Lima Group Research Articles

Peru’s leadership in the Lima Group: emergence and failure (2015–2021)

This research seeks to understand the factors that enabled Peru, a medium-sized country in Latin America, to take on a leadership role in the creation and development of the Lima Group—a multilateral mechanism whose main objective was to find a resolution to the democratic crisis in Venezuela—as well as the subsequent failure of this leadership. Using the analytical tools provided by Neoclassical Realism, this study argues that Peru’s leadership was made possible not only by the growing importance of the Venezuelan crisis on the international agenda since 2015 but also by changes in regional and global power dynamics. These shifts allowed a country like Peru to exercise such leadership and garner greater interest from the United States. Additionally, the presence of a favorable domestic political climate and clear national interests led the Peruvian government to view this leadership as beneficial, not only for the region but also for Peru itself. However, despite the Venezuelan crisis remaining on the international agenda due to Nicolás Maduro’s continued hold on power, the hardening of the U.S. stance on Venezuela and political changes at the regional level since 2018 fractured the Lima Group and undermined Peru’s leadership. Moreover, domestically, political changes led to heightened instability, weakening Peru’s external position. Finally, the surge in Venezuelan migration and the onset of the COVID-19 pandemic shifted the priorities of Peru’s foreign policy, diminishing its focus on the Venezuelan crisis and contributing to the failure of its leadership.

DESCRIÇÃO PETROGRÁFICA DAS ROCHAS MÁFICAS E ULTRAMÁFICAS DA UNIDADE OURO FINO, DO GRUPO NOVA LIMA, EM CAETÉ E SANTA BÁRBARA, MG

The petrographic study of rocks of Ouro Fino unit’s Nova Lima group, Greenstone Belt Rio das Velhas, on interior of Quadrilatero Ferrifero, cointains few affected samples by alterations processes. Many rocks have the primary features like lavas in pillow, tonsils and pox. The petrographic analysis classifies the rocks in tholeiitic basalt and magnesium tholeiite. Unidade Ouro Fino’s metabasalts are from ocean bottom ambiente shown by structures (pillow lava) and lithologic associations (banded iron formation and metachert). A few minerals was recognized in petrographic thin section allowing the description of its mineral paragenese and a needed classification.The geochemical characteristics consulted in the literature indicate that part of the metabasalts was formed by a mantle plume (P-MORB). These characteristics infer a submarine plateau ambient.

The Lima Group

The Lima Group

Latin American Integration against the Backdrop of a Conservative Wave: Between Irrelevance and the Search for New Meanings

The evolution of regionalism in Latin America has historically been greatly influenced by changes in governments and their ideological programs. In this article, looking at the courses of the administrations of Jair Bolsonaro in Brazil, Ivan Duque in Colombia, Sebastian Piñera (second term, 2018-2022) in Chile, Mario Abdo Benítez in Paraguay, it is proposed to examine transformations in integration against the backdrop of increased influence of right-wing forces in regional alignments in the mid-2010s to the early 2020s. Analyzing foreign policy steps, rhetoric, content of conceptual documents, the authors focus on the peculiarities of the national views of various right forces on integration initiatives. The paper concentrates on the decline of Union of South American Nations (UNASUR) and Community of Latin American and Caribbean States (CELAC), creation of Forum for the Progress and Integration of South America (PROSUR) and Lima Group, activities of the Pacific Alliance and Southern Common Market (MERCOSUR) in the context of the demand for flexible and pragmatic integration “without politicization.” The authors come to several conclusions which show ambiguity of the right turn impact on the integration landscape in Latin America. First, although the rise of liberal-conservative forces has reformatted philosophy, key ideas of integration, participation in multilateral groups remains important for governments that seek to solve pressing problems and expand their own reputational and political capital with its help. Second, despite the fluidity of electoral dynamics, which may soon put an end to the dominance of the right, they have raised a number of issues of long-term relevance in the context of the future path of integration associations in the region. The contribution of the work is explained by its appeal to the very factor of ideology in the development of Latin American integration, detailed analysis of specific country experiences and new conclusions based on it.

Study of the relationship between mineralogical types and potential radon in rocks of the region of Serra do Gandarela – Quadrilátero Ferrífero-MG.

