Chemical and boron isotopic compositions of tourmaline from the Paleoproterozoic Houxianyu borate deposit, NE China: Implications for the origin of borate deposit

Accepted Manuscript Chemical and boron isotopic compositions of tourmaline from the Paleoproterozoic Houxianyu borate deposit, NE China: Implications for the origin of borate deposit Xue-long Yan, Bin Chen PII: DOI: Reference:

S1367-9120(14)00239-9 http://dx.doi.org/10.1016/j.jseaes.2014.05.021 JAES 1964

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

18 December 2013 3 May 2014 27 May 2014

Please cite this article as: Yan, X-l., Chen, B., Chemical and boron isotopic compositions of tourmaline from the Paleoproterozoic Houxianyu borate deposit, NE China: Implications for the origin of borate deposit, Journal of Asian Earth Sciences (2014), doi: http://dx.doi.org/10.1016/j.jseaes.2014.05.021

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Chemical

and

boron

isotopic

compositions

of

tourmaline

from

the

Paleoproterozoic Houxianyu borate deposit, NE China: Implications for the origin of borate deposit Xue-long Yan, Bin Chen* School of Earth and Space Sciences, Peking University, Beijing 100871, People’s Republic of China *Corresponding author: [email protected] Abstract The Houxianyu borate deposit in northeastern China is one of the largest boron sources of China, hosted mainly in the Paleoproterozoic meta-volcanic and sedimentary rocks (known as the Liaohe Group) that are characterized by high boron concentrations. The

borate

ore-body

has

intimate

spatial

relationship

with

the

Mg-rich

carbonates/silicates of the Group, with fine-grained gneisses (meta-felsic volcanic rocks) as main country rocks. The presence of abundant tourmalinites and tourmaline-rich quartz veins in the borate orebody provides an opportunity to study the origin of boron, the nature of ore-forming fluids, and possible mineralization mechanism. We report the chemical and boron isotopic compositions of tourmalines from the tourmaline-rich rocks in the borate deposit and from the tourmaline-bearing fine-grained gneisses. Tourmalines from the fine-grained gneisses are chemically homogeneous, showing relatively high Fe and Na and low Mg, with δ11B values in a narrow range from +1.22‰ 1

to +2.63‰. Tourmalines from the tourmaline-rich rocks, however, commonly show compositional zoning, with an irregular detrital core and aeuhedral overgrowth, and have significantly higher Mg, REE (and more pronounced positive Eu anomalies),V (229-1852 ppm) and Sr (208-1191 ppm) than those from the fine-grained gneisses. They show varied B isotope values ranging from +4.51‰ to +12.43‰, which plot intermediate between those of the terrigenous sediments and arc rocks with low boron isotope values (as represented by the δ11B = +1.22‰ to +2.63‰ of the fine-grained gneisses of this study) and those of marine carbonates and evaporates with high boron isotope values. In addition, the rim of the zoned tourmaline shows notably higher Mg, Ti, V, Sn, and Pb, and REE (particularly LREEs), but lower Fe, Co, Cr, Ni, Zn, Mn, and lower δ11B values than the core. These data suggest that (1) the sources of boron of the borate ore-body are mainly the Paleoproterozoic meta-volcanic and sedimentary rocks, and (2) the ore-forming fluids should be the high temperature metamorphic fluids related to the amphibolite-facies metamorphism of the Paleoproterozoic foldbelt, which leach boron from the boron-rich meta-volcanic and sedimentary rocks of the Liaohe Group, and the boron-rich metamorphic fluids subsequently interacted with the marine Mg-rich carbonates and evaporates, forming borate deposit, the tourmaline overgrowth in the rim and the tourmaline-rich rocks. Key words: tourmaline; borate; boron isotope; Paleoproterozoic; N.E. China

1.

Introduction Boron is not a major element in the earth’s crust and waters, averaging only 3-20 2

ppm in the upper continental crust and is a minor component in seawater (Garrett, 1998). However, boron tends to accumulate in the last phase of magmatic flows or geothermal fluids due to its mobile and volatile features. Most economically exploited borate deposits are formed by leaching of hydrothermal waters circulating through boron-rich lithologies, and subsequent evaporation of surface and near-surface waters in volcanic terrains characterized by active geothermal springs (Warren, 2006). Most borate deposits are commonly situated in active volcanic terrains, and have a close relationship with tuff, basalts and volcanic sediments. In these areas, the subduction processes may have played an important role in the formation of borate-rich series (Floyd et al., 1998). Boron is enriched in continental crust, clastic sediments, and seawater-altered oceanic crust (Leeman and Sisson, 1996). Thus, the initial source and enrichment of boron was likely in a subduction environment via metasomatism of lithosphere by boron-rich fluids released from down-going altered oceanic slabs (Palmer, 1991; Ryan and Langmuir, 1993) and possible boron-rich pelagic sediments (You et al., 1993). The Houxianyu borate deposit of northeastern China is one of the major boron sources in China. The borate deposits in China are unusually hosted in the Palaeoproterozoic metamorphic volcanic-sedimentary rocks (known as the Liaohe Group; Zhang, 1984), while most other major economic borate deposits on Earth are contained in Cenozoic (or younger) sediments (Kistler and Helvaci, 1994). Origin of the Chinese borate deposits remains controversial, focusing on three issues:(1) the source of ore-forming fluids,(2) the origin of boron, and (3) the mechanism of borate formation. 3

The Houxianyu borate deposit is one of the largest borate deposits in the Liaohe Group of northeastern China. It has experienced the most extensive hydrothermal alteration, and resulted in the formation of a complex borate-silicate-carbonate assemblage (Jiang et al., 1997), in which tourmaline-rich rocks are ubiquitous. Tourmaline has received great interest in recent years and is believed to be a useful petrogenetic indicator for ore genesis (Henry and Dutrow, 1996; Slack, 1996; Jiang, 2001; Jiang et al., 1999, 2004;Marschall and Jiang, 2011; Slack and Trumbull, 2011). Tourmaline-rich rocks have a close spatial relationship with the borates ore bodies, providing an opportunity to study the origin of boron, the nature of ore-forming fluids, and possible mineralization mechanism (Marschall and Jiang, 2011; Slack et al., 1993; Slack and Trumbull., 2011). However, few studies have been done on the origin and significance of tourmaline-rich rocks in the Houxianyu borate ores (Jiang et al., 1997; Peng and Palmer, 2002; Xu et al., 2004). In this paper, we present new mineralogical and geochemical data of the tourmalines from the borate ores, particularly, the in-situ analyses of boron isotopic and chemical compositions of tourmalines from different rock types, in an attempt to clarify the origin of the borate ores.

2.

Geological setting The borate deposits are hosted in the Paleoproterozoic Jiao-Liao-Ji Belt (JLJB) that

lies between two Archean cratons, the Longgang Block to the north and the Nangrim 4

Block to the south (Bai, 1993; Fig. 1a). The two Archean blocks are largely made up of granitoids and greenstone belts. The NE-trending 300 km-long Paleoproterozoic belt consists mainly of volcanic sedimentary rocks and associated granitic and mafic intrusions of greenschist to lower amphibolite facies (Zhang, 1984, 1988; Fig. 1b). The Li’eryu Formation, being at the lower part of the Paleoproterozoic Liaohe Group, is made up of meta-volcanic sedimentary rocks, and is characterized by high boron concentration (known as boron-bearing sequence; Zhang, 1988), in which several borate deposits are found (Fig. 1b). The Li’eryu Formation is overlain by graphitic gneiss of the Gaojiayu Formation, magnesium marbles of the Dashiqiao Formation, and metamorphosed clastic sediments of the Gaixian Formation. Several Paleoproterozoic granitic plutons (known as the Liaoji granites) occur in the Liaohe Group, including the deformed gneissic granites (~2.16 Ga; Li et al., 2005; Lu et al., 2006; Li and Zhao, 2007) and un-deformed post-collisional granites (~1.85 Ga; Li et al., 2005; Lu et al., 2006; Li and Zhao, 2007). The dynamic setting and tectonic evolution of the JLJB remain controversial (e.g., Zhao et al., 2011, 2013). Some workers believe that the Paleoproterozoic fold-belt represents a sequence of volcanic-sedimentary rocks (and associated granite intrusions) in a continental rifting setting, which finally were metamorphosed and folded due to the collision between the Longgang and Nangrim blocks at ca. 1.9 Ga (Li et al., 2001, 2004; Luo et al., 2004; Li and Zhao, 2007;Li et al., 2012; Tam et al., 2011, 2012). Others suggest that the belt represented Paleoproterozoic continental arc series, and was finally 5

metamorphosed during an arc-continent collisional process (Bai, 1993; Peng and Palmer, 1994, 1995; He and Ye, 1998; Faure et al., 2004; Lu et al., 2006; Wang et al., 2011; Zhao et al., 2011, 2013).

