Chemical Changes in Quartz and Micas During Greisenization: Examples from European Variscan Plutons

Abstract

Metals like Li, Sn, W, Nb and Ta accumulate mostly during the magmatic–hydrothermal transition and subsequent hydrothermal alteration of highly fractionated granites, especially greisenization. Evaluation of about 450 bulk-rock analyses, 1500 LA-ICP-MS analyses of quartz and 1600 EPMA and LA-ICP-MS analyses of mica from parental granites and related greisens and quartz–mica veins from four typical areas of European Variscan granite plutons with greisen mineralization (Beauvoir, France; Panasqueira, Portugal; and Cínovec and Nejdek, Erzgebirge, Czech Republic) illustrate diversity in initial magma composition (S- vs. A-types), in style of greisenization (pervasive greisenization in granite cupolas vs. vein-like greisen strings along joints), and in chemical evolution of quartz and micas during magmatic–hydrothermal transition. The contents of all monitored elements in quartz and mica from greisen and veins are of very high variability, with principal differences among studied localities. Generally, very low contents of Al (<100 ppm), Ti (<1 ppm) and Li (<10 ppm) or, on the contrary, extremely high contents of Al (>1000 ppm) or Li (>100 ppm) in quartz may indicate its hydrothermal origin. Contents of Sn, W, Nb, and Ta in micas tend to become depleted during greisenization, this trend is more pronounced in Nb and Ta than in Sn and W. Transition from magmatic to hydrothermal crystallization leads to an increase in the Ta/Nb values in mica: from 0.20 to 0.24 in S-type magmatic systems, and from 0.13 to 0.34 at Cínovec as a representative of A-type granites. Whether granite belongs to the S- or A-type is not essential for the development of greisenization.

1. Introduction

Strongly fractionated granites (rare metal granites, RMGs) are, together with complex pegmatites, a crucial source of a number of technologically important metals like Li, Rb, Cs, Sn, W, Nb and Ta. From the chemical point of view, these elements belong to two groups with very different chemical properties: rare alkali metals (Li, Rb, Cs), and high field strength elements (HFSE: Sn, W, Nb, Ta, U, Th). While the elements of the first group are mainly found as minor constituents of rock-forming silicates, the elements of the second group mainly form simple or complex oxides. However, both groups have a similar ability to concentrate during the fractionation of granitic magma, especially if this magma is enriched in water and fluorine, and possibly also P and B [1,2,3]. The named elements accumulate during the fractionation of F-rich magma with only small deviations caused by high peraluminosity (favoring the concentration of Cs, Ta and U) or peralkalinity (favoring the concentration of Nb and Th). Although the ability of magma to concentrate these elements during fractional crystallization is high, the achievement of economically interesting concentrations during this process is rather rare. This usually occurs only during the magmatic–hydrothermal transition and especially during subsequent hydrothermal alteration. The most common type of high-temperature alteration of peraluminous and subaluminous RMG is greisenization [4,5,6], i.e., replacement of magmatic feldspars by hydrothermal quartz and mica (+ minor topaz, fluorite, etc.). The term “greisen” has long been used by Erzgebirge miners [4] for a hard rock ore composed predominantly of quartz and mica and containing cassiterite. From the terminological point of view, it is important that the greisen bodies occurred within the granite pluton; i.e., greisenizing fluids reacted with the parent rock during the crystallization of which they were formed. The movement of fluids and the changes they caused were limited to the interior of the magmatic body. This process is sometimes referred to as autometasomatism [7,8]. This term has also entered the scientific literature in the Erzgebirge in connection with the transformation of granites by their own unmixed solutions [9].

Tin ores formed by granite intrusion-related hydrothermal alteration of exocontact rock types (rhyolite, gneiss, mica schist, phyllite) were traditionally called “zwitter”. Today’s understanding of the term greisen is broader and is often used for acid, quartz-rich metasomatites in a variety of primary rocks. In this study, we use the term greisen in its original meaning defined in the Erzgebirge, referring to dominantly quartz–mica metasomatite.

The main greisen minerals are quartz and micas, both in a combination of relics of primary magmatic domains (crystal cores), mantled with metasomatized domains, and new hydrothermally grown mineral grains (Figure 1 and Figure 2). Typical greisen may also contain relics of both feldspars (albite and orthoclase), and relict and/or newly formed topaz, apatite, tourmaline, rutile, monazite, xenotime and zircon. And, of course, predominantly newly formed ore minerals cassiterite, wolframite, rarely also columbite or microlite, common sulfides and native Bi.

According to their shape, greisens can be divided into two basic groups that differ mainly structurally and morphologically, but also mineralogically and chemically: (i) pervasive greisens and (ii) vein greisens. Pervasive greisens form relatively large plates, lenticular or irregular, mostly flat and essentially homogeneous bodies tens of m thick and up to hundreds of m across. They are developed exclusively in the upper parts of granite cupolas, which they partially or completely fill (Figures S1–S3 in Supplement S1). Typical examples have been described and have been mined on a large scale in the Erzgebirge. They are associated with both cupolas of A-type granites (Cínovec, Sadisdorf, [10,11,12]) and cupolas of strongly peraluminous granites of S-type (Ehrenfriedersdorf [13], Krásno [14]). All examples in the Erzgebirge are linked to explosive degassing and hydrofracturing in shallow intrusions; the fluids penetrated through a dense system of irregular, omnidirectional fractures, practically homogeneously throughout the entire volume of the rock (Figures S5 and S6 in Supplement S1). Major constituents of pervasive greisens are quartz and mica (+ topaz > fluorite) in very variable ratios (compare Figure S12 in the Supplement S1). Contents of Li and F, and thus the type of mica, depend on the degree of fractionation of the parent granite: from zinnwaldite at Cínovec, Krásno and similar deposits in the Erzgebirge, to muscovite at Panasqueira, Portugal. However, the extent of Sn or W mineralization does not generally correlate with the composition of micas and the content of Li and F in greisen (see further text). The concentration of Sn in greisen usually does not exceed 0.5 wt%, but locally, in Li-rich greisens, there was an enormous enrichment of cassiterite in the form of bonanzas (Figure S10 in Supplement S1).

