Adauto Souza Neto

ABSTRACT Este artigo descreve os efeitos de incêndio na estrutura populacional de Casearia sylvestris Swartz (Salicaceae). Foram analisados a estrutura de diâmetro e altura e o padrão de distribuição espacial antes e após o fogo. Além disso, foi realizada a caracterização morfológica e anatômica do sistema subterrâneo para determinar a natureza e origem dos brotos oriundos desse sistema. Neste estudo foram amostrados 10 ha de fevereiro a novembro de 2006, em agosto, um incêndio queimou cerca de 80% da área de estudo, permitindo a comparação de dados coletados antes e após este evento. Foram verificados que os sistemas subterrâneos gemíferos são de natureza radicular e que os brotos se originam próximo ao câmbio vascular. A distribuição espacial dos brotos de raiz tornou-se agregada após o incêndio e houve maior concentração de brotos de raiz na primeira classe de tamanho indicando que esses brotos são responsáveis pela recomposição dessa espécie em áreas queimadas.

The chemical composition of marbles is a limiting factor for common mineral geothermometry. But carbon isotope fractionations occur in the system carbonate-CO2-graphite-CH4 both at low and high temperatures. Thus, the fractionation of the 13C and 12C isotopes between calcite and graphite has become a useful geothermometer for a large range of temperatures. In this work, the results of the calcite-graphite geothermometer are reported to estimate the temperature of the metamorphic peak of the Itajubatiba region, Borborema Province, Northeastern Brazil. Calibration curves were used that are valid for a range of temperatures (0-700 °C and 400-680 °C) that include those (572-627 °C; mineral geothermometry in mica schists) considered as the peak of the regional metamorphism. The temperatures (560-656 °C) obtained for the metamorphic peak are coherent with those previously known from the regional scenario. The agreement between the temperatures found and those already established indicates that calcite and graphite are in isotopic equilibrium, as
corroborated by the intimate physical contact between both minerals. The 13C/12C isotopic ratios of calcite and graphite are a good tool to estimate metamorphic peak temperatures in marbles, and therefore can be applied as a geothermometer in other regions.

With the purpose of evaluating inorganic contamination in the two main oil fields located in the on-shore part of the Potiguar basin, sediments from streams crosscutting the Canto do Amaro and Alto da Pedra fields were sampled in rainy (30 samples) and dry seasons (55 samples). Fine fractions (< 63 microns) of the sediments were digested in acqua regia and analyzed for a multi-element package using an ICP-AES. Among the elements investigated, arsenic and antimony showed anomalous concentrations (6 to 69 ppm, and 5 to 28 ppm, respectively) when compared to background levels established for the area. A highly positive correlation was observed between these two elements, suggesting a common source for them. Oil and/or combustion of fossil fuels in the area could be the source of As and Sb detected. Alternatively, these elements could be related to a particular chemical composition of the rock substrate in the area.

At the Bonfim mine, cyanide processing used to extract gold from abandoned tailings represents an important environmental concern. In order to evaluate cyanide concentrations, twenty samples of sediments of the main streams crossing the mine site, and ten samples of the
five mainly tailings were analyzed for total cyanide. Cyanide was found in two samples of tailings (0.83 and 1.44mg/Kg of total cyanide). Cyanide found indicates that its natural degradation
could not occur after nine years of exposition of the tailings under surface conditions. Cyanide is probably present as insoluble metallic complexes formed by reactions between solution used in the gold recovery and metals from ore minerals. Cyanide detected at Bonfim shows concentrations
below of those reported to contaminated tailings of the U.S. gold mines.

