Joachim Karfunkel

Late Proterozoic glaciation in central-eastern Brazil: Synthesis and model

Abstract In the surroundings of the Middle Proterozoic Sao Francisco Craton of central-eastern Brazil, numerous traces of an ancient glaciation can be found. Almost all criterua necessary to prove an ancient glaciation occur, including scratched and facetted pebbles, a striated surface, quarrying, outwash plains, eskers, varvites, dropstones, “decollement” structures etc. Poorly sorted tillites with varying pebble composition are widely distributed. Continental tillites and glaciomarine sequences interfinger in the east and southeast of the craton. Traces of ice movement suggest flow directions of the ice towards the east and the southeast of the craton. At the coast, the wet-base glaciers passed over to floating shelf ice. In the northeast, glaciers also came from the Serrinha Craton in northeastern Bahia. The glaciation covered an area of at least 1300 × 700 km. The sequences containing traces of glaciation discordantly overlie Archean to Middle Proterozoic rocks. They in turn are overlain by Late Proterozoic calcareous and psammopelitic sediments of the Bambui Group. Present data indicate that glaciation took place around 1000 Ma ago.

Spectroscopic characterization of transition metal impurities in natural montebrasite/amblygonite

Abstract Natural single-crystal specimens of the montebrasite/amblygonite series from Brazil, with general formula LiAlPO4(F,OH), were investigated by electron microprobe, Raman spectroscopy, X-ray diffraction, and infrared absorption. Since little is known about impurities and their local symmetries, electron paramagnetic resonance (EPR) was applied. Six different paramagnetic impurities and radiation defects were detected by EPR. Three of them, all substituting for Al3+ ions, namely, iron (Fe3+), vanadium (V4+), and niobium (Nb4+) impurities were characterized in this work. The Fe3+ (3d5)-related EPR spectra and angular dependencies show occupation of low-symmetry sites that are revealed in the high asymmetry parameter of the electronic fine structure, E/D = 0.27. Vanadium and niobium impurities are identified through their typical strong hyperfine interactions. Both form interesting examples for which the properties of 3d1 ion (V4+) and 4d1 ion (Nb4+) in the same host matrix can be compared. It is shown that both ions form complex defects of type VO2+ (vanadyl) and NbO2+ (niobyl), showing superhyperfine interaction with two equivalent hydrogen ions and not to fluorine. The EPR rotation patterns are analyzed in detail for three mutually perpendicular crystal planes. Spin Hamiltonian parameters are calculated and discussed.

The Basal Conglomerate of the Capacete Formation (Mata da Corda Group) and its Relation to Diamond Distributions in Coromandel, Minas Gerais State, Brazil

The diamond bearing district of Coromandel is located in the northwestern part of Minas Gerais, within the Alto Paranaiba Arch, famous for the discovery of most of Brazils large diamonds above 100 ct. Detailed mapping, aimed at characterizing the Mata da Corda Group of Upper Cretaceous age of Coromandel, has been carried out. This Group was divided into the Patos Formation, composed of kimberlitic and kamafugitic rocks, and the Capacete Formation, presented by conglomerates, pyroclastic rocks, arenite and tuffs. Exposures of the latter Formation have been studied in detail at the small abandoned mine called Canastrel, as well as in the headwater of Santo Antonio do Bonito River. The results have been compared to studies of the kimberlite bodies in the nearby Douradinho River. Kimberlite indicator minerals from these localities show the same compositional trend. Moreover, in the basal conglomerate of the Garimpo Canastrel two diamonds diamonds have been recovered and described. The Garimpo Wilson, situated in the headwater of the river Santo Antonio do Bonito in paleo-alluvium, is composed of material exclusively derived from the erosion of the Capacete Formation and Precambrian (sterile) Canastra quartzites and schists. These detailed investigations suggest that the basal conglomerates of the Capacete Formation represent the main source rock of the alluvial diamond deposits in the Coromandel region.

Resources for a growing city - sand extraction north of Belo Horizonte (Brazil)

To integrate essential mass resources for urbanisation into a sustainable land use planning, their amount and location and the consequences of their exploitation have to be analysed beforehand. A multi-temporal case study regarding the sand extraction in a small, exemplary area north of Belo Horizonte has been performed which analyses the dynamic of sand extraction during the last decades and its consequences for environment and land use change. Additionally, the sedimentological architecture of the river system has been studied in an open sand pit to estimate the sand content and the fluvial style of the Quaternary sediments. Based on these case studies, a rough assessment has been made regarding the total sand resources available in this fluvial system. Nevertheless, more detailed studies of the Quaternary sediments are necessary to quantify and locate important sand yielding deposits.

Fluid phases in carbonado and their genetic significance

Using the method of pyrolysis—gas chromatography, the composition of fluid inclusions in Brazilian carbonado and associating monocrystalline diamonds were investigated for the first time. Temperature trends were obtained for the composition of the released gas phase; based on this data, it was concluded that the diamonds include two generations of inclusions: those decrepitating at temperatures below 500°C and above 500°C. It was found that the fluid released from carbonado at high temperatures is anomalously enriched in carbon monoxide, which is strongly different from the characteristics of both the products of intracrustal mineral, rock, and ore formation and the results of crust—mantle interaction.

