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41.
The Chelopech deposit is one of the largest European gold deposits and is located 60 km east of Sofia, within the northern
part of the Panagyurishte mineral district. It lies within the Banat–Srednegorie metallogenic belt, which extends from Romania
through Serbia to Bulgaria. The magmatic rocks define a typical calc-alkaline suite. The magmatic rocks surrounding the Chelopech
deposit have been affected by propylitic, quartz–sericite, and advanced argillic alteration, but the igneous textures have
been preserved. Alteration processes have resulted in leaching of Na2O, CaO, P2O5, and Sr and enrichment in K2O and Rb. Trace element variation diagrams are typical of subduction-related volcanism, with negative anomalies in high field
strength elements (HFSE) and light element, lithophile elements. HFSE and rare earth elements were relatively immobile during
the hydrothermal alteration related to ore formation. Based on immobile element classification diagrams, the magmatic rocks
are andesitic to dacitic in compositions. Single zircon grains, from three different magmatic rocks spanning the time of the
Chelopech magmatism, were dated by high-precision U–Pb geochronology. Zircons of an altered andesitic body, which has been
thrust over the deposit, yield a concordant 206Pb/238U age of 92.21 ± 0.21 Ma. This age is interpreted as the crystallization age and the maximum age for magmatism at Chelopech.
Zircon analyses of a dacitic dome-like body, which crops out to the north of the Chelopech deposit, give a mean 206Pb/238U age of 91.95 ± 0.28 Ma. Zircons of the andesitic hypabyssal body hosting the high-sulfidation mineralization and overprinted
by hydrothermal alteration give a concordant 206Pb/238U age of 91.45 ± 0.15 Ma. This age is interpreted as the intrusion age of the andesite and as the maximum age of the Chelopech
epithermal high-sulfidation deposit. 176Hf/177Hf isotope ratios of zircons from the Chelopech magmatic rocks, together with published data on the Chelopech area and the
about 92-Ma-old Elatsite porphyry–Cu deposit, suggest two different magma sources in the Chelopech–Elatsite magmatic area.
Magmatic rocks associated with the Elatsite porphyry–Cu deposit and the dacitic dome-like body north of Chelopech are characterized
by zircons with ɛHfT90 values of ∼5, which suggest an important input of mantle-derived magma. Some zircons display lower ɛHfT90 values, as low as −6, and correlate with increasing 206Pb/238U ages up to about 350 Ma, suggesting assimilation of basement rocks during magmatism. In contrast, zircon grains in andesitic
rocks from Chelopech are characterized by homogeneous 176Hf/177Hf isotope ratios with ɛHfT90 values of ∼1 and suggest a homogeneous mixed crust–mantle magma source. We conclude that the Elatsite porphyry–Cu and the
Chelopech high-sulfidation epithermal deposits were formed within a very short time span and could be partly contemporaneous.
However, they are related to two distinct upper crustal magmatic reservoirs, and they cannot be considered as a genetically
paired porphyry–Cu and high-sulfidation epithermal related to a single magmatic–hydrothermal system centered on the same intrusion. 相似文献
42.
