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Lena Virgínia Soares Monteiro Jorge Silva Bettencourt Caetano Juliani Tolentino Flvio de Oliveira 《Gondwana Research》2007,11(3):362-381
The Vazante Group hosts the Vazante nonsulfide zinc deposit, which comprises high-grade zinc silicate ore (ZnSiO4), and late-diagenetic to epigenetic carbonate-hosted sulfide-rich zinc deposits (e.g. Morro Agudo, Fagundes, and Ambrósia). In the sulfide-rich deposits, hydrothermal alteration involving silicification and dolomitization was related with ground preparation of favorable zones for fluid migration (e.g. Fagundes) or with direct interaction with the metalliferous fluid (e.g. Ambrósia). At Vazante, hydrothermal alteration resulted in silicification and dolomite, siderite, jasper, hematite, and chlorite formation. These processes were accompanied by strong relative gains of SiO2, Fe2O3(T), Rb, Sb, V, U, and La, which are typically associated with the nonsulfide zinc mineralization. All sulfide-rich zinc ores in the district display a similar geochemical signature suggesting a common metal source from the underlying sedimentary sequences.Oxygen and carbon isotope compositions of hydrothermally altered rocks reveal a remarkable alteration halo at the Vazante deposit, which is not a notable feature in the sulfide-rich deposits. This pattern could be attributed to fluid mixing processes involving the metalliferous fluid and channelized meteoric water, which may control the precipitation of the Vazante nonsulfide ore. Sulfide deposition resulted from fluid–rock interaction processes and mixing between the ascending metalliferous fluids and sulfur-rich tectonic brines derived from reduced shale units. 相似文献
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The Tres Marias carbonate-hosted Zn–Ge deposit in Chihuahua, Mexico contains sphalerite with the highest average Ge (960 ppm)
and willemite with the highest reported Ge contents of Mississippi-Valley-type (MVT) deposits worldwide. This has prompted
current exploration efforts to focus on the deposit as a high-grade source of germanium. The sulfide-rich ore type (>125,000 t
at 20% Zn and 250 g/t Ge) contains Fe-rich botryoidal sphalerite (type I) associated with solid hydrocarbons. This type exhibits
distinctive intimately intergrown lamellar texture of high-Fe sphalerite (average 9.9 wt.% Fe and 800 ppm Ge) and a somewhat
less Fe-rich sphalerite phase (average 5.5 wt.% Fe and 470 ppm Ge). Reddish-brown banded sphalerite (type II, average 5.7 wt.%
Fe and 1,320 ppm Ge) is subordinately followed by galena and pyrite. The sulfide-poor “oxidized” zinc ore (up to 50 wt.% Zn;
250 to 300 ppm Ge) is a fine-grained, often friable, alteration product of the sulfide ore and associated limestone and breccia
host. While some areas are dominated by carbonates and sulfates, others are enriched in silicates such as hemimorphite and
willemite. The gangue assemblage includes goethite, hematite, and amorphous silica or quartz. Minor wulfenite, greenockite,
cinnabar, and descloizite also occur. Willemite occurs as interstitial replacement of sphalerite and fracture fillings in
the oxidized ore and can be unusually rich in Pb (up to 2.0 wt.%) and Ge (up to 4,000 ppm). Oscillatory zonation reflects
trace element incorporation into willemite from the oxidation of primary Ge-bearing sphalerite and galena by siliceous aqueous
fluids. The Tres Marias deposit has hybrid characteristics consisting of a primary low-temperature MVT Ge-rich Zn–Pb sulfide
ore body, overprinted by Ge-rich hemimorphite, willemite, and Fe oxide mineralization. 相似文献
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