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1.
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.  相似文献   

2.
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.  相似文献   

3.
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.  相似文献   

4.
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.
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.
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5.
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 ( 18Oinit) 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.  相似文献   

6.
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.  相似文献   

7.
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.  相似文献   

8.
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.  相似文献   

9.
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.  相似文献   

10.
Chemistry dependence of D of chlorites is inferred from data for natural chlorites. D of water equilibrated with those chlorites is estimated to be –2–8.  相似文献   

11.
A large difference of about 5 in the O-isotopic composition of quartz characterizes the transition from very low grade or even anchimetamorphic slates to pelites metamorphosed at greenschist facies conditions. A further increase in temperature does not result in systematic isotopic shifts (mean around +15.5). However, immediately at the isograds marking the transition from greenschist to amphibolite facies and amphibolite facies to higher amphibolite facies the quartz 18O values are significantly higher by >1. This increase is restricted to the area immediately at the isograds.It can be shown that during the low grade transformation a fluid phase is present in excess; the metamorphism of this stage therefore can be described as water present metamorphism. This fluid phase leaves the system during pervasive deformation of the rocks, during which large scale homogenization processes are possible.At higher temperatures a free fluid phase is only present at specific isograds, where it is produced by mineral dehydration reactions. If the production rate of the fluid phase is sufficiently slow, this results in a re-equilibration of the mineral phases with the newly generated fluid phase, the isotopic composition of which is completely controlled by the primary mineral phases.In intermediate regions with enlarged fluid production and escape rates, a free fluid phase is present only for short periods of time at fractures which form due to fluid overpressure. The high escape rate inhibits the solid phases from re-equilibration with the generated fluid. The term water absent metamorphism can be used to describe this state of reduced H2O activity within a rock.  相似文献   

