LA-ICP-MS UTh-Pb Dating and Trace Element Geochemistry of Allanite : Implications on the Different Skarn Metallogenesis between the Giant Beiya Au and Machangqing Cu-Mo-( Au ) Deposits in Yunnan , SW China

The giant Beiya Au skarn deposit and Machangqing porphyry Cu-Mo-(Au) deposit are located in the middle part of the Jinshajiang–Ailaoshan alkaline porphyry metallogenic belt. The Beiya deposit is the largest Au skarn deposit in China, whilst the Machangqing deposit comprises a well-developed porphyry-skarn-epithermal Cu-Mo-(Au) mineral system. In this paper, we present new allanite U-Th-Pb ages and trace element geochemical data from the two deposits and discuss their respective skarn metallogenesis. Based on the mineral assemblage, texture and Th/U ratio, the allanite from the Beiya and Machangqing deposits are likely hydrothermal rather than magmatic. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) allanite U-Th-Pb dating has yielded Th-Pb isochron ages of 33.4 ± 4.6 Ma (MSWD = 0.22) (Beiya) and 35.4 ± 9.8 Ma (MSWD = 0.26) (Machangqing), representing the retrograde alteration and magnetite skarn mineralization age of the two deposits. The Beiya and Machangqing alkali porphyry-related mineralization are synchronous and genetically linked to the magmatic hydrothermal activities of the Himalayan orogenic event. Major and trace element compositions reveal that the Beiya allanite has higher Fe3+/(Fe3+ + Fe2+) ratios, U content and Th content than the Machangqing allanite, which indicate a higher oxygen fugacity and F content for the ore-forming fluids at Beiya. Such differences in the ore-forming fluids may have contributed to the different metallogenic scales and metal types in the Beiya and Machangqing deposit.


Introduction
Allanite ((Ca,REE,Th) 2 (Fe,Al) 3 Si 3 O 12 (OH)) is a rare earth element (REE)-rich epidote-group mineral, and is commonly found in many magmatic/metamorphic rocks and hydrothermal mineral systems, especially in skarn and IOCG (iron-oxide copper gold).Allanite typically contains high U, Th and REE concentrations, which can be used for age dating and tracing various magmatic, metamorphic and ore-forming processes [1].Allanite is a highly resistate mineral and has high closure temperature, rendering it a useful age dating material [2].Allanite could crystallize in a wider range of metamorphic P-T conditions than zircon and monazite [3], and its common larger size enables the preservation of internal chemical and isotopic zoning [4].Allanite has been increasingly used to date metamorphic events [5][6][7][8][9][10].Compared to igneous and metamorphic petrogenetic research, in-situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) allanite dating and trace element analyses have not been widely used to investigate hydrothermal metallogenesis.In recent years, there are attempts to directly constrain skarn mineralization age using allanite U-Th-Pb dating [11,12], and to use allanite as tracer of the multiple hydrothermal fluid phases [13].Although allanite commonly contains higher initial common Pb than zircon (which makes it harder to yield a concordant age) [4,14], allanite U-Th-Pb can still yield geological meaningful ages given that appropriate common Pb correction methods are adopted [4,[14][15][16][17][18].
The giant Beiya Au-polymetallic skarn deposit is situated in the middle part of the Jinshajiang-Ailaoshan alkaline porphyry metallogenic belt.The deposit is the largest Au skarn deposit in China, with estimated ore reserves of 130.8 million tonnes (Mt) Au @ 2.47 g/t, 170 Mt Fe @ 33.3 wt %, 125 Mt Cu @ 0.52 wt %, along with considerable amounts of Pb, Zn and Ag [19,20].The Jinshajiang-Ailaoshan alkaline porphyry metallogenic belt is an important Cu-Au province in SW China, and contains a large number of deposits that are spacetime associated with the Cenozoic (Himalayan) alkaline magmatism [21][22][23][24].Ore deposit geology, ore fluid composition and metallogenic studies of the Beiya Au mineralization all suggest that the Beiya deposit resembles typical porphyry-related Au skarn deposits worldwide [25][26][27][28][29].
The Machangqing porphyry Cu-Mo-(Au) deposit near Beiya is also located in the middle part of the Jinshajiang-Ailaoshan metallogenic belt, and the mineral system also contains porphyry-related skarn and epithermal Cu-(Au) mineralization [23,[30][31][32].The two deposits likely share common post-collisional metallogenic settings and alkali porphyry-related mineralization style, but they differ greatly in terms of the ore deposit size and types of metals.Previous age dating studies have focused on the Machangqing intrusive complex and the molybdenite of the porphyry mineralization [23,[32][33][34][35][36], whereas age constraints on the skarn mineralization are lacking.
Direct and precise dating of the skarn mineralization is pivotal to reveal the hydrothermal ore-forming processes of the Beiya and Machangqing deposits, to establish any genetic link with the regional alkaline magmatism, and to correlate the skarn mineralization in various parts of the Jinshajiang-Ailaoshan metallogenic belt [37].In this study, we present new U-Th-Pb age and trace element data on the hydrothermal allanite from the Beiya and Machangqing deposits.Our new data are integrated with published data to discuss the ore-forming environment and magmatic-hydrothermal evolution of these two porphyry-related mineral systems.