3.

Geological features of the Houxianyu borate deposit The Houxianyu borate deposit is one of the three main borate deposits hosted in

the Li’eryu Formation (boron-bearing sequence) of the Paleoproterozoic rocks. Four main rock units are present in the Houxianyu borate deposit (Fig. 2), including: (1) finegrained gneisses; (2) tourmalinite and tourmaline-rich quartz veins (tourmalite); (3) magnesium-rich series; and (4) borate ore bodies. (1) Fine-grained gneisses The fine-grained gneisses show banding structures with foliated biotite and amphibole (Fig. 3a), being in conformable contact with the borate-hosting rocks. They are mainly composed of quartz (40 vol.%), microcline (25 vol.%) and plagioclase (10 vol.%), and small amounts of hornblende, biotite, magnetite and tourmaline (Fig. 3b), etc., representing metamorphosed felsic volcanic rocks (Peng and Palmer, 1995). Tourmaline is minor (less than 10 vol.%) in the rock, and appears to have formed during the metamorphism as indicated by the mosaic contact with quartz and feldspar (Fig. 3b). (2) Tourmaline-rich rocks The tourmaline-rich rocks include tourmalinites and tourmaline-rich quartz 6

(±amphibole) veins (tourmalite). Tourmalinites occur as stratiform (Fig. 3c), being the direct hanging walls of the borate ore bodies. Tourmalinite contains more than 90 vol.% of the tourmaline and minor quartz, feldspar, diopside, muscovite, magnetite and zircon (Fig. 3d). Tourmaline appears as both euhedral and subhedral crystal, ranging from 0.1 to 3 mm in diameter. Most tourmaline grains show compositional zoning, with either a yellow brown rim on a dark green/yellowish brown core or a greenish blue rim on a yellowish core (Fig. 3d). The boundary between core and rim is sharp. The tourmaline core is characterized by the presence of many tiny detrital phases such as quartz, tourmaline and zircon, which contrasts with the scarcity of such detrital grains in the rim. Tourmaline-rich quartz (± amphibole) veins are generally restricted to the borate bodies or their stratigraphic hanging wall (Fig. 3e), being made up mainly of quartz (40 vol.%), and tourmaline (40 vol.%), amphibole (15 vol.%), and minor feldspar (Fig. 3f). (3) Magnesium-rich series The borate ore bodies are exclusively hosted in the magnesium-rich series (Fig.4a, b) that consist mainly of Mg-silicate (forsterite-diopside-phlogopite-tremolite rocks) and subordinate magnesium-rich carbonate. The peridotiteis variedly serpentinized, and comprises more than 90 vol.% forsterite (Fig. 4c, d). Forsterite (olivine) occurs in two forms. The smaller forsterite grains (about 50-500um in diameter) suffered no alteration, commonly coexisting with szaibelyite and ludwigite (Fig. 4e, f). The larger forsterite grains (up to >2mm) show mosaic textures, generally being altered to serpentine and tremolite, and coexist with apatite and magnetite (Fig. 4c, d). Phlogopite is present in 7

carbonates and Mg-silicate rocks. (4) Borate ore bodies The ore bodies are stratiform or lenticular in shape, and show conformable contact with the host rocks. The borate phases are dominated by szaibelyite (Mg2B2O5·H2O) and suanite (Mg2B2O5), with minor ludwigite ((Fe, Mg)4Fe2B2O10). Szaibelyite occurs either as massive platy crystals or as fibrous form (Fig. 4e). Suanite is commonly hydrated to szaibelyite during subsequent metamorphism. Ludwigite is less abundant, and generally being alternated to szaibelyite and magnetite. Both szaibelyite and ludwigite coexist with the smaller forsterite grains (Fig. 4e, f).

4.

Sampling and analytical methods

4.1. Sampling Most of the samples are taken from the borate ore body and nearby, including the tourmalinite/tourmaline-rich rocks, Mg-silicates, and borate ores. In addition, two tourmaline-bearing fine-grained gneisses away from the borate ore body were collected.

4.2. Electron microprobe analysis Major elements of tourmaline were determined on polished and carbon-coated thin sections using a JEOLJXA-8100 instrument, at the Key Laboratory of Orogenic Belts and Crustal Evolution (KLOBCE), Peking University, Beijing. We applied a beam 8

current of 10 nA, an acceleration voltage of 15 kV and a beam of 5 µm diameter. Natural oxide and silicate mineral reference materials were used for calibration, when a phi (rho z) curve was applied to improve the accuracy of the results. Our procedure for microprobe analysis was routinely analyzing two or more points from the core to rim portions of zoned grains to check for chemical variations.

4.3. Laser Ablation ICP-MS (LA-ICP-MS) In situ trace elements analyses were carried out on representative tourmalines, using an Agilent 7500ce inductively coupled plasma mass spectrometry (ICP-MS) connected to a Coherent COMPexPro102 laser ablation system with an ArFExcimer laser at KLOBCE, Peking University. The laser has an output wavelength of 193 nm and a spot size of 90 μm. Helium combined with argon was used as a carrier gas, and ablation was carried out with a pulse rate of 5 Hz and an energy density of 6 J/cm2. The software GLITTER (Macquarie University) was used for data reduction with NISTSRM 610 (Pearce et al., 1997) as the external standard and Si as the internal standard using microprobe data. The accuracies of concentrations for the reference glass NISTSRM 612 and 614 (Pearce et al., 1997) were generally within 5% and 10%, respectively. Detection limits for most elements were between 0.002 and 0.2 ppm. Higher detection limits were achieved for Cr, Cu and Zn (0.66 ppm, 0.35 ppm and 0.30 ppm, respectively).

4.4. Boron isotope 9

In situ boron isotopic compositions were measured in polished sections, in order to reveal isotopic variation within individual tourmalines. The analyses were carried out on a Neptune Plus multicollector ICP-MS and a Newwave UP193 laser ablation system, at the State Key Laboratory for Mineral Deposits Research, Nanjing University. Analytical condition is 8 Hz laser output frequency, 75 um ablation diameter, and energy density of about 11J/cm2. The data was collected statically and simultaneously in cycles of 100 with an integration time of 0.131s. The detailed analytical procedure is similar to that described by Hou et al. (2010) and Yang and Jiang. (2012). Mass bias of the instrument and the fractionation of isotopes were calibrated in this study using the standard-sample-bracketing (SSB) method. IAEA B4, the B isotope standard of tourmaline from the International Atomic Energy Agency (Tonarini et al., 2003), was used as the external standard. Instrumental mass fractionation (IMF) and analytical quality were assessed by replicate analyses of tourmaline reference materials dravite (HS #108796) and schorl (HS#112566) from the Harvard Mineralogical Museum (Dyar et al., 2001). The similarity of IMF values (Table 3) determined for the chemically distinct tourmaline standards demonstrates a lack of significant chemical matrix effect with our analytical setup. The observed individual uncertainties for reference samples were typically below ± 0.5‰ (2 SD). The variations of δ11B for both references were 1.0‰ and 1.3‰, respectively, which we believe to be the best estimate for the trueness of the data set. Boron isotope data are reported in δ11B notation (δ11B = ((11B/10BSample)/(11B/10BIAEA B4)-1) ×1000+δ11B IAEA B4) using the δ11B value of IAEA B4 10

of -8.71‰ ± 0.18‰ (Tonarini et al., 2003).

