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G. Ros A.D. SupanitskyG.A. Medina-Tanco L. del PeralJ.C. D’Olivo M.D. Rodríguez Frías 《Astroparticle Physics》2011,35(3):140-151
A new family of parameters intended for composition studies in cosmic ray surface array detectors is proposed. The application of this technique to different array layout designs has been analyzed. The parameters make exclusive use of surface data combining the information from the total signal at each triggered detector and the array geometry. They are sensitive to the combined effects of the different muon and electromagnetic components on the lateral distribution function of proton and iron initiated showers at any given primary energy. Analytical and numerical studies have been performed in order to assess the reliability, stability and optimization of these parameters. Experimental uncertainties, the underestimation of the muon component in the shower simulation codes, intrinsic fluctuations and reconstruction errors are considered and discussed in a quantitative way. The potential discrimination power of these parameters, under realistic experimental conditions, is compared on a simplified, albeit quantitative way, with that expected from other surface and fluorescence estimators. 相似文献
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The Santa Rita gold deposit (Central Goiás, Brazil) is hosted by Middle to Upper Proterozoic carbonate-pelite sequences of the Paranoá Group that have been metamorphosed in the greenschist facies. The ore is contained in pyrite-bearing quartz-carbonate veins. The mineralization is structurally controlled by WNW-ESE high-angle faults and fractures resulting from the reactivation of older NE-SW lineaments. Pyrite is the sole sulphide and it shows growth zones enriched in Co, Ni and As (up to 4 wt%). Hydrothermal alteration zones are enriched in Co, Ni and As and are characterized by diffuse albitization, carbonatization, silicification and pyritization.A fluid inclusion study on quartz from pyrite-bearing quartz-carbonate veins led to the identification of two fluids: (1) a highly saline CO2---N2-rich aqueous fluid with halite and ± sylvite daughter minerals, and (2) a CO2---N2 rich aqueous fluid with moderate salinity. The two fluid types occur in the same quartz domain and display great variation in the degree of filling and notable dispersion of the microthermometric data. On heating, all the inclusions decrepitate between 200° and 300°C. Raman spectrometry detected high concentrations of N2 in the gas phase, with a
molar ratio between 1 and 19 and a small proportion of CH4 (up to 2 mole %).The simultaneous entrapment of compositionally variable fluids in the system H2O---CO2---N2---NaCl---KCl allow us to propose a mechanism of heterogeneous trapping. The entrapment may result from the mixing of a high-salinity fluid (H2O---NaCl---KCl system) with a carbonic fluid (H2O---CO2---N2 system) produced by the devolatilization process of carbonate and phyllitic host rocks. Considering the absence of spatially and temporally related igneous activity and the low P-T regional metamorphism in the Paranoá Group, the brines are inferred to result from leaching of evaporites occurring in the lower part of the Paranoá lithostratigraphic column.Gold was probably initially transported as an AuCl2-complex (T>300°C, low pH, moderate ƒO2-pyrite field stability). As temperature decreased below 290°C, the “switch-over” process would lead to the predominance of Au(HS)2− in the fluid. Pyrite precipitated in this temperature interval. The oscillatory zoning of the As---Co---Ni-bearing pyrites indicates episodic fluctuation of the fluid composition. Such changes in fluid composition are favoured by a mechanism of fluid mixing by intermittent supplies in the hydrothermal system. The proposed mechanism of heterogeneous trapping of two separate fluids in the system H2O---CO2---N2---NaCl---KCl and the resulting changes in the physicochemical conditions caused by the fluid mixing appears as a conspicuous process for the Santa Rita hydrothermal fluid evolution.A model based on the existence of a Proterozoic geothermal system involving the regional thermal gradient is proposed. 相似文献
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The Athabasca Basin hosts many world-class unconformity-related uranium deposits. Recently, uranium reserves for the Eagle
Point basement-hosted deposit have increased with the discovery of new mineralized zones within secondary structures. A paragenetic
study of Eagle Point reveals the presence of three temporally distinct alteration stages: a pre-Athabasca alteration, a main
alteration and mineralization comprised of three substages, and a post-main alteration and mineralization stage that culminated
in remobilization of uraninite from primary to secondary structures. The pre-Athabasca alteration stage consists of minor
amounts of clinochlore, followed by dolomite and calcite alteration in the hanging wall of major fault zones and kaolinitization
of plagioclase and K-feldspar caused by surface weathering. The main alteration and uranium mineralization stage is related
to three temporally distinct substages, all of which were produced by isotopically similar fluids. A major early alteration
substage characterized by muscovite alteration and by precipitation Ca–Sr–LREE-rich aluminum phosphate-sulfate minerals, both
from basinal fluids at temperatures around 240°C prior to 1,600 Ma. The mineralization substage involved uraninite and hematite
precipitated in primary structures. The late alteration substage consists of dravite, uranophane-beta veins, calcite veins,
