Genesis of the Nanyangtian scheelite deposit in southeastern Yunnan Province,China:evidence from mineral chemistry, fluid inclusions,and C–O isotopes

Qianru Cai?Yongfeng Yan?Guangshu Yang?Fuju Jia?Chao Li

Abstract The Nanyangtian skarn-type scheelite deposit is an important part of the Laojunshan W–Sn polymetallic metallogenic region in southeastern Yunnan Province,China.The deposit comprises multiple scheelite ore bodies;multilayer skarn-type scheelite ore bodies are dominant,with a small amount of quartz vein-type ore bodies.Skarn minerals include diopside,hedenbergite,grossular,and epidote.Three mineralization stages exist:skarn,quartz–scheelite,and calcite.The homogenization temperatures of fluid inclusions in hydrothermal minerals that formed in different paragenetic phases were measured as follows:221–423 °C (early skarn stage),177–260 °C (quartz–scheelite stage),and 173–227 °C(late calcite stage).The measured salinity of fluid inclusions ranged from 0.18%to 16.34%NaCleqv(skarn stage),0.35%–7.17%NaCleqv(quartz–scheelite stage),and 0.35%–2.24%NaCleqv(late calcite vein stage).Laser Raman spectroscopic studies on fluid inclusions in the three stages showed H2O as the main component,with N2present in minor amounts.Minor amounts of CH4were found in the quartz–scheelite stage.It was observed that the homogenization temperature gradually reduced from the early to the late mineralization stages;moreover,δ13CPDBvalues for ore-bearing skarn in the mineralization period ranged from-5.7‰ to-6.9‰and the corresponding δ18OSMOWvalues ranged from 5.8‰to 9.1‰,implying that the ore-forming fluid was mainly sourced from magmatic water with a minor amount of meteoric water.Collectively,the evidence indicates that the formation of the Nanyangtian deposit is related to Laojunshan granitic magmatism.

Keywords Fluid inclusion ·C–O isotopes·Skarn ·Scheelite deposit·Nanyangtian

1 Introduction

The Laojunshan W–Sn metallogenic district is an important mineral resource base in Yunnan Province.Various W–Sn polymetallic deposits surround the Laojunshan granitic pluton:the Nanyangtian-Chayeshan W metallogenic belt in the east,the Dazhushan-Xinzhai Sn–W metallogenic belt in the north,and the Manjiazhai-Tongjie Zn–Sn–In metallogenic belt in the southwest.Ten years ago,a large Nanyangtian scheelite deposit containing high-grade ore with considerable prospecting potential was discovered;it has since been the subject of several studies.Previous research has focused on the geological setting,geochemistry,and fluid inclusions of minerals in this deposit and has produced a detailed chronology of events(Zeng et al.1999;Shi et al.2011;Wang et al.2012;Feng et al.2011a,b),thereby paving the way for further prospecting.

Previous work in the field of fluid inclusions includes:Feng et al.(2011a,b)work on quartz and garnet in the Nanyangtian scheelite deposit;Li(2013)research of fluid inclusions in quartz,garnet,and calcite in the Yanggouhe deposit;and Zhang et al.’s(2014)study of inclusions in skarn mineral and quartz in the Guanfang scheelite deposit using a thermometer and laser Raman spectrometer.Previous research on the source of ore-forming fluids and ore genesis by fluid inclusion analysis has not been comprehensive.As gangue minerals and ore minerals have different characteristics,the condition of ore minerals cannot be gleaned by analyzing gangue(Campbell and Robinson-Cock 1987;Campbell and Panter 1990;Giamello et al.1992).All three publications mentioned above focused on the characteristics of fluid inclusions in gangue minerals rather than in ore;by analyzing fluid inclusions of ore minerals,the current work aims to provide more relevant information on the properties of metallogenic fluids.

Following a description of the geological setting and lithology of the Nanyangtian scheelite deposit,we discuss skarn mineralogical characteristics and the associated evolutionary process.Next,we report data from fluid inclusions in gangue and ore minerals,focusing particularly on the analysis of scheelite fluid inclusions.Finally,information about the ore-forming fluid and a logical explanation of the genesis of the Nanyangtian skarn scheelite deposit are provided.

1.1 Regional geology

Laojunshan is located at the intersection of the peri-Pacific and Tethyan Himalayan tectonic syntaxes and at the junction of the South China fold system,Ailaoshan fold system,North Vietnam Block,and Yangtze Plate(Fig.1).The study area is in the southeastern Yunnan–western Guixi metallogenic belt,which lies to the west of the Nanling W–Sn polymetalloic metallogenic belt.Laojunshan therefore experiences frequent geologic activity,contains many active faults,and has excellent metallogenic conditions.

