The age and thermal history of Cerro Rico de Potosi, Bolivia   总被引：1，自引：0，他引：1  

C. G. Cunningham  R. E. Zartman  E. H. McKee  R. O. Rye  C. W. Naeser  O. Sanjinés V  G. E. Ericksen  F. Tavera V 《Mineralium Deposita》1996,31(5):374-385

 Cerro Rico de Potosi, Bolivia, is the world’s largest silver deposit and has been mined since the sixteenth century for silver, and for tin and zinc during the twentieth century, together with by-product copper and lead. The deposit consists primarily of veins that cut an altered igneous body that we interpret to be a dacitic volcanic dome and its underlying tuff ring and explosion breccia. The deposit is compositionally and thermally zoned, having a core of cassiterite, wolframite, bismuthinite, and arsenopyrite surrounded by a peripheral, lower-temperature mineral assemblage consisting principally of sphalerite, galena, lead sulfosalt, and silver minerals. The low-temperature assemblage also was superimposed on the high-temperature assemblage in response to cooling of the main hydrothermal system. Both the dacite dome and the ore fluids were derived from a larger magmatic/hydrothermal source at depth. The dome was repeatedly fractured by recurrent movement on the fault system that guided its initial emplacement. The dome was extruded at 13.8±0.2 Ma (2σ), based on U-Th-Pb dating of zircon. Mineralization and alteration occurred within about 0.3 my of dome emplacement, as indicated by a ⁴⁰Ar/³⁹Ar date of 13.76±0.10 Ma (1σ) for sericite from the pervasive quartz-sericite-pyrite alteration associated with the main-stage, high-temperature, mineralization. The last thermal event able to reset zircon fission tracks occurred no later than 12.5±1.1 Ma (1σ), as indicated by fission-tract dating. Minor sericite, and magmatic-steam alunite veins, were episodically formed around 11 Ma and between 8.3 and 5.7 Ma; the younger episodes occurring at the time of extensional fracturing at Cerro Rico and widespread volcanism in the adjacent Los Frailes volcanic field. None of these younger events appear to be significant thermal/mineralizing events; the exceptionally flat thermal release pattern of ³⁹Ar from sericite and the results of the fission-tract dating of zircon show that none of the younger events was hot enough, and lasted long enough, to cause significant loss of Ar or annealing of zircon fission tracks. U-Th-Pb dating of zircon cores indicates a Precambrian progenitor for some zircons, and REE analyses of dated samples of hydrothermally altered dacite show the presence of a prominent positive Eu anomaly, which constrains interpretations of the origin and evolution of the magmatic/hydrothermal system. Received: 14 October 1995/Accepted: 29 January 1996  相似文献   

Investigating the Halogen Chemistry From High-Latitude Nighttime Stratospheric Measurements of OClO and NO2     

Emmanuel Damien Rivière  Michel Pirre  Gwenaël Berthet  Jean-Baptiste Renard  Franck Lefèvre 《Journal of Atmospheric Chemistry》2004,48(3):261-282

Two cases of simultaneous nighttime measurements of NO₂ and OClO in the winter polar stratosphere are analyzed in order to test our present knowledge of halogen chemistry in the presence of high amount of NO₂ at low temperature. Comparisons with Lagrangian model calculations using several hypotheses are performed. First simulations, using the admitted constant rates of chemical reaction, strongly underestimate the measured OClO while the NO₂ profiles are correctly reproduced. If uncertainties in actinic fluxes calculations are taken into account, simulation results do not show a significant reduction of the underestimation. A better agreement can be achieved if the formation of unstable isomers of ClONO₂ and of BrONO₂ occurs in the cold conditions of the polar stratosphere. An approximate value of the branching ratios of the channels leading to ClONO₂ and ClOONO, and to BrONO₂ and BrOONO, necessary to reproduce both OClO and NO₂ is given and discussed.  相似文献   

地球化学场精细结构解析方案与应用   总被引：2，自引：2，他引：2  

陈明何凯  王全明Jacques Yvon 《地质通报》2004,23(2):147-153

为满足勘查地球化学对化探数据处理越来越精细的要求，把人工神经网络和统计分析结合起来形成地球化学场精细结构解析方案。该方案包括对地球化学样品的分类和对各类样品性质的详细研究，目的在于提供便捷的数学和计算机软件工具，以提取化探数据中的深层信息。可以用于：①研究各组样品中元素含量水平和组合特征以解释可能的矿种；②研究元素组合特征与指示元素的关联和区别以揭示可能的成矿作用过程；③研究异常样品的空间组合属性以揭示地球化学场的精细结构；④研究异常样品之间的差异性以缩小找矿靶区；⑤综合分析以确定剥蚀深度和找矿前景。该方案也适用于环境地球化学及矿产资源预测研究。  相似文献   

Time varying gravimetry, GGP, and vertical crustal motions     

Gerhard Jentzsch  David Crossley  Jacques Hinderer  Shuzo Takemoto 《Journal of Geodynamics》2004,38(3-5):223

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On Some Simplifications of Cokriging Neighborhood   总被引：2，自引：0，他引：2  

