Trace Elements And Sr Isotopes Of Fluorite And Scheelite Altered Granite Type Copper Tin Polymetallic Deposit And Their Indicative Significance In Gejiu Xi’ao, Yunnan Province

Fluorite and scheelite,as two common calcium containing minerals,are very common in hydrothermal deposits and different types of tin deposits.Because they are rich in trace elements,they are often widely used to study the characteristics of ore-forming fluid,inverse the law of fluid evolution,trace the source of ore-forming fluid and finely describe the ore-forming process.Gejiu tin deposit,as the largest tin deposit in the world,enjoys a good reputation all over the world.It has attracted much attention because of its unique metallogenic geological conditions.In particular,a large number of fluorite and scheelite are developed in the metallogenic process of the deposit.Taking the large copper tin polymetallic deposit of altered granite,a newly discovered mineralization type in the western depression of Laoka granite body in Gejiu tin mine,as the research object,this paper analyzes the structural lithofacies alteration characteristics of key tunnels and boreholes in the middle sections of 1800m,1700 m and 1650 m,and compares it with the altered granite in Gaofengshan ore section of Laochang ore field,Fluorite（quartz tourmaline vein type and skarn type）and scheelite（quartz tourmaline vein type and altered granite type）with different occurrence are selected to carry out in-situ and dissolved sample trace element,Sr isotope and Sr-Nd isotope analysis,so as to find out the geochemical characteristics of fluorite and scheelite,and explore the source of metallogenic materials and metallogenic process.Finally,the following main understandings are obtained:（1）The structural lithofacies alteration characteristics of the tunnels and boreholes of the altered granite type copper tin polymetallic deposit in the West concave belt show that star point pyrite brass ore quartz veins are mostly developed in the contact zone between the altered granite and the surrounding rock limestone,and skarn is developed in only a few areas,which is obviously inconsistent with the extremely thick skarn ore body in other contact zones of Laoka granulated granite.From the center to the edge of the ore body,there are vein alteration characteristics of fluorite potassium silicified massive tin copper deposit→sericitization infected pyrite brass mineralization tin petrification→silicified star shaped pyritization→non mineralized kaolinite montmorillonite biotite granite,indicating that the deposit is a typical magmatic hydrothermal vein type deposit.（2）The two occurrence fluorites are horizontally distributed in the Y/Ho La/Ho diagram,and mainly fall into the hydrothermal origin area in the Tb/La-Tb/Ca diagram,suggesting that the two fluorites may be formed in the near same period and are of magmatic hydrothermal origin.FluoriteδEu is 0.01～0.17,showing the characteristics of reduction conditions,which may inherit the Eu negative anomaly of granite,andδCe is 0.56～1.05,showing the characteristics of weak oxidation conditions,suggesting that both may be formed in the environment of transition from oxidation to reduction.（3）Among them,quartz tourmaline fluorite(⁸⁷Sr/⁸⁶Sr)_iis 0.709 53～0.711 47,which has a small variation range and is consistent with Laoka isogranular granite.Skarn fluorite(⁸⁷Sr/⁸⁶Sr)_iis 0.708 70～0.715 53.There are high and low end elements,the high end element is close to Laoka rock body,and the low end element is close to Gejiu formation carbonate rock.The REE distribution patterns of the two types of fluorites are flat"Seagull type",which is similar to Laoka isogranular granite,suggesting that the Ca in skarn fluorite may mainly come from Laoka isogranular granite and carbonate rock stratum,and the Ca in quartz tourmaline vein fluorite mainly comes from Laoka isogranular granite.（4）Quartz tourmaline vein scheeliteΣREE is（474～2121）×10^-6,with an average of 1035×10^-6,Y/Ho value is between 12.34～24.13,the average value is 17.65,and its(⁸⁷Sr/⁸⁶Sr)_ivalue is（0.7091～0.7115）;Altered granite scheeliteΣREE is（155～501）×10^-6,349 on average×10^-6,the Y/Ho value is between 20.66～38.51,the average value is 28.80,and its(⁸⁷Sr/⁸⁶Sr)_ivalue is（0.7094～0.7132）.The(⁸⁷Sr/⁸⁶Sr)_ivalues are similar to those of Laoka isogranular granite,but the altered granite scheeliteΣREE is significantly lower than that of quartz tourmaline vein type,and the Y/Ho value also varies greatly.It is suggested that the two occurrence scheelite minerals may mainly come from Laoka granite,and the altered granite type scheelite minerals may contribute to some porphyry granite.At the same time,quartz tourmaline vein scheelite is likely to be formed in the early stage,while altered granite scheelite is likely to be formed in the late stage.（5）The REE distribution patterns of scheelite in the two occurrences in the west sag belt are mree enrichment type,and the content of Mo is high,ranging from（2.84～42.67）×10^-6,with an average of 19.67×10^-6.Quartz tourmaline vein scheeliteδEu is（0.01～0.1）,altered granite scheeliteδEu is（0.08～0.14）,showing strong negative anomalies,suggesting that the scheelite of the two occurrences are of magmatic hydrothermal origin and formed in a strong oxidizing environment.（6）Based on the above structural lithofacies alteration and the related geochemical characteristics of fluorite and scheelite,it is considered that the ore-forming fluid of the altered granite type copper tin polymetallic deposit in the west depression of Laochang is magmatic hydrothermal,the fluid has the characteristics of high temperature,rich F and rich Na,and has experienced the evolution of oxygenation→reduction in the process of migration and cooling.It is speculated that there may be a two-stage magmatic emplacement process in the deposit,that is,the emplacement and condensation of porphyritic granite in the early stage formed a nearly East-West extensional fault or joint emplacement channel,but did not provide a source of ore-forming materials;The ore-forming fluid formed by Laoka isogranular granite in the late stage is located in the early fracture structure to form magmatic hydrothermal vein type deposit.At the same time,combined with fluorite Sm-Nd isochron age,granite zircon age and previous research results,it is considered that the diagenetic and metallogenic age of the deposit is about 82 Ma.