Navegando por Autor "SOUZA, Valmir da Silva"

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Desconhecido
Field and petrographic data of 1.90 to 1.88 Ga I- and A-type granitoids from the central region of the Amazonian Craton, NE Amazonas State, Brazil
(2012-12) VALERIO, Cristóvão da Silva; MACAMBIRA, Moacir José Buenano; SOUZA, Valmir da Silva
The SW Presidente Figueiredo district, which is located in the northeastern Amazonas State of the central Amazonian Craton, Brazil, consists of 1890 to 1898 Ma I-type granitoids (Terra Preta Granite, Água Branca Suite), A-type hornblende-bearing syenogranites (Canoas Syenogranite, Mapuera Suite), felsic to intermediate volcanic rocks (Iricoumé Group), and 1883 to 1889 Ma rapakivi granites (São Gabriel Granite, Mapuera Suite) and related rocks (quartz-gabbro-anorthosite and diorite), in addition to Castanhal quartz-monzonite, mylonites, and hornfels. The quartz-diorite facies of Terra Preta Granite were formed by mingling processes between a synplutonic quartz-gabbro dike and a hornblende granodiorite. Partially assimilated globules of Canoas hornblende-bearing syenogranite and their clear contacts with Terra Preta hornblende granodiorite suggest that Canoas Syenogranite is slightly younger than Terra Preta Granite. Canoas Syenogranite xenoliths inside São Gabriel Granite show that the granite is younger than the Canoas Syenogranite. New geologic and petrographic evidence improve the petrological understanding of these rocks and suggest that, in addition to fractional crystallization, assimilation and magma mingling played a role, at least at the local scale, in the evolution and compositional variation of the plutons. Such evidence is found in Terra Preta Granite mingled quartz-diorite, felsic material associated with the Canoas Syenogranite and in the intermediate microgranular enclaves, which exhibit primary biotite in hornblende-bearing rocks, plagioclase dissolution, corrosion of feldspars rims, alkali feldspar mantles, second apatite generation, and high xenocrystal contents in intermediate enclaves formed from the fragmentation of mafic intrusions. Petrographic analyses show that a deformational event recorded in the western part of the study area (with progressive deformation from E to W) is dated between the 1.90 Ga postcollisional magmatism and the intrusions of the São Gabriel Granite and related mafic/intermediate rocks (intra-plate). However, it is extremely necessary to obtain absolute ages for this metamorphic event.
Desconhecido
Geologia e mineralizações auríferas da área do Garimpo do Manelão, região do Bacajá (Pa)
(Universidade Federal do Pará, 1995-03-14) SOUZA, Valmir da Silva; KOTSCHOUBEY, Basile; http://lattes.cnpq.br/0096549701457340
The Manelão area is part of the Itacaiunas Shear Belt located in the center-west of the State of Pará. The Archean-Proterozoic lithostratigraphic units recognized in this area are Xingu Complex granites and gneisses, amphibolites, micaschists and quartzites from the São Manoel volcano-sedimentary sequence and the Felicio Turvo intrusive stratoid monzogranite. Mesozoic gabbro and diabase dykes, Cenozoic laterites and recent colluvial and alluvial deposits complete the lithostratigraphic picture. The main structures identified in the area were originated by a sinistral transcurrent deformation of a ductile to ductile-ruptile nature that involved a rock mass transport from ESE to WNW. Such tectonism caused low to medium easy amphibolite metamorphism, although locally more intense hydrothermalism caused alteration of high greenschist facies. Gold occurs both in veins lodged in amphibolites and schists of the São Manoel volcano-sedimentary sequence, as well as in lateritic cover and in alluvial/colluvial deposits. Two systems of veins were identified, one with direction N70E and dip of 80° to NW, the other with direction N23E and dip equal to the previous one. In these bodies the gold is essentially in the free form, in particles of low purity (about 870) that fill fractures in the quartz or quartz-albitic gangue. Furthermore, it is believed that the pyrite disseminated both in the veins and in the mylonitized and heavily altered host rocks also contains sub-microscopic gold. The hydrothermal alteration resulted in a rough zoning in the host rocks. Upon contact with the veins, a philic alteration is normally observed, while the intermediate zone exhibits propylitization. The outer zone is marked above all by an intense epidotization of the rocks. The metallic minerals, pyrite (two generations), pyrrhotite, chalcopyrite, ilmenite and rutile are found in the form of crystals or small scattered aggregates and are more abundant in the host rocks than in the veins themselves. The fluid inclusion study revealed that the mineralizing fluids were essentially low-temperature (minimum temperature 160-180°C) aqueous solutions of low to moderate salinity (probably H2O-KCl-CaCl2 and H2O-NaCl-CaCl2) and low density. (0.9 - 1.1 g/cm 3 ). Rare three-phase inclusions suggest, however, that high temperature and high salinity solutions equally participated in the mineralizing process. Although CO2 was not detected in this preliminary assessment, its presence in subordinate amounts cannot be ruled out. Considering the predominance in the sulfide system and low temperatures of fluid trapping, the gold seems to have migrated mainly in the form of thio-complexes. In the lateritic cover developed over the São Manoel sequence, gold occurs in fine particles or small nuggets of high purity (about 985) with frequencies intergrown with iron oxyhydroxide both in the mottled zone and in relict fragments of the ferruginous crust. The high degree of purity of lateritic gold suggests that the mobilization of the metal occurred mainly after complexation with organic and thiosulfate ligands. Silver leaching into primary gold particles may also have accounted for the significant difference in purity. Gold particles and nuggets are also dispersed in the clayey matrix of colluvial deposits. Finally, gold forms concentrations of relevant economic value in the lower gravel horizon of alluvial accumulations.
Desconhecido
Oscilações climáticas Plio-Pleistocênicas e sua influência na prospecção de ouro na área do garimpo do Manelão (PA)
(2001-07) SOUZA, Valmir da Silva; KOTSCHOUBEY, Basile
In the Manelão deposit the gold is associated with quartz veins hosted in the São Manoel metavolcano-sedimentary sequence. At period Plio- Pleistocene, under a humid climate, an autochthonous and immature lateritic cover was developed on the São Manoel sequence. This lateritic cover contains gold particules of high purity suggesting lixiviation or remobilization and redeposition processes of Au-Ag in a lateritic enviroment. During Pleistocene the climate changed to arid or semi-arid favouring the erosion of the lateritic profile through colluvial processes associated with periodical floods. This colluvial deposit cover lateritic profile destroying a possible geochemical disperson of pattern of the supergenic gold and harming the geochemical exploration surface. At the end of Pleistocene and beginning of Holocene the humid climate condition returned and associated with the intemperic processes form stone line and latosols. The lateritic and colluvial cover were the source area for the current alluvial material of the São Manoel river, in which the gold occurs free in the lower sedimentary strata and forms concentrations around 10 g/ton.