Surface transformations of platinum grains from Fifield, New South Wales, Australia

Abstract

A growing literature is demonstrating that platinum (Pt) is transformed under surface conditions; yet (bio)geochemical processes at the nugget-soil-solution interface are incompletely understood. The reactivity of Pt exposed to Earth-surface weathering conditions, highlighted by this study, may improve our ability to track its movement in natural systems, e.g., focusing on nanoparticles as a strategy for searching for new, undiscovered sources of this precious metal. To study dissolution/re-precipitation processes of Pt and associated elements, grains of Pt-Fe alloy were collected from a soil placer deposit at the Fifield Pt-field, Australia. Optical-and electron-microscopy revealed morphologies indicative of physical transport as well as chemical weathering. Dissolution "pits," cavities, striations, colloidal nano-particles, and aggregates of secondary Pt platelets as well as acicular, iron (Fe) hydroxide coatings were observed. FIB-SEM-(EBSD) combined with S-m-XRF of a sectioned grain showed a fine layer of up to 5 mm thick composed of refined, aggregates of 0.2 to 2 mm sized crystalline secondary Pt overlying more coarsely crystalline Pt-Fe-alloy of primary magmatic origin. These results confirm that Pt is affected by geochemical transformations in supergene environments; structural and chemical signatures of grains surfaces, rims, and cores are linked to the grains' primary and secondary (trans)formational histories; and Pt mobility can occur under Earth surface conditions. Intuitively, this nanophase-Pt can disperse much further from primary sources of ore than previously thought. This considerable mineral reactivity demonstrates that the formation and/or release of Pt nanoparticles needs to be measured and incorporated into exploration geochemistry programs.

Supergene neoformation of Pt-Ir-Fe-Ni alloys: multistage grains explain nugget formation in Ni-laterites

“…Bowles et al 1994;Varajão et al 2000;Azaroual et al 2001;Colombo et al 2008), but there are only a few descriptions from natural sites showing evidence for PGE mobilization and reprecipitation (e.g. Talovina and Lazarenkov 2001;Melcher et al 2005;Cabral et al 2007Cabral et al , 2009Suárez et al 2010;Garuti et al 2012;Oberthür et al 2003Oberthür et al , 2014Campbell et al 2015). However, in a recent study, Reith et al (2016) reported that Pt is mobile under surface conditions and can become bioavailable.…”

Microscale data to macroscale processes: a review of microcharacterization applied to mineral systems

“…Once the reactions and the reaction mechanisms are understood, they can be used to bracket fluid compositions and the volumes responsible for mineralization from numerical modelling (Pearce et al 2015) or analogous experiments (Etschmann et al 2014). Campbell et al (2015) used EBSD combined with chemical analysis to demonstrate that a layer of pure supergene platinum formed on the surface of magmatic nuggets was microcrystalline and interpreted this in terms of the reactivity of platinum in the soil environment. Another common use of EBSD data in rock deformation is to examine the effects of crystallographic orientation on rock properties such as seismic anisotropy (Mainprice & Humbert 1994), piezoelectricity (Mainprice et al 2014) and magnetic anisotropy.…”

Evaluating the potential of capillary rise for the migration of Pt nanoparticles in Luvisols and Phaeozems (Western Siberia)

“…Among the metal NPs, platinum nanoparticles (PtNPs) deserve special attention. On the one hand, PtNPs are produced by humans for further application in the medical and chemical industries (Alex and Tiwari, 2015), and on the other hand, they are formed under natural conditions during the weathering of Au/Pt deposits (Hough et al, 2008(Hough et al, , 2011Campbell et al, 2015). Stabilized metal NPs can be subjected to dispersion and further transportation in natural waters, and changes in the geochemical conditions of the environment can lead to their deposition and the formation of secondary enrichment zones (Southam et al, 2009;Reith et al, 2010;Reith and Cornelis, 2017;Korshunova and Charykova, 2019).…”