Chronology and geochemical characteristics of quartz diorite in Gegen' aobao, Dong Ujimqin Banner, Inner Mongolia, and its geological significance

The newly found Gegen'aobao lead-zinc deposit is within the quartz diorite. Because the exploration is insufficient, the metal resources are unclear. As a newly found deposit, there is no previous research data, and hence systematic isotope dating must be done to determine the age of metallogenesis for the deposit. The U-Pb dating shows that the quartz diorite has the ages of 299±1~301±1Ma. Major element geochemistry shows that the quartz diorite is characterized by high SiO₂ and K₂O, whereas trace element geochemistry shows that the quartz diorite is a "right-inclined" shape in the chondrite-normalized REE patterns. The rocks are rich in large ion lithophile elements (K, Rb, Ba) and LREEs and depleted in high field strength elements (Th, Nb, Sr, P, Ti) and HREE, with the loss of high field strength elements (HFSEs) such as Th, Nb, P and Ti. The quartz diorite is of high-K calc-alkaline series and has a little negative Eu anomaly (Eu/Eu^*=0.80~0.91), with low A/CNK (0.79~0.95). These characteristics shows that the Gegenaobao quartz diorite is mantle source granite. The in situ zircon Hf isotopic data are in agreement with U-Pb age data of zircon grains, and the ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratio of the quartz diorite is 0.282906~0.282984, with ε_Hf(t) values >0, indicating that the source region of the quartz diorite magma was depleted mantle or newly created lower continental crust beneath the study area. Based on geochemistry, isotopes and regional tectonic evolution history, the authors hold that the quartz diorite was derived from a depleted mantle, which was probably related to an active continental margin arc environment.