The Geochemical Characteristics And Their Indication Of Mineralization Process Of Scheelite In Typical Tungsten Deposits In Yunnan Province

As a common metal mineral,scheelite is very common in different types of tungsten deposits and orogenic gold deposits.Because of its rich trace elements,it is widely used in inversion of the evolution of ore-forming fluid,tracing the source of ore-forming fluid and fine description of ore-forming process.In recent years,LA-ICPMS technology has played an important role in the in-situ geochemical study of scheelite with its advantages of fast,low cost and high spatial resolution.In this paper,two typical tungsten deposits of similar age in Yunnan Province(xiuwa promoted tungsten molybdenum deposit in Northwest Yunnan and Yangtian tungsten deposit in Southeast Yunnan)are selected as the research objects to carry out detailed field geological investigation of petrology,mineralogy and mineralogy.In situ microanalysis and LA-MC-ICPMS are used for Scheelite in Yunnan Province by combining with scanning electron microscope Based on the in-situ isotope analysis and other advanced techniques,the geochemical characteristics of scheelite are discussed to indicate the evolution of ore-forming fluid,the source of ore-forming material and the genesis of the deposit.Finally,we got the following main understandings:(1)Xiuwacu is a typical hydrothermal quartz vein type tungsten molybdenum deposit.According to the occurrence of scheelite and Cl images of cathodoluminescence,this study shows that there are three stages in the early,middle and late stages,in which scheelite is the most developed.The REE distribution of early stage scheelite is similar to that of porphyry monzogranite,showing a right leaning pattern of light REE enrichment.Eu has a medium negative anomaly(?Eu=0.42),the average content of Mo is 3.0%,and the average value of ⁸⁷Sr/⁸⁶Sr of early stage scheelite is 0.7098,which is close to that of porphyry monzogranite(0.7075?0.7098).Compared with early stage,the light REE content of middle stage scheelite is lower,and Eu also has a medium negative abnormal(?Eu=0.37),Mo content decreased to an average of 2445?g/g,⁸⁷Sr/⁸⁶Sr ratio increased to 0.7113 in the middle stage,which was closer to ⁸⁷Sr/⁸⁶Sr value(0.7092-0.7144);rare earth distribution of scheelite in the late stage showed a relatively enriched arched pattern,Eu was basically normal(?Eu=0.93),Mo content decreased to 56?g/g,and the average ⁸⁷Sr/⁸⁶Sr value in the late stage was 0.7083,which was the lowest.From early to late,the gradual depletion of light rare earth elements,especially La and Ce,in scheelite indicates the existence of crystallization of bastnaesite;the increase of?Eu and the sharp decrease of Mo content indicate the transition of ore-forming fluid from oxidation to reduction.The change of Sr isotopic composition indicates the change of the source of ore-forming materials.The contribution of magmatic fluid in the early stage is huge.The magmatic hydrothermal fluid and the surrounding rock strata in the middle stage act on a large scale.The strata provide a large amount of Ca for the formation of scheelite,indicating that the strong interaction of water and rock plays an important role in the formation of the deposit.(2)There are two types of scheelite in Nanyangtian tungsten deposit,skarn type and feldspar quartz vein type.The feldspar quartz vein type ore body obviously cuts through the skarn type ore body,and there is a stage of non mineralized granite porphyry in its lower part.The U-Pb weighted average age of zircon in granite porphyry is 82.19±0.33Ma,and the Re-Os isotopic isochron age of generated molybdenite and scheelite in feldspar quartz vein is 151.0±1.3Ma.To sum up,with previous studies on metallogenic chronology of Nanyangtian tungsten deposit,the formation time of skarn type scheelite should be about 214Ma,interpenetrating with skarn type scheelite feldspar quartz vein type scheelite formed in 151.0Ma,and then there is a stage of mineralization free granitic porphyry magmatic activity.(3)The light rare earth enrichment and heavy rare earth intensive depletion of skarn type scheelite show obvious negative anomaly(?Eu=0.46),?REE content is65.6?g/g,Mo content is 240?g/g,Sr content is 883?g/g,and the ⁸⁷Sr/⁸⁶Sr value of scheelite is relatively low and relatively uniform,ranging from 0.71319 to 0.71491,indicating that the ore-forming fluid is mainly from magmatic hydrothermal solution;feldspar quartz vein type scheelite rare earth content is relatively low The average value of ⁸⁷Sr/⁸⁶Sr of scheelite is 0.71537-0.72803,which shows the characteristics of metamorphic fluid.The difference of trace and rare earth element contents between the two shows that they have different fluid sources.Eu anomaly indicates that skarn type scheelite is formed in the environment with high oxygen fugacity,feldspar quartz vein type scheelite is formed in the reducing environment,and Sr isotopes of the two different types of scheelite have binary mixing characteristics,indicating that feldspar quartz vein type scheelite is of skarn type The scheelite is superimposed and reformed,and the strong metasomatism of ore-forming fluid and surrounding rock is the key to the formation of scheelite.However,the Yanshanian intrusive body played a superimposition on the Nanyangtian deposit,rather than a direct cause of its formation.(4)The geochemistry of scheelite has a great application prospect in ore deposit science.The content of trace elements as,Sr,Mo,REE and the degree of Eu anomaly can determine the type of ore deposit.The orogenic type of scheelite has the characteristics of low Mo and high?Eu and Sr,while the skarn type of Scheelite is characterized by high Mo,low REE and Sr.at the same time,it is not easy to divide it by combining with cathodoluminescence map In the early stage of mineralization,Mo and REE contents of scheelite are generally high,and their contents decrease with mineralization.In addition,Mo and?Eu can indicate the change of oxygen fugacity of ore-forming fluid.High Mo(>100?g/g)and low?Eu indicate the environment of ore-forming fluid with high oxygen fugacity.Low Mo and high?Eu(positive Eu anomaly)indicate the relatively reduced ore-forming fluid ring Environment.