Onset and duration of Zn—Pb mineralization in the Talate Pb—Zn (—Fe) skarn deposit, NW China: Constraints from spessartine U—Pb dating

Accepted Manuscript Onset and duration of Zn-Pb mineralization in the Talate Pb-Zn (Fe) skarn deposit, NW China: Constraints from spessartine U-Pb dating

Dengfeng Li, Yu Fu, Xiaoming Sun PII: DOI: Reference:

S1342-937X(18)30169-2 doi:10.1016/j.gr.2018.05.013 GR 1994

To appear in:

Gondwana Research

Received date: Revised date: Accepted date:

28 February 2018 23 May 2018 27 May 2018

Please cite this article as: Dengfeng Li, Yu Fu, Xiaoming Sun , Onset and duration of ZnPb mineralization in the Talate Pb-Zn (-Fe) skarn deposit, NW China: Constraints from spessartine U-Pb dating. Gr (2018), doi:10.1016/j.gr.2018.05.013

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ACCEPTED MANUSCRIPT

Onset and duration of Zn-Pb mineralization in the Talate Pb-Zn (-Fe) skarn deposit, NW China: Constraints from spessartine U-Pb dating

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Dengfeng Lia, b *, Yu Fu a, b, Xiaoming Suna, b, c * a. School of Marine Sciences, Sun Yat-sen University, Guangzhou 510006, China b. Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, Guangzhou 510006, China

*Corresponding author: D.F. Li and X.M. Sun

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c. School of Earth Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China

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Address: Sun Yat-sen University, No. 135, Xingang Xi Road, Guangzhou, 510275, P. R. China

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E-mail: [email protected] (D.F. Li) & [email protected] (X.M. Sun)

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ACCEPTED MANUSCRIPT Abstract Distal Pb-Zn skarn commonly displays ambiguous relationships with the nearby intrusions, leading to poorly constrained and often controversial skarn metallogenic models. In this study, we discuss the potential usage of spessartine, a common U-bearing gangue mineral in many Pb-Zn skarn deposits, in U-Pb dating. Spessartine from the Talate skarn Pb-Zn deposit (Xinjiang, NW China) can be divided into two types (GI

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and GII) based on mineral assemblage. GI spessartine (Sp56.5 Al28.3 Gr12.4 to Sp66.0 Al22.3 Gr8.5) is coarsegrained with core-rim texture occasionally well-preserved, disseminated magnetite and magmatic TI titanite grains are hosted within the spessartine GI core, indicating that the spessartine GI is slightly postdate the magmatic titanite TI and disseminated magnetite. GII massive spessartine (Sp64.2 Al24.8 Gr2.7 to Sp69.6 Al20.1 Gr1.2) occurs with hydrothermal titanite TII and shows a close relationship with coarse-grained hydrothermal

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magnetite, and is cut by quartz or fine sulfide (e.g., pyrite, pyrrhotite and chalcopyrite) veins. The flat time-resolved signals obtained from the depth profile analyses and inclusion-free trace element

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mapping of U in spessartine indicate that the U is structurally bounded in the lattice. Both GI and GII spessartine show clear positively correlation between U, Al and Mn, but no apparent correlation is present

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between U and Fe, which indicates that Al and Mn are more important in the U incorporation in spessartine. Integrating with the positive correlations with U and LREEs in spessartine, which suggest U is incorporated

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in the lattice by a substitution of [U4+]VIII + 2[Al3+]IV − [Ca2+]VIII + 2[Si4+]IV. In-situ U-Pb dating of the two spessartine types yielded a weighted average 206Pb/238U age of 231.7 ± 7.2 Ma (GI; MSWD = 0.56; n = 35)

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and 211.5 ± 5.8 Ma (GII; MSWD = 1.1; n = 47). Considering the errors caused by common Pb and matrix mismatch, the corrected GI and GII spessartine ages are consistent with the TI (228.0 ± 4.6 Ma; MSWD =

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2; n = 28) and TII (209.8 ± 3.6 Ma; MSWD = 1.03; n = 39) titanite ages, suggesting that spessartine U-Pb dating is robust and reliable. The least 8.1 My gap between the formation of the two titanite and spessartite types reflects two mineralization events, suggesting an influx of magmatic fluids during the retrograde alteration/mineralization. Our study represents a new way of directly constraining the timing of skarn alteration/mineralization. Key words: spessartine-rich garnet; U-Pb dating; distal skarn Pb-Zn deposit; Altay; NW China

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Introduction

Directly constraining the timing of the early-stage (prograde/retrograde) skarn alteration/ mineralization has been a long unresolved issue. In many studies, the timing is constrained by U-Pb dating of zircon and/or monazite. However, this reflects largely the emplacement age of the causative intrusions (not the skarn), not to mention that identifying the causative intrusions (if any) is often problematic (Meinert

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et al., 2005). This is especially true for many Pb-Zn skarn deposits, e.g., the Talate Pb-Zn (-Fe) deposit in Xinjiang (NW China) (Li et al., 2017, 2014), whose low-temperature ore mineral assemblage (e.g., magnetite, sphalerite, galena and pyrite) means that the mineralization was likely distal from the intrusions (Chang, 2017). At Talate, the ore-forming age was previously constrained by biotite Ar-Ar dating to be ca. 228 and 214 Ma (Li et al., 2014), corresponding to the quartz-magnetite and quartz-sulfide stage mineralization.

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Again, these ages may not represent the onset of skarn alteration/mineralization, particularly because the biotite Ar-Ar systematics (with low closure temperature) can easily be reset by later thermotectonic events.

