The Granulite-Facies Rocks of the Northern Segment of the Ribeira Belt, Eastern Minas Gerais, SE Brazil

Granulite rocks are exposed in eastern Minas Gerais, Brazil. Its early neoproterozoic evolution is characterised by a history of an active continental margin, including the accretion of suspect terranes. The Manhuaçu Terrane is one of those which is represented by a granitic continental plutonic arc and terrigeneous metasediments reflecting a continental margin. A metasedimentary gneiss belt at this margin with shallow to deep marine clastic lithologies as well as metavolcanic and metaplutonic mafic rocks was interpreted as an extensive tectonic segment with suspect development in a back-arc setting. Fragments of a volcanic arc are identified and interpreted as an evidence for a probable island-arc domain. The granulites occur as massive rocks as well as high-grade gneisses and show lithological, structural and metamorphic attributes consistent with their host belt type. In the western portion granulites derived from sedimentary protoliths, have been deposited, deformed and metamorphosed together with the mafic intrusions and as well as with their crystallization. Regional uplift exposed these rocks probably immediately after the metamorphism. In these belts the metamorphic grade is not uniform, especially where uplifting has exposed oblique cross sections over the granulitic rocks. Geothermobarometric calculations indicate that the granulites has been generated under T conditions between 800 and 990 ± 50 C and from medium (4.8 kb) to relatively high (10.0 kb) pressures.     

Geochemistry Of The GuaxupÉ Granulites, Minas Gerais, Brazil

Geochemical data for granulite terrain are presented in the northernmost portion of the Guaxupé Massif, at Mantiqueira Province, SE Brazil. Several types of granulites are recognized in the area: basic, intermediate and acid granulite. Major and trace elements (including REE) point to only one magma source for these granulites generated at different times. Geochemical data point to plagioclase and apatite fractionation as the responsable by the REE behaviour in intermediate and more basic rocks. Overall composition of the Guaxupé Granulites is similar to average described worldwide in the literature to the lower crust.     

Stable Isotope Investigations on the Granulite Facies Terrains of Brazil: A Review

Stable isotope data from three areas in the granulite facies terrains of Brazil are assembled and discussed. All the three areas (Jequié, Guaxupé and São José do Rio Pardo) are from the São Francisco Craton. The carbon isotope composition of the fluid inclusion CO₂ in the Archean granulite terrain of Jequié indicate the participation of two distinct sources, an upper mantle source and an internal source of Archean organic matter. The isotope data may be interpreted in terms of the granulite genesis due to the intrusion of magma in the lower crust. The singularly uniform carbon isotope data for CO₂ fluid inclusion of plutonic granulites is a clear indication of the magmatic addition of CO₂-rich volatiles from deeper crustal sources. In the São José do Rio Pardo area sulfur and carbon isotope data of scapolites from the granulites imply the derivation of volatiles from internal sources. The whole rock oxygen isotope data of the amphibolite and granulite facies gneisses from Guaxupé indicate a small scale variation of d¹⁸O values, compatible with the chemical data, suggesting the preservation of pre-metamorphic oxygen isotope composition. The isotope data of the granulites from São Francisco Craton indicate non-pervasive fluid flow during metamorphism.     

Recognition features of felsic pyroclastics of Serra do Tombo formation,Minas Gerais,Brazil

In this contribution we report a study of poorly exposed, rhyodacitic welded-ignimbrite deposit from Minas Gerais. A petrographic study of textures indicate high temperature of emplacement. Key features include eutaxitic texture, flattened and agglutinated lapilli and glass menisci. Most of the feldspar minerals and glass are extensively altered to clay minerals, which pseudomorph the original volcanic textures. Glass menisci and spherules suggest a possible process of liquid immiscibility. Immobile trace element distribution indicates a possible link with other post-Palaeozoic felsic volcanic rocks in Brazil, a magmatism interpreted as due to basaltic underplating and partial melting of a hydrous continental crust. A peculiar feature is a high Light REE/Heavy REE ratio. Depletion in heavy rare earth elements is possibly due to a residual HREE-bearing phase in the source. The geologic context of these rocks suggests a Lower Cretaceous age and a tectonic relationship with a continental rifting event.     

