Navegando por Assunto "Metalogenia - Tapajós, Região (PA)"

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Acesso aberto (Open Access)
Aspectos geológicos e metalogenéticos do depósito de ouro hospedado em metaconglomerados e metarenitos paleoproterozoicos Castelo de Sonhos, Província Tapajós, sudoeste do Pará
(Universidade Federal do Pará, 2015-04-06) QUEIROZ, Joana D’arc da Silva; KLEIN, Evandro Luiz; http://lattes.cnpq.br/0464969547546706
Castelo de Sonhos, located in the central-south sector of the Amazonian Craton, near the boundary between the Tapajós and Xingu-Iriri tectonic domains, is a gold deposit hosted in metaconglomerates and metasandstones of the Castelo dos Sonhos Formation (<2080 Ma U-Pb SHRIMP). Subvolcanic rocks and granitoids were identified in boreholes that drilled the deepest parts of the deposit area. Some of these rocks are intrusive into the Castelo dos Sonhos Formation, while for others rocks, the contact relationships could not be determined with confidence. In general, these rocks show calc-alkaline to alkaline affinities and their geochemical patterns indicate that they are related to volcanic arc or post-collisional tectonic settings. The subvolcanic rocks are represented by a porphyritic dacite with age of 2011 ± 6 Ma (U-Pb LA-ICP-MS).The granitoids were classified as biotite granodiorite, biotite monzogranite, muscovite monzogranite, respectively dated at 1976 ± 7 Ma, 1918 ± 9 Ma and 1978 ± 6 Ma (U-Pb SHRIMP), and an undated syenogranite. These ages represent three to four distinct magmatic events and indicate that the studied rocks are coeval to four major units from Tapajós Domain: the Cuiú-Cuiú Complex (2033-2005 Ma), the Comandante Arara Formation (2020-2012 Ma), the Creporizão Intrusive Suite (1998-1957 Ma), and the Tropas Intrusive Suite (1907-1892Ma). Despite the temporal correspondence, the geochemical data show no direct correspondence with the units cited above. Notwithstanding, the intrusion relationship between some of the studied rocks and the metasedimentary rocks of the Castelo dos Sonhos Formation establishes a temporal, spatial and possibly stratigraphic relationship between this formation and the Tapajós Domain. The intrusive contact relationship between the porphyritic dacite and metasandstones of the Castelo dos Sonhos Formation allowed us to determine at 2011 ± 6 Ma the minimum sedimentation age of this unit. The primary gold mineralization at Castelo de Sonhos deposit is stratabound and restricted to a metaconglomerate package and interlayered metasandstones. The mineralization distribution is erratic and does not seem to follow special features or structural control. In the matrix of the metaconglomerates, gold occurs as intergranular particles, occasionally associated with magnetite, and also within quartz grains (medium to coarse sand), which probably represent fragments of auriferous veins. In general, the gold particles show subrounded to rounded shapes, mild to moderately rough surfaces. The particles seldom contain inclusions, and only of magnetite. The chemical composition is homogeneous and characterized by high Au/Ag ratios. These characteristics indicate a syngenetic origin for gold within the metaconglomerates package. Therefore, the age of mineralization is limited by the time x interval of deposition of the Castelo dos Sonhos Formation (2011 ± 6 Ma to ca. 2080 Ma). On the other hand, the occurrence of gold in fracture planes of metasandstones indicates an epigenetic origin for this style of mineralization. The epigenetic mineralization is related to concurrent metamorphic, magmatic and deformational processes that affected the sedimentary sequence of the Castelo dos Sonhos Formation and caused the remobilization of gold originally hosted in metaconglomerates. It is likely that the interaction of these processes associated with infiltration of meteoric waters contributed to the generation and flow of oxidizing hydrothermal fluids, which have percolated through the metaconglomerates package and were able to solubilize some of the gold, and re-precipitate it accompanied by ferruginous films, in fracture planes of the metasandstones. As a conclusion, a modified paleoplacer model is proposed here to explain the hybrid nature (syngenetic and epigenetic) of the gold mineralization in the Castelo de Sonhos deposit.
