Geochrobology,geochemistry And Genesis Of Orogenic Gold Deposits In The Wuyishan Metallogenic Belt

Lode gold deposits in metamorphic terranes are among the most economically important types of gold deposits in the world.This family of gold deposits is represented by orogenic gold deposits,and their mineralization processes and ore genesis have always been the hotspot and frontier field of economic geology.The Wuyishan area is an important polymetallic belt in the interior of the Cathaysia Block that hosts numerous Au,Ag,Cu,Pb-Zn,and Fe deposits.In the last two decades,several lode Au deposits have been discovered within the Precambrian basement rocks in the Wuyishan metallogenic belt,making this area a significant Au producer throughout the South China Block.Researchers have conducted some research work on these gold deposits and accumulated some geological data.Still,the research level of gold deposits in the Wuyishan area is generally relatively low compared with the adjacent Jiangnan orogenic belt.More importantly,the ore genesis,formation age,fluid evolution,fluid and metal sources,and tectonic settings of these lode Au deposits have been controversial.The Shuangqishan and Wuyishan lode gold deposits in the Wuyishan metallogenic belt have large reserves and representative mineralization styles.The gold orebodies of both deposits are hosted by Neoproterozoic metamorphic basement rocks and may record the metallogenic processes of the Wuyishan area in different ages.As such,these two deposits are ideal test cases for studying the“mineralization process and genetic mechanism of lode gold deposits in metamorphic terranes.”In this thesis,I selected the representative Shuangqishan and Hebaoshan Au deposits as the main research objects through systematic field investigation,mineralogy,geochronology,multiple isotopes（H-O-C-S-Pb-He-Ar）,and novel in-situ analytical techniques,aiming to provide a better understanding of nature,timing,detailed ore-forming processes,and tectonic settings of gold vein formation that in turn lead to a robust genetic model.This study will also provide theoretical guidance for prospecting and exploring the gold deposits in the Wuyishan metallogenic belt.The Shuangqishan gold deposit is located in the Dehua-Youxi-Yongtai“Golden Triangle”gold district in the central section of the Wuyishan metallogenic belt.The Shuangqishan deposit has a proven reserve of ca.20 t gold at an average grade of 9.2 g/t and is one of the largest gold deposits in the area.It is hosted by the amphibolite-facies metamorphosed volcano-sedimentary sequences of the Neoproterozoic Daling Formation.It consists of auriferous quartz-sulfide veins mainly controlled by brittle-ductile shear zones.The formation history of this deposit can be divided into metamorphic and hydrothermal periods.The metamorphic period is marked by the emplacement of many of milky-white gold-barren quartz veins.Gold mineralization mainly took place in the hydrothermal period,which can be further subdivided into four stages:（I）the disseminated pyrite stage,（II）the quartz-pyrite stage,（III）the quartz-pyrite-chalcopyrite stage,and（IV）the quartz-carbonate stage,of which stages II-III are the main gold-producing stages.In this study,a detailed investigation of U-Pb isotope geochronology,fluid inclusions,and H-O-C-S-Pb-He-Ar isotopes of ore and gangue minerals has been carried out to determine the age of gold mineralization,sources of ore-forming fluids and metals of this deposit,the mineralization processes of this deposit have also been discussed.The U-Pb isotope ages of metamorphic apatite and hydrothermal apatite closely associated with auriferous sulfides are 461±9 Ma and425±15 Ma,representing regional amphibolite facies metamorphism and gold mineralization,respectively.The fluid inclusion study reveals that the metamorphic and hydrothermal stages II-III quartz mainly contains three-phase CO₂-H₂O fluid inclusions.In contrast,only H₂O-Na Cl fluid inclusions are recognized in the stage IV quartz.The homogenization temperatures of fluids from metamorphic