Mercury in itabirite-hosted soft hematite ore in the Quadrilátero Ferrífero of Minas Gerais

Mercury contents in Precambrian banded iron formation-hosted hematite ores are virtually unknown. In an attempt to provide information on the abundance and distribution of Hg in Fe ore, we present analyses for Hg in samples of high-grade soft hematite ore from Gongo Soco, Minas Gerais, Brazil. Bulk samples contain from <  5 to 25  ppb Hg without obvious correlation with major elements. Granulometric fractions of follow-up samples have amounts of Hg from 6 to 48  ppb and display positive linear correlations with total Mn as MnO (r = 0.87), LOI (r = 0.87) and SiO₂ (r = 0.76), as well as a negative linear correlation with total Fe as Fe₂O₃ (r = −  0.87). The correlations suggest that Hg is associated with a hydrated ferruginous groundmass bearing residual Mn, Al and Si, which replaced gangue minerals in itabirite in the process of formation of the Gongo Soco soft hematite ore.     

Decompression reactions and P–T conditions in high-pressure granulites from Casares-Los Reales units of the Betic-Rif belt (S Spain and N Morocco)

High-pressure granulites are exposed in the Casares-Los Reales group (internal zones of Betic-Rif belt, S Spain–N Morocco) as part of the crustal envelope of Beni Bousera-Ronda Peridotites. They are mostly metapelitic but include intercalations of mafic composition. The metamorphic history is marked by the preservation of early high-pressure assemblages together with secondary low-pressure assemblages suggesting a state of textural and compositional disequilibrium. The P–T path constrained by geothermobarometry and reaction textures from mafic and pelitic lithotypes passes from 800 °C/15 kbar to 600 °C/5 kbar, to indicating a strong decompression related to cooling, followed by a near-isobaric cooling 430 °C and 4 kbar. Such P–T evolution of granulites is thought to reflect some sort of rapid tectonic collapse of crust previously thickened through collision.     

The pegmatites of the Nova Era–Itabira–Ferros pegmatite district and the emerald mineralisation of Capoeirana and Belmont (Minas Gerais, Brazil): geochemistry and Rb–Sr dating

The chemical composition and Rb–Sr ages of mica, feldspar, and whole rock samples from the emerald mineralisation of Capoeirana and Belmont, from emerald-barren pegmatites and of the Borrachudos granitic gneiss, Monlevade banded and granitic gneisses from the area of Nova Era–Itabira–Ferros (Minas Gerais, Brazil) as well as from the Guanhães gneiss (Minas Gerais, Brazil) have been determined. The Borrachudos granitic and Monlevade banded gneiss with connected pegmatitic schlieren and pegmatite veins, as well as the Guanhães gneiss, got their actual textures and mineralogical composition at about 1.9 Ga in the context of the Transamazonic tectonothermal event.The Monlevade banded gneiss belongs to a volcano-metasedimentary greenstone belt sequence, which is the typical country rock of the emerald occurrences. The main emerald-forming event at Belmont and Capoeirana was a metasomatic reaction of Be-rich anatectic pegmatites with Cr-rich ultrabasic rocks during the Transamazonic event. At Capoeirana, K–feldspar-bearing Be-rich pegmatites were transformed during the emerald-forming process into plagioclase–quartz rocks. Rb–Sr ages on biotite of about 480 Ma from the emerald mineralisation result from the rejuvenation of Transamazonic biotite by the Brasiliano event.The widespread macroscopically unmetamorphosed pegmatites of the study area formed in the Brasiliano event at 477±14 Ma. These pegmatites resulted to be emerald-barren although the differentiation degree, as given by diagrams such as Cs vs. K/Rb for muscovite and K–feldspar, starts from ceramic and ends with Be pegmatites. Some pegmatites display a marked internal differentiation.

