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61.
62.
华北陆块北缘西段狼山—白云鄂博裂谷系中元古代形成的大型多金属矿床 ,是我国重要的铁、稀土、铜、铅、锌矿产地。矿床为产于沉积岩中热水沉积型多金属矿床。其形成受地层、构造控制 ,与裂谷作用有着密切的关系。裂谷系内外分支的地质环境及矿床矿种不同 ,导致矿床特征及成因具有一定差异。白云鄂博铁、稀土矿床为钾、钠、磷等含量较高的热水沉积型 ,霍各乞铜、铅、锌多金属矿床为与中基性火山岩有关的过渡热水沉积型 ,东升庙和甲生盘等铅、锌多金属矿床为热水沉积型。文中将对上述矿床分别进行论述。 相似文献
63.
Geochronology of Gold Deposits and Its Implication for Metallogenesis in the Fengxian-Lixian Area, Qinling Orogenic Belt, China 总被引:2,自引:0,他引:2
Abstract: A series of super large‐scale and large‐scale Pb and Zn, and Au deposits are distributed in the Qinling orogenic belt, China. Gold deposits were generally ascribed to Carlin‐type originated from circular meteoric water. Visible and coarse‐grained gold (up to over 3mm in grain size) was recently identified in some gold deposits in the Fengxian‐Lixian area, Qinling. Au‐bearing quartz lodes related to magmatism were discovered in the Xiaogouli gold deposit. Two types of Au‐bearing quartz veins, i.e., NW‐trending quartz veins and NE‐trending quartz veins cutting strata are widely present in the Baguamiao gold deposit. Both are spatially associated with each other. The former is generally snake–like, S‐shape or zigzag, which was resulted from plastic deformation by ductile shearing, being generally cut by the latter. The latter is generally linear with widely developed bleaching alteration zones in its adjacent wall rocks, which symbolizes the superimposition of brittle deformation and filling and metasomatism of magmatic hydrothermal solution in ductile shear zones after uplifting of the shear zones near the surface. The NW‐trending quartz veins contain Au of lower than 3ppm. The NE‐trending quartz veins contain Au of more than 3 ppm, so that NE‐trending quartz veins and the adjoining altered rocks are important ores. The NW‐trending Au–bearing quartz vein was dated as 210.61.26 to 232.581.59 Ma by 40Ar–39Ar method, i.e., late Indosinian epoch (Triassic). The NE‐trending Au–bearing quartz vein was dated as 131.910.89 to 197.451.13 Ma by 40Ar–39Ar method, i.e., Yanshanian epoch (Jurassic). The 40Ar–39Ar age of the NW‐trending Au–bearing quartz veins represents the age of the ductile shear formation. The isotope data of the NE‐trending quartz veins indicate that gold mineralization was closely related to Indosinian and Yanshanian granite intrusives not only in time and space, but also in origin. 相似文献
64.
Ahmed Barakat Christian Marignac Marie-Christine Boiron Mohamed Bouabdelli 《Comptes Rendus Geoscience》2002,334(1):35-41
The gold showings at Bleida are hosted in Late Pan-African N50–80 °E quartz–hematite–chlorite 1 tension lenses that are related to the activity of major sinistral sub-east–west thrusts. Ores result from three superimposed stages of fluid migration. Gold occurs in microcracks offsetting the earlier minerals. Fluids evolved from COHN compositions with a saline component to boiling aqueous fluids. Pressure and temperature decreased from 50 MPa and 300 °C to less than 4 MPa and 150 °C. Thus, the gold showings at Bleida were formed in a typical geothermal (epithermal) setting, likely controlled by the Late Pan-African magmatism. To cite this article: A. Barakat et al., C. R. Geoscience 334 (2002) 35–41 相似文献
65.
