Geochronology, geochemistry, Hf isotope, and their geological significance of the tonalite and fine-grained diorite from Kushuiquan gold deposit, North Qaidam

The Kushuiquan gold deposit, located in the middle part of the North Qaidam structural belt, is a newly discovered gold deposit in recent years, which has the characteristics of the orogenic gold deposit, and whose ore bodies are distributed in tonalite along the fault structure and are closely related to fine-grained diorite vein in space. In this article, investigations in terms of geochemistry, zircon U-Pb dating and Hf isotope data have been carried out on the tonalite and the fine-grained diorite in the Kushuiquan gold deposit. Bulk-rock analyses show that the tonalite is similar to adakite, for it is rich in Na and poor in K (Na2O/K2O = 6.24 similar to 13. 09), high in Sr and low in Y (Sr/Y = 205 similar to 335), which characteristics are also very similar to the adakitic felsic veins within the Xitieshan eclogite. The fine-grained diorites are rich in Al, Ca and Fe, but depleted in Mg. Also, they are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and depleted in high field strength elements (HFSEs) and low in Ni and Co. It is thus a typical rock formed in the lower continental crust. Zircon U-Pb dating shows that tonalite and fine-grained diorite were formed at 429.9 +/- 2.5Ma and 428.0 +/- 1.9Ma, respectively. Hf isotopic analysis shows that the zircon epsilon(Hf) (t) values of the tonalite are from +9.3 to +11.9, and the two-stage model age (t(DM2)) is from 613Ma to 747Ma, the zircon epsilon(Hf) (t) values of finegrained diorite are from -31.4 to -9.9, and the two-stage model age (t(DM2)) is from 1722Ma to 2803 Ma. The comprehensive analysis shows that the North Qaidam is in the stage of continental crust subduction and exhumation in Early Silurian, and the Kushuiquan tonalite were the products of partial melting of eclogite formed by subduction oceanic crust metamorphism during exhumation of continental crust, and the fine-grained diorite was originated from the partial melting of the ancient basaltic lower crust materials. The grade of the gold orebody distributed in the hanging wall and footwall of the fine-grained diorite is much higher than the average grade, which indicates that the fine-grained diorite provides the thermal power and hydrothermal solution for gold mineralization, and may also provides some ore -forming materials, which makes the grade of gold ore body locally rich. Therefore, the age of fine-grained diorite is approximately regarded as the metallogenic age of Kushuiquan gold deposit (similar to 428Ma). The determination of the metallogenic age and tectonic setting of the Kushuiquan gold deposit indicates that there is a period of gold mineralization during the exhumation of continental crust in North Qaidam in Early Silurian.