Distribution characteristics of platinum-group elements in the Daba-Xiugugabu ophiolite zone, Tibet

Abstract：Platinum-group element (PGE) contents and distribution patterns of different mantle peridotites from ophiolite masses (blocks) are to some extent different. Relative to things of the primitive mantle, PGE in Langcuo and Dongpo masses don't show distinct anomalies. The curves of primitive mantle-normalized distribution patterns are approximately even, and there exists no strong fractionation between PPGE and IPGE. On the whole, the PGE characteristics are similar to those of mantle peridotites from Xigaze, Alps and Eastern Mediterranean regions. However, PPGE contents of Dangqiong masses vary in a wide range and don't show a strong fractionation between PPGE and IPGE. PGE contents and distribution patterns of clinopyroxenite dykes differ greatly from those of the mantle peridotite：Os, Ir, Ru and Rh are depleted whereas Pt and Pd are enriched. The pattern curve declines to the left. These characteristics suggest that clinopyroxenite dykes were probably formed by the high degree of partial melting as well as melting segregation. PGE contents of chromitite are high and vary in a very wide range of 19.7-5526 ppb. The average ∑PGE (729.33ppb) is equal to 25.6 times its value in primitive mantle. Os, Ir, Ru and Rh are enriched but Pt and Pd have experienced both enrichment and depletion as compared with their values in the primitive mantle. The pattern curves are scattered and decline to the right. There exists a strong fractionation between IPGE and PPGE, and Pt in most of samples assumes negative anomalies. These characteristics are similar to those of chromitite in ophiolites from China's Luobusha, Greece, Yugoslavia, Cyprus and some other regions in the world. PGE contents of chromitite are not related to base metal sulfides (BMS) contents in the same samples. PGE don't mainly exist in BMS. It is thought that PGE mostly occur in platinum-group element minerals (PGM), which remain to be investigated and found in future.