Mineralization of the Weilasituo rare metal-tin-polymetallic ore deposit in Inner Mongolia: Insights from fractional crystallization of granitic magmas

内蒙古维拉斯托稀有金属-锡多金属矿床位于大兴安岭南段西坡，是一个以锡为主，共伴生锌、钨、铜、钼、铌、钽、锂和铷的大型矿床。该矿床以发育铌、钽、铷、锂矿化有别于大兴安岭南段的其他锡多金属矿床。本文对维拉斯托地区的北大山岩体和维拉斯托岩体开展了LA-ICP-MS锆石U-Pb定年、全岩地球化学研究和锆石Lu-Hf同位素分析。北大山岩体和维拉斯托岩体的结晶年龄分别为140±2Ma和137±1Ma~138±1Ma。2个岩体均具有高硅、富碱、贫铁镁钙特征，主体为高钾钙碱性的弱过铝质碱长花岗岩，轻重稀土分馏不明显，呈现出显著的负铕异常。2个岩体均富集Rb、Th、U、K、Ta、Hf和轻稀土元素，强烈亏损Ba、Sr、P、Eu和Ti等元素。与北大山岩体相比，维拉斯托岩体有更低的稀土总量，更明显的Eu负异常，展示了明显的稀土元素四分组效应；Ba、Sr、Eu、Ti和Y亏损更强烈。二者的时空关系和地球化学特征表明2个岩体为同一期岩浆活动不同演化阶段的产物，岩浆演化过程经历了钾长石、斜长石、黑云母、独居石、榍石、褐帘石、磷灰石、磷钇矿、锆石和钛铁矿的分离结晶，维拉斯托岩体还发生了熔体-流体相互作用。北大山岩体和维拉斯托岩体锆石的ε_Hf（t）值分别为5.4~8.6和4.2~9.9，两阶段Hf同位素模式年龄分别为643~847Ma和556~921Ma，表明它们的岩浆源区为起源于亏损地幔的新元古代新生地壳物质。北大山岩体属于高分异I型花岗岩，维拉斯托岩体发育钠长石、天河石、黄玉、锂云母和萤石，属于超分异的Li-F花岗岩。花岗质岩浆的高程度分离结晶和熔体-流体相互作用是形成维拉斯托超分异花岗岩并发生稀有金属-锡多金属成矿作用的重要控制因素。维拉斯托稀有金属-锡多金属矿床属于岩浆-热液矿床。维拉斯托地区的早白垩世花岗质岩石形成于伸展环境。;The Weilasituo rare metal-tin-polymetallic deposit of Inner Mongolia, located on the west slope of the southern Great Xing'an Range, NE China, is a large-sized Sn deposit associated with Zn, W, Cu, Mo, Nb, Ta, Li, and Rb. The deposit is different from other Sn-polymetallic deposits in the southern Great Xing'an Range by developing Nb, Ta, Rb, and Li mineralization. In this paper, LA-ICP-MS zircon U-Pb dating, whole-rock geochemistry, and zircon Lu-Hf isotopic analysis for the Beidashan and Weilasituo intrusions in the Weilasituo area were carried out. The crystallization ages of the Beidashan and Weilasituo intrusions are 140±2Ma and 137±1Ma~138±1Ma, respectively. Both of them have high SiO₂ and total alkali contents and low Fe₂O₃, FeO, MgO, and CaO contents, mainly belonging to weakly peraluminous alkali-feldspar granite with a high-K calc-alkaline affinity. The two intrusions are characterized by enrichment of Rb, Th, U, K, Ta, Hf, and LREE and strong depletion of Ba, Sr, P, Eu, and Ti, with extremely negative Eu anomalies and weak fractionation between LREE and HREE. Compared with the Beidashan pluton, the Weilasituo stock has lower total REE content, more obvious negative Eu anomalies, and more strong depletion of Ba, Sr, Eu, Ti, and Y, showing an obvious tetrad effect of rare earth elements. The temporal-spatial relationship and geochemical characteristics of the two intrusions indicate that they were the products of different evolution stages of the same magmatic activity. The magma experienced the fractional crystallization of K-feldspar, plagioclase, biotite, monazite, sphene, allanite, apatite, xenotime, zircon, and ilmenite during its evolution. In addition, the interaction between melt and fluid occurred during the formation of the Weilasituo stock. The zircon ε_Hf(t) values of the Beidashan pluton and the Weilasituo stock are 5.4~8.6 and 4.2~9.9, respectively, and the two-stage Hf isotopic model ages are 643~847Ma for the Beidashan pluton and 556~921Ma for the Weilasituo stock, indicating that their magma sources were Neoproterozoic juvenile crustal materials originated from depleted mantle. The Beidashan pluton is highly fractionated I-type granite, and the Weilasituo stock belongs to ultra fractionated Li-F granite, which is characterized by albite, amazonite, topaz, lepidolite, and fluorite typomorphic minerals. The high degree fractional crystallization of granitic magma as well as fluid-melt interaction is the important controlling factors for the formation of the Weilasituo ultra fractionated granite and the rare metal-Sn-polymetallic deposit. The Weilasituo rare metal-Sn-polymetallic deposit genetically belongs to magmatic-hydrothermal deposit. The Early Cretaceous granitic rocks in the Weilasituo area formed in an extensional environment.