12.
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.
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.
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13.
Information on tectonic evolution can be obtained from the succession of magmatic types represented (I-type and S-type granitoids and volcanics, basic dykes and alkali complexes). The earliest Proterozoic magmatic activity resulted in the formation of greenstone belts within the Karelian continent area, probably in the age range 2500-2100 Ma. This was followed by an event similar to Andinotype orogeny at around 1900 Ma with I-type granitoids and calc-alkaline volcanics. This progressed to an event of Hercynotype which was concluded by about 1750 Ma. Subsequent magmatic activity (I-type granitoids and volcanics) suggests a new phase of Andino-type orogeny until about 1630 Ma (S and C Sweden) or 1500 Ma (N Sweden). Tension-related magmatism at around 1550 Ma (basic dykes, alkaline complexes and gabbro) may relate to rifting prior to the independant evolution of SW Sweden which includes intrusion of I-S-type granitoids, metamorphism, followed by continental rotation and collision with N America at about 1050 Ma. Basic dykes and S-type granites are related to subsequent uplift 950-850 Ma ago.
Zusammenfassung Information über die tektonische Entwicklung kann aus der Folge der dargestellten Magmentypen (I-Typ und S-Typ granitähnlicher Gesteine und Vulkanite, basische Gänge und Alkalikomplexe) erhalten werden. Die früheste proterozoische magmatische Aktivität bildete die Formation von Grünsteingürteln innerhalb des Gebietes des Karelian-Kontinents, wahrscheinlich in der Zeit vor 2500-2100 Ma. Vor etwa 1900 Ma folgte daraufhin ein Ereignis, ähnlich der Andinotypen-Orogenese, mit I-Typ-Granitoiden und Kalk-Alkali-Vulkaniten. Dies schritt weiter fort zu einem Hercynotyp-ähnlichen Ereignis, welches vor ungefähr 1750 Ma abgeschlossen war. Nachfolgende magmatische Aktivität ruft eine neue Phase Andinotyp-ähnlicher Orogenese bis ungefähr 1630 Ma (Süd- und Mittelschweden) oder 1500 Ma (Nordschweden) hervor. Zerrspannungsabhängiger Magmatismus um ungefähr 1550 Ma (basische Gänge, Alkali-Komplexe und Gabbro) können zu Rissen führen vor der unabhängigen Entwicklung SW-Schwedens, welche die Intrusion von I-S-Typen granitähnlicher Gesteine, Metamorphose, gefolgt von kontinentaler Rotation und Kollision mit N-Amerika vor ungefähr 1050 Ma einschließt. Basische Gänge und S-Typ-ähnliche Granite hängen mit der nachfolgenden Hebung vor 950-850 Ma zusammen.
Résumé L'information portant sur l'évolution tectonique peut être déduite de l'étude de la succession des types magmatiques représentés (Séries granitiques de type »I«ou»S« et volcanites, dykes basiques, complexes alcalins etc.). La première manifestation magmatique du Protérozoique est représentée par les ceintures de roches vertes, au sein du continent »Karélien«, probablement mises en place entre 2500 et 2100 Ma. Elle a été suivie par un événement orogénique à caractère «Andinotype» et qui correspond à l'intrusion de granites (Iato sensu) et de roches volcaniques, datée d'environ 1900 Ma. Cet événement fut suivi par un autre, de caractère «Hercynotype» qui se termina au tour de 1750 Ma. L'activité magmatique subséquente peut être caractérisée comme nouvelle phase Andinotype; elle est terminée à environ 1630 Ma pour la Suède méridionale et centrale, et à environ 1500 Ma pour le Nord du pays. Un épisode magmatique, en relation avec une phase d'extension, pourrait être dû à la formation de rift. Il est caractérisé par l'intrusion de dykes basiques, de complexes alcalins et de gabbros et précède l'évolution indépendante du sud-ouest de la Suède comprenant des granites de type «I» et «S» ainsi que leur métamorphisme ultérieur. Ceci fut suivi par une rotation continentale ainsi que par la collision avec l'Amérique du Nord à environ 1050 Ma. Enfin, des dykes basiques ainsi que des granites de type «S» se sont mis en place en relation avec une phase de soulèvement ultérieure datée d'environ 950-850 Ma.
( I S , ). , - 2500–2100 . 1900 , I - . , , 1750 . , , 1630 , 1500 . 1550 ( , ), - , I S, 1050 . , S, , , , 950–850 .
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14.
A systematic study of the auriferous quartz veins of the Val-dOr vein field, Abitibi, Quebec, Canada, demonstrates that the C, O, S isotope composition of silicate, carbonate, borate, oxide, tungstate and sulphide minerals have a range in composition comparable to that previously determined for the whole Superior Province. The oxygen isotope composition of quartz from early quartz–carbonate auriferous veins ranges from 9.4 to 14.4 whereas later quartz-tourmaline-carbonate veins have 18Oquartz values ranging from 9.2 to 13.8 . Quartz-carbonate veins have carbonate (18O: 6.9–12.5 ; 13C: –6.2– –1.9 ) and pyrite (34S: 1.2 and 1.9 ) isotope compositions comparable to those of quartz-tourmaline-carbonate veins (18O: 7.9–11.7 ; 13C: –8.0 – –2.4 ; 34S: 0.6–6.0 ). 18Oquartz values in quartz-tourmaline-carbonate veins have a variance comparable to analytical uncertainty at the scale of one locality, irrespective of the type of structure, the texture of the quartz or its position along strike, across strike or down-dip a vein. In contrast, the oxygen isotope composition of quartz in quartz-tourmaline-carbonate veins displays a regional distribution with higher 18O values in the south-central part of the vein field near the Cadillac Tectonic Zone, and which 18O values decrease regularly towards the north. Another zone of high 18O values in the northeast corner of the region and along the trace of the Senneville Fault is separated by a valley of lower 18O values from the higher values near the Cadillac Tectonic Zone. Oxygen isotope isopleths cut across lithological contacts and tectonic structures. This regional pattern in quartz-tourmaline-carbonate veins is interpreted to be a product of reaction with country rocks and mixing between (1) a deep-seated hydrothermal fluid of metamorphic origin with minimum 18O=8.5 , 13C=0.6 and 34S=–0.4 , and (2) a supracrustal fluid, most likely Archean seawater with a long history of water-rock exchange and with maximum 18O=3.9 , 13 C=–5.6 and 34S=5.0 .  相似文献   

15.
Ion microprobe analysis of magnetites from the Adirondack Mountains, NY, yields oxygen isotope ratios with spatial resolution of 2–8 m and precision in the range of 1 (1 sigma). These analyses represent 11 orders of magnitude reduction in sample size compared to conventional analyses on this material and they are the first report of routinely reproducible precision in the 1 per mil range for analysis of 18O at this scale. High precision micro-analyses of this sort will permit wide-ranging new applications in stable isotope geochemistry. The analyzed magnetites form nearly spherical grains in a calcite matrix with diopside and monticellite. Textures are characteristic of granulite facies marbles and show no evidence for retrograde recrystallization of magnetite. Magnetites are near to Fe3O4 in composition, and optically and chemically homogeneous. A combination of ion probe plus conventional BrF5 analysis shows that individual grains are homogeneous with 18O=8.9±1 SMOW from the core to near the rim of 0.1–1.2 mm diameter grains. Depth profiling into crystal growth faces of magnetites shows that rims are 9 depleted in 18O. These low 18O values increase in smooth gradients across the outer 10 m of magnetite rims in contact with calcite. These are the sharpest intracrystalline gradients measured to date in geological materials. This discovery is confirmed by bulk analysis of 150–350 m diameter magnetites which average 1.2 lower in 18O than coarse magnetites due to low 18O rims. Conventional analysis of coexisting calcite yields °18O=18.19, suggesting that bulk 18O (Cc-Mt)=9.3 and yielding an apparent equilibration temperature of 525° C, over 200° C below the temperature of regional metamorphism. Consideration of experimental diffusion data and grain size distribution for magnetite and calcite suggests two contrasting cooling histories. The data for oxygen in calcite under hydrothermal conditions at high P(H2O) indicates that diffusion is faster in magnetite and modelling of the low 18O rims on magnetite would suggest that the Adirondacks experienced slow cooling after Grenville metamorphism, followed by a brief period of rapid cooling, possibly related to uplift. Conversely, the data for calcite at low P(H2O) show slower oxygen diffusion than in magnetite. Modelling based on these data is consistent with geochronology that shows slow cooling through the blocking temperature of both minerals, suggesting that the low 18O rims form by exchange with late, low temperature fluids similar to those that infiltrated the rock to serpentinize monticellite and which infiltrated adjacent anorthosite to form late calcite veinlets. In either case, the ion microprobe results indicate that two distinct events are recorded in the post-metamorphic exchange history of these magnetites. Recognition of these events is only possible through microanalysis and has important implications for geothermometry.  相似文献   