Because of a lack of upper Ordovician,Silurian,Jurassic,and Cretaceous strata,other strata are fully exposed in the area.According to lithology,the Laojunshan rock body can be divided into seven intrusive bodies:the Tuantian medium-to fine-grained granitic,Laochengpo fine-grained granitic,Huashitou two-mica adamellite,Malutang medium-to fine-grained two-mica adamellite,Taipingtou medium-to fine-grained two-mica adamellite,Qingjiao micrograin two-mica adamellite,and Kouha granite-porphyry intrusions.The Cambrian strata are exposed in the external contact zone of the Laojunshan rock body,which is mainly composed of schist,plagioclasite hornblende schist,amphibolite,and carbonate.Undermagmatic hydrothermalism,carbonate altered to stratiform skarn,in which the scheelite ore body occurs.

The Cambrian strata comprise the Chongzhuang Formation in the Lower Cambrian seriesand the Longha()and Tianpeng formationsin the Middle Cambrian series.The Chongzhuang Formation exists mainly in the Nanyangtian mining area and can be classif i ed into three lithologic members:the lower Geling section(),comprising granite gneiss and plagioclase gneiss;the middle Nanyangtian Sectioncomprising verdelite biotite granulite,biotite plagioclase gneiss,two-mica schist,and tremolite-zoisite-diopside skarn;and the upper Taiyangping section(),comprising muscovite monzonitic gneiss.The Longha Formation is exposed mainly in Wenshan and Maguan as a hoary-white medium-embedded dolomite marble with a middle layer containing a small amount of phyllite and a bottom layer containing muscovite schist.The Tianpeng Formation occurs exclusively in the Dulong mining area.Its bottom layer is gneiss;its middle layer contains ore-bearing rocks mainly comprising actinolite,diopside,and chlorited skarn;and the rocks in its upper stratum are dolomitic phyllite,mica quartz schist,and interbedded marble(Jia 2010).

The Wenshan-Malipo Fault lies to the north of the Laojunshan granite body,the Maguan-Dulong Fault lies to the south,and the Nanwenhe Fault is an almost east–west direction.The secondary fault of Laojunshan provides conditions suitable for W–Sn ore formation.Regional metamorphism is strong,owing to multistage tectonism.A low-temperature regional dynamo metamorphism of granulite and schist occurred during the Caledonian-Indosinian.In addition,contact metamorphism occurred with the intrusion of Laojunshan granite during the Yanshanian,turning many early-deposited carbonate rocks into impure marble.

1.2 Geology of Nanyangtian scheelite deposit

In the mining area,the exposed strata are of the Lower Cambrian Chongzhuan Formation,outlined briefly above:comprises muscovite-biotite plagioclase gneiss,muscovite-biotite schist,and tourmaline quartz schist with skarn lentils.Some parts of this section include mere granulite,and their thickness increases from the center margin.Эconsists mainly of ore-hosting strata.Its top is composed of thin biotite plagioclase gneiss and tourmaline feldspar quartzite;the middle of tourmaline biotite plagioclase gneiss,muscovite quartz schist,and phacoidal skarn;and the bottom of muscovite quartz schist,muscovite quartz schist with epidote tremolite skarn,and actinolite diopside skarn.consists of granite gneiss and plagioclase gneiss with skarn lenticules,where skarn includes nonuniform and unstable dot and vein scheelite.

The main fault in the mining area is the Nanwenhe nappe structure,outcropping in the Nanyangtian District for approximately 10 km.The nappe structure is separated by the nappe fault in an imbricate arrangement consistent with direction of motion(from southeast to northwest).It is thought that the nappe fault may have been formed by the decoupling detachment of the Nanwenhe metamorphic core complex.The Nanyangtian sliding fracture is the highest sliding fracture within the nappe structure and is exposed to the east of the mining area near Nanyangtian.The southwest section of the fault is intruded by Dulong super-unit granite;the upper wall of the fault relates to the Laochengpo intrusion and contains Nanwenhe sequence gneissic fine-grained granite.The major fracture surface has strong mylonitization,and a quartz vein is sheared into a ‘‘Z’’fold and a rootless fold by the fault.The hanging wall mainly consists of schist within the Chongzhuang Formation,and the cleavage tendency is southeast.The schist and gneiss have a small asymmetric fold plunging toward 200°–220°at an angle of 8°–15°.

