The Skarn In Hongniu Copper Deposit, Shangri-La,Yunnan,and Geochronology

The Hongniu copper deposit is located in Gezan Town,Shangri La County of Yunnan Province.It is one of the newly discovered copper deposits in the south part of the Triassic Yidun arc.The current copper reserves have reached a large scale,which is still in exploration.Skarns are widely exposed in Hongniu copper deposit,however,their mineralogy and petrology characteristics,the relationship between them and buried rock,and their connection with copper mineralization are not very clear.This paper focused on the skarns and carried out observation and investigation about their mineralogy and petrology characteristics,measured new formation ages of Skarns,intrusions and ores,which provides a new reference for the understanding of mineralization.The main understandings and progresses are as follow,The garnets and diopsides in early skarns in Hongniu copper deposit have two obvious stage,early garnets are mainly andradites with a few grossulars and andradite-grossular solid solution series,associated with small amount of diopsides;there are clear oscillatory zoning belt and the TFeO,Al₂O₃ contents are negatively correlated from the core to the edge;late garnets are almost all andradites,with a large number of diopsides associated;all above reflect the ore-forming fluid changed from acid to alkaline and the oxygen fugacity changed from low to high.The skarn minerals in degenerate alteration stage are mainly tremolites and actinolites with a large-scale development of metal sulfides mineralization,which reflect that the oxygen fugacity has declined so that the S in ore-forming fluid could maintain its reducibility and made the metal materials settling down.The Sm-Nd isochron age of garnets in skarns is 79±15Ma;the Re-Os isochron age of molybdenites in ores is 79.7±3.1Ma?its weighted average age is 79.5±1.3Ma?;the weighted average zircon U-Pb ages of quartz monzonite porphyry in Hongniu copper deposit is 73.4±0.7Ma.These three ages are very close,which proves that the formation of early skarn,the diagenesis of quartz monzonite porphyry and the copper mineralization were simultaneous,and it is the late Yanshan period that is the main metallogenic period of Hongniu copper deposit.The Re content of molybdenites in is 13.65-63.91?g/g,the average¹⁴³Nd/¹⁴⁴Nd_?i?of garnets in skarn is 0.512078±0.000013 and the average?_Nd?t?is-8.95,which show that the metamorphic hydrothermal fluid belongs to crust-mantle mixed source,and the ore-forming materials mainly come from the crust,with a small part of additional mantle materials.