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Item Acesso aberto (Open Access) 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/1406458719432983The 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.Item Acesso aberto (Open Access) Alteração supergênica das rochas básicas do grupo Grão-Pará: implicações sobre a gênese do depósito de bauxita de N5 - Serra dos Carajás(Universidade Federal do Pará, 1981-11-19) LEMOS, Vanda Porpino; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983The present study deals with the supergene alteration of the basic volcanic rocks of the Grão Pará Group and gathers evidences that support the N5 bauxite deposit (Carajás Sierra) to be an extreme product of this alteration. In the lack of a continuous profile, this hypothesis was tested with a composed profile using the subsurface information available for the contiguous N4 plateau where the whole weathered sequence could be observed. The bauxite-rich and lateritic horizons, only developed in N5, were then assumed to be a part of that sequence and taken as its more superficial levels. The basic volcanic rocks are of precambrian age and were classified as tholeiitic basalts compositionally similar to those of the calc-alkaline suites that occur in modern island ares (TiO2-Zr/P2O5 diagram). The primary mineral assembly is dominantly composed of labradorite and pigeonite-augite and has zircon, ilmenite and opaque minerals as the major accessory phases. Hydrothermal activity caused mineralogical changes in theses rocks producing chlorite, epidote, calcite, sericite, amphibole and quartz. Significant chemical and mineralogical changes were induced by the weathering in these rocks and could be evaluated to depths up to 140 m. The serbidecomposed material showed substantial losses of CaO, Na2O and FeO (this due to partial oxidation to Fe+3) and less important lasses of SiO2, MgO and K2O. In contraposition there was relative enrichment of Fe2O3, Al2O3, TiO2 and P2O5 besides major gains of H2O. This new chemical environment favored the formation of compatible stable phases represented, in order of abundance, by chlorite, smectite-chlorite, opaque minerals and quartz. The totally decomposed basalts revealed an aimost complete loss of alkalls, MgO and CaO, with SiO2 contents dimishing to values of approximately 40% of its initial quantities. This enhanced great relative gains of Fe2O3, Al2O3, TiO2 and P2O5 and H2O with respect to the previous alterations stage. The resulting mineral assembly turned out be dominated by kaolinite, goethite and titanium oxides, and secondarily by gibbsite and quartz. Determinations of Cr, Ni, Co and Zr were done for both the bas.alts and its weathered equivalents. Enrichment factors of the order of 1.5 to 5.0 generally progressive towards the surface indicate the greater or lesser mobility of these elements within the supergene environment. Cr, Ni and Co were retained by coprecitpitation with iron hidroxides while Zr was accounted for by the preservation of zircon as a residual mineral. The N5 bauxite deposit consists of a gibbsite-rich upper layer with an average thickness of 4.7 m and average chemical composition of 3-.13% SiO2, 2.3% TiO2, 47.0% Al2O3, 23.0% Fe2O3 and 24.0% volatiles. Mineralogically it is composed of gibbsite, kaolinite, titanium oxides and iron hidroxides (goethite). The subjacent layers show distinct chemical constitution but the mineral suites differ in the proportions rather than in the kinds of the phases present. Downwards it is observed 1) a lateritic crust as thick as 10 m, 2) a gibbsite-rich clay that do not exceed 35 m in thickness and 3) an argillaceous horizon of indefinite thickness. The lateritic crust has an average chemical composition of 3.6% SiO2, 2.0% TiO2, 28.0% Al2O3, 47.0% Fe2O3 and 19.0% volatiles, and a mineralogy dominated by hematite, kaolinite, iron hidroxides, titanium oxides and subordinate quantities of gibbsite. The gibbsite-rich clay has average proportions of 24.0%, 2.0%, 28.0%, 32.0% and 13.0.% for SiO2, TiO2, Al2O3, Fe2O3 and volatiles respectively while the argillaceous horizon shows average proportions of 47.0%, 1.5%, 20.0%, 22.0% and 7.5% for these same components in that order. The mineral assembly of these two last levels is dominated by kaolinite, gibbsite, titanium oxides and iron hydroxides although hematite appears only in the gibbsite-rich clay while goethite and quartz appear, only ,in, the argillacebus horizon. The identification of heavy minerals in samples of both the decomposed basalt and the bauxite material revealed the same suite consisting of ilmenite, zircon and tourmaline, the latter mineral being found in greater abundances in the bauxite. Boron determinations from samples of the various horizons of both sequentes (N4 and N5) indicated contente that varied from 70 to 100 ppm, justifying /the probable presence of tourmaline even in the rocks from which was not possible to separate heavy minerals. The integration of all these data allowed the interpretation of the N5 bauxite plateau as a residual deposit of the supergene alteration of the volcanic basic rocks of the Grão Pará. Group with basis on 1) the chemical and mineralogical identities between the two sequences, especially the decomposed basalt and the gibbsite-rich clay, 2) the chemical correspondente that suggests the argillaceous horizon to be an intermediate stage of alteration between the semidecomposed and the decomposed basalt, 3) the presence of gibbsite in the decomposed basalt suggesting a stage of evolution which, given enough time and the appropriate conditions, could produce a material progressively rich in alumina and 4) typical trace elements of basic rocks present in relatively high concentrations in the bauxite deposit and, taken the genetic link for granted, showing enrichment or impoverishment factors along a common trend from the unaltered basalt to the bauxite material. Special attention was paid to the lateritic crust that formed underlying the bauxite deposit. It has been interpreted as a result of the relativo mobility of Fe and Pd, under Eh-pH conditions that favored the descending movement of Fe and the fixation of in the upper horizons.Item Acesso aberto (Open Access) Alvo Borrachudo, Serra dos Carajás (PA): rochas ígneas ricas em magnetita e apatita com mineralizações de sulfetos associada(Universidade Federal do Pará, 1996-05-25) FARIAS, Edielma dos Santos; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983Item Acesso aberto (Open Access) Ambiente