5.

Results

5.1. Chemical compositions of tourmaline Tourmaline is a complex borosilicate mineral with the general structural formula of XY3Z6(T6O18)(BO3)3V3W (Hawthorne and Henry, 1999). where X = Ca, Na, K, vacancy; Y = Mg, Fe2+, Mn2+, Al, Li, Cr3+, V3+, Fe3+, (Ti4+); Z = Mg, Al, Fe3+, V3+, Cr3+; T = Si, Al, (B); B = B; V = OH, O; W = OH, F, O (Henry andDutrow, 1996; Hawthorne and Henry, 1999). Tourmaline structure formulae were calculated by normalizing to 15 cations atoms per formula unit (a.p.f.u) in the tetrahedral and octahedral sites (T + Z + Y) following the procedures of Henry andDutrow (1996). The structural formulae are approximate, since we did not determine B, H2O, ferrous/ferric iron ratio, and minor elements such as Li and Zn, which may be present at sub-wt.% level. Microprobe analyses of tourmalines from the Houxianyu deposit are given in Table 1. All tourmalines, including those from the borate ore body (tourmalinite and tourmaline-rich quartz vein) and those from the fine-grained gneisses, plotin field of alkali group of Henry et al. (2011) based on the X-site occupancy (Fig. 5). The concentration of X-site vacancy is very low, with a maximum of 0.11 a.p.f.u. The majority of tourmalines plot in the field #6 (Fe3+-rich quartz-tourmaline rocks, calc silicate rocks, and metapelites) and/or field #10 (Ca-poor metapelites, metapsammites, 11

and quartz tourmaline rock) of Henry & Guidotti(1985) (Fig. 6). Most tourmalines plot along the schorl-dravite solid-solution line with Fe/(Fe+Mg) ratios ranging from 0.16 to 0.60 (Fig. 6, 7a). However, there is a considerable deviation of Al from the ideal schorldravite, with most tourmalines being Al-deficient (<6 a.p.f.u) (Fig. 6, Fig. 7b). Tourmalines from the fine-grained gneisses and those from the tourmaline-rich rocks (tourmalinite and tourmaline-rich vein) have different trends in Fig. 7b, which, along with other differences in major and trace element compositions as below, suggesting they have different origins. (1) Tourmalines from the fine-grained gneisses show no compositional zoning, and are characterized by relatively higher Fe and Na contents (a.p.f.u) than those from tourmaline-rich rocks, with Fe/(Fe+Mg) and Na/(Na+Ca) ratios ranging from 0.51 to 0.60 and 0.75 to 0.93, respectively (Fig. 7a). These tourmalines show LREE enrichment and positive Eu anomalies (Eu/Eu* = 1.9-6.1) in the chondrite-normalized REE patterns (with total REE contents =3.9-15 ppm) (Table 2, Fig. 8a). The highly positive Ce anomalies of some analyzed spots may be related to the presence of minute crystals of Ce-rich accessories like allanite. Overall, these tourmalines have relatively low contents of V (16-180 ppm), Cr (4.5-33 ppm), Co (3.9-14 ppm), Ni (6.0-21 ppm), Sn (3.3-14 ppm), and Sr (45-110 ppm) (Table 2). (2) Tourmalines from tourmaline-rich rocks of the borate deposit are generally Mgrich and Na-poor (as dravite) (Fig. 7a). Theses tourmalines exhibit higher REE contents (0.71-48 ppm) and more pronounced positive Eu anomalies (Eu/Eu* = 1.99-75.26) than 12

those from the fine-grained gneisses, and are characterized by variably significant enrichment of LREEs (Fig. 8b; Table 2), with the LaCN/YbCN ratios ranging from 2.7 to 174.7. They show higher contents of V (229-1852 ppm), Cr (2.0-557 ppm), Co (8.0-37 ppm), Ni (12-78 ppm), Sn (2.9-122 ppm), and Sr (208-1191 ppm) than those from the fine-grained gneisses (Table 2). Note that the rim of the zoned tourmaline shows higher Mg (Mg/Fe ratios = 1.27~1.86), Ti, V (338-1852 ppm), Sn (13-41 ppm), and Pb (4.5-28 ppm), but lower Fe, Li (4.8-18 ppm), Cr (2.0-97 ppm), Co (8.0-21 ppm), Ni (12-78 ppm), Zn (36-147 ppm), Mn (50-193 ppm), Sc (5-68 ppm), and to a less extent, Al, than the core (Fig. 9b, c, e, g, h). No significant variation was found for Si, Ca, Na, and K (Fig. 9c, d, e). In addition, the tourmaline rim has notably higher REE contents(3.5-48 ppm) than the core (0.71-7.3 ppm), especially the LREEs (Fig. 8b, 9f).

5.2. Boron isotopic compositions of tourmalines The boron isotope data are listed in Table 4 and shown in Fig. 10. The δ11B values of tourmalines from the fine-grained gneisses away from the borate ore-body range from+1.22‰ to +2.63‰ (Table 4; Fig. 10a). No significant intra-crystalline isotopic variation was found for these tourmalines, being consistent with the fact that no compositional zoning was found for them. Tourmalines from the tourmaline-rich rocks within the borate ore body have δ11B values ranging from +4.51‰ to +12.43‰ (Table 4;Fig. 10b), which are significantly higher than those of the tourmalines from the fine-grained gneisses. In addition, zoned tourmalines show large variation of boron isotope data from core to rim. One euhedral 13

grain has δ11B = 11.23‰ for the core and 7.82‰ for the rim (sample HX-4, Fig. 11a); the two figures are 7.75‰ and 4.76‰, respectively, for another tourmaline (sample HX-5, Fig. 11b).

6.

Discussion

6.1. Origin of ore-forming fluids Chemical compositions of tourmaline can be used to constrain the nature and evolution of ore-forming fluids/melts from which it crystallized (Slack and Coad, 1989; Slack, 1996; Jiang et al., 1999, 2004, 2008; Slack and Trumbull, 2011). In a recent experimental study, van Hinsberg (2011) found that tourmaline does not fractionate specific trace elements to significant degree from melt, with the majority of the tourmaline-melt partition coefficients of trace elements ranging from 0.4 to 1.1 at 800 ℃and 7.5 kbar. Similarly, Klemme et al. (2011) concluded that most trace elements show tourmaline-mica partition coefficients close to one at ~600℃ and 3~5 kbar, with only Ni and the light REE partitioning strongly (i.e., Dtourmaline/mica>10) into tourmaline, whereas large lithophile elements such as Ba and Rb, and W, Sn, Nb, and Ta, portioning strongly into the coexisting fluids (i.e., Dtourmaline/mica<0.1). Therefore, tourmaline that crystallized from ore-forming fluids can monitor the change of trace elements of fluids. Compositional zoning is common for tourmalines from the tourmalinites (Fig. 3d, 9, 11). The boundary between core and rim is sharp. The tourmaline core is 14

characterized by the presence of many tiny detrital phases such as quartz, tourmaline and zircon, which contrasts to the scarcity of such detrital grains in the rim (Fig. 3d). In addition, the core shows high Fe/Mg ratios (generally >1.0) and low REE contents, in contrast to the Mg-rich and high REE features of the rim. These data suggest that the tourmaline core formed probably during the volcano-sedimentation process (at 2.2Ga2.1Ga; Bai, 1993; Sun et al., 1993; Jiang, 1987), related to circulating and leaching of hot-spring water through the Paleoproterozoic boron-rich volcanic-sedimentary rocks. Mg-rich carbonates could be formed simultaneously due to intense evaporation (Sheila and Khangaonkar, 1989; Hänchen et al., 2008).The presence of the abundant detrital grains in the core probably helps overcome the nucleation barrier at low temperatures (e.g., Henry and Dutrow, 1996, 2012; van Hinsberg et al., 2011). This is supported by the very low REE contents compared with the rim (Fig. 8b, 9f), which is consistent with the low REE solubility/contents in fluids at relatively low temperatures (Alibo and Nozaki, 1999; Soyol-Erdene and Huh, 2013).The tourmaline rim is typically euhedral, and represents an overgrowth in a late stage. The rim is characterized by significantly higher Mg (and Mg/Fe ratios = 1.27-1.86), Ti, V, REEs, Sn, and Pb contents, and lower Fe, Co, Ni, Zn, Mn and Sc contents than the core (Fig. 9). The notably high REEs, in particular, LREEs (preferentially fractionated into metamorphic fluids; Plimer et al., 1991; Slack et al., 1993; Slack, 1996; Jiang et al., 2004), and the pronounced positive Eu anomalies of the tourmaline rims suggest that they could have formed from a high temperature fluid (>250℃; Sverjensky, 1984;Bau, 1991). We believe that such a high 15

