Claudio Nery Lamarão

Geology, geochemistry, and Pb–Pb zircon geochronology of the Paleoproterozoic magmatism of Vila Riozinho, Tapajós Gold Province, Amazonian craton, Brazil

Abstract Pb–Pb zircon geochronologic data define two different periods of intense igneous activity in the Vila Riozinho region, Tapajos Gold Province (TGP), south-central Amazonian craton. At ∼2.00–1.97 Ga, the Vila Riozinho volcanic sequence (2000±4 Ma, 1998±3 Ma) and the older Sao Jorge granite (1981±2 Ma, 1983±8 Ma) were formed. At ∼1.89–1.87 Ga, the younger Sao Jorge (1891±3 Ma), Jardim do Ouro (1880±3 Ma), and Maloquinha (1880±9 Ma) granites and the Moraes Almeida volcanic sequence (1890±6 Ma, 1881±4 Ma, 1875±4 Ma) were emplaced. Similar age intervals are registered throughout the TGP that is thus a little younger than the Paleoproterozoic terranes of the Maroni-Itacaiunas Province in the Guiana Shield. Geochemical and geochronologic data demonstrate that the Sao Jorge pluton is composed of two different granitoids, the older and younger Sao Jorge granites. These, as well as the Jardim do Ouro granite, are I-type and magnetite series. The Sao Jorge granites are high-K calc-alkaline and show significant geochemical variation, the Jardim do Ouro granite is more iron-rich and less oxidized. The Vila Riozinho Formation is calc-alkaline to shoshonitic and geochemically similar to the Sao Jorge granites. The Maloquinha granite and the Moraes Almeida Formation differ from the other studied rocks in petrographic and geochemical characteristics, have aluminous A-type affinities, and were probably derived by low-temperature crustal melting. The volcanic sequences of the studied area, formerly included into the Iriri Group of the Uatuma Supergroup, are divided into the Vila Riozinho and Moraes Almeida sequences. The presence of these two sequences in the same tectonic domain demonstrates the heterogeneity of the Uatuma Supergroup. The TGP registers accretionary processes related to the formation of the Atlantica supercontinent at ∼2.00 Ga. This was followed (at ∼1.88 Ga) by an intracontinental taphrogenic event that lasted throughout the Mesoproterozoic. The tectonic setting of the TGP was thus transitional between a subduction-related magmatic arc and a stable continental block undergoing extension.

Geologia, petrografia e geoquímica das rochas vulcânicas Uatumã na área sul de São Félix do Xingu (PA), Província Carajás

As rochas vulcânicas da area sul de Sao Felix do Xingu, esta- do do Para, estao inseridas no contexto geologico da provincia geocrono- logica Amazonia Central, sudeste do craton Amazonico. Estas rochas sao dominantemente relacionadas a Formacao Sobreiro e, subordinadamente, a Formacao Santa Rosa, ambas pertencentes ao Grupo Uatuma de idade Paleoproterozoica. A Formacao Sobreiro apresenta tres facies: facies de fluxo de lavas subaerea de composicao subalcalina; facies de fluxo de lavas subae- rea de composicao calcioalcalina a shoshonitica; facies vulcanoclastica su- baerea. As rochas da Formacao Santa Rosa sao enquadradas em uma unica facies denominada facies de fluxo de lavas subaerea. Na Formacao Sobreiro sao encontrados andesitos basalticos, andesitos, traquiandesitos, traquitos, tufos de cristais felsicos, lapili-tufos e brechas polimiticas. Os litotipos da Formacao Santa Rosa sao riolitos. Os dados geoquimicos mostram que os conteudos de SiO 2 das rochas da Formacao Sobreiro variam de 52,14 a 69,21% e as razoes K 2 O/Na 2 O de 0,16 a 1,62. Por outro lado, os vulcanitos da Formacao Sao Rosa formam uma serie evoluida com teores de SiO 2 entre 72,27 e 77,14% e razoes K 2 O/Na 2 O entre 1,50 e 2,12. A Formacao So- breiro tem carater essencialmente calcioalcalino, discretamente transicional de calcioalcalino a shoshonitico, composicao metaluminosa a fracamente peraluminosa e assinatura tectonica de ambiente de arco vulcânico. A For- macao Santa Rosa apresenta composicao peraluminosa a fracamente meta- luminosa, assinatura tipo A e afinidade tectonica intraplacas. As rochas vul- cânicas da area sul apresentam perfeita correlacao petrografica, geoquimica e tectonica com os vulcanitos da area oeste/sudoeste de Sao Felix do Xingu.

