Study On Genesis Of Chedu Magnetite Quartzite Type Gold Deposit In Mianluening Block

There are many magnetite quartzite type gold deposits in the world,but the genesis of these deposits is controversial.There are obvious differences between the magnetite quartzite type gold deposits in the Mianluening block and the typical magnetite quartzite type gold deposits in the world.The gold mineralization in the typical magnetite quartzite type gold deposits in the world is mainly related to the quartz sulfide vein superimposed in the later metamorphic deformation process,and the carrier minerals of gold are mainly pyrite,pyrrhotite and other sulfides.However,the magnetite quartz type gold deposits in the Mianluening block do not develop quartz sulfide veins superimposed in the later stage,and the main carrier mineral of gold is magnetite.The Chedu gold deposit is a magnetite quartzite type gold deposit with large scale and typical metal ogenic characteristics in the mianluening block.Based on the detailed field geological investigation and indoor microscopic ultramicroscopic observation,combined with the mineral inclusion,metasomatic relationship and metamorphic degree of rocks(ores)to divide the metal ogenic stages,this paper analyzes the gold bearing minerals of different metal ogenic stages by micro in situ trace element analysis,and then the genesis of the deposit is discussed,which provides basic data for further study on the metal ogenic mechanism of this type of gold deposit.In addition,zircons from magnetite quartzite type ore bodies were selected for U-Pb geochronology study to determine the sedimentary age of magnetite quartzite and then define the gold mineralization age.According to the metamorphic degree of rock(ore)and the interspersed inclusion relationship of minerals,the mineralization can be divided into the following three stages:(1)sedimentary metamorphism stage,accompanied by the deposition metamorphism of magnetite quartzite,formed a high-grade layered gold ore body,a large number of natural gold distributed between magnetite,magnetite and quartz particles;(2)hydrothermal activity stage,divided into two metal ogenic substages,the first stage is the smoky gray quartz sulfide vein.In this substage,the gold is mainly distributed in pyrite and quartz,The second substage is sulfide bearing carbonate vein ang the gold is mainly distributed in chalcopyrite and calcite;(3)in the epigenetic stage,the magnetite quartzite has strong limonized,according to the structure and spatial distribution characteristics of limonite,limonite is divided into two generations.In the first generation,the gold is mainly distributed in the inner part of limonite and the contact part between limonite and residual magnetite.The gold in the second generation limonite is mainly distributed in the inner part and the edge of limonite.Based on the classification of metal ogenic stages of Chedu gold deposit,the gold bearing minerals of different stages are analyzed by LA-ICP-MS in situ.The elements associated with Au in magnetite(Mt)of sedimentary metamorphic stage are Ag-Cu-Pb-Bi-Mo,the elements associated with Au in pyrite(Py1)are Ag-Co-Cu-Zn-Bi-Mo,and the elements associated with Au in pyrite(Py2)of hydrothermal stage are Ag-Co-Cu-Zn-Bi-Mo-Te.The elements association associated with Au in epigenetic flocculent limonite(Lm2)are Ag-Cu-As-Pb-Bi.Through in-situ microanalysis,the content of invisible gold in the lattice of magnetite and pyrite in the magnetite quartzite is relatively low,but a large number of large particles of natural gold are observed under the microscope,mainly distributed in the second generation of flocculent limonite(Lm2).The content of Ti in flocculent limonite(Lm2)is several orders of magnitude higher than that in the first generation of limonite,which is close to the content of Ti in magnetite,the association of Au related elements(Ag-Cu-As-Pb-Bi-Mo)is similar to that of Au related elements(Ag-Cu-As-Pb-Bi)in flocculent limonite(Lm2),which has certain inheritance.Through LA-ICP-MS in-situ microanalysis of pyrite in magnetite quartzite and quartz sulfide vein,it is found that there is invisible gold in both stages of pyrite.Most of invisible gold occurs in the form of solid solution(Au~+),a small amount of invisible gold occurs in the form of nanoparticles.Through zircon U-Pb dating of the Chedu magnetite quartzite ore body,the age is about2.5 billion years.Based on the previous studies on the geochronology of bikou group and yudongzi group in Mianluening block,the age of Chedu magnetite quartzite is close to that of yudongzi group.Therefore,it is considered that the Chedu magnetite quartzite may be be a remnant of the yudongzi group magnetite quartzite in the Bikou Group.