Platinum-Group Element Geochemical Characteristics of the Picrites and High-Ti Basalts in the Binchuan Area, Yunnan Province.

The Binchuan area of Yunnan is located in the western part of the Emeishan large igneous province in the western margin of the Yangtze Block. In the present study, the Wuguiqing profile in thickness of about 1440 m is mainly composed of high-Ti basalts, with minor picrites in the lower part and andesites, trachytes, and rhyolites in the upper part The picrites have relatively higher platinum-group element (PGE) contents (ΣPGE=16.3-28.2 ppb), with high Cu/Zr and Pd/Zr ratios, and low S contents (5.03-16.9 ppm), indicating the parental magma is S-unsaturated and generated by high degree of partial melting of the Emeishan large igneous province (ELIP) mantle source. The slightly high Cu/Pd ratios (11 000-24 000) relative to that of the primitive mantle suggest that 0.007% sulfides have been retained in the mantle source. The PGE contents of the high-Ti basalts exhibit a wider range (ΣPGE=0.517-30.8 ppb). The samples in the middle and upper parts are depleted in PGE and have ε_Nd (260 Ma) ratios ranging from −2.8 to −2.2, suggesting that crustal contamination of the parental magma during ascent triggered sulfur saturation and segregation of about 0.446%-0.554% sulfides, and the sulfide segregation process may also provide the ore-forming material for the magmatic Cu-Ni-PGE sulfide deposits close to the studied basalts. The samples in this area show Pt-Pd type primitive mantle-normalized PGE patterns, and the Pd/Ir ratios are higher than that of the primitive mantle (Pd/Ir=1), indicating that the obvious differentiation between Ir-group platinum-group elements (IPGE) and Pd-group platinum-group elements (PPGE) are mainly controlled by olivine or chromites fractionation during magma evolution. The Pd/Pt ratios of most samples are higher than the average ratio of mantle (Pd/Pt=0.55), showing that the differentiation happened between Pt and Pd. The differentiation in picrites may be relevant to Pt hosted in discrete refractory Pt-alloy phase in the mantle; whereas the differentiation in the high-Ti basalts is probably associated with the fractionation of Fe-Pt alloys, coprecipitating with Ir-Ru-Os alloys. Some high-Ti basalt samples exhibit negative Ru anomalies, possibly due to removal of laurite collected by the early crystallized chromites. [ABSTRACT FROM AUTHOR]