小秦岭华阳川地区吕梁期和燕山期铀成矿作用:来自含铀伟晶岩年代学的启示

Uranium Mineralization of Luliangian and Yanshanian in Huayangchuan Area of Xiaoqinling, China:Implications from Geochronology of U-bearing Pegmatite

华阳川铀矿位于小秦岭构造带西部,是一个以U、Nb共生为主,并伴生Pb、稀土的多金属矿床。已有的年代学数据表明华阳川地区存在印支期和燕山期两期铀成矿作用。近年来,在华阳川矿区及外围发现了大量含铀伟晶岩。对位于华阳川地区草坪沟一带的含铀伟晶岩进行岩相学、矿物学、锆石及独居石U-Pb年代学等研究。结果表明:含铀伟晶岩中锆石U-Pb年龄为(1 922.5±6.0)Ma,锆石与含铀矿物共生表明存在成岩期成矿现象,反映吕梁期(2.0~1.8 Ga)华北克拉通南缘基底拼接过程中的铀成矿作用;含铀伟晶岩中独居石的稀土元素含量具有总量较高且轻、重稀土元素分馏十分明显,球粒陨石标准化稀土元素配分模式具有Eu强烈亏损和右倾特征,显示出高度分异岩浆的晚期热液结晶成因特征;独居石U-Pb年龄((141.9±1.8)Ma)表明存在燕山期花岗岩浆热液作用对已有富铀地质体的叠加改造成矿作用。总之,华阳川地区除印支期铀成矿外,至少还存在吕梁期U预富集成矿作用和燕山期叠加改造成矿作用。

Huayangchuan uranium deposit in the west of Xiaoqinling intracontinental orogenic belt, the southern margin of North China Craton, is a polymetallic deposit, which is dominated by U and Nb, and associated with Pb and REE. The existing chronological data show that there are two stages of Indosinian and Yanshanian uranium mineralization in Huayangchuan area. In recent years, a large number of U-bearing pegmatites have been found in Huayangchuan mining area. The petrography, mineralogy and U-Pb chronology of zircon and monazite from U-bearing pegmatites located in Caopinggou of Huayangchuan area were carried out. Zircon U-Pb dating in U-bearing pegmatites is(1 922.5±6.0)Ma, and the mineral assemblage of U-bearing pegmatites shows that there is mineralization in diagenetic stage, indicating that Luliangian(2.0-1.8 Ga) uranium mineralization occurs in the process of basement splicing in the southern margin of North China Craton. The monazites in U-bearing pegmatites have high REE contents with obvious fractionation between LREE and HREE, and the chondrite-normalized REE pattern is characterized by a tilt to right with strong negative Eu anomaly, showing the characteristics of late hydrothermal crystallization genesis of highly differentiated magma. Monazite U-Pb age((141.9±1.8)Ma) indicates that there is a superimposed transformation mineralization of the existing U-rich geological body by Yanshanian granite magmatic hydrothermal process. Therefore, in addition to Indosinian uranium mineralization, there are at least Luliangian uranium pre-enrichment integration mineralization and Yanshanian superimposed transformation mineralization in Huayangchuan area.