The Geologic Characteristics, Minerogenetic Conditions And Deposit Model Of The Sandaowanzi Gold Deposit, Heihe City, Helongjing Province

The Sandaowanzi gold deposit lies on the northeastern margin of the DaHinggan Mountain metallogenic belt of Yanshanian epoch. It is a typical low-sulfidation gold-telluride-bearing epithermal deposit, which consists of sets of quartz-filled fractures, with a maximum gold grade of 20 000×10^-6. It was hosted in andesite and volcanic breccia of the early Cretaceous Longjiang formation, and contains economic concentrations of petzite, sylvanite, hessite, calaverite, krennerite, stutzite, and native gold. Sulfides content is less than 1.79 percent, and dominated by pyrite and small amounts of chalcopyrite, sphalerite and galena. Wall-rock alteration consist mainly of silicification, sericitization, pyritization, K-feldsparization, kaolinitizatin, chloritization, epidotization and calcitization.The deposit locates in a range of high background values of Au, Ag and Te.During the alteration period, the wall rocks lossed Na₂O、Al₂O₃、CaO、MgO、Fe₂O₃,CaO and REE, meanwhile SiO2 and K₂O entered. The wall rocks enriched in K, Rb, Pb , Sr and Th,but depleted in Nb、Ta、Ti and P. There are some similarities to the island-arc volcanic rocks.Isotopic studies obtainedδ³⁴S values between -2.18 and 1.91 per mil for sufides, suggesting a mamatic origin. Theδ¹⁸O values of quartz range from -15.3 to -9.9 per mil, theδD V-SMOW values of quartz range from -110 to -85. Measured and calculated O and H isotope compositions indicate that meteoric water was dominant during Au-Ag-Te mineralization. ²⁰⁶Pb/²⁰⁴Pb values is between 18.223 and 18.353,²⁰⁷Pb/²⁰⁴Pb between 15.464 and 15.53, ²⁰⁸Pb/²⁰⁴Pb between 37.919 and 38.145;⁸⁷Sr/⁸⁶Sr（i） between 0.70481 and 0.70504 andεNd（t） values from 0.3167 to 1.1623. The rocks have positiveεHf（t） values of 0.3167 - 1.1623. The isotope data imply that the primary magmas were probably derived from the mantle.The homgenization temperatures of fluid inclusions in quartz ranged from 142 to 403℃.The main mineralization stage formed at temperatures 260℃t o 280℃. The salinity are estimated to be 1.27 to 10.53 per cent （Nacl eqv.）, the density ranged from 0.54 to 0.95 g/cm³, the pressure ranged from 0.4 to 29 Mpa.The depth of mineralization is estimated to be 0.04 to 2.9 km. The ore-forming fluid changed from acidic to weakly alkaline with decreasing temperature, Eh value was raised, but fugacities of O2,S2 and Te2 was decreased with decreasing temperature.The migration of Au was likely in the form of Au（HS）₂^-.The mineralization formed during the early Cretaceous epoch, constrained by U-pb ages of 115 .4±1.6Ma to 111.7±1.2Ma for wall rock andesite and diorite porphyrite, respectively.It is deduced that the ore-forming fluid of the Sandaowanzi gold deposit transported from the northwest to the southeast and correspondingly from deeper to shallower level. The ore bodies occur along dipping direction. It is believed that there will be a considerable potential for prospecting in its deep and adjacent area.