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 (10(2)-10(3) mu g 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 (delta Se-82/76(NIST3149) =- 17.4 parts per thousand to -15.4 parts per thousand +/- 0.2 parts per thousand, 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 similar to 70 degrees 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.