Geochemistry of the garnets in the Baiganhu W–Sn orefield, NW China

Accepted Manuscript Geochemistry of the garnets in the Baiganhu W–Sn orefield, NW China Jianhou Zhou, Chengyou Feng, Daxin Li PII: DOI: Reference:

S0169-1368(16)30386-9 http://dx.doi.org/10.1016/j.oregeorev.2016.11.019 OREGEO 2024

To appear in:

Ore Geology Reviews

Received Date: Revised Date: Accepted Date:

2 July 2016 13 November 2016 13 November 2016

Please cite this article as: J. Zhou, C. Feng, D. Li, Geochemistry of the garnets in the Baiganhu W–Sn orefield, NW China, Ore Geology Reviews (2016), doi: http://dx.doi.org/10.1016/j.oregeorev.2016.11.019

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Geochemistry of the garnets in the Baiganhu W–Sn orefield, NW China Jianhou Zhou a,*, Chengyou Feng a, Daxin Li a a

MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, People’s Republic of China

Abstract The Baiganhu W–Sn orefield in the southeastern Xinjiang Uygur Autonomous Region is associated with Caledonian S-type syenogranites and metasediments of the Paleoproterozoic Jinshuikou Group. Four types of garnets have been identified in the orefield using petrographic and major and trace element data. Grt-I garnets are generally present as inclusions within magmatic quartz in the syenogranites, with end-member formulas of Sps45–53Alm46–53Adr0–1Grs0–1Prp0–1 and rare earth element (REE) patterns that are enriched in heavy REE (HREE) and contain strong negative Eu anomalies. Grt-II garnets are associated with tourmaline and quartz and occur in interstices between feldspars within the syenogranites.

In

general,

the

Grt-II

garnets

have

end-member

formulae

(Sps64–70Alm29–34Adr0–1Grs0–2Prp0) and REE patterns that are similar to the Grt-I garnets although they are more spessartine-rich and contain higher concentrations of HREE. Grt-III garnets coexist with clinopyroxenes and Mo-rich scheelites within skarns developed along the syenogranite and marble contact. Their compositions are Adr62–88Grs1–18Sps3–12Alm0–8Pyr0 and they have relatively flat REE patterns with no negative Eu anomalies. Grt-IV garnets are present as massive aggregates that are often cross-cut by Mo-poor scheelite-bearing calcite veins. Their end-member formulas are Adr4–22Grs62–73Sps5–16Alm2–10Pyr0 and they have slightly domed REE patterns without negative Eu anomalies. Both Grt-III and Grt-IV garnets contain lower concentrations of the HREE (2–3 and 4–32 ppm, respectively) than Grt-I and Grt-II garnets (682–1352 ppm with Y = 1558–2159 ppm, and 6051–12831 ppm with Y =9663–13333 ppm, respectively). The occurrences, mineral assemblages, major element compositions, and REE patterns *

Corresponding author. Tel.: +86 156 5230 4137. E-mail: [email protected].

1

of the Grt-I and Grt-II garnets suggest they have magmatic origin and crystallized at relatively low temperatures and pressures, whereas the Grt-III and Grt-IV garnets have hydrothermal origin. The association of magmatic garnet (Grt-II) with tourmaline suggests that boron in S-type magmas has an important role in the formation of W–Sn mineralization during magmatic and subsequent hydrothermal processes. All four types of garnets contain low concentrations of W (≤1.64 ppm) and Mo (<0.77 ppm), but relatively high and variable concentrations of Sn (22.96–8364 ppm). A positive correlation between SnO2 and andradite molecule contents confirms the substitution of Sn4+ for Fe3+ within these garnets. These data suggest that Sn-rich andradite–grossular garnets in skarns may be used as an indicator mineral for W–Sn exploration when combined with other geological, geophysical, and geochemical signatures. Keywords: magmatic and hydrothermal garnet; W–Sn mineralization; temperature–pressure conditions; tourmaline and boron; Sn-rich garnets; indicator minerals.

1. Introduction Tungsten mineralization is generally associated with granitoids that may contain variable amounts of garnet (Xu et al., 1984; Tan, 1985; Li and Yan, 1991; Yang et al., 2013). Recent research has focused on the genesis and significance of garnets in granitic rocks (e.g., Taylor and Stevens, 2010; Lackey et al., 2012; Hönig et al., 2014; Samadi et al., 2014), suggesting five potential sources for these garnets: (A) restitic garnet formed as a restite phase during partial melting (e.g., White and Chappell, 1977; Chappell et al., 1987; Stone, 1988; Wang and Wang, 1989; Clarke and Rottura, 1994; Krippner et al., 2014); (B) garnet produced by magmatic crystallization, including late-stage occurrences in aplites and pegmatites (e.g., Miller and Stoddard, 1981; Li and Yan, 1991; Harangi et al., 2001; Dahlquist et al., 2007; René and Stelling, 2007; Yang et al., 2013; Hönig et al., 2014; Samadi et al., 2014); (C) xenocrystic garnet derived from the metamorphism of the surrounding country rock (e.g., Allan and Clarke, 1981; Clarke, 1981, 2007; Erdmann et al., 2009; Lackey et al., 2011; Scallion et al., 2011; Gao et al., 2012; Krippner et al., 2014); (D) peritectic garnet 2

derived from wallrock or xenolith material that reacted with the host magma (e.g., Stevens et al., 2007; Villaros et al., 2009; Taylor and Stevens, 2010; Dorais and Tubrett, 2012; Lackey et al., 2012); and (E) secondary metasomatic garnet formed by the interaction between post-magmatic hydrothermal fluids and the hosting granitoids (e.g., Kontak and Corey, 1988; Clarke and Rottura, 1994; Deer et al., 2013). Combining petrography and major and trace element data could provide insights into the origin of garnets formed in these settings (Jung and Hellebrandt, 2006; Krippner et al., 2014). Skarn-hosted tungsten and tin forms an important part of the total W–Sn resources of China, as exemplified by the recently discovered Zhuxi W–Cu polymetallic skarn deposit in northeast Jiangxi Province, which may be the largest W deposit in the world. This deposit contains 2,860,000 t of WO3 resources at an average grade of 0.504 wt% that are associated with 224,400 t of Cu resources at an average grade 0.98 wt% (Chen et al., 2016; Department of Land and Resources of Jiangxi Province, 2016). Tin-bearing garnets within W–Sn skarns are well-known (e.g., Saksela, 1951; Deer et al., 1982; Chen et al., 1992a, 1992b, 1992c), with an andradite (Grs36.6) containing 1.44 wt% SnO2 reported from a polymetallic cassiterite deposit at Pitkaranta, Finland (Saksela, 1951) and andradite-rich garnets containing up to 5.14 wt% SnO2 reported from the Gejiu Sn skarn in Hunan Province, China (Chen et al., 1992a). A recently discovered new mineral named toturite (a Sn-rich garnet: Ca3Sn2Fe2SiO12), may contain up to 38.51 wt% SnO2, with almost all of the Sn located in octahedral sites (Galuskina et al., 2010). The Baiganhu W–Sn orefield, located ~250 km south of the Ruoqiang County in the southeast Xinjiang Uygur Autonomous Region (Fig. 1a) and consisting of the Kekekaerde, Baiganhu, Bashierxi, and Awaer deposits, is a newly discovered large W–Sn orefield associated with Caledonian granitic rocks and metasediments of the Paleoproterozoic Jinshuikou Group (Li et al., 2012; Li et al., 2013a; Wang et al., 2014b; Zheng et al., 2016; Zhou et al., 2016). These deposits comprise mainly three main types of W–Sn mineralization, namely early-stage skarn-, middle-stage greisen-, and late-stage quartz vein-types (Li et al., 2013a; Zhou et al., 2016). This paper reports new major and trace element data for four differing types of garnets from parental syenogranites and associated skarns as well as mineral assemblage and textural data, all of which provides insights into the genesis of these 3

garnets and their significance for the W–Sn mineralization in this area. We also report the concentrations of elements such as W, Sn, and Mo in these garnets and conclude that the presence of Sn-rich andradite–grossular garnet within skarns may be a useful indicator mineral for W–Sn exploration. Mineral abbreviations throughout the text are according to Whitney and Evans (2010). 2. Geological setting The geological characteristics of the Baiganhu W–Sn orefield have been previously described by Zhou et al. (2016). The principal features from this study are described below and have been integrated with our new observations and data. The Baiganhu W–Sn orefield is located at the junction of the Arkyn Tagh and the East Kunlun terranes (Fig. 1a) and is bound by the Arkyn Tagh Fault to the north (~20 km from the orefield) and by the Baiganhu Fault to the east (~2–3 km from the orefield; Fig. 1b). Rocks that outcrop in the Baiganhu orefield consist mainly of the Xiaomiao Formation of the Paleoproterozoic Jinshuikou Group and the Baiganhu Formation of the Silurian Qimantage Group. These two formations are separated by the NE-trending Baiganhu Fault (Fig. 1b). The Xiaomiao Formation, which crops out to the north of the Baiganhu Fault, consists mainly of low-grade metamorphosed secondary muscovite–quartz schist and marble with minor plagiogneiss, and thus the protoliths were either siliciclastic or carbonate sedimentary rocks with minor volcanic rocks; this formation is the major host rock of the W–Sn deposits. The Baiganhu Formation, which crops out to the south of the Baiganhu Fault, consists mainly of low-grade metamorphosed graptolite-bearing shale facies. The major constituents of this formation are slate and phyllite derived from mudstone and siltstone. The major magmatic Caledonian Bashierxi intrusions were emplaced into the Xiaomiao Formation as stocks, batholiths, or apophyses and comprising 20 major intrusions with a total exposed surface area of ~275.5 km2 (Li et al., 2013b). These intrusions are classified as monzogranites, alkali feldspar granites, and syenogranites (Fig. 1b), with recent research (Wang et al., 2014b; Zhou et al., 2016) suggesting that these intrusions were emplaced in a post-orogenic setting at ca. 432–413 Ma with a nearly contemporaneous evolution from A-type monzogranites and alkali feldspar granites (~432–421 Ma) to S-type syenogranites 4

(~428–413 Ma), where the latter are more closely linked to the W–Sn mineralization in this area. The A-type monzogranite and alkali feldspar granite units in this area are present as stocks or batholiths in the northern part of the orefield (Fig. 1b). Both of these types of granite have subhedral to anhedral coarse-grained textures and are generally quite fresh, not associated with either economic W–Sn mineralization or associated alteration. The monzogranites contain K-feldspar (>10 mm, 35%–40% by volume), plagioclase (25%–30%), quartz (25%–30%), biotite (5%–10%), and hornblende (±5%), whereas the alkali feldspar granites contain K-feldspar (>10 mm, 60%–65%), quartz (25%–30%), biotite (5%–10%), and plagioclase (<5%). Both units contain accessory titanite, apatite, and zircon, without inherited zircons identified during cathodoluminescence imaging (Li et al., 2012). Petrographic observations show that the mafic minerals (hornblende and/or biotite) formed later than or contemporaneously with the felsic minerals in these intrusions, with both types of granite free of both garnet and tourmaline. These intrusions contain high total alkali (Na2O+K2O = 8.30–9.69 wt%) and total rare earth element (TREE: 203–413 ppm) concentrations, have elevated high field strength element (HFSE: Zr + Nb + Ce + Y = 562–1006 ppm) concentrations, and have relatively high zircon saturation temperatures (857°C–917°C), with A/CNK (Al2O3/(CaO + Na2O + K2O), by mole) ratios of 0.92–1.02. These intrusions therefore belong to metaluminous or weakly peraluminous shoshonite series (Zhou et al., 2016). The S-type syenogranites in the study area were emplaced as stocks into the Xiaomiao Formation and have an exposed surface area of ~2.5 km2 in the area around the Baiganhu and Bashierxi deposits but are not exposed in the area around the Kekekaerde and Awaer deposits barring small apophyses (<1 m) that crop out at the surface (Figs 1b, 2). The contact zone between the syenogranites and the Xiaomiao Formation contains greisen-type scheelite ± wolframite, skarn-type scheelite, and quartz vein-type wolframite ± cassiterite ± scheelite mineralization (Figs 1b, 2). The fresh syenogranite contains K-feldspar (2–5 mm, 40%–45%), quartz (2–5 mm, 30%–35%), plagioclase (2–5 mm, 20%–25%), and minor amounts of biotite (1–3 mm, ±5%) and magmatic muscovite (1–3 mm, ±5%), with accessory zircon, garnet, tourmaline (locally abundant), apatite, and Mn-rich ilmenite. Inherited zircons have also been 5

