Iron ore mining promotes iron enrichment in sediments of the Gualaxo do Norte River basin,Minas Gerais State,Brazil

The objective of the present study is to assess the influence of iron ore mining on Fe concentrations in fluvial sediments of the Gualaxo do Norte River basin (Minas Gerais State, Brazil). Initially background values were determined for Fe by means of the boxplot representation, the iterative 2σ technique and the calculated distribution function, using geochemical data obtained for alluvial terraces. Geochronologic analyses in carbonaceous materials attested for the proposed background values. After that, chemical analyses of active drainage sediments collected along the entire basin were carried out and a geochemical map of the study area was prepared. The mean background value for Fe is relatively high (8.2 wt %), when compared to the Fe concentration in the Earth’s crust (5.6 wt %). Such background value is strongly associated with the geological characteristics of the region. It was observed that most of the high Fe concentrations were detected in active sediments collected in the main river of the basin and not in its tributaries. This suggests that Fe enrichment is not only linked to the local geology but also to possible increase resulting from accidental leakage of iron ore mining tailings from containment dams and lakes upstream. It is suggested that measures that help stop or minimize such impact be adopted. The large capacity of Fe to adsorb and form complexes with toxic metals implicates in environmental hazards that may not be restricted only to the Gualaxo do Norte river basin, but may affect the other basins downstream.     

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.     

Sea level fluctuation during the past 6000 yr at the coast of Mauritania

In an area regarded to be very favorable for the study of Holocene sea level changes one or several eustatic (?) oscillations of sea have been found using sedimentological and ecological methods. After a maximum of +3 m during the Nouakchottian stage (= Middle Flandrian or Late Atlantic) about 5500 YBP a drop of sea to ?3.5 ± 0.5 m about 4100 YBP is testified by stromatolitic algae indicating the former sea level within the tidal zone with high accuracy. This evidence is supported by the observation of post-Nouakchottian regressive and transgressive geologic sequences, by buried beach deposits and flooded hardgrounds, post-Nouakchottian marine terraces of different height and age, the cutting off of one large and several small bays from the open sea, etc. Possibly one or two smaller oscillations followed between 4000 and 1500 YBP (derived sea level curve Fig. 3).     

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.     

The Diamantina Monazite: A New Low‐Th Reference Material for Microanalysis

Most monazite reference materials (RMs) for in situ U‐Pb geochronology are rich in Th; however, many hydrothermal ore deposits contain monazite that is low in trace element contents, including Th, U and Pb. Because of potential problems with matrix effects and the lack of appropriate matrix‐matched RMs, such variations can bias dating of hydrothermal deposits. In this paper, we describe a polycrystalline low‐U and low‐Th Diamantina monazite from the Espinhaço Range, SE Brazil. It has a U‐Pb ID‐TIMS weighted mean ²⁰⁷Pb^*/²³⁵U ratio of 0.62913 ± 0.00079, ²⁰⁶Pb^*/²³⁸U of 0.079861 ± 0.000088 and ²⁰⁷Pb^*/²⁰⁶Pb^* of 0.057130 ± 0.000031, yielding a weighted mean ²⁰⁶Pb^*/²³⁸U date of 495.26 ± 0.54 Ma (95% c.l.). In situ dates acquired with different methods (LA‐(Q, SF, MC)‐ICP‐MS and SIMS) are within uncertainty of the ID‐TIMS data. U‐Pb LA‐(Q, MC)‐ICP‐MS runs, using Diamantina as a primary RM, reproduced the ages of other established RMs within < 1% deviation. The LA‐MC‐ICP‐MS analyses yielded homogeneous Sm‐Nd isotopic compositions (¹⁴³Nd/¹⁴⁴Nd = 0.511427 ± 23, 2s; ¹⁴⁷Sm/¹⁴⁴Nd = 0.1177 ± 13, 2s) and εNd_(495 Ma) of ?18.7 ± 0.5 (2s). SIMS oxygen isotope determinations showed measurement reproducibility better than ± 0.3‰ (2s), confirming Diamantina's relative homogeneity at test portion masses below 1 ng.     

