Geological And Geochemical Characteristics And Genesis Of The Jinlu Gold Deposit,Inner Mongolia

The Jinlu gold deposit is located in the convergence of North China Craton and central Asian orogenic belt.Based on a synthetical research methods of fluid inclusions,electron probe of the gold minerals,trace elements of the pyrite,pyrite thermoelectricity,H-O-S-Pb isotopes,we proposed a definitive interpretation of the characteristics,evolution and source of ore-forming fluids,the genesis and the metallogenic model of this deposit.Its ore-bodies controlled by the NE trending ductile-brittle faults,are hosted in metamorphic rocks of the Archean Xiaotazigou Formation and gneissic monzonitic granite.The ore-forming process can be divided into four metallogenic stages,respectively named the white quartz stage（Ⅰ）,the quartz-pyrite stage（Ⅱ）,the quartz-polymetallic sulfide stage（Ⅲ）and carbonate stage（Ⅳ）.The gold minerals is mainly produced in the quartz-polymetallic sulfide stage（III）,which is dominated by native gold,subordinate electrum.The fineness of native gold are high（varies from802 to 963）.The crystal form of pyrites are mainly characterized by cubic,pentadododecahedral and poly-type.The thermoelectric conduction type of pyrite is characterized by P?N in theⅠstage,P≈N in theⅡstage,and P>>N in theⅢstage.A calculatuon with empirical formula implies that the ore deposit is a high-medium temperature gold deposit,whose pyrite crystallization temperature is in the range of240³90℃.The total content of rare earth elements is low,and the REE distribution reveals an obvious light REE enrichment pattern.The Hf/Sm,Nb/La and Th/La ratios are less than 1,indicating the ore-forming fluids might be Cl-rich reduced fluids.The Co/Ni ratios of auriferous pyrite range from 0.71 to 6.6.The Co/Ni ratios are close to average values of those from magmatic-hydrothermal type gold deposit.The fluid-inclusion study shows that the ore-forming fluid is medium-high temperature and medium-high salinity fluid.Petrographic observation indicates that fluid inclusions of the quartz can be divided into three types,i.e.,aqueous inclusions,CO₂-bearing aqueous inclusions and daughter mineral-bearing multiphase inclusions.TheⅠ-stage quartz mainly contains aqueous inclusions and CO₂-bearing aqueous inclusions.The homogenization temperatures and salinities are in the ranges of365℃to 405℃and 7.9%to 11.7%.All three types of inclusions are trapped in theⅡ-stage quartz.The homogenization temperatures and salinities vary from 335℃to390℃and 4.3%to 35.5%.All three types of inclusions are found in theⅢ-stage quartz.The homogenization temperatures and salinities have a range of 270℃to367℃and 1.7%to 37.8%.TheⅣ-stage ankerite mainly contains aqueous inclusions.The homogenization temperatures and salinities are in the ranges of 182℃to 245℃and 3.1%to 5.4%.The study of fluid inclusions in three metallogenic stages indicate that the ore-forming fluid belongs to H₂O-NaCl-CO₂ system and the metallogenetic temperature gradually decreased.Based on fluid inclusion and geologic characteristics,it is suggested that fluid boiling may be the main mechanism for the gold precipitation and enrichment in this deposit.This study systematically analyses the characteristics of H-O and S-Pb isotopes of hydrothermal mineral to discuss the source of ore-forming materials.The H-O isotopes of quartz indicate that the ore-forming fluid is magmatic water.The sulfide isotope shows that the sulfur is related to mantle.The lead isotope is crust-mantle mixed Pb.According to ore body characteristics,microthermometry of fluid inclusions,trace elements of the pyrite and stable isotopes research,the genesis of the Jinlu gold deposit is a magmatic hydrothermal gold deposit.