新疆库尔勒市上户稀土矿床成矿机制研究

The Shanghu rare earth element deposit is located in western of Kuluketage continental margin block, which is located in northern margin of Tarimu plate, the Tarim block is in the south, the Eerbin late Paleozoic remnant basin is in the north-east, the Kunlun tectonic belt is in the south-west. They are strong tectonic activities in this area, especially, the low Proterozoic stratigraphy, the intermediate-acidic rock intrusive bodies of Proterozoic and middle variscan, the pegmatite are well-developed, showed that they are very excellent geological conditions of REE element. Based on the petrography, major and trace element geochemistry, radiogenic isotopic and stable isotopic geochemistry, geochronology study, this program has carried out the electron-probing analysis of the fluid inclusions and microscopic measurement research. According to the systemic study on the tracing country rock and different pegmatite lithofacies belt formed in substance composition and evolution characteristics of magmatic system; Discussion on the coupling relation of fluid evolution and enrichment of rare earth elements, reveal the genetic connection of deposit and its surrounding rocks – proterozoic granite, limited restriction of magma evolution process to rare earth element enrichment and mineralization, determined the genesis of the Shanghu rare earth ore deposit, discussed metallogenic mechanism of it’s; further enrich and improve the existing rare-earth element metallogenic theory and to the further exploration of ore deposits and regional prospecting to provide technical support. So, it has important theoretical significance and econimical value.

库尔勒上户北稀土矿床位于塔里木陆块北缘库鲁克塔格陆缘地块西段，北邻额尔宾晚古生代残余盆地，南接塔里木中央地块，构造活动强烈，尤其是下元古界地层和元古代及华力西期中酸性岩体及伟晶岩脉非常发育，稀土元素成矿地质条件十分有利。本项目在系统的岩石学、元素地球化学、同位素地球化学和年代学研究工作基础上，重点开展流体包裹体的微区微束分析和显微测温研究。通过系统研究示踪围岩及伟晶岩不同岩相带形成时岩浆体系的物质组成及演化特征；探讨流体演化与稀土元素富集的耦合关系，揭示该矿床与其围岩-元古代的花岗片麻岩之间的成因联系，限定岩浆演化过程对稀土元素富集和成矿作用的制约，确定上户稀土矿床的成因类型、阐述其形成机制，进一步丰富和完善已有的稀土元素成矿理论，为矿床的进一步勘探和区域找矿提供技术支撑，具有重要的理论意义和较高的实际应用价值。

库尔勒上户北稀土矿床位于塔里木陆块北缘的库鲁克塔格陆缘地块西段，仅在2平方千米范围内就已发现了21条稀土矿化的褐帘石伟晶岩脉。伟晶岩脉主要分布在元古代的混合岩中，矿化伟晶岩脉体走向以北东东-南西西向为主，与区域总体构造线方向平行。伟晶岩脉体一般长80~1000米，宽0.2~20米；最长者达1100米，最短者仅为20米，脉体倾角为45~87°。伟晶岩脉具伟晶结构，矿石矿物主要是褐帘石、钛铁矿和锆石，其中褐帘石一般呈浸染状分布于由长石和石英组成的集合体中。上户稀土矿床赋矿围岩-黑云母混合岩含有典型变质成因的锆石，样品中锆石边部207Pb/206Pb加权年龄分别为1924±27 Ma和1944±15 Ma，暗示混合岩化峰期年龄约为1930 Ma；黑云母混合岩中锆石的初始176Hf/177Hf比值为0.281131~0.281466，其对应的εHf(t)为-15~-3.1，平均值-6.5，Hf同位素的其二阶段模式年龄tDM2 (Hf) 则介于2791~3501 Ma 之间；研究认为，库尔勒北缘在~1930 Ma发生了陆-陆碰撞，导致了陆壳俯冲到深部（麻粒岩相，石榴子石稳定区），进而发生熔融形成了黑云母混合岩，其形成可能与哥伦比亚超大陆聚合有关。.在上户稀土矿床的含褐帘石伟晶岩中存在着具有明显的韵律环带、且Th/U比值与岩浆成因锆石相似的颗粒较大的自形锆石，这表明它形成于熔体体系，暗示伟晶岩可能是熔体结晶的产物；但显微岩相学研究也显示，部分锆石受到了后流体的交代，这可能暗示形成含稀土矿化的伟晶岩的母岩浆为富水熔体。通过元素地球化学、同位素地球化学和同位素年代学的综合研究，在~1930 Ma，库尔勒地区发生了陆-陆碰撞，导致了陆壳的深俯冲并发生熔融形成了黑云母混合岩；在1810 Ma，区域地球动力学体制由挤压转为伸展，混合岩的易熔组分减压重熔为富含稀土和水等挥发分的熔体，早期结晶形成锆石、磷灰石等副矿物，随着斜长石等硅酸盐矿物的结晶，体系由富水熔体转为热液体系，结晶出褐帘石、钛铁矿等热液成因矿物。因此，上户稀土矿床的形成与富水等挥发分的低温熔体有关，而稀土矿化则发生在热液阶段。.总之，库尔勒上户稀土矿床是伟晶岩型稀土矿床，它应是塔里木克拉通北缘库鲁克塔格地区~18.1 Ga构造热事件的产物。