Optical spectroscopy study of variously colored gem-quality topazes from Ouro Preto,Minas Gerais,Brazil

Variously colored gem-quality topazes from Ouro Preto, Minas Gerais, Brazil, were studied by optical absorption spectroscopy and photoluminescence methods. In the near infrared range (750–2500 nm) the absorption spectra display an identical pattern of narrow intense absorption lines caused by overtones and combination vibrations of OH groups, which do not relate to the coloration of the topazes studied. Their colors were found to be caused by combination of three sets of absorption features, (1), (2), and (3) in the visible and near-UV range, which are due to different color center. (1) denotes a pair of broad split bands with maxima 18 000 and 25 000 cm^–1 caused by electronic spin-allowed dd transitions of Cr³⁺ ions. They cause a light rose to deep violet color and characteristic pleochroism of Cr³⁺-containing topazes. Photoluminescence evidences of at least three different types of Cr³⁺ complexes which, most probably, differ by ligand surroundings, O₄F₂, O₄F(OH) and O₄(OH) (2) Corresponds to the intense weakly polarized UV absorption edge. Two different parts, the thermally stable one, caused by ligand-to-metal charge transfer, and the thermally unstable one, caused by some defect center(s), contribute to the edge. (3) denotes a system of two broad unstructured bands with maxima around 19 000 cm^–1 (X>Y Z) and 24 000 cm^–1 (Y Z X). They cause the unique orange color and characteristic pleochroism of Brazilian Imperial topazes. Combinations of (1) and (3) absorption features cause various yellow-rose colors of the samples. Investigations of natural irradiated and thermally treated topazes show that the color centers (1) and (3) transform to each other at annealing and X- or gamma irradiation. The color of natural orange-red Imperial topazes is assumed to be caused by Cr⁴⁺, stabilized by other impurity ions and/or defect irradiation EPR centers. At T=300 °C Cr⁴⁺ reduces to Cr³⁺, the color of Imperial topazes changes to pale rose, caused by spin-allowed bands of Cr³⁺. In artificially irradiated crystals the (3)-center, Cr⁴⁺, may be induced according to the reaction 2Cr³⁺ Cr⁴⁺ + Cr²⁺, which involves chromium pairs in adjacent Al sites of the structure. Such artificially induced color is unstable at room temperature and in daylight. The process of the decay of (3)-centers may be described as a recombination Cr⁴⁺+Cr²⁺ 2Cr³⁺ that results in vanishing of the (3)-bands accompanied by the appearance or increase in Cr³⁺ dd bands, the original orange color turning to a pale rose.     

The origin of diamonds in western Minas Gerais,Brazil

Important diamond mineralized alluvium/colluvium occurs in western Minas Gerais (WMG) in the Brasília Orogenic Belt (700-450 my). Diamonds are geographically related to glacial and periglacial sediments (Upper Proterozoic and Permo-Carboniferous), wadi and debris flow conglomerates (Lower Cretaceous and Upper Cretaceous), and ultramafic alkaline rocks (intrusive and extrusive, including kimberlites, leucitites, kamafugites, carbonatites, etc.) of Cretaceous age. Petrological data indicate that the rocks originally petrographically classified as kimberlites present mineralogical, chemical, and isotopical differences with Group I and II kimberlites and more closely resemble kamafugites. The Brasília Orogenic Belt presents features of a Wilson Cycle implying that WMG does not fulfil the geotectonic and geothermometric requisites to host primary diamond sources. An analysis of field relations and sedimentology in WMG shows that the majority of the diamonds have been transported by glacial events from the São Francisco Craton further east.     

Natural analogues for the transuranic actinide elements: An investigation in Minas Gerais,Brazil

A highly weathered deposit of thorium and rare earth elements located near the summit of a hill in the state of Minas Gerais, Brazil, is being studied as an analogue for a radioactive waste repository that sometime in the distant future may be eroded to the surface or intruded by groundwater Thorium serves as an analogue for Pu⁴⁺, and La³⁺ as an analogue for Cm³⁺ and Am³⁺ The mobilization rate constants of the analogue elements by groundwater are so slow (of the order of 10⁻⁹ per year) as to suggest that essentially complete radioactive decay of the transuranic actinides would occur in place even under the relatively unfavorable conditions that exist at a site such as this     

Ultramafic complexes and associated mineral deposits in the Precambrian of eastern Minas Gerais,Brazil

Based on field work, prospection and petrographic studies ultramafic complexes and their mineral deposits from eastern Minas Gerais in southeastern Brazil are generally outlined to form a basis for future investigations in this region. The bodies dealt with occur at Ipanema, Córrego Novo, Bela Vista de Minas, Rio Pomba and Liberdade. These ultramafic bodies are generally enclosed in high-grade gneisses and consist of serpentinized peridotites and harzburgites which were metamorphosed together with their country rocks in upper amphibolite to granulite facies. Weathering of these rocks gives rise to nickeliferous laterite, while metamorphism has resulted in anthophyllite asbestos and talc deposits.     

