Navegando por Orientadores "KLEIN, Evandro Luiz"

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Aspectos geológicos e metalogenéticos do depósito de ouro hospedado em metaconglomerados e metarenitos paleoproterozoicos Castelo de Sonhos, Província Tapajós, sudoeste do Pará
(Universidade Federal do Pará, 2015-04-06) QUEIROZ, Joana D’arc da Silva; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706
Castelo de Sonhos, located in the central-south sector of the Amazonian Craton, near the boundary between the Tapajós and Xingu-Iriri tectonic domains, is a gold deposit hosted in metaconglomerates and metasandstones of the Castelo dos Sonhos Formation (<2080 Ma U-Pb SHRIMP). Subvolcanic rocks and granitoids were identified in boreholes that drilled the deepest parts of the deposit area. Some of these rocks are intrusive into the Castelo dos Sonhos Formation, while for others rocks, the contact relationships could not be determined with confidence. In general, these rocks show calc-alkaline to alkaline affinities and their geochemical patterns indicate that they are related to volcanic arc or post-collisional tectonic settings. The subvolcanic rocks are represented by a porphyritic dacite with age of 2011 ± 6 Ma (U-Pb LA-ICP-MS).The granitoids were classified as biotite granodiorite, biotite monzogranite, muscovite monzogranite, respectively dated at 1976 ± 7 Ma, 1918 ± 9 Ma and 1978 ± 6 Ma (U-Pb SHRIMP), and an undated syenogranite. These ages represent three to four distinct magmatic events and indicate that the studied rocks are coeval to four major units from Tapajós Domain: the Cuiú-Cuiú Complex (2033-2005 Ma), the Comandante Arara Formation (2020-2012 Ma), the Creporizão Intrusive Suite (1998-1957 Ma), and the Tropas Intrusive Suite (1907-1892Ma). Despite the temporal correspondence, the geochemical data show no direct correspondence with the units cited above. Notwithstanding, the intrusion relationship between some of the studied rocks and the metasedimentary rocks of the Castelo dos Sonhos Formation establishes a temporal, spatial and possibly stratigraphic relationship between this formation and the Tapajós Domain. The intrusive contact relationship between the porphyritic dacite and metasandstones of the Castelo dos Sonhos Formation allowed us to determine at 2011 ± 6 Ma the minimum sedimentation age of this unit. The primary gold mineralization at Castelo de Sonhos deposit is stratabound and restricted to a metaconglomerate package and interlayered metasandstones. The mineralization distribution is erratic and does not seem to follow special features or structural control. In the matrix of the metaconglomerates, gold occurs as intergranular particles, occasionally associated with magnetite, and also within quartz grains (medium to coarse sand), which probably represent fragments of auriferous veins. In general, the gold particles show subrounded to rounded shapes, mild to moderately rough surfaces. The particles seldom contain inclusions, and only of magnetite. The chemical composition is homogeneous and characterized by high Au/Ag ratios. These characteristics indicate a syngenetic origin for gold within the metaconglomerates package. Therefore, the age of mineralization is limited by the time x interval of deposition of the Castelo dos Sonhos Formation (2011 ± 6 Ma to ca. 2080 Ma). On the other hand, the occurrence of gold in fracture planes of metasandstones indicates an epigenetic origin for this style of mineralization. The epigenetic mineralization is related to concurrent metamorphic, magmatic and deformational processes that affected the sedimentary sequence of the Castelo dos Sonhos Formation and caused the remobilization of gold originally hosted in metaconglomerates. It is likely that the interaction of these processes associated with infiltration of meteoric waters contributed to the generation and flow of oxidizing hydrothermal fluids, which have percolated through the metaconglomerates package and were able to solubilize some of the gold, and re-precipitate it accompanied by ferruginous films, in fracture planes of the metasandstones. As a conclusion, a modified paleoplacer model is proposed here to explain the hybrid nature (syngenetic and epigenetic) of the gold mineralization in the Castelo de Sonhos deposit.
Acesso aberto (Open Access)
Caracterização da alteração hidrotermal no alvo Coelho Central, depósito aurífero Pedra Branca, sequência Serra das Pipocas, maciço de Troia, com base em estudos isotópicos (O, H, S e C) e inclusões fluidas.
