Genesis,metallogenic model,and prospecting prediction of the Nibao gold deposit in the Guizhou Province,China

Weifang Song · Lulin Zheng · Jianzhong Liu · Shengtao Cao ·Zhuojun Xie

Abstract Southwestern Guizhou province is one of China’s most important distribution areas of Carlin-type gold deposits.The Nibao deposit is a typical gold deposit in southwestern Guizhou.To elucidate the genesis of the Nibao gold deposit,establish a metallogenic model,and guide prospecting prediction,we systematically collected previously reported geological,geochemical,and dating data and discussed the genesis of the Nibao gold deposit,based on which we proposed the metallogenic model.Earlier works show that the Nibao anticline,F1 fault,and its hanging wall dragged anticline (Erlongqiangbao anticline) were formed before or simultaneously with gold mineralization,while F2,F3,and F4 faults postdate gold mineralization.Regional geophysical data showed extensive low resistivity anomaly areas near the SBT (the product of tectonic slippage and hydrothermal alteration)between the P2/P3 and the strata of the Longtan Formation in the SSE direction of Nibao anticline in the lower plate of F1 and hanging wall dragged anticline (Erlongqiangbao anticline),and the anomaly areas are distributed within the influence range of anticlines.Simultaneously,soil and structural geochemistry show that F1,Nibao anticline,Erlongqiangbao anticline,and their transition areas all show good metallogenic elements (Au,As,and S) assemblage anomalies,with good metallogenic space and prospecting possibilities.There are five main hypotheses about the source of ore-forming fluids and Au in the Nibao gold deposit: (1) related to the Emeishan mantle plume activity;(2) source from the Emeishan basalt;(3) metamorphic fluid mineralization;(4) basin fluid mineralization;(5) related to deep concealed magmatic rocks;of these,the mainstream understanding is the fifth speculation.It is acknowledged that the ore-forming fluids are hydrothermal fluids with medium-low temperature,high pressure,medium-low salinity,low density,low oxygen fugacity,weak acidity,weak reduction,and rich in CO2 and CH4.The fluid pressure is 2-96.54 MPa,corresponding to depths of 0.23-3.64 km.The dating results show that the metallogenic age is～141 Ma,the extensional tectonic environment related to the westward subduction of the Pacific Plate.Based on the above explanation,the genetic model related to deep concealed magmatic rocks of the Nibao gold deposit is established,and favorable prospecting areas are outlined;this is of great significance for regional mineral exploration and studying the genesis of gold deposits.

Keywords Nibao gold deposit · Source of ore-forming fluids and Au·Genesis·Metallogenic model·Prospecting prediction

1 Introduction

The southwestern Guizhou province is an important golddeposit distribution area in the Guizhou-Guangxi-Yunnan‘‘Golden Triangle’’at the southwest margin of the Yangtze Craton (Su et al.2009a).There are many similar characteristics between Carlin-type gold deposits (CTGDs) in southwest Guizhou and Nevada,such as (1) similar extensional basin metallogenic background related to plate subduction(Hofstra et al.1999;Hu et al.2002;Cline et al.2005;Ressel et al.2006;Muntean et al.2011;Xie et al.2018a;Su et al.2018).(2) Orebodies are mainly hosted in sedimentary rocks and controlled by strata and structures(Hofstra et al.1999;Cline et al.2005;Ressel et al.2006;Su et al.2009a;Xie et al.2018a;Su et al.2018;Song et al.2018).(3) Gold mainly occurs in arsenic sulfides in invisible forms (Hofstra et al.1991;Arehart 1996;Simon et al.1999;Cline et al.2005;Reich et al.2005;Su et al.2008;Chen et al.2011,2015;Cline et al.2013;Xie et al.2018a;Su et al.2018).(4) These two districts contain all abnormal element assemblages of Au,As,Sb,Hg,and Tl(Simon et al.1999;Zhang et al.2003;Cline et al.2005;Wang 2013;Xie et al.2018a,b;Song et al.2018;Yan et al.2018).(5) All alterations contain decarbonization,silicification and sulfidation,the precipitation of realgar,and orpiment and stibnite formed in the late ore stage(Su 2002;Huang et al.2001;Peters et al.2007;Liu et al.2010;Li et al.2014;Chen et al.2015;Pi et al.2016;Li 2021).In addition to the above similar characteristics,there are important differences in gold-carrying fluid characteristics,metallogenic age,and the spatiotemporal relationship between magmatic rocks and gold orebodies.These scientific problems are also important reasons for some scientists to question whether the gold deposits in the Youjiang Basin are typical CTGDs (Su et al.2018;Xie et al.2018b;Wei et al.2020).

