Study On Diagenesis And Mineralization Of The Granite Porphyry,Yangla Copper Deposit,Yunnan,China

The Yangla Cu deposit located in the Yangla Township,Deqin County,Yunnan Province of Southwestern China.Its geotectonic location is in the central part of the Jinshajiang tectonic belt.It is the largest Cu deposit in the Jinshajiang tectonic belt and a most typical Cu deposit in the“Sanjiang”region(Jinshajiang,Lancangjian,and Nujia ng).The ore-bodies are mostly stratiform and lenticular,which are obviously controlled by stratum-structure-magma.Based on the field geological survey,this paper studies the geological characteristics,diagenetic chronology,characteristics of major-trace elements and rare earth elements(REE),diagenetic types,ore-forming fluids source,property,evolution and diagenesis-mineralization mechanism of the granite porphyry in the Yangla Cu deposit.The mian achievements as follows:The granite porphyry was dike and emplaced the Linong Formation,and the overall strike was a NW inclination at a dip angle of 40°.The contact boundary line between the granite porphyry dike and surrounding rocks was irregular,and mineralization-alterations were evident on the internal and external contact zones of the dike.There are silicification,carbonation,pyritization,chalcopyritization,skarnization,and chloritization in the internal and external contact zone.The diagenetic ages of the three granite porphyry samples were 213±15 Ma(MSWD=1.9),198.40±8.6 Ma(MSWD=3.7),and 195.3±6.4 Ma(MSWD=3.3).The diagenesis occurred later than that of the Yangla Cu deposit granodiorites 208-239Ma,which were supported by the fact that magmatic activities in the Yangla mining area might have lasted 44 Ma.The geochemical characteristics of the granite porphyry showed that belonged to metaluminous-strong peraluminous S-type granite in the calc-alkaline series.The granite porphyry relatively enriched large ion lithophile elements such as Rb,Ba,Th and U,and relatively deficient high field strength elements such as Nb,Ce,Zr and Hf,and significantly enriched Pb element,loss of P element.The values of metallogenic elements shows a trend of Cu>Pb>Zn.The total rare earth elements(?REE)values are 87.09×10^-6-106.11×10^-6,with a weak negative Eu anomaly and weak negative Ce anomaly characteristics.The granitic porphyry may have formed in the late or post-collision tectonic environment of the Zhongza microplate and the Jiangda-Weixi continental margin,and may be the products of post-granodiorite residual magma that underwent diagenesis in an epithermal setting.The fluid inclusion studies have shown that have medium temperature-salinity(280?320?,14?21 wt.%Na Cl_eq)and low temperature-salinity(160?200?,4?19 wt.%Na Cl_eq)two kinds of properties of Na Cl-H₂O system ore-forming fluids,and the ore-forming fluids unisothermal mixing action may be primary mechanism of metal mineral deposits in the porphyry pluton metallogenic stage.The H-O isotope composition characteristics in the porphyr y ore-bodies indicate that the value s of?¹⁸O_H2O are-1.91‰?-1.02‰and the?D_SMOW values are-143.10‰?-110‰.The results show that H₂O in ore-forming fluids mainly derived from the mixture of magmatic water and meteoric water.The S isotope composition characteristics of the pyrite and chalcopyrite in the porphyry ore show that the?³⁴S_V-CDT values are-6.10‰?0.80‰and the total sulfur?³⁴S_?S=0.34‰.It indicates that sulfur mainly derived from the magmatic rocks Yangla mining area.The Pb isotope composition characteristics of the pyrite and chalcopyrite in the porphyry ore-bodies show that the ²⁰⁸Pb/²⁰⁴Pb=38.8208-38.9969,the²⁰⁷Pb/²⁰⁴Pb=15.7079?15.7357,and the ²⁰⁶Pb/²⁰⁴Pb=18.5363?18.7045.It indicated the lead mainly derived from upper crust.The porphyry ore-bodies and skarn ore-bodies in the Yangla Cu deposit should be the products of the same ore-forming fluids system.The evolution trend of ore-forming fluids from skarn metallogenic stage to porphyry metallogenic stage are high temperature-high capture pressure-high salinity to low temperature-low capture pressure-low salinity,and the overlapping regions of temperature,capture pressure and salinity,which indicates the continuity evolution of the ore-forming fluids.The metallogenic process may be the formation of skarn ore-bodies by metasomatic carbonate rocks in the early stage of ore-forming fluids,and the main metallogenic mech anism is the boiling action of the ore-forming fluids.The porphyry ore-bodies were formed by the late stage of the ore-forming fluids unisothermal mixing and cooling actions.