Navegando por Assunto "Metalogenia"

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Acesso aberto (Open Access)
Alterações hidrotermais associadas às rochas máfico-carbonatíticas do depósito de fosfato Serra da Capivara, região de Vila Mandi (PA), extremo sul do Cráton Amazônico.
(Universidade Federal do Pará, 2019-07-14) VIEIRA, Danilo Amaral Strauss; FERNANDES, Carlos Marcello Dias; http://lattes.cnpq.br/9442875601862372
Near the border of the states of Pará and Mato Grosso, in the Amazonian Craton, about 90 km west of the Vila Mandi district, Santana do Araguaia (PA) city, there is an unprecedented volcano–plutonism named Santana mafic-carbonatitic Complex. It is formed by a lower maficultramafic member with plutono–volcanic and other volcaniclastic lithofacies; besides an upper carbonatitic member with plutonic, effusive, and volcaniclastic lithofacies originated in a volcanic caldera environment with large areas of hydrothermal alterations and genetically related circular structures. The severe Amazon weathering partially affected this cluster, producing the Serra da Capivara Phosphate deposit supergenically. Although speculative, the Santana mafic-carbonatitic Complex is Paleoproterozoic in age, because it invades the Paleoproterozoic volcano-plutonic sequences Cinco Estrelas and Vila Mandi formations (1980–1880 Ma) and it is capped by sedimentary rocks from the same Era. The lower maficultramafic member has lithofacies with slabs of pyroxenite, and minor isolated metric blocks of ijolite and apatitite. They are medium-grained ceylonite-bearing (MgAl2O4) pyroxenites with augite (~ 90% vol.), magnesio-riebeckite, and olivine crystals replaced by clay minerals (saponite). The ijolite is composed of clinopyroxene and nepheline phenocrysts immersed in a fine-grained groundmass with nepheline, calcite, and interstitial magnetite. Apatitite blocks are composed of medium-grained apatite grains (~ 98% vol.) and calcite. The volcanic rocks of this lithofacies comprise isolated metric blocks of alkali basalt and rare associated outcrops of finegrained apatitite. This basalt rock presents plagioclase-rich groundmass and acicular augite phenocrysts as essential mineralogy. Aphyric samples have primary spherules filled with calcite and quartz, besides interstitial pyrite, iron oxides, apatite, barite, rutile, celestine, and monazite. This textural feature suggests silicate and carbonatitic melts immiscibility process. An explosive to autoclastic mafic volcaniclastic lithofacies encompasses poor sorting deposits of massive polymictic breccia, lapilli-tuff, crystal-rich tuff, and ash tuff. The autoclastic rocks reveal volcaniclastic texture comprising centimetric angular clasts sourced from autofragmentation of the mafic-plutonic plutono–volcanic lithofacies. Epiclastic sedimentary volcanogenic deposits usually cover all previous lithofacies. The upper carbonatitic member reveals coarse-grained carbonatite (sövite) lithofacies comprising reddish-yellow sövite (calcite carbonatite) composed of subhedral to euhedral calcite (85–90% vol.), with variations to magnesium-ferriferous calcite and dolomite. Primary accessories are magnetite, hematite, potassic feldspar, and pyrite. These lithotypes show hydrothermalized medium- to fine-grained carbonatite veins. Rare coarse-grained apatitite bodies occur associated with this lithofacies, which represents part of the proto-ore. An effusive carbonatite (alvikite) lithofacies reveals finegrained calcite-rich (80–85% vol.) to porphyritic alvikite, besides hematite, magnetite, potassic feldspar, and pyrite. Fragment-rich explosive carbonatitic volcaniclastic lithofacies encompassing poor sorting and texturally variable massive crystal-rich tuff, lapilli-tuff, and massive polymictic breccia formed by angular clasts sourced from host rocks and the complex. Syenitic stocks and dikes invade these rocks. The main hydrothermal magmatic alteration of the complex is represented by hydrothermalized carbonatitic rocks of reddish, brownish, and yellowish colors. The mineral paragenesis found was barite + fluorapatite + dolomite ± quartz ± rutile ± chalcopyrite ± pyrite ± monazite ± magnetite ± hematite. This alteration occurs in three distinctive ways; 1) in the deeper zones, where the minerals found were barite, fluorine apatite, and dolomite in pervasive to fracture-controlled alteration associated with deep fine carbonatites. 