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The Canadian Mineralogist Vol. 46, pp. 41-58 (2008) DOI : 10.3749/canmin.46.1.41

CHEMICAL AND TEXTURAL FEATURES OF TOURMALINE FROM THE SPODUMENE-SUBTYPE KOKTOKAY No. 3 PEGMATITE, ALTAI, NORTHWESTERN CHINA: A RECORD OF MAGMATIC TO HYDROTHERMAL EVOLUTION Ai Cheng ZHANG State Key Laboratory for Mineral Deposits Research and Department of Earth Sciences, Nanjing University, Nanjing 210093, P.R. China, and Laboratory for Astrochemistry and Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, 210008, P.R. China

Ru Cheng WANG§, Shao Yong JIANG and Huan HU State Key Laboratory for Mineral Deposits Research and Department of Earth Sciences, Nanjing University, Nanjing 210093, P.R. China

Hui ZHANG Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550002, P.R. China

Abstract The Koktokay No. 3 pegmatite, Altai, northwestern China, is a spodumene-subtype granitic pegmatite. In this study, we report textural and chemical features of tourmaline from the altered country-rock, the contact zone, and the pegmatite. The tourmaline in the altered country-rock, Ca- and Fe-rich dravite, shows an obvious chemical heterogeneity within individual grains. Tourmaline in the contact zone consists of two generations: zoned Ca- and Fe-rich dravite and in interstitial foitite–schorl solid solution. Tourmaline from the altered country-rock and the contact zone reflects interaction between the country rock (metagabbro) and the pegmatite-forming melt or fluids derived from it. The chemical variation of these tourmalines depends on various contributions of components from the country rock and the pegmatite. The tourmaline in the outer zones (zones I to IV) of the pegmatite is elbaite–schorl solid solution with an intermediate composition between the end members; it is generally homogeneous within individual grains. In the inner zones (zones V, VI, and VIII), the tourmaline is dominantly elbaite with rare rossmanite in zone V. Elbaite is either abruptly zoned within individual grains, or has a replacement texture. Chemically, elbaite in the inner zones has a higher proportion of X-site vacancy than elbaite–schorl in the outer zones. Chemical trends of tourmaline compositions in the spodumene-subtype Koktokay No. 3 pegmatite are generally similar to those in other pegmatite subtypes (lepidolite, petalite, and elbaite subtype). Systematic variations in the internal textures of tourmaline from the outer zones to the inner zones suggest that exsolution of fluids occurred between zone IV and zone V. The outer zones crystallized from a volatile-unsaturated pegmatiteforming magma, whereas the inner zones crystallized from a hydrothermal system. This evolution process is consistent with the London model of internal evolution of granitic pegmatites. Keywords: tourmaline, magmatic to hydrothermal evolution, spodumene subtype, granitic pegmatite, Koktokay No. 3, Altai, China.

Sommaire La pegmatite granitique Koktokay No. 3, dans la chaîne Altaï, du nord–ouest de la Chine, fait partie du sous-type à spodumène. Nous décrivons les attributs texturaux et chimiques de la tourmaline provenant des roches-hôtes, de la zone de contact, et de la pegmatite elle-même. La tourmaline des roches altérées de l’exocontact est une dravite riche en Ca et en Fe; les cristaux montrent une hétérogénéité évidente. Deux générations de tourmaline sont présentes dans la zone de contact, dravite riche en Ca et en Fe, et une solution solide interstitielle entre foïtite et schorl. La tourmaline de ces deux zones résulte d’une interaction entre les roches hôtes métagabbroïques et le magma duquel la pegmatite a cristallisé, ou bien la phase fluide issue de ce magma. La variation chimique de ces tourmalines dépend des diverses contributions de composantes du socle ou de la pegmatite même. La tourmaline des zones externes (zones I à IV) de la pegmatite est une composition intermédiaire de la solution solide elbaïte– §

E-mail address: [email protected]