(Universidade Federal do Pará, 2019-05-08) LIMA, Rafael Guimarães Corrêa; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706
The gold mineralization of Pedra Branca deposit is associated with rhyacian metavolcanic rocks of Serra das Pipocas Greenstone Belt, at the Archean–Paleoproterozoic Troia Massif, in Borborema Province, NE of Brazil. The deposit comprises four mineralized targets, Mirador, Queimadas, Igrejinha and Coelho Central and has been classified as a hypozonal orogenic gold deposit based on the geological context, types of hydrothermal alterations and isotopic data obtained for Mirador and Queimadas targets. At Coelho Central target, the main gold host rocks are ilmenite- and garnet amphibolites (after mafic volcanic rocks). Some gold occurrences has been recognized in altered metadacites and metatonalites intrusive lenses. Hydrothermal and mineralized zones show mineral assemblages formed under amphibolite facies conditions. (i) calc-silicate veinlets (diopside, titanite, calcite, epidote and sulfides), (ii) hornblende, albite, biotite and garnet veinlets with pyrrhotite and gold, (iii) biotite-rich potassic alteration containing pyrrhotite, gold and tellurides and (iv) quartz veins are the main types of ore-related alterations. In addition, (v) epidote, titanite and calcite pockets, (vi) chloritization and (vii) pyrite-rich fissural carbonatization are the late and no ore-related alterations that marks a ductile-brittle deformation, probably under greenschist facies conditions. Gold occurs predominantly as inclusions in pyrrhotite, chalcopyrite, pentlandite and Co-pentlandite indicating their association with sulfur species and subordinately as free-milling microparticles in quartz veins. The metallic association of sulfidation zones also includes abundant Ag, Bi, Ni and Pb tellurides. Oxygen and hydrogen geothermometry for hydrothermal silicates yielded temperatures from 484 to 586 ºC for the gold-related alterations. The fluid isotopic composition in equilibrium with hydrothermal silicates (quartz, hornblende, biotite, tourmaline and titanite) shows values of δ18O (+6.8 to + 10.7 ‰) and δD (-58.4 to -35.5 ‰), as well as the values of δ34S in sulfides (-3.1 and +2.7 ‰) and δ13C for calcite (-11.1 to -5.8 ‰), indicative of deepseated magmatic-hydrothermal fluid, with possible interaction and mixing with fluids of the greenstone sequence. Fluid inclusions assemblages in quartz veins show the predominance of CO2 inclusions, with a density up to 1.15 g/cm3 and up to 15 mol% of CH4, coexisting with N2 inclusions, and also with low salinity (< 9.7 wt% NaCl equiv.) H2O-CO2-NaCl and H2O-NaCl-FeCl2±MgCl2 varieties. The petrographic and microthermometric criteria suggest the immiscibility of a CO2-H2O-NaCl-N2-CH4 fluid as responsible for the generation of the observed inclusions. Desestabilization of complexes such as Au(HS)-2 and precipitation of gold and associated metals occurred by immiscibility, fluid-rock interaction, and changes in redox conditions and fluid pH, between 2.2 and 5,5 kbar (6.3 to 16.0 km). A H2O-NaCl-CaCl2 latter fluid was the responsible for the calcite, with pyrite and sphalerite precipitation in fractures and faults. The characteristics presented above allow us to ratify the Coelho Central target and Pedra Branca deposit as a hypozonal gold mineralization formed under amphibolite facies conditions, from CO2-rich deep-seated magmatic fluids that interacted with the metamorphic sequence of Serra das Pipocas Greenstone Belt and precipitated gold and associated metals.
