Metallogenesis Of Porphyry Molybdenum Deposits In Southern Jining Area,Inner Mongolia

The north margin of North China Craton?NCC?molybdenum ore belt is the third largest Mo ore belt in China,only second to the East Qinling and the Xiao Hinggan Mountains-Zhangguangcai Mountains Mo ore belt.The Mo deposits in this belt,namely as Yanliao molybdenum ore belt,are mainly located in Yanshan area that is in the north of Hebei province and Beijing,as well as western Liaoning province.In recent years,the discovery of the Dasuji large porphyry Mo deposit in the Zhuozi county,the Caosiyao super-large porphyry Mo deposit in the Xinghe county,and the Quanzigou large porphyry Mo deposit in the Fengzhen county,has made the Jining area,Inner Mongolia,which is the westernmost part of the Yanliao belt,a new large Mo ore-concentrated area in the northern margin of the NCC.The origin,migration,enrichment and sedimentation mechanism of mineralization of Mo deposits in this area and its relationship with Mesozoic magmatic activities is still unclear,which has seriously affected the understanding of Mo mineralization mechanism and the arragement of further ore-prospecting work.Therefore,systematic research is urgently needed.Based on the detailed study of ore deposit geology characteristics,in order to discuss the diagenesis and mineralization process in the southern Jining area,two porphyry Mo deposits?the Caosiyao and Quanzigou deposit?in southern Jining area were studied in this research,mainly focused on the ore-forming fluid system,the origin,precipitation,migration and enrichment mechanism of ore-forming fluids,as well as genesis of porphyries.The orebodies in the Caosiyao deposit,with 1.75 Mt molybdenum resources,are mainly produced in the external contact zone between the Caosiyao complex and wall rocks,and molybdenite mainly occurs in the leptite,granulite of the Jining Group,Neoproterozoic diabase and the Yanshanian granitic porphyries as veinlets and stockworks.The ⁴⁰Ar/³⁹Ar isochronology age of sericite from the syenogranite porphyry and the Rb-Sr isochronology age of sulfides from Pb-Zn-Au-?Ag?orebodies in the periphery of the mining area are 146.4±2.2 Ma and 145.3±1.0 Ma,respectively,suggesting that they belong to the same mineralization system.The sulfides?pyrite,molybdenite,copper,galena and sphalerite?LA-ICP-MS trace elements results indicate that from the early stage to late,the ore-forming fluid of the Caosiyao deposit evolved from high temperature hydrothermal fluid,related to magmatic activity,to low temperature hydrothermal fluid.The LA-ICP-MS trace element composition of melt inclusions indicates that the initial granitic magma of syenogranite porphyry in the Caosiyao deposit that is closely related to mineralization is mainly silicate rich in Mg,Mn,Fe,Ti and other elements.The volatiles?H₂O,CO₂,CH₄,S,Cl,etc.?in the magma are oversaturated by crystallization differentiation.When the magma rich in volatiles migrated upward from deep chamber,exsolution between melt phase and fluid phase occurred because of the decrease in temperature and pressure,then the ore-forming elements?Mo,Fe,Cu,etc.?enriched in the volatiles precipitated at the favorable position,while the melt phase gradually got more deprived in Mg,Mn,Fe,Ti,and relatively rich H₂O and CO₂,eventually evolved into fluid only composed of SiO₂ and H₂O,as well as a small amount of ore-forming elements.The monzogranitic porphyry invaded after mineralization show little obvious mineralization,which may be related to the lack of S,Cl and other volatiles in its primary magma.The Quanzigou large porphyry molybdenum deposit locates in Fengzhen City,Inner Mongolia.A Yanshanian granitic complex,namely Hongiangshan complex,is exposed in the deposit,mainly composed of medium-coarse grained granite,porphyritic granite and quartz porphyry.The Mo orebodies occur mainly in the porphyritic granite.The results of molybdenite Re-Os and zircon U-Pb dating show that the Quanzigou Mo deposit formed in the late Jurassic?¹60Ma?.The Hongniangshan complex are featured by rich in SO₂ and K₂O,and deprived in MgO,CaO,FeO and Fe₂O₃,belonging to peraluminous high-K calc-alkaline series I-type granite,formed in the orogenic extension environment,which is closely related to Mongolian-Okhotsk orogeny and the remote effect of subduction of the ancient Pacific plate to Eurasia in the east;Zircon Lu-Hf and K-feldspar Pb-O isotopic compositions indicate that the hongniangshan complex is mainly the product of partial melting of continental crustal materials in late paleoproterozoic.The results of fluid inclusions study show that the ore-forming fluid in the main ore-forming stage of the Quanzigou deposit is medium-high temperature and medium-high salinity fluid,whereas the fluid in the late stage belongs to medium temperature and medium-low salinity fluid,generally belonging to H₂O-NaCl-CO₂ system.The H-O isotopic composition of quartz indicates that the fluid in the main metallogenic stage in the Caosiyao deposit is dominated by magmatic water,with a small amount of meteoric water added in the late metallogenic stage.The S-Pb isotope composition of sulfides indicates that the sulfur and lead in the fluid are mainly from magma.The LA-ICP-MS analysis results of trace composition of sulfide show that W,Re,Se,V,Te and other elements were enriched in molybdenite,with little difference of trace composition in different stages.Pyrite in the quanzigou deposit can be divided into 5generations-Py1a,Py2a and Py3,which are primary pyrite,as well as Py1b and Py2b,which are pyrite altered by hydrothermal process in late stage.From Py1a to Py2a,the contents of Co,Ni and Co/Ni decreased,while the contents of As,Se and Te increased.Up to Py3,the contents of Co,Ni,Se,Te and Co/Ni ratio are very low,suggesting that pyrite crystallization environment evolved from high temperature magmatic hydrothermal condition to medium-low temperature magmatic hydrothermal condition.Studies on fluid inclusion and mineralogy show that fluid boiling is the main mechanism of metal sulfide precipitation in the Quanzigou deposit.By the studies on the Caosiyao and Quanzigou deposits,combining previous research on the Dasuji deposit in southern Jining area,it is concluded that,during the Indosinian,Wuliji-Linxi orogenic belt turned into the orogenic evolution stage,the study area was in extension environment,and then quartz porphyry,syenogranitic porphyry invaded,forming the large-scale Dasuji porphyry molybdenum deposit.In the early Yanshanian period,the area may be affected by both the post orogeny of the Mongol-Okhotsk orogenic belt in the north and the subduction of the paleo-pacific plate in the east,making it an extensional environment and causing partial melting of the lower crust and the formation of the Quanzigou and Caosiyao porphyry Mo deposits.During the late Jurassic-early Cretaceous period,this area may be mainly affected by the subduction of the paleo-pacific plate in the east,resulting in extensive continental volcanic eruptions,causing the formation of epithermal Pb-Zn-Ag-Au deposits in the area.