Dissertações em Geologia e Geoquímica (Mestrado) - PPGG/IG

URI Permanente para esta coleçãohttps://repositorio.ufpa.br/handle/2011/2604

O Mestrado Acadêmico pertence ao Programa de Pós-Graduação em Geologia e Geoquímica (PPGG) do Instituto de Geociências (IG) da Universidade Federal do Pará (UFPA).

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As águas subterrâneas de Belém e adjacências: influência da Formação Pirabas e parâmetros físico-químicos para medidas de qualidade
(Universidade Federal do Pará, 1996-04-08) SAUMA FILHO, Michel; LIMA, Waterloo Napoleão de; http://lattes.cnpq.br/1229104235556506
In the Metropolitan Region of Belém (PA) the water supply to the population comes from springs (physiographic area of Utinga) and from a network of positional tube wells, in general, in more remote urban areas or where pumping is precarious. This work evaluates the groundwater used in the Metropolitan Region of Belém, correlating data of physical, physical-chemical and chemical parameters, in an attempt to compose an understandable picture about the quality of these waters, and to verify the influence that they suffer from the geological units in which they are located. the aquifers that preserve them are located. To carry out the work, water samples were collected in two different seasonal periods: dry and rainy. After exhaustive consultation of the files of companies, institutions and researchers, 17 tubular wells were selected, 9 in Belém, 5 in Icoaraci, 2 in Mosqueiro and 1 in Ananindeua (Annex A). The most frequent turbidity indices were between 9 and 14 units (ppm of SiO2), but some wells showed higher values (33, 41 and 71 ppm of SiO2. Only in some cases, this turbidity can be immediately correlated with the silica content obtained by chemical analysis. The most frequent color measurements are in the range from zero to 7.5 U.C., with the zero index predominating. However, some wells showed a value above 100 U.C. and others, less frequent, with indices varying between 20 and 60 U.C. The pH and electrical conductivity were quite different parameters. Thus, the highest pH and electrical conductivity indices were verified in the aquifers of the Pirabas Formation. In these cases, the pH was around 6 .4 to 7.6 and conductivity between 231 and 362 µS/cm, with a discontinuity at 87.5 µS/cm, also attributed to a well associated with the aforementioned Formation. More acidic waters (pH below 6.38 and above of 4.01) are certainly attributed to the aquifers of the Barreiras and Post-Barreiras Group. The chemical constituents, notably the contents of Ca2+, Mg2+, Na+ and K+, are consistent with the interpretation of the numerical values of pH and electrical conductivity. Without exception, the concentrations of Ca2+ are higher than those of the other cations, establishing a decreasing order according to Ca2+> Mg2+> Na+>K+, with some inversion between Na+ and Mg2+. The highest concentrations of Ca2+ (soon followed by Mg2+) result from the dissolution of carbonates present in the Pirabas limestone. In fact, confirming this assertion, the concentrations of HCO-3 are also much higher than the concentrations of Cl- and SO2-4. It is to be expected, therefore, that the dissolution of Pirabas sediments produces higher concentrations of Ca2+ and HCO-3. The silica and iron contents also discriminate such waters. In general, higher silica contents correspond to greater depths, as would be expected, taking into account the action of chemical weathering on silicate minerals. As for iron, this constitutes a differentiating parameter of the waters of the Pirabas Formation, almost always at much lower levels than the corresponding values associated with the Barreiras and Post-Barreiras aquifers, with, however, exceptions, in which appreciable indices of iron related to Pirabas sediments. It should be noted that the Pirabas Formation appears in the Metropolitan Region of Belém almost always at depths greater than 100 m, although there are records of smaller depths, but these are apparently rarer situations, as is the case with well number 3. , on the University Campus, near the Guamá River, with a depth of 76 m, and the 94 m well of the Museu Paraense Emílio Goeldi, in the central area of the city (Annex A). The exhaustive consultation of the aforementioned archives of institutions, companies and researchers led to the realization that many tubular wells installed in the urban area use water associated with the Barreiras and Post-Barreiras aquifers, where the pH values are almost always, below 6 units, and electrical conductivity measurements rarely reach 100 µS/cm. Finally, it appears that there is a need for greater investments in order to increase the prospection and use of groundwater in the region, as these, in addition to dispensing with treatment prior to distribution, are still a source of resources, not dimensioned, but of great potential.
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Alteração hidrotermal e potencial metalogenético do vulcanoplutonismo paleoproterozoico da região de São Félix do Xingu (PA), Província Mineral de Carajás
(Universidade Federal do Pará, 2015-08-27) CRUZ, Raquel Souza; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983
The region of Sao Felix do Xingu, south-central Pará, exposes a volcano-plutonic system exceptionally well preserved and grouped in the Sobreiro and Santa Rosa formations, in which hydrothermal alteration and mineralization associated were recognized. The Sobreiro Formation consists of lava facies flow of andesitic, basaltic andesite, and dacitic composition, according to the proportions or absence of clinopyroxene and/or amphibole phenocrysts. Volcaniclastic facies is genetically associated and is represented by mafic crystals tuff, lapilli-tuff, and massive polymictic breccia. Santa Rosa Formation is fissure-contolled and composed of lava flow facies and associated volcaniclastic facies of felsic crystal tuffs, ignimbrites, lapilli-tuff, and massive polymictic breccia. Part of this system is interpreted as ash-flow caldera partially eroded and developed in several stages. Conventional petrography, X-ray diffraction (XRD), scanning electron microscopy (SEM), and infrared spectroscopy show hydrothermal alteration paragenesis occurring in these rocks. In general, the alteration minerals develop subeuhedral anhedral crystals and replace magmatic minerals. The types of hydrothermal alteration identified are incipient the pervasive and are distinguished propylitic, sericitic, intermediate argillic, and potassic, which overlap, and fracture-controlled silicification associated with hematite and carbonate. Propylitic alteration, prevalent in Sobreiro Formation, presents both pervasive and fracture-controlled styles. The paragenesis consists of epidote + chlorite + carbonate + quartz + sericite + clinozoisite ± albite ± hematite ± pyrite, which is overlapped by pervasive potassic alteration or fracture-controlled, mainly represented by potassic feldspar + biotite ± hematite. Locally, fracture is filling with prehnite-pumpellyite association that suggests geothermal low-grade metamorphism conditions. The sericitic alteration is marked by the occurrence of mainly sericite + quartz + carbonate ± epidote ± chlorite ± muscovite. It is manifested mainly in mafic crystal tuff. However, the overlap of these types of changes is evidenced by relics of propylitic chlorite alteration and textures of rocks, partially obliterated, in which there were only pseudomorphs of sericitized plagioclase. In the Santa Rosa Formation the sericitic alteration is pervasive and characterized by the occurrence of sericite + quartz + carbonate. Also presents fracture-controlled, which is represented by sericite + quartz. It is the main type of change identified in this unit by assigning the whitish rocks. SEM data show that, associated with the sericitic alteration occur lead phosphate, gold, rutile, and barite. The potassic alteration is more subordinate, generally associated with granitic porphyry and locally to rhyolites. Paragenesis is given by microcline + biotite + chlorite + carbonate + sericite ± albite ± magnetite. The intermediate argillic alteration was recognized in rhyolites and possibly corresponds to the final stages of hydrothermal alteration. It is characterized by the presence of montmorillonite + illite + chlorite + sericite ± kaolinite ± halloysite ± quartz ± hematite, which were identified by infrared spectroscopy and XRD. It gives whitish to whitish pink to the rocks. The hydrothermal alteration types were mainly controlled by temperature, fluid composition, and fluid/rock ratios. They are compatible with thermal anomalies related to magma, and possible temperature decrease due to mixing and neutralization with meteoric water, similar to that described in low- and intermediate-sulfidation mineralization. Gold identification and compatible accessories phases provide important information for prospective studies in the region, especially for potential intermediate- and low-sulfidation epithermal deposits of precious metals (gold and silver) in volcano-plutonic systems with related ash flow calderas, as well the Au(Cu) and Mo porphyry-type deposits.
