Navegando por Autor "MEIRELES, Henrile Pinheiro"

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Geologia e ocorrências auríferas da área do Morro do Lajeado, Porto Nacional – TO
(Universidade Federal do Pará, 1995-06-26) MEIRELES, Henrile Pinheiro; KOTSCHOUBEY, Basile; http://lattes.cnpq.br/0096549701457340
The gelogical setting of the Morro do Lajeado region includes Precambrian and Phanerozoic formations. The first ones comprise 1- a gneissic basement of tonalitic to granodioritic composition; a volcano-sedimentary sequence (Morro do Aquiles Formation) made up of quartz-amphibolites, mylonitic schists, metapelites and gondites, and tonalitic bodies (Torre granitoid) intrusive into the former units. Folded metapsammites of the Natividade Group occur locally. Ductile-brittle NE-SW striking shear zones crosscut all these units and constitute an anastomosing system which encloses preserved, undeformed or weakly mylonitized cores of tonalitic bodies. Paleozoic sediments of the Serra Grande and Pimenteiras formations belonging to the Parnaiba Basin, Mesozoic doleritic dykes and silis, Cenozoic laterites and recent colluvial and alluvial deposits complete the lithostratigrafic framework of the area. Quartz veins, concordant and sub-concordant with the regional foliation, occur within the shear zones. They are massive and lenticular in shape (boudin-like) and are up to several meters thick. All veins are oriented N30E, except the Torre vein whose direction is N75W, and crop out over a few meters up to 200 m along the strike. The mineralization consists of native gold and pyrite aggregates. Gold occurs as sub-millimetric particles concentrated in dimimite geodes and druses or disseminated in frature planes close to the ends of the quartz lenses. Microprobe analyses of'these particles revealed a fineness of approximately 908. Chemical analyses showed that the inner parts of the veins are barren or contain only traces of gold, whereas their borders are enriched in this metal, mainly in the extremities of the mega- boudins and at the contact with the host rocks. Microthermometric study evidenced the presence both, of aqueous salihe solutions (4-15 equiv. wt % NaCl) containing NaCl and MgClh,, and CO; - rich solutions, the latter confirmed by Raman microprobe spectroscopy. Minimum trapping temperatures of 219ºC to 260ºC were obtained for the aqueous solutions, while a temperature of approximately 350ºC was determined for the carbonic ones. The intensely mylonitized host rocks belonging either to the Morro do Aquiles Formation or to the Torre granitoid exhibit a pervasive hydrothermal alteretion. Sericitization and/or epidotization of plagioclase, chloritization of biotite and amphibole, kaolinization of K-feldspar and neoformation of cabonate are the principal alteration processes. The geological evolution of the Morro do Lajeado area, after the formation of the gneissic basement, recorded the accumulation of clastic and chemical sedimentas associated to volcanic flows, as well as the later emplacement of tonalitic intrusive bodies. All these rocks underwent a high greenschist facies regional metomorphism followed by an intense shearing and hydrothermalism. Low- to middle greenschist facies transformations affected the highly deformed rocks in the shear zones while the mobilized fluids caused leaching of gold probably out of the volcano-sedimentary pile. The mineralizing fluids are believed to have been mainly of metamorphic origin though magmatic and/or meteoric contributions should not be ruled out. Thus, the precipitation of gold together with quartz and pyrite most likely was caused by the reaction between fluids of different sources.