LA-MC-ICP-MS Sulfur Isotope Analysis Of Sulfide Minerals:Methodology And Applications To The Study Of The Jinlongshan Au-Sb-Hg Deposit,South Qinling Orogenic Belt

The sediment-hosted gold deposit is one of the most important type of gold deposits all over the world.It thus has become a hot topic in the past decades.The ore-forming elements,such as Au,Sb,Hg and As,have been recorded in the mineralized area,leading to a spatially genetic relationship among gold,antimony and mercury deposits.Ore minerals in this type of deposit are arsenian pyrite,arsenopyrite,stibnite and cinnabar.Therefore,a systematic sulfur isotope determination on sulfide minerals could provide crucial constraints on sulfur and ore-forming elements sources,as well as ore genesis.However,it is hard to determine different domains of sulfide minerals by conventional method due to a low formation temperature of this type of ore deposit and a complex texture of sulfide minerals.In order to provide a solution,this thesis has developed an in-situ sulfur isotopic analytical method of sulfide minerals.This method has been further applied in the investigation of the Jinlongshan Au-Sb-Hg ore field,South Qinling orogenic belt.Meanwhile,on the basis of a detailed work of petrography,as well as trace elements and Pb isotope of sulfide minerals,this study has provided a deep discussion on the source of ore-forming elements and the ore genesis in the Jinlongshan Au-Sb-Hg ore field,aiming to a refined reappraisal of the Jinlongshan Au-Sb-Hg ore field.Firstly,the laser-and ICP-induced isotopic fractionation in sulfides with different matrices was investigated by using 257 nm femtosecond?fs?and 193 nm ArF excimer nanosecond?ns?laser ablation systems coupled to a Neptune Plus MC-ICP-MS.Compared to ns-LA-MC-ICP-MS,higher sensitivity?1.4-2.4 times?at similar instrumental conditions and better precision?¹.6-fold?at the same signal intensity condition were achieved by fs-LA-MC-ICP-MS.In addition,fs-laser provides less fluence and matrix dependent of S isotopic fractionation,and more stable transient isotopic ratios compared to ns-laser.Better results acquired by fs-LA-MC-ICP-MS were attributed to the smaller size of particles and less thermal effect produced by fs-laser,which were evidenced by the morphologies of the ablation craters and ejected aerosol particles of P-S-1?the pressed powder pellet of IAEA-S-1?and PPP-1?a pyrite single crystal from the Sukhoi Log deposit?.Thus,fs-LA-MC-ICP-MS was chosen to in-situ analyze the S isotopic composition of sulfide.Secondly,in order to overcome the matrix effect between sulfides with different matrices,the effects of different make-up gas flow rate on the matrix effect were investigated.The results showed a significant reduction of matrix effect was obtained using a robust plasma condition at lower makeup gas flow rate(0.52-0.54 l min^-1)relative to the maximum sensitivity condition(0.6 l min^-1)for S isotopes analysis.These could be ascribed to the particles not only pass into the higher temperature ICP for a longer residence time at lower makeup gas flow rate that resulted in more efficient vaporization of the particles,but also experience a more robust plasma induced by adding 4-6 ml min^-1 N₂ into the plasma.Furthermore,under the robust condition,the results of six reference materials with different matrices obtained by fs-LA-MC-ICP-MS with non-matrix matched calibration at the spot size of 20-44?m showed excellent agreement with the reference values and the mass-dependent fractionation line,validating the applicability of the proposed approach for providing high-quality in situ sulfur isotopic data(?³³S and?³⁴S)of sulfides.Located in the southern part of the Zhen'an-Banyanzhen fault,the northern margin of the Zhen'an-Xunyang basin,the southern part of the South Qinling orogenic belt,the Jinlongshan ore field consists of two parts.From west to east in sequence,the Qiuling-Jinlongshan gold-antimony mineralized belt in the western part is composed of the Guloushan,Qiuling,Yaojian,and Jinlongshan ore sections,and the Ding-Ma antimony-mercury mineralized belt in the eastern part consists of the Xipoling,Dingjiashan,and Shijiashan ore sections.The ore body is mainly controlled by the east-west striking Jinjiling anticline.The gold ores are controlled by the E-W striking and the N-W striking faults,whereas the antimony-mercury ores are related to the N-E striking and the N-W striking faults.The