Abstract

Foram determinadas as composições químicas e as idades Rb–Sr de mica branca, feldspato potássico e de rochas totais das mineralizações de esmeraldas de Capoeirana e Belmont, de pegmatitos sem esmeraldas e dos gnaisses Borrachudos, Monlevade e Guanhães da região de Nova Era–Itabira–Ferros (Minas Gerais, Brazil). Os gnaisses graníticos Borrachudos, os gnaisses bandados Monlevade, seus respectivos pegmatitos e veios/schlieren pegmatóides, e os gnaisses Guanhães, adquiriram suas texturas e composições mineralógicas atuais há cerca de 1.9 Ga no contexto do evento Transamazônico.As rochas regionais encaixantes típicas das ocorrências de esmeraldas são os gnaisses Monlevade que pertencem a uma sequência metavulcano-sedimentar de tipo greenstone belt. O evento principal de formação de esmeraldas em Belmont e Capoeirana foi uma reação metassomática dos pegmatitos anatéticos ricos em Be com rochas ultrabásicas ricas em Cr durante o evento Transamazônico em torno de 1.9 Ga. Em Capoeirana nesse contexto os pegmatitos com feldspato potássico ricos em Be foram transformados em rochas de plagioclasio–quartzo. As idades Rb–Sr de cerca de 480 Ma de minerais das mineralizações de esmeralda resultaram da reequilibração de biotitas e feldspatos Transamazônicos durante o evento Brasiliano.Os pegmatitos não-metamórficos e sem esmeralda da região estudada foram formados no evento Brasiliano há 477±14 Ma. O grau de diferenciação dos pegmatitos, estudado em diagramas indicadores específicos como por exemplo Cs vs. K/Rb de micas brancas e feldspatos potássicos, varia desde pegmatitos cerámicos até muscovita-pegmatitos, à pegmatitos de metais raros e até berilíferos. Alguns dos pegmatitos apresentam marcante diferenciação interna.     

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 D₁, D₂, D₃, 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.     

Lead isotope constraints on the genesis of Pb–Zn deposits in the Neoproterozoic Vazante Group, Minas Gerais, Brazil

The Neoproterozoic Vazante Group at the western border of the São Francisco Craton, Brazil, hosts the largest Zn–Pb district in South America. Several authors have classified this mineral district as Mississippi Valley-type (MVT), based on the intimate association with carbonates and the epigenetic character of most ore bodies. In this paper, we present 47 new lead isotope data from four deposits located along the 300 km N–S Vazante–Paracatu–Unai linear trend. Pb isotope ratios indicate sources with relatively high U/Pb and Th/Pb ratios. Considering the ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios as indicative parameters for the source, we suggest an upper crustal source for the metals. The small variation on the Pb isotope ratios compared to those observed in the classical MVT deposits, and other geological, fluid inclusion and sulphur isotopic data indicates a metallogenic event of long duration. It was characterized by focused circulation of hydrothermal fluids carrying metals from the basement rocks and from the sedimentary pile. The data obtained are more compatible with an evolution model similar to that of IRISH-type deposits. The existence of three Pb isotopic populations could be the result of regional differences in composition of the source rocks and in the fluid–rock interaction since the mineralization is a long-term process.     

An epigenetic origin for the Passagem de Mariana gold deposit, Quadrilátero Ferrífero, Minas Gerais, Brazil