Several studies have been conducted on the Nibao gold deposit in southwestern Guizhou.Orebodies in Nibao can be divided into fault-controlled,strata-controlled,and SBT(the structural alteration body)-controlled (Zhang et al.2003;Peters et al.2007;Xie et al.2018a,b;Yan et al.2018).The SBT occurs in the unconformity between the Longtan and Maokou Formations and is the product of tectonic slippage and hydrothermal alteration.The breccia in SBT is mainly composed of the Longtan Formation and a small amount of bioclastic limestone from the Maokou Formation.The ore-forming fluids is a hydrothermal fluid with medium-low temperature,ultra-high pressure,medium-low salinity,low density,low oxygen fugacity,weak acidity,weak reduction,and rich in CO2and CH4.The oreforming fluid pressure is 2-96.54 MPa,and the corresponding depth is 0.23-3.64 km.However,due to significant differences in trace (including rare earth) elements,isotope geochemistry,and dating data,five different speculations on the genesis and metallogenic model of the Nibao gold deposit have been reported;these are related to(1) the Emeishan mantle plume activity (Nie 2007;Nie et al.2008;Zheng et al.2014);(2)Emeishan basalt(Sheng et al.2016;Qi et al.2019,2020);(3) metamorphic fluid mineralization;(4) basin fluid mineralization (Xie et al.2016;Wei et al.2020);(5)deep concealed magmatic rocks(Zhang et al.2016;Chen et al.2018;Song et al.2018;Li et al.2019;Shao et al.2019;Wu et al.2019;Zheng et al.2019a);however,to date,a unified metallogenic model has not been formed,severely restricting the genetic study and prospecting works in the Nibao gold deposit and other CTGDs in the Dian-Qian-Gui district.

To clarify the genesis of the Nibao gold deposit,establish a reasonable genetic model of the deposit,and effectively guide the prospecting prediction in the mining area,we systematically collected the existing research data on the Nibao gold deposit,analyzed the fluid inclusions and H-O isotopes of quartz veins in orebody IV,discussed the genesis of Nibao gold deposit,established the metallogenic model,and outlined prospecting target areas.This study provides new theoretical support for genesis research and prospecting prediction in mining areas.

2 Geological background

Dian-Qian-Gui ‘‘Golden Triangle’’ is located at the convergence of the Cathaysian Plate and Yangtze Platform,and it is a major CTGD province in Southwestern China(Fig.1) (Zhang et al.2003).Nibao gold deposit is located northwest of Dian-Qian-Gui ‘‘Golden Triangle,’’ a typical CTGD in this area.The exposed strata include the Proterozoic to Quaternary,of which Permian and Triassic are the most widely distributed.The lithology is mainly calcareous clastic rocks,calcareous mudstone,mudstone,bioclastic limestone,pure limestone,alkaline volcanic rock(Emeishan Basalt Formation),and tuff.In addition to pure limestone and mudstone,all other rock types can be used as ore-hosted rocks.The calcareous clastic rocks interbedded with carbonate rocks formed in shallow water platform,distributed in the northwestern Triassic Youjiang basin and the deep-water basin facies turbidite sequences in the southeast of the Triassic Youjiang basin (Fig.1;Su et al.2009a;Wang 1990).From the Triassic to the Early Jurassic,southwest Guizhou was under the nappe tectonic background northwest of Yanshanian subduction tectonic movement (Li et al.2007).When the Pacific Plate subducted westward to the Eurasian Plate,in the sedimentary sequence of the Yunnan,Guizhou,and Guangxi regions in the convergence zone of the Cathaysian and Yangtze Blocks,the strata bent,rose,overlapped,and formed a series of fold structures,creating a favorable metallogenic space for gold mineralization (Hu et al.2002).

Fig.1 Distribution map of low-temperature hydrothermal deposits in the ‘‘Golden Triangle’’ area of Yunnan,Guizhou,and Guangxi (Zheng et al.2019a).1 Proterozoic,2 Paleozoic,3 Triassic,4 Granite,5 Quartz porphyry,6 Alkaline mafic-ultramafic rock,7 Fault,8 Gold deposit,9 Antimony deposit,10 Mercury deposit,11 Arsenic deposit,12 Lead-zinc deposit,13 Nibao gold deposit,and 14 County town

Except for the Permian Emeishan basalt (about 260 Ma),which is widely distributed in this area,other igneous rocks are scarce,containing a small amount of intrusive alkaline granites and several felsic and ultramafic dikes,77789and the ages of the granites,ultramafic dikes,and felsic dikes range from 77-99 Ma (Hu et al.2002,2018;Large et al.2009;Su et al.2009a,2018,2019;Chen et al.2015;Xie et al.2018a,b).Previous studies have proposed that the ages of quartz porphyry veins present in some CTGD ore fields are similar to that of the regional gold mineralization (Zhu et al.2016).Additionally,aeromagnetic surveys discovered plutons in the Triassic Youjiang basin (Wang et al.2009,2015),implying that the quartz porphyry veins and the gold mineralization are genetically related (Ma et al.2013;Li et al.2014;Chen et al.2016;Zhu et al.2016).

3 Ore deposit geology

The exposed stratum in the mining area mainly includes the Middle Permian Maokou,Upper Permian Longtan,Lower Triassic Feixianguan,and Middle Triassic Guanling Formations(Fig.2).The lithology of the Maokou Formation is medium-thick bioclastic,dolomitic,and micrite limestones.The Longtan Formation can be divided into three sections based on lithological characteristics: 1) The first section from the bottom to the top consists of tuff and volcanic breccia,sandstone intercalated with carbonaceous mudstone,argillaceous siltstone intercalated,tuffaceous siltstone,and carbonaceous mudstone intercalated with siltstone.2) The second section from bottom to top contains argillaceous siltstone intercalated with carbonaceous mudstone,gravel tuff,tuffaceous siltstone,argillaceous siltstone intercalated with carbonaceous mudstone,and gravel sandstone intercalated with limestone lens.3) The third section consists of argillaceous siltstone intercalated with carbonaceous mudstone and siltstone.Purple-red and gray-green thin argillaceous siltstone intercalated with purple-red mudstone is observed in the Feixianguan Formation.The Guanling Formation is medium-thick layered limestone intercalated with dolomite,mudstone,and sandy mudstone in the lower part.Orebodies in Nibao can be divided into fault-controlled,strata-controlled,and SBTcontrolled(Mao 1991;Xie et al.2018a,b;Yan et al.2018).The strata-controlled orebodies(orebody I,II,VI)occur in the Longtan Formation of Upper Permian,and the host rocks are mainly bioclastic limestone and calcareous clastic rocks.The fault-controlled orebodies (orebody III) mainly occur in F1,and the compositions of structural breccia are mainly from the Maokou and Longtan Formations.The SBT-controlled orebodies (orebody IV) occur in the unconformity surface between the Longtan and Maokou Formations (Su et al.2009a,2018),and the source compositions of breccia are mainly from the first section of the Longtan Formation,and a small amount of the Maokou Formation.Simultaneously,intense mineralization alterations,including silicification,sulfidization,carbonation,and argillization,generally occur in breccia ore.