Acesso aberto (Open Access)
O Depósito Aurífero Piaba no fragmento ratônico São Luis (NW-Maranhão): petrografia das rochas hospedeiras e fluidos mineralizadores
(Universidade Federal do Pará, 2012-08-29) FREITAS, Saney Cecílio Ferreira de; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706; https://orcid.org/0000-0003-4598-9249
The Piaba gold deposit became, in 2010, the first gold mine in operation in the São Luís Cratonic Fragment, northwestern of the State of Maranhão, Brazil. The tectonic setting comprises the metavolcano-sedimentary rocks of the Aurizona Group and the granitoids of the Tromaí Intrusive Suite, along with minor units. All these rocks formed in island arc setting between 2240 and 2056 Ma. Gold mineralization at Piaba is hosted in a fine-grained granophyric granodiorite (Piaba Granophyre) and in a subvolcanic andesitic rock belonging to the Aurizona Group. The ore bodies are hosted in the Piaba fault, which is an ENE-WSWtrending brittle-ductile structure. These ore bodies consist of stockwork veins and veinlets and their hydrothermal haloes. The petrographic study was undertaken in drill cores of the mineralized zone. This study has shown that the host rocks have been affected by strong hydrothermal alteration that provoked intense mineralogical transformations of the rocks. The most prominent alteration processes have been the sericitization and chloritization of the host rocks, occurring in pervasive and fissure-filling modes. In addition, carbonatization, silicification and sulfidation occurred in response to fluid flow and veining along fractures and small faults associated with the Piaba fault. These latter types of alteration appear to be associated with gold precipitation. Petrographic, microthermometric and Raman microspectroscopic studies have been undertaken in quartz from mineralized veins from a stockwork zone. These studies identified two and three phase aqueous-carbonic fluid inclusions that have been interpreted as produced by heterogeneous trapping during immiscible phase separation. Late aqueous inclusions have also been identified. The mineralizing aqueous-carbonic fluid is composed of CO2 (5 – 24 mol%, density of 0.96-0.99 g/cm3), H2O (74 – 93 mol%), N2 (≤1 mol%), CH4 (≤1mol%), and has 5.5 wt. % NaCl equivalent. The gold ore deposited at 267-302ºC and 1.25-2.08 kbar, corresponding to depths of 4 to 7 km. Fluid composition and the P-T interval of the mineralizing fluid, its relatively reduced character (log ƒO2 -31.3 to -34.3), and the strong sulfidation of the host rocks indicate that gold has been transported as a sulfur complex and was deposited in response to phase separation and lowering of ƒO2 and of the sulfur activity during fluid-rock interaction. These conditions are consistent with the brittle regime and mesozonal to hypozonal conditions. Comparing the geological and fluid P-T-X characteristics of Piaba with those of other gold prospects already investigated in the same region, it is possible to define Piaba as an orogenic gold deposit.
Acesso aberto (Open Access)
Estudos de inclusões fluidas e de isótopos estáveis no depósito Moreira Gomes do campo mineralizado do Cuiú-Cuiú, Província Aurífera do Tapajós, Estado do Pará
(Universidade Federal do Pará, 2013-08-29) ASSUNÇÃO, Rose de Fátima Santos; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706
Moreira Gomes is a recently discovered deposit (preliminary resources of 21.7 t Au) of the Cuiú-Cuiú goldfield, an importante and historical mining área of the Tapajós Gold Province, Amazonian Craton. The mineralized zone is about 1200 m long, 30-50 m wide, and is followed to at least 400 m in depth. The zone is controlled by a subvertical, east-west-trending structure that is related to a left-handed strike-slip fault system. The host rocks in the deposit are predominantly tonalites dated at 1997 ± 2 Ma that are attributed to the magmatic arc or post-collision Creporizão Intrusive Suite. Hydrothermal alteration and mineralization are predominantly of the fissure-filling type and locally