Regional Geology
The Jinshajiang-Ailaoshan alkali porphyry metallogenic belt is located in the Sanjiang Tethyan domain in SW China (Figure 1) [21,38], and is bounded by the South China Block to the east along the Jinshajiang-Ailaoshan suture zone [39,40].Many large porphyry and skarn deposits are associated with this belt, which makes it one of the most important metallogenic belts in China.The Jinshajiang-Ailaoshan alkali porphyry metallogenic belt includes two subbelts: the Yulong porphyry subbelt in the north and the Ailaoshan-Red River porphyry subbelt in the south [28].[41,42].
The Beiya and Machangqing deposits are located in the eastern side of the Jinshajiang suture (Figure 1).Major igneous rocks emplaced in/around the Beiya ore district include the Late Permian Emeishan continental flood basalt and the Cenozoic alkaline porphyries.The Emeishan flood basalt is exposed in the southeastern part of the mining area.The regionally-widespread Cenozoic alkaline porphyries include those of (quartz) syenite, biotite-K-feldspar and quartz-albite [26,28,[43][44][45]], yet they are rarely exposed in the Beiya mining district.Major exposed rocks in the Beiya deposit include the Lower Triassic Qingtianbao Formation (175-350 m thick), the Middle Triassic Beiya Formation and Quaternary sedimentary rocks (Figure 2).The Lower Triassic Qingtianbao Formation comprises arkose, hornfelsed greywacke, sandstone and basaltic volcaniclastic rocks, whereas the main orebearing Middle Triassic Beiya Formation comprises dolomitic, ferruginous, bioclastic and argillaceous limestone (ca.138-531 m thick).The Beiya Formation is deposited in a NS-trending basin upon which a broad NS-trending syncline is developed [28].[41,42].
The Beiya and Machangqing deposits are located in the eastern side of the Jinshajiang suture (Figure 1).Major igneous rocks emplaced in/around the Beiya ore district include the Late Permian Emeishan continental flood basalt and the Cenozoic alkaline porphyries.The Emeishan flood basalt is exposed in the southeastern part of the mining area.The regionally-widespread Cenozoic alkaline porphyries include those of (quartz) syenite, biotite-K-feldspar and quartz-albite [26,28,[43][44][45]], yet they are rarely exposed in the Beiya mining district.Major exposed rocks in the Beiya deposit include the Lower Triassic Qingtianbao Formation (175-350 m thick), the Middle Triassic Beiya Formation and Quaternary sedimentary rocks (Figure 2).The Lower Triassic Qingtianbao Formation comprises arkose, hornfelsed greywacke, sandstone and basaltic volcaniclastic rocks, whereas the main ore-bearing Middle Triassic Beiya Formation comprises dolomitic, ferruginous, bioclastic and argillaceous limestone (ca.138-531 m thick).The Beiya Formation is deposited in a NS-trending basin upon which a broad NS-trending syncline is developed [28].Outcropping rocks in the Machangqing deposit include the Lower Ordovician Xiangyang Formation (O1x), dolomitic limestone of the Lower Devonian Kanglang Formation (D1k) and limestone of the Lower Devonian Qingshan Formation (D1q) (Figure 3).The Xiangyang Formation comprises littoral-facies clastic rocks and contains four members, among which the third (O1x 3 ) and fourth (O1x 4 ) member are exposed in the ore field.O1x 3 comprises grayish-green fine-grained sandstone with dark-gray shale in the upper part, and white fine-grained quartz sandstone with minor shale in the lower part.Unit O1x 4 comprises quartz siltstone with limestone bands/lenses (main ore host) in the upper part, and coarse-grained feldspar-quartz sandstone with