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In-situ LA-ICP-MS dating on skarn-related hydrothermal minerals, such as garnet and titanite, can provide direct age constraints on the skarn alteration/mineralization (Burton et al., 1995; Burton and O’Nions,

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1992; Deng et al., 2017; Jung and Mezger, 2003; Seman et al., 2017; Vance and Holland, 1993; Yang et al., 2018; Zhang et al., 2018). Many of these minerals contain considerable amount of U and Th, have high

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closure temperature (garnet > 850 °C, Mezger et al. 1989; titanite > 700 °C; Cherniak 1993), and are thus ideal for U-Pb dating. Furthermore, many of these minerals are commonly larger than zircon and monazite,

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enabling the usage of larger spot size during the analysis. Titanite is mainly formed during the magma crystallization as magmatic titanite or during retrograde

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alteration (Fu et al., 2016), while garnet is a major prograde skarn mineral. Direct dating of these minerals can constrain not only the onset of the skarn alteration/mineralization, but also the duration of it. In this contribution, we conduct LA-ICP-MS U-Pb dating and trace element analysis on these hydrothermal minerals from the Talate Pb-Zn (-Fe) skarn deposit, and discuss the ore-forming age and clarify the ore genesis.

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Geological setting

The Central Asian Orogenic Belt (CAOB) extends from the Urals in the west to the Pacific coast in the east. It is the world's largest Phanerozoic accretionary orogen, and has an evolution history that spans across ca. 800 m.y. (ca. 1000–250 Ma; Fig. 1a; Jahn et al. 2000a, b, c; Jahn 2004; Windley et al. 2007; Safonova et al. 2011; Chen et al. 2012; Pirajno et al., 2009).

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The Chinese Altay orogen in the northern margin of Xinjiang (NW China) is an important part of the CAOB (Fig. 1b). The orogen contains four tectonic units, including (from NW to SE) (Chen et al., 2012): (1) the Late Devonian–Early Carboniferous Nurt volcanic basin developed on a pre-Devonian crystalline basement; (2) the Keketuohai Paleozoic magmatic arc (or the Central Altay terrane) that contains Precambrian high-grade metamorphic rocks, Neoproterozoic to Early Triassic granites, and the giant

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Keketuohai pegmatite ore field; (3) the Devonian-Carboniferous Kelan basin developed on the southern margin of the pre-Devonian metamorphic rocks; and (4) the Armantay–Erqis accretionary complex with

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high-grade metamorphic rocks, Devonian–Carboniferous fossiliferous sedimentary rocks, and post-orogenic diorite dikes (276.7 ± 2.9 and 273.2 ± 4.3 Ma; Cai et al., 2016) and No. 3 granitic pegmatite (220 ± 9 Ma,

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198 ± 7 Ma and 213 ± 6 Ma, respectively; Wang et al., 2007). The NW-trending faults in the Kelan Basin commonly separate different stratigraphic units: for example, the Keyingong Fault separates the Kulumuti

formations (Fig. 1b).

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Group and the Kangbutiebao Formation, whereas the Abagong Fault separates the Kangbutiebao and Altay

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The Devonian-Carboniferous Kelan basin is bounded by the Abagong Fault in the north and Erqis Fault in the south, and is composed mainly of volcano-sedimentary rocks of the Lower Devonian Kangbutiebao

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Formation, the Middle-Upper Devonian Altay Formation, with subordinate Carboniferous volcanosedimentary units. Middle-Upper Silurian schist and gneiss, and Devonian marine volcanic rocks are distributed in the Ashele, Chonghu’er, Kelan and Maizi volcano-sedimentary basins (Fig. 2). The Middle Ordovician Kulumuti Group comprises mainly migmatite and schist. The metamorphic rocks are only locally exposed in the northwestern vicinity of the Talate mine and unconformably overlie the Habahe Group metamorphosed clastic rocks (Windley et al., 2002). Detrital zircon dating has yielded youngest ages of ca. 465–576 Ma with a few older grains (ca. 766–972 Ma and 1321–2572 Ma; Wang et al., 2014).

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ACCEPTED MANUSCRIPT The Lower Devonian Kangbutiebao Formation (zircon U-Pb ages: ca. 412−388 Ma; Chai et al., 2009; Zheng et al., 2013) hosts many Pb-Zn (-Cu-Fe) deposits in the Ashele, Chonghu’er, Kelan and Maizi basins (Fig. 1b; Geng et al., 2012; Li et al., 2012; Shan et al., 2012). Rocks of the Kangbutiebao Formation were deformed and metamorphosed to greenschist-facies during the late Paleozoic Hercynian Orogeny, and present with a typical metamorphic mineral assemblage of biotite + chlorite + epidote + actinolite. The

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overlying Middle Devonian Altay Formation (zircon U-Pb ages: ca. 380–354 Ma) comprises metamorphosed sandstone, siltstone and limestone (Geng et al., 2010). The sediments were interpreted to have been deposited in a fore-arc basin environment (Wang et al., 2006). The Au–Cu–Pb–Zn mineralization in the Kelan and Maizi basins is consistent with the post-organic activity, as indicated by their

40Ar/39Ar

plateau ages ranging from 260 to 204 Ma (Zheng et al., 2017), the Late Permian–Triassic mineralization

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Talate Pb-Zn (-Fe) deposit

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3.1 Mineralization style

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region (Chen et al., 2014, 2007; Pirajno et al., 2009)

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episode in the Chinese Altay Orogen might have corresponded to a major tectono-thermal reactivation in the

The Talate Pb-Zn (-Fe) deposit an important deposit in the Abagong Pb-Zn-Fe ore district, and contains