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schorl; les cristaux sont généralement homogènes. Dans les zones internes (zones V, VI, et VIII), la tourmaline est surtout l’elbaïte, avec la rossmanite comme composante rare de la zone V. Les cristaux d’elbaïte sont soit zonés de façon abrupte, où montrent une texture de remplacement. L’elbaïte des zones internes possèdent une proportion plus importante du lacunes au site X que l’elbaïte–schorl des zones externes. Les tracés marquant l’évolution de la tourmaline dans la pegmatite du sous-type à spodumène Koktokay No. 3 sont semblables aux tracés de la tourmaline dans les autres sous-types de pegmatite (lépidolite, pétalite, et elbaïte). Des variations systématiques des textures intracristallines de la tourmaline dans les zones externes en direction des zones internes font penser que l’exsolution d’une phase fluide a eu lieu entre la zone IV et la zone V. La tourmaline des zones externes a cristallisé à partir d’un magma sous-saturé en phase volatile, tandis que celle des zones internes a cristallisé à partir d’un système hydrothermal. Ce schéma d’évolution concorde avec le modèle pétrogénétique de London pour décrire l’évolution interne des pegmatites granitiques. (Traduit par la Rédaction) Mots-clés: tourmaline, évolution magmatique et hydrothermale, sous-type à spodumène, pegmatite granitique, Koktokay No. 3, Altaï, Chine.

Introduction On the basis of the dominant Li-bearing minerals, rare-element granitic pegmatites have been divided into five subtypes, namely spodumene, lepidolite, petalite, elbaite, and amblygonite (Černý 1991, Novák & Povondra 1995). Tourmaline is one of the most important accessory minerals in these pegmatites and has been considered as a monitor of the evolution history of pegmatites (Jolliff et al. 1986, London & Manning 1995, Roda et al. 1995, Federico et al. 1998, Keller et al. 1999, Selway et al. 2000a, b, Roda-Robles et al. 2004). Chemical evolutions of tourmaline from lepidolite-, petalite-, and elbaite-subtype pegmatites have been extensively studied (Selway 1999, Tindle et al. 2002, Novák et al. 1999), but compositional trends of tourmaline from spodumene- and amblygonite-subtype pegmatites remain incompletely known. The Koktokay No. 3 pegmatite is one of the most interesting pegmatites in the Altai pegmatite field, northwestern China. Černý (1991) classified this pegmatite as a spodumene-subtype pegmatite. Tourmaline is an important accessory mineral in the Koktokay No. 3 pegmatite. Wang et al. (1981) reported occurrences and quasi-quantitative compositions of schorl and elbaite in this pegmatite. Recently, Zhang et al. (2004a) described occurrences and mineral compositions of two alkalideficient tourmaline species, foitite and rossmanite. In this paper, we intend to report systematic features of internal texture and the composition of tourmaline from the internal zones of the Koktokay No. 3 pegmatite, the altered country-rock, and the contact zone, and further discuss possible implications with respect to the magmatic to hydrothermal evolution of the pegmatite and interaction between the pegmatite-forming melt and surrounding country-rock.

Geological Setting and Mineralogy of the Pegmatite The Altai is one of the major orogens in central Asia, and the Altai Mountains lie at the junction of

