A silver-palladium alloy from the Bahia lateritic gold deposit, Carajas, Brazil

Abstract

A silver-palladium alloy with structural formula close to AgPd has been found in laterite from the Bahia lateritic gold deposit. The alloy occurs in a void of an iron oxide nodule, associated with goethite and hematite. The angular shape and protuberances of the alloy grains suggest crystal growth in a lateritic environment, indicating that the alloy is a secondary mineral precipitated during lateritisation. The oxidation of sulphides of the parent rocks probably favoured the migration of palladium and silver as transient thiosulphate and sulphite complexes. Destruction of the thiosulphate and sulphite ligands could result in precipitation of both palladium and silver as an alloy. Eh-pH phase diagrams for PdHzO-C1 and Ag-H20-C1 systems show that both palladium and silver are stable in lateritic environments under lower redox potentials. Such an environment may exist at the top of the ferruginous zone due to the abundant organic matter near the surface.

The Influence of Acetate and Oxalate as Simple Organic Ligands on the Behavior of Palladium in Surface Environments

“…Wood et al (1994) also showed that the presence INTRODUCTION The potential for mobility of palladium in surface environments is relevant to the use of geochemical methods of exploration (e.g., soil and biogeochemical surveys) for platinum-group element (PGE) ores and to the behavior of these elements as environmental contaminants. Several authors have interpreted field evidence in terms of the mobility of Pd in soils and laterites (Fuchs & Rose 1974, Kothny 1978, Bowles 1986, Riese & Arp 1986, Taufen & Marchetto 1989, Wood & Vlassopoulos 1990, Coker et al 1991, Cook et al 1992, Zang et al 1992, Cook & Fletcher 1993, Bowles et al 1994, Salpéteur et al 1995. Other authors have demonstrated uptake of PGE by plants and have suggested their possible use in biogeochemical exploration surveys (Kothny 1978, 1979, 1987, Dissanayake & Kritsotakis 1984, Dunn 1986, Riese & Arp 1986, Dunn et al 1989, Hall et al 1990, Rencz & Hall 1992, Roger & Dunn 1993, Fletcher et al 1995.…”

Advances in Amazonian Biogeochemistry

“…Palladium and Pt, which are not included in the cluster analysis, also show a similar enrichment pattern as Fe, suggesting that they may also belong to this association. Two Ag-Pd alloy grains were identified in the ferruginous zone (111).…”