16.
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  相似文献   

17.
Zusammenfassung In einem Vorbericht werden acht detailliert aufgenommene Profile aus dem marinen Perm Ost- und Zentralafghanistans durch Fusuliniden grob gegliedert und mit der Permfolge Russisch-Mittelasiens parallelisiert. An Hand eigener Geländeaufnahmen und der Literatur wird versucht, die paläogeographische Entwicklung dieses Gebietes zu skizzieren.Unterperm (Karatschatyr-Stufe, Pseudoschwagerina- Zone) tritt in Nordost-und Zentralafghanistan auf. Unterschiedliche Fazies und Faunenführung erschweren den stratigraphischen Vergleich beider Vorkommen. Das untere Mittelperm (Darwas-Stufe, Parafusulina- Zone) ist in Zentral- und Südostafghanistan weit verbreitet. Die Überflutung des südlichen Hindukusch beginnt und wird im oberen Mittelperm (Murghab-Stufe, Neoschwagerina- Zone) offenkundig, dessen gleichförmige Ablagerungen im Westhindukusch weit nach Norden übergreifen. Die Funde von Oberperm (Pamir-Stufe, Codonofusiella- Reichelina- Zone) beschränken sich auf das östliche Zentralafghanistan sowie Südostafghamstan und deuten einen Rückzug der Tethys nach Südosten an.
Eight sections of the marine Permian of eastern and central Afghanistan have been studied in detail. Preliminarily these sections have been subdivided by means of fusulinids and correlated with the standard section of Permian in Russian Central Asia. The recent fieldwork and literature are evaluated to trace the paleogeographical evolution of this region.Lower Permian (Karatschatyr-stage,zone of Pseudoschwagerina) is present in northeast and central Afghanistan. Stratigraphic correlation of these two localities, separated by the Hindu-Kush, is difficult because of their different faciès and faunal content. The lower middle Permian (Darwas-stage, zone ofParafusulina) is widely distributed in the central and south-eastern parts of the country. In this era starts inundation of the southern Hindu-Kush, which becomes obvious in the upper middle Permian (Murghab-stage, zone ofNeoschwagerina), when uniform sediments are extending wide northward in the western Hindu-Kush. Deposits of upper Permian (Pamir-stage, zone ofCodonofusiella-Reichielina) are only found in the eastern part of central Afghanistan and in the south-eastern Afghanistan. A retreat of the Tethys towards southeast is indicated.
Résumé Dans un avant-propos, le Permien marin de l'Afghanistan oriental et central est divisé grossement à l'aide des fusulines dans huit coupes, prises bien en détail. Cette série est mise en parallèle aux coupes du Permien dans l'Asie centrale russe.D'après mes recherches en Afghanistan et dans la littérature il est possible de tracer l'évolution paléogéographique de cette région.Le Permien inférieur (étage de Karatschatyr, zone avecPseudoschwagerina) existe dans la région nord-est et central de l'Afghanistan, il manque dans le Hindou-Kouch, séparant les deux régions. La comparaison stratigraphique est rendue difficile, parce que le faciès et les faunes des deux gisements sont de caractère différent. Le Permien moyen inférieur (étage de Darwas, zone ayecParafusulina) est répandu dans l'Afghanistan central et sud-est. Dans cette époque-là l'inondation commence dans le bord méridional du Hindou-Kouch. Pendant le Permien moyen supérieur (étage de Murghab, zone avecNeoschwagerina) elle s'étend avec des sédiments uniformes jusqu'au nord du Hindou-Kouch occidental. Les gisements du Permien supérieur (étage de Pamir, zone avecCodonofusiella etReichelina) se trouvent seulement en Afghanistan central-oriental et en Afghanistan sud-est et indiquent le retrait de la Tethys vers le sud-est.
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18.
Eclogites from the Roberts Victor mine, Kaapvaal craton are classic examples of subducted Achaean oceanic crust brought up as xenoliths by kimberlite. New in situ trace element and oxygen isotope data (18O=3.09–6.99 SMOW) presented here reemphasise their origin from seawater-altered plagioclase-rich precursors. Their Hf–Nd isotopic compositions are not in agreement with compositions predicted by geochemical modelling of the isotopic composition of aged subducted oceanic crust. Instead, Hf isotopic compositions are very heterogeneous, varying between 0.281625 and 0.355077 (–37.8 and +2561 Hf) at the time of kimberlite emplacement (128 Ma) in keeping with equally variable Nd isotopic compositions (0.511124–0.545092; –26.3 to +636 Nd). However, most samples plot on the terrestrial array. The isotopic compositions of some samples are too extreme to play a major role in mixed peridotite-eclogite melting in basalt source regions, whereas the isotopic composition of other samples is reconcilable with a contribution of up to ca. 15% of eclogite partial melt to the MORB source. Most importantly, our results show that ancient subducted oceanic crust is not isotopically homogeneous and should not be treated as a component or reservoir during geochemical modelling. The heterogeneity reflects radiogenic in-growth starting from small compositional heterogeneities in gabbroic protoliths, followed by modification during sea-floor alteration, subduction and emplacement into the subcratonic lithosphere.  相似文献   

