Chem ical and boron isotopic compositions of tourmaline in the Longtoushan hydrothermal gold deposit,Guangxi,China:im plications for gold m ineralization

Lihua Qian·Jianqing Lai·Shugen Zhang·Lifang Hu·Rong Cao·Shilong Tao

Abstract The Longtoushan hydrothermal gold deposit is located in the southwestern region of the Dayaoshan Uplift.Tourmaline isw idespread in the Longtoushan gold deposit and is mainly distributed in the rhyolite porphyry and associated cryptoexplosive breccia.The spatialdistribution of tourmaline enrichment is similar to that of the gold orebody.Feldspar has been largely replaced by tourmaline in the rhyolite porphyry and cryptoexplosive breccia.Electron m icroprobe analysis revealed that tourmalines in the Longtoushan depositbelongmainly to the alkaligroup and partly to the X-vacant group;they mostly fell in the schorl-dravite series f ield.Two distinct sets of dom inant substitutionswere observed:MgFe-1 and Alh(NaR2+)-1,where R=Fe,Mg.In addition,minor substitutions include(CaMg)(NaAl)-1 and FeAl-1.The calculatedδ11B value for them ineralizing f luids ranged from-12.8 to-9.7‰,which is typical of S-type granites,and boron-enriched f luids predom inantly derived from rhyolitic melt.Part of the tourmaline from the rhyolite porphyry crystallized during the magmatic-hydrothermal stage,whereas most tourmalines from the deposit formed in the post-magmatic hydrothermalstage.The tourmalineswere deposited from a relatively reduced and acidic f luid system,and the gold predominantlyprecipitatedduringthepost-magmatic hydrothermal stage in the Longtoushan deposit.?Jianqing Lailjq@csu.edu.cn

Keywords Longtoushan gold deposit·Tourmaline·Oreforming f luids·Boron isotopes

1 Introduction

Tourmaline forms in a variety of settings,including as a signif icant m ineral crystallizing in fractionated igneous bodies;as a diagenetic m ineral in buried sedimentary basins;and as a ganguem ineral in ore deposits,associated w ith contact,regional,and subduction-related metamorphism in metasomatism(e.g.,Henry and Dutrow 1996;Slack 1996;Marschall et al.2009;Hinsberg et al.2011).Tourmaline is stable under an extensive range of pressures and temperatures thatcovermostof the geological settings found in Earth'scrust,and itcan exist in equilibrium w ith a variety of geological f luids(e.g.,Huang etal.2008;Nova′k etal.2011;Dutrow and Henry 2011;Hinsberg etal.2011).The major and trace element chem istry of hydrothermal tourmaline is controlled by the host rock,the external hydrothermal f luids,and the pressure—temperature conditionsof tourmaline crystallization(e.g.,Henry and Guidotti 1985;Codec?o et al.2017).In f luid-dom inated systems,such as breccia pipes and veins,tourmaline chemistry is generally buffered by the f luid phase(Slack and Coad 1989;Slack and Trumbull 2011).M ineralogical,chem ical and boron isotopic analyses of tourmaline have been used tobetter constrain ore-form ing processes(Slackand Trumbull 2011;Xiong et al.2014).

The Longtoushan hydrothermal gold deposit,which is associated with cryptoexplosive breccias,is located in the southwestern region of the Dayaoshan Uplift(Fig.1).Gold mineralization can be divided into two types in the deposit:(1)dissem inations or veins in the rhyolite porphyry and cryptoexplosive breccia,and(2)veinsor stockworks in theclastic rocks of the Lianhuashan Formation(Xie and Sun 1993;Yang et al.2008;Wang 2011).Tourmaline,mainly distributed in the rhyolite porphyry and cryptoexplosive breccia,is closely related to gold m ineralization,and crystallized in the early stage of the pneumatic-hypothermal episode(Xie and Sun 1993;Huang et al.1999).Nevertheless,few studies have been performed on the tourmaline from this deposit.Lu(2008)brief ly described the occurrence and composition of tourmaline in the gold orebody of the Longtoushan deposit,suggesting that it belongs to the schorl-dravite series.Based on petrographic and electron m icroprobe analysis of tourmalines from the Longtoushan deposit,Wang(2011)emphasized that tourmalines above the 420-m level are rich in Fe and depleted in Mg,and those below 420m are rich in Mg.However,previous studies of tourmaline fromthe Longtoushan deposit did not concentrate on its genesis and relationship to mineralization.This paper presents mineralogical-textural characteristics and chem ical and boron isotopic compositions of tourmaline from the Longtoushan gold deposit.These results,combined w ith relevant data from Wang(2011),were used to constrain the early evolution of ore-form ing f luid in the Longtoushan gold deposit.