Supergene leaching and formation of platinum in alluvium: evidence from Serro, Minas Gerais, Brazil

Summary The Córrego Bom Sucesso alluvial deposit near Serro, Minas Gerais, probably provided the specimens from which the element palladium was first discovered. Its Pt–Pd nuggets are characteristically botryoidal, arborescent and coralloidal, and exhibit an external halo with the composition of palladiferous platinum to virtually pure platinum. X-ray mapping of an arborescent Pt–Pd nugget from the historical occurrence documents selective palladium depletion, similar to the high-fineness gold haloes developed on detrital Au–Ag grains under supergene conditions. The Pd-depleted alteration zone truncates inclusions of crystals stoichiometrically close to PdPt within the Pt–Pd aggregate. Selective metal leaching in the weathering environment can account for the frequently observed Pt-enriched rims on alluvial platiniferous alloys which may lead to the formation of platinum nuggets at an advanced degree of weathering. Present address: Department of Geology: Exploration Geology Rhodes University PO Box 94, Grahamstown, 6140 South Africa     

Titanosaur (Dinosauria: Sauropoda) Osteoderms from the Maastrichtian of Uberaba,Minas Gerais State,Brazil

Titanosaur remains are common findings at the paleontological site of Peirópolis in Uberaba. Among those remains, two osteoderms referred to titanosaur sauropods were reported. Both dermal bones share many features, such as the coarse texture, parallel ventral grooves, lack of the cingulum and presence of a ventral ridge (crest). Armored titanosaurs had wide distribution in the Upper Cretaceous, and have been reported from Argentina, Brazil, France, Madagascar, Malawi, Romania and Spain. In the present work we describe and compare the Brazilian titanosaur osteoderms found thus far.     

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.     

Characterization of beryl (aquamarine variety) from pegmatites of Minas Gerais,Brazil

Eight samples of the beryl variety aquamarine were selected from four pegmatites in the Governador Valadares and Araçuaí regions in northeastern Minas Gerais State, Brazil. These samples were fully characterized by chemical analysis, infrared and UV-visible spectroscopy, thermal analyses, and high-temperature X-ray diffraction (from room temperature up to 800 °C). Several physical and chemical properties of beryl were found to depend on the amount of water and ions residing in the structural channels. The thermal expansion coefficients from room temperature to about 800 °C are temperature-independent, with αa ? ?3.2 × 10^?6 ° C^?1 and αc ? ?8.7 × 10^?6 ° C^?1. The contraction of both a and c unit-cell parameters with increasing temperature and the shift of the infrared band centered at about 1200 cm^?1 were tentatively ascribed to interactions between channel water and the silicate rings.The color of beryl seems to be dictated by the relative proportions of Fe³⁺ in the octahedralsites and of fe²⁺ in the channels. Thus, deep-blue samples have little Fe³⁺, whereas greener samples have more Fe³⁺ or less channel Fe²⁺.     

Arsenic in the groundwater of Ouro Preto (Brazil): its temporal behavior as influenced by the hydric regime and hydrogeology

In the city of Ouro Preto (MG), water catchment for public supply originates from superficial drainage, springs, old abandoned mines and some driven wells. In the rocks of the region, As is originally found in gold-enriched sulphide-bearing mineral deposits. The weathering process introduces As into the hydrological system by dissolution of this element into the leachate. Measurement of the As content in the groundwater of some catchments was carried out during 1 year and these measurements demonstrated high As content—up to 224 μg L⁻¹ of As(V)—during the rainy season (the maximum concentration limit according to World Health Organization is 10 μg L⁻¹). Lower values were observed during the dry season and in some sampling stations, As was not even detected. The As concentration variability during 1 year shows a strict and direct relationship to seasonal and hydrological conditions. For city authorities, responsible for public water supply, it is necessary to perform a complete inventory of the water sources used and constantly monitor the As content in the water.     