Figure 1. Mineral maps of granites and greisen constructed with TIMA technology [15]. (a–c) granite and vein greisen, Horní Blatná, Nejdek pluton, Czech Republic: (a) biotite granite; (b) mica-rich greisen; (c) topaz-rich greisen; (d,e) Panasqueira deposit, Portugal: (d) muscovite granite from deeper part of the cupola; (e) pervasive muscovite greisen forming the upper part of the cupola. Size of all images is ca. 35 × 23 mm.

Figure 1. Mineral maps of granites and greisen constructed with TIMA technology [15]. (a–c) granite and vein greisen, Horní Blatná, Nejdek pluton, Czech Republic: (a) biotite granite; (b) mica-rich greisen; (c) topaz-rich greisen; (d,e) Panasqueira deposit, Portugal: (d) muscovite granite from deeper part of the cupola; (e) pervasive muscovite greisen forming the upper part of the cupola. Size of all images is ca. 35 × 23 mm.

Vein greisens form thin sheet-like metasomatic (!) bodies along parallel steep fractures. Individual cm sized strips can combine into bands (sheeted-vein complex) in tectonic zones up to several meters thick (Figure S4 in Supplement S1). Swarms of greisen veins up to a kilometer long and 50 m thick are typical of the inner parts of more deeply eroded plutons like the central part of the Nejdek-Eibenstock pluton in the Western Erzgebirge [16,17] and in Land’s End and Cligga Head granites, Cornwall (Figures S7–S9 in Supplement S1) [18]. Vein greisens are composed of quartz and Li-poor mica (+ some tourmaline, apatite or topaz) and are mostly mineralized only with cassiterite; wolframite is sparse and Nb + Ta are completely absent (Figure S13 in Supplement S1).

Besides greisens, another granite-related form of mineralization is represented by hydrothermal veins showing different temporal and spatial relationships to the greisens. Early intragranitic quartz–zinnwaldite lodes with greisen rims at Cínovec [19] (Figure S5 in Supplement S1), and late distal quartz–muscovite veins at Suchot in the Beauvoir granite, France [20], represent extreme types.

Figure 2. Mineral composition of greisen: (a) back-scattered electron (BSE) image of quartz–zinnwaldite greisen, Cínovec, Czech Republic; (b) the same area in cathodoluminescence. Remnants of primary magmatic quartz (relatively light Qz-r) are easily distinguishable from newly formed hydrothermal quartz (dark Qz-n); (c) relics of magmatic zinnwaldite surrounded by hydrothermal muscovite (Cínovec, Czech Republic). Znw—zinnwaldite, Ms—muscovite, Toz—topaz. Scale bars 0.5 mm in all images.

Figure 2. Mineral composition of greisen: (a) back-scattered electron (BSE) image of quartz–zinnwaldite greisen, Cínovec, Czech Republic; (b) the same area in cathodoluminescence. Remnants of primary magmatic quartz (relatively light Qz-r) are easily distinguishable from newly formed hydrothermal quartz (dark Qz-n); (c) relics of magmatic zinnwaldite surrounded by hydrothermal muscovite (Cínovec, Czech Republic). Znw—zinnwaldite, Ms—muscovite, Toz—topaz. Scale bars 0.5 mm in all images.

Due to the growing interest in the greisen type of rare metal deposits, a number of greisen bodies have been subjected to detailed mineralogical research in the last decade. Most often, fluid inclusions have been investigated and p-T conditions of greisen genesis have been modeled [21,22,23]. A number of studies have investigated the evolution of minerals in RMG during the fractionation and transition to hydrothermal processes. Descriptions of minerals from greisen alone are sparser, and studies involving both granites and their hydrothermal transformation products are rather rare [24].

During typical greisenization, the contents of Li and metals (Sn, W) increase, but sometimes these metals are carried away and dispersed into the exocontact. In greisen, besides ore minerals, micas are potentially the most important metal carriers. Evolutionary models have been proposed with mica as a temporary concentrator of metals during magmatic fractionation, and, subsequently, as their donor to ore minerals [25]. However, opposite processes of enrichment of micas during greisenization are presented in this paper, especially in the case of muscovite. Both models are certainly possible, depending on the composition of fluids and the surrounding environment [6,26].

Quartz, as the main mineral component of both greisen and postgranitic veins, is not able to accommodate significant amounts of rare metals into its structure [27,28]. However, trace amounts of some elements offer relevant information about the environment from which magmatic and hydrothermal quartz crystallized [24,29].

The aim of this work is to show the diversity of chemical changes during greisenization and their record in the composition of quartz and micas. This is why I selected 4 typical areas of European Variscan RMG with greisen mineralization that demonstrate different initial magma types (S- vs. A-types), different styles of greisenization (pervasive greisenization in cupolas vs. vein-like metasomatized zones along joints), different styles of chemical evolution during greisenization, and different relationships between greisen and quartz veins.

2. Brief Geology of the Studied Granite/Greisen Systems and the Studied Samples

The Beauvoir and Colette granites together with the La Bosse quartz–wolframite stockwork and several distal quartz veins form the late Variscan Echassieres granite system in the northern part of the Variscan Massif Central, France (Figure S1 in Supplement S1). The geologically relatively older porphyritic two-mica Colette granite forms a roughly round outcrop, ca. 2 km in diameter, while the Beauvoir leucogranite (313 Ma, [30]) forms a small steep stock (outcrop < 0.2 km²) at its southern contact. The Beauvoir granite is strongly peraluminous and enriched in P, F, Li, Rb, Nb, Ta, Sn and W. Borehole GPF1, 900 m deep, revealed a strong vertical zoning from the least evolved biotite granite at the bottom (unit B3) to extremely fractionated lepidolite granite at the top of the stock (unit B1) [31,32,33]. Quartz–muscovite–apatite greisen zones with indistinct contacts and rare quartz veins are widespread, especially in the uppermost part of the intrusion [29,34]. The La Bosse quartz–wolframite stockwork found at the base of borehole GPF1 is newly considered early Variscan, pre-metamorphic [20]. The studied samples comprise granites from the whole vertical profile of borehole GPF1 (author’s data) and the Colette granite, the La Bosse stockwork and the Suchot distal quartz vein [29,34].

