Abstract
Abstract 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 to −15.4 ± 0.2, 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.
Highlights
The origin of platinum (Pt) nuggets in placer deposits and lateritic profiles worldwide remains debated, with two contrasting origins being proposed: (1) magmatic, formed in ultramafic rocks (Hattori and Cabri, 1992; Oberthür et al, 2017); and (2) neoformation under supergene conditions (Cousins and Kinloch, 1976; Bowles and Suárez, 2021)
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
Summary
The origin of platinum (Pt) nuggets in placer deposits and lateritic profiles worldwide remains debated, with two contrasting origins being proposed: (1) magmatic, formed in ultramafic rocks (Hattori and Cabri, 1992; Oberthür et al, 2017); and (2) neoformation under supergene conditions (Cousins and Kinloch, 1976; Bowles and Suárez, 2021). The former is based on the mechanical resistance of Pt-rich alloys and their low solubility, essential in causing residual concentrations in placer deposits, whereas the latter requires dissolution and precipitation processes, with or without biogenic interactions. High abundances of iodine in the Pt-Pd matrix (102 μg g−1 I; Cabral et al, 2011b), where organic carbon and nitrogen are heterogeneously distributed (Reith et al, 2016), as well as nugget-covering biofilms (Fig. 1E; Reith et al, 2016), suggest a biogenic origin
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