temperature fluid is likely the metamorphic fluids related to the extensive amphibolitefacies metamorphism in the JLJB at ca. 1.9 Ga (Luo et al., 2004; Lu et al., 2006; Li and Zhao, 2007; Tam et al., 2011, 2012). This is supported by the following lines of evidence. (1) The high Mg contents of tourmaline rim resulted from high fluid flux and intense water-rock interaction during the metamorphism, because the tourmalinites exclusively occur near the Mg-rich series (peridotite and Mg-carbonate). (2) The high contents of V and Sn in the rim could be related to the dissolution of biotite (main host of the two elements; Tischendorfet al., 2001) during the metamorphism, leading to enrichment of V and Sn in the metamorphic fluids (Harlavan and Erel, 2002). Pb is enriched in the rim due probably to its high mobility in the metamorphic fluids. And (3) the sharp decrease of the δ11B values from tourmaline core (+11.23‰～+7.82‰) to rim (+7.75‰～+4.76‰) (Fig. 11a, b) indicates involvement of metamorphic fluids in the overgrowth of tourmaline. The boron-bearing volcanic-sedimentary rocks commonly have relatively low δ11B values (<5‰; Fig. 10d; Chaussidon and Albarède, 1992; Palmer and Swihart, 1996; Jiang et al., 1999), as is suggested by the δ11B values of 1.22.6‰ for tourmalines from the fine-grained gneiss (meta-felsic volcanic rocks). Circulating and leaching of hot-spring water through the volcanic-sedimentary rocks could lead to concentration of boron in hot-spring water from which the tourmaline core precipitated, and the core should have significantly higher δ11B values than the volcanicsedimentary rocks, because 11B is preferentially fractionated into the fluids and thus is preferentially concentrated in the hot-spring water. Consequently, the volcanic16

sedimentary rocks could be depleted in

11

B relative to

10

B after leaching. Therefore,

metamorphic fluids released from the “leached” volcanic-sedimentary rocks during the late-stage metamorphism (at ca. 1.9Ga) are expected to have low δ11B values, which coincide with the relatively low δ11B values of the tourmaline rim.

6.2. Redox state of ore-forming fluids The chemical compositions of tourmalines can record the change of redox state of fluids, e.g., the ferric/ferrous ratios and the V/Sc ratios of tourmalines (Mlynarczyk and Williams-Jones, 2006; Li and Lee, 2004; Lee et al., 2005; Slack and Trumbull, 2011). In the Houxianyu borate deposit, the notable aluminium-deficiency (Fig. 6, 7b) and the strong negative correlation between Al (a.p.f.u) and Fe (a.p.f.u) (Fig. 7b) of tourmalines suggest a substitution of Al by Fe3+ (e.g.,Slack, 2002). Within the zoned tourmalines, the rim shows lower Al (a.p.f.u) at Z site than the core (Table 1), suggesting higher values of Fe3+ needed to occupy the vacancies at Z site (Henry et al., 2011; Slack, 2002). Combined with the lower Fetotal (Fe2+ + Fe3+) values in the rim (Table 1, Fig. 9b), it is inferred that the tourmaline rim has higher Fe3+/Fe2+ ratios than the core. Therefore, the ore-forming metamorphic fluids are more oxidizing than the earlier fluids from which the tourmaline core was derived. This is supported by the variation of V/Sc ratios from core to rim. Vanadium is a redox-sensitive element and occurs simultaneously as V3+, V4+ and V5+ in silicate melts or dominantly as V5+ in the fluids (e.g., Toplis and Corgne, 2001; Fan et al., 2005), while Sc speciation is not influenced by redox conditions. Because of their otherwise similar geochemical behavior, V/Sc ratios can reflect the 17

redox state of the fluids (Li and Lee, 2004; Lee et al., 2005). As shown in Fig. 9g, h, the V/Sc ratios of tourmaline rim are significantly higher than those of the core. The decoupling of V and Sc was likely related to the change of redox state of fluids. Hence, the high V/Sc ratios and high V concentrations in tourmaline rims support the idea that the ore-forming metamorphic fluids of the borate ore body are highly oxidizing.

6.3. Sources of boron The δ11B values of tourmaline could be used to calculate those of the original fluids/melts from which tourmaline crystallized, which can track the origin of boron. Experimental studies indicate that suanite and ludwigite likely form at 450-500 °C under high CO2 fugacity (Wang and Xu, 1964 and refs in that paper), which is consistent with the regional amphibolite-facies metamorphism and abundance of carbonate in the ore-body (Zhang, 1988). Thus, we take 450 °C as the minimum temperature of ore-forming fluids in the Houxianyu deposit in the calculation. Calculatedδ11B values for the ore-forming fluids near the borates are in the range from +6.80‰ to +14.72‰ (Fig. 10d), based on the fractionation factor of about -2.29‰ at 450 °C (Meyer et al., 2008), which agrees with the B isotopic range of borates of the same deposit (e.g., Jiang et al., 1997; Peng and Palmer, 2002; Fig. 10c). Note that the calculated boron isotope data of the ore-forming fluids plot intermediate between those of the terrigenous sediments, continental hydrothermal solutions and arc rocks and those of the marine carbonates/evaporates (Fig. 10d). The former normally show low δ11B values (<+5‰ to as low as -10‰; Palmer and Swihart, 1996; Jiang et al., 1999; Palmer, 18

1991;Peacock and Hervig, 1999; Nakano and Nakamura, 2001), as is represented by the δ11B = +1.22‰ ~ +2.63‰ of the fine-grained gneisses of this study, and the latter typically have high δ11B values (>+10‰ to as high as +30‰; Palmer and Slack, 1989; Palmer and Swihart, 1996; Jiang et al.,1998). We thus suggest that the Paleoproterozoic meta-volcanic and sedimentary rocks (the Liaohe Group) and the marine Mg-rich carbonates/silicates could be the principal boron sources for the Houxianyu borate deposit. This is strongly supported by the close spatial relationship between borate orebody and the Mg-carbonates and Mg-silicates (Zhang, 1988; Fig. 2, 4a, 4b), and by the high boron contents of the Mg-rich carbonates (B2O3 =3670 ppm; Zhang, 1994)and the Paleoproterozoic meta-volcanic and sedimentary rocks (B2O3= 500 ppm to 4000 ppm; Wang et al., 2008). Considering the high solubility of boron in water (boric acid/water solubility: 57g/L) and the high boron abundance in all rocks of the Liaohe Group, we believe that the metamorphic fluids (derived from dehydration of hydrous minerals during metamorphism) are capable of transporting huge amounts of boron for a long distance. The

boron-rich

metamorphic

fluids

subsequently

reacted

with

the

Mg-

carbonate/silicates, forming the borate orebodies. The Mg-carbonate/silicates (host rock of borate orebodies) can provide fractures for fluid flow and space for ore accumulation due to its chemical activity and fragility, which is similar to the situation in carbonaterelated skarn-type deposits. The borate minerals have significantly higher δ11B values (+9.4‰~+12.1‰;Jiang et al., 1997; Peng and Palmer, 2002) than aforementioned 19