period and stages II to IV are316～337°C,271～310°C,249～295°C,and 186～228°C,respectively,with the corresponding salinities of 5.2～7.4 wt%,4.6～7.1 wt%,4.4～7.4 wt%,and 2.9～5.1wt%Na Cl equiv.Fluid immiscibility leads to the precipitation of gold-bearing sulfide minerals at Shuangqishan.The pre-ore milky-white ore-barren bulk quartz veins show strong evidence of ductile deformation and haveδ¹⁸O_fluidandδD values of 10.3 to11.2‰and-67 to-66‰,respectively,consistent with a metamorphic fluid source.Stages II and III auriferous quartz veins have stable H-O isotope compositions that overlap the metamorphic and magmatic fields in theδD vs.δ¹⁸O_fluiddiagram,illustrating a deep-seated source for the mineralizing fluids.δ³⁴S values of sulfides,including pyrite and chalcopyrite,mainly range between-7.9～-0.1‰.The near zero or slightly negative sulfur isotope compositions illustrate a deep-seated sulfur source,most likely of magmatic or mantle origin.The Pb isotope compositions of undeformed pyrite crystals from various stages suggest a mixed curst-mantle source for the ore-forming materials.The gold-related,undeformed pyrite grains have less radiogenic Pb isotope compositions,with²⁰⁶Pb/²⁰⁴Pb=17.114～17.394,²⁰⁷Pb/²⁰⁴Pb=15.441～15.490,and²⁰⁸Pb/²⁰⁴Pb=37.904～38.176,than those of the Caledonian granites in the mining area which have ²⁰⁶Pb/²⁰⁴Pb=18.143～18.292,²⁰⁷Pb/²⁰⁴Pb=15.588～15.616,and²⁰⁸Pb/²⁰⁴Pb=38.530～38.862.Auriferous pyrites have³He/⁴He ratios of 0.06 to 0.23,implying a mixed crust-mantle source for the mineralizing fluids.Emplacement ages determined by U-Pb zircon data are 439±2 Ma for the local granitic intrusion and 427±2 Ma for a mafic dike.Geochronological data indicates that the Au mineralization at Shuangqishan postdates the metamorphism of local host rocks and granitic magmatism（427±2 Ma）by～35 Myr and～14 Myr,respectively,negating that the mineralizing fluid was sourced via devolatilization of hosting supracrustal sequences or derived from a granite-related hydrothermal fluid.A deep-seated,externally-derived ore-fluid source is required.Combined with regional tectonic evolution and isotopic characteristics of the Shuangqishan deposit,the devolatilization of the lithospheric mantle,which was previously metasomatized by slab-derived fluids during Neoproterozoic oceanic subduction,was the most likely source for the mineralizing fluid at Shuangqishan.It is proposed that the transition from lithospheric transpression to extension in the late Caledonian intracontinental orogenesis triggered asthenosphere upwelling and subsequent devolatilization of this metasomatized lithosphere to generate the auriferous fluid that ascended along the regional lithospheric-scale fault,and finally precipitated as auriferous quartz-sulfide veins in the brittle-ductile shear zones at shallower crustal levels.Mineralogy and sulfide LA-ICP-MS trace element analysis show that seven textural types of pyrite are developed at Shuangqishan:of which Py1,Py2,and Py3 are primary pyrite types that are deposited in stages I-III,respectively;Py2a and Py3a grains are porous and inclusion-rich,which are the modified products of Py2 and Py3by later hydrothermal fluids,respectively;Py2b and Py3b are the products of brittle deformation of Py2b and Py3b.LA-ICP-MS analysis indicates that Py1 is Au-poor,whereas invisible gold mainly exists in Py2 and Py3 in the form of nano-to micro-scale Au-Ag-Pb-Bi-Te inclusions,and visible Au mainly occurs as larger inclusion within Py2a and Py3a,or as gold veins filling the microfractures in Py2b or Py3b.Au,Ag,Pb,Te,and Bi are relatively enriched in Py2a,Py2b,Py3a,and Py3b compared to other pyrite types,and Au shows an apparent positive correlation with Ag,Pb,Te,and Bi.The appearance of Au-Ag-Te-Bi enrichment at Shuangqishan also suggests a mantle affinity for the mineralizing fluids.Gold distribution in different syn-to late-ore pyrite generations indicate initial evenly deposition of nano-to micro-inclusions of Au-Ag-Pb-Bi tellurides within pyrite grains and subsequent Au remobilization following brittle deformation and fluid-mediated coupled dissolution-reprecipitation（CDR）reactions.Gold dissolved from the pyrite during the reworking event became re-concentrated and re-precipitated as visible gold either as inclusions in pyrite（Py2a,Py3a）or larger free gold grains within microfractures of pyrite（Py2b,Py3b）.Early,undeformed pyrite（Py2,Py3）has the least radiogenic Pb isotopic compositions.Late,porous and inclusion-rich pyrite（Py2a,Py3a）,fractured and brecciated pyrite（Py2b,Py3b）,have variable,but in general,more radiogenic Pb isotope ratios,which are attributed to modification and precipitation from a more evolved crustal fluid during the reprecipitation of native gold.SHRIMP U-Pb dating of hydrothermal xenotime coexists with fractured pyrite and chalcopyrite,indicates that Au remobilization and reprecipitation may take place at ca.416 Ma,and a concealed granitic intrusion may provide heat and fluid that drives gold remobilization at Shuangqishan.The Hebaoshan Au deposit（34 t Au averaging 3.4 g/t）,situated at the central segment of the Chongan-Shancheng Fault Zone in the middle-north section of the Wuyishan metallogenic belt,represents the largest active gold producer in the Chongan-Shancheng Fault Zone.It is hosted by Neoproterozoic volcano-sedimentary rocks of the Jiaoxi Formation that experienced Triassic（peak at ca.235 Ma）greenschist-to amphibolite-facies metamorphism.Disseminated-type gold orebodies are mainly distributed in the NE-trending fracture zones.Three hydrothermal stages can be distinguished:（I）the quartz-pyrite stage,（II）the quartz-carbonate-polymetallic sulfide stage,and（III）the quartz-carbonate stage.The sulfides in stage I are dominated by pyrite,followed by minor chalcopyrite and pyrrhotite.The main metallic minerals in stage II are pyrite,chalcopyrite,and a small amount of sphalerite and magnetite;gold occurs mostly as native gold along microfractures of pyrite in this stage.Stage III metallic minerals are dominated by pyrite,followed by magnetite and hematite.This study focuses on the geochronology,fluid inclusions,and multiple isotopes of gangue and ore minerals in different paragenetic stages of the Hebaoshan gold deposit.We aim to better constrain the timing of mineralization,nature,origin,and evolution of the ore-forming fluids and source of ore-forming materials.Auriferous pyrite samples yielded a Rb-Sr isochron age of 234±3 Ma,and ore-related hydrothermal sericite yielded a⁴⁰Ar-³⁹Ar plateau age of 226.4±2.0 Ma,which are distinctly younger than ca.430 Ma granitic rocks in the mining district and older than 170～130 Ma granites in the region,ruling out any genetic link between granite emplacement and gold mineralization.Three types of fluid inclusions in the Hebaoshan Au deposit are recognized as CO₂-H₂O,pure CO₂,and H₂O-Na Cl-solution types.Stage I quartz contains all three types of fluid inclusions,and the homogenization temperatures of CO₂-H₂O and H₂O-Na Cl-solution are 290 to 350°C,and the salinities are 2.4～10.1wt%Na Cl equiv.Stage II quartz contains primary CO₂-H₂O and H₂O-Na Cl fluid inclusions with homogenization temperatures clustering at 230 to 280°C and the salinities of 2.2～8.0 wt%Na Cl equiv.Fluid inclusions in stage III quartz and calcite are H₂O-Na Cl type with low homogenization temperatures（159～228°C）and low salinities（0.8～5.1 wt%Na Cl equiv.）.The H-O isotope compositions of samples from stages I-II quartz veins are close to the metamorphic water field;this,together with the medium-to low-temperature,low salinity,and CO₂-rich fluid inclusion characteristics,indicates that the mineralizing fluids are most likely metamorphic in origin with addition of minor amounts of meteoric water in stage III.The calcite has C-isotopes ranging from-6.1 to-3.8‰and O-isotopes from 2.8 to 13.6‰.,and it is similar to typical orogenic gold deposits worldwide.H-O-C isotope data are consistent with a metamorphic fluid derived from the devolatilization of Neoproterozoic basement rocks during regional metamorphism in the Late Triassic Indosinian period.Theδ³⁴S values of sulfides for stages I,II,and III are 0.1 to 7.8‰,-10.6 to 