Fig.1 a Sketchmap show ing the location of the Dayaoshan Uplift in the Qinzhou—Hangzhoumetallogenic belt,South China.b Sketchmap of the Dayaoshan Uplift.Modif ied after Chen etal.(2015)

2 Geological setting

2.1 Regional geology

The Qinzhou—Hangzhou metallogenic belt,located in the Neoproterozoic collisional orogen between the Yangtze and Cathaysia Blocks,isamajor polymetallic(Cu,Mo,W,Sn,Pb,Zn,Au,and Ag)belt in South China(Mao et al.2011;Zhou et al.2012).The Dayaoshan Uplift consists of weakly metamorphosed basement,and is situated in the southwestern margin of the Qinzhou—Hangzhou metallogenic belt(Fig.1a).The strata exposed in this area consist mainly of the Sinian Peidi Formation and the Cambrian Xiaoneichong and Huangdongkou Formations,composed dom inantly of greywackesw ith some interlayers of argillites and silty shales(Bureau of Geology and M ineral Resources of Guangxi Zhuang Autonomous Region 1985).Themain structures in the region include the EW-trending Dayaoshan anticlinorium and the Dalideep fault,on which NE-,NW-,andSN-trendingstructures superimposed(Fig.1b).W idespread magmatic intrusions in the Dayaoshan area include granite,biotite granite,monzogranite,and granodiorite,formed during four stages:Caledonian(470—430 Ma),Hercynian—Indosinian(270—240 Ma),early Yanshanian(170—150 Ma),andlateYanshanian(110—90 Ma)(Chen et al.2015).The Caledonian and Yanshanian granitoidsare themostabundant felsic igneous rocks in the Dayaoshan area,and are accompanied byabundantigneousrelatedW—Mo—Cu—Au—Ag—Pb—Zn polymetallic m ineralization(Huang et al.2003).

2.2 Local geology

The main lithostratigraphic units that outcrop in the Longtoushan gold deposit include the Cambrian Huangdongkou Formation and the Lower Devonian Lianhuashan Formation(Fig.2a).The former is composed of low-grade metamorphic f ine-grained sandstone,argillaceoussiltstone,carbonaceous slate,and spotted slate;the latter consists of conglomerate,sandy conglomerate,quartz sandstone,f inegrained sandstone,and argillaceous siltstone.The faults in this depositare NW-,NS-,NE-,or EW-striking.The NWstriking faults,which are characterized by long extensions,deep depths,and steep dips,are themain ore-controlling structures in the deposit(Xie and Sun 1993).The Longtoushan granitic complex has an irregular,oval shape horizontally and a pipe shape vertically(Fig.2b),dipping steeply to the northwest.It ismainly composed of biotite granite porphyry and subvolcanic sequences,including cryptoexplosive breccia,rhyolite porphyry,monzogranite porphyry,and felsic dykes.Zircon SHRIMPU—Pb ages of monzograniteporphyryandrhyoliteporphyryyield 100.3±1.4 Ma and 103.3±2.4Ma,respectively(Chen et al.2008),suggesting that the gold mineralization at the Longtoushan deposit occurred in the late Yanshanian.Additionally,the granites are considered to be S-type granitoids(Duan et al.2011;Wang 2011).

