Metallogenic Potential Of Magmatic Related Gold Deposit In The Great Xing’an Range

The Great Xing’an Range（GXR）in the northeastern China,east of the Central Asian Orogenic Belt（CAOB or Altaid）,is world famous for the unique characteristics of soft collision and weak orogeny and the largest young crustal growth.The Great Xing’an Range metallogenic condition is superior with various types of deposits.As magmatic gold deposit named Bilihe identify and found which is located in the north rim of North China Craton（NCC）,a new action would be promoted for deposits prospecting.On the basic of magmatic gold deposit,this paper take the magmatic rocks as the research object.Combination with 1988 Hf istopes published and datas from samples picked in Yiminhe,Wulagai,Sunitezuoqi and Bilihe,temperature of mineralization,origin of substance,age of mineralization and magmatic rocks,geodynamics,petrogenesis and regional characteristics of Hf isotopic distribution,we have a discussion on potential area and target of magmatic related gold deposit and preliminary calculate the crust growth of GXR.The result of field geological survey,petrographic study,LA-ICP-MS zircon U-Pb dating and Hf isotopes show that ages of granodiorite and granite porphyry in Yiminhe are 217±1.6Ma（MSWD=2.1,n=23）and 222±2.8Ma（MSWD=9.0,n=33）,which are the products of late indosinian.Combined with the regional geological and geochemical characteristics of aluminum,high potassium,low MgO content and Sr,negative Eu anomaly（0.9 0.5）,loss of Nb,P,Ti,HREE and enrichment of LREE,we reckon that these belongs to A-type granites.The ages of gabbro and quartz diorite in Wulagai are 357±3.1Ma（MSWD=3.2,n=9）and 470.6±1.8（MSWD=0.84,n=20）with geochemical characteristics of high sodium,aluminum,calcium alkaline-high potassium calc alkali,high Mg#,loss of Ba,Sr,P,Ti,LREE,enrichment of Th,Nd,Zr and Hf,HREE,a delta Eu slight positive anomaly of Eu（1.1¹.2）and origin of mantle region.This has a significant differernces from A1-type granites in late Jurassic and indicate that the tectonic environment is the subduction of oceanic crust,which belongs to I-type granites.The granites in Sunitezuoqi could be divided into thress groups: caledonian（15S-2CN,438.4±3.4Ma,early silurian）,variscan（15S-1CN,282.0±3.9Ma,early permian）and indosinian（15S-3CN,238.8±4.7Ma,early triasic）.According to the regional geologic background and rock groups,this area is in the stage of oceanic crust subduction and collision from early Paleozoic to late Mesozoic and has a transition to intraplate evolution stage after late Triassic.Combined with geochemical characteristics of high SiO2>70%,aluminum,low MgO content,high Yb,low Sr,loss of Ba,Sr,P,Ti,enrichment of Th,Nd,Zr and Hf,significant negative anomaly Eu（0.2⁰.9）,we reckon that the granites in Sunitezuoqi belong to I-type granites.According to the 429 ages of granites in GRK and spatial analysis,the granites are younger from North to Middle,from Middle to South and from west to east.The magmatic activity is last from ⁵00Ma to ¹00Ma and to the peak at ⁴80Ma（O1,early Ordovician）,300Ma（C2,late Carboniferous）and ¹20Ma（K1,early Cretaceous）.The peak at ⁴80Ma is response to crust growth in early Paleozoic of Erguna Block.The peak at ⁴80Ma is response to collision between Siberian Block and NCC and process of late orogenic,could be stated as evidence of orogenic of GXK and activation of NCC.The peak at ¹20Ma is response to the subduction of east Pacific and delamination of thicken crust.On the basic of previous and this study,we infer that the originated granites in west and south of GXR are mainly affected by the closure of paleo-Asian ocean and the originated granites in east and north of GXR are mainly affected by the subduction of east Pacific ocean.The genesis of magmatic rocks in GXR has the characteristic of regional.In Paleozoic of GXR,the magmatic rocks which ages are earlier than ⁵00Ma in Erguna Block,which ages are earlier than ³23Ma in Northern Great Xing’an Block,which ages are earlier than ³10Ma in Southern Great Xing’an Block,which ages are earlier than ²50Ma in Northern Margin Block of NCC,have the characteristics of I-type granites or Adakite granites and later than the ages provided has the characteristics of A-type granites.In Mesozoic of GXR,the magmatic rocks which ages in middle-late Jurassic has the characteristics of I-type granites or Adakite,the magmatic rocks which