Chemical and mineralogical evidence of the occurrence of mantle metasomatism by carbonate-rich melts in an oceanic environment (Santiago Island, Cape Verde)

Lavas from Santiago Island attest to a complex magmatic history, in which heterogeneous mantle source(s) and the interactions of advecting magmas with thick metasomatised oceanic lithosphere played an important role in the observed isotopic and trace element signatures. Young (<3.3 Ma) primitive lavas from Santiago Island are characterised by pronounced negative K anomalies and trace element systematics indicating that during partial melting D_K>D_Ce. These features suggest equilibration with an oceanic lithospheric mantle containing K-rich hydrous mineral assemblages, consistent with the occurrence of amphibole + phlogopite in associated metasomatised lherzolite xenoliths, where orthopyroxene is partially replaced by newly formed olivine + (CO₂ + spinel + carbonate inclusion-rich) clinopyroxene. Metasomatism induced a decrease in $ a ^{{{\text{melt}}}}_{{{\text{SiO}}_{{\text{2}}} }} $ and Ti/Eu ratios, as well as an increase in fO ₂ , Ca/Sc and Sr/Sm in the Santiago magmas, suggesting a carbonatitic composition for the metasomatic agent. Santiago primitive lavas are highly enriched in incompatible elements and show a moderate range in isotopic compositions (⁸⁷Sr/⁸⁶Sr?=?0.70318–0.70391, ¹⁴³Nd/¹⁴⁴Nd?=?0.51261–0.51287, ¹⁷⁶Hf/¹⁷⁷Hf?=?0.28284–0.28297). Elemental and isotopic signatures suggest the involvement of HIMU and EM1-type mantle end-members, in agreement with the overall isotopic characteristics of the southern Cape Verde Islands. The overall geochemical characteristics of lavas from Santiago Island allow us to consider the EM1-like end-member as resulting from the involvement of subcontinental lithospheric mantle in the genesis of magmas on Santiago.     

Surface water quality and sediment geochemistry in the Gualaxo do Norte basin,eastern Quadrilátero Ferrífero,Minas Gerais,Brazil

This paper describes the geochemistry of water and sediment samples downstream from two iron mines in the Gualaxo do Norte River basin, north of Mariana city, Minas Gerais state, Brazil. Electric conductivity, total dissolved solids, pH, E_h, Na, K, Ca, Mg and trace elements (Fe, Mn, Al, Cd, Ni, Cr, Zn, As, Pb, Co, Sb, Cu, Mo, Rb, Sr, Ti, V, Y, Zr and Ba) have been analysed in water and sediments. The observed Na anomalies are mainly associated with the iron mine processing plants. Enriched concentrations of iron and manganese were only observed in rudimentary gold exploitation areas (prospectors) and where supergenic alteration of the banded iron formation has occurred. Iron and manganese background values are in some places higher than the limits established by Brazilian legislation. Mn, As, Sb and Ba anomalies have been observed in sediments. Their main source is associated with the sulphide-rich mineral deposits in the spring of the Gualaxo do Norte River and not directly related to the present iron exploitation in the region. Background values are proposed for the studied region.     

Evaluation of the mineral exploration influence on sediment composition in the Gualaxo do Norte River Basin (MG-Brazil) based on geochemical and stratigraphic data

The aim of this study was to investigate the influence of mining on the geochemical composition of the alluvial sediments of the Gualaxo do Norte River Basin, located in the eastern-southeastern portion of the Quadrilátero Ferrífero (Minas Gerais State, Brazil). The analysis of the sedimentological succession was carried out using 16 stratigraphic sections (cutbanks and alluvial terraces) and analysis of 111 sediment samples for major- and trace-elements by ICP-OES. The stratigraphic analysis revealed the deposition history of the collected sediments. Anomalous concentrations of certain elements such as As and Pb were found, which are associated with stratigraphic facies with evidences of gold artisanal mining, and Fe anomalies associated with iron ore mining. The classification of the sections was possible by means of principal component analysis. From the sediment characteristics, three groups were identified: (1) those influenced by iron ore mining and gold artisanal mining; (2) those influenced by iron ore mining and (3) those with no influence of human activities.     

