Discussion On Ore-forming Material Sources And Deposit Genesis Of Manaoke Gold Deposit,Northwestern Sichuan

The Manaoke gold deposit,which is one of the large Carlin type gold deposits in south China,is located in Jiuzhaigou,Sichuan Province,discovered by the 205 Geological Team of Sichuan Bureau of Geology and Mineral Resources in early 1990 s.It has attracted interests of researchers many years.The ore deposits are hosted in Zhagashan Formation of Middle Triassic,a set of clastic rock-carbonate formation,and ore body distribution and occurrence are controlled by Yangbuliang thrust-fault belt.The thrust-fault belt is a part of regional fault system,which extends to join the Maqu-Lueyang fault zone in the west and extends to join the Mianlue suture belt in the east.Affected by the NS-striking faults,the ore deposit is divided into 3 ore sections,namely,Lianghekou,Manaoke,Qingshanliang from west to east.This study investigated the major-trace elements characteristics of rock,mineralogical characteristics of Au-bearing minerals and characteristics of pyrite in situ sulfur isotope,based on the field investigation and systemically thin sections observation under optical microscope,using x-ray fluorescence spectrometry(XRF),inductively coupled plasma mass spectrometry(ICP-MS),electron probe microanalysis(EPMA),nanometer secondary ion mass spectroscopy(Nano-SIMS).Research shows,three ore types are identified in Manaoke,including altered sandstone,altered slate and altered carbonate.The chemical composition of rock is mainly SiO2,CaO and Al2O3,followed by MgO,FeO,Fe2O3,K2 O,etc,and here is a negative correlation between SiO2 and CaO,MgO.It shows that during mineralization,some elements were added to the ore while others show the characteristics of being removed from the wallrock,and decarbonation may provides favor environment for ore forming.Arsenian pyrite and arsenopyrite are the main Au-bearing minerals.The pyrites are the As-rich minerals,containing 1-10wt% As in the overgrowth rim.Because of the difference of As,Back-scattered electron(BSE)imagines of arsenian pyrite showed clear zoned texture,which imply multiperiodic hydrothermal activities.The elemental mapping of arsenian pyrite showed that distribution of Au correlates positively with As and negatively with S,Cu.Ore-forming material sources and deposit genesis of Manaoke gold deposit are controversial in previous studies.In Carlin-type gold deposit,the occurrence of gold are“invisible”,and arsenian pyrite shows clear zoned texture in common.So it is very important to find out the source of sulfur in each micro-areas of Au bearing minerals.To this end,We have obtained sulfur isotope results of each micro-areas by nanometer secondary ion mass spectroscopy(Nano-SIMS).The detailed analysis shows that The ?34S values of arsenian pyrite has a wider range,and according to it,pyrite of the deposit can be divided into three types:(1)the ?34S value is-12.07‰,mainly derived from seawater sulfate reduced by bacteria in open environment;(2)the ?34S value of rim is-1.88‰,showing certain features of magmatic origin;(3)pyrites are characterized by enrichment in heavy sulfur(18.93‰~26.45‰),and it has been confirmed that the sulfur is mainly derived from sea-water sulfate by bacterium fractionation in a close or half-close environment.The micro-areas which shows certain features of magmatic origin(-1.88‰)contain higher Au and As than others.It shows that intermediate hydrothermal activities may be related to the main mineralization events.In conclusion,we suggest that the ore-forming of Manaoke gold deposit may be related to the Late Indosinian-Early Yanshanian magmatic activity,and the metallogenic materials mainly came from the magma and host wallrock.The ore-forming fluid is rich in As,Au,and it was a mixture of magmatic fluid,formation water metamorphic water and meteoric water.Therefore,the deposit belongs to a medium-low temperature mixed hydrothermal gold deposit...