Geochemical characteristics and geological significance of apatites in the Pulang porphyry copper deposit, NW Yunnan Province

The Pulang porphyry copper deposit is located at the south segment of the Yidun island arc of the Sanjiang-Tethys Orogen, which is the product of the subduction of the Garze-Litang oceanic plate in Late Triassic. The major and trace elements including Sr and Ga, rare earth elements ( REE) and halogens in apatite from the ore-related intrusions have been determined using electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to evaluate the potential of apatite as a petrogenetic-metallogenic indicator. In this study, both unaltered and altered apatite grains have been selected from the ore-related quartz diorite porphyry and quartz monzonite porphyry. Generally, the altered apatite shows variable but lower Na, S, Mn, Ca, Y and REE concentrations than the unaltered apatite. Most of the apatite analyses are characterized by high Sr, delta Eu and low Y, indicating that the parental magma has the adakite-like signature. The results from this study show that halogens, Sr and REE in apatite can be used to track parental magma compositions, oxidation states and crystallization process. The apatite in the Pulang deposit shows higher delta Eu, lower Mn and delta Ce, and relatively negative correlation between delta Eu and delta Ce compared with the Triassic Xiuwacu barren plutons. This indicates that the oxidation state of the ore-bearing porphyries from Pulang is higher. In addition, the decreases in (La/Sm)(N), (La/Yb)(N) and (Sm/Yb) (N) ratios are likely a result of fractionation of monazite or the exsolution of bearing-Cl hydrothermal fluids. The Cl/F ratios in the Pulang apatite suggest that it might form in a typical subduction condition.