Non-linear time domain site response analysis is widely used in evaluating local soil effects on propagated ground motion. This approach has generally provided good estimates of field behavior at longer periods but has shortcomings at relatively shorter periods. Viscous damping is commonly employed in the equation of motion to capture damping at very small strains and employs an approximation of Rayleigh damping using the first natural mode only. This paper introduces a new formulation for the viscous damping using the full Rayleigh damping. The new formulation represents more accurately wave propagation for soil columns greater than 50 m thick and improves non-linear site response analysis at shorter periods. The proposed formulation allows the use of frequency dependent viscous damping. Several examples, including a field case history at Treasure Island, California, demonstrate the significant improvement in computed surface response using the new formulation. 相似文献
66.
This paper deals with ground-hugging, gas–pyroclast currents from explosive volcanic eruptions and their deposits. Key field observations and laboratory determinations are proposed to relate specific deposit types with flow regimes and particle concentration in the transport and depositional systems. Three relevant flow scenarios and corresponding deposit types have been recognized from a survey of pyroclastic successions of the Vulsini Volcanic District (central Italy): (1) dilute, turbulent, pyroclastic currents producing normally or multiply graded beds by direct suspension sedimentation; (2) concentrated bedload regions beneath suspension currents, depositing inversely graded beds by traction carpet sedimentation; (3) self-sustained, high particle concentration, laminar, mass flows developing massive, poorly sorted bodies, with opposite grading of coarse lithic and pumice clasts, overlying fine-grained, inversely graded, basal layers. Main distinguishing criteria include the occurrence and pattern of clast grading, clast–thickness relationships, grain size, ash matrix componentry and pyroclast size–density relationships. Downcurrent and temporal transitions among identified flow scenarios are likely to occur for changing energy conditions and gas–pyroclast ratio both on regional and local scales. The nature and efficiency of magma fragmentation, volatile content, conduit geometry (which determine the characteristics of the erupted mixture and possible lateral blast component at the vent), and the angle of incidence of the column collapse, are suggested as the main factors controlling the generation of one type over the other at flow inception. Dilute, fine-grained, overpressured eruption clouds are thought to favor the formation of low particle concentration turbulent currents. Column collapse over slightly inclined volcano slopes, causing a high degree of compression of the collapsing mixture and of gas expulsion, would favor the generation of high particle concentration pyroclastic currents. 相似文献
67.
Two end-member types of pyroclastic density current are commonly recognized: pyroclastic surges are dilute currents in which particles are carried in turbulent suspension and pyroclastic flows are highly concentrated flows. We provide scaling relations that unify these end-members and derive a segregation mechanism into basal concentrated flow and overriding dilute cloud based on the Stokes number (ST), the stability factor (ΣT) and the dense-dilute condition (DD). We recognize five types of particle behaviors within a fluid eddy as a function of ST and ΣT: (1) particles sediment from the eddy, (2) particles are preferentially settled out during the downward motion of the eddy, but can be carried during its upward motion, (3) particles concentrate on the periphery of the eddy, (4) particles settling can be delayed or ‘fast-tracked’ as a function of the eddy spatial distribution, and (5) particles remain homogeneously distributed within the eddy. We extend these concepts to a fully turbulent flow by using a prototype of kinetic energy distribution within a full eddy spectrum and demonstrate that the presence of different particle sizes leads to the density stratification of the current. This stratification may favor particle interactions in the basal part of the flow and DD determines whether the flow is dense or dilute. Using only intrinsic characteristics of the current, our model explains the discontinuous features between pyroclastic flows and surges while conserving the concept of a continuous spectrum of density currents. 相似文献
68.