The Yanshanian Laojunshan granitic pluton outcrops to the east of the mining area.Local igneous activity exhibits multiphase and multistage characteristics.Evolutionary periods(from ancient times to the current day)have resulted in three rock types:medium-coarse monzogranite,medium- fine monzogranite,and granite porphyry(the spread of which is reduced from the first rock body to the third,with the late-stage rock body intruding into the early rock body).The main part of the outcropped bedrock in the mining area is gneissic fine-grained granite porphyry with a mineral composition of 40%plagioclase,40%quartz,and 10%biotite.The gneiss composition tends to be uniform and shows strong metasomatism.The rock has a metasomatic,relict,and perthitic texture.Although scheelite is not found in the granite,the W content is high.

All three stages of granites have relatively high Si(13.9%–70.4%),high A/NCK(1.11%to 1.26%),low Na2O(＜ 4.0 wt%),and high K2O(4.4 wt%–6.8 wt%).In ad‘dition,they are enriched in P and depleted in Zr,Th,Y,and heavy rare earth elements,suggesting that Laojuanshan granites are S-type(Chappell 1999;Clemens 2003;Collins and Richards 2008;Xu 2015).The Nanyangtian phlogopite Ar–Ar average age is 118 Ma,consistent with the age of Laojunshan first-stage granites(Tan et al.2011).From the above,tungsten mineralization and the first-stage Yanshanian granites are closely related(Liu et al.2011).

The ore-bearing layer in the Nanyangtian mining area mainly occurs in the Nanyangtian section within the Lower Cambrian Chongzhuan Formation,where it lies within the upper,middle,and bottom strata.The ore body can be divided into skarn layers,a schist and gneiss layer,a quartz-tourmaline rock layer,and a schist and gneiss layer.Skarn-like rocks are embedded within granite gneiss from the top to the bottom of the strata.The ore body occurs in the bottom of the skarn-like layer and in the upper tourmaline-quartzite and gneiss layers(Figs.2 and 3).

Fig.2 mining area geological map(a)cross section A–B of Nanyangtian scheelite deposit(b)(after Zijin Tungsten Industry Group Co.Ltd,2014)

According to the gallery sketch presented in Fig.4,the following conclusions can be drawn.First,the shape of the ore body is generally stratiform or stratiform-like.The ore body is limited by the skarn,and the boundary is represented by spotted dissemination within the skarn.The quartz vein contains symbiotic vein-like scheelite and is very highly mineralized;W stems from the hydrothermal fluid.Second,in relation to the wall rock alteration characteristics,skarnization and siliconization are closely related to the mineralization of the surrounding rock,and the degree of mineralization is higher at locations where these processes have been more active.Third,according to the paragenetic association of minerals,the ore body layer is closely related to the genesis of skarn minerals and the vein ore body to the genesis of the quartz vein.

According to the interrelation of veins,ore texture and structure,and mineral assemblage,the mineralization stages can be divided into(1)the skarn stage,(2)the quartz–scheelite stage,and(3)the calcite stage.The skarn stage is characterized by garnet,diopside,tremolite,andepidote.The quartz–scheelite stage is characterized by quartz and scheelite.The calcite stage is characterized by calcite and sulfides,such as pyrite,chalcopyrite,and pyrrhotite(Fig.5).

Fig.3 The orebody distribution diagram of Nanyangtian scheelite deposit(after No 317 Geological Team of Yunnan,1984)

Skarn minerals observed in the Nanyangtian deposit include diopside,tremolite,zoisite,and garnet.Pyroxene was observed to be the main mineral in the Nanyangtian deposit skarn.Two types of pyroxene were observed in hand specimen:garnet at an early stage replaced by granular pyroxene,and pyroxene in a granular or stumpy form within skarn.Under a microscope,columnar and irregular pyroxene were observed in contact with garnet and hornblende(Fig.6b,e).In addition,some pyroxene was noted to host fine-grained quartz and plagioclase,which impart a metasomatic relict texture.Rhombic dodecahedron or octahedron garnet crystals,rufous in color,were widespread.Garnet had experienced serious alteration,and scheelite was found between garnet crystals.Herein,zonation of garnet was observed using a microscope.Epidote is the most important hydrous skarn mineral observed in the deposit and formed by garnet alteration in the late skarn stage.Epidote was mainly seen as an idiomorphic granular aggregate,colorful under the microscope,and occasionally replaced with pyroxene (Fig.6c, f).Hornblende is another important hydrous skarn mineral found in the deposit(Fig.6a,d)and was observed mainly as a radial or fibrous aggregate replacing pyroxene.

Fig.4 Sketch ore body and wall rock contact relations in 1158 gallery 150 m

Fig.5 Mineral-forming sequence in the NanYangtian deposit

2 Sampling and analytical methods

Eight samples were collected from tunnels 25,27,and 1158 in the Nanyangtian deposit.The samples collected contain diopside skarn,tremolite skarn,dioside-tremolite skarn,zoisite-diopside skarn,and garnet skarn.Electron microprobe analysis(EMPA)was performed for skarn minerals at Chang’an University using a JXA-8100 with an acceleration voltage of 15 kV and a beam current of 10 nA.Samples were also collected from tunnels 27 and 1158 in the Nanyangtian ore district to analyze fluid inclusions.Fluid-inclusion analysis was conducted on samples containing garnet,epidote,quartz,and scheelite in the skarn stage;quartz and scheelite in the quartz–scheelite stage;and calcite in the calcite stage.The procedure involved polishing the samples into thin sections prior to observation with a microscope in order to determine differentiating forms,types,and distributions of fluid inclusions.Fluid-inclusion analysis was conducted using a Linkam THMSG-600 heating-freezing apparatus and a Renishaw inViaTMlaser Raman spectrometer installed on a Leica DM2500 M polarizing microscope at the Fluid Inclusion Laboratory of Kunming University of Science and Technology,Yunnan Province.A Spectra-Physics argon-ion laser was used to excite the laser Raman spectra with a 514.5-nm laser line at an incident power of 20 mW,spatial resolution of 1–2 μm,and an integral time of 60 s.The sample stage was connected to a thermal control unit to maintain the temperature in the range of 196–600 °C,and the measurements were considered accurate to± 0.2°C for ice-melting and CO2homogenization temperatures,± 0.5 °C for clathrate melting temperatures,± 1.5 °C for eutectic temperatures,and± 5°C for homogenization temperatures(Li et al.2016a,b).

Five calcite samples were collected from the calcite vein of the Nanyangtian ore district for C–O isotope research.First,calcite samples were crushed and selected using a microscope.They were then powdered to a 200-mesh using an agate mill.C–O isotope analysis was then conducted using a Delta Plus XP continuous- flow isotope ratio mass spectrometer(CF-IRMS)at the Canada ALS mineral Laboratory(analytical precision was generally higher than 0.2‰).

3 Analysis results

3.1 Mineral chemistry analysis

3.1.1 Pyroxenes

Data on typical pyroxenes obtained from EMPA are shown in Table 1.The main components of pyroxene are diopside(55%–82%)and hedenbergite(18%–41%),with a small amount of johannsenite(0.3%–6%)(Fig.7).As shown in Fig.8, field-determined pyroxene components in the Nanyangtian deposit are typical of skarn deposits found throughout the world.

3.1.2 Garnet

Fig.6 Microphotograph of skarn minerals in the NanYangtian deposit;a,d tremolites of early stage replaced by quartz of late stage;b,e diopsides and tremolites dis-tributed in skarn;c,f scheelites is surrounded by diopsides and zoisites.Qz quartz,Tr tremolite,Di diopside,Zo zoisite,Sch scheelite

The EMPA data obtained for typical garnet are shown in Table 2.In general,the end-member composition of garnet varies,providing important information about the formation environment(Ai and Jin 1981;Zhao et al.1983;Meinert et al.2005).As shown in Table 2,the endmember of Nanyangtian garnet samples was mainly grossular(Gro;82.84%–88.51%)with some andradite(And;3.61%–5.26%)and grossular(Alm+Spe+Pyr)(Fig.8 and Fig.9a),while the endmember of aluminum garnet samples was mainly almandine with small amounts of pyrope.

3.1.3 Epidote

The EMPA data obtained for typical epidote are shown in Table 3.The contents are as follows:SiO2ranges from 37.43%to 39.66%with an average of 38.78%,CaO from 22.71%to 23.15%with an average of 22.92%,Al2O3from 25.53%to 29.84%with an average of 28.40%,and FeOTfrom 4.31%to 8.64%with an average of 5.62%.Overall,epidote from the Nanyangtian deposit was Fe-poor with alumina-rich minerals.Figure 9b shows epidote mainly composed of clinozoisite with a small amount of piedmontite.

3.1.4 Hornblende

The EMPA data obtained for typical hornblende are shown in Table 4.The Al2O3content ranges from 0.90%to 5.22%,CaO from 11.82%to 12.40%,FeOTfrom 8.68%to 15.41%,MgO from 12.96%to 17.62%,and Na2O from 0.11%to 0.78%.According to the standard classification(Leake et al.1997),the hornblende within the Nanyangtian deposit plot determined in the field is of the tremoliteactinolite type(Fig.10).