The Seridó Mobile Belt (SMB) is located in the Borborema Province in north-eastern Brazil and consists of a gneiss basement (Archean to Neo-Proterozoic), a meta-sedimentary sequence (marble, quartzites and schists) and the Brasiliano igneous suite, both of Neo-Proterozoic age. In this region, skarns occur within marble and at the marble-schist contact in the meta-sedimentary sequence. Most of the skarn deposits have been discovered in the early 1940’s, and since then they have been exploited for tungsten and locally gold. Recently, the discovery of gold in the Bonfim tungsten skarn has resulted in a better understanding of the skarn mineralization in this region. The main characteristics of the SMB skarns are that they are dominantly oxidized tungsten skarns, with the exception of the Itajubatiba and Bonfim gold-bearing skarns which are reduced based on pyrrhotite as the dominant sulfide, garnet with high almandine and spessartine component, and elevated gold contents. In the Bonfim deposit, pressure estimates indicate that the skarns formed at 10-15 km depth. The mineralized skarns present the prograde stage with almandine, diopside, anorthite, and actinolite-magnesio-hornblende, and titanite, apatite, allanite, zircon, and monazite as accessory minerals. The retrograde stage is characterized by alkali feldspar, clinozoisite-zoisite-sericite, calcite and quartz. Scheelite occurs in four ore-shoots distributed within the marble and at the marble-schist contact. The main ore body is 5-120 cm wide and contains an average of 4.8 wt% WO3, which occurs in the basal marble-schist contact. Fold hinges appear to control the location of high-grade scheelite. The late stage gold mineralization contains bismite (Bi2O3), fluorine-bearing bismite, native bismuth, bismuthinite (Bi2S3) and joseite (Bi4(Te,S)3), but also chlorite, epidote, prehnite, chalcopyrite, and sphalerite. This gold-bismuth-tellurium mineralization exhibits a typical late character, and occurs as a black fine-grained mineral assemblage controlled by conjugate brittle-ductile faults (and extensional fractures) that cross-cut not only the banding in prograde skarn, but also the retrograde alkali feldspar and clinozoisite-zoisite-sericite assemblage. The Au-Bi-Te bearing minerals are intergrown with retrograde epidote, prehnite, chlorite, chalcopyrite, and sphalerite, indicating that gold mineralization at Bonfim is linked to a late-stage skarn event. The polymetallic nature of the Bonfim deposit can be used as an important guide for the exploration of this type of skarn deposit in the Borborema Province, which potentially contains significant new, undiscovered gold and polymetallic deposits.

The Itajubatiba deposit represents a gold skarn, in northeastern Brazil. Since its discovery in the 1940s, it was exploited during about 30 years and it has produced 5 tons of gold. Its average ore grade ranges from 0.5 to 2.0 ppm and has reached 6.3 ppm. Regional geologic units consist of a gneissic-migmatitic basement (Archean to Paleoproterozoic), a meta-sedimentary sequence (marble and schist) and Neoproterozoic intrusive igneous rocks. Later a metasomatism transformed marble, meta-tonalite (gneissic-migmatites), and meta-syenogranite into skarns. Gold mineralization is typically later, associated with sulfides, which appear either filling veins crosscutting the skarns or disseminated throughout the skarns. In order to verify this hypothesis regarding the formation of gold, a detailed search for the metal was carried out using several polished thin sections. Gold, however, was not observed. A subsequent detailed investigation of trace elements in sulfides of the Itajubatiba ore was conducted. Proton-Induced X-ray Emission (PIXE) analyses revealed that some elements occur in relatively high amounts in these sulfides, such as Mn (up to 0.9 wt.%), Au (up to 690 ppm), As (up to 1,360 ppm), Bi (0.5 wt.%), Pb (up to 1,870 ppm), Ni (up to 3,050 ppm), Co (0.6 wt.%), Se (up to 1,000 ppm), and W (up to 1,420 ppm). Pyrite shows the highest amounts of gold among the sulfides studied. Considering the quantity of gold detected in pyrite, the pyrite abundance in the study rocks, and the average densities of the main minerals occurring in skarns, their gold grade varies from 1.5 to 10.4 ppm (for gold hosted in pyrite only). The results reveal economically significant gold amounts linked to sulfides, and could provide a new perspective for the exploitation of the Itajubatiba deposit.