identified during cathodoluminescence imaging (Zhou et al., 2016). Biotite is present as euhedral crystals or as inclusions in quartz, both of which are indicative of relatively early crystallization compared to the felsic minerals in these intrusions. The syenogranites contain relatively low concentrations of the HFSE (Zr + Nb + Ce + Y = 176–225 ppm) and have relatively low zircon saturation temperatures (751°C–791°C), with A/CNK ratios and K2O values of 1.07–1.12 and 2.88–4.23 wt%, respectively. This indicates these intrusions are peraluminous and high-K calc-alkaline series (Zhou et al., 2016). 3. Ore deposit geology The Kekekaerde, Baiganhu, and Bashierxi deposits in the northeast part of the orefield have been intensely explored, with a total of 76 discrete W–Sn orebodies identified up to 2013 yielding indicated reserves of 174,913 t WO3 and 79,091 t Sn. The deposits are classified as skarn-, greisen-, and quartz vein-type orebodies based on mineralization and alteration characteristics (Figs 2–5). Skarn-type orebodies are localized in the contact zone between the syenogranite and the marble wallrock that is interbedded in the Xiaomiao Formation. They are generally bed-like or lenticular, strike NE–SW, and dip to the SE. Ore thicknesses vary from several to tens of meters and the mineralization in this area is dominated by scheelite and is hosted by skarn or skarn-altered marble. At least two types of scheelite have been recognized in the skarn, an earlier scheelite that is present as coarse tabular crystals coexisting with garnet, pyroxene, and later stage replacements of fluorite, quartz, molybdenite, and calcite (Figs 5a–b), and later scheelite that are present as fine-grained disseminations within late-stage calcite veins that cross-cut garnets (Figs 5c–f). No oxides such as magnetite, cassiterite, or hematite have been identified in these skarns although they do contain local molybdenite, arsenopyrite, pyrrhotite, pyrite, and chalcopyrite accumulations; however, no economic Mo or Cu orebodies have been identified to date. The mineralization is hosted by a pyroxene, amphibole, calcite, quartz, phlogopite, fluorite, zoisite, and garnet gangue, with garnets relatively rare throughout the area but are locally abundant within massive units in contact zones (e.g., within the Bashierxi deposit; Fig. 5c). The skarn-type orebodies typically have grades of 0.15–1.07 wt% WO3 (mean of 0.37 wt%), as exemplified by the Baiganhu deposit. 6

Greisen-type orebodies are generally located in the endocontact zone of intrusions, especially within the upper part of the buried intrusion cupola, and within aplitic apophyses that emanate from cupolas (Fig. 2b). These orebodies are generally tabular or lenticular and are also present as vein-type orebodies within the metasedimentary wallrocks above the cupola (Fig. 2b). The mineralization is generally hosted by pervasively greisen-altered syenogranites or greisen zones and is dominated by scheelite (Fig. 3c) with small amounts of wolframite, cassiterite, pyrite, arsenopyrite, pyrrhotite, molybdenite, and sphalerite, all of which are hosted by a quartz, K-feldspar, plagioclase, muscovite, abundant tourmaline, and rare fluorite and topaz gangue. These greisen deposits are exemplified by the Kekekaerde deposit, where grades range from 0.15 to 0.42 wt% WO3 (mean of 0.24 wt%). Quartz vein-type orebodies are common in intrusion exocontact zones, within fractures and fissures in the overlying metasediments, and to a lesser extent in greisen-altered zones in the parental syenogranites (Figs 2b, 3b). These orebodies have highly variable shapes and occurrences, including veins that bifurcate and recombine as well as expand and contract. These orebodies (especially those in the exocontact zone) are controlled by fractures that were most likely the result of regional tectonic activity enhanced by the intrusion of the granitic rocks in this area. These deposits generally strike NE–SW and dip to the SE, with lengths, widths, and depths of 500–800, 0.5–2, and 40–150 m, respectively. They contain wolframite and cassiterite (Figs 3b, d) with minor amounts of scheelite (Fig. 3e–f), small amounts of chalcopyrite and pyrite, and rare molybdenite, all of which are hosted by a quartz, muscovite, tourmaline, K-feldspar, Mn-rich apatite, rare fluorite gangue. These orebodies are relatively high grade and contain 0.15–1.77 wt% WO3 (mean of 0.30 wt%) and 0.10–0.56 wt% Sn (mean of 0.30 wt%). Skarn mineral assemblages such as garnet and clinopyroxene are locally cross-cut by scheelite-bearing greisen stringers typically with quartz and muscovite (Fig. 3a). This indicates that greisen alteration and related mineralization formed after the skarns and associated mineralization. Some wolframite-bearing quartz veins in the endocontact zone cross-cut greisen-altered parental syenogranites (especially in the cupola; Fig. 3b) where infilled fractures may be related to primary fractures formed during the syenogranite cooling process, to fluid pressure that generated hydraulic fracturing, or both. This suggests that the 7

skarn-type mineralization roughly formed early, followed by the middle-stage greisen-type of mineralization, and the late-stage quartz vein-type of mineralization. However, the complexity of the hydrothermal fluids in this area means that these three stages of mineralization are not completely isolated and most likely overlapped temporally. 4. Analytical methods Systematic petrographic studies were performed on more than 150 hand specimens, polished sections, and thin sections from outcrops and drill holes. Where garnets were recognized, more detailed petrographic examinations such as textural, mineral assemblage, and metasomatic replacement identifications were undertaken, with representative samples were selected for major and trace element analyses. In situ garnet oxygen isotopic analyses by SIMS at the Beijing SHRIMP Center, China, could not be pursued due to the lack of standard materials. 4.1. Major element analyses Major element compositions were determined at the Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, China, using a JOEL JXA-8230 electron microprobe analyzer (EPMA) with an accelerating voltage of 15 kV, a beam current of 20 nA, and a beam diameter of 5 µm. Natural and synthetic oxides were used as standards (jadeite for Si, Na and Al; forsterite for Mg; hematite for Fe, orthoclase for K, wollastonite for Ca, rutile for Ti, topaz for F, and synthetic MnO and NaCl for Mn and Cl, respectively). Matrix corrections were undertaken using the ZAF correction program supplied by the manufacturer, with the precision of the analyzed elements better than 1.5%. The proportions of ferric and ferrous iron in the garnets were calculated based on charge balances with garnet end-members calculated according to Locock (2008). 4.2. Trace element analyses In situ trace element concentrations were determined using polished thin sections and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) at the National Research Center for Geoanalysis, Beijing, China using a New Wave UP 193 laser ablation 8

system coupled to a Thermo Element II single collector high-resolution magnetic sector ICP–MS. The laser was operated at 10 Hz, with a laser fluency of 15 J/cm2, a spot size of 40 µm, and background and analysis counting times of 20 and 40 seconds, respectively. The analytical methods and procedures used are the same as described by Hu et al. (2008). A National Institute of Standards (NIST) 612 glass standard was used for external calibration and was measured twice at the beginning and end of each analytical batch (approximately 8 analyses). Garnet SiO2 contents determined by EPMA were used for internal standardization, with the limit of detection (LOD) of the analyzed elements given in the results table. The precision and accuracy of the analyses are better than 10% for most trace elements based on replicate analyses of the reference material. The LA–ICP–MS analyses are generally adjacent to the EPMA spots used for internal standardization, but some rims and mantles of the garnets were not analyzed by LA–ICP–MS as a result of their small grainsize. 5. Garnet petrography Four types of garnets have been recognized in the Baiganhu W–Sn orefield. Grt-I garnets are hosted by syenogranites, are euhedral, are very fine-grained (0.1–0.2 mm), and are often enclosed in magmatic quartz (Figs 4a, b). No optical zonation is present in these garnets. Grt-II garnets also occur in the syenogranite although they are subhedral, are larger (0.4–0.8 mm), often coexist with tourmaline and quartz, and are located in interstitial spaces between feldspars (Figs 4c–f). These garnets contain well-developed cracks that host secondary muscovite that has locally replaced the primary garnet. No deformation is present in either Grt-II or Grt-I garnets. Grt-III garnets are hosted by skarns between the syenogranite intrusion and the marble wallrock and are present as 5–10 mm crystals associated with disseminated Mo-rich yellow scheelite (MoO3: 11–15 wt%) and clinopyroxene, both of which are replaced by late–stage fluorite, calcite, quartz, and molybdenite (Figs 5a, b). Grt-IV garnets are also hosted by skarns developed between the syenogranite intrusion and the marble wallrock but are often present as massive aggregates associated with disseminated molybdenite and fluorite (Figs 5c–f). These garnets are commonly cross-cut by 9

Mo-poor gray–white scheelite-bearing calcite veins (Figs 5d–f). Grt-I and Grt-II garnets were identified in different drill holes meaning that there is no direct evidence of their relative crystallization ages. Spatially, Grt-II garnets are located in the uppermost parts of syenogranite intrusions (about 10–20 m from the contact between the intrusions and the hosting metasediments) whereas the Grt-I garnets are located 130–160 m from the contact boundary. Both Grt-III and Grt-IV garnets occur in outcrops but no direct cross-cutting relationships were observed between these garnets. However, Grt-III garnets coexist with pyroxenes and are replaced by fluorite that often coexists with the pyroxenes-absent Grt-IV garnets, suggesting that Grt-III garnets formed before the Grt-IV garnets. The petrographic characteristics and mineral assemblages described above suggest that the Grt-I garnets are magmatic and the Grt-III and Grt-IV garnets are hydrothermal. Grt-II garnets are more complicated and are examined in more detail in the discussion. 6. Mineral chemistry 6.1. Major elements The major element compositions and end-member formulas of the four types of garnets in the Baiganhu W–Sn orefield are given in Table 1 and Fig. 6. Grt-I garnets contain MnO (19.12–21.62 wt%),

FeO

(21.12–23.58 wt%),

Al2O3

(20.16–20.56 wt%),

and

SiO2

(35.32–36.27 wt%), with lesser amounts of CaO (0.25–0.34 wt%) and MgO (0.11–0.21 wt%), yielding an end-member formula of Sps45–53Alm46–53Adr0–1Grs0–1Prp0–1. Compared with Grt-I, the Grt-II garnets contain higher concentrations of MnO (27.18–29.34 wt%)

and

CaO

(0.38–0.61 wt%)

but

lower

(12.98–15.58 wt%)

and

MgO

(<0.04 wt%),

yielding

an

concentrations end-member

of

formula

FeO of

Sps64–70Alm29–34Adr0–1Grs0–2Prp0. No specific compositional zoning is present between the cores and rims of either Grt-I or Grt-II garnets (Fig. 6), although the Grt-II garnets have much higher MnO/(FeO + MnO) ratios than the Grt-I (Table 1) garnets, suggesting they are more evolved (MÜller et al., 2012). Grt-III garnets contain CaO (27.70–33.31 wt%), FeO (21.89–26.78 wt%), Al2O3