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higher Zn than Pb grade (Fig. 1b; Li et al., 2014). Talate currently hosts an ore reserve of 2.52 million tonnes (Mt) at 7.93% Pb + Zn and 27% Fe (0.35% cut-off; Yuan et al., 2011). The Lower Devonian Kangbutiebao

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Formation, which comprises granulite, marble, metamorphosed rhyolite and tuff, is the main ore host of the Talate and other Pb-Zn-Fe deposits in this area (Fig. 1b). The orebodies are located in the contact zone

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between the meta-conglomerate and the felsic lavas (Figs. 1c and 2). These orebodies present as veins and situate along the Abagong Fault. Alteration minerals at Talate include skarn-related garnet, epidote, tremolite, titanite, allanite and actinolite, as well as pyrite, calcite and quartz. The Pb-Zn mineralization is spatially related to skarn alteration, and the sulfides are mainly disseminated but locally occur as bands and veinlets. Li et al. (2014) divided the alteration/mineralization at Talate into four stages based on mineral assemblage and vein crosscutting relationships, i.e., the early skarn (I: E-Skarn), quartz-magnetite (II: QM), quartz-sulfide (III: QS) and carbonate (IV: QC) stage.

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ACCEPTED MANUSCRIPT Stage I could be divided into two sub-stages as I-1 and I-2, the stage I-1 is made up of some primary igneous minerals such as disseminated magnetite, cassiterite, tourmaline, magmatic titanite, biotite, plagioclase and minor garnet etc. There is some disseminated magnetite which was originally magmatic or magmatic-hydrothermal, since it co-precipitated with cassiterite and tourmaline, and is characterized by higher Ti and Sn concentrations than the hydrothermal massive magnetite (Duchoslav et al., 2016; Li et al.,

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2016). These disseminated magnetite and titanite were found inside the garnet (GI) and pyroxene, which indicates that these magnetite and titanite grains are slightly predate the garnet of stage I-1. The stage I-2 is primarily made up of anhydrous minerals (e.g., garnet and pyroxene) with minor pyrite. The sulfur isotope composition (δ34S) of the stage I is 2.6‰ (Li et al., 2014), close to the average magmatic sulfur (i.e., −3‰ to + 3‰; Hoefs, 1997). The garnet is occurred with hydrothermal massive magnetite and show no correlation

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with disseminated magnetite, indicated that this kind of garnet is paragenetically postdate the GI, they were abbreviated as GII. But due to the limit samples, the crosscutting relationship of GI and GII are not clarified

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in this study.

Stage II hydrothermal alteration contains hydrous minerals, e.g., epidote, allanite, tremolite, actinolite,

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hornblende and titanite with abundant magnetite and pyrite. The coarse-grained, massive anhedral titanite shows a close relationship with massive magnetite, they are feature with lower Ti and Sn concentration

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compared with the disseminated magnetite and classified as hydrothermal magnetite (Li et al., 2016). The ore fluid temperatures were estimated (based on fluid inclusion studies) to be 271 to 426 °C, and the δ34S

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values range from 5.0 to 5.2‰, indicative of fluid-rock interactions with metasedimentary host rocks that contain heavier sulfur isotopes (Hoefs, 1997). Ar-Ar dating of Stage II biotite yielded 227.6 ± 2.2 Ma (Li et

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al., 2014).

Stage III alteration is characterized by sphalerite, galena, pyrite, chalcopyrite and pyrrhotite and trace arsenopyrite. The ore fluid temperatures were estimated to be 204 – 269 °C, with negative δ34S values reported (−1.7 and −6.2‰) (Li et al., 2014). The biotite in Stage III is coexisted with large volumes of sulfides and yielded an Ar-Ar age of 214.1 ± 2.1 Ma (Li et al., 2014). Stage IV alteration is dominated by calcite and some pyrite, the fluid temperatures were estimated to be 175 to 211 °C, with a δ34S value reported to be −2.2‰ (Li et al., 2014). The Talate alteration/mineralization paragenetic sequence and a detailed description can be found in Li et al. (2014).

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3.2 Sample description and paragenesis All the samples were collected from the skarn alteration zone in the drill core ZK144-4 (Fig. 1c). The titanite from Talate can be divided into two types, i.e., magmatic (TI) and hydrothermal (TII) based on mineralization assemblage and crosscutting relationships. The TI titanite coexists with igneous accessory

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minerals such as the magmatic biotite, K-feldspar, plagioclase and quartz and/or as interstices or inclusions within magmatic minerals such as biotite (Fig. 3a). Euhedral TI titanite hosts within hornblende and garnet (Fig. 3b), whilst some fine-grained euhedral to subhedral titanite grains are included by primary quartz (Fig. 3c). The coarse-grained, massive anhedral TII titanite shows a close relationship with massive magnetite. The TII titanite grains are dominantly anhedral and usually correlates to the coarse-grained massive

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magnetite (Fig. 3d). The titanite TII occurs either as tabular grains in quartz (Fig. 3e) or as thin titanite rims around massive magnetite (Fig. 3f). Garnet GI at Talate was formed in the stage I-1, and occurs with

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disseminated magnetite (Fig. 3g). Subhedral coarse-grained garnet (GI, featured by core-mantle-rim texture) is bounded by tiny discrete quartz grains, and its rim is altered to quartz and biotite (Fig. 3g). Euhedral garnet

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GI core usually contains disseminated magnetite (Fig. 3h), and the fractures (if present) are usually filled with tiny quartz grains (Fig. 3i). GII garnet is characterized by core-rim texture, with the core and rim

