『Abstract
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. Chemical 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 saprolite
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.
Keywords: Lateritic weathering; ultramafic rocks; mass-balance
calculation; Pt-chromite; New Caledonia』
Introduction
Geological setting
Material and methods
Results and discussion
Physical and geochemical characterization
Density and porosity change
Petrolic and geochemical patterns
Mass-balance calculations
Supergene dissolution of chromite
Residual origin of platinum
Concluding statement
Acknowledgments
References cited