Nilson Francisquini Botelho

Granites and the geodynamic history of the neoproterozoic Brası́lia belt, Central Brazil: a review

Abstract Recent field and geochronological studies have demonstrated the importance of granitic magmatism in the evolution of the Neoproterozoic Brasilia Belt, in Central Brazil. This is an orogenic belt developed in response to the convergence between the Amazon, Sao Francisco–Congo and Parana continental blocks. The presence of Neoproterozoic juvenile arc rocks and syn-collisional peraluminous granites challenged previous intracontinental evolution models for the belt. The granitoid intrusions reviewed in this paper record the different stages of evolution of the orogen and their field and isotopic characteristics can be used to reconstruct the tectonic history of the belt. The main field and isotopic characteristics of four granite suites associated with the Brasilia Belt are reviewed: (i) 1.77–1.58 Ga old rift related A-type granite intrusions, (ii) ca. 0.8–0.7 syn-collisional granitoids, (iii) arc metatonalites and metagranodiorites (ca. 0.9 to 0.63 Ga), and (iv) bimodal post-orogenic suite ranging in age from ca. 0.59 to 0.48 Ga. These rocks suggest that during most of the Neoproterozoic the western margin of the Sao Francisco continent faced a large oceanic basin, where subduction and oceanic lithosphere consumption started at ca. 0.9 Ga, roughly coeval with the initial stages of the break up of Rodinia. Final ocean closure happened at ca. 0.63–0.60 Ga with crustal thickening, uplift and erosion. Post-orogenic extension-related magmatism took place between ca. 0.6 and 0.5 Ga and was partially contemporaneous with the deposition of the Paraguay and Tucavaca sedimentary successions, resulting from the rifting event related to the break up of Laurentia from southwestern Gondwana.

Granite-ore deposit relationships in Central Brazil

Ore deposits related to granitic rocks in Central Brazil are associated with different Paleo- to Mesoproterozoic granite types. Sn and Au are the economic ore elements with important indium concentrations in some tin deposits. Tin mineralization is related to an A-type granite province, in the State of Goias, composed of two distinct granite groups of 1.77 Ga and 1.58 Ga ages. The most important tin deposits are closely related to the younger group, hosted in or near to Li-mica±topaz leucogranites. Despite the chemical differences, both groups are enriched in F, Sn, Rb, Y, Th, Nb, Ga and REE. Their Nb/Ta>1 and their high F/Li ratios allow their classification as a NYF fertile granite association. Recent studies reveal indium concentrations that might be recoverable as by-product of tin. Besides indium minerals and In-bearing phases, cassiterite is the most important indium carrier (0.2–0.4% In). Primary gold mineralization is hosted in oxidized I-type calc-alkaline plutons in the northern Mato Grosso State, with characteristics either of volcanic arc or post-collisional granites. Gold occurs in small high grade vein type deposits or is disseminated in widespread hydrothermal zones with alteration such as sericitization, feldspathization and pyritization. δ34S values between +1.3 and +3.5‰ of associated sulfides are typical of magmatic deposits. The association of gold with oxidized I-type granites and the style of hydrothermal alteration are analogous to those associations present in world-class porphyry-style deposits. Although the tectono-magmagtic setting of these granites is not well understood, such an association constitutes an important target in the search for gold in the northern region of the State of Mato Grosso.

Petrogenetic and mineralization processes in Paleo- to Mesoproterozoic rapakivi granites: examples from Pitinga and Goias, Brazil

Abstract The 1.8 Ga Pitinga granites—Agua Boa and Madeira massifs—in Amazonas (northern Brazil) are within-plate, shallow-level rapakivi granites associated with an extensional fracture system. They comprise an early facies of pyterlitic to wiborgitic rapakivi granite, a fine- to medium-grained biotite granite, as well as topaz granite (Agua Boa massif) and albite granite (Madeira massif). The granites are usually metaluminous to peraluminous, the albite granite, however, is peralkaline. They are enriched in SiO2, K2O, Na2O, F, Rb, Th, Nb, Y, Zr and the rare-earth element (REE) and impoverished in MgO, TiO2, P2O5 and Sr, as the majority of Sn-mineralized granites. Sn contents range from ∼1 ppm in the rapakivi facies to ∼3400 ppm in the albite granite. eNd (at 1.8 Ga) values vary from −2.2 to +0.4 and Nd model ages lie between 2.4 and 2.1 Ga. Mineralization in the Madeira massif includes disseminated magmatic cryolite, zircon, cassiterite, pyrochlore, columbite-tantalite and xenotime and massive cryolite bodies in the F-rich peralkaline albite granite. In the Agua Boa massif, Sn mineralization is associated with greisens and episyenite. The 1.77 Ga (g1) and 1.58–1.57 Ga (g2) rapakivi granites of Goias (central Brazil) are coeval with the Arai rift basin. Granites of the g1 suite are metaluminous and alkaline, while the g2 suite is metaluminous to peraluminous. Both are enriched in F, Sn, Rb, Y, Th, Nb, Ga and the REE. Primary micas range from Fe-rich biotite to zinnwaldite. The micas of the more evolved granites and the metasomatic micas are strongly enriched in F, with F/Li between 2 and 10. The initial eNd values of both suites show a considerable range (∼−14 to 0) and indicate substantial compositional variation in source. Sn deposits in Goias are hosted mainly by greisens. Indium is concentrated in quartz–topaz rock and albitized g2d granite of the Mangabeira massif and is always related to a cassiterite-sulfide association. This quartz–topaz rock is of metasomatic origin, probably generated by a hydrothermal fluid derived from the topaz–albite granite. Mineralization in the studied deposits was essentially associated with F enrichment. In the peralkaline Madeira albite granite, extremely high F contents favored disseminated mineralization, while in the Goias Tin Province (GTP) and Agua Boa massif greisenization was related to early fluid saturation. The GTP and Pitinga granites display tectonic, petrogenetic, geochemical, isotopic and metallogenic similarities that can be applied in search of Sn and rare-metal deposits.

