Academic literature on the topic 'West Peko deposit'

Abstract The Stemnic river (Buda) is a right side affluent of Bârlad river, with the surface of the catchment basin of 15,662.5 ha. Situated in the central part of the Central Moldavian Plateau, the catchment of the Stemnic (Buda) is characterized by an oblong form (30.5 km, respectively 8.5 km), having a lithologic homogeneity, but also by a diversity of the superficial deposits (eluvia, diluvia, colluvia and proluvia, alluvia). The fields in the catchment basin of the Stemnic (Buda) have been analyzed from the point of view of the soils’ quality, that have been classified into five quality classes. Besides the intrinsic characteristics of the soils, their classification included also the pedo-chemical properties of the lands, geomorphologic or climatic properties of the area. First, second and the third quality classes are predominant in the lower half of the catchment basin, less fragmentary, with prolonged cuesta reverses, corresponding to the distribution area of the chernosols. The lands that form part of the fourth quality class are distributed, in a great percentage, on the same types of soil, but represent greater constrains because of the abrupt cliffs. The fourth class is made up of the lands with severe limitations that reduce the range of agricultural crops or that need special measures or work in order to protect and ameliorate the soil’s resources. This class cumulates a percentage of 9%, being characteristic for the area affected by landslides, prevalent mainly in North-West part of the catchment of the Stemnic (Buda). In the fifth class there are included soils with major constrains for agricultural use. From this perspective, in the catchment, there are predominant the soils in the third quality class (37%), being followed by the second class (26%) and the first class (23%). The main limitative factors for the agricultural production of the lands in the catchment of the Stemnic (Buda) are the erosion in surface, the landslides, humidity excess and the small quantity of nutritive elements.

Resultados de datações U-Pb (LA-ICP-MS) sobre zircões e análises litoquímicas de amostras de paragnaisse e xisto do Grupo Andrelândia, coletadas desde sua área-tipo até a região de alto grau metamórfico do sistema orogênico Araçuaí-Ribeira, evidenciam que esta extensa unidade estratigráfica inclui representantes de bacia precursora de margem continental passiva, representada pelo Grupo Andrelândia Inferior (paragnaisse de Madre de Deus, Seqüência Carrancas), com idade máxima em ca. 950 Ma e fontes sedimentares diversificadas, datadas do Toniano ao Neoarqueano; e de bacias orogênicas, representadas pela Unidade Santo Antônio do Grupo Andrelândia Superior e paragnaisses de alto grau metamórfico da região de Ubá-Carangola, com idade máxima em torno de 640 Ma e extrema concentração de áreas-fontes neoproterozóicas. As análises litoquímicas apontam para larga predominância de fontes sedimentares situadas em arcos magmáticos, mas a amostra que melhor indica a bacia precursora (o paragnaisse de Madre de Deus, Seqüência Carrancas) tem assinatura similar a depósitos de margem continental passiva. Amostras de paragnaisse migmatítico revelam a idade de ca. 565 Ma para o clímax metamórfico-anatético na zona de fronteira Araçuaí-Ribeira. Destaca-se que o Grupo Andrelândia Superior, na forma como é representado em mapas atuais, incluiria pelo menos duas bacias orogênicas, i.e., uma, na Faixa Brasília e, outra, no sistema Araçuaí-Ribeira.Palavras-chave: geocronologia U-Pb, litoquímica, Grupo Andrelândia ABSTRACT: PRECURSOR BASIN VERSUS OROGENIC BASINS: EXAMPLES FROM THE ANDRELÂNDIA GROUP BASED ON ZIRCON U-Pb (LA-ICP-MS) AND LITHOCHEMICAL ANALYSIS. Results from zircon U-Pb (LA-ICP-MS) and lithochemical analysis on samples from the Andrelândia Group, collected from its type-area to the high grade core of the Araçuaí-Ribeira orogenic system, suggest that this extensive stratigraphic unit includes a precursor passive margin basin, represented by the Lower Andrelândia Group (Madre de Deus paragneiss, Carrancas sequence), with maximum depositional age around 950 Ma and diversified sediment sources dated from the Tonian to Neoarchean; and orogenic basin(s), represented by the Santo Antônio unit of the Upper Andrelândia Group and high grade paragneisses of the Ubá-Carangola region, with a maximum depositional age around 640 Ma and provenance extremely restricted to Neoproterozoic sources. Lithochemical analysis point to a large predominance of sediment sources located in magmatic arcs, but the sample representing the precursor basin shows signature similar to passive margin deposits. High grade paragneisses reveal the age of ca. 565 Ma to the metamorphic-anatectic climax in the Araçuaí-Ribeira boundary zone. It is important to notice that the Upper Andrelândia Group, as represented in current maps, includes at least two orogenic basins, i.e., the older, to the west, in the Brasília belt, and the younger, to the east, in the Araçuaí-Ribeira system.Keywords: U-Pb geochronology, lithochemistry, Andrelândia Group.