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bounded by tiny discrete quartz grains (Fig. 3j). The garnet coexists with massive magnetite, and locally pyrite grains and fine pyrite veins (Fig. 3k). Massive magnetite, garnet, and quartz also coexist with allanite

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(Fig. 3l). and the paragenesis of Talate is present in Figure 4

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3.3 Methodology and common Pb corrections Electron probe microanalysis (EPMA), including in-situ major element analysis, elemental mapping and back-scattered electron (BSE) imaging, was carried out at the School of Geosciences and Info-Physics of the Central South University, using a 1720 EPMA (Shimadzu Corporation, Japan). Analytical conditions include 15 kV (accelerating voltage), 2.0 × 10-8 A (probe current) and 1 μm (spot size). Natural silicate minerals were used as standards for garnet, including quartz [Si], almandine [Fe, Mg], corundum [Al], yttrium aluminium garnet [Al], diopside [Ca] and metasilicate [Mn]. For titanite, rutile [Ti] and fluorite [F] were used for calibration. The spectral lines, peak time (s), and off-peak background time (s) used for the

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ACCEPTED MANUSCRIPT wavelength-dispersive spectroscopy (WDS) analyses were as follows: Si (Kα, 10, 5), Al (Kα, 10, 5), Mg (Kα, 10, 5), Ca (Kα, 10, 5), Ti (Kα, 10, 10), Fe (Kα, 10, 10), Mn (Kα, 10, 10), F (Kα, 10, 5). Detection limits for the elements are below 0.01 wt %. and with 0.01% detection limit. Data were corrected using the internal ZAF3 correction program. LA-ICP-MS U–Pb dating and trace element analyses of garnet and titanite were performed at the Key

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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. A 32 µm spot size was used with an energy density of 5 J/cm2 and a repetition rate of 5 Hz. The trace element compositions were calibrated against the standard NIST610 (Pearce et al., 1997), using Si determined by EPMA as the internal standard for garnet and titanite. Zircon 91500 (Wiedenbeck et al., 1995)

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was used as the external standard for the garnet U–Pb dating, and the matrix effect has been proved to be minor for garnet U-Pb dating (Deng et al., 2017; Mezger et al., 1989; Seman et al., 2017), we use the garnet

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grain QC04 and OH-1 to monitor the garnet dating, yielded discordia intercept age of 131 ± 2 Ma and 1022 ± 20 Ma, consisted with the published data (Deng et al., 2017; Seman et al., 2017), which indicated the error

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caused by matrix mismatch issue could be neglect in this study. Titanite OLT-1 (Aleinikoff et al., 2002) was used as the external standard for the titanite U–Pb dating. Each analysis consists of a 20 s background

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measurement (laser-off) followed by 45s of data acquisition. Data reduction was performed using ICPMSDataCal software (Liu et al., 2009). ISOPLOT 3.0 software (Ludwig, 2003) was used to construct

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the Tera-Wasserburg diagram, isochrons and weighted mean calculations. For the titanite and garnet that contains negligible common Pb, the concordant age can be obtained

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using the model by Stacey and Kramers (1975) for Pb isotope correction. For the samples with high common Pb (common

206Pb

> 10%) and young age, caution should be taken in the Pb isotope correction, and the

choice of initial Pb isotope compositions would significantly influence the calculated ages (Frost et al., 2000). Therefore, we adopt the 207Pb correction method in this study, which is commonly used to determine the age of Proterozoic and younger samples (Aleinikoff et al., 2002; Kirkland et al., 2016; Stern, 1997). The uncorrected data are plotted in the Tera-Wasserburg diagram, and a regression through these analyses yields a lower intercept that represents the approximate age. The y-intercept represents the initial

207Pb/206Pb

(Aleinikoff et al., 2002), which can be used in an algorithm to allow a form of 207Pb-correction (Frost et al.,

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ACCEPTED MANUSCRIPT 2000; Stern, 1997). Subsequently, the individual 207Pb-corrected 206Pb/238U ages can be used to calculate the weighted average age, which represents the real age of titanite and garnet. Garnet and titanite common contain high initial common Pb contents, for the related intrusion and coexisted sulfide Pb isotopic compositions are unavailable at Talate, and the whole rock powder Pb isotope composition are not consistent with the in situ analysis here, therefore, the initial 207Pb/206Pb ratio are adopt

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based on the 207Pb correction method, which has been widely used to determine the age of relatively young titanite and garnet (Williams, 1998). The 204Pb correction is less precise, because the counting error on 204Pb is larger than on the more abundant 207Pb. 204Pb 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 age. The y-intercept represents the initial which can be used for the

207Pb-correction.

The amount of common

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207Pb/206Pb,

206Pb

is expressed as a

fraction of the total 206Pb (f206), which can be calculated from: 206

207

𝑃𝑏⁄ 206𝑃𝑏𝑚 − 207𝑃𝑏⁄ 206𝑃𝑏

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𝑃𝑏𝑐 𝑓206 = 206 = 𝑃𝑏𝑡𝑜𝑡𝑎𝑙

207

𝑃𝑏⁄ 206𝑃𝑏𝑐 − 207𝑃𝑏⁄ 206𝑃𝑏

∗

∗

where 207Pb/206Pbm is the measured ratio, 207Pb/206Pb* is the expected radiogenic ratio for the inferred

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age and 207Pb/206Pbc is the common Pb composition (the initial 207Pb/206Pb). This approach has been shown to be effective for correcting common-Pb in titanite and garnet minerals. This approach has been shown to

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Results

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be effective for correcting common-Pb in these U-bearing minerals.

