北秦岭丹凤地区含铀伟晶岩地质地球化学特征

Geological and Geochemical Characteristics of Uranium-Bearing Pegmatite in the Danfeng Area,North Qinling

北秦岭丹凤地区含铀伟晶岩脉沿北西西向含榴二长花岗岩体和片麻状花岗岩体内外接触带展布。本文以该区含铀黑云母伟晶岩为研究对象,在研究其地质地球化学特征的基础上,分析探讨其与含榴二长花岗岩及片麻状花岗岩之间的成因关系。结果显示,含铀伟晶岩脉与含榴二长花岗岩具有一致的副矿物和岩石地球化学(高K、低Na)特征,而与片麻状花岗岩(高Na、低K)相差较大;相对富集元素Rb、Th、Sm,亏损Ti、P、K、Ba,负Eu异常(δEu=0.03~0.12),与含榴二长花岗岩的稀土和微量元素配分模式一致,元素表现出随着不相容性增强(Sm→La→Th→Rb),逐渐向伟晶岩富集的特征;含铀伟晶岩脉Rb/Ti和Zr/Hf比值(Rb/Ti=0.09~0.62、Zr/Hf=21.82~28.89)与含榴二长花岗岩(Rb/Ti=0.27~0.62、Zr/Hf=18.56~28.54)基本一致,与片麻状花岗岩(Rb/Ti=0.01~0.03、Zr/Hf=27.06~42.58)差别较大。因此,认为含铀伟晶岩脉由含榴二长花岗岩的残余岩浆结晶而成,含榴二长花岗岩体的内外接触带黑云母伟晶岩密集区是伟晶岩型铀矿找矿有利区。

Uranium-bearing biotite-granite-pegmatite dikes spread along the NW-trending outer contact zone between garnet monzogranite and gneissic granites in the Danfeng area,North Qingling.This work focused on the uranium-bearing biotite pegmatite in this area.On the basis of research of its geological and geochemical characteristics,the genetic relationships between this pegmatite and garnet monzogranite and gneissic granite were examined.Results show that the uranium bearing pegmatite and garnet monzogranite have the same assemblages of accessory minerals and geochemical characteristics(high K and low Na),and distinct difference with gneissic granite(high Na and low K).It has relative enrichment of Rb,Th,Sm,deficit of Ti,P,K,and Ba,and an Eu negative anomaly(δEu=0.03~0.12).In trace elements and REE distribution patterns,it is consistent with the garnet monzogranite.With the increase of incompatibility(Sm→La→Th→Rb),the elements gradually enrich to the pegmatite.The ratios of Rb/Ti and Zr/Hf of uranium bearing pegmatite dikes(Rb/Ti=0.09~0.62,Zr/Hf=21.82~28.89)are largely consistent with those of the garnet monzogranite(Rb/Ti=0.27~0.62,Zr/Hf=18.56~28.54),while obviously different from the gneissic granite(Rb/Ti=0.01~0.03,Zr/Hf=27.06~42.58).Therefore,it is considered that the uranium bearing pegmatite dike resulted from the residual magma of garnet monzogranite.The internal and external contact zones of the rock mass are the favorable areas for pegmatite-type uranium deposit prospecting.