| The Taipingcun molybdenum deposit is located in 30 km southeast to Xinglong,Hebei Province.It belongs to the southern central section of the Yanshan-Liaoning molybdenum(copper)Metallogenic belt and is a newly discovered large-sized molybdenum deposit in the Eastern Hebei Province.By now,this deposit is still under exploration with a few theoretical researches.This paper is based on the preliminary exploration results and field investigations,through the system analysis of metallogenic geological characteristics,diagenetic and metallogenic age.Metallogenic physical and chemical conditions,ore material resource and dynamic background has been determined.Combining contrasting with similar deposits,genetic type has been constrained,which provides the theoretical basis for Mo prospecting potential and further ore-prospecting.The orebodies are mainly hosted by the metamorphic rocks and cryptoexplosive breccias which located in external contact zone of the granite,and is controled by complex fracture system,which is formed by multiphase granitic magma intrusions and fault activities.The dominant mineralization types include veinlet and stockwork,followed by disseminated Mo mineralizations.The main metal minerals in the ore are molybdenite,pyrite,and gangue minerals include quartz,potashfeldspar and sericite,etc.The wall rock alteration includes potassium feldspathization silicified belt?silicide sericitization zone?prophylitic zone,from inside to outside,among which silicification,potassic alteration and sericitization have close relationship with the Mo mineralization.The mineralization is divided into the late magmatic gas-liquid stage,and the hydrothermal stage are divided into five phases: quartz-K-feldspar ? quartz-molybdenite ? quartz-polymetallic sulfide?late quartz-vein? carbonate,and the quartz-molybdenite phase is the main metallogenic period.According to the whole-rock geochemistry,the concealed porphyritic monzonitic granite,which closely related to the mineralization,is enriched in SiO2 and Al2O3,ascribed to metaluminousperaluminous K-high calc-alkaline series I-type granite and belongs to oxidation and high degree differentiation of magnetite series,which is favourable to Mo mineralization.The porphyritic monzonitic granite is characterized by a right “L”-shape REE pattern with a weak negative Eu anomaly,it is rich in LREE,LILE(Rb)and HFSES(Th and U),and depleted of HREE,Ba,Sr,Nb,Ta,with obvious fractionation.It suggest that the rock formed in the syn-collision and post-collision setting,belongs to the geological background of tectonic regime transformation,namely,changed from compression to extension,during A curtain or middle curtain stage of the Yanshanian movement and related to the subduction of the ancient Pacific plate.The rock was generated from deep melting or remelting of the middle-lower crust,and mixing part of the upper mantle materials.Geochronological data show that the acidic magma intrusion activities in the deposit mainly occur in late middle Jurassic.The U-Pb age of the zircom from the concealed porphyritic monzonitic granite,which closely correlated with mineralization,is(164.11±0.92)Ma and the molybdenite Re-Os weighted mean age is(164.11±0.92)Ma.Obviously,diagenetic and metallogenic epochs are basically identical,both taking place in late middle Jurassic,which indicate that the rock and deposit maybe result from the third tectonic-magma-metallogenic events of the Yanshan-Liaoning molybdenum(copper)metallogenic belt.The Re in the molybdenite varies between 18.79×10-6 and 66.65×10-6,averaging at 41.86×10-6,indicating that the ore-forming materials might have been derived mainly from the crust with the participation of small amounts of mantle materials,and that coincide well with magma source of the rock,all further reflecting the closely relationship between the magmatism and mineralization.The fluid inclusion study indicated that liquid-rich phase inclusions(type L),gas-rich phase inclusions(type V),CO2 –bearing three-phase inclusions(type C)and daughter mineral-bearing polyphase inclusions basically existed in the main metallogenic phase,with various types and close coexistence,which showing the existence of fluid immiscible and boiling phenomena.Their homogenization temperature were concentrated at 298.4~485? and 108.9~200.3?;corresponding salinities were concentrated in 2.06%~11.23% and 40.80%~54.54%NaCleqv.Their ore-forming pressures range is divided into 29.008~50.048 and 75.73~117.226 MPa,represented the relatively closed and relatively open conditions,further confirmed that the ore-forming fluid experienced the boiling.Their ore-forming depths ranged from 3.0 to 5.3km,belong to middle-deep facies.The ore-forming fluid system changed from the early middle-high temperature and high-salinity NaCl-H2O-CO2 system to the late middle-lower temperature and lower salinity NaCl-H2 O system.The ore-forming fluid immiscibility or boiling is the major mechanism of the metal sulfides precipitation.Compared with typical porphyry molybdenum deposit,Taipingcun Mo deposit is similiar in the tectonic setting,ore controlling factors,ore body characteristics,mineral fabric,wall-rock alteration characteristics,metallogenic rock petrochemistry and ore-forming physicochemical conditions,but it has its own uniqueness of the spatial distribution of ore bodies and mineralization characteristics,which is related to metallogenic rock emplaced depth,metallogenic depth and wall-rock physicochemical conditions.The comprehensive analysis reveals that Taipingcun Mo deposit is a “medium-plutonic porphyry molybdenum deposit ”. |
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