The Panasqueira granites form a completely hidden pluton emplaced in the Lower Paleozoic Beira schist, west of the city of Fundao, central Portugal (Figure S2 in the Supplement S1). The pluton comprises peraluminous porphyritic biotite granite at its periphery and geochemically more evolved muscovite leucogranite with partly greisenized cupola in its center [6,35,36]. Pervasive quartz–muscovite greisen in the cupola is generally poor in ore elements, while coeval flat quartz lodes with muscovite rims, up to 50 cm thick, cutting the surrounding schists, are mineralized with wolframite [37,38,39]. Presented are mineral compositions from hidden granites, greisen cupola, and quartz–wolframite lode [36].

The Nejdek-Eibenstock pluton is the most typical example of strongly peraluminous Variscan magmatism in the Western Krušné Hory/Erzgebirge area (Czech Republic/Germany, [40,41]). This pluton is composed of an older intrusive complex (OIC) represented by biotite granites followed by a younger ore-productive intrusive complex (YIC) of Li, F-enriched albite–biotite granites with topaz (328.6 ± 2 Ma and 320 ± 1 Ma, respectively, [42]). The most fractionated, extremely F- and P-rich batches of residual magma crystallized in the form of layered rock with comb quartz layers at Podlesí [43]. Small pervasive greisen bodies are located at cupola-like elevations of the YIC granites near Jáchymov and Vykmanov [44], while numerous steep metasomatized greisen zones, traditionally termed “vein greisens”, are widespread especially in the central parts of the YIC granite intrusions [5,16,45,46] (Figure S3 in Supplement S1). The most fractionated Podlesí intrusion with extremely high primary contents of rare elements is free of post-magmatic alteration and ore-bearing processes. Presented data comprise minerals from the main types of OIC and YIC granites, and from cupola and vein greisens (authorʼs data). Quartz data from the Podlesí granite stock have already been published by Breiter and Müller [47].

The Altenberg-Teplice Caldera, located in the Eastern Krušné Hory/Erzgebirge (Czech Republic/Germany), represents the largest complex of A-type rhyolites and granites in Variscan Europe, covering an area of about 500 km² [48,49]. After the caldera collapse, several small plutons of A-type biotite granites and albite–zinnwaldite–topaz granites intruded its volcanic fill. The subvolcanic character of these intrusions is demonstrated by the existence of pipes of explosive breccia [50]. Among several intrusions of fractionated granites, the Cínovec cupola is the most voluminous (Figure S4 in Supplement S1). Its upper part down to a depth of 735 m is formed by partly greisenized, dominantly medium-grained zinnwaldite granite with Li-Sn-W mineralization of greisen and vein type [11,51]. The deeper part is composed of barren porphyritic coarse-grained biotite granite. Arched quartz–zinnwaldite (+ topaz, wolframite, cassiterite) veins, roughly parallel to the upper granite contact, up to 50 cm thick (Figure S5 in Supplement S1), were historically mined in the upper-central part of the cupola, while less mineralized but huge pervasive quartz–zinnwaldite greisen bodies with cassiterite are widespread especially in the southern part of the cupola. Presented data represent minerals from all granite types down to the depth of 1600 m [46,52] and from pervasive greisen and vein-type mineralization (unpublished authorʼs data, Müller et al. [24] and Hreus et al. [53]).

3. Methods

Bulk-rock compositions of granites vs. greisen are evaluated based on published analytical data [5,11,31,32,33]. Most of the mineral data used in this work were acquired by the author in the Institute of Geology of the Czech Academy of Sciences in Praha. A larger part has already been published in works devoted to individual localities; some data, especially from the Beauvoir granite and Nejdek pluton, were acquired specifically for this study.

3.1. Quartz

The contents of trace elements Al, B, Be, Fe, Ge, Li, Mn, P, Rb, Sn, Sr and Ti in quartz from the author’s samples from Cínovec, Nejdek pluton, Panasqueira and the Beauvoir granite were determined at the Institute of Geology of the CAS using LA-ICP-MS system composed of Thermo-Finnigan Element 2 sector field mass spectrometer coupled with an Analyte Excite 193 nm excimer Laser (Photon Machines, Belgrade, MT, USA). The laser was fired at a repetition rate of 10 Hz, a laser fluency of 4–5 J/cm² and a beam size of 100 μm. The ablated material was transported by high-purity He gas. Time-resolved signal data were processed using the Glitter^4.4.2. software (http://www.glitter-gemoc.com/). Data were calibrated against the external standard of synthetic silicate glass NIST SRM 612. Isotope ²⁹Si was used as the internal standard based on the assumption that the analyzed quartz contains 99.95 wt% SiO₂. For details, see [54]. Data taken from the literature were obtained in a similar manner; for details, see [47] (Podlesí granite) and [29] (Beauvoir greisen and mineralized veins and stockwork).

3.2. Mica

The contents of major elements Si, Ti, Al, Fe, Mn, Mg, Rb, Cs, Na, K and F in mica were analyzed using the Jeol JXA—8230 electron microprobe (JEOL Ltd., Akishima, Tokyo, Japan) housed at the Institute of Geology of the CAS, Praha, operated in the wavelength-dispersive mode. The contents of minor and trace elements Li, Sc, Ti, Mn, Fe, Ga, Ge, Rb, Nb, In, Sn, Cs, Ta, W, and Tl in mica were determined at Faculty of Chemistry, Brno University of Technology using ArF* excimer laser ablation system Analyte Excite+ (Teledyne CETAC Technologies, Omaha, NE, USA) emitting the laser beam at the wavelength of 193 nm, connected to quadrupole ICP mass spectrometer Agilent 7900 (Agilent Technologies, Inc., Santa Clara, CA, USA). Individual spots were ablated by a 50 μm laser beam with a fluence of 3 J/cm² and a repetition rate of 10 Hz. The ablated material was carried using helium carrier gas (0.5 + 0.3 L/min) and mixed with argon (~1 L/min) prior to the torch. The detection limits were calculated as 3 × SD/b, where SD means standard deviation of the He-Ar gas blank and b is the sensitivity. The spectrometer was tuned using SRM NIST 612 with respect to the maximum sensitivity and minimum doubly charged ions oxide formations. Accuracy control was performed by repeated measurements of the mica in-house test sample measured together with all the samples included in this paper. For additional details, see [55].