The gold–tourmaline quartz–vein deposit of Passagem de Mariana, in the southeastern part of the Quadrilátero Ferrífero, produced more than 60 tonne of gold, from the end of the 17th Century until 1954. The mine has not operated since 1985. Orebodies are veins composed of white quartz (> 60 vol.%), carbonate (ankerite), tourmaline, sericite and sulfides. Tourmaline (dravite), up to 10 vol.% of the vein, occurs as subhedral, coarse, commonly zoned crystals, and is concentrated along vein boundaries and on the edges of host rock inclusions in the veins. Tourmaline is present in all rock types in the mine, but the chemical composition of the host rocks determinates the intensity of tourmalinization, with the alteration being greater in sericitic phyllites, graphite–sericite phyllites, and calcareous rocks. The most abundant sulfide is arsenopyrite, which is normally associated with pyrite and pyrrhotite. Minor amounts of chalcopyrite, galena, löllingite, berthierite, and maldonite are present throughout the deposit. Sulfides are concentrated at veins boundary or are dispersed in the veins. Arsenopyrite is associated, most commonly, with calcareous rocks, and graphite–sericite phyllite. Pyrrhotite is usually found at the base of itabirites. Gold abundance is directly proportional to sulfide concentration. Hydrothermal alteration associated with the veins includes silicification, tourmalinization, and sulfidation. The mineralized zone is a shear zone associated with a bedding-parallel thrust fault that juxtaposes the itabirite (Lower Proterozoic Minas Supergroup) over other units. This shear zone/thrust fault extends for tens of km beyond the Passagem mine and hosts numerous gold deposits. The richest orebodies are along the itabirite footwall contact and within the graphite–sericite phyllite (Main orebody). Although many lithologic units were mineralized the graphite–sericite phyllite appears to have been most favorable for gold deposition.The area underwent three phases of deformation, D₁, D₂ and D₃. Mineral assemblages indicate upper-greenschist to lower-amphibolite conditions of regional metamorphism. Retrograde metamorphism, characterized by chloritization of biotite and chloritization and biotitization of garnet, developed locally. The gold-bearing veins crosscut the main foliation and lithologic contacts at low angle and occur within, or are in contact with, all lithotypes. Field and laboratory data indicate that gold mineralization at Passagem de Mariana is epigenetic. Gold deposition occurred after the peak of metamorphism, within the late- to post-D₂ period of deformation, which is correlated with second set of structures of Trasamazonian age of Alkmim and Marshak [Alkmim, F.F., Marshak, S., 1998. Transamazonian Orogeny in the Southern São Francisco Craton Region, Minas Gerais, Brazil: evidence for Paleoproterozoic collision and collapse in the Quadrilátero Ferrífero. Precambrian Research 90, 29-58.], indicating that the gold mineralization occurred between 2.124 and 2.04 Ga. We choose to regard Passagem de Marina as an orogenic gold deposit as defined by Groves et al. [Groves, D. I., Goldfarb, R.J., Gebre-Mariam, M., Hagemann, S.G., Robert, F., 1998. Orogenic gold deposits: A proposed classification in the context of their crustal distribution and relationship to other gold deposit types. Ore Geology Reviews 13, 7-27.], i.e., an epigenetic, structurally-hosted lode–gold vein system in a deformed metamorphic terrane.     

南天山基性麻粒岩中变形石英和石榴子石的TEM研究

基性麻粒岩产于大陆地壳深部,它的研究可以为大陆岩石圈的性质、组成、演化和深部动力学过程提供重要信息。南天山库米什榆树沟地区发育总厚达1000多米的基性麻粒岩构造岩片,走向南东,朝北东推覆。剖面上和石炭纪蛇绿混杂岩共存,并发生了石榴子石的旋转变形和强烈的石英-长石矿物相韧性变形。然而它们的变形条件至今尚未被研究。本文运用透射电镜(TEM)超微组构技术对榆树沟基性麻粒岩中石英和石榴子石进行了详细研究,结果表明,石英中存在着丰富的超显微变形构造,自由位错、亚颗粒和位错网非常发育,这表明研究区基性麻粒岩在折返抬升过程中曾遭受过较强的石英-长石相韧性变形作用;石榴子石中的超微变形构造则极为少见,只在其中发现几根自由位错,并在其颗粒边缘发现少量亚颗粒构造,说明该矿物主体属于脆性变形。这两种矿物变形性质的差异,反映了石榴子石的晶体结构在该条件下比石英更稳定,而且说明石榴子石的变质温度没有超过900℃,也就是说,本区麻粒岩的变质作用并非发生在极高温度的条件下。     

Genesis of itabirite-hosted Au–Pd–Pt-bearing hematite-(quartz) veins, Quadrilátero Ferrífero, Minas Gerais, Brazil: constraints from fluid inclusion infrared microthermometry, bulk crush-leach analysis and U–Pb systematics