10

(2.22–8.12 wt%), SiO2 (34.75–36.93 wt%), and minor amounts of MnO (1.15–5.02 wt%), yielding an end-member formula of Adr62–88Grs1–18Sps3–12Alm0–8Prp0. In comparison, the Grt-IV garnets contain higher concentrations of Al2O3 (16.97–19.92 wt%), lower concentrations of FeO (5.94–9.32 wt%), and minor amounts of MnO (2.05–7.31 wt%), yielding an end-member formula of Adr4–22Grs62–73Sps5–16Alm2–10Prp0. All four types of garnets contain extremely low concentrations of chlorine (≤0.07 wt%) and Grt-I and Grt-II garnets contain extremely low concentrations of fluorine, with Grt-III garnets having generally low fluorine (≤0.31 wt%), and Grt-IV garnets containing the highest concentrations of fluorine (0.66 wt%). 6.2. Trace elements The ore metal and trace element compositions of garnets associated with the Baiganhu W–Sn orefield are given in Table 2 and are compared to the compositions of magmatic and hydrothermal garnets from elsewhere around the world in Fig. 7. All four types of garnets contain very low concentrations of W (≤1.64 ppm), Mo (<0.77 ppm), and Cu (<2.12 ppm), but contain higher and variable concentrations of Sn, with Grt-I and Grt-II garnets containing 23.22–79.83 and 22.96–89.67 ppm Sn, respectively. In comparison, the skarn-hosted Grt-III and Grt-IV garnets contain 2544–8364 and 51.7–269 ppm Sn, suggesting that the hydrothermal fluids that formed these skarns contained Sn. Grt-I garnets contain high TREE concentrations (686–1357 ppm), are enriched in the HREE (682–1352 ppm; Y: 1558–2159 ppm) and are LREE depleted (4–5 ppm), with both LREE/HREE and (LaN/YbN) ratios approaching zero. The Grt-I garnets also contain extremely low concentrations of Eu (<0.04 ppm) and have chondrite-normalized REE patterns that are HREE enriched with significantly negative Eu anomalies (Fig. 7a). Grt-II garnets have REE patterns similar to the Grt-I garnets (Fig. 7a), but have much higher TREE (6073–12867 ppm), LREE (22–36 ppm) and HREE (6051–12831 ppm; Y: 9663–13333 ppm) values. No core–rim trace element variations are present in either Grt-II or Grt-I garnets (Fig. 7a). The Grt-III and Grt-IV garnets contain similar LREE concentrations (4–10 and 4–26 ppm, respectively) as the Grt-I and Grt-II garnets but contain much lower 11

concentrations of the HREE (2–3 and 4–32 ppm, respectively) and higher concentrations of Eu (0.22–0.43 and 1.14–3.22 ppm, respectively). The Grt-III and Grt-IV garnets also have flat or slightly domed chondrite-normalized REE patterns (Fig. 7e) that are free of the negative Eu anomalies present within the Grt-I and Grt-II garnets (Fig. 7a). The HREE-enriched REE patterns of both the Grt-I and Grt-II garnets with their significantly negative Eu anomalies are similar to the REE patterns of other magmatic garnets (Figs 7a–d), such as those within biotite and two-mica granites associated with the Xihuashan W deposit (Yang et al., 2013), within leucogranites in the Himalayan orogenic belt (Gao et al., 2012), and within aplite and pegmatite units in the Khajeh Morad region of Iran (Samadi et al., 2014). The fractional crystallization of plagioclase is thought to be a major factor in the generation of Mn-rich garnets with significantly negative Eu anomalies that form during the later stages of magmatism (Gao et al., 2012; Samadi et al., 2014). Skarn-related garnets from the study area and other regions contain much lower HREE concentrations and have general positive but variable Eu anomalies (Figs 7e–h), compared to magmatic garnets within the studied syenogranites (Grt-I) and other granites (Figs 7a–d). The majority of experimental research into the partitioning of REE between magmatic hydrothermal fluids and silicate melts (e.g., Flynn and Burnham, 1978; Ayers and Eggler, 1995; Kravchuk et al., 1995; Bai and van Groos, 1999; Reed et al., 2000) suggest that an initial episode of boiling can fractionate the REE, with magmato-hydrothermal fluids generally being LREE-enriched, HREE-depleted, containing low concentrations of the REE, and having a general positive but variable Eu anomaly. 7. Discussion 7.1. Genesis of garnets within syenogranites in the study area The Grt-I garnets in the syenogranites in the study area are magmatic, as evidenced by their presence as individual euhedral fine-grained inclusions within magmatic quartz and a lack of replacement textures with other minerals (Figs 4a–b). In comparison, the origin of Grt-II, with the end-member formulas of (Sps64–70Alm29–34Adr0–1Grs0–2Prp0) and coexisting with the tourmaline and quartz, is not easy to determine. The following discussion will help to

12

better understand its origin. The Grt-II garnets are subhedral, angular, are free of metamorphic mineral inclusions, and contain extremely low concentrations of MgO (<0.04 wt%) and high concentrations of MnO (27.18–29.34 wt%). These characteristics indicate these garnets do not have xenocrystic or restite origins and were not derived from wallrock metasediments. They contrast sharply with garnet xenocrysts within granitoids of the South Mountain batholith in Nova Scotia, which are anhedral, rounded, inclusion-rich, commonly biotite-rimmed micro- or macro-crysts, and contain 1.50–4.26 wt% MgO (mean of 2.76 wt%; n = 25; Lackey et al., 2011). In addition, residual garnets in the Cerberean Cauldron of Australia coexist with cordierite and contain 2.5–7.5 wt% MgO (predominantly 5–7 wt%; n = 62; Birch and Gleadow, 1974), whereas those in the peraluminous Darongshan granite in the Guangxi Autonomous Region in China also coexist with cordierite and contain 4.87–6.44 wt% MgO (n = 6; Wang and Wang, 1989). The Grt-II garnets are also only associated with feldspar, quartz, and tourmaline rather than biotite, sillimanite, orthopyroxene, or ilmenite. However, these minerals are commonly present in peritectic garnets as a result of fluid-absent biotite melting that involves the following reactions: Bt + Qz + Pl + Sil = Grt + melt + Ilm ± Kfs in metapelites, and Bt + Qtz + Pl = Grt + Opx + melt + Ilm ± Kfs in metapsammites (Stevens et al., 2007). Peritectic garnets also generally have high Mg# values [10 * Mg/(Mg + Fe) = 12–27] and have low spessartine molecule contents (1–4 mol%; Taylor and Stevens, 2010), contrasting with the composition of the Grt-II garnets (Table 1). This suggests that the Grt-II garnets did not form by peritectic fluid-absent biotite melting. Garnet in W skarns commonly have higher Sps + Alm molecule contents than garnets developed in other metal-bearing skarns (such as, Sn, Mo, Au, Zn, Cu, and Fe; Meinert et al., 2005; Fig. 8a). And the Grt-II garnets are located in the upper parts of the intrusion and are close to the contact with metasediments interbedded with marbles. These can lead us to consider the Grt-II garnet as a skarn mineral. However, a lack of coexisting skarn minerals such as clinopyroxene and idocrase and the low CaO and Grs + Adr contents of these garnets suggests that they did not crystallize from metasomatic solutions during skarn formation. They contrast with Sps + Alm-rich garnets from the Kuga, Fujigatani and Kagata scheelite 13

skarn deposits in Japan (Sps5.8–51.2Alm5–31.5Adr1.8–12.7Grs28.7–83.2Prp0–0.3; n = 13) that contain 13.3–32.9 wt% CaO and coexist with Ca-clinopyroxene, vesuvianite, wollastonite, actinolite, and epidote (Shimazaki, 1977). Other examples of skarn-associated garnets are also described by Chen et al. (1992a) and Yao et al. (2013). In fact, the Sps + Alm values of Grt-II garnets plot close to the Sps + Alm pole of Fig. 8a and are outside the range of garnets associated with W skarns elsewhere in the world (Zhao and Barton, 1988; Chen et al., 1992a; Meinert et al., 2005). Moreover, the elevated HREE contents and significantly negative Eu anomalies of the Grt-II garnets (Fig. 7) are similar to the REE compositions of the magmatic Grt-I garnets as well as other magmatic garnets, such as those within in the Xihuashan granites and in the leucogranites of the Himalayan orogenic belt, but contrast sharply with hydrothermal Grt-III and Grt-IV garnets in the study area as well as other hydrothermal garnets, such as those associated with the Hehuaping Sn, Xintianling W, Xitian Sn, and Xieertala Fe–Zn skarns (Fig. 7). An in the discrimination diagram of Mange and Morton (2007), all of the Grt-II garnets fall into the magmatic filed (Fig. 8b), suggesting a magmatic rather than hydrothermal origin. The geochemistry and texture of the tourmaline associated with or included within the Grt-II garnets (Figs 4c–f, 8c–d) also supports the above conclusion (Zhou et al., in preparation). Although the majority of the tourmaline in peraluminous granites have subsolidus metasomatic replacement origins, experimental research suggest that some granites may contain magmatic tourmaline (e.g., Chorlton and Martin, 1978; Clarke, 1981). Research undertaken on the parental granitoids associated with Sn–W mineralization in southern China led Chen et al. (2000) to suggest that tourmaline in granitoids can form in magmatic or sub-solidus conditions. In addition, a detailed study of magmatic and hydrothermal tourmaline within granites in southwest England (London and Manning, 1995) indicated that magmatic tourmalines have high Fe/Mg ratios, whereas hydrothermal tourmaline contains more Mg and has compositions closer to the schorl–dravite solid-solution. Study by Neiva et al. (2007) on the tourmaline from Variscan granites associated with Sn–W and Au deposits from northern Portugal also shows that hydrothermal tourmaline is lower in Fe/(Fe+Mg) than magmatic tourmaline. Similar characteristics were also has been reported for tourmalines from Erongo granites in Namibia (Trumbull et al., 2008). As for trace elements, Sr is expected to be low in tourmalines forming in a late magmatic environment 14

due to its compatible nature in feldspars, but high in the hydrothermal tourmaline due to its affinity to high salinity aqueous fluids (Bai and van Groos, 1999). An increase in Sr from magmatic to hydrothermal tourmaline has been recently confirmed by Drivenes et al. (2015), with a similar trend for V reported by Marks et al (2013). In our previous work, tourmalines in the Baiganhu W–Sn orefield were only regarded as hydrothermal origin (Zhou et al., 2016). However, our new geochemical data indicate that the tourmalines associated with or included within the Grt-II garnets (Figs 4c–d) have magmatic signatures because they are rich in FeO (11.74–14.77 wt%; n = 9; Table 3) with low concentrations of MgO (0.08–0.53 wt%), and extremely low concentrations of Sr and V (≤0.86 and ≤1.10 ppm, respectively). These tourmalines plot on the left side of the schorl field and have very high Fe/(Mg + Fe) ratios (Table 3; Figs 8c–e). All of these features contrast with hydrothermal tourmalines in the quartz veins (Fig. 3e), which contain low concentrations of FeO (8.47–9.67 wt%; n = 9), high concentrations of MgO (4.16–4.89 wt%), are Sr and V enriched (66–94 and 124–181 ppm, respectively), and plot on the right side of the schorl field approaching the boundary between the schorl and dravite fields (Table 3; Figs 8d–e). All of these data suggest that both Grt-I and Grt-II garnets crystallized in a magmatic environment. The Grt-II garnets have much higher MnO/(FeO + MnO) ratios than the Grt-I garnets, suggesting that they formed after and/or from a more evolved melt than the Grt-I garnets (MÜller et al., 2012). Volatile enrichments in very late but still water-unsaturated melts (e.g., during the magmatic transition from later solidus to early subsolidus crystallization) may explain the elevated HREE contents and imperfect crystal shapes of the Grt-II garnets (e.g., by garnet resorption when conditions approach the solidus; Villaros et al., 2009; MÜller et al., 2012). However, further work such as melt/fluid inclusion and isotopic studies is needed. 7.2. Pressure and temperature constraints on magmatic garnet genesis The discussion above indicates that both Grt-I and Grt-II garnets are magmatic. Magmatic garnet compositions are a function of melt composition, pressure, and temperature,

15

and thus can provide constraints on these parameters (e.g., Green 1976, 1977; Harangi et al., 2001 and references therein; Deer et al., 2013 and references therein). Experimental studies indicate that high pressure (>7 kbar) garnet formation can occur during the early stages of magmatism, forming phenocryst within silicic melts that are Mn-poor but are Ca-rich (MnO <4 wt%, CaO >4 wt%; Green, 1976, 1977; Clemens and Wall, 1981; Conrad et al., 1988; Harangi et al., 2001; Yuan et al., 2009). Examples of garnet phenocrysts forming at high pressure include (i) garnets in calc-alkaline volcanic rocks of the Northern Pannonian Basin in Eastern–Central Europe that contain 4.5–8.1 wt% CaO and 0.8–3.0 wt% MnO (estimated temperature and pressure of 800°C–940°C and 7–12 kbar; Harangi et al., 2001); (ii) Alm56–76Grs24–30Prp9–18Sps1–7 garnets in an I-type porphyritic tonalite from East Kunlun containing 5.27–11.41 wt% CaO and 0.40–2.96 wt% MnO (n = 47; estimated

minimum

pressure:

8–10

kbar;

Yuan

et

al.,

2009);

and

(iii)

Alm62-75Grs11-16Prp 7-16Sps4-7 garnets in an I-type porphyritic granite associated with subduction-related magmatism in Western Junggar, China containing 4.37–6.77 wt% CaO and 1.79–3.03 wt% MnO (n = 16; estimated pressure: >7 kbar; Wang and Zhang, 2015). Medium to low pressure garnets can crystallize in situ within highly fractionated peraluminous granitic melts in a process that generates Mn-rich but Ca-poor garnets (e.g., Hall, 1965; Hsu, 1968; Cawthorn and Brown, 1976; Allan and Clarke, 1981; Miller and Stoddard, 1981; Harrison, 1988; Harangi et al., 2001). For example, experimental research indicates that Mn-rich garnets containing 20–25 mol% spessartine may crystallize in equilibrium with a granitic liquid at pressures of ~3 kbar, with even lower pressure crystallizing generating garnets with increasing spessartine contents (Green, 1977). Garnets within pegmatites are commonly Mn-rich almandine–spessartine solid-solutions (Manning, 1983). In addition, garnets within the granitoids associated with wolframite–quartz vein mineralization in the

