Differentiated rare-element mineralization in an ongonite-topazite composite dike at the Xianghualing tin district, Southern China: An electron-microprobe study on the evolution from niobium-tantalum-oxides to cassiterite
Differentiated rare-element mineralization in an ongonite-topazite composite dike at the Xianghualing tin district, Southern China: An electron-microprobe study on the evolution from niobium-tantalum-oxides to cassiterite
复制标题
华南香花岭锡区红辉石-黄玉石复合脉中的差异性稀有元素矿化：铌钽氧化物到锡石演化的电子探针研究
DOI：
10.1016/j.oregeorev.2014.08.008
复制
发表时间：
2015
影响因子：
3.3
通讯作者：
Zhang Rong-Qing
中科院分区：
文献类型：
--
作者：
Huang Fang-Fang;Wang Ru-Cheng;Xie Lei;Zhu Jin-Chu;Erdmann Saskia;Che Xu-Dong;Zhang Rong-Qing
本文详细描述了一个高度演化的、复合的红长岩-黄玉岩脉的矿物学、结构和成分特征及其岩浆分异历史。我们目前的组成数据收集的既定技术，即通过电子探针和湿化学分析，这为未来的研究，采用国家的最先进的分析技术提供了一个详细的框架。431号岩墙出露于南岭香花岭地区，与侏罗纪锡铌钽花岗岩岩体空间关系密切。431堤岩样采自构造较低的钻孔和较高水平的探槽。在整个岩脉中均发现了奥长岩，但黄玉岩仅沿着上部近地表岩脉的边缘露出。全岩常量和微量元素分析结果表明，431脉岩石为强过铝质，平均ACNK值为：奥长岩~ 1.5，黄玉> 3.9。它们富含F，1.7重量%和5.4重量%平均分别为红云母和黄玉。岩石具有较低的Zr/Hf和Nb/Ta比值，较高的Nb、Ta、Sn、W等成矿元素含量。硅酸盐和氧化物矿物组合，纹理和成分也不同的两种岩石类型的研究。在岩脉下部的钾长岩中，有丰富的钾长石、石英和钠长石斑晶，以及在石英、钾长石和钠长石为主的基质中的黄玉和锌钠长石微斑晶。特征氧化物是铌钽铁矿、钽铁矿和微晶石，但不含铌钽铁矿。岩脉上部的钙长岩具有与下部钙长岩相似的硅酸盐组合;铌铁矿-（Mn）、铀微晶和少量的钙长石是主要的副矿物。黄玉的特征是大量的黄玉和锌硅锌矿与石英共生，而钾长石、钠长石和石英斑晶呈圆形，相对罕见。锡石是最丰富的矿石矿物，而铌钽氧化物矿物较少。我们解释了全岩成分趋势，矿物纹理，组合和成分，以反映分化的演化，最初均匀的岩浆，分离成铝硅酸盐和水盐熔体，分别对应于结晶的长角闪岩和黄玉。含铌钽锡矿物的结晶受两种熔体相分离的强烈控制。我们假设，最初的堤坝就位后，堤坝传播/扩大可能已经驱动分离的铝硅酸盐和水盐熔体相，结晶为ongonite在核心和黄玉沿着的边缘的结构较高的一部分的堤坝。
Our study characterizes in detail the mineralogical, textural and compositional features of a highly evolved, composite ongonite–topazite dike and its magmatic differentiation history. We present compositional data collected by established techniques, i.e. by electron microprobe and wet-chemical analysis, which provide a detailed framework for future studies that employ state-of-the-art analytical techniques. The studied dike (referred to as the No. 431 dike) crops out within the Xianghualing area in the Nanling Range of southern China, in close spatial association with Jurassic Sn–Nb–Ta granite plutons. The rock samples in the No. 431 dike were collected from a structurally lower drill hole and a trench at higher level. The ongonite is encountered throughout the dike, but the topazite is only revealed along the margin of the upper, near-surface dike. The results of whole-rock major and trace element analyses show that the rocks of the No. 431 dike are strongly peraluminous with an average ACNK value of ~ 1.5 for ongonite and > 3.9 for topazite. They are enriched in F, 1.7 wt.% and 5.4 wt.% on average for ongonite and topazite, respectively. The rocks have low Zr/Hf and Nb/Ta ratios, and high levels of ore-forming elements including Nb, Ta, Sn, and W. Silicate and oxide mineral assemblages, textures, and compositions are also distinct for the two rock types studied. In the lower ongonite of the dike, there are abundant phenocrysts of K-feldspar, quartz, and albite, and microphenocrysts of topaz and zinnwaldite in a matrix dominated by quartz, K-feldspar, and albite. Characteristic oxides are columbite–tantalite, tapiolite, and microlite, but cassiterite is absent. The upper ongonite of the dike has a silicate assemblage similar to the lower ongonite; columbite-(Mn), uranomicrolite, and limited amounts of cassiterite are the dominant accessory minerals. The topazite is characterized by large amounts of topaz and zinnwaldite intergrown with quartz, while K-feldspar, albite, and quartz phenocrysts have rounded shapes and are relatively rare. Cassiterite is the most abundant ore mineral, while Nb–Ta oxide minerals are less abundant. We interpret the whole-rock compositional trends, mineral textures, assemblages, and compositions to reflect the differentiation of an evolved, initially homogeneous magma that separated into aluminosilicate and hydrosaline melts, corresponding to crystallization of ongonite and topazite, respectively. The crystallization of Nb–Ta- and Sn-bearing ore minerals was strongly controlled by the separation of the two melt phases. We hypothesize that dike propagation/widening subsequent to the initial dike emplacement may have driven the separation of the aluminosilicate and hydrosaline melt phases that crystallized to ongonite in the core and topazite along the margins of the structurally higher part of the dike.