The gold orebodies are dom inantly hosted w ithin the contact zone of the rhyolite porphyry and cryptoexplosive breccia(Fig.2b),and partly distributed in the fault or fracture zone of thewall rock.Additionally,porphyry-style Cu mineralization has been discovered in the granite porphyry at depth(～70m above sea level)(Huang et al.1999).Gold orebodies,which are vein-like,lenticular,and cystic w ith dip angles of 76°to 90°,are mostly NWtrending in the deposit.Inf lation,contraction,branching,compounding,pinching,and recurring occur along the strike and dip of the orebodies.Themainmetallicminerals are native gold,pyrite,chalcopyrite,chalcocite,arsenopyrite,and bismuthinite,while the major nonmetallic minerals are quartz,tourmaline,sericite,and kaolinite.Wallrock alteration is pervasive in the Longtoushan gold deposit.Tourmalinization and silicif ication are predom inantly distributed in the rhyolite porphyry and cryptoexplosive breccia and their contact zone,while K-feldspar,sericite,and argillic alteration are mainly hosted in the granite porphyry.

A number of f luid inclusion studies have been performed on the Longtoushan deposit.According to Xie and Sun(1993),Huang etal.(1999),and Zhu(2002),the ore-forming temperature of the Longtoushan gold depositmay range from 160 to 320°C,170 to 560°C,or180 to 290°C,respectively.Salinitiesof the f luid inclusions vary between 9.4 and 53.6%NaCl eq(Huang et al.1999).Group f luid inclusionsanalysessuggest that the f luidsare dom inated by Na+,K+,Ca2+,and Mg2+cations;Cl-,F-,and Sanions;and H2O,CO2,CO,and CH4molecules(Xie and Sun 1993;Huang et al.1999;Zhu 2002).Theδ34S values of pyrites range from+0.6 to+2.5‰,indicating deeply sourced magmatic sulfur(Xie and Sun 1993;Zhu 2002;Tao et al.2017).The ore-form ing materials are directly derived from the remelting of granitic magma;the f luids originatemainlyfromamagmaticsourcew ithan involvement of meteoric water as indicated by hydrogen and oxygen isotopes(Xieand Sun 1993;Huang etal.1999;Zhu 2002).

Fig.2 a Geologic map and b cross-section prof ile of the Longtoushan gold deposit.a ismodif ied after Xie and Sun(1993)

3 Tourmaline occurrences

Tourmaline,generally f ine-grained,occurs as spot-like,veined and dissem inated structures or f illings in geodes with quartz in the Longtoushan gold deposit.According to Xie and Sun(1993),native gold is generally distributed in quartz-tourmaline veinlets.Tourmaline in the depositwas classif ied by its occurrence:(1)dense dissemination in the rhyolite porphyry(Fig.3a,b);(2)dissemination in the cement of cryptoexplosive breccia(Fig.3c);(3)inhomogeneous dissem inationinthe monzogranite porphyry(Fig.3d);(4)veinlet-dissem ination in the biotite granite porphyry(Fig.3e);and(5)dissem ination in quartz—pyrite veins in sandstone of the Lianhuashan Formation(Fig.3f).

Feldspar in the rhyolite porphyry was observed to be completely replaced by euhedral to subhedral tourmaline(Fig.4a),making up 25—40 vol.%of the rhyolite porphyry.A minor amountof corroded quartz phenocrysts displayed grow th rim containing f ine-grained tourmaline(Fig.4b).The content of tourmaline in cryptoexplosive breccia,chief ly distributed in the cement,wasalso high.Biotite and feldspar in thebrecciaswere commonly replaced by tabular and radial aggregates of tourmaline.Biotite and feldspar phenocrysts in the monzogranite porphyry were locally replaced by tourmaline,quartz,sericite,and pyrite;tourmalinization was weak in thematrix and associated with sericitization.Moreover,tourmalinepartlyexhibited oscillatory zoning in optical and backscattered electron(BSE)images(Fig.4c).Tourmalinization in the sandstones of the Lianhuashan Formation intensif ied toward the Longtoushan subvolcanic rocks,reaching contentsof up to 25%.Tourmaline generally displayed acicular-prismatic habit in quartz—pyrite veins in the sandstone(Fig.4d).In addition,rutile associated w ith tourmaline(Fig.4e)wasmainly distributed in the monzogranite porphyry and quartz—pyrite—tourmaline veins.