conglomerate, gravelbearing gritstone and argillaceous siltstone lenses in the lower part.Permian basalts are also exposed in the northern part of the Machangqing deposit (Figure 3).During the Cenozoic Himalayan orogeny, multiple intrusions were emplaced in the area from 55.5 to 23.2 Ma [32].These intrusive rocks are mainly composed of alkaline intrusions including syenite porphyry, quartz syenite porphyry, monzonite porphyry, granite porphyry, porphyritic granite and lamprophyre [30,35,36].Among these porphyries, the alkaline granite porphyry (37.9-35.0Ma) is considered to be ore-related [34,35,47,48].The Machangqing intrusive complex was emplaced in the Xiangyang Formation at the onset of the mineralization.
Cenozoic tectonic evolution of the Jinshajiang-Ailaoshan suture is generally interpreted to comprise three stages: (1) India-Asia continent-continent collision started at ca. 55-50 Ma [49,50]; (2) northward indentation of the Indian plate caused the transition from "soft" to "hard" collision at 45 Ma, as evidenced by a reduction in the convergence rate (e.g., [51]); and (3) the collisional strain was accommodated by the lateral extrusion (i.e., escape tectonics) of the Indochina-Simao Block from ca. 32 Ma onward, and the Jinshajiang-Ailaoshan suture was reactivated as a regional strike-slip fault [52].The Beiya and the Machangqing deposits were products of this Himalayan collisional tectonic event.
Outcropping rocks in the Machangqing deposit include the Lower Ordovician Xiangyang Formation (O 1 x), dolomitic limestone of the Lower Devonian Kanglang Formation (D 1 k) and limestone of the Lower Devonian Qingshan Formation (D 1 q) (Figure 3).The Xiangyang Formation comprises littoral-facies clastic rocks and contains four members, among which the third (O 1 x 3 ) and fourth (O 1 x 4 ) member are exposed in the ore field.O 1 x 3 comprises grayish-green fine-grained sandstone with dark-gray shale in the upper part, and white fine-grained quartz sandstone with minor shale in the lower part.Unit O 1 x 4 comprises quartz siltstone with limestone bands/lenses (main ore host) in the upper part, and coarse-grained feldspar-quartz sandstone with conglomerate, gravel-bearing gritstone and argillaceous siltstone lenses in the lower part.Permian basalts are also exposed in the northern part of the Machangqing deposit (Figure 3).During the Cenozoic Himalayan orogeny, multiple intrusions were emplaced in the area from 55.5 to 23.2 Ma [32].These intrusive rocks are mainly composed of alkaline intrusions including syenite porphyry, quartz syenite porphyry, monzonite porphyry, granite porphyry, porphyritic granite and lamprophyre [30,35,36].Among these porphyries, the alkaline granite porphyry (37.9-35.0Ma) is considered to be ore-related [34,35,47,48].The Machangqing intrusive complex was emplaced in the Xiangyang Formation at the onset of the mineralization.
Cenozoic tectonic evolution of the Jinshajiang-Ailaoshan suture is generally interpreted to comprise three stages: (1) India-Asia continent-continent collision started at ca. 55-50 Ma [49,50]; (2) northward indentation of the Indian plate caused the transition from "soft" to "hard" collision at 45 Ma, as evidenced by a reduction in the convergence rate (e.g., [51]); and (3) the collisional strain was accommodated by the lateral extrusion (i.e., escape tectonics) of the Indochina-Simao Block from ca. 32 Ma onward, and the Jinshajiang-Ailaoshan suture was reactivated as a regional strike-slip fault [52].The Beiya and the Machangqing deposits were products of this Himalayan collisional tectonic event.