The Quadrilátero Ferrífero (QF) is one of the most well-known metallogenetic provinces in the world and has its geological context well studied since the end of the 17th century. The Serra do Gandarela is positioned in the northeast of the QF and is supported by units of the Minas Supergroup. The Moeda Formation (MF) in the Serra do Gandarela hosts an uranium (U) occurrence which was recently characterized in terms of mineralogical context. Uraninite, coffinite and brannerite are the main U minerals present. It is expected that mineralogy and lithology contribute to radon emanation and exhalation, the radon (Rn) transport from the soil to the atmosphere. In this work, the Rn exhalation was correlated to the lithological types. Samples from the MF conglomerates and Nova Lima group schists were collected from the Serra do Gandarela. Rn exhalation measurements were performed on the samples with AlphaGUARD detector. These values were compared to the U content of samples obtained via INAA. The result of INAA was more expressive in the MF conglomerate sample from the NUCLEBRÁS drillcore, with a U concentration value of 24 µg.g-1. The sample that had the highest average exhalation rate was also the same. The lowest value presented for both U concentration and average exhalation rate was shale, which were, 2 µg.g-1 and 180 Bq.m-3, respectively. The results are relevant for exploration and mining activities, during which Rn can accumulate in galleries. Additionally, the knowledge of the characteristic emanometric can be an important tool for geological and environmental studies.

Sources of auriferous fluids associated with a Neoarchean BIF-hosted orogenic gold deposit revealed by the multiple sulfur isotopic compositions of zoned pyrites

Internal textures, multiple sulfur isotopic compositions, and contents of gold, selenium, and molybdenum of ore-related pyrites from a Neoarchean carbonate-facies BIF-hosted gold deposit in Quadrilatero Ferrifero were investigated to elucidate the source(s) of sulfur. Sodium hypochlorite etching and BSE imaging revealed pervasive zoning in pyrites. Five different growth zones (Py1a, Py1b, Py2, Py3, and Py4) and six types of zoning (Type A–Type F) were identified. Two pyrite generations were distinguished in ores: G1 (Py1a and Py1b) and G2 (Py2, Py3, Py4). Both G1 and G2 have positive Δ33S, but the magnitudes of G1 are higher. The G1-relevant fluids can be shallow-sourced, whereas the G2-related auriferous fluids are most likely deep-sourced metamorphic fluids derived from devolatilization of the lower succession of the Nova Lima Group (metavolcanics and metasedimentary rocks) during metamorphism, with fluid–rock interactions during fluid ascent and at the depositional site. The negative δ34S, higher selenium contents, and carbonates inclusions of Py1a contrast with the positive δ34S, lower selenium contents, and carbonaceous material inclusions of Py2, Py3, and Py4. The Lamego system possibly started with shallow-sourced oxidized and low-gold fluids (Py1a), closely followed by mixing in of deep-seated reduced auriferous fluids (Py2, Py3, Py4) initiated by tectonic activities. The oscillatory zoning of Py2 with the highest gold contents consists of alternating gold-rich and gold-poor laminae, indicating that fault-valve activity is a trigger of gold deposition. The sulfidation of siderite in BIF (desulfidation of auriferous fluids) related to the formation of ore-related pyrites also contributed to gold precipitation.

Caracterização dos Sistemas Aquíferos do Distrito Ferrífero de Itabira, Minas Gerais

Este estudo pretende contribuir para o conhecimento hidrogeológico da região e o melhor entendimento do funcionamento deste sistema aquífero e de suas inter-relações com os demais aquíferos. A escolha desta região para o estudo baseou-se em um conjunto de características incluindo: a representatividade geológica, as grandes reservas de minério de ferro e os grandes empreendimentos minerários, existência de estudos realizados pelas mineradoras e importância das águas subterrâneas para o abastecimento de água do município. Foram identificados os seguintes sistemas aquíferos: Sistema Aquífero Coberturas Detríticas, Sistema Aquífero das Rochas Ultrabásicas, Sistema Aquífero Galho do Miguel, Sistema Aquífero Membro Campo Sampaio, Sistema Aquífero Sopa Brumadinho, Sistema Aquífero Gnáissico-Granítico, Sistema Aquífero Piracicaba, Sistema Aquífero Itabira, Sistema Aquífero Nova Lima, Sistema Aquífero Gnáissico-Granítico. Os poços mais produtivos são os perfurados nas formações ferríferas do Cauê, (Qmédia=41,75 m3/h)), seguidos pelos poços perfurados dos quartzitos do Grupo Piracicaba (Qmédia=34,01 m3/h) e, por fim, os poços menos produtivos são os perfurados nas rochas do Grupo Nova Lima e Complexo Granito-Gnáissico (Qmédia=13,30 m3/h).