China, Russia, Kazakhstan, and Mongolia. Li et al. (2003) divided the Altai Mountains into three blocks: the North Altai, the Central Altai, and the South Altai. Koktokay is located in the Central Altai and mainly composed of high-grade metamorphic rocks, abundant granites, and some metasedimentary rocks (see Zhu et al. 2006). Pegmatite dikes in the Koktokay region occur both in granites and metamorphic rocks. Thousands of pegmatite dikes have been found (Zhu et al. 2006). The Koktokay No. 3 pegmatite is one of the most highly evolved and strongly zoned (Wang et al. 1981). It was emplaced in metagabbro (Fig. 1), which is cross-cut by gneissic biotite granite (Wang et al. 1998, Liu & Zhang 2005). Chen et al. (2000) reported 40Ar/39Ar isochron ages (177.9–140.8 Ma) of some textural zones of the Koktokay No. 3 pegmatite; however, they did not date the nearby granites. Recently, Zhu et al. (2006) analyzed chemical compositions and established Rb–Sr isochron ages of the Koktokay No. 3 pegmatite (218.4 ± 5.8 Ma), a biotite granite (248.8 ± 7.5 Ma), and a nearby two-mica granite (247.8 ± 6.3 Ma), and concluded that the Koktokay No. 3 pegmatite, along with the biotite and two-mica granites, were derived from a common magma. The Koktokay No. 3 pegmatite has an unusual shape: it looks like a hat. It consists of two major parts: a gently dipping “plate” and a steeply dipping cupola protruding from the plate upward. The pegmatite shows a well-developed internal zonation both in the cupola and in the “plate”. From the outermost textural zone inward, the following nine textural zones have been distinguished in the cupola (Fig. 2): I graphic pegmatite zone, II saccharoidal albite zone, III blocky microcline zone, IV muscovite – quartz zone, V platy albite – spodumene zone, VI quartz – spodumene zone, VII platy albite – muscovite zone, VIII lepidolite – platy albite zone, IX blocky quartz and microcline core (Zhu et al. 2000, Zhang et al. 2004a, b). At the margin of the pegmatite, some dispersed masses of fine-grained albite granite and aplite occur, with widths up to 3 m. There is a contact zone 1–2 cm wide between the pegmatite and surrounding metagabbro, at some localities. Adjacent to



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Fig. 1.  Geological map of the Koktokay No. 3 pegmatite, Altai, northwestern China (modified after Zhu et al. 2000).

the contact zone, the metagabbro was strongly altered, with the alteration halo enriched in Li, Rb, Cs, F and B (Liu & Zhang 2005). Both the altered country-rock and the contact zone are dominated by black tourmaline. In the altered country-rock, tourmaline grains are euhedral to subhedral and are fine to medium in size, some forming blocky aggregates. They contain subhedral to euhedral crystals of fluorapatite with minor inclusions of phlogopite. Minor quartz grains occur in the interstices of dravite grains. In the contact zone, most of the finegrained, elongate tabular grains of dravite have grown perpendicular to the contact, and commonly exhibit a strong pleochroism from light yellow to bluish green under the microscope. In the portion of the contact zone close to the pegmatite, anhedral foitite–schorl fills the interstices and fractures of early euhedral grains of dravite. Associated minerals are quartz, oligoclase, muscovite, and fluorapatite (Zhang et al. 2004a). In the Koktokay No. 3 pegmatite, zones I to IV (the outer pegmatite zones) are dominant in volume (~70%), but the main rare-metal mineralization is localized in the inner pegmatite zones (V to VIII). The various

internal zones are distinguished according to mineral assemblages. Modal abundances of some minerals in the internal zones are listed in Table 1. Zone I is characterized by a typical microcline + quartz graphic texture. In this zone, elbaite–schorl is scarce and occurs as euhedral black to dark blue fine-grained prisms. Minor albite, muscovite, and spessartine–almandine also are encountered. Zone II consists of dominant white and fine-grained albite. Other minerals include muscovite, manganocolumbite–manganotantalite, spessartine– almandite, fluorapatite, and zircon. No tourmaline grains were found in this zone. At some localities, zone II is absent, resulting in a direct contact of zones I and III. In zone III, microcline crystals are large (~30 cm), and rare black elbaite–schorl aggregates are also found enclosed by blocky microcline. Zone IV is mainly composed of muscovite and quartz, locally with euhedral white crystals of beryl. Spessartine–almandine, fluorapatite, elbaite–schorl, manganocolumbite, and zircon occur as accessory minerals. The size of elbaite– schorl in this zone varies from several hundred mm to several cm. The crystals are black to dark blue in hand

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Fig. 2.  Internal textural zonation of the Koktokay No. 3 pegmatite, Altai, northwestern China (modified after Zhu et al. 2000).



