Study On The Enrichment Regularities Of Mineralization Of Jinchang Gold Deposit, Dongning In Heilongjiang Province

Jinchang gold deposit is one of the large hydrotherm gold deposits in the northeasternmargin of China. The deposit is located in the Dongning county of Heilongjiang province,the tectonic position of which is the binging site of the Taipingling Ridge and LaoheishanRift in the east of Xingmeng orogenic. The northwest of the area is Dunmi crustal-cuttingfaults. The study area experienced the closure of the Paleoasian ocean and the subduction ofthe Pacific plate and located in the superposition and conversion parts of the two tectonicdomain since the late Paleozoic, which led to complex mineralization background anddiverse types of mineralization.On the basis of the study on the regional metallogenic conditions and geologicalcharacteristics of the deposit, we carried on a systematic comprehensive identification on theminerals in the ores of J0and J1, as well as conducted a series of works on the ziron U-Pbdating, fluid inclusions research, stable isotope analysis and geochemical testing in order toexplore the genesis of Jinchang gold deposit, mineralization process and summarize theenrichment regularities of mineralization to guide prospecting work.This article was first to identify and confirm the existence of the tetradymite, whichcoexists with gold and can be considered as a strong evidence of mantle source. Themineralization was mainly related to quartz, creep chlorite, adularia, chalcedony, calcite,greigite and marcasite with the mainly useful element of Au and associated Cu. Mineral assemblages show the characteristics of low sulphidation epithermal mineralization.This study obtained the zircon U-Pb age of granodiorite, gaphic granite and dioriticporphyrite of194Ma,192Ma and123Ma. Combining with the results of previous studies onthe chronology of the rocks, we divide the magmatic activities in the region into five stages:diorite in the late Indosinian, biotite-granite in the late Indosinian, granodiorite and gaphicgranite in the early Jurassic, dioritic porphyrite in the early Cretaceous and granite porphyryin the early Cretaceous. Considering the characteristics of major and trace elements, it issuggested that the diorite was formed in the environment of stretch after the closure of thePaleoasian; granodiorite and gaphic granite were formed in the activitive continental marginrelated to the subduction of the Pacific plate; dioritic porphyrite was formed in thetransitional environment of the stress system conversion from compression to tension, andthe granite porphyry was formed in the tectonic setting of continuous extention of back-arc.The microthermometry results of the fluid inclusions in quartz from orebodies showthat the homogeneous temperature ranges from181℃to465℃, and relativly concentratingon the ranges of200~280℃; There is an obvious differentiation on the fluid salinity, whichfocuses on two intervals of40~48wt%NaCl and4~1wt%NaCl, concentrating on8~14wt％NaCl overall; The density of the fluid ranges from0.5~1.2g/cm3, concentrating on0.8~1.0g/cm3. The ore-forming fluids is mid-low temperature, mid-low salinity and lowdensity fluids. We got the mineralization pressure focusing on2~30MPa, and calculated thecorresponding depth of0.2~3.0km. Combining with the coexistence of the gas-liquidtwo-phase inclusions and the vapor-rich inclusions, the broad range of the homogenizationtemperature and salinity indicates the presence of two immiscible fluids and the phenomenonof fluid boiling.The results of the S isotope analysis from J0and J1orebodies shows that the valuesvaried from2.2‰~3.6‰in the pyrite with the value of one galena sample is0.1‰, closingto the meteorite sulfur and exhibit the characteristics of magmatic sulfur isotopiccompositions. The isotopic composition of S and Pb varies little in different types of ores,indicating that they had the same source of minerals and should be the product of the sameperiod of mineralization. Hydrogen and oxygen isotope studies indicate that ore-forming fluid exhibits features of mantle-newborn liquids, and there was an addition of atmosphericwater in the late stage of hydrothermal evolution.On the base of studies on ore bodies’ characteristics, ore fabric, mineral assemblages,fluid inclusions’ microthermometry, and characteristics of rock alteration, we believe that theores controlled by hydrothermal breccia barrels and ring-radial tensile fractures mainlydeveloped epithermal mineralization while the18th orebody in the deep of Banjiegou mainlydeveloped porphyry mineralization. And mineralization should be related to another magma-hydrothermal activity rather than the previous perceived dioritic porphyrite or graniteporphyry.Based on the above, this paper argues that the genetic type of Jinchang gold deposit is atransition of porphyry to epithermal, dominated by low-sulphidation epithermalmineralization. The deposit was formed in the background of continuing tension of theback-arc associated with the subduction of the Pacific plate, the upper limit mineralizationage of which is113Ma(ie the emplacement age of granite porphyry). Summarizing theenrichment regularities of mineralization as follows:The faults before mineralization controlled the output of the orebodies indirectly bycontrolling the output of the hydrothermal breccia barrels.The ring-radial tensile fractures caused by the intrusive activity of the magma and thedome-shaped fracture zone formed by a series of small cracks were important leading andhosting structures for mineralization.The hydrothermal breccia barrels and the ring-radial vein-type orebodies are outputequidistantly and the grade become poor to the deep with little prospecting potential. Thealtered rock type orebodies in the fracture zone show significant features of side volt to thesouthwest and southeast, showing that there may have a greater exploration potential to thesouthwest and southeast in the deep.