Bingeing on High-Fat Food Enhances Evoked Dopamine Release and Reduces Dopamine Uptake in the Nucleus Accumbens.

ObjectiveBinge‐eating disorder (BED) disrupts dopamine neuron function, in part by altering dopamine transporter (DAT) activity. This study characterized the effects of high‐fat bingeing on presynaptic dopamine terminals and tested the hypothesis that acute low‐dose amphetamine would restore DAT function.MethodsC57BL/6 mice were given limited access (LimA) to a high‐fat diet (2 h/d, 3 d/wk) or standard chow (control). After 6 weeks, ex vivo fast‐scan cyclic voltammetry was used to characterize dopamine‐terminal adaptations in the nucleus accumbens. Prior to undergoing fast‐scan cyclic voltammetry, some mice from each group were given amphetamine (0.5 mg/kg intraperitoneally).ResultsEscalation of high fat intake, termed bingeing, occurred in the LimA group and coincided with increased phasic dopamine release, reduced dopamine uptake rates, and increased dopamine receptor 2 (D2) autoreceptor function. Acute amphetamine selectively reversed dopamine uptake changes in the LimA group and restored the potency of amphetamine to inhibit uptake.ConclusionsHigh‐fat bingeing enhanced dopaminergic signaling in the nucleus accumbens by promoting phasic dopamine release and reducing clearance. This study’s data show that amphetamine was efficacious in restoring impaired DAT function caused by high‐fat bingeing but did not reduce dopamine release to normal. These presynaptic changes should be considered if amphetamine‐like dopamine releasers are used as treatments for BED.

Crisis in South American regionalism and Brazilian protagonism in Unasur, the Lima Group and Prosur

Abstract This paper discusses the crisis in regional governance in South America from 2017 onwards by analyzing Unasur’s disintegration and the emergence of new arrangements (the Lima Group, Prosur, and the Leticia Pact), as well as the regional and global conditions that enabled them. We assessed their performance in responding to different crises in South America, concluding that the constraints and challenges they faced jeopardize their aim to prosper in the future. Special attention is given to changes in Brazilian foreign policy guidelines toward South America, which contributed to these outcomes.

Venezuela: to be or not to be

The article deals with the most crucial components of the current political and institutional crisis in Venezuela. A brief historical deflection into the events of 1989-2020 was made, which indicate, among other things, deep-seated left-wing political views and feelings amidst the majority of the population. This situation serves as a solid platform for strengthening the role of the ruling PSUV party and the present head of State, N. Maduro, in the country’s domestic politics. The political and institutional crisis in the above-mentioned oil-rich South American country has expanded sharply, after being elected Juan Guaidó, a representative of the opposition party VP («People’s Will») as a Chairman of the National Assembly on January 5, 2019. He challenged the prevailing President, declaring himself as the Acting head of State just on January 23, 2019. In this context, the key social and economic aftermaths of the crisis in present-day Venezuela are revealed, in particular, such as the GDP contraction, hyperinflation, growth of the population living in extreme economic poverty, an increase of Venezuelans forced to leave the country due to, above all, obvious signs of a humanitarian crisis. Deepening of the Venezuelan society in destructive processes, escalating of the political debates at the level of parties’ leaders, a lack of the transparent discussion in the public arena concerning the country’s future, to be a necessary condition for realizing the citizen’s rights to democratic choice, appear as consequences of the failure of Guaidó’s strategy. It was built in particular on the desired, but not achieved cessation the usurpation of power by Maduro. The postures of the main international players (USA, EU, OAS, «Lima Group», International Contact Group) are outlined with regard to the assessments of the domestic situation in Venezuela and suggestions for resolving the political crisis and restoring democracy in this country, in particular via introducing sanctions against Venezuelan officials and legal entities, as well as establishing a constructive dialogue between Maduro and the opposition. The main items of Ukraine’s official position on the «Venezuelan issue» had also been notified. In a nutshell, a conclusion is made that for a certain category of Ukrainian voters and for the majority of Venezuelans, despite the obvious differences, the issue of the public administration model remains rhetorical, authoritative, as an outcome of the improvement of democratization, or authoritarian one.