Acesso aberto (Open Access)
Ambiente geológico e mineralizações associadas ao granito Serra Dourada (extremidade meridional) Goiás
(Universidade Federal do Pará, 1983-08-29) MACAMBIRA, Moacir José Buenano; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983
The Serra Dourada granite belongs to a set of domic structures, generally mineralized in cassiterite, located in the center-east of Goiás, where rocks of the Uruaçu and Brasília folding belts and the Goiás median massif dominate. In order to contribute to the petrological, metallogenetic and stratigraphic knowledge of these granitic rocks, the southern tip of Serra Dourada was selected for this study. The methodology adopted was mapping at a 1:45,000 scale, petrographic, minerographic and geochronological studies, in addition to the determination of the contents of major elements in rocks and some minerals, and of trace elements in rocks. The granitic rocks of Serra Dourada were classified as syenogranites, presenting three varieties: amphibole-biotite granite, muscovite-biotite granite and biotite granite, the latter being dominant. The K-Rb graph indicates an advanced degree of fractionation for these rocks and suggests a trend that starts from granite to amphibole and ends in muscovite. In turn, the systematic variation of the contents and ratios of some trace elements reveals an intimate relationship between these varieties, meaning multiple intrusions that correspond to different degrees of partial fusion of the original material. In attempts at dating by the Rb-SR method, it was observed that the phenomena subsequent to the initial lodging in the crust introduced possible isotopic rejuvenations. However, these granitic rocks provided maximum conventional ages close to 2 b.a. The last magmatic phases of the Serra Dourada granite were the pegmatites which, in the core of the batholith, are zoned and contain aquamarine, while at the edge they bear tantalite-columbite, emerald, muscovite and monazite. Then, large amounts of hydrothermal solutions enriched in Sn and F reached both the granite and its host, changing them to greisens. Upon contacting the enclaves, the solutions precipitated cassiterite, magnetite, fluorite and sulfides. Veins with wolframite and rutile lodged in the nearest enclaves. At lower temperatures, these solutions generated kaolin when reaching the pegmatites of the contact range. Several types of enclaves have been identified in the granite: biotitite, soda-gneiss, xenoliths of schists and quartzites, and amphibolites. The soda-gneiss enclaves are trondhjemitic in nature and also have amphibole and biotite, biotite and biotite and muscovite varieties. The similarity of the assemblage and chemistry of some mineralogical phases suggests a consanguinity between soda-gneiss and granite, with the possibility that they are partially intact fragments of the rocks that gave rise, by anatexia, to the granitic material. On the other hand, the contents and anomalous ratios of some elements of soda-gneiss indicate reaction with magma, which is emphasized by the position of these rocks in the K-Rb graph. This reaction certainly affected the isotopic ratios, allowing only to suggest an Archean age. In turn, the biotites are possibly restricted. The sequence where the Serra Dourada granite was lodged is composed of intercalations of schists and quartzites from the Serra da Mesa Group. The typical mineralizations of acid magmatism, greissens, pegmatites and granitic sills in the metasediments, in addition to xenoliths from the enclosing rocks and pronounced foliation at the edges of the body, testify to the intrusive character of the granite in these metamorphites, whose late-syntectonic event is associated with the formation of the brachyanticlinium, which agrees with the regional structural pattern. Through the mineralogical assemblage of these rocks, conditions of low amphibolite facies were attested for its formation, where pressures above 4.5 Kb and temperatures around 550°C.
Acesso aberto (Open Access)
Aspectos geológicos e metalogenéticos do depósito de ouro hospedado em metaconglomerados e metarenitos paleoproterozoicos Castelo de Sonhos, Província Tapajós, sudoeste do Pará
(Universidade Federal do Pará, 2015-04-06) QUEIROZ, Joana D’arc da Silva; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706
Castelo de Sonhos, located in the central-south sector of the Amazonian Craton, near the boundary between the Tapajós and Xingu-Iriri tectonic domains, is a gold deposit hosted in metaconglomerates and metasandstones of the Castelo dos Sonhos Formation (<2080 Ma U-Pb SHRIMP). Subvolcanic rocks and granitoids were identified in boreholes that drilled the deepest parts of the deposit area. Some of these rocks are intrusive into the Castelo dos Sonhos Formation, while for others rocks, the contact relationships could not be determined with confidence. In general, these rocks show calc-alkaline to alkaline affinities and their geochemical patterns indicate that they are related to volcanic arc or post-collisional tectonic settings. The subvolcanic rocks are represented by a porphyritic dacite with age of 2011 ± 6 Ma (U-Pb LA-ICP-MS).The granitoids were classified as biotite granodiorite, biotite monzogranite, muscovite monzogranite, respectively dated at 1976 ± 7 Ma, 1918 ± 9 Ma and 1978 ± 6 Ma (U-Pb SHRIMP), and an undated syenogranite. These ages represent three to four distinct magmatic events and indicate that the studied rocks are coeval to four major units from Tapajós Domain: the Cuiú-Cuiú Complex (2033-2005 Ma), the Comandante Arara Formation (2020-2012 Ma), the Creporizão Intrusive Suite (1998-1957 Ma), and the Tropas Intrusive Suite (1907-1892Ma). Despite the temporal correspondence, the geochemical data show no direct correspondence with the units cited above. Notwithstanding, the intrusion relationship between some of the studied rocks and the metasedimentary rocks of the Castelo dos Sonhos Formation establishes a temporal, spatial and possibly stratigraphic relationship between this formation and the Tapajós Domain. The intrusive contact relationship between the porphyritic dacite and metasandstones of the Castelo dos Sonhos Formation allowed us to determine at 2011 ± 6 Ma the minimum sedimentation age of this unit. The primary gold mineralization at Castelo de Sonhos deposit is stratabound and restricted to a metaconglomerate package and interlayered metasandstones. The mineralization distribution is erratic and does not seem to follow special features or structural control. In the matrix of the metaconglomerates, gold occurs as intergranular particles, occasionally associated with magnetite, and also within quartz grains (medium to coarse sand), which probably represent fragments of auriferous veins. In general, the gold particles show subrounded to rounded shapes, mild to moderately rough surfaces. The particles seldom contain inclusions, and only of magnetite. The chemical composition is homogeneous and characterized by high Au/Ag ratios. These characteristics indicate a syngenetic origin for gold within the metaconglomerates package. Therefore, the age of mineralization is limited by the time x interval of deposition of the Castelo dos Sonhos Formation (2011 ± 6 Ma to ca. 2080 Ma). On the other hand, the occurrence of gold in fracture planes of metasandstones indicates an epigenetic origin for this style of mineralization. The epigenetic mineralization is related to concurrent metamorphic, magmatic and deformational processes that affected the sedimentary sequence of the Castelo dos Sonhos Formation and caused the remobilization of gold originally hosted in metaconglomerates. It is likely that the interaction of these processes associated with infiltration of meteoric waters contributed to the generation and flow of oxidizing hydrothermal fluids, which have percolated through the metaconglomerates package and were able to solubilize some of the gold, and re-precipitate it accompanied by ferruginous films, in fracture planes of the metasandstones. As a conclusion, a modified paleoplacer model is proposed here to explain the hybrid nature (syngenetic and epigenetic) of the gold mineralization in the Castelo de Sonhos deposit.
