湖南仁里矿田锂辉石白云母伟晶岩地质特征及其找矿意义

Geological characteristics and the prospecting significance of the spodumene-muscovite pegmatite in the Renli ore-field,Hunan Province

仁里矿田位于扬子陆块与华夏陆块交汇的江南隆起造山带中段、幕阜山岩体西南缘的舌状体区域。稀有金属伟晶岩矿化类型在空间上具分带性,矿田北东部为白云母伟晶岩,西南部为锂辉石白云母伟晶岩。本文对矿田内3条代表性的锂辉石白云母伟晶岩开展了地质和地球化学特征研究。测试结果为:SiO2含量为62.0%~78.0%(平均71.8%);Al2O3为14.8%~18.0%(平均16.1%),Al2O3/(K2O+Na2O+CaO)摩尔比(A/CNK=1.52~13.0)大于1.1,分异指数(DI)89.7~97.5,∑REE=2.07~14.8μg/g,LREE/HREE=6.90~15.9。研究表明:①仁里矿田伟晶岩为过铝质岩浆系统下高分异、富碱、富稀有金属、富铪锆型花岗质伟晶岩,锂铌钽等稀有金属矿化与磷灰石化关系密切。②锂辉石白云母伟晶岩具有良好的分带性,其结晶分异时间晚于白云母伟晶岩,我们推测伟晶岩岩浆的结晶、分异时间越长越有利于Li元素的富集;矿田稀有金属矿化组合呈现Be+Nb+Ta→Be+Nb+Ta+Li→Be+Li的演化规律。③仁里矿田锂矿资源丰富,主要集中在矿田西南部,有望在进一步增加矿田钽铌资源量的基础上,提交一处高品位的大型锂矿床;在锂矿找矿工作中,尤其注意伟晶岩中的贫Nb、Ta锂辉石白云母伟晶岩地段。

The Renli ore field is located on the southwest margin of the tongue like section of the Mufushan batholith in the middle part of the Jiangnan orogenic belt between the Yangtze and the Cathaysian blocks.Rare metal pegmatites in the Renli ore field display spatially zonal mineralization.The pegmatite type in the northeast of the ore field is the muscovite pegmatite,but is spodumene-muscovite in the southwest.In this paper,the geological and geochemical characteristics of the three representative spodumene muscovite pegmatites in the ore field are studied.The analysis data shows that the average value of SiO2 is 71.8%,Al2 O3 is 16.1%,the mole ratio of Al2 O3/CaO+Na2 O+K20(A/CNK=1.52~13.0)is more than 1.1,the differentiation index(DI)is 89.7~97.5,and the content of F is 0.04%~0.84%.These samples exhibit moderate Eu anomalies(δEu=0.27~0.87),∑REE=2.07~14.8,and LREE/HREE=6.90~15.9.Based on the above data we propose that:(1)the granitic pegmatites in the Renli ore field are typical highly differentiated composite granitic pegmatites formed under the peraluminous magmatic system,which is rich in alkali,rare metals,and Hf-Zr.The mineralization of lithium,niobium,tantalum and other rare metals is closely related to phosphorization.(2)Spodumene-muscovite pegmatite has good internal zoning,and its crystallization differentiation time is later than muscovite pegmatite.We speculate that the longer crystallization and differentiation time of pegmatite magma is more favorable for the enrichment of Li element.The mineralized combination of rare metal in the ore field exhibit evolution trend of Be+Nb+Ta→Be+Nb+Ta+Li→Be+Li.(3)The Renli ore field is rich in lithium ore resources that are mainly concentrated in the southwest of the ore field.It is expected to further increase the tantalum and niobium resources in the ore field and has the potential to become a large high-grade lithium deposit.Furthermore,more attention should be paid to the poor Nb-Ta concentration area of the spodumene pegmatites during the process of lithium exploration in the Renli ore-field.