Study On Genesis And Metallogenic Prognosis Of Lanjia Gold Deposit,Jilin Province

Lanjia gold deposit, located in Lanjia Country,Changchun City, Jilin Province,tectonically, is situated in the gold and silver polymetallic metallogenic belt of middleDaheishan horst, which belongs to the southwestern Jihei fold zone of Tianshan–Xing′anfold belt, in the eastern active continental margin of the Eurasian plate. This metallogenicbelt is an important Au-Fe and nonferrous metal metallogenic zone in northeast China, andhas ascendance on regional metallogenetic condition.The strata exposed in the mine area is sample, mainly the fiest segment of DyasFanjiatun group and part of Yangjiagou group. The faults and fold structures are relativelydeveloped. The faults are in small scale and can be divided into three groups, respectivelyto NW, NNW, NNE. Fold structures mainly developed in Fanjiatun group, include Lanjiaoverturned syncline and Lanjia syncline. In addition, there is an annular structure in themine area. Regional magmatic activity is relatively frequent and magmatic rocks arewidely distributed in Lanjia area, mainly Yanshan period magmatic rocks.TheNanquanyan diorite of middle Jurassic has the closest relations with the formationevolution of Lanjia gold deposit.The main ore types of Lanjia are garnet-magnetite skarn type, actinolite skarn typeand garnet skarn type. The main metallic mineral are pyrite, magnetite, hematite,chalcopyrite, pyrrhotite, sphalerite, galena, arsenopyrite, cobaltite, bismuthinite andgustavite.Elemental minerals are native gold, native platinum,etc., it also containselectrum. Ore textures include allotriomorphic granular texture, hypidiomorphic granulartexture,granular texture and emulsion droplets containing texture. Ore structures are massive structure and disseminated structure and thin vein structure. Wall rock alterationare mainly skarnization, silicatization, chloritization, epidotization, albitization, hydromica,tourmalinization, carbonatization, zeolite–fluoritization. skarnization,silicification andchloritization are close relationship with gold mineralization.According to the results of metal ore mineral electron probe analysis，the S/Fe ratioof pyrite in ths area is1.143on average, less than the theoretical value of1.148, as sulfurloss type, Co content is close to0.05%on average, the Co/Ni ratio is greater than1, Ascontent is high, all the characteristics reflect that the pyrite is hydrothermal genesis.Element Te appears in galena, it reflects the mantle-derived magma’s contribution to itsformation to some extent. High Bi and Ag content is consistent with the typomorphiccharacteristic of galena in gold deposits. The Fe content in Sphalerite is4.593%, high Fecontent suggests that the forming temperature is relatively high.The compositioncharacteristic of arsenopyrite shows depletment of As and rich enrichment in S, reflectingit is a typical auriferous arsenopyrite.Nanquanyan diorite, which is the metallogenic intrusion of the Lanjia sharn type golddeposit,is belonging to calc-alkaline and metaluminous series. The trace elements ofdiorite show enrichment in LILE(such as K、Rb、Sr), more mobile incompatibleelements(such as Th, U) and LREE, relative depletment of HFSE(such as Nb, Ta, Ti, P)and HREE, and show obviously negative Eu anomalies(δEu=0.19～0.25). LA-ICP-MSU-Pb zircon data of the diorite give weighted average ages of170.21±0.73Ma(MSWD=0.14) and concordant age of170.21±0.39Ma (MSWD=0.018), which arein the middle Jurassic period. It is suggested that during the early Yanshan, subduction ofPacific plate leaded to extension of continental arc and lithospheric thinning, andasthenosphere mantle magma underplated subsequently and provided thermopower,finally partial melting of the immature lower crurst formed Nanquanyan diorite.The fluid inclusion research of Lanjia gold deposit shows that the main metallogenictemperature range is200℃~320℃, salinity range is8~12wt%NaCl, fluid density is0.743~0.989g/cm~3, metallogenic pressure is26.93~62.18Mpa, depth is between0.9km with2.07km. The δ34S value from Lanjia gold deposit changes in the range of-3.91～+3.37×10-3, R=+7.28x10-3, the change range is relatively narrow and near the meteorite sulfur standard,this reflects the sulfur of gold deposit comes from deep source. the δ18OQ(‰) value ofgas-liquid inclusions in mineralization area of Lanjia gold deposit is10.2~12.84, theδ18OH2O(‰) value is+5.30~+6.26and6.78‰on average, the δD(‰) value is-83.9‰.These values show that the ore-forming fluid mainly comes from magmatic water,and there is a small amount of atmospheric water joining later. The Nd value and147Sm/144Nd value are less than chondrite, εNd（t=170Ma） values are between+0.6with+1.5.These suggest that the magma was derived from depleted mantle and contaminated byimmature crustal material. The appearing of Natural platinum shows that mantle-derivedmagmatism had happened in Lanjia region.The mineralizing process of Lanjia gold deposit can be divided into two metallogenicphase, five mineralization stages. Skarn phase:early skarn stage and late skarnstage.Hydrothermal phase (quartz–sulfide phase): oxide stage, early sulphide stage andlate sulfide stage. Lanjia gold deposit is mainly formed in quartz-sulfide stage.The metallogenic process is relatively complex and is characteristic of multi-stage.Mesozoic early Yanshanian tectono-magmatic activities are the main factor ofmineralization. In consideration of the diagenetic and metallogenic background,we holdthat the deposit belongs to skarn-type gold deposits.According to the analysis of geological conditions, ore-controlling conditions, andthe distribution of gold mineralization(body) in this area, and combine with itsgeophysical-geochemical anomaly characteristics, we predict Jiangjiayao andXiaomoujiagou as the advantageous prospecting target areas surrounding Lanjia golddeposit.