Infrared and Mössbauer study of Brazilian tourmalines from different geological environments

Abstract Infrared (IR) spectra in the OH stretching region and Mössbauer spectra (MS) have been acquired for natural tourmalines from magmatic and hydrothermal deposits in the state of Minas Gerais, Brazil. In the magmatic environment, samples of tourmaline were collected from granitic pegmatites and granitoids. In the former, the elbaite-schorl series is prevalent, while in the latter the schorl species is dominant. Tourmalines from hydrothermal gold deposits are typically intermediate members of the dravite-schorl series, whereas the uvite and Fe-bearing uvite species are present in tourmaline samples from a magnesite deposit. Different behaviors are discussed in terms of local lattice environment, geological history, fluid/rock interaction or magma evolution, and degree of crystallinity.

Sr and Nd isotopic characteristics of 1.77-1.58 Ga rift-related granites and volcanics of the Goiás tin province, central Brazil

As rochas supracrustais do Grupo Arai, e os granitos tipo-A associados, representam um rift continental paleo-mesoproterozoico. Duas familias de granitos sao identificadas: a mais antiga (ca. 1,77 Ga) forma pequenos plutons circulares enquanto a mais jovem (ca. 1,58 Ga), constitui corpos maiores e deformados. Dados isotopicos Sr-Nd indicam que o magmatismo felsico e predominantemente o produto de re-fusao de crosta de idade paleoproterozoica. Razoes 87Sr/86Sr iniciais das duas familias sao ca. 0,726 e 0,720. A maioria das idades modelo TDM caem no intervalo entre 2,58 e 1,80 Ga e os valores de eND(T) se distribuem entre +3.6 e -11.9. Rochas vulcânicas do Grupo Arai sao bimodais, com basaltos e dacitos/riolitos intercalados em sedimentos continentais. As vulcânicas felsicas mostram caracteristicas isotopicas de Nd muito semelhantes as dos granitos e tambem sao interpretadas como produtos da re-fusao de crosta paleoproterozoica. Sedimentos do Grupo Arai indicaram TDM entre 2,4 e 2,16 Ga, demonstrando que sao o produto da erosao de crosta paleoproterozoica. Os dados apresentados indicam que o rift Arai se instalou em crosta recem estabilizada, apos a orogenia Paleoproterozoica.

Discriminação de áreas de espesso regolito do leste do Estado do Amazonas usando estatística multivariada, algoritmo hiperespectral e modelagem de dados espaciais

Areas with deep regolith profiles are widely distributed in the Amazonian region. Research on regolith geology in Brazil has concentrated, mainly, in geochemical and mineralogical aspects and where possible utilizing these in exploration. Although airborne geophysics and spatial data integration methods are being used increasingly for mineral resource assessments, the application in regolith-dominated terrains is new. The objective of this paper is to show the potential of a systematic approach, including the use of regional gamma-ray spectrometry, digital elevation model and image digital processing techniques for the identification of deep regolith areas characterized by potential economic interest in less well explored region of the eastern portion of Amazonas State. The application of multivariate statistics techniques (Principal Components Analysis and K-means) in airborne gamma-ray data allows the individualization of regoliths in comparison to the geologic units of this region. How exploratory guide, the following patterns were observed: low values of K (easily lixivied during the weathering) and K/eU ratio; high values of eTh, eTh/K ration and medium values of eU, besides the higher altitude (plateau). Based on these images, the hyperspectral algorithm named Spectral Angle Mapper (SAM) and the boolean and fuzzy logic spatial modeling techniques were applied to produce regolith potential maps that were compared with previous geological data in 1:250.000 scale. The quantitative relationships between geophysical and altimetric datasets have shown great capability for linking geological and geophysical interpretations, indicating potential areas that can direct new ways for mineral prospecting at the study area.

