The Characteristics Of Magmatism And Giant Sn-Cu Enrichment Analysis Of The Gejiu Ore Field,Yunnan

The geochemical behaviors of Sn and Cu in acidic magma differ greatly.Thus,the Sn mineralization is related to high differentiation of F-rich reductive magma and Cu deposit is related to the formation and evolution of water-rich highly oxidized magma.Most of the granite-type Sn deposits in the world contain little or no Cu,while the Cu deposits related to intermediate-basic rocks also include very little Sn.Moreover,the large-scale and high-grade Sn-Cu deposits such as Gejiu copper-tin polymetallic ore field in Yunnan province,southern China are relatively rare in the world.The previous studies mainly focused on rock-and ore-forming ages and lithogeochemical feature.It was found that these deposits are closely associated with the same Cretaceous magmatism.However,the genesis of the deposits and the large-scale accumulation of Sn and Cu remain controversial.In addition,the key factors forming the large-scale Sn-Cu mineralization in Gejiu have not been well identified.The Cretaceous magmatic activity is strong in the Gejiu ore field.The gabbro,monzonite and granite,which are not mineralized,are mostly exposed in the west side of the Gejiu fault.By contrast,the main deposits are almost concentrated in the east side of the Gejiu fault.Five mineralization centers,including Malage,Songshujiao,Gaosong,Laochang and Kaofang,are formed from north to south around these concealed equigranular granites and porphyritic granites.The major mineralization types include the greisen-and skarn-type Sn-Cu mineralization,basalt-host layered Cu mineralization,and quartz-veined Sn mineralization.In order to solve the above-mentioned questions,this paper referenced the previous researches,and analyzed systematic geochronology,whole-rock geochemistry,the mineral compositions（e.g.zircon,biotite and apatite）on the quartz monzonite,mafic microgranular enclaves（MMEs）,porphyritic granite,and equigranular granite by EMPA and LA-ICP-MS.Furthermore,it investigated oxygen fugacity and volatile compositions of main rock types during magma evolution,revealed whether both Cl-rich oxidized and F-rich reduced magmas favorable for Cu and Sn mineralization are developed in the Gejiu ore field,analyzed ore forming element anormaly patterns between Cretaceous pluton,Triassic basalt and the adjacent Dongchuan type Cu deposits hosted in the Preterozoic basement,and verified the sources of Cu.In addition,this research revealed possible relations between the Sn-Cu mineralization and different types of intrusions and summarized the key factors controlling the formation of large and giant deposits with element association of Sn and Cu.Finally,the conclusions below were obtained:（1）Zircon U-Pb ages of the main plutons in Gejiu ore field are concentrated in the range of 79.6±1.8 Ma to 84.6±1.2 Ma.The pluton derived from different sources show different biotite compositions:the biotites of quartz monzonite and MMEs derived from a crust-mantle mixed source have lower Al₂O₃,Fe O and higher Mg O contents than equigranular granite from the crust source,while the biotite geochemical characteristics of the porphyritic granite are somewhere in between,indicating that the extensional settings induced mantle upwelling,decompression melting,and mafic magma production in the Gejiu ore field in the late Cretaceous.Meanwhile,the emplacement of the intermediate-basic magmas induced partial melting of the crust,to generate fractionated granites.（2）Compared with porphyritic granites,equigranular granites have higher fractionated degree.The contents of Mn O,Al ^Tand Rb in biotite gradually increase from porphyritic granite to equigranular granite,whereas the contents of Ti O₂and Ba gradually decrease.The content of Sr and ratio of（La/Yb）_N,（La/Sm）_Nand（Sm/Yb）_Nin apatite decreased gradually,whileδEu increased gradually.The equigranular granites representative“tetrad effect”as well as strongly negative Eu anomalies,and Rb/Sr and Nb/Ta ratios are higher,indicating that porphyritic granites are relatively lowly fractionated.（3）The characteristics of biotite major elements and zircon trace elements indicate that intermediate-basic magma in Gejiu ore field is water-rich,Cl-rich and high oxidized,equigranular granite is water-poor,F-rich and reduced,while the porphyritic granitic magma is somewhere in between.Similar to the Cu-rich porphyry in the northern Chile,MMEs and quartz monzonite have relatively high zircon Ce⁴⁺/Ce³⁺and（Eu/Eu*）_Nratios.Most biotites from the equigranular granite fall on or above the nickel-nickel oxide（NNO）oxygen buffer,whereas those from the porphyritic granite are mostly clustered around the magnetite-hematite（HM）buffer or between the NNO and HM buffer in the Fe³⁺-Fe²⁺-Mg²⁺plot.In addition,the difference of volatile（F and Cl）contents in biotite and apatite indicates that the equigranular granites are F-rich and Cl-poor,which can exsolve the fluid with lower Cl/F value,MMEs and quartz monzonite are relatively Cl-rich with the higher Cl/F value fluid,while the porphyritic granitic magma is somewhere in between.The zircon of MMEs and quartz monzonite have higher 10000*（Eu/Eu*）/Y ratios（>1）and（Ce/Nd）/Y ratios（>0.01）and lower Dy/Yb ratios（<0.3）,revealing that the intermediate-basic magmatic rocks are water-rich.The equigranular granite contains no hornblende,theporphyritic granite contains a few of hornblende,while MMEs and monzonite contain generally hornblende.By using the whole-rock zircon saturation temperature and the phase diagram of intermediate-acid magmatic-hydrothermal system,the range of H₂O content in 800 MPa melt can be roughly estimated:the H₂O content of equigranular granites is less than 2.8%,while that of porphyritic granites,quartz monzonite and MMEs is 4.0-9.2%,8.8-11.7%and 9.1-11.8%,respectively.（4）Although the equigranular granites and porphyritic granites have similar source regions,but F/Cl values and water contents are different.The former has higher F/Cl value and lower water content,while the latter has relatively lower F/Cl value and higher water content.Combined with the water-rich and Cl-rich intermediate-basic magma in the area,this research suggested that the volatile phase composition characteristics of porphyritic granitic magma might be related to the degassing of the intermediate-basic magma chamber in Gejiou ore field.Specifically,the degassing of deep intermediate-basic magma chamber led the exsolving oxidized Cl-rich fluid to flow into the granitic magma and form porphyritic granite.（5）The S-Pb isotopic composition of Cu mineralization in Gejiu deposit was different from that of Triassic basalt related to laryed Cu ore body,which did not support that the Cu came from basalt.Combining with the occurrence of"Dongchuan type Cu deposit"in the Neoproterozoic and the possible degassing of the deep mafic magma chamber in the Gejiu ore field,this research found that the fluid formed by the degassation could extract Cu from the basement,which provided material basis for the formation of Sn-Cu rich ore in Gejiu ore field.（6）At Late Cretaceous,SE Yunnan ore-forming belt occupied the extensional setting,and the lithospheric delamination induced asthenosphere upwelling and crust melting to form oxidized basic magma chamber and reduced acidic magma chamber,respectively.The deep basic magma chamber produced water-rich and Cl-rich volatiles through degassing.These volatiles extracted Cu from possible sources,such as Proterozoic basement containing“Dongchuan-type copper deposit”and entered the acidic magma chamber,resulting in the composition difference between porphyritic granite melt and equigranular granite melt.The F-rich,reduced and highly evolved granitic melt was related to quartz vein type Sn mineralization,while the Cu-rich fluid dissolved Sn from early-crystallized Ti-rich minerals such as biotite in the porphyritic granite melt to form greisen type-skarn type Sn-Cu deposit.The formation of the layered type Cu mineralization was related to the metasomatism between Cu-rich fluid and basalt.