Metallogenic Mechanism And Ore Forecasting Of Huaixi Copper-gold Deposit, Pingyang Country, Zhejiang Province

The Huaixi copper-gold deposit,a hydrothermal vein filling-metasomatic type,is situated in Shanmen volcanic bottomland in Pingyan county,southeast depression of Zhejiang.Volcanic rocks occurred widely in the bottomland,which range from Late Jurassic to Early Cretaceous in Mesozoic.Host strata of ore-bodies are the Early Cretaceous Chaochuan Formation.27 Late Yanshanian intrsuions of quartz-diorite,K-feldspar granite and granite-porphyry have been identified and were controlled by NE basic structure.Volcanic apparatus and NW faults are prevalent in this area.Intersection parts of the former and locations of later faults crosscutting the former control the distribution of deposits.Attitude and shape of ore-bodies are consistent with the NW faulsts.Over 37 mineral deposits have been investigated in studying area,including 11 mineral types of gold,silver,copper,lead,zinc,pyrite,alunite and pyrophyllite.The first(K₁c¹)and second(K₁c^2-1)segment of Chaochuan Formation outcropped broadly in the district and belong to continental intermediate-felsic and felsic volcanic lava,and volcaniclastic rock interbeding with minor sedimentary rocks.Faults are dominated by NW direction with subordinate NE,NNE NNW and NS ones.Intrusions are chiefly composed of quartz-diorite,K-feldspar granite-porphyry,with minor porphyritic alkali-feldspar granite and diabase dike.Lode ore bodies are controlled by the NW faults,and their occurrences are consistent with these faults.Gold mineralization upper and copper lower is obvious.Ore minerals are mainly consisted of chalcopyrite,galena,sphalerite,with minor native gold,jamesonit and chalcocite.Gangue minerals are quartz,pyrite,calcite,chlorite,epidotes.Ore textures mainly have granular-euhedral pyrite,metasomatic,poikilitic ore texture.Wall alterations develop silicification,chloritization,ericitization,epidotization and argillation.Mineralization is divided into 4 phases in 2 stages.The first stage endogenetic stage is with 3 mineralization phases,while the second one is epidiagenetic stage.The quartz-diorite is characterized of high-K calc-alkaline metaluminous granite,and its CIPW standard minerals have high contents of diopside and magnetite,indicating a feature ofâ… -type granite.The intrusion is enriched in LREE,Rb,Th,U,Pb,depleted in Nb,Ta,Ti and HREE and has the characteristics of syncollision granites.These results suggest the quartz-diorite intruded at the process of the collisional orogeny.K-feldspar granite porphyry is featured by calc-alkaline peraluminous granite with high kalium.It is depleted in Ca and Mg while enriched in Na,K,and high field strength elements(HFSE)such as Th,Ta,Hf.This porphyry exhibits features of A2-type granite and intruded at the end stage of the collisional orogeny.The consistent trace elements and rare elements distribution patterns,and similar Pb isotopic compositions between these rocks,indicate that both of them came from a same deep source and were productions of comagmatic evolution of the same origin at different stages.The homogeneous temperature of fluid inclusions in quartz mostly clustered between 380-410℃,320-360℃and 230-300℃three interzones.Freezing point is from -6.8 to -8.2℃and salinity varies from 10.2 to 11.9ω(NaCl)%.The sphalerite fluid inclusions' homogeneous temperature is 230-271℃,freezing point from -8.3 to -8.9℃and salinity between 12.0 and 12.7ω(NaCl)%.These suggest that metallogenic fluid is low-middle salinity.Based on detail estimation and calculation,the density of mineralization fluid is 0.68-0.91g/cm3 and depth is 0.51-1.67 km,indicating a supergene condition.Fractionation between LREE and HREE in quartz increase from 2.96 to 10.39.Hydrogen and oxygen isotopic compositions of quartz's fluid inclusions are thatδD= -53.60‰tO -68.50‰andδ¹⁸O_H2O=-6.88‰to -8.55‰,which located between the magmatic water and regional Mesozoic meteoric water(δD=-65‰,δ¹⁸O=-9.4‰)in the southeasten Zhejiang Province, indicating that ore-forming fluid was derived from mixing source of magmatic and meteoric. Eight pyrites' sulfur isotopic compositions are from -2.14‰to +4.14‰,with mean of +1.67‰and range of 6.28‰.δ³⁴S values of galena and sphalerite are 2.3‰and 2.2‰respectively,δ³⁴S values of sulpur minerals are close to those of aerolite and granite,showing the sulfur of Huaixi copper-gold deposit from the deep magma.Value of initial Sr(I_sr)of 5 quartzs' fluid inclusions samples is 0.70938,which is similar with those of fluorite and calcite(0.70971 and 0.70927)and more than the upper mantle(0.702-0.706).However,it is close to the continental crust(0.710), indicating that metallogenic materials are dominantly from continental crust.Similar Pb isotopic compositions between ore sulfides and whole-rock intrusive rocks,which are all around the extended line of upper crust and orogenic belt,indicate the diagenetic and metallogenic materials are affinitative and dominantly from continental crust,probably inputted with some mantlederived materials.Cations in fluid inclusions are dominated by Ca²⁺,Na⁺ and K⁺ with minor Mg²⁺,while anions are chiefly composed of HCO₃^- with subordinate Cl^- and SO₄^2-and minor F^-.pH values of fluids are weak acid to approximately neutral.Copper and gold are transported in ore-forming fluids as chloride complex and sulfide complex respectively.Due to meteoric water inputting, fluid cooling and Cl^- concentration deceasing result in the main mechanism of copper precipitation.Along with ore-bearing fluids ascending through fractures,water-rock interaction, Fe²⁺,Zn²⁺and Pb²⁺precipitation,and pH,fO₂ and total sulfur concentration decreasing,involve in Au sulfide complex being destroyed and Au precipitated.The diagenesis age of quartz-diorite is 110Ma,and LA-ICPMS zircon U-Pb age of K-feldspar granite porphyry is(101.57±0.86)Ma(MSDW=1.3).Five quartz fluid inclusions samples yield a perfect Rb-Sr isochrone age of(101±1)Ma with I_Sr=0.70938±0.00001, repesenting the mineralization age.The similar diagenesis and minerogenetic ages suggest the genetic relationship between them.The tectonic setting of Huai-xi Cu-Au deposit is related to subduction of Pacific Ocean plate to Asia continent.During110-100Ma the dominant stress direction was SN lightly leaning to west,the syn-collisional granitoids and the compresso-twist faults of NE and NW occured.While 100Ma±the tectonic stress fields exchanged to extension, Caomen K-feldspar granite porphyry and other A2-type granites formed.The intrusion of granitoids supply the metallogenic fluid and materials,which filled in NW faults to form ore-bodiesBased on comprehensive investigations of ore-body geology,mineralization mechanism and controlling factors,the metallogenic model and ore-body occurrence model of Huaixi Cu-Au deposit were established.With the help of geophysical exploring information,rock fracture geochemistry,primary halo and statistical character,granularity-thermoelectric properties-trace elements geochemistry of pyrites,six forecast targets in the deposit are pointed finally.