甘肃龙首山地区碱交代型铀钍成矿作用：单矿物原位元素和同位素研究

The Longshoushan U-Th metallogenic belt is located at the southwestern margin of the Alxa Block and hosts tens of alkali-metasomatism type U-Th deposits or occurrences of different tonnages, indicating that the ore belt is one of the most favorable locations to study the alkali-metasomatism type U-Th mineralization process. The origin of both the uranium and ore-forming fluids, the relationship between U and Th mineralization, the essential connections between the alkali metasomatism and U-Th mineralization, however, still remain controversial. On the basis of traditional ore geology and ore geochemistry studies of the Jiling U and Xinshuijing U-Th deposits, together with the in-situ LA-ICP-MS or SIMS element and isotope analyses, we intend to determine the nature and origin of the ore-forming fluids and ore components, to delicately illustrate the mineralization process and obtain accurate mineralization ages. By the comprehensive studies of the aforementioned factors, we will reveal the U-Th mineralization mechanism and ore genesis, and finally establish the metallogenic model for the Longshoushan metallogenic belt. The result is not only significant for the understanding of the metallogenic theories of the alkali-metasomatism type U-Th deposits, but also can provide important constraints for the U-Th exploration at depth or in surrounding areas of the known deposits.

龙首山铀钍成矿带位于阿拉善地块西南缘，发育数十个不同规模的碱交代型铀钍矿床/点，是研究碱交代型铀钍成矿作用的绝佳场所。成矿带内碱交代型铀钍矿床的成矿流体和成矿物质来源、铀钍成矿作用之间的关系、碱交代蚀变与铀钍成矿作用的内在成因联系等关键科学问题长期存在争议。有鉴于此，本申请选取成矿带内的芨岭单铀矿床和新水井铀钍共生矿床开展重点对比研究，在传统的矿床学和矿床地球化学研究基础上，重点开展不同阶段铀矿物和共生脉石矿物的LA-ICP-MS（或SIMS）原位元素和同位素研究，以查明成矿流体和成矿物质来源，精细反演铀钍成矿作用过程，精确厘定成岩成矿时代，深入揭示铀钍成矿机理和矿床成因，构建成矿模式并总结成矿规律。相关研究不仅有望深化和完善碱交代铀钍成矿理论，还可为铀钍综合利用、以及成矿带内深部和外围同类型矿床的找矿工作提供理论依据，因此具有重要的科学理论意义和潜在经济价值。

甘肃龙首山成矿带发育数十个不同规模的钠交代型铀矿床/点，是研究钠交代型铀成矿作用的绝佳场所。本项目针对区内成矿流体和成矿物质来源不清、成矿作用过程长期存在争议等关键科学问题，以芨岭和新水井铀（钍）矿床为研究对象，在传统的矿床学和矿床地球化学研究基础上，对不同成矿阶段关键蚀变矿物（方解石、绿泥石、钠长石）开展流体包裹体和C-H-O稳定同位素分析，确定成矿流体以大气降水热液或层间建造水为主；通过对矿石、蚀变岩、赋矿花岗岩和主成矿期黄铁矿开展Sr-Nd同位素组成对比，结合主成矿期黄铁矿LA-ICP-MS原位微区S同位素分析、主成矿期磷灰石LA-ICP-MS原位微区Sr-Nd同位素分析，综合探讨和限定了本区的铀源可能以赋矿芨岭花岗岩体为主，区域元古宙老地层亦可能贡献了部分成矿物质；本项目首次采用蚀变钠长石Ar-Ar和热液锆石U-Pb定年方法的联用，限定了龙首山铀成矿带的主成矿时代约为370Ma左右，明显晚于赋矿花岗岩的成岩时代（~440Ma）；开展综合研究，建立了龙首山成矿带的钠交代型铀成矿模式，并总结了成矿规律。为深入了解钠交代型铀成矿作用，还对世界最大的乌克兰地盾中部钠交代型铀成矿省、以及岩浆成因的赛马碱性岩型铀钍稀土稀有金属矿床和青海查查香卡钠长岩脉型铀钍稀土稀有金属矿床开展了初步研究，探讨了与钠交代蚀变相关的多种不同类型含铀矿床地质特征和矿床成因异同，并建立了鉴别标志和找矿标志。本项目取得的相关研究成果不仅有利于丰富和完善钠交代铀成矿理论，所凝练的成矿规律和找矿标志也为区域找矿提供了依据，兼具重要的理论研究意义和潜在经济价值。