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981.
Darren A. Bennetts John A. Webb Malcolm McCaskill Reto Zollinger 《Hydrogeology Journal》2007,15(6):1197-1210
Detailed study of a localised saline discharge zone in southeastern Australia shows that the salinisation is mostly due to
the shallow water table (<1–2 m from the surface). Direct evaporation, particularly in summer, leads to extremely high soil–water
salinities at the surface, even though the underlying groundwater is moderately fresh. Groundwater discharge is localised
at a break of slope, where the water table intersects the surface, and where the transition from permeable sands to clay-rich
sediments inhibits lateral groundwater flow. Higher salt concentrations build up in the clays because of the long residence
times during which soil-waters are exposed to evapotranspiration and the reduced potential for salts to be flushed from the
sediments. As a result the saline discharge area does not correspond to the part of the site with the largest salt store.
Results of the study demonstrate that for dryland salinisation to occur, the groundwater beneath the discharge zone need not
be saline, and the presence of a large salt store does not necessarily lead to problems of dryland salinisation if, as in
the clay-rich sediments at the site, the salt lies below the pasture root zone. Furthermore, mobilisation of salt stores within
low permeability sediments by rising groundwater may be minor.
Résumé L’étude détaillée d’une zone localisée d’émergence salée au sud-est de l’Australie montre que la salinisation est surtout due à la nappe phréatique (<1–2 m sous la surface). L’évaporation directe, particulièrement durant l’été, conduit à des salinités de l’eau du sol extrêmement élevées à la surface, même si l’eau souterraine sous-jacente est modérément douce. L’émergence de l’eau souterraine est localisée à la rupture de pente, là où la nappe phréatique rencontre la surface du sol et où la transition entre sables perméables et sédiments riches en argiles inhibe les écoulements d’eau souterraine latéraux. Les plus fortes concentrations en sel s’accumulent dans les argiles du fait de temps de résidence élevés, durant lesquels les eaux du sol sont exposées à l’évapotranspiration et à un lessivage réduit des sédiments. Il en résulte que l’aire d’émergence des eaux salées ne correspond pas à la partie du site rencontrant la réserve de sel la plus importante. Les résultats de l’étude démontrent que pour que la salinisation d’une zone aride devienne effective, l’eau souterraine sous la zone ne doit pas être forcément salée, et la présence d’une zone étendue de réserve de sel ne conduit pas forcément à des problèmes de salinisation de zones arides si, comme dans les sédiments argileux du site, le sel ne repose pas sous la zone de pature. De plus, la mobilisation des réserves de sel dans les sédiments peu perméables par la montée du niveau de l’eau souterraine devrait être mineure.
Resumen El estudio detallado de una zona local de descarga salina en el sudeste de Australia, muestra que la salinización es principalmente debido a un nivel freático poco profundo (<1–2 m de la superficie). La evaporación Directa, particularmente en verano, conlleva a salinidades de suelo-agua sumamente altas en la superficie, aunque el agua subterránea subyacente es moderadamente dulce. La descarga de Agua subterránea se localiza en una interrupción de la ladera, dónde el nivel freático intercepta la superficie, y donde la transición de las arenas permeables a los sedimentos ricos en arcilla inhibe el flujo lateral del agua subterránea. Las concentraciones de sal más altas se forman en las arcillas debido a los tiempos de residencia largos durante los cuales se exponen el conjunto suelo-agua a la evapotranspiración y también por el potencial reducido para las sales de ser expulsadas de los sedimentos. Como resultado el área de la descarga salina no corresponde a la parte del sitio con el contenido de sal más grande. Los resultados del estudio demuestran que para que ocurra la salinización en terrenos secos, el agua subterránea bajo la de zona de descarga no necesita ser salina, y que la presencia de un almacenamiento de sal grande, no necesariamente lleva a los problemas de salinización en terrenos secos si, como en los sedimentos ricos en arcilla del sitio, la sal yace debajo de la zona de raíz de la pastura. Además, la movilidad de depósitos de sal dentro de los sedimentos de permeabilidad baja pueden ser menores, por causa del agua subterránea ascendente.相似文献
982.
983.
984.