Fig.2 Geological Map of the Nibao gold deposit (modified after Zheng et al.2019a). a Geotectonic map, b The distribution of the Emeishan basalt

The Nibao is a giant-sized gold deposit (＞70 t of gold resource),which is located in the northwestern wing of NESW-trending Erlongqiangbao dragged anticline in Pu’an city.The Erlongqiangbao dragged anticline is the main orecontrolling structure formed before or simultaneously with Au mineralization.The faults in the Nibao mining district include NEE-trending faults(F1,F2,F3,and F4)and NWtrending faults (F6,F8,F10,and F11).The F1 fault is the main ore-controlling fault in the mining area,with the largest gold orebody (orebody III;Fig.3).The SBT was thought to be the structural conduit that fed ore fluids into the cores of the anticlines(Tan et al.2015),and it controls the orebody IV of the Nibao gold deposit (Fig.3).Nibao anticline is also an important ore-bearing structure of the Nibao gold deposit,but due to the low degree of exploration,it is still a favorable prospecting space in the mining area.

Fig.3 Diagram of the cross-section along the 9460 exploration line of the Nibao gold deposit (modified after Zheng et al.2019a)

Hydrothermal alterations in the Nibao gold deposit include decarbonatization,sulfidation,silicification,argillization,and dolomitization.The gold-carrying minerals are mainly arsenian pyrite,with a few arsenopyrite,realgar,orpiment,and stibnite.Gangue minerals mainly include quartz,calcite,fluorite,rutile,and clay (e.g.,sericite and illite).The formation of quartz throughout the whole gold mineralization process,with calcite,fluorite,and clay minerals,was mainly in the middle to late stage of gold precipitation.The pyrite consists of diagenetic and hydrothermal pyrites according to their structures and chemical compositions.The hydrothermal pyrite is goldbearing arsenian pyrite,and most of the gold occurs in zoned pyrite with As-poor cores (Fig.4e),mantled by Asrich auriferous rims that contain either a solid solution or submicroscopic Au (Zhang et al.2016;Zheng et al.2017;Chen et al.2018).According to the ore textures,crosscutting relationships,and mineral assemblages,the hydrothermal mineralization process can be divided into three stages: the early stage of quartz-pyrite,the main stage of quartz-pyrite-arsenopyrite,and the late stage of quartz-calcite-realgar-orpiment-stibnite and clay minerals(Jin 2017;Zheng et al.2017,2019a;Song et al.2018).

Fig.4 Photographs depicting the microscopic characteristics of ores in Nibao gold deposit. a Realgar and orpiment fill in quartz veins and fissures,b Quartz veins and pyrites in ores,c Irregular pyrite and acicular and columnar arsenopyrite,d Quartz particles enclosed in calcite veins,e Rim-pyrite, f Short columnar and rhombic arsenopyrite grows along the edge of pyrite.rlg-realgar,orp-orpiment,bre-breccia,py-pyrite,asparsenopyrite,qtz-quartz,and cc-calcite

4 Sampling and analytical methods

In this study,eight ore samples were collected from the orebodies in the SBT of the Nibao gold deposit,and microthermometry and H-O isotopes analysis of inclusions were conducted.Fluid inclusion testing and laser Raman spectroscopy experiments were conducted in the Laboratory of Fluid Inclusions,Institute of Geochemistry,Chinese Academy of Sciences,and the instrument used was the British Linkam THMSG600 heating and freezing stage,which uses pure synthetic CO2and pure water inclusions for temperature calibration before temperature measurement.The temperature measurement error was ± 0.2 °C below+30 °C and ± 2 °C above 100 °C.The laser Raman experiment for the composition of a single fluid inclusion was completed using the Renishaw inViaTMReflex laser Raman spectrometer at the State Key Laboratory of Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences.The laser wavelength was 514.5 nm,the laser power was 20 mW,the spectral line resolution was 2 cm-1,and the spatial resolution was 1 μm.The H-O isotopes of three quartz samples were tested this time,and the test experiment was carried out in the Isotope Laboratory of the Institute of Mineral Resources,Chinese Academy of Geological Sciences.The instrument used was MAT-253,and oxygen isotope analysis adopted the BrF5method.Hydrogen isotope analysis adopts the burst method to extract water and zinc to produce hydrogen.The burst temperature was 550°C,and the analysis accuracy of hydrogen and oxygen isotopes was ± 2% and ± 0.2%,respectively.The results were standardized by Standard Mean Ocean Water (SMOW).