O trabalho apresenta uma resenha da obra “Geognostisches Gemälde von Brasilien und wahrscheinlichesMuttergestein der Diamanten “ de Wilhelm Ludwig von Eschwege por ocasião do sesquicentenário da suamorte em 1o de fevereiro de 2005. A obra foi publicada, em pequena tiragem, em 1822. Trata inicialmentedos grandes divisores de água: um de direção aproximada leste – oeste, separando a bacia do rio Amazonasdas dos rios Paraná e Paraguai, que Eschwege batiza de “Serra das Vertentes”, o outro divisor separa a baciado Rio São Francisco dos rios que correm diretamente ao Oceano Atlântico o qual chama de “Serra doEspinhaço”, incluindo nela a Serra da Mantiqueira. Em segundo lugar apresenta um esquema estratigráficobaseado nos modelos usados na Europa, como, por exemplo, aquele proposto em 1787 por AbrahamGottlieb Werner, professor da Academia de Minas de Freiberg na Alemanha. A Primeira FormaçãoPrimitiva é formada pelo embasamento cristalino, a Segunda Formação Primitiva corresponde às seqüênciassupracrustais dobradas (representadas pelos Supergrupos Rio das Velhas, Minas e Espinhaço), a Terceira oude Transição abrange essencialmente o atual Grupo Bambui e uma quarta subdivisão reúne depósitossuperficiais como aluviões e coberturas terciárias e quarternárias. Percebe-se que suas idéias a respeito dageologia do Brasil são fortemente influenciadas pela escola netunista de Werner. Descreve aindamacroscopicamente os principais tipos de rocha encontrados no Brasil, define os novos termos “itacolumito”e “itabirito” e introduz o termo “tapanhoacanga” na nomenclatura geológica, todos com suas localidadestipo.Tapanhoacanga, hoje reduzida para canga, é de origem indígena de tapanhu = escravo negro e acanga= cabeça (ou a = cabeça e canga = osso). A última parte do “Quadro geognóstico... “ trata da ocorrência dosdiamantes no Brasil e de sua possível rocha matriz, na sua opinião formados em concreções limoníticasoriginadas das rochas ferruginosas da Segunda Formação Primitiva.Palavras-chave: História da Geologia, Quadrilátero Ferrífero, Serra do Espinhaço, estratigrafia precambriana,itabirito, itacolumito, canga, diamantes ABSTRACT: ESCHWEGE’S “GEOGNOSTICAL SKETCH OF BRAZIL AND THE PROBABLE SOURCE ROCK OFDIAMONDS”: BRIEF COMMENTS ON HIS VISION OF BRAZILIAN GEOLOGY. This small brochure waspublished in 1822 by the German geologist Wilhem Ludwig von Eschwege (1777 – 1855) and is nowtranslated to Portuguese for the first time completely as a memorial of his passing away 150 years ago.Initially, Eschwege reports on the physical geography of Brazil and suggests the names “Serra das Vertentes”(Watershed Mountains) and “Serra do Espinhaço” (Backbone Ridge), running East – West the first and North– South the second, separating the great hydrographic basins in Brazil. A main chapter is dedicated to a veryfirst proposal of a stratigraphic scheme based on European models of the time and heavily influenced by A.G. Werner, the principal protagonist of the neptunism, which interpreted all rocks as being precipitated fromaqueous solutions. He distinguishes four stratigraphic divisions: the First Primitive Formation containinggranite, gneiss, and mica schist, corresponding in more modern terms to the crystalline basement; the SecondPrimitive Formation is formed by itacolumite (quartzite), itabirite (iron formation) and schist, representedby the Rio das Velhas, Minas, and Espinhaço supergroups. The third or Transitional Formation composed byslates, quartz schist, greywacke, and massive limestone corresponds to the Macaúbas and Bambui groups. Thefourth and uppermost formation encloses all superficial deposits, such as alluvial, river gravels and a peculiarferruginous conglomerate called by the native tapanhoacanga, which means Negro head. His argumentationis heavily influenced by neptunistic thinking. Eschwege still describes in great detail the principal rock types,as known at this time in Brazil and introduces the terms itacolumite, itabirite and (tapanhoa)canga into thegeological nomenclature. The second part is dedicated to the occurrence, distribution and origin of Braziliandiamonds. He considers that they are formed within any rock of his Second Primitive Formation, due to theoccasional founding of limonitic concretions with inclusions of diamonds.Keywords: History of geology, Quadril

