Textural and fluid inclusion constraints on the origin of the banded-iron-formation-hosted gold deposits at Maevatanana,central Madagascar     

Prosper Andrianjakavah  Stefano Salvi  Didier Béziat  Damien Guillaume  Michel Rakotondrazafy  Bernard Moine 《Mineralium Deposita》2007,42(4):385-398

The Maevatanana deposits consist of gold-bearing quartz–sulphide veins crosscutting banded iron formation (BIF) within a metamorphosed 2.5 Ga greenstone belt. The host rocks are dominated by a sequence of migmatites, gneisses, amphibolites, magnetite-rich quartzites and soapstones, intruded by large granitoid batholiths (e.g. the 0.8 Ga Beanana granodiorite). In the mineralised rocks, pyrite is the dominant sulphide, in addition to accessory chalcopyrite and galena. Outside the immediate ore zone, the BIF is dominated by quartz + magnetite ± hematite, accompanied by cummingtonite, albite and biotite. Gold occurs as globular grains (usually <500 μm) within quartz crystals close to the sulphides and as invisible inclusions within pyrite and chalcopyrite (up to 2,500 ppm Au content). Fluid inclusion textural and microthermometric studies indicate heterogeneous trapping of a low-salinity (∼3.6 wt.% eq. NaCl) aqueous fluid coexisting with a carbonic fluid. Evidence for fluid-phase immiscibility during ore formation includes variable L/V ratios in the inclusions and the fact that inclusions containing different phase proportions occur in the same area, growth zone, or plane. Laser Raman spectroscopy confirms that the vapour phase in these inclusions is dominated by CO₂ but shows that it may contain small amounts of CH₄ (<1 mol%), H₂S (<0.05 mol%) and traces of N₂. Fluid inclusion trapping conditions ranged from 220 to 380°C and averaged 250°C. Pressure was on the order of 1–2 kbar. The abundant CO₂ and low salinity of the inclusions suggest a metamorphic origin for the fluid. Likewise, the presence of H₂S in the fluid and pyritisation of the wall-rock indicate that gold was likely transported by sulphide complexing. Fluid immiscibility was probably triggered by the pressure released by fracturing of the quartzites during fault movements due to competence differences with the softer greenstones. Fracturing greatly enhanced fluid circulation through the BIF, allowing reaction of the sulphide-bearing fluids with the iron oxides. This caused pyrite deposition and concomitant Au precipitation, enhanced by fluid phase separation as H₂S partitioned preferentially into the carbonic phase.  相似文献   

Synergie entre la télédétection multispectrale et les données de terrain pour la conception d'un nouveau modèle géodynamique d'ouverture du bassin paléozoïque des Jebilet centrales (Maroc)     

Abderrazak El Harti  Abderrazak Bannari  El Mostafa Bachaoui  El Mostafa Aarab  Guillaume Girouard  Abderrahmen El Ghmari 《Comptes Rendus Geoscience》2004,336(14):1311-1320

The geodynamic model of the Palaeozoic basin opening of central Jebilet has been unknown before this study in spite of the abundance in the geological studies carried out in the studied sector using conventional methods. This is due to the scarcity of the key beds and synsedimentary structures. Using the synergy between the image data of the Landsat satellite TM sensor and the ground data, we have highlighted, herein, new structural data allowing the design of a new model of the Palaeozoic basin opening of central Jebilet. This opening could have been made according to dextral submeridian transverse faults with the individualization of subequatorial normal faults. To cite this article: A. El Harti et al., C. R. Geoscience 336 (2004).  相似文献   

Quantitative precipitation forecasts: a statistical adaptation of model outputs through an analogues sorting approach   总被引：2，自引：0，他引：2  

Charles Obled  Guillaume Bontron  Rmy Garon 《Atmospheric Research》2002,63(3-4)

Medium-term quantitative precipitation forecasts (QPFs) up to several days ahead are required to issue early flood warnings and to allow optimum operation of hydraulic structures or reservoirs. This paper describes an approach which can be seen as an adaptation of deterministic meteorological model outputs. It involves searching for a sample of past situations similar to the current one from a long meteorological archive. The analogy is considered in terms of general circulation patterns over a window covering western Europe. For this restricted sample of days similar to the day at hand, the corresponding sample of observed daily precipitation is extracted for each catchment. The rainfall to be observed during the current day is assumed to follow the same distribution, known from this empirical sample. This provides a probabilistic forecast expressed, for example, by a central quantile and a confidence range. This paper describes the many choices underlying the optimisation of this approach: choice of predictor variables to characterise a meteorological situation, choice of similarity criterion between two situations, criterion for performance evaluation between two versions of the algorithm, etc. This method was calibrated over about 50 catchments located in France, Italy and Spain, using a meteorological and hydrological archive running from 1953 to 1996. Comparisons carried out over a validation sample (1995–1996) with three poor-man methods prove the interest of this approach, in a perfect prognosis context. In real-time operation, the use of forecast instead of observed predictor variables, essentially geopotential fields, produces only a minor decrease in performance. The use of the single-valued central quantile supplemented by the confidence interval provided a QPF that has proved effective and informative on the potential for extreme values.  相似文献   