Fig.3 Photographs show ing tourmaline occurrences in the Longtoushan deposit.a,b Tourmaline disseminated in the rhyolite porphyry;c Tourmaline mainly distributed in the cement of cryptoexplosive breccias;d Tourmaline in the monzogranite porphyry(inhomogeneous dissem ination);e Biotitegranite porphyryw ith tourmalinization;and f Tourmaline in thequartz—pyriteveins from sandstone.Tur tourmaline,Py pyrite,Qtz quartz

Fig.4 Photomicrographs of tourmaline from the Longtoushan deposit.a Tabular aggregate of tourmaline(Tur)from rhyolite porphyry is pseudomorphic after feldspar phenocryst,transm itted light(+).b The grow th rim of quartz phenocryst in rhyolite porphyry contains f ine tourmaline,transm itted light(+).c Photom icrograph[left hand side,transm itted light(-)]and backscattered electron(BSE)image showing opticaland compositional zoning in tourmaline.d Euhedral tourmaline from sandstone,transm itted light(-).e Rutile is chief ly dissem inated in tourmaline,transm itted light(-).f BSE image show ing compositionalzoning in tourmaline.‘‘-''meansunder plane-polarized light;‘+''means under cross-polarized light.Tur tourmaline,Qtz quartz,Fsp feldspar,Py pyrite,Rt rutile

Tourmaline was observed to be dominantly distributed on the edge of the biotite granite porphyry and occurred in veinlet-dissem inated morphologies.Biotite and feldspar phenocrysts were observed to be partly replaced by tourmaline,sericite,and pyrite,or completely replaced by tourmaline,quartz,and muscovite.Additionally,the tourmaline generally exhibited oscillatory zoning(Fig.4f)in the biotite granite porphyry.

4 Sam p ling and methods

We performed a geological survey on levels between 300 and 420m a.s.l.(above sea level)in the southern part of the Longtoushan deposit(Fig.2).In total,165 samples were collected from those sections.Among them,127 samples were collected from the 380-m level at 10m intervals.The petrography of these samples was studied with a conventional polarizing microscope.Duplicate samples from the 380-m levelwere crushed to a size of 200 mesh and were subm itted to the Guilin Research Instituteof Geology for M ineral Resources for boron analysis by em ission spectroscopy w ith a 2-m grating spectrograph(Oberkochen,GER).

Table 1 Locations and host rocks of samples

Based on the petrographic analysis of tourmaline,we selected f ive representative samples w ith different occurrences.Positions and descriptions of these samples are shown in Table 1.Chem ical analyses of tourmaline were performed on polished thin sections using a Shimadzu EPMA-1720 electronm icroprobe at the Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring(Central South University),M inistry of Education of China.Them icroprobewas operated using an accelerating voltage of 15 kv,a beam currentof10 nA,abeam sizeofapproximately 1μm,and a detection lim itof0.01%.The follow ing referencematerials wereused:SiO2(Si),TiO2(Ti),Y3Al5O12(Al),NaAlSi3O8(Na),Fe3O4(Fe),MnSiO3(Mn),MgO(Mg),CaF2(Ca),and KA lSi3O8(K).Tourmaline structural formulae were calculated based on 15 cations in the tetrahedral and octahedral sites(T+Z+Y)(Henry and Dutrow 1996).