The Au-Cu-Bi- deposits of the Proterozoic Tennant Creek Inlier share geological and geochemical characteristics that indicate strong links in their genesis, yet the diversity in alteration assemblages, metal ratios and zonation patterns reflect variations in ore forming processes that previously have not been explained in detail. The West Peko deposit is representative of Cu-rich, pyrrhotite-bearing mineralisation with intermediate gold grades, in magnetite+ hematite-rich syntectonic ‘ironstones’. By contrast, the high grade Eldorado Au deposit contains minor sulfides and very low Cu grades, similar to several of the larger gold producers in the field (e.g. Juno, White Devil, Nobles Nob), and is also hematite-rich. Au, Chalcopyrite and Bi-sulfosalts were introduced into pre-existing ironside during progressive shearing, either late in the first regional deformation event (D1) or during a second phase of deformation. The occurrence of some Au zones outside ironstones suggests the ore fluids in part followed different flow paths to hose of the ironside-forming fluids. Three chemically and isotopically distinct fluids have been characterised. (i) Ironstone-forming fluids at West Peko and Eldorado were Ca-Na-Cl (-Fe?) brines containing 12-20 weight % total dissolved salts, and reached temperatures of 350-400°C during magnetite deposition. Oxygen and hydrogen isotope compositions of minerals formed at the ironside stage are consistent with an origin of ironstones from formation or metamorphic waters. (ii) The inferred Au-Bi+Cu transport fluid in he Cu- and sulphide-rich West Peko deposit was of low to moderate salinity (3-10 eq. wt. % Na Cl), ~300-350°C and N2 + CH4 – rich. Newly represented phase equilibria among the Fe-silicates stilpnomelane and minnesotaite, chlorite, biotite, sulfides, oxides and carbonates as well as fluid inclusion vapour compositions indicate that the Au-Bi+Cu transport fluid was relatively reducing with near-neutral pH and total dissolved sulphur contents of 0.001m to 0.01m. In the Eldorado Deeps Au- and hematite-rich deposit the Au-transporting fluid also may have been of low-moderate salinity, with Au deposition occurring at ~300°C. The reducing Au-Bi+Cu transport fluid at West Peko resembles primary magmatic or metamorphic water in oxygen and hydrogen isotopic composition. Carbon isotope ratios of Au-sulfide stage carbonates at West Peko point to involvement f organic carbon, probably sourced outside the host Warramunga Formation. (iii) A regionally distributed, oxidising Ca-Na-Cl brine with 20-35 weight percent total dissolved salts, was present prior to, after and probably during ore deposition. Mixing with lower salinity reducing Au-Bi+Cu transport fluid is inferred at West Peko and us suggested to have caused effervescence of N2+CH4 by ‘salting out’, relatively late in the Au depositional stage. An hypothesis of metal transport and deposition is proposed for the Tennant Creek deposits in which gold, copper and bismuth were transported in a reducing fluid and were deposited in the Cu- and sulphide-rich deposits dominantly by oxidation, desulfidation and initial pH increase as the reducing fluid reacted with magnetite+hematite ironstone. Mass transfer modelling indicates that relatively small amounts of ironstone are required to precipitate Au + Bi-sulfides, such as Eldorado, the oxidising brine may have played a significant role in ore deposition either by mixing with a reducing Au-Cu-Bi-transporting fluid, or by producing hematite oxidant additional to any already present in the ironstones. The greater extent of oxidation of the ore fluid in such deposits may have generally prevented saturation of copper minerals, resulting in low Cu grades. Gold is inferred to have been transported dominantly as uncharged bisulfide complexes, although biselenide complexes were potentially important. New thermodynamic data estimated for bismuth complexes are consistent with bismuth transport as uncharged S-H-O-bearing species in the Tenant Creek ore fluids. The existence of high grade Au-Bi deposits outside ironstones is predicted by chemical modelling of mixing between reducing and oxidising fluids, located where structures allowed focused flow of both fluids.