Nanling region have

spessartine-rich almandine–spessartine

solid-solution compositions (Tan, 1985; Li and Yan, 1991; Yang et al., 2013) and are thought to have formed at 550°C–860°C and pressures of 1–4 kbar (Tan, 1985). The Sps44–57Alm43–56 magmatic garnets in granitic rocks within southeastern Fujian Province, China, are thought to have formed at 600°C–700°C and 2–3 kbar based on garnet–biotite and garnet–muscovite geobarometers as well as petrographic constraints (Yu et al., 2004). 16

Zircon saturation geothermometry provides an upper temperature limit of 751°C for the studied syenogranites since some inherited zircons are present (Watson and Harrison, 1983; Milker et al., 2003; Zhou et al., 2016). The Ti-in-biotite geothermometer of Henry et al. (2005) can also be used with peraluminous igneous rocks (Sarjoughian et al., 2014), yielding a biotite crystallization temperatures between 598°C and 654°C (mean of 631°C; n = 10; Table 4) within the peraluminous syenogranites in the study area, a range of temperatures that is considered to roughly represent the temperature of the magma. The total-Al-in-biotite geobarometer of Uchida et al. (2007) also yields a solidification pressure of 1.71–3.15 kbar (mean of 2.46 kbar; n = 10; Table 4) for the syenogranites, values that are consistent with the estimated average solidification pressure for W deposits elsewhere (2–3 kbar; Uchida et al., 2007). The low-grade metamorphism recorded by the Xiaomiao Formation country rocks within the Baiganhu W–Sn orefield also suggest that the garnet-bearing syenogranite intrusion was emplaced at pressures <4–5 kbar. These data indicate that the magmatic Grt-I and Grt-II garnets (Mn-rich almandine–spessartine solid-solutions) in the Baiganhu W–Sn orefield crystallized under relatively low temperature and pressure conditions (e.g., 650°C–550°C and 1.5–3 kbar). This provides temperature–pressure constraints for the formation of the W–Sn mineralization in this area as all of the mineralization in the orefield develop in or close to the parental syenogranites. 7.3. Significance of magmatic garnet and tourmaline associations The association of magmatic garnet with fluorite is common, as exemplified by the Xihuashan W deposit (Yang et al., 2013) and within granitic pegmatites in the Froland and Evje–Iveland areas of southern Norway (MÜller et al., 2012). However, there is little information available on the association of magmatic garnet with tourmaline, although this association has been mentioned in the Calamity Peak Layered granite–pegmatite complex in the Black Hills of South Dakota (Duke et al., 1992), in the Dora Maira massif in Italy (Dutrow and Henry, 2011), and in leucogranites within the Himalayan orogenic belt (Gao et al., 2012), but the significance of this association for ore-forming processes remains unclear. The association between magmatic garnet and tourmaline (Figs 4c–f, 8f) in the Baiganhu 17

W–Sn orefield probably suggests that boron has an important role in the formation of W–Sn mineralization, consistent with the results of previous research (Zhou et al., 2016). Although boron is probably not directly involved in the transport of W–Sn in the form of W–B or Sn–B complexes (Liu and Ma, 1987; Taylor and Wall, 1993; Chen et al., 2000), a close association between tourmaline and W–Sn mineralization in the Baiganhu orefield and other deposits around the world (e.g., Pollard et al., 1987; Slack, 1996; Lehmann et al., 2000; Williamson et al., 2000) suggests that some other effects of boron on the W/Sn mineralization probably exist during the magmatic and hydrothermal processes. The boron isotopic composition of tourmaline within quartz veins provides evidence of magmatic-hydrothermal origin of ore-forming fluids within the Baiganhu orefield (Zheng et al., 2016). The ability of boron to lower melting temperatures and melt viscosities in silicate systems has long been used in glass technology (London, 1992). Experimental studies indicate that the solubility of H2O in magmas may be enhanced in B-rich melts, whereas F does not have this effect (Pichavant, 1981). The behavior of B and other volatiles such as F and Cl in a melt is similar to that of water in that these elements react and break the Si–O bridges within the silicate tetrahedral, depolymerizing the melt (e.g., Burnham, 1979). This reduces the viscosity of the melt and lowers its solidus temperature (e.g., a minimum melt temperature for the haplogranite system with boron has been suggested at or below 600°C at 1 kbar fluid pressure; Chorlton and Martin, 1978; London, 1992). Decreasing both viscosity and solidus temperature contributes to fractional crystallization, concentrating volatiles and incompatible elements such as W and Sn in residual melts and post-magmatic hydrothermal fluids (Chen et al., 2000; Pirajno, 2009, page 87). As mentioned above and in our previous work (Zhou et al., 2016), the Bashierxi magmatic series consists of two main types of intrusions: the tourmaline-absent A-type granites (monzogranite and alkali feldspar granite), and the tourmaline-bearing S-type granites (syenogranite). Field and microscope observations together with whole-rock and mineral geochemistry (Zhou et al., 2016) indicate that the tourmaline-absent A-type granites have relatively higher zircon saturation temperatures (TZr: 857°C–917°C; n = 10), are free of inherited zircons, and contain lower amounts of water. These lower water contents are evidenced by the crystallization sequence of water-bearing mafic minerals (i.e., biotite and/or 18

hornblende) that crystallize later than or nearly contemporaneously with felsic minerals in A-type granites but crystallize earlier in syenogranites. Economic W–Sn ore systems and accompanying alteration are rarely associated with the A-type granites in this area. The tourmaline-bearing S-type granites contain inherited zircons, have lower TZr values (upper temperature limit: 751°C; n = 10), and contain more H2O than the A-type granites in this region. The greisen-, skarn-, and quartz vein-type W–Sn mineralization and associated alteration in the study area is focused around these S-type granites, and is especially located in intrusion cupolas. Ignoring other factors such as pre-enrichment of source rocks and/or redox effects suggests that these differences between tourmaline-absent A-type and tourmaline-bearing S-type granites associated with magmatic garnet and tourmaline reflect the involvement of boron on W–Sn mineralization in the Baiganhu W–Sn orefield as follows: (i) enhancing melt H2O contents, (ii) lowering melt solidus temperatures, and (iii) acting as non-structural media enabling the transporting of W and Sn to intrusion cupolas and overlying metasedimentary wallrocks through fractures and fissures around the intrusion. 7.4. Significance of garnet Sn contents As mentioned above, all four types of garnets in the Baiganhu W–Sn orefield contain relatively high concentrations of Sn (>20 ppm), especially the Grt-III within the skarn (i.e., andradite-rich grandite that contain the highest Sn concentrations of 2544–8364 ppm). As shown in Table 5, tin-bearing garnets (especially andradite-rich grandite garnets) within skarns have been known for a significant amount of time, with Saksela (1951) reporting an andradite (Grs36.6) containing 1.44 wt% SnO2 within a polymetallic cassiterite deposit in Pitkaranta, Finland (also see Deer et al., 1982). Stannian andradite associated with pyroxene in the Plavno Mine of the Krusne Hory Mountains of the Czech Republic also contain up to 1.07 wt% SnO2 (Dadak and Novak, 1965). In addition, garnets from Davib Ost in Namibia contain up to 5.8 wt% SnO2 and are presumed to have formed during the contact metamorphism of calcareous metasediments by an associated granite intrusion (McIver and Mihalik, 1975). Plimer (1984) also described andradites containing 0.5–3.5 wt% SnO2 associated with clinopyroxene in the Doradilla area of New South Wales, Australia. A comprehensive mineralogical study of the important Sn-bearing Yaogangxian, Dachang, 19

Xinlu, Debao, Gejiu, and Shizhuyuan skarns in southern China (Chen et al., 1992a, 1992b, 2000) also indicate that skarn-hosted garnet SnO2 contents range from 0.02 to 2.0 wt%, reaching a maximum of 5.14 wt% SnO2 within andradite-rich garnets in the Gejiu skarn. These Sn data were obtained by spectrographic, X-ray fluorescence, or EPMA (Table 5), all of which have relatively low sensitivities and detection limits. An extensive review of garnet compositions did not yield much information on tungsten and tin contents obtained by ICP–MS, the exception being recent analyses of pure garnet powders from the Hehuaping, Xitian, and Xintianling W–Sn deposits (Yao et al., 2013). The new in situ LA–ICP–MS analysis of garnets from Baiganhu W–Sn orefield presented in this study confirms that andradite-rich grandites are Sn-enriched (Tables 2, 5). Chemical composition, unit-cell dimension, and Mossbauer spectral studies have demonstrated that the incorporation of Sn in the lattice of garnet is the result of the substitution of Sn4+ for Fe3+ in octahedral sites (e.g., McIver and Mihalik, 1975; Butler, 1978; Amthauer et al., 1979; Plimer, 1984; Chen and Wu, 1988; Chen et al., 1992a, 1992b). Research into Ca–Sn–Ga garnets by Rulmont et al. (1993, 1995) indicates that Sn can be incorporated into all three structural sites within the garnet structure simultaneously. The andradite-rich Grt-III grandite garnets from the Baiganhu W–Sn orefield

have

Adr62–88Grs1–18Sps3–12Alm0–8Prp0 compositions and contain the highest Sn concentrations of any garnets in the study area (2544–8364 ppm), whereas the common andradite grandites (i.e., Grt-IV, Adr4–22Grs62–73Sps5–16Alm2–10Prp 0) contain lower but still elevated Sn contents (51.7–269 ppm), and the andradite-poor Grt-I and Grt-II garnets are Sn depleted relative to the other garnets (23.22–79.83 and 22.96–89.67 ppm, respectively). The generally positive correlation between SnO2 contents and garnet andradite components within Baiganhu W–Sn orefield and other W–Sn deposits in China suggests that Sn4+ substitutes for Fe3+ in garnet octahedral sites (Figs 9, 10). The substitution of Sn4+ for Fe3+ in the octahedral sites of the garnet lattice requires a compensatory substitution to maintain the charge balance. Two different conditions are generally considered (Fig. 10), namely either (i) the majority of cases, where a coupled substitution of the type Sn4+(oct) + Fe2+(oct) = 2Fe3+(oct) occurs (Mulligan and Jambor, 1968; McIver and Mihalik, 1975; Amthauer et al., 1979; Plimer, 1984), or (ii) more rarely this 20

substitution causes a deficiency of cations in the octahedral site, leading to an excess of bivalent cations for the dioctahedral sites that could be explained either by a substitution of the type 3Sn4+(oct) + □(oct) = 4Fe3+(oct) (Nekrasov, 1971) or by allowing some other bivalent cation (e.g., Mg2+) to enter the octahedral site (e.g., Sn4+(oct) + Mg2+(oct) = 2Fe3+(oct); Chen et al., 1992a). The majority of the octahedral sites in the garnets from the study area are fully occupied, indicating that the first type of substitution dominates in the Baiganhu W–Sn orefield (Table 1). For garnets with a deficiency in the octahedral sites in this study, the rather low Mg2+ values (apfu<0.003) seems more likely to support the substitution of the type 3Sn4+(oct) + □(oct) = 4Fe3+(oct) (Nekrasov, 1971). The substitution of Sn4+ for Fe3+ within andradite–grossular garnets is also controlled by the physico-chemical parameters of the hydrothermal fluids that formed the skarn. For example, relatively high oxygen fugacity conditions (e.g., between the magnetite–hematite (MH) and nickel–nickel oxide oxygen buffers (NNO)) will be dominated by ferric-rich andradite that contains lattice-bound Sn (Eadington and Kinealy, 1983; Chen et al., 1992a, 2000). Temperatures above 400°C (i.e., within the stability field of the typical skarn minerals andradite and hedenbergite) will bind Sn in silicates, predominantly within andradite lattices (Chen and Wu, 1988). Other factors such as pH and component abundances (e.g., Sn, Fe, and Mg) can also influence the behavior of Sn during garnet formation (Burt, 1978; Eadington and Kinealy, 1983; Chen et al., 2000). Remarkably, the Grt-III garnets (Adr-rich) that coexist with Mo-rich scheelites (MoO3: 11–15 wt%; WO3: 61–64 wt%; Fig. 5b) contain high concentrations of Sn but extremely low concentrations of W and Mo (W: 0.48–0.83 ppm; Mo: <0.77 ppm; Table 2). This suggests that differences in iron radii and related ligand likely caused the disparity of the substitution for Fe3+ in garnets (e.g., the effective ionic radii are: Fe3+ = 0.65Å, Sn4+ = 0.69Å, W6+ = 0.60Å and Mo 6+ = 0.59Å; the coordination number = 6; Shannon, 1976). Although there is some uncertainty, the Sn contents of the andradite-rich grandites in the majority of the W–Sn deposits in China are generally relatively high (e.g., Sn ≥1000 ppm when andradite contents are >40 mol%; Fig. 9; Table 5). This is probably because the substitution of Sn4+ for Fe3+ can more readily take place when the hydrothermal fluids responsible for the W–Sn mineralization contain elevated concentrations of Sn irrespective of 21