5.1‰,and-18.9 to-14.1‰,respectively,indicating an increase in oxygen fugacity during fluid ascent which resulted in more negative sulfur isotope values of sulfides and precipitation of gold.Fluids in equilibrium with sulfide minerals of different stages haveδ³⁴S values of 1.4～12.3‰,showing a much wider range than deep-seated sulfur（such as magmatic-or mantle-derived sulfur）.Pyrite grains have ²⁰⁶Pb/²⁰⁴Pb ratios of 17.306 to 17.949,²⁰⁷Pb/²⁰⁴Pb ratios of 15.532 to 15.620,and ²⁰⁸Pb/²⁰⁴Pb ratios of 37.696 to 39.199.The S-Pb isotope data suggest the derivation of S and Pb from the Neoproterozoic metamorphic country rocks.Collectively,H-O-C-S-Pb isotopes demonstrate that the ore-forming fluids and metals of the Hebaoshan deposit were derived from the metamorphic devolatilization of ore-hosting supracrustal sequences.Auriferous pyrite yielded elevated ³He/⁴He ratios（0.78–1.46 Ra）,which suggest a mantle component as can be expected from fluids derived from a subduction setting.Although the metallogenic ages and the sources of ore-forming fluids and materials are different,the Shuangqishan and Hebaoshan could be classified as orogenic gold deposits.They are the mineralization responses to the Early Paleozoic and Early Mesozoic orogenic events in the Cathaysia Block,respectively.The Au mineralization in the Shuangqishan deposit occurred in the Late Silurian,which is consistent with the time of the Caledonian intracontinental orogeny in South China.The occurrences of orebodies and characteristics of ore-forming fluids of Shuangqishan are very similar to orogenic gold deposits.The geochronological and isotope data indicate the Shuangqishan deposit may be related to the mantle-derived fluids.However,this does not deny it is an orogenic gold deposit,as mantle fluids could have a significant contribution to the mineralizing fluids in some orogenic gold deposits.All available data demonstrate that the Shuangqishan deposit is the first reported orogenic gold deposit formed during the Early Paleozoic intracontinental orogeny in the Cathaysia Block.The Late Triassic Hebaoshan gold deposit was contemporaneous or slightly later than the regional peak metamorphism and is similar to the orogenic gold deposits in styles of ore-controlling structures,features of hydrothermal alteration,assemblages of ore minerals,characteristics of ore-forming fluids,and compositions of multiple isotopes,both indicating that the Hebaoshan gold deposit is a typical orogenic gold deposit in the Wuyishan area.Collectively,the Wuyishan metallogenic belt in the interior of the Cathaysia Block has witnessed two separate periods of tectono-magmatism and orogenic gold mineralization in the Early Paleozoic and Early Mesozoic.This is also the case for the Jiangnan Orogenic Belt immediately to the west of the Wuyishan area,indicating that these two-stage mineralization events are related to the formation and evolution of the South China Block.The Early Paleozoic gold mineralization and magmatism were temporally coeval with the middle-to late Caledonian intracontinental orogenesis in South China.The transition from lithospheric transpression to extension in the Early Paleozoic triggered asthenosphere upwelling and subsequently devolatilization of the paleo-subduction modified metal-rich metasomatized lithosphere to generate the required amounts of auriferous H₂O-CO₂fluids.This fluid carries metals to shallow crustal levels with fluid flow focused into shear zones,precipitating auriferous quartz-pyrite veins.The geodynamic background of the Early Mesozoic（Triassic）is related to the northwest flat-slab subduction of the paleo-Pacific plate underneath the South China Block,during which the thickening of the curst triggered the metamorphic devolatilization of the regional Neoproterozoic volcano-sedimentary sequences to generate the ore fluids.At sufficiently deep levels,a mantle component in the metamorphic devolution of a subducting oceanic slab and overlying sedimentary rocks may also contribute to the Late Triassic gold mineralization in the Wuyishan metallogenic belt.