Research paper Iodine in alluvial platinum–palladium nuggets: Evidence for biogenic precious-metal fixation Alexandre R. Cabral a,b, ⁎, Martin Radtke c , Frans Munnik d , Bernd Lehmann b , Uwe Reinholz c , Heinrich Riesemeier c , Miguel Tupinambá e , Rogerio Kwitko-Ribeiro f a Department of Geology, Exploration Geology, Rhodes University, PO Box 94, Grahamstown, 6140, South Africa b Mineral Resources, Technische Universität Clausthal, Adolph-Roemer-Str. 2A, 38678 Clausthal-Zellerfeld, Germany c BAM Federal Institute for Materials Research and Testing, Richard-Willstätter-Str. 11, 12489 Berlin, Germany d Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf, P.O. Box 510119, 01314 Dresden, Germany e Tektos-Geotectonic Research Group, Faculdade de Geologia, Universidade do Estado do Rio de Janeiro, Rua S. Francisco Xavier 524 s. A4016, 20550-050 Rio Janeiro-RJ, Brazil f Centro de Desenvolvimento Mineral, Companhia Vale do Rio Doce, Rodovia BR 262/km 296, Caixa Postal 09, 33030-970 Santa Luzia-MG, Brazil abstract article info Article history: Received 12 July 2010 Received in revised form 19 November 2010 Accepted 5 December 2010 Available online 11 December 2010 Editor: J.D. Blum Keywords: Iodine Pt–Pd nuggets Córrego Bom Sucesso Minas Gerais Brazil Synchrotron radiation-induced X-ray fluorescence (SR-XRF) spectrometry was applied to determine iodine concentrations in alluvial Pt–Pd aggregates with delicate morphological features from Córrego Bom Sucesso, Minas Gerais, Brazil. The millimetre-sized botryoidal and rod-shaped grains of alluvial Pt–Pd–Hg intermetallic compounds have surprisingly high concentrations of iodine, in the range from 10 to ~ 120 μg/g. Because iodine is a strongly biophile element, known to be enriched in peatlands and plant remains in soils by microbial activity, its concentration in the Pt–Pd nuggets suggests biogenic precious-metal fixation in the aqueous alluvial milieu. Biogenic and inorganic processes, such as bioreduction and electrochemical metal accretion, could mutually have contributed to the growth of nanoparticles, formed on organic templates, to millimetric Pt–Pd nuggets. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Because of its strong biophilic character, iodine concentrations in waters are controlled by microbial activity (e.g. Collins and Egleson, 1967; Elderfield and Truesdale, 1980; Worden, 1996). In marine settings, microbial decomposition of organic matter in buried sediments not only produces high concentrations of iodine, not rarely over 1 mM, in pore waters (e.g. Fehn et al., 2006; Gieskes and Mahn, 2007; Muramatsu et al., 2007), but also a pore-water signature of biophilic iodine that can be recognised in mantle-wedge peridotite (Sumino et al., 2010). Such an association with microbial activity has made iodine a tracer of organic matter (e.g. Fehn et al., 2007; Tomaru et al., 2009; Scholz et al., 2010). In terrestrial settings, organoiodine formation during microbial decomposition of organic matter is a key process in the storage of iodine in peatlands, which are a major reservoir of iodine on land (Keppler et al., 2004). The close link between iodine and organic matter suggests that iodine should be a relatively abundant trace element in minerals formed in environments of intense microbial activity. Indeed, bacterioform morphologies of precious-metal aggregates, particularly of Au, have been used as evidence for mineral formation mediated by micro-organisms in the weathering environment (Watterson, 1991; Mann, 1992; Reith et al., 2006; Lengke and Southam, 2007). However, no data on iodine contents have been reported from precious-metal aggregates. One of the main reasons for the scarcity of data is the paucity of analytical techniques having iodine detection levels sufficient for microscale determinations. Apart from bacterial biofilms on naturally occurring secondary Au aggregates (Reith et al., 2006), evidence for the biogenic accumulation of precious metals within residual weathering deposits and alluvia has been based on morphological features only. A geochemical fingerprint indicating biogenic precious-metal fixation in natural systems has not been established. Here we present the first in situ measurements for iodine in alluvial Pt–Pd aggregates. These aggregates have delicate morphological features that are suggestive of in situ growth within the alluvium (Hussak, 1904, 1906). 2. Geological background and sample material Platinum–palladium nuggets were recovered from an alluvial deposit, Córrego Bom Sucesso, in the southern Serra do Espinhaço, Minas Gerais, Brazil. The mountain range of Serra do Espinhaço has Chemical Geology 281 (2011) 125–132 ⁎ Corresponding author. E-mail address: alexandre.cabral@tu-clausthal.de (A.R. Cabral). 0009-2541/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2010.12.003 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo