Genesis Of The Donggou Porphyry Mo Deposit And Wangpingxigou Pb-Zn Deposit In The Southern Margin Of The North China Craton

Late Mesozoic granitic magmatism are widespread in the southern margin of the North China Craton?NCC?,contemporary with large-scale Mo mineralization.With a total reserve of >5 million tons of Mo metal,the southern margin of the NCC is the largest Mo ore belt in the world.Most of the Mo deposits are closely associated with granites widely distributed in the belt.However,there are more than 100 granitic plutons,and only half of them might be associated with Mo deposits or Mo mineralization.Although many granites including large batholiths and small porphyritic bodies are geochemically similar,most of the Mo deposits are closely associated with small porphyritic bodies.This phenomenon may be related to the Mo behavior during magmatic-hydrothermal evolution.However,the Mo behavior in the formation of Mo deposits in the southern margin of the NCC is still poorly constrained.Moreover,large-scale Pb-Zn mineralization also developed in the southern margin of the NCC.The genesis of Pb-Zn deposits has long been controversial.Some scholars suggested that the Pb-Zn deposits may represent distal facies of the porphyry Mo deposits,and they belong to a magmatic-hydrothermal system.However,others considered that the Pb-Zn deposits formed in a collision regime and the ore-forming fluid was metamorphic in origin.To better confine the formation of Mo and Pb-Zn deposits in the southern margin of the NCC,I have conducted the following studies.Firstly,a systematic geochemical study of biotite from the Taishanmiao batholith and the Donggou porphyry associated with the porphyry Mo mineralization in the southern margin of the NCC was investigated,in order to explore the control factors of Mo enrichment during the magmatic-hydrothermal evolution,to identify the geochemical indexes of ore-forming granites and to determine the relationship between Mo mineralization and the granites.Secondly,a systematic study on the trace elements of pyrite and S-Pb isotopic compositions of sulfides from the Donggou porphyry Mo deposit and the peripheral Wangpingxigou Pb-Zn deposit were conducted to study their genetic relationship.Finally,we compiled and synthesized published data in the literatures in order to better confine the metallogenic mechanism of Mo deposits,and the genetic relationship between Mo and Pb-Zn deposits in the southern margin of the NCC.The main innovations and new viewpoints are summarized as following:1.Controlling factors of Mo enrichment during the magmatic-hydrothermal evolutionGeochemical characteristics of the Taishanmiao batholith and Donggou porphyry,and the magmatic biotites in the granites reveal that these granites are highly fractionated I-type granites.Trace element features of biotites suggest a magma differentiation trend from the Taishanmiao batholith to the Donggou porphyry,as also revealed by systematically decreasing K/Rb ratios and compatible element contents?Co,Ba,V and Ti?,and increasing incompatible element contents?such as Cs,Li,Ta and Tl?.Combining with published studies on geochronology and isotopic geochemistry,we consider that the Donggou porphyry is the highly differentiated product of the Taishanmiao batholith.The Fe³⁺/Fe²⁺ values of the biotite gradually increase from the batholith to the porphyry,indicating progressively increasing f O2 during magmatic differentiation.The compositions of the magmatic biotite mostly plot above the NNO buffer,indicating that Mo6+ is the predominant phase in magma.In addition,F variations in the biotite indicate that the magma is F-rich,which promotes the high degree of magma fractionation.Biotite compositions suggest that the earlier fluids associated with the Taishanmiao batholith are relatively F-poor with negative log?f HF/f HCl?values,whereas the later fluids associated with the Donggou porphyry are relatively F-rich with positive log?f HF/f HCl?values.In summary,the high oxygen fugacity in magma increases the incompatibility of Mo.Meanwhile,the high F contents in the magma extends the duration of magma crystallization and promotes high degree of magma differentiation.Therefore,Mo is highly concentrated in the residual melt,which provides a preliminary basis for final Mo mineralization.Afterwards,the relatively F-rich fluids are able to extract the Mo from the melt to form the super-large Donggou