Abbreviations of mineral names follow the IMA rules [56].

4. Results

4.1. Bulk-Rock Chemical Changes During Greisenization

Even though greisen deposits are a frequent subject of research, surprisingly few whole-rock analyses of greisens have been published. For scientists, greisens are probably too inhomogeneous to be subject to bulk analyses, and exploration companies tend to determine only the contents of elements of interest. A statistically significant dataset is available from Cínovec [11], smaller datasets come from Beauvoir [31,32,33] and Nejdek [5]. No Li data are available from Panasqueira [36]. The whole dataset of bulk-rock analyses is available in Supplement S2.

Since the ratio of the main mineral components of greisen, quartz and mica varies widely, the results of the analyses pose a very inhomogeneous set (

Table 1. Chemical composition of granites (medians); major elements in wt%, traces in ppm (free space—no data).

Pluton	Unit	n	SiO2	TiO2	Al2O3	Fe2O3	MnO	MgO	CaO	Li2O	Na2O	K2O	P2O5	n	F	LOI	n	Li	Rb	Nb	Sn	Ta	W	Sources
Beauvoir	greisen	17	73.50	0.02	16.50	0.59	0.07	0.30	0.08		0.10	4.62	0.10	17		2.36	17		1820	94	1004	143	31	[31,32,33]
Beauvoir	B1	53	68.68	0.01	17.33	0.27	0.04	0.03	0.54	0.89	4.71	3.21	1.16	53	2.00	2.00	53	5110	3320	101	943	156	42
Beauvoir	B2	26	69.90	0.01	16.70	0.40	0.09	0.02	0.40	0.69	4.47	3.66	1.25	26	1.98	1.48	26	3201	2290	50	669	54	33
Beauvoir	B3	17	70.90	0.01	16.10	0.50	0.09	0.02	0.42	0.38	4.42	3.63	1.04	17	1.69	1.40	17	1796	1705	44	355	37	25
Beauvoir	Colette	25	72.80	0.12	15.10	1.01	0.03	0.23	0.46	0.11	3.29	4.74	0.38	25		1.22	25	493	563	27	71	10	17
Panasqueira	porphyritic granite	8	73.80	0.16	13.83	1.98	0.03	0.38	0.51		2.64	3.95	0.33	8	0.37	2.15	8		550	15	46	4	17	[36]
Panasqueira	Ms granite	7	74.07	0.02	14.92	1.33	0.03	0.09	0.35		3.19	3.40	0.31	7	0.37	1.60	7		721	26	57	12	18
Panasqueira	greisen. Granite	8	75.02	0.03	14.04	1.64	0.03	0.19	0.34		2.52	2.80	0.22	8	0.33	2.10	8		600	22	50	9	20
Nejdek	granite	25	74.98	0.11	13.27	1.51	0.12	0.04	0.39	0.07	3.03	4.83	0.26	25	0.37	1.00	25	303	932		16			[5] +
Nejdek	greisen.granite	6	74.40	0.11	13.02	1.82	0.04	0.22	0.38	0.05	1.64	4.10	0.27	6	0.59	1.77	6	236	575		57			author’s
Nejdek	vein greisen	10	75.24	0.10	11.77	4.48	0.08	0.17	0.41	0.08	0.18	3.88	0.30	10	0.79	2.08	10	369	832		854			data
Cínovec	greisen	37	80.78	0.01	8.61	3.18	0.25	0.00	0.52	0.84	0.06	3.29	<0.01	15	0.48	0.87	63	4111	2802	67	36	26	37	[11] +
Cínovec	cupola granite	52	78.11	0.01	11.26	0.94	0.08	0.08	0.39	0.22	4.00	3.47	<0.00	8	0.66	0.72	52	1005	1665	102	67	39	15	author’s
Cínovec	feldspar-rich rocks	29	76.04	0.01	11.83	0.29	0.02	0.05	0.28	0.01	4.36	5.46	<0.00	7	0.73	1.13	29	30	1708	85	4	30	5	data
Cínovec	zinnwaldite granite	70	77.04	0.02	11.07	0.96	0.05	0.02	0.35	0.16	4.19	4.82	<0.00	5	0.15	0.75	70	762	1620	80	24	20	37
Cínovec	biotite granite	98	77.53	0.07	10.60	1.30	0.04	0.10	0.53	0.05	3.24	5.18	0.01	3	4.18	2.31	98	246	722	51	21	8	14

, Figure 3, Figure 4 and Figure 5).

In general, however, greisenization is accompanied by an increase in Si (often up to 85, locally 90 wt% SiO₂, Figure 3a,c,e) and a decrease in Na contents, i.e., a destruction of albite, often to less than 0.1 wt% Na₂O (Figure 3b,d,f). The contents of Al₂O₃ and K₂O may not change much since both of these elements are simply transferred from magmatic K-feldspar to newly formed mica. Lithium and the usually positively correlated F (Figure 4a,c,e) have two behavior scenarios during greisenization. In typical pervasive greisens in the Erzgebirge, like Cínovec, Li and F contents increase significantly: up to 1.5 wt% Li₂O (Figure 3c) and 2.5 wt% F; both elements are hosted in newly formed zinnwaldite mica. At other places, however, newly formed mica is of muscovite composition, being associated with a decrease in Li and F contents. This is typical for vein greisens in the Nejdek pluton (Figure 3e and Figure 4e) and also for cupola-hosted greisens at Beauvoir and Panasqueira, and the late stage of Cínovec greisen (Figure 2c). Although lepidolite granite B1 from Beauvoir contains more Li than zinnwaldite granite from Cínovec (1.0–1.5 vs. 0.2–0.4 wt% Li₂O), this ratio becomes reversed during greisenization; this is mineralogically expressed by the contrast between zinnwaldite at Cínovec and muscovite at Beauvoir.

