Palladium Oxides in Ultramafic Complexes Near Lavatrafo, Western Andriamena, Madagascar

L. McDonald1, *, D Ohnenstetter2, M. Ohnenstetter2, 3 and D. J. Vaughan1
1 Dept. of Earth Sciences, University of Manchester, Manchester M13 9PL, UK
2 CNRS-CRPG, 15 Rue Notre Dame des Pauvres, BP20 54501 Vandoeuvre les Nancy, France
3 BRGM, Avenue Claude Guillemin, 45060 Orleans, Cedex 2, France
*Present address: School of Earth & Environmental Sciences, University of Greenwich, Chatham Maritime, Kent ME4 4TB, UK.

Abstract: Small (<2 km diameter), ultramafic intrusive complexes in the Andriamena region of Madagascar contain zones with disseminated chromite and sulphides which carry high concentrations of platinum-group elements (PGE). Assay of drill core from one complex, designated UM2, revealed three zones showing consistently high PGE grades. Mineralogical investigation of the UM2 core reveals that a small number of low reflecting, Pd-bearing platinum-group minerals (PGM) are present — always in association with a poorly characterized Pd stibio-arsenide [Pd3(Sb,As)] phase — in the shallowest of the three zones. Electron microprobe analysis of these PGM indicates the presence of oxygen and that at least two species exist. The resulting stoichiometries suggest that at least one species could be a hydrated form of palladinite [PdO.(H2O)n]. The other phase could be a hydroxide [Pd(OH)2] or a less strongly hydrated form of palladinite. Textural evidence suggests that the Pd-O species formed via the replacement of a precursor Pd-rich PGM, with only a limited removal of Pd, rather than via precipitation of a Pd-O PGM from a fluid that was Pd-bearing. The limited thermal stability of hydrated Pd oxides and the apparent restriction of the Pd-O phases to a shallow zone which is affected by the seasonal movement of groundwater, suggests that they may have formed at low temperatures via the leaching and replacement of other elements from a precursor Pd-rich PGM by oxygen and water during alternating water-saturated and dry conditions. If this is the case, the water table interface might be another environment, in addition to the surface, in which to look for the development of PGE oxides.

Keywords: platinum-group elements • palladium • oxygen • hydrated oxides • Andriamena • Madagascar

Mineralogical Magazine; June 1999 v. 63; no. 3; p. 345-352; DOI: 10.1180/002646199548565
© 1999, The Mineralogical Society
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