Beiya
The Beiya Au-polymetallic deposit (~90 km north of Dali) is the largest gold skarn deposit in China.The deposit is located in the middle part of the Jinshajiang-Ailaoshan alkaline porphyry metallogenic belt [53] (Figure 1) and contains six ore segments, namely the Weiganpo, Bijiashan and Guogaishan segments in the east and the Wandongshan, Hongnitang and Jingouba segments in the west (Figure 2a).The Wandongshan (mainly the KT52 ore body) ranks the biggest, containing most of the Au resources (99 Mt @ 2.61 g/t Au [25]) and is currently being mined for gold and iron.
The orebodies in the Beiya deposit mostly occur in and/or along the contact zones between the intrusions and the carbonate host rocks of the Beiya Formation.Some stratiform orebodies are locally distributed along the flat interlayer fractures and breccia zones within the Beiya Formation carbonates, as well as at the contact of the carbonates and the underlying Qingtianbao Formation sandstone.Vein-like orebodies occur within the porphyritic intrusions, and laterite-hosted orebodies have also been documented [54].Based on previous studies, the mineralization style in the Beiya deposit can be divided into porphyry, skarn and supergene styles [55].Of these, skarn is the major mineralization type at Beiya deposit, with orebodies mostly found along the intrusive contact

Beiya
The Beiya Au-polymetallic deposit (~90 km north of Dali) is the largest gold skarn deposit in China.The deposit is located in the middle part of the Jinshajiang-Ailaoshan alkaline porphyry metallogenic belt [53] (Figure 1) and contains six ore segments, namely the Weiganpo, Bijiashan and Guogaishan segments in the east and the Wandongshan, Hongnitang and Jingouba segments in the west (Figure 2a).The Wandongshan (mainly the KT52 ore body) ranks the biggest, containing most of the Au resources (99 Mt @ 2.61 g/t Au [25]) and is currently being mined for gold and iron.
The orebodies in the Beiya deposit mostly occur in and/or along the contact zones between the intrusions and the carbonate host rocks of the Beiya Formation.Some stratiform orebodies are locally distributed along the flat interlayer fractures and breccia zones within the Beiya Formation carbonates, as well as at the contact of the carbonates and the underlying Qingtianbao Formation sandstone.Vein-like orebodies occur within the porphyritic intrusions, and laterite-hosted orebodies have also been documented [54].Based on previous studies, the mineralization style in the Beiya deposit can be divided into porphyry, skarn and supergene styles [55].Of these, skarn is the major mineralization type at Beiya deposit, with orebodies mostly found along the intrusive contact between porphyries and the Beiya Formation carbonates.Orebody KT52 (around the Wandongshan porphyry) is the largest skarn orebody (Figure 2b), with proven reserves of 87.2 Mt Au @ 2.35 g/t, along with 90.27 Mt Fe @ 34 wt %, and 111.8 Mt Cu @ 0.34 wt % [54].
In the Beiya deposit, gold mainly occurs as native gold and electrum in fractures and/or as vein-filling in pyrite, magnetite, limonite and quartz.In the retrograde alteration and quartz-sulfide stages, the gold is also hosted by magnetite and sulfides (pyrite, chalcopyrite and bismuthinite), respectively.Compositions of the Beiya skarn minerals suggest a typical oxidized skarn system [28].Fluid inclusion studies indicate medium to high temperatures (186-372 • C) and high salinities (7.5-19.8wt % NaCl equiv.)for the ore-forming fluids, which were likely originated from the magma with late-stage meteoric water involvement [62].The δ 34 S values of the quartz-sulfide stage range from −2.4‰ to 4.5‰, suggesting that the ore-forming fluids were derived from and/or modified by the magma [54,62,63].The δ 18 O and δD compositions of the ore-forming fluids in the quartz-sulfide stage are −0.85‰ to 3.52‰ and −78.6‰ to −88.6‰, respectively [62].The isotope evidence suggests that the Beiya ore-forming materials were derived from deep-sourced magmas [54,62].
The mineralization of the Machangqing deposit includes porphyry Cu-Mo mineralization (the Baoxingchang and Luandongshan ore segments), skarn Cu-Mo-Fe mineralization along the intrusive contacts (the Baoxingchang and Luandongshan ore segments) and epithermal Au-Pb-Zn mineralization in the Xiangyang and Kanglang formations (the Shuangmacao, Rentouqing and Jinchangqing ore segments [30,32]).These three mineralization styles form a complete alkalic porphyry-related mineral system, and show a systematic alteration and mineralization style transition from the intrusion to the wall rocks [30,32].In the Machangqing deposit, the porphyry ores usually occur as Cu-Mo veinlets within the intrusion associated with potassic, phyllic and argillic alterations [23].The skarn mineralization mainly comprises three major stages: (1) prograde skarn; (2) retrograde alteration; and (3) a quartz-sulfide mineralization.Prograde skarn minerals are mainly anhydrous (e.g., garnet and pyroxene) (Figure 4e), whereas the retrograde alteration minerals are mainly hydrous (e.g., epidote-group minerals, biotite and chlorite) (Figure 4f).Magnetite mineralization mainly occurred during the late retrograde alteration (quartz-magnetite) stage, whilst the quartz-sulfide stage contains mainly pyrite, chalcopyrite and molybdenite (Figure 4g,h), and constitutes the main Cu-Mo mineralization stage.Previous studies have shown that the ore-forming temperature, pressure and salinities of the ore-forming fluids decreased from the porphyry, through skarn and to the epithermal mineralization [32,64].The metal and fluid sources also changed from dominantly magmatic to hydrothermal [32].The molybdenite Re-Os ages of the porphyry mineralization range from 33.9 ± 1.1 Ma to 35.8 ± 1.6 Ma [23,[33][34][35]65].