3.2 Fluid inclusion

3.2.1 Characteristics and microscopic temperature of fluid inclusions

Nanyangtian scheelite deposits can be mainly divided into three metallogenic stages:the skarn,quartz–scheelite,and calcite stages.Most Nangyangtian fluid inclusions are liquid-rich two-phase aqueous solutions(Lu et al.2004).In this study,a homogenization temperature was observed when fluid inclusions were uniformly heated to a liquid phase,and a freezing-point temperature was observed when the temperature was lowered.Salinity was calculated using WNaCl=0.00+1.78Tm-+0.000557-(Bodnar 1993),whereWdenotes the percentage composition of NaCl andTmdenotes the freezing-point temperature in Fahrenheit.Results are shown in Table 5.

3.2.1.1 Skarn stageFluid inclusions of garnet,epidote,quartz,and scheelite in skarn were mainly two-phase aqueous inclusions;these were mainly observed and analyzed using temperature measurements(Fig.11a–c).At room temperature(25°C),the primary,secondary,and pseudo secondary inclusions were observed to be elliptical,long,or irregular.Secondary inclusions and pseudo secondary inclusions were mainly observed as negatives orround crystals with a linear distribution.Two-phase aqueous inclusions were mainly colorless,with liquid/vapor ratios approximately 75%and lengths of 3–18 μm.The homogenization temperature of the inclusions in this stage ranged from 221 to 423°C,with homogenization temperatures of garnet and epidote higher than those of quartz and scheelite.Salinity ranged from 0.18%to 16.34%(Fig.12).

Table 1 EPMA composition of pyroxene from Nanyangtian scheelite deposit

Fig.7 Composition of pyroxene(a)and composition of pyroxene(b).Jo johannsenite,Hd hedenbergite,Di diopside,Wo wollaetonite,En magnesium enstenite,Fs iron enstenite,Hd hedenbergite,Pi pigeonite,ClEn Clinoenstatite,ClFs clinoferrosilite,Au augite

3.2.1.2 Quartz–scheelitestageThe fluid inclusions observed in quartz included gas-and liquid-rich inclusions,with lengths of 5–30 μm and liquid/vapor ratios of approximately 75%(Fig.11e).Most liquid-rich inclusions measured 10–18 μm,with very few up to 30 μm.Most two-phase inclusions were elliptical,long,or irregular.In addition,the fluid inclusions in scheelite were mainly two phase,and mostly elliptical or irregular in shape,with length 3–14 μm(Fig.12d)and liquid/vapor ratio greater than 85%.One-phase liquid inclusions were occasionally observed,usually in a clumped distribution and smaller than the two-phase inclusions.The homogenization temperatures of the inclusions in this stage ranged from 177 to 260°C,with homogenization temperature of quartz higher than that of scheelite.Salinity ranged from 0.35%to 7.17%(Fig.12).

Table 2 EPMA composition of garnet from Nanyangtian scheelite deposit

Fig.8 Ternary plots of garnet and pyroxene compositions from major large skarn deposits(after Meinert 1992).And andradite,Alm almandine,Spe spessartite,Gro grossular,Di diopside,Hd hedenbergite,Jo johannsenite

3.2.1.3 Quartz–scheelite stageFluid inclusions in calcite were mainly observed as two-phase,round,sporadically distributed aqueous inclusions,with liquid/vapor ratios greaterthan 90%,length 3–6 μm (Fig.11f),homogenization temperatures of 173–227 °C,and salinity from 0.35%to 2.24%(Fig.12).

Owing to the later metamorphism,the fluid inclusions of garnet in the early skarn stage are secondary inclusions;however,as they were destroyed,they could not contribute to the results of this study.In contrast,the fluid inclusions of epidote in the regressive alteration skarn stage were not destroyed and are composed of H2O.In the quartz–scheelite stage, fluid inclusions generally displayed good integrity and a linear and clumped distribution.In general,the fluid inclusions in quartz were bigger than those in other stages.However,there was a lack of primary inclusions observed in scheelite.In addition, fluid inclusions in the calcite stage were less than 3 μm in length,making it difficult to obtain temperature readings.

3.2.2 Composition of fluid inclusions using laser Raman spectroscopy

In the skarn stage,the gas and liquid phases of the fluid inclusions in quartz and in epidote were composed of H2O(Fig.13g,h).In the quartz–scheelite stage;the composition of the gas and liquid phases of the representative fluidinclusions of the first type of quartz was H2O,N2,and CO2(Fig.13a,b);of the second type,H2O and N2(Fig.13c,d).The composition of the fluid inclusions in scheelite was H2O,N2,and CH4(Fig.13e,f)and that of calcite,H2O.