We selected four samples from those thatwere analyzed by m icroprobe for the determ ination of the boron isotopic compositions of tourmaline.Boron isotopic compositions weremeasured in situ on polished thin sectionsusing laser ablation—multicollector—inductively coupled plasma—mass spectrometry(LA-MC-ICP-MS)at the State Key Laboratory of Geological Processesand M ineralResources,China University of Geosciences,Wuhan.A Resonetics-S155 laser-ablation system(Boston,USA)and Nu Instruments Nu Plasma II ICP-MS(W rexham,Wales,UK)were combined for these experiments.A 193-nm ArF excimer laser,homogenized by a setof beam delivery systems,was focused on the tourmaline surface.The ablation protocol employed a spotdiameterof 33μm and a repetition rate of 8 Hz for 40 s(equal to 320 pulses).The aerosol produced during ablation was transported using a carrier gas of helium m ixed w ith argon and a small amount of nitrogen,and the m ixture was carried into the plasma of the MCICP-MS.The11B/10B ratios of standard and test samples were obtained using the abovemethod,and theδ11B values were calculated using external calibration(SSB method).The tourmaline standard IAEA B-4(δ11B=-8.7‰)from Tonarinietal.(2003)wasused asan externalstandard.The tourmaline standards IMR RB1(Hou)and Dai of the Laboratory were used formonitoring.Theδ11B values of tourmaline IMR RB1 obtained during this analysis ranged between-13.2 and-13.1‰,consistent w ith the previous result(-12.96±0.49‰,1σ)(Hou et al.2010).

5 Results

5.1 Distribution of tourmaline

Cryptoexplosive breccia was observed to be dom inantly distributed in the contact zone of the rhyolite porphyry,with cement mainly composed of rhyolite porphyry or aggregates of pyrite,tourmaline,f ine-grained quartz,and rock debris.Clasts of quartz sandstone,siltstone,and granite porphyry ranged from 0.3 to 4 cm in diameter.The cryptoexplosive breccia gradually transitions into rhyolite porphyry,and the interface between them is irregular.Therefore,we could not completely distinguish cryptoexplosive breccia from rhyolite porphyry at the 380-m level in the southern part of the deposit.

Boron anomalies indicate the distribution of tourmaline inthe Longtoushandeposit.Gridding using Kriging method in Surfer 8(Golden Software,Golden,Colorado,USA)was applied to the boron contents of samples from the 380-m level to produce a boron contourmap(Fig.5).The boron contourmap suggests that tourmaline is abundant in the contactzone between the rhyolite porphyry and cryptoexplosive brecciasalong NW—SE striking faults.The pattern of tourmaline enrichment is similar to the extentof the gold orebody in the Longtoushan deposit.

5.2 Chem ical com positions of tourm aline

Crystal-chem ical formulae of tourmaline were calculated on the basis of the general structural formula XY3Z6T6-O18(BO3)3V3W,where X=Na,K,Ca,□;Y=Li,Mg,Fe2+,Mn,Al,Cr,V,Ti,Fe3+;Z=A l,Fe3+,Mg,V,Cr;T=Si,Al,B;V+W=OH-+F-+Cl-=4 atomsper formula unit(apfu)(Henry etal.2011;Nova′k etal.2011).

Fig.5 Boron contourmap of the 380-m level in the southern part of the Longtoushan deposit

Representative chemical compositions of tourmalines from the Longtoushan deposit are shown in Table 2.In general,tourmalines were found to belong mostly to the alkali group(Fig.6a).Tourmalines from the rhyolite porphyry exhibited highly variable X-site vacancies,whereas tourmalines fromthe cryptoexplosive breccia,biotite granite porphyry,and monzogranite porphyry fell w ithin the alkali group.A m inor proportion of tourmalines from veins in sandstone fell in thevacancy group.On the Al—Fe—Mg discrimination diagram(Henry and Guidotti 1985),sampled tourmaline plotted mainly w ithin the f ields of Lipoor granitoids and metasedimentary rocks,and predom inantly above the schorl-dravite line(Fig.6b).In addition,tourmaline compositions from the rhyolite porphyry show two distinct clusters(Fig.6b),and tourmaline from veins in sandstone plotted partly w ithin the f ield of Fe3+-rich quartz-tourmalinerocks,calc-silicaterocksand metapelites.

Tourmaline from the Longtoushan deposit displayed considerable variations in Fetot/(Fetot+Mg)(Table 2).Except foranumberof analyses from the rhyolite porphyry and individual sample points from veins in sandstone,which fellw ithin the foititeor‘‘Oxy-foitite''and Mg-foitite f ields,tourmalinecompositionplottedpredom inantly within the f ield of the schorl-dravite series(Fig.7).