Samples and Analytical Methods
Allanite samples for the in-situ LA-ICP-MS U-Pb dating and trace element analysis were collected from the mineralized skarn (exoskarn) in the Beiya deposit (Wandongshan ore segment) and Machangqing deposit (Baoxingchang ore segment).The allanite samples from the Beiya deposit occur in a mineral assemblage of garnet, pyroxene, magnetite, epidote, feldspar and quartz, with minor titanite and scheelite, whereas those from the Machangqing deposit occur in a mineral assemblage of pyroxene, magnetite, feldspar, quartz, chalcopyrite and calcite, with minor titanite and scheelite.Petrographic thin sections were prepared for all the samples, and were studied with optical microscopy and Back-Scattered Electron (BSE) imaging, using a Zeiss SIGMA field-emission Scanning Electron Microprobe (SEM) (Oberkochen, Germany) at the School of Earth Science and Geological Engineering of Sun Yat-sen University (Guangzhou, China).Prior to the LA-ICP-MS analysis, major element abundance of the allanite samples was determined on thin sections using a JEOL JXA-8230 electron probe micro-analyzer (EPMA) (Tokyo, Japan) at the Key Laboratory of Mineralogy and Metallogeny in the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS).Operating conditions include 15 kV accelerating voltage, 20 nA beam current and a beam diameter of 2 µm.
In-situ U-Pb dating and trace element analyses of allanite were performed by LA-ICP-MS at the Key Laboratory of Marine Resources and Coastal Engineering, Sun Yat-sen University.The analyses were performed using a 193 nm ArF excimer laser ablation system (GeoLasPro) coupled with an Agilent 7700x ICP-MS (Santa Clara, CA, USA).A 32 µm spot size was used with an energy density of 5 J/cm 2 and a repetition rate of 5 Hz.The trace element compositions of allanite were calibrated against the Standard NIST610 [66], using Si determined by EPMA as the internal standard.Zircon 91500 [67] was used as the external standard for the U-Pb dating.A sample-standard bracketing method was used for instrumental drift correction.Each analysis consists of a 20 s background measurement (laser-off) followed by 45 s of data acquisition.Data reduction was performed using ICPMSDataCal software [68].Time-dependent drifts of U-Th-Pb isotopic ratios were corrected using a liner interpolation (with time) for every five analyses based on the variations of 91500 zircon standard, and the detailed procedure can be found in literature [68].ISOPLOT 3.0 software [69] was used to construct the Tera-Wasserburg diagram and isochrons.
Allanite commonly have large initial common Pb contents.Therefore, attention should be paid in correcting Pb isotopic data, and the choice of initial Pb isotopic composition would significantly influence the calculated ages [4].We adopted the 207 Pb correction method, which has been widely used to determine the age of relatively young allanite [4,70].The 204 Pb correction is less precise, because the counting error on 204 Pb is larger than on the more abundant 207 Pb. 204 Pb is also more susceptible to measurement error due to isobaric interference.The uncorrected data are plotted on the Tera-Wasserburg diagram, and a regression through these analyses yields a lower intercept that represents the apparent allanite age.The y-intercept represents the initial 207 Pb/ 206 Pb, which can be used for the 207 Pb-correction.The amount of common 206 Pb is expressed as a fraction of the total 206 Pb (f 206), which can be calculated from: where 208 Pb/ 206 Pb c is the initial common Pb composition, which can be determined from the intercept of the isochron.The age of allanite can be calculated from the slope of the isochron.This approach has been shown to be effective for correcting common-Pb in allanite.
The allanite in the Beiya and Machangqing mineralized skarn comprise euhedral to subhedral crystals (Figure 5a-f).Allanite grains in the Beiya deposit are typically 50-300 µm in length (Figure 5a,c,d) and exhibit obvious pleochroism under the microscope.They associate with skarn minerals such as garnet, pyroxene, magnetite, titanite, fluorite, feldspar and quartz, and mostly occur in the core of altered garnet (Figure 5c).Allanite from the Machangqing mineralized skarn is similar with the Beiya deposit, most of which is 50-400 µm in length and occurs with magnetite, chlorite, chalcopyrite, calcite and quartz (Figure 5b,e,f).All the allanite grains from both the Beiya and Machangqing deposits are often closely intergrown with magnetite (Figure 5a,b), and some fractures in the magnetite are filled with sulfides (Figure 4d).This indicates that the allanite was syn-magnetite mineralized, followed by sulfide mineralization.No obvious patchy zoning or overgrowth texture was observed in the allanite from the Beiya and Machangqing deposits under BSE imaging, suggesting a single growth phase (Figure 5c-f).
where 208 Pb/ 206 Pbc is the initial common Pb composition, which can be determined from the intercept of the isochron.The age of allanite can be calculated from the slope of the isochron.This approach has been shown to be effective for correcting common-Pb in allanite.
The allanite in the Beiya and Machangqing mineralized skarn comprise euhedral to subhedral crystals (Figure 5a-f).Allanite grains in the Beiya deposit are typically 50-300 μm in length (Figure 5a,c,d) and exhibit obvious pleochroism under the microscope.They associate with skarn minerals such as garnet, pyroxene, magnetite, titanite, fluorite, feldspar and quartz, and mostly occur in the core of altered garnet (Figure 5c).Allanite from the Machangqing mineralized skarn is similar with the Beiya deposit, most of which is 50-400 μm in length and occurs with magnetite, chlorite, chalcopyrite, calcite and quartz (Figure 5b,e,f).All the allanite grains from both the Beiya and Machangqing deposits are often closely intergrown with magnetite (Figure 5a,b), and some fractures in the magnetite are filled with sulfides (Figure 4d).This indicates that the allanite was syn-magnetite mineralized, followed by sulfide mineralization.No obvious patchy zoning or overgrowth texture was observed in the allanite from the Beiya and Machangqing deposits under BSE imaging, suggesting a single growth phase (Figure 5c-f).