tourmaline rim (δ11B = +7.75‰~+4.76‰) that likely formed during the metamorphism, because the borate minerals were formed due to reaction of B-rich metamorphic fluids with Mg-rich marine carbonate that is expected to have very high δ11B values (+10‰~+30‰;Palmer and Slack, 1989; Palmer and Swihart, 1996; Jiang et al., 1998). The marine environment of Mg-carbonates is supported by the following lines of evidence. (1) The borate orebodies are associated with the Mg-carbonate, silicalite and stratiform tourmalinite in the Paleoproterozoic Liao-Ji Belt. Previous workers suggest that stratiform tourmalinite is commonly precipitated in a submarine environment as hydrothermal sedimentary rocks (Palmer et al., 1989; Slack et al., 1993). This is consistent with the presence of anhydrite, an indication of marine environment, in Mgcarbonate from the Zhuanmiao borate deposit (Peng and Palmer, 1995). (2) Zhang (1994) report δ18O = +10‰~+21‰ (+14‰ for marine carbonates), δ13C = 3.4‰~+2.1 ‰ (0 for present-day seawater) and δ34S= +10‰~+14‰ for the Mgcarbonate and borate phases, which, along with the δ13C values of -2.6‰~ +2.69‰ reported for the carbonates of the Yangmugan borate deposit by Xie et al. (1998), suggests a marine environment for the borate deposits and related Mg-carbonates. In addition, the δ34S values of pyrites from the borate ore-body range from +9.1‰ to +17.3‰ and those of pyrites from the host rocks in the range from +2.5‰ to + 16.1‰ (Wang and Han, 1989). The strong positive δ34S values suggest a marine environment for pyrites, because marine evaporate sulfate commonly have δ34S values ranging from +10‰ to +35‰ (Hölser, 1977). (3) We obtained the δ11B values of tourmaline and 20

borate in the range from +4.51‰ to +12.43‰, which are intermediate between those for arc-related volcanic-sedimentary rocks (<5‰) and marine carbonate/evaporate (+18.2‰ to +31.7‰; Swihart et al., 1986). This could be reasonably explained by a process of interaction of B-rich metamorphic fluids derived from the arc-related volcanic-sedimentary rocks with the marine Mg-carbonate/evaporate; none-marine evaporates (with δ11B = -30.1‰ to +7‰; Swihart et al., 1986) are unlikely to elevate the δ11B values of borate minerals to so high levels.

6.4. Metallogenic model Three models have been proposed for the origin of the Houxianyu borate deposits. (1) Wang and Xu (1964) suggest that the deposits belong to the skarn-type related to the nearby Paleoproterozoic granites, with boron being introduced from magmatic fluids. (2) Zhang (1988) and Feng et al. (1994) conclude that the deposits formed due to a submarine volcanic exhalative process, and were further remobilized and enriched during the Paleoproterozoic metamorphism. (3) More recently, some workers suggest that the borate deposits represent metamorphosed evaporates, in which primary borates were deposited in playa lakes fed by geothermal springs (Peng and Palmer, 1994, 1995, 2002; Jiang et al., 1997). Model (1) is simply rejected, because the borates from the Houxianyu deposit have δ11B values of ~+10‰ that are much higher than those for the granitic rocks (with a maximum δ11B values of +3‰; Jiang et al., 1997). Model (2) is not favored, because the formation of tourmalinite from exhalative hydrothermal fluids requires transport of 21

significant Al in the fluids over large distance (Slack et al., 1993), which is inconsistent with the typically very low Al concentrations in submarine hydrothermal fluids (e.g., Von Damm et al., 1985). In addition, it is less likely for borate minerals to precipitate from aqueous fluids that did not experience significant evaporation or extensive interaction with boron-rich evaporates, due to their high solubility (e.g., Kemp, 1956). We believe that model (3), the metamorphosed evaporate model, can best explain the origin of the Houxianyu borate deposit. Moreover, our new data suggest that the concentration of boron was likely related to the metamorphism of a marine evaporate, rather than non-marine evaporate as previously proposed by some workers (Jiang et al., 1997; Peng and Palmer., 2002; Xu et al., 2004).The ore-forming mechanism could be summarized as below: (1)

During the period of ~2.2Ga to ~2.1Ga, massive continental arc magmas

and related volcanic-sedimentary rocks were formed responding to the subduction of oceanic crust beneath the Archean Nangrim block. These arc rocks are characterized by high boron contents (Zhang, 1988; Zhang, 1994; Wang et al., 2008), probably being related to the boron-rich mantle source that was previously metasomatized by subduction zone boron-rich fluids released from seawater-altered oceanic crust and/or pelagic sediments (Palmer, 1991; Ryan and Langmuir, 1993; You et al, 1993). The boron-enriched Paleoproterozoic volcanic and sedimentary rocks subsequently experienced intensive regional geothermal activities (hot-spring), which leach the boron-rich volcanic-sedimentary sequence, and supply boron to local playa lakes. 22

Original water-bearing borate minerals such as inderite (Mg2B6O22·15H2O) and pinnoite (MgB2O4·3H2O), and tourmalines (represented by the low-REE tourmaline core in Fig. 8b, 9f), could be formed after long-term supplies of boron and evaporation of the boronrich fluids; Mg-carbonates could have been formed during this period. (2)

The Paleoproterozoic boron-rich rocks experienced strong regional

deformation and metamorphism (to amphibolite facies; Zhang, 1988), caused by the collision between the Longgan and Nangrim blocks at ~1.9Ga (Peng and Palmer, 1995; Li et al., 2001, 2004; Luo et al., 2004; Li and Zhao, 2007; Tam et al., 2011, 2012). The metamorphism could cause dehydration of early formed water-bearing borate minerals. In addition, extensive metamorphic fluid activity could result in intense fluid-rock interaction, leaching boron from the boron-rich sequence (the meta-volcanic and sedimentary rocks of the Liaohe Group). The highly boron-enriched high temperature metamorphic fluids subsequently interacted with the marine Mg-rich carbonates and evaporates, leading to the formation of the borate orebodies, tourmalinites, and tourmaline-rich quartz veins.

7.

Conclusions (1) Tourmalines from the fine-grained gneisses and those from the tourmalinite and

tourmaline-rich quartz veins of the borate deposit have different origins. Tourmalines from the fine-grained gneisses show no compositional zoning and low δ11B values 23

(+1.22‰ to +2.63‰), with chemical and B isotopic compositions controlled mainly by the host rocks. Tourmalines within the borate ore bodies have significantly higher δ11B values (+4.51‰ to +12.43‰), and show compositional zoning, with notably higher contents of Mg, Ti, V, Sn and REE, and δ11B values we well, in the rim than in the core, suggesting a genetic link with high temperature metamorphic fluids. (2) The ore-forming fluids should be the high temperature metamorphic fluids related to the amphibolite-facies metamorphism of the Paleoproterozoic foldbelt, which leach boron from the boron-rich meta-volcanic and sedimentary rocks of the Liaohe Group, and subsequently interacted with the marine Mg-rich carbonates and evaporates, forming the tourmaline-rich rocks and the borate deposit. (3) The sources of boron of the borate ore-body are mainly the Paleoproterozoic meta-volcanic and sedimentary rocks (with low δ11B values) and the marine Mg-rich carbonates/evaporates (with high δ11B values).

Acknowledgements We thank the editor and two anonymous reviewers for their suggestion and comments, which lead to significant improvement of the manuscript. This work was supported by a National Key Basic Research Program of China (Grant 2012CB416603).

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Zhao, G.C., Zhai, M.G, 2013. Lithotectonic elements of Precambrian basement in the North China Craton: Review and tectonic implications. Gondwana Research 23, 1207-1240.

37

Table Captions Table 1. Compositions of tourmaline from the Houxianyu area. Table 2. Minor and trace elements (ppm) of tourmalines from different rock types. Table 3. LA-MC-ICP-MS boron isotope analyses of reference tourmaline samples. Table 4. LA-MC-ICP-MS boron isotope analyses of tourmaline from Houxianyu.

Figure Captions Fig. 1Geologic map of (a) the North China Craton (after Zhao et al., 2005) and (b) the Liao-Ji Paleoproterozoic orogenic belt, showing the location of borate deposits (modified after Peng and Palmer, 1995).

Fig. 2 Simplified geological map of the Houxianyu borate deposit; also shown are the sampling locations. Modified after Wang et al. (2006).