pervasive. Sericitization, chloritization, sulfidation, silicification, carbonatization and epidotization are the observed alteration types. Pyrite is by far the predominant sulfide mineral and bears inclusions of chalcopyrite, galena, sphalerite and minor hesite and bismuthinite. Gold occurs predominantly as inclusions in pyrite and subordinately in the free-milling state in quartz veins. Ag, Pb and Bi have been detected by semi-quantitatiive analysis. Three types of fluid inclusions, hosted in quartz veins and veinlets, have been identified. (1) Type 1: one- and two-phase CO2 inclusions; (2) Type 2: two- and three-phase H2O-CO2-salt inclusions, and (3) Type 3: two-phase H2O-salt inclusions. CO2 is largely the predominat volatile phase in the CO2-bearing inclusions. The CO2-bearing types 1 and 2 are interpreted as the product of phase separation of an immiscible fluid. This fluid presentes low to moderate density, low to moderate salinity (1.6 to 11,8 wt.% NaCl equivalent) and was trapped chiefly at 280° to 350°C. In Type 3 fluid, the chemical system may contain CaCl2 and/or MgCl2, salinitye varies from zero to 10.1 wt.% NaCl equivalent. Only locally salinities up to 25% have been recorded. This fluid was trapped mainly between 120° and 220°C and is interpreted as resulting from mixing of a hotter and more saline aqueous fluid (in part derived from phase separation of the H2O-CO2 fluid) with a cooler and dilute aqueous fluid. As a whole, the fluid inclusions indicate phase separation, pressure fluctuations, mixing, and reequilibration during trapping. The isotopic composition of inclusion fluids and of the fluid in equilibrium with hydrothermal minerals (quartz, chlorite, and calcite) show δ18O and δD values that range from +0.5 to +9.8‰, and from -49 to -8‰, respectively. The δ34S values of pyrite (-0.29‰ to 3.95‰) are probably related to magmatic sulfur. Mineral pairs show equilibrium isotopic temperatures that are compatible with the fluid inclusion homogenization temperatures and with textural relationships of the hydrothermal minerals. Isotopic results combined with mineralogical and fluid inclusion data are interpreted to reflect a magmatic-hydrothermal system that evolved in at least three stages. (1) Exsolution of a CO2-bearing magmatic fluid between 400°C and 320-350°C and up to 2.1 kbars (6-7 km in depth) followed by phase separation and main precipitation of the hydrothermal assemblage composed of chlorite-sericite-pyrite-quartz-gold. (2) Cooling and continuous exolution of CO2 producing a CO2-depleted and slightly more saline aqueous fluid that was trapped mainly at 250°-280°C. The predominant hydrothermal assemblage of stage 1 continued to form, but epidote is the main phase at this stage. (3) Mixing of the stage 2 aqueous fluid with a cooler and dilute aqueous fluid of meteoric origin, whis was responsible for the main carbonatization phase. The composition of the hydrothermal assemblage and the fluid and isotopic composition indicate that the mineralizing fluid was neutral to slightly alkaline, relatively reduced (only locally, more oxidezed conditions have been attained, resulting in the precipitation of barite). H2S (and/or HS-) might have been the main súlfur species in the fluid and Au(HS)-2 was probably the gold transporting complex. Gold deposition occurred as a consequence of a combination of mechanisms, such as phase separation, mixing and fluid-rock interaction. The Moreira Gomes is a granite-hosted gold deposit that interpreted to be a product of a magmatic-hydrothermal gold system. The age of ore formation (~1.86 Ga) is consistent with the final stages of evolution of the widespread high-K, calc-alkaline Parauari Intrusive Suite, although the ttransitional to predominantly alkaline Maloquinha Intrusive Suite cannot be ruled out. Notwithstanding, the deposit does not show the classic features of (oxidized or reduced) intrusion-related gold deposits of Phanerozoic magmatic arcs.
Acesso aberto (Open Access)
Estudos de inclusões fluidas e isotópicos (Sr, C, O, H) e implicações para a mineralização aurífera no alvo Enche Concha, Cinturão Gurupi.
