Metallogenesis Of Gold,Mercury And Antimony Deposits In The Zhen'an-Xunyang Basin,South Qinling Orogen

The Zhen'an-Xunyang basin is a Late Paleozoic rifted basin in the Southern Qinling orogenic belt(SQB).A large number of medium-low temperature hydrothermal gold,mercury,and antimony deposits are spatially concentrated in the basin.The Au-Sb-Hg mineralization develops in the northern part of the basin,including the Jinlongshan,Xiajiadian,Ertaizi,Xipoling,Dingjiashan and Shijiashan deposits.The Hg-Sb-Au mineralization develops in the middle of the basin,including the Gongguan,Qingtonggou,Luojia,Shatonggou,Huijiagou and Xiaohe deposits.The paragenesis and separation of Au,Sb,Hg elements are ubiquitous in the basin,but the element-control mechanism is ambiguous.The genesis and sources of ore-forming fluids and metals in these Au-Sb-Hg deposits are controversial.In addition,the geodynamic setting and structural evolution of mineralization are rarely studied.These factors restrict the efficiency of mineral exploration and the evaluation of metallogenic potential in the basin.This dissertation starts off from the coupling of metallogenic system of Au-Sb-Hg and pivotal orogenic events in SQB,and selects representative Jinlongshan world-class gold deposit,Gongguan and Qingtonggou giant Hg-Sb deposits in Zhen'an-Xunyang basin for systematic studies.In this study,we first conducted detailed field geological surveys of the basin and deposits,focusing on structural features and the relation of evolving deformation patterns to Au-Sb-Hg mineralization.Second,the sources of ore-forming fluids and metals were determined by optical and scanning electron microscopy,in-situ trace element analysis of pyrite(LA-ICP-MS),in-situ sulfur and lead isotope analyses of sulfides(LA-MC-ICP-MS)as well as fluid inclusion geochemistry and C-H-O isotope analysis.Finally,we clarified the genesis of the deposits and the metallogenic mechanism of gold,mercury and antimony in the orogenic process of the Southern Qinling Orogen.Detailed fieldwork reveals that the Zhen'an-Xunyang basin has mainly undergone three deformation events.The first stage deformation(D₁)is mainly represented by the EW-NWW trending fold structures formed in the collisional orogenic stage.The second stage deformation(D₂)is dominated by EW trending ductile-brittle shear zones with a reverse sense of movement in the northern part of the basin and EW-NWW trending brittle-ductile shear zones with a dextral oblique-slip reverse sense of movement in the middle of the basin,which formed in the post-orogenic collapse and depression.The third stage deformation(D₃)consists of the EW-NWW trending thrust faulting,the NE and NNW trending strike-slip faulting,which formed in the intracontinental orogenic compression and thrusting stage.The D₁ and D₂ deformations are pre-ore stage stuctures,the D₃ deformation is closely related to the metallogenesis and can be subdivided into ore-stage and post-stage stuctures.The ore-stage structures of the Jinlongshan gold deposit(D₃^JLS)consist of the NE-NEE trending dextral oblique-slip reverse faulting,the NW trending reverse faulting and the EW-NWW trending sinistral oblique-slip normal faulting,which formed during an approximate NWW-SEE directed compression.The ore-controlling structures of the Gongguan and Qingtonggou Hg-Sb deposits(D₃^Hg-Sb)are both the re-active of the EW-NWW trending brittle-ductile shear zones and the secondary SN,NE,NW trending faults of D₂ stage.The NWW trending dextral oblique-slip reverse faulting and the SN trending sinistral oblique-slip normal faulting are the main ore-controlling structures of the Gongguan deposit and formed under the local NW-SE orientated compression.The Qingtonggou deposit is dominated by the SN trending sinistral oblique-slip normal faulting,which formed in the local NE-SW orientated compression.Taken together,the D₃^Hg-Sb stage structures were formed by S-N directed compression.The predominant ore minerals of the Jinlongshan gold deposit are pyrite,arsenopyrite and stibnite.Three stages of pyrite are identified based on morphology and paragenesis.Pre-ore stage pyrites(Py₁,Py₂)contain