Depositional setting and U-Pb detrital record of rift-related deposits in the Moeda Formation (Minas Supergroup) at the Gandarela and Ouro Fino synclines, Quadrilátero Ferrífero, Brazil

Abstract The Moeda Formation (Caraca Group, base of Minas Supergroup) registers the initial rift stages of the Minas Basin in the Quadrilatero Ferrifero, Southern Sao Francisco craton, SE-Brazil. We present stratigraphic and U-Pb detrital zircon dating analyses of the Moeda Formation that contribute to the comprehension of its sedimentary evolution in the southernmost Gandarela and Ouro Fino synclines (Central-Eastern Quadrilatero Ferrifero). The Moeda Formation sequence consists of a basal proximal alluvial fan with clast-supported conglomerates, grading upward into an alluvial floodplain with quartz arenites, and upper massive sandstones from distal fluvial fans. Because the Moeda Formation is normally limited by similar lithologies at both its base (Nova Lima Group, Rio das Velhas Supergroup) and top (Batatal Formation, uppermost Caraca Group) and stratigraphic inversion has occurred in the study area, U-Pb detrital zircon dating was also extended to these units to differentiate them stratigraphically. The youngest clusters of detrital zircon ages in the Nova Lima Group and Moeda and Batatal formations were 2716, 2777 and 2786 Ma, respectively. Based on our data and relevant literature, the depositional age of the Moeda Formation was interpreted as between 2716 and 2520 Ma with the Mesoarchean continental crust, the Rio das Velhas Supergroup and the Archean TTG complexes as its main source areas.

The Archean Rio das Velhas greenstone belt revisited: new insights into the stratigraphy

The Neoarchean Rio das Velhas greenstone belt (RVGB), southeast Brazil, contains major tectonic and stratigraphic discontinuities that allow its division into three fault-bound tectonostratigraphic blocks named Santa Barbara, Nova Lima-Caete and Sao Bartolomeu. Detailed 1:25,000 scale mapping of the RVGB northern part revealed that each block contains important particularities and were subdivided into different lithostratigraphic formal units. The Santa Barbara block comprises the basal ultrabasic rocks of the Quebra Osso Group, followed by metavolcano-sedimentary successions of the Nova Lima Group (Vigario da Vara, Santa Quiteria, and Corrego do Sitio formations), and metasedimentary rocks of the Maquine Group (Palmital and Casa Forte formations). The Nova Lima-Caete block comprises the basal ultrabasic/basic volcanism (Ouro Fino and Morro Vermelho formations), metavolcano-sedimentary sequences (Mestre Caetano and Ribeirao do Brumado formations) of the Nova Lima Group, and the upper clastic sedimentary rocks of the Maquine Group (Palmital and Casa Forte formations). The Sao Bartolomeu block has a distinct rock assemblage given by metasedimentary rocks of the Nova Lima Group (Fazenda Velha, Corrego da Paina, and Pau D'Oleo formations), and clastic sedimentary rocks of the Maquine Group (Andaimes Formation). Despite some remarkable differences between these blocks, it is possible to recognize stratigraphic correlations and similar depositional evolution. The authors suggest volcanic architecture and sedimentary depositional models for the studied area, which includes: i) fissure eruption of komatiite lavas of the Quebra Osso Group in the SB block with subordinate clasto-chemical sedimentation; ii) ultrabasic/basic volcanism related to multiple volcanic centers in a subaqueous environment, characteristic from the basal portion of the Nova Lima Group in the SB and NLC blocks; iii) intermediate to acid volcanic and volcaniclastic rocks from the Nova Lima Group, present only in the NLC block, related to explosive eruptions; iv) clasto-chemical sedimentation, well recorded in the SB block; v) thick sequence of clastic sediments recorded in the three blocks with a predominance of greywacke-argillite cycles, deposited by turbidity currents in tectonically active submarine basins; vi) metasediments formed in alluvial-fluvial, coastal and shallow marine environments of the Maquine Group.

