Geological Characteristics And Genesis Of Mingsai Gold Deposit In Longzi County,Tibet

Mingsai gold deposit is a newly discovered independent gold deposit in zhaxikang ore concentration area,which is located in the west of zhaxikang Pb-Zn-Ag-Sb superlarge deposit.It has important economic value and scientific research significance.However,the geological characteristics of the deposit have not been systematically summarized,and the genesis of the deposit is not clear.Therefore,on the basis of detailed field geological investigation,this paper systematically summarizes the geological characteristics of Mingsai gold deposit,and discusses its genesis in combination with the geochemical characteristics of the deposit in order to enrich the mineralization types of zhaxikang ore concentration area,and provides clues for similar gold deposits in the region.The ore body of Mingsai gold deposit is located in the intercalated altered tuff in the carbonaceous calcareous slate of the Jurassic terrestrial thermal group.There are Au-?and Au-?ore bodies,which are generally layered,relatively stable in thickness,with an overall dip angle of 5°,2.14°-18.39°,and the two ore bodies are nearly parallel.The total elevation of Au-?ore body and Au-?ore body is between 4671m and 4851m,but the elevation of Au-?ore body is 65-75m higher than that of Au-?ore body.Pyritization,arsenopyrification,silicification,sericitization,chloritization,carbonation and other alteration are developed in the deposit.Among them,pyritization,arsenopyritization,silicification and sericitization are the most developed,which are closely related to gold mineralization.Pyrite and arsenopyrite are infective,distributed in tuff or where silicification develops locally;silicification and carbonation are veined quartz and calcite penetrated in tuff.Based on the SEM analysis of sulfide and gangue mineral systems,it is found that the gold bearing minerals in Mingsai gold deposit are mainly pyrite,followed by arsenopyrite,quartz and other clay minerals.In the study of the occurrence state of gold,it is found that the characteristics of different gold bearing minerals are different.Through detailed microscopic identification and field geological investigation,it is identified that there are four phases of pyrite(py1-py4)in Mingsai gold deposit:the first phase is the earliest pyrite,belonging to the stratum pyrite;the second phase is dark in color,with cracks,mostly paragenetic structure with arsenopyrite,indicating that it is damaged by the later tectonic stress;the third phase pyrite is mostly developed with holes,but it can be seen that zinc flash.Ore,tetrahedrite and other minerals coexist with pyrite,indicating that they are obviously metasomatized or corroded by later fluids;pyrite in the fourth stage is very clean,without any metasomatism or corrosion effect.The first stage pyrite does not contain gold,the second stage pyrite contains little or no gold,and the third and fourth stages pyrite are the main gold bearing minerals.In order to further understand the generation and source of pyrite,in-situ S-isotopic determination and trace element analysis of surrounding rock and gold bearing pyrite particles of different stages are selected in this paper.The analysis results show that the characteristics of trace elements are quite different in pyrite of different ages.The?³⁴S value of pyrite(Py1)in the surrounding rock is between 8.19-15.86‰,while that of gold bearing pyrite(Py2-Py4)is between1.68-3.92‰.The S-isotope of py1 pyrite is obviously heavy sulfur rich,which is consistent with the S-isotope of surrounding rock strata,indicating that the s-source is stratums,while the S-isotopic composition of Py2-Py4 gold bearing pyrite is near 0,which is basically consistent with the sulfur composition of mantle source,indicating that the gold material of Mingsai gold deposit comes from deep mantle.The results show that the homogenization temperature is between 292-366?,the salinity is between 0.2-8.3(wt%Na Cl equiv);the homogenization temperature is between 247-320?,the salinity is between 0.2-13.1(wt%Na Cl equiv);the homogenization temperature is between 206-272?,the salinity is between 0.8-15.2(wt%Na Cl equiv).The fluid has the characteristics of evolution from high temperature low salinity fluid to medium high temperature medium low salinity fluid to medium temperature medium salinity fluid.Raman shows that the fluid is rich in CO₂ and does not contain organic components such as N₂ and CH₄.It is also worth noting that the CO₂-H₂O three-phase inclusions and the rich liquid phase H₂O solution inclusions coexist at different homogenization temperatures,but they have similar salinity,indicating that the ore-forming fluid of the Mingsai gold deposit has fluid mixing rather than fluid boiling,and fluid mixing can be the main reason for gold precipitation.Through Ar-Ar dating of Sericite in arsenopyrite bearing alteration tuff of Mingsai gold deposit,the test result is 19.20±0.27Ma,indicating that Mingsai gold deposit is the product of Miocene mineralization.The comprehensive study shows that the North-South rift in the same period may provide a channel for the upward migration of materials.The extensional detachment structure in the periphery of the evolution process of the structure of the dome of the Cuo-na cave provides a space for the migration and precipitation of gold fluid,while the mixing of the fluid promotes the gold precipitation and enriches the integrated ore.Based on the analysis of ore controlling factors,fluid evolution and metallogenic geological background of Mingsai gold deposit,combined with the comparative study of geological characteristics of typical orogenic gold deposits,this paper holds that Mingsai gold deposit is orogenic.