A Silver—Palladium Alloy from the Bahia Lateritic Gold Deposit, Carajas, Brazil

Weisheng Zang, William S. Fyfe and Robert L. Barnett
Department of Geology, University of Western Ontario, London, Ontario, Canada N6A 5B7

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 Pd-H2O-Cl and Ag-H2O-Cl 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.

Keywords: silver-palladium alloy • thiosulphate complex • chloride complex • laterite • Bahia gold deposit • Brazil

Mineralogical Magazine; March 1992 v. 56; no. 382; p. 47-51; DOI: 10.1180/minmag.1992.056.382.05
© 1992, The Mineralogical Society
Mineralogical Society (www.minersoc.org)