Characteristics and significance of aegirine and arfvedsonite in Boziguoer Nb-Ta-Zr-Rb-REE deposit related to alkaline granite, Xinjiang

Abstract

碱性造岩矿物能够记录碱性岩源区特征、岩浆演化以及晚期成矿的重要信息，是开展碱性岩成岩成矿研究的有效手段。波孜果尔碱性花岗岩型铌-钽-锆-铷-稀土矿床位于塔里木北缘-中亚南天山晚古生代造山带，是塔里木地块北缘铌成矿带中典型的碱性岩型矿床。本文通过对含矿岩体中的霓石和钠铁闪石开展矿物学研究，结合全岩成分揭示波孜果尔稀有-稀土金属矿床含矿岩体的岩石类型、演化特征、构造背景及成矿条件。研究发现，含矿碱性岩体由霓石钠铁闪石碱长花岗岩、霓石钠铁闪石石英碱长正长岩、霓石钠铁闪石碱长正长岩组成。不同岩相岩石均表现出相似的稀土和微量元素配分模式，以富集轻稀土元素和高场强元素，亏损大离子亲石元素和重稀土元素为特征，具有显著的负Eu异常，指示三种岩石类型是同源岩浆演化的产物。含矿岩体中的辉石为霓石-霓辉石，角闪石为钠铁闪石。霓石和钠铁闪石稀土和微量元素配分模式相似且含量都较低，富集重稀土、亏损轻稀土，具有显著的负Eu异常，并富集Zr、Hf等高场强元素，亏损Ba、Sr等大离子亲石元素。霓石和钠铁闪石微量元素特征指示三个岩相单元的演化程度由低到高依次为霓石钠铁闪石碱长花岗岩→霓石钠铁闪石石英碱长正长岩→霓石钠铁闪石碱长正长岩，且结晶分异作用控制了岩体的形成。同时，早期霓石钠铁闪石碱长花岗岩的钠铁闪石呈现Ce正异常，晚期岩相中则无异常，指示演化的早期阶段氧逸度较高，随着磁铁矿等氧化物的结晶熔体趋于还原。波孜果尔含矿岩体成矿元素除Rb外分布并不均匀，表现为演化早期的岩相富集Nb，而演化晚期的岩相更富集稀土和Zr。进一步研究发现，塔里木北缘碱性岩带发育的铌、稀土、钽、锆、铷、铀等稀有稀土金属矿化与地幔柱引起的幔源岩浆底侵有关，其构造背景可能为地幔柱对造山带的叠置。;Alkaline minerals can record the important information of source area characteristics, magmatic evolution and late mineralization of alkaline rocks, which is an effective means for the study of diagenesis and mineralization of alkaline rocks. Boziguoer Nb-Ta-Zr-Rb-REE deposit is a typical deposit related to alkaline rocks, which is located in the Late Paleozoic orogenic belt of the southern Tianshan-the northern margin of Tarim block. Based on mineralogical study of aegirine and arfvedsonite in the ore-bearing intrusion, combining with its whole rock compositions, this paper reveals the rock types, evolution characteristics, tectonic setting and metallogenic conditions of the intrusion in the Boziguer rare-earth metal deposit. The rock types of ore-bearing intrusion are aegirine arfvedsonite alkali feldspar granite, aegirine arfvedsonite quartz alkali feldspar syenite and aegirine arfvedsonite alkali feldspar syenite. All of the three types of rocks show similar REE and trace element distributions, with enriched LREE and HFSE, depleted HREE and LILE, and a significant negative Eu anomaly, indicating that they are the products of homologous magmatic evolution. The pyroxene group minerals are aegirine, and the amphibole group minerals are arfvedsonite. Aegirine and arfvedsonite show similar distribution patterns of trace and rare earth elements, which are characterized by enriched HREE and depleted LREE with a significant Eu negative anomaly. They all show enrichment in HFSE (Zr, Hf), depletion in LILE, such as Ba and Sr. Trace element and REE distributions of these minerals indicate that the degree of their evolution from low to high is: aegirine arfvedsonite alkali feldspar granite→aegirine arfvedsonite quartz alkali feldspar syenite→aegirine arfvedsonite alkali feldspar syenite, showing the formation of the intrusion is controlled by crystallization and differentiation. The arfvedsonite of aegirine arfvedsonite alkali feldspar granite shows a positive anomaly of Ce, indicating a high oxygen fugacity environment at the early stages of its evolution. The crystallization of magnetite in the early lithofacies make the evolution environment of the melt tend to be reduced. The Rb in all rock types is equally distributed, and the rocks of early evolution stage is more enriched in Nb, while that of the late-stage more enriched in REE and Zr. The mineralization of rare earth metals such as Nb, REE, Ta, Zr, Rb and U developed in the alkaline rock belt in the northern margin of Tarim basin is related to the mantle magma underplating caused by mantle plume, and its tectonic setting may be the superposition of mantle plume to orogenic belt.