5 Results

5.1 Petrography of fluid inclusions

Fluid inclusions in host minerals(mainly quartz and calcite veins) were relatively abundant,with sizes varying from 1 to 30 μm (Fig.5),and were mainly primary inclusions,occurring as isolated,scattered,grouped,or linearly distributed,with rounded,oval,rhombic,and irregular shapes.According to their components and phase behavior at room temperature(20°C),as well as the transition characteristics during the cooling process,three types of fluid inclusions were recognized,including aqueous inclusions (type W),CO2-bearing aqueous inclusions (type C),and CO2inclusions (type PC).CO2-bearing aqueous inclusions are most abundant in host minerals.Fluid inclusions were dominated by H2O-rich CO2-H2O inclusions with vapor-to-liquid ratios of 5-45%.CO2-bearing aqueous inclusions could be further divided into CO2three-phase inclusions (type C1)and CO2two-phase inclusions(type C2)according to their phase transition during freezing.CO2-H2O three-phase inclusions consist of liquid and vapor CO2and liquid H2O.A small bubble appeared in type C2 inclusions and continued to grow during the cooling process.The bubble became smaller when the temperature increased,eventually disappearing as a homogeneous liquid phase.No inclusions containing salt crystals were found.

Fig.5 Photographs showing the petrography of inclusions in Nibao gold deposit. a CO2-H2O two-phase inclusions (type C2) and aqueous inclusions (type W), b CO2-H2O three-phase inclusions (type C1) and CO2-H2O two-phase inclusions (type C2), c CO2-H2O two-phase inclusions (type C2), d CO2-H2O two-phase inclusions (type C2) and CO2 inclusions (type PC), e CO2-H2O two-phase inclusions (type C2)

5.2 Fluid inclusions micro-thermometry

The microthermometric results are summarized in Table 1.Table 1 and the histogram of homogenization temperature(Fig.6) indicate that the fluid inclusion parameters of the three orebodies could be summarized as follows: the homogeneous temperature of the fluid inclusions in the orebodies was 103.4-385.1 °C (peak value: 200-240 °C),salinity ranged from 0.00 to 8.00 wt% NaCleqv,with adensity of 0.54-1.03 g/cm3.Table 1 shows that the distribution range of homogenization temperature peak value of fluid inclusions in different gold orebodies was roughly the same.Further,the melting temperature of solid phase CO2and that of CO2clathrate,as well as the partial homogenization temperature of CO2,freezing point,salinity,and density,were consistent.The metallogenic pressure was～2-96.54 MPa,and the mineralization depth was 0.23-3.64 km.

Fig.6 Histogram of homogenization temperature of inclusions in Nibao gold deposit

Table 1 Collected microthermometric results of fluid inclusions in the Nibao gold deposit

The above characteristics of fluid inclusions indicate that the gold-bearing fluids have boiling and/or multisource fluids mixing in the process of Au mineralization.They cause CO2overflow in the high-pressure fluids and reduce the acidity of hydrothermal fluids and the solubility of the gold complex,finally leading to gold precipitation.The static rock pressure estimated using the average pressure gradient value of the upper crust of 26.5 MPa/km and the cumulative thickness of the overlying strata in the mining area (852-2324 km) was 22.58-61.59 MPa,which was lower than the calculated pressure value of fluid inclusions,indicating that the ore-forming fluid area consisted of high-pressure fluids.Simultaneously,it confirmed that the ore-forming fluids did not come from circulating atmospheric precipitation.The metallogenic temperature range was significantly higher than that of the basin paleotemperature range (160-250 °C) (Zhuang 1995) calculated and restored based on the vitrinite reflectance of coal and conodont color change index in the sedimentary strata of the study area.It was also significantly higher than the homogenization temperature of fluid inclusions in the Upper Permian paleo oil reservoir in this region (73-175°C),showing that the regional paleo-geothermal temperature was not sufficient to provide the heat source for the mineralization of CTGDs,and the ore-forming hydrothermal fluids did not come from the basin fluids.Based on the above,we believe that the ore-forming fluids have a deep origin.

5.3 Ore-forming fluid compositions

Earlier works on the gas phase compositions of fluid inclusions in the Nibao gold deposit showed a small amount of gas components,such as CH4and N2,in the CO2-H2O inclusions,in addition to H2O and CO2(Fig.7).The existence of CH4gas indicated the reductive character of ore-forming fluids.The results were the same as those indicated by no anomaly to weak positive anomaly of Ce and significant positive anomaly of Eu of calcite veins.The ion compositions of fluid inclusions in the Nibao gold deposit show that the ion components in different minerals(quartz,calcite,fluorite,and realgar) were mainly Cl-,Na+,Ca2+,SO42-,Mg2+,K+,and Br-,while no Fe was detected in the analysis results,further verifying that Fe in gold-bearing pyrite in CTGDs mainly came from the surrounding rock (dissolution and release of Fe containing carbonate minerals during decarbonization).

Fig.7 Plots showing the laser Raman spectroscopy of fluid inclusions in Nibao gold deposit. a, b Liquid and gas phase of type C2 inclusions,c Type PC inclusions, d Gas phase of type C2 inclusions

5.4 H-O isotopes of fluid inclusions

The H-O isotopic compositions of quartz in the main metallogenic stage of the Nibao gold deposit are listed in Table 2.The results showed that the δ DV-SMOWvalues of fluid inclusions in three quartz samples are -78%,-75%,and -76%,respectively,and the δ18OV-SMOWvalues of mineral quartz are 22.8%,23.3%,and 22.3%.According to the equilibrium exchange and fractionationequation of oxygen isotope between the tested mineral and water (Clayton et al.1972),according to the corrected homogenization temperature of the ore-forming fluid (≈220 °C),the calculated value of δ18OH2Ois 11.802%,12.302%,and 11.302% respectively.