Exploring the impact of diagenesis on (isotope) geochemical and microstructural alteration features in biogenic aragonite
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Ann‐Christine Ritter Vasileios Mavromatis Martin Dietzel Ola Kwiecien Felix Wiethoff Erika Griesshaber Laura A. Casella Wolfgang W. Schmahl Jennifer Koelen Rolf D. Neuser Albrecht Leis Dieter Buhl Andrea Niedermayr Sebastian F. M. Breitenbach Stefano M. Bernasconi Adrian Immenhauser 《Sedimentology》2017,64(5):1354-1380
For the Quaternary and Neogene, aragonitic biogenic and abiogenic carbonates are frequently exploited as archives of their environment. Conversely, pre‐Neogene aragonite is often diagenetically altered and calcite archives are studied instead. Nevertheless, the exact sequence of diagenetic processes and products is difficult to disclose from naturally altered material. Here, experiments were performed to understand biogenic aragonite alteration processes and products. Shell subsamples of the bivalve Arctica islandica were exposed to hydrothermal alteration. Thermal boundary conditions were set at 100°C, 175°C and 200°C. These comparably high temperatures were chosen to shorten experimental durations. Subsamples were exposed to different 18O‐depleted fluids for durations between two and twenty weeks. Alteration was documented using X‐ray diffraction, cathodoluminescence, fluorescence and scanning electron microscopy, as well as conventional and clumped isotope analyses. Experiments performed at 100°C show redistribution and darkening of organic matter, but lack evidence for diagenetic alteration, except in Δ47 which show the effects of annealing processes. At 175°C, valves undergo significant aragonite to calcite transformation and neomorphism. The δ18O signature supports transformation via dissolution and reprecipitation, but isotopic exchange is limited by fluid migration through the subsamples. Individual growth increments in these subsamples exhibit bright orange luminescence. At 200°C, valves are fully transformed to calcite and exhibit purple‐blue luminescence with orange bands. The δ18O and Δ47 signatures reveal exchange with the aqueous fluid, whereas δ13C remains unaltered in all experiments, indicating a carbonate‐buffered system. Clumped isotope temperatures in high‐temperature experiments show compositions in broad agreement with the measured temperature. Experimentally induced alteration patterns are comparable with individual features present in Pleistocene shells. This study represents a significant step towards sequential analysis of diagenetic features in biogenic aragonites and sheds light on reaction times and threshold limits. The limitations of a study restricted to a single test organism are acknowledged and call for refined follow‐up experiments. 相似文献
43.
D. H. Garrison D. D. Bogard A. A. Albrecht S. Vogt G. F. Herzog J. Klein D. Fink B. Dezfouly-Arjomandy R. Middleton 《Meteoritics & planetary science》1992,27(4):371-381
Abstract— Core samples were obtained from various locations of the ~ 105-kg Chico, NM, L6 chondrite in order to study the effects of large shielding on the production rates of cosmic-ray-produced nuclides. Relations between measured abundances of cosmogenic nuclides (10Be, 26Al, and stable isotopes of He, Ne, and Ar) and the cosmogenic 22Ne/21Ne ratio were determined and compared with recent model predictions of production rates. The measured 22Ne/21Ne ratios (1.06-1.08) and significant variations observed in concentrations of cosmogenic 21Ne and 3He suggest an ~40-cm shielding gradient across Chico and irradiation within a large object (> 100-cm radius). Noble gas data indicate that Chico experienced greater shielding than chondrites Knyahinya or Keyes and similar to Jilin. Values of 10Be (average = 20.7 dpm/kg) and 26Al (average = 71.1 dpm/kg) are nearly constant, however, and show no correlation with either 22Ne/21Ne or 21Ne. Activities of 10Be and 26Al suggest irradiation in a smaller object (~40–80 cm radius). The 26Al activity and the 26Al/10Be ratio (average value = 3.42) are both significantly larger than values for most other chondrites. These results could indicate a two-stage irradiation with t1 ~ 104 Ma and t2 ~ 4 Ma and a second-stage body the size of Knyahinya. The single stage, 10Be/21Ne exposure age for Chico is 65 Ma. The 22Ne/21Ne ratio apparently becomes insensitive to shielding for objects the size of Chico. No substantial evidence exists for chondrites with 22Ne/21Ne ratios significantly less than ~ 1.055. 相似文献
44.
45.
Christiana Photiadou Bart van den Hurk Aarnout van Delden Albrecht Weerts 《Climate Dynamics》2016,46(1-2):187-203
An adapted statistical bias correction method is introduced to incorporate circulation-dependence of the model precipitation bias, and its influence on estimated discharges for the Rhine basin is analyzed for a historical period. The bias correction method is tailored to time scales relevant to flooding events in the basin. Large-scale circulation patterns (CPs) are obtained through Maximum Covariance Analysis using reanalysis sea level pressure and high-resolution precipitation observations. A bias correction using these CPs is applied to winter and summer separately, acknowledging the seasonal variability of the circulation regimes in North Europe and their correlation with regional precipitation rates over the Rhine basin. Two different climate model ensemble outputs are explored: ESSENCE and CMIP5. The results of the CP-method are then compared to observations and uncorrected model outputs. Results from a simple bias correction based on a delta factor (NoCP-method) are also used for comparison. For both summer and winter, the CP-method offers a statistically significant improvement of precipitation statistics for subsets of data dominated by particular circulation regimes, demonstrating the circulation-dependence of the precipitation bias. Uncorrected, CP and NoCP corrected model outputs were used as forcing to a hydrological model to simulate river discharges. The CP-method leads to a larger improvement in simulated discharge in the Alpine area in winter than in summer due to a stronger dependence of Rhine precipitation on atmospheric circulation in winter. However, the NoCP-method, in comparison to the CP-method, improves the discharge estimations over the entire Rhine basin. 相似文献
46.
