Chemical and physical transfers in an ultramafic rock weathering profile: Part 1. Supergene dissolution of Pt-bearing chromite
Résumé
Chemical weathering and supergene dissolution processes of Pt-bearing chromite have been studied
in a lateritic weathering profile developed on ultramafic rocks in New Caledonia (southwest Pacific).
The chemical distributions of alkaline earth, transition metals, and precious metals (including Pt and
Pd) were determined in a weathering profile varying from bedrock at the base upward through coarse
and fine saprolites, and capped by a mottled zone and a lateritic colluvial nodular horizon. Chemi-
cal analyses and mass-balance calculations suggest that progressive weathering of the parent rock
is characterized by an enrichment of Fe, Co, and Mn, a segregation of Ni at the boundary between
the bedrock and the coarse sapolite and in the lower part of the fine saprolite, and a depletion of Mg,
Ca, Si, Al, and Cr. The higher concentration of transition metals at the interface between the coarse
and fine saprolite is due to vertical transfer and precipitation at the base of the weathering profile.
In such a lateritic environment, the Pt-bearing chromite grains are progressively dissolved and the
Pt-group minerals (PGM) are released in the weathering mantle with a preferential depletion of Pd
with regard to Pt.
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