The Political Crisis in Venezuela and the Role of the Lima Group: Stocktaking and Challenges of its Diplomatic Action

The Political Crisis in Venezuela and the Role of the Lima Group: Stocktaking and Challenges of its Diplomatic Action

Відносини США-Венесуела на сучасному етапі: геополітичний вимір

Relations with Venezuela play a significant role in the US policy towards Latin America. Venezuela is an important resource state and a promising sales market, therefore it attracts the attention not only of the United States, but also of other important geopolitical players such as the Russian Federation and the People’s Republic of China, which view the situation in Venezuela as an opportunity to challenge the United States and to advance their regional interests in Central and Eastern Europe and the South China Sea and Taiwan, respectively. Therefore, the crisis that has developed in Venezuela is acquiring geopolitical significance, because the state is becoming an area of confrontation between the leading powers. The objective of the article is to study and analyze the geopolitical dimension of bilateral relations between the United States and Venezuela at the current phase and find out how these relations affect the geopolitical situation on the regional and global scale, as well as predict possible scenarios for their further development. The article provides an analysis of the American-Venezuelan political and economic relations at the point and concludes that the United States have imposed a number of sanctions and restrictions that put significant pressure on the Venezuelan economy. Having studied the role and position of the United States in resolving the Venezuelan crisis, the authors have indicated that the US support J. Guaido and are working on a plan to resolve the political crisis. The study also identifies the role of third parties in relations between states, in particular the strategy of the Russian Federation regarding the Venezuelan crisis. Attention is also focused on the fact that the events in Venezuela caused the migration crisis – a significant geopolitical problem and requires the attention of not only the United States, but also the world community. Taking into account the geopolitical interests of the United States in Venezuela and concerns about the emergence of global geopolitical players in the state, the most favorable way for the United States would be to strengthen its role in the negotiation process to resolve the Venezuelan crisis, as well as join coordinated actions with the Lima Group.

Geophysical analysis of natural caves in iron lithotypes in the region of Mariana, Southeastern Quadrilátero Ferrífero, Brazil

Abstract In Brazil, natural caves in iron lithotypes have increasingly been discovered in iron mining areas and its surroundings. Mining companies attempt to study them according to the Federal Decree 6640/2008 and the Normative Instruction 2/2009 of the Ministry of the Environment, which is concerned with the preservation of the Brazilian Speleological Heritage. This research is within one of those studies in which a geophysical campaign methodology is established for an attempted prospection of caves, in an area located on the north flank of Mariana's Anticlinal, Quadrilatero Ferrifero. The flank, which is sustained by iron formations from the Itabira Group and covered by the product of alteration and sedimentary redistribution, experienced extensive and compressive tectonics, generating, respectively, the main structuration in the NW-SE direction and schist thrust slices from the Nova Lima Group. Caves are mainly developed in the lithological contacts and in fault zones. In the regional stage, the methodology uses aeromagnetometric (identification of iron lithotypes, structural and statistical analysis of magnetic lineaments) and aeroradiometric (structural-geological mapping and setting of lithological contacts) methods. Results have shown some caves in lateral contact zones between lithologies, and the structural control imposed by a shear zone in the area. At the local stage, the electrical resistivity method - EL was used, dipole-dipole array, data mathematical inversion, presentation of pseudo-depth sections and tridimensional modeling. The results, in accordance with the topographic data, show the possible limits of the geometric contour and the tendency for cave deepening. However, studies of the moisture, of similar caves with different spacings, and the use of Ground Penetration Radar - GPR are suggested.

The use of geological handmade models in the teaching of Geosciences

The Projeto de Educação Tutorial Engenharia Geológica (PET Engenharia Geológica) of UFOP (Federal University of Ouro Preto) is a student entity basically structured in the three pillars: extension activities, scientific research and education. Further, during extension/educational activities leaded by the group, it has always been a challenge to teach geological matters in a sim-ple way to student of secondary and high school levels. Therefore, the PET Engenharia Geológica decided to build a few mod-els to better illustrate some of the geological processes and explain what may not be so easily understood by the students. The first model made last year and consists of a geologic prototype of Serra de Ouro Preto, showing the topography and lithology aspects of its surface. Geologically, it is characterized by Archean and Paleoproterozoic rocks of Rio das Velhas Supergroup (Maquiné Group and Nova Lima Group), Minas Supergroup (Caraça, Itabira, Piracicaba and Sabará groups) and the lithologies of Itacolomi Group. It´s main structure signature is the occurrence of the Mariana´s Anticlinal and its topographic response. There were also completed a second prototype, a volcano model. The mentioned models was exposed it in two different educa-tion/academic events already in 2017 regarding very positive feedbacks as a result of the initiative. Now, the project aims to continue and follow the education track, aiming to help in the teaching of geosciences – in public high schools of Ouro Preto – with the handmade models.

