Study On Chronology And Petrochemistry In Dongping And Hougou Gold Orefields, Northern Hebei Province

The Dongping and Hougou gold deposits are located in Zhangjiakou-Xuanhua area of Hebei Province, on the northern margin of North China Craton. Those two gold deposits mainly occur in the endocontact and exocontact of the Shuiquangou alkaline complex intrusion with its wall rocks. The gold ores in the deposits can be divided into two types: namely quartz vein and K-feldspar alteration types. The Shuiquangou alkaline complex is a syenitic intrusion , and intruded into metamorphic rocks of the Archean Sanggan Group, with an outcropping area of about 340 km². LA-ICP-MS dating of zircon U-Pb has ages range from 372.7±2.4 to 383.4±2.8 Ma, indicating that the complex formed during the Late Devonian.In order to look into the genetic relationship between Dongping gold deposit and the Shuiqiangou alkaline complex intrusion, the author took samples from No. 1 quartz vein and No. 70 gold-contained K-altered rock in the middle part of the intrusion, studied the zircons systematically in them, and found two groups of zircons with clearly different characteristics. One group is characterized by euhedral-subhedral crystal and prismatic form with clear magmatic oscillatory zones on cathodeluminescence （CL） image, which shows that they are inherited magmatic zircons. And their LA-ICP-MS zircon U-Pb weighted mean ages are 380.5±2.6 Ma、382.8±3.3 Ma, respectively, which means the middle part of the Shuiqiangou alkaline complex intrusion formed in the late Devonian. The other group zircon is featured with mainly irregular form filling inside the inherited magmatic zircon or lying in the edge of the zircons. And they appear dark-black color （non-luminescent） on the CL image. There are a lot of gas-fluid inclusions inside the zircons. So, these zircons should mainly belong to hydrothermal reformed type zircons. Their LA-ICP-MS zircon U-Pb weighted mean ages are 140.2±1.3 Ma and 140.3±1.4 Ma, respectively, which means the Dongping gold deposit formed in the Early Cretaceous. Considering the fact that there is a time gap about 240 Ma between the rock-forming and ore-forming, the author suggest there is no direct genetic relationship between the Shuiquangou alkaline complex and the Dongping gold deposit. The author find out there are brittle-ductile shear zones with near NE direction in the Hougou gold deposit field. In order to study its forming age, the authors made a mineralogical and U-Pb geochronologic research on the zircons from the mylonitized alkali-feldspar granite to the north of the Shiduokou village in the shear zones. Two types of zircons, magmatic and hydrothermal ones are recognized. The magmatic zircons are characterized by euhedral-subhedral form with clear magmatic oscillatory zones on cathodeluminescence （CL） image. Their U-Pb analysis data gave a weighted mean ²⁰⁶Pb/²³⁸U age of 383.4±2.8 Ma. The hydrothermal reformed zircons are mainly irregular form filling in the magmatic zircons, and show dark-black color （non-luminescent） on the CL image. Their LA-ICP-MS zircon U-Pb age is 154.4±1.3 Ma, which indicates a Late Jurassic tectonic event. Combining with the geological facts, the author suggests the former age indicates intrusive time of the eastern part of the Shuiquangou alkaline complex intrusion, and the latter age implies the forming age of the NE direction brittle-ductile shear zones. For the time interval of the ore forming age （140 Ma） and the brittle-ductile shear zones age （154 Ma） is not very long, the author thinks that there are two stages of hydrothermal ore-forming events in the Dongping and Hougou deposit fields.Synthesis study on the geology, petrochemistry, whole rock Nd isotope and zircon Hf isotope demonstrates that the Shuiqiangou alkaline complex intrusion was formed in a extensional tectonic setting during the transition stage from the late-orogenic to post-orogenic episode, and the complex was mainly derived from the partial melting of the Late Archean crustal materials with the involvement of mantle materials.