赣西北洞上铀矿床产铀花岗岩的时代、岩石地球化学特征及其地质意义

Geochronology and petrogeochemical characteristics of U-bearing granites in the Dongshang deposit,northwestern Jiangxi,China and its geological significance

洞上铀矿床位于赣西北九岭岩基南部甘坊岩体内,产铀花岗岩以中粗粒斑状黑(二)云母花岗岩为主。通过锆石及独居石U–Pb年代学、岩相学和岩石地球化学研究,确定产铀花岗岩的形成时代、源区属性与岩石成因,探讨其铀成矿潜力。LA–ICP–MS分析结果显示,锆石U–Pb下交点年龄和加权平均年龄均为152±1 Ma,独居石U-Pb下交点年龄和加权平均年龄分别为151±1 Ma和151±2 Ma,表明产铀花岗岩形成于燕山早期。主量元素具有高硅(SiO_(2)含量为72.1%~75.6%)、高碱(K_(2)O+Na_(2)O含量为7.26%~8.43%)、富钾贫钠(K_(2)O/Na_(2)O=1.07~1.42)、高铝(A/CNK=1.12~1.29)、低钛(TiO_(2)含量为0.07%~0.17%)、贫铁镁(FeO^(T)含量为0.75%~1.28%、 MgO含量为0.19%~0.31%)特征,属高钾钙碱性系列过铝质花岗岩;微量元素Ba、Sr、Nb、Ti亏损,Rb、U、Pb、Ta富集,属典型的低Ba、Sr花岗岩;稀土总量较低(∑REE=21.6×10^(-6)~50.7×10^(-6)),配分曲线为右倾的轻稀土富集型,Eu负异常明显,属S型花岗岩。结合年代学和岩石地球化学特征,认为洞上产铀花岗岩形成于燕山早期同碰撞造山的主挤压阶段,是新元古界双桥山群安乐林组富白云母的变泥质岩部分熔融的产物。富铀、Rb/Sr比值高、Th/U比值小于3、锆石铀含量高等特征指示该花岗岩为产铀花岗岩,具有提供铀源的条件与潜力。

The Dongshang Uranium Deposit is situated in the southern section of the Ganfang Pluton in the Jiuling Orogenic Belt of northwestern Jiangxi Province.The U-bearing granites consist mainly of medium-to coarse-grained,porphyritic biotite(binary)granite.Through zircon and monazite U-Pb geochronology,petrology,and rock geochemistry studies,the U-bearing granites'age,source characteristics,and rock genesis were determined,and their uranium metallogenic potential was also discussed.The LA-ICP-MS analysis showed that the zircon U-Pb intercept and weighted average ages are both 152±1 Ma,and the monazite U-Pb intercept and weighted average ages are 151±1 Ma and 151±2 Ma,respectively,indicating the formation of the U-bearing granites during the early Yanshan period.The major elements exhibit the characteristic of high silica content(SiO_(2) ranging from 72.1%to 75.6%),high alkalis content(K_(2)O+Na2O ranging from 7.26%to 8.43%),potassium-rich and sodium-poor(K_(2)O/Na2O=1.07 to 1.42),high aluminum(A/CNK=1.12 to 1.29),low titanium content(TiO_(2) ranging from 0.07%to 0.17%),and iron-poor magnesium(FeO^(T) ranging from 0.75%to 1.28%,MgO ranging from 0.19%to 0.31%),classifying the U-bearing granites as high potassium calcalkaline peraluminous granites.Trace elements Ba,Sr,Nb,and Ti are depleted,while Rb,U,Pb,and Ta are enriched,representing a typical low Ba,Sr granite.The total rare earth elements(ΣREE)are relatively low(∑REE=21.6×10^(-6) to 50.7×10^(-6)),exhibiting a right-dipping light rare earth enrichment pattern with a prominent negative Eu anomaly,which belongs to S-type granites.Based on geochronology and rock geochemical features,it's suggested that the Dongshang U-bearing granites were formed during the syn-collision compressional setting,resulting from the partial melting of the muscovite-rich metapelites of the Anlelin Formation in the Neoproterozoic Shuangqiaoshan Group.High uranium content,high Rb/Sr ratios,Th/U ratios less than 3,and high zircon uranium contents indicate the potential for uranium ore-forming conditions within these granites.