other factors such as redox, pH, and temperature conditions. This suggests that the presence of Sn-rich grandite in skarns is a useful indicator mineral for W–Sn exploration when combined with other geological, geophysical, and geochemical observations. However, a better understanding of the genesis of unusually Sn-rich grandites, including fluid inclusion and isotopic research on garnets and coexisting minerals, is needed, including the determining of ore metal contents (especially Sn) in garnets within other W–Sn skarns using techniques such as in situ LA–ICP–MS. 8. Conclusions (1) Both Grt-I (Sps45–53Alm46–53Adr0–1Grs0–1Prp 0–1 enclosed by magmatic quartz) and Grt-II (Sps64–70Alm29–34Adr0–1Grs0–-2Prp 0 coexisting with tourmaline) in S-type syenogranites are magmatic. (2) Spessartine-rich magmatic garnets crystallized under relatively low temperature and pressure conditions (e.g., at 650°C–550°C and 1.5–3 kbar), providing temperature–pressure constraints for the W–Sn mineralization in the study area. (3) The association of magmatic garnet and tourmaline indicates that boron probably plays an important role in W–Sn mineralization during magmatic and hydrothermal processes by: (i) enhancing melt H2O contents, (ii) lowering melt solidus temperatures, and (iii) acting as a non-structural media involved in the transportation of W and Sn. (4) Sn substitutes into the garnet structure by Sn4+ replacing Fe3+ as both have similar effective ionic radii (Fe3+ = 0.65, Sn4+ = 0.69; coordination number = 6), with skarn-hosted Sn-rich andradite–grossular garnets potentially a useful indicator mineral for W–Sn exploration when combined with other geological, geophysical, and geochemical signatures. Acknowledgments This study was financially supported by the grant No. 41172076 from the National Natural Science Foundation of China and the Program of High-Level Geological Talents (201309) and Youth Geological Talents (201112) of the China Geological Survey. We thank Anshun Zhou, Shuo Wang, Ye Xiao, Jiannan Liu, and Miao Yu for their help during field work. We also appreciate the assistance of Zhenyu Chen and Jingting Li with electron microprobe analyses, Mingyue Hu and Dongyang Sun

22

with LA–ICP–MS analyses at Chinese Academy of Geological Sciences (CAGS), Beijing, China. We are grateful to Prof. Shan Qin from Peking University, Prof. Xinyou Zhu from Beijing Institue of Geology for Mineral Resources and Prof. Georges Beaudoin from Laval University for stimulant discussion on the garnet crystal structure and genetic implications. We are grateful to the two anonymous reviewers for their thoughtful reviews and their constructive and stimulating comments, which helped to significantly improve the manuscript. We also thank the Editor-in-Chief, Franco Pirajno, for providing prompt feedback and information on the status of reviews of our paper, and for his assistance during the revision and submission process.

References Allan, B.D., Clarke, D.B., 1981. Occurrence and origin of garnets in the South Mountain Batholith, Nova Scotia. The Canadian Mineralogist. 19, 19–24. Amthauer, G., McIver, J.R., Viljoen, E.A., 1979.

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Figure captions Fig. 1. Distribution of the main geotectonic units in the Altyn Tagh and East Kunlun (a, after Wang et al., 2014a) and schematic geological map of the northeastern Baiganhu W–Sn orefield in Xinjiang (b, after Zhou et al., 2016). In figure (a): NASB = North Altyn Tagh subduction–collision belt, BGHF = Baiganhu Fault, HS–NLGLF = Heishan–Nalinggele Fault. References in the figure (b): (1), Gao et al., 2010; (2), Gao and Li, 2011; (3), Li et al., 2012; (4), Li et al., 2013b; (5), Wang et al., 2014b; (6), Zhou et al., 2016. Fig. 2. Representative geological map of and cross-section (A–B) of the Kekekaerde deposit in Baiganhu W–Sn orefield (after Zhou et al., 2016). Fig. 3. Photographs and micrographs showing the three types of W–Sn mineralization and the temporal relationships between them (Figs a and c, cross polarized light; Fig. f, plane polarized light; the image at lower right in (e) was taken under ultraviolet light). (a) Scheelite-bearing greisen stringers cross-cut the skarn mineral assemblages (mainly cosisiting of clinopyroxene). (b) Wolframite-bearing quartz veins cross-cut the strongly greisenized parent syenogranite (in the cupola, SGS = strongly gresenized syenogranite). (c) Greisen-type scheelite mineralization. (d) Quartz vein-type cassiterite mineralization. (e) and (f) Quartz-vein-type scheelite mineralization associated with massive tourmaline, note that the scheelite shows a plain contact with tourmalines. Mineral abbreviations (also for Figs 4 and 5): calcite (Cal), clinopyroxene (Cpx), cassiterite (Cst), fluorite (Fl), garnet (Grt), K-feldspar (Kfs), molybdenite (Mol), muscovite (Ms), plagioclase (Pl), quartz (Qz), scheelite (Sch), tourmaline (Tur), and wolframite (Wol). Fig. 4. Micrographs of the Grt-I and Grt-II garnets within the parent syenogranite from the Baiganhu W–Sn orefield (e, cross polarized light; the rest, plane polarized light). (a) and (b) Grt-I garnets are single euhedral and enclaved by the magmatic quartz. (c) to (f) Grt-II garnets are subhedral and located in the interstices among the feldspars and quartz. They are associated with tourmalines and cracks develop well with some secondary muscovite replacing the garnets along the cracks. Spots of the LA–ICP–MS analyses are also shown in Figs b, d, e, and f. Mineral abbreviations as Fig. 3. All minerals labeled have been confirmed by EPMA, the same below. Fig. 5. Photographs and micrographs of the Grt-III and Grt-IV garnets within the skarn from the Baiganhu W–Sn orefield (b, d and f, plane polarized light; e, reflected light). (a) and (b) Grt-III garnets are associated with clinopyroxenes and disseminated Mo-rich yellow scheelites, all of 34

which are replaced by later–stage fluorites and molybdenites. (c) to (f) Grt-IV garnets occur as massive aggregation and are associated with disseminated molybdenites and fluorite. These garnets are cross-cut by some Mo-poor-gray-white-scheelite–bearing calcite veins. Spots of the LA–ICP–MS analyses are also shown in Figs b and f. Mineral abbreviations as Fig. 3. Fig. 6. Major elements features of the garnets from Baiganhu W–Sn orefield. (a) and (b) Major elements and end-member molecules of Grt-I and Grt-II garnets from the core to the rim. (c) to (f) The contour lines of five major elements (wt%) showing the major compositions of the garnets. Fig. 7. Chondrite-normalized REE patterns of the garnets from Baiganhu W–Sn orefield (Figs. a and e). Magmatic garnets data in Figs b, c, and d from Yang et al. (2013), Gao et al. (2012), and Samadi et al. (2014) respectively. Hydrothemal garnets in Fig. f from Yao et al. (2013) and Guo et al. (2016), in Figs g and h from Peng et al. (2015) and Zhai et al. (2014) respectively. Samples were normalized to the C1 chondrite values of McDonough and Sun (1995). The limit of detection (LOD) is used for the normalized REE patterns, when one element value is below the LOD. Fig. 8. Discrimination diagrams showing the origin of the four types of garnets (a, b) and the classification of the two types of tourmalines (c, d). (a) Ternary diagram of Meinert et al. (2005) showing the values of the Sps+Alm molecules of Grt-II garnets are beyond all the ranges of garnets in W/Sn skarn around the world (garnet data in Sn and W skarn in South China are from Chen et al., 1992a). (b) Ternary diagram of Mange and Morton (2007) showing all the Grt-II garnets plotting in the “Type-Bi” field whereas almost all hydrothermal Grt-III and Grt-IV garnets plotting in the “Type-B” or “Type-D” fields. Type-A, mainly from high-grade granulitefacies metasediments or charnockites and intermediate felsic igneous rocks; Type-B, amphibolite-facies metasedimentary rocks; Type-Bi, intermediate to felsic igneous rocks; Type-Ci, mainly from high-grade mafic rocks; Type-Cii, ultramafics with high Mg (pyroxenites and peridotites); Type-D, metasomatic rocks, very low-grade metamafic rocks and ultrahigh temperature metamorphosed calc-silicate granulites. (c) to (e) Classifcation diagrams of tourmaline based on X-site occupancy (c), on the dravite–schorl–elbaite subsystem of the alkali group (d), and on Mg content and vacancies in the X-site (e), showing the tourmaline associated with Grt-II garnets have much higher Fe/(Mg+Fe) ratios than those of hydrothermal origin within quartz vein. Based map from Drivenes et al. (2015). Data of tourmaline within quartz vein from Zheng et al. (2016) also plotted. (f) A micrograph (plane polarized light) showing intergrowth of the tourmaline and Grt-II garnets. 35

Fig. 9. Positive correlation between SnO2 (wt%) and andradite molecules (mol%) in garnets from the W/Sn deposits of China. Data from this study, Yao et al. (2013) and Chen et al. (1992a, 1992b). Fig. 10. Portion of the garnet structure projected down z showing the major end-members and the substitution of Sn4+ for Fe3+ of garnet (after Novak and Gibbs, 1971; Feneyrol, 2012; effective ionic radius (coordination number = 6) according to Shannon, 1976).

Table captions Table 1. Representative EPMA analyses (wt%) and end-member molecules (mol%) of the garnets from the Baiganhu W–Sn orefield. Table 2. Representative LA–ICP–MS analyses (ppm) of garnets from the Baiganhu W–Sn orefield. Table 3. Representive EPMA analyses (wt%) of tourmalines from the Baiganhu W–Sn orefield. Table 4. Representive EPMA analyses (wt%) and estimated temperature and pressures of biotites within parental syenogranites from the Baiganhu W–Sn orefield. Table 5. A brief summary on the SnO2 (wt%) or Sn (ppm) contents of Sn-bearing garnets around the world.

36

Table 1. Representative EPMA analyses (wt.%) and end-member molecules (mol%) of the garnets from the Baiganhu W–Sn orefield. Grt-I

Type

Grt-II

14804-20

14804-20

14804-20

14804-20

14804-20

14804-20

14804-20

14804-20

14804-20

17601-19

17601-19

17601-19

17601-19

17601-19

17601-19

17601-19

17601-19

17601-19

_1

_2

_3

_4

_5

_6

_7

_8

_9

_1

_2

_3

_4

_5

_6

_7

_8

_9

Position

C

M

R

C

M

R

C

M

R

C

M

R

C

M

R

C

M

R

SiO 2

35.32

35.84

35.87

35.75

36.08

36.25

36.27

36.06

36.12

35.48

35.52

35.11

35.92

36.14

36.16

35.80

35.30

35.88

TiO 2

0.12

0.10

bdl

bdl

0.14

bdl

0.06

bdl

0.07

0.13

0.10

0.09

bdl

0.15

0.10

0.14

0.08

bdl

Al2O3

20.23

20.24

20.42

20.39

20.45

20.33

20.37

20.56

20.16

20.75

20.64

20.10

20.51

19.87

20.81

20.70

20.65

20.01

FeO T

21.12

23.05

23.30

23.26

22.74

23.58

23.43

22.78

22.71

14.25

14.81

14.85

14.79

15.13

15.05

12.98

15.58

15.58

MnO

21.62

20.05

19.87

19.47

20.19

19.12

19.55

19.93

20.05

27.93

27.99

28.11

27.65

27.95

27.23

29.34

27.18

27.31

MgO

0.12

0.13

0.20

0.13

0.11

0.21

0.13

0.14

0.13

0.02

bdl

0.02

0.02

bdl

0.04

0.02

0.04

0.04

Sample

CaO

0.33

0.29

0.25

0.27

0.29

0.34

0.25

0.31

0.28

0.61

0.60

0.52

0.52

0.50

0.48

0.52

0.48

0.38

Na2O

0.17

0.05

bdl

0.11

0.04

0.09

bdl

0.08

0.09

0.17

0.19

0.02

0.08

0.02

0.08

0.10

0.10

0.06

K2O

0.04

bdl

bdl

0.03

0.01

bdl

bdl

bdl

bdl

0.03

0.01

bdl

bdl

bdl

0.01

0.01

0.01

bdl

F

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Cl

0.07

bdl

0.01

0.01

0.01

0.01

bdl

bdl

bdl

0.03

bdl

bdl

bdl

bdl

bdl

bdl

0.01

bdl

O=(F+Cl)