47.
Rivalenti Giorgio Mazzucchelli Maurizio Vannucci Riccardo Hofmann Albrecht W. Ottolini Luisa Bottazzi Piero Obermiller Walter 《Contributions to Mineralogy and Petrology》1995,122(3):275-288
Field, mineralogical and petrological data are presented on a newly found carbonatite occurrence associated with “kamafugite”
lava at Cupaello, central Italy. This carbonatite occurrence is part of the Late Pleistocene Umbria-Latium ultra-alkaline
district (ULUD) which extends southwards within the Apennines to Mount Vulture, delineating an important magmatic province
along the most peripheral belt of the Tyrrhenian extensional tectonic system. This province is distinct, but probably related
genetically with the more abundant and common leucite-bearing assemblages of the Roman Comagmatic Region and represents the
first reported occurrence of carbonatite assemblages in the Mediterranean Basin. The Cupaello suite indicates that primary
or near-primary mantle silicate melts of “kamafugitic” composition are transitional with Ca-carbonatite liquid and provides
direct evidence of immiscibility of carbonatite from “kamafugite” magma. It is inferred that a primary mantle origin of Ca-carbonatites
is conditional upon a potential silicate magma that may be coupled with the carbonatite, but may not have reached the surface.
The data indicate a strong genetic link between ULUD Ca-carbonatites and some African analogues, supporting the view that
their genesis depends on similar source and associated tectonic conditions.
Received: 17 January 1995 / Accepted: 14 June 1995 相似文献
48.
ABSTRACTHydrogeological hazards are increasingly causing damage worldwide due to climatic and socio-economic changes. Building resilient communities is crucial to reduce potential losses. To this end, one of the first steps is to understand how people perceive potential threats around them. This study aims at exploring how risk awareness of, and preparedness to, face hydrological hazards changes over time. A cohort study was carried out in two villages in the northeastern Italian Alps, Romagnano and Vermiglio, affected by debris flows in 2000 and 2002. Surveys were conducted in 2005 and 2018, and the results compared. The survey data show that both awareness and preparedness decreased over time. We attribute this change to the fact that no event had occurred in a long time and to a lack of proper risk communication strategies. The outcomes of this study contribute to socio-hydrological modelling by providing empirical data on human behaviour dynamics. 相似文献
49.
Hans J. Albrecht 《Pure and Applied Geophysics》1956,34(1):196-206
Summary Following a brief discussion of known methods of electronic amplification, this paper describes new thermocouple amplifiers using junction-type transistors, and their theoretical and practical design. The apparatus amplify a thermocouple voltage or other input signals of any impedance such that an ordinary pointer-type galvanometer or recorder can be used as indicator. The problem of temperature compensation has also been dealt with. Details of two transistor amplifiers are given as examples. In conclusion geophysical applications are mentioned.
Zusammenfassung Nach einer kurzen Behandlung bekannter elektronischer Verstärkungsmethoden beschreibt diese Arbeit neuartige Thermoelementverstärker mit Schichttransistoren und ihre theoretische und praktische Konstruktion. Die Geräte verstärken eine Thermoelementspannung oder andere Eingangssignale irgendeiner Impedanz, sodass ein normales Zeigergalvanometer oder Registriergerät als Anzeigegerät benutzt werden kann. Ausserdem wird die Frage der Temperaturkompensation behandelt. Einzelheiten von zwei Transistorverstärkern werden als Beispiele eingehend beschrieben. Abschliessend werden geophysikalische Anwendungen erwähnt.相似文献
50.