Characteristics Of Metallogenic Fluids Of The Laerma Deposit,Western Qinling

The Laerma gold deposit is located north of the Qinling fold and connected with the North China block through the Qilian Caledonian fold.Based on the in-depth study of the geological characteristics of the deposit,the geochemistry of magmatic rocks and the characteristics of fluid inclusions,the geological and geochemical characteristics and metallogenic mechanism of the Laerma deposit are further discussed in this paper.The main understandings are as follows:?1?The Laerma gold deposit mainly occurs in the Laerma formation and Cambrian strata of the Sun Top Group of the Upper Sinian.Through field and laboratory studies,the metallogenic stage is divided into sericite-quartz stage,stibnite-pyrite-quartz stage,natural gold-barite-antimony stage.Metal minerals mainly include pyrite,antimony,natural gold,realgar,female yellow,sphalerite and so on.Non-metallic minerals mainly include quartz,sericite,barite,fluorite and so on.The ore structure has radial structure,tuberculosis structure,self-shaped-semi-self-shaped grain structure,its grain structure and gap filling structure-vein filling structure and so on.The ore structure has complex breccia structure,breccia fragmentary structure,disseminated structure vein-reticulated structure and striped structure.?2?The Laerma gold deposit is a H₂O-NaCl system with medium and low temperature?144³02??and low salinity?2.1⁹.7NaClwt%?,density?0.79⁰.97g/cm³?.The metallogenic pressure range is 10²9MPa,.Formed in medium-shallow?1.1³.0km?environment.From the early stage to the late stage of mineralization,the oxygen fugacity of the fluid decreases gradually,the pH value decreases gradually,and the Eh value increases gradually.It shows that gold exists in the form of Au?HS?₂^-in the early stage of mineralization and HAu?HS?₂⁰ in the late stage of mineralization.Immiscibility?boiling?occurs in the process of fluid migration,which leads to gold precipitation and mineralization.?3?The study of hydrogen and oxygen isotopes shows that the ore-forming fluid mainly comes from the mixed fluid of magmatic water and precipitation.The sulfur isotope study shows that the sulfur source mainly comes from the surrounding rock,from the sedimentary sulfur source and seawater sulfate.?4?It is considered that the Laerma gold deposit is a fine disseminated deposit related to magmatism.Mineralization has gone through the formation period of source layer,hydrothermal mineralization period and supergenesis and evolution period.The deposit is mainly formed by the convective circulation of ore-bearing solution fluid.Tectonic activities lead to the activation and migration of ore-forming materials in the source layer and adjacent strata,and the immiscibility?boiling?of ore-forming fluid in the process of migration,which leads to mineral precipitation.The orebody is enriched and formed.