Magma Emplacement Sequence And Mineralization Of The Jiawula Pb-Zn-Ag Deposit,Inner Mongolia,NE China

The Jiawula Ag-Pb-Zn deposit is located in the northern part of the Da Hinggan Mountains metallogenic belt in the eastern section of the Central Asian Orogenic Belt.Sphalerite,galena,pyrite,chalcopyrite,and arsenopyrite are the major sulfide minerals occurring in this deposit.Here we report results from electron probe micro-analysis(EPMA),thermoelectricity,and sulfur isotope studies of the constituent silver minerals and sulfide phases of the Jiawula deposit.Petrographic observations and EPMA study reveal abundant silver mineralization in the ore,especially within sphalerite and galena.Discrete grains of silver minerals(including argentite,pyrargyrite,and canfieldite)and isomorphism in silver-bearing sulfides are identified.Silver and tellurium contents in galena are relatively high and show inter-relationship.Significant substitution of S by Te in the galena lattice facilitates silver entering the galena structure,which might have been promoted by relatively low lgfS2.The thermoelectric coefficient of pyrite shows a marked gradient from N-type to P-type from pre-to post-metallogenic stages.Pyrites in Jiawula are enriched in Co and As and in the absence of Ni,displaying features typical of epithermal deposits.The isotopic data suggest that the sub-volcanic intrusions might have significantly contributed to the mineralization.New U-Pb zircon geochronology,whole-rock geochemistry,mineral chemistry,and Sr-Nd isotope data are presented for the intrusions in the Jiawula deposit in order to evaluate the timing,petrogenetic type of the granitoid rocks,origin and evolution of magmatism,geodynamics,and to establish its relationship with lead-zinc mineralization.Zircon SHRIMP U-Pb analyses yield weighted mean ages of 150.1±1.8 Ma for quartz porphyry,148.8±2.2 Ma for syenite porphyry,and 145.3±1.9 Ma for monzonite porphyry,indicating a Late Jurassic(Yanshanian)magmatic event.An earlier magmatic event(Indosinian)occurred during the Late Permian to Early Triassic from ca.254 Ma to ca.247 Ma and is represented by granodiorite(254±2 Ma),dacite porphyry(252.9±4.8 Ma),and diorite porphyry(247±4.1 Ma).Both the Indosinian and Yanshanian igneous rocks are classified as I-type granitoids.The late Jurassic intrusions are highly fractionated and characterized by negative anomalies of Eu,Sr,P,and Ti.The hypabyssal intrusions have initial 87Sr/86 Sr values between 0.70458 and 0.70522,and ?Nd(t)values of-3.4 to-0.2,indicating relatively older crust in Jiawula among more juvenile crust in this area.Magma generation in Jiawula is linked to juvenile lower crustal and slightly enriched mantle sources.The ~250 Ma magmatic episode in Jiawula might be related to the subduction of the Mongol-Okhotsk oceanic plate towards the south beneath the Erguna massif.The ~150 Ma magmatic event occurred after the closure of the Mongol-Okhotsk Ocean followed by the change in subduction direction of the Paleo-Pacific plate.Varying temperature,stronger fractionation and higher oxygen fugacity related to the magmatic-hydrothermal transition caused Pb-Zn mineralization.Studies on fluid inclusions are also carried out to discuss the physical and chemical conditions of fluid evolution.Fluids in the Jiawula deposit are mainly NaCl-H2 O system.The fluid of this deposit are composed of that with relatively low temperature(155.3~286.1 ?,averaging 202.6 ?)and low salinity(0.4~2.4%,averaging 1.4%).There is no direct evidence of fluid mixing.Since the fluid is low temperature and low salinity fluid,the fluid is thought to be meteoric water or groundwater.For the Jiawula deposit,the process of fluid evolution is likely to occur with both cooling and decompression.