Acesso aberto (Open Access)
Aspectos lito-estruturais das minas de ferro N4E e manganês do azul, Serra dos Carajás-Pará
(Universidade Federal do Pará, 1991-08-06) MARÇAL, Mônica dos Santos; COSTA, João Batista Sena; http://lattes.cnpq.br/0141806217745286
The areas of iron (N4E) and manganese (Azul) mines in the Serra dos Carajás region were investigated with emphasis on structural geology, aiming to define the geometric arrangement of the various ore bodies and the kinematic framework, as well as their relationships with the regional structuring. The iron mine (N4E) is subdivided into two segments called Aba Leste and Aba Sul. The lithotypes occurring in this area are represented by friable itabitite (soft hematite HM, hard hematite HD), metajaspilite, metavolcanic rocks and canga. . Most of the rock units are distributed in the form of lenses and discontinuous and wedged bands, with a general orientation N-S on the East Rim and E-W on the South Rim, drawing a “J” with concavity facing northwest. The analysis of the structural elements present in the rocks of the N4E mine, in line with the information available for the Serra Norte region, allowed the characterization of three sets of main structures: the oldest set comprises the mylonitic foliation, the shear zones with oblique riding character, stretch lineation and associated folds; the second set refers to the NW-SE, E-W and N-S ductile transcurrent shear zones that cut and limit the body of N4; the last set corresponds to the folds and crenulations present throughout the entire ore body. These sets of structures are interpreted in the context of a single deformational event. The lithotypes that make up the manganese mine (Blue) are classified into three types: Protomines, Surface Deposits and Subsurface Deposits. Mining is currently being carried out in the central part of the deposit, between the LT00 and LT800E lines, where the ores related to surface and subsurface deposits were identified. The first includes pisolites (PIS), blocks (BL), platelets (PLT) and manganese breccias (BLM), and the second encompasses manganeseiferous mudstones (PM), granulated manganese material (MMG) and massive manganese material (MMM). The various types of manganese ore are wavy, defining synformal and antiformal structures oriented in the E-W direction and associated with strongly inclined thrusts to the north. The N4E and Azul mines are located on the northern edge and in the central-north region of the divergent macrostructure of Serra dos Carajás, respectively. Such a regional geometric arrangement is interpreted as a positive flower structure associated with an E-W sinistral binary. In this sense, the lithostructural units of the two studied areas must be understood as fractions of an inverted transcurrent volcano-sedimentary basin.
Acesso aberto (Open Access)
Caracterização da alteração hidrotermal no alvo Coelho Central, depósito aurífero Pedra Branca, sequência Serra das Pipocas, maciço de Troia, com base em estudos isotópicos (O, H, S e C) e inclusões fluidas.
(Universidade Federal do Pará, 2019-05-08) LIMA, Rafael Guimarães Corrêa; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706
The gold mineralization of Pedra Branca deposit is associated with rhyacian metavolcanic rocks of Serra das Pipocas Greenstone Belt, at the Archean–Paleoproterozoic Troia Massif, in Borborema Province, NE of Brazil. The deposit comprises four mineralized targets, Mirador, Queimadas, Igrejinha and Coelho Central and has been classified as a hypozonal orogenic gold deposit based on the geological context, types of hydrothermal alterations and isotopic data obtained for Mirador and Queimadas targets. At Coelho Central target, the main gold host rocks are ilmenite- and garnet amphibolites (after mafic volcanic rocks). Some gold occurrences has been recognized in altered metadacites and metatonalites intrusive lenses. Hydrothermal and mineralized zones show mineral assemblages formed under amphibolite facies conditions. (i) calc-silicate veinlets (diopside, titanite, calcite, epidote and sulfides), (ii) hornblende, albite, biotite and garnet veinlets with pyrrhotite and gold, (iii) biotite-rich potassic alteration containing pyrrhotite, gold and tellurides and (iv) quartz veins are the main types of ore-related alterations. In addition, (v) epidote, titanite and calcite pockets, (vi) chloritization and (vii) pyrite-rich fissural carbonatization are the late and no ore-related alterations that marks a ductile-brittle deformation, probably under greenschist facies conditions. Gold occurs predominantly as inclusions in pyrrhotite, chalcopyrite, pentlandite and Co-pentlandite indicating their association with sulfur species and subordinately as free-milling microparticles in quartz veins. The metallic association of sulfidation zones also includes abundant Ag, Bi, Ni and Pb tellurides. Oxygen and hydrogen geothermometry for hydrothermal silicates yielded temperatures from 484 to 586 ºC for the gold-related alterations. The fluid isotopic composition in equilibrium with hydrothermal silicates (quartz, hornblende, biotite, tourmaline and titanite) shows values of δ18O (+6.8 to + 10.7 ‰) and δD (-58.4 to -35.5 ‰), as well as the values of δ34S in sulfides (-3.1 and +2.7 ‰) and δ13C for calcite (-11.1 to -5.8 ‰), indicative of deepseated magmatic-hydrothermal fluid, with possible interaction and mixing with fluids of the greenstone sequence. Fluid inclusions assemblages in quartz veins show the predominance of CO2 inclusions, with a density up to 1.15 g/cm3 and up to 15 mol% of CH4, coexisting with N2 inclusions, and also with low salinity (< 9.7 wt% NaCl equiv.) H2O-CO2-NaCl and H2O-NaCl-FeCl2±MgCl2 varieties. The petrographic and microthermometric criteria suggest the immiscibility of a CO2-H2O-NaCl-N2-CH4 fluid as responsible for the generation of the observed inclusions. Desestabilization of complexes such as Au(HS)-2 and precipitation of gold and associated metals occurred by immiscibility, fluid-rock interaction, and changes in redox conditions and fluid pH, between 2.2 and 5,5 kbar (6.3 to 16.0 km). A H2O-NaCl-CaCl2 latter fluid was the responsible for the calcite, with pyrite and sphalerite precipitation in fractures and faults. The characteristics presented above allow us to ratify the Coelho Central target and Pedra Branca deposit as a hypozonal gold mineralization formed under amphibolite facies conditions, from CO2-rich deep-seated magmatic fluids that interacted with the metamorphic sequence of Serra das Pipocas Greenstone Belt and precipitated gold and associated metals.