Allanite U-Th-Pb Ages
The corrected U-Pb isotope results of allanite from the Beiya and Machangqing deposits are presented in Table A3.In the Tera-Wasserburg diagram, the common Pb-uncorrected data of the Beiya and Machangqing allanite define a linear array, giving a lower-intercept age of 42.1 ± 4.7 Ma (MSWD = 4.2) and 35.4 ± 5.5 Ma (MSWD = 2.3), respectively.The Y-intercept of initial 207 Pb/ 206 Pb is 0.864 and 0.826, respectively (Figure 9).The Th-Pb isochrons of the Beiya and Machangqing allanite samples are shown in Figure 10

Allanite U-Th-Pb Ages
The corrected U-Pb isotope results of allanite from the Beiya and Machangqing deposits are presented in Table A3.In the Tera-Wasserburg diagram, the common Pb-uncorrected data of the Beiya and Machangqing allanite define a linear array, giving a lower-intercept age of 42.1 ± 4.7 Ma (MSWD = 4.2) and 35.4 ± 5.5 Ma (MSWD = 2.3), respectively.The Y-intercept of initial 207 Pb/ 206 Pb is 0.864 and 0.826, respectively (Figure 9).The Th-Pb isochrons of the Beiya and Machangqing allanite samples are shown in Figure 10

Reliability of Allanite U-Th-Pb Dating
Previous studies [4,14,16,73] show that precise allanite U-Th-Pb dating is influenced by several factors: (1) high and variable common Pb content; (2) excess 206 Pb from 230 Th decay in high-Th samples; and (3) lack of suitable matrix-matched allanite standards due to the large compositional variations caused by the solid solution with epidote.
Both the U-Pb and Th-Pb systems can be used to date allanite, although, in many cases, the latter is preferable due to its high Th content [14].The correction for incorporation of 230 Th during the mineral's growth is often necessary for young allanite, because the excess 206 Pb from the 230 Th decay

Allanite U-Th-Pb Ages
The corrected U-Pb isotope results of allanite from the Beiya and Machangqing deposits are presented in Table A3.In the Tera-Wasserburg diagram, the common Pb-uncorrected data of the Beiya and Machangqing allanite define a linear array, giving a lower-intercept age of 42.1 ± 4.7 Ma (MSWD = 4.2) and 35.4 ± 5.5 Ma (MSWD = 2.3), respectively.The Y-intercept of initial 207 Pb/ 206 Pb is 0.864 and 0.826, respectively (Figure 9).The Th-Pb isochrons of the Beiya and Machangqing allanite samples are shown in Figure 10

Reliability of Allanite U-Th-Pb Dating
Previous studies [4,14,16,73] show that precise allanite U-Th-Pb dating is influenced by several factors: (1) high and variable common Pb content; (2) excess 206 Pb from 230 Th decay in high-Th samples; and (3) lack of suitable matrix-matched allanite standards due to the large compositional variations caused by the solid solution with epidote.
Both the U-Pb and Th-Pb systems can be used to date allanite, although, in many cases, the latter is preferable due to its high Th content [14].The correction for incorporation of 230 Th during the mineral's growth is often necessary for young allanite, because the excess 206 Pb from the 230 Th decay

Reliability of Allanite U-Th-Pb Dating
Previous studies [4,14,16,73] show that precise allanite U-Th-Pb dating is influenced by several factors: (1) high and variable common Pb content; (2) excess 206 Pb from 230 Th decay in high-Th samples; and (3) lack of suitable matrix-matched allanite standards due to the large compositional variations caused by the solid solution with epidote.
Both the U-Pb and Th-Pb systems can be used to date allanite, although, in many cases, the latter is preferable due to its high Th content [14].The correction for incorporation of 230 Th during the mineral's growth is often necessary for young allanite, because the excess 206 Pb from the 230 Th decay in Th-rich minerals can affect the 206 Pb/ 238 U and 207 Pb/ 235 U ages of the young samples [4,14,18].Excess 206 Pb results in older apparent concordia intercept ages and lower 207 Pb/ 206 Pb values for the common Pb component in the y-intercept.The elevated intercept age reflects the variable 206 Pb excess (Figure 9).Therefore, we use the Th-Pb system and choose the Th-Pb isochron ages as the real age of the allanite samples.Compared to the Beiya allanite, the Machangqing allanite yielded an intercept age closer to the Th-Pb isochron age because of its lower Th content (Figures 9 and 10).In addition, the larger analytical uncertainty of Th-Pb isochron ages for the Machangqing allanite may have resulted from the lower Th and 208 Pb contents than the Beiya allanite (Table A2).
Some studies suggest that LA-ICP-MS analysis is less sensitive to matrix effect than ion-microprobe analysis [4].In many cases, using an allanite standard for the external calibration of U-Th-Pb ages may not be imperative due to the compositional difference between the samples and the reference materials.Completely matrix matched materials are rare because allanite has a much greater range of elemental substitution than many other minerals used for U-Th-Pb dating (e.g., monazite), and even the most widely-used allanite reference materials (e.g., AVC and Tara) have large variations in common-Pb content [4].Many previous studies have shown that accurate and precise allanite U-Th-Pb ages can be obtained by using non-matrix matched external standards, such as zircons (91500 and GJ-1) [11,12,14,17] and synthetic glass (NIST610) [18].Therefore, the non-matrix matched external standardization approach with the use of zircon standard 91500 in this study is likely to be reliable.

