Geochemical characteristics and significance of apatite from the Koktokay pegmatitic rare-metal deposit, Altay, Xinjiang

Abstract

磷灰石可以记录和保存岩浆和热液活动的信息。可可托海伟晶岩型稀有金属矿床磷灰石发育，为研究该矿床伟晶岩成岩成矿过程提供了优良的条件。已有对可可托海伟晶岩型稀有金属矿床磷灰石的研究集中在其稀土元素特征，较少讨论其对伟晶岩成岩成矿过程的制约。本文选取可可托海伟晶岩型稀有金属矿床富矿伟晶岩脉（3号脉）和相对贫矿伟晶岩脉（1号、2b号和3a号脉）中的磷灰石作为研究对象，进行磷灰石岩相学和地球化学研究。岩相学分析表明，磷灰石主要与钠长石、石英、白云母、锰铝榴石等伴生。EPMA分析显示，磷灰石F含量为3.67%~4.41%，Cl含量小于0.67%，较低的Cl含量表明伟晶岩熔体出溶的流体Cl含量较低；大部分磷灰石MnO含量为4.67%~8.71%，但2b号脉磷灰石MnO含量变化较大（1.23%~14.28%），这是由于2b号脉磷灰石具有分带结构，暗示其遭受后期热液作用，促使磷灰石溶解-再沉淀，导致MnO含量发生较大变化。LA-ICP-MS分析显示，贫矿伟晶岩脉磷灰石的稀土元素含量较低（<180×10^-6）；相反，富矿伟晶岩脉磷灰石的稀土元素含量较高（>700×10^-6），并具有明显的四分组效应（TE_1-3平均值为1.7）。1号脉和3a号脉磷灰石均显示轻稀土元素富集，反映其形成过程中有含Cl热液的参与。3号脉磷灰石显示强烈Eu负异常和Ce正异常，而2b号脉磷灰石显示强烈Ce负异常和中等Eu负异常，这种Eu、Ce异常的差异可能与岩浆-热液阶段大量流体出溶密切相关。磷灰石的沉淀将导致热液中HF含量的降低，促使磷灰石周围铌钽矿结晶和Nb、Ta进入磷灰石中。可见，在伟晶岩形成过程中，磷灰石并非保持稳定，其分带结构和主微量成分变化记录了后期热液活动，暗示后期热液活动对伟晶岩的成矿具有重要作用。;Apatite records and preserves the information of magmatic and hydrothermal processes. There is much apatite in Koktokay pegmatitic rare-metal deposit, which provides excellent conditions for studying the diagenetic and metallogenic process of pegmatite in the deposit. Previous studies on apatite from the deposit have focused on the characteristics of rare earth elements, and less on its constraints on the diagenesis and mineralization of pegmatite. In this paper, apatite from rare-metal enriched pegmatite (No.3) and relatively barren pegmatite (No.1, No.2b, and No.3a) of the Koktokay pegmatitic rare-metal deposit is selected as the research object to study their petrography and geochemistry. Petrographic analysis shows that the apatite is mainly associated with albite, quartz, muscovite, and spessartine. EPMA analysis shows that the F content of apatite is 3.67%~4.41%, and the Cl content is less than 0.67%. The low Cl content indicates that the Cl content of the fluid exsolved from pegmatite melt is low; The MnO content of most apatite is 4.67%~8.71%, but the MnO content of apatite from No.2b pegmatite changes greatly (1.23%~14.28%), which is due to the zoning structure of the apatite, suggesting that it was affected by later hydrothermal process, which promoted apatite dissolution and reprecipitation, resulting in great changes in MnO content. LA-ICP-MS analysis shows that the rare earth element content of apatite from the relatively barren pegmatite is low (<180×10^-6); On the contrary, the content of rare earth elements in apatite of the enriched pegmatite is high (>700×10^-6) and has obvious REE tetrad effect (the average value of TE_1-3 is 1.7). The apatite from No.1 and No.3a pegmatite shows enrichment of light rare earth, reflecting the participation of bearing Cl hydrothermal fluid in their formation. The apatite of No.3 pegmatite shows extreme negative Eu anomaly and positive Ce anomaly, while the apatite of No.2b pegmatite shows strong negative Ce anomaly and medium negative Eu anomaly. The difference between Eu and Ce anomalies may be closely related to a large number of fluids dissolved by the melt in the magmatic-hydrothermal stage. The precipitation of apatite leads to the decrease of HF content in hydrothermal fluid, promotes the crystallization of columbite-tantalite group mineral around apatite, and the entry of Nb and Ta into apatite. Therefore, apatite is not stable during the formation of pegmatite, and its zoning structure and changes of major and trace composition record the later hydrothermal process, suggesting that the process plays a vital role in the mineralization of pegmatite.