Acesso aberto (Open Access)
O Depósito Aurífero Piaba no fragmento ratônico São Luis (NW-Maranhão): petrografia das rochas hospedeiras e fluidos mineralizadores
(Universidade Federal do Pará, 2012-08-29) FREITAS, Saney Cecílio Ferreira de; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706; https://orcid.org/0000-0003-4598-9249
The Piaba gold deposit became, in 2010, the first gold mine in operation in the São Luís Cratonic Fragment, northwestern of the State of Maranhão, Brazil. The tectonic setting comprises the metavolcano-sedimentary rocks of the Aurizona Group and the granitoids of the Tromaí Intrusive Suite, along with minor units. All these rocks formed in island arc setting between 2240 and 2056 Ma. Gold mineralization at Piaba is hosted in a fine-grained granophyric granodiorite (Piaba Granophyre) and in a subvolcanic andesitic rock belonging to the Aurizona Group. The ore bodies are hosted in the Piaba fault, which is an ENE-WSWtrending brittle-ductile structure. These ore bodies consist of stockwork veins and veinlets and their hydrothermal haloes. The petrographic study was undertaken in drill cores of the mineralized zone. This study has shown that the host rocks have been affected by strong hydrothermal alteration that provoked intense mineralogical transformations of the rocks. The most prominent alteration processes have been the sericitization and chloritization of the host rocks, occurring in pervasive and fissure-filling modes. In addition, carbonatization, silicification and sulfidation occurred in response to fluid flow and veining along fractures and small faults associated with the Piaba fault. These latter types of alteration appear to be associated with gold precipitation. Petrographic, microthermometric and Raman microspectroscopic studies have been undertaken in quartz from mineralized veins from a stockwork zone. These studies identified two and three phase aqueous-carbonic fluid inclusions that have been interpreted as produced by heterogeneous trapping during immiscible phase separation. Late aqueous inclusions have also been identified. The mineralizing aqueous-carbonic fluid is composed of CO2 (5 – 24 mol%, density of 0.96-0.99 g/cm3), H2O (74 – 93 mol%), N2 (≤1 mol%), CH4 (≤1mol%), and has 5.5 wt. % NaCl equivalent. The gold ore deposited at 267-302ºC and 1.25-2.08 kbar, corresponding to depths of 4 to 7 km. Fluid composition and the P-T interval of the mineralizing fluid, its relatively reduced character (log ƒO2 -31.3 to -34.3), and the strong sulfidation of the host rocks indicate that gold has been transported as a sulfur complex and was deposited in response to phase separation and lowering of ƒO2 and of the sulfur activity during fluid-rock interaction. These conditions are consistent with the brittle regime and mesozonal to hypozonal conditions. Comparing the geological and fluid P-T-X characteristics of Piaba with those of other gold prospects already investigated in the same region, it is possible to define Piaba as an orogenic gold deposit.
Acesso aberto (Open Access)
Estudo de inclusões fluidas e química mineral do depósito aurífero do alvo Jerimum de Baixo, campo mineralizado do Cuiú-Cuiú, província aurífera do Tapajós, Pará
(Universidade Federal do Pará, 2018-03-06) OLIVEIRA, Helder Thadeu de; BORGES, Régis Munhoz Krás; http://lattes.cnpq.br/4220176741850416
The Jerimum de Baixo gold target is located in the Cuiú-Cuiú golfield, central region of the Tapajós Gold Province, Amazonian Craton. The target comprises monzogranitic rocks, essentially isotropic, that were weak to strongly hydrothermal and carriers of Fe-rich biotite. Chloritization, sericitization, sulfidation, silicification and carbonatization are the most important types of alteration. The produced chlorite is enriched in Fe of the chamosite type and was formed mainly between 280 and 315°C, whereas the white mica assumes muscovitic compositions. The mineralization is represented by quartz veinlets with low sulfide content (pyrite + pyrrhotite ± chalcopyrite ± galena ± sphalerite) in which gold occurs as free-milling particles and in more fragil and altered zones, usually associated with pyrrhotite. The petrographic and microtermometric study of fluid inclusions hosted in quartz veinlets defined aqueou-carbonic, carbonic and aqueous inclusions. The fluids with CO2 represent the probable mineralizing fluid and were generated by phase separation processes between 280 and 380°C, mainly. Further infiltration and mixing processes are indicated for the later aqueous fluids. Temperatures <400°C and the reduced character of the environment (pyrrhotite compounding the ore) point to H2S as the major ligand in the mineralizing fluid and Au (HS)-2 as the primary gold transporting complex. Phase separation, changes in pH conditions, and fluid/rock interaction were the important mechanisms for Au precipitation, which occurred at the brittle to locally brittle-ductile level of the crust (between 2 and 6 km). In general terms, Jerimum de Baixo presents similarities among the other deposits/targets previously studied in terms of hydrothermal alteration, fluid types and mineralization. The features observed in Jerimum de Baixo do not allow a classificatory framework absolutely adequate to any of the classical metalogenetic typological models. Characteristics such as type and style of hydrothermal alteration, type and low content of sulfides, types of fluids involved, estimated depth for mineralization, metallic association (e.g., S, Bi, Te), together with the good correspondence between the data collected in other deposits/targets in the Cuiú-Cuiú goldfield indicate for the Jerimum de Baixo target a gold deposit with magmatic-hydrothermal affiliation, presenting greater similarity to those deposits classified as belonging to Reduced Intrusion-Related Gold Systems (RIRGS).
Acesso aberto (Open Access)
Estudo dos fluidos hidrotermais relacionados às "brechas" mineralizadas com sulfetos de ouro da área Bahia-Carajás
(Universidade Federal do Pará, 1998-11-23) ALMADA, Maria do Carmo Oliz; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983
The Bahia deposit is hosted by the Igarapé Bahia Group, of Archean age, which occurs in the Mineral Province of Carajás (SE of Pará). This group consists of basic metavolcanic, metapyroclastic and clastic metasedimentary rocks, in addition to banded iron formations and breccias, being cut by dikes of basic composition. All this rocky package is intensely altered by hydrothermalism that generated mineral associations compatible with the greenschist facies. The Bahia deposit is made up of the bodies Acampamento Norte, Acampamento Sul, Furo Trinta and Alemão. In the first three, mineralization is widespread and is mainly located in breccias. The latter, more recently discovered, is formed by massive sulphide lenses. The present work was based on testimonies of seven drill holes that were executed in Corpo Acampamento Sul. The breccias occur in the contact, today verticalized, between the basic metavolcanic rocks and clastic metasedimentary rocks and generally present gradational contacts with the host rocks. The clasts originate from metavolcanic rocks of basic composition and from banded iron formations, constituting angular to subangular fragments with dimensions between 1 and 3 cm most frequently. The matrix consists of chlorite, siderite, chalcopyrite, quartz, magnetite, tourmaline and calcite, with a predominance, in general, of one or two of these minerals. The matrix/clast ratio is variable and some breccias show slight matrix foliation and clast orientation. Sometimes, interspersed with breccias, massive beds of magnetite and sulfides occur. These breccias are suggested to have a phreatic origin, whose fragments were reworked and transported to deeper zones by debris currents. In breccias, mineralization also occurs in veins and pockets where sulphides join quartz and/or siderite and are more enriched in copper and gold than in host rocks. Chalcopyrite and pyrite are the main sulfides in all lithological types, but in the stratiform massive beds of magnetite and sulfides, bornite is also present. Magnetite is an abundant mineral, occurring in fragments of banded iron formation, disseminated in the matrix of breccias or in massive beds. Chloritization, carbonation, magnetization and sulfidation are the most important types of hydrothermal alteration, with silicification and tourmalinization also being recorded in a subordinate manner. Chloritization has affected almost all rocks to a greater or lesser extent, lending them a characteristic greenish tinge. Carbonation, magnetization and sulfidation are represented, respectively, by the precipitation of siderite and calcite, magnetite and chalcopyrite, pyrite and bornite. Studies on quartz crystals revealed biphasic and triphasic aqueous inclusions chemically represented by the H2O-NaCl-CaCl2 system, with variable salinity (5.3 - 41.5 % eq. NaCl). Inclusions with pure CO2 were also found, some with traces of N2. Homogenization temperatures were more frequent in the ranges of 110-140ºC for biphasic inclusions and 150-225ºC for aqueous triphasic inclusions. With the pressure correction made, the prevailing conditions for the formation of the Bahia deposit were estimated at 160-240°C and 1-2 kbar. The aqueous fluids were interpreted as seawater modified as a result of cognitive movement that made it circulate and leach metals from the volcanic sedimentary package for later deposition in the form of sulphides. On the other hand, the carbonic fluids are a probable mantle source and are responsible for the precipitation of siderite. The chlorite geothermometer proved to be inadequate to define the paleotemperatures, given the very different results obtained according to the equation and correction for Al (IV) used. The geological characteristics of the Bahia deposit favor its interpretation as a Besshi-type volcanogenic deposit, which may have incorporated, during later events, uranium and rare earths that occur in it at anomalous levels.
