Platinum-group element geochemistry of mafic rocks from the Dongchuan area, southwestern China

Mafic intrusions and dykes are well preserved in the Yinmin and Lanniping districts, located within the western margin of the Yangtze Block, SW China. Although these mafic rocks from the two areas formed during different periods, they share similar ranges of PGE concentration. Most of the Yinmin gabbroic dykes contain relatively high PGE concentrations （PGEs=13.9-87.0 ppb） and low S contents （0.003 %-0.020 %）, higher than the maximum PGE concentrations of mafic magmas melting from the mantle. Two exceptional Yinmin samples are characterized by relatively low PGE （PGEs = 0.31-0.37 ppb）and high S （0.114 %-0.257 %） contents. In contrast, most samples from the Lanniping gabbroic intrusion have low PGE con- centrations （PGEs = 0.12-1.02 ppb） and high S contents （0.130 %-0.360 %）, except that the three samples exhibit relatively high PGE （PGEs=16.3-34.8 ppb） and low S concentrations （0.014 %-0.070 %）. All the Yinmin and Lanniping samples are characterized by the enrichment of PPGE relative to IPGE in the primitive-mantle normalized diagrams, and the high-PGE samples exhibit obvious Ru anomalies. This study suggests that during the ascent of the parental magma, removal of Os-Ir-Ru alloys and/or chro- mite/spinel leads to high Pd/Ir ratios and Ru anomalies for the Yinmin high-PGE samples and relatively lower Pd/lr ratios and Ru anomalies for the Lanniping low-PGE samples. We propose that the magmas parental to the Yinmin gabbroic dykes are initially S-unsaturated, and subsequently, minor evolved magma reached sulfur saturation and led to sulfide segregation. Although the Lanniping parental magmas are originally not saturated in S, the high Cu/Pd ratios （3.8× 10^4 to 3.2 × 10^6） for most of the Lanniping samples indicate the S-saturated state and sulfide segregation. A calculation shows that the PGE-poor magmas might have experienced 0.01%-0.1%sulfidesegregationinthemagma chamber. Therefore, our study provides a possible opportunity to discover PGE-enriched sulfide mineralization some- where near or within the Lanniping mafic intrusion.