Table 3 EPMA composition of epidote from Nanyangtian scheelite deposit

Table 4 EPMA composition of hornblende from Nanyangtian scheelite deposit

Fig.9 a End members of garnets,b composition of epidote from the Nanyangtian scheelite deposit(Gro grossular,And andradite,Spe spessartite,Alm almandine,pyr pyrope)

3.3 C and O isotopic composition

The C isotopic compositions of the calcite samples in the Nanyangtian deposits have a modest distribution(δ13CPDBwas between-5.7‰,and-6.9‰,with an average value of-6.3‰).However,the oxygen-isotope values of the calcite samples have a larger distribution(δ18OSMOWwas between 5.8‰,and 9.1‰,with an average value of 7.45‰)(Table 6).

Fig.10 Classification of amphiboles in the Nanyangtian deposit(base map after Leake et al.1997)

4 Discussion

4.1 Skarn type and formation environment

Skarn often forms within the metasomatic zone between intermediate intrusive rock and carbonate rock or between Ca/Mg enriched clastic rocks.It is generally believed that skarn is closely related to large amounts of metal accumulation(Zhao et al.1990;Cheng and Zhao 1994);determining skarn type is significant because has implications for ore-bearing rock.

Table 5 Microthermometric data for fluid inclusions in the Nanyangtian tungsten deposit

Fig.11 Fluid inclusions in ore and gangue minerals of different altered stages of Nanyangtian scheelite deposit;a primary aqueous inclusion in the garnet from skarn;b primary aqueous inclusion in the epidosite from skarn;c primary aqueous inclusion in the quartz from quartz–scheelite vein;d primary aqueous inclusion in the scheelite from quartz–scheelite vein;e primary aqueous inclusion in the quartz from quartz–scheelite vein;f primary aqueous inclusion in the calcite from calcite vein

Based on mineral composition,skarn is categorized as either magnesium or calcium skarn(Einaudi et al.1981).Magnesium skarn mainly consists of forsterite,diopside,spinel,phlogopite,and tremolite minerals,and the surrounding rock is dolomite.Calcium skarn mainly consists of andradite-grossularite,diopside-hedenbergite,wollastonite,scapolite,epidote,idocrase,and hornblende,and the surrounding rock is limestone.The mineral assemblage of the Nanyangtian deposit is mainly diopside+hedenbergite+grossularite+hornblende+epidote,indicating a primarily calcareous skarn.

During the process of skarn formation,the oxidation–deoxidation regime and the pH of the ore-forming fluid are very important for mineralization(Chen et al.2012);moreover,the mineral composition of skarn provides important information about the ore-formation setting(Einaudi et al.1981;Ahmed and Hariri 2006).Furthermore,the endmembers of garnet and pyroxene indicate the environment of skarn formation.Kwak(1994)and Lu et al.(2003)suggest that hessonite and hedenbergite usually form in a deoxidizing environment.The results of this study show that the end member of Nanyangtian garnet is mainly calcium–aluminum garnet and that the components of pyroxene are mainly diopside and hedenbergite.These results indicate that the early skarn stage occurred in a reducing environment.

Fig.12 Histogram of homogenization temperature and salinity in fluid inclusions from the Nanyangtian scheelite deposit

4.2 Nature and evolution of ore-forming fluids

Certain fluids can dissolve metallic elements;moreover,it is widely believed that they are effective carriers and transportation media for ore-forming elements(Li 2013).Scheelite in the Nanyangtian deposit spans skarn,quartz–scheelite,and calcite stages.In general,scheelite and Mo minerals are associated with each other,and the phenomena of rich W and deficient Mo mineralization in the Nanyangtian scheelite deposit suggest an ore-forming fluid with low oxygen fugacity(Candela and Bouton 1990).Under such conditions,a greater amount of W is transferred to the fluid phase during the magmatic crystallization process.Two distinct fluids were found in quartz inclusions of the Nanyangtian scheelite deposit:one containing CO2,N2,and H2O(Fig.13a,b)and the other only N2and H2O(Fig.13c,d)—an inhomogeneous fluid distribution.However,the low salinity of the fluid inclusions suggests a weak association between migration of scheelite in the Nanyangtian deposit and chlorine and fluorine content.The fluid inclusions within scheelite contained N2,H2O,and a certain amount of CH4(Fig.13e,f).Although scheelite is a hypothermal ore-forming element,its actual ore-forming temperature is not high in some deposits,which is the main reason for tungstic acid migration in the ore-forming hydrothermal solution(Cattalani and Williams-Jones 1991;Wood and Samson 2000;Liu et al.2008).