The Hercynian basement of the Iberian Peninsula was uplifted by the Alpine orogeny during the Tertiary. It gave rise to a set of block mountains and tectonic grabens, one of which is the Ciudad Rodrigo Basin. It is located in Western Spain and forms a westward extension of the great Tertiary Duero Basin. The sediments filling this graben are of continental origin, their ages ranging from Palaeogene to Quaternary. Morphologically, the southern part of this basin forms a set of piedmont surfaces (the “Raña” surfaces) appearing above the terrace system of the present rivers. This paper examines the weathering processes developed over these old piedmont surfaces using micromorphological, XR diffraction, scanning electron microscopy (SEM) and microporosity techniques.The Raña sediments are rich in quartzite pebbles and gravel within a clayey matrix. Once deposited, these materials underwent important in situ weathering processes under somewhat hydromorphic conditions, of which hydrolysis, ferrolysis and xerolysis were the most important. All these processes gave rise to: (1) transformation of most of the clasts of shists and slates into a matrix causing the destruction of the original sedimentary structures and a relative concentration of the resistant lithologies (quartzites and quartz); (2) important changes in the clay fraction, leading to a predominance of kaolinite in the upper levels of the profiles, (3) release of elements from primary minerals, Fe being one of the most important, and (4) redistribution of matter, mainly clay and Fe oxyhydroxides, within the profiles.The coexistence of seasonal periods with pF higher than 4.2 repeated over a long time, together with poor internal and external drainage conditions, are the cause of the special features displayed by Raña deposits. 相似文献
69.
One of the main factors that affects the performance of MLP neural networks trained using the backpropagation algorithm in mineral-potential mapping isthe paucity of deposit relative to barren training patterns. To overcome this problem, random noise is added to the original training patterns in order to create additional synthetic deposit training data. Experiments on the effect of the number of deposits available for training in the Kalgoorlie Terrane orogenic gold province show that both the classification performance of a trained network and the quality of the resultant prospectivity map increasesignificantly with increased numbers of deposit patterns. Experiments are conducted to determine the optimum amount of noise using both uniform and normally distributed random noise. Through the addition of noise to the original deposit training data, the number of deposit training patterns is increased from approximately 50 to 1000. The percentage of correct classifications significantly improves for the independent test set as well as for deposit patterns in the test set. For example, using ±40% uniform random noise, the test-set classification performance increases from 67.9% and 68.0% to 72.8% and 77.1% (for test-set overall and test-set deposit patterns, respectively). Indices for the quality of the resultant prospectivity map, (i.e. D/A, D × (D/A), where D is the percentage of deposits and A is the percentage of the total area for the highest prospectivity map-class, and area under an ROC curve) also increase from 8.2, 105, 0.79 to 17.9, 226, 0.87, respectively. Increasing the size of the training-stop data set results in a further increase in classification performance to 73.5%, 77.4%, 14.7, 296, 0.87 for test-set overall and test-set deposit patterns, D/A, D × (D/A), and area under the ROC curve, respectively. 相似文献
70.
Use of GIS layers, in which the cell values represent fuzzy membership variables, is an effective method of combining subjective geological knowledge with empirical data in a neural network approach to mineral-prospectivity mapping. In this study, multilayer perceptron (MLP), neural networks are used to combine up to 17 regional exploration variables to predict the potential for orogenic gold deposits in the form of prospectivity maps in the Archean Kalgoorlie Terrane of Western Australia. Two types of fuzzy membership layers are used. In the first type of layer, the statistical relationships between known gold deposits and variables in the GIS thematic layer are used to determine fuzzy membership values. For example, GIS layers depicting solid geology and rock-type combinations of categorical data at the nearest lithological boundary for each cell are converted to fuzzy membership layers representing favorable lithologies and favorable lithological boundaries, respectively. This type of fuzzy-membership input is a useful alternative to the 1-of-N coding used for categorical inputs, particularly if there are a large number of classes. Rheological contrast at lithological boundaries is modeled using a second type of fuzzy membership layer, in which the assignment of fuzzy membership value, although based on geological field data, is subjective. The methods used here could be applied to a large range of subjective data (e.g., favorability of tectonic environment, host stratigraphy, or reactivation along major faults) currently used in regional exploration programs, but which normally would not be included as inputs in an empirical neural network approach. 相似文献