Navegando por Autor "VILLAS, Raimundo Netuno Nobre"

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Contexto geológico, estudos isotópicos (C, O e Pb) e associação metálica do depósito aurífero Tocantinzinho, domínio Tapajós, Província Tapajós-Parima
(2013-03) VILLAS, Raimundo Netuno Nobre; SANTIAGO, Érika Suellen Barbosa; CASTILHO, Marília Portela
The Tocantinzinho ore deposit is located along a NW–SE-trending lineament, southwestern of Itaituba (Pará, Brazil), and is the largest known gold deposit of the Tapajós Province. The host Tocantinzinho granite is essentially isotropic and dominated by syenogranites and monzogranites that have been weakly to moderately altered by hydrothermal fluids. Microclinization (earliest), chloritization, sericitization, silicification and carbonatization (latest) are the main types of alteration. Most mineralization was contemporaneous with the sericitization/silicification and is represented by sulfide- and gold-bearing veinlets which locally occur as stockwork. Pyrite, chalcopyrite, sphalerite and galena are the most common sulfides. Among the ore metals, Cu, Pb and Zn present the highest contents, but Mo, As and Bi locally show anomalous concentrations. The relationship of Au with Cu, Pb or Zn is at random and the Au/Ag ratios range from 0.05 to 0.5. The higher the sulfide contents, the higher the Au concentrations, though it occurs mainly included in pyrite. Zircon monocrystals from the Tocantinzinho granite yielded an average Pb-Pb age of 1982 ± 8 Ma and may represent an earlier event of the Creporizão magmatic arc. δ13CPDB values for calcite from the carbonatization stage fall dominantly between -3.45 and -2.29‰, being compatible with a deep crustal source that may include carbonatite reservoirs. In turn, δ18OSMOW values vary from +5.97 to +14.10‰, being indicative of magmatic derivation, although the less positive values suggest contribution from surficial waters. Unpublished fluid inclusion study reveals the presence of aquo-carbonic fluids, whose CO2 could have been dissolved in the granitic magma rather than being related to the shear zone. The available data allow the Tocantinzinho deposit to be classified as a granite-hosted, intrusion-related gold deposit.
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Depósito Cu-Au Visconde, Carajás (PA): geologia e alteração hidrotermal das rochas encaixantes
(2012-09) CRAVEIRO, Gustavo Souza; VILLAS, Raimundo Netuno Nobre; SILVA, Antonia Railine da Costa
Visconde Cu-Au deposit, Carajás (PA): geology and hydrothermal alteration of the host rocks. The Cu-Au Visconde deposit is located 15 km east of the Sossego mine (Canaã de Carajás, in the State of Pará) and it lies in the contact zone between the Itacaiúnas Supergroup (2.76 Ga) and the basement (> 3.0 Ga). The main rocks present in the deposit are the Serra Dourada granite, gabbros/quartzdiorites and rhyodacites, all variably altered and deformed. The Planalto Intrusive Suite has also been identified in the neighborhood, but it shows distinct alteration features. Mafic and felsic dikes crosscut the whole rock pile. The hydrothermal alteration has evolved, under rheological conditions that changed from ductile-brittle to brittle, from an early and pervasive calcic-sodic type (albite, scapolite or amphiboles) to a potassic one (K-feldspar and Cl-biotite), restoring, though locally, the sodic-calcic characteristics (albite, epidote, apatite, tourmaline and fluorite) to finally acquire a calcium-magnesium signature (clinochlore, actinolite, carbonates, and subordinate talc). In the Serra Dourada granite, albitization, epidotization, and tourmalinization were dominant in contrast with scapolitization, biotitization, amphibolitization and magnetitization in gabbros/quartzdiorites or K-feldspatization in rhyodacites. The main ore bodies are represented by chalcopyrite- and bornite-rich breccias and veins, besides chalcopyrite + pyrite ± molybdenite ± pentlandite disseminations. The basic metal suite is Fe-Cu-Au ± REE. Abundant sulfide was precipitated in the transition from potassic to calcic-magnesian alteration, along with different proportions of apatite, scapolite, actinolite, epidote, magnetite, tourmaline, calcite, gypsum, and fluorite. Metals were transported by hydrothermal fluids enriched in Na, Ca, K, Fe, and Mg, in addition to P, B, F, and S species. The similarities overcome the differences, so that the Visconde and Sossego deposits can be considered co-genetic.