-0.02

-0.00

-0.00

-0.00

-0.00

-0.00

-0.00

-0.00

-0.00

-0.01

-0.00

-0.00

-0.00

-0.00

-0.00

-0.00

-0.00

-0.00

Total

99.12

99.75

99.92

99.42

100.06

99.93

100.06

99.86

99.61

99.39

99.86

98.82

99.49

99.76

99.96

99.61

99.43

99.26

Based on 12 oxygens Si

2.941

2.967

2.964

2.966

2.977

2.991

2.994

2.977

2.992

2.938

2.929

2.935

2.974

2.994

2.978

2.959

2.926

2.984

Al

0.060

0.033

0.036

0.034

0.023

0.010

0.007

0.024

0.008

0.063

0.072

0.066

0.026

0.006

0.022

0.041

0.074

0.017

T-site sum

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

Al

1.926

1.942

1.953

1.960

1.966

1.967

1.975

1.977

1.960

1.963

1.935

1.914

1.976

1.934

1.998

1.976

1.943

1.945

Ti

0.008

0.006

0.000

0.000

0.009

0.000

0.004

0.000

0.005

0.008

0.006

0.006

0.000

0.010

0.006

0.009

0.005

0.000

0.146

0.087

0.084

0.092

0.047

0.057

0.025

0.060

0.054

0.111

0.155

0.144

0.064

0.057

0.025

0.064

0.137

0.082

O-site sum

2.080

2.035

2.036

2.052

2.021

2.024

2.003

2.037

2.018

2.082

2.096

2.063

2.039

2.000

2.029

2.049

2.085

2.026

Mg

0.015

0.016

0.025

0.016

0.014

0.026

0.016

0.017

0.016

0.003

0.000

0.003

0.003

0.000

0.005

0.003

0.005

0.005

2+

1.325

1.510

1.527

1.523

1.523

1.570

1.593

1.513

1.520

0.876

0.867

0.895

0.961

0.991

1.012

0.833

0.943

1.002

1.525

1.406

1.391

1.369

1.411

1.336

1.367

1.394

1.407

1.959

1.955

1.990

1.939

1.961

1.900

2.054

1.908

1.924

Fe

Fe

3+

Mn 2+

37

Ca2+

0.030

0.026

0.022

0.024

0.026

0.030

0.022

0.028

0.025

0.054

0.053

0.047

0.046

0.045

0.043

0.046

0.043

0.034

D-site sum

2.893

2.958

2.964

2.931

2.973

2.962

2.997

2.951

2.968

2.891

2.874

2.934

2.949

2.997

2.959

2.936

2.899

2.964

Prp

0

1

1

1

0

1

1

1

1

0

0

0

0

0

0

0

0

0

Alm

46

51

51

52

51

53

53

51

51

31

30

30

33

32

34

29

33

34

Sps

53

48

47

47

48

45

46

47

48

68

68

69

66

67

64

70

66

65

Adr

0

0

1

0

0

0

0

0

0

0

0

1

0

1

0

0

0

1

Grs

1

0

0

0

1

1

0

1

0

1

2

0

1

0

2

1

1

0

0.51

0.47

0.46

0.46

0.47

0.45

0.45

0.47

0.47

0.66

0.65

0.65

0.65

0.65

0.64

0.69

0.64

0.64

1

1

2

1

1

2

1

1

1

0

0

0

0

0

0

0

0

0

BS-03_1

BS-03_2

BS-03_3

BS-03_4

BS-03_5

BS-03_6

BS-03_7

BS-03_8

BS-03_9

37.90

37.74

37.87

38.13

38.29

37.93

38.01

38.70

38.85

MnO/ (FeOT+Mn O) Mg# Notes: C = core, M = mantle, R = rim, bdl = below detection limit. FeOT-total iron measured by electron microprobe; Mg# = 10*Mg/(Mg+Fe), atomic ratio, ato; Adr = andradite, Alm = almandine, Grs = grossular, Sps = spessartine, Prp = pyrope.

Table 1 (Continued). Grt-III

Type

Grt-IV

Sample

BS-02_1

BS-02_2

BS-02_3

BS-02_4

BS-02_5

BS-02_6

BS-02_7

BS-02_8

SiO2

36.93

36.55

36.17

35.24

35.65

35.75

36.09

34.75

BS-02_9 34.79

TiO 2

0.13

0.11

0.09

0.08

bdl

0.08

bdl

bdl

bdl

0.24

0.13

0.06

0.36

bdl

0.39

0.02

0.03

0.03

Al2O3

8.12

5.82

3.52

3.53

3.40

3.29

2.22

6.36

5.35

18.21

18.38

17.08

17.01

19.27

16.97

19.92

17.91

18.91

FeO T

22.02

24.40

26.78

26.29

26.04

25.87

23.64

21.89

22.32

6.79

7.67

8.70

9.32

5.94

9.24

6.12

8.93

7.52

MnO

5.02

4.81

4.59

1.31

1.25

1.15

4.72

1.96

2.28

3.94

7.31

2.10

2.19

6.88

2.05

6.80

2.05

4.43

0.10

0.07

0.01

0.05

0.04

0.02

bdl

0.04

bdl

bdl

0.02

0.01

27.12

32.78

31.34

27.98

31.45

26.66

30.50

28.58

0.02

0.06

0.03

bdl

bdl

0.01

0.01

bdl

0.01

0.01

bdl

bdl

bdl

bdl

MgO

bdl

bdl

bdl

0.08

bdl

0.02

CaO

27.70

28.24

28.77

32.41

32.44

32.53

31.24

33.21

33.31

32.11

Na2O

0.05

0.05

0.05

0.00

0.03

bdl

0.27

0.02

0.02

0.03

0.02

bdl

0.02

0.02

bdl

0.01

K2O

bdl

bdl

bdl

bdl

bdl

bdl

38

F

bdl

bdl

bdl

bdl

0.10

bdl

bdl

0.31

0.10

0.62

0.08

0.08

0.11

0.42

0.14

0.66

0.22

0.44

Cl

bdl

bdl

bdl

bdl

0.01

bdl

bdl

bdl

0.01

bdl

bdl

bdl

0.01

bdl

0.01

bdl

0.01

0.01

O=(F+Cl)

-0.00

0.00

-0.00

-0.00

-0.04

-0.00

-0.00

-0.13

-0.04

-0.26

-0.03

-0.03

-0.05

-0.18

-0.06

-0.28

-0.09

-0.19

Total

99.97

99.98

99.99

98.94

98.98

98.74

98.18

98.39

98.15

99.63

98.47

98.68

98.49

98.68

98.12

97.91

98.29

98.60

Based on 12 oxygens Si

2.983

2.974

2.965

2.895

2.924

2.941

2.993

2.834

2.854

2.927

2.980

2.960

2.994

2.997

2.990

2.997

3.000

3.000

Al

0.017

0.026

0.035

0.106

0.076

0.060

0.007

0.167

0.146

0.073

0.021

0.040

0.006

0.004

0.011

0.003

0.000

0.000

T-site sum

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

3.000

Al

0.756

0.532

0.305

0.236

0.259

0.260

0.210

0.465

0.378

1.623

1.695

1.538

1.574

1.778

1.575

1.851

1.657

1.745

Ti

0.008

0.007

0.006

0.005

0.000

0.005

0.000

0.000

0.000

0.014

0.008

0.004

0.022

0.000

0.023

0.001

0.002

0.002

Fe3+

1.253

1.488

1.727

1.770

1.767

1.774

1.506

1.403

1.427

0.390

0.308

0.493

0.391

0.182

0.381

0.071

0.240

0.117

O-site sum

2.017

2.027

2.038

2.011

2.027

2.039

1.716

1.868

1.805

2.026

2.011

2.035

2.000

2.000

2.000

2.000

1.963

1.959

Mg

0.000

0.000

0.000

0.010

0.012

0.009

0.000

0.003

0.001

0.006

0.005

0.003

0.000

0.005

0.000

0.000

0.003

0.001

2+

0.090

0.105

0.050

0.199

0.076

0.222

0.207

0.229

0.333

0.347

0.375

0.235

0.172

0.109

0.036

0.019

0.006

0.134

2+

0.344

0.331

0.319

0.091

0.087

0.080

0.332

0.135

0.158

0.258

0.489

0.139

0.146

0.456

0.137

0.454

0.137

0.294

Ca2+

2.398

2.462

2.527

2.852

2.851

2.867

2.776

2.902

2.928

2.657

2.294

2.745

2.637

2.346

2.656

2.252

2.565

2.397

D-site sum

2.977

2.965

2.955

2.989

2.969

2.962

3.242

3.130

3.192

2.970

2.987

2.963

3.004

3.014

3.022

3.039

3.050

3.067

Fe

Mn

Prp

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Alm

8

6

4

1

1

0

1

3

3

2

7

2

7

6

7

9

10

10

Sps

12

11

11

3

3

3

10

4

5

9

16

5

5

16

5

16

5

11

Adr

62

73

85

88

87

87

86

75

79

18

15

22

20

9

19

4

13

6

Grs

18

10

1

7

9

10

3

18

13

71

62

71

68

69

69

71

72

73

39

Table 2. Representative LA–ICP–MS analyses (ppm) of the garnets from the Baiganhu W–Sn orefield. Type

