Magmatism And Ag-polymetallic Mineralization In Shuangjianzishan Deposit And Its Periphery,Inner Mongolia

The super-large Shuangjianzishan Ag-polymetallic deposit is a newly discovered deposit located in the Huanggang-Ganzhuermiao polymetallic metallogenic belt of Inner Mongolia,NE China.Ag-polymetallic mineralization is closely related to the emplacement of the Late Jurassic magmatic activity.Ag-polymetallic mineralization is mainly hosted by strongly deformed and altered volcano-sedimentary sequences in the Permian Dashizhai Formation that are unconformably overlain by Jurassic volcanic rocks.The polymetallic mineralization hosted in the volcano-sedimentary rock series entirely,they always comeposes the layered,?long?lenticular,wedge-shaped and cystic orebodies in the form of disseminated,vein,sheet,veinlet.Three types of mineralization have been recognized with the disseminated sulfides accompanied by densely spaced stringers sulfide,relatively later quartz?-carbonate?-veins orientated in NE-and NW-trending shears and faults and breccia mineralization occurred within faults.Preliminary studies show that the Shuangjianzishan deposit might represent a mixed ore system where an early disseminated and stringer mineralization has been overprinted by later hydrothermal vein-type and breccia mineralization.Zircon LA-ICP-MS U-Pb dating study shows that the age of felsic magmatic rocks in the Shuangjianzishan Ag-polymetallic district are 262²38 Ma,179¹59 Ma,142¹36 Ma and 131 Ma,respectively,these dating results indicate that magmatism occurred in late Permian-early Triassic,middle-late Jurassic and early Cretaceous.It is important that the isotope ages of muscovite and sulfides with the growth of the silver polymetallic ore are?147±2?Ma and?159±6?Ma respectively,and sulfide samples?arsenopyrite and pyrite?from the vein-type mineralization has Re-Os isochron ages of 159±6 Ma,suggestting the mineralization is associated with late Jurassic magmatism in the Shuangjianzishan district basically..Stable isotopes(?¹⁸OH₂O and ?D)in fluid inclusions from vein-type and breccia ores are characterized by very systematic and small variation,and this difference reflects the higher fluid fluxes and smaller amount of rock-buffering for the quartz veins.The C and O stable isotopes for hydrothermal carbonates form NW-and NE-trending veins also subtly vary with ?13C values between-10.6 and-14.3‰,indicating that the carbon in the ore-forming fluid was sourced from a mixed magmatic and highly evolved meteoric fluid.In addition,the S-isotopic composition of sulfides at the deposit is markedly close in range of granitic rocks and mantle,which is consistent with a magmatic source for the sulfur.Lead-isotopes values for sulfides are indicative of a mixed mantle and crustal source.The Shuangjianzishan Ag-polymetallic deposit is therefore interpreted as a medium-temperature,intermediate sulfidation deposit and the mineralization was deposited from a mixed source during a phase?temperature or pressure?change,probably during fluid mixing and boiling.The Late Jurassic magmatism in the Shuangjianzishan district are related to subduction of the Mongolia–Okhotsk Ocean and subduction of the Paleo-Pacific Ocean Plate,respectively.