The Metallogeneses And Geodynamic Settings Of Molybdenum (Copper) Deposits In Yan-Liao Metallogenic Belt

The Yan-Liao(Yanshan-western Liaoning) area is an important metallogenic belt for Au,Ag,Pb-Zn on the northern margin of the North China Block and is also the second largest Mo metallogenic belt in China where more than 20 Mo(Cu) deposits have been explored.Those Mo(Cu) deposits are present in the northern Hebei province and western Liaoning province and are mainly porphyry types,skarn types and quartz vein types.Base on the pervious researchers and detail field investigation in Yan-Liao area,we selected three types of representative Mo deposits such as Lanjiagou(quartz vein types),Xiaojiayingzi (skarn type) and Sadaigoumen(porphyry type) to detail study.Focusing on the geological characteristics, fluid inclusions analyses,stable isotope,chronology and synthetically comparative study etc,we studied the source and evolution of ore fluids of representative Mo deposit,summarized the metallogenic regularities of Mo(Cu) deposits,discussed the metallogenic mechanism and the relation between geodynamics evolution and the Mo(Cu) mineralization of the Yan-Liao area,and compared with the Mo deposits in east Qinling Mo metallogenic belt in the southern margin of North China Block.The main advance achievements from this study are as followings:1.Located in the Yan-Liao area,the Mo(Cu) deposits occurred in an arc structure belt in the northern margin of North China Block,which were controlled by the EW-trending,NE-trending and NNE-trending regional faults.The host rocks of these deposits are Mesoproterozoic Wumishan Formation,comprising dolomitic limestone and Cambrian-Ordovician limestone and shale.Most these Mo(Cu) deposits were temporally and spatially associated with intermediate-felsic rocks(except Xiaojiayingzi deposit),and occurred in the inner of felsic rocks or the contact zone between the felisc with host rocks.Intrusive rocks and host rocks play an important role for the metal mineralization.Those Mo(Cu) deposits,occurring in the inner of granite porphyry,felsic rocks and intermediate-felsic volcanic rock are mainly porphyry type, whereas the Mo(Cu) deposits,occurring in the contact zone between felsic rocks and carbonate rocks are mainly skarn type or porphyry type deposits.2.The Lanjiagou Mo deposit is located in Liaoning province.Its major molybdenite orebodies are situated at the inner of fine granite of Hongluoshan and the contact zone between fine granite and coarse granite.Molybdenite quartz large veins are the primary mineralization type in Lanjiagou deposits.The wallrock alteration consists of potassium alteration,silicification,greisenization and hydromuscovite alteration.Petrographic observation shows that aqueous two-phase inclusion is the mainly fluid inclusion type in molybdenite quartz veins and the homogenization temperatures and salinities of the inclusion in ore-forming stage range from 160 to 405℃,2.4～16.5%NaCl equivalent,respectively.Laser Raman Spectroscopic analysis of fluid inclusions suggest that the ore-forming fluids once were of NaCl-H₂O±(CO₂) system.The hydrogen and oxygen isotopic components of typical ores indicate that theδD values of ore-forming fluids from -81‰to -101‰and theδ¹⁸O_H2O values vary from -0.1‰to 4.5‰,which suggested that the ore-forming fluids could be produced by mixing magmatic and meteoric water.During mineralization stages,the fluid inclusions with lower salinities underwent mixing processes with the fluid inclusion with middle salinities,which brought positive influence to molybdenite mineralization and concentrate.Comparative studies on geological features,mineralization,wallrock alteration,fluid inclusions characteristics and H,O isotope,the author confirm that there are many differences between Lanjiagou Mo deposit and porphyry Mo deposit.The genesis of Lanjiagou deposits is of quartz vein type Mo deposit.3.The Xiaojiayingzi Mo(Fe) deposit is temporally and spatially associated with intermediate to mafic rocks and the mineralization hosted in skarn.Detail fluid inclusion studies show that there are mainly two types primary fluid inclusion occurred in this deposit.The fluid inclusion in granet and chondrodite from garnet skarn stage and magnetite mineralization stage are mainly daughter crystal-bearing polyphase,which homogenized to the liquid phase by disappearance of vapor phase and homogenization temperatures from 505 to 560℃and 435 to 541℃,salinity from 43.4 to 50.0%NaCl equiv and 44.9 to 56.9%NaCl equiv, respectively.The fluid inclusions occured in calcite and quartz from molybdenite