5.3 Boron isotopic compositions of tourmaline

Tourmalines from the Longtoushan deposit displayed a narrow range ofδ11B values(Table 3).Theδ11B valuesof tourmalines from the rhyolite porphyry,cryptoexplosive breccia,biotite granite porphyry and sandstone ranged from-14.0 to-12.3‰(average=-13.2‰),-15.4 to-14.3‰(average=-14.7‰),-14.4 to-12.3‰(average=-13.7‰),and-15.2to-13.3‰(average=-14.4‰),respectively.

6 Discussion

6.1 Boron isotope signatures

Theδ11B value of tourmaline is chief ly controlled by the composition of the boron sources(e.g.,Palmer and Slack 1989;Jiang et al.1999).The entrapment stage of f luid inclusions in Xie and Sun(1993)and Zhu(2002)is unclear,thus the relatively low homogenization temperature presented by them may correspond to late stages of mineralization in the Longtoushan deposit.According to Huang et al.(1999),tourmaline formed during two early ore-form ing stagesw ith temperatures ranging from 320 to 560°C.The homogenization temperature peaks of the two stages are 370 and 460°C,respectively,and the average value of them is approximately 410°C.Using this as a reasonable estimate for tourmaline formationinthe Longtoushan deposit,the hydrothermal f luid would be 2.6‰heavier inδ11B than the coexisting tourmaline based on the factors of Meyer et al.(2008).Calculatedδ11B values for the mineralizing f luids ranged from-12.8 to-9.7‰,which is typicalof S-type granites(TrumbullandSlack 2018).A 240°C gradient in the temperature of tourmaline grow th corresponds to a difference of 3.8‰in the fractionation effect,which largely corresponds to the observed variation in the boron isotope compositions of tourmaline.In addition,11B partitions preferentially into H2O f luid and10B into illite,muscovite,and silicatemelt(Hervig et al.2002),which may have led to the slightly heavierδ11B values of tourmalines from the rhyolite porphyry and biotite granite porphyry than those from the cryptoexplosive breccias and sandstone.

Table 2 Average compositions of tourmaline from the Longtoushan deposit

Table 2 continued

Fig.6 Composition of tourmaline from the Longtoushan deposit.a Ca-X-site vacancy-Na(+K)triangle(Henry et al.2011).b Fetot—Mg—Al triangle(Henry and Guidotti1985).In plot b:(1)Li-rich granitoid pegmatites and aplites;(2)Li-poor granitoid rocksand associated pegmatites and aplites;(3)Fe3+-rich quartz—tourmaline rocks(hydrothermally altered granites);(4)Metapelites andmetapsammites coexistingw ith an Alsaturating phase;(5)Metapelites and metapsammites not coexisting w ith an A l-saturating phase;(6)Fe3+-rich quartz—tourmaline rocks,calcsilicate rocks,andmetapelites;(7)Low-Cametaultramaf ic rocks and Cr,V-richmetasediments;(8)Metacarbonates and meta-pyroxenites

Fig.7 Compositional discrim ination between tourmalines from the Longtoushan deposit in termsof Fe/(Fe+Mg)versus X vacancy/(Na+X vacancy),atoms or atom equivalents

The modal percentage of tourmaline(up to about 40 vol.%)in the rhyolite porphyry and cryptoexplosive breccia implies that the high boron concentration must have derived externally and that the isotopic composition of boron in tourmaline is dom inated by that of the hydrothermal f luid.Furthermore,the tourmaline enrichment zone is concentrated in the contact between the rhyolite porphyry and cryptoexplosive breccia,and the rhyolite porphyry evolved from S-type melt(Duan et al.2011).Consequently,boron-rich ore-form ing f luids that ledto the formation of tourmalinewere predom inantly derived from the rhyoliticmelt.