Influence of geology in the geochemistry signature of Itacolomi State Park waters,Minas Gerais-Brazil

In all geological scenarios, mineral water reactions will affect the water chemistry. As such, water resources in different rocks commonly involve different hydrogeological compartments. The aim of this work is to evaluate the influence of geology in the geochemistry signature of Itacolomi State Park waters. To do so, a survey of the geological units in the area was carried out, a geological/stratigraphic division was made, and its correlation with the main geological events was determined. Using the advantages of GIS, all the catchments were delimited. Based on this division, near 30 stream and lake segments were chosen for analyses. In each point, all physiochemical properties of the water were measured, and samples were collected to determine the concentrations of major and trace elements by ICP–OES. The dynamics of the Itacolomi State Park rock-soil and stream water solutions suggest that mixing of drainage waters from different bedrock and soil sources regulates stream water physical–chemical parameters and solute concentrations. The analytical data showed a clear correlation between the chemical compositions of the solute and the geological characteristics of the catchment. Units that are covered by iron oxide hardpan (Manso unit) and iron-banded formations (Custódio unit) show a large amount of soluble elements, including high values of Fe and Mn. On the other hand, the presence of high values of Al and K (Itacolomi unit) are a direct consequence of the presence of quartzite associated with low pH values.     

Zur Mikrofazies kalkiger Sedimenteinschaltungen in Vulkaniten der andinen Geosynklinale Mittelchiles

Zusammenfassung In den vorherrschend vulkanischen Serien des Neokoms im westlichen Teil der andinen Geosynklinale Mittelchiles kam es nur untergeordnet zur Ausbildung kalkiger Sedimente. Die bedeutenden Faziesunterschiede dieser Flachwassersedimente lassen sich durch ein intensives, durch unregelmäßige Lavaströme erzeugtes Kleinrelief erklären. Der, die Biozönosen bestimmende ökologische Hauptfaktor war die Sauerstoffversorgung. Biozönosen des Hyperpneustons (Brandungszonen auf Lavarücken), umgeben von grobem Schalenschutt, lagen dicht neben euxinischen, lebensfeindlichen Schlammzonen (Apneuston). Die Kalkbildung beruhte weitgehend auf Schalendetritus. Anzeichen einer anorganogenen Kalkfällung fehlen im Hauptgebiet des Vulkanismus. Westlich Santiago bildeten die Ausbruchszentren geschlossene Inselbarrieren mit nur kleinen Meeresbuchten, in denen jedoch der Anteil planktonischer Organismen bemerkenswert ist. Nach Norden zu, mit zurücktretendem Vulkanismus, herrschten größere zusammenhängende Meeresbecken mit zunehmend besseren Lebensbedingungen vor.

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The carbonatic sediments are poorly developed in the predominant volcanic series of the Neocomian Age in the western part of the Andean Geosyncline. The significant facies-differentiations of these shallow water sediments can be explained by an intensive fine-relief formed by irregular lava-flows. The supply of oxygen was the main factor involved in the distribution of life associations. Communities of a hyperpneuston (a zone of strong water agitation on the crests of lavaflows) surrounded by coarse fossil debris, joined euxinic, abiotic muddy-bottom zones. The formation of carbonatic sediments is mainly based on detritical shelldebris. In the main region of volcanism the anorganic precipitation of carbonatic sediments is lacking.West of Santiago the centers of eruptions formed closed barriers of islands, in which the sea formed small bays containing an astonishing amount of planktonic organisms. Towards the north more extensive marine basins dominated with better conditions for life, because the volcanism diminished.

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Resumen Los sedimentos calcáreos quedan bastante subordinados como intercalaciones en las secuencias principalmente volcánicas del Neocomiano de la parte occidental del geosinclinal andino en Chile Central. Los cambios laterales de las facies, que son muy notables en estos sedimentos de aguas someras, se explican por un relieve muy intenso, producido por coladas irregulares de lava. La aireacíon (abasto de oxígeno) fue el factor ecológico principal en la distribucíon de las biocenosis. Las biocenosis de un hyperpneuston (zona de resaca en las crestas de las coladas) se encontraban rodeadas de conchuela gruesa, y se ubicaban muy cerca de zonas euxínicas y fangosas, sin vida (apneuston). La formatión de calizas dependía principalmente de la producción de conchas calcáreas. La precipitación anorgánica de carbonato de calcio, ausente en la zona de volcanismo dominante, existió sin embargo más al Norte, Al oeste de Santiago los volcanes formaban barreras de islas y el mar penetraba solamente en bahías y engolfamientos restringuidos. Sin embargo, el contenido de restos organicos planctónicos es notable. Hacia al Norte, junto con una disminuación del volcanismo, se extendían cuencas marinas más amplias, con un mejoramiento de las condiciones para los organismos.

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Vortrag gehalten auf der Jahrestagung der Geologischen Vereinigung in Wien, Februar 1966.