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specimen. Some tourmaline grains contain small inclusions of quartz. Zone V is mainly composed of platy albite, pink spodumene, quartz and minor muscovite. Some albite crystals grow radially, nucleating from pink spodumene laths. Other minerals are amblygonite, fluorapatite, pollucite, elbaite (+ rossmanite), manganotantalite, hafnian zircon, and rare microlite. Euhedral grains of elbaite in this zone are pink in hand specimen and coarse-grained. Zone VI is mainly composed of quartz and pink spodumene. Minor phases include platy albite, microcline, elbaite, manganotantalite, rare fluorapatite, pollucite, and “Cs-dominant lepidolite”. Elbaite in this zone exhibits pink or green coloration and is coarsegrained. Some elbaite grains are closely associated with “Cs-dominant lepidolite” (Wang et al. 2007). Zone VII is mainly composed of platy albite and muscovite. Accessory minerals are white or milky beryl, manganotantalite, uranmicrolite, hafnian zircon, amblygonite, pollucite, and very rare ferrotapiolite. No tourmaline grains are found in this zone. Zone VIII occurs as pods with muscovite, fine- to medium-grained lepidolite, “Cs-dominant lepidolite”, platy albite, pollucite, and beryl. Green euhedral crystals of coarse-grained elbaite are found closely coexisting with muscovite. Zone IX consists of dominant quartz and microcline, and the proportion of quartz is slightly higher than that of microcline. In addition, minor muscovite and spodumene were reported in previous investigations (e.g., Wang et al. 1981, Zhu et al. 2000).

Sampling and Analytical Methods Tourmaline samples used in this study were collected from most textural zones of the pegmatite, as well as from the altered country-rock and the contact zone. In previous investigations (Wang et al. 1981), tourmaline has been reported existing in zones II and VII; however, no tourmaline grains were observed in our samples from these two zones. Mineral analyses were performed in the State Key Laboratory for Mineral Deposits Research at the Nanjing University with a JEOL JXA–8800M electron microprobe operating in wavelength-dispersion mode. An accelerating voltage of 15 kV and 10 nA or higher (only for X-ray mapping) beam currents were used for quantitative analyses, back-scattered electron (BSE) imaging, and X-ray mapping. The following materials were used as standards: hornblende (Na, Mg, Si, Ca, Ti, and Fe), fluorapatite (F), cordierite (Al), orthoclase (K), olivine (Mn), and ZnO (Zn), and the detection limits are: 250 ppm Ti, 500 ppm Fe, 300 ppm Mg, 550 ppm Mn, 300 ppm Zn, 150 ppm Ca, 350 ppm Na, 130 ppm K, and 2500 ppm F. Peaks and backgrounds were measured for all elements, with counting times of 10 seconds and 5 seconds, respectively. All data were reduced using the ZAF correction program.

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Structural formulae were calculated on the basis of 31 anions (O, OH, and F), assuming stoichiometric amounts of H2O as (OH), i.e., OH + F = 4 apfu (atoms per formula unit), B2O3 (3 apfu B) and Li2O (as Li+) (MacDonald et al. 1993, Burns et al. 1994). The amount of Li assigned to the Y site corresponds to the ideal sum of the cations occupying the T + Z + Y sites (15 apfu) minus the sum of the cations actually occupying these sites [Li = 15 – (Si + Al + Mg + Fe + Mn + Zn + Ti)]; the calculation was iterated to self-consistency. Calculations of the structural formula were performed by using the Microsoft ExcelTM worksheet for tourmaline from the following web address: http://www.open.ac.uk/ earth-research/tindle/. All Fe and Mn were assumed to be divalent because the existing spreadsheet cannot be used for tourmaline species that contain various valences of iron and manganese; Fe3+ and Mn3+ could occur to various extents in some tourmaline species.