Acesso aberto (Open Access)
Estudo hidrogeoquímico das regiões de Salobo e Pojuca, Carajás-PA
(Universidade Federal do Pará, 1992-07-20) SILVA, José Francisco Berrêdo Reis da; RAMOS, José Francisco da Fonseca; http://lattes.cnpq.br/8189651755374537
This study aims to verify the use of the hydrogeochemical prospecting method in waters draining metasedimentary and metavolcanic rocks from the Salobo and Pojuca areas, mineralized to Cu and Cu-Zn, respectively. As a result, the chemical composition of these waters is bicarbonated, with important contributions of Mg, Na and Ca and high levels of silica, which characterize evident geochemical relationships between the aqueous medium and the geology of these regions. The pH of the waters is weakly acidic, with low concentrations of sulfate, contrary to what is observed in areas of similar mineralogy. The rainfall regime contributes to the maintenance of total dissolved solids levels, removing soluble salts accumulated in dry periods, aided by topography. The use of dissolved heavy metals to locate mineral deposits is quite limited, with large concentrations linked to stream sediments, probably iron and manganese oxyhydrate phases. However, zinc has high levels in solution, above the established background, capable of delineating important hydrogeochemical halos in the “Pojuca Area”.
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Estudos de inclusões fluidas e isotópicos (Sr, C, O, H) e implicações para a mineralização aurífera no alvo Enche Concha, Cinturão Gurupi.
(Universidade Federal do Pará, 2019-08-19) MELO JUNIOR, Reinaldo Fontoura; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706
The Gurupi Belt is a Neoproterozoic orogen of NNW-SSE orientation developed at the southsouthwest margin of the San Luis Cratonic Fragment, of Paleoproterozoic age. The belt is composed of metavolcano-sedimentary sequences, gneisses and several generations of plutonic rocks representing reworked fragments of the cratonic border and portions of the basement on which the Gurupi Belt developed in the neoproterozoic. The Chega Tudo Formation, the main host for gold mineralization in the Gurupi belt, is a metavolcanosedimentary sequence that is characterized by being a narrow and elongated band in the NWSE direction. The rocks of this unit comprise an alternation metavolcanic, metavolcanoclastic and metassedimentary varieties, which are invariably schistose and/or mylonitic, and locally folded. Enche Concha is one of the gold prospects that occur in the Chega Tudo Formation and a better definition of it’s geological characteristics and genetic aspects is presented here. The host rocks are phyllites, dacites and breccia zones. The phyllites are rocks of blackish greenish colors, consisting essentially of quartz, micas and carbonate. Dacites are whitish gray-colored rocks consisting primarily of plagioclase and quartz. The breccias are fragmented, non-cohesive rocks, with no obvious tectonic fabric consisting of fragments of quartz and carbonate veins, and of fragments of phyllites. The types of hydrothermal alteration defined in the target was as follows: (i) silicification, (ii) sericitization, (iii) carbonatization and (iv) sulfidation, along with precipitation of gold. The sulfides are mainly pyrite, along with sphalerite and chalcopyrite. Gold occurs as inclusions in the three sulfide minerals, with concentrations of up to 6%, as identified by analyzes of Scanning Eletron Microscopy. The fluid inclusion were classified as type 1 (biphasic aqueous-carbonic - H2Oliquid - CO2liquid and H2Oliquid - CO2vapor) and type 2 (aqueous, single-phase and twophase H2Oliquid and H2Oliquid - CO2vapor). Type 1 inclusions provided CO2 melting temperature values (TfCO2) between -56.6 and -57.3ºC, which allows inferring that the main volatile component of the carbonic phase of these inclusions is CO2. The CO2 homogenization temperature values (ThCO2) are between 12 and 25.5ºC (global density between 0.7 and 1 g/cm3, CO2 density between 0.2 and 1 g/cm3). As for the melting temperature of the clathrate (TfClat), results were obtained between 6 and 8.4ºC, which gives salinities between 4.4 and 5.5 wt% NaCl equiv. The total homogenization temperature (Tht) of these inclusions varied between 251 and 369º, with homogenization occurring both for the liquid and vapor phases. Type 2 inclusions presented values of melting temperature of the ice (Tfg) between -0.1 and - 4.1 ºC, and Tht values between 167 and 342ºC,with homogenization occurring for the liquid phase. These inclusions presented low values of salinity, between 0.18 and 6.3 wt.% in NaCl equiv. weight, and global density between 0.7 and 0.9 g/cm3. The coexistence of aqueous and aqueous-carbonic inclusions over the same range of Tht, in addition to inclusions that homogenized for the liquid phase and for the vapor phase, suggest fluid immiscibility. The isotopic composition of oxygen and carbon was determined hydrothermal quartz and calcite, whereas hydrogen isotopic compositions were obtained from inclusion fluids. The values of δ18O in quartz vary from +15.25 to +17.74‰. The values of δ18O and δ13C in calcite range, respectively, from +14.32 and +16.24‰ and from -9.83 to -15.12‰. Values of δ13C bellow - 10‰ suggest contribution of organic C from the host rocks. Values of δD extracted from the inclusion fluids gave results of -25 and -28‰. The isotopic results suggest metamorphic composition for the mineralizing fluids at Enche Concha. The strontium isotopic composition was determined in hydrothermal calcite. The analyzed samples have low, little radiogenic 87Sr/86Sr ratios, ranging from 0.702699 and 0.705141. These results indicate origin of deep sources of the lower crust or mantle and should not have contribution of the regional rocks of the Gurupi Belt. As a whole, the integrated data indicate that the Enche Concha shares characteristics whit those of orogenic gold deposits, which are widespread in the Gurupi belt.
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Estudos de inclusões fluidas e isótopos estáveis nos alvos Jerimum de cima e Babi, campo mineralizado do Cuiú-Cuiú, Província Aurífera do Tapajós, Cráton Amazônico: implicações para os processos genéticos
(Universidade Federal do Pará, 2015-08-18) SILVA JUNIOR, Carlos Alberto dos Santos; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706
The Cuiú-Cuiú goldfield is located near the central portion of the Tapajós Gold Province in the south-central portion of the Amazonian Craton. This goldfield is one of the oldest prospecting areas of the province and holds multiple more or less developed prospects and gold deposits (Central, Raimundinha, Pau da Merenda, Guarim, Jerimum de Cima, Jerimum de Baixo, Nho, Moreira Gomes, Babi and other less known). As contribution to the understanding of the metallogenic evolution of the Cuiú-Cuiú goldfield in general, this study focused on the mineralized Jerimum de Cima and weakly mineralized Babi targets and aimed: (1) to define the sulfide mineralogy associated with gold mineralization and its textural relationships with the host rocks; (2) to define the physical and chemical characteristics of the mineralizing/hydrothermal fluids through petrographic, fluid inclusions and stable isotopes (C, O, S) studies trying to identify what caused the hydrothermal alteration in rocks from these targets and that enabled more significant mineralization at Jerimum de Cima (and other targets/deposits), whereas Babi is only weakly mineralized (not economic). The petrographic study identified strongly hydrothermally altered host rocks, with obliterated primary characteristics. In the Jerimum de Cima target the host rocks are biotite-hornblende tonalite, monzogranite and granodiorite. In the Babi target, titanite monzogranite, biotite monzogranite, biotite-hornblende tonalite, and brecciated monzogranite are the hydrothermally-altered rocks. Sericitization, silicification and sulfidation occur strongly in the Jerimum de Cima target, whereas carbonatization and chloritization occur usually in both targets. Pyrite, sphalerite, chalcopyrite and galena, in decreasing order of abundance, are the sulfide minerals, with large predominance of pyrite. Fluid inclusions (FI) trapped in quartz crystals occur in small groups, in isolation, or in trails. In decreasing order of abundance, there are three types of FI: two-phase aqueous (Type 1), aqueous-carbonic (Type 2) and carbonic (Type 3). The microthermometric results show that the aqueous FI at Jerimum de Cima homogenized between 105 and 387°C, and have salinities that range from 0,0 to 18 wt.% NaCl equivalent; whereas the aqueous-carbonic type has final homogenization temperatures between 144 and 448°C, salinities of 1,0 to 7,8 wt.% NaCl equivalent, and bulk density ranging from 0,6 to 1,0 g/cm3. At Babi the aqueous FI are the only type present. These FI homozenized between 136 and 410°C and show salinities from 0,7 to 13,2 wt.% NaCl equivalent. The aqueous-carbonic FI are interpreted as a product of fluid immiscibility (phase separation). The absence of CO2–bearing inclusions in the Babi target is possibly a consequence of late-timing of fluid trapping during the evolution of the hydrothermal system, after the CO2 consumption, with only aqueous FI being trapped. Stable isotope analyses of hydrothermal minerals present in veins and alteration zones indicate mineral precipitation between 305 and 330°C and between 108 and 205°C, which is in line with the fluid inclusion honogenization temperatures and indicate more than one stage of mineral precipitation. The data also suggest magmatic and meteoric sources for the fluids. As a whole, our data are compatible with a magmatic-hydrothermal gold systems (intrusion-related), and with mixing of magmatic and meteoric fluids. The lack of CO2 at Babi might explain the weak mineralization in this target.
