Juliana Charão Marques

Estratigrafia e geocronologia do greenstone belt de Guarinos, Goiás

The Guarinos greenstone belt is one of the three low metamorphic grade supracrustal rocks assemblage rimmed by orthogneisses of Central Brazils Archean Block northern limits. The investigation of the stratigraphy of those greenstone belts started by the end of 1970s and underwent improvements during the years as a result of detailed geological mapping. The latest and accepted stratigraphic model for the Guarinos greenstone belt refers its supracrustal rocks under the Guarinos Group, which is subdivided into the Serra do Cotovelo (metakomatiites), Serra Azul (metabasalts), Sao Patricinho (mafic metaturbidites), Aimbe (BIF) and Cabacal (carbonaceous phyllites and impure metarenites) formations. Detailed geologic mapping (1:10,000) and drill-cores during an exploration program by Yamana Desenvolvimento Mineral S.A. in the area allowed a better definition of the metasedimentary package of the Cabacal Formation, which is here formally proposed to be subdivided into a Lower Member of carbonaceous phyllites with basalt lava flows and gondite lenses interlayers, an Intermediate Member of gondite, iron formation, metachert and massive barite lenses, and an Upper Member of carbonaceous phyllites with minor metachert lenses. The impure metarenites, formerly considered as part of the Cabacal Formation, are proposed under the Mata Preta Formation, which is laterally interfingered with the Cabacal Formation. U-Pb LA-ICP-MS geochronological data of detrital zircon grains from the Sao Patricinho mafic metaturbidites and the impure metarenites of the Mata Preta formations indicate that the major source-area of the clastic load had a Siderian to Rhyacian age, with minor contribution from Archean rocks. From the rock assemblage of both units and their contact relationships it is concluded that the basin stage they represent evolved during the world-wide Anoxic Oceanic Event (AOE) that took place during the 2.2 to 2.06 Ga, represented by the carbonaceous phyllites of the Cabacal Formation, coeval with the erosion of a magmatic arc that derived the immature clastic load of the Mata Preta Formation.

Geocronologia U-Pb em zircão de rochas intrusivas e de embasamento na região do Vale do Jacurici, Cráton do São Francisco, Bahia

The Jacurici Complex, located in the NE of the Sao Francisco Craton, is constituted by several Cr-mineralized mafic-ultramafic N-S bodies, possible fragments of a single sill disrupted during deformation. Some works suggest it is intruded on the Serrinha Block while others consider it in the Salvador-Curaca Belt. The basement on this region is informally divided into paragneisses and orthogneisses; the latter is supposed to be younger considering it is less deformed. Petrography revealed that some of the paragneisses are alkali-feldspar granite strongly milonitized. The orthogneisses occur at the north and consist, at least in part, of monzogranites with heterogeneous deformation, locally of low temperature. U-Pb zircon dating were performed for five representative samples. Just three provided good concordia ages. A mafic rock produced a 2102 ± 5 Ma age and it is petrographically similar to the metanorites described in the Jacurici Complex, being interpreted as the record of the first pulses of the mafic magmatism. A monzogranite yielded a 2995 ± 15 Ma age, older than expected, related to the Serrinha Block. The alkalifeldspargranite yielded a 2081 ± 3 Ma age. The Itiuba Syenite and the pegmatites that crosscut the JacuriciComplex have similar ages. Considering the lack of information about the supracrustal sequence that hosts the intrusive alkaline and mafic-ultramafic rocks at the Ipueira and the Medrado areas, it is possible that part of the terrain belongs to the Salvador-Curaca Belt. We suggest that the Jacurici Complex could be intruded after the tectonic amalgamation between the Serrinha Block and the older part of the Salvador-Curaca Belt and, therefore, could be hosted by both terrains.

O corpo Várzea do Macaco e as mineralizações de cromo, níquel e cobre, Complexo Máfico-ultramáfico Jacurici, Cráton São Francisco, Bahia

The Jacurici Mafic-ultramafic Complex, located in the northeastern portion of the Sao Francisco craton, consists of several N-S oriented layered bodies that host the largest chromite deposit in Brazil. The Varzea do Macaco body is at the northern part and also host a Ni-Cu sulfide mineralization. This study describes this body and its mineralization and compare with the southern intrusions (Ipueira-Medrado) where a petrological evolution was previous established. The Varzea do Macaco is stratigraphically inverted and disrupted in five blocks, laterally dislocated by late faults. It is constituted by dunite, lherzolite, ol-webesterite, chromitite and gabbronorite with variable serpentinization. The sulfide ore (Po ± Pn ± Cpy) is concentrated close to the main thick chromitite layer and occurs as: primary magmatic with interstitial sulfides associated with olivine and pyroxene; and as a remobilized ore, with sulfides associated to metasomatic veinlets or lenses that crosscut the primary layering. The interval where magmatic ores occur is characterized by the presence of magmatic amphibole that possible favoured late metamorphism and metasomatism transformation, stronger in this interval. The remobilized sulfide is enriched in chalcopyrite showing an increase in the Cu/Ni ratio. Comparing to Ipueira-Medrado, the Varzea do Macaco is enriched in clinopyroxene, but it can be subdivided in the same Ultramafic and Mafic zones. Possibly, both bodies are part of a single intrusive system characterized by a primitive magma with high Mg and Ni contents. The chromite mineralization is considered to be triggered by crustal contamination. At Varzea do Macaco, sulfur saturation was reached.

Development of a microwave-assisted ultraviolet digestion method for biodiesel and subsequent trace elements determination by SF-ICP-MS

A microwave-assisted ultraviolet digestion (MW-UV) method for biodiesel is proposed for subsequent determination of Ba, Co, Cr, Cu, Mn, Ni, Pb, Sr, and V by sector field inductively coupled plasma mass spectrometry (SF-ICP-MS). It is the first time that the combination of microwave and ultraviolet radiation is proposed for biodiesel pretreatment achieving high efficiency of digestion even when dilute acid and high sample mass were used. Variable concentrations of nitric acid (2, 4.5, and 7 mol L−1), hydrogen peroxide (0, 1, and 2 mol L−1), and sample mass (ranging from 500 to 1000 mg) were evaluated to optimize digestion (i.e., low residual carbon content and low residual acidity). Using the proposed MW-UV method, residual carbon content (RCC) of 19.2% and residual acidity (RA) of 17.4% were obtained for digestion of 950 mg of biodiesel using 7 mol L−1 nitric acid. Samples were also digested by microwave-assisted digestion (MW-AD) using the same conditions and final digests were not suitable for analytes determination due to the high dissolved carbon content in solution (RCC about 27.4%). A study was performed in order to evaluate the maximum concentration of carbon that allowed accurate 52Cr, 60Ni, and 51V determinations without influence from polyatomic interferences by SF-ICP-MS. It was verified that interferences on 52Cr occurred when C concentration was higher than 150 mg L−1, requiring the use of medium resolution mode. The formation of polyatomic interference over 51V and 60Ni species in low resolution mode was not observed varying the C concentration. Analysis of a certified reference material (CRM, HU-1 used oil) by the proposed method agreed with certified values (t-test, 95% confidence level). Analyte recovery studies were in the range of 95–108%.