攀西碱性岩型Nb-Zr-REE矿床的矿物学与地球化学研究

Mineralogy and geochemistry of the alkaline rocks-related Nb-Zr-REE deposits in the Panzhihua-Xichang area,Sichuan,China

峨眉山大火成岩省中攀枝花-西昌线性构造带内的炉库和白草碱性岩型Nb-Zr-REE矿床的含矿岩脉侵入于红格层状辉长岩体中。榍石LA-ICPMS U-Pb定年结果为(258±2.1)Ma,表明该矿床成矿作用与峨眉山地幔柱活动密切相关。本文主要通过研究矿物学特征和副矿物的原位主量-微量元素组成,对仍存在较大争议的碱性岩型Nb-Zr-REE矿床成矿机制进行了更为深入的探讨。研究认为,炉库和白草矿床存在岩浆和热液两期成矿活动。岩浆期钾长石、钠长石等造岩矿物的结晶分异导致残余熔体中富集高场强元素(HFSE),形成众多富Nb、Zr、REE等高场强元素的矿石矿物(如Ce-U烧绿石、锆石、榍石等),是控制Nb-Zr-REE矿化的主要因素。在热液期时,热液交代Ce-U烧绿石、锆石、榍石等早期矿石矿物,导致高场强元素重新活化形成热液锆石和Ta烧绿石等。富集REE且有较低Nb/Ta比值的烧绿石结晶,会导致残余熔体与流体呈现低REE含量与高Nb/Ta比值,岩浆结晶分异是该区Nb-Zr-REE矿化的关键,而后期热液过程仅仅导致了局部质量分数升高。

The ore-hosting syenitic dikes of the Luku and Baicao alkaline rocks-related Nb-Zr-REE deposits in the Panzhihua-Xichang linear structural belt of the Emeishan Large Igneous Province intruded into the Hongge layered gabbros.LA-ICPMS U-Pb dating results of titanite grains show that the titanite was formed at 258±2.1 Ma,indicating that the mineralization of this deposit was closely related to the Emeishan mantle plume activity.In this paper,the controversial ore-forming mechanism of the alkaline rock related Nb-Zr-REE deposit has been further discussed based on mineralogical characteristics and in-situ analyzed major and trace elemental compositions of accessory minerals.This study shows there are magmatic-stage and hydrothermal-stage mineralization in the Luku and Baicao deposits.At the magmatic-stage,the crystallization differentiation of rock-forming minerals,such as K-feldspar and albite,had resulted in the enrichment of high field strength elements(HFSE)in the residual melt which then formed abundant ore minerals enriched in Nb,Zr and REE,such as Ce-U pyrochlore,zircon and titanite.This is the main factor controlled the Nb-Zr-REE mineralization.At the hydrothermal-stage,the matasomatism of early ore minerals,such as Ce-U pyrochlore,zircon,and titanite,by hydrothermal fluid resulted in the reactivation of high field strength elements from these minerals to have formed hydrothermal zircon and Ta-pyrochlore.Therefore,it is suggested that the crystallization of pyrochlore with high REE content and low Nb/Ta ratio could result in the low REE concentrations and high Nb/Ta ratios in residual melt and fluid.The crystallization differentiation is the key factor resulting in the Nb-Zr-REE mineralization in this area,while the late stage hydrothermal metasomatism only lead to the increase of the Nb-Zr-REE grades of local ore bodies.