Thermometry data show that cooling occurred between the skarn and calcite stages,indicating a wide ore-forming temperature range for scheelite,similar to the thermal evolutionary history of other skarn scheelite deposits such as the CanTung and Kara scheelite ores(Singoyi and Zaw 2001).As shown in the homogenization temperature and salinity correlogram(Fig.14),there is no obvious correlation between homogenization temperature and salinity.When a fluid experiences cooling,it may also experience mixing.The salinity of inclusions captured during different ore-forming periods within the Nanyangtian scheelite deposit ranged from 0%to 10%NaCleqv,consistent with the measurements of fluid inclusions in the Gannan scheelite ore(less than 10%Nacleqv)(Wang et al.2012).The coexistence of vapor-rich and liquid inclusions indicates that fluid boiling was not a key factor in the ore-forming processes(Wu et al.2014);therefore,researchers have concluded that scheelite ore formation partially depends on the wall rock type instead of on high Cl/H2O content(Candela and Bouton 1990).However,the low salinity of the fluid inclusions in the ore and gangue minerals in the Nanyangtian scheelite deposit proves that the metasomatic reaction between the scheelitic fluid and carbonatite was the mechanism for the formation of the scheelite deposit.

Fig.13 Laser Raman spectra of fluid inclusions in the Nanyangtian scheelite deposit.a,b.Vapor and liquid phase composition of quartz inclusion.c,d Vapor and liquid phase composition of quartz inclusion.e,f Vapor and liquid phase composition of scheelite inclusion.g,h Vapor and liquid phase composition of epidosite inclusion

Table 6 C–O isotopes of carbonates from Nanyangtian scheelite deposit

Fig.14 Plot of homogenization temperatures versus salinities of fluid inclusion in Nanyagtian scheelite deposit

Fig.15 δ18O–δ13C diagram(after Ray et al.2000)

5 Source of ore-forming fluids

Fig.16 δD versus δ18OH2Odiagram of the Nanyangtian scheelite deposit(after Sheppard 1986)

C–O isotopic systems have been widely used for tracing the source and evolution of ore-forming fluids(Zhou et al.2013a,b).In general,carbon in hydrothermal fluids can be derived from the mantle,marine carbonate rocks,or sedimentary organic matter(Taylor et al.1967;Veizer and Hoefs 1976;Demény and Harangi 1996;Liu and Liu 1997;Demény et al.1998;Zhou et al.2016).The mantle,marine carbonates,and organic matter have δ13CPDBvalues of-4.0‰ to-8.0‰ (Taylor et al.1967),-4.0 to+4.0‰(Veizer and Hoefs 1976),and-30.0‰ to-10.0‰ (Liu and Liu 1997),respectively and δ18OSMOWvalues of+6.0‰ to+10.0‰ (Taylor et al.1967),+20.0‰ to+30.0‰ (Veizer and Hoefs 1976),and+24.0‰ to+30.0‰ (Liu and Liu 1997),respectively.The δ13CPDBvalues of calcite from Nanyangtian,compared with main natural carbon reservoirs,are consistent with those of mantle.In the δ13CPDBversus18OSMOWdiagram(Fig.15),calcite samples from Nanyangtian fall into the area of igneous calcite.Meanwhile,the δ13CPDBvalues of calcite separates are also in accordance with those of magmatic source(from–9‰to 3‰,Li et al.2016a,b),indicating an isotope exchange between granitic fluid and country-rock carbonate during the ore-forming processes.

As the main component of ore-forming fluid,water is one of the most researched topics in ore genesis(Wei et al.2011).H and O isotopes have historically been used to trace the origin of ore-forming fluid of hydrothermal deposits(Deines 1980;Hoefs 1997;Pirajno 2009).Based on the measurement of H and O isotope values in garnet,calcite,and quartz,Feng et al.(2011a,b)suggested that the water in ore formation is mainly magmatic,with a small amount of meteoric water,based on δD versus δ18OH2Odiagrams(Fig.16).In conclusion,C–O isotope features indicate that the calcite in ore-forming fluid is mainly derived from granitic magmatic water with a small amount of meteoric water,and H–O isotope features indicate that the water in ore formation mainly comes from magmatic water with a small amount of meteoric water.