Mobilité des terres rares au cours des altérations hydrothermales : l'exemple du granite de Serra Branca, Brésil central

The Middle Proterozoic granitic massif, in the Goias Tin Province, is affected by pervasive post-magmatic hydrothermal alterations, chiefly by eastward, increasingly developed greisenization. Hydrothermal alteration results in strong mineralogical and chemical modifications of granite composition, but nevertheless the effect of fluid circulations is probably limited to a dilution of the initial REE content in magmatic rocks without any important fractionation, as suggested by the similarity between chondrite-normalized patterns for all rocks, from the less altered granite to the muscovite-topaz-bearing greisen.

Pirometamorfismo ígneo na Bacia Potiguar, Nordeste do Brasil

In the Potiguar Basin (NE Brazil), cretaceous rocks (sandstones, siltstones, shales, limestones) are intruded by Paleogene to Neogene basic bodies. As a result, were formed buchites, pyrometamorphic rocks indicating very low pressures and very high temperatures. Field descriptions permitted distinguishing light buchites (LB) and dark buchites (DB), which were investigated throughout petrographic, electron microprobe and X-ray diffraction studies. LBs contain abundant clasts of quartz surrounded by radial tridymite needles, besides phenocrysts of sanidine and clinopyroxene included in a vitreous groundmass. DBs have mainly microcrystals of Fe-cordierite (sekaninaite), mullite, armalcolite, ilmenite and spinel, dispersed within a black cryptocrystalline matrix. Chemically, LBs are richer in SiO2 (~76.7%) and K2O (~5.7%) and poorer in Al2O3 (~12.8%) when compared to DBs (respectively ~51.5, ~0.2 and ~42.7%). Based on phase diagrams published in the literature, the habit of crystals (acicular, elongated sometimes hollow crystals) and the significant content of glassy material we consider that the liquid formed by melting at ~1100 - 1150oC of sedimentary material cooled quickly at very high temperatures and pressures below 1 kbar. The results obtained are relevant in petrological terms, and may also have economic implications since a large number of basic bodies intrude rocks with hydrocarbon reservoirs.

Análise prospectiva para ouro nas regiões Ouro Roxo-Cantagalo e Chico Torres, Província Mineral do Tapajós

Spatial modeling (fuzzy logic) was used to predict key areas for gold exploration in the Tapajos Mineral Province (Brazilian Amazon), specifically in the regions here named Ouro Roxo-Cantagalo and Chico Torres. The data used were regional airborne geophysical and SAR-R99B data. The airborne geophysical data processing allowed the generation of evidence layers for modeling approaches. The best evidences in current models were anomalous potassium, F parameter, radioelement ratios, radioelement normalization, and amplitude of the analytical signal of the magnetic anomalous field. The SAR images were used in the photointerpretation stage. In the case of regions of the Ouro Roxo-Cantagalo and Chico Torres shear zones, geophysical images and interpretations (when necessary) were used to application of fuzzy logic, and the gold locations were used to evaluate the quality of the modeling results. The main contributions of this research include the observation of potential gamma-ray spectrometry signatures, the refining interpretation of the digital data, and improving the information of the current geological maps. The prospective Ouro Roxo-Cantagalo and Chico Torres modeling results using the conceptual fuzzy logic method exemplify the generation of regional exploration models for the cartography and selection of gold potential areas. This methodology can be extended to others regions of the Tapajos Mineral Province using airborne geophysical data current available by CPRM and the SAR images of the Centro Tecnico Operacional do Sistema de Protecao da Amazonia (CENSIPAM).

The Mata Azul pegmatitic field, Tocantins/Goiás, central Brazil: geology, genesis and mineralization

In Goias and Tocantins States, Central Brazil, several granitic pegmatites were characterized and grouped for the first time. These pegmatites had been intensely explored by hand in the past, producing mainly gemstone varieties of tourmaline and beryl. Barren, beryl- and tourmaline-bearing pegmatites occur across an area of 2,000 km 2 where they intrude regional metasedimentary rocks and peraluminous granites. K‑feldspar (mostly altered to kaolin), quartz and mica (mainly muscovite) are the major minerals. The main accessory minerals are beryl, tourmaline, garnet, albite, Fe-Mn phosphate aggregates, and trilithionite. The paragneiss surrounding the barren pegmatites was affected by thermal metamorphism and later hydrothermal alteration, producing Ca-silicates, Ti-Nb-Y oxides and sulfides. Leucogranites of the Mata Azul Suite are peraluminous and syn- to post-orogenic with geochemical characteristics of the LCT granite-pegmatite group. LA-ICP-MS U-Pb geochronology in monazite yields an age of 519 ± 2.8 Ma. Additionally, U-Th-Pb chemical dating of uraninite reveals a maximum age between 500 and 560 Ma. These ages, the field relationships, the mineralogy and the geochemical data suggest that the granites of the Mata Azul Suite are the probable sources of the studied pegmatites. The mineral associations and the mineral chemistry are used to define the degree of fractionation of the pegmatites. We propose that the group of studied pegmatites represents a pegmatitic field, called the Mata Azul Suite Pegmatitic Field.