排序方式: 共有56条查询结果,搜索用时 11 毫秒
21.
The giant Imiter silver deposit: Neoproterozoic epithermal mineralization in the Anti-Atlas, Morocco
Alain Cheilletz Gilles Levresse Dominique Gasquet Moulay Azizi-Samir Rachid Zyadi Douglas A. Archibald Edward Farrar 《Mineralium Deposita》2002,37(8):772-781
The world-class Imiter silver deposit, in the Anti-Atlas Mountains of Morocco, is a Neoproterozoic epithermal vein deposit genetically associated with a felsic volcanic event, and formed within a regional extensional tectonic regime. Rhyolitic volcanism related to ore formation has been dated at 550Dž Ma by ion-probe U/Pb on zircons. The economic silver mineralization is superimposed on an older, discrete base-metal assemblage associated with calc-alkaline granodioritic magmatism. The magmatism is dated at 572LJ Ma by ion-probe U/Pb dating on zircons, and by 40Ar/39Ar dating on hydrothermal muscovites. In the Anti-Atlas Mountains, the Precambrian-Cambrian transition appears as an important period for the formation of major, productive precious-metal deposits associated with volcanic events and extensional tectonics. The Imiter silver deposit constitutes a Precambrian analogue to modern epithermal deposits. 相似文献
22.
Rachid Zayane Abderrahim Essaifi René C Maury Alain Piqué Edgard Laville Mohamed Bouabdelli 《Comptes Rendus Geoscience》2002,334(2):97-104
The Middle Jurassic plutonism of the Central High Atlas (Morocco) was emplaced in N45° trending anticlinal ridges. It is characterised by various petrographic facies including mafic rocks (troctolites), intermediate rocks (diorites, monzodiorites), and evolved rocks (syenites), together with heterogeneous facies resulting from mixing between acidic and the intermediate magmas. Mineralogical and chemical data show (i) the transitional character of the Jurassic magmatic series of the Central High Atlas and (ii) the implication of continental crust as a contaminant during fractional crystallization. To cite this article: R. Zayane et al., C. R. Geoscience 334 (2002) 97–104. 相似文献
23.
24.
Moussa Dia Rachid Zentar Nor-edine Abriak Ange Nzihou Guy Depelsenaire Alain Germeau 《国际泥沙研究》2019,34(5):486-495
Dredging operations produce considerable quantities of materials, to be managed and this opens an opportunity for valorization in civil engineering. However, the contamination of the dredged sediments has become a major problem to solve. The major contaminants are heavy metals and organic compounds. This study focuses on the use of phosphoric acid (H3PO4) to stabilize heavy metals from sediments and destroy organic matter by calcination at 650 °C with a goal of using sediments in roadworks. Several studies have been conducted in this eld. The stabilized materials obtained have been used in civil engineering. The main purpose of this work is to discuss the environmental behavior of marine sediment treated by phosphatation and calcination. Two types of phosphoric acids were used. The pH dependence leaching test has been used as the basic characterization to evaluate the effect of the type of phosphoric acid on the metals behavior in a valorization scenario. The standard leaching test and the Toxicity Characteristic Leaching Procedure (TCLP) were conducted as compliance tests. In regards of the obtained results, the environmental assessment has also shown a reduction in the availability of targeted heavy metals in alkaline environment whatever the type of acid used for treatment. This opens opportunities for co-valorization. 相似文献
25.
After the last glacial maximum (LGM; 18,000 years. bp) plants migrated onto areas made free by the retreating ice sheets. Pollen records document changes in ecosystems and are an important proxy for the reconstruction of climate parameters. Here, we quantify changes in January temperature (Tjan) over the past 14,000 years using 216 European pollen records. We used functional principal component analysis to evaluate the patterns of change over this period of time. Between 14 and 12 ka, Tjan records show comparable changes across Europe. After this time period, northwest and southeast Europe experienced differing trends in Tjan, trends that became pronounced by 9 ka. The spatial gradient of Tjan records is well-correlated with the overall change from steppe to increasingly diverse forest ecosystems. In particular, differential incorporation of conifer vs. deciduous species into forest ecosystems, based on differential colonization out of refugia, likely resulted in albedo gradients over Europe that affected regional Tjan. 相似文献
26.
Clemente Irigaray Fernndez Toms Fernndez Del Castillo Rachid El Hamdouni Jos Chacn Montero 《地球表面变化过程与地形》1999,24(6):537-544
A methodology is proposed for mapping susceptibility to landsliding and validating the results. Heavy rains in late 1996 and early 1997 led to a large number of landslids in the Rute sector (Córdoba, Southern Spain), where landslide susceptibility mapping had previously been carried out using a ‘matrix’ method developed with a Geographical Information System (GIS). Analysis of the distribution of the new landslides (or reactivated earlier ones) enabled the methodology to be validated by calculating association coefficients and determining the closeness of the match between subsequent field evidence and the previously defined susceptibility levels. From the data obtained, it can be concluded that the susceptibility mapping effectively explained the spatial distribution of landslides, thus providing valuable information on stability conditions over a widespread area. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
27.