mineralization stage: daughter crystal-bearing polyphase inclusion and aqueous two-phase inclusion.The former homogenized to liquid phase by dissolution of daughter minerals,with homogenization temperatures from 203 to 440℃and salinity ranging from 32.0%to 52.0%NaCl equiv.The latter homogenized to the liquid phase by disappearance of the vapor bubble at homogenization temperatures from 146 to 402℃,salinity between 4.3%to 18.4%NaCl equiv.Fluid inclusion and H,O isotope analysis show that the ore-forming fluid from pre-ore stage is characterized by high temperature and high salinity.The ore-forming fluid from molybdenite mineralization stage are characterized by mixed fluid of high temperature and high salinity fluid end and medium to lower temperature and lower salinity fluid end.During the evolutional processes of ore-forming fluid,the transformed of the pressure and density of high temperature and high salinity fluid play an important role to magnetite minealization,and the mixing of fluid between high salinity and lower salinity take a positive influence to molybdenite deposition.4.The Sadaigoumen deposit lies in Hebei province and is the largest molybdenum deposit in this region thus far reported.Its major molybdenite orebodies are mainly hosted inside monzogranite and occurred as veinlet,disseminated structure or banding structure.Molybdenite mineralization is closely associated with microclinization,silicification and muscovitization.Fluid inclusion studies suggest that three types of fluid inclusion are present in this deposit:aqueous two-phase inclusions,CO₂-bearing three-phase inclusions and CO₂ phase inclusions.Fluid inclusions in quartz in the pre-ore stage and post-ore stage are mainly aqueous two-phase.The homogenization temperatures and salinities of the former stage vary from 280 to 452℃and from 5.4%to 18.4%NaCl equivalent respectively,and those of the latter stage vary from 153 to 279℃and from 3.9%to 9.7 wt%NaCl equivalent respectively.During the mineralization stage,all the three types of fluid inclusion were formed in ores.Microthermometric studies indicate that their homogenization temperatures and salinities range from 170 to 370℃and from 4.3 to 14.4 wt%NaCl equivalent respectively.Laser Raman spectrometric and hydrogen and oxygen isotope studies of the fluid inclusions in ores indicate that quartz in the Sadaigoumen deposit contains -82 to -98‰δD and 0.1 to 6.2‰δ¹⁸O_H2O and that the ore-forming fluids once belonged to the immiscible NaCl-H₂O-CO₂ system and were mainly derived from magmatic water,which were mixed with meteoric water in the post-ore stage.During the formation of the ore-forming fluids,three stages of fluid evolution occurred in the Sadaigoumen deposit:magma dehydrating and degassing in the pre-ore stage,fluid immiscibility in the mineralization stage and meteoric waters mixing in the post-ore stage,respectively.The decrease of temperatures and fluid immiscibility are the dominant factors for molybdenite deposition.5.SHRIMP zircon U-Pb,zircon U-Pb and molybdenite Re-Os isotopic dating methods are utilized in order to determine the timing of the mineralization and its relationship intrusive rock.We obtained the SHRIMP zircon U-Pb weighted mean age of(188.9±1.2) Ma for the Lanjiagou fine-grained granite and (169.9±1.4) Ma for the Xiaojiayingzi fine-grained diorite.The molybdenite Re-Os isochron age is (165.5±4.6) Ma for Xiaojiayingzi deposit.The molybdenite Re-Os model age ranges from 237 to 238 Ma for the Sadaigoumen deposit and 136 to 146 Ma for the Dacaoping deposit.The zircon U-Pb for monzonitic granite from Sadaigoumen deposit and for granodiorite from Dacaoping deposit yield modle age of 227 Ma and 137 Ma,respectively.After collecting and documenting the precise radiometric age of the Mo(Cu) deposits in Yan-Liao area,there appear three Mo(Cu) mineralization pulses,i.e.ca.225 Ma,ca. 185 Ma and ca.145 Ma in Mesozoic.Base on the Mesozoic tectonic evolution,we propose these three pulses of large-scale mineralization are the consequences of the short time compression during the postcollisional process of the Siberian Block with the North China Block,the closure of Mongolo-Okhotsk, and the late stage of tectonic regime changing its main stress from NS-trending to EW-trending.6.Based on comprehensive studies and comparison of these Mo deposits between Yan-Liao metallogenic belt and east Qinling Mo mineralization belt,we confirmed that there were significant differences exist in regional geochemical anomaly,the tectonic system,the magmatic composition,the age of magma emplacement and the mineralization pulse.