Jacques Rivoirard 《Mathematical Geology》2004,36(8):899-915

Choosing the cokriging neighborhood is often difficult. A poor choice, ignoring influent data, can result in a loss of information as well as in artifacts in simulations based on cokriging. Then it is convenient to use if possible, or to refer to models that lead to simplified cokriging neighborhood. We essentially consider the case of two stationary variables, a target variable and an auxiliary one. By examining possible simplifications, we set up a list of models (essentially models with residuals) that, in general or under specific configurations, lead to simplifications of cokriging neighborhood. Collocated, dislocated, and other types of neighborhood are identified, that are optimal in some models and configurations. Possible extensions to cokriging with unknown means, and to more variables, are included.  相似文献   

Environmental impact of an urban landfill on a coastal aquifer (El Jadida, Morocco)     

Amina Chofqi  Abedelkader Younsi  El Kbir Lhadi  Jacky Mania  Jacques Mudry  Alain Veron 《Journal of African Earth Sciences》2004,39(3-5):509

The El Jadida landfill is one among many uncontrolled dumping sites in Morocco with no bottom liner. About 150 tons/day of solid wastes from mixed urban and industrial origins are placed directly on the ground. At the site of this landfill, the groundwaters circulate deeply (10–15 m) in the Cenomanian rock (calcareous–marl), which is characterised by an important permeability from cracks. The soil is sand–clay characterized by a weak coefficient of retention.The phreatic water ascends to the bottom of three quarries, which are located within the landfill. These circumstances, along with the lack of a leachate collection system, worsen the risks for a potential deterioration of the aquifer.To evaluate groundwater pollution due to this urban landfill, piezometric level and geochemical analyses have been monitored since 1999 on 60 wells. The landfill leachate has been collected from the three quarries that are located within the landfill. The average results of geochemical analyses show an important polluant charge vehiculed by landfill leachate (chloride = 5680 mg l⁻¹, chemical oxygen demand = 1000 mg l⁻¹, iron = 23 000 μg l⁻¹). They show also an important qualitative degradation of the groundwater, especially in the parts situated in the down gradient area and in direct proximity to the landfill. In these polluted zones, we have observed the following values: higher than 4.5 mS cm⁻¹ in electric conductivity, 1620 and 1000 mg l⁻¹ respectively in chlorides and sulfate (), 15–25 μg l⁻¹ in cadmium, and 60–100 μg l⁻¹ in chromium. These concentrations widely exceed the standard values for potable water.Several determining factors in the evolution of groundwater contamination have been highlighted, such as (1) depth of the water table, (2) permeability of soil and unsaturated zone, (3) effective infiltration, (4) humidity and (5) absence of a system for leachate drainage. So, to reduce the pollution risks of the groundwater, it is necessary to set a system of collection, drainage and treatment of landfill leachates and to emplace an impermeable surface at the site of landfill, in order to limit the infiltration of leachate.  相似文献   

Une controverse que l'on croyait oubliée     

Jacques Touret 《Comptes Rendus Geoscience》2004,336(2):167

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Effet du vieillissement des céramiques poreuses sur leur capacité à évaluer la concentration de pesticide en solution     

Nicolas Domange  Caroline Grégoire  Véronique Gouy  Michèle Tremolières 《Comptes Rendus Geoscience》2004,336(1):49-58

The soil solution sampling by ceramic cups allows pesticide transfer monitoring in the soil during long times. The ageing of material involves a bias in the sampling results. In laboratory, the comparison of two types of ceramic suction cups, new and installed in situ during four years, shows a modification of the hydrodynamic properties and a possible evolution of the adsorption capacity of the matrix. The passage rate, as well qualitative as quantitative, is better for the old material. Recommendations about site management are finally exposed. To cite this article: N. Domange et al., C. R. Geoscience 336 (2004).  相似文献   

Field and petrological evidence for a Late Palaeozoic (Upper Carboniferous–Permian) age of the Erfaulet orthogneiss (Gran Paradiso,western Alps)     

Benjamin Le Bayon  Michel Ballèvre 《Comptes Rendus Geoscience》2004,336(12):1079-1089

In the Gran Paradiso massif (western Alps), the boundary between the Erfaulet orthogneiss and the overlying metasediments (Money Complex) is interpreted as a Late Palaeozoic intrusive contact. Major arguments in favour of this hypothesis are: (i) the obliquity of the sedimentary layering with respect to the contact; (ii) the presence of aplitic dykes within the Money Complex; (iii) the lack of a mylonitic zone; and (iv) rare relics of an early generation of garnet in the Money metasediments, interpreted as evidence of the contact metamorphism of the Erfaulet granite. To cite this article: B. Le Bayon, M. Ballèvre, C. R. Geoscience 336 (2004).  相似文献   

Particularités de la contamination crustale des phonolites : exemple du Velay oriental (Massif central)     

Jean-Marie Dautria  Jean-Michel Liotard  Danièle Briot 《Comptes Rendus Geoscience》2004,336(11):971-981

Sr and Nd isotopic compositions of one trachyte, eight phonolites and five basalts have been measured. The isotopic characteristics of the trachyte can be explained by a combined assimilation–fractional crystallization process within an upper crustal magmatic chamber. Some phonolites display isotopic signatures identical to basalts, suggesting that they have been protected against any crustal assimilation during their formation. Some others have low Sr contents, whereas they are enriched in radiogenic Sr (0.70451<⁸⁷Sr/⁸⁶Sr_i<0.71192), and display basaltic ¹⁴³Nd/¹⁴⁴Nd ratios. Both observations could be explained by very strong alkali feldspar fractionation and by subsequent very low assimilation of surrounding rocks (between 0.3 and 4%) during intrusion. To cite this article: J.-M. Dautria et al., C. R. Geoscience 336 (2004).  相似文献