The Lower Cretaceous series, in the Western Saharan Atlas, shows important detritic deposits, particularly continental to nearshore red sandstones. Lithologically, it is an alternation of red clay and sandstone intercalated with some carbonate levels at the lower part of the series. These series can be subdivided into three lithostratigraphic units corresponding to Valanginian-Hauterivian, Barremian, and Aptian-Lower Albian. The lithostratigraphic analysis used to reconstitute the important subsidence in the Lower Cretaceous period, particularly in the Lower Albian. The Lower Cretaceous series could be considered as a geodynamic model characterized by individualization of independent depocenter filled with different detritic material and separated by Jurassic structures. This new situation appeared in the beginning of Lower Cretaceous (Valanginian-Hauterivian), following the Neocimmeriean tectonic phase, and it is increasing in the Early Albian, where differences of thicknesses are more important and allow the distinction of the depocenter. The biostratigraphic analysis of the clay samples yielded some foraminifera and palynomorphs. Benthic and planktonic foraminifera confirmed the Hauterivian age of the basal formation, also, two dominant forms of palynomorphs were recognized; according to the large stratigraphic distribution of the identified forms, it can be attributed to the Lower Cretaceous. 相似文献
28.
Pavel E. Tarasov Rachid Cheddadi Joel Guiot Sytze Bottema Odile Peyron Jordina Belmonte Vittoria Ruiz-Sanchez Fatima Saadi Simon Brewer 《第四纪科学杂志》1998,13(4):335-344
An objective method for the assignment of pollen spectra to appropriate biomes has been published recently. The aim of this paper is to improve the distinction between warm and cool steppes, thus refining vegetation and climate reconstruction, particularly during the Last Glacial Maximum. A set of modern pollen spectra from the Mediterranean and Kazakhstan regions, dominated today by open vegetation types, has been analysed statistically in order to relate pollen taxa abundances to warm and cool grass/shrub plant functional types (PFTs). A statistical test using modern pollen data shows that the method is able to distinguish between cool and warm steppe biomes with a high degree of confidence. The method has been applied to two fossil pollen records. The results of this exercise showed that cool steppe dominated in central Greece between 18 000 and 13 000 yr BP, while in western Iran the vegetation was at the boundary between cool and warm steppes. These vegetation types were replaced by warm mixed forest in Greece and warm steppe in Iran after that time span. © 1998 John Wiley & Sons, Ltd. 相似文献
29.
Environmental factors structuring fish composition and assemblages in a small macrotidal estuary (eastern English Channel) 总被引:3,自引:1,他引:3
The fish assemblage structure was analyzed along an estuarine gradient of a small macrotidal estuary (the Canche, France). Fishes were collected every two months between May 2006 and July 2007 from 12 sampling stations using a 1.5-m beam trawl with a 5 mm mesh size in the cod end. To complement this information, sampling was also performed using 15-m fyke nets (8 mm mesh size in the cod end). For each sample, abiotic (temperature, salinity, pH, oxygen, turbidity, river flow, wind speed and depth) and biotic (macro crustacean species abundances) were recorded. Throughout the study, 28 fish species belonging to 20 families were collected. Fish catches were dominated by juveniles, especially Young-Of-the-Year (YOY) for the majority of the species. According to the Index of Relative Importance (IRI), common goby Pomatoschistus microps, flounder Platichtys flesus, sprat Sprattus sprattus, sea-bass Dicentrarchus labrax and plaice Pleuronectes platessa were the most abundant species, together accounting for 99.2% of the total IRI. Estuarine residents (ER = 66.2%) and marine juvenile migrants species (MJ = 31.4%) were the most important ecological guilds. The structure of the fish assemblage and its relationship to environmental variables was examined using multivariate techniques. Cluster and non-metric multidimensional scaling (nMDS) analysis defined six distinct groups in the Canche estuary, which are discriminated by specific species (SIMPER). Spatio-temporal variations in fish assemblage structure reflect the density peaks of the most abundant species. Spearman rank correlations and canonical correspondence analysis (CCA) showed that among the ten environmental variables examined, temperature, salinity and Crangon crangon (a potential predator for YOY fish or prey for older ones) are the three most important factors influencing fish species richness and abundances. Our observations reinforce the idea that certain fish species may have different life history styles in different geographic areas. The present study highlights the necessity of a better knowledge of the connectivity between estuaries and adjacent marine areas. The Canche constitutes an important ecosystem for fishes and as it is subject to little anthropogenic disturbance; its ichthyofauna can be viewed as a reference or normal assemblage for small temperate macrotidal estuaries. 相似文献
30.
Delphine Vantelon Rachid Belkhou Isabelle Bihannic Laurent J. Michot Emmanuelle Montargès-Pelletier Jean-Louis Robert 《Physics and Chemistry of Minerals》2009,36(10):593-602
X-PEEM images and XPS were collected on isolated layers of three synthetic swelling clays, one hectorite and two saponites
with various charge, recording the Si(2p), Al(2p) and Mg(2p) core level spectra from the clay sheets. Spectra were fitted
to determine the different components of the core levels. Due to their large full width at half maximum, Si XPS spectra were
fitted using two to three doublets. It appears that, for a given clay mineral, Si, Al and Mg binding energies (BE) were constant,
for all the observed layers. However, variations of the Si BE were observed depending on the nature of the mineral investigated.
The various components obtained from the fit of Si spectra could be assigned to different substitution rates; binding energy
shifting to lower values with substitution increase in the layer. Furthermore, variations in Si BE according to charge location
were assigned to the influence of exchangeable cation. 相似文献