Age of Skarn Mineralization in the Beiya and Machangqing Deposits
Different origins of allanite can be discriminated by their paragenetic mineral assemblages and geochemistry [9].In general, hydrothermal allanite contains lower Th/U (mostly < 50) than magmatic allanite (mostly > 50) due to the higher mobility of U in hydrothermal fluids than Th [74].Both the Beiya and Machangqing allanite yielded relatively low Th/U ratios (average: 25.5 and 12.9, respectively), which rules out a magmatic origin.Besides, the allanite from the Beiya and Machangqing deposits commonly coexists with skarn minerals (e.g., magnetite, garnet, feldspar and pyroxene), suggesting a hydrothermal origin.
Paragenetic evidence indicates that the Beiya and Machangqing allanite were formed during the retrograde alteration stage before the quartz-sulfide stage, which implies that the hydrothermal allanite ages at Beiya and Machangqing were broadly coeval with the magnetite mineralization but slightly older than the sulfide mineralization.Thus, the U-Th-Pb ages of the hydrothermal allanite from the Beiya and Machangqing deposits provide an upper age limit of mineralization at 33.4 ± 4.6 Ma and 35.4 ± 9.8 Ma, respectively.
Previous studies suggested that the Beiya ore-related porphyry were formed at 36.1 ± 0.4 Ma [24], and that the titanite U-Pb age and the molybdenite Re-Os age have altogether constrained the Beiya Au skarn mineralization to ca. 33.1-34.1 Ma [61].In this study, the skarn mineralization is determined to be largely coeval with the Au mineralization, and slightly younger than the porphyry emplacement.Meanwhile, the ore-related granite porphyries in the Machangqing deposit were zircon U-Pb dated to be ca.35.0-37.9Ma [34,35,47,48], and the porphyry mineralization was molybdenite Re-Os dated to be ca.33.9-35.8Ma [23,[33][34][35]65], which are broadly coeval with the skarn mineralization (determined by allanite U-Th-Pb dating in this study).The similar ages between the porphyry and skarn mineralization in the Machangqing deposit suggest that they were likely formed in the same metallogenic system, and that the alkaline granite porphyry had likely supplied the ore-forming materials, fluids and heat that drove the mineralization.As a whole, the skarn mineralization in the Beiya and Machangqing deposits are synchronous and temporally related to the magmatic hydrothermal activity in the Himalayan orogenic event.Besides, the marked similarity between our new Beiya allanite U-Th-Pb age (33.4 ± 4.6 Ma) and the published Beiya titanite U-Pb age (33.1 ± 1.0 Ma) [61] suggests that our U-Th-Pb dating and data reduction approach are reliable.

Metallogenic Implications
The U contents of allanite can be used to evaluate the oxygen fugacity (f O 2 ) of the ore-forming fluids [12,13].Uranyl (U 6+ ) complexes are strongly soluble in hydrothermal fluids [13], and therefore highly U contents may indicate more oxidized fluids [12].The Beiya allanite has higher U contents (average: 171 ppm) than the Machangqing allanite (average: 78 ppm) (Figure 7a), suggesting that the ore-forming fluids in the Beiya deposit may have been more oxidized.This conclusion is also supported by the higher Fe 3+ /(Fe 3+ + Fe 2+ ) ratios of the Beiya allanite (Figure 11).The allanite in the Beiya deposit is co-precipitated with magnetite during the retrograde stage, which predate the gold discharge at quartz sulfide stage [75], and the oxygen fugacity revealed by allanite should represent for the quartz-magnetite stage that recharging and/or migration the gold-bearing fluid.Similarly, the mineral assemblage of allanite and magnetite in the Machangqing deposit predated the precipitation of copper during the sulfide stage.The oxygen fugacity of allanite from the Machangqing deposit could provide constrain for the oxidization state of Cu-bearing fluid in the quartz-magnetite (retrograde) stage.