ages in early cretaceous has the characteristics of A-type granites or transition between I-type and A-type granites.These characteristics indicated that the subduction of the Pacific Block is from east to west and tectonic regime is from extrusion to stretch.According to the spatial distribution characteristics of magmatic zircon epsilon Hf（t）and relationship between epsilon Hf（t）and gold gold deposites,the GXR could be divided into three Hf isotopic provinces: high positive epsilon Hf（t）values and young second stage model ages,medium epsilon Hf（t）values and medium second stage model ages,low epsilon Hf（t）and old second stage model ages.In the area of high positive epsilon Hf（t）values and young second stage model ages,epsilon Hf（t）is between +11.6 and +17.5 and second stage model ages are no more than 660 Ma.This area is mainly located in North of Erlianhaote.In the area of medium epsilon Hf（t）values and medium second stage model ages,epsilon Hf（t）is bwteen +11.6 and +17.5 and second stage model ages are bound from 660 Ma to 745 Ma.This area is mainly located in North of Xilinhaote and west of Zhalantun.In the area of low epsilon Hf（t）values and old second stage model ages,epsilon Hf（t）is bwteen-3.3 and +8.6 and second stage model ages are higher than 745 Ma.This area is very important for various types of gold deposit and is mainly located in the line of Manzhouli-Mohe-Nenjiang,surrounding of Balinzuoqi and the north of Daerhanqi.Extremely low epsilon Hf（t）which is no more than-3.3 is rarely exposed in GXR and could only be seen in northeast of Wulagai and west of Mohe accidental.This will not be as a evidence for the forth area of Hf isotopic province.The Hf isotopes in chemical storage and spheres of earth have a signicance differences and are the linear relation ships with the evolution of time.Combined with the regional background and dynamics metallogenic conditions,we have a discuss on crust growth and metallogenic potential of gold deposit.The result shows that new crust is accounted for 0.2%（TDM2<0.3Ga）,crust originated from the mantle is accounted for 40.9%（0.3Ga<TDM2<0.8Ga）,young crust is accounted for 41.9%（0.8Ga<TDM2<1.1Ga）and crust originated from old crust（TDM2>0.3Ga）is accounted for 17.0%.This imply that the structure of deep crust in Great Xing’an Range and could infer that the growth rate of crust is 41.1% in Phanerozoic.The results of Zr saturation thermometer calculation from whole rocks analysis of Bilihe gold deposit indicate that Zr content in bearing-rocks is stability and saturated.The Zr saturation thermometer of quartz-diorite is between 820℃and 880℃ while the Zr saturation thermometer of syenite porphyry is bound from 880℃and 960℃,which is significantly higher than other types of rocks,and is accordance with the temperature of gold and beta-quartz.The microscopic observation and temperature analysis of fluid inclusions in quartz and oblique of granodiorite show that the temperature and salinity are following into two groups: 150-200℃,0.87-7.15 wt.%-NaCl and 350-400℃,11.84-14.20 wt.%-NaCl.Primary and secondary inclusions have a relatively consistent temperature and salinity,belong to high temperature and low salinity of fluid,which is significantly lower than the gold metallogenic temperature.This implys that the fluid activity is after the magmatic gold mineralization.From the aspect of global,micro blocks of GXR have theirs own metallogenic characteristics of gold deposit.At Northern Margin Block of NCC,gold metallogenic period is from ²60Ma to ²80Ma.At Northern GXR,the mineralization age of Xiaoliangba gold deposit is ²60Ma.At Erguna Block,gold metallogenic period is between ¹20Ma and ¹40Ma.These characteristics imply that the mineralization age of gold deposits in GXR is younger from south to north.There is a great potienal for magmatic related gold deposit in GXR.Those gold mineralization points with following characteristics are the first target to identify magmatic gold deposits: Au-bearing magmatic rock is mafic with high calcium alkaline or potassium calcium alkaline and originat from lower crust and upper mantle,ε_Hf（t）is bound from-3.3 to +8.6,tectonic regime is from subduction into a stretch,geological and metallogenic conditions in polarity reversal.In the Great Xing’an Range,southwest of Balinzuoqi and line of Mohe-Tahe-Nenjiang areas are the potential of the target for magmatic related gold deposits.