The amount of water bound in the form of OH⁻ and/or crystal water in minerals, approximately expressed as loss on ignition (LOI), can indicate the extent of hydrothermal alteration during greisenization (Figure 4b,d,f). This water content increases significantly in Cínovec and Nejdek systems, being usually <1 wt% in fresh granites and reaching 1.5–2.5 wt% in greisen (Figure 4d,f). In contrast, at Beauvoir, LOI is high already in granites, and no further enrichment was encountered during greisenization (Figure 4a).

Rubidium is bound, besides mica, also to K-feldspar. During simple pervasive or vein-type greisenization, the ratio of Rb and Li remains roughly the same as at Beauvoir and Nejdek. In contrast, the initial melt at Cínovec with the Rb/Li value of ca 1 became separated into a residual quartz–feldspathic melt enriched in Rb, and mica-rich greisens enriched in Li (Figure 5a,c,e) [11].

Tin enrichment does not depend on the increase/decrease in Li and F contents, and occurs in both of the above-mentioned scenarios, i.e., in greisens with Li-mica (Cínovec) and in greisens with muscovite or muscovitized biotite (Nejdek) (Figure 5d,f). Of course, the existence of greisens with very low contents of Sn and other metals (Beauvoir, Figure 5b) confirms the assumption that greisenization itself and the enrichment in Sn are two largely independent processes or stages of post-magmatic granite evolution.

4.2. Quartz

The contents of trace elements in quartz are shown in Figure 6 and Figure 7 (all analyses) and in

Table 2. Trace-element contents in quartz (medians in ppm). Free spaces—no data available.

Pluton	Rock Type	n	Li	Be	B	Ge	Rb	Sn	Al	P	Ti	Mn	Fe
Beauvoir	Colette granite	28	62.4	0.50	4.1	1.00	0.14	0.15	352		69.5
Beauvoir	leucogranite	128	103	0.55	2.1	2.01	1.37	0.28	657	7.9	5.8	0.4	1.1
Beauvoir	greisen	49	271	0.50	2.5	3.06	0.05	0.04	2068		4.1
Beauvoir	La Bosse stockwork	82	33.2	0.50	3.4	2.07	0.10	0.13	256		20.1
Beauvoir	distal vein	20	89.5	2.87	8.8	2.55	13.0	0.17	5129		10.7
Panasqueira	granite	133	13.4	0.12	1.1	1.16	0.34	0.17	289	6.3	36.4	0.3	1.9
Panasqueira	cupola greisen	40	12.9	<0.1	0.8	3.12	0.11	0.17	172	7.1	2.2	<0.2	0.5
Panasqueira	vein	38	4.6	<0.1	0.9	2.52	<0.1	0.18	70	8.6	1.4	<0.2	<0.5
Nejdek	OIC granite	49	32.8	0.11	1.15	0.71	<0.05	0.18	170	4.6	46.5	<0.2	1.8
Nejdek	YIC granite	196	46.6	0.29	1.3	1.08	0.05	0.23	378	12.9	37.7	0.2	2.2
Nejdek	Podlesí granite	40	67.7	0.19	5.3	1.59	3.11	<0.03	663		13.5	<0.1	1.5
Nejdek	cupola greisen	86	36.0	0.43	1.5	1.03	0.18	0.37	398	14.3	17.5	0.2	2.5
Nejdek	vein greisen	42	38.2	0.31	0.9	0.92	1.54	0.14	341	9.2	40.1	0.4	13.3
Cínovec	Bt granite	108	23.1	0.28	0.5	0.9	0.22	0.03	140	7.5	30.6	0.2	1.6
Cínovec	Znw granite	221	21.2	0.67	1.0	1.66	0.82	0.08	247	8.2	7.5	0.3	1.6
Cínovec	cupola greisen	212	2.4	0.14	0.5	1.11	0.08	0.04	52	3.1	0.7	0.1	<0.1
Cínovec	Qtz-Znw vein	53	9.5	0.08	0.4	0.90	0.01	0.03	34	4.9	1.5	<0.1	<0.1

(medians). The whole dataset of analyses is available in Supplement S3.

Al, Ti and Li are generally the most common trace elements in quartz of magmatic origin, and a decrease in the Ti/Al value is a reliable indicator of magmatic fractionation [27,28,29,47,54]. As opposed to the decreasing Ti contents during greisenization in all studied cases (to <40 ppm at the Nejdek pluton, <20 ppm at Cínovec and <10 ppm at Beauvoir and Panasqueira, Figure 6a–d), the behavior of Al and Li is ambivalent. The contents of both elements strongly increase up to 3500 ppm Al and 400 ppm Li at Beauvoir but clearly decrease at Cínovec (<150 ppm Al, <15 ppm Li). At Panasqueira and Nejdek, the changes are insignificant (Figure 6e–h). As for other monitored elements, greisen quartz from Beauvoir is relatively depleted in Sn (Figure 7a) but enriched in Ge (Figure 7b), B and Be (Table 2). No clear changes are seen at other localities.

Quartz from associated hydrothermal veins is mostly characterized by high Al contents (up to 6000 ppm Al in the Suchot vein at Beauvoir) but low Ti, Li, Ge and Sn contents.

4.3. Mica

Chemical compositions of micas from the studied RMG and related greisens vary within wide limits among biotite (Bt), muscovite (Ms) and lepidolite (Lpd), and evolutionary trends typical for individual regions differ significantly (Figure 8,

Table 3. Chemical composition of micas (medians in wt%). Free spaces—no data available.

