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21.
Francisco González-Sánchez Antoni Camprubí Eduardo González-Partida Rafael Puente-Solís Carles Canet Elena Centeno-García Viorel Atudorei 《Mineralium Deposita》2009,44(3):343-361
Northeastern Mexico hosts numerous epigenetic stratabound carbonate-hosted low-temperature hydrothermal deposits of celestine,
fluorite, barite and zinc-lead, which formed by replacement of Mesozoic evaporites or carbonate rocks. Such deposits can be
permissively catalogued as Mississippi Valley-type (MVT) deposits. The deposits studied in the state of Coahuila are associated
with granitic and metasedimentary basement highs (horsts) marginal or central to the Mesozoic Sabinas Basin. These horsts
controlled the stratigraphy of the Mesozoic basins and subsequently influenced the Laramide structural pattern. The Sabinas
Basin consists of ~6,000-m-thick Jurassic to Cretaceous siliciclastic, carbonate and evaporitic series. The MVT deposits are
mostly in Barremian and in Aptian-Albian to Cenomanian formations and likely formed from basinal brines that were mobilized
during the Laramide orogeny, although earlier diagenetic replacement of evaporite layers (barite and celestine deposits) and
lining of paleokarstic cavities in reef carbonates (Zn–Pb deposits) is observed. Fluid inclusion microthermometry and isotopic
studies suggest ore formation due to mixing of basinal brines and meteoric water. Homogenization temperatures of fluid inclusions
range from 45°C to 210°C; salinities range from 0 to 26 wt.% NaCl equiv., and some inclusions contain hydrocarbons or bitumen.
Sulfur isotope data suggest that most of the sulfur in barite and celestine is derived from Barremian to Cenomanian evaporites.
Regional geology and a compilation of metallogenic features define the new MVT province of northeastern Mexico, which comprises
most of the state of Coahuila and portions of the neighboring states of Nuevo León, Durango and, perhaps extends into Zacatecas
and southern Texas. This province exhibits a regional metal zonation, with celestine deposits to the south, fluorite deposits
to the north and barite and Zn–Pb deposits mostly in the central part. 相似文献
22.
Patricia Vázquez Ana Luque Francisco J. Alonso Carlota M. Grossi 《Environmental Earth Sciences》2013,69(4):1237-1248
This study assesses the changes on the surface of crystalline stones due to salt crystallisation. Efflorescence was forced to grow on the surface of granite and marbles through 60 cycles of salt crystallisation with sodium sulphate. Changes on surface roughness, gloss and colour were measured every 15 cycles and the specimens were examined with naked eye and SEM. Sodium sulphate produces damage which depends on mineral composition. Results show that granites experience a mechanical decay with an increase in roughness. Peaks of mica can be observed on the surface and cracks widen and grow deeper. Colour and gloss do not show any significant change, although gloss decreases with an increase in surface roughness. In marbles, the decay is mainly chemical. Surface roughness increases due to dissolution of the calcite. White marbles exhibit yellowing. Gloss decreases during the first cycles—as grain boundaries become more visible—but tends to regain almost its initial value as the number of cycles increases. In this case, gloss does not show any relation with surface roughness. 相似文献
23.
José F. Noguera Lluís Rivero Xavier Font Andrés Navarro Francisco Martínez 《Environmental Geology》2002,41(8):898-905
24.
Javier Fernández-Suárez Fernando Corfu Ricardo Arenas Alberto Marcos José R. Martínez Catalán Florentino García Jacobo Abati Francisco J. Fernández 《Contributions to Mineralogy and Petrology》2002,143(2):236-253
. A isotope dilution thermal ionisation mass spectrometry U-Pb geochronological study was carried out on the high-pressure and high-temperature units (HP-HT units) overlying the oceanic suture in the Allochthonous Complexes of the NW Iberian Variscan Belt. The rocks investigated are seven granulite- to eclogite-facies paragneisses and one leucosome within mafic high-pressure granulites in the Ordenes and Cabo Ortegal Complexes of NW Spain. U-Pb dating of zircon, monazite, titanite and rutile reveal the presence of a pervasive Early Ordovician metamorphic event at ca. 500-480 Ma and a later Early Devonian event at ca. 400-380 Ma. The U-Pb ages, in conjunction with petrological and structural data, indicate that the high-pressure event recorded by these rocks is Early Ordovician in age. Monazite ages in the paragneisses suggest that peak metamorphic conditions were reached at ca. 500-485 Ma. Subsequently, the rock ensemble underwent exhumation accompanied by partial melting and zircon growth at ca. 485-470 Ma. Melting of mafic granulites was coeval with this latter episode as indicated by zircon crystallisation age in the leucosomes dated at ca. 486 Ma. Based on these data and on the general features of magmatism and metamorphic evolution, it is proposed that this process took place at a convergent plate boundary within a peri-Gondwanan oceanic domain. Monazite, titanite and rutile data in some of the samples studied show evidence of a second metamorphic episode that took place between ca. 400 and 380 Ma (with a peak at ca. 390-385 Ma). This Early Devonian event, at variance with the previous one, was not pervasive, but, rather, was localised in areas of intense Variscan tectonothermal reworking. It is claimed that this later metamorphic event was recorded by the U-Pb system in areas where monazite and titanite growth was enhanced by fluid circulation in highly strained rocks (Variscan shear zones). According to previous structural studies and Ar-Ar dating of fabrics, this Early Devonian episode took place as the HP-HT units were deformed and thrusted upon the ophiolitic units in the early stages of the Variscan collision. 相似文献
25.
