Sundaram S. Iyer

Variations in the boron isotope composition of Coffea arabica beans

Abstract Significant B isotope abundance variations were found for a variety of Coffea arabica beans from a number of coffee-growing regions around the world. This may be attributed to the influence of local sources of B (including soil, water and fertilizer) each having a characteristic B isotope abundance ratio of its own. The results of this preliminary study indicate that B isotope abundance data can be used to study the biogeochemical cycle of B, an important micronutrient. In addition, the isotopic data can be employed in quality assurance programs of commercial coffee as the quality depends to a large extent on the genetic and geographic origin of the coffee.

Geological and Isotopic Constraints on the Metallogenic Evolution of the Proterozoic Sediment-Hosted Pb-Zn (Ag) Deposits of Brazil

Abstract Integrated studies of seven Proterozoic sediment-hosted, Pb-Zn-Ag sulfide deposits of Brazil, permit the estimation of the age of the hosting sequence and the mineralization, the nature of the sulfur and metal sources, the temperature range of sulfide formation and the environment of deposition. These deposits can be classified into three groups, according to their ages. (a) Archean to Paleoproterozoic: the Boquira deposit, in Bahia state, consists of stratiform massive and disseminated sulfides hosted by parametamorphic sequences of grunnerite-cummingtonite+magnetite that represent a silicate facies of the Boquira Formation (BF). Lead isotope data of galena samples indicate a time span between 2.7 and 2.5 Ga for ore formation, in agreement with the stratigraphic position of the BF. The relatively heavy sulfur isotope compositions for the disseminated and stratiform sulfides (+8.3 to +12.8 ‰ CDT)suggest a sedimentary source for the sulfur. (b) Paleo to Mesoproterozoic: stratiform and stratabound sulfides in association with growth faults are present in the Canoas mine (Ribeira, in Parana state) and in the Caboclo mineralization (Bahia state). They are hosted by calcsilicates and amphibolites in the Canoas deposit, whereas in the Caboclo area the mineralization is associated with hydrothermally altered dolarenites at the base of the 1.2 Ga Caboclo Formation. The interpreted Pb-Pb age of the Canoas mineralization is coeval with the 1.7 Ga host rocks. Sulfur isotopic data for Canoas sulfides (+1.2 to +16 ‰ CDT) suggest a sea water source for the sulfur. The range between −21.1 and +8.8 ‰ CDT for the Caboclo sulfides could suggest the action of bacterial reduction of seawater sulfates, but this interpretation is not conclusive. (c) Neoproterozoic: stratiform and stratabound sulfide deposits formed during the complex diagenetic history of the host carbonate rocks from the Morro Agudo (Bambui Group), Irece and Nova Redencao (Una Group), yield heavy sulfur isotope values (+18.9 to +39.4 ‰ CDT). The uniform heavy isotope composition of the barites from these deposits (+25.1 to +40.9 ‰) reflect their origin from Neoproterozoic seawater sulfates. The late-stage, and most important, metallic concentrations represent sulfur scavenged from pre-existing sulfides or from direct reduction of evaporitic sulfate minerals. Lead isotope data from the Bambui Group suggest focused fluid circulation from diverse Proterozoic sediment sources, that probably was responsible for metal transport to the site of sulfide precipitation. (d) Late Proterozoic to Early Paleozoic: lead-zinc sulfides (+pyrite and chalcopyrite) of Santa Maria deposits, in Rio Grande do Sul, form the matrix of arkosic sandstones and conglomerates, and are closely associated with regional faults forming graben structures. Intermediate volcanic rocks are intercalated with the basal siliciclastic members. Lead isotope age of the mineralization (0.59 Ga) is coeval with the host rocks. Sulfur isotopic values between −3.6 and +4.1 are compatible with a deep source for the sulfur. Geological, petrographic and isotopic data of the deposits studied suggest that they were formed during periods of extensional tectonics. Growth faults or reactivated basement structures probably were responsible for localized circulation of metal-bearing fluids within the sedimentary sequences. Sulfides were formed by the reduction of sedimentary sulfates in most cases. Linear structures are important controls for sulfide concentration in these Proterozoic basins.

Isotopic Evidence for the Late Brasiliano (500-550 Ma) Ore-Forming Mineralization of the Araés Gold Deposit, Brazil