Acesso aberto (Open Access)
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)
Rochas hospedeiras, alteração hidrotermal e avaliação do balanço geoquímico de massa do Depósito Aurífero Tocantinzinho, Província do Tapajós-PA
(Universidade Federal do Pará, 2012-03-05) SANTIAGO, Érika Suellen Barbosa; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983
The Tocantinzinho deposit is located in the central part of Tapajós Gold Province (TGP), within the Tocantinzinho shear zone (NW-SE), at about 200 km southwest of Itaituba city, Pará state. The host granite (ca. 2.0 Ga) is made up of several granitoid rocks, though monzogranites are the dominant type and represent together with subordinate syenogranites and alkali feldspar granites the most evolved facies. It is an elongated NW-SE-trending stock that has been interpreted as a late-orogenic to post-collisional intrusion, related to the final stages of the Cuiú-Cuiú orogeny likewise the coeval Jamanxim Monzogranite. The magma was emplaced at depths of 6 to 9 km and crystallized under intermediate ƒO2 conditions (oxidized type of the ilmenite series). Two varieties of monzogranite have been recognized according to the alteration degree: a weakly altered variety (5 to 10% of hydrothermal minerals) and a moderately altered variety (10 to 30% of hydrothermal minerals), the latter referred to informally as salami and smoky. In general, the weakly altered samples reveal medium- to coarse granulation and hypidiomorphic to allotriomorphic texture, with local poikilitic and rapakivi features. Essentially isotropic, they are composed of microcline (41 to 50%), quartz (21 to 33%) and oligoclase (An28-29) (22 to 36%), in addition to biotite (1,5 to 8%) and Fe-edenite (0 to 2%). Zircon, magnetite, apatite, allanite, monazite, U-thorite and titanite are the main magmatic accessory phases. The monzogranites are metaluminous to peraluminous and present shoshonitic character, low CaO (<1,6%) and MgO (<0,5%) contents, besides Fe2O3/FeO and K2O/Na2O ratios that range from 0,44 to 0,55 and from 1,22 to 1,57, respectively. The salami and smoky varieties show remarkable macroscopic differences, but they are mineralogically and chemically very similar, aside the Fe2O3/FeO ratios and Na2O and MgO contents. The magmatic textures have been moderate to severely masked, especially in cataclastic zones. These rock varieties have been altered at different degrees, the most noticeable types being chloritization, sericitization, silicification and carbonatization. The first two are ubiquitous, whereas the others are represented by scattered filling veins/veinlets. The secondary minerals replace commonly primary minerals or are constituents of mono and polymineralic veins/veinlets. The mineralization is represented by gold, pyrite, chalcopyrite, sphalerite and galena, being closely related to sericitization. Stockwork is the most significant mineralization style. The hydrothermal stage started with chloritization at temperatures around 315-330°C when chamosite was produced. Then sericitization was set forth at the same time that the ore-bearing fluids precipitated pyrite, chalcopyrite, sphalerite, galena and gold in response to the increase of both solution pH and sulfur species activities. As the alteration advanced, silica-saturated solutions moved into fractures where decreasing temperature and H+ activity favored the deposition of quartz. Later on, aqueous and aqueous-carbonic fluids might have mixed, allowing Ca2+ e CO2 to r eact toform calcite (carbonatization). Considering the relatively low amounts of hydrothermal products, the Tocantinzinho paleosystem seems to have evolved under low fluid/rock ratios. Chlorite formed continuously with distinct composition most likely controlled by the nature of the replaced mineral, fluid composition and temperature. Mass balance calculations showed that the Tocantinzinho hydrothermal palaeosystem did not evolve isovolumetrically, but may have experienced volume changes no greater than 10%. The transfer of components depended upon the alteration type and rock variety, but, in general, losses of Al2O3, FeO, Na2O, CaO, Ba and Sr, and gains of Fe2O3, S, volatiles and Rb are recorded. Potassium was largely conserved during chloritization and sericitization, whereas significant losses occurred during silicification and carbonatization. SiO2 was the most sensitive component to the volume factor chosen. The fluids seemed to have had low capacity of mobilizing REE, whose distribution patterns are very similar despite the alteration degree. Estimates of mass losses or gains per m3 of rock yielded 210 to 330 kg, the larger amounts being detected in the salami variety, except for chloritization. SiO2, Al2O3, Fe2O3 and CaO were the components that have mostly contributed to the mass transfer between the fluids and the granitic intrusion. The Tocantinzinho deposit share many similarities with the Batalha and São Jorge deposits, and some prospects of the Cuiú-Cuiú goldfield of the TGP. From the typological point of view, it can be more properly classified as an intrusion-related gold deposit.