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4.1 Geochemistry 4.1.1 Garnet

The EPMA major element compositions of garnet are summarized in Table A1. The GI garnet has relatively uniform SiO2 (34.8–38.6 wt.%), Al2O3 (18.6–20.8 wt.%), MnO (25.1–27.7 wt.%), FeO (11.5–13.3 wt.%), MgO (0.239–0.723 wt.%) and CaO (2.54–4.51 wt.%) contents, with their compositions varies from Sp56.5 Al28.3 Gr12.4 to Sp66.0 Al22.3 Gr8.5 (Sp = Spessartine; Al = Almandine; Gr = Grossular), with minor pyrope (1.0–3.0 %) and andradite (0–1.9 %). GII garnet shows similarly uniform SiO2 (35.0–36.6 wt.%), Al2O3 (19.0–20.0 wt.%), FeO (11.4–12.8 wt.%), MgO (0.202–0.661 wt.%) and CaO (2.86–3.28 wt.%) contents, but with higher MnO content (27.5–28.8 wt.%) than GI garnet. GII garnet has compositions ranging 9

ACCEPTED MANUSCRIPT from Sp64.2 Al24.8 Gr2.7 to Sp69.6 Al20.1 Gr1.2, with minor pyrope (0.8–2.6 %) and andradite (0.4–1.0 %). The REE concentrations of GI and GII garnet range from 60 to 292 and 51 to 92 ppm, respectively (Table A2). GI garnet is LREE-depleted HREE-enrich (chondrite-normalized), with no apparent Eu anomalies (Eu/Eu* = 0.72 to 1.29) (Fig. 5a). GII garnet shows similar REE patterns with GI garnet, but with an even lower HREE concentrations (GI = 35 to 242 ppm; GII = 8 to 54 ppm; Fig. 5b) and apparent positive

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Eu anomaly (Eu/Eu* = 1.72 to 4.33) (Fig. 5a). Uranium contents range from 1.3 to 7.1 and 3.3 to 5.7 ppm in GI and GII garnet, respectively, and they show a positive correlation with the total REE concentration (Fig. 5c). GI garnet shows less pronounced Eu anomaly but more distinct Ce anomaly than GII garnet (Fig. 5d). Th/U ratios of GI and GII garnet are of 0.19 to 0.36 and 0.28 to 0.66, respectively.

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4.1.2 Titanite

The EPMA major element compositions of the titanite from Talate are summarized in Table A3. The TI

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titanite contains relatively uniform SiO2 (29.3–32.6 wt.%), TiO2 (35.3–37.9 wt.%), CaO (26.1–27.4 wt.%), Al2O3 (0.89–2.05 wt.%) and FeO (0.95–2.31 wt.%). TII titanite also exhibits uniform SiO2 (31.0–33.2 wt.%),

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TiO2 (32.1– 36.2 wt.%), CaO (26.3 – 28.1 wt.%), Al2O3 (1.44 – 3.12 wt.%) and FeO (2.16–3.37 wt.%). Although the SiO2 and CaO contents are very similar in these two titanite types, TII titanite contains higher

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F (average: 0.78 wt.%) and FeO (average: 2.62 wt.%), and lower TiO2 (average: 34.1 wt.%) than TI titanite (average: 0.35 wt.%, 1.51 wt.% and 36.3 wt.%, respectively).

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The total REE concentrations of TI titanite range from 1052 to 5445 ppm, higher than those of TII titanite (675 to 2015 ppm). Both TI and TII titanite are featured by LREE-depletions and HREE-enrichments

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(Fig. 6a; Table A4), yet TI titanite contains higher HREEs (average: 2171 ppm) than TII titanite (average: 76 ppm). Uranium contents of TI (28 to 184 ppm) and TII (13 to 111 ppm) titanite are similar. Uranium show clear positive correlation with the LREEs in both TI and TII titanite (Fig. 6b), and similar positive correlation trend is also present between U and HREEs in TII titanite, but absent in TI titanite (Fig. 6c). These two types of titanite are featured by different degrees of Ce and Eu anomalies, with TI titanite containing lower Ce (1.22–1.33; average: 1.28) but higher Eu (0.85–1.76; average: 1.22) anomalies than TII titanite (1.33–1.49 (average: 1.43) and 0.60–0.78 (average: 0.65), respectively) (Fig. 6d).

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ACCEPTED MANUSCRIPT 4.2 U-Pb geochronology 4.2.1 Garnet age The corrected U-Pb isotope results of GI and GII garnet from Talate are presented in Table A5. In the Tera-Wasserburg diagram, the common Pb-uncorrected data of GI garnet define a linear array, giving a lower-intercept age of 237.3 ± 7.5 Ma, and a y-intercept of initial 207Pb/206Pb of 0.8913 (Fig. 7a). Using this

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common Pb composition, common Pb correction was conducted using the 207Pb-based method of Frost et al. (2000) and Stern (1997). The 35 analyses of GI garnet yielded a weighted average 206Pb/238U age of 231.7 ± 7.2 Ma (MSWD = 0.56), representing the formation age of GI garnet and is consistent within the error range of the lower-intercept age (Fig. 7a). GII garnet gave a lower-intercept age of 213.7 ± 9.9 Ma and a y-intercept initial

207Pb/206Pb

of 0.807 (Fig. 7b), and yielded a weighted average

206Pb/238U

age of 211.5 ± 5.8 Ma

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(MSWD = 1.1; n=47), representing the formation age of G II garnet and consistent within the error range of the lower-intercept age (Fig. 7b)