5.1 Ore genesis and formation process

Previous studies have drawn different conclusions about the metallogenic mechanism.For example,Zeng et al.(1999)and Zhou et al.(1998)researched different types of scheelite rare earth elements and skarn and considered that the existence of scheelite in the Nanyangtian deposit to be related to metamorphic genesis.Feng et al.(2010,2011)conducted comparative studies and determined that scheelite mineralization could be connected with magmatic movement during the Indo-Chinese epoch and that the formation of scheelite ore could be the result of a magmatic hydrothermal fluid reaction during the Indo-Chinese epoch.The Xianglushan scheelite deposit is representative of a magmatic hydrothermal-type skarn deposit.As the Nanyangtian scheelite deposit is of the same metallogenetic period and stage as the Xianglushan,this study compares their fluid geochemistry.The homogenization temperatures of the inclusions in the Xianglushan deposit at various stages are as follows.In the skarn stage:209–383 °C,with a salinity of 0.35%–5.26%NaCleqv;in the quartz–scheelite stage:163–278 °C,with a low salinity of 0.35%–5.86% NaCleqv; in the calcite stage:143–235 °C,with a low salinity of 0.35%–2.07%NaCleqv(Wu et al.2014).A comparison of the Xianglushan and Nanyangtian deposit inclusions shows similar homogenization temperatures and salinities;the Nanyangtian scheelite deposit has characteristics similar to those of a magmatic hydrothermal-type skarn deposit.The results of the field work and experiments conducted in this study show that scheelite mineralization occurred in the Laojunshan granite body and that it underwent multiphase ore formation.Previous studies have discussed the genesis of the Laojunshan granite body and concluded that it was formed by partial fusion of the lower crust rock,where hot mantle material upwelling was triggered during the Late Yanshanian(Liu et al.2014).This conclusion is in accordance with the evidence provided by the present study on C and O isotopes and ore-forming fluids.The skarn W–Sn deposit is generally considered to be closely related to granites.Granitic magma is mainly related to a molten continental crust,which occurs during an intraplate rift(Richards 2011)when the continental crust is thicker owing to an earlier collision.It then experiences an extensional setting,during which the underplating of mantle-derived magma and direct heating of the asthenosphere crust cause the overlying lower crust materials to partially melt and form Nanyangtian primary magma(Hildreth 1981).In general,continental crust provides all or most of the metal,and the W–Sn deposit is related to S-type granite(Hedenquist and Lowenstern 1994;Mao et al.2011).As shown in Fig.8,garnet components in the Nanyangtian deposit are not in the range of typical skarn deposits found throughout the world.This indicates that garnet having high aluminum and low iron contents formed through a metasomatic reaction between minerals and S-type granitic magmatic hydrothermal fluids.

The nappe structure not only causes spatial super-positioning of the granite and the granulite but also causes shear sliding of the rock strata,which forms an interlayer fracture zone.The fracture zone is not only a migration pathway for ore-forming fluids but also provides an important host space.Through this migration channel,the acidic fluid migrates to the carbonate formations that formed during the early stage when metasomatism was completed(Bi et al.2015).

The conclusions that follow are based on analyses of the material composition during the mineralization stage and the fluid properties.Mantle magma upwelled at an early stage and was stored at the bottom of the crust.The mantle heat source led to large-scale metamorphism and anatexis in the lower crust,which then formed a mass of granitic magma that crystallized and cooled as a result of the exsolution of Fe–Al-rich fluid.Garnet skarn and diopside skarn were formed by the metasomatism of a fluid with a temperature higher than that of the surrounding rock,consequently releasing a tungstate fluid.The temperature of this fluid was subsequently lowered during a reaction with the country rock,which caused scheelite to subside and mineralize.As the rock mass cooled,the released fluid partially transformed into Si–W–H2O and continued to over flow.Vein-like faults formed between the skarn and the wall rock in relation to pressure;moreover,a vein ore body formed,which is why the vein-like quartz–scheelite ore overlies the skarn.Late stage Ca-rich fluid migrated along large faults and in- filled to form calcite veins.

6 Conclusion

Based on previous research,we analyzed fluid inclusions and measured C–O isotopes to determine the genesis of metal ores,the source of ore-forming fluids,and the metallogenic mechanism involved in the formation of skarn minerals within the Nanyangtian scheelite deposit.The following conclusions are drawn:

(1) Early-stage skarn was formed in a reducing environment.

(2) Early-to late-stage mineralization occurred over a wide temperature range.

(3) The metasomatic reaction between the scheelitic fluid and carbonatite was the effective mechanism underlying the formation of the scheelite deposit.

(4) Based on fluid inclusion analysis and the results of stable isotope tracing,ore-forming fluids of the Nanyangtian scheelite deposit are mainly related to a magmatic source and contain both stratigraphic water and atmospheric precipitation.

(5) The Nanyangtian deposit evolved through the Caledonian and Indo-Chinese but was formed by Laojunshan granite through magmatic contact and metasomatic action during the Yanshanian period.

AcknowledgementsThis study was supported by the Natural Science Foundation of China(No.41373050).The authors are extremely grateful for the support and help provided by staff of the Nanyangtian ore deposit.