gold ores are mainly hosted by the Upper Devonian Nanyangshan Formation,including siltstone,shale and limestone,whereas the antimony-mercury ores are mainly hosted in the Lower Carboniferous Yuanjiagou Formation,such as limestone and shale.The sulfide minerals in the gold-antimony mineralized belt are disseminated auriferous pyrite and arsenopyrite,as well as a few stibnite.The wall rock alteration mainly includes ankerite,siderite,sericite,and quartz.The Ding-Ma antimony-mercury mineralized belt has sulfide mineral assemblage of massive stibnite and veined cinnabar.The alteration in the wall rock includes calcite and quartz.Based on the mineral paragenetic assemblages and textural relationships,the ore-forming process of Au deposit can be divided into three stages:diagenetic stage?S1?,early-ore stage?S2?,and main-ore stage?S3?.The trace elemental and sulfur isotopic feature of these stages is dramatically different.Diagenetic pyrite?Py1?can be divided into two subtypes.The first one is the framboidal pyrite?Py1a?from the shale and siltstone.This pyrite has low of Au concentrations less than 1 ppm.However,other trace elements concentrations could be much higher.The As,Cu,Co,Ni,and Pb concentrations range from 100 to 1000 ppm,while the contents of Sb,Hg,Bi,Ag,and Zn are up to 100ppm.Moreover,the sulfur isotope composition of this pyrite shows an extremely negative value(?³⁴S:-38.2‰^-43.0‰;n=10).The second one is the coarse-grained pyrite from the limestone.This pyrite is characterized by an extremely low concentrations of trace elements.Most of them are below or marginally higher than the detection limit.And its sulfur isotopic compositions are from-4.52‰to-8.91‰?n=5?.The pyrite grains?Py2?from the early-ore stage could be classified into three subtypes,indicating an extremely heterogeneous distribution on textures and geochemistry.The first one is the euhedral pyrite?Py2a-1?from the shales.It shows high concentrations of As,Co,and Ni?100¹000ppm?.Copper,Sb and Pb could be up to 100 ppm.Other trace elements,such as Au,Hg,Bi,Ag,and Zn,are lower than 10 ppm.This type of pyrite has displayed an extremely positive sulfur isotopic value(?³⁴S:18.2‰³2.2‰;n=5).The second one is subhedral-anhedral pyrite?Py2a-2?found in the shale and siltstone.It shows a similar pattern of trace-element with that of the framboidal pyrite?Py1a?.The main difference is that the arsenic concentrations have increased by an order of magnitude.And this type of pyrite has a broad range of sulfur isotopic composition(?³⁴S:-24.4‰⁸.9‰;n=15).The third one characterized by moderate alteration is the coarse-grained pyrite from limestone.The concentrations of As,Cu,Co,Ni,Pb,and Zn in this pyrite have broad ranges mainly from10 to 200 ppm.Other trace elements concentrations,such as Au,Sb,Hg,Bi,and Ag,are still below or marginally higher the detection limit.The sulfur isotopic composition of this pyrite is ranging from 0‰to 7.1‰?n=6?.All of the pyrite grains from the main-ore stage?Py3a and Py3b?show a relatively uniform pattern of the trace-element signature.The As,Cu and Au concentrations have reached maximum values,whereas the contents of Hg,Co,Ni,Bi,Pb,Ag,and Zn show a simultaneous depletion trend.The As concentrations are at least higher than 1.0 wt.%.The Au contents in this pyrite grains have been up to 200 ppm,and its Cu content is more than 1000 ppm.Moreover,Py3a and Py3b have almost the same sulfur isotopic composition(?³⁴S:9.7‰¹6.0‰;n=74).The diagenetic pyrite has an extremely negative sulfur isotopic value in the host sediments.However,the pyrite formed in the early-ore stage has shown a distinct elevation compared to the diagenetic one.This trend has continued to occur until the sulfur isotopic value has reached a homogeneous distribution.Therefore,it could be speculated that the ore-forming fluid should have an extremely positive sulfur isotopic value?>30‰?.In the early-ore stage,when the ore-bearing fluid has reacted with the hosted sediments,the fluid-rock interaction style is heterogeneous due to a limited porosity and permeability.The diagenetic pyrite has gone through recrystallization,resulting in a broad range of sulfur isotopic value?-24‰³0‰?and a narrow range of trace-element concentration,except for arsenic.This geochemical signature is also strongly controlled by the hosted lithology and mineral textural features.However,as a continuous fluid-rock interaction,the fluid/rock ratio has increased gradually in the main-ore stage.Calcite?unit cell volume:367.21?