Navegando por Assunto "Metalogênese"

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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)
Evolução geológica pré-cambriana e aspectos da metalogênese do ouro do cráton São Luís e do Cinturão Gurupi, NE-Pará/ NW-Maranhão, Brasil
(Universidade Federal do Pará, 2004-07-06) KLEIN, Evandro Luiz; GIRET, André; HARRIS, Christopher; MOURA, Candido Augusto Veloso; http://lattes.cnpq.br/1035254156384979
In the Gurupi region, located at the border between the Pará and Maranhão states in northern Brazil, igneous and metamorphic rocks crop out as part of the Parnaíba Structural Province. Early geochronological studies, based on the Rb-Sr and K-Ar methods have shown two geochronological domains. The rocks that crop out towards the Atlantic margin showed a Paleoproterozoic signature, around 2000 Ma, whereas the rocks that crop out towards the inner portions of the continent showed a Neoproterozoic signature, especially between 800 and 500 Ma. These domains have been then defined as the São Luís Craton and Gurupi Belt, respectively. Several lithostratigraphic propositions have been developed throughout more than two decades. However, these propositions always lack robust geochronological support. Geotectonic models discussed a one- or two-phase evolution for the Gurupi Belt, also lacking robust geochronological and isotopic data to consubstantiate the interpretations. Furthermore, among the several gold deposits that occur in both the cratonic and belt areas, only a few have geological and genetic information. These subjects are addressed in more or less depth by this thesis. New propositions for the regional lithostratigraphy and geological evolution have been achieved in this work by revaluating the available geological, geochemical, geochronological and isotopic dataset, as well as by adding new geochronological data on zircon (Pb-evaporation, U-Pb ID-TIMS, and LAM-ICP-MS) for most of the igneous and orthometamorphic rocks in the region. Whole rock Nd isotope data have also been obtained, allowing the discussion of crustal accretion and reworking. The results show a rather complex geological evolution with intensive and extensive crustal growth between 2.24-2.15 Ga and crustal reworking, involving melting, migmatization, metamorphism, and deformation around 2.10 Ga. The following results have been obtained for the São Luís Craton: Aurizona Group, metavolcano-sedimentary sequence, maximum age of 2241 Ma (juvenile) that possibly evolved until c.a. 2200 Ma; Tromaí Intrusive Suite, calc-alkaline, metaluminous tonalites of oceanic island arc, 2168 Ma (juvenile); Areal Granite, calc-alkaline, weakly peraluminous, 2150 Ma (mixing of juvenile and arc materials). In the Gurupi Belt, the following results have been obtained: Igarapé Grande Metatonalite, small and localized granoblastic tonalite, 2594 Ma; Itapeva Complex, weakly migmatized tonalitic orthogneiss, 2167 Ma (mostly juvenile); Chega Tudo Formation, metavolcano-sedimentary sequence (back-arc basin?), 2150-2160 Ma; Maria Suprema Granite, syntectonic, peraluminous muscovite-bearing granite, 2100 Ma (similar to other peraluminous granitoids in the Gurupi Belt). The Gurupi Group is tentatively placed in the Paleoproterozoic (>2160 Ma), but this must still be proved. The above data are interpreted on a plate tectonics basis, as follows. An oceanic basin is open at ca. 2260 Ma and is followed by the onset of subduction, formation of island arcs and voluminous calc-alkaline magmatism in oceanic settings, and concomitant reworking of the arcs between 2170-2150 Ma. This set of oceanic terranes has been accreted (soft-collision) onto an Archean continental margin to southwest (Archean part of the Amazonian Craton or a present day concealed cratonic nuclei). The collision provoked the metamorphism, deformation, and partial melting of the newly formed Paleoproterozoic crust and of part of the Archean bloc, or their erosive detritus, migmatization, and emplacement of peraluminous granitoids at 2100-2080 Ma. The region has been the locus of a second event in the Neoproterozoic. A continental rift developed in the bloc that was assembled in the Paleoproterozoic, as attested by the intrusion of a nepheline syenite (Boca Nova) at 732 Ma. Sedimentary rocks that filled this rift (Marajupema Formation) have detrital zircon crystals that show the youngest ages around 1100 Ma. The rift evolved probably to an oceanic basin, as suggested by the widespread occurrence of detrital zircons with ages around 550 Ma in small sedimentary basins that have been filled with immature sediments. The precise time of orogenesis climax that followed basin closure, with mass transport from SSW to NNE and accompanying metamorphism, is not yet constrained. Equivocal