Abstract

The Alto Condoto Complex, dated at 20 Ma, is a zoned ultramafic intrusion with a magmatic rock sequence ranging from dunite to hornblendite. The different petrographic units are cumulates formed by fractional crystallization processes.Parts of the central dunite are anomalously enriched in PtFe alloys, and to a minor degree, in IrOs alloys. All other lithologic units of the complex display normal fractionation trends of platinum-group elements (PGE), with a general increase of total PGE from wehrlite to horn-blende clinopyroxenite and hornblendite. This normal trend is best described by increasing Pd and decreasing Ir abundances.The controlling factor for PGE mineralization in zoned ultramafic complexes is probably an effective PtFe alloy precipitation during the crystallization of olivine. This may be initiated by rapid tectonic movements causing pressure decrease and overheating of the magma, with PGE alloy formation as a result of the f (sub O 2 ) shift during olivine crystallization.

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