Geology of the Pitangui greenstone belt, Minas Gerais, Brazil: Stratigraphy, geochronology and BIF geochemistry

Based on drillcore description of rocks at the São Sebastião gold deposit and surroundings a new geological map and a detailed stratigraphic column are presented for the Pitangui greenstone belt, which is located northwest of the Quadrilátero Ferrífero, southern São Francisco craton, Brazil. This belt comprises three units: a Lower Unit predominantly of metavolcanic ultramafic and mafic rocks interbedded with thinner metasedimentary intervals of banded iron formations (BIFs), turbiditic metasandstones and metapelites; a Middle Unit of turbiditic metasediments interbedded with metavolcanic intermediary rocks, minor BIFs and quartzites; and an Upper Unit composed of quartzites and polymictic metaconglomerates. Rocks of all three units are metamorphosed under upper greenschist to lower amphibolite facies conditions and are bounded by domes (TTGs and late- to post-tectonic alkaline granitoids). The Pitangui greenstone belt records the geologic evolution of an ancient oceanic basin that evolved from a thick succession of metavolcanic ultramafic and mafic rocks interbedded with metasedimentary intervals of BIFs and turbidites; the latter representing moments of quiescence in volcanic activity. This basin experienced a transient period dominated by andesitic volcanism, turbidite deposition and minor BIF, and a final stage of clastic-only sedimentation at the same time when volcanic activity ceased. Combined U–Pb and Lu–Hf isotope data of detrital zircon from Lower Unit metasandstone revealed a maximum depositional age of 2859±11Ma. This age is similar to those found in the basal unit of the Nova Lima Group (2.90–2.78Ga) in the Rio das Velhas greenstone belt, located approximately 100km southeast. Both belts can be correlated on the basis of similarities in stratigraphy and isotopic ages, indicating that they possibly represent parts of an ancient larger domain. Geochemical data of BIF coupled with stratigraphic analysis place constraints on paleoenvironmental conditions during the development of the Mesoarchean Pitangui greenstone belt. Shale-normalized rare-earth elements (REE+Y) in some BIF samples display negative cerium (Ce) anomalies, which may be interpreted as evidence for the oldest oxygenation pulse documented in Brazil to date (2.86–2.78Ga). Such anomalies endorse the presence of episodic periods of seawater oxygenation before the Great Oxidation Event.

Rochas ultramáficas plutônicas do greenstone belt Rio das Velhas na porção central do Quadrilátero Ferrífero, Minas Gerais, Brasil

Em Amarantina, distrito de Ouro Preto, encontram-se rochas ultramáficas expostas em duas áreas com cerca de 500 m² cada. As rochas afloram no Complexo do Bação, que é o embasamento gnáissico do greenstone belt Rio das Velhas, na porção central do Quadrilátero Ferrífero (QF). O interesse no estudo petrogenético desses corpos deve-se à preservação parcial de minerais ígneos, ausentes na maior parte das rochas ultramáficas totalmente metamorfizadas do QF. Entre essas rochas, destacam-se os esteatitos e os serpentinitos, devido a sua importância econômica. As rochas ultramáficas de Amarantina possuem textura equigranular, fato que as caracteriza como tendo origem plutônica, isto é, trata-se de metaperidotitos. Possuem grãos maiores de olivina, piroxênio e espinélio da rocha ígnea original distribuídos em matriz metamórfica fina com talco, serpentinas, cloritas, anfibólios e minerais opacos. Escassas arita (NiSbAs) e breithauptita (NiSb) foram formadas a partir de pentlandita durante o metamorfismo associado a hidrotermalismo. A comparação da composição química com a de um metakomatiito com textura spinifex do QF, bem como com rochas komatitiiticas de outras partes do mundo, mostra que os metaperidotitos são, quimicamente, semelhantes aos komatiitos não-desfalcados em alumínio. Portanto é provável que as rochas ultramáficas estudadas correspondam à porção plutônica do magmatismo komatitiitico do Grupo Nova Lima, que é a unidade basal do greenstone belt Rio das Velhas.

