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

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.

Metal recycling tracked by mercury and helium isotopes in platinum–palladium nuggets from Córrego Bom Sucesso, Brazil

“…The age coincides with the final stage of rapid exhumation that brought basement granite-gneissic rocks to ~70°C until the Eocene-Oligocene (Amaral-Santos et al, 2019). The nugget Pt-Os age coincides with the emplacement of Transminas dolerite dykes that occur in the study area (Cabral et al, 2021) and in south-eastern Brazil, related to the South Atlantic opening (Chaves and Correia Neves, 2005).…”

“…Platinum-osmium age determinations of 5 nuggets are indistinguishable within error (181 ± 6 Ma; Cabral et al, 2021). The age coincides with the final stage of rapid exhumation that brought basement granite-gneissic rocks to ~70°C until the Eocene-Oligocene (Amaral-Santos et al, 2019).…”

“…Microbial reduction of aqueous Hg(II) has experimentally induced mass-dependent fractionation only, leading to isotopically lighter Hg(0) (Kritee et al, 2007). The data dispersion in Figure 3A Assuming a 190 Pt-186 Os age of 181 Ma (Cabral et al, 2021), less than 2% of the 4 He released from the Pt-Pd nuggets could be derived from in situ radiogenic decay of U and Th, implying that the nugget-trapped He comes from an external source. Low 3 He/ 4 He ratios are typical of aquifers in Li-poor (<15 µg/g Li) crustal rocks (e.g., Castro, 2004).…”

“…This view has recently been disputed on the basis of Pt-Os age of ca. 180 Ma and Se isotopes, which suggest nugget formation in the deep biosphere (Cabral et al, 2021). The Δ 199 Hg and Δ 201 Hg signals and reconnaissance He isotopic determinations provide new constraints on the geochemical processes responsible for the generation of the nuggets.…”

“…3B), suggesting that the MIF was driven by aqueous Hg(II) photoreduction. On the other hand, the Pt-Pd nuggets appear to have been formed by microbial mediation (Reith et al, 2019;Cabral et al, 2021).…”