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排序方式: 共有149条查询结果,搜索用时 722 毫秒
91.
Amos Fety Michel Rakotondrazafy Gaston Giuliani Daniel Ohnenstetter Anthony E. Fallick Saholy Rakotosamizanany Alfred Andriamamonjy Thogne Ralantoarison Madison Razanatseheno Yohann Offant Virginie Garnier Henri Maluski Christian Dunaigre Dietmar Schwarz Voahangy Ratrimo 《Ore Geology Reviews》2008,34(1-2):134
Madagascar is one of the most important gem-producing countries in the world, including ruby and sapphires. Gem corundum deposits formed at different stages in the geological evolution of the island and in contrasting environments. Four main settings are identified: (1) Gem corundum formed in the Precambrian basement within the Neoproterozoic terranes of southern Madagascar, and in the volcano-sedimentary series of Beforona, north of Antananarivo. In the south, high-temperature (700 to 800 °C) and low-pressure (4 to 5 kbar) granulites contain deposits formed during the Pan-African orogenesis between 565 and 490 Ma. They accompany mafic and ultramafic complexes (ruby deposits of the Vohibory group), skarns at the contact between Anosyan granites and the Proterozoic Tranomaro group (sapphire deposits of the Tranomaro–Andranondambo district), and shear-zone corridors cross-cutting feldspathic gneisses, cordieritites and clinopyroxenites in the Tranomaro, Vohimena and Androyan metamorphic series (biotite schist deposits of Sahambano and Zazafotsy, cordieritites of Iankaroka and Ambatomena). The circulation of fluids, especially along discontinuities, allowed in-situ alkaline metasomatism, forming corundum host rocks related to desilicified granites, biotitites, “sakenites” and “corundumites”. (2) Gem corundum also occurs in the Triassic detrital formations of the Isalo group, as giant palaeoplacers in the Ilakaka–Sakaraha area. Here, sapphires and rubies may come from the metamorphic granulitic terranes of southern Madagascar. (3) Gem corundum deposits occur within the Neogene-Quaternary alkali basalts from Ankaratra (Antsirabe–Antanifotsy area) and in the Ambohitra Province (Nosy Be, Ambato and Ambondromifehy districts). Primary deposits are rare, except at Soamiakatra where ruby in gabbroic and clinopyroxenite xenoliths within alkali-basalts probably derive from mantle garnet peridotites. The blue-green-yellow sapphires typical of basaltic fields are always recovered in palaeoplacer (in karst formed upon Jurassic limestones from the Montagne d'Ambre, Antsiranana Province) and alluvial and soil placers (Ankaratra volcanic massif). (4) Deposits occur within Quaternary eluvial, colluvial and alluvial concentrations, such as high-quality rubies from the Andilamena and Vatomandry deposits. 相似文献
92.
Hydrogeochemical modeling of organo-metallic colloids in the Nsimi experimental watershed,South Cameroon 总被引:1,自引:0,他引:1
M. Sekhar Jean-Jacques Braun K. V. Hayagreeva Rao Laurent Ruiz Henri Robain Jérôme Viers Jules Rémy Ndam Bernard Dupré 《Environmental Geology》2008,54(4):831-841
This paper presents a hydrogeochemical modeling code HYDROS, which combines the multi-component transport model with equilibrium
speciation module MINTEQA2. The processes of adsorption, aqueous speciation and mineral precipitation/dissolution are represented
in the model. The numerical model uses a sequential iterative approach for solving the solute transport and the equilibrium
geochemistry modules. Further the transport part is solved using an operator split approach wherein a finite volume method
is used for solving the advective equations while a classical finite difference method is employed for solving the dispersive
equations. The model performance is evaluated by comparing it with MINTOX for a literature problem. HYDROS is then applied
to the case study of the transfer of transition metals with organic colloids in the swamp groundwater system of the experimental
Nsimi watershed, representative of the humid tropical ecosystem of the South Cameroon Plateau. Field observations at the site
swamp system suggest that the carbon is mainly transferred as organic colloids (i.e., dissolved organic carbon) produced by
the slow biodegradation of the swamp organic matter. Using HYDROS, the behaviour of Al(III) and Fe(III) elements in the base
flow system is simulated during inter rain events of a short rainy season (May–June 1996). The elemental time-series for Al,
Fe, Cl, pH compare well with the simulation results. The colloids are found to have a strong impact on the mobilization and
transfer of Al(III) and Fe(III), which are considered to have low mobility in weathering environment. 相似文献
93.
94.
Henri Robain Yves Albouy Michel Dabas Marc Descloitres Christian Camerlynck Pierre Mechler Alain Tabbagh 《Journal of Applied Geophysics》1999,41(4):588
In 2D-multielectrode electrical surveys using the pole–pole array, the distance to ‘infinite electrodes' is actually finite. As a matter of fact, the available cable length generally imposes a poor approximation of theoretical location of these electrodes at infinity. This study shows that in most of the cases, the resulting apparent resistivity pseudosection is strongly distorted. Numerical simulation validated by field test also shows that a particular finite array provides results that are as close as possible to the ones of the ideal pole–pole array. This is achieved when two conditions that are weaker than an infinite location are fulfilled: (i) the ‘infinite electrodes' are placed symmetrically on both sides of the in-line electrodes with a spread angle of 30° and (ii) the length of ‘infinite lines' is at least 20 times the greatest distance between in-line electrodes. The electrical 2D image obtained with this enhanced array is the least distorted one with respect to the pole–pole image. The apparent resistivities are generally underestimated, but this deviation is almost homogeneous. Though the shift cannot be determined a priori, the interpretation of such an image with direct or inverse software designed for pole–pole data provides an accurate interpretation of the ground geometry. 相似文献
95.