Table 2 The composition of H-O isotopes of fluid inclusions in quartz and calcite of Nibao gold deposit

6 Discussion

6.1 Rare earth elements (REEs) characteristics of Ca-bearing minerals

Ca-bearing minerals(calcite,dolomite,fluorite,apatite)are hydrothermal gangue minerals widely distributed in medium-low-temperature hydrothermal deposits.These minerals are often used to study the source and evolution of ore-forming fluids,metallogenic environmental characteristics,and metallogenic age of the deposits.The corresponding data play an irreplaceable role in understanding the genesis,metallogenic process,metallogenic age,and corresponding tectonic background of the deposits.The main hydrothermal Ca-bearing minerals in the Nibao gold deposit are calcite,dolomite,fluorite,and apatite (Chen et al.2018;Song et al.2018;Li et al.2019;Shao et al.2019;Wu 2019).Earlier studies have shown that calcite,fluorite,and apatite related to gold mineralization in southwest Guizhou have the characteristic features of Middle Rare Earth Elements (MREE) enrichment (chondrite-normalized distribution curve),which,unrelated to gold mineralization,are characterized by the enrichment of Light Rare Earth Elements (LREEs;Wang et al.2010;Zhang et al.2010;Tan et al.2015;Chen et al.2018;Su et al.2018),which in turn can be used as an important index to determine the genesis of the above minerals and the most direct prospecting indicator.However,the formation mechanism of this MREE enrichment feature is still unclear.Calcite,fluorite,and apatite are common in Nibao;mineralography and SEM observation show that these three minerals are closely related to gold mineralization (Liu et al.2006;Chen et al.2018;Zheng et al.2019b).Simultaneously,they all show significant MREE enrichment characteristics like other gold deposits in southwest Guizhou (Fig.8),indicating a hydrothermal origin.The studies on the genesis of Ca-bearing minerals are of positive significance to further study the genesis and metallogenic age of the deposit.The hydrothermal calcite of the Nibao gold deposit has the characteristics of weak negative anomaly to weak positive anomaly of Ce,and Eu has the characteristics of a weak negative anomaly to significant positive anomaly.As the gold-bearing fluids are rich in Eu,the fluids have experienced a shift from reduction to oxidation in the middle to late ore stage,and the changing properties of ore-forming fluids may explain the mechanisms underlying gold precipitation.

Fig.8 Plots showing the chondrite-normalized REE distribution curves of apatite and calcite from the Nibao gold deposit (Ji et al.2020)

6.2 C-O isotopes of calcite veins

The C-O isotopic system of carbonate minerals is of practical significance for discussing the formation of carbonate minerals and tracing the source of ore-forming fluids of corresponding deposits.The main carbonate minerals in the Nibao gold deposit are calcite and dolomite.The MREE enrichment characteristics of calcite and its symbiosis with gold-carrying minerals such as realgar and stibnite in the late mineralization suggest hydrothermal genesis.Therefore,mainly the C-O isotopic characteristics of calcite are discussed here.The C-O isotopic data of gold-bearing calcite veins in the Nibao gold deposit were collected.The results show that the variation range of δ13CPDBand δ18OV-SMOWof hydrothermal calcite veins in the Nibao gold deposit were -7% to 3% and 10-25%,respectively.The δ13CPDBvalues were distributed in the range of magmatic carbon and marine carbonate rocks,while the δ18OV-SMOWvalues were in the evolution zone from the magma to the marine carbonate rocks.From the carbon and oxygen isotopic data of calcite veins(Xie et al.2016;Zheng et al.2017;Jin 2017;Ji et al.2020)andin-situoxygen isotopic data of apatite (Chen et al.2018) in the metallogenic period (Fig.9),indicating that although the carbon and oxygen isotopic values of some samples were distributed in the range of magmatic rocks and marine carbonate rocks respectively,there were still a large amount of data distributed in the transition zone between them,showing the multi-component mixing characteristics of hydrothermal fluids.The geology of the mining area shows that the Nibao gold mining area is widely distributed with Emeishan basalt,Upper Permian marine continental transitional carbonate rock,clastic rock assemblage,and the extremely thick marine carbonate rock from the Cambrian to Middle Permian Maokou Formation in the lower part.Combined with carbon and oxygen isotopes and geological characteristics,we propose two explanations for the source of ore-forming fluids.First,the ore-forming fluids originate from the sedimentary strata under the subduction background of the Pacific Plate,and the formation water was released,which reacted with the Emeishan basalt to form gold deposits.Second,the oreforming fluids originate from the deep concealed granite.The gold-carrying fluids reacted with the deep carbonate strata in the rising process and reacted with the host strata(including basalt) when reaching the favorable position of shallow crust,precipitating gold under changing physicochemical conditions.Earlier studies showed that although the strata of the Longtan Formation (Au of 0.009-0.07 ppm;As of 28-5990 ppm) and Emeishan Basalt Formation (Au of 0.005-0.211 ppm;As of 26-5230 ppm) have certain Au and As contents (Zheng et al.2017;Song et al.2018),the contents of Sb,Hg,and Tl are lower.Furthermore,the basalt widely distributed in the mining and its northwest area has not undergone significant alterations,and hence,the conditions to form Sb,Hg,Tl,and super-large Au deposits do not exist.Therefore,we prefer the hypothesis that the gold-bearing fluids are sourced from the deep concealed granite.

