Geology,Geochemistry And Genesis Of Daheishan Molybdenum Deposit In The Central Jilin Province

Daheishan Molybdenum Deposit in the Eastern Xing-Meng Orogenic Belt,is located in the southern Lesser Xing'an-Zhangguangcai Ranges,the area between the Yitong-Yilan and Dunhua-Mishan faults.The nearly EW trending basement faults and NNE trending faults dominate in this mining area.Mo mineralizations,mainly veinlet-disseminated and veinlet types,are found in the Qiancuoluo granodiorite porphyry and Qiancuoluo anisometric granodiorite.Veinlet-disseminated,quartz veinlet-stockwork and breccia types of ores are common.The wall rock alteration is seen to be surrounding the porphyry in the intrusions as concentrated rings and beresitization zone,quartz-sericitization zone and propylitization zone appear in turn outwards.Potassic feldspathization zone and quartz core-quartz stockwork zone and beresitization zone overlapped roughly in the center of the porphyry.Mo mineralization occurred in the hydrothermal metallogenic period,which can be furtherly divided into quartz-molybdenite-pyrite stage,quartz-polymetallic sulphidemolybdenite stage and quartz-carbonate-fluorite stage.The ?34S is 2.2‰~3.2‰ in molybdenite and pyrite,which indicates that the sulfur originated from deep seated magma.The fluid inclusions are mainly composed of daughter mineral-bearing three-phase inclusions and aqueous two-phase inclusions.The ore-forming fluids,containing a small amount of CO2,belonging to medium-high temperature and salinity H2O-NaCl-CO2 hydrothermal solutions,are suggested to come mainly from magmatic water.The fluid boiling is thought to be the main cause of ore mineral precipitation.Petrological and petrographic studies have shown that: Daheishan complex,related to the ore-formation of molybdenum,is comprised of Changgangling and Qiancuoluo rock bodies.The zircon U-Pb age of the Changgangling monzonitic granite is 178.4±0.9 Ma,the Qiancuoluo granodiorite porphyry is 169.9±0.9 Ma,the porphyraceous granodiorite is170.3±0.3 Ma,and the results indicate that the metallogenic age is in Early Yanshanian.They are all I-type granites of the high-K cale-alkalic series,and enriched in large-ion lithophile elements(such as Rb and K)and LREE,depleted in high field strength elements(such as Nb,Ta,P and Ti),which is similar to the characteristics of the island arc or continental margin arc magma.(87 Sr /86Sr)i is 0.70472~0.70487,?Nd(t)is 1.3~1.9.Comprehensive analysis shows that the magmatic source of Daheishan complex is closely related to the mantle.The formation of the rocks has a connection with the partial melting of the thickened juvenile crust in the subduction of the plate.The partially melting of the lower-middle crustal materials in lower pressure formed the monzonitic granite with the properties of the classical island arc magmatic rocks in early stage,a bit later,the partially melting of the lower crustal materials in higher pressure formed the granodiorite porphyry with the nature of the adakites.Comparing with other typical Mo deposits in this region in the age of diagenesis and mineralization,the source of ore-forming materials and the characteristics of ore-forming fluid,we found that,the magmatism related to the mineralization of porphyry molybdenum deposits in the middle-east of Jilin Province mainly occurred at 180~170Ma,with two peaks of 180~178Ma and 172~168Ma.Early(180~178Ma),after the magmatic emplacement and differentiation crystallization in Daheishan,Fu'anpu and Jidetun mines,the monzonitic granite and granodiorite occurred and became the the early ore-bearing magmatic rocks in each mine.In the late stage(172~168Ma),the magmatism continued and the ore-forming material was further enriched,brought a large-scale ore-bearing porphyry in Daheishan deposit,sporadic granite-porphyry dykes in Fu'anpu deposit and the granite-aplite interspersed in granodiorite and monzonitic granite in Jidetun deposit.It is speculated that the porphyry with close genetic relationship with the molybdenum mineralization may be distributed in the lower part or the periphery of the ore-bearing intrusions in Fu'anpu and Jidetun deposits.The major metallogenetic matters of the porphyry molybdenum deposits in the middle-east of Jilin Province mainly came from the mixture of crust and mantle and partly dominated from shells.The metallogenetic processes are related to the melting of thelower crustal materials.The Re of Molybdenite in the Daheishan mine had obviously improved from the regional comparing,suggesting that there are more mantle-derived materials involved in the ore-forming process.In summary,the Daheishan Molybdenum Deposit is thought to be a typical porphyry molybdenum deposit based on the present geological and geochemical studies,and the formation mechanism of this deposit is suggested as the follows: in 178 Ma,influenced by the subduction of the Pacific plate,the crust of the middle-east of Jilin Province thickened.When the lower-middle crustal materials melted after the upwelling and underplating of the mantle-derived basaltic magma,the classical island arc magma was formed under lower pressure and intruded upwards to form the large-area Changgangling granites,the peripheral part of the Daheishan complex.During 170~168Ma,when the asthenospheric materials continued to rise under influence of the continuous subduction of the Pacific plate,increased pressure resulted in the adakitic magma by partially melting of the new basaltic lower crustal materials.The adakitic magma crystallized and resulted in the formation of the Qiancuoluo granodiorite porphyry and anisometric granodiorite.The ore-bearing fluid and the volatiles separated from the magmatic system continued to rise along structural fissures,then filled and enriched in the fractures in the Qiancuoluo Granodioritic intrusions.For the Qiancuoluo granodiorite porphyry was more closely related to Mo mineralization,it is called the metallogenic rock.The Changgangling intrusions provided a part of metallogenic materials only,it can be called as the favorable wall-rocks.