Characteristics Of Fluid Inclusions And Metallogenic Depth Estimation Of Gold Deposits In The Northern Margin Of North China Platform

This paper is concerned with estimation of metallogenic depth of gold deposits in the northern margin of North China Platform.It takes the relevant thermodynamic parameters of fluid inclusions of 4 typical gold deposits in this area as the main research objects.Magmatic tectonic activity is complex and intense.There are all kinds of structure development.And there are many intrusions exposed.It is a good place to study the vein type gold deposits.In the past,the study focused on the metallogenic epoch and genetic mechanism of single ore deposit.The metallogenic depth and spatial distribution rule of each deposit are neglected.The types of gold deposits are complex and diverse,and affected by many factors including geological structures,magmatic rock and wall rock.Finally,the author indicated that the reasonable metallogenic depth which corresponds to geologic facts could be acquired based on thermobarometer of fluid inclusion,combined with other mineral geologic thermometer and manometer.Through the petrographic observation of fluid inclusions in the study area,most of them are between 3μm and 8μm.Through the laser Raman test of fluid inclusions,it is found that the fluid inclusions are NaCl-H₂O inclusions.And some inclusions contain a small amount of volatile（C₂H₄）.The homogenization temperature and salinity of the fluid inclusions are obtained by microthermomentry of fluid inclusions.The salinity range is 4.95%(NaCl_eq)to 12.10%(NaCl_eq),and the homogenization temperature is between 240℃and 325℃.The data of homogenization temperature and salinity of fluid inclusions are compared with previous data.The data in this paper are basically the same as previous data,but slightly different.It is mainly due to different sample location and the samples in this paper are all ore samples.Through the formula method and the combined independent thermometer equal volume line method,the metallogenic pressure range of the study area has been obtained.The metallogenic pressure ranges from 43MPa to 101MPa in the eastern Hebei Province,and the mineralization pressure is between 42MPa to 71MPa in western Liaoning.Based on hydrogen and oxygen isotopes,the ore-forming fluid of the gold deposit in this study area is mainly composed of rock slurry water.Meanwhile the structural environment of gold deposits in this study area is mostly ductile fracture.It is considered that the ore-forming fluids of some typical gold deposits in the northern margin of the North China platform are in the super hydrostatic pressure range,and are more inclined to the static rock pressure.Because of the inability to calculate the accurate pressure gradient,this paper selects the average value of the hydrostatic pressure gradient and the value of the gradient of static pressure（18.25MPa/km）to calculate the metallogenic depth.It is estimated that the best metallogenic depth of gold deposits in the study area is between 2.4km and3.9km.