Rock mass dating, geochemical characteristics and metallogenic role of Laoliwan granite in Xiaoshan area, west He'nan Province

Laoliwan granite lies in Xiaoshan area on the southern margin of North China Block. It is a granite body discovered re-cently by Henan Provincial Bureau of Geology and Mineral Resources. This granite mainly consists of adamellite. In this granite body several ore deposits have been found. In order to investigate its formation mechanism and metallogenic role, the authors carried out a study in such aspects as petrology, chronology, geochemistry and mineralization. The formation time of Laoliwan granite is 149 ±11Ma, as shown by Rb-Sr dating. It is highly K calc-alkaline and metaluminous-peraluminous, enriched in silica (SiO₂=65.81%~68.52%), aluminum (Al₂O₃=14.52%~14.75%), alkali (K₂O+Na₂O=6.52%~7.49%) and depleted in MgO (0.21%~0.73%); LREE is rel-atively enriched while HREE is depleted (ΣLREE/ΣHREE=15.69~16.38), with medium negative Eu anomalies (δEu=0.63~0.66). In addition, it is rich in large ion lithophile elements, such as K, U, and Th, poor in high field elements, such as Nb, Ta, P and Ti, and contains medium Sr (265×10^-6~415×10^-6, averagely 315×10^-6), low Y (14.7×10^-6~18.8×10^-6, averagely 16.7×10^-6) and Yb (1.14×10^-6~1.37×10^-6, averagely 1.26×10^-6), suggesting it was formed during the transition from thickened crust to normal crust, with garnet and plagioclase relics. The inner mineralization is similar to feature of porphyry deposits and its mentallogenic age is Late Jurassic. Its metallogenic role is a geological response to the Late Jurassic mineralization event in Yuxi area. So the Laoliwan granite has a close relationship with mentallization. The ore-searching prospect is good, and the area around Laoliwan granite should be con-sidered to be one of the prospecting targets in the east Xiaoshan region.