Alteration And Ement Migration Characteristics Of The Orebearing Porphyry In The Super-large Beiya Gold Deposit,Yunnan Province,SW China

The Beiya gold polymetallic deposit located in the northwest of the Yangtze block,the middle of the Jinsha River-Ailao Mountain-Honghe shear zone is one of the representative deposits of the Cenozoic Alkali-rich porphyry mineralization.At present,the proven gold reserves are over 200t and associated with iron and copper and other polymetallic minerals.There are quartz monzonitic porphyry,biotite monzonitic porphyry and lamprophyre etc.in the mining area,which quartz monzonitic porphyry has the greatest influence on mineralization.Quartz monzonitic porphyry are characterized by w?Si O₂?ranging from 67.10%to 68.45 wt.%,w?K₂O+Na₂O?from 9.76to 10.71 wt.%,w?Al₂O₃?>14 wt.%,TFe₂O₃ ranging from 4.11 to 4.50 wt.%,which belongs to high potassium calc alkaline series.The content of REE ranges from 28.39to 62.50ppm and has a clear fractionation between LREE and HREE.It shows a right rare-earth distribution pattern,LREE/HREE=2.42³.76,?La/Yb?_N=4.47¹1.33,and a weak Eu/Eu*.The rocks are rich in LILE?K,Pb,Sr,etc.?,and have a loss of HFSE?Nb,Ta,Yb,Y,etc.?.The rock has a high Sr/Y ratio,which shows the attribute of the adakite rock.The biotite monzonitic porphyry in the mine area shows similar geochemical characteristics to quartz monzonitic porphyry.Zircon SHRIMP U-Pb shows the diagenetic age of quartz monzonitic porphyry is?36.03+0.62?Ma?n=15,MSWD=1.8?.The electron probe test shows that the quartz monzonitic porphyry developed two species of feldspar with different sizes.The larger size of the feldspar was more than2mm.From the internal to external electron probe data,the mineral composition of the feldspar was all sanidine,and the feldspar surface was strongly kaolinitization.The particle size of small feldspar is mostly about 1mm,and the composition of feldspar from center to side is changed from sanidine to albite to sodium sanidine.It is presumed that the crystallization time of small particle feldspar is short and fast,and it may be formed in the period of rapid magma intrusion.Through field and indoor observation and short wave infrared experiment,it is found that the mineralized quartz monzonitic porphyry is characterized by potassium alteration?potassicfeldspathization?,silicification,quartzsericitization,argillation?montmorillonitizationandkaolinization?,propylitization,and carbonicacidation.The quartzsericitization and argillation are the most widely distributed alteration.,Other alteration is sporadic,not banded.There is no typical albitization.From the center of the rock mass,alteration from quartzsericitization to argillation zone is consistent with porphyry alteration.Due to the lack of typical albitization,it is significantly different from the IOCG type deposit.From the center to the edge of the rock,the element migration of the quartz monzonitic porphyry was calculated in Wandongshan.In the process of alteration,the main elements like K and Si migrate,and the Na moves out.The trace elements U,Th,La,Ce,Ni,W,Pb moved in,and Cu,Zn moved out.The HREE are inactive during the alteration process.The absence moving in of Na accords with field and microscopic observation.It indicates that the Beiya should be a typical porphyry deposit instead of IOCG deposit.The initial magma of porphyry gold copper deposit is considerde by high temperature,high oxygen fugacity and low water/rock ratio generally.During the quartzsericitization stage,the temperature,pressure,magmatic pH values changed during the magma evolution process and there is meteoric precipitation and in the fluid.Here are the reasons for the migration of elements.From the characteristics of migration of the elements in the alteration zone,the LREE?such as La and Ce?have been brought into large quantities during the alteration process,while the HREE can basically remain inert.Rb,Cr,U,Th and other elements highly concentrated in some specific minerals migrated obviously in mineral alteration.The increase of Rb is related to the formation of sericite and other K-riched minerals;Sr moves out with the plagioclase alteration;most of Cr enriched in heavy minerals?such as rutile?,which is easily retained in altered rocks,and may also enter the altered mica minerals.In altered mass,the contents of metal elements such as Pb,W and Ni have increased,suggesting that the metallogenic fluid and the fluid causing the alteration come from the same system.But the Cu comes out and the Au content is far from the industrial grade in the altered rock,indicating that the enrichment of Au and Cu is not in the porphyry.The Au-Cu mineralization of the porphyry of the Beiya Au deposit is not strong.