LOD

Sample

Garnet

Position

Grt-I

Grt-I

Grt-I

Grt-I

Grt-I

Grt-II

Grt-II

Grt-II

Grt-II

Grt-II

Grt-II

Grt-II

Grt-II

Grt-II

14804-20

14804-20

14804-20

14804-20

14804-20

17601-19

17601-19

17601-19

17601-19

17601-19

17601-19

17601-19

17601-19

17601-19

_1

_2

_3

_4

_5

_1

_2

_3

_4

_5

_6

_7

_8

_9

C

R

C

R

C

C

R

C

R

C

R

C

R

R

La

0.00

0.04

0.01

0.02

bdl

bdl

0.12

bdl

0.02

0.00

bdl

0.03

0.01

0.02

bdl

Ce

0.03

0.15

bdl

0.06

bdl

bdl

0.43

0.04

0.04

0.11

0.06

0.05

0.06

0.07

0.04

Pr

0.07

bdl

bdl

bdl

bdl

bdl

0.11

0.08

bdl

0.13

0.08

bdl

0.12

0.10

bdl

bdl

bdl

0.56

bdl

2.34

2.70

1.67

4.40

2.75

1.49

2.49

2.92

2.94

0.39

bdl

Sm

0.34

4.00

4.54

4.44

4.64

4.56

23.77

24.69

21.47

31.65

27.34

21.68

28.85

32.79

28.87

Eu

0.04

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Gd

0.36

48.78

44.24

47.63

49.75

47.75

178.60

187.20

163.20

205.90

181.80

164.20

200.60

208.60

189.00

Tb

0.05

33.08

28.00

30.42

30.93

29.65

93.79

97.57

85.33

107.10

89.32

84.91

100.50

108.80

98.63

Dy

0.00

324.90

284.00

283.38

272.60

252.00

1033.00

1141.00

939.20

1232.00

1002.00

988.70

1157.00

1289.00

1116.00

Ho

0.00

69.82

62.36

57.87

55.56

43.74

308.50

346.30

269.90

391.10

297.90

293.60

338.80

408.50

324.60

Er

0.09

236.10

207.60

183.60

175.10

115.60

1332.00

1572.00

1081.00

1844.00

1284.00

1240.00

1440.00

1946.00

1365.00

Tm

0.02

54.84

50.69

41.29

38.95

20.69

366.80

462.30

276.70

554.90

341.90

351.40

398.70

587.10

369.80

Yb

0.00

508.00

495.50

381.68

368.00

155.20

4004.00

5405.00

2787.00

6739.00

3601.00

3895.00

4334.00

7017.00

3934.00

Lu

0.02

76.58

74.59

54.71

49.90

17.76

669.90

949.50

448.70

1216.00

589.00

671.60

709.30

1266.00

633.20

Y

0.11

2159.00

1900.00

1842.50

1753.00

1558.00

9942.00

10533.00

9663.00

11747.00

9822.00

9707.00

11661.00

13333.00

11239.00

Nd

W

0.21

1.64

bdl

0.50

bdl

bdl

0.31

0.24

bdl

0.23

bdl

bdl

bdl

0.22

bdl

Sn

2.59

23.22

34.64

42.22

31.19

79.83

42.02

61.04

22.96

89.67

56.12

29.49

49.52

48.74

45.69

Mo

0.77

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Cu

2.12

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Pb

0.43

1.03

bdl

0.66

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Zn

4.13

99.03

102.20

103.03

109.40

101.50

80.95

72.03

91.83

67.90

93.53

92.56

84.19

81.79

82.55

Li

2.10

207.00

183.30

192.33

190.80

188.20

642.00

656.10

533.40

679.50

487.70

616.60

707.60

780.10

657.90

Be

3.32

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

V

0.47

1.51

0.82

0.87

1.00

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Cr

8.16

101.60

bdl

27.62

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Co

0.29

0.99

0.94

0.70

0.56

0.31

bdl

bdl

bdl

bdl

0.35

bdl

bdl

bdl

bdl

40

Ga

2.37

30.08

29.23

30.33

29.97

32.05

77.58

85.86

68.94

97.02

69.92

68.23

80.21

92.92

76.58

Ge

3.02

85.57

86.91

90.11

76.97

111.00

79.49

84.32

88.96

74.28

69.31

90.82

89.21

82.53

86.28

As

1.39

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Zr

3.45

9.26

11.42

10.61

10.16

11.61

4.36

6.19

5.36

10.24

8.93

3.67

6.79

11.34

7.02

Hf

0.25

0.72

0.94

0.87

0.75

1.09

0.84

0.76

0.68

1.38

0.81

0.95

0.72

1.02

0.74

Nb

0.12

1.96

1.45

2.10

0.93

4.05

0.35

0.40

0.24

0.80

2.62

0.21

0.45

1.06

0.35

Ta

0.12

1.14

1.56

1.94

0.92

4.15

0.97

1.31

0.72

2.30

1.63

0.89

0.95

2.15

1.01

Ag

1.71

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Cd

2.35

7.25

7.96

8.99

8.05

12.71

12.36

13.31

10.46

12.38

6.78

10.59

10.56

11.61

12.71

In

0.13

0.22

0.27

0.28

0.25

0.37

1.41

1.92

0.92

2.26

1.15

0.94

1.72

2.14

1.88

Sb

0.30

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

11.73

bdl

bdl

bdl

bdl

bdl

Cs

0.43

bdl

bdl

bdl

bdl

bdl

1.13

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Bi

0.06

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Th

0.10

1.83

bdl

0.66

0.10

bdl

bdl

bdl

bdl

bdl

0.25

bdl

bdl

bdl

bdl

U

0.00

0.32

0.11

0.17

0.08

0.17

0.15

0.14

0.06

0.17

0.28

0.07

0.08

0.16

0.08

Rb

0.63

bdl

bdl

bdl

bdl

bdl

18.49

3.69

4.57

4.30

2.91

10.31

2.90

4.40

2.62

Sr

0.23

0.26

0.43

0.45

0.69

0.41

2.73

3.81

1.65

4.68

2.51

2.56

2.77

5.25

2.67

Ba

0.44

bdl

bdl

bdl

bdl

bdl

1.72

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Notes: C = core, R = rim, LOD = Limit of detection, bdl = below detection limit.

Table 2 (Continued). Type

Grt-III

Grt-III

Grt-III

Grt-III

Grt-III

Grt-IV

Grt-IV

Grt-IV

Grt-IV

Grt-IV

BS-02

BS-02

BS-02

BS-02

BS-02

BS-03

BS-03

BS-03

BS-03

BS-03

Sample _1

_2

_3

_4

_5

_1

_2

_3

_4

_5

La

0.32

0.30

0.08

0.61

0.17

0.02

0.00

bdl

0.03

0.06

Ce

2.88

2.10

0.74

3.56

1.23

0.18

bdl

bdl

0.48

0.94

Pr

0.74

0.49

0.25

0.72

0.26

0.19

bdl

bdl

0.35

0.67

Nd

4.14

2.52

1.46

3.13

1.35

1.47

0.67

bdl

3.51

9.66

Sm

1.05

0.73

0.73

0.69

0.47

3.93

2.36

1.65

5.25

11.75

Eu

0.43

0.30

0.25

0.28

0.22

1.76

1.37

1.14

1.91

3.22

41

Gd

1.00

0.80

0.60

0.70

0.88

5.57

2.46

4.63

6.62

12.76

Tb

0.14

0.11

0.09

0.11

0.10

0.75

0.17

0.94

1.01

1.93

Dy

0.81

0.66

0.53

0.70

0.59

3.47

0.50

5.84

5.25

9.40

Ho

0.17

0.15

0.08

0.18

0.16

0.69

0.11

1.38

1.01

1.55 3.35

Er

0.42

0.35

0.14

0.40

0.44

1.57

0.19

3.83

2.46

Tm

0.05

0.05

0.03

0.07

0.03

0.18

bdl

0.50

0.30

0.40

Yb

0.49

0.28

0.20

0.19

0.25

1.56

0.29

4.02

2.21

2.30

Lu

0.06

0.05

0.03

0.04

0.06

0.20

0.03

0.53

0.29

0.30

Y

9.35

7.91

4.67

9.68

7.94

16.92

1.53

40.34

29.15

45.59

W

0.63

0.67

0.48

0.83

0.74

0.75

0.65

0.60

0.77

1.06

Sn

2544.00

3542.55

5070.20

8364.00

2755.00

195.07

421.10

269.00

247.27

51.70

Mo

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Cu

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Pb

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Zn

7.31

10.19

13.77

8.32

11.37

12.32

10.46

11.64

12.44

15.21

Li

bdl

bdl

bdl

2.77

bdl

bdl

bdl

bdl

bdl

2.77

Be

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

V

4.64

6.93

8.25

6.60

8.22

13.38

7.70

25.06

14.68

11.29

Cr

43.88

38.19

13.70

8.81

86.36

bdl

bdl

bdl

bdl

bdl

Co

0.31

0.30

bdl

0.49

bdl

bdl

bdl

bdl

bdl

bdl

Ga

60.63

69.55

52.76

95.73

69.08

53.57

73.38

67.20

56.68

29.47

Ge

112.80

132.27

65.18

228.40

122.70

50.21

103.00

93.60

68.73

9.59

As

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Zr

bdl

bdl

bdl

bdl

bdl

22.77

10.72

7.14

37.33

94.14

Hf

bdl

bdl

bdl

bdl

bdl

0.51

bdl

bdl

0.78

1.89

Nb

8.68

8.99

14.53

3.72

9.02

24.57

21.87

20.06

25.03

33.17

Ta

bdl

bdl

bdl

bdl

bdl

0.20

bdl

0.31

0.25

0.37

Ag

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Cd

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

In

30.15

33.66

14.85

60.33

29.31

2.02

2.35

1.76

2.03

2.00

Sb

bdl

bdl

bdl

0.43

bdl

bdl

bdl

bdl

bdl

bdl

Cs

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

42

Bi

bdl

bdl

bdl

bdl

bdl

Th

0.11

bdl

bdl

0.24

bdl

bdl

bdl

bdl

bdl

bdl

U

2.57

2.38

1.21

4.42

1.33

0.70

0.27

0.23

1.22

3.15

Rb

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Sr

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

Ba

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

bdl

43

bdl

bdl

bdl

bdl

bdl

Table 3. Representive EPMA analyses (wt.%) of tourmalines from the Baiganhu W–Sn orefield. Tourmaline associated with Grt-II garnets

Type

Tourmaline within quartz vein

No.

Tur-1

Tur-2

Tur-3

Tur-4

Tur-5

Tur-6

Tur-7

Tur-8

Tur-9

Tur-10

Tur-11

Tur-12

Tur-13

Tur-14

Tur-15

Tur-16

Tur-17

Tur-18

SiO2

35.56

36.14

34.63

35.80

34.79

36.31

34.97

35.58

35.29

36.46

36.65

35.59

35.68

35.74

35.55

35.94

35.72

36.50

TiO2

0.00

0.02

0.02

0.00

0.01

0.00

0.00

0.00

0.06

0.11

0.06

0.03

0.03

0.03

0.00

0.02

0.00

0.03

Al2 O3

32.83

31.41

32.45

33.06

32.20

33.18

32.56

33.58

33.09

30.45

31.37

30.98

30.48

31.69

31.38

31.27

31.87

32.09

FeO

13.28

14.47

14.29

13.24

13.62

11.74

14.77

13.73

12.52

9.10

8.47

8.96

9.67

9.25

8.78

9.55

8.73

8.97

MgO

0.53

0.51

0.25

0.21

0.28

0.10

0.35

0.28

0.08

4.38

4.16

4.36

4.36

4.42

4.89

4.50

4.29

4.29

CaO

0.05

0.05

0.13

0.02

0.11

0.12

0.13

0.39

0.16

0.44

0.32

0.44

0.47

0.43

0.56

0.78

0.39

0.34

MnO

0.05

0.02

0.08

0.04

0.06

0.09

0.04

0.05

0.13

0.02

0.01

0.00

0.05

0.00

0.06

0.04

0.00

0.00

Na2O

1.59

1.67

2.00

1.62

2.04

1.83

1.90

1.97

1.96

2.30

2.16

2.19

2.13

2.14

2.03

2.10

2.18

2.07

K 2O

0.04

0.04

0.07

0.04

0.06

0.03

0.06

0.04

0.03

0.18

0.14

0.13

0.06

0.05

0.05

0.08

0.10

0.08

F

0.43

0.31

0.43

0.65

0.72

0.72

0.35

0.78

0.50

0.45

0.26

0.53

0.53

0.64

0.60

0.42

0.45

0.41

Cl

0.00

0.00

0.01

0.00

0.01

0.00

0.00

0.01

0.01

0.01

0.00

0.01

0.00

0.00

0.00

0.00

0.01

0.01

H2 O*

3.32

3.37

3.27

3.23

3.12

3.22

3.34

3.20

3.27

3.35

3.46

3.26

3.26

3.25

3.25

3.37

3.33

3.41

B2O3 *

10.20

10.21

10.07

10.24

10.05

10.31

10.15

10.35

10.18

10.33

10.39

10.20

10.19

10.30

10.26

10.34

10.29

10.45

Li2 O*

0.55

0.74

0.41

0.66

0.55

1.06

0.34

0.56

0.73

0.83

0.87

0.57

0.54

0.42

0.36

0.49

0.51

0.59

Total

98.41

98.96

98.10

98.80

97.60

98.72

98.95

100.51

98.01

98.41

98.31

97.27

97.45

98.34

97.77

98.90

97.88

99.23

O=F

0.18

0.13

0.18

0.27

0.30

0.30

0.15

0.33

0.21

0.19

0.11

0.22

0.22

0.27

0.25

0.18

0.19

0.17

Total*

98.23

98.83

97.92

98.53

97.30

98.42

98.80

100.18

97.80

98.22

98.20

97.05

97.22

98.07

97.51

98.72

97.69

99.06

Structural formula based on 31 anions (O, OH, F) T: Si Al

6.06

6.15

5.97

6.07

6.02

6.12

5.99

5.97

6.03

6.14

6.13

6.06

6.09

6.03

6.02

6.04

6.03

6.07

0.00

0.00

0.03

0.00

0.00

0.00

0.01

0.03

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

B

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

Z: Al

6.00

6.00

6.00

6.00

6.00

6.00

6.00

6.00

6.00

6.00

6.00

6.00

6.00

6.00

6.00

6.00

6.00

6.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Mg 3+

Fe

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Y: Al

0.59

0.30

0.57

0.61

0.57

0.59

0.56

0.62

0.66

0.04

0.19

0.22

0.13

0.30

0.27

0.19

0.34

0.29

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.01

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Ti

44

Fe3+

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Mg

0.14

0.13

0.06

0.05

0.07

0.03

0.09

0.07

0.02

1.10

1.04

1.11

1.11

1.11

1.23

1.13

1.08

1.06

Mn

0.01

0.00

0.01

0.01

0.01

0.01

0.01

0.01

0.02

0.00

0.00

0.00

0.01

0.00

0.01

0.01

0.00

0.00

Fe2+

1.89

2.06

2.06

1.88

1.97

1.66

2.11

1.93

1.79

1.28

1.18

1.28

1.38

1.30

1.24

1.34

1.23

1.25

Li*

0.37

0.50

0.29

0.45

0.38

0.72

0.23

0.38

0.50

0.56

0.58

0.39

0.37

0.28

0.25

0.33

0.34

0.39

Sum-Y

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

3.00

0.01

0.01

0.02

0.00

0.02

0.02

0.02

0.07

0.03

0.08

0.06

0.08

0.09

0.08

0.10

0.14

0.07

0.06

Na

0.52

0.55

0.67

0.53

0.68

0.60

0.63

0.64

0.65

0.75

0.70

0.72

0.70

0.70

0.67

0.68

0.71

0.67

X: Ca

K

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.04

0.03

0.03

0.01

0.01

0.01

0.02

0.02

0.02

Vacancy

0.46

0.43

0.29

0.46

0.28

0.37

0.33

0.28

0.31

0.13

0.21

0.17

0.20

0.21

0.22

0.16

0.19

0.25

OH

3.77

3.83

3.76

3.65

3.60

3.61

3.81

3.58

3.73

3.76

3.87

3.71

3.71

3.66

3.68

3.78

3.76

3.78

F

0.23

0.17

0.23

0.35

0.39

0.38

0.19

0.41

0.27

0.24

0.13

0.29

0.28

0.34

0.32

0.22

0.24

0.22

Cl

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Schorl

Schorl

Schorl

Schorl

Schorl

Schorl

Schorl

Schorl

Schorl

Schorl

Schorl

Schorl

Schorl

Schorl

Schorl

Schorl

Schorl

Schorl

Mineral Name Notes:

The analysis condition is the same to garnet. Structural formulae and site allocation of tourmaline were calculated using an unpublished spreadsheet by Selway and Xiong (http://www.open.ac.uk/earth-research/tindle/AGTWebData/Tourmaline.xls, 2014) calculating B2 O3, H2 O and Li2 O by stoichiometry for B = 3 apfu, OH + F = 4 apfu and Li = 15-total(T + Z + Y) and normalizing to 31 anions. Nomenclature follows the guidelines of Hawthorne and Henry (1999).

