Paulo Beljavskis

Revisão da litoestratigrafia da faixa São Roque/Serra do Itaberaba - SP

A great number of publications on the geology of the Sao Roque/Serra do Itaberaba Belt (Sao Paulo State, Brazil) introduce and use diverse lithostratigraphic denominations, creating difficulties to a clear understanding of the geological evolution and lithostratigraphy of the metamorphic belt. This paper seeks an organization of the various nomenclatures and litostratigraphic suggestions for the Belt. A review of theessential publications about the subject is made, including those where the first litostratigraphic denominations were cited. After acomparative and critical analysis of the bibliography and, taking as a basis the geologic map 1:25.000 of the Itaberaba and Pedra Branca inountains, a sepatation of the supracrustal mcks which constitute the Belt in two major litostratigraphic units is proposed: Serra do Itaberaba Group and Sao Roque Group. The Serra do Itaberaba Group is subdivided into the Morro da Pedra Preta, Nhangucu and Pirucaia formations. The Morro da Pedra Preta Fotmation is a volcano-sedimentary sequence, and is basal to the Nhangucu, mainly composed by andalusite-bearing and manganesiferous metapelites, with some smalllenses of carbonatic and calc-silicatic rocks. The Pirucaia Formation is predominantly quariizitic and probably represents lhe lithoracies deposited in the margins of the two other formations, partieularly oft he Nhangucu Formation. In the Sao Roque Group the following formations can be recognized: Piragibu, composed by metapelites with few lenses of metaconglomerates and metavolcanics; Pirapora do Bom Jesus, essentialy volcanic/volcaniclastic, with bioherms and calc-phyllites; Estrada dos Romeiros, with rhythmic metapelites and metarenites and, finally, the metarenitic arcosean . Boturuna Formation. The Piragibu Formation is basal in the area of the Itaberaba, Pedra Branca and Morro Doce Mountains. In the Pirapora Synclinorium, the homonymic formation is basal to the Estrada dos Romeiros Formation, which corresponds to the more distal facies of the Piragibu Formation, covered by the Boturuna Formation.

Geochemistry of tourmalines associated with iron formation and quartz veins of the Morro da Pedra Preta Formation, Serra do Itaberaba Group (São Paulo, Brazil)

Tourmalines of intermediate schorl-dravite composition occur in iron formation (including metachert and tourmalinites), metasediments, calc-silicate and metabasic/intermediate rocks of the Morro da Pedra Preta Formation, a volcanic-sedimentary sequence of the Serra do Itaberaba Group (northeast of Sao Paulo City, southeastern Brazil). The Morro da Pedra Preta Formation is crosscut by quartz veins that contain both intermediate schorl-dravite and an alkali-deficient, Cr-(V-)bearing tourmaline, in which the occupancy of the X-site is ϑ0.51Ca0.33Na0.15, characterizing it as intermediate to foitite and magnesiofoitite end-members. Mg# values for this tourmaline are higher than those for intermediate schorl-dravite. Raman spectroscopy also confirms the presence of two groups of tourmalines. Stable isotope data indicate sediment waters as fluid sources, rather than fluids from magmatic/post-magmatic sources. Delta18O compositions for tourmalines, host metachert, and quartz veins are similar, showing that fluid equilibration occurred during crystallization of both quartz and tourmaline. Syngenetic, intermediate schorl-dravite tourmalines were formed under submarine, sedimentary-exhalative conditions; amphibolite-grade metamorphism did not strongly affect their compositions. Younger tourmalines of compositions intermediate to foitite and magnesiofoitite reflect the composition of the host rocks of quartz veins, due to fluid percolation along faults and fractures that caused leaching of Cr (and V) and the crystallization of these alkali-deficient, Cr-(V-)bearing tourmalines.

Turmalinitos vulcanogênicos da formação Morro da Pedra Preta do Grupo Serra do Itaberaba (SP): petrografia, composição química da turmalina e implicações metalogenéticas