Routes of Agulhas rings in the southeastern Cape Basin     

Guillaume Dencausse  Michel Arhan  Sabrina Speich 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(11):1406-1421

Using weekly sea surface height data, Agulhas rings from the period October 1992 to December 2006 are detected and tracked, from their formation dates and throughout the Cape Basin. While 102 of them formed at the Agulhas Current retroflection, their subsequent subdivisions and junctions led to 199 trajectories. The rings geographical probability of presence shows two maxima. One, related to numerous ring passages, lies in the submarine bight formed by the Erica seamount, the Schmitt-Ott seamount, and the northeastern tip of the Agulhas Ridge. The other one, to be ascribed to topographic blocking of the eddies, is southeast of the latter obstacle. On the basis of topographic effects three routes for Agulhas rings are distinguished, a Northern route for rings that enter the south-Atlantic northeast of the Erica seamount, a Central one for those passing westward between this seamount and the tip of the Agulhas Ridge, and a Southern one farther south. Despite its bathymetric obstacles, the central route is the dominant one, both in terms of percentage of eddy crossings at its definition segment, and in terms of conveyed volume transport. Specific behaviours of rings along each route are described, referring to observations in previous studies. Some rings from the Northern route interact with the flow regime of the South African continental slope. The southernmost trajectories of the Central route are thought to settle the location of the climatological Subtropical Front in that region. The rings of the Southern route experience important core property alteration as they transit through the subantarctic domain.  相似文献   

Plio-Quaternary reactivation of the Neogene margin off NW Algiers, Algeria: The Khayr al Din bank     

Abdelkarim Yelles  Jacques Déverchère  Rabah Bracène  Pierre Strzerzynski  Guillaume Bertrand  Thierry Winter  Pascal Le Roy  Hamou Djellit 《Tectonophysics》2009,475(1):98-116

The Algiers region, northern Algeria, is known to be seismically active, with recurrent large (M>6) earthquakes. Because of the lack of high-resolution bathymetry, the offshore structures remained for a long time poorly known. Thanks to a new marine data base (MARADJA 2003 cruise), the offshore part of the margin is accurately mapped, and new active and recent structures are described. West of the bay of Algiers, the margin enlarges, forming the Khayr al Din bank, interpreted as a tilted block of the passive margin born during the opening of the Algero-Provençal basin. At the slope break, a 80 km-long fault-tip Quaternary fold, namely the Khayr al Din fault, extends at the foot of the margin off NW Algiers and represents the largest active structure of the coastal area, together with the Sahel anticline. We also map for the first time a set of overlapping, en echelon active folds in the upper part of the Khayr al Din bank, located off previously known active structures on land. Most of these faults represent actually a threat for the Algiers region in terms of seismic hazard but also geological hazards, such as tsunamis, as most of them depicts significant dimensions and slip rates. The highest long-term horizontal shortening rate is found on the Khayr al Din fault and is estimated at 0.5 ± 0.1 mm/yr, with a maximal magnitude of 7.3, which provides one of the highest seismogenic potential in the region.A new tectonic framework for the Algiers region is proposed, in which the main south-dipping offshore structure, of opposite vergence relative to most thrusts on land, appears to be nowadays the main driving fault system, as also found further east in the Boumerdès (M 6.8) 2003 rupture zone. The overall apparent pop-up structure of the recent and active faults may result from a progressive migration of the plate limit from the Late Miocene, north-dipping suture zone on land, to the Quaternary, south-dipping main Khayr al Din fault at sea, suggesting a process of subduction inception.  相似文献   

Virtual organisation in the SIMDAT meteorological activity: a decentralised access control mechanism for distributed data     

Baudouin Raoult  Guillaume Aubert  Marta Gutiérrez  Cristina Arciniegas-Lopez  Ricardo Correa 《Earth Science Informatics》2009,2(1-2):63-74

The EU funded SIMDAT project is aimed at applying generic grid technology for the solution of complex application problems in several representative fields including automotive, aerospace, pharmaceutical and meteorology. To satisfy the requirements of the World Meteorological Organization (WMO) and the WMO Information Systems (WIS), the partners in the meteorology activity within SIMDAT (ECMWF, Deutscher Wetterdienst, the UK Met office, EUMETSAT and Météo-France), have developed grid-enabled software that provides generic distributed access to distributed meteorological data repositories via web-based portals, through a series of nodes organized in a mesh network. However, granting access to such an infrastructure, especially considering its fully distributed nature, is a serious challenge and a risk to the security of the overall grid infrastructure. SIMDAT solves this problem by implementing a security model based on a decentralized fine-grained access control mechanism that federates data providers and security issues using the notion of “trust domains”. In this paper we highlight the main features of the SIMDAT grid application and describe its security model in detail.  相似文献   

Diagenetic controls on the isotopic composition of carbonate‐associated sulphate in the Permian Capitan Reef Complex,West Texas     

Theodore M. Present  Melissa Gutierrez  Guillaume Paris  Charles Kerans  John P. Grotzinger  Jess F. Adkins 《Sedimentology》2019,66(7):2605-2626

