滇西普朗斑岩铜矿床中磷灰石的地球化学特征及其地质意义

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

滇西普朗斑岩铜矿床位于三江特提斯构造带义敦岛弧南端,是晚三叠世甘孜-理塘洋壳俯冲背景下的成矿作用产物。本文选取普朗矿床含矿石英闪长玢岩和石英二长斑岩中蚀变和未蚀变磷灰石进行研究,以探讨其对成岩、成矿作用的指示意义。结果显示,相比于未蚀变磷灰石,蚀变磷灰石的Na、S、Mn、Ca、Y及REE等元素含量均有不同程度的降低。总体而言,普朗矿床中磷灰石均具有高Sr、δEu,低Y的特征,指示了其母岩浆具似埃达克质特点。本次研究表明磷灰石的卤素、Sr和REE可以用来追踪成矿母岩浆成分、氧化状态和结晶演化过程。与三叠纪不成矿的休瓦促岩体相比,普朗矿床成矿岩体的磷灰石具高δEu、低Mn和δCe的特点,且δEu和δCe总体呈负相关趋势,表明普朗矿床成矿岩体氧逸度相对较高。此外,磷灰石中(La/Sm)_N、(La/Yb)_N和(Sm/Yb)_N比值降低可能是岩浆中独居石结晶或含Cl热液出溶的结果。与邻区的春都矿床、白垩纪休瓦促成矿岩体对比研究表明,普朗斑岩成矿岩浆具有高Cl/F特征,可能形成于典型的大洋俯冲环境。

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,δEu 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 higherδEu,lower Mn and δCe,and relatively negative correlation between δEu and δCe 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）Nand（ Sm/Yb）Nratios 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.