Ag Polymetallic Mineralization And Peripheral Metallogenic Prediction Of Bayanbaolege Deposit In The Southern Great Xing'an Range

The large Bayanbaolege Ag polymetallic deposit is one of newly discovered deposits in the Tuquan-Linxi Fe?Sn?-Cu-Pb-Zn-Ag-Nb?Ta?metallogenic belt of the Southern Great Xing'an Range,Chifeng area,NE China.Its major ore bodies,sulfide-quartz veins,are distributed within granodiorite porphyry and in adjacent Late Permian strata,and controlled by the NE faults.From west to east and from shallow to deep,the major mineralization element in this deposit changes from Ag to Zn.Ore structures of this deposit consist of major vein,minor banded,brecciated and disseminated structures.This deposit can be classified as the low-to-moderate temperature hydrothermal vein-type.Three mineralization stages have been recognized,namely,arsenopyrite-pyrite-quartz?stage I?,polymetallic sulfides?stage II?and calcite-sulfides-silver minerals?stage III?,and the stage III is the major silver mineralization stage.Microthermometric measurements and laser Raman analyses of four types of fluid inclusions formed in the three stages show that the ore-forming fluid belongs to the H₂O-CO₂-NaCl system,with medium temperatures and medium-low salinities.C-H-O isotope data indicate that ore-forming fluid was derived largely from magmatic fluid,with the mixture of meteoric water in the later stage.Fluid immiscibility which can significantly cause the phase equilibrium change of a ore-fluid with the temperature and pressure dropping might be the major mechanism of metal sulfide precipitation.The data of S,Pb and Sr isotope compositions of the sphalerite and pyrite show that the part of ore-forming materials were derived from the intermediate-acid magma mixed with crust and mantle sources,and the other part of materials originated from the Permian Linxi Formation.Detailed field investigation and laboratory petrographic analysis indicate that Ag polymetallic mineralization in the Bayanbaolege deposit is spatially,temporally and genetically related to granodiorite porphyry.The data of the LA-MC-ICP-MS zircon U-Pb dating show that the ages of the granodiorite porphyry and the post-ore diorite porphyrite are 130±1Ma and 125±1Ma,and the granite porphyry and alkali feldspar granite collected from adjacent the ore deposit,are 135±1Ma and 133±1Ma,respectively.The isochron age of eight sphalerite and pyrite samples from ore is 130.4±2.3 Ma,and that of six sphalerite samples is 129.9±2.9 Ma,indicating that the diagenesis and mineralization concordantly took place the early Cretaceous.The petro-geochemical studies show that the granodiorite porphyry and the diorite porphyrite belong to the I-type granites,and the granite porphyry and alkali feldspar granite are the A-type granites.Sr-Nd-Hf isotope data indicate that the Cretaceous granodiorite porphyry magmatisms were resulted from partial melting of the Neoproterozoic juvenile crust.Combined with the ages of diagenesis and mineralization and the regional geological data,it can be suggested that this deposit formed under the extensional environment associated with the westward subduction of the Paleo-Pacific Plate.Base on the discussed above,the genetic model of Bayanbaolege Ag polymetallic deposit and the regional metallogenic model of polymetallic deposit in the Southern Great Xing'an Range has been established.Based on the systematic collection and analysis of the medium-large Ag polymetallic deposits,such as,the metallogenic geological conditions,the geological characteristics,the ore genesis,the ages of diagenesis and the mineralization,in the Southern Great Xing'an Range,the spatio-temporal distribution regularities of the regional Ag polymetallic deposit are summarized.In time,Ag polymetallic deposits are mainly formed at 270²30 Ma and 150¹30 Ma,and most of the deposits were formed in the late Jurassic-early Cretaceous?150¹30 Ma?.In space,the Ag polymetallic orebody are controlled by NW or NE faults,and mainly hosted in the Permian sedimentary rocks or Mesozoic volcanic rocks,implying mostly related to the early Cretaceous intermediate-acid magmatisms.Ore elements,including Au,Ag,Cu,Pb,Zn,Mo,W,Sb and Sn,were analyzed for the upper Permian Linxi Formation rocks,upper Jurassic Manketouebo Formation rocks and multiple intermediate-acid magmatic vein rocks in the Bayanbaolege Ag polymetallic deposit area.And we studied the variation rules of the ore-bearing properties of these rocks.According to the comprehensive analysis,it can be concluded that the granodiorite porphyry was the major contributor to the polymetallic mineralization,and the Linxi Formation also played the significant part in the ore-forming process.Structurally,the NE-and NW-trending faults are the main structures in the district,and the NE-trending faults serve as the major intrusive body-and ore-controlling structures.In addition,some prospecting indications of the Ag polymetallic deposit in Bayanbaolage mining area and its periphery are summarized.On the basis of a detailed summary of the characteristics of the ore-prospecting criteria and the regional geochemical anomaly,comprehensive multiple anomalies clues,including of the mineralization and wall-rock alteration clues,the induced polarization anomalies,the high precision magnetic anomalies and their relationship with mineralization,are all considered.Two prospecting target areas in the southern periphery of the Bayanbaolege Ag polymetallic deposit area were delineated.