deposicional, composição mineralógica e estudo isotópico Pb-Pb das zonas sulfetadas do Alvo São Martim, Cinturão Araguaia, sul do Pará(Universidade Federal do Pará, 2004-07-07) LIMA, Aderson David Pires de; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983Item 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/1406458719432983The 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.Item Acesso aberto (Open Access) Estudo da alteração hidrotermal,com ênfase no metamorfismo sódico, de rochas granitóides e máficas da região de Canaã de Carajás, Província Mineral de Carajás(Universidade Federal do Pará, 2007-06-25) SOUZA, Francisca D'ávila Soares de; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983The Sossego and Serra Dourada areas are located in the southern part of Carajas ridge, at the contact zone between the basement rocks (Xingu Complex; ~2.8 Ga) and the Grão Pará Group (2.76 Ga). Serra Dourada lies 15 km east of the Sossego deposit, at the transition strip that bounds the Itacaiúnas block and the Rio Maria granite-greenstone terrane. In the Sossego deposit granitoids, metavolcanic rocks, biotite-rich and mafic rocks were investigated, all of them were hydrothermalized and/or deformed to a greater or lesser extent. In Serra Dourada granitoids, their enclaves and mafic rocks were studied, the former being represented by unaltered, altered and milonitized varieties. The likely protoliths of the apogranitoids and metavolcanic rocks that occur in Sossego were syenogranites/granodiorites and porphyritic quartzodiorite/diorite, respectively, while the mafic altered rocks were derived from microgabros. The porphyroclastic texture of the biotite-rich rocks may be an evidence for their protoliths being, at least in part, the same as those of the metavolcanic rocks. Apogranitoids, metavolcanic and biotite-rich rocks have similar REE patterns, suggesting that they might be comagmatic, although some samples of the biotite-rich rocks show higher La/Yb ratios. Concerning the contents of less mobile elements (Nb and Zr), the metavolcanic and biotite-rich rocks are much more similar. Albitization, scapolitization, amphibolitization, chloritization, biotitization, epidotization and potassic feldspatization were the most important hydrothermal processes that affected those protoliths. Albitization is more common in the apogranitoids and acid metavolcanic rocks, whereas scapolitization is more typical of the biotite-rich and mafic rocks. In the apogranitoids two albite generations (I and II) were identified. The albite I represents crystals with chessboard texture produced by the replacement of potassium feldspar. Albite II is present in monomineralic veinlets which correspond to one of the latest stage of hydrothermal activity. Only albite I occurs in all granitoid varieties. In the mafic rocks two scapolite generations were recognized. One refers to crystals aggregates that replaced the primary plagioclase and the other to fine sinuous veinlets of scapolite+Mg-hornblende that, in these rocks, mark the latest stage of the hydrothermal alteration. In the biotite-rich rocks only scapolite I was identified. The scapolite that occurs in both the mafic and biotite-rich rocks have similar meionite (Me=27-28%) and Cl (3-4%) contents, and were most likely formed by a NaCl-rich fluid, with little influence of the rock composition. This fluid might have also been responsible for the production of albite in all analyzed samples. This albite is purer in the apogranitoids (Ab=98.5-99.3%) and the metavolcanic rocks (Ab=99-99.3%) than in the other rocks. In Serra Dourada area, the granitoids include syenogranites, granodiorite and tonalites as well as enclaves of tonalitic composition. Their mineralogical characteristics and calc-alkaline affinity allow them to be classified as I-type granitoids related to subduction with some influence of extensional regime. Most geochemical data indicate that they are cogenetic and have been produced by fractional crystallization. Albitization, scapolitization and minor biotitization were the most important types of alteration that affected those rocks, the first being restricted to the syenogranites, while the other two types are recorded only in the tonalites. The syenogranites present different degrees of alteration ranging from aposyenogranite without hydrothermal albite to albitite with ~75% of hydrothermal albite. In all granitoids, the hydrothermal albite is purer than the magmatic albite. Mass-balance calculations shows that, in relation to the average composition of syenogranites and for volume factors of 0.8-1.1, the albitites gained Na2O and Cu, and lost K2O, Ba, Rb, Sr and W. The scapolitized rocks resulted from the alteration of tonalites and are mainly characterized by scapolite-rich (up to ~70%) veins/veinlets that cross-cut the rocks. In the moderately altered varieties, the scapolite composition is richer in Na (Me=24%) and Cl (4%) in comparison to that present in veins and in the mylonitized granitoids. Massbalance calculations indicate that the formation of the scapolitized rocks was accompanied by losses of Al2O3, MgO, CaO, Sr and Zr and gains of Fe2O3(t), Na2O, K2O, volatiles, Rb, Ba and Cu. The fluids that caused albitization and scapolitization of the rocks were Na and Cl-rich and, apparently, the albitization of the syenogranites preceded the scapolitization of the tonalites. The scapolite composition seems to have been strongly controlled by the composition of primary plagioclase, as evidenced by its more calcic character in the mafic rocks than in the apotonalites. Estimates of Cl content (1-2%) of rocks from both areas were based on its concentrations in modal scapolite, biotite and amphiboles. The stability of scapolite requires high salinity fluids as those that have been reported in the Sossego deposit. No fluid inclusion data concerning the Na-rich fluids responsible for albitization and scapolitization in the Serra Dourada area are yet available, although the evidences, notably the abundance of scapolite, suggest an evaporitic or similar source.Item Acesso aberto (Open Access) Estudo do campo Urucum do depósito Amapari, Amapá, com base em dados petrográficos, de química mineral e microtermométricos(Universidade Federal do Pará, 2001-05-14) MELO, Luciana Viana de; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983Item 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/1406458719432983The 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.Item Acesso aberto (Open Access) Estudo geoquímico do sistema hidrotermal relacionado à mineralização cuprífera da área Bahia, Serra dos Carajás, Pará(Universidade Federal do Pará, 1989-10-24) RIBEIRO, Ana Maria Rodrigues; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983Item Acesso aberto (Open Access) Estudos geológicos, geoquímicos e microtermométricos da jazida de sulfetos de Cu-Zn do corpo 4-E/Pojuca, Serra dos Carajás.