Fluid inclusions hosted in quartz and specular hematite from auriferous (jacutinga) and barren veins in the Quadrilátero Ferrífero (QF) have been studied using conventional and near infrared microscopy, respectively. The mineralization consists of veins that cross-cut metamorphosed iron formation (itabirite) of the Paleoproterozoic Itabira Group. The sample suite comprises hematite from veins from the low-strain domain in the W and SW of the study area, as well as hematite samples from the eastern high-strain domain in the central and NE parts of the QF. Halogen ratios of fluid inclusions in quartz and hematite from all studied deposits are consistent with a fluid evolved from dissolving and reprecipitating halite that was subsequently diluted. Fluid inclusions hosted in quartz and hematite are characterized by consistent Na/K ratios and considerable SO₄ contents, and suggest similar formation conditions and, perhaps, fluid origin from a common source. Na/K and Na/Li fluid mineral geothermometers indicate water–rock interaction at approximately 340±40°C. Hematites from the high-strain domain contain fluid inclusion assemblages of high-temperature aqueous-carbonic and multiphase high-salinity, high-temperature aqueous inclusions probably due to fluid immiscibility in the system H₂O–NaCl–CO₂. Fluid inclusions hosted in hematite from barren veins in the low-strain domain, as well as in hematite from jacutinga-type mineralization from the central part of the QF, only host multiphase aqueous fluid inclusions all showing narrow ranges of salinity (7.2–11.7 wt.% NaCl equiv.) and homogenization temperatures (148 to 229°C). Lower homogenization temperatures and the absence of CO₂-rich inclusions in specular hematite from these occurrences are attributed to carbonate precipitation and/or CO₂ escape due to cooling during fluid migration from the high- to the low-strain domain. Pb–Pb and U–Pb systematics of gold, hematite and hematite-hosted fluid inclusions in combination with geochemical evidence indicate distinct sources for Pd, Au, and Pb. The formation of specular hematite veins may be related to retrograde metamorphic fluids being released during the Brazilian orogenic cycle (600–700 Ma). The Pb isotopic characteristics of all samples are readily reconciled in a simple model that involves two different Paleoproterozoic or Archean source lithologies for lead and reflects contrasting depths of fluid percolation during the Brasiliano orogeny.     

K–Ar dating of authigenic minerals in siliciclastic sequences: an example from the south Sanfranciscana Basin (Western Minas Gerais,Brazil)

K–Ar dating was applied on authigenic potassic minerals which are abundant in sandstones from the south of the Sanfranciscana Basin, Western Minas Gerais State, central Brazil. The Quintinos Member fluvial sandstones (Três Barras Formation, Areado Group) contain significant amounts of authigenic K‐feldspar as microcrystals of adularia and sanidine habits. The ages of these microcrystals cluster into three groups: 106.1 ± 2.2, 89.9 ± 1.9 and 88.8 ± 1.8 Ma (from Albian to Coniacian). The older age of 106.1 ± 2.2 Ma was obtained from the coarse fraction analysed (10–20 µm) that can contain a mixture of detrital potassic minerals (K‐feldspar, muscovite, biotite and illite) and different authigenesis of K‐feldspar (overgrowths and microcrystals). Thus, only the younger ages were interpreted as precipitation of K‐feldspar microcrystals during the Late Cretaceous into the Quintinos Member sandstones. Moreover, these ages can document the formation of microcrystals within a few million years after deposition of the sandstones. The ages of authigenic illite from the Capacete Formation epiclastic sandstones (Mata da Corda Group) range from 88.5 ± 1.9 to 71.5 ± 1.9 Ma (Coniacian–Campanian). These results suggest the timing of the illitization event in these sandstones as well as a synchrony with K‐feldspar authigenesis in the Quintinos Member sandstones. These results are well constrained and are in agreement with stratigraphic, biostratigraphic and radiometric ages previously reported for the Sanfranciscana Basin. Copyright © 2014 John Wiley & Sons, Ltd.     