Table 3 Boron isotopic compositions of tourmaline from the Longtoushan deposit

6.2 Com positional variations in tourmaline

Tourmaline in the Longtoushan deposit belongsmainly to the schorl-dravite series,and can be divided into f ive types w ith respect to host rock.One cluster of tourmaline analyses from the rhyolite porphyry fell in f ield 2 in the Fetot—Mg—A l triangular diagram,suggesting a formation environment sim ilar to Li-poor granitoids.Moreover,some tourmalines in this cluster had high Fe/(Fe+Mg)and X vacancy/(Na+X vacancy),while others had relatively low Fe/(Fe+Mg)and X vacancy/(Na+X vacancy)(see Fig.7).Based on petrographic features,a large number of potassium feldspar crystals have been replaced by tourmaline in the rhyolite porphyry rock.Hence,like tourmaline nodules describedbyYanget al.(2015),the tourmalines with high Fe/(Fe+Mg)ratios from the rhyolite porphyry exhibited magmatic-hydrothermal features related to the exsolution,phase separation,and entrapment of imm iscible aqueous boron-rich f luids.However,the tourmalinesw ith low Fe/(Fe+Mg)ratios and tourmaline from the cryptoexplosive breccia,monzogranite porphyry,biotite granite porphyry,and sandstone fell dominantly within f ields 4 and 5(Fig.6b).Thismay be the result of post-magmatic hydrothermal alteration caused by the inf iltration of boron-rich f luids.Additionally,a smallgroup of tourmalines from sandstone fell w ithin f ield 6,suggesting the presence of Fe3+(Henry and Guidotti 1985).The presence of Fe3+indicates that those tourmalines crystallized in a relatively oxidized f luid system.

A w ide variation in the Fe/(Fe+Mg)ratio ref lects the exchange vector MgFe-1in tourmaline w ith different occurrences,whereas values of∑(Fe+Mg)＜3 apfu(atoms per formula unit)reveal the negligible presence of Fe3+(Fig.8a—d).Substantial amounts of Al were incorporated in the Y-site via the Al□(NaR2+)-1substitutions in tourmaline,where R=Fe,Mg(Fig.8e—h).The X-site vacanciesversus A ldiagram(Fig.8e—h)perm itsseparating the inf luence of the A lO(R2+OH)-1and FeA l-1exchanges from the A l□(NaR2+)-1exchange for the different types of tourmaline.The inf luence of the FeAl-1exchange mechanism was negligible in tourmaline from the rhyolite porphyry,cryptoexplosive breccia,biotite granite porphyry,and monzogranite porphyry,although relatively large in tourmaline from the sandstone.W ith respect to the correlation between Na and Ca at the X-site(Fig.8i—l),tourmaline from the depositwas predom inately controlled by(□A l)(NaMg)-1.In addition,as shown in Fig.8i—l,there was locally a vector corresponding to(CaMg)(NaAl)-1in tourmaline from the cryptoexplosive breccias and sandstone,whichmay be controlled by the host rock.

Tourmaline that crystallized in the post-magmatic-hydrothermal stage generally shows dravitic compositions and exhibits some compositional zoning in BSE images(e.g.Fig.2c).The Longtoushan gold deposit is controlled by breccia pipe and faults(Huang etal.1999),suggesting a f luid-dom inated system,which indicates that tourmaline chem istry isgenerally buffered by the f luid phase.Electron microprobe analyses revealed that there are considerable variations in Ti,Fe,Mg,and Na contents in the oscillatory zones,and negligible variations of other elements(see Fig.9).Darker-colored zones exhibit relatively higher Mgcontent and lower Ti and Fe content,whereas lighter-colored zones exhibit the opposite.In Fig.9,the increase in Na content from core to rim in tourmaline is consistent w ith experimental results(Goerne et al.2001)indicating that Na content increases w ith decreasing temperature.Moreover,the higher Fe content in the rims suggests a temperature decrease(M lynarczyk and W illiams-Jones 2006).

