赣南石头坪重稀土矿床赋矿母岩地质特征及成矿意义探讨

Geological Characteristics of Metallogenic Host Rock and Their GeneticSignificance of Shitouping Heavy Rare Earth Depositin Southern Jiangxi Province

赣南石头坪离子吸附型重稀土矿床是近期新发现的超大型矿床,由于缺乏精确的赋矿母岩矿物学、年代学和地球化学研究,目前该矿床成因机理尚不清楚。对该矿床赋矿母岩开展岩相学、地球化学、锆石U-Pb年代学、锆石Lu-Hf同位素分析,结果表明,赋矿母岩具有较高的SiO_(2)(76.11%~77.96%)和全碱(K_(2)O+Na_(2)O)(7.18%~7.86%)含量及较低的P_(2)O_(5)含量(0.01%~0.02%),其Zr/Hf(18.73~26.97)、Nb/Ta(5.29~11.10)和K/Rb(52.22~57.28)值相对较低,属于高分异I型花岗岩和富钇轻稀土型花岗岩(LREE/HREE=1.05~1.90,Y/REE=20.5%~32.0%)。赋矿花岗岩形成于早白垩世早期((145.7±4.5) Ma),具有明显高于区域古老地壳基底的锆石Hf同位素组成(ε_(Hf)(t)=-0.21~+2.01),表明岩浆源区明显存在新生陆壳物质的加入,与区域古太平洋板块俯冲后撤引起的伸展环境有关。赋矿花岗岩中存在大量与萤石共生的热液成因的氟碳酸盐类稀土矿物,风化过程中易于分解,为离子吸附型重稀土矿床提供了主要的成矿物质来源。石头坪重稀土矿床的发现,表明华南地区燕山晚期富钇轻稀土型花岗岩在风化作用下能形成重稀土矿床。

The Shitouping ion-adsorptive heavy rare earth deposit in southern Jiangxi province is a newly discovered large deposit.Due to the lack of accurate mineralogy,chronology,and geochemical studies,the genetic mechanism of the deposit is still unclear.Lithography,geochemistry,zircon U-Pb chronology and zircon Lu-Hf isotope analysis are carried out on the ore-forming host rock of the deposit.The results show that the ore-forming host rock was highly differentiated I-type granite and yttrium-rich light rare earth type granites(LREE/HREE=1.05~1.90,Y/REE=20.5%~32.0%)with high content of SiO_(2)(76.11%~77.96%)and total alkali(K_(2)O+Na_(2)O)(7.18%~7.86%)contents,lower content of P_(2)O_(5)(0.01%~0.02%)and low value of Zr/Hf(18.73~26.97),Nb/Ta(5.29~11.10)and K/Rb(52.22~57.28)ratios.The ore-forming granites were formed in the Early Cretaceous((145.7±4.5)Ma),and the zircon Hf isotopic composition(ε_(Hf)(t)=-0.21~+2.01)is significantly higher than that of the regional ancient crustal basement,indicating that there is a significant addition of neocontinental crust material in the magmatic source area,which is related to the extensional environment caused by the subduction and withdrawal of the regional paleo-Pacific plate.There are a large number of hydrothermal fluoro-carbonate rare earth minerals associated with fluorite in ore-forming granite,which are easy to decompose during weathering,providing the main source of ore-forming materials for ion-adsorbed heavy rare earth deposits.The discovery of Shitouping heavy rare earth deposit indicates that heavy rare earth deposit can be formed after weathering of yttrium-rich light rare earth type granite in late Yanshanian in south China.