(Universidade Federal do Pará, 1985-06-25) MEDEIROS FILHO, Francisco Augusto de; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983Item Acesso aberto (Open Access) Estudos hidrogeoquímicos e geofísicos na região da Braquidobra de Monte Alegre-PA(Universidade Federal do Pará, 2005-04-11) LOPES, Elem Cristina dos Santos; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983The Monte Alegre brachyanticline is located in the central-western Pará and is one of the most striking structures of the Paleozoic Amazon sedimentary basin. The dome outcropping surface is elliptical with axes of 30 and 20 km trending NESW and NW-SE, respectively. At the present erosion level, rocks of the Ererê, Barreirinha, Curiri, Oriximiná, Faro, Monte Alegre, Itaituba and Alter do Chão formations, as well as the Penatecaua diabase are exposed. Thermal springs with temperatures from 29 to 37ºC issue out of the Ererê Formation. The present study focuses on the physico-chemical characterization of surface and groundwater that occur within the dome and on the interaction with their mineral environment. In addition, the thermal waters are also characterized isotopically and their subsurface temperatures estimated with basis on the silica and Na-Ca-K geothermometers. Gravimetric and resistivity methods were applied aiming at detecting potential sources that could account for the heating of the thermal waters. The Piper diagrams show a wide chemical variation for both the surface and groundwater which spread over the bicarbonate, calc-sulfate and sodic-chloride fields. The thermal waters are chemically more homogeneous and fall on the bicarbonate and sodic-chloride fields. This variability reflects the composition of the rocks through which the waters migrate, whose components result mainly from hydrolysis and redox reactions involving pyrite and sulfate minerals. Most water samples are in equilibrium with kaolinite as deduced from activity diagrams constructed at 25ºC, 1 atm and at quartz saturation. The thermal water samples cluster near the boundary kaolinite-sericite, whereas a few samples of surface waters that drain the Itaituba Formation plot near the line separating the kaolinite and leonhardite stability fields. The samples of the Menino Deus thermal spring collected in October/2002 reached equilibrium with sericite but failed to equilibrate with paragonite despite their Na/K ratios being higher than unity. Furthermore, most water samples are saturated with quartz. During the dry season, sample FT-27 and those from the Menino Deus thermal spring presented higher silica contents causing them to equilibrate with pyrophyllite. Isotopic data show that the Monte Alegre thermal waters have a meteoric origin with δ18O and δD values coincident with the global meteoric water line, although slightly enriched in deuterium. The excess of deuterium varies seasonally, being recorded values of 11,8-14,8 ‰ in the dry season and 4-9,5 ‰ in the wet season. Sub-surface temperatures for the thermal waters estimated with the silica geothermometer yield mean values of 71°C (wet season) and 83°C (dry season). Average water circulation depths ranging from 1560 m (wet season) to 1900 m (dry season) were calculated assuming a geothermal gradient of 30°/km and using the estimated sub-surface temperatures. After infiltrating into the ground, these meteoric waters are heated and rise back to the surface through a channel way network, particularly NE-SW and N-S-trending faults which truncate the brachyanticline flanks. Gravimetric surveys allowed to infer the presence of a lacolith-like body at a depth of about 1.3 km. This body is thicker close to the central portion of the brachyanticline. Contact relationships between sedimentary units and the laccolith could also be defined. Likewise, fractures and faults could be mapped at depth whose traces are visible on SRTM radar images. Resistivity profiles did not identify any thermal anomaly in the area, but they confirm the lithological diversity of the geological units as well as faults that may have served as conduits to the fluids.Item Acesso aberto (Open Access) Estudos isotópicos (Pb, O, H, S) em zonas alteradas e mineralizadas do depósito cupro-aurífero Visconde, Província Mineral de Carajás(Universidade Federal do Pará, 2013-06-05) SILVA, Antonia Railine da Costa; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983The Cu-AuVisconde deposit is located in the Carajás Mineral Province, northern Brazil, about 15 km east of the world-class Sossego deposit. It lies within a regional WNW–ESE-striking shear zone that marks the contact between the ~2.76 Ga metavolcano-sedimentary rocks of the Carajás Basin and the basement units. Other Cu- Au deposits with similar characteristics (Bacaba, Castanha, Alvo 118, Cristalino, Jatobá) occur along this shear zone. They have been included in the IOCG class, although much controversy exists regarding their genesis, particularly with respect to the mineralization age and source of fluids, ligands and metals. TheVisconde deposit is hosted by Archean rocks, mainly felsic metavolcanic rocks (2968 ± 15 Ma), the Serra Dourada granite (2860 ± 22 Ma), and gabbro/diorites. These rocks are variably sheared and reveal various types of hydrothermal alteration with strong structural control. The earliest types are the sodic (albite-scapolite) and sodic-calcic alterations (albiteactinolite ± tourmaline ± quartz ± magnetite ± scapolite ± epidote), which promoted ubiquitous replacement of the rock primary minerals and precipitaton of disseminated chalcopyrite, pyrite, molybdenite and pentlandite. Oxygen isotope data of representative minerals from these stages show that the hydrothermal fluids were hot (410 – 355°C) and 18O-rich (δ18OH2O = +4.2 to +9.4‰). The following potassic stage is characterized by intense biotitization of the rocks, which developed concomitantly a mylonitic foliation highlighted by the remarkable orientation of biotite flakes. This mica precipitated from fluids with similar oxygen isotope signature to that of the previous stages (δ18OH2O = +4.8 to +7.2‰, at 355°C). Microcline and allanite are other typical minerals of this stage, in addition to chalcopyrite that deposited along the foliation planes. At lower temperatures (230 ± 11°C), 18O-depleted fluids (δ18OH2O = -1.3 to +3.7‰) generated a calcic-magnesian mineral assemblage (albite + epidote + chlorite ± actinolite ± calcite) present mostly in veins and contemporaneous with the main mineralization. The δ18OH2O and δDH2O data indicate that the hydrothermal fluids were initially formed by metamorphic and formation waters, possibly with some contribution of magmatic water. At later stages, there was a considerable influx of surface water. The resulting fluid dilution and cooling might have accounted for the abundant precipitation of sulphides (chalcopyrite ± bornite ± chalcocite ± digenite) mainly in tectonic breccias, whose matrix contains up to 60% sulphides. These breccias represent the most important ore bodies, although sulphides also occur in veins together with sodic-calcic minerals. The mineral associations assign a Cu-Au-Fe-Ni-ETRL-B-P signature to the ore. The sulphur isotope composition (δ34SCDT= -1.2 to 3.4‰) is compatible with a magmatic source for sulphur, which could have been either exsolved from a crystallizing granitic magma or dissolved from sulphides originally present in preexisting igneous rocks. Additionally, it indicates relatively reducing conditions for the fluid. Dating of chalcopyrite by Pb leaching and total dissolution techniques yielded ages of 2736 ± 100 Ma and 2729 ± 150 Ma. Despite the large errors, they point to a Neoarchean age for the mineralization and preclude a Paleoproterozoic mineralizing event. The age of 2746 ± 7 Ma (MSDW = 4.9; Pb evaporation on zircon), obtained for a non-mineralized granitic intrusion present in the deposit area and correlated to the Planalto Suite, was considered as the minimum age for the mineralization. Thus, the Visconde deposit genesis could be related to the 2.76-2.74 Ga transpressive tectonothermal event that was responsible for the inversion of the Carajás basin and generation of granitic magmatism in the Carajás and Transition domains. Such an event should have triggered devolatilazion reactions in the Itacaiunas Supergroup rocks, producing metamorphic fluids or even driving off water trapped in the pores of the basin rocks. These fluids migrated along regional shear zones and reacted with both the basin and basement rocks through which they moved during the ductile regime. The subeconomic concentrations of the Visconde deposit might be the result of the absence of prominent structures that would otherwise favor a greater influx of fluids, as it seems to have been the case in the Sossego and Alvo 118 deposits.Item Acesso aberto (Open Access) Estudos isotópicos (Pb-Pb, Sm-Nd, C e O) do depósito Cu-Au do Sossego, Província Mineral de Carajás(Universidade Federal do Pará, 2006-10-30) NEVES, Marcely Pereira; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983The Sossego deposit is located at about 25 km NW of Canaã de Carajás, southeast of Pará, at the contact zone between the Grão Pará Group and basement rocks. This deposit comprises two main ore bodies, Sequeirinho and Sossego, which were formed in an environment where deformational processes, emplacement and cooling of intrusive bodies led to an intense hydrothermal fluid circulation. This work intended to characterize the ore host rocks, the mineralization styles and the types of hydrothermal alteration, as well as to date the sulfide mineralization and assess potential sources for metals and hydrothermal fluids. Invariably altered and deformed granitoids and mafic rocks, in addition to biotiterich rocks, hydrothermalites (magnetitites and ore bodies) and breccias occur in the deposit area. All these rocks are crosscut by mafic and felsic dikes. Albitization, epidotization, chloritization, silicification, amphibolitization and scapolitization were the main alteration processes recognized in the deposit. The first is more common in the granitoids while the last two are more typical of the mafic rocks. In these mafic rocks the concentration of actinolite increases towards the ore bodies resulting in the formation of actinolitites. A strong iron metasomatism was also identified, being responsible for the magnetitite generation. Abundant calcite-rich veinlets mark the final stage of the hydrothermal activity in the area. Although present in the Sequeirinho ore body, it was more intense in the Sossego ore body. Mineralization was subsequent to the shearing event and took place contemporaneously with the epidotization, chloritization and actinolization processes, allowing one to infer that mineralizating fluids, besides Cu, Au and P, were also enriched in Ca and Fe. Pb isotopic data in chalcopyrite from ore samples yielded ages of 2530 ± 25 Ma, 2608 ± 25 Ma (Sequeirinho ore body) and 1592 ± 45 Ma (Sossego ore body). The Archaean ages were discussed considering the hypotheses of the mineralization having or not a genetic link with the granitoids. In the first case, it could be related to the 2.76 – 2.74 Ga granitogenesis and the ages would represent the opening of the Pb isotopic system caused by subsequent thermal or deformational events. Alternatively, those ages could be related to the ~2.6 Ga magmatic event that is accounted for the dikes present at the neighboring Cu-Au 118 deposit, but its unexpressive significance in the Carajás region makes it unlikely to be responsible for the production of great cuprousauriferous deposits, like the Sossego. In the absence of a genetic link with granitoids, the mineralization could be related to metamorphic processes and the Pb-Pb ages would record the accompanying shearing event that caused the granitoid mylonitization and might have induced the infiltration of high salinity fluids, resulting in biotite-rich rocks with high Cl contents. No geological significance was given to the Mesoproterozoic age. A Sm-Nd isochron of 2578 ± 29 Ma was also obtained for ore samples. It is comparable to the Pb-Pb age of 2608 ± 25 Ma and most likely reflects an isotopic reequilibrium in response to later events. Model-ages (3.16 – 2.96 Ga) for the ore are similar to those of the deposit granitoid (3.12 – 2.98 Ga) and also to the Grão Pará Group basalts (2.76 – 3.09 Ga), suggesting that the ore could have been, at least in part, derived from these rocks. εNd values (-4.09 to -0.94) indicate an essentially crustal origin for both the host rocks and the ore. Based on the