Fig.9 Plot showing the C-O isotopes compositions of calcite veins from the Nibao gold deposit(modified after Xie et al.2016;Jin 2017;Zheng 2017;Ji et al.2020.The base map and other representative source areas after Hu et al.2002).MW-local atmospheric precipitation;MC-marine carbonate rocks;CMX-carbonatite and mantle xenoliths;BUR-basic ultrabasic rocks;Gran-granite;Sedim Orgsedimentary organic matter

6.3 H-O isotopes of fluid inclusions

The isotopic compositions of fluids from different sources are noticeably different (White 1974).The H-O isotopic compositions of quartz veins in the main metallogenic stage of the Nibao gold deposit are listed in Fig.8.To more intuitively discuss the source of the ore-forming fluids in the Nibao gold deposit,the H-O isotope values of fluid inclusions in quartz and calcite of different orebodies are projected on the δDH2O-δ18OH2Odiagram (Fig.10).The results show that the δDV-SMOWof quartz samples in orebody IV were from -75 to -78%;further,the δ18OV-SMOWranged from 22.3 to 23.3%.According to the equilibrium exchange and fractionation equation of oxygen isotope between the tested mineral and water (Clayton et al.1972),and based on the corrected homogenization temperature of the ore-forming fluid (≈220 °C),the calculated value of δ18OH2Oranged from 11.302 to 12.302%.The δ18OH2Odata were significantly higher than the range of atmospheric precipitation and hot water in this area(-8.7 to -14%) (Li et al.1989;Hu et al.2002) and distributed in the range of granitic magma water (7-13%)(Han and Ma 2003).The δDV-SMOWdata of orebody III was -85% to -69.7%,and the δDV-SMOWvalue of fluid inclusions in calcite was -64 to -55%.The distribution of δDV-SMOWvalues of orebody VI was-85%to-70%,which was in the range of original magma water (Taylor 1974).The distribution range of δ18OH2Oof orebody III was 11.2-14.6%,and simultaneously,the distribution range of δ18OH2Oin calcite was 8.7-9.6%.The distribution range of δ18OH2Oof orebody VI was 10.9-11.2%(Xie et al.2016;Zheng et al.2019b),which was within the range of granitic magma water (7-13%) (Han and Ma 2003).The H-O isotopes of fluid inclusions indicate that the goldbearing fluids exhibited magmatic origin,but the possibility of metamorphic fluid participating in mineralization cannot be excluded.

Fig.10 Plot of δDH2O vs.δ18OH2O for ore-forming fluids in Nibao gold deposit.(Range of magmatic water and metamorphic water from Taylor 1974.H-O isotopic data after Wang 2013;Xie et al.2016;Jin 2017;Zheng 2017,and this study)

6.4 Source of ore-forming fluids

In the process of mineralization,fluids play a role in transporting gold and other mineralized elements from the source area to an effective metallogenic space.In this process,the fluids react with the rocks on the channel walls and also mix with fluids from other sources.These actions eventually cover up the real sources of ore-forming fluids to varying degrees,which are unfavorable for the meticulous study of the ore-forming process,deposit genesis,and prospecting and exploration.Therefore,more geological and geochemical evidence is needed to effectively identify the real source of gold-bearing fluids.The C,H,and O isotopes are important tracers to distinguish different sources of fluids.According to the C-H-O isotope characteristics of the main-stage quartz and the late-stage calcite veins (Figs.6 and 10),and combined with the fluid inclusions,micro-thermometry showed that the homogeneous temperature,salinity,and density of the fluid inclusions in different orebodies of the Nibao gold deposit are relatively concentrated and consistent,indicating the same source of ore-forming fluid.The REE distribution curve of ore-forming calcite veins is not only consistent with the host strata of the Emeishan basalt and Longtan Formation but also differs from that of the sedimentary basin fluid,showing a special genesis.The H-O isotopes of fluid inclusions were mainly in the range of primary magmatic and/or metamorphic waters,with almost half distributed below primary magmatic and/or metamorphic waters.Earlier explanations of ore-forming fluids were mixtures of basin fluid and atmospheric precipitation (Xie et al.2016;Wei et al.2020),primary magmatic water,metamorphic fluid(Zhang et al.2016;Chen et al.2018;Song et al.2018;Li et al.2019;Shao et al.2019;Wu et al.2019;Zheng et al.2019a),and basin organic fluid(Xie et al.2016).However,our comprehensive analysis considers that gold and goldcarrying fluids mainly come from deep magmatic and/or metamorphic waters,which are mixed with basin water and atmospheric precipitation in varying degrees in the process of upward migration.This understanding can reasonably explain not only the complex metallogenic process but also the C-H-O isotopic data.Earlier studies have shown that although the strata of the Longtan Formation (Au of 0.009-0.07 ppm;As of 28-5990 ppm) and the Emeishan Basalt Formation (Au of 0.005-0.211 ppm;As of 26-5230 ppm) have certain Au and As contents (Zheng et al.2017;Song et al.2018),the contents of Sb,Hg,and Tl are lower.Furthermore,the basalt widely distributed in the mining and its northwest area has not undergone significant alterations,and hence,the conditions to form Sb,Hg,Tl,and super-large Au deposits do not exist.Simultaneously,the ore-forming temperature and fluid pressure are higher than those of the basin fluid and deep circulation atmospheric precipitation,validating our hypothesis of deep source ore-forming fluid.

