Study Of The Carbonatite Type U-REE Mineralization In The Huayangchuan-dashigou Ore-concentrated Area,east Qinling Orogen,China

The Qinling orogenic belt is one of the most important and strategically significant ore-concentrated areas in China,with a series of multi-metal deposits including Mo,U,REE,Au,W,Fe,and Pb.The Huayangchuan-Dashigou ore-concentrated area,a typical carbonatite type U-REE ore-concentrated area in the eastern Qinling orogenic belt,has gained extensive attention from geological researchers both domestically and internationally due to its abundant mineralization types,large scale,diverse mineral species,and significant exploration potential.Although some previous studies have been conducted on this ore-concentrated area,there is still a lack of systematic research on the genesis of ore-bearing carbonate rocks,the source of mineralize materials,and the mineralization processes,which to some extent limits the understanding of the U-REE mineralization and the establishment of mineralization models within this area.Therefore,this paper aims to explore the carbonate-hosted U-REE mineralization and establish a mineralization model by conducting systematic studies of element geochemistry,isotope geochemistry,mineralogy,and geochronology of the ore-bearing carbonate rocks within the ore-concentrated area.The following new findings and insights have been obtained in this paper:（1）The U-REE mineralization in the carbonatite rocks of the Huayangchuan-Dashigou ore-concentrated area is characterized by enrichment in incompatible elements such as U,Sr,Ba,and REE,indicating a typical igneous carbonatite.The whole-rock geochemical features indicate that the mineralization materials may have originated from the same source areas.The carbonatites have Sr-Nd isotopic compositions close to EM1-type enriched mantle and Pb isotopic compositions between mantle and lower crust.In addition,they also show low negativeε_Nd（t）values and(¹⁴³Nd/¹⁴⁴Nd)_icharacteristics,implying a minor contribution of crustal materials.The in-situ Nd isotope characteristics of the rare earth minerals also reflect a high similarity in the mineralization material.Therefore,it is preliminarily inferred that the ore-bearing carbonatites in the Huayangchuan-Dashigou ore-concentrated area may have originated from a mixed source of enriched mantle（EM1）and partial lower crust.（2）In the ore-concentrated area,the uranium minerals in the Huayangchuan deposit are mainly uraninite（Ur-1,Ur-2 type）,pyrochlore（Pcl-1 to Pcl-4）,with minor amounts of coffinite,uranothorite,and uranium-bearing Changbai ore.Meanwhile,in the Dashigou deposit,the uranium minerals in the ore are primarily composed of uraninite（Ur-1,Ur-2 types）,brannerite,and pyrochlore（Pcl-1 to Pcl-3）.Notably,pyrochlore exhibits clear magmatic-hydrothermal evolution processes（i.e.,primary to alteration modification）in both deposits.LA-ICP-MS in situ U-Pb dating of uraninite shows two distinct uranium mineralization events with ages of～223 Ma（early）and～144 Ma（late）,respectively.（3）The REE minerals in the Huayangchuan deposit are primarily composed of monazite,allanite,and fergusonite,while in the Dashigou deposit,they consist of monazite,allanite,bastnasite,parisite,synchysite,xenotime,and ancylite.Notably,monazite and allanite were formed during the magmatic to late-magmatic stage.While fergusonite,bastnasite,parisite,synchysite,xenotime,and ancylite were formed during the hydrothermal（alteration）stage.LA-ICP-MS U-Pb dating of monazite in the ore-bearing carbonate rocks yielded ages of 224.7±2.4 Ma（Huayangchuan deposit）and 219.4±6.0 Ma（Dashigou deposit）,indicating the two ages are essentially consistent.Furthermore,LA-ICP-MS U-Pb dating of bastnaesite and xenotime yielded ages of 145.9±5.3 Ma and 142.59±1.82 Ma,respectively,indicating the existence of Yanshanian REE mineralization in the Dashigou deposit.Thus,this paper concludes that there are two stages of REE mineralization events in the ore-concentrated area,with ages of 224.7 to 219.4 Ma（early）and 145.9 to 142.6 Ma（late）,respectively,and these two ages are consistent with the two uranium mineralization events.Moreover,the study discovered the first evidence of late-stage rare earth mineralization in the ore-concentrated area,which is also the first confirmation of Yanshanian rare earth mineralization event in the Qinling area.（4）Early U-REE mineralization formed in a post-collisional extensional setting during the Late Triassic,and the mineralize materials were derived from alkaline silicate-carbonate magma enriched in the mantle（EM1）,which subsequently migrated along deep-seated faults in the area to shallow surface and intruded into the basement rocks of the lower crust.The alkaline rock-carbonate magma was rich in ore-forming materials（such as U,Nb,REE,Mo,etc.）in the late stage of crystallization differentiation,resulting in the formation of mineralized carbonate veins in the ore-concentrated area and the development of early U-REE mineralization.Late U-REE mineralization was driven by the subduction of the ancient Paleo-Pacific plate,which was accompanied by extensive Yanshanian magmatic activity in the region（Laoniushan and Huashan）that provided sufficient heat and fluid sources.This caused the reactivation and recrystallization of REE in the early carbonate rocks and rare earth minerals,resulting in more intense late-stage U-REE mineralization in the area,which formed during the post-orogenic and extensional environment of the collisional orogeny.Additionally,other minerals in the carbonate veins（such as calcite,apatite,etc.）may have also contributed to this process.The present study provides further insight into the two-stage U-REE mineralization in the Huayangchuan-Dashigou ore-concentrated area,which has enhanced the understanding of the ore-forming theory of carbonatite-type deposits in the area.The discovery of late-stage U-REE mineralization（145.9 to 142.6 Ma）is particularly significant for uranium and rare earth mineralization in carbonate-type deposits in the Qinling orogenic belt,providing new insights for future exploration of carbonatite-type U-REE minerals in the Huayangchuan-Dashigou ore-concentrated area and even the Qinling orogenic belt.