Acesso aberto (Open Access)
Estudos isotópicos (Pb e Nd) e de química mineral do depósito aurífero Cipoeiro, Cinturão Gurupi, estado do Maranhão
(Universidade Federal do Pará, 2018-04-27) EL-HUSNY NETO, Chafic Rachid; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706
The Cipoeiro orogenic gold deposit, located in the Gurupi Belt, Maranhão State, Brazil, is hosted by tonalite of the Tromaí Intrusive Suite (2148 Ma), and shows the higher concentration of gold known to date (61.9 t Au). In order to contribute to the knowledge of the metallogeny of this deposit, this work looked for: to define the composition and temporal sequence of the hydrothermal mineralogy and/or types of hydrothermal alteration; to investigate the chemical composition of the ore; to identify potential sources of Pb and Nd in the mineralization; and to estimate the age of the mineralizing event. The studies have shown that the tonalite is strongly altered and locally deformed, which caused the obliteration of the primary mineralogy and textures. The hydrothermal alteration has distal and proximal variation and occurs in the pervasive and fissural/venular forms. The distal alteration is pervasive and comprises chlorite and sericite. The proximal alteration is pervasive and fissural/venular and is composed of quartz, chlorite, sericite, calcite, pyrite and subordinate amounts of chalcopyrite, sphalerite and galena, in addition to the gold mineralization and a set of tellurides. The gold occurs in three forms: (1) particles included in pyrite, (2) precipitated in pyrite fractures, and (3) free-milling, in quartz veins. The telluride mineralogy comprises petzite (Ag-Au), hessite (Au) and sylvanite (Au-Ag), and subordinate coloradoite (Hg), kochkarite (Pb-Bi) and volynskite (Ag-Bi). The mineralization are compatible with the greenschist facies conditions. The chlorite-pyrite-sphalerite equilibrium along with the telluride composition allowed the estimation of log fO2 in the range of -29.6 to -33.2, and log fS2 ranging from -9.6 to -10.6, indicating relatively reduced fluid conditions. This values, along with available physico-chemical data, suggest gold transportation as a reduced sulfur complex. Isotopic studies, Pb in pyrite and Nd in calcite, allowed considering that the source of the fluid is likely a mixture of different regional sources, caused by the fluid-rock interaction during the ascent of the fluid through structures to the site of ore deposition. The age of mineralization could not be defined unequivocally, but the Pb and Nd isotopes indicate the Paleoproterozoic as the most probable age of the mineralization.
Acesso aberto (Open Access)
A Formação Palmeiral (Proterozóico Superior) na serra dos Pacaás Novos, oeste de Rondônia
(Universidade Federal do Pará, 1997-03-18) BAHIA, Ruy Benedito Calliari; TRUCKENBRODT, Werner Hermann Walter; http://lattes.cnpq.br/5463384509941553
A Serra dos Pacaás Novos, localizada na região oeste do Estado de Rondônia, nas adjacências da cidade de Guajará Mirim, e encaixada na porção ocidental do Cráton Amazônico, é constituída mormente de conglomerados e arenitos da Formação Palmeiral, do Proterozóico Superior. Esta idade é baseada na datação Rb/Sr de rochas vulcânicas básicas intercaladas na base desses depósitos, da parte oriental da serra. A Formação Palmeiral, objeto deste trabalho, foi estudada sob os pontos de vista faciológico e tectono-estrutural. A análise faciológica distinguiu 6 litofácies que incluem: (1) ortoconglomerado maciço ou com estratificação incipiente, como fácies predominante (Gm); (2) arenito com estratificação horizontal (Sh); (3) arenito com estratificação cruzada acanalada (St); (4) arenito com estratificação cruzada tabular (Sp); (5) arenito com estratificação sigmoidal (Sl); e (6) arenito maciço (Sm). A fácies conglomerática, interpretada como barras longitudinais ou lençóis empilhados, gerados durante eventos de alta descarga, contém níveis de lentes arenosas, que representam depósitos formados durante períodos de baixa descarga em canais menores. Esta variação na descarga, aliada com a capacidade transportadora variável do fluxo, e a pouca profundidade do leito sugerida pela estruturação dos lençóis conglomeráticos, aponta para os sedimentos Palmeiral, um estilo fluvial “braided”, semelhante ao tipo “Scott” de MIALL (1977). Entretanto, ficam dúvidas se este padrão relaciona-se a um sistema fluvial proximal ou leque fluvial. Corpos conglomeráticos com fácies comparáveis àquelas da Serra dos Pacaás Novos, encontradas a mais de 100 km a norte da serra mostrando o mesmo padrão de paleocorrente para SW, parecem favorecer a interpretação de um sistema fluvial proximal. A proveniência dos depósitos Palmeiral é indicada pelos diferentes tipos de seus eixos; ocorrem seixos de quartzo, quartzito, arenito e rochas vulcânicas, os quais apenas em parte são oriundos do embasamento cristalino. Contudo, fragmentos de rochas do embasamento mais próximo (Complexo Jamari), que bordeja a serra, não foram encontrados. A Formação Palmeiral passou por vários processos diagenéticos incluindo sobrecrescimento de quartzo, formação de caulinita e ilitização. Podem ocorrer pseudomorfos de ilita a partir de caulinita. A dissolução por pressão é bem documentada por concavidades em seixos da fácies conglomerática. Na Serra dos Pacaás Novos, A Formação Palmeiral está confinada numa estrutura de graben, que possivelmente no Cenozóico sofreu inversão de relevo,, responsável pela atual configuração morfológica da serra. A análise estrutural dessa formação possibilitou a separação de três setores. O setor I, situado no extremo norte da serra, apresenta a forma da cunha, com direção geral NE-SW, caracterizada pelas falhas/fraturas rúpteis-dúcteis verticalizadas. O setor II, com orientação WNW-ESE, ocupa a porção central da serra. Encoberto parcialmente por sedimentos recentes, é constituído por zonas de cisalhamento dúcteis que interligam-se formando amêndoas, verticalizando o acamamento das rochas. O setor III, localizado na parte sul da serra, apresenta duas zonas de cisalhamento dúcteis, na direção E-W, resultando num duplex transcorrente.