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4.2.2 Titanite age

The corrected U-Pb isotope results of TI and TII titanite from Talate are listed in Table A6. In the Tera-

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Wasserburg diagram, the common Pb-uncorrected data of TI titanite define a linear array, giving a lowerintercept age of 229 ± 15 Ma, and a y-intercept of initial

207Pb/206Pb

at 0.729 (Fig. 7c). With this common

a weighted average

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Pb composition, common Pb correction was conducted using the 207Pb-based method. All analyses yielded 206Pb/238U

age of 228.0 ± 4.6 Ma (MSWD = 2; n = 28) (Fig. 7c), representing the

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formation age of Talate titanite and consistent with the lower-intercept age within the error range. The common Pb-uncorrected data of type II titanite define a linear array, giving a lower-intercept age of 210 ±

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10 Ma, and the y-intercept of initial

207Pb/206Pb

at 0.699 (Fig. 7d). With this common Pb composition,

common Pb correction was conducted using the

207Pb-based

method. All analyses yielded a weighted

average 206Pb/238U age of 209.8 ± 3.6 Ma (MSWD = 1.03; n = 39) (Fig. 7d), representing the formation age of Talate titanite and consistent with the lower-intercept age within the error range.

5

Discussion

5.1 Petrogenesis of titanite and garnet The TI titanite is likely magmatic, as evidenced by its fine-grained euhedral to subhedral shape, and its

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ACCEPTED MANUSCRIPT coexistence with igneous accessory minerals such as the biotite, K-feldspar, plagioclase and quartz (Fig. 3a). Meanwhile, TII titanite is likely hydrothermal, as supported by its coarse-grained anhedral texture and coexistence with massive hydrothermal magnetite and quartz. Moreover, TI titanite (commonly as interstices or inclusion in primary biotite) contains similar major element compositions to typical igneous titanite, and lower Al (avg. 0.06 apfu (atoms-per-formula-unit)) and Fe (avg. 0.04 apfu) and higher Al/Fe ratio (avg. 1.31)

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than TII titanite (Fig. 8a). TI titanite is also characterized by its lower Al + Fe (avg. 0.11 apfu) and F (avg. 0.04 apfu), and higher Ti (avg. 0.90 apfu) values than those of TII titanite (0.15, 0.08 and 0.07 apfu, respectively) (Figs. 8b and c). TI titanite is Th-rich (23–168 ppm) and has higher Th/U (0.53–1.15) ratio than TII titanite (Fig. 8d). The TI magmatic titanite contains higher total REE (1052–5445 ppm), Yb (114– 764 ppm) and HFSE contents (e.g., Nb = 774–2932 ppm) than those of the TII hydrothermal titanite (675–

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2015 ppm, 0.04–3.59 ppm and 51–588 ppm, respectively).

GI garnet shows close relationship with TI titanite as GI hosts the magmatic TI, which indicated that

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the formation of GI is followed with TI, and both of them are coarse-grained, some with well-preserved core-rim texture, with some disseminated magnetite hosted with the garnet cores. The coarse-grained garnet

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(GII) was partial replaced by massive magnetite grains and TII titanite and were cut by the quartz veins and/or fine sulfide (pyrite, pyrrhotite and chalcopyrite) veins.

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GI garnet is LREE-depleted and HREE-enriched when normalized to the chondrite (Sun and McDonough, 1989), with no apparent Eu anomalies, while GII garnet shows similar REE patterns to GI but

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with apparent positive Eu anomaly (Fig. 5a). This could be due to relatively lower oxygen fugacity of the fluid at the time of GI generation. As the higher abundance of grossular (gro: 12.4) in GI relative to that of

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in GII (gro: 2.7) may imply that the GI has been formed by fluids with relatively higher oxygen fugacity (Shimazaki, 1977). Speciation can extend the Eu3+ stability in hydrothermal fluids at low temperatures (< 250 ºC), whereas Eu2+ ions dominate at above 250 ºC (Sverjensky, 1984). Geothermometric studies of the Talate skarn deposit indicate that the ore-forming fluids (quartz-magnetite stage) were of 271 to 426 ºC, suggesting that Eu2+ is dominant in the hydrothermal fluids. Fluid inclusion studies at Talate indicate that the hydrothermal fluids contain Cl- (0.76 to 0.97 wt. % by EDS; Li et al. 2014), which can enhance the stability of soluble Eu2+ except for REE3+ and form distinct positive Eu anomalies (Allen and Seyfried, 2005; Gaspar et al., 2008; Mayanovic et al., 2007, 2002). These garnet grains were likely formed at a higher

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ACCEPTED MANUSCRIPT temperature than that of the quartz-magnetite stage (426 ºC; Li et al. 2014). Sulfur isotopes (δ34Sfluid = 2.6– 5.2 ‰) of the quartz-magnetite stage indicate a magmatic-related hydrothermal system, which is consistent with a skarn origin of this garnet type. The garnet species at Talate are dominated by spessartite, almandine and grossular, distinct from most Fe-Cu skarn deposits (which contain mainly Fe- and Al-rich garnet (Meinert, 1992; Zhang et al., 2017b, 2017a) but similar to many Pb-Zn skarn deposits (Hammarstrom et al.,

5.2 Onset and duration of skarn mineralization

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1986; Newberry et al., 1991).