6.5 Source of sulfur and gold

Sulfur isotopes were used to trace the source of sulfur in sulfides of hydrothermal deposits.The total sulfur isotope value (δ34S∑S) is an important parameter in studying the source of materials in ore deposits.However,whether the sulfur isotope value of sulfides can represent the total sulfur isotope composition of the ore-forming fluid is key for discussing the source of ore-forming materials(Zheng et al.2000).Au in the Nibao gold deposit is mainly hosted in arsenian pyrite and arsenopyrite,and simultaneously,the‘‘core-rim’’ structure is widely developed in arsenian pyrite.While selecting single pyrite minerals for sulfur isotope analysis,it was impossible to distinguish hydrothermal and sedimentary pyrites.Therefore,the δ34S value obtained from pyrite single minerals analysis is a mixed value;thus,this value does not reflect the δ34S composition of the deposit (Chen et al.2009;Su et al.2012;Zheng et al.2017).

In this study,we collected the δ34S data ofin-situarsenian pyrite evaluated by laser ablation inductively coupled plasma mass spectrometry (LA-MC-ICP-MS).Zheng (2017) analyzed the δ34S value of the gold-bearing rim-pyrite in the Nibao gold deposit,and the data ranged from-3.62 to-2.65%,which is based on the three sources of sulfur in hydrothermal deposits,indicating that the main source of the sulfur in the Nibao gold deposit is the mantle and deep crust (δ34S∑S ≈0 ± 3%),and simultaneously,it is a hybrid of sedimentary sulfur.The sulfur isotopic data of hydrothermal pyrite in the Nibao gold deposit is between-5.26 and+4.9%,with an obvious turritus effect,indicating that the main source of sulfur is the mantle(Fig.11)(Jin 2017;Li et al.2019;Wei et al.2020).Although the Emeishan basalt in the area has a relatively high gold background value (Zheng 2017;Song et al.2018),as the regional basalt has not undergone extensive alteration and there is still a large number of gold deposits outside the basalt distribution area,we believe that the regional gold mineralization is not necessarily related to the Emeishan basalt.Based on the above studies,we conclude that the source of sulfur of the sulfides in the Nibao gold deposit is the mantle or deep crust,while ore-bearing fluids are contaminated by sedimentary sulfur when rising to the shallow crust;therefore,the source of materials of the Nibao gold deposit is the mantle or deep crust.

Fig.11 Plot depicting the in-situ S isotopic compositions of Aubearing arsenic pyrite from Nibao gold deposit.(in situ S isotopic data after Jin 2017;Zheng 2017;Wei et al.2020)

6.6 Metallogenic age

Based on the studies on the geochronology of CTGDs in southwest Guizhou,various scholars obtained different data using different testing methods and reported a large number of testing methods and dating data.By systematically collecting and summarizing earlier research data,we believe that the data obtained by the Sm-Nd method of calcite,the Rb-Sr method of quartz,and the Th-Pb method of apatite can represent the reliable age of mineralization.Liu et al.(2006) obtained the metallogenic age of the stratum-controlled gold orebodies of the Nibao gold deposit by using the Rb-Sr isochron of quartz,which was 142 ± 2 Ma.Zheng et al.(2017) used the gold-bearing quartz Rb-Sr isochron to obtain the mineralization ages of the stratum-controlled and fault-controlled orebodies of the Nibao gold deposit,which were 141 ± 2 Ma and 142 ± 3 Ma,respectively.Chen et al.(2018) obtained the mineralization age of 141 ± 3 Ma by using the hydrothermal apatite symbiotic with pyrite Th-Pb method.Although the Rb-Sr dating method of quartz inclusions has some limitations,such as an unclear fractionation mechanism,the dating data still have important reference values.Furthermore,hydrothermal apatite is closely associated with gold-bearing pyrite.Simultaneously,their rare earth distribution pattern also confirms a hydrothermal origin,and hence,the dating data are considered to truly reflect the gold precipitation time.Most dating data indicate that mineralization occurred in the Early Cretaceous,which corresponds to the continental lithospheric extension geotectonic background.Therefore,we speculated that it could effectively represent the metallogenic age of the Nibao gold deposit.

7 Genesis,metallogenic model,and favorable prospecting areas

According to fluid inclusion micro-thermometry,C-H-O stable isotopes combined with earlier gold-bearing pyrite in situ S isotope and geochronology data,no metamorphic rocks were found in this area,although some studies believed that the Au-carrying fluids came from the lowgrade metamorphic rocks of deep Precambrian basement stratum (Su et al.2009a,2018).However,earlier studies stated that this low-grade metamorphic water could not extract Au from the basement rocks (Goldfarb and Groves 2015).Simultaneously,an aeromagnetic survey recognized deep-seated intrusions in this area (Wang et al.2009,2015),which are spatially associated with the CTGDs,including the Nibao gold deposit.Furthermore,many granite-related hydrothermal Sn,Pb,Zn,W,and Mo deposits are distributed in the western,southwestern,and eastern margin Gu of the Dian-Qian-Gui district.Felsic dikes in some Carlin-type gold fields and their ages are similar to those of the regional gold mineralization (captured zircon U-Pb age: 140-130 Ma) (Zhu et al.2016).Based on the above discussion,we speculated that the main source of the ore-forming fluids and Au of the Nibao gold deposit is deep magmatic water.When the fluids rise to the favorable ore-forming space,the pressure drops quickly,and H2O,HCL,H2S,CO2,and other gases are separated from the fluids.Simultaneously,boiling and water-rock reaction consumes CO2and H2S,and the equilibrium of Au complexes in hydrothermal fluids is destroyed,causing the fluid pH to rise.With the continuous water-rock reaction,the change in the physicochemical conditions of the fluids causes the decomposition of metal complexes,as well as precipitation and mineralization.Ore-bearing fluids mixed with the basin and atmospheric waters,the water-rock reaction between ore-bearing fluid and wall rocks,and the continuous consumption of CO2in gold-carrying fluids are all of the great significance for gold precipitation.