Pluton	Rock Type	Mica	n	SiO2	TiO2	Al2O3	FeO	MnO	MgO	ZnO	Rb2O	Li2O	Na2O	K2O	F
Colette	two-mica granite	Ms	10	45.7	0.54	33.2	3.07	0.03	1.11		0.17	0.52	0.77	9.68	3.05
Colette	two-mica granite	Znw	9	38.4	1.04	22.5	17.7	0.24	2.37		0.27	1.37	0.28	9.19	4.93
Beauvoir	granite	Lpd	52	50.5	0.09	23.2	4.38	0.84	0.02	0.21	1.20	5.55	0.34	10.5	7.78
Beauvoir	greisen	Ms	5	48.7	0.02	36.5	0.20	0.05	0.08		0.38	0.01	0.03	8.34	0.71
La Bosse	stockwork	Ms	11	49.1	0.02	29.3	1.83	0.10	1.37		0.27	0.04	0.04	8.94	1.93
Suchot	distal vein	Ms	15	49.8	0.54	32.3	2.06	0.05	2.14		0.08	0.22	0.40	9.93	2.36
Panasqueira	granite	Bt	3	35.9	2.60	19.1	22.6	0.52	3.92	0.17	0.23	0.57	0.11	9.37	2.03
Panasqueira	granite	Ms	25	46.8	0.38	32.5	3.51	0.05	0.76	0.03	0.12	0.21	0.65	10.8	1.55
Panasqueira	cupola greisen	Ms	15	46.5	0.07	32.9	3.99	0.08	0.09	0.05	0.19	0.19	0.52	10.7	1.36
Panasqueira	lode	Ms	3	46.6	0.22	32.7	3.75	0.07	0.42	0.04	0.16	0.32	0.58	10.7	1.46
Nejdek OIC	granite	Bt	8	35.5	3.18	17.0	21.2	0.31	7.04		0.10	0.33	0.15	9.66	<0.1
Nejdek YIC	granite	Li-Bt	60	39.4	0.70	21.5	20.4	0.33	1.13	0.11	0.58	1.76	0.21	9.88	2.16
Nejdek-Podlesi	granite	Znw	5	45.4	0.27	20.9	11.9	0.09	0.31		1.02	4.08	0.30	9.69	7.84
Nejdek-Vykmanov	cupola greisen	Li-Bt	40	39.1	0.21	22.7	20	0.27	0.19	0.03	0.5	1.20	0.21	9.93	1.70
Nejdek	vein greisen	Ms	16	46.6	0.19	31.0	7.78	0.09	0.32		0.19	0.84	0.22	10.6	1.76
Cínovec	Znw granite	Znw	19	45.9	0.18	20.2	12.6	0.91	0.04		1.11	3.77	0.23	9.82	8.07
Cínovec	Bt granite	Li-Bt	10	38.2	1.21	20.0	22.4	0.60	0.88		0.53	1.42	0.30	9.48	4.35
Cínovec	cupola greisen	Znw	12	46.9	0.09	19.7	12.7	1.14	0.02		1.31	3.69	0.23	9.70	7.87
Cínovec	lode	Znw	8	46.7	0.04	20.1	11.4	1.46	0.01	0.14	1.02	3.64	0.18	10.3	8.46

). Within the Beauvoir igneous system, the older Colette granite contains Al-enriched biotite and Fe-enriched muscovite, both with very low Li contents (Figure 8a), while the Beauvoir RMG itself evolved from Fe-enriched lepidolite in unit B3 to pure lepidolite in unit B1. Surprisingly, micas in the associated greisen, stockwork and veins are Li-poor. Li-poor are also all micas in the Panasqueira deposit (Figure 8b). Mica evolution from biotite to zinnwaldite is typical for all RMG plutons in the Erzgebirge (Figure 8c,d). New hydrothermal mica from pervasive cupola greisens at Cínovec and Nejdek is similar to magmatic micas in neighboring granites. Mica in the Nejdek vein greisen originated via muscovitization of primary granitic biotite (Figure 8c).

The contents of some minor and trace elements in micas are presented in

Table 4. Trace-element contents in micas (medians in ppm). Free spaces—no data available.

Pluton	Rock	Mica	n	Li	Sc	Ga	Ge	Nb	In	Sn	Cs	Ta	W
Colette	two-mica granite	Ms	10	2432			4	247		297	98	39	161
Colette	two-mica granite	Znw	9	6375			6	468		158	657	78	54
Beauvoir	leucogranite	Lpd	220	25,569		74	15	165	0.1	186	984	38	191
Beauvoir	greisen domains	Ms	10	34			2	7		1131	199	29	2
Beauvoir	stockwork La Bosse	Ms	14	496			2	12		57	176	3	6
Beauvoir	Suchot distal vein	Ms	20	672			4	12		251	299	2	118
Panasqueira	leucogranite	Bt	15	2585	29	74	3	86	0.4	71	288	8	8
Panasqueira	leucogranite	Ms	46	953	8	129	6	88	1	130	104	17	83
Panasqueira	cupola greisen	Ms	72	855	1	136	7	153	2	170	119	25	97
Panasqueira	lode	Ms	22	1416	3	112	6	52	2	300	128	17	84
Nejdek OIC	Bt granite	Bt	41	1219	63	94	16	150	0.6	173	115	11	9
Nejdek YIC	Bt granite	Li-Bt	158	6344	30	126	3	232	0.7	190	657	31	51
Nejdek Podlesí	Znw granite	Znw	81	19,074	13	109	7	125	0.5	104	1191	25	126
Nejdek Vykmanov	cupola gresen	Li-Bt	45	5483	25	167	3	277	1	162	1471	45	52
Nejdek	vein greisen	Ms	24	861	4	83	2	15	17	457	296	1	23
Cínovec	Bt granite	Li-Bt	203	6081	82	109	8	522	0.2	243	480	66	32
Cínovec	Znw granite	Znw	371	16,585	72	80	6	91	0.3	125	437	28	37
Cínovec	cupola gresen	Znw	293	17,614	63	105	6	51	0.6	103	410	21	23
Cínovec	lode	Znw	52	17,389	50	109	5	38	0.7	119	297	12	24

, and the whole dataset is in Supplement S4. Figure 9 shows the relation between the contents of Li and rare metals Sn and W, well illustrating the contrasting nature of these two elements. For example, Sn contents at Beauvoir increase in greisen mica but remain roughly the same in the veins. On the other hand, W contents decrease in all cases. The contents of Sn and W in greisen mica are similar to those in neighboring granite in the Panasqueira and Nejdek cupola greisens, and both elements tend to slightly decrease at Cínovec. Both Sn and W are enriched in Li-poor mica from the Nejdek vein greisen.

