Extreme fractionation of selenium isotopes and possible deep biospheric origin of platinum nuggets from Minas Gerais, Brazil

Platinum-rich nuggets offer an opportunity for understanding how precious metals accumulate. We analyzed the selenium (Se) isotopic composition of Se-rich (102-103 µg g-1) platinum-palladium (Pt-Pd) nuggets from a recent placer deposit in Minas Gerais, Brazil, for which a biogenic origin has been in...

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Bibliographic Details
Published in:Geology (Boulder) 2021-11, Vol.49 (11), p.1327-1331
Main Authors: Cabral, Alexandre Raphael, König, Stephan, Eickmann, Benjamin, Brauns, Michael, Tupinamba, Miguel, Lehmann, Bernd, Varas-Reus, María Isabel
Format: Article
Language:English
Subjects:
absolute age
biogenic processes
Biological activity
Biosphere
Brazil
Chemical composition
Composition
Corrego Bom Sucesso
Diamantina Brazil
Economic geology
Fractionation
Geochronology
Geology
Heavy metals
Iodine
Isotope composition
Isotopes
Jurassic
Lower Jurassic
Mesozoic
metal ores
Metals
Microbial activity
Microorganisms
Minas Gerais Brazil
mineral deposits, genesis
Minerals
Organic carbon
Palladium
palladium ores
placers
Platinum
platinum group
platinum ores
Pt/Os
Se-82/Se-76
Selenium
Selenium isotopes
South America
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Summary:Platinum-rich nuggets offer an opportunity for understanding how precious metals accumulate. We analyzed the selenium (Se) isotopic composition of Se-rich (102-103 µg g-1) platinum-palladium (Pt-Pd) nuggets from a recent placer deposit in Minas Gerais, Brazil, for which a biogenic origin has been inferred. We obtained Se isotopic values with a relatively narrow range (δ82/76SeNIST3149 = -17.4 ppm to -15.4 ppm ± 0.2 ppm, two standard deviations [2 SD]). The Pt-Os age of the nuggets is 181±6 Ma (2 SD). The data indicate that the nuggets did not form in the recent placer deposit, but by replacement of hydrothermal vein minerals at ∼70°C and at least 800 m below the surface. The high abundance and extreme isotopic composition of Se as well as the presence of other biophilic elements like iodine, organic carbon, and nitrogen within the nugget matrix are consistent with a microbial origin. Although abiogenic reduction of Se oxyanions cannot be ruled out, the nuggets plausibly record Se-supported microbial activity in the deep biosphere.
ISSN:0091-7613
1943-2682
DOI:10.1130/G49088.1