geochronological information point to 650-520 Ma (zircon of the nepheline syenite, Rb-Sr and KAr ages in minerals). The metallogeny of selected gold deposits occurring in both the São Luís Craton and the Gurupi Belt is addressed using varied information, such as geology, chlorite chemistry, fluid inclusion geochemistry, and stable (O, H, C, S) and radiogenic (Pb) isotopes. Structural and textural relationships, and Pb isotope data indicate a post metamorphic peak and late- to posttectonic timing for the gold mineralization with respect to the Paleoproterozoic events (post 2080 Ma). At a regional scale, the deposits show a similar signature characterized by formation temperatures between 280° and 380°C; pressures of 2-3 kbars; low-salinity (5 mass % NaCl equiv), reduced and moderately dense aqueous-carbonic (CO2 <20 mol%, traces of CH4 and N2), showing strong evidence for phase separation. Stable isotope studies suggest distinct sources for fluids and solutes. The carbonate, graphite, and fluid inclusion carbon comes from two sources: a depleted organic source, and an unknown source that may be magmatic, metamorphic or mantlederived (or both). Sulfide sulfur derived directly from magmas or from the dissolution of magmatic sulfides. Combined oxygen and hydrogen isotopes attest a metamorphic source for the fluids. Therefore, dehydration and decarbonization reactions during the metamorphism of the Paleoproterozoic metavolcano-sedimentary sequences appear to have produced the mineralizing fluids. Gold was transported as a reduced sulfur complex, such as the Au(HS)2 - and precipitated in response to the breakdown of this complex due to phase separation and fluid-rock interactions. The geological and genetic constraints are consistent with the orogenic gold model, found in metamorphic belts of all ages. As a whole the results of this study have implications for the understanding of the Paleoproterozoic and Neoproterozoic orogenies that built up the South American Platform and for the assembly and break-up of the Atlantica, Rodinia, and West-Gondwana supercontinents. The geological scenario outlined here for the Paleoproterozoic shows good correlations with those found especially in the southeastern Guyana Shield and in the southern portion of the West- African Craton. For the Neoproterozoic, the available information is still insufficient to draw major correlations.
Acesso aberto (Open Access)
Gênese do depósito aurífero do Cuca/Tucumã-PA, com base nos fluidos hidrotermais e isótopos de chumbo.
(Universidade Federal do Pará, 2000-08-04) CORREIA JUNIOR, Firmino Coutinho; SANTOS, Márcio Dias; http://lattes.cnpq.br/6977793618030488
Acesso aberto (Open Access)
Geologia e metalogênese do depósito aurífero Ouro Roxo, Província Tapajós, Jacareacanga-PA.
(Universidade Federal do Pará, 2011-05-25) VELOSO, Ângela Suélem Rocha; SANTOS, Márcio Dias; http://lattes.cnpq.br/6977793618030488
The OuroRoxo gold deposit is located near São José Village at Pacuriver margin, far around 36 km from Jacareacanga city, western Tapajós Gold Province, SW portion of Para State. The OuroRoxo deposit comprises a hydrothermal system of sulfide quartz veins hosted by mylonitegranitoidTropas Intrusive Suitof Paleoproterozoic age. The ore bodies are controlled by a N-S trending shearing known as OuroRoxo-CantaGalo shear zone. The host granitoids are oxidized calc-alkaline granodiorite and tonalite with magnetite and sphene, characterized as magmatic arc granitoids.The OuroRoxoCantaGalo shear zone is a sinistral oblique ductile-brittle shear of low to middle angle which has turned the granitoids into protomylonites and mylonitesinterlayed with breccias zones. The protomylonites are characterized by predominance of porfiroclasts over the matrix composed by phengite, chlorite and quartz which grade into mylonites and minor ultramylonits with almost 100% of matrix. The mylonitic foliation is defined by chlorite, biotite and phengite along NNE-SSW direction dipping to ESE. Stretched quartz grains define a lineation which indicates the oblique tectonic movement to NNW. Shear features such as mica-fish,shadow pressure, tail recristalization and microboudinage are ubiquitous in theses rocks and indicate a sinistral kinematics for shearing. The ore bodies are quartz veins and pipes hosted by mylonites and breccias, surrounded by prominent alteration halos and concordant to mylonitic foliation. Besides silicification and sulfidation more concentrated in the ore bodies, three types of hydrothermal wall-rock alteration were recognized in alteration halos: 1) propilitic alteration: chlorite+phengite+carbonate. Four chlorite generations were identified, the last one being veinlets; 2) phylic alteration: phengite+quartz+carbonate+pyrite, with three phengite