relatively high concentrations of Co,Ni,Cu,Zn,Pb,Mn.Ore stage pyrites(Py_3a,Py_3b)contain high concentrations of As,Sb,Au,Cu,Ag,Pb,Bi,Hg,Tl.Post-ore stage pyrite(Py₄)is characterized by low concentrations of As,Au,Sb,but relatively high concentrations of Pb,Cu,Ni.Gangue minerals consist of quartz,calcite,sericite,and carbonaceous material.Hydrothermal alterations mainly consist of decarbonatization,silification,sericitization,sulfidation,and carbonatization.The main ore minerals of the Gongguan and Qingtonggou Hg-Sb deposits are cinnabar and stibnite.The mineralization process of Hg-Sb can be divided into four stages.Stage?mainly consists of euhedral quartz(Qtz₁)and stibnite(Snt₁)with a small amount of cinnabar(Cin₁),which is the main mineralization stage of antimony.Stage?is characterized by multiple types of gangue minerals,including dolomite(Dol₂),quartz(Qtz₂),barite(Brt)and calcite(Cal₁),and metallic minerals of stibnite(Snt₂),cinnabar(Cin₂)and pyrite(Py₂),representing the main mineralization stage of mercury.Stage?is fine-grained calcite(Cal₂)and cryptocrystalline cinnabar(Cin₃).Stage?mainly consists of coarse-grained calcite and ferrocalcite(Fe-Cal).The ore-forming fluid of the Jinlongshan gold deposit is characterized as a NaCl-H₂O system with medium-low temperature(210-270?),low salinity(6.0-8.4 wt.%NaCl_eqv)and low density(0.852-0.903 g/cm³).The carbon,hydrogen and oxygen isotope compositions of the ore-forming fluids indicate they are dominated by metamorphic fluids.The ore-forming fluids of the Gongguan and Qingtonggou Hg-Sb deposits have features of medium-low temperature(180-230?),low salinity(0.5-5.8 wt.%NaCl_eqv)and low density(0.841-0.925 g/cm³),belonging to a NaCl-H₂O system.The?D_H2O-?¹⁸O_H2O values of the fluid inclusions(in Qtz₁)are linearly distributed between the metamorphic fluids and basinal fluids(formation water),indicating the initial ore-forming fluids are mixed fluids dominated by basinal fluids.The?D_H2O-?¹⁸O_H2O values of stage?ore-forming fluids vary widely and are close to the meteoric water,implying the incursion of circulating meteoric water into the ore fluids.Pre-ore stage pyrites(Py₁,Py₂)of Jinlongshan gold deposit have a wide range of?³⁴S values,ranging from-43‰-+63.8‰and 0.76‰-69.6‰,which were formed by bacterial and thermochemical sulfate reduction of Devonian marine sulfate(BSR and TSR),respectively.The?³⁴S values of ore stage sulfides(Py₃,Apy and Snt)vary from11.3‰-13.8‰,which differ significantly from those of pre-ore stage pyrites(Py₁,Py₂)and are higher than those of typical igneous rocks.The limited range of positive?³⁴S values of ore stage sulfides may source from the decomposition of pyrite in the underlying low-grade metamorphic Paleozoic sedimentary rocks.Similarly,the Pb isotope compositions of ore stage sulfides resemble that of underlying low-grade metamorphic Paleozoic sedimentary rocks,whereas differ from those of Precambrian metamorphic basement rocks and granitoids around the basin.Therefore,the metals of the Jinlongshan gold deposit are from the underlying Paleozoic sedimentary rocks.The?³⁴S values of stage?sulfides(Snt₁,Cin₁)of Gongguan and Qingtonggou deposits vary from 8.9‰-18.3‰which is significantly higher than those of typical igneous rocks.The Pb isotope compositions of stage?sulfides are consistent with those of ore-bearing Devonian dolomite.Combing with the composition study of stage?ore-forming fluid,the reduced sulfur were likely originated from thermochemical sulfate reduction of seawater sulfates and ore-forming metals were mainly leached from Devonian dolomite by basinal fluid.The?³⁴S values of sulfides(Snt₂,Cin₂)and barite(Brt)in stage?range from 4.1‰-16.6‰and 24.8‰-34.8‰respectively,showing the sulfur isotopic fractionation between sulfides(Snt₂,Cin₂)and barite(Brt).The calculated total sulfur isotope values of the stage?ore-forming fluids(?