Titanosaur (Dinosauria: Sauropoda) Osteoderms from the Maastrichtian of Uberaba, Minas Gerais State, Brazil

Titanosaur remains are common findings at the paleontological site of Peirópolis in Uberaba. Among those remains, two osteoderms referred to titanosaur sauropods were reported. Both dermal bones share many features, such as the coarse texture, parallel ventral grooves, lack of the cingulum and presence of a ventral ridge (crest). Armored titanosaurs had wide distribution in the Upper Cretaceous, and have been reported from Argentina, Brazil, France, Madagascar, Malawi, Romania and Spain. In the present work we describe and compare the Brazilian titanosaur osteoderms found thus far.     

Influence of the natural oxidation of the leachate organic fraction from a landfill on groundwater quality, Belo Horizonte: Minas Gerais, south-eastern Brazil

The objective of this work was to evaluate the influence of the natural degradation of the leachate organic fraction from the Belo Horizonte landfill on groundwater chemical evolution. The work focused on the modifications introduced by redox reactions in the presence of terminal electron acceptors. Twenty-one sampling points distributed along 13 monitoring wells were selected for the analysis of the chemical indicators of interest (TDS, SO ₄ ^?2 , Fe⁺², Ba, pH and Eh). The behaviour of the variables involved in the alteration of the water quality was assessed by the spatial distribution of target parameters, elaboration of redox diagrams and chemical modelling that focused on the determination of mineral saturation indexes. The study showed a trend toward pyrite precipitation, which leads to the removal of chemical species such as divalent iron and sulphur as sulphide from the system. This removal disturbs the chemical equilibrium, typically by moving the reactions to replenish the sulphate concentration present in the groundwater. This process occurs primarily through the dissolution of compounds that have sulphate in their chemical composition, such as barite, suggesting that part of the barium concentration in the subsurface can be of geogenic origin. This study demonstrated the importance of knowing the nature of the geochemical processes in groundwater contaminated by urban solid waste.     

An application of lithogeochemistry to the evaluation of the Ni-sulphide ore potential of weathered serpentinites in the Fortaleza de Minas Greenstone Belt,Minas Gerais,Brazil

A practical application of lithogeochemistry to the classification of weathered serpentinites as to whether or not serpentinites are host rocks to Ni-sulphide ore is presented. An effort is made to apply existing concepts used to distinguish between fertile and sterile unweathered ultramafics to explain the observed chemical differences between fertile and sterile weathered serpentinites in southwestern Minas Gerais, Brazil.The data comprise total-attack chemical analyses for Al, Ca, Cr, Cu, Fe, Mg, Mn, Ni, Si, Ti, V, and partial-attack determinations of Cu and Ni as ascorbic acid, hydrogen peroxide-soluble metal. Student's t-tests are used to identify element concentration differences between the two serpentinite groups. Discriminant analysis is used to classify the weathered fertile and sterile serpentinites. Non-linear mapping and fuzzy c-means clustering help to assess the validity of the discriminant analysis. Finally, chemical element associations provide extra information on geological and mineralogical causes that can be responsible for the identified differences.It is found that a clear separation and classification of weathered serpentinites is possible using the procedures presented. The observed chemical differences between fertile and sterile serpentinites are interpreted in terms of original magmatic differentiation, the degree of sulphur saturation of the original magma and the behavior of the derived ultramafic rocks in a tropical weathering environment.     

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.     

Characterization and origin of spongillite-hosting sediment from João Pinheiro,Minas Gerais,Brazil

Spongillite from João Pinheiro, Minas Gerais, Brazil is mainly known for its use in brick production and in the refractory industry. Very few studies have focused on its geological context. Spongillite-rich deposits occur in shallow ponds on a karstic planation surface developed on rocks of the Neoproterozoic São Francisco Supergroup. Cenozoic siliciclastic sediments are related to this surface. A field study of these deposits and analysis of multispectral images showed a SE–NW preferential drainage system at SE, suggesting that Mesozoic Areado Group sandstones were the source area of the spongillite-hosting sediments. Mineralogical and textural characterization by optical microscopic analysis, X-ray diffraction (XRD), differential and gravimetric thermal analysis (DTA-GTA), infrared spectroscopy (IR) and scanning electron microscopy (SEM) of seven open-pit spongillite-rich deposits (Avião, Carvoeiro, Vânio, Preguiça, Divisa, Severino, Feijão) showed a sedimentological similarity between the deposits. They are lens-shaped and are characterized at the bottom by sand facies, in the middle by spicules-rich muddy-sand facies and at the top by organic matter-rich muddy-sand facies.Petrographically, the spongillite-hosting sediments and the siliclastic sediments of the Areado Group show detrital phases with similar mineralogical and textural features, such as the presence of well-sorted quartz grains and surface features of abrasion typical of aeolian reworking that occurred in the depositional environment in which the sandstones of the Areado Group were formed. Detrital heavy minerals, such as staurolite, zircon, tourmaline, and clay minerals, such as kaolinite, low amounts of illite, scarce chlorite and mixed-layer chlorite/smectite and illite/smectite occur in the spongillite-hosting sediments and in sandstones from the Areado Group. In both formations, staurolite has similar chemical composition. These mineralogical and textural features show that the sediments of the Areado Group constitute the main source of the pond sediments that host spongillite.     

