Navegando por Assunto "Hydrothermalism"

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Acesso aberto (Open Access)
Caracterização mineralógica com espectroscopia de reflectância por infravermelho (SWIR): exemplo do Complexo máficocarbonatítico Santana, sul do Cráton Amazônico
(Universidade Federal do Pará, 2021-09-21) COSTA, Jhoseph Ricardo Costa e; FERNANDES, Carlos Marcello Dias; http://lattes.cnpq.br/0614680098407362; https://orcid.org/0000-0001-5799-2694
On the border of the Pará and Mato Grosso states, in the Amazonian Craton, municipality of Santana do Araguaia (PA), there is a volcano-plutonism named Santana mafic-carbonatitic complex. This set houses the Serra da Capivara phosphate deposit. A lower mafic-ultramafic member reveals plutono-volcanic lithofacies with pyroxenite, ijolite, apatitite, and alkaline basalt. Autoclastic lithofacies contains poorly selected deposits of massive polymictic breccia, lapilli-tuff, crystal tuff, and ash tuff. Volcanogenic epiclastics rocks cover these lithofacies. The upper carbonatite member contains plutonic lithofacies with coarse calcite-carbonatite (sövite). Fine carbonatite veins with pervasive carbonatic and apatitic alterations crosscut this lithotype. Minor thick apatitite occcurs associated to this member and represents the protore of the deposit. Effusive volcanic lithofacies reveals fine calcite-carbonatite (alvikite) with porphyritic, equigranular, or aphanitic textures. A poorly sorted lithofacies of crystals tuff, lapilli-tuff, and massive polymictic breccia completes this member. Stocks and syenitic dykes invade these lithofacies. Detailed mapping suggests that the complex is a volcanic caldera in which large zones of hydrothermal alterations occur with reddish, brownish red, and yellowish carbonatitic rocks. Petrographic observations reveal paragenesis of barite + fluorapatite + calcite + dolomite ± quartz ± rutile ± chalcopyrite ± pyrite ± monazite ± magnetite ± hematite. The application of short wave infrared spectroscopy (SWIR) revealed the chemical characteristics and their importance in the crystallinity of most of these hydrothermal minerals, such as radicals (OH- and CO3), H2O molecule, and cation-OH bonds such as Al-OH, Mg-OH, and Fe-OH. The main mineral phases identified were dolomite, calcite, serpentine, chlorite, muscovite with low, medium, and high aluminum, montmorillonite (Ca and Na), illite, nontronite (Na0.3Fe2((Si,Al)4O10) (OH)2·nH2O), and epidote. The data suggest a control by temperature, fluids composition, and fluid/rock ratio during the evolution of the Santana mafic-carbonatitic complex. This low-cost exploratory technique, which is applied in hand-held samples or drill holes on a large scale, is promising in characterization of volcano-plutonic centers in regions subjected to severe weathering conditions, as well as helping to develop models for prospecting mineral deposits of Rare Earth Elements (e.g. Nd, La) associated with alkaline-carbonatitic complexes. We can even combine this tool with artificial intelligence algorithms for more robust and faster results.
Acesso aberto (Open Access)
Petrografia, alterações hidrotermais e eventos mineralizantes do Bloco Norte do depósito aurífero Volta Grande, Domínio Bacajá (PA), Cráton Amazônico
(Universidade Federal do Pará, 2021-09-22) SOUZA, Hugo Paiva Tavares de; VASQUEZ, Marcelo Lacerda; http://lattes.cnpq.br/4703483544858128; https://orcid.org/0000-0003-2729-9404; FERNANDES, Carlos Marcello Dias; http://lattes.cnpq.br/0614680098407362; https://orcid.org/0000-0001-5799-2694
The southeastern region of the Amazonian Craton has been the target of several mineral survey programs over the past few years, which have recently led to the identification of the world-class Volta Grande gold deposit, with reserves of ~3.8 Moz at 1.02 g/t, which provides an expectation of 17 years of operation. The deposit is in the municipality of Senador José Porfírio in Pará and is housed in Rhyacian granitoids (2.15 Ga) that occur associated with the volcano-sedimentary Siderian sequence (2.45 Ga) of the Três Palmeiras Group. These units are in the Bacajá Domain, which is formed by belts of high-grade para- and orthoderived rocks and greenstone belt of Archean to Siderian protoliths, reworked during the orogenesis of the Transamazonian Cycle (2.26–2.06 Ga). Granitoids and charnockites sectioned this set in Rhyacian. Part of the mineralization at the Volta Grande is housed in granitoids metamorphosed under medium to high-grade conditions. Local kinematic indicators suggest dip-slip movement in which the greenstone moves up relative to the intrusive rocks. Petrographic descriptions carried out in this work revealed: 1) gray to greenish mylonitic granodiorite, with intense deformation of the main minerals that make up them, such as quartz, biotite, and feldspars. The texture in this lithotype is mainly porphyroclastic. Main metamorphic foliation (S1) is defined by biotite and amphibole, as well as reveals concordant quartz veins and venules. The highest gold contents are distributed in upper amphibolite facies zones. In these, the ore occurs mainly as isolated grains in cm-sized quartz veins and venules associated with pervasive carbonatic alteration that was synchronous to dynamic metamorphism, as well as in a fracture-controlled style. Part of the gold is also associated with a low sulfides content disseminated in the veins and host rock; 2) The metamafic rocks comprise foliated fine- to medium-grained amphibolite and andesite with a dark grayish-green color and nematoblastic texture. Chlorite, calcite, sericite, and opaque minerals are the main secondary phases. These relationships are compatible with lode-type gold systems, usually developed in the transition between greenschist to amphibolite metamorphic facies. Lava flows and dykes of isotropic rhyodacite, rhyolite, and plutonic rocks such as quartz monzonite, granodiorite, monzodiorite, and minor microgranite cut the mineralizing event previously described. Plutonic rocks are medium- to coarse-grained, have a gray color with reddish and greenish portions throughout the profiles, inequigranular texture with quartz, feldspar, biotite, and amphibole. Apatite, zircon, calcite, epidote, and opaque minerals are primary accessories. In turn, volcanics have light gray, black or dark red colors, porphyritic to aphyric texture, and microlithic or felsophyric groundmass. They reveal phenocrysts of plagioclase, amphibole, potassic feldspar, and quartz. This volcano-plutonic system contains potassic, propylitic, intermediate argillic, and/or carbonate hydrothermal alterations in selective, pervasive, or fracture-controlled styles. In hydrothermalized zones, gold occurs as isolated grains disseminated or associated with sulfides, as well as in cm-sized quartz veins in a stockwork arrangement. These characteristics are like those of shallow intermediate- to lowsulfidation epithermal systems already identified in the Amazonian Craton. The Volta Grande data suggest a second overprinted mineralizing event, common in high-tonnage productive gold deposits in China, Finland, and other areas of the planet and represents a new exploration guide for the Bacajá Domain. Several mineralizing events are critical to the economic feasibility and longevity of world-class gold deposits. Thus, new geochemical, geochronological, microthermometric, and stable isotope data will be obtained to better define the genetic modeling of the Volta Grande gold deposit.