Facies analysis of tertiary alluvial fan deposits in the Jundiaí region, São Paulo, southeastern Brazil

This article presents an analysis of facies of sedimentary sequences that occur as discontinuous bodies in the Jundiaí region, west of the main Tertiary continental basins of the southeastern Brazil continental rift. Nine identified sedimentary facies, grouped into four associations, suggest the existence of an ancient alluvial fan system whose source area was the Japi mountain range (Serra do Japi). The deposits are considered Tertiary in age and chronocorrelated with those identified in the Atibaia region and at other sites up to 100 km east and northeast of Jundiaí. The depositional model adopted to explain the filling of the basin proposes that the alluvial fans, which directly derive from the source area, terminated in a braided channel longitudinal to the basin axis that flowed to northwest, in a similar configuration to that of the present day. This basin may have extended to the Atibaia region or formed a set of small basins laterally contiguous to the faults associated with the rift. Such occurrences show that the formation of rift basins was broader than the area presently occupied by the main deposits.     

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.     

Denudation rates of the Quadrilátero Ferrífero (Minas Gerais, Brazil): Preliminary results from measurements of solute fluxes in rivers and in situ-produced cosmogenic Be

This paper investigates the denudation rates in the Quadrilátero Ferrífero, Minas Gerais State (Brazil). The aim is to compare chemical weathering rates from measurements of solute fluxes in rivers and long-term mean erosion rates deduced from in situ-produced cosmogenic ¹⁰Be concentrations measured in fluvial sediments. Both water samples and sediments were collected in fifteen stations (checkpoints) located in four hydrographic basins with low anthropogenic perturbations.Depending of the type of substratum, three degrees of chemical denudation rates from water samples are observed: (i) high rates in marbles; (ii) medium rates in schists, phyllites, granites, gneisses and migmatites; (iii) low rates in quartzites and itabirites. Preliminary results of long-term erosion rates deduced from in situ-produced ¹⁰Be are comparable with those of chemical rates.     

Nonsulfide and sulfide-rich zinc mineralizations in the Vazante, Ambrósia and Fagundes deposits, Minas Gerais, Brazil: Mass balance and stable isotope characteristics of the hydrothermal alterations

The Vazante Group hosts the Vazante nonsulfide zinc deposit, which comprises high-grade zinc silicate ore (ZnSiO₄), and late-diagenetic to epigenetic carbonate-hosted sulfide-rich zinc deposits (e.g. Morro Agudo, Fagundes, and Ambrósia). In the sulfide-rich deposits, hydrothermal alteration involving silicification and dolomitization was related with ground preparation of favorable zones for fluid migration (e.g. Fagundes) or with direct interaction with the metalliferous fluid (e.g. Ambrósia). At Vazante, hydrothermal alteration resulted in silicification and dolomite, siderite, jasper, hematite, and chlorite formation. These processes were accompanied by strong relative gains of SiO₂, Fe₂O_3(T), Rb, Sb, V, U, and La, which are typically associated with the nonsulfide zinc mineralization. All sulfide-rich zinc ores in the district display a similar geochemical signature suggesting a common metal source from the underlying sedimentary sequences.Oxygen and carbon isotope compositions of hydrothermally altered rocks reveal a remarkable alteration halo at the Vazante deposit, which is not a notable feature in the sulfide-rich deposits. This pattern could be attributed to fluid mixing processes involving the metalliferous fluid and channelized meteoric water, which may control the precipitation of the Vazante nonsulfide ore. Sulfide deposition resulted from fluid–rock interaction processes and mixing between the ascending metalliferous fluids and sulfur-rich tectonic brines derived from reduced shale units.     

Multipurpose geochemical maps produced by integration of geochemical exploration data sets in The Paraná Shield, Brazil

Old geochemical datasets from the Paraná Shield of southern Brazil have been integrated into a new geochemical database and the results are presented as a series of multipurpose geochemical maps. Although the 24 datasets retrieved were analysed by three different laboratories, the maps produced correlate surprisingly well with bedrock geological features.The retrieval of old geochemical exploration data sets for the purpose of generating maps showing element depletions and enrichments is a powerful tool with a wide range of applications: trace elements such as Zn, Cu and Mo can significantly affect crop productivity; areas with high abundances of harmful elements such as As and F can be delineated in order to identify public health hazards; heavy metals such as Cd, Pb and Hg can be measured in order to better constrain knowledge of chemical background levels for environmental monitoring. The integration and manipulation of different data sets can however be complicated by the heterogeneity in sampling and analytical procedures.