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Estudo petrográfico e geoquímico dos ironstones da base da Formação Pimenteiras, Borda Oeste da Bacia do Parnaíba, Tocantins
(2012-06) AMARO, Gabriel de Jesus Lavareda; VILLAS, Raimundo Netuno Nobre; KOTSCHOUBEY, Basile
Ironstones that occur at the base of the Devonian Pimenteiras Formation (northwestern border of the Parnaíba basin) were investigated along the Xambioá-Vanderlândia and Colinas do Tocantins-Couto Magalhães sections. The ironstones consist of discontinuous decimeter-thick layers intercalated in sandstones and shales that underlie rocks of the Neoproterozoic Araguaia belt. The Xambioá-Vanderlândia ironstones are texturally and chemically quite distinct from those of the Colinas do Tocantins-Couto Magalhães area. Besides the presence of oolites, the former shows smaller amounts of terrigenous material (especially quartz) and higher iron oxide-hydroxides proportions. They also are more enriched in V, Sr, Zr and ΣREE, and poorer in Al2O3 and Rb. The North American Shale Composite (NASC) normalized REE distribution pattern is also different, especially concerning the IREEN values, which, being higher in the oolithic ironstones, display convex curves and, being lower in the non-oolithic ironstones, display concave ones. In the field, spatial relationships could not be defined between both ironstone types. It is then suggested that they represent different lithological facies of the same iron formation. Probably, the deposition of the non-oolithic facies occurred in deeper and quieter waters away from the continental border, where larger amounts of detrital sediments were discharged, whereas the deposition of the oolithic facies took place in shallower and more agitated waters, with less supply of terrigenous material. Iron was largely derived from the erosion of continental areas where reduced environments favored its mobilization and transport by rivers as suspended particles, colloids, and organic complexes.
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The Tocantinzinho gold deposit, Tapajós province, state of Pará: host granite, hydrothermal alteration and mineral chemistry
(2013-03) SANTIAGO, Érika Suellen Barbosa; VILLAS, Raimundo Netuno Nobre; OCAMPO, Ruperto Castro
This paper presents geological, petrographic and mineralogical data about the granite that hosts the Tocantinzinho gold deposit and aimed at contributing to the understanding of the hydrothermal processes related to the ore genesis. The host granite is a late to post-tectonic biotite monzogranite that fits in the oxidized sub-type of the ilmenite series. It was emplaced at depths of 6 - 9 km and reveals no deformation other than fracturing and brecciation. This intrusion has undergone mild to moderate hydrothermal alteration that generated two main rock varieties (salami and smoky), with no significant mineralogical or chemical differences, though quite distinct macroscopically. Several types of hydrothermal alteration have been recognized in the granitic rocks, greatly represented by filling veins and/or replacement of primary minerals. The hydrothermal history started with microclinization, during which the granitic protolith was in part transformed into the salami variety. This process was followed by chloritization when the temperature dropped to ~330 ºC producing chamosite with XFe in the 0.55 - 0.70 range. Then it evolved to sericitization, at the same time that ore-bearing fluids precipitated pyrite, chalcopyrite, sphalerite, galena and gold. As alteration advanced, solutions saturated in silica and formed quartz veinlets. At the latest stage (carbonatization), aqueous and aqueous-carbonic fluids might have mixed, allowing Ca2+ e CO2 to react to form calcite. Most sulfides are present in veinlets that crosscut the granite, some arranged as stockwork. Gold is normally very fine-grained and occurs mainly as submicroscopic inclusions or along microfractures in pyrite and quartz. The Tocantinzinho deposit is very similar to the Batalha, Palito and São Jorge deposits, and to those of the Cuiú-Cuiú goldfield. Topologically, it has been classed as an intrusion-related gold deposit.