中国西北地区北部蚀源区铀含量特征及其地质、成矿意义

Characteristics of uranium content and its geological and mineralization significance for the provenance areas,northern Northwest China

中国西北地区北部主要盆地蚀源区包括银额盆地周缘、北山盆地群周缘、三塘湖盆地周缘、准噶尔盆地周缘,主体位于中亚构造域中西部南缘(部)或与之毗邻区。通过对研究区1 416件中性、酸性及碱性岩浆岩样品的年龄和铀含量数据进行分区统计处理,计算其在地史上的铀迁移量和古铀含量,结果显示,研究区岩浆活动主要发生在早石炭世—早三叠世(349～248 Ma),其次在中奥陶世—早泥盆世(470～396 Ma)。这分别与古亚洲洋开始俯冲消减(O_2-S_1)和最终关闭、中亚—蒙古碰撞褶皱带形成(C—T_1)的大地构造演化阶段及重大事件相对应。各区富铀岩石主要形成于早石炭世—早三叠世(334～250 Ma),其与晚古生代岩浆活动具有时间上的一致性。在地域上,两者有从西向东时代变新的趋势。这亦与古亚洲洋关闭、中亚构造域形成西早东晚的演化特征相一致。各蚀源区岩石的铀含量不尽相同,综合对比和评价表明:北山柳园区、准噶尔盆地东北缘阿勒泰—蕴都地区和东南部清河地区岩石铀含量与宗乃山区铀含量相当或更富;准噶尔盆地东北缘卡拉麦里区岩石铀含量偏富;银额盆地南缘雅布赖山、巴彦诺日公及庆格勒地区及其北部的沙拉扎山区,北山盐滩及马鬃山区,三塘湖盆地周缘,以及准噶尔盆地东北缘的阿勒泰西北区和西缘的北部区铀含量中等;其他地区铀含量相对较贫。研究区现今富铀蚀源区的大部地区,在晚三叠世开始隆升,此后进一步发展,为邻近中新生代盆地提供了丰富的沉积物和铀物质,成为相邻中新生代陆相盆地砂岩型、煤岩型及泥岩型铀矿床的主要成矿物质来源;盆内深部烃源岩层富含铀元素,可降低烃源岩的生烃门限温度并增加烃源岩生烃总量,具有重要的地质、成矿意义。

The provenance areas of major basins in northern Northwest China,including the peripheral regions of the Yin’e,Beishan group,Santanghu,and Juggar basins,are situated primarily on the south margin of the mid-western Central Asian Tectonic Zone and neighboring areas.Dividing 1416 collected samples,including intermediate,acidic and alkaline rocks in the study area into subgroups,we analyzed rock age and uranium contents of all samples and calculated their paleo-uranium contents and the amount of uranium migration during geological history.The result showed that major and lesser magmatic activities occurred in the Early Carboniferous-Early Triassic(349-248 Ma)and Middle Ordovician-Early Devonian(470-396 Ma),respectively,which correspond to the tectonic evolution stages and major events of the beginning subduction(O2-S1)and closure(C-T1)of the Paleo-Asian Ocean and formation of the Central Asian-Mongolian collision fold belt(C-T1),respectively.The uranium-rich rocks were mainly formed in the Early Carboniferous-Early Triassic(334-250 Ma),consistent with the Late Paleozoic magmatic activity.And regionally,the younging of magmatic and uranium-rich rock age to the east matches the evolution history of the Paleo-Asian Ocean closure and formation of the Central Asian Tectonic Zone from west to east.Variable uranium contents were observed for several provenance areas.Comprehensive comparison and evaluation showed that the areas including the Liuyuan in Beishan,and Altay-Yundu and southeastern Qinghe along the northeast margin of the Junggar Basin,had no less uranium than the Zongnaishan area;the Kalamaili area on the northeast margin of the Junggar Basin was relatively rich in uranium.In addition,the periphery of the Yin’e Basin including Yabulaishan,Bayannuorigong,Qinggele and Shalazhashan,Yantan and Mazongshan areas in Beishan,the Santanghu Basin periphery,and part of the Junggar Basin including northwestern Altay on the northeast and northern west margins,had moderate uranium content.The rest of areas had poor uranium content.Our study indicates the uranium-rich provenance areas in the study area began to rise in the Late Triassic and developed later on to provide abundance of sediments and uranium to the adjacent Mesozoic-Cenozoic basins and became the dominant uranium source for the sandstone host,coal-type and mudstone-type uranium deposits in the basins.The deep source rocks in the basin are rich in uranium,which can reduce the threshold temperature of hydrocarbon generation and increase hydrocarbon production,and both aspects have significant geological and mineralization implications.