The genesis of the Nibao gold deposit can be explained as follows: in the middle Yanshanian,the continental lithosphere at the juncture of the Cathaysian plate and Yangtze Platform was in an extensional background (the Pacific Plate rollback).Simultaneously,the mantle magma upwelling to the shallow crust resulted in the re-melting of the deep gold-rich crust,and the gold-bearing fluid relatedto the magma rose to the upper crust along large deep faults.Due to the mixing of ore-bearing fluids,the waterrock reaction between the ore-bearing fluid and wall rocks,as well as the continuous consumption of CO2and H2S in gold-carrying fluids,changed the physicochemical conditions,destroying the equilibrium of Au complexes of goldbearing hydrothermal fluids.Therefore,the gold precipitated in the favorable metallogenic spaces.The metallogenic age was～142 Ma,and the metallogenic epoch was probably in the Early Cretaceous.

The metallogenic model related to deep concealed magmatic rocks was proposed (Fig.12).Combined with earlier studies on the metallogenic conditions of the Nibao gold deposit,recognizing that the F1 fault was formed before or simultaneously with Au mineralization(while F2,F3,and F4 were post-mineralization structures),and combined with soil geochemical survey (data of Guizhou 105 geological party),we proposed that the favorable metallogenic spaces in the mining area are anticlines,F1 fault,the Longtan Formation within the anticlines,and the SBT.Simultaneously,it is a requirement that the host rocks must have carbonate mineral compositions and a large number of Fe-carrying minerals so that the gold-carrying fluids can dissolve the carbonate minerals,consume CO2and other components in the fluids,reducing the acidity of the fluids with the occurrence of extensive gold sulfidation and precipitation.We consider that there is still a huge prospecting space in the NE extension parts of the F1 fault and Erlongqiangbao anticline,the Nibao anticline in the north of the F1 fault and the SBT in its upper part,and the extension position of the F1 fault in the lower plate F3 fault and the SBT in its upper part in the Nibao gold mining area.Although no pre-metallogenic fault structures were reported in the Nibao anticline,the orebodies were exposed by drilling in the Longtan Formation strata and the SBT at the southeast wing of the Nibao anticline.Furthermore,it is well-known that the Nibao gold mining area has complex structures and widely developed joint and fracture systems,and these microstructures often play a positive role in mineralization.Therefore,we believe that Nibao anticline still has a large prospecting space.Based on the above understanding,the favorable prospecting locations of the Nibao gold mining area are delineated,which provide theoretical support for the realization of new prospecting progress in the mining area.

Fig.12 Diagram representing the metallogenic model and prospecting target areas of Nibao gold deposit (modified after data of Guizhou 105 geological party)

8 Conclusions

1.The Nibao gold deposit and other gold deposits in southwest Guizhou are typical CTGDs,and the oreforming fluids mainly originated from deep concealed magmatic rocks.The ore-forming fluid is a hydrothermal fluid with medium-low temperature,high pressure,medium-low salinity,low density,low oxygen fugacity,weak acidity,weak reduction,and rich in CO2and CH4.

2.The main controlling factors of gold precipitation are the boiling of gold-bearing hydrothermal fluids,decrease of fluids acidity,increase of oxidation degree,decrease and decomposition of gold-complex solubility caused by multi-source fluids mixing,and waterrock reaction.

3.According to the geological background,ore deposit and fluid inclusions characteristics,multiple isotopes,and geochronology data,the Nibao gold deposit mainly originated from a magmatic-hydrothermal system,and thus,a metallogenic model of the Nibao gold deposit was established.Combined with geophysical and geochemical measurements,considering that the NE extension parts of F3 fault and the Erlongqiangbao anticline,and the Nibao anticline in the north of F3 fault of the Nibao gold mining area are favorable prospecting areas.

AcknowledgementsThis paper was financially supported by the National Natural Science Fund of China(41962008),the Talent Team Program of Science and Technology Foundation of Guizhou Province(Qian Ke He Ping Tai Ren Cai CXTD [2021]007),the National Natural Science Fund of China (U1812402),and the Guizhou Province Graduate Research Fund (YJSCXJH[2020]095).We thank the No.105 Geological Party of the Guizhou Bureau of Geology and Mineral Exploration &Development for field works.

FundingThis study was supported by the National Natural Science Fund of China (41962008),the Talent Team Program of Guizhou Science and Technology Fund (Qianke Pingtairen Caixintang[2021]007),the Geological Exploration Fund Project of Guizhou Province (520000214TLCOG7DGTDRG),the National Natural Science Foundation of China(U1812402),Scientific Research Project of Hubei Geological Bureau (KJ2022-21),and the Graduate Research Fund of Guizhou Province (YJSCXJH [2020]) 095).

Declarations

Conflict of interestThe authors declare no conflict of interest.