Geochronology And Geochemistry Of Alkaline Rocks In Zhongshangou Au Deposit,Northern Hebei,and Its Geological

In this paper,the whole-rock geochemistry,chronology and Hf isotope geochemical characteristics of the alkaline rocks in the Zhongshangou gold deposit were systematically investigated to identify the rock types,rock intrusion dates,rock genesis and the nature and age of the source area of the alkaline rocks in the mining area,and to explore their relationship with gold mineralisation.Zircon LA-ICP-MS U-Pb dating of the alkaline rocks in the study area yielded weighted mean ages of 393±3 Ma and 391±2.2 Ma for the magmatic zircons of the two rock samples,indicating that the alkaline rocks were intruded during the Devonian.The zircons in the samples are all high in rare earth elements,deficient in HREE and relatively enriched in LREE,showing strong positiveδCe anomalies and insignificant weak negativeδEu anomalies.The zircons have a ¹⁷⁶Hf/¹⁷⁷Hf ratio ranging from 0.2820 to 0.2822 and a two-stage Hf model age(T_DM2)conversion range from 2005 to 2499 Ma,with a mean value of2314 Ma.According to whole-rock geochemical data,the alkaline rocks are a high-silica,alkali-rich,quasi-aluminium calc-alkaline rock series,which is a high-potassium calc-alkaline type A granite.The alkaline rocks are a high potassium-calcium-alkaline A-type granite.The alkaline samples are all enriched in light rare earth elements and large ionophile elements（Rb,Ba,Th）and deficient in heavy rare earth elements and high field strength elements（Ta,Nb,Zr,Hf）.Based on the isotopic geochemistry of rare earth elements,trace elements and Hf,the magma source of the alkaline rocks is related to the lower crust and mixed with some upper mantle materials,and the magma source area may be on the transition zone between the lower crust and the upper mantle.The alkaline rocks are the direct ore-bearing enclosing rocks of the Zhongshangou gold mine,and they are closely related to each other in terms of genesis.