and sudoite alteration from Mg–Ca-rich chemically modified basinal fluids with temperatures around 180°C. The post-main alteration
and mineralization stage is characterized by remobilization of main stage uraninite from primary to secondary structures at
a minimum age of ca. 535 Ma. U–Pb resetting events recorded on primary and remobilized uraninites are coincident with fluid
flow induced by distal orogenies, remobilizing radiogenic Pb to a distance of at least 225 m above the mineralized zones. 相似文献
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A regional sedimentological and sequence stratigraphic analysis is presented in this paper to provide new insights into the morphological complexity and sedimentary mechanisms of unconformities giving shape to basin margins. Although the development of unconformities entails significant sediment delivery to be considered when analysing ‘source to sink’ systems, as well as large palaeogeographical changes related to basin reconfiguration episodes, the complexity that unconformities can reveal towards basin margins has been scarcely explored. A sedimentological and architectural analysis combining outcrop and subsurface datasets shows five palaeoenvironmental evolutionary stages and four key stratigraphic surfaces in a constrained ca 2 Ma Valanginian interval on the southern margin of the Neuquén back-arc basin (Argentina). A complex-type subaerial unconformity was identified, composed in turn of two subaerial unconformities exhibiting both single and composite motifs, different morphologies and facies shifts representative of large palaeogeographical changes. In the studied stratigraphic interval, two erosional stages occur linked to combinations of exclusively non-marine-driven processes involved in unconformities developing. The two subaerial unconformities differ in nature and distribution, representing a novel case of complex unconformities and stratigraphic architectures in non-marine lowstand wedges. The Valanginian complex subaerial unconformity entails a high diachroneity along strike and depositional dip, implying that the hiatus created in landward settings occurred during relative sea-level fall and rise stages during a period longer than in basinward areas. Disagreeing with classical sequential models, two third-order sequence components of a complex lowstand wedge are preserved in proximal settings. Subsidence-controlled accommodation and interplay between second-order and third-order cycles were combined, increasing the prospects of sediment storage and preservation potential of composite sequences towards landward areas. This work improves current comprehension about complex subaerial unconformities formation and related lowstand architectures in proximal settings, providing criteria to understand and revaluate lowstand wedge geometries, particularly for more complex examples, such as the case reported in this contribution. 相似文献
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Miriela María Ulloa Santana Márcia Abrahão Moura Gema R. Olivo Nilson Francisquini Botelho T. Kurtis Kyser Bernhard Bühn 《Mineralium Deposita》2011,46(1):91-104
The Camagüey district, Cuba, is known for its epithermal precious metal deposits in a Cretaceous volcanic arc setting. Recently,
the La Unión prospect was discovered in the southern part of the district, containing gold and minor copper mineralization
interpreted as porphyry type. Mineralization is hosted in a 73.0 ± 1.5 Ma calc–alkaline I-type oxidized porphyry quartz diorite
intrusive within volcanic and volcaniclastic rocks of the early Cretaceous Guáimaro Formation. The porphyry is affected by
propylitic alteration and crosscut by a network of quartz and carbonate veinlets and veins. Chlorite, epidote, sericite, quartz,
and pyrite are the main minerals in the early veins which are cut by late carbonate and zeolite veins. Late barite pseudomorphously
replaces pyrite. Gold is associated with pyrite as disseminations in the altered quartz diorite and in the veins, occurring
as inclusions or filling fractures in pyrite with 4 g/t Au in bulk samples, and up to 900 ppm Au in in pyrite. Fluid inclusion
and oxygen isotope data are consistent with a H2O–NaCl–(KCl) mineralizing fluid, derived from the quartz diorite magma, and trapped at least at 425°C and 1.2 kbar. This primary
fluid unmixed into two fluid phases, a hypersaline aqueous fluid and a low-salinity vapor-rich fluid. Boiling during cooling
may have played an important role in metal precipitation. Pyrite δ34S values for the La Unión prospect range between 0.71‰ and 1.31‰, consistent with a homogeneous magmatic sulfur source. The
fluids in equilibrium with the mineralized rocks have estimated δ18O values from 8‰ to 11.8‰, calculated for a temperature range of 480–505°C. The tectonic environment of the La Unión prospect,
its high gold and low copper contents, the physical–chemical characteristics of the mineralizing fluids and the isotopic signature
of the alteration minerals and fluids indicate that the La Unión gold mineralization is similar to the porphyry gold type,
even though the ore-related epidote–chlorite alteration can be classified as propylitic and not the classic potassic and/or
phyllic alteration. The low copper contents in the prospect could be due to a mineralizing fluid previously saturated in copper,
which is indicated by trapped chalcopyrite crystals in high-temperature fluid inclusions. The low-temperature paragenesis,
represented by carbonate, zeolite and barite, indicates epithermal overprint. The study shows the potential for other gold
porphyry-type deposits in the Cretaceous volcanoplutonic arc of Cuba. 相似文献
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