³⁴S_?S)is 24.9‰,indicating the reduced sulfur most likely sources from the underlying Cambrian sedimentary rocks with heavy sulfur.The Pb isotope compositions of stage?sulfides(Snt₂,Cin₂,Py₂)are not completely consist with those of the stage?and differ from those of Precambrian metamorphic basement rocks and granitoids around the basin,but resemble those of underlying Paleozoic sedimentary strata.In addition,LREE-enriched patterns of the dolomite(Dol₂)in stage?also suggest that REEs were leached from Devonian dolomite,but barite(Brt)has different REE patterns and Y/Ho values to those of Dol₂,indicating the addition of other fluids and metals in stage?.Therefore,the metals of the stage?source from Devonian dolomite,and the metals originated from underlying Paleozoic sedimentary rocks were added in the ore-forming processes of stage?.The increasing of altered intensity and the contents of metallogenic elements from wall rocks to ore-bearing fractures indicate that the fluid-rock interaction was generally developed in the Jinlongshan gold deposit.The gradually raised?³⁴S values in ore stage sulfides indicate that the consumption of H₂S during mineralization leads to the enrichment of?³⁴S in residual ore fluids.Therefore,the sulfidation of H₂S-rich fluids reactive with Fe-rich wall rocks is the most likely reason for gold precipitation.Different stages of veined-and brecciated-structures developed in the Gongguan and Qingtonggou Hg-Sb deposits indicate that the pressure of ore-forming fluids changed regularly.Coexisting liquid-rich and vapor-rich W-type fluids in the Qtz₁ and raised?D_H2O values in the residual fluids at shallow depths of stage?indicate that fluid boiling possibly has occurred during the mineralization.Fluid boiling might result in the Hg⁰ migrated with gas phase and deposited in the wall rock to form fine-veined ores.The escaping of H₂S caused an imbalance of antimony-sulfur complexes in the liquid phase and the precipitation in the faults to form veined ores.The positive Eu anomalies of dolomite(Dol₂)and the precipitation of barite(Brt)indicate the increasing of oxygen fugacity in stage?.Therefore,the mixing between reduced basinal fluids and oxidized circulating meteoric water had triggered the mineralization of stage?.In conclusion,the Jinlongshan gold deposit and the Gongguan and Qingtonggou Hg-Sb deposits were formed in different tectonic setting in the Yanshanian intracontinental orogeny of SQB,they belong to different metallogenic systems.The Jinlongshan gold deposit formed in a local extensional environment where the underlying sedimentary rocks udergone a metamorphism during the shear deformation(D₂^JLS)and formed the initial ore-bearing fluid reservoir by the devolatilization of pyrite and the following Au and S releasion into metamorphic fluids.The mineralization of gold occurred during the transition of tectonic setting from compression to extension(D₃^JLS),which reactivated the shear zones and transported deep ore-bearing fluids into NE-trending brittle faults.The fluid-rock interaction leads to the precipitation of gold.Therefore,we propose that the Jinlongshan gold deposit is a Carlin-type gold deposit in the SQB.However,the Gongguan and Qingtonggou Hg-Sb deposits formed in a compressional setting which resulted in the relatively high temperature of basinal fluid and further triggered TSR to reduce the aqueous sulfate.The basinal fluid contains with the reduced sulfur and leached metals from sedimentary strata and likely migrated through the interformational faults during compression and thrusting process.The SN and NE trending sinistral oblique-slip normal faulting(D₃^Hg-Sb)formed in a N-S orientational compression,which resulted in the basinal fluid boiling and the mineralization of stage?.With the continuous extension of the SN and NE trending faults,oxidized circulating meteoric water carrying the metals of underlying sedimentary rocks mixed with reduced basinal fluids and formed the mineralization of stage?.Thus the Gongguan and Qingtonggou Hg-Sb deposits are medium-low temperature hydrothermal-reformed deposits.The differences in geology,ore-forming fluids,metal sources,and ore precipitation mechanisms have led to the formation of gold mineralization represented by the Jinlongshan deposit in the northern part of the basin,and mercury-antimony mineralization represented by the Gongguan and Qingtonggou deposits in the the middle of the basin.