云南临沧岔河稀土矿床矿物学和微量元素地球化学特征

Mineralogical and Trace Element Geochemical Characteristics of the Chahe REE Deposit in Lincang,Yunnan

岔河矿床位于临沧岩体(临沧花岗岩体)中段临沧城附近,是近年新发现的风化壳型稀土矿床。对岔河稀土矿床ZK11-50、ZK01-36和ZK01-60三个钻孔中风化壳样品进行矿物学、微量和稀土元素地球化学研究表明:(1)黏土层、全风化层和半风化层样品中主要矿物有黏土矿物、石英、长石、云母、以及少量赤铁矿和锆石等,随着风化程度增加,黏土矿物含量增多,云母和长石类矿物含量减少,石英含量变化不明显;(2)全部样品中稀土矿物主要有独居石、硅铈石、方铈石、锰铈矿和磷铝镧矿等,且稀土矿物多分布在黏土矿物集合体内或石英、长石和云母间隙中;(3)从风化壳底部向上,Rb、Ba和Sr逐渐亏损,而Ga和Y在全风化层中下部强烈富集,稀土元素总量(ΣREE)呈现中间富、上下贫的分布规律;(4)大部分样品具Eu负异常(δEu=0.35~0.80,除3个样品δEu=1.02~1.17),黏土层和部分全风化层样品具Ce正异常(δCe=1.02~2.91),大部分全风化层和半风化层样品以负Ce异常为主(δCe=0.17~0.99,除3个样品δCe=1.05~1.41),表明Eu异常主要受原岩控制,少量Eu弱正异常可能与局部富斜长石有关,而Ce异常主要与导致氧化还原环境变化的潜水面波动有关。综上,临沧岔河稀土矿床的形成主要受到外生风化作用的制约,风化作用导致了REE在风化壳中间部位(全风化层中下部和半风化层上部)富集。

The Chahe REE deposit is a large ion-exchangeable REE deposit discovered recently in the middle part of the Lincang batholith.In this paper,we report the mineralogical,trace element geochemical characteristics of the weathering crust of the Chahe REE deposit.Mineralogical and geochemical analyses of samples from three boreholes show that:(1)The main minerals in the weathering crust include clay minerals,quartz,feldspar,mica and minor hematite,zircon,etc.Moreover,the content of clay minerals increases with increasing degree of weathering,while the contents of biotite and feldspar decrease,and without significant change of quartz content;(2)REE-bearing minerals in the weathering crust include monazite,cerite,cerianite,magnetite,and La-bearing florencite.These REE-bearing minerals commonly occur in clay mineral aggregates or in the interstices of quartz,feldspar and mica;(3)From the bottom of the weathering crust upwards,the contents of Rb,Ba,and Sr gradually decrease,while Ga and Y are strongly enriched in the middle and lower parts of the whole weathered layer.REEs are enriched in the middle part of the weathering crust;(4)Most of the samples show negative Eu anomalies(δEu in range of 0.35 to 0.80),except 3 samples with weak positive Eu anomalies(δEu in range of 1.02 to 1.17).The samples from the clay layer and part of the whole weathered layer have positive Ce anomalies(δCe values vary from 1.02 to 2.91).Most samples from the whole weathered and semi-weathered layers show negative Ce anomalies(δCe values vary from 0.17 to 0.99),and only 3 samples haveδCe values of 1.05 to 1.41.It indicates that Eu anomalies are mainly controlled by protoliths,the positive Eu anomalies may indicate enrichment of plagioclase.The Ce anomalies are mainly related to phreatic surface fluctuations that lead to changes in the redox environment.In summary,the formation of the Chahe REE deposit is mainly controlled by exogenous weathering.