porphyry deposit.2.The genetic relationship between Mo and Pb-Zn depositsPyrite grains from the Wangpingxigou deposit have high Co/Ni ratios?mostly >1?with Au and As contents similar to those in the typical epithermal systems.Moreover,the sulfides from this deposit have 206Pb/204Pb?17.345-17.417?,207Pb/204Pb?15.454-15.514?,and 208Pb/204Pb?38.138-38.364?ratios similar to those of the Donggou deposit?17.380-17.523,15.435-15.576 and 38.210-38.507?,indicating that they have been formed from the same source.Pb,Ag,Sn,Zn,Sb,Mn,Au and As in pyrite decrease gradually from the Wangpingxigou deposit to the Donggou deposit,indicating gradually increasing oxidation states during fluid evolution.Combining with the ore-forming age,we consider that Donggou porphyry Mo deposit and the Wangpingxigou Pb-Zn deposit are the products of different evolution stages of an ore-forming fluid system,and they form a metallogenetic system.Sulfides from these two deposits have S isotopic compositions?7.4 to 10.3‰ for the Donggou deposit and 5.9 to 8.7‰ for the Wangpingxigou deposit?broadly overlapping those of the magnetite-series granites.The relatively high ?34S values could be inherited from their magmatic source.Combining with the lead isotopic compositions of various sulfides,we consider that the metal source for Mo and Pb-Zn mineralization involves both the crust and mantle.3.The metallogenic mechanism of Mo deposits and the genetic relationship between Mo and Pb-Zn deposits in the southern margin of the NCCCompilation of published geochemical data on the ore-forming granites in the southern margin of the NCC indicates that most of them are characterized by high Si O2 contents?>70%?,and belong to highly fractionated granites,indicating that the formation of Mo deposits is mostly associated with highly fractionated magmas.Calculations based on the biotite compositions in granites associated with the Yuchiling,Nannihu and Leimengou Mo deposits indicate that these magmas are all F-rich?0.24-0.76%?.In addition,fluorite is a common mineral in many Mo deposits in the southern margin of the NCC,such as the Jinduicheng,Shangfanggou and Shibaogou deposits.Therefore,the magmatism associated with the porphyry Mo deposits in the southern margin of the NCC is commonly characterized by F enrichments.Most of the granite associated with the porphyry Mo deposits have a long magmatic activity time?>8 Ma?,which may be related to either the F enrichments in the magma or the injection of mafic magma.Based on this work and published studies on the magmatic oxygen fugacity and ore-forming fluids in the granites associated with the Mo deposits in the southern margin of the NCC,we suggest that the formation of porphyry Mo deposits is related to both high magmatic oxygen fugacity?> NNO?and relatively F-rich fluids.High oxygen fugacity?>NNO?increases the incompatibility of Mo,and the F enrichments in the magma extend the duration of magma crystallization to form highly fractionated melts.Both processes would increase the Mo contents in the residual melt,and afterwards,the relatively F-rich fluids are able to extract the Mo from the melt to form the abundant Mo deposits in the southern margin of the NCC.Compilation on the H-O isotopic compositions of the ore-forming fluids in the Pb-Zn deposits in the southern margin of the NCC shows that the ?DH2O and ?18OH2O values range from-67 to-111‰?peak value-79.8‰?and from-3.4 to 13.9‰?peak value 4.0‰?,respectively,indicating that the ore-forming fluids are magmatic-hydrothermal in origin.The S isotopic compositions of the sulfides in these Pb-Zn deposits from-9.4 to-12.1‰?peak value 4.9‰?are comparable to those of the Mo deposits?-6.0⁹.8‰,peak value 3.5‰?,overlapping with those of the magnetite-series granites?1-9‰?.Moreover,the Pb-Zn deposits has similar Pb isotopic compositions to the Mo deposits and granites in the southern margin of the NCC,indicating that the ore-forming materials in the Pb-Zn and Mo deposits are all from the granites with a mixed source of both crust and mantle.Additionally,the ore-forming ages of the Pb-Zn deposits in the southern margin of the NCC are from 144 to 117 Ma,broadly coeval with the age of the Mo deposits?155-116 Ma?and granites?158-112 Ma?in this area.In summary,we consider that these Pb-Zn and Mo deposits in the southern margin of the NCC derived from a common magmatic-hydrothermal system,and they form a Mo-Pb-Zn metallogenitic system.