(Universidade Federal do Pará, 2019-08-19) MELO JUNIOR, Reinaldo Fontoura; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706
The Gurupi Belt is a Neoproterozoic orogen of NNW-SSE orientation developed at the southsouthwest margin of the San Luis Cratonic Fragment, of Paleoproterozoic age. The belt is composed of metavolcano-sedimentary sequences, gneisses and several generations of plutonic rocks representing reworked fragments of the cratonic border and portions of the basement on which the Gurupi Belt developed in the neoproterozoic. The Chega Tudo Formation, the main host for gold mineralization in the Gurupi belt, is a metavolcanosedimentary sequence that is characterized by being a narrow and elongated band in the NWSE direction. The rocks of this unit comprise an alternation metavolcanic, metavolcanoclastic and metassedimentary varieties, which are invariably schistose and/or mylonitic, and locally folded. Enche Concha is one of the gold prospects that occur in the Chega Tudo Formation and a better definition of it’s geological characteristics and genetic aspects is presented here. The host rocks are phyllites, dacites and breccia zones. The phyllites are rocks of blackish greenish colors, consisting essentially of quartz, micas and carbonate. Dacites are whitish gray-colored rocks consisting primarily of plagioclase and quartz. The breccias are fragmented, non-cohesive rocks, with no obvious tectonic fabric consisting of fragments of quartz and carbonate veins, and of fragments of phyllites. The types of hydrothermal alteration defined in the target was as follows: (i) silicification, (ii) sericitization, (iii) carbonatization and (iv) sulfidation, along with precipitation of gold. The sulfides are mainly pyrite, along with sphalerite and chalcopyrite. Gold occurs as inclusions in the three sulfide minerals, with concentrations of up to 6%, as identified by analyzes of Scanning Eletron Microscopy. The fluid inclusion were classified as type 1 (biphasic aqueous-carbonic - H2Oliquid - CO2liquid and H2Oliquid - CO2vapor) and type 2 (aqueous, single-phase and twophase H2Oliquid and H2Oliquid - CO2vapor). Type 1 inclusions provided CO2 melting temperature values (TfCO2) between -56.6 and -57.3ºC, which allows inferring that the main volatile component of the carbonic phase of these inclusions is CO2. The CO2 homogenization temperature values (ThCO2) are between 12 and 25.5ºC (global density between 0.7 and 1 g/cm3, CO2 density between 0.2 and 1 g/cm3). As for the melting temperature of the clathrate (TfClat), results were obtained between 6 and 8.4ºC, which gives salinities between 4.4 and 5.5 wt% NaCl equiv. The total homogenization temperature (Tht) of these inclusions varied between 251 and 369º, with homogenization occurring both for the liquid and vapor phases. Type 2 inclusions presented values of melting temperature of the ice (Tfg) between -0.1 and - 4.1 ºC, and Tht values between 167 and 342ºC,with homogenization occurring for the liquid phase. These inclusions presented low values of salinity, between 0.18 and 6.3 wt.% in NaCl equiv. weight, and global density between 0.7 and 0.9 g/cm3. The coexistence of aqueous and aqueous-carbonic inclusions over the same range of Tht, in addition to inclusions that homogenized for the liquid phase and for the vapor phase, suggest fluid immiscibility. The isotopic composition of oxygen and carbon was determined hydrothermal quartz and calcite, whereas hydrogen isotopic compositions were obtained from inclusion fluids. The values of δ18O in quartz vary from +15.25 to +17.74‰. The values of δ18O and δ13C in calcite range, respectively, from +14.32 and +16.24‰ and from -9.83 to -15.12‰. Values of δ13C bellow - 10‰ suggest contribution of organic C from the host rocks. Values of δD extracted from the inclusion fluids gave results of -25 and -28‰. The isotopic results suggest metamorphic composition for the mineralizing fluids at Enche Concha. The strontium isotopic composition was determined in hydrothermal calcite. The analyzed samples have low, little radiogenic 87Sr/86Sr ratios, ranging from 0.702699 and 0.705141. These results indicate origin of deep sources of the lower crust or mantle and should not have contribution of the regional rocks of the Gurupi Belt. As a whole, the integrated data indicate that the Enche Concha shares characteristics whit those of orogenic gold deposits, which are widespread in the Gurupi belt.