2) In the sövite, of weak interstitial form with mineralogy similar to the deep alterations. 3) in the alvikite with intense interstitial changes and formation of hydrothermal quartz associated with barite, fluorapatite, dolomite, monazite, celestine, and rutile. The mineral assemblage of the deeper alterations suggests initially sulphate-rich, magnesium, phosphorus, and CO2 fluids with possible transitional source between the late magmatic and the hydrothermal stages. In transition to more superficial phases of the volcanism, there was an assimilation of SiO2 from the country rocks evidenced by the formation of fine interstitial quartz crystals in alvikite. The interpreted environment of volcanic caldera occurs in the interception of regional NE-SW and NW-SE faults with up to 40 km of extension and that served as deep conduit of the precursor magma of the complex. The root of the system is represented by maficultramafic rocks and plutonic carbonatites. The pre-caldera phase involved intense degasification and hydrothermal activities as a function of magmatic evolution, and ascending by lithic faults and placing on the surface of large volume of carbonate lava (alvikites) that built the extinct volcanic building. The collapse of this structure and the topographic landslide coincided with explosive volcanism and formation of the volcanoclastic lithotypes, representing the intra-caldera filling. The late syenites may represent the post-caldera phase and sealing of these structures. The hydrothermal paragenesis identified in the Santana maficcarbonatitic Complex shows important metallogenetic potential for rare earth elements and phosphate and represents a prospective guide on Proterozoic terrains of the Amazonian Craton, like other areas of the planet.
Acesso aberto (Open Access)
Geologia e metalogênese do ouro do greenstone belt da Serra das Pipocas, Maciço de Troia, Província Borborema, NE - Brasil
(Universidade Federal do Pará, 2018-12-13) COSTA, Felipe Grandjean da; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706
At the Archean–Paleoproterozoic Troia Massif, in Borborema Province, NE–Brazil, two major Paleoproterozoic greenstone belts are recognized (Algodões and Serra das Pipocas). These share similar ages and lithostratigraphic characteristics with other 2.2–2.1 Ga greenstone belts of the surrounding cratonic domains (e.g. Guiana shield and São Luis–West Africa craton), and also host gold mineralization. In this thesis, a U–Pb zircon age of 2185 Ma was obtained for a pre–collisional metatonalite (Mirador tonalites) with geochemical affinity similar to adakites–like rocks. For syn– to post–collisional potassic plutons (Bananeira suite) we obtained U–Pb zircon ages of 2079 Ma for a deformed quartz monzonite and of 2068 Ma for the less–deformed equigranular granite. These granitoids of the Bananeira suite are both of high–K calc–alkaline affinity, and probably derived from partial melting of crustal sources. Zircon Hf crustal model ages of all granitoids range between 2800 and 2535 Ma, indicating that Archean crustal components contributed to their magma genesis. However, two analyzed c. 2.3 Ga old inherited zircon grains showing Ɛ_Hf (t) values of c. +4.9, indicate that crustal reworking of less–radiogenic Paleoproterozoic sources also participated. Gold mineralization in the Serra das Pipocas greenstone belt is associated with a regional NE-trending shear