Since TI titanite is magmatic, its crystallization can constrain the timing of the magmatic-dominated, early-stage hydrothermal mineralization. TI titanite dating yielded a corrected U-Pb isotope 206Pb/238U age

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of 228.0 ± 4.6 Ma (Fig. 7a). These euhedral TI titanite grains host within hornblende and garnet (Fig. 3b), whilst some fine-grained euhedral to subhedral titanite grains are included by primary quartz (Fig. 3c). The

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TI titanite should formed slight earlier than the GI garnet, even the mean age of titanite is younger than the garnet GI (231.7 ± 7.2 Ma; Fig. 6c). When considering the errors, they are overlap with each other and

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represent for the most possible initiation time of skarn alteration. Hydrothermal TII titanite coexists with GII garnet, and is dated to be 209.8 ± 3.6 Ma (Fig. 7d). This

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age represents the timing of the sub-economic massive magnetite mineralization, with which GII garnet of (211.5 ± 5.8 Ma) coexists (Fig. 7b). Comparing the ages of TI titanite/GI garnet with TII titanite/GII garnet,

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a 18 My (or 8.1 My considering the analytical uncertainty) age gap is present (Fig. 9). Therefore, these two titanite/garnet types were likely formed during two distinct mineralization events. This conclusion is

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consistent with the phenomenon that retrograde alteration/mineralization is usually accompanied by the brecciation led by magmatic-hydrothermal fluid influx, which would generate several pulses of mineralization (Li et al., 2018; Meinert et al., 2005), as being invoked to account for the triggering of the porphyry-related mineralization processes (Hattori and Keith, 2001; Hollings et al., 2013). The second mineralization is followed by the onset of Talate skarn alteration, which indicates that the initial hydrothermal alteration may have faded, until it was influenced and overprinted by the second pulse of magma heating event. The garnet, titanite and biotite (Zheng et al., 2017) ages indicate that the mineralization commenced in the Late Triassic (~ 230 Ma), significant later than the formation of the

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ACCEPTED MANUSCRIPT Devonian Kangbutiebao volcanic host rock (Chai et al., 2009). The two distinct mineralization events here suggest that the skarn mineralization could be affected by several heating events and lasted for several million years, which is consistent with many important skarn deposits in the world, such as the Coroccohuayco porphyry-skarn deposit in Peru (Chelle-Michou et al., 2015) and Tengtie skarn system in

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Nanling range, South China (Zhao et al., 2016).

5.3 Element mobility and U incorporation in spessartine

A major concern of utilizing the garnet chronometer is that uranium may be incorporated as mineral inclusions (e.g., zircons or monazite) rather than in the garnet crystal lattice (Deng et al., 2017), especially for andradite-rich garnet due to the decreasing incorporation energy of U in the dodecahedral site (with the

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Fe3+ increase in the neighboring tetrahedral site) (Rák et al., 2011). But for the case of Talate, spessartine dominates and the incorporation regime remains unknown. BSE imaging indicates that the garnet (both GI

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and GII types) has no zoning texture, and the EPMA elemental mapping and flat LA-ICP-MS time-resolved signals indicate that the garnet bear no U-rich inclusions (details shown in the U map in Fig. 10a and b).

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These textural and geochemical features are also shared by TI and TII titanite, indicating that the U in both garnet and titanite is incorporated in the crystal lattice, and that their U-Pb ages can represent the timing of

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crystallization during the skarn alteration/mineralization. These trace element maps of garnet have revealed the element mobility and incorporation regime: Both

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GI and GII garnet show clear positive correlation between U, Al and Mn, and the high U, Al and Mn areas overlap in elemental maps, yet such features are absent for Fe (Figs. 10a and b). This may reflect a coupled

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substitution of [Al3+]IV in the equation of [U4+]VIII + 2[Al3+]IV − [Ca2+]VIII + 2[Si4+]IV, which is different from the regime of [U4+]VIII + 2[Al3+, Fe3+]IV − [Ca2+]VIII + 2[Si4+]IV suggested by Deng et al. (2017). Similar patterns have also been found in other andradite-rich garnet elemental maps from the Beiya giant Fe-Au and Xinqiao Fe skarn deposits (author’s unpubl. data), which indicate that the U incorporation may have been closer correlated with [Al3+]IV than [Fe3+]IV (Figs. 10a and b). Similar correlations are also found between U and LREEs (Figs. 10a and b), implying a similar substitution mechanism in grandite garnet (Gaspar et al., 2008; Park et al., 2017; Smith et al., 2004), andradite garnet (Deng et al. 2017; author's unpubl. data) and spessartine (this study). The correlation between U with HREE are not obvious, probably because the U

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ACCEPTED MANUSCRIPT incorporation into hydrothermal garnet is also dependent on surface sorption during the rapid crystal growth led by fluid infiltration (Jamtveit and Hervig, 1994; Smith et al., 2004).

5.4 Implications for spessartine chronometers Recent studies suggest that Fe-rich garnet can incorporate abundant uranium, and is a potentially useful

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chronometer for Fe skarn, iron-oxide-copper–gold (IOCG) and REE-U deposits (Deng et al. 2017 and reference therein). However, Fe-rich garnet is unsuitable to date Pb-Zn skarn deposits because the latter is commonly intrusion-distal and of significantly lower temperature.