Acesso aberto (Open Access)
Estudos de inclusões fluidas e isótopos estáveis nos alvos Jerimum de cima e Babi, campo mineralizado do Cuiú-Cuiú, Província Aurífera do Tapajós, Cráton Amazônico: implicações para os processos genéticos
(Universidade Federal do Pará, 2015-08-18) SILVA JUNIOR, Carlos Alberto dos Santos; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706
The Cuiú-Cuiú goldfield is located near the central portion of the Tapajós Gold Province in the south-central portion of the Amazonian Craton. This goldfield is one of the oldest prospecting areas of the province and holds multiple more or less developed prospects and gold deposits (Central, Raimundinha, Pau da Merenda, Guarim, Jerimum de Cima, Jerimum de Baixo, Nho, Moreira Gomes, Babi and other less known). As contribution to the understanding of the metallogenic evolution of the Cuiú-Cuiú goldfield in general, this study focused on the mineralized Jerimum de Cima and weakly mineralized Babi targets and aimed: (1) to define the sulfide mineralogy associated with gold mineralization and its textural relationships with the host rocks; (2) to define the physical and chemical characteristics of the mineralizing/hydrothermal fluids through petrographic, fluid inclusions and stable isotopes (C, O, S) studies trying to identify what caused the hydrothermal alteration in rocks from these targets and that enabled more significant mineralization at Jerimum de Cima (and other targets/deposits), whereas Babi is only weakly mineralized (not economic). The petrographic study identified strongly hydrothermally altered host rocks, with obliterated primary characteristics. In the Jerimum de Cima target the host rocks are biotite-hornblende tonalite, monzogranite and granodiorite. In the Babi target, titanite monzogranite, biotite monzogranite, biotite-hornblende tonalite, and brecciated monzogranite are the hydrothermally-altered rocks. Sericitization, silicification and sulfidation occur strongly in the Jerimum de Cima target, whereas carbonatization and chloritization occur usually in both targets. Pyrite, sphalerite, chalcopyrite and galena, in decreasing order of abundance, are the sulfide minerals, with large predominance of pyrite. Fluid inclusions (FI) trapped in quartz crystals occur in small groups, in isolation, or in trails. In decreasing order of abundance, there are three types of FI: two-phase aqueous (Type 1), aqueous-carbonic (Type 2) and carbonic (Type 3). The microthermometric results show that the aqueous FI at Jerimum de Cima homogenized between 105 and 387°C, and have salinities that range from 0,0 to 18 wt.% NaCl equivalent; whereas the aqueous-carbonic type has final homogenization temperatures between 144 and 448°C, salinities of 1,0 to 7,8 wt.% NaCl equivalent, and bulk density ranging from 0,6 to 1,0 g/cm3. At Babi the aqueous FI are the only type present. These FI homozenized between 136 and 410°C and show salinities from 0,7 to 13,2 wt.% NaCl equivalent. The aqueous-carbonic FI are interpreted as a product of fluid immiscibility (phase separation). The absence of CO2–bearing inclusions in the Babi target is possibly a consequence of late-timing of fluid trapping during the evolution of the hydrothermal system, after the CO2 consumption, with only aqueous FI being trapped. Stable isotope analyses of hydrothermal minerals present in veins and alteration zones indicate mineral precipitation between 305 and 330°C and between 108 and 205°C, which is in line with the fluid inclusion honogenization temperatures and indicate more than one stage of mineral precipitation. The data also suggest magmatic and meteoric sources for the fluids. As a whole, our data are compatible with a magmatic-hydrothermal gold systems (intrusion-related), and with mixing of magmatic and meteoric fluids. The lack of CO2 at Babi might explain the weak mineralization in this target.
