Metallogenic Correlation Of The Laojiezi Lead Deposit And Baimaju Gold Deposit In Yao'an,Yunnan Province

The Yao'an alkali-rich porphyry area is located in the middle section of Jinshajiang-Ailaoshan-Honghe alkali-rich porphyry metallogenic belt and in the central and northern part of Chuxiong basin.There are Laojiezi Pb-Ag deposit and Baimaju Au(Cu)deposit,which associated with Himalayan stage alkali-rich porphyry and only 2 kilometers apart from each other in space,forming the Yao'an Pb-Ag-Au polymetallic mine district.However,the two deposits in the same space present two mineralization centers dominated by Pb-Ag and Au(Cu).The difference of porphyry and the deep structure of the two mineralization centers are unclear,and the genetic mechanism of the deposits are also unknown,which restrict the theoretical knowledge and deep exploration of the polymetallic deposits in the Yao'an alkali-rich porphyry area.Based on field geological surveys,this paper summarized and studied the fabric and geochemical characteristics of ore-bearing syenite porphyry,structure of mineralization,elemental zoning,iron oxide,ore-forming material source,ore-forming fluid properties and their evolution in the two deposits.Then the diagenesis-mineralization mechanisms of the two deposits were further compared.The main achievements as follows:(1)The contrast of rock features shown the difference of formation depth:The fabric and geochemical characteristics of ore-bearing syenite porphyry in the two deposits indicate that they are both enrichment in large-ion lithophile elements,and depletion in high field strength elements.Both of the syenite porphyry belonged to alkali-rich,high-K and metaluminous-peraiuminous magma series,which are associated with syenite and quartz monzonite.All this characteristics indicate that these two syenite porphyry are the products of homologous and contemporaneous magmatic evolution.However,the formation depth of Baimaju syenite porphyry was deeper than that of Laojiezi syenite porphyry.(2)The zonal regularity of mineralization elements was summarized and the structure of mineralization was established:From the Laojiezi deposit to Baimaju deposit,the mineralization elements have the zoning characteristics of Pb-Ag anomaly to Au-Cu-Mo anomaly in the plane.The metal zonation from epithermal Pb-Ag to porphyry-type Cu-Mo-Au in Laojiezi deposit indicates that the exploration potential for porphyry-type Cu-Mo-Au deposits.(3)We are the first to confirm the constituents of iron oxide in the deep of the Laojiezi deposit:The iron oxide was identified as specularite by the X-ray diffraction analysis and electron microprobe analysis.The similarity of crystal structure,chemical constituents and spatial distribution between the specularite in Laojiezi deposit and Baimaju deposit indicates that they have similar genesis and fluid source,which is significant for the deep gold mineralization.(4)Ore-forming materials and characteristics of ore-forming fluids:According to the summary of the previous results and the experimental analysis,the characteristics of rare earth elements,trace elements and isotopes of the metallic minerals indicate that the metallogenic fluids of Pb,Ag,Cu and Mo in Laojiezi deposit and the ore-forming fluids in the early stage of Maimaju are relatively simple,mainly due to the magmatic fluids differentiated in the diagenetic process of syenite.While the ore-forming fluids in the late stage of the Laojiezi specularite and Baimaju deposit are complex,which may be a mixed fluid of magmatic fluid and meteoric water that rich in ³⁴S.During the process of migration and precipitation,the mixed fluid may also be mixed with ore-forming materials in some sedimentary strata.(5)Metallogenic correlation:A synthetically comparative study of the diagenesis-mineralization in the two deposits,shown that the Pb,Ag,Au,Cu,Mo polymetals in the Yao'an alkali-rich porphyry area were genetically related,controlled by the same magmatism and were the product of different parts of the same geological tectonic environment and rock formations.These two deposits had a"temporal-spatial-material-evolution"mineralization correlation.(6)Polymetallic mineralization model and metallogenic system:Based on the metallogenic correlation studies of Laojiezi Pb-Ag deposit and Baimaju Au(Cu)deposit,a Pb-Ag-Au polymetallic mineralization model in the Yao'an alkali-rich porphyry region was constructed.And these two deposits and their deep parts jointly constitute an epithermal-porphyry type Pb-Ag-Au(Cu-Mo)polymetallic mineralization system,which indicates exploration potential for porphyry-type Cu-Mo-Au deposits.(7)Ore-forming process:In the early stage of metallogenesis(?33Ma),the syenite-porphyry magma which rich in ore-forming materials such as Pb,Ag,Au,Cu,Mo and so on were located in the Mesozoic strata.During the diagenesis,hydrothermal fluids which rich in ore-forming materials were separated into the volcanic pipe and directional fissures at the top of the porphyry,forming the epithermal Pb-Ag and deep porphyry-type Cu-Mo(Au)mineralization in the Laojiezi deposit and the hydrothermal vein gold deposit Au-Cu ore bodies in the Baimaju syenite porphyry.Then,in the later stage of metallogenesis(?30Ma),a large amount of meteoric water was mixed into the ore-forming fluid,forming a mixed fluid of magmatic hydrothermal and meteoric water,which may be also mixed with some ore-forming material in strata(such as pyrite)at the process of migration and precipitation.Finally,the fluid condensed and crystallized to form the specularite in the deep part of the Laojiezi deposit and the hydrothermal vein gold and iron polymetallic ore in the contact zone and surrounding rock of the Baimaju syenite porphyry.