Zircon U-Pb geochronology, geochemistry and geological significance of diorite porphyrites in Qijia gold deposit, Aohan in Inner Mongolia
Wang Yishen1,2, Liu Jianming1, Liu Hongtao1, Chu Shaoxiong1, Gao Shuai1,2, Chen Yong'an3, Xu Teng4, Wu Zonglin4, Gao Yuyou5
1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, CAS, Beijing 100029;
2. University of Chinese Academy of Sciences, Beijing 100049;
3. Aohan Long Wu Mining Co., Ltd., Chifeng, Inner Mongolia 024313;
4. China University of Geosciences, Beijing 100083;
5. No. 243 Geological Party of Nuclear Industry, Chifeng, Inner Mongolia 024006
Qijia gold deposit, which is quartz-vein type gold deposit discovered recently, is located in the binding site between the North China Craton and the Xingmeng orogenic belt. It is near the side of Xingmeng orogenic belt. Diorite porphyrites, which are closely related to quartz-vein type orebody in space, are mainly occurred in this area. According to the interspersed relationship between dykes and orebody, it will be divided into pre-mineralization and post-mineralization diorite porphyrites. LA-ICP-MS zircon U-Pb geochronology shows that the weighted average age of the pre-mineralization diorite porphyrites is 166.3±3.4 Ma, while the weighted average age of the post-mineralization diorite porphyrites is 128.6±4.5 Ma. The results approximately coincide with the two tectonic-magmatic activity time(respectively around about 160 Ma and 128 Ma)of this area. Characteristics of the major elements show that both of them belong peraluminous and high-K calc-alkaline series. Characteristics of the trace elements show that both of them are enriched in LREE, LILEs(Rb and K)and lively incompatible elements(U and Pb), while they are relatively depleted in HREE and HFSEs(Nb, Ta and Ti). These all reflect that the characteristic of the magma source is crustal source. All of the characteristics indicate that both of them have geochemical affinity of arc magmas. In addition, post-mineralization diorite porphyrites have high Nb/Ta(18.66~20.27)and Zr/Hf(37.16~39.23)ratio, which suggests obviously that the magma source is mantle origin. Geochemical characteristics indicate that pre-mineralization diorite porphyrites magma may originate from partical melting of the lower crust caused by the Pacific plate subduction and post-mineralization diorite porphyrites magma may be derived from melting of mantle wedge metasomatized by subduction fluid, accompanied with contamination of crustal material in the process of emplacement. The diagenetic age, petrogenesis of diorite porphyrites, combined with regional tectonic evolution, suggest that the formation of Qijia gold deposit is related to the subduction of the paleo-Pacific Ocean. The deposit was formed in the conversion process from crustal compression thickening to extension thinning.