Chronology,Geochemical Characteristics And Geological Significance Of The Granite-porphyry In Caijiaying Lead-zinc Mining Area

The Caijiaying lead-zinc deposit is located in the Zhangxuan area on the northern margin of the North China Craton.It is a large-scale lead-zinc polymetallic deposit and a major breakthrough in prospecting for the northern margin of the North China Craton.This article takes the granite porphyry exposed on the north side of the mining area as the research object.On the basis of previous studies,through field investigations and geological features of the area,rock petrography,geochemistry,geochronology and fluid inclusions deep analysis of other characteristics,combined with the geological characteristics of the deposit,further explores the relationship between the Caijiaying deposit and the ore-bearing granite porphyry or magmatic hydrothermal activity,so as to provide theoretical guidance for prospecting in the Caijiaying area and even the Zhangxuan area.The granitic porphyry in Caijiaying lead-zinc mining area is widely distributed in the mining area,and most of the granitic porphyry intrudes into the ancient Archean Hongqiyingzi rock group in vein form.The granite porphyry has porphyritic-matrix textural structure and massive structure.The phenocrysts are composed of potassium feldspar(10%),quartz(10%)and a small amount of mafic minerals,and the matrix(80%)is composed of felsic minerals.Zircon U-Pb dating results show that the two granite porphyry samples were formed at140.9±0.8 Ma and 143.6±0.7 Ma,respectively,belonging to the Early Cretaceous,and are the product of Yanshanian magmatic activity.The geochemical characteristics show that the granite porphyry is rich in silicon,potassium,and total alkali content.The aluminum saturation index A/CNK is between 1.05 to 1.24,and A/NK is between 1.15 to 1.41,which belongs to the high Potassium-calcium-alkaline peraluminous granite.Rock trace elements show positive anomalies in U,Ta,Nd,Hf,and Sm in the standardized spider web map of the primitive mantle,while Ba,K,P,and Ti show obvious negative anomalies.The distribution curve of rare earth is relatively flat and shows a"seagull"distribution.The fractionation of heavy and light rare earth is not obvious and has strong negative Eu anomaly.The average zirconium saturation temperature is about 853.895?,which suggests that the granite porphyry has the characteristics of A-type granite.Zircon Hf isotopic data show that:The?_Hf(t)of the CG-4 and CG-14 samples ranges from-0.08 to-13.49 and-18.37 to-16.32,and the corresponding two-stage depletion mantle model ages of T_DM2range from 1196 to 2042 Ma and from 2224 to 2350 Ma,indicating that the magma originated from the Paleoproterozoic crustal remelting.The data of fluid inclusions show that the ore-forming fluid in the ore-bearing vein samples is H₂O-Na Cl system of medium temperature,medium salinity and medium density.Combined with previous studies on fluid inclusions and H-O,S and Pb isotopes of the ore in Caijiaying lead-zinc mining area,the following conclusions can be drawn:The ore-forming materials originated from the mixing of the lower crust and mantle in deepth,so it is speculated that the Caijiaying granite porphyry may be closely related to the mineralization of the Caijiaying Pb-Zn deposit.