Geological Characteristics And Discussion On Genesis Of The Yalong Skarn Iron Deposit In The West Of Ban-Nu Suture Zone,Tibet

The Bangong Lake-Nujiang metallogenic belt is one of the most important metallogenic belts on the Qinghai-Tibet Plateau,and many skarn-type iron-rich deposits have been discovered.However,the research on the genesis and metallogenic regularity of skarn-type iron deposits in the metallogenic belt is weak,which restricts further prospecting breakthroughs in this area.This article takes the Yalong deposit in the metallogenic belt as the main research object,based on the synthesis of previous research data and results,through the main trace geochemistry,zircon U-Pb dating and Hf isotope,molybdenite Re-Os dating,The garnet HO isotope and other research methods,discussed the genesis of the deposit,summarized the metallogenic regularity of skarn-type iron deposits in the region,and pointed out the direction for the next step of regional prospecting.Through research,the following achievements and understandings are obtained:The Yalong skarn-type iron deposit is located on the southern edge of the Qiangtang block in the western section of the Bangong Lake-Nujiang metallogenic belt.The ore body is produced in the late Jurassic granodiorite and the late Triassic Rigan mismatched group muddy strips.In the skarn with limestone and sandstone contact zone,the surrounding rock develops skarnization and hornification.The ore minerals are mainly magnetite and molybdenite.Granite diorite,monzonite granite and potash feldspar granite are developed in the deposit.The zircon U-Pb dating results show that the ²⁰⁶Pb/²³⁸U-²⁰⁷Pb/²³⁵U harmonic ages of granodiorite,monzonite and potassium feldspar granite are 158.05Ma,157.27Ma and 161.27Ma,respectively.The Re-Os isotope age of the molybdenite deposit in the granodiorite is 153.7 Ma,and the weighted average age is 153.9 Ma.Field geological characteristics and chronological research results show that the ore-forming rock body of the Yalong deposit is granodiorite and the metallogenic age is late Jurassic.Geochemical analysis results show that the three types of rock masses are type I granites of the meta-alumina-high-potassium-calcium-alkaline series,which are enriched in large ion lithophilic elements?LILE?and deficient high field strength elements?HFSE?,indicating evolution in magma.There is participation of terrestrial crustal materials in the process.Regarding the characteristics of rare earth elements,all rock samples showed the characteristics of enrichment of light rare earth elements,loss of heavy rare earth elements,and strong Eu negative anomalies,indicating that the granite experienced strong crystalline separation of plagioclase during the formation process.The zircon Hf isotope analysis results show that the granite zircon?Hf?t?value is-4.3².83,and the corresponding two-stage model age is 1.03¹.48Ga,indicating that the diagenesis and mineralization materials originated from the remelting or diagenesis of ancient crust-derived materials There is a mixture of sediments.The H-O isotope analysis of garnet indicates that the ore-forming fluid should be mainly derived from granodiorite.Combined with the previous research results in Caima and Fuye,the intrusive rocks of the three deposits all have arc magmatic properties,which are different lithologic rocks formed by the same magma chamber in the same period,have similar tectonic environments,and are similar to Bangong Lake-The northward subduction of the Nujiang Ocean is related to magmatic activities with island arc properties.Therefore,the late Jurassic skarn-type iron deposits such as Yalong,Fuye and Caima were formed under the dynamic background of the northward subduction of Bangong Lake-Nujiang Ocean.