Chemical and Textural Features of Tourmaline In this paper, twelve samples containing a tourmaline-group phase are available for study. Representative electron-microprobe compositions of tourmaline are given in Table 2. Table 3 lists the tourmaline species important for this study and their ideal formula. Generally, tourmaline in the Koktokay No. 3 pegmatite shows a wide compositional range, and significant variations were observed for Ca, Na, X-site vacancy (abbreviated as X hereafter), Mg, Fe, Al, and Li (Figs. 3, 4). Manganese and Ti also show variations from the outer to the inner pegmatite zones (Fig. 5, Table 2). The contents of other elements, such as Zn and F, are very low, and no systematic variations were observed. The concentrations of F in tourmaline from the pegmatite are generally higher than those in the altered country-rock and the contact zone (Table 2). The altered country-rock Tourmaline grains in the altered country-rock are quite heterogeneous (Fig. 6A); a few grains show simple zonation, with a lighter rim and a darker center on BSE images. Compositionally, it is generally alkalirich and dravitic, with a relatively high proportion of calcic and ferrous components (Figs. 3A, 4, 7, Table 2); some compositions are uvitic (Figs. 4, 7). At the X site, Ca and Na show slight variations, and consequently a change of occupancy of the X site is also observed (Figs. 3A, 7). At the Y + Z sites, the contents of Al, Mg, and Fe vary simultaneously in “field 6” of Figure 4, whereas variation in the ratio Mg/(Mg + Fe) is limited (Fig. 7). Distinct variation in the ratio of Na/(Na + X) is observed (Fig. 7). The contents of Ti and Mn are fairly low, even down to the detection limits (Table 2), and no significant variations are observed.

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a

b

c

Fig. 3.  Classification of the principal groups of tourmaline based on X-site occupancy (from Hawthorne & Henry 1999). A) The altered country rock and the contact zone, B) the outer pegmatite (I, III, and IV), and C) the inner pegmatite (V, VI, and VIII). Contact-zone G1 corresponds to generation 1 in the contact zone, and contact-zone G2 corresponds to generation 2 in the contact zone.

The contact zone Tourmaline grains in the contact zone can be divided into two generations. Euhedral to subhedral tourmaline grains and their fragments are defined as generation 1 and abbreviated as G1. The other, anhedral tourmaline (defined as generation 2 and abbreviated as G2), filled interstices and fractures in G1 grains. The G1 tourmaline commonly shows a compositional zonation on BSE images (Figs. 7, 8A), whereas compositional heterogeneity of G2 tourmaline is not very clear on BSE images (Zhang et al. 2004a). The G1 tourmaline is dravite and has high contents of Ca and Fe (Table 2). The G2 tourmaline is an intermediate solid-solution between

foitite and schorl. It has higher Al, Fe, X-site vacancy, and Na than G1 tourmaline (Figs. 3A, 4, 7). Variation in the ratio Mg/(Mg + Fe) of generation-1 tourmaline is limited, whereas that of G2 tourmaline shows a positive correlation with the ratio Na/(Na + X) (Fig. 7). G1 tourmaline contains the highest concentration of Ti (up to 0.085 apfu) in this study (Table 2). As compositional zonation is a general character of G1 tourmaline in the contact zone, one zoned tourmaline grain was X-ray-mapped for Al, Fe, and Ca, and chemical zonation was observed (Fig. 8). The darker core has higher contents of Al, whereas the lighter colored rim is enriched in Fe and Ca.



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Fig. 4.  Al – Fe(tot) – Mg diagram (in molar proportions) for tourmalines (after Henry & Guidotti 1985). (1) Li-rich granitic pegmatites and aplites, (2) Li-poor granitic rocks and associated pegmatites and aplites, (3) Fe-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) Fe-rich quartz–tourmaline rocks, calc-silicate rocks, and metapelites, (7) low-Ca meta-ultramafic and Cr-, V-rich metasedimentary rocks, (8) metacarbonates and metapyroxenites. Contact-zone G1 corresponds to generation 1 in the contact zone, and contact-zone G2 corresponds to generation 2 in the contact zone.

Fig. 5.  Mn and Ti versus YAl/(YAl + Fe) of tourmaline in the Koktokay No. 3 pegmatite.

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The outer pegmatite (zones I, III, and IV) Tourmaline in the outer zones of the pegmatite shows no obvious compositional heterogeneity on BSE images. They belong to the elbaite–schorl solid solution intermediate between the two end-members (Fig. 9), and a general overlap of major elements could be observed for many of the data points (Figs. 3B, 4). The X site is mainly occupied by Na, very low Ca (