Late Palaeozoic‐age strata from the Capitan Reef in west Texas show facies‐dependent heterogeneity in the sulphur isotopic composition of carbonate‐associated sulphate, which is trace sulphate incorporated into carbonate minerals that is often used to reconstruct the sulphur isotopic composition of ancient seawater. However, diagenetic pore fluid processes may influence the sulphur isotopic composition of carbonate‐associated sulphate. These processes variously modify the sulphur isotopic composition of incorporated sulphate from syndepositional seawater in shelf crest, outer shelf, shelf margin and slope depositional settings. This study used a new multicollector inductively‐coupled plasma mass spectrometry technique to determine the sulphur isotopic composition of samples of individual depositional and diagenetic textures. Carbonate rocks representing peritidal facies in the Yates and Tansill formations preserve the sulphur isotopic composition of Guadalupian seawater sulphate despite alteration of the carbon and oxygen isotopic compositions by meteoric and dolomitizing diagenetic processes. However, sulphur isotopic data indicate that limestones deposited in reef and slope facies in the Capitan and Bell Canyon formations largely incorporate sulphate from anoxic marine‐phreatic pore fluids isotopically modified from seawater by microbial sulphate reduction, despite generally preserving the carbon and oxygen isotopic compositions of Permian seawater. Some early and all late meteoric calcite cements have carbonate‐associated sulphate with a sulphur isotopic composition distinct from that of Permian seawater. Detailed petrographic and sedimentary context for carbonate‐associated sulphate analyses will allow for improved reconstructions of ancient seawater composition and diagenetic conditions in ancient carbonate platforms. The results of this study indicate that carbonate rocks that diagenetically stabilize in high‐energy environments without pore fluid sulphate gradients can provide a robust archive of ancient seawater's sulphur isotopic composition.  相似文献   

H2O Content Measurement in Phengite by Secondary Ion Mass Spectrometry: A New Set of Reference Materials     

Cindy Luisier  Lukas Baumgartner  Guillaume Siron  Torsten Vennemann  Martin Robyr 《Geostandards and Geoanalytical Research》2019,43(4):635-646

Five new natural white mica reference materials (RMs) were developed for in situ H₂O content analyses by secondary ion mass spectrometry at the SwissSIMS laboratory of Lausanne University, Switzerland. The white mica reference materials cover a large part of the natural muscovite–phengite compositional range and are therefore suitable as reference materials for the analysis of natural rocks as well as individual minerals. The independent H₂O content of the reference materials UNIL_WM1 to UNIL_WM5 was obtained by thermal conversion elemental analyser and corresponds to 4.35 ± 0.02, 4.33 ± 0.03, 4.30 ± 0.07, 4.50 ± 0.02 and 4.42 ± 0.11 (% m/m, ± 1s), respectively. SIMS determinations of H₂O content revealed a matrix effect correlated to the FeO content of white mica. The compositional range in FeO of the reference materials that were calibrated for H₂O determination is from 1.13% to 3.67% m/m. No crystallographic orientation dependency was observed at the level of homogeneity of these reference materials. An analytical precision of 0.02% to 0.08% m/m (1SE) is expected for the final uncertainty on measurements of unknown white micas in natural samples.  相似文献   

Automatic correction and simplification of geological maps and cross-sections for numerical simulations     

Pierre Anquez  Jeanne Pellerin  Modeste Irakarama  Paul Cupillard  Bruno Lévy  Guillaume Caumon 《Comptes Rendus Geoscience》2019,351(1):48-58

Incorporating prior geological knowledge in geophysical process models often meets practical meshing challenges and raises the question of how much detail is to be included in the geometric model. We introduce a strategy to automatically repair and simplify geological maps, geological cross-sections and the associated meshes while preserving elementary consistency rules. To identify features breaking validity and/or the thin features potentially problematic when generating a mesh, we associate an exclusion zone with each model feature (horizon, fault). When these zones overlap, both the connectivity and the geometry of the geological layers are automatically modified. The output model enforces specific practical quality criteria on the model topology and geometry that facilitates the generation of a mesh with lower bounds on minimum angles and minimum local entity sizes. Our strategy is demonstrated on an invalid geological cross-section from a real-case study in the Lorraine coal basin. We further explore the impacts of the model modifications on wave propagation simulation. We show that the differences on the seismograms due to model simplifications are relatively small if the magnitude of simplifications is adapted to the physical problem parameters.  相似文献   

The Planeterrella,a pedagogic experiment in planetology and plasma physics     

Jean Lilensten  Mathieu Barthélemy  Cyril Simon  Philippe Jeanjacquot  Guillaume Gronoff 《Acta Geophysica》2009,57(1):220-235

We present here a plasma physics experiment which makes it possible to simulate, in a naive yet useful way, the formation of polar lights. It involves shooting electrons at a magnetized sphere placed in a vacuum chamber. This experiment, inspired by K. Birkeland’s Terrella, built at the turn of 19th century, allows the visualization of very many geophysical and astrophysical situations. Although delicate, it is feasible at undergraduate level.  相似文献