Geological Characteristics And Prospecting Direction Of The Shangmachang Gold Deposit In Heihe Area,Heilongjiang Province

The Shangmachang gold deposit is located on the eastern edge of the Xing'an Massif,which is one of China's important Au,Ag,Cu,Pb,Zn and other polymetallic metallogenic regions.The study area is tectonicallylocated in the Daxinganling Early Paleozoic continental accretionary margin and the eastern section of the Xingmeng orogenic belt,the southeastern margin of the Duobaoshan Ordovician island arc active belt is belong to the eastern segment of the northeast trending Bainaimiao-Duobaoshan Cu,Au,Mo and Fe ore belt.The region experienced intense Mesozoic magmatism and provided very favorable conditions for mineralization.The outcropped strata in the mining area are mainly the Early Cretaceous Longjiang Formation andesite,mottled andesite,roughfaced andesite,basaltic andesite,andesite porphyrite,andesitic tuffs,andesitic tuff lava,trachyte and rhyolite.The magmatic rocks in the area are not developed,but only the monzogranite,biotite granite,and granite porphyry;faults in the area is mostly tensile with NE and NEE trending,and the northeastward fault structure is Mine control ore-controlling structure.The mining area is divided into seven gold ore belts and a number of gold ore bodies.The ore body is NE trending,showing strips and wedges.The ore bodies are mainly produced in andesite and quartz-carbonate veins,and less in the altered andesite,andesite breccia lavas and rhyolitic tuff lavas.The ore minerals are mainly pyrite,sphalerite and silver-gold deposits,with chalcopyrite partially found;gangue minerals are mainly epidote,chlorite,quartz,calcite and plagioclase.Ore is enhedral-subhedral,anhedral,metasomatic,anhedral intersertal and poikilitictexture,ore structure is mainly disseminated and honeycomb structure.The alteration types of deads include silicification,carbonation,chlorite and epidotization,among which silicification and carbonation are closely related to mineralization and can be used as important ore-prospecting signs.The mineralization hydrothermal period is divided into quartz-pyrite stage,quartz-calcite-sulfide-gold stage and quartz-carbonate stage,and the second stage is mainly mineralization stage.The LA-ICP-MS zircon U-Pb dating method was used to study the formation age of volcanic rock in the mining area.The results show that the U-Pb age of zircon for the Andesite in the Longjiang Formation is 123.4±2.1 Ma,andU-Pb age of the rhyolite is 123±3.7 Ma,both of which belong to the Early Cretaceous and represent an important magmatic evolution event.Andesite is the main surrounding rock,andthe metallogenic epoch is inferred to be later than the age of diagenesis.Combined with the results of the same type of deposit in the region,the Shangmachang gold deposit was formed in the Early Cretaceous.The geochemical characteristics of the rocks indicate that the volcanic rocks in the mining area are characterized by alkali-rich and poor-aluminum features,and are rich in large-scale ionic-lithium elements such as Rb,Ba,Th,and K are enriched,and depleted in high-field strength elements such as Nb,Ta,and Ti,indicative a source from the mantle.All volcanic rocks fall into the volcanic arc environment,belong to an orogenic belt volcanic rock,and is the product of plate subduction.Combining with the characteristics of regional tectonic evolution,it is believed that the Shangmachang gold deposit and its hostile rock may form under the lithospheric thinning background caused by the subduction of the Izenazaki plate,which is the product of the transformation process from the regional extrusion background to the stretching background,and is closely related to magmatic activities.The results of the study on fluid inclusions in the mining area show that the fluid inclusions in the gold-bearing quartz-carbonate veins in the Shangmachang gold deposit are gas-liquid phase.At the main metallogenic stage,the homogenization temperature is 176.2~215.5°C,the salinity is concentrated at 0.87~5.25wt% Nacleqv,the density is 0.86~0.92g/cm3,the ore-forming pressure is about 11.89~17.15 MPa,and the ore-forming depth is about 1.19~1.72 km,showing the characteristics of hypothermia.A summary study of the geological features of the Shangmachang mining area confirmed that the Shangmachang gold deposit is an epithermal gold-type gold deposit.Summarized on geological feature and analysises on metallogenic geologicall conditions of Shangmachang gold deposit.The Longjiang formation andesite is a favorable ore-forming strata,which enrich materalies for ore-forming;the ore veins occur in the intersection of NW and NE geotectonic systems,and the single vein is controlled by the NE trending structure,and the volcanic action makes the metal elements enriched and mineralizing.The study shows that the Shangmachang gold deposit formed the multiple action of three conditions.The study identifies the prospecting criteria and prospecting direction for the Shangmachang gold deposit.Silicification,carbonation are closely related to Au mineralization.Gold is hosted in quartz carbonate vein,besides electrum is existed in the gold deposit and it is the most important prospecting criteria.In addition,low magnetic and low resistivity anomalies can be used as a geophysical signatures and the abnormal inner and middle zone are the geochemical signs.Further prospecting direction is determined according to the development degree of metallogenic geological conditions and the favorable degree of ore prospecting indicators.