The relative evolution of Sn vs. W, and also Nb vs. Ta, is shown in Figure 10. At Beauvoir, Sn contents increased, and W, Nb and Ta contents decreased in all hydrothermal micas (Figure 10a,b); a similar behavior was also found in the Nejdek pluton (Figure 10e,f). At Panasqueira, some grains of greisen mica roughly retain granitic contents of all metals (relics?), while some are enriched in Sn and Ta (newly formed grains, Figure 10c,d). A trend towards depletion in Sn, W and Nb and a small relative enrichment in Ta was found at Cínovec.

5. Discussion

5.1. Characteristic Chemistry of Magmatic and Hydrothermal Quartz: Is Simple and Reliable Discrimination Possible?

As already shown in Figure 6 and Figure 7, the dispersion in the chemical compositions of hydrothermal quartz from greisen and veins is wide, significantly wider than the dispersion in the compositions of magmatic quartz. The trend towards the enrichment or depletion of an element during hydrothermal processes varies from deposit to deposit. For example, Al and Li can be strongly enriched during greisenization, as in the case of Beauvoir, or significantly depleted, as at Cínovec. The Ge contents sometimes increase (Beauvoir) but usually remain roughly the same. Neither the Sn contents in quartz correlate demonstrably with the occurrence of real Sn mineralization.

The earliest attempt to classify quartz on a chemical basis was made by Schrön in 1988 [59]. At that time, in proportion to the existing analytical methods, he used atomic emission spectrometry of 62 pulverized bulk samples of pegmatitic, granitic and rhyolitic quartz and proposed the Ti-(Al/50)-(10*Ge) ternary diagram for a basic classification of quartz from these three lithologies: according to this pioneering study, Ti dominates in rhyolitic, Al in granitic, and Ge in pegmatitic quartz. Despite the somewhat primitive analytical method, from today’s point of view, the applicability of this diagram has been repeatedly confirmed [47,60,61]. Unfortunately, hydrothermal quartz behaves very differently in the Ti-Al-Ge system (Figure 11): while greisen quartz from the Beauvoir complex and Nejdek pluton keeps the composition of parental granitic quartz (near the Al apex, Figure 11a,c), greisen quartz from Panasqueira and Cínovec is shifted to the Ge apex (pegmatite field, Figure 11b,d).

The composition of quartz from veins is even more irregular, even within the same system; this is mainly due to the wide fluctuation of Al contents and its growth especially in the late low-temperature stages of crystallization. The one order of magnitude difference in Al contents in individual stages of magmatic–hydrothermal evolution reflects the pH-dependent Al solubility in the fluid. At low temperatures, Al solubility increases dramatically in a very acidic environment of distal quartz veins (pH > 4), while at temperatures around 500 °C (greisen stage), Al solubility is pH-independent and low [62]. As a result, in the environment of greisenized rare-metal granites, the Ti-Al-Ge diagram generally does not allow distinguishing between magmatic and hydrothermal quartz.

Recently, Shah et al. [63] proposed a simple Ge/Al ratio as a usable discriminant of magmatic (Ge/Al < 0.008) and hydrothermal quartz (Ge/Al > 0.008). If we insert our data into the Shah diagram (Figure 12a), we see that the proposed discrimination line transects all the clusters of granitic-, greisen- and vein-quartz analyses: S-type rocks, relatively richer in Al, lie in the proposed “magmatic” field, while A-type rocks lie predominantly in the “hydrothermal” field, regardless of their actual origin.

Another frequently studied type is quartz from porphyry Cu(Au) deposits and other types of gold deposits [64,65]. Large datasets confirm a decrease in the Ti/Al values in quartz, proportional to the decrease in crystallization temperatures; the discrimination diagram proposed by Rusk [64] (Figure 12b) has been often applied recently [66,67]. However, the relationship between magmatic and greisen + vein quartz does not correspond to a simple model of Ti/Al dependence of temperature decrease from magmatic to hydrothermal to epithermal (distal veins) processes: (1) S-type granites span from ca. 200 to 0.2 ppm Ti, being distributed across all temperature-related classes of Au deposits, and (2) many S-type greisen bodies and veins keep Ti and Al contents nearly identical to those of their granitic precursors although they must have been formed at significantly lower temperatures (Figure 12b).

Among many other options tested, the Al vs. Li diagram seems to be relatively the most useful, at least in the setting of A-type granites. Contents of Al and Li in most quartz from both A- and S-type granites range between 100–1000 and 10–100 ppm, respectively, while greisen quartz and vein quartz of A-type from Cínovec display almost always low contents of both elements: <100 ppm Al and <10 ppm Li, being clearly distinguishable from quartz of their magmatic predecessor (Figure 12c). In the case of S-type systems, most quartz samples from greisen bodies and veins fall within the field of magmatic quartz, and only a smaller part of the samples show significantly higher or lower Al and Li contents, and can thus be distinguished from their magmatic predecessor relatively reliably. Most analyses of quartz from S-type greisen cannot be distinguished from magmatic quartz (Figure 12d).

5.2. Mica Composition

Micas are a very diverse group of minerals with variable chemical compositions that are sensitive to the type and chemistry of the host rock [68]. Micas, due to their crystal structure, can contain high levels of a number of minor and trace elements like Li, Rb, Sn, W, Nb, Ta, etc. This is especially evident in highly fractionated rocks such as rare-metal granites and pegmatites [6,26,34,55,68,69,70,71,72,73]. Fe-rich micas (biotite) are able to selectively incorporate Nb into their lattice and enrich the residual melt with Ta [74]. This opens a question whether there are any general rules for changes in the chemistry of micas during the magmatic–hydrothermal transition, i.e., during greisenization. A look at Figure 13 suggests that they are very difficult to define: differences between the trends in individual plutons/deposits (compare Figure 9 and Figure 10) are fundamental.