Geochemistry and origin of lower crustal granulite facies rocks in the Serra do Caparaó region, Espirito Santo/Minas Gerais, Brazil

Rocks of the Serra do Caparaó region, on the border between the Brazilian states of Espirito Santo and Minas Gerais, comprise granulitic (charnockitic) sequences that have undergone Brasiliano-aged metamorphism (586 ± 2 Ma.). Thermobarometric results document pressures between 8.1 kb and 10.6 kb and temperatures up to 700 ± 50°C. Chemical analysis show a calc-alkaline (CAC) trend with granodioritic, tonalitic-trondhjemitic chemistry. K₂O/Rb data represent low- to mid-pressure charnockites corresponding to the undepleted granulite types. Discriminant analyses and diagrams indicate a source rock characteristic of sedimentary origin. REE patterns show slight to moderate fractionation and also slight negative as well as positive Eu anomalies. REE trends and the total amounts of REE of the acidic to basic rock series are similar to those of North American and Australian post-Archaean shales. The granulite facies rock series in the Serro do Caparaó region are interpreted as having originated from supracrustal rock series—i.e., graywackes, which represent sedimentary sequences derived from a Lower Proterozoic bimodal magmatic suite.     

Minor and trace-element data of iron oxides from iron-formations of the Iron Quadrangle,Minas Gerais,Brazil

Summary Chemical investigations of 192 hand-picked iron-oxide concentrates from banded iron-formation of the Iron Quadrangle, Brazil, did not yield any significant difference in minor and trace-element contents between itabirite and iron-ore samples (Fe >63%). Mean values (171–177 samples) are: Ca 82 ± 12 ppm; Co 120 ± 8; Cr 77 ± 8; Cu 40 ± 2; Mg 40±5; Mn 170±L28; Ni 167±7; P 905±23; Ti 114±13; V 138±8; Al 1068 ± 174 ppm (52 samples) and Au 33 ± 8 ppb (50 samples). Microprobe results indicate different trends of Fe-substitution by minor elements depending on the different lattice types of magnetite and hematite. Mg and Mn are much more concentrated in magnetite than in hematite whereas hematite is distinctly enriched in titanium due to the isostructural nature of the hematite and ilmenite lattices.

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Zusammenfassung Chemische Analysen von 192, unter dem Binokular ausgelesener Eisenoxidkonzentrate von Proben aus den Eisenerzlagerstätten des Eisernen Vierecks in Brasilien, ergaben keinerlei Unterschiede in den Neben- und Spurenelementgehalten zwischen den Itabiriten und den Reicherzen (Fe >63%). Die Mittelwerte von jeweils 171–177 Elementbestimmungen ind folgende: Ca 82± 12 ppm; Co 120± 8; Cr 77± 8; Cu 40± 2; g 40±5; Mn 170±28; Ni 167±7, P 905±23; Ti 114:L13; V 138±8, Al 1068 ± 174 ppm (52 Proben) und Au 33 ± 8 ppb (50 Proben).Aus Mikrosondenuntersuchungen ergeben sich in Abhängigkeit von den unterschiedlichen Kristallgittertypen des Magnetits und Hämatits verschiedene Substitutionsraten des Eisens durch Nebenelemente. So zeigen Mg und Mn in Magnetiten um eine Vielfaches höhere Werte als in Hämatiten, während Ti im Hämatit wegen der Isotypie des Hämatit-Ilmenit-Gitters deutlich angereichert ist.

