Geochemistry And Ore-forming Mechanism On Zaozigou Gold Deposit In Xiahe-hezuo,West Qinling,China

The Zaozigou gold deposit,Gansu province is located in the West Qinling Orogen,China.It is one of the large gold deposits,and the ore bodies are hosted in fine-grained slates intercalated with limestone of the Middle-Triassic Gulangdi Formation and varied dikes.The gold ore bodies are strictly controlled by the NE,NW and SN trening tensional and shearing faults with high dipping angle.Based on the observation of geological characteristics and lithofacies of the Zaozigou gold deposit in the West Qinling Orogenic Belt,LA-ICP-MS zircon U-Pb dating,whole rock major elements,trace elements and Rb-Sr-Sm-Nd isotope and in-situ Hf isotope of zircon of granodiorite porphyry,and the geochemical characteristics of pyrite and arsenopyrite,inclusions and H-O-S isotopes of the granodiorite porphyry and marble of the Zaozigou gold deposit have been carried out in this study.The data will be used to obtain the formation age of granodiorite porphyry,and further to constrain the metallogenic age,to estimate the characteristics of metallogenic fluid and metallogenic physical and chemical conditions,and to discuss the genesis of the granodiorite porphyry and pyrite,the source of the ore-forming materials and gold occurrence,and then to determine the genesis of the Zaozigou gold deposit and elucidate the Triassic tectonic dynamic evolution of the West Qinling orogenic belt.(1)The rocks of the Zaozigou granodiorite porphyry dykes are metaluminous to weakly peraluminous with A/CNK values in range of 0.94–1.34 and high-K calc-alkaline series with low Si O2(60.71–67.70 %),high Mg O contents(1.56–3.24%)concentrations and high Mg#(42–60).Zr+Nb+Ce+Y(<350 ppm)is low,Fe O*/Mg O < 4,and(K2O+Na2O)/Ca O <7,which indicate that they belongto undifferentiated granites.The granodiorite porphyry of the Zaozigou gold deposit is characterized by light rare earth enrichment in the standardized diagrams of chondrites and negative Eu anomaly(Eu/Eu*=0.81-0.89).They showenrichment in LREE,Th,U,K and Pb,and depletion in Ba,Nb,Ta,Sr,P and Ti.Negative initial ?Nd values of-14.85 to-10.06,low initial 87Sr/86 Sr ratios of 0.711058 to 0.711428,and low ?Hf(t)values of-11.5 to-10.1 with the corresponding Nd and Hf model age of 1908 Ma~1998 Ma and 1822 Ma~2210 Ma suggest that the magma was derived from the partial melts of Paleoproterozoic basaltic basement rocks which had a mixed source between depletedmantle-and crust-derivedmeltsand oceanic sediments covering the oceanic crust.The calculated temperatures using the Ti contents in zircon are between 598°C and 761°C.Therefore,the magma of Zaozigou granodiorite porphyry may originate from partial melting of Paleoproterozoic mafic crust under high pressure and low temperature.The granodiorite porphyry dikes mainly fall into the range of volcanic arc granite in the Nb versus Y and(Y + Nb)versus Rb diagrams,indicating that they they have magmatic arc geochemical characteristics and that the source rocks are the result of partial melting of basic rocks related to island arc magmatism.The zircon LA-ICP-MS U-Pb age of the Zaozigou granodiorite porphyry are 233.4±1.5 Ma and 242.6±1.5 Ma,indicating that the intermediate-acid dikes of the Zaozigou gold deposit are the product of large-scale magmatism in the Xiahe-Hehe area.Intermediate-acid dikes are cut by gold ore bodies,some of which undergo intense gold mineralization and even become gold ore bodies.Therefore,the metallogenic age of the Zaozigou gold deposit is later than 233 Ma.(2)The Au contents in pyrite range from 99 to 1149 ppm,and in arsenopyrite vary from 99 to 1149 ppm.Arsenic occurs in its anionic As1-state in pyrite under the reduced conditions.Au in arsenopyrite is present as cationic Au in solid solution,wheareas Au in pyrite is in solid solution and nanoparticles of native metal(Au0).Pyrite has relatively high Co(~472 to 2248 ppm)but relatively low Ni(~10 to 1156 ppm)contents with Co/Ni ratios ranging from0.07 to 76,indicating the Zaozigou deposit was overprinted by a hydrothermal event related to volcanic activity.(3)The texture characteristics and trace-element geochemistry among cores,transition zone and rims of pyrites reveal that there are at least 4 pluses offluid participating in the generation of pyrite in the Zaozigou deposit.The first pulse of magatic fluid is enriched in Au,which is the most important Au deposition age.The second pulse of hydrothermal fluid is enriched in As,and the third pulse of fluid is enriched in Pb but deplted in Au,and the fourth pulse of hydrothermal fluid is enriched in Au and Fe,which is another important Au deposition event.(4)The homogenization temperatures of fluid inclusions in Zaozigou gold deposit ranged from 120.8 to 257.5°C,mainly from 150 to 230 °C(average 177.94 °C)and from-3.6 to-0.1 °C(average-1.55 °C).Hydrogen and oxygen isotope measurements of quartz samples show that the ?DV-SMOW values are between-99.8‰ and-91.40‰,and the ?18OH2Ovalues are between-2.53 and 4.08,indicating that theore-forming fluids may be formed by mixing magmatic water/metamorphic water with atmospheric precipitation or groundwater.(5)The pyrite ?34S values in Zaozigou gold deposit are all negative,and limited in a narrow distribution ranging from-14.2 to-7.6,indicating that the ore-forming materials may be originated from the regional magmatic or metamorphic rocks.Six pyrite samples from the granodiorite porphyry in the Zaozigou gold deposit exhibitlimited Pb isotopic compositional variety with 206Pb/204 Pb values of 18.190 to 18.727(average 18.531),207Pb/204 Pb values of 15.555 to15.682(average 15.634),and 208Pb/204 Pb values of 38.250 to 39.103(average 38.645).The Pb isotopic compositionssuggest that the metallogenic materials of the Zaozigou deposit were chiefly originated from hybrid magma of themantle and upper crust.(6)The calculated formation temperatures of the Zaozigou deposit vary from 148°C to 304°C,with an average temperature of 213°C based on Au contents in pyrite.The range of the mole percentage of Au and As concentrations of pyrite from the Zaozigou gold deposit overlapped with the range of the epithermal deposits and the Carlin-type hydrothermal deposit.Combined with the wall rock characteristics and Au supersaturation in ore-forming fluidof Zaozigou gold deposit,it is considered that the Zaozigou gold deposit may be classified as the epithermal deposit.