陕西小花岔铀矿床岩浆演化及其对铀成矿作用的制约

The Magmatic Evolution and Its Constraints on Uranium Mineralization in the Xiaohuacha Uranium Deposit,Shaanxi Province

陕西小花岔铀矿床位于北秦岭造山带的东北部,矿体产于黑云母花岗伟晶岩和黑云斜长片麻岩的接触带同化混染区。为了厘定研究区花岗质岩浆演化与铀矿化作用的关系,对矿区内出露的花岗岩、花岗伟晶岩开展了详细的年代学和岩石地球化学研究。LA-ICP-MS锆石U-Pb定年结果表明:灰池子花岗岩体的形成年龄为(444±4.0)Ma;高山沟花岗岩株的形成年龄为(422±0.82)Ma;产铀黑云母花岗伟晶岩的形成年龄为(417±2.6)Ma;非含矿黑云母花岗伟晶岩的形成年龄为(413±1.8)Ma。地球化学数据表明:矿区花岗质岩石富集大离子亲石元素Rb、Ba、K,亏损高场强元素Nb、Ta。含矿黑云母花岗伟晶岩由于同化混染作用及与围岩的元素交换等原因,其基性组分Fe、Mg和挥发分F-含量较高,同一条黑云母花岗伟晶岩脉元素组成的差异是由于伟晶岩浆的同化分离结晶所致,在伟晶岩-黑云斜长片麻岩的接触带发生化学组分的元素交换,使得伟晶岩浆中U-F络合物发生分解,并且在良好的成矿条件下(围岩的铀含量较高、较好的构造环境)使得铀饱和沉淀形成铀矿物,如晶质铀矿等。小花岔铀矿床的形成主要受到了伟晶岩浆、围岩成分、岩浆热液中挥发分共同的作用,最终导致了铀矿床的形成。

The Xiaohuacha uranium deposit is located in the northeastern section of the North Qinling Orogen.The uranium mineralization occurs within the assimilation and contamination area in the contact zone between the biotite granitic pegmatite and biotite plagioclase gneiss.To investigate the relationship between the granitic magma evolution and uranium mineralization,this paper presents detailed geochronological and geochemical studies of the granitoids.The results show that LA-ICP-MS zircon U-Pb ages of the Huichizi,Gaoshangou granites,and biotite pegmatites （uraniferous and non-mineralized zones） are （444 ± 4.0） Ma,（422 ± 0.82） Ma,（417 ± 2.6） Ma,and （413 ± 1.8） Ma,respectively.The granitoids are characterized by enrichment of largeion lithophile elements （LILE）,such as Rb,Ba,and K,but depletion of high field-strength elements （HFSE）such as Nb and Ta.High concentrations of Fe,Mg,and F in the uraniferous zone are interpreted to indicate chemical exchange between the pegmatitic magma and wall rocks in the assimilation-contamination area.The compositional heterogeneity in the individual biotite pegmatites is ascribed to the assimilation-fractional crystallization of pegmatitic magma.Interaction with the wall rocks in the contact zone triggers the destabilization of U-F complexes and precipitation of uraninite from the pegmatitic magma （ca.417 Ma） under favorable emplacement conditions.The assimilation of biotite plagioclase gneiss by pegmatitic magma during volatile saturation plays a main role in the formation of the Xiaohuacha uranium deposit.