Geological Characteristics And Metallogenic Model Of Shijing Gold-silver Deposit In Yanbian Area

Shijing gold-silver deposit is located in the east margin of the Xing'an-Mongolia Orogenic belt,west side of the Tumen River fault and Yanbian Mesozoic fault basin of western Pacific continental margin.Desipte this deposit was found twenty years ago,it has been paid little attention,especially on its theoretical research.On the basis of the collection and analysis of regional and mine district geology,in this paper,the metallogenic geological conditions,geological characteristics,mineralization and ore-forming fluid as well as ore-forming source of Shijing gold-silver deposit have been basically ascertained with the field survey,indoor analysis and comprehensive comparison.According to comprehensive studies and comparison with the typical similar gold deposits both in China and abroad,the ore genesis of Shijing gold-silver deposit has been determined and the metallogenic model has been established.Strata in Shijing gold-silver mining area include mainly epimetamorphic rock series of Permian Kedao formation,early Cretaceous Tuntianying formation and Dalazi formation,as well as loose Quaternary sediments.Gold and silver ore bodies,in this deposit,occur within the volcanic rocks of Tuntianying formation.The main ore-hosting rock types are andesite,hornblende andesite,andesitic breccia and andesitic tuff.The structures in the ore field are dominated by the faults trending in EW,EN,NW and NWW directions.In Shijing gold-silver deposit,two mineralization alteration zone?I and II?and 14gold-silver ore bodies have been delineated.All ore bodies are controlled by the NW and NWW trending faults,occurring in the poor sulfide quartz veins and quartz-calcite veins in the volcanic clastic rocks of Tuntianying formation.There are obvious boundaries between the gold-silver ore bodies and their surrounding rocks.The contents of metal sulfides in ores are relatively low.The main metallic minerals include pyrite,chalcopyrite,sphalerite,galena,native gold.The gold-bearing mineral exists mainly in quartz particles as allotriomorphic granular isomorphism style.Metal minerals are characterized by euhedral to idiomorphic-allotriomorphic granular,subhedral-granular,dissolution,replacement remnant,metasomatic pseudomorph and solid solution separation textures,etc.The ore is characterized by?net?vein,disseminated,massive,banded,geode-crystal cluster and brecciated structures.The silicification,sericitization,chlorinization,epidotization,adularization and carbonatization are mainly developed in orebodies and wall-rocks,among which the silicification,adularization and carbonatization are closely related to the gold-silver mineralization.Comprehensive analysis shows that,Shijing gold-silver deposit has experienced hydrothermal mineralization period and oxidative mineralization periods.And the hydrothermal mineralization period can be divided into three stages,namely,quartz fine-grained pyrite stage,quartz-pyrite?polymetallic?sulfide stage,calcite stage.And the major mineralization stage is quartz-pyrite?polymetallic?sulfide stage.The fluid inclusions in auriferous quartz-calcite veins of Shijing gold-silver deposit are single type,which are mainly gas-liquid two-phase inclusions.The gas-liquid ratios are generally less than 40%.The main ore-forming fluid belongs to the H₂O-NaCl system,which has medium-low temperature?240?²80??,low salinity??0.18¹8.10?wt%NaCl?,low density?0.57⁰.97g/cm³?and low pressure?10.79⁴8.19MPa?.The metallogenic depth is estimated at 0.36¹.61km.The H-O isotope compostions in the quartz and the C-O isotope compostion in the calcite show that the ore-forming fluid was mainly derived from meteoric water and the carbon from deep magma.Comparison shows that the ore genesis of Shijing gold-silver deposit is similar to Wufeng-Wuxingshan gold deposit,Ciweigou gold deposit and so on.It can be concluded that the Shijing deposit belongs to the low-sulfidation epithermal gold-silver deposit,formed in the late Early Cretaceous.In the late Yanshanian period,Yanbian area was situated in the extensional tectonic setting afterward intensive subduction of the Paleo Pacific Plate.Large-scale volcanic eruptions resulted in a series of back arc faulted basins in this area.In the late stage of volcanic magma activity,the meteoric water was permeated along the volcanic vents and fractures,mixed with the upward magmatic fluid,heated the circulation,and continuously extracted the metallogenic material in the surrounding rock,forming the medium-low temperature,middle-low salinity and low density gold-bearing hydrothermal fluid.The fluid rose up to the intersection of shallow fracture structures.Due to cooling and decompression,useful components were deposited and enriched.