zone. The mineralized areas (the Pedra Branca gold deposit) are located near–parallel to the stratigraphy, siting on shear zones, between metavolcanic and metasedimentary unit boundaries. The main stage of gold mineralization is found in association with quartz veins, high–temperature calc–silicate alteration (diopside, K–feldspar, amphibole, titanite, biotite, pyrite, albite, magnetite ± carbonates) and albitization. Free–milling gold commonly precipitates in close association with magnetite and gold/silver tellurides. Two fluid inclusion assemblages were identified in mineralized quartz veins. Assemblage 1 is characterized by pseudo–secondary trails that show the coexistence of CO₂–rich and low salinity (0 to 8 wt% NaCl equiv.) CO₂–H₂O–NaCl and H₂O–NaCl inclusions, suggesting formation during phase separation (fluid immiscibility). The mean isochores intersection of CO₂–rich and H₂O–NaCl inclusions of assemblage 1 suggests PT conditions of 495 °C and 2.83 kbar (c. 10.5 km depth), akin to hypozonal orogenic gold deposits. Assemblage 2 is represented by late secondary low–temperature (Th<200°C) H₂O–NaCl inclusions, probably unrelated to gold mineralization. The δ¹⁸O, δD and δ¹³C values of hydrothermal minerals (quartz, calcite, biotite, hornblende and magnetite) define fluid δ¹⁸O values ranging from +8.3 to +11.0‰ (n=59), fluid δD from -98 to -32‰ (n=24) and δ¹³C values of calcite from -6.35 to -9.40‰ (n=3). Oxygen isotope thermometry for quartz–magnetite pairs gave temperatures from 467 to 526°C (n=7, average 503°C), which probably represents the temperature of gold deposition. The association of gold with magnetite and tellurides strongly suggests an ore–forming fluid sourced by oxidized magmas, similar to those interpreted as ‘orogenic oxidized intrusion– related gold deposits’ in other Precambrian greenstone belts (e.g. Abitibi and Eastern Goldfields). Four deformation events (Dn, Dn+1, Dn+2 and Dn+3) are recognized in the Serra das Pipocas greenstone belt. The Dn event is responsible for the early Sn foliation, parallel to bedding (So) of the greenstone pile. The Dn+1 event is characterized by a pervasive, southeasterly–dipping Sn+1 foliation that is axial–planar to a number of asymmetric, tight to isoclinal and recumbent folds. The Dn+2 event represents a transcurrent deformation phase and the late Dn+3 event is characterized by ductile–brittle deformation. The main stage of gold mineralization is found as deformed quartz veins and associated high–temperature alteration, but some lower temperature gold (±Te, Ag) occurrence along the late stage brittle structures (Dn+3 event) is also observed. The U–Pb titanite age of 2029 ± 28 Ma for the high– temperature calc–silicate alteration (and gold mineralization) is presented here. However, the strong Pb loss of titanite grains defines a 574 ± 7 Ma lower intercept age, evidencing that early gold mineralization were broadly affected by Neoproterozoic deformational events and metamorphism (Brasiliano/Pan–African orogeny). The U–Pb zircon age of 575 ± 3 Ma for syn–tectonic diques bracketed the age of late Dn+3 deformation event. Then, the progressive deformation recorded (Dn+1, Dn+2 and Dn+3) is probably of Neoproterozoic age, with the maximum compressive stress (ζ1) in the WNW–ESE direction. However, at local scale, Paleoproterozoic deformation records (Dn) still preserved. The genetic model for the Pedra Branca gold deposit is suggested here by a two–stage exhumation–drive gold mineralization; represented by a (1) early oxidized hypozonal orogenic gold mineralization (main stage) that occurred at c. 2029 Ma, shortly after the high–grade Paleoproterozoic metamorphism and first exhumation processes of the greenstone pile, and later on, at c. 580 Ma, a (2) late gold mineralization (remobilization?) occurred at shallow levels (second exhumation process) associated to late Neoproterozoic Brasiliano/Pan–African orogeny.
