Research On Mineralization Of The Bianjiadayuan Tin Polymetallic Deposit, Inner Mongolia, China

The Bianjiadayuan tin polymetallic deposit is located in the southern Daxing'an Mountains in the eastern part of the Central Asian orogenic belt.It is a typical magmatic hydrothermal tin polymetallic deposit.But there are still some controversies such as metallogenic geological body and metallogenesis and metallogen1ic model.In this paper,by field investigation and systematically studying the geological and geochemical features of Bianjiadayuan deposit,the forming factors of deposit are explored,and the metallogenic model of deposit is discussed.Detailed information is provided for the establishment of tin-polymetallic metallogenic system in the southern part of Daxing'anling Mountains.At the same time,the metallogenic regularity of tin-polymetallic deposits in the region is supplemented and verified.?1?The ore bodies of Bianjiadayuan deposit are divided into four types:greisen-type copper-tin ore bodies,cryptoexplosive breccia-type tin-copper-lead-zinc ore bodies,quartz vein-type copper-zinc ore bodies and vein-type Lead-zinc-silver ore bodies.The greisen-type copper-tin ore bodies occur in granite-porphyry dolomitized belt,while the other types of ore bodies are controlled by weak tectonic surfaces of rock bodies,dikes and strata.The four ore bodies belong to the same metallogenic system,and there is a post-metallogenic thrust fault in the western part of the mining area.?2?The metallogenic geological body of Bianjiadayuan deposit is concealed granite porphyry in the western part of the mining area.The magma intrusion age is?138.2±0.8?Ma in the Early Cretaceous.The granite porphyry has crystal cave structure and microscopic identification shows that all feldspars are alkaline feldspars.The granite porphyry is characteristics with high silicon,alkali and calcium and belongs to the high potassium calc-alkaline-potassium basalt series.The chondrite-normalized REE pattern exhibit right-dipping V-type curve with strongly depleted Eu anormaly.The granite pophyre also enrich with the LILE,and deplete with HFSE and transitional elements.The lithofacies and geochemical characteristics show that the granite porphyry belongs to A₂-type alkali feldspar granite with high evolution characteristics.It was formed in the extensional environment of post-orogenic crustal stretching and thinning.The Hf isotope characteristics show that the material originated from partial melting of neogenic lower crustal material.The zircon age of pyroxene diorite is?137.4±0.7?Ma,but the field phenomena show that pyroxene diorite is formed earlier than granite porphyry and has nothing to do with mineralization.?3?S-Pb isotopes show that S comes from deep-source magma and Pb is mixed lead from upper crust and mantle.Primary fluid inclusions are divided into gas-liquid two-phase inclusions and three-phase inclusions.They belong to mesothermal inclusions,and ore-forming fluids belong to medium-salinity NaCl-H₂O system fluids formed in medium-low pressure and Medium-Shallow depth environments.The gas components in inclusions are mainly CH₄ and CO₂.?4?It is presumed that the upwelling of mantle material in tensional and thinning environment causes partial melting of the crust to form parent magma.After long-term crystallization differentiation and evolution of magma,fluids containing a large amount of volatile and ore-forming elements accumulate continuously at the top of rock mass,forming ore-hosting hydrothermal solution.When the internal pressure of ore-hosting hydrothermal solution exceeds the static pressure of surrounding rock,concealed explosion occurs at the top of rock near consolidation,and the ore-forming hydrothermal solution follows.With the addition of cassiterite crystallization and atmospheric precipitation,vein-like ore bodies were formed,and finally Bianjiadayuan tin polymetallic deposit was formed.