Fig.8 Chem ical compositions of tourmaline from the Longtoushan deposit expressed in terms of atom ic ratios and atoms per formula unit(apfu).a—d Mg(apfu)versus Fe(apfu);e—h A l(apfu)versus X-site vacancies(apfu);i—l Ca(apfu)versus Na(apfu)

6.3 Im p lications for gold m ineralization

Alkaline solutions produce boron complexes that are typically tetrahedrally coordinated w ith oxygen in[B(OH)4]-anionic complexes.Nevertheless,boron is triangularly coordinated w ith three oxygens in a B(OH)3complex in neutral to acidic conditions,which is consistent w ith the coordination of boron in tourmaline(Meyer et al.2008;Dutrow and Henry 2011).The amountof boron required to stabilize tourmaline increases w ith increasing pH,andtourmaline has not yet been synthesized under alkaline conditions(Morgan and London 1989).Thus,tourmaline is stable in highly acidic to neutral solutions(Morgan and London 1989;Henry and Dutrow 1996).In the Longtoushan deposit,a large number of potassium feldspar crystals have been replaced by tourmaline in the cryptoexplosive breccia and rhyolite porphyry.The reaction can be described as follow.

Fig.9 Line plot of the content of Ti,Fe,Mg,and Na in tourmaline zonation from monzogranite porphyry

Gold is deposited both w ith tourmaline and partly after tourmalinization(Xie and Sun 1993;Huang et al.1999).The tourmaline precipitated from f luids w ith temperature ranging from 320 to 560°C,and the anions in the f luids were dom inated by Cl-(Huang et al.1999).According to W illiams-Jones et al.(2009),AuCl2-is likely to be the main control on the solubility of gold in chloride-rich systems at temperatures above 350°C,and cooling is effective in causing large-scale deposition of gold transported as chloride complexes.Therefore,AuCl2-was the dom inant form of gold in hydrothermal solutions during the early stages of mineralization in the Longtoushan deposit,and cooling was effective in causing low-grade gold m ineralization in the rhyolite porphyry and its wall rock.However,in tourmalines from the rhyolite porphyry,cryptoexplosive breccia,monzogranite porphyry,and biotite granite porphyry,negligible substitution of Fe3+for Al indicates that the tourmalineswere deposited from a relatively reduced f luid system(Trumbulletal.2011).H+was released during the replacement of potassium feldspar by tourmaline,leading to a decrease in pH of the f luids.Furthermore,[Au(HS)2]-and Au(HS)are the dom inant forms of gold in hydrothermal solutions atm ineralization temperatures below350°C(W illiams-Jones et al.2009).Consequently,the solubility of gold may have increased due to the increased stability of bisulf ide species after an initial decrease in the Longtoushan deposit.In addition,abundant coexisting liquid-rich and vapor-rich inclusions are indicative of boiling,and the occurrence of native gold is closely related to pyrite in the Longtoushan deposit(Huang et al.1999).The gold would be transported to the fractures and faults,and deposited by other controlling factors,such as boiling and sulf idation,which is consistent w ith the occurrences of native gold and gold orebodies in the Longtoushan deposit(Xie and Sun 1993).

7 Conclusions

(1)Tourmaline is enriched in the contact zone between rhyolite porphyry and cryptoexplosive breccia in the Longtoushan deposit.The NW-striking trend of the tourmaline enrichment zones is sim ilar to thatof the gold orebody.

(2)Tourmalines dom inantly plotw ithin the f ield of the schorl-dravite series.The tourmalinesbelongmainly to the alkaligroup and partly to the X-vacantgroup.The bimodality in chem ical compositions of tourmaline from the rhyolite porphyry ref lects that one group formed in the magmatic-hydrothermal stage and the other in the post-magmatic hydrothermal stage.However,most tourmalines from the Longtoushan deposit crystallized during the post-magmatic hydrothermal stage.

(3)Calculatedδ11B values for the mineralizing f luids ranged from-12.8 to-9.7‰,which is typical of S-type granites,and the boron-enriched f luids predom inantly derived from the rhyolitic melt.

(4)The tourmalines were deposited from a relatively reduced and acidic f luid system,and the gold in the Longtoushan depositwaspredom inantly precipitated in the post-magmatic hydrothermal stage.

Acknow ledgem entsThis study was supported by the Project of Innovation-driven Plan in Central South University(Project No.2015CX008)and the Fundamental Research Funds for the Central Universities of Central South University(Project No.2015zzts071).M iao Yu and Jeff Dick are thanked for their critical review of themanuscript.Moreover,we w ish to thank two anonymous reviewers and editors for their constructive comments.