³?in the hosted sediments has been replaced by dolomite and ankerite?unit cell volume:325.83?³?.This reaction could remarkably reduce the carbonate volume to at least 10%,leading to an improvement of porosity and permeability of the hosted sediments.Therefore,it is not surprise to see that all of the pyrite grains in the main-ore stage show a homogeneous distribution of sulfur isotope?10‰¹5‰?and trace element,which is independent of the hosted lithology or mineral texture.At the same time,as a continuous interaction between the fluid phase and the pyrite grains,which is so called dissolution-reprecipitation reaction,arsenic has been enriched into the pyrite structure from the fluid phase.This scenario could improve the compatibility of gold in pyrite structure,leading to the formation of the economic ores.In other words,even the initial concentrations of gold and arsenic in the fluid phase is moderate,it can evolve to gold-bearing arsenian pyrite due to a protracted fluid-rock interaction.In fact,the sulfur isotopic trend in pyrite is consistent with the lead isotopic trend.As a continuous fluid-pyrite interaction,auriferous pyrite has shown a remarkable decrease in the ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb ratios compared with that of the framboidal pyrite.Therefore,the Pb isotopic signature of the ore-forming fluid can be confirmed by this mixing trend.Consequently,it can be further compared with the geochemical reservoirs around the Jinlongshan ore field,South Qinling orogenic belt.The result shows that neither magmatic source?late Triassic or late Jurassic?nor Precambrian basement shows a genetic relationship with ore-forming fluid.Conversely,there are three lines of evidence to suggest that the Cambrian strata could be the source of the ore-forming elements.First and foremost,a large number of barite deposits have been reported in the adjacent area of the Jinlongshan ore field,which could provide an extremely positive sulfur isotopic value.Furthermore,previous research suggests that the sulfur isotopic value in the Cambrian ocean has reached a peak point of³0‰since Phanerozoic.Finally,gold concentration in the diagenetic pyrite from the Cambrian strata is also one of the peak values?up to 2 ppm on average?in the Earth history.In a word,in combination with the Pb and S isotopic signatures of the ore-forming fluid,as well as previous research,we contend that the auriferous pyrite from the Cambrian strata has been transformed to pyrrhotite due to regionally tectonic stress,providing a stable source of ore-forming elements for the Qiuling-Jinlongshan gold mineralized belt.The antimony-mercury mineralized belt in the Jinlongshan ore field consists of two parts.The first one is the Jinlongshan antimony ore section,and the second one is Ding-Ma mercury-antimony mineralized belt.The mineral assemblage is relatively simple in the Jinlongshan antimony ore section,including calcite,quartz,stibnite,and a few pyrite grains.The pyrite in Jinlongshan Sb ore section can be divided into three subtypes:the pyrite?Py1c?formed in the diagenetic stage,the pyrite?Py2c?formed in the early-ore stage,and the pyrite?Py3c?formed in the main-ore stage.The trace elemental and sulfur isotopic feature of these stages is dramatically different.The diagenetic pyrite is enriched in Cu,Ni,and Zn,which could be up to thousands ppm.The concentrations of As,Sb,Hg,Co,and Pb range from 100 to 1000 ppm.Other trace elements,such as Au,Bi,and Ag,are generally less than 10 ppm.The diagenetic pyrite shows an extremely negative value of sulfur isotope(?³⁴S:-33.3‰^-46.3‰;n=5).The euhedral-subhedral ore-related pyrite?Py2c and Py3c?shows a distinct zonation from the early-ore stage(Py2c;the core:As=¹00 ppm;?³⁴S:-6.6‰¹.4‰;n=2)to the main-ore stage(Py3c;the rim:As=¹000 ppm;?