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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.
Acesso aberto (Open Access)
Geologia e petrologia da região central da Serra da Cordilheira-Goiás
(Universidade Federal do Pará, 1984-09-21) TEIXEIRA, Nilson Pinto; DALL'AGNOL, Roberto; http://lattes.cnpq.br/2158196443144675
The area in focus was mapped at a scale of 1:100,000 and is located west of the city of Araguaína (Goiás) in a region located in the districts of Aragominas (formerly Pé do Morro) and Muricilândia and its surroundings. It is limited by the parallels of 7°00'00" and 7°25'23" of south latitude and by the meridians of 48°23'36" and 48°40'00" of west longitude, making a total of approximately 1395km². Geologically, it is located, for the most part, within the Araguaia Folding Belt. In this area occur the domic structures of Grota Rica and Cocalândia, in whose cores outcrop gneiss rocks, amphibolites and alkali-alumino-magnesian rocks, which constitute the Colméia Complex. These structures are similar to those previously described in the regions of Xambioá, Lontra, Colméia, Cantão, Rio Jardim and Rio Cunhãs, being enhanced by the quartzites and mica-schists of the Morro do Campo Formation. This gradually passes to mica-schists of the Xambioá Formation, which are the regionally dominant lithological types. The Xambioá Formation also contains, locally, amphibolite lenses. The set of constitutive rocks of the Morro do Campo Formation (lower) and Xambioá (upper) make up the Estrondo Group. Metamorphosed ultrabasic bodies occur locally, forming aligned hills with a submeridian disposition. These ultrabasic bodies, together with the units of the Estrondo Group, constitute the Baixo Araguaia Super Group. The Estrondo Group is covered to the east by sediments from the Pimenteiras Formation of the Parnaíba Sinéclise and to the west by Tertiary-Quaternary Covers. Among the gneiss rocks of the Colméia complex, trondhjemitic and granitic gneisses were identified, the former being largely dominant. In Trondhjemitic gneisses, two bands (1 and 2) have been described. Banding 1 has restricted occurrences and is folded in at least two deformation phases, with the first folding, absent in the other units, presenting orientation and relating to an older thermotectonic cycle (presumably from Archean), while the second with N-S orientation, it is related to the thermo-tectonic cycle that generates the constitutive rocks of the Baixo Araguaia Super Group (Upper Precambrian). Banding 2 is associated with this cycle, it is strongly penetrative, truncates locally and banding 1, presenting a parallel disposition to the axial trace (in plan) of the second fold. Banding 2 is generally shaped to domic structures. Associated with the first cycle (Archaean) and in agreement with the banding 1, there are quartz-feldspathic veins, apparently related to the oldest phase. There are also other quartz-feldspathic veins, sometimes concordant, sometimes discordant, with banding 2. Chemically, trondhjemitic gneisses almost always have a slightly variable composition, the same being true for granitic gneisses. Quartz-feldspathic veins concordant and discordant with banding 2 have their origin attributed to anathexic processes, which occurred at depth, during the second cycle (of the Upper Precambrian). These processes were partial and restricted, and allied to other factors, allowed the diapiric ascent of the gneisses of the Colméia Complex, whose lodging generated the domic structures, under metamorphic conditions equivalent to the average degree. In the Xambioá Formation, the dominant rocks - mica-schists - revealed, through their petrographic and chemical characteristics, a predominance, among the original sediments, of pelites and greywackes, with smaller contributions of carbonates. In the petrographic study, a close relationship was observed between the absence of garnet and the presence of epidote and vice versa, although there is incompatibility between them. Chemical analyzes showed that these relationships are controlled by the CaO content of each sample. When it exceeds a certain value, epidote crystallization is favored and garnet crystallization is inhibited, the opposite occurring in the opposite case. This compositional variation is observed even on a millimeter scale and is reflected in the area through the erratic distribution of the garnet, which makes it impossible to trace any isograde based on the appearance of this mineral. A similar phenomenon is observed in relation to staurolite, whose crystallization is only possible in rocks of adequate composition, a condition that is not satisfied by almost all the samples of the analyzed shales. In this case, the critical factor is the FeO content of the rock, which must be high to allow the formation of staurolite. The paragenesis present in the different lithological types of the Baixo Araguaia Super Group indicate that the metamorphism reached minimum conditions equivalent to the upper weak degree and maximum conditions consistent with the beginning of the medium degree, with a reasonably high pressure regime (5 to 6kb). The crystallization of garnet, epidote and staurolite was controlled by the composition of the original sediments. This fact was also observed a few kilometers to the north in the Xambioá region and a few kilometers to the south in the regions of Cantão, Rio Jardim and Rio Cunhãs. The structural study of the Baixo Araguaia Super Group allowed the characterization of at least five phases of deformations within a progressive deformation process, with the first four developed in a compression regime, generating different folds and inverse faults (?), while the latter developed in a stage of relaxation of efforts. A major accident is the Inverse Fault (?) of Aragominas of N-S orientation, which coincides with the Serra das Cordilheiras, which cuts the mica-schists of the Xambioá Formation, affecting them intensely. This unit outcrops both east and west of the escarpment, and it can be assumed that the other units have also been affected by the fault, although they are not exposed along the same current level of erosion. Other expressive lineaments with NNW-SSE and NNE-SSW orientations, better marked in the Xambioá Formation domains, also occur, some of which may correspond to directional faults along S2. From the interpretation of the data obtained and their integration with the existing ones, it is concluded that the geological evolution of the studied region was polycyclic, distinguishing different phases in each cycle.
Acesso aberto (Open Access)
Geologia e petrologia da região de Serra Negra do Norte (RN-PB)
(Universidade Federal do Pará, 1984-08-25) GONÇALEZ, Maria das Graças Bonfin; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983
The Serra Negra do Norte area belongs to the geographic domain of Seridó, located in the Northeastern folding region, consisting of orthogneisses, probably from the Lower Proterozoic. These rocks have a granitic and quartz-monzonitic composition and comprise an association predominantly of augen gneisses, housed in the gneisses of the Caicó Group, belonging to the Archean basement, and in the supracrustal metasediments, represented in the area by the Jucurutu Unit, which is the base of the Seridó Group. Cutting through the orthogneisses and metasediments, an Intrusive Suite occurs, related to the tectonic events of the Brasiliano cycle, composed of three distinct temporal placement units and represented by granites, granodiorites, quartz-monzonites, quartz-monzodiorites and quartz-diorites. Three deformational events are recorded in the Serra Negra do Norte area. The oldest, dating back to the Trans-Amazonian cycle, affected both the cover sediments and the intrusion that metamorphosed into the heights of the gneisses. In the rocks of the Intrusive Suite, whose geological evolution culminated with the end of the Brasiliano cycle, the last two deformational events are recorded, while in the peak gneisses only the effects corresponding to the last of these events were observed. The petrographic evidences and the geochemical behavior of the major elements and, mainly, of the traces support the idea that the metamorphism, related to the first deformational event, was of the allochemical type, and may have been responsible for the k-feldspatization of the pre-augen gneisses, whose intensity, although not fully known, was not enough, however, to completely eliminate some primary textural and chemical characteristics. ”. However, the behavior of some major elements and traces is not consistent with the generation of the several units of this suite through continuous processes of magmatic differentiation, although they have acted in each unit individually. On the other hand, based on comparisons made with rocks from similar geological provinces, a petrogenetic evolution is suggested for the rocks of Units 1 and 2 and those of the Southern Body of Unit 3, via partial melting processes, either from sources or from a single source, probably located in the lower crust and undergoing different degrees of partial melting. As for the North Body of Unit 3, an origin is also suggested by partial melting, from a material where the peak gneisses made an important contribution. Significantly, the rocks of the Serra Negra do Norte area reveal striking structural, stratigraphic and chemical similarities, with rocks from Proterozoic magmatic provinces of Nigeria, indicating that these areas underwent analogous evolutionary processes provided by the physical continuity that existed between them in pre- Mesozoic drift.