Petrography, magnetic susceptibility and geochemistry of the Rio Branco Granite, Carajás Province, southeast of Pará, Brazil

The Paleoproterozoic Rio Branco stock intrudes the RESUMO: Petrografia, suscetibilidade magnetica e geoquimica do Archean Cruzadao biotite monzogranite. It occurs west of the Canaa dos Carajas city, close to the Sossego copper mine in the Carajas Prov- ince. It is constituted by undeformed and isotropic, hololeucocratic syenogranites, showing equigranular texture. The granite is formed essentially by perthitic alkali feldspar, quartz, and plagioclase, with additional variable amounts of chloritized biotite. Fluorite, allanite, and zircon are common accessory minerals, whereas pyrite and chal- copyrite are scarces. Albitization and subordinate greisenization are the main alteration processes that affected the granite. The secondary mineralogy is represented by albite, fluorite, topaz, chlorite, musco- vite, siderophyllite, and iron oxides. Magnetic susceptibility values are systematically low and vary between 1.3 x 10 -5 a 6.9 x 10 -4 (SI). The Rio Branco Granite is metaluminous to peraluminous and shows high FeOt/(FeOt + MgO). It has geochemical affinities with ferroan, reduced A2-subtype granites. The rare earth element (REE) patterns are flat with little heavy REE fractionation and show accentuated negative Eu anomalies (Eu/Eu* = 0.08 – 0.13). These characteristics are typical of evolved granites, derived from volatile-enriched liq- uids responsible by the intense subsolidus hydrothermal alteration. The dating of the Rio Branco granite did not give conclusive results, but the obtained data, associated with geological evidence, suggest that it is of Paleoproterozoic age. The comparison between the Rio Bran- co granite and Paleoproterozoic A-type suites of the Carajas Province suggests that the Rio Branco granite has more affinity with the Velho Guilherme suite and, in lesser degree, with the Serra dos Carajas suite. On the other hand, is clearly different from oxidized A-type granites of the Jamon suite. Despite its similarities with tin-specialized gran- ites, the Rio Branco Granite is not associated with tin mineralizations.

Quartzo e zircão como marcadores da evolução magmático-hidrotermal do Granito Antônio Vicente, Suíte Intrusiva Velho Guilherme, Província Carajás

Four morphological and textural types of quartz, informally labeled Qz1, Qz2, Qz3 and Qz4, were identified in the different facies of the Antonio Vicente Granite, Carajas Province, by scanning electron microscope-cathodoluminescence (SEM-CL) images. In the less evolved rocks, containing amphibole and biotite, well developed anhedral to subhedral, luminescent and intensely fractured crystals dominate, named Qz1. Hydrothermal fluids that percolated the granite modified the magmatic quartz (Qz1) into Qz2 and Qz3 through processes of alteration, dissolution and recrystallization, with these changes much more evident in the intensely altered syenogranite rocks. Qz4 constitute medium-to-coarse grained crystals, usually luminescent and comparatively little fractured. Its occurrence is restricted to strongly hydrotermalized syenogranite rocks and bodies of greisens, suggesting the beginning of the greisenization process. In the greisens, medium-to-coarse grained euhedral, concentrically zoned quartz crystals dominate, with typical features of hydrothermal origin (Qz5). Fine crystals of zoned cassiterite (≤ 100 µm) are common and fill cavities in the types Qz4 and Qz5. Zircon crystals dominantly anhedral, corroded, with the highest contents of Hf and the lower Zr/Hf ratios belong to more evolved and hydrothermally altered rocks and to associated greisens, both carriers of Sn mineralization. This fact suggests that the geochemical signature of zircon, especially Zr/Hf ratio, can be used for the preliminary assessment of metallogenic potential of tin granites.

