华北式(G层)铝土矿稀土元素地球化学特征及其地质意义

GEOCHEMICAL CHARACTERISTICS OF THE RARE EARTH ELEMENTS IN NORTH CHINA PATTERN(G STRATA)ALUMINA ORES AND THEIR GEOLOGICAL SIGNIFICANCE

华北式铝土矿REE总量为441.43～1085.34(10^(-6)),LREE/HREE为2.67～6.64;山西式铁矿REE为300.08×10^(-?),LREE/HREE为1.4～4.6。两者具有不甚明显的Eu亏损,LREE略富集,且分馏程度不大。组成模式极为相似。但矿层底板中奥陶统石灰岩REE为14.735×10^(-?),LREE/HREE为2.326,Eu,Ce均无异常,这与华北式铝土矿稀土元素组成模式迥然不同,而与五台山隆起之前寒武纪花岗岩类和变质岩系稀土元素组成模式极为相似。这表明,华北式铝土矿(含山西式铁矿)的成矿物质来源主要为古风化壳的前寒武纪花岗岩类及变质岩系。成矿模式为古风化壳经红土化阶段—迁移就位沉积成矿阶段—表生富集阶段。

The total amounts of REE of North China Pattern alumina ores areestimated to 441.43～1085.34 (10^(-6)), the LREE/HREE=2.67～6.64; whilethose of the Shanxi Pattern iron ores 300.08～446.15(10^(-6)),the LREE/HREE=1.4～4.6. There are some similarities between the two patterns, forexample, both show slight depletion in Eu,slight enrichment in LREE and lowfractionation degree. However, in the footwall (Ordovician limestone)of ore beds, REE equals to 14.735×10^(-6), LREE/HREE=2.326 and noobvious variation in Eu and Ce. This composite Pattern of rare earthelements is quite different from that of North China Pattern alumina ores,but similar to those of Pre-Cambrian granitoid and metamorphic series inthe Wutai Rise. All these show that the metallogenetic elements of NorthChina Pattern alumina ores(including Shanxi Pattern iron ores)areorginated from the Pre-Cambrian granitoid and the metamorphic series ofthe Palaeo-weathered crust,and the mineralization processes have experiencedstages such as Palaeo-weathered crust-deposition-superficial enrichment etc.