96.
P. O. Petrucci G. Ponti G. Matt L. Maraschi J. Malzac M. Mouchet C. Boisson A. Longinotti K. Nandra P. Ferrando G. Henri 《Astronomische Nachrichten》2006,327(10):1043-1046
Mkn 841 has been observed during 3 different periods (January 2001, January 2005 and July 2005) by XMM‐Newton for a total cumulated exposure time of ∼108 ks. We present in this paper a broad band spectral analysis of the complete EPIC‐pn data sets. These observations confirm the presence of the strong soft excess and complex iron line profile known to be present in this source since a long time. They also reveal their extreme and puzzling spectral and temporal behaviors. Indeed, the 0.5–2 keV soft X‐ray flux decreases by a factor 3 between 2001 and 2005 and the line shape appears to be a mixed of broad and narrow components, both variable but on different timescales. The broad‐band 0.5–10 keV spectra are well described by a model including a primary power law continuum, a blurred photoionized reflection and a narrow iron line, the blurred reflection fitting self‐consistently the soft excess and the broad line component. The origin and nature of the narrow component is unclear. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
97.
The relationship between seismic and aseismic motions induced by forced fluid injections 总被引:1,自引:1,他引:0
Fran?ois Henri Cornet 《Hydrogeology Journal》2012,20(8):1463-1466
The injection of fluid into a rock mass results in variations of effective stresses that sometimes generate induced seismicity. These effective stress field variations depend on the diffusion process, which depends, in turn, on the magnitude of the pore pressure variation relative to the total stress. Four diffusion mechanisms are distinguished: diffusion through a poroelastic rock mass, and diffusion in preferential directions controlled either by slip along preexisting fractures, or by the development of fresh shear zones, or by hydraulic fracturing. More importantly, in some instances, this diffusion process also generates non-seismic motions that, in turn, influence the seismic activity, in particular when injection stops. 相似文献
98.
Laurent Zimmermann Pierre Henri Blard Pete Burnard Sarah Medynski Raphael Pik Nicolas Puchol 《Geostandards and Geoanalytical Research》2012,36(2):121-129
Helium‐3 is a stable cosmogenic isotope that can be used to determine the time interval during which a rock sample has been at or close to the Earth’s surface. As a result of the high production rate of ‘cosmogenic’3He (≈ 130 at g?1 year?1) and the low detection limit of modern mass spectrometers, it is possible to date exceptionally young surfaces (≈ 1000 years). The precision and accuracy of cosmogenic 3He measurements depend critically on the passive helium blank (produced by the metalwork of the extraction furnace) which can be significant relative to the sample signals. We have developed and constructed, at the CRPG (Nancy, France), a new high temperature furnace (< 1500 °C) to extract helium in minerals such as apatite, pyroxene and olivine at 1050, 1350 and 1450 °C, respectively. The furnace demonstrated an excellent helium extraction yield (> 99% for olivine and pyroxene for heating times of 20–30 min and temperatures in the range 1050–1450 °C) and low residual helium contributions (the blank, obtained under the same analytical conditions as for sample extraction: 1 × 10?15 mole 4He and < 4 × 10?21 mole 3He). This is approximately an order of magnitude lower than those reported by other laboratories using conventional furnaces. 相似文献
99.
In the Western French Massif Central, the Argentat fault is a major structure through which As–Au fluids percolated in the Late Carboniferous along brittle fractures. New petrostructural investigations show that an early ductile normal-dextral faulting, coeval to leucogranite emplacement took place during the Late Visean syncollisional extension of the belt and was accompanied by a hydrothermal event marked by the growth of muscovites whose 40 Ar/39 Ar ages cluster around 335 Ma. This early fluid channelling is associated with brittle deformation only in the hangingwall of the Argentat fault, whereas ductile deformation is restricted to the footwall. These results provide new evidence for the upper crust implication during the syncollisional extension in the French Massif Central. This study stresses the interest of a detailed multimethod analysis to characterize hydrothermal processes, especially in basement areas where the tectonic, plutonic and metamorphic evolution is polyphased. 相似文献
100.
Sand banks have a wavelength between 1 and 10 km, and they are up to several tens of meters high. Also, sand banks may have
an impact on large-scale human activities that take place in the North Sea like sand mining, shipping, offshore wind farms,
etc. Therefore, it is important to know where sand banks occur and what their natural behavior is. Here, we use an idealized
model to predict the occurrence of sand banks in the North Sea. The aim of the paper is to research to what extent the model
is able to predict the occurrence of sand banks in the North Sea. We apply a sensitivity analysis to optimize the model results
for a North Sea environment. The results show that the model correctly predicts whether or not sand banks occur for two thirds
of the North Sea area. 相似文献