The Araés gold deposit, located in eastern Mato Grosso State, central Brazil, is hosted in Neoproterozoic volcanosedimentary rocks of the Paraguay belt, which formed during collision of the Amazonian craton and the Rio Apa block. 40Ar/39Ar geochronology and Pb and S isotopic analyses constrain the timing and sources of mineralization. Three biotite flakes from two samples of metavolcanic host rock yield 40Ar/39Ar plateau ages between 541 and 531 Ma, interpreted as cooling ages following regional metamorphism. Clay minerals from a hydrothermal alteration zone yield an 40Ar/39Ar integrated age of 503 ± 3 Ma. Galena grains from ore-bearing veins yield values of 206Pb/204Pb from 17.952 to 18.383, 207Pb/204Pb from 15.156 to 15.811, and 208Pb/204Pb from 38.072 to 39.681. Pyrite grains from ore-bearing veins yield values of 206Pb/204Pb from 18.037 to 18.202, 207Pb/204Pb from 15.744 to 15.901, and 208Pb/204Pb from 38.338 to 38.800. Pb isotope variations may be explained in terms of mixing a less radiogenic lead component (μ ~ 8.4) from mafic and ultramafic basement host-rocks (Nova Xavantina metavolcanosedimentary rocks) and a more radiogenic lead component (μ ~ 9.2) probably derived from supracrustal rocks (Cuiabá sedimentary groups). Sulfur isotope compositions are homogeneous, with δ34S values ranging from -1.1‰ to 0.9‰ (galena) and -0.7‰ to 0.9‰ (pyrite), suggesting a mantle-derived reservoir for the mineralizing solutions. Based on the Ar, Pb, and S isotope data, we suggest that the precious metals were remobilized from metavolcanic host rocks by hydrothermal solutions during Brasilide-Panafrican regional metamorphism. The Araés gold deposit probably formed during a late stage of the orogeny, coeval with other mineralization events in the Paraguay Belt.

Lead- and Sulfur-Isotope Investigations of the Boquira Sediment-Hosted Sulfide Deposit, Brazil

Lead- and sulfur-isotope investigations were performed at the Boquira sediment-hosted lead-zinc deposit in Bahia state, Brazil. This deposit, hosted by banded iron formation of metamorphosed oxide, carbonate, and silicate facies, was the most important lead mine in Brazil for nearly 40 years, until 1991, when mine operations ceased as a result of inadequate proven reserves. The lead-isotope data indicate an Archean-Paleoproterozoic age (2.5 to 2.7 Ga) for the deposit, and the lead is inferred to have been derived from crustal sources, probably from the basement rocks. The δ34S values for the layered sulfides are in the range from +8.3 to +12.8‰. The high positive values indicate that the sulfur was derived from the sediments by thermochemical reduction of Archean/Paleoproterozoic marine seawater; Boquira is a rare sediment-hosted galena-sphalerite deposit of this age. The study emphasizes the need to prospect marine sedimentary areas of Archean age for possible economic deposits of Pb and Zn.

Sulfur Isotopes in Komatiite-Hosted Ni-Cu Sulfide Deposits from the Morro do Ferro Greenstone Belt, Southeastern Brazil

In the Morro do Ferro greenstone belt in southeastern Brazil, Ni-Cu massive sulfides hosted by komatiites occur in the vicinity of the town of Fortaleza de Minas. Sulfur-isotope abundances were analyzed in sulfides from the main orebody in order to determine the source of the sulfur. The main sulfide minerals in the ore are pyrrhotite (65%), pentlandite (30%), and chalcopyrite (5%). The δ34S values of the ores fall within a relatively narrow range, but are slightly higher than values for similar occurrences in other greenstone belts. As there does not seem to be evidence of thermal erosion of country rocks and sulfur assimilation, the heavier sulfur possibly is the result of crustal contamination of the ascending komatiite magma and separation of an immiscible sulfide liquid already at this stage.

Lead isotope evidence for recent uranium mobility in geological formations of Brazil: implications for radioactive waste disposal

Abstract Lead–lead isotope data from whole rock samples are used to investigate the recent (last few million years) mobility of U and Th. The method is based on the comparison of the calculated present day U and Th concentrations required to yield the Pb isotope composition in the samples with the actual present day concentrations of U and Th obtained by direct measurement. The geological formations studied include the Neoproterozoic carbonate sediments of the Bambui Group, Archean/Paleoproterozoic granite–greenstone terrain of the Contendas–Mirante Complex and a Proterozoic ortho-gneisses hosting U deposit in Lagoa Real. All these formations are in the Sao Francisco Craton, Brazil. The data show high U mobility in the carbonate sediments and in the deformed ortho-gneisses set in a ductile shear zone. Infiltration of groundwater through fault zones seems to have facilitated the U mobility. The Pb isotope approach is a useful technique complementing U-series disequilibrium studies and may be included for site characterization studies for radioactive waste disposal.

Low Density Carbonic Fluids in the Archean Granulite Facies Terrain of the Jequié Complex, Bahia, Brazil

The 2.7 Ga old high-grade metamorphic rocks of the Jequi Complex of Bahia include charnockites, enderbites, and late pink potassic granites. Mineralogic P-T determinations for the granite terrain yield 4-5 kb/750-850C for the peak of metamorphism. Microthermometry of fluid inclusions in quartz was performed on different rock types. Early

Petrological And Geochemical Characteristics Of The Granulitic Terrain Of Brejses, Bahia, Brazil

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