Marcelo R. R. Da Silva

Fluid inclusion microthermometry in Be–Ta–(Li–Sn)-bearing pegmatites from the Borborema Province, Northeast Brazil

Abstract Fluid inclusions in Be–Ta–Li–Sn-bearing pegmatites have been reported in the literature as hypersaline aqueous brines or low-salinity aqueous or aqueous–carbonic fluids. The evolution paths of volatile fluids in equilibrium with the different crystallizing phases of rare-element pegmatites do not seem to be unique. The aim of this work is to contribute to this discussion, on the basis of fluid inclusion information from the Borborema Pegmatite Province (BPP) in Northeast Brazil. Some well-known heterogeneous Be–Ta–(Li–Sn) mineralized pegmatites of this province were systematically sampled along cross sections with the purpose of studying the fluid inclusions in the different pegmatite zones (I, muscovite-rich contact zone; II, homogeneous or graphic pegmatite; III, blocky feldspar zone; and IV, quartz core). The observed fluid inclusions (mainly in quartz) were almost always secondary or pseudosecondary, grouped along healed fractures with random orientation. Even some trails paralleling quartz grain limits were not undoubtedly primary. Almost all these fluid inclusions were filled with low-salinity aqueous or aqueous–carbonic fluids. Primary inclusions were recognized only in some well-formed crystals from druses with erratic distribution in the pegmatites. Microthermometric data from these samples confirm that the various crystallizing phases of the pegmatites were in equilibrium with low-salinity aqueous or aqueous–carbonic fluids (with insignificant N 2 or CH 4 contents) with frequent accidentally trapped solids. The various types of primary inclusions found in these crystals compare well with the types of secondary inclusions of the samples from the systematic cross sections. The relative age of the crystals hosting primary fluid inclusions, the relative age of primary inclusion types in successive crystal-growth zones, the relative age of secondary fluid inclusion trails and the corresponding microthermometric data allowed to establish the most probable evolution of the fluid characteristics during pegmatite crystallization. An early low-salinity aqueous–carbonic fluid with CO 2 contents of 20 mol% was present during the crystallization of early minerals, such as garnet and tourmaline of the border zone and graphic quartz, beryl, manganotantalite, and blue tourmaline of zones II and III. The end of zone III crystallization occurred in the presence of a low-salinity, aqueous fluid with very low CO 2 contents, as indicated by inclusions in the smoky core of quartz crystals at the limit between zones III and IV. Later aqueous fluids barren in CO 2 and with salinities increasing from 5% to 20% coexisted with the formation of the pegmatite nucleus and replacement bodies, as indicated by inclusions in the border of these crystals and in euclase. The estimated trapping conditions of 3.5 kbar and 580°C for the earliest inclusion types agree with the regional metamorphic conditions and with the known liquidus conditions of rare-element-bearing pegmatite crystallization. The trapping conditions of inclusions in euclase and coexisting quartz crystal rims in the pegmatitic core, at 3.5 kbar and 400°C are slightly below the conditions admitted for the solidus of rare-element pegmatites and indicate an isobaric cooling during pegmatite crystallization.

Auriferous Quartz Veins from Northeastern Brazil: A Fluid-Inclusion Study

Auriferous quartz veins are known to exist in more than two dozen prospects, encompassing an area of 500 km2 northward from Serrita township (state of Pernambuco) in northeastern Brazil. Gold-bearing veins occur either with a strike of 70° to 110°, crosscutting muscovite schists of the Middle Proterozoic Salgueiro Group, or with a strike of 330° in granodiorite intrusions in the same schists. Small amounts of pyrite, galena, arsenopyrite, chalcopyrite, and sphalerite commonly are observed. Sericite, chlorite, and epidote are the most common wall-rock alteration products. Fluid inclusions were studied in samples of mineralized quartz veins from the Barra Verde III prospect in a small granodiorite body, and from the Inga, Sabura, and Riacho do Meio prospects in the Salgueiro schists. Some samples of barren quartz veins also were studied for comparison. Primary and pseudosecondary inclusions in the mineralized veins are triphasic or biphasic aqueous-carbonic at room temperature. The wide range of the CO2/H2O...

VARIEDADES GEMOLÓGICAS DE MINERAIS DA PROVÍNCIA PEGMATÍTICA DA BORBOREMA, NE DO BRASIL: UMA SÍNTESE

Pegmatitos graniticos sao fonte de numerosos minerais com elevado potencial gerador de variedades de boa qualidade gemologica, com destaque para berilo, varias especies do super-grupo da turmalina, topazio, espodumenio, quartzo, feldspatos, alem de outros minerais menos comuns {p. ex. granadas, euclasio, gahnita, tantalita-(Mn), microlitas}. Na Provincia Pegmatitica da Borborema (PPB), alvo deste estudo, destacam-se as turmalinas, incluindo a famosa “turmalina Paraiba, berilo, quartzo, espessartita, gahnita, euclasio, como minerais de bom potencial gemologico. A formacao destas gemas na PPB, assim como na maioria das Provincias Pegmatiticas mundiais, e localizada em cavidades situadas preferencialmente na transicao entre a zona intermediaria interna e o nucleo de quartzo de pegmatitos heterogeneos mais diferenciados. A descricao das principais ocorrencias de gemas da PPB sera objeto deste trabalho.

Identificação gamaespectrométrica de placeres rutilo-monazíticos neoproterozóicos no sul da Faixa Seridó, nordeste do Brasil

Aerial gamma-ray survey data covering Neoproterozoic supracrustal sequences in the Serido Belt were processed and analyzed together with ground gamma-ray data, air photos and geological data for lithogeophysical characterization and mapping of granitic rocks, related pegmatites fields and lithological units of Serido Group. Interpretation was based on individual and ternary images of the three radio-elements and the eU/eTh and eTh/K ratios, and allowed the discovery of thorium anomalies associated with coarse-grained metarenites and metaconglomerates facies intercalated with quartzites of the Equador Formation. High contents of iron oxides, ilmenite, monazite, rutile, titanite and zircon were identified by ore microscopy of polished sections in the metaconglomerates matrix. Semiquantitative scanning electron microscopy (SEM) analyses in minerals of two samples revealed up to 79.4% of Th02 and 87.7% of REE in monazites; up to 99.2% of Ti02 in ilmenite and rutile and up to 1.81 % of HfO2 in zircon. Gamma-ray anomalies due to thorium were also identified in association with sediments of Cenozoic age in the region.