In the case of the Cínovec deposit (A-type granite), all monitored rare metals in micas tend to become depleted during greisenization. This trend is more pronounced in Nb and Ta than in Sn and W. In the Nejdek pluton, micas from greisens are depleted in W, Nb and Ta but enriched in Sn (Figure 10e,f). At Panasqueira, Sn, Nb and Ta contents increased during greisenization while W contents remained unchanged (Figure 10c,d). At Beauvoir, Nb, Ta and W contents decreased and only Sn contents in muscovite greisen increased. In general, muscovite has a greater ability to absorb Sn from fluids than biotite. Sn enrichment of hydrothermal muscovite was also reported by Launay et al. [6] from Panasqueira and by Monnier et al. [34] from the Beauvoir greisen muscovite. The general decrease in the contents of rare metals from granitic to greisen micas can be attributed to the saturation of the fluid with respect to cassiterite, wolframite and possibly columbite under greisen crystallization conditions. A detailed analysis of the mineral share of rare metals at the Cínovec deposit [52] showed that in less fractionated granites, ca. 90% of Sn, W, Nb and Ta are hosted by mica (Li biotite). In more fractionated facies, this share decreases with the onset of crystallization of finely disseminated cassiterite. In greisen, due to extreme mineral inhomogeneity, zinnwaldite hosted 1–90% of the total rare metal contents in samples of the order of kilogram weight.

No correlation can be found between Sn and W due to their different geochemical properties (Figure 13a). In contrast, Nb and Ta correlate significantly (Figure 13b). The experiments of Stepanov [74] have shown that the transition from magmatic to hydrothermal crystallization leads to an increase in the Ta/Nb values in mica. Our data confirms this: the Ta/Nb ratio in mica increases from 0.20 to 0.24 in S-type systems, and from 0.13 to 0.34 at Cínovec as a representative of A-granites. Dispersion among the values is, however, wide and the results should therefore be taken with caution despite the large number of evaluated analyses.

5.3. The Role of Mica in Tin Accumulation

The most objective data to address this issue is available for the pervasive greisens of the Cínovec deposit. Some time ago, Johan et al. [25] presented a model in which Li-micas throughout the volume of the granitic intrusion were the primary carrier of Sn in the magmatic stage, and only during the hydrothermal stage was tin released from the micas and transported by fluids to the dome where cassiterite crystallized. This hypothesis has been questioned because detailed research of the micas in the entire available 1.6 km deep profile has not shown any alterations of the micas leading to depletion of Sn, nor of W, Nb and Ta [52]. Field and mineralogical evidence from Cínovec is consistent with the experiments of Zhao et al. [75] demonstrating direct Sn transition from fractionated melt to hydrothermal fluid. The D_Sn^fluid/melt, at the corresponding ASI = 1.05–1.08, is 1.9–35 [75], which demonstrates very effective mixing of Sn into the fluid.

The study of fluid inclusions in the Cínovce greisen demonstrated the boiling of the fluid and its separation into vapor and liquid [21,23]. The increase in volume after vapor separation triggered explosive brecciation and hydrofracturing [11,50]. F-rich vapor advanced upwards along hairline cracks more rapidly, causing the decomposition of feldspars, the crystallization of quartz, topaz and mica, and increasing the volume of pores [23]. This allowed later fluids to permeate the entire volume of the rock, forming pervasive greisen. Tin transport occurred in a fluid environment, probably in multiple episodes. The last, relatively low-temperature Sn influx by an already F, Li-poor fluid was associated with the muscovitization of greisen zinnwaldite (Figure 2c). This is consistent with the empirical finding from the mine that muscovitization increases the amount of cassiterite in the ore [76]. During muscovitization, the Sn content in mica increased on average from 145 to 1408 ppm, while the F and Li contents decreased from 7.0 to 0.4 wt% F, and from 3.5 to 1 wt% Li₂O.

The vein greisens have been studied rarely in modern times [5,77]. These greisens were formed when the flow of hydrothermal fluid was focused into systems of vertical fractures. The vertical extent of this greisen type reaches up to 800 m, but the source from which the fluids were tapped has not yet been identified. Models involving both possibilities, i.e., deeper parts of the surrounding intrusion itself, or the existence of a younger, more fractionated intrusion hidden at depth, have been considered but not confirmed [5,17].

5.4. Differences Between S- and A-Type Greisen Systems

The Erzgebirge, where equally old RMGs of both types, i.e., S- and A-type, occur only a few tens of km apart, is seemingly a suitable area for assessing the influence of the genetic type of granite on the style and extent of Sn-mineralization. Although both types of granites have specific chemical features that point to different source rocks [78,79], the Sn-deposits in the domes of A- and S-granites are very similar, both chemically and structurally [11,12,14]. Granites of initially different chemistry converge with advancing fractionation. Moreover, in the case of the Erzgebirge, the general enrichment of Li, Sn, and W indicates a similar material source melted under different conditions rather than different source lithologies [80]. Dolejš and Štemprok [81] found rather smaller initial delta¹⁸O_WR in A-type Cínovec than in S-type Krásno RMGs (5.2–8.4 vs. 10.4–10.7 ‰ SMOW), but the water responsible for the greisenization was of meteoric origin in both cases. Current knowledge suggests that specific emplacement conditions, i.e., the very shallow depth of intrusion (1–2 km) and the resulting explosive degassing and development of the hydrothermal process in an environment of high pressure and temperature gradient, were decisive for the structural and material development of mineralization in the granite cupolas. Inherited chemical specificities, such as differences in peraluminity or phosphorus content, did not affect the development of mineralization.

6. Conclusions

The contents of all monitored elements in quartz and mica from greisen and associated veins are of very high variability, significantly higher than in parent granites. The trends in elemental contents in minerals during the magmatic–hydrothermal transition are different at each studied locality. Nevertheless, some generally valid trends can be stated:

• Quartz: Extremely low contents of Al (<100 ppm), Ti (<1 ppm) and Li (<10 ppm) or, on the contrary, extremely high contents of Al (>1000 ppm Al) and/or Li (>100 ppm) are strong arguments for a hydrothermal origin of the studied quartz. This discriminator is very reliable in A-type systems, but only limited in S-type systems.
• Micas: (1) The contents of all monitored elements fluctuate strongly, often within two orders of magnitude; (2) the transition from magmatic to hydrothermal crystallization leads to a statistical increase in the Ta/Nb values in mica; (3) all monitored rare metals usually tend to become depleted in greisen micas. This trend is more pronounced in Nb and Ta than in Sn and W. The only exception is the increase in Sn contents at localities with muscovite as the only hydrothermal mica; (4) transition from magmatic to hydrothermal crystallization leads to an increase in the