19.
Dalradian metamorphic rocks, Lower Ordovician meta-igneous rocks (MGS) and Caledonian granites of the Connemara complex in SW Connemara all show intense retrograde alteration. Alteration primarily involves sericitization and saussuritization of plagioclase, the alteration of biotite and hornblende to chlorite and the formation of secondary epidote. The alteration is associated with sealed microcracks in all rocks and planes of secondary fluid inclusions in quartz where it occurs, and was the result of a phase of fluid influx into these rocks. In hand specimen K-feldspar becomes progressively reddened with increasing alteration. Mineralogical alteration in the MGS and Caledonian granites took place at temperatures 275±15°C and in the MGS Pfluid is estimated to be 1.5 kbar during alteration. The °D values of alteration phases are:-18 to-29 (fluid inclusions),-47 to-61 (chlorites) and-11 to-31 (epidotes). Chlorite 18O values are +0.2 to +4.3, while 18O values for quartz-K-feldspar pairs show both positively sloped (MGS) and highly unusual negatively sloped (Caledonian granites) arrays, diverging from the normal magmatic field on a - plot. The stable isotope data show that the fluid that caused retrogression continued to be present in most rocks until temperatures fell to 200–140°C. The retrograde fluid had D -20 to-30 in all lithologies, but the fluid 18O varied both spatially and temporally within the range-4 to +7. The fO2 of the fluid that deposited the epidotes in the MGS varied with its 18O value, with the most 18O-depleted fluid being the most oxidizing. The D values, together with low (<0) 18O values for the retrograde fluid in some lithologies indicate that this fluid was of meteoric origin. This meteoric fluid was probably responsible for the alteration in all lithologies during a single phase of fluid infiltration. The variation in retrograde fluid 18O values is attributed to the effects of variable oxygen isotope shifting of this meteoric fluid by fluid-rock interaction. Infiltration of meteoric fluid into this area was most likely accomplished by convection of pore fluids around the heat anomaly of the Galway granite soon after intrusion at 400 Ma. However convective circulation of meteoric water and mineralogical alteration could possible have occurred considerably later.  相似文献   