Metallogenic Implications
The U contents of allanite can be used to evaluate the oxygen fugacity (fO2) of the ore-forming fluids [12,13].Uranyl (U 6+ ) complexes are strongly soluble in hydrothermal fluids [13], and therefore highly U contents may indicate more oxidized fluids [12].The Beiya allanite has higher U contents (average: 171 ppm) than the Machangqing allanite (average: 78 ppm) (Figure 7a), suggesting that the ore-forming fluids in the Beiya deposit may have been more oxidized.This conclusion is also supported by the higher Fe 3+ /(Fe 3+ + Fe 2+ ) ratios of the Beiya allanite (Figure 11).The allanite in the Beiya deposit is co-precipitated with magnetite during the retrograde stage, which predate the gold discharge at quartz sulfide stage [75], and the oxygen fugacity revealed by allanite should represent for the quartz-magnetite stage that recharging and/or migration the gold-bearing fluid.Similarly, the mineral assemblage of allanite and magnetite in the Machangqing deposit predated the precipitation of copper during the sulfide stage.The oxygen fugacity of allanite from the Machangqing deposit could provide constrain for the oxidization state of Cu-bearing fluid in the quartz-magnetite (retrograde) stage.Previous studies show that in chloride-rich hydrothermal fluids, Th is much less soluble than U, yet both U and Th are highly soluble in F-rich fluids [11,76].Thus, enrichments of F − ligands in the hydrothermal fluids would cause high-Th allanite precipitation [11,13,77].The hydrothermal allanite from the Machangqing deposit contains much lower Th content (average: 914 ppm) than that from the Beiya deposit (average: 3630 ppm), suggesting higher F activities in the Beiya ore-forming fluids.This conclusion is consistent with the presence of fluorite in the Beiya Au skarn ores (Figure 5c).In summary, our new allanite geochemical data indicate a higher F content and oxygen fugacity for the ore-forming fluids in the Beiya deposit than those in the Machangqing deposit.
Although the giant Beiya Au deposit and Machangqing Cu-Mo deposit are both located in the middle part of the Jinshajiang-Ailaoshan metallogenic belt (about 100 km apart), they differ greatly in terms of the ore deposit size and types of metals.The higher oxygen fugacity in the Beiya magmatic fluids had likely prevented early sulfide precipitation and the consequent metal removal, as well as increased the metal solubility in the hydrothermal fluids [78].As a result, the Beiya ore-forming fluids could carry more metals than its Machangqing counterparts, leading to a larger deposit size of the former.Besides, gold can be effectively transported as gold (III) fluoride in high-temperature hydrothermal fluids [79], and the higher F − content in the Beiya hydrothermal fluids (cf. the Machangqing ones) may lead to the Au-dominated mineralization in the Beiya deposit (and the subeconomic gold mineralization in the Machangqing deposit).Previous studies show that in chloride-rich hydrothermal fluids, Th is much less soluble than U, yet both U and Th are highly soluble in F-rich fluids [11,76].Thus, enrichments of F − ligands in the hydrothermal fluids would cause high-Th allanite precipitation [11,13,77].The hydrothermal allanite from the Machangqing deposit contains much lower Th content (average: 914 ppm) than that from the Beiya deposit (average: 3630 ppm), suggesting higher F activities in the Beiya ore-forming fluids.This conclusion is consistent with the presence of fluorite in the Beiya Au skarn ores (Figure 5c).In summary, our new allanite geochemical data indicate a higher F content and oxygen fugacity for the ore-forming fluids in the Beiya deposit than those in the Machangqing deposit.
Although the giant Beiya Au deposit and Machangqing Cu-Mo deposit are both located in the middle part of the Jinshajiang-Ailaoshan metallogenic belt (about 100 km apart), they differ greatly in terms of the ore deposit size and types of metals.The higher oxygen fugacity in the Beiya magmatic fluids had likely prevented early sulfide precipitation and the consequent metal removal, as well as increased the metal solubility in the hydrothermal fluids [78].As a result, the Beiya ore-forming fluids could carry more metals than its Machangqing counterparts, leading to a larger deposit size of the former.Besides, gold can be effectively transported as gold (III) fluoride in high-temperature hydrothermal fluids [79], and the higher F − content in the Beiya hydrothermal fluids (cf. the Machangqing ones) may lead to the Au-dominated mineralization in the Beiya deposit (and the sub-economic gold mineralization in the Machangqing deposit).
(3) Major and trace element characteristics of the allanite from the two deposits suggest that the ore-forming fluids in the Beiya deposit may have had higher F content and oxygen fugacity than those in the Machangqing deposit.
Appendix A