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Petrografia, caracterização química e significado geológico dos metassilexitos e formações ferríferas do Grupo Tocantins, centro oeste do Cinturão Araguaia
(Universidade Federal do Pará, 2013-11-19) COSTA, Nivia Oliveira da; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983
Metacherts and iron-formations from de Agostinho, Grande, Jabuti, Pau Ferrado and Salto hills that occur in the central-western portion of the Araguaia fold belt in the Tocantins state have been investigated. These rocks are interlayered with slates, phyllites and metagreywackes (Tocantins Group), derived from siliciclastic sediments that were deposited in a Neoproterozoic proto-oceanic basin. They are locally associated with chloritites and more commonly with lenses of serpentinized or talcified ultramafic rocks. Metabasalts occur near Araguacema town, in the western part of the studied area, where they exhibit well preserved pillowed structures. The metacherts are massive to laminated and dominantly grey-colored. They show microcrystalline texture and normally are cross cut by quartz veinlets. Quartz is the chief mineral, although subordinate amounts of magnetite, hematite, talc, chlorite, chromite, muscovite, kaolinite and rutile are present. In the banded iron-formations, mm- to cm- scale bands rich in microcrystalline quartz alternate regularly with bands rich in hematite and magnetite, associated with minnesotaite, stilpnomelane, rutile, chromite and tourmaline. Goethite replaces hematite to a lesser or greater extent. Chemically, the metacherts present SiO2 contents higher than 90% and Fe2O3 contents that range from 2 to 8.4%. Also variable are the MgO concentrations that reach 9.55%, although most values are less than 0.1%. Al2O3 contents are normally below 0.6%, but in three samples they record >2%. These variations reflect basically the modal composition of these rocks. Concerning the trace elements, only Ni, Co and Cu have some significance with averages of 222 ppm, 122 ppm e 40 ppm, respectively. Gold reveals low values, except in a sample (27.4 ppb). REE total (in general, ≤ 67 ppm) is lower than the standard North American Shale Composite (NASC). Only two samples contain higher ETR values (154 and 237 ppm). The REE distribution pattern, normalized to the NASC, varies significantly from a metachert sample to another. The LREE tend to be enriched with respect to the HREE, although the contrary is also observed. Most samples present important Ce negative anomalies, whereas others show slightly positive or no anomalies at all. Similar finding is valid for Eu, although most values are positive. In the ironformations, the average Fe2O3(total) content is 76%. Silica concentrations are higher than 14% (average of 21,3%), but in just one sample it is as low as 2,95%. Al2O3 contents vary from 0.21% to 2.35%. Other major components reveal very low contents. Regarding the trace elements, the iron-formations differ from the metacherts in that the contents of Ni and Co are highly variable (averages of 20 and 30 ppm, respectively) and Co averages 23 ppm. Copper concentrations are low, but exceed 120 ppm in a few samples. REE total in iron- formations is comparable to that of the metacherts (≤ 89 ppm). However, the distribution of these elements, normalized to the Post-Archean Average Australian Sedimentary Rocks (PAAS) standard, displays a more systematic pattern than in the metacherts. The geological context related to a proto-oceanic basin, the anomalous contents of trace elements such as Ni, Zn, Co and Cr, the negative Ce and positive Eu anomalies frequent in both metacherts and iron-formations, suggest that the protoliths of these rocks formed by exhalative activity in a submarine environment. Nevertheless, the chemical sediments did not precipitate from high temperature hydrothermal fluids, but underwent some contamination from terrigenous sediments. The positive Y anomalies, also detected in the iron-formations samples, are common in modern seawater and suggest that the precipitation of the ferruginous material was relatively fast and favored by the migration of reducing, slightly acid marine waters to shallow environments where the water was more alkaline and oxidizing. The association of some metacherts and iron-formations with chloritites, serpentinites and hydrothermally altered mafic/ultramafic rocks supports the interpretation of a ocean-floor environment and allows considering the investigated sequences as slices of ophiolitic complexes that have been tectonically dismembered and emplaced into the Tocantins Group rocks. As such characterized, this portion of the Araguaia fold belt is potentially favorable to the exploration of both exhalative and podiform chromitite deposits.
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Petrologia do maciço alcalino de Peixe-Goiás
(Universidade Federal do Pará, 1983-12-21) LEMOS, Ronaldo Lima; ALBUQUERQUE, Carlos Alberto Ribeiro de
The Alkaline Fish Massif consists of a nepheline syenite gneiss body, leucocratic, foliated and banded. Foliation is accentuated at the edges and incipient or absent at the center, while banding is restricted to the edges. The Massif is surrounded by a narrow band of quartz syenite and granite gneisses, in contact with pre-Serra da Mesa Group metasediments, predominantly pelitic (graphite-sillimanite micaschist and sillimanite-quartz micaschist). The nepheline syenite gneiss consists essentially of nepheline, mesopertite, peristerite and microcline, with biotite being the mafic mineral and, more rarely, magnetite. Its agpaitic coefficient is typical of miaskitic rocks, and its mineralogy is also of this type. The different textural types of nepheline show variation in the Si/Al ratio, while the Na/K ratio is approximately constant. Peristerite has composition An4.0-5.1 -An0.5-2.8 and, in a narrow band of the edge, it is An7.0-10.7 -An0.4-4.0. The , microcline and potassium phase of mesopertite (microcline) have composition Or8 3.0-9 4.4, while the sodium phase of mesopertite is predominantly peristerite (An4.0-5.1 -An0.7-2.8) and sometimes it is albite (An0.4-2.6). Biotite is of the lepidomelane-siderophyllite type. The composition of nepheline syenite gneiss, similar to the minimum point in the Ne-Ks-Qz diagram, the variation in the ratio, Si/Al. of nepheline, typical of nepheline crystallized from a nepheline-like syenite magma under plutonic conditions, as well as the trends of the elements Si, Al, Na and K and the K/K + Na ratio in differentiation diagrams, which are coincident with the variation observed in series of alkaline igneous rocks, indicate that the nepheline syenite gneiss resulted from a nepheline syenitic magma, generated in the lower crust or upper mantle, in a tectonically stable environment. During the ascent of this magma, the increase in the geothermal gradient caused fusions in rocks of the crust, generating magmas of quartz syenitic and granitic composition. The nepheline syenite body, the quartz syenite and granite belt and the pre-Serra da Mesa Group metasediments were later affected, together with the sediments of the Uruaçú Development Belt (Serra da Mesa Group), by a regional medium pressure type metamorphism. in the amphibolite facies, with a temperature of 650 ± 30°C and a pressure of 6 ± 0.5 kb. The metamorphism took place in the presence of a fluid phase rich in H2O and CO2, in addition to S03, F and P originated by the decomposition of late-magmatic minerals from nepheline syenite, rich in volatile components, which had great importance during the metamorphism in the formation and recrystallization of minerals and, in local processes, of autometasomatism. This Massif presents striking analogies with the alkaline complexes of Blue Mountain, Bigwood and Darkainle.

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