辽南大东沟金矿床流体包裹体和C-H-O同位素特征及其成因探讨

Fluid inclusions and C-H-O isotopic characteristics as well as genesis of Dadonggou gold deposit in southern Liaoning Province

大东沟金矿床位于华北克拉通北缘东段,辽东古裂谷的西端。矿体主要产于古元古界盖县岩组二段绢云千枚岩内,呈似层状、透镜状、褶皱状、脉状,受NWW向褶皱构造和NE向断裂构造共同控制。围岩蚀变主要为黄铁矿化、硅化、绢云母化、绿泥石化及少量碳酸盐化。金属矿物以黄铁矿为主,次为毒砂和磁黄铁矿,另有少量白铁矿、黄铜矿、方铅矿、自然金、银金矿等。岩浆热液期为主成矿期,可划分为4个阶段:早期无矿石英阶段(1)、石英-金-毒砂-黄铁矿阶段(2)、石英-金-多金属硫化物阶段(3)、石英-碳酸盐阶段(4),其中(2)和(3)阶段为金主要成矿阶段。该矿床岩浆热液期石英中发育5种类型的流体包裹体:富液相包裹体(Ⅰ型)、富气相包裹体(Ⅱ型)、含子矿物三相包裹体(Ⅲ型)、CO2-H2O三相包裹体(Ⅳ型)和单相包裹体(Ⅴ型)。Ⅲ-1阶段主要发育Ⅰ型、Ⅱ型和Ⅲ型包裹体,均一温度155~482℃,w(NaCleq)5.86%~34.51%;Ⅲ-2阶段主要发育Ⅰ型、Ⅱ型和Ⅲ型包裹体,均一温度111~450℃,w(NaCleq)1.91%~33.59%;Ⅲ-3阶段主要发育Ⅰ型、Ⅱ型、Ⅲ型和Ⅳ型包裹体,均一温度158~420℃,w(NaCleq)2.41%~33.31%。包裹体激光拉曼光谱分析结果显示,各阶段包裹体气相成分类似,主要为CO2、CH4以及少量N2。岩浆热液期成矿流体的C、H、O同位素特征显示:δ13CV-PDB为-18.6‰^-9.6‰,δDV-SMOW为-98.3‰^-77.1‰,δ18OH2O为2.97‰~7.43‰,暗示成矿流体主要为岩浆来源,并混有部分大气降水和围岩有机碳组分。流体包裹体特征表明,流体的不混溶作用为矿质沉淀的主要机制;其次,地层中早期形成的碳质、黄铁矿层可能为成矿物质沉淀的有利成矿结构面。该矿床成因类型为中低温岩浆热液型。

The Dadonggou gold deposit is located in the east of northern margin of North China Craton,at the western end of the ancient rift of eastern Liaoning.Its orebodies occur in the sericite phyllite of the second member of Gaixian Formation in stratoid,lenticular,plicated and veined forms,and are controlled by the intersection of NWW trending fold structure and NE trending fault structure.The main alteration of the surrounding rocks includespyritization,silicification,sericitization,chlorinization and a small amount of carbonation.The metallic mineral is mainly pyrite,followed by arsenopyrite and pyrrhotite.In addition,there is a small amount of pyrite,chalcopyrite,galena,natural gold,electrum,etc.The magmatic hydrothermal period is the primary ore-forming period,and it can be divided into four stages:early barren quartz stage(1),quartz-Au-arsenopyrite-pyrite stage(2),quartz-Au-polymetallic sulfide stage(3)and quartz-carbonate stage(4),with stages 2 and 3 as the main Au mineralization stages.Fluid inclusions in the quartz during magma hydrothermal period of the deposit can be divided into five types,i.e.,liquid-rich aqueous inclusions(Ⅰtype),vapor-rich aqueous inclusions(Ⅱtype),daughter mineral-bearing three-phase inclusions(Ⅲtype),CO2-H2O three-phase inclusions(Ⅳtype)and monophase inclusions(Ⅴtype).The fluid inclusions in the stageⅢ-1 containⅠ,ⅡandⅢtypes,and their homogenization temperatures and salinities w(NaCleq)vary from 155℃to 482℃and 5.86%to 34.51%,respectively,whereas the fluid inclusions in the stageⅢ-2 contain alsoⅠ,ⅡandⅢtypes,and their homogenization temperatures and salinities w(NaCleq)vary from 111℃to 450℃and 1.91%to 33.59%,respectively.In addition,the fluid inclusions in the stageⅢ-3 containⅠ,Ⅱ,ⅢandⅣtypes,and their homogenization temperatures and salinities w(NaCleq)vary from 158 to 420℃and 2.41%to 33.31%,respectively.The results of Laser Raman analysis showed that gas compositions of the ore-forming fluids were similar at all stages,mainly CO2,CH4 and a little N2.The C,H and O isotopic compositions of the ore-forming fluids in the magmatic hydrothermal period show thatδ13CV-PDB=-18.6‰^-9.6‰,δDV-SMOW=-98.3‰^-77.1‰,andδ18OH2O=2.97‰~7.43‰,which indicates that the oreforming fluid was mainly derived from magmatic water and mixed with some components of meteoric water and organic carbon of surrounding rocks.The characteristics of fluid inclusions indicate that fluid immiscibility seems to have been the dominant mechanism for mineral precipitation.Secondly,the early-formed carbon and pyrite layer in the formation might have been conducive metallogenic structures for the minerals precipitate.The Dadonggou gold deposit can genetically be defined as middle-low temperature magmatic hydrothermal type deposit.