³⁴S:6.54‰¹4.86‰;n=11).The Cu,Sb,and Ni concentrations in the ore-related pyrite are generally higher than 100 ppm,whereas Au,Hg,Co,and Pb concentrations ranges from 10 to 100 ppm.Other trace elements,such as Bi,Ag,and Zn concentrations,are less than 10 ppm.The Dingjiashan Hg-Sb ore section is the representative one of the Ding-Ma Hg-Sb mineralized belt.The mineralized rock consists of quartz,calcite,stibnite,cinnabar and pyrite.Pyrite?Py3d?in the Dingjiashan Hg-Sb ore section is homogeneous without any zonation.It commonly coexisted with cinnabar and stibnite,and has low of Au concentrations less than 1 ppm.The arsenic,Cu,Co,and Pb concentrations range from 10 to 100 ppm,whereas the mercury,Bi,Ag,and Zn concentrations are less than 10 ppm.Moreover,the sulfur isotopic composition of Py3d is between 0.7‰and 5.3‰?n=13?.The sulfur isotopic value of sulfide minerals from different ore sections is similar with each other,probably indicating a same mineralized system.However,there is still a slightly different of the sulfur isotopic value.For instance,the Jinlognshan ore field spatially shows gold,gold-antimony,and antimony-mercury mineralization in sequence from west to east.In terms of sulfur isotope,pyrite formed in the main-ore stage shows a gradually decreased trend from west to east?gold:9.7‰¹6.0‰;gold-antimony:6.5‰¹4.9‰;antimony-mercury:0.7‰⁵.3‰?.Even with the application of our new developed sulfur isotopic analytical method above on other sulfide minerals,a similar trend can also be found.Specifically,the sulfur isotopic value of arsenian pyrite,arsenopyrite,and stibnite in the gold or gold-antimony mineralized belt has a narrow range?mainly 10‰¹5‰?.On the contrary,the sulfur isotopic value of stibnite from the Hg-Sb mineralized belt ranges from 4.5‰to 14.5‰,and that of cinnabar ranges from3.5‰to 10.0‰.Given the fact that gold ores are mainly hosted in the Upper Devonian Nanyangshan Formation,whereas antimony-mercury ores are distributed in the Lower Carboniferous Yuanjiagou Formation,we suggest that a gradual elevation of sedimentary sulfur contamination could account for this scenario during the ascent of the ore-forming fluid from deep?Au mineralization?to shallow zone?Hg-Sb mineralization?.In fact,other lines of evidence,such as trace-element evolution,nanoparticles,and so on,indicate that the Jinlongshan Au-Sb-Hg ore field is affiliated to a same mineralized system.When the ore-forming fluid has reacted with the framboidal pyrite in the hosted sediments,the Au,As,and Cu in the fluid phase will dramatically decrease due to their incorporation into pyrite structure.Meanwhile,some elements may have been released from the framboidal pyrite to the fluid phase,leading to an enrichment of Pb,Zn,Sb,and Hg in the evolved fluid.At this moment,antimony in this fluid phase has gradually reached to supersaturated state,as evidenced by the formation of Pb-Sb sulfosalt nanoparticles enclosed by the auriferous pyrite.As a continuous elevation of the ore-forming fluid,temperature has decreased,leading to the precipitation of stibnite.The simultaneous precipitated pyrite shows a distinct depletion of As and Au compared with that in the gold ores.At this stage,Pb-Sb sulfosalt enclosed by stibnite has grown to micron meter.Finally,the residual fluid could allow cinnabar to precipitate.Cinnabar is characterized by sphalerite exsolution.First and foremost,this thesis has developed an in-situ method of sulfur isotopic determination on sulfide minerals,providing a technical support on the investigation of the Jinlongshan Au-Sb-Hg ore field.Secondly,the new method has been applied in the ore genesis.Based on a detailed petrography work,sulfide minerals?e.g.,pyrite,arsenopyrite,stibnite,and cinnabar?have been determined with sulfur isotope.In combination with in-situ Pb isotope and trace element of pyrite,we content that the ore-forming elements are originated from the metamorphic fluid that has been released during the transformation from auriferous pyrite to pyrrhotite in Cambrian strata.However,some lines of evidence suggest that a part of Sb and Hg may have originated from the remobilization of the framboidal pyrite in the hosted sediments.Further investigation of this scenario should be taken in the future.It is also a weakness of this thesis.In summary,based on our investigation of the ore genesis of the Jinlongshan ore field,we suggest that the gold-antimony mineralized belt in the western part?e.g.,Guloushan,Qiuling,Yaojian,Jinlongshan ore sections?could represent the deep zone of this mineralized system.On the contrary,the Ding-Ma mercury-antimony mineralized belt has represented the shallow one.In the eastern part of this ore field,the Ding-Ma mercury-antimony mineralized belt has been preserved,whereas that in the western part has been denuded,leading to the exposure of the gold-antimony mineralized belt.Therefore,there is a huge potential of gold mineralization in the deep zone of the Ding-Ma mercury-antimony mineralized belt,especially in the Upper Devonian strata.