Acesso aberto (Open Access)
Estudos isotópicos (Pb e Nd) e de química mineral do depósito aurífero Cipoeiro, Cinturão Gurupi, estado do Maranhão
(Universidade Federal do Pará, 2018-04-27) EL-HUSNY NETO, Chafic Rachid; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706
The Cipoeiro orogenic gold deposit, located in the Gurupi Belt, Maranhão State, Brazil, is hosted by tonalite of the Tromaí Intrusive Suite (2148 Ma), and shows the higher concentration of gold known to date (61.9 t Au). In order to contribute to the knowledge of the metallogeny of this deposit, this work looked for: to define the composition and temporal sequence of the hydrothermal mineralogy and/or types of hydrothermal alteration; to investigate the chemical composition of the ore; to identify potential sources of Pb and Nd in the mineralization; and to estimate the age of the mineralizing event. The studies have shown that the tonalite is strongly altered and locally deformed, which caused the obliteration of the primary mineralogy and textures. The hydrothermal alteration has distal and proximal variation and occurs in the pervasive and fissural/venular forms. The distal alteration is pervasive and comprises chlorite and sericite. The proximal alteration is pervasive and fissural/venular and is composed of quartz, chlorite, sericite, calcite, pyrite and subordinate amounts of chalcopyrite, sphalerite and galena, in addition to the gold mineralization and a set of tellurides. The gold occurs in three forms: (1) particles included in pyrite, (2) precipitated in pyrite fractures, and (3) free-milling, in quartz veins. The telluride mineralogy comprises petzite (Ag-Au), hessite (Au) and sylvanite (Au-Ag), and subordinate coloradoite (Hg), kochkarite (Pb-Bi) and volynskite (Ag-Bi). The mineralization are compatible with the greenschist facies conditions. The chlorite-pyrite-sphalerite equilibrium along with the telluride composition allowed the estimation of log fO2 in the range of -29.6 to -33.2, and log fS2 ranging from -9.6 to -10.6, indicating relatively reduced fluid conditions. This values, along with available physico-chemical data, suggest gold transportation as a reduced sulfur complex. Isotopic studies, Pb in pyrite and Nd in calcite, allowed considering that the source of the fluid is likely a mixture of different regional sources, caused by the fluid-rock interaction during the ascent of the fluid through structures to the site of ore deposition. The age of mineralization could not be defined unequivocally, but the Pb and Nd isotopes indicate the Paleoproterozoic as the most probable age of the mineralization.
Acesso aberto (Open Access)
Estudos isotópicos e de inclusões fluidas no depósito central do campo mineralizado do Cuiú-Cuiú, província aurífera do Tapajós, estado do Pará
(Universidade Federal do Pará, 2014-01-09) ARAÚJO, Ana Claudia Sodré; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706
Central is a gold deposit of the Cuiú-Cuiú goldfield, located in the Tapajós Gold Province, Amazonian Craton. The deposit is hosted in a NW-SE-trending structure and the mineralized zone is followed by 800 m along the strike and 450 m along the dip, and is 50-70 m thick. The ore bodies are hosted in a monzogranite dated at 1984±3 Ma and ascribed to the Parauari Intrusive Suite. Resources are estimated in 18.6 t Au. The hydrothermal alteration is predominantly of the fissure-filling type and sericitization, chloritization, silicification, carbonatization and sulfidation are the main alteration types. Pyrite is the predominant sulfide mineral, whereas chalcopyrite, sphalerite and galena are subordinated phases occurring in fractures and rims of pyrite. Gold particles occur in fractures of pyrite and contain subordinate amounts of silver. Three types of fluid inclusions are hosted in quartz veins and veinlets. Type 1 is the least abundant and is composed of one- (CO2vapor) and two-phases (CO2liq-CO2vapor) inclusions; Type 2 comprises two- (H2Oliq-CO2liq) and three-phases (H2Oliq-CO2liq-CO2vapor) inclusions; Type 3 is the most abundant type and consists of two-phases (H2Oliq-H2Ovapor) inclusions. CO2 is the volatile phase in CO2-bearing inclusions (types 1 and 2) and these inclusions were produced by phase separation of an aqueous-carbonic fluid. The density of this fluid is low to moderate (0,33 - 0,80 g/cm³), as is the salinity (11.15 - 2.42 wt.% NaCl equiv.). The homogenization temperatures show a peak at 340ºC. Type 3 inclusions have NaCl as the main salt component, the global density varies from 0.65 to 1.11 g/cm³, and the salinity ranges from 1.16 to 13.3 wt.% NaCl equiv. The homogenization temperature shows bimodal distribution, with peaks at 120-140ºC and 180ºC. Fluid inclusion and mineral (quartz, chlorite, calcite) isotopic compositions show δ18O and δD values of +7.8 to +13.6 ‰ and -15 a -35 ‰, respectively. Pyrite shows δ34S values of +0.5 to +4.0 ‰ and δ13C values ranging from -18 to -3.7 ‰ were obtained in calcite and CO2 inclusion fluids. The fluid δ18OH2O and δDH2O values plot in the field of metamorphic waters with a weak shift towards the meteoric water line. However, considering the absence of known metamorphic event at the time of mineralization at Central, the fluids are interpreted as belonging to a magmatic-hydrothermal system. Accordingly, the aqueous-carbonic fluids were exsolved from felsic magmas related to the latest phases of evolution of the Parauari Intrusive Suite and the carbonic and aqueouscarbonic fluid inclusions were trapped in this phase, predominantly at 340°C. The continuous exsolution lead to progressive decrease in the CO2 contents of the magmas and to increasing predominance of aqueous fluids. At this time, the fluids might have interacted with meteoric waters and most of the low-temperature aqueous inclusions were trapped. It is possible that part of the aqueous fluid inclusions (those with the highest trapping temperatures) represent local mixing of the different fluid sources. These observations allow to interpret Central as a magmatic-hydrothermal gold deposit related to the final stages of evolution of the Parauari Intrusive Suite.