Fig. 3 (a) Field photograph of the fine-grained gneiss near the Houxianyu boron deposit. (b) Microphotograph of the fine-grained gneiss, showing the mosaic contact between tourmaline (Tou), quartz (Q) and microcline (Mic). (c) Field photograph of tourmalinite and tourmalite 38

(tourmaline-rich quartz vein) from the hanging wall of the borate ore-body. (d) Microphotograph of tourmalinite, showing the zoned tourmaline. The core is characterized by the presence of many tiny detrital phases such as quartz, tourmaline and zircon. (e) Field photograph of tourmaline-quartz vein near the borate ore body. (f) Microphotograph of tourmalite, composed mainly of tourmaline, quartz, and tremolite (Tr).

Fig. 4 (a) Field photograph of Mg-silicates (peridotite) that host the borate ore body, and the contact relationship between peridotite and fine-grained gneiss. (b) Field photograph shows the relationship between borate ore and peridotite. (c) Microphotograph of peridotite, composed mainly of olivine (Ol), partially altered to serpentine (Srp) and magnetite (Mt), and accessory apatite (Ap). (d) Microphotograph of Mg-silicates, composed mainly of olivine, tremolite (Tr) and carbonate mineral (Cb). (e) Microphotograph of borate ore-body, containing szaibelyite (Sz) and ludwigite (Lg) and forsterite (Ol); szaibelyite occurs either as massive platy grain or as tiny fibrous crystal. (f) Microphotograph of borate ore-body; note that ludwigite coexists with peridotite vein.

Fig. 5 Classification of tourmalines from the Houxianyu borate deposit (Henry et al., 2011). Tourmalines from fine-grained gneisses (open square) tourmaline-rich rocks near the borate orebodies (open circle) plot in the field of alkali group. Tourmaline core and tourmaline rim of 39

zoned tourmalines near the borates are shown as solid diamond and solid circle, respectively. Note that most data are X-site saturated, and data of fine-grained gneiss have largest concentrations of Na (+K). The unzoned tourmalines show large variations in Ca and Na (+K). No striking variations in Ca and Na (+K) show for zoned tourmaline from core to rim.

Fig. 6Al-Fe-Mg and Ca-Fe-Mg ternary diagrams after Henry and Guidotti (1985) for tourmalines from the Houxianyu boron deposit. Note that these tourmalines plot near the schorl-dravite line, and most of them are Al-depleted. The majority of tourmalines plot in the field #6 and/or field #10. Tourmaline from fine-grained genies is Fe-rich, with some data plot in the field #2.The unzoned tourmalines from tourmaline-rich rocks are highly varied in Fe, Mg and Al. Tourmaline core of zoned tourmaline has higher Fe(tot)/Mg ratios than tourmaline rim, but they show similar Ca and Al concentrations. Labeled fields are: (1) Li-rich granitoids and associated pegmatites and aplites, (2) Li-poor granitoids and associated pegmatites and aplites, (3) Fe3+-rich quartz-tourmaline rocks (hydrothermally altered granites), (4) Metapelites and metapsammites coexisting with an Al-saturating phase, (5) Metapelites and metapsammites not coexisting with an Al-saturating phase, (6) Fe3+-rich quartz-tourmaline rocks, calc silicate rocks, and metapelietes, (7) Low Ca metaultramafics and Cr-V-rich metasediments, (8) Metacarbonates and metapyroxenites, (9) Ca-rich metapelites, metapsammites, and calc-silicate rocks, (10) Ca-poor metapelites, metapsammites, and quartz-tourmaline rocks, (11) Meta-

40

carbonates, and (12) Meta-ultramafics. Symbols same as in Fig. 5.

Fig. 7 Chemical compositions of tourmalines shown as atomic ratio and atoms per formula unit (a.p.f.u). (a) Fe/(Fe+Mg) versus Na/(Na+Ca). Note that tourmalines from fine-grained gneiss plot in the field of schorl, while the unzoned tourmalines from the tourmaline-rich rock (tourmalinite and tourmaline-bearing vein) plot in the field of dravite, and show large variations in both Fe/(Fe+Mg) and Na/(Na+Ca) ratios. Tourmaline core is typically Mg-rich schorl while the tourmaline rim is Fe-rich dravite. Tourmaline core and rim show similar Na/(Na+Ca) ratios. (b) Total Al (a.p.f.u) versus Fe (a.p.f.u.) diagram. Note that tourmalines from the fine-grained gneiss and those from the tourmaline-rich rock plot in different fields, and show negative correlation between Fe (a.p.f.u) and Al (a.p.f.u), suggesting a substitution of Al by Fe3+ (Slack, 2002). Data in both (a) and (b) show that the tourmaline rim has similar chemical compositions with that of the unzoned tourmalines. Symbols same as in Fig. 5.

Fig. 8 Chondrite-normalized REE patterns of (a) tourmalines from the fine-grained gneiss and (b) zoned tourmalines from the tourmalinites. Note that all tourmalines show positive europium (Eu) anomalies. The tourmaline rim shows higher concentrations of REE than the core.

Fig. 9 Chemical compositions of a zoned tourmaline grain from tourmalinite of the Houxianyu 41

borates. (a) A black-scattered electron (BSE) image of the zoned tourmaline (#HX-4); in-situ analyses for the tourmaline are carried out along the dotted line. (b) - (g) The compositional profiles of the zoned tourmaline grain. Note that, compared with the core, the rim has higher Mg, Ti, REEs, V, Sn, and Pb values, and lower concentrations of Fe, Al, Cr, Co, Ni, Zn, Mn, Sc and Li.

Fig. 10 Histograms of boron isotopic compositions (δ11B) of tourmalines from (a) the finegrained gneiss, and (b) the tourmaline-rich rocks.(c) Also shown for comparison are the boron isotope data of borate minerals from the Houxianyu deposit(Jiang et al., 1997;Peng and Palmer, 2002). (d) Plot of boron isotopic compositions for different rock types on Earth (Chaussidon and Albarède, 1992; Palmer and Slack, 1989; Palmer, 1991; Palmer and Swihart, 1996; Jiang et al., 1999; Peacock and Hervig, 1999; Nakano and Nakamura, 2001), in which the boron isotope data of the fine-grained gneiss and fluids from which tourmalinites crystallized are shown.

Fig. 11 Boron isotopic variations of zoned tourmaline grains from tourmalinite. The numbers of the analyzed spots and the δ11B values are shown.

42

Table 1. Compositions of tourmaline from the Houxianyu area. Rock

Fine-grained

Tour-Amp-

Tour-

types

gneiss

Q vein

Q vein

Analy

H

sis

X- X- X- X- X- X- X- X- X- X- X- X- X- X- X- X- X- X-

No.

1-

1-

2-

2-

1

1

1

2

2

0

0

0

0

0

0

0

1

1

0

0

1-

5-

2-

2-

3-

4-

4-

4-

4-

4-

4-

4-

4-

4-

0-

0-

3

8

2

1

0

0

2

1

8

0

0

0

1

2

2

2

0

0

1-

1-

8

1

2

8

3

2

3

9

2-

2-

1

2

1

2

H

H

H

H

H

H

H

H

Tourmalinite

H

H

H

H

H

H

H

H

H

c

ri

ri

c

ri

ri

c

ri

c

or

m

m

or

m

m

or

m

or

e

e

e

e

wt.%a SiO2

TiO2

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

5.

6.

4.

4.

4.

5.

6.

6.

6.

6.

5.

6.

5.

6.

6.

6.

4.

4.

4

4

8

2

0

2

2

9

9

4

8

2

7

3

8

0

9

6

5

0

0

2

0

4

0

7

5

2

6

3

3

9

4

7

9

5

0.

0.

1.

0.

0.

1.

0.

0.

1.

1.

0.

1.

0.

1.

1.

1.

1.

0.

7

3

1

5

8

2

5

9

1

1

9

3

7

1

8

0

4

6

3

8

0

3

7

2

2

9

4

2

1

0

3

3

0

9

6

6

43

Al2O3

MgO

MnO

FeOb

CaO

Na2O

K2O

3

3

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

0.

1.

6.

9.

6.

6.

8.

5.

5.

5.

5.