The Archean BIF-hosted Cuiabá Gold deposit, Quadrilátero Ferrífero, Minas Gerais, Brazil

The Cuiabá Gold Deposit is located in the northern part of the Quadrilátero Ferrífero, Minas Gerais State, Brazil. The region constitutes an Archean granite–greenstone terrane composed of a basement complex ( ca. 3.2 Ga), the Rio das Velhas Supergroup greenstone sequence, and related granitoids (3.0–2.7 Ga), which are overlain by the Proterozoic supracrustal sequences of the Minas (< 2.6–2.1 Ga) and Espinhaço (1.7 Ga) supergroups. The stratigraphy of the Cuiabá area is part of the Nova Lima Group, which forms the lower part of the Rio das Velhas Supergroup. The lithological succession of the mine area comprises, from bottom to top, lower mafic metavolcanics intercalated with carbonaceous metasedimentary rocks, the gold-bearing Cuiabá-Banded Iron Formation (BIF), upper mafic metavolcanics and volcanoclastics and metasedimentary rocks. The metamorphism reached the greenschist facies. Tectonic structures of the deposit area are genetically related to deformation phases D 1, D 2, D 3, which took place under crustal compression representing one progressive deformational event (E n). The bulk of the economic-grade gold mineralization is related to six main ore shoots, contained within the Cuiabá BIF horizon, which range in thickness between 1 and 6 m. The BIF-hosted gold orebodies (> 4 ppm Au) represent sulfide-rich segments of the Cuiabá BIF, which grade laterally into non-economic mineralized or barren iron formation. Transitions from sulfide-rich to sulfide-poor BIF are indicated by decreasing gold grades from over 60 ppm to values below the fire assay detection limit in sulfide-poor portions. The deposit is “gold-only”, and shows a characteristic association of Au with Ag, As, Sb and low base-metal contents. The gold is fine grained (up to 60 μm), and is generally associated with sulfide layers, occurring as inclusions, in fractures or along grain boundaries of pyrite, the predominant sulfide mineral (> 90 vol.%). Gold is characterized by an average fineness of 0.840 and a large range of fineness (0.759 to 0.941). The country rocks to the mineralized BIF show strong sericite, carbonate and chlorite alteration, typical of greenschist facies metamorphic conditions. Textures observed on microscopic to mine scales indicate that the mineralized Cuiabá BIF is the result of sulfidation involving pervasive replacement of Fe-carbonates (siderite–ankerite) by Fe-sulfides. Gold mineralization at Cuiabá shows various features reported for Archean gold–lode deposits including the: (1) association of gold mineralization with Fe-rich host rocks; (2) strong structural control of the gold orebodies, showing remarkable down-plunge continuity (> 3 km) relative to strike length and width (up to 20 m); (3) epigenetic nature of the mineralization, with sulfidation as the major wall–rock alteration and directly associated with gold deposition; (4) geochemical signature, with mineralization showing consistent metal associations (Au–Ag–As–Sb and low base metal), which is compatible with metamorphic fluids.

U–Pb SHRIMP monazite ages of the giant Morro Velho and Cuiabá gold deposits, Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Minas Gerais, Brazil

U–Pb SHRIMP results of 2672 ± 14 Ma obtained on hydrothermal monazite crystals, from ore samples of the giant Morro Velho and Cuiabá Archean orogenic deposits, represent the first reliable and precise age of gold mineralization associated with the Rio das Velhas greenstone belt evolution, in the Quadrilátero Ferrífero, Brazil. In the basal Nova Lima Group, of the Rio das Velhas greenstone belt, felsic volcanic and volcaniclastic rocks have been dated between 2792 ± 11 and 2751 ± 9 Ma, coeval with the intrusion of syn-tectonic tonalite and granodiorite plutons, and also with the metamorphic overprint of older tonalite–trondhjemite–granodiorite crust. Since cratonization and stable-shelf sedimentation followed intrusion of Neoarchean granites at 2612 + 3/− 2 Ma, it is clear that like other granite–greenstone terranes in the world, gold mineralization is constrained to the latest stages of greenstone evolution.