45

Table 4. Representive EPMA analyses (wt%) of the magmatic biotite in the parental syenogranite from Baiganhu W–Sn orefield. No.

B01

B02

B03

B04

B05

B06

B07

B08

B09

B10

SiO2

38.67

37.97

37.98

38.04

38.46

37.81

38.66

37.75

37.92

37.75

TiO2

2.32

2.43

2.48

2.32

2.07

2.74

2.53

2.73

2.51

2.44

Al 2O3

17.86

17.95

16.59

16.99

17.14

15.44

15.17

15.47

16.19

16.79

FeOT

20.12

21.17

21.63

20.61

21.27

21.41

21.02

21.26

21.90

21.64

MnO

0.61

0.65

0.48

0.53

0.37

0.27

0.28

0.33

0.43

0.48

MgO

6.51

6.63

7.34

7.49

7.53

8.49

8.72

8.61

7.58

7.21

CaO

0.00

0.00

0.02

0.00

0.00

0.00

0.00

0.00

0.02

0.02

Na2 O

0.09

0.05

0.11

0.17

0.13

0.11

0.10

0.12

0.11

0.11

K 2O

9.56

9.63

8.89

9.10

9.14

9.13

9.01

9.07

8.69

8.92

F

0.60

0.56

0.01

0.68

0.74

0.55

0.25

0.30

1.02

0.85

Cl

0.03

0.02

0.91

0.03

0.02

0.05

0.06

0.05

0.05

0.04

O=(F+Cl)

-0.26

-0.24

-0.39

-0.29

-0.31

-0.24

-0.11

-0.14

-0.44

-0.26

Total

96.11

96.82

96.05

95.67

96.56

95.76

95.69

95.55

95.98

95.99

Si

5.841

5.733

5.790

5.786

5.804

5.782

5.878

5.772

5.794

5.791

Al IV

2.159

2.267

2.210

2.214

2.196

2.218

2.122

2.228

2.206

2.209

T-site sum

8.000

8.000

8.000

8.000

8.000

8.000

8.000

8.000

8.000

8.000

AlVI

1.020

0.929

0.771

0.833

0.853

0.564

0.597

0.561

0.710

0.766

Ti

Based on 22 oxygen atoms

0.264

0.275

0.284

0.266

0.235

0.315

0.289

0.314

0.289

0.285

3+

0.000

0.069

0.029

0.053

0.078

0.317

0.165

0.325

0.049

0.115

Fe2+

2.541

2.604

2.729

2.569

2.607

2.420

2.508

2.393

2.750

2.643

Mn

0.078

0.084

0.061

0.068

0.048

0.035

0.036

0.043

0.056

0.061

Fe

Mg

1.467

1.493

1.669

1.698

1.694

1.935

1.976

1.962

1.727

1.670

O-site sum

5.370

5.454

5.543

5.487

5.515

5.586

5.571

5.598

5.581

5.540

Ca

0.000

0.000

0.003

0.000

0.000

0.000

0.000

0.000

0.004

0.003

Na

0.026

0.016

0.031

0.049

0.039

0.034

0.030

0.036

0.034

0.031

K

1.843

1.856

1.730

1.766

1.759

1.780

1.749

1.769

1.694

1.727

46

Interlayer

1.869

1.871

1.764

1.815

1.798

1.814

1.779

1.805

1.732

1.761

F

0.286

0.265

0.440

0.328

0.352

0.265

0.118

0.145

0.493

0.444

Cl

0.008

0.005

0.011

0.007

0.004

0.014

0.015

0.012

0.012

0.011

OH

3.706

3.730

3.549

3.665

3.644

3.721

3.866

3.844

3.495

3.545

XMg

0.366

0.358

0.377

0.393

0.387

0.414

0.425

0.419

0.382

0.377

Al T

3.179

3.196

2.981

3.047

3.049

2.782

2.719

2.789

2.916

2.975

T(°C)

617

624

632

622

598

654

642

654

635

631

P(kbar)

3.10

3.15

2.50

2.70

2.71

1.90

1.71

1.92

2.30

2.63

Notes: The analysis condition is the same to garnet. FeOT means total iron measured by EPMA. The proportion of ferric and ferrous iron in biotite is calculated based on charge balance. T(°C) = {[ln(Ti)-a-c(XMg) 3]/b}0.333, where Ti is the number of atoms per formula unit (apfu) normalized on the basis of 22 O atoms, XMg is Mg/(Mg + Fe), a = -2.3594, b = 4.6482 × 10–9 and c = -1.7283. The calibration range for this expression is XMg = 0.275–1.000, Ti = 0.04–0.60 apfu, and T = 480–800 °C. This geothermometer initially proposed for graphitic and peraluminous metapelites (Henry et al., 2005) but suggeted also suitable for peraluminous igneous rocks (Sarjoughian et al. 2014). P(kbar) = 3.03*Al T-6.53 (±0.33), where P means the solidification pressure, AlT designates the total Al contents in biotite on the basis of 22 O atoms (Uchida et al., 2007).

47

Table 5. A brief summary on the SnO2 (%) or Sn (ppm) contents of Sn-bearing garnets around the world. Location

Garnet origin, occurrence, end-member formulas, and SnO2 (%) or Sn (ppm) contents

Analysis methods for Sn

References

Hydrothermal garnets in cassiterite-polymetallic deposit;

Spectrographic

Saksela, 1951;

Pitkaranta, Finland

Andradite (Grs36.6) with SnO2 1.44%

(?)

also see Deer et al., 1982

Plavno Mine in the Krusne

Hydrothermal garnets occurs with pyroxene and carbonates;

Hory Mts., Czechoslovakia

Andradite (Adr89.7Grs6.4Pyr0.9Sps0.7) with SnO2 1.07%

Spectrographic analysis

Dadak and Novak, 1965

Red-a-ven Mine, Devonshire,

Hydrothermal garnets associated with hedenbergite and wollastonite;

X-ray

El-Sharkawi

England

Andradite (Adr47-65Grs27-35Pyr6-13Sps1Alm1-6) with SnO2 0.14-1.15%

analysis

Dearman,1966

Cassiar District of northern

Hydrothermal garnets associated with clinopyroxenes;

British Columbia, Canada

Andradite (Adr99.95 ) with SnO2 0.9%

Spectrographic analysis

Mulligan and Jambor, 1968

Davib Ost near Usakos, South

Hydrothermal garnets associated with grossular and vesuvianite;

West Africa

Andradite (Adr95.4-98.5) with max SnO2 5.82%

EPMA

McIver and Mihalik, 1975

Doradilla, New South Wales,

Hydrothermal garnets associated with clinopyroxene;

Australian

Andradite (Adr51.4-89.6Grs5.4-47.1) with SnO2 0.5-3.5%

EPMA

Plimer, 1984

El Hammam mining district,

Hydrothermal garnets associated with wollastonite and malayaite (CaSnSiO5 );

Central Morocco

Andradite (Adr52.5-79.1Grs1.0-47.5) with SnO2 0.2-5.6%

EPMA

Sonnet and Verkaeren, 1989

Yaogangxian Sn–polymetallic

Hydrothermal garnets associated with pyroxene, etc.;

deposit, China

Grossular (average = Adr13Grs65Sps17Alm4Pyr0, n=9) with max SnO2 0.02%

EPMA

Chen et al., 1992a

EPMA

Chen et al., 1992a

EPMA

Chen et al., 1992a

EPMA

Chen et al., 1992a

EPMA

Chen et al., 1992a

Sn-polymetalilc

deposit

in

analysis

fluorescence

Hydrothermal garnets associated with pyroxene, etc.; Dachang Sn deposit, China

Grossular (average = Adr47Grs51Sps1Alm0Pyr0, n=5) with mean SnO2 0.22% and max SnO2 0.38% Hydrothermal garnets associated with pyroxene, etc.;

Xinlu Sn deposit, China

Andradite (average = Adr83Grs 14Sps1Alm1Pyr1, n=8) with mean SnO 2 1.33% and max SnO2 1.90%

Debao Cu–Sn deposit, China Gejiu Sn deposit, China

Hydrothermal garnets associated with pyroxene, etc.; Andradite (Adr>60%) with max SnO2 1.92% Hydrothermal garnets associated with pyroxene, etc.; Andradite (average = Adr90Grs3Sps1Alm0Pyr1, n=21) with mean SnO2 2.54% and max SnO2

48

and

5.14% Shizhuyuan W–Sn–polymetallic

deposit,

China Hehuaping

Sn–polymetallic

deposit, China Xitian

Hydrothermal garnets associated with pyroxene, idocrase, and wollastonite; Andradite (Adr21-74Grs20-39Sps4-10Alm0-6 Pyr0-1 , n=10) with SnO2 0.09-0.67% Hydrothermal garnets associated with pyroxene, etc.; Grossular (average = Adr26Grs65Sps4Alm2Pyr2, n=12) with SnO2 0.130-0.138% (Sn = 964–1087 ppm)

W–Sn–polymetallic

deposit, China Xintianling W deposit, China

Pure garnet powders analyzed by ICP-MS

Hydrothermal garnets associated with pyroxene, etc.;

Pure garnet powders

Andradite (average = Adr48Grs39Sps11 Alm2Pyr0, n=14) with SnO2 0.045% (Sn = 357.9 ppm)

analyzed by ICP-MS

Magmatic garnets associated with tourmaline (Grt-II); Sps 64-74Alm29-37 Adr0-1Grs0-2Prp0 with SnO2 0.003–0.011% (Sn = 22.96–89.67 ppm) China

orefield,

analyzed by ICP-MS

Grossular (average = Adr21Grs67Sps0 Alm0Pyr6, n=12) with SnO2 0.130% (Sn = 1022 ppm)

Sps 45-53Alm46-53 Adr0-1Grs0-1Prp0-1 with SnO2 0.003–0.010% (Sn = 23.22–79.83 ppm)

W–Sn

Pure garnet powders

Hydrothermal garnets associated with pyroxene, etc.;

Magmatic garnets enclaved by quartz (Grt-I);

Baiganhu

EPMA

Chen et al., 1992b

Yao et al., 2013

Yao et al., 2013 Yao et al., 2013

In situ LA-ICP-MS

This study

In situ LA-ICP-MS

This study

In situ LA-ICP-MS

This study

In situ LA-ICP-MS

This study

Hydrothermal garnets associated with pyroxene and Mo-rich scheelite (Grt-III); Andradite (Adr62-88Grs1-18Sps3-12Alm0-8Pyr0, n=5) with SnO2 0.323–1.062% (Sn = 2544–8364 ppm) Hydrothermal garnets associated with fluorite and crosscut by calcite veins (Grt-IV); Grossular (Adr4-22Grs62-73Sps5-16Alm2-10Pyr 0, n=5) with SnO2 0.007–0.034% (Sn = 51.7–269 ppm)

49

50

51

52

53

54

55

56

57

58

59

Highlights: (1) Magmatic and hydrothermal garnets varied in texture, major and trace elements; (2) Mn-rich magmatic garnet crystallized at relatively low temperature and pressure; (3) Association of garnet and tourmaline presented and discussed for W/Sn mineralization; (4) Sn-rich grandite confirmed and suggested as an indicator mineral for W/Sn exploration.

60

Positive correlation between SnO2 (wt%) and andradite molecules (mol%) in garnets from the W/Sn deposits of China. Data from this study, Yao et al. (2013) and Chen et al. (1992a, 1992b).

61