Diamonds from Minas Gerais, Brazil: An Update on Sources, Origin, and Production

The State of Minas Gerais (MG) was the main diamond producer worldwide for almost 150 years. Today, Brazil contributes about 1.5% to the world diamond production, with 220,000 carats (ct) coming from MG; the two principal source areas are located in the Diamantina region of Espinhaco province and in western MG. Although many theories have been advanced to explain the primary source of diamonds in MG, all commercial production is from secondary deposits. This paper reviews the geology of these deposits and emphasizes the importance of the redistribution of diamonds in Cretaceous and younger sediments. The Espinhaco diamond province, with distinct diamond districts and fields, varies in average grade from 0.008 to over 0.2 ct/m3; however, immense reserves, such as the Jequitinhonha River gravels, locally can reach 400,000,000 m3 or more. However, the largest stones weigh only several tens of carats, unlike in Western MG (which supplied practically all of Brazils large stones), where many weigh over 100 ct....

GIS-based analysis of geo-resources and geo-hazards for urban areas - the example of the northern periphery of Belo Horizonte (capital of Minas Gerais, Brazil)

Easily understandable thematic maps of geo-scientific parameters are important for land use decision making. If several parameters are relevant and have to be compared, it is important that they are consistent with each other, available at the same spatial range and detail and normed to a common data range. In the current study, geological and topographical data have been used to derive a set of 90 geo-scientific maps for an area of 400 km² in the northern part of the metropolitan area of Belo Horizonte. Each parameter has been transferred to a common data range between 0 and 1 using a Semantic Import Model strategy and afterwards combined to derive new parameters for soil hydrology and hydrogeology. From these, many intermediate geo-scientific parameters, maps of geo-resources (sand/gravel, carbonates, fertile soils) and geo-hazards (erosion, groundwater pollution) have been derived that they can be used as base information for a participatory and sustainable land use planning. The workflow is transparently stored in GIS-tools and can be modified and updated if new information is available.

Geoquímica e proveniência sedimentar da Formação Uberaba (sudeste do Triângulo Mineiro, MG)

Manuscript ID: 20170032. Received in: 01/12/2017. Approved in: 04/26/2017. RESUMO: A Formação Uberaba, de idade campaniana e ocorrência restrita à região da cidade homônima, é composta de arenitos líticos e conglomerados de cor esverdeada, com possível contribuição vulcanoclástica. Nessa unidade ocorrem minerais pesados como ilmenita, granada, perovskita e magnetita, além de clinopiroxênio, plagioclásio, quartzo, calcita e apatita. A Formação Uberaba é marcada geoquimicamente por altos teores em Ba, Ta, La, Nb e Th, com padrões de elementos terras raras normalizados ao condrito fortemente fracionados (LaN/YbN = ca. 128) e planos (sem anomalia de Eu). Análises de química mineral das granadas detríticas da Formação Uberaba indicam predominância de chorlomita com afinidade com granadas crustais (G3). A proveniência sedimentar da Formação Uberaba corresponde a uma mistura de materiais de distritos alcalinos do Alto Paranaíba e derivados da erosão da Formação Serra Geral e dos grupos Canastra e Araxá. Os conglomerados produtores de diamante que ocorrem proximamente, em Romaria, foram considerados por alguns autores como correlatos à Formação Uberaba, porém a composição das granadas retiradas desses conglomerados é bastante diferente, plotando no campo de granadas mantélicas (G9/G10). Descarta-se, dessa forma, correlação entre essas duas unidades, e a Formação Uberaba, portanto, não é uma fonte provável dos diamantes aluvionares encontrados no rio homônimo.

Diamonds from Borrachudo river, São Francisco basin (Tiros, MG): morphologic and dissolution aspects

A representative set of diamonds from the Borrachudo River (114 stones) was described for the first time as to their physical characteristics, among them weight, morphology, dissolution figures, and abrasion. Most samples are low in weight (~51% less than 0.30 ct) and only ~3.5% are above 3.0 ct. However, considering the total weight, ~47% of it is concentrated in the range above 3 ct. On the other hand, there even occur diamonds with dozen of carats. The most common crystallographic forms are originated from the octahedral form by dissolution, although irregular shapes like chips and flats also occur. The diamonds show differences in form compared to their weight ranges; those up to 0.30 ct have various shapes; in the range of 0.31 to 1 ct the most common are octahedral forms and their descendants; and in stones larger than 1 ct chips and flats predominate. The general habitus of the crystals, identified by their final tetrahexahedroid shapes, chips and flats, corroborated by the dissolution figures, indicate that the diamond were submitted to strong dissolution in a magmatic environment. Residual hillocks and holes represent the final stage of dissolution. The study indicates that the abrasion by the fluvial transport was not expressive enough to cause mechanical wear, thus ca. 97% shows no sign of wear. This fact suggests a proximal source for most of these diamonds.