Geological And Geochemical Characteristics And Ore Genesis Of The Gold Deposits In The Kunlun River District,Qinghai Province

Kunlun River district is located in the west part of East Kunlun metallogenic belt,where several gold deposits have been discovered in recent years,such as Heicigou,Dazaohuo,Heihainan and Heihaibei.In this paper,we conducted detailed analyses on the Heicigou gold deposit and the Dazaohuo gold deposit,by discussing their geology,features of the ore-forming fluids,sources of the ore-forming material.Combined with results from previous studies,we provide insights of the ore-forming mechanism,and illustrate the mineralization pattern of these deposits,which will provide guidance for future prospecting in this region.The Heicigou gold deposit and the Dazaohuo gold deposit are located in the south of the Kunzhong Fault.The ore-bodies are hosted in the Ordovician Nachitai group,the Lower Triassic Hongshuichuan formation and the Minddle Triassic Naocangjiangou formation,where wall-rocks were metamorphosed into greenschist facies.Ore bodies occur as nearly EW-trending,which are mainly controlled by faults.Ore types are divided into altered-rock type and quartz vein type.Metal minerals,which are low in content,are mainly pyrite and a few arsenopyrite,chalcopyrite.Nonmetal minerals are mainly quartz and calcite.Ore textures include crystalline texture,reaction-rim texture,metasomatic relict texture,metasomatic pseudomorph texture and crushing structure,etc.Ore structures include vein-fine vein,comb and disseminated structure.Wallrock alterations are predominantly silicification,pyritization,arsenopyritization,carbonatization and localized chloritization.Petrographic observations and laser Raman spectroscopy results of fluid inclusions in the mineralizing stage quartz veins of the Heicigou deposit and the Dazaohuo deposit show that there are manily four types of fluid inclusions:(?)liquid aqueous inclusions,(?)CO₂-bearing three-phase inclusions,(?)pure gaseous hydrocarbon inclusions and(?)pure gaseous CO₂ inclusions.Micro-thermometry measurements indicate that the homogenization temperatures of fluid inclusions from the Heicigou gold deposit and the Dazaohuo gold deposit are concentrated at 190?220?C and 200?230?respectively.The salinities for fluids in the Heicigou deposit are2.8?10.2wt%while fluids in the Dazaohuo deposit have salinities at 3.9?9.9wt%.These results indicate that the ore-forming fluids belong to the Na Cl-H₂O-CO₂±CH₄system and are characterized by low salinity and mid to high temperatures.H-O istope studies show that the ore-forming fluids of the Heicigou gold deposit have?D values of-51‰?-97‰and?¹⁸O values are 0.5‰?6.0‰,and for the Dazaohuo gold deposit,?D values for the ore-forming fluids are-64.6‰?-62.4‰and?¹⁸O values are 5.3‰?6.6‰.The results show that the ore-forming fluids are mainly originated from metamorphic water.?¹³C_V-PDB of carbonate minerals are 3.5‰?5.5‰and-3.8‰?-0.1‰,suggestting that carbon in the ore-forming fluids should come from the wallrock.?³⁴S values of pyrite from the Dazaohuo gold deposit are4.4‰?8.1‰,suggesting that sulfer may have originate from strata.Pb isotopic compositions of the pyrites from the Dazaohuo gold deposit indicate the Pb should have a mixed source,and magmatism may contribute certain amount of Pb into the ore-forming system.Comprehensive studies show that the Heicigou gold deposit and the Dazaohuo gold deposit share the typical features of orogenic gold deposits in terms of their geological features,ore-controlling factors,wallrock alterations,mineral components and ore-texture.Isotope studies also indicate a siminlar source of ore-forming material with that of orogenic gold deposits.The mineralization processes are closely related to the orogeny buildup.Hence the Heicigou gold deposit and the Dazaohuo gold deposit can be classified as orogenic gold deposits.