generations; 3) carbonatization with three generations of carbonate, the last one beingveinlets. Besides magmatic quartz and microcrystalline quartz of mylonites, five generations of hydrothermal quartz were recognized in the lodes, the last one beingveinlets.The ore shoots are mainly made up by pyrite and chalcopyrite, with minor bismutinite, native bismuth and gold. Besides the pyrite associated to gold and an earlier pyrite magmatic generation, late pyrite veinlets occur. Two generations of chalcopyrite were identified and the second one substitutes the pyrite 2 associated to ore. The main gange minerals are quartz, chlorite, phengite and carbonate. Azurite, bornite and covellite are copper supergenic minerals derived from alteration of chalcopyrite.Three kinds of fluids were identified in vein quartz of the OuroRoxo deposit: 1) low-to-middle salinity aqueous fluid within the H2O-NaCl-MgCl2-FeCl2systemwhich occurs in quartz 3, with Th=180-280°C; 2) brines within the H2O-NaCl-CaCl2 system which occur in quartz 4 and 6, with Th=270-400°C, and underwent dilution brought about by mixing with meteoric water, lowering its salinity and temperature (Th=120-380); 3) middle-salinity aquocarbonic fluid which occurs in quartz 3 and 4, with Th=230-430°C. The aquocarbonic fluid was interpreted as ore fluid related to shear zone with higher Th in ductile conditions. The brines origin was related to a magmatic event coeval to shearing, with lower Th in the end of the residual magmatic process.The temperature and pressure conditions of OuroRoxo deposit formation were estimated using the hydrothermal chlorite geothermometer and the isochors calculated from the fluid inclusion microthermometric data. Values range from 315 to 390°C and 2,0 to 4,2 k b. Two mechanisms were regarded to be the main cause for gold deposition. 1) Fluid- rock interaction brought about increasing of fO2 and reduction of fS2 during hydrolysis (propylitic and phylic alteration) and sulfidation reactions; 2) Mixing between aquocarbonic fluid and magmatic brines induced an increasing of fO2anddecreasing of pH. These two mechanisms would favor gold deposition in transtension sites of the shear zone.The age of ore formation, obtained by Pb-Pb method in pyrite, was estimated around 1858±130Ma. The large error of that age turn it as reference age only which shows a time relationship between the mineralization, the shearing and the Maloquinha granite event. The orogenic magmatic arc environment, the veining style, the structural control by the shear zone, the hydrothermal alteration (propylitic+philic+carbonatization), the metallic association (Au+Cu+Bi), the middle-salinity aquocarbonic ore fluid and the participation of magmatic brines in ore deposition, are all consistent to a hybrid model (orogenic with magmatic participation) for the genisis of the OuroRoxo gold deposit.
Acesso aberto (Open Access)
The mineralizing fluid in the Piaba gold deposit, São Luís cratonic fragment (NW-Maranhão, Brazil) based on fluid inclusion studies in quartz veins
(2013-03) FREITAS, Saney Cecílio Ferreira de; KLEIN, Evandro Luiz
Piaba is the first gold mine to operate in the São Luís cratonic fragment, NW-Maranhão, northern Brazil. The geological setting comprises chiefly metavolcano-sedimentary sequences (Aurizona Group) and subduction-related granitoids (Tromaí Intrusive Suite), formed in island arc between 2240 and 2150 Ma. Gold mineralization is hosted in a fine-grained granophyric granodiorite (Piaba Granophyre) and in a subvolcanic andesite of the Aurizona Group. The mineralized zone is confined within the limits of the Piaba fault (ENE-WSW-trending brittle-ductile shear zone) and consists of quartz veins and veinlets and accompanying hydrothermal haloes (chlorite + muscovite + carbonate + pyrite + chalcopyrite + gold) disposed in stockwork geometry. Petrographic, microthermometric and microRaman spectroscopic studies of quartz have defined two- and three-phase aqueous-carbonic fluid inclusions produced by heterogeneous trapping during phase separation, in addition to late aqueous fluids. The mineralizing solution is an aqueous-carbonic fluid composed of CO2 (5 - 24 mol%), H2O (74 - 93 mol%), N2 (< 1 mol%) and CH4 (< 1mol%). It presents low salinity (5.5 wt.% NaCl equivalent) and density of 0.96 - 0.99 g/cm3). Ore deposition occurred at 267 - 302ºC and 1.25 - 2.08 kbars, corresponding to 4 - 7 km in depth, in agreement with the structural information. The P-T-X and reduced characteristics (log ƒO2 -31.3 to -34.3) of the fluid, combined with host rock sulfidation, altogether indicate that gold has been transported as a sulfur complex and that ore deposition occurred in response to phase separation and lowering of the sulfur activity and ƒO2 during fluid-rock interaction.