华北陆块北缘哈毕力格铀矿床S、Pb同位素组成及对成矿作用的指示

Sulfur and Lead Isotopic Compositions of Habilig Uranium Deposit in Northern Margin of North China Block:Implication for Uranium Mineralization

哈毕力格铀矿床位于华北陆块北缘中段,主要受乌兰哈达-猴儿山背斜和区内断裂控制。铀矿化主要产于新太古界乌拉山群第二岩段石英岩中,一直被认为是变质成因铀矿床。在分析该矿床成矿地质背景和矿化特征的基础上,系统研究了矿石与围岩中黄铁矿的硫、铅同位素特征。数据表明,硫同位素组成变化于-4.7‰~12.9‰之间,暗示成矿流体主要来自岩浆热液,同时遭受了地层物质的混染。铅同位素组成(208Pb/204Pb=36.147~42.968,207Pb/204Pb=15.919~34.268,206Pb/204Pb=19.488~168.032)远高于单阶段演化模式组成,不同样品的207Pb/204Pb-206Pb/204Pb线性关系良好,为典型的二阶段铅同位素演化体系,表明变质地层为成矿作用提供了铀源。通过放射性207Pb/206Pb计算,结合区域岩浆演化,认为古元古代(~1 805 Ma)区域变质作用促使乌拉山群铀发生初步富集,晚古生代(374 Ma)花岗闪长质岩浆分异出大量流体活化萃取变质地层中的铀,在有利构造空间富集成矿。

Habilig uranium deposit is located in the middle part of northern margin of North China Block,which is mainly controlled by the Ulanhada-Houershan anticline and regional faults.Uranium mineralization occurs mainly in quartzite of the second member of the Wulashan Group in the Neoarchaean,and has long been consi-dered to be a metamorphic uranium deposit.Based on the study of the geological setting and mineralization cha-racteristics of the deposit,the sulfur and lead isotope compositions of pyrite in ores and surrounding rocks are systematically analyzed.The analysis results show that the sulfur isotope value varies from-4.7‰to 12.9‰,suggesting that source of ore-forming fluids are mainly from magmatic hydrothermal fluids and are contaminated by stratigraphic materials as well.Lead isotope composition(208Pb/204Pb=36.147-42.968,207Pb/204Pb=15.919-34.268,206Pb/204Pb=19.488-168.032)is much higher than that of single-stage evolution model.207Pb/204Pb-206Pb/204Pb in different samples have a good linear relationship and represent a typical two-stage lead isotope evolution system,indicating that metamorphic strata provide uranium source for ore-forming fluid.Based on 207Pb/206Pb equation and history of regional magmatic evolution,it is considered that uranium was preliminarily enriched in the Wulashan Group because of the regional metamorphism in Paleoproterozoic(~1805 Ma).In Late Paleozoic(374 Ma),a large amount of magmatic fluids generated from granodiorite magma,which have activated and extracted uranium from metamorphic strata and contributed to mineralization in favorable structure.