Petrological And Geochemical Characteristics Of Yao'an Alkali-rich Porphyry And Its Metallogeny In Yunnan Province

The Yao'an Pb-Ag deposit,located in the Chuxiong Basin,western Yangtze Block,is an important component of the Jinshajiang-Ailaoshan alkaline porphyry-related polymetallic intrusive belt.There is a series of alkali-rich porphyry composed of trachyte,syenite porphyry,syenite-aplite with spot and pseudoleucite porphyry.Based on the proecessors studies,this paper reconstructed the Yao'an syenite porphyry,syenite-aplite and pseudoleucite porphyry in detail.Comprehensive and comparison studies on spatial distribution,fabric characteristics,elemental geochemistry,isotope geochemistry and their mineralization effect were taken.Several conclusions were drawn as below.(1)We are the first to dated pseudoleucite porphyry samples by means of LA-ICP-MS U-Pb isotopic dating and a weighted average 206Pb/238 U age of(34.1 ± 0.3)Ma.(2)Spatial distribution:According to intercalated relationship,the mineral sequence of alkali porphyry is as follow:Trachyte?Syenite porphyry?Syenite-aplite?pseudoleucite porphyry.Syenite porphyry produced in the form of rock mass and dike,which is ore-hosting rock.Syenite-aplite with spot intrusive into syenite porphyry and mud-sandstone fragments in vein by NW direction and NE direction.Pseudoleucite porphyry produced in vein,breccia or a small rock branch nearby the syenite porphyry volcanic breccia and it suited on the northwest of the breccia-type lead mine,northeast side of F4.Pseudoleucite porphyry and syenite aplite with spot are the later vein rock after syenite porphyry.(3)Petrological characteristics:syenite porphyry can be classified into several categories.According to different mineral composition,it can be divided into syenite porphyry,biotite syenite porphyry and amphibole-bearing syenite porphyry.According to the color difference,it is divided into tan syenite porphyry,gray-white syenite porphyry,purple red-gray purple syenite porphyry and reseda syenite porphyry.According to the different enrichment degree of feldspar phenocryst,it is divided into phenocryst which is relatively enrichment and another relatively few.According to the different size of feldspar phenocryst,it can be divided into fine phenocryst-syenite porphyry,medium grain phenocryst-syenite porphyry and coarse grain phenocryst-syenite porphyry.Spot-bearing syenite-aplite,mauve and greyish-white,is in porphyritic and mass.The phenocryst includes fresh sanidine and minor biotite.Sanidine phenocrysts look like a wide plate,with long axis orientation.Matrix was fine-grain structure and was mainly composed of fine grained alkali feldspar form a rough structure which was inter strip alignment or orientation arranged.Pseudoleucite porphyry,greyish-white,and greyish-green,in porphyritic and mass,of which phenocrysts are composed of pseudoleucite,alkali feldspar and biotite,and the matrix is cryptocrystalline.The study area of pseudoleucite has two generations like the early rounded and late tetragonal trisoctahedron.(4)Geochemical characteristics of alkali-rich porphyry:Based on an integrated study rare earth element geochemistry,Sr-Nd-Pb isotopes as well as zircon U-Pb isotopic age on the various types of syenite porphyry,spot-bearing syenite-aplite and pseudoleucite porphyry,the results indicate that:? Syenite porphyry belongs to intermediate-acid rock and spot-bearing syenite-aplite belongs to intermediate rock,pseudoleucite porphyry belongs to intermediate-basic rock.?Petrochemistry of syenite porphyry,spot-bearing syenite-aplite and pseudoleucite porphyry is characterized by rich alkali,high potassium,high aluminum,and spot-bearing syenite-aplite has richer alkali and higher potassium.? They are enriched in LILE(such as Rb,Th,K,Ba etc),depleted in HFSE(such as Ta,Nb,P,Ti etc),and with strongly negative Ta,Nb,and Ti(TNT),anomalies-all characteristics of subduction-zone mantle-derived rock.? They are rich in light rare earth elements.They have light and heavy REE fractionation significantly and negative Eu anomalies slightly,and pseudoleucite porphyry is richer light and heavier REE.? Trace elements and REE characteristics showed that they are partially melted from EMII with character of crust and mantle.?They are all formed in an intraplate extensional tectonic environment after the collision of the Eurasian and Indian Plates.(5)Ore-forming effect:The Pb-Ag orebody was identified primarily within the syenite porphyry and its outer contact zones,and the upper and lower of the spot-bearing syenite aplite veins.It has a close relationship with NE-bearing veins especially.Yao'an alkali-rich porphyry and Pb-Ag ore(mineralized)body has a close time-space-source contact,and both of them formed under the dynamics background of the collision of the Eurasian and Indian Plates and Ailaoshan-Jinshajiang fracture.