Previous biotite Ar-Ar dating indicates that the late-skarn alteration and quartz-magnetite mineralization at Talate occurred at ca. 228 Ma, while the quartz-sulfide mineralization occurred at ca. 214

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Ma (Li et al., 2014). Sulfur isotope compositions of the two mineralization phases are of 5.0–5.2‰ and – 1.7‰ (Li et al., 2014), respectively, indicating that the two mineralization phases are not continuous. These

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ages are similar to the spessartine ages obtained (ca. 232 Ma and 212 Ma), suggesting that the LA-ICP-MS spessartine dating method presented in this study is a rapid and reliable technique to constrain the timing of

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skarn alteration/mineralization, and is less susceptible to post-mineralized thermal resetting than biotite or muscovite. Conclusions

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6

Magmatic titanite from the Talate skarn Pb-Zn deposit is characterized by lower (Al + Fe) and F

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contents, but with high Ti values, the TII titanite coexists with massive magnetite and garnet. Both TI and TII titanite are featured by high (Al + Fe) and F contents and low Ti content. Titanite U-Pb dating indicates

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two distinct mineralization events at 228.0 ± 4.6 Ma and 209.8 ± 3.6 Ma. Corresponding to the two titanite types (TI and TII), there are two spessartine garnet types (GI and GII), which yielded a weighted average 206Pb/238U age of 231.7 ± 7.2 Ma (GI; MSWD = 0.56; n = 35) and 211.5 ± 5.8 Ma (GII; MSWD = 1.1; n = 47). Considering the common Pb and matrix mismatch caused errors, the corrected spessartine U-Pb ages are consistent with those of the titanite, indicated that the spessartine-rich garnet could be independent chronometer that suitable for the skarn Pb-Zn deposit.

Acknowledgements

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ACCEPTED MANUSCRIPT This work was financially supported by the National Natural Science Foundation of China (NSFC) (41702067, U1302233 and 41602067), the Fundamental Research Funds for the Central Universities (20174200031610052), and the Higher School Specialized Research Fund for Doctoral Program (200805580031). Dr. Jianping Liu was thanked for the EPMA analysis. The Profs. Franco Pirajno, Ilkay Kuşcu and an anonymous reviewer are thanked for constructive reviews that greatly improved the quality of

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this paper.

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Gondwana Res. 43, 4–16.

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ACCEPTED MANUSCRIPT Figure captions Fig. 1.

(a) Sketch map showing the tectonic framework of North Xinjiang. (b) Geologic map of the Abagong polymetallic ore belt (modified after Geological Team 706, 2000). (c) sketch geologic profile for the Talate Exploration Line 144. Generalized stratigraphic columns of the Kelan Basin (modified after Chai et al., 2009).

Fig. 3.

BSE imaging, showing the mineral assemblage of titanite and spessartine. (a) TI titanite coexists

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Fig. 2.

with plagioclase, pyroxene, disseminated magnetite and quartz. (b) euhedral titanite hosts within hornblende and garnet; (c) subhedral titanite hosts within hornblende, both included by quartz; (d) anhedral titanite show close relationship with the massive magnetite and quartz; (e) tabular titanite is host within quartz; (f) thin titanite rim around coarse-grained magnetite; (g) subhedral coarse-

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grained garnet, which is characterized by core-mantle-rim texture. Garnet is bounded by tiny discrete quartz grains, the rims are altered into quartz and biotite; (h) euhedral garnet with its core

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filled with disseminated magnetite; (i) massive garnet with disseminated magnetite. Fractures in garnet filled with quartz; (j) the core-rim textured garnet, with the core and rim bounded by tiny

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and discrete quartz grains. Garnet coexists with massive magnetite, and locally pyrite and tiny pyrite veins; (k) massive magnetite coexists with coarse-grained garnet, both cut by tiny pyrite

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veins; (l) coexisting massive magnetite, garnet, allanite and quartz. (Abbreviations: Mgt 1 = disseminated magnetite, Mgt 2 = massive magnetite, Ttn 1 = titanite TI, px = pyroxene, Plg =

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plagioclase, Ttn 1 = magmatic titanite TII; Ttn 2 = hydrothermal titanite, Qtz = quartz, Bt = biotite, Py = pyrite, Aln = allanite, Grt 1 = garnet GI, Grt 2 = garnet GII). Paragenetic sequence of the Talate Pb-Zn (-Fe) deposit (modified after Li et al., 2016).

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Fig. 4.

Abbreviations: M-H-Mgt = magmatic-hydrothermal disseminated magnetite; M-Mgt = massive magnetite; QM = quartz magnetite stage; QS = quartz sulfide stage; QC = quartz carbonate stage. Fig. 5.

Diagrams showing (a) chondrite-normalized REE pattern; (b) U vs. HREE; (c) U vs. LREE; (d) Eu anomaly vs. Ce anomaly of garnet.

Fig. 6.

Diagrams showing (a) chondrite-normalized REE pattern; (b) U vs. LREE; (c) U vs. HREE; (d) Eu anomaly vs. Ce anomaly of titanite.

Fig. 7.

The Tera–Wasserburg diagram for (a) GI and (b) GII garnet, and for (c) TI and (d) TII tianite.

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ACCEPTED MANUSCRIPT Fig. 8.

(a) ternary Al-Fe-10*F (apfu) diagram; (b) binary Al+Fe (apfu) vs. F (apfu) diagram; (c) diagram showing obvious negative relationship between TiO2 (wt. %) and F (wt. %); (d) U vs. Th diagram for the Talate titanite.

Fig. 9.

The geochronological constraints of two distinct mineralization events.

Fig. 10.

Representative trace elemental maps (Al, Fe, Mn, LREE, HREE and U) of (a) GI and (b) GII

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garnet. (abbreviations are as Figure 3).

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Highlights Distal Pb-Zn skarn commonly displays ambiguous relationships with intrusions; Spessartine are well developed in that of distal Pb-Zn skarn deposit;

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U is incorporated within the spessartine lattice by a substitution regime;

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Our study represents a new way of directly dating of skarn Pb-Zn deposit.

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Graphics Abstract

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