6.

6.

6.

6.

6.

5.

6.

4

1

2

1

9

6

2

2

4

8

4

1

4

3

2

2

7

5

9

1

1

4

6

7

8

7

5

6

6

8

1

0

1

7

2

3

5.

5.

6.

5.

9.

9.

9.

9.

9.

9.

8.

9.

7.

9.

9.

7.

9.

8.

7

5

8

7

3

4

1

0

8

3

6

3

0

1

9

4

4

8

9

4

2

2

2

0

6

1

7

2

9

3

8

3

7

8

1

7

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0

0

1

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

7

5

2

5

2

3

2

7

5

4

0

4

4

0

1

6

4

2

1

1

1

1

1

8.

8.

1

9.

1

1

1

1

1

9.

1

9.

9.

2.

1.

3.

2.

0.

3

4

0.

2

1.

1.

0.

4.

0.

4

3.

2

3

0

0

6

8

1

1

5

4

1

1

7

2

6

7

0

2

2

1

6

3

9

3

0

3

6

7

5

3

0.

0.

1.

0.

2.

1.

1.

1.

1.

1.

2.

1.

1.

2.

1.

2.

2.

1.

7

3

3

4

3

8

6

9

8

9

1

8

9

0

6

0

2

6

3

7

0

0

0

9

7

0

3

3

1

1

5

5

0

1

2

6

2.

2.

2.

2.

1.

1.

2.

2.

2.

1.

1.

2.

1.

2.

2.

2.

1.

1.

6

7

3

7

6

9

4

4

1

9

8

0

9

0

2

0

8

8

3

4

0

0

6

1

3

0

5

8

1

3

8

0

6

1

1

5

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0

1

1

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

7

0

0

8

7

6

4

6

6

5

6

3

4

7

6

6

7

8

3

a.p.f. u.c 44

2

Si

5.

6.

5.

5.

5.

5.

5.

6.

6.

6.

6.

6.

5.

6.

6.

6.

5.

5.

8

0

9

8

7

9

9

1

1

0

0

0

9

0

0

0

9

9

6

1

1

2

4

7

9

8

4

0

2

0

4

1

2

0

5

4

Al(T

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

)

1

0

0

1

2

0

0

0

0

0

0

0

0

0

0

0

0

0

4

0

7

7

4

3

1

0

0

0

0

0

4

0

0

0

4

4

Al(Z

5.

6.

5.

5.

5.

5.

5.

4.

4.

5.

5.

5.

5.

5.

5.

5.

5.

5.

)

8

0

1

6

1

3

5

9

9

0

0

1

1

1

0

1

1

3

1

0

8

7

3

0

0

8

8

2

4

1

3

2

4

5

2

3

Al(Y

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

)

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

5

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0

0

1

0

1

1

0

1

1

1

1

1

0

1

2

1

1

0

9

5

4

7

1

6

7

2

4

4

2

6

9

4

2

4

9

9

1.

1.

1.

1.

2.

2.

2.

2.

2.

2.

2.

2.

1.

2.

2.

1.

2.

2.

4

3

7

4

3

3

2

2

4

2

1

3

7

2

4

8

3

2

3

6

3

5

4

7

6

5

5

9

7

0

5

5

3

6

9

7

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

1

1

2

1

0

0

0

1

1

1

0

1

1

0

0

1

1

0

1.

1.

1.

1.

1.

1.

1.

1.

1.

1.

1.

1.

2.

1.

1.

1.

1.

1.

6

5

9

8

4

1

1

4

2

5

6

4

0

4

2

8

3

3

7

2

5

2

3

8

7

6

8

3

5

2

4

8

8

4

1

4

Ti

Mg

Mn

Fe

45

Ca

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

1

0

2

0

4

3

3

3

3

3

3

3

3

3

2

3

4

3

3

7

4

7

2

4

0

4

3

4

8

2

5

6

8

6

1

1

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

8

8

7

8

5

6

7

7

6

6

5

6

6

6

7

6

6

6

4

8

6

9

4

3

8

8

9

3

9

5

4

4

2

5

0

2

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

2

2

2

2

2

1

1

1

1

1

1

1

1

2

1

1

2

2

X-

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

vacac

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

y

1

4

0

2

3

2

0

0

0

2

2

2

1

0

0

0

0

6

Fe/(

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

Fe+M

5

5

5

5

3

3

3

3

3

4

4

3

5

4

3

5

3

3

g)

4

3

3

6

8

3

4

9

4

0

3

8

4

0

5

0

6

7

Na/(

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

0.

Na+C

8

9

7

9

5

6

7

7

6

6

6

6

6

6

7

6

6

6

a)

7

3

6

2

7

5

3

0

8

5

1

7

5

4

2

4

0

7

Na

K

aChemical

btotal

analyses by electron microprobe.

Fe reported as FeO.

cstructural

based on 15 cations in T, Z and Y sites (Henry and Dutrow 1996).

46

Table 2. Minor and trace elements (ppm) of tourmalines from different rock types. Fin

Tourmali

Tour

Ref

e-

nite(N=

-

eren

grai

50)

amp

ce:

ned

hibol

NIS

gne

ite-q

T

iss

vein

SR

(n

(n=

M

=14

7)

612

) core

rim

(N=22

(N

)

=28 )

Ele Ra me

% Range

nge edi an

nt

Li

M

5.6

16

M

% Ra nge

edi b

an

M

ge

edi an

b

% Ran

b

M

% TR

Me

er

sur

ro

b

ed

r (

UEb

edi an

d

d

d

d

(N

la

l

l

l

=16 %

0

11.0-30

19

0 4.8- 9. 47

0 11-

12

0

41

)

)

40

-

-17

18

9

14

1. 77

Be

0.4

0.

6-

83

0

0.56-1.9

1

5 0.3 2-

0.

0 0.17

0.

58

-

33

36

39

7. 92

0.54

0.9

2.5

0

9 Sc

16-

44

0

4.7-82

39

70

0 5.0- 11 68

0 6.111

7.

0

40

37

7

7. 41

V

42-

11

0

180 4

229-

29

0 337

60

1553

5

-

2

0 908-

10

0

38.3 37

1378 55

2.

185

53

2 Cr

4.5

8.

-33

4

0

8.0-197

16

0 2.0- 11 97

0 11-

66

0

39

37

136

5. 40

Mn

146 45 -

0

116-314

3

28 2

0 50193

16 4

0 175236

18

0

37

37

3

0. 07

613 Co

3.9

12

0

17-37

35

-14

0 8.0- 26 21

0 18-

22

0

35

34

27

2. 37

Ni

6.0 -21

16

0

20-52

48

0 1278

38

0 2133

27

0

38

37

1. 93

48

Cu

0.3

0.

4

0.23-

0.

7 0.2

0.

6 0.3-

0.

8

-

52

7

0.61

31

3 5-

33

4 0.3

3

6

1.2

37

36

1. 67

0.5 5

Zn

49-

15

0

97-233

211 6 Ga

38-

88

20 2

0

35-61

37

105 Rb

0 36147 0 31-

13 1 35

74

0 67-

74

0

39

98 0 41-

49

0

37

37

67

0. 18

0.

3

0.02-

0.

4 0.0

0.

3 0.02

0.

4

2-

12

3

0.31

09

5 2-

06

6 -

03

3

0.3

2. 76

0.0

100

38

32

32

0.

0.04

22

4 Sr

45-

59

0

208-718

110

24

0 213

29

0 277-

52

0

2

-

2

670

2

0.

7 0.02

0.

1

05

-

06

4

75

76

0. 80

119 1 Y

0.0

0.

2-

07

0

0.01-

0.

0.28

04

0.6

5 0.0 13.9

38

38

0.

0.11

10

8 Zr

0.0

0.

7-

25

0

0.11-

0.

0.45

28

0 0.1 8-

0. 32

0 0.24.1

0.

0

37

37

59

1. 02

0.6

86

8 Nb

0.1

0.

0

0.02-

0.

0 0.0 49

0.

0 0.04

0.

0

36

34

-

3-

Mo

Sn

21

0.11

05

5-

0.5

0.3

3

1

13

-2.7

14

4. 30

0.0

0.