Acesso aberto (Open Access)
Greisens e Epi-sienitos potássicos associados ao granito água boa, Pitanga (AM): um estudo dos processos hidrotermais geradores de mineralizações estaníferas
(Universidade Federal do Pará, 2002-10-23) BORGES, Régis Munhoz Krás; DALL'AGNOL, Roberto; http://lattes.cnpq.br/2158196443144675
Three stanniferous greisen types were characterized in the western border of Água Boa pluton, Pitinga mine (AM), associated with the rapakivi granite facies: greisen 1 (Gsl), composed mainly by quartz, topaz, brown siderophyllite and sphalerite; greisen 2 (Gs2), composed essentially by quartz, phengite and chlorite; greisen 3 (Gs3), composed of quartz, fluorite and phengite, with minor green siderophyllite. Besides these rocks, a potassic episyenite (EpSK) was identified associated with the Gs2. In spite of the compositional and petrographic differences, all of these hydrothermal rocks derived from a same protholith, a hornblende biotite aikali feldspar granite to syenogranite. The Gsl shows an inner mineralogical zoning defined by topaz or siderophyllite predominance. Along drill cores, the siderophyllite-rich zone occurs near the contact with the greisenized grafite and the topaz-rich zone is situated far from the grafite contact. The brown siderophyllite displays moderated Al contents, and its compositional changes can be explained by Fe+2 substitution for A1+3 and Li in octahedral sites, with a coupled Al+3 substitution for Si+4 in tetrahedral sites. The mineralogical zones in the Gs2 are physicaliy separated in leveis with phengite or chlorite predominance. The mica of Gs2 is a phengite, whose chemical variation is due to substitution of viAl for Fe+2, coupled with Si+4 enrichment. The calculated Li contents in phengites are lesser than those estimated in siderophyllite. The green siderophyllite from Gs3 is VIAl richer and F poorer than Gs1 brown siderophyllite, and the phengite displays two compositional types: an early Fe+2-poor aluminous phengite and a later Fe+2- F-rich one whose chemical variation is similar to that of Gs2 phengite. The chlorite from the three greisen is a Fe-rich daphnite, and its compositional range is due to VIAl substitution for R+2 cations, coupled with Si+2 enrichment. The aluminous chlorite displays higher temperature formation than ferrous one, according to the geothermeter proposed in the literature. The Pitinga greisens were formed by different processes of interaction among three main fluids: (1) low salinity, F-rich, aquo-carbonic fluid, with initial temperatures between 400° -350°C, present during Gsl and Gs3 formation; (2) low salinity aqueous fluid, with a temperature around 300°C, which during a progressive salinity increasing process, originates a moderate to high salinity residual fluid, with temperatures between 200° - 100°C, present during the Gs2 formation and silicification stage of EpSK; (3) low salinity aqueous fluid, with temperatures between 200° - 150°C, which interplayed with the others two fluids in differents grades, contributing to the formation of ali the hydrothermal rocks. The first two fluids has seemingly an orthomagmatic origin while the latter has a surface characteristic (meteoric water?). Moreover, the data suggests that the fluid responsible by the initial stage of the episyenitization process was not registered in the studied samples. These fluids were trapped in pressure conditions around 1 Kbar, representing high crustal levels conditions, similar to that of the stanniferous granites from Pitinga. Both episyenitization and greisenization processes occurred without volume changes in the granitic protholith, and the density differences of the altered rocks were caused by the mass variations along the alteration processes. The greisenization process caused a extensive loss of Na2O and K2O, while SiO2 showed a immobile behaviour in Gsl but was parcially removed in Gs2. The Al2O3 was depleted during the Gs2 formation but added in Gsl. The Fe2O3 (Fe total), Sn, S, volatiles LOl and F were the responsible by the mass increase at greisenization. In the Gsl, the chemical changes in the fiuids were caused by F activity decrease and fO2 increase during cooling. These changes also originated the differentiation between the ZT, in the inner portions of the fratures/conducts, and the ZS, nearest to surrounding gravite. The Gs3 was formed in more oxidizing conditions by F-poorer fiuids than those trapped in the ZS. The dissolution cavities generated during the episyenitization process increased the permeability of the altered rocks, providing an increase of fluid/rock ratios in the EpSK and Gs2 sites. The interaction between aqueous fluid and EpSK feldspar, during the Gs2 formation, caused a continuous salinity increase. The ZF was formed in the early stages of this interaction, at higher temperatures, while the ZC was originated by the more cold and saline, residual fluid. The latter was also trapped in the quartz filling cavities in the EpSK during the later silicification stage. In this way, the greisens and the potassic episyenites were generated from interactions among, at least, three fluids of seemingly independent origin, from a same protholith, in shallow crust conditions. The fO2, F activity and salinity variations, during the hydrothermal system cooling, and the contrast in fluid/rock ratios caused by permeability differences, were very important factors to greisen differentiation. These factors controlled greatly the fluids compositional changes, and caused the cassiterite and sulphides precipitation in the greisens and the Sn- S-enrichment during later greisenization of EpSK.