Acesso aberto (Open Access)
Geologia e Metalogênese do Depósito Au-Ag (Pb-Zn) do Coringa, Sudeste Província Mineral Tapajós, Pará.
(Universidade Federal do Pará, 2020-09-16) GUIMARÃES, Stella Bijos; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706; https://orcid.org/0000-0003-4598-9249
The Tapajós Mineral Province (TMP) is located in the south-central portion of the Amazonian Craton and is considered one of the main metallogenic provinces of Brazil. A significant part of the province comprises felsic volcanic and volcanoclastic rocks and granites, which formed predominantly in two intervals, 2.02 to 1.95 Ga and 1.91 to 1.87 Ga, belonging to several stratigraphic and lithodemic units. Fieldwork, petrography, and high-resolution airborne geophysics allowed us to produce a new map at the 1:100,000 scale for the southeastern portion of the TMP, where the gold and silver (Cu-Pb-Zn) Coringa deposit is located. We identified two new geological units: (1) the volcanic and pyroclastic rocks of the Vila Riozinho Formation, previously attributed to the Iriri Group, including a facies defined here of this formation, which comprises a group of rocks with the largest magnetic content in the region (Vila Riozinho Formation - magnetic pyroclastic facies), and (2) the Serra Alkali Feldspar Granite, which intruded into the Vila Riozinho Formation (VRF). These units are the host rocks of Coringa deposit. The FVR rocks represent a magmatic arc with high K calcalkaline to shoshonitic affinities. There are similarities in the patterns of LILE and HFSE and the multielementar diagrams with the granitic rocks from Creporizão Intrusive Suite (CIS). The contemporaneousness between these units reinforce a possible petrogenetic correlation and converge to the hypothesis of similar sources, of probable remelting of arc rocks. Isotopic data revealed similar behavior between VRF, SAFG and Maloquinha Intrusive Suite expose similar behavior and present negative εNd values; however, it indicates rocks derived from enriched sources (ancient crustal rocks). Therefore these units had the same source during tectonic setting and crustal evolution of TMP. It is a transcurrent post-collisional stage that followed the collision of the Cuiú-Cuiú Magmatic Arc related to the Orosian volcano-plutonic event (2033-2005 Ma). Based on available geochronological information these units can be associated with a volcano-plutonic event that occurred in the Orosirian period, at about 1.98 Ga. The Au-Ag (Cu-Pb-Zn) Coringa deposit, occurs essentially in veins and veinlets whose match the regional trend (NNW-SSE). The host rocks are volcanic and pyroclastic rocks of the Magnetic Pyroclastic Facies (MPF) from the Vila Riozinho Formation (ignimbrites, tuffs, and breccia), and the Serra Alkali Feldspar Granite, with a predominance of the supracrustal rocks The hydrothermal processes affected all lithotypes associated with mineralization, producing distal alteration (carbonate-chlorite-epidote), intermediate-proximal alteration (sericite-pyrite) and proximal alteration (chlorite-hematite). The mineralized veins are generally composed of quartz + pyrite + chalcopyrite + galena + sphalerite + electrum + chlorite + sericite. Gold grains occur as inclusions or fractures in pyrite. The fluids presents low salinity, rich in H2O and poor in CO2, with evidence of mixing (magmatic-meteoric), and the presence of adularia and Mn-carbonate are outstanding features of this deposit. All characteristic converge to confirm an intermediate- sulfidation epithermal deposit as a genetic model to Coringa deposit.
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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.