20.
In the eastern border area of the Andes of north-western Argentina two structural units are represented: the Cordillera Oriental and the Sierras Subandinas.The Cordillera Oriental represents a structural unit of faulted blocks strongly uplifted in relation to the Sierras Subandinas. In the Cordillera Oriental, the metamorphic basement has been partially folded together with the sedimentary cover. In some cases the cover has been detached away from its substratum and folded independently of it.It was generally accepted that the Sierras Subandinas coincide with big asymmetrical anticlines produced in the cover rocks by tilting of rigid basement blocks. The detailed geological mapping of the region situated between the parallels 24° 45 and 26° latitude south and the meridians 64° 30 and 66° west has provided data that permit to modify considerably this scheme.The existence of important strike-slip faults running highly oblique to the main regional structural pattern and controlling the shape of the Sierras Subandinas folds has been verified. These highly oblique faults with strike-slip movements, that affect the whole region, are probably ancient lineaments reactivated during Andean diastrophism in Upper Pliocene to Early Pleistocene time.
Zusammenfassung Im östlichen Grenzgebiet der Anden im nordwestlichen Argentinien liegen zwei strukturell unterschiedliche Einheiten vor, die Cordillera Oriental und die subandinen Sierren (Sierras Subandinas).Die Cordillera Oriental besteht aus Leistenschollen eines leicht metamorphisierten Sockels. Sie ist als Ganzes im Verhältnis zu den subandinen Sierren stark herausgehoben worden. In der Cordillera Oriental wurde dieser Sockel zum Teil zusammen mit seinen sedimentären Deckschichten verfaltet, zum Teil wurden sie aber auch von ihrem Unterlager abgeschert und unabhängig von ihm gefaltet.Bisher wurde allgemein angenommen, daß die subandinen Sierren aus großen assymmetrischen Falten aufgebaut werden, die in den Deckschichten durch Kippung starrer Blöcke im tieferen Untergrund hervorgerufen wurden. Die genaue geokartographische Aufnahme eines Gebietes zwischen den Breitenkreisen 24° 45 und 26° südl. Breite und den Längenkreisen 64° 30 und 66° westl. Länge erbrachte Daten, die es erlauben, diese Vorstellung erheblich zu modifizieren.Weit durchhaltende Seitenverschiebungen, die die übergeordneten Strukturen des gesamten Gebietes schräg durchziehen und die Gestalt der Falten in den Sierren erheblich beeinflussen, wurden nachgewiesen. Sie stellen wahrscheinlich alte, präandine Lineamente dar, die während der Anden-Orogenese im Jungtertiär bis Altpleistozän reaktiviert Wurden.
Resumen En el borde oriental de los Andes del Noroeste Argentino están representadas dos unidades estructurales: la Cordillera Oriental y las Sierras Subandinas.La Cordillera Oriental es una unidad constituida por bloques fallados y, en conjunto, elevada con respecto a las Sierras Subandinas. En la Cordillera Oriental el basamento metamórfico ha sido parcialmente plegado junto con la cobertura sedimentaria. En algunos casos lo cobertura ha sido despegada de su substrato y plegada independientemente.En general se aceptaba que las Sierras Subandinas coincidian con grandes anticlinales asimétricos producidos en la cobertura sedimentaria por el basculamiento de bloques rígidos de basamento situados en el subsuelo. La cartografía geológica-estructural detallada de la región comprendida por los paralelos 24° 45 y 26° S y los meridianos 64° 30 y 66° W ha proporcionado datos que permiten modificar considerablemente este esquema.Se ha comprobado la existencia de importantes fallas transcurrentes, fuertemente oblicuas con respecto a la estructura regional, que han tenido profunda influencia sobre la forma de los pliegues subandinos. Estas fallas oblicuas son probablemente lineamientos antiguos en los que el diastrofismo Andico (Plioceno-Pleistoceno) ha producido considerables desplazamientos paralelos al rumbo de los mismos.
Résumé Dans la bordure orientale des Andes du Nord-ouest de l'Argentine, deux unités structurales différentes sont présentes: les Sierras subandines et la Cordillère orientale.La Cordillère orientale est composée d'un socle faiblement métamorphique faillé en blocs fortement surélevés par rapport aux Sierras subandines. Dans la Cordillère orientale, ce socle métamorphique a été en partie plissé avec sa couverture sédimentaire; mais parfois celle-ci a été décolée de son substratum et plissé indépendentement.Jusqu'à présent, on admettait que les Sierras subandines sont constituées de grands anticlinaux assymétriques produits dans la couverture par l'ascension de blocs faillés du socle. La cartographie géologico-structurale detaillée de la région comprise entre les parallèles 24° 45 et 26° de latitude sud et les méridiens 64° 30 et 60° ouest conduit à modifier considérablement ce schéma.D'importants décrochements très obliques par rapport à la direction de la structure régionale et qui ont fortement influencé la forme des plis subandins sont probablement des accidents anciens rajeunis pendant le diastrophisme andin au cours du Pliocène supérieur et du Pléistocène inférieur.
- : Cordillera Oriental Sierras Subandinas. . . ; . , , . 24°45 26° 64°30 66° . , , . , , , , .
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