7

0.02--

0.

6 0.0

0.

8 0.08

0.

8

4-

04

3

0.10

04

8 4-

05

6 -

08

6

0.0

0.0

8

7

3.3

6.

-14

5

0

2.9-10

7.

0 13-

7

35

33

6.

0.08

18

0 14-

41

-

03

21

0

40

36

122

11 .1 3

Cs

Ba

0.0

0.

4

0.01-

0.

6 0.0

0.

5 bdl

1-

03

7

0.35

05

4 1-

03

7

0.9

0.1

5

3

0.2

0.

1-

56

0

2.1-4.7

2.

6.7

41

41

0

0. 80

0

0 1.7- 2.

8

bdl 1

0 1.6-

2.

3

2

7

0

37

37

0.

383

81

ΣR

3.9

9.

-

EE

-15

2

Hf

0.0

0.

2

2-

02

0

0.71-7.3

1.

-

3.5- 11

-

3.9-

5.

18

3

-

-

-

-

35

35

1.

5

48

0.02-

0.

2 0.0

0.

2 0.06

0.

4

0.08

03

3 1-

03

9 -0.6

14

3

0.

3 0.03

0.

4

23

0.0

2.9

8 Ta

0.0

0.

0

0.0-0.01

0.

6 050

36

32

-

1-

04

01

0.1

4 0.1

01

6 -1

15

3

12

2

.2

1 W

Pb

2

0.0

0.

6

0.02-

0.

3 0.0

0.

6 0.06

0.

8

1-

03

0

0.17

06

6 3-

04

8 -

06

6

5.

0

0.0

0.0

4

6

2.5

3.

1-

1

0

3.07-

4

6.37

36

38

17

0.06

0 4.5- 9

0 3.8-

28

18

38

37

1

1. 98

4.2 Th

4.

0.0

0.

3

0.01-

0.

8 0-

0.

6 0.01

0.

5

1-

17

3

0.02

01

2 0.0

01

4 -

02

7

0.5

2

37

38

1. 80

0.06

5 U

0.0

0.

2

0.01-

0.

4 0.0

0.

4 bdl

0-

07

7

0.33

02

1 1-

01

3

0.7

0.1

7

3

a

below detection limit.

b

from Pearce et al. (1997).

51

bdl 1 0 0

37

37

0. 84

Table 3. LA-MC-ICP-MS boron isotope analyses of reference tourmaline samples. Analysis

11B/10B(measured) IMFa

δ11B (‰）

2σ(‰)b

Schorl (HS #112566, 11B/10B = 3.9994c, δ11B= -12.5‰) 1

4.5547

1.1389

-14.06

0.45

2

4.5552

1.1390

-13.96

0.46

3

4.5703

1.1427

-14.17

0.40

4

4.5702

1.1427

-14.11

0.37

5

4.5747

1.1438

-14.76

0.44

6

4.5770

1.1444

-14.95

0.35

7

4.5737

1.1436

-14.13

0.43

8

4.5731

1.1435

-14.21

0.41

Mean

4.5686

-14.29

Dravite (HS #108796, 11B/10B = 4.0233c, δ11B = -6.6‰) 1

4.5904

1.1410

-6.12

0.46

2

4.5939

1.1418

-5.49

0.63

3

4.6077

1.1453

-6.34

0.53

4

4.6058

1.1448

-6.81

0.62

5

4.6012

1.1437

-6.81

0.53

52

6

4.6115

1.1462

-6.64

0.32

7

4.6094

1.1457

-6.70

0.43

Mean

4.6029

-6.42

a

Instrumental mass fractionation [(11B/10Bmeasured)/(11B/10Bstandard)].

b

Internal precision in permil for single analysis from 100 cycles ( standard deviation/ mean)

×1000. c11 10 B/ B 11

ratios of the reference tourmaline samples were calculated from the δ11B values and

B/11B = 4.05003 for standard NIST SRM 951 (Ishikawa and Tera, 1997; Ishikawa et al.,

2001).

Table 4. LA-MC-ICP-MS boron isotope analyses of tourmaline from Houxianyu. 11B/10B

δ11B

2σ(‰)

HX-1-03

4.6395536

1.64

0.70

HX-1-04

4.6394792

1.81

0.40

HX-1-05

4.638825

1.81

0.34

HX-1-06

4.6416393

2.49

0.61

Analysis no. HX-1, fine-grained gneiss

53

HX-1-07

4.6422483

2.63

0.52

HX-1-08

4.6398905

2.16

0.52

HX-1-09

4.636131

1.22

0.37

HX-1-10

4.6384671

1.57

0.42

HX-1-11

4.6372769

1.37

0.55

HX-3-01

4.6871902

10.42

0.87

HX-4-01

4.6855883

12.43

0.33

HX-4-02

4.6836632

10.26

0.54

HX-4-03

4.6756701

8.76

0.38

HX-4-04

4.6725861

8.34

0.53

HX-4-05

4.6685851

7.82

0.62

HX-4-06

4.6727454

8.85

0.65

HX-4-07

4.673016

8.87

0.45

HX-4-08

4.671919

8.77

0.60

HX-4-09

4.6783257

10.21

0.56

HX-4-10

4.6811486

10.85

0.54

HX-4-11

4.6822526

11.23

0.55

HX-4-12

4.6724215

9.02

1.08

HX-4-13

4.6820552

10.93

0.62

HX-4-14

4.6831305

11.16

0.52

HX-4, tourmalinite

54

HX-4-15

4.6749676

9.36

0.59

HX-4-16

4.6730279

8.80

0.50

HX-4-17

4.6793816

10.05

0.58

HX-4-18

4.6736982

8.87

0.67

HX-4-19

4.6749643

9.15

0.63

HX-4-20

4.6736644

8.75

0.63

HX-4-21

4.6743575

8.91

0.60

HX-4-22

4.6749418

9.07

0.89

HX-4-23

4.682037

10.60

0.66

HX-4-24

4.6792175

9.92

0.53

HX-4-25

4.6552928

4.69

2.49

HX-4-26

4.6682895

7.52

1.61

HX-4-27

4.6680182

7.45

2.93

HX-4-28

4.6757991

9.15

2.36

HX-4-30

4.673158

8.42

0.38

HX-4-29

4.6760457

9.09

0.50

HX-4-31

4.678346

9.56

0.37

HX-4-32

4.6778163

9.40

0.48

HX-4-33

4.6736111

8.39

0.73

4.6688269

7.43

1.50

HX-5, tourmalinite HX-5-01

55

HX-5-02

4.6760398

8.18

3.49

HX-5-03

4.6776747

8.49

1.00

HX-5-04

4.6768535

8.81

0.99

HX-5-05

4.6767194

9.27

0.85

HX-5-06

4.6735272

8.64

0.95

HX-5-07

4.6726885

8.42

0.83

HX-5-08

4.6777081

9.36

0.73

HX-5-09

4.6566982

4.76

1.71

HX-5-10

4.6557834

4.61

4.47

HX-5-11

4.6709505

7.75

1.12

HX-5-12

4.6563707

4.51

1.10

HX-5-13

4.6646095

6.18

10.70

HX-5-14

4.6767647

8.65

1.43

HX-5-15

4.6772022

8.75

1.08

HX-13-01

4.6804304

7.86

0.36

HX-13-02

4.6825407

8.12

0.37

HX-13-03

4.6807524

8.07

0.39

HX-13-04

4.6810448

8.58

0.37

HX-13-05

4.6756814

7.75

0.40

HX-13, tour-amp-q vein

56

HX-24, tour-q vein HX-24-01

4.6817926

8.66

0.50

HX-24-02

4.6829543

8.72

0.47

HX-24-03

4.6862834

9.00

0.51

HX-24-04

4.6851545

8.36

0.57

Highlights 

Two types of tourmaline occur in the Houxianyu area, NE China.



B-isotopic zoning is common in tourmaline based on in-situ analyses.



High temperature metamorphic fluids are the ore-forming fluids.



Boron isotope data suggest two boron sources for the origin of the Houxianyu borate ore.

57

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11