西南三江甭哥金矿床磷灰石地球化学特征及地质意义

甭哥金矿床位于西南三江造山带北段的义敦弧南缘,属于与富碱侵入岩有关的金矿床。目前,对其成矿机理认识仍较为薄弱,制约了资源评价和找矿勘查进展。本文选取甭哥金矿床强矿化的正长斑岩和弱矿化的黑云辉石正长岩中的磷灰石作为研究对象,详细剖析磷灰石的地球化学特征,探讨其记录的成岩成矿信息。结果表明,磷灰石的稀土元素含量特征及配分模式显示富碱岩浆主要来自于壳幔混合的源区,黑云辉石正长岩中磷灰石的(La/Sm)N、(La/Yb)N、(Sm/Yb)N值和Sr含量呈正相关,说明长石结晶对岩浆结晶分异有重要的影响;正长斑岩中磷灰石具有高Sr/Y、Ce/Pb值,而Th/U、(Sm/Yb)N值较低,指示强烈的流体活动参与了岩浆结晶过程;磷灰石挥发分(F、Cl)含量及比值特征指示金矿成矿流体主要来自地幔源区,成矿与富碱、高氯的成矿流体有关。磷灰石Mn氧逸度计估算结果显示,甭哥富碱侵入岩具有高氧逸度特性,但两种不同岩性岩石的氧逸度具有差异性。其中,正长斑岩的logf_(O_2)值为-12～-10. 3,黑云辉石正长岩的logf_(O_2)值为-15. 5～-11. 1,磷灰石中SO_3含量及Ga含量也暗示正长斑岩的氧逸度高于黑云辉石正长岩的特征;结合磷灰石低Mn、Ga含量和高的Cl、SO_3含量,反映甭哥金矿床金的成矿是在高氧逸度条件下金氯络合物迁移、富集而沉淀的结果。因而,磷灰石的地球化学特征对金矿床成矿过程示踪和勘查评价具有重要的指示意义。

The geochemical characteristics and their geological significance of apatite from the Bengge gold deposit in Sanjiang region, SW China

The Bengge gold deposit located at southern Yidun arc margin along the Sanjiang orogenic belt in southwestern China belongs to the hydrothermal-type gold-polymetallic deposit related to the alkali-rich intrusive rocks. Some predecessors have carried out much preliminary research for its geology, however the advances are weak of constrains on the ore genesis and its metallogenesis conditions. So, there still obstacles of the ore-forming mechanism and mineral resource exploration. In this contribution, the authors choose apatite of the strong mineralized syenite porphyryies and the weak mineralized biotite pyroxene syenites from the Bengge gold deposit, respectively, to recover the authentic ore-forming processes records through the mineral geochemical tracing. The REE content and chondrite-normalized model of apatite show that the alkali-rich magma mainly originates from the crust-mantle mixing source. The ratio of (La/Sm)_N, (La/Yb)_N, (Sm/Yb)_N and Sr content of the biotite pyroxene syenite shows a positive correlation distribution, indicating the feldspar crystallization play an important role on the crystallization differentiation process. The apatite from the syenite porphyry has higher ratio value of Sr/Y and Ce/Pb, but lower ratio value of Th/U and (Sm/Yb)_N hinted that the syenite porphyry underwent strongly fluid participation during crystallization process. The volatile components variation including F and Cl of apatite indicates that the ore-forming fluids mainly originate from the mantle, and the gold mineralization is mainly of the alkali-rich and chlorine-rich fluids. The oxygen fugacity variation of apatite through quantitative calculated of Mn content show that, the alkali-rich intrusive rocks are generally characterized by different high oxygen fugacity for the different types. That is, the logf_O2 value of the syenite porphyries is from -12 to -10.3, and the logf_O2 value of the biotite pyroxene syenites is from -15.5 to -11.1. The variation tendency is similar with the SO₃ content and Ga content in apatites, indicates that the oxygen fugacity of the syenite porphyries is higher than that of the biotite pyroxene syenites. In summary, the geochemical characteristics of apatite can definite the metals mineralization processes, and the content of trace elements in apatites can be used as the distinguishing indicator to symbol the gold mineralization of the Bengge gold deposit. In addition, the lower content of Mn, Ga, and higher content of Cl, SO₃ in apatites is also a key indicator for mineralization intensity. The migration and precipitation of gold in the form of gold-chlorine complex with high oxygen fugacity is the critical ore formation mechanism for this deposit.