Characterization and Distribution of Pyrogenic Carbon in a Fraction of Archaeological Black Earth from Caxiuanã

This study aims to determine the atomic ratio of O/C in an archaeological black earth (ABE) profile of the Ilha de Terra site, a region of Caxiuana in the Para State, Brazil, to determine the types of pyrogenic carbon (PyC) particles and to infer the source of biomass and burning temperature necessary to produce the PyC. The O/C ratios were monitored using scanning electron microscopy combined with energy dispersive spectroscopy (SEM/EDS). The results indicated atomic ratios for clay, silt and fine sand fractions that were between those registered for the PyC particles types: condensed combustion (CC) (0.09, 0.1, 0.13), charcoal (0.32, 0.31, 0.34) and char (0.43, 0.45, 0.52). CC is the predominant type of particle found because of the high firing temperature (> 350 °C), which is consistent with the probable biomass sources of wood, cellulose and lignin.

Quartzo magmático e hidrotermal do depósito de ouro São Jorge, Província Aurífera do Tapajós, Pará: petrografia, microscopia eletrônica de varredura-catodoluminescência e implicações metalogenéticas

Studies in crystals of quartz present in the mineral associations to the area of the Sao Jorge gold deposit, Tapajos Gold Province, southwest of the Para state, identified four morphological and textural types (Qz1, Qz2, Qz3 and Qz4) by scanning electron microscopy-cathodoluminescence images. In the more preserved rocks of the Younger Sao Jorge Jovem granite, rich in amphibole and biotite (associations 1 and 2), anhedral crystals of magmatic quartz with high to moderate luminescence (Qz1) dominate. In the partly altered rocks (associations 2 and 3), post-magmatic to hydrothermal fluids affected the granite, and filled fractures in Qz1 and crystallized not luminescent (dark) Qz2. In the most intensely altered rocks (association 4), successive alteration, dissolution and recrystallization processes gave rise to typically hydrothermal zoned, subhedral (Qz3) and euhedral (Qz4) quartz crystals. Images by backscattered electrons and semiquantitative analysis by energy dispersive spectroscopy identified two generations of gold: Au1, enriched in Ag (4.3 to 23.7%) and associated to pyrite crystals; Au2, enriched in Te (1.1 to 17.2%) and included or associated to Qz4. The scanning electron microscopy-cathodoluminescence study provided important information that was preserved in the quartz structure. The morphological and textural evolution of this mineral in different stages shows the gradual action of the hydrothermalism in the rocks and minerals associations of Sao Jorge deposit. Gold mineralization of the deposit was chemically (energy dispersive spectroscopy) and paragenetically (pyrite, sphalerite and Qz4) characterized, and it can be divided into different generations or mineralizing events. The effectiveness of the methodology used in this study was established, allowing its application in studies of other hydrothermal deposits.

Variação morfológica e composicional de zircão e suas implicações metalogenéticas: o exemplo das suítes Jamon, serra dos Carajás e Velho Guilherme, Cráton Amazônico

Zircon from granites of the Jamon suite (JS), Serra dos Carajas suite (SCS) and Velho Guilherme suite (VGS) were studied by scanning electron microscope (SEM) through backscattered electron and cathodoluminescence images and energy dispersive spectroscopy (EDS) analyses. Granites and greisens of the VGS have predominantly anhedral zircons that are altered and intensely corroded, are enriched in Hf and have the lowest Zr/Hf ratios of the studied suites. In the granites, these ratios decrease towards the more evolved varieties. Zircons of the JS are euhedral to subhedral, zoned and slightly altered; they are also comparatively depleted in Hf and display the highest Zr/Hf ratios, indicating limited potential for tin-associated mineralization. Zircons from granites of the SCS are subhedral to anhedral, altered and corroded, and show Hf contents and Zr/Hf ratios intermediate to those of the JS and VGS. The granites of the VGS with Sn-, Ta- and W-associated mineralization contain zircons with Zr/Hf ratios varying from 7 to 22. It is concluded that ratios of similar magnitude can be used as a prospecting guide for specialized granites. Zircons from the greisens associated with the Cigano granite of the SCS have average Zr/Hf ratios of approximately 23, but no cassiterite was found in these rocks, indicating that the zircons preserved their magmatic geochemical signature. This study distinguished the three granitic suites in terms of zircon composition and demonstrated the importance of their geochemical signature, especially in terms of their Zr/Hf ratio, in the identification of specialized granites. EDS-SEM analysis can thus be used in a preliminary assessment of the metallogenic potential of tin granites.