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With 1 Figures     

Metamorphism and PGE-Au content of chromitite from the Ipanema mafic/ultramafic Complex, Minas Gerais, Brazil

Summary The Santa Cruz massif, which forms part of the Ipanema mafic/ultramafic Complex, Minas Gerais, Brazil, has an exposed upward sequence of metadunite, metaharzburgite (including three separate chromitite layers), metapyroxenite, metagabbro, and meta-anorthosite. Primary igneous chromite grains in the main chromitite layer are poikiloblastic and tectonically fragmented, and have a narrow (10–20 μm) margin of chromian spinel. Cataclased chromite fragments are extensively replaced and mantled by chromian spinel; they have a composite margin comprised of an inner zone of more aluminous spinel and an euhedral outer zone of more Cr-rich spinel, representing granulite and amphibolite facies metamorphic events, respectively. The contents of platinum-group elements (PGE) and Au in chromite separates are relatively high (Os 45, Ir 23, Ru 136, Rh 19, Pt 98, Pd 63, and Au 83 ppb), and significantly enriched (∼ 4x) over whole rock values. Platinum-group minerals are not observed and micrometre-sized inclusions of sulfide minerals (chalcopyrite and pentlandite) in relict chromite are rare. However, comparison of mineral proportions in the separated chromite and whole rock shows that the precious metals are hosted predominantly in the relict igneous chromite grains, rather than the secondary chromian spinel and primary and secondary Mg-rich silicates. The major element composition and average chondrite-normalized PGE pattern of the separated chromite correspond to S-poor stratiform chromitite. We suggest that the precious metals accumulated with chromite during crystallization of a S-poor magma, and were not remobilized in the relict chromite during the subsequent high grade metamorphism.

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Zusammenfassung Metamorphose und PGE-Au-Gehalte von Chromitit aus dem mafischen/ultramafischen Ipanema Komplex, Minas Gerais, Brasilien Das Santa Cruz Massiv, das Teil des mafischen/ultramafischen Ipanema Komplexes, Minas Gerais, Brasilien, ist, beinhaltet eine (von unten nach oben) Abfolge von Metadunit, Metaharzburgit (mit drei Chromititlagen), Metapyroxenit, Metagabbro und Metaanorthosit. Prim?re magmatische Chromitk?rner in der Chromit-Hauptlage sind poikiloblastisch, tektonisch fragmentiert und werden von einem dünnen (10–20 m) Saum von Chromspinell umgeben. Kataklastische Chromitfragmente sind intensiv umgewandelt zu und werden ummantelt von Chromspinell. Dieser mehrphasige Saum beinhaltet eine innere Zone mit Al-reicherem Spinell und eine euhedrale Au?enzone mit Cr-richerem Spinell, die sich bei granulit- bzw. amphibolitfaziellen Metamorphoseereignissen gebildet haben. Die Gehalte an Platingruppen-Elementen (PGE) und Au in Chromit-Separaten sind relativ hoch (Os 45, Ir 23, Ru 136, Rh 19, Pt 98, Pd 63, Au 83 ppb) und signifikant angereichert (∼ 4-fach) im Vergleich zum Gesamtgestein. Platingruppen-Minerale sind nicht zu beobachten und Mikrometer gro?e Einschlüsse von Sulfiden (Chalcopyrit und Pentlandit) in den Chromitrelikten sind selten. Der Vergleich der Mineralverh?ltnisse in den separierten Chromiten und Gesamtgesteinen zeigt aber, dass die Edelmetalle haupts?chlich in den reliktischen Chromitk?rnern sitzen und nicht so sehr in den sekund?ren Chromspinellen und Mg-reichen Silikaten. Die Hauptelement-Zusammensetzung und die Chondrit-normierten PGE Muster der separierten Chromite stimmen mit denen S-armer stratiformer Chromite überein. Wir vermuten, dass die Edelmetalle sich mit Chromit bei der Kristallisation eines S-armen Magmas akkumulierten und w?hrend der anschlie?enden hochgradigen Metamorphose nicht remobilisiert wurden.

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Received April 3, 2000; revised version accepted October 12, 2000     

A discussion on the lead isotope geochemistry of galenas from the Bambui group,Minas Gerais — Brazil

Lead isotope data on galenas from the Bambui group, Minas Gerais have been assembled and discussed. The isotope data, when plotted on the Single Stage Model III lead isotope development curve of Cumming and Richards (1975), yield 207/206 ages of 650±50 m.y and a linear array of data points with negative model ages, which intercept the growth curve at 650±60 m.y and 1850±150 m.y. The radiometric dates coincide with the two major orogenic episodes, the Brazilian and the Transamazonian. The data are interpreted in terms of a two stage model, where the source of lead for galenas, formed during Brazilian orogeny, is the basement rocks of Transamazonian age. The calculated Th/U value for the source rock is 2.9±0.3. The geographical distribution of lead isotope ratios shows a zonation in Vazante Paracatu region, an area containing large deposits of zinc. The implication of the distribution on the ore formation is discussed.