Zircon SHRIMP U-Pb Dating of K-Feldspar Granite Samples from the Aobaotu Granite Stock in the Su-Cha ( Sumoqagan Obo) Fluorite Ore District, Inner Mongolia
苏-查萤石矿区钾长花岗岩锆石SHRIMP年龄 及其地质意义

Located on the northern margin of the North China continental massif, the Su-Cha (Sumoqagan Obo) fluorite ore district is the largest independent fluorite ore district in the world. Fluorite mineralization occurs in both Late Paleozoic volcano-sedimentary sequences of the Xilimiao Group and Mesozoic granitoid intrusions. Among these granitoid intrusions, the Aobaotu granitoid stock shows an intimate spatial relationship with the fluorite deposits. It intruded into Lower Permian volcano-sedimentary rocks of the Xilimiao Group, with an outcrop of 66 km2. The stock consists mainly of K-feldspar granite in the central facies and marginal facies porphyry- like granites. Petrology, geochemistry and isotopic geology of the Aobaotu stock are similar to those of Paleozoic and Mesozoic granitoid intrusions, with positive Nd (t) values existent within the Da Hinggan Mountains-Mongolia orogenic belt. A fresh K feldspar granite sample from the Aobaotu granitoid stock was selected for SHRIMP U-Pb age dating. The zircon grains separated from the fresh K-feldspar granite sample give a weighted mean 206Pb/238U age of 138±4 Ma, with a MSWD value of 3.2. Based on this zircon SHRIMP U-Pb age and combined with other geological evidence, it is suggested that the Aobaotu granitoid stock and its related fluorite deposits were formed within a reformed rift basin located along the collision zone of the North China continental massif and Siberian platform. The Aobaotu granitoid stock was probably a product of Mesozoic Yanshanian tectonic- magmatic activities, and played an important role in the ore-forming process of the Su-Cha fluorite deposit. The rock-forming materials of the Aobaotu granitoid stock were derived from a mixed source composed of mantle- and crust-related components.