zusammenfassung Die Autoren benutzten alle chemischen, vulkanologischen, chronologischen und tektonisch-physikalischen Daten, die von einer bestimmten Anzahl von kontinentalen Massiven (Mont-Dore und andere) entnommen wurden.Das Ergebnis dieser Studien zeigt, daß es zwei voneinander unabhängige kontinentale Magmen gibt.Das basaltische Magma, vom Sima stammend, und das rhyolitische Magma aus der oberen Sial-Kruste. Die Vermischung dieser beiden Magmen und die zusätzliche Wirkung der Gravitation und Pneumatolyse erklären auf natürliche Weise die Gesamtheit der geochemischen Phänomene des kontinentalen Vulkanismus.

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Zur Tektonik im westlichen Fichtelgebirge

Zusammenfassung Es wird zu einigen tektonischen Problemen im westlichen Fichtelgebirge (Gradabteilungsblätter Weidenberg und Ebnath) Stellung genommen, wobei andere Deutungsmöglichkeiten zur Diskussion gestellt werden. Vor allem wird eine schichtparallele Verschieferung einer älteren Schiefrigkeit abgelehnt und das Umlaufen der Schieferung am SW-Rand des Fichtelgebirges durch sekundäre Kippungen erklärt.

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The author expresses his opinion on some tectonic problems in the western Fichtelgebirge (Topographical maps: Gradabteilungsblätter Weidenberg und Ebnath).He proposes different ways of interpretation. Above all he rejects that an older schistosity became again foliated parallel to the strata, and he ascribes the bending of the schistosity on the south-west border of the Fichtelgebirge to secundary tilts.

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Résumé L'auteur critique quelques problèmes tectoniques qui se posent à l'ouest du Fichtelgebirge. (Tebles topographiques: Gradabteilungsblätter Weidenberg und Ebnath). Il indique d'autres interprétations possibles. Avant tout il réfute qu'une ancienne schistosité au sud-ouest du Fichtelgebirge est attribuée à un basculement secondaire.

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Fichtelgebirge. .

     

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).