Turmalinitos associados a anfibolitos, metachert, formacao ferrifera, metatufos, rochas metassedimentares, metavulcanoclasticas e calciossilicaticas sao encontrados na Formacao Morro da Pedra Preta, sequencia metavulcano-sedimentar basal do Grupo Serra do Itaberaba (SP - Brasil). A deformacao da rocha, que e formada por leitos ricos em turmalina alternados com leitos ricos em quartzo, a presenca de clastos do tipo rip-up e a quimica de rocha total semelhante a das rochas metassedimentares comprovam a origem sin-sedimentar dos turmalinitos. Analises quimicas da turmalina por microssonda eletronica mostram que, para a turmalina dos turmalinitos associados a metatufos, formacao ferrifera, metachert e rochas metavulcanoclasticas e metassedimentares, as composicoes sao intermediarias a serie schorlita-dravita, enquanto para os turmalinitos associados a anfibolitos, metatufos e a rochas calciossilicaticas, as composicoes sao intermediarias a serie dravitauvita. As primeiras apresentam conteudos baixos de Ca e elevados de Al e Na, com variacoes nos teores de Fe e Mg (serie schorlita-dravita), e as ultimas sao caracterizadas por teores baixos de Na e Al e altos de Mg e Ca, contendo adicionalmente fluor. A componente schorlita da turmalina dos turmalinitos situados em porcoes proximais aos centros de atividade vulcano-exalativa de fundo oceânico e indicativa do potencial para ouro da Formacao Morro da Pedra, o que contrasta com as condicoes requeridas para a formacao de depositos de sulfetos macicos (fluidos hipersalinos e associacao com turmalinitos magnesianos). A turmalina dos varios turmalinitos da Formacao Morro da Pedra Preta nao e tipicamente magnesiana e sua composicao foi mais fortemente influenciada pela composicao das rochas circundantes do que por aquela dos fluidos hidrotermais, especialmente no caso em que o turmalinito se depositou em uma posicao intermediaria a distal em relacao ao centro de atividade vulcano-exalativa.

Composição Química da Turmalina de Turmalinitos Estratiformes da Mina de Passagem de Mariana, Sudeste do Quadrilátero Ferrífero (MG)

The Passagem de Mariana Mine in the southeastern part of the Quadrilatero Ferrifero, Minas Gerais, Brazil, is one of the oldest Brazilian gold mines. Gold is associated with stratiform tourmalinites and tourmaline-bearing quartz and carbonate veins and breccias. The highest gold contents (up to 200 g/t) have been recorded in the stratiform tourmalinites, which belong to the Batatal Formation (Caraca Group, Minas Supergroup). Three petrographic types of tourmaline have been defined and analyzed by electron microprobe and ICP-MS. Type 1 corresponds to the very fine-grained tourmaline that composes homogeneous tourmalinites. They occur as continuous levels or boudins, and as fragments in quartz-carbonate veins and breccias. Type 2 zoned tourmaline is coarser-grained and composes both homogeneous and banded tourmalinites. Type 3 coarse-grained, zoned tourmaline grows perpendicularly to the contact between the tourmalinite or its fragments and the quartz-carbonate veins and breccias. The main chemical variations observed in the three tourmaline types are decrease of SiO2 and Al2O3 and increase of MgO, Na2O and F contents with increasing CaO and TiO2 contents. Additionally, types 2 and 3 tourmalines also contain higher concentrations of LREE, Rb, Sr, Y, Cs, Th and U in relation to type 1. These chemical variations are coherent with the geochemical conditions that prevail during diagenesis and metamorphism. No evidence was found of the participation of external (e.g., magmatic) fluids in the generation of the stratiform tourmalinites, for which the main source for boron could have been the Batatal Formation carbonaceous metapelites. Therefore, the syngenetic origin of the tourmalinites cannot be completely ruled out. The tourmalinites could have evolved during the prograde metamorphism of the beginning of the Transamazonian Event. Type-3 tourmaline, grown in the contact between the tourmalinite and quartz-carbonate veins and breccias, could have been generated after the metamorphic peak and during the extensional phase of the Transamazonian Event, with gold remobilization in veins and breccias

Rochas ricas em turmalina da formação Morro da Pedra Preta: relações com os turmalinitos portadores de ouro do Grupo Serra do Itaberaba, SP

A turmalina e um mineral comum no Grupo Serra do Itaberaba, particularmente na Formacao Morro da Pedra Preta. Dentre as rochas ricas em turmalina, destacam-se: turmalinitos, metatufos basicos, metavulcanoclasticas intermediarias a acidas, metapelitos e veios de quartzo. A presenca de ouro disseminado em turmalinito do prospecto Tapera Grande estimulou a investigacao da relacao entre os ambientes de formacao dos turmalinitos e as mineralizacoes do Grupo Serra do Itaberaba. A investigacao e ampliada neste trabalho, incluindo metatufos basicos, metavulcanoclasticas intermediarias a acidas e metapelitos que, alem de conter turmalina, apresentam indicios de silicificacao, carbonatizacao, potassificacao, cloritizacao e sulfetizacao. A quimica da turmalina revelou que mesmo nas porcoes mais proximais ao centro exalativo, a razao fluido/rocha foi baixa, pois a turmalina preserva muitas das caracteristicas da rocha hospedeira do que propriamente as dos fluidos ricos em boro que a geraram. Os fluidos, de origem hidrotermal-metamorfica, promoveram a redistribuicao de ouro sin-sedimentar ou singenetico; entretanto, sua composicao (aquo-carbonica, baixa salinidade), nao favoreceu a formacao de depositos de sulfetos macicos.