The equation of motion written down using a metric obtained from the Kerr metric allows us to get both the deflection angle of light ray bending by the gravitational field of the Sun and its corresponding time delay. We reproduce the experimental data of Epstein-Shapiro and also it is in accordance with the theoretical results obtained by Brumberget al. (1990) using another approach to this problem.Presented at the 2nd UN/ESA Workshop, held in Bogotá, Colombia, 9–13 November, 1992. 相似文献
26.
Diagnosis of trace metal contamination in sediments: the example of Ensenada and El Sauzal, two harbors in Baja California, Mexico 总被引:1,自引:0,他引:1
Huerta-Diaz MA Delgadillo-Hinojosa F Hernández-Ayón M Segovia-Zavala JA García-Esquivel Z López-Zárate H Siqueiros-Valencia A Galindo-Bect S 《Marine environmental research》2008,66(3):345-358
Total metal concentrations in sediments from within Ensenada and El Sauzal Harbors are generally higher than at the mouths. Grain-size analyses suggested that this enrichment could be due to the presence of fine-grained sediments in the inner part of the harbors rather than to anthropogenic perturbations. The (Me/Al)(sample) ratios for Pb, Co, Ni and Fe were significantly higher for Ensenada Harbor relative to El Sauzal Harbor, whereas the ratios for Cd, Mn, Zn and Cu were statistically equivalent for both harbors. Calculated enrichment factors [EF(Me)=(Me/Al)(sample)/(Me/Al)(shale)] indicated that the metals showing slight enrichment were those associated with anthropogenic contamination (Pb, Zn), or probably related to primary productivity in the water column (Cd, Co). The levels of most of the metals were not greatly enriched, a consideration that is of the utmost importance when contamination issues are at stake. 相似文献
27.
José De La Cruz-Agüero Francisco Javier García-Rodríguez Víctor Manuel Cota-Gómez Felipe Neri Melo-Barrera Rogelio González-Armas 《Ocean Science Journal》2012,47(2):147-153
Fresh and preserved (type material) specimens of the black ghost chimaera Hydrolagus melanophasma were compared for morphometric characteristics. A molecular comparison was also performed on two mitochondrial gene sequences (12S rRNA and 16S rRNA gene sequences). While significant differences in measurements were found, the differences were not attributable to sexual dimorphism or the quality of the specimens, but to the sample size and the type of statistical tests. The result of the genetic characterization showed that 12S rRNA and 16S rRNA genes represented robust molecular markers that characterized the species. 相似文献
28.
Francisco M. Wallenstein Marlene R. Terra Joana Pombo & Ana I. Neto 《Marine Ecology》2009,30(S1):113-117
Studies on macroalgal communities of the Azores report algal turfs as one of the most conspicuous ecological entities occurring on the rocky shores of these islands. The present study investigates the influence of shore height and substratum on turf composition. Data were analysed using the software PRIMER and results confirmed the previous distinction between calcareous, dominated by articulated coralline algae, and non-calcareous turfs, characterized mainly by small red algae. However, no differences in species composition were found in either type of turf occurring on different substrata or at different shore levels. 相似文献
29.
Robert S. Pomeroy Michael D. Pido John Francisco A. Pontillas Benjamin S. Francisco Alan T. White Eva Marie C. Ponce De Leon Geronimo T. Silvestre 《Marine Policy》2008
The live reef food fish (LRFF) trade has an estimated annual retail value of US$1 billion in the Asia-Pacific region and at least US$30 million in the Philippines. Palawan Province is the trade's center accounting for as much as 55% of the commodity's total export. Concern about the LRFF trade has caused the Palawan Provincial government to take action to regulate the trade. Four key policy options to address the short-term policy objective of developing a sustainable LRFF industry through the reduction of threats associated with destructive fishing and overfishing were proposed. These four policy options and associated analyses were presented to stakeholders at three separate policy consultations. The stakeholders reached a consensus that a regulated LRFF trade was the most sensible option. As a result, the Provincial Government of Palawan took action to enact “The Palawan Live Reef Fish Ordinance of 2006”, which provides for the sustainable and integrated regulation of the LRFF industry. This provincial ordinance will assist in crafting a national policy for the LRFF trade and may also be useful in revising the 1998 Philippine Fisheries Code. The key challenge is how to translate the ordinance provisions into effective on-the-ground actions. 相似文献
30.