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

Cabral, A. R., Deng, C., Yin, R., Yakubovich, O. V., Stuart, F. M. , Tupinambá, M. and Lehmann, B. (2022) Metal recycling tracked by mercury and helium isotopes in platinum–palladium nuggets from Córrego Bom Sucesso, Brazil. Chemical Geology, 593, 120752. (doi: 10.1016/j.chemgeo.2022.120752)

Abstract

The enigmatic botryoidal nuggets of platinum (Pt) and palladium (Pd) from Córrego Bom Sucesso in the southern Serra do Espinhaço, Minas Gerais, Brazil, are considered to have formed during supergene alteration of placer deposits. This requires hitherto unidentified precious-metal enrichment processes and is inconsistent with Pt–Os age of 180 Ma that entails formation at depth, as recently proposed. Here we report the first mercury (Hg) and helium (He) isotopic determinations of Pt–Pd nuggets. Mercury isotopic compositions have a mass-independent fractionation (MIF) signature with an odd-mass deficit (Δ199Hg = − 0.22 ± 0.04; 1SD, n = 15), which requires aqueous photochemical reduction of Hg (II). Extremely low 3He/4He (<0.001 Ra) and extremely high concentrations of He (up to 1.9 × 1017 at/g) are indicative of nugget formation from He-enriched fluids within the quartzite sequence of the Espinhaço basin, not from meteoric surface water. The data are consistent with a nugget-forming setting in the deep biosphere, as a result of groundwater interaction with Pt–Pd–Hg minerals in Pan-African-Brasiliano post-orogenic veins. We propose that the negative Hg-MIF signature was inherited from the vein minerals that originally acquired their Hg from Earth's surface during the intracratonic sedimentation of the Proterozoic Espinhaço basin.

• Item Type: Articles
• Additional Information: This research was supported by the SUERC, State Task of the IPGG RAS (FMNU-2022-0003).
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Stuart, Professor Fin
• Authors: Cabral, A. R., Deng, C., Yin, R., Yakubovich, O. V., Stuart, F. M., Tupinambá, M., and Lehmann, B.
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Chemical Geology
• Publisher: Elsevier
• ISSN: 0009-2541
• ISSN (Online): 1872-6836
• Published Online: 02 February 2022
• Copyright Holders: Copyright © 2022 Elsevier B.V.
• First Published: First published in Chemical Geology 593: 120752
• Publisher Policy: Reproduced in accordance with the publisher copyright policy