Acesso aberto (Open Access)
O Magmatismo granítico e os seus efeitos na região de Xambioá/Araguanã, To
(Universidade Federal do Pará, 2009-05-22) POINSIGNON, Janaina Reis; KOTSCHOUBEY, Basile; http://lattes.cnpq.br/0096549701457340
The Xambioá region is located in the eastern part of Arguaia belt, in the northern portion of the domain of Estrondo Group (northwest of the State of Tocantins). The granites of Ramal do Lontra and Serra da Ametista, intrusive in the rocks of Estrondo Group, were until recently, the only felsic bodies known in the region. More recent geological observations, during detailed geological mapping, identified several intrusive granitic bodies, generally small, as well as in many sectors, unequivocal signs of albitization, greisenization and caulinization in both granitic bodies and host rocks. Quartz vein systems, host of hyaline quartz and eventually amethyst are commonly associated with granitic intrusions and/or with altered host rocks. The felsic magmatism expressed by the emplacement of alkali-granites (Ramal do Lontra and Serra da Ametista granites and occurrences of Fazenda Novo Horizonte, Fazenda Bela Vista, Fazenda Belém, Morro das Antenas and adjacency) and of albite-granites (Araguaci and sector of Pedra Preta granites). Fine to medium-grained alkali-granites, are usually deformed. These bodies have been considered late-tectonic and correspond most likely to apical zones of larger granitic intrusions, not yet exposed. Fine to medium-grained albite-granites consist essentially of albite, like euhedral crystals, and quartz do not show deformation. It has been interpreted as products of extreme granitic magma differentiation, which originated the alkali-granite. The postmagmatic context had a complex development, resulting in the formation of quartz vein systems and marked alteration of both the intrusive garnitic rocks and host rocks of Estrondo Group formations and the Archean basement. Based on fluid inclusions studies of vein quartz, two types of fluids have been recognized: aqueous- carbonic and aqueous. The first, with high salinity (38 to 53% eq of NaCl) and temperature (340 to 500°C) of metamorphic origin with important magmatic contribution, may have caused the formation of the early milky quartz, and probably the metasomatic alterations at high temperature, namely the albitization and greisenization of both granitic intrusions and host rocks. The second type, of moderate to low temperature and salinity (to 120 from 200°C and 1 to 18% eq of NaCl), is interpreted as having a magmatic origin and after mixing with meteoric waters caused its temperature and salinity decrease. It is possible that the drops in temperature and salinity may have been, in part, resulted from the natural cooling of hydrothermal process. These fluids were responsible for the formation of hyaline quartz, in distensive context of decreasing pressure. They also caused the alteration of igneous rocks and host rocks at low temperature, essentially kaolinization. The albitization was the earliest metasomatic alteration. At high temperature and alkaline conditions, it affected both alkali-granites and albite-granites, and the nearby host formations. Albitite with riebeckite and aegirine resulted also from this process. Strong signs of albitization were also detected in the Estrondo Group rocks and Complexo Colméia, regardless of any magmatic influence, what suggests the possibility of this process not be local but regional and related to the regional metamorphism or dynamometamorphism that acted during the structural evolution of Araguaia belt. The greisenization, even more local, succeeds to albitization, at high temperature but in acidic conditions. It is restricted to immediately host rocks of the intrusive granitic bodies and probably to their marginal portions. At last, at lower temperatures and acidic conditions, kaolinization occurred and their effects have been observed only in the host schists, although this alteration may have also affected the intrusive bodies. The total destruction of all primary minerals but not quartz, resulted in the formation of pure kaolin consisting of kaolinite of high crystallinity.The geochemical data reveal coherence between several processes, like the concentration of trace elements (Th, U, Hf, Nb, Zr, V and W) very common in granitic environment.The results obtained in this study suggest that magmatic/ hydrothermal manifestations have been more intense that believed before, being recommended the continuation of investigations, in more meridian zones of the Estrondo Group domain.