diagram εNd x t, it is plausible to admit that ore metals were derived from both granitoids and mafic rocks, especially the basalts from the Grão Pará Group. The δ 13CPDB values for calcite indicate a homogeneous source for C, probably mantle-derived. They show a negative linear correlation with the δ 18O values, suggesting a common evolution process. In terms of C and O natural reservoirs, the isotopic data are more consistent with sources of carbonatitic affiliation as indicated by the anomalous P and ETR contents found in the ore bodies. Calculation of δ 18Ofluid reveals that most values fall, respectively, in the ranges of -7.24 to -5.17‰ and +1.14‰ to +3.21‰ at 150° and 350ºC, being consistent with the participation of meteoric water in the Sossego hydrothermal system. However, higher values (+9.79‰ at 250ºC and +12.77‰ at 350ºC) suggest a magmatic or metamorphic water signature. Despite the speculations, integrating field, mineralogical, isotopic (stable and radiogenic) and fluid inclusion data allows to sustain that mineralization occurred at 2.76-2.60 Ga and was related to metamorphic-deformational processes, which triggered the migration of high saline fluids that had previously reacted with salt-rich sequences and became capable of transporting great amounts of metals, including Cu and Fe.Item 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/1406458719432983The 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.Item Acesso aberto (Open Access) Geologia, alteração hidrotermal e gênese do depósito IOGG Cristalino, Província Mineral de Carajás, Brasil.(Universidade Federal do Pará, 2018-10-22) CRAVEIRO, Gustavo Souza; XAVIER, Roberto Perez; http://lattes.cnpq.br/6388495537140928; 6388495537140928; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983; 1406458719432983The Archean Cu-Au Cristalino deposit is located 40 km east from Sossego mine in the eastern end of the regional WNW-ESW Carajás shear zone, in the Serra do Rabo region. Its host rocks are mainly the mafic volcanic rocks of the Parauapebas Formation, and subordinately the BIF of the Carajás Formation. Field-work, petrographic data seconded by SEM-EDS, in addition to microprobe analysis and fluid inclusion and stable isotope (O, H, C and S) systematics, allowed characterizing a hydrothermal system that was responsible for the development of successive alteration zones in the wall rocks. Sodic metasomatism (650°C ≥ T > 400°C, and P > 1.8 Kbar) formed near pure albite, schorlitic tourmaline, REE-rich minerals (allanite-Ce, monazite) and minor calcite and quartz. It was followed by a more pervasive calcic-ferric alteration which produced abundant actinolite (XMg = 0.9-0.7 and up to 0.6 wt. % Cl), allanite-Ce (up to 0.6 wt. % Cl) and magnetite, associated with sulfide disseminations and replacement breccia-like bodies composed of chalcopyrite-pyrite-magnetite-Au (early ore association). Locally, Fe-edenite (XMg=0.7-0.4, Cl up to 2.9 wt. %) replaced calcic-ferric assemblages within restrict sodic-calcic alteration halos. From 410o down to 220o C and 1.8 Kbar > P > 0.6 Kbar, the previous alteration assemblages were overprinted by potassic (Kfeldspar, minor biotite) and propylitic/carbonatic (epidote, chlorite, calcite) alterations, in this order. K-feldspar is practically stoichiometric, but with high contents of BaO (up to 1.2 wt. %). Chlorite shows the greatest compositional variation among all minerals and seems to have been controlled by the type of host rock, chemistry of the hydrothermal fluid and temperature. Both chamosite and clinochlore (XFe=0.4-0.8) are present, the former being more common. Chlorine contents are in general < 0.02 wt. % and a little more significant in chlorites that replaced chessboard albite (up to 0.06 wt. %). The late ore association (chalcopyrite±Au±pyrite±hematite) is contemporaneous with the potassic and propylitic alterations and bears evidence that the Cristalino system evolved to the final stages with increase in oxygen fugacity. The ore fluid was hot (> 550°C), hypersaline and chemically approached by the system H2O-NaCl-CaCl2-CO2±MgCl2±FeCl2. Salinity exceeded 55.1 wt. % NaCl equiv. in the early stages but decreased progressively to 7.9 wt. % NaCl equiv. from 250o C on, after incursion of surficial water into the system. Initially 18O-enriched/D-depleted (δ18Ovsmow =+9.7 to +6.5 ‰; δDvsmow= -30.8 to -40.2 ‰) and most likely derived from magmatic sources, the fluid became relatively 18O-depleted/D-enriched (δ18Ovsmow =+5.57 to - 0.28‰; δDvsmow= -19.15 to -22.24‰) as result of dilution caused by mixing with meteoric water. δ13CVPDB values for vein and breccia calcite (-6.5 to -3.8‰) are consistent with a deep source for CO2, which was probably released from an underlying magma chamber. The δ34SVCDT values for chalcopyrite show narrow variation (+1.6 to +3.5 ‰) and indicate a homogeneous reservoir for sulfur, which was likely of igneous origin. Although most data point to a magmatic affiliation, a few samples reveal significant influence of sedimentary rocks on their isotope composition. Mostly transported as chloride complexes (>350oC), Cu and Au precipitated in response to decrease in temperature and Cl- activity and increase in pH. An aqueous, colder (200-150o C) and less saline (21-3.1 wt. % NaCl equiv.) fluid appears to have circulated in the Cristalino deposit area, being trapped as secondary fluid inclusions. The origin is unknown, but it could be related to a nearby Paleoproterozoic granitic intrusion. The data presented here support previous interpretations that consider Cristalino as of IOCG typology. In comparison with other Archean Carajás IOCG deposits, particularly those that lie in the southern sector of the Carajás Domain, the Cristalino deposit shows many similarities regarding ore fluid composition and evolution, as well as the isotopic signature of sulfides and carbonates.Item Acesso aberto (Open Access) Geologia, fluidos hidrotermais e origem do depósito cupro-aurífero Visconde, Província Mineral de Carajás.(Universidade Federal do Pará, 2011-08-24) CRAVEIRO, Gustavo Souza; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983The Cu-Au Visconde deposit lies at the contact zone between the basement (>3.0Ga) and the Grão Pará Group (2.76 Ga) within the so-called Transition Domain of the Mineral Carajás Province. It is located at about 15 km east of the Cu-Au Sossego mine in the county of Canaã de Carajás, Pará state. Felsic metavolcanic rocks, probably belonging to that group, as well as granitic and mafic intrusions, dominate in the deposit and neighboring area. Subordinate ultramafic bodies occur within the mafic units. All these rocks are moderately to strongly hydrothermally altered and show varying degrees of deformation. Mafic dikes and a granitoid isotropic body, the latter probably related to the granitogenesis of 1.88 Ga, represent the last igneous activity in the area and cut the pre-existing rock package. Despite the mineralogical and textural changes, it was possible to infer a monzogranitic to granodioritic composition to the original granitoids largely based on the amounts of chess-board albite. Moreover, the scarcity of primary mafic minerals makes these granitoids similar to the Planalto Granite, which crops out approximately 7 km east of the deposit area. The mineral content and the partially preserved subophitic texture are suggestive that gabros and/or quartz diorites were potential protoliths of the mafic intrusions. On the other hand, the recognition of plagioclase, quartz and K-feldspar primary phenocrysts and the use of trace elements with limited mobility in the hydrothermal environment allowed discriminating a riodacitic composition for the protolith of the felsic volcanic rocks. Grading from ductile to brittle regimes, the hydrothermal alteration changed from early sodic-calcic assemblages, characterized by ubiquitous albitization, scapolitization or amphibolitization, to late potassic assemblages, in which the K-feldspar and Cl-biotite are the diagnostic minerals. Then the alteration restored its sodic-calcic character as indicated by albite, epidote, apatite, tourmaline and fluorite that replaced pre-existing minerals or filled open spaces. At last, it is recorded a calcic-magnesian stage during which clinochlore/Fe-clinochlore, actinolite, carbonate and subordinate talc were equilibrated. In the granitoids, albitization, epidotization and tourmalinization were the most prominent alteration processes, whereas scapolitization, biotitization, amphibolitization and magnetization were more remarkable in gabros/quartz diorites and K-feldspatization in the felsic metavolcanic rocks. The ore, whose formation began at the final stages of the potassic alteration, was essentially controlled by brittle structures. Initially represented by weak chalcopyrite, molybdenite and pyrite disseminations in the zones altered to tremoliteactinolite, scapolite, albite and magnetite, the mineralization evolved to sulfide concentrations in veins and breccias. Among the sulfides chalcopyrite, bornite, molybdenite are dominant, but pyrite and pentlandite also occur together with apatite, scapolite, actinolite, epidote, magnetite, martite, hematite, calcite, and gypsum or fluorite as the main gangue minerals. The typical metallic suite of the sulfide breccia is Fe–Cu–Ni–ETR±Au±Zn±Y±Co±Se, with ƩETR as high as 1030 ppm. Fluid inclusions trapped in quartz, scapolite, apatite and calcite crystals unraveled at least three aqueous fluids. Fluid 1, simplified by the system H2O-NaCl-CaCl2±MgCl2 and present in all host minerals, was hot (450–500ºC) and very saline (up to 58 wt% equivalent NaCl). The alteration and mineralization haloes should have resulted from the interaction of the host rocks with this fluid, which might have experienced cooling and dilution probably due to mixing with surficial waters. After the mineralization event, the deposit recorded the successive inflow of fluid 2 (H2O-NaCl-FeCl2±MgCl2, up to 30 wt % equiv. NaCl) and fluid 3 (H2O-NaCl±KCl, up to 18 wt % equiv. NaCl), both cooler than fluid 1. With a restrict circulation and preserved only in quartz and apatite crystals, fluid 2 might have been related to the intrusion of the late mafic dikes, whereas fluid 3 migration would have taken place in response to the emplacement of the alkali granite (1.88 Ga?), being trapped, similarly to fluid 1, in all host minerals, but as secondary IF. The high salinity and no evidence of boiling, coupled with the presence of Cl-rich minerals, suggest that a purely magmatic source is unlike for fluid 1. As an alternative, it is assumed a mixed source, involving the migration of magmatic or metamorphic fluids throughout carbonatic-evaporitic sequences from which Na, Ca and Cl have been largely leached. The mineralogical, chemical and microthermometric data allowed to characterize the mineralizing fluid as an aqueous solution consisting of NaCl, CaCl2, KCl, FeCl2 and MgCl2(?) that also carried P, B, F, Y, Ba, Sr, Rb and ETR, Cu, Ni and Co, besides S species. The Sossego and the Visconde deposits present similarities in terms of (1) the nature of the host rocks (felsic metavolcanics, granitoids, and mafic intrusions), (2) the types of alteration, highlighting the intense and widespread sodic-calcic metassomatism, (3) the occurrence of the major ore bodies in brecciated zones and (4) the Fe-Cu-Ni-ETR±Au±Co as the metallic signature of the ore. Regarding the main differences, the sub-economic sulfide accumulations and the smaller amounts of massive magnetitites of the Visconde deposit could be listed.Item Acesso aberto (Open Access) O granito central: fácies petrográficas e alteração hidrotermal do setor norte.(Universidade Federal do Pará, 1991-09-06) JAVIER RIOS, Francisco; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983Item Acesso aberto (Open Access) Ironstones da base da Formação Pimenteiras, borda noroeste da bacia do Parnaíba, região Xambioá-Colinas do Tocantins(Universidade Federal do Pará, 2011-01-05) AMARO, Gabriel de Jesus Lavareda; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983The present study deals with the ironstones that occur at the base of the Devonian Pimenteiras Formation in the northwestern border of the Parnaíba basin. Samples were collected along the Xambioá-Vanderlâdia and Colinas do Tocantins-Couto Magalhães sections about 180 km away from each other. In both sections the sedimentary package lies unconformably over the Proterozoic basement represented by rocks of the Araguaia belt. The ironstones consist of discontinous decimeter-thick layers intercalated in sandstones and shales. The ironstones of the Xambioá-Vanderlândia show abundant oölites immersed in an iron-oxide/hydroxide-rich matrix. The size of the oölites ranges from 0.2 to 0.5 mm, reaching exceptionally 1.0 mm, while the shape is generally spherical to ellipsoidal. Most oolites present concentric internal structure and only a few are broken. Their nucleus is commonly occupied by detrital grains of quartz, zircon or monazite around which iron-oxide/hydroxide concentric lamellae were formed. Locally some laminae are composed of chemically precipitated monazite. Goethite/hematite (75-80%), quartz (5-15%) and smectite /kaolinite (0-10%) are the main mineral constituents. Chemically the oolithic ironstones are made up of SiO2 (3-15%), Al2O3 (4.4-7.7%), Fe2O3 (62.5-79%), P2O5 (0.5-3,2%), TiO2 (0.2-0.8%); other components total less than 0,2% and LOI values vary from 9.4 to 13.5%. P2O5 contents are high enough to be accounted for the amounts of the phosphate minerals (monazite and goyasite), so that some P was most likely adsorbed on the iron oxides-hydroxides. Regarding the trace elements, the higher average concentrations were determined for V (780-1990 ppm), Zr (125-600 ppm), Sr (15-296 ppm), Ba (25-266 ppm), Ni (22-225 ppm),Y (39-181ppm) and REE (144-1630 ppm, with Ce>NdLa), which reflect the variable amounts of zircon, monazite and goyasite. The vanadium values in particular may be due to the presence of interbedded clay minerals and /or adsorption on the iron oxides-hydroxides. When normalized to the North American Shale Composite (NASC), the oölithic ironstone samples are enriched in REE by factors greater than 2 few of them exhibiting discrete positive Ce anomalies. In general, the distribution pattern for REE is convex between La and Dy, but towards Lu it tends to become sub-horizontal resembling a “capsized spoon”. Unlike the ironstones described above, those of Colinas do Tocantins-Couto Magalhães area do not present oolithic texture, although they are also basically composed of iron oxides-hydroxides, which not only imparts them a dark red coloration but also acts as a cement for the randomly distributed detrital grains of several minerals. These non-oölithic ironstones consist of hematite/goethite, quartz, clay minerals, muscovite and accessory amounts of monazite and zircon. Their chemical composition reveals Fe2O3 (41-60%), SiO2 (16-39%), Al2O3 (6-11%), K2O (0.5-1,2%), P2O5 (0.32%) and H2O (6-11%) recorded as LOI. MgO, CaO, Na2O, MnO and Cr2O3 are minor components and represent less than 0.4% of the rock. V (83-3.488 ppm), Zr (62-372ppm), Ba (166-347 ppm), Rb (26-62 pm), Zn (19-868 ppm), Ni (3-106 ppm) e Sr (31-51 ppm) are the trace elements with more expressive contents. REE values range from 100 to 300 ppm. LREE (Ce> La> Nd) contents are usually >15 ppm whereas the other REE are in general below of 5 ppm. Although the chemical composition of these ironstones is largely controlled by the amounts of minerals present, the high concentrations of some trace elements may indicate incorporation in the phyllosilicate lattice (Ni and Ba) and adsorption on the iron oxides-hydroxides (V). Comparatively to NASC, the nonoölithic ironstone samples are poorer in REE, whose distribution pattern is dominantly concave and marked by the fractionation of the IREE in relation to both LREE and HREE. 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 show smaller amounts of terrigenous material (especially quartz) and higher iron oxide-hydroxides proportions. They also are more enriched in V, Sr, Zr and REE, and poorer in Al2O3 e Rb. The NASCnormalized REE distribution pattern is also different, especially concerning the IREEN values which, being higher in the oölithic ironstones, display convex curves and, being lower in the nonoolithic ironstones, display concave curves. In the field, however, spatial relationships could not be defined between the two ironstones types. It is then suggested that they represent different lithological facies of the same iron formation. Probably, the deposition of the non-oölith 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 oölith 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 transported by rivers along with suspended particles, colloids, and organic complexes.Item Acesso aberto (Open Access) A jazida de wolframita de Pedra Preta, granito Musa, Amazônia Ooriental (PA): estudo dos fluidos mineralizantes e isótopos estáveis de oxigênio em veios hidrotermais(Universidade Federal do Pará, 1995-11-14) JAVIER RIOS, Francisco; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983The Pedra Preta wolframite deposit contains the main known tungsten reserves of the Brazilian Amazon. It is Iocated near the western border of the 1.88Ga old Musa grafite, in the Rio Maria region, south of the Carajás Mineral Province. The mineralization occurs in a vein system thats cuts at depht the cupola of the granitic body and above it rocks of the Andorinhas Supergroup whose meta-sandstones are of archean age (2.9 Ga). At least four hydrothermal events have been identified in the Pedra Preta area which are related to severa' quartz vein generations. The first event is represented by the early EHV veins that are basically made up of quartz 1 and have been generated, prior to the emplacement of the Musa grafite, from metamorphic aquo-carbonic fluids. CH4 was the dominant carbonic phase. Fluid inclusions from the quartzite quartz grains showed H2O + CH4 with lesser amounts of CO2. The second event was associated to the Musa intrusion and involved F-poor aqueous fluids exsolved from the erystallizing magma. Once the granite was broken by hydraulic fracturing, fluids that circulated around the pluton moved towards it, mixed with the magmatic aqueous solutions and flowed through the open spaces where quartz 3 was precipitated. The late hydrothermal veins (LHV) began then to be formed. These Ca-free fluids had moderate salinity and were virtually devoid of carbonic phases. δ18O values for quartz 2 (present in the grafite) and quartz 3 (present in the LHV at greater depths) are comparable (7.6‰) indicating reequilibration with dominantly magmatic fluids. The third event was induced by the tectonic reopening of the fracture planes where the quartz 3 had been deposited. They served as escape tone for metamorphic fluids composed of different proportions of CH4, CO2 and H2O. The aqueous phase rnight have been of low salinity although containing Ca++. Temperatures varied from 230 to 400°C and pressures estimates fell in > 2,5 Kbar. Oxygen fugacity values of 10-38 to 10-37 bar indicated reducing conditions. As the metamorphic fluids entered the Pedra Preta system, they were oxidized, though, at least initially, the process had been less complete in the upper part of the deposit. Oxygen fugacities dropped to 10-27 bar by the time the wolframite began to precipitate from acidic solutions (pH 4-5) under therrnal conditions of 300-400°C and pressures > 2.5Kbar. δ18O values for quartz 3 of the LHV (9.0-9.6‰) at lower depths suggested reequilibration with fluids having more metamorphic components than those of greater depths. Soon after or partially contemporaneous with the wolfrarnite deposition, occurred a F-metasomatism brought about by a hypothetical magmatic pulse. Granitic rocks were then greisenized in the lower part of the deposit to a mineral assemblage in which topaz, fluorite and sericite are present, whereas in the upper part these minerais precipitated within the LHV as well as in the host walls. The fluids of this hydrothermal stage were aquo-carbonic, suggesting that mixing with the metamorphic solutions continued, but the carbonic phase was exclusively composed of CO2. Xco2 dropped to values below 0.01 by the time fluorite was formed. Aqueous phase was enriched in Ca++ and Na+. Temperatures did not change much from the deposition of wolframite to the deposition of topaz (300-350°C), but fell to about 250°C when fluorite started precipitating. Despite similar prevailing conditions both in the lower and higher parts of the deposit, irnportant features are recorded that differentiate these two domains. The most striking difference is the much more abundant wolframite precipitation in the upper part. Besides the structural control, the mineralization might also have been controlled by the more frequent metavolcanic lens of the Babaçu Group in the upper part, from which the W-bearing aquo-carbonic solutions leached iron for the precipitation of wolframite. The last hydrothermal event, that resulted from tectonic relaxation probably of Brasiliano age, gave origin to the so-called final veins (FHV) which constitute a net of microveinlets composed of quartz 4, chlorite, sulfides, carbonates and quartz 5. High salinity fluids (30 weigth % NaCl) with high concentration of Ca++ and Na+ acted upon the rocks at conditions of 1.5 Kbar and temperatures beiow 250°C, and may represent connate waters or even deep groundwaters. Chioritization and sulfidization were the most important processes related to this hydrothermal event which ied to the precipitation of chlorite (that replaced feldspars and micas in the host rocks or filled intergranular spaces within the veins) as well as sulfides (mainly chalcopyrite and pyrite). As the system finally died out, drusy quartz 5 was formed trapping low salinity fluids (<5 weigth % NaCl). Pressures were around 5 bar and temperatures reached no more than 100°C suggesting contribution of superficial meteoric waters.Item Acesso aberto (Open Access) Mineralização aurífera de Montes Áureos (Maranhão): rochas hospedeiras, controles deposicionais e fluidos mineralizantes.(Universidade Federal do Pará, 2000-04-03) YAMAGUTI, Humberto Sabro; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983The Montes Áureos gold mineralization occurred within the Tentugal Shear Zone (ZCT) which is a + 100km long, 15km wide NW-SE trending structure that marks the south-southwest boundary of the São Luís craton and may represent a collisional suture between the Belém block and this craton. This zone is characterized by tectonites produced by compressive/transpressive and strike-slip movements, and corresponds to the most deformed rocks of the Gurupi mobile belt. The ore bodies are hosted by Proterozoic Gurupi Group metavolcanic and metasedimentary rocks which have been metamorphosed under conditions of low (chlorite + sericite), medium to high greenschist (chlorite + biotite + muscovite + epidote + actinolite + Mg- hornblende + Fe-hornblende) and low amphibolite facies (biotite + plagioclase + edenite + pargasite + ferrotchermakite). They have been also deformed under a brittle-ductile regime, leading to the formation of structures with different shapes, styles and degree of strain. A superimposed hydrothermal event generated assemblages composed of chlorite, carbonates and epidote that partially replaced metamorphic amphiboles, plagioclase and biotite. It also produced a system of vein and veinlets both concordant and discordant with respect to the rock foliation. Gold occurs in up to 2 cm thick, late-tectonic quartz + carbonates veins or veinlets associated with arsenopyrite, pyrite and minor chalcopyrite. The spatial distribution of the mineralized zones forms lenticular to tabular bodies with gold contents less than 2 ppm. Texture features and the time relationship between hydrothermal and metamorphic assemblages indicate that mineralization followed the metamorphic peak and that gold occurs in at least two different forms: 1) granular gold deposited simultaneously with sulfides; and 2) in microfractures in arsenopyrite. Gold had been most likely transported by the sulphur thio- complex [Au(HS)"] in an aqueous-carbonic, low salinity fluid (2 to 10 wt% equiv. NaCl) at temperature < 450ºC. Deposition occurred in a temperature range of 260 to 350C. For the prevalent value of 300ºC, obtained by the chlorite geothermometer, pressure estimates fall between 1,3 and 2,8 kb, corresponding to depths of 5-10 km. Desestabilization of that complex as temperature dropped and as the fluids interacted with the host rocks brought about gold precipitation. Fluids related to the systems CO; + CH,, H,0-C0O,-NaCl + CH, + MgCl e/ou FeCl, and H;0-NaCl + MgCl e/ou FeCl, circulated through Montes Áureos rocks. The aqueous-carbonic fluids are considered to be products of dehydration and decarbonization of carbon- fluids are considered to be products of dehydration and decarbonization of carbon-bearing sedimentary rocks at temperature probably above 500ºC. At first homogeneous, these fluids have undergone imiscibility and were then trapped in fluid inclusions with different H,)0/CO; ratios, some almost pure H,O or CO>. As the metamorphic thermal regime decreased and carbonates precipitated, the aqueous-carbonic fluids became progressively impoverished in CO, and less saline. Mixture with cooler and low salinity superficial waters may have occurred by the end of the evolution of the Montes Áureos hydrothermal system. The geotectonic setting, the structural control of the mineralization by shear zone, the hydrothermal alteration features, the time relations between the metamorphic peak and hydrothermal alteration, the gold ore mineral association and the physical-chemical characteristics of the mineralizing fluids allow the Montes Áureos gold deposit to be classified in the lode category, as many others that are formed at convergent plate margins.