Peter M. Downes 《Resource Geology》2007,57(1):1-23
Felsic volcanic units of the Early Devonian Bindook Volcanic Complex host the Yerranderie epithermal silver–gold–lead district 94 km west–southwest of Sydney. Mineralization in the district forms part of a fault‐controlled, intermediate sulfidation, epithermal silver–gold–base metal vein system that has significant mineral and alteration zonation. Stage 1 of the mineral paragenesis in the veins developed quartz and carbonate with early pyrite, whereas stage 2 is a crustiform banded quartz–pyrite–arsenopyrite assemblage. Stage 3, the main stage of sulfide deposition, comprises early sphalerite, followed by a tetrahedrite–tennantite–gold assemblage, then a galena–chalcopyrite–native silver–pyrite assemblage, and finally a pyrargyrite–polybasite–pearceite assemblage. Stage 4 involves the deposition of quartz veins with minor (late) pyrite and stage 5 is characterized by siderite that infilled remaining voids. Mineral zonation occurs along the Yerranderie Fault, with bornite being restricted to the Colon Peaks–Silver Peak mine area, whereas arsenopyrite, which is present in both the Colon Peaks–Silver Peak and Wollondilly mine areas, is absent in other lodes along the Yerranderie Fault. The Yerranderie Fault, which hosts the major lodes, is surrounded by a zoned alteration system. With increasing proximity to the fault the intensity of alteration increases and the alteration assemblage changes from an outer quartz–muscovite–illite–(ankerite) assemblage to a quartz–illite–(pyrite–carbonate) assemblage within meters of the fault. 40Ar/39Ar dating of muscovite from the alteration zone gave a 372.1 ± 1.9 Ma (Late Devonian) age, which is interpreted to be the timing of the quartz–sulfide vein formation. Sulfur isotope values for sulfides range from 0.1 to 6.2‰ with one outlier of ?5.6 δ34S‰. The results indicate that the initial ore‐forming fluids were reduced, and that sulfur was probably sourced from a magmatic reservoir, either as a direct magmatic contribution or indirectly through dissolution and recycling of sulfur from the host volcanic sequence. The sulfur isotope data suggest the system is isotopically zoned. 相似文献
985.
Rock relationships in the Mogok metamorphic belt, Tatkon to Mandalay, central Myanmar 总被引:5,自引:0,他引:5
A.H.G. Mitchell Myint Thein Htay Kyaw Min Htun Myint Naing Win Thura Oo Tin Hlaing 《Journal of Asian Earth Sciences》2007,29(5-6):891-910
The Mogok metamorphic belt (MMB), over 1450 km long and up to 40 km wide, consists of regionally metamorphosed rocks including kyanite and sillimanite schists and granites lying along the Western margin of the Shan Plateau in central Myanmar and continuing northwards to the eastern Himalayan syntaxis. Exposures in quarries allow correlation of Palaeozoic meta-sedimentary, early Mesozoic meta-igneous and late Mesozoic intrusive rocks within a 230 km long northerly-trending segment of the MMB, from Tatkon to Kyanigan north of Mandalay, and with the Mogok gemstone district 100 km to the northeast. Relationships among the metamorphic and intrusive rocks, with sparse published radiometric age controls, indicate at least two metamorphic events, one before and one after the intrusion of Late Jurassic to early Cretaceous calc-alkaline rocks. These relationships can be explained by either of two possible tectonic histories. One, constrained by correlation of mid-Permian limestones across Myanmar, requires early Permian and early Jurassic regional metamorphic events, prior to an early Tertiary metamorphism, in the western part of but within a Shan-Thai – western Myanmar block. The second, not compatible with a single laterally continuous Permian limestone, requires pre-Upper Jurassic regional metamorphism and orogenic gold mineralization in the Mergui Group and western Myanmar, early Cretaceous collision of an east-facing Mergui-western Myanmar island arc with the Shan Plateau, and early Tertiary metamorphism in the MMB related to reversal in tectonic polarity following the arc-Plateau collision. 相似文献
986.
Structural and geological characteristics of a “seismic gap” in the central part of the Kuril Island Arc 总被引:1,自引:0,他引:1
R. G. Kulinich B. Yu. Karp B. V. Baranov E. P. Lelikov V. N. Karnaukh M. G. Valitov S. M. Nikolaev T. N. Kolpashchnikova I. B. Tsoi 《Russian Journal of Pacific Geology》2007,1(1):3-14
The results of the cruise of R/V Akademik M.A. Lavrentiev conducted by the Pacific Institute of Oceanology, Far East Division of the Russian Academy of Sciences and the Shirshov Institute of Oceanology, Russian Academy of Sciences in August to September 2005 are considered. The aim of the works was to specify the tectonic structure, seismogenic potential, and tsunamigenic hazard of the central segment of the Kuril Island Arc. The complex studies involved single-channel seismic profiling, gravimetry, magnetometry, detailed bathymetry, dredge sampling of sea-floor rocks and sediments, and gas geochemistry. Geophysical and geological data are reported. It was demonstrated that the target area is an active tectonic destruction zone, the zone boundaries were outlined, and the main internal structural and compositional heterogeneities were identified. 相似文献
987.
Effect of brittle fracture on the rheological structure of the lithosphere and its application in the Ordos 总被引:2,自引:0,他引:2
An empirical formula for the fracture strength of the principal rock type in the lithosphere is obtained based on the experimental data from previous studies, in which the effects of the confining pressure, size of the rock sample, temperature, strain rate and the pore pressure are taken into account, the empirical formulae for the effects of them are also presented. By comparing the frictional strength to the fracture strength calculated using the new empirical formula, it is shown that frictional sliding is dominant in the upper crust but brittle fracture is dominant in the lower part of the crust and the lithosphere beneath the crust. Therefore the fracture mechanism must be taken into account in the study of the rheological structure of the lithosphere. The empirical formula for the fracture strength is applied to study the rheological structure of the lithosphere in the Ordos block. Brittle regime in the rheological structure can be divided into two sub-regions, in one of which brittle fracture and in the other frictional sliding are dominant, respectively, unlike previous conventional studies in which frictional sliding is assumed to be the only factor; the magnitude of the rheological strength of the lithosphere calculated by the empirical formula is also lower than that obtained in previous conventional studies. 相似文献
988.
Clive E. Willman 《Mineralium Deposita》2007,42(5):449-463
The Bendigo and Castlemaine goldfields are classic examples of structurally controlled orogenic gold deposits in the Bendigo
Zone of central Victoria, SE Australia. Detailed mapping and biostratigraphic interpretation has led to a better understanding
of the regional structural controls of this type of gold-quartz mineralisation. Mineralised quartz veins are hosted by the
Castlemaine Group, an Early-to-Middle Ordovician turbidite succession at least 3,000 m thick. Gold deposits are controlled
by low-displacement faults that are clustered into several belts (the goldfields) indicating a regional structural control.
The timing of mineralisation overlapped with that of the major period of deformation including folding, cleavage development
and regional faulting. The Bendigo and Castlemaine goldfields are located in an area termed the Whitelaw thrust sheet bounded
by two unmineralised, high-displacement, regional-scale faults. Mapping has revealed an interrelationship between the regional-scale
faults, regional structural style and goldfield location. The goldfields lie immediately west of the boundary between the
upper and lower portions of the thrust sheet and are characterised by symmetric folds with sub-horizontal to synclinal enveloping
surfaces, relatively low co-axial strains and moderate cleavage development. The non-gold-bearing areas immediately east of
each goldfield correspond with the lower part of the Whitelaw thrust sheet and are characterised by higher non-coaxial strains,
stronger cleavage and folds with wide west-dipping limbs giving rise to easterly vergent sections and steeply west-dipping
enveloping surfaces. That mineralisation was an integral part of the thin-skinned style of deformation in the central Bendigo
Zone is indicated by timing relationships and the interrelationship between local-scale mineralised structures and regional-scale
features such as large-displacement unmineralised faults, regional variations in fold style and overall thrust sheet geometry.
The work supports previous models that suggest mineralised fluids were focussed along a linked system of deep-seated faults.
The primary conduits may have been major regional-scale ‘intrazone’ faults, which are inferred to sole into detachments near
the base of the Castlemaine Group. It is proposed that these structures linked with minor intrazone faults and then with networks
of low-displacement mineralised faults that were strongly controlled by folds. The location of minor intrazone faults was
probably controlled by internal thrust sheet geometry. The distribution of gold deposits and of gold production suggests that
maximum fluid flow was concentrated along the eastern margins of networks of low-displacement faults. 相似文献
989.
Elemental and Sm-Nd isotopic geochemistry on detrital sedimentary rocks in the Ganzi-Songpan block and Longmen Mountains 总被引:1,自引:0,他引:1
Chen Yuelong Liu Fei Zhang Hongfei Nie Lanshi Jiang Liting 《Frontiers of Earth Science》2007,1(1):60-68
Systematic results of major and trace element geochemistry and Sm-Nd isotopic geochemistry on detrital sedimentary rocks of
Precambrian to Triassic in the Ganzi-Songpan block and Longmen Mountains are presented. The rocks are classified into greywackes
or feldspar sandstones, grains of which are the mixtures of mafic rocks, felsic rocks, and quartz+calcite. Total rare earth
elements (REE) contents of the rocks increase gradually and negative Eu anomalies become more obvious from Precambrian to
Triassic, which may indicate intensifying crustal anatexis. Tectonic setting was stable during the Late Paleozoic, therefore
there are obvious negative Ce anomalies. Nd model ages are between 1.6 Ga and 2.4 Ga, which are very similar to those of the
Yangtze craton, South Qinling and North Qinling belts and quite different from those of the North China craton. Therefore,
provenance of the sedimentary rocks in the Ganzi-Songpan block and Longmen Mountains was the Yangtze craton and/or the Qinling
orogen, which evolved on the basis of the Yangtze craton. The correlation between provenances and tectonostratigraphic strata
of the western Yangtze craton shows that the source materials should be primarily from Neoproterozoic. Secondary sources were
Archean and Paleoproterozoic strata. Triassic clastic sedimentary rocks contain Late Paleozoic mantle-derived materials, represented
by the Emeishan Permian flood basalts. Spatial distribution of initial Nd isotopic compositions indicates that denudating
areas were in the east and the north and depositing areas of deep water were in the west and the south for the Ganzi-Songpan
basin during Triassic.
Translated from Geology in China, 2006, 33(1): 109–118 [译自: 中国地质] 相似文献
990.
The active role of fluids in the formation of ore-localizing structures expressed in dilatancy and hydraulic fracturing is suggested from the results of detailed comprehensive study of the Tokur gold deposit in the Upper Amur region, Russia, and the surveying of the published data on fluid pressure that characterizes formation of other hydrothermal deposits different in depth, morphology of orebodies, and their localization with respect to rocks of various permeability. Development conditions and possible implications of hydraulic fracturing for ore formation are considered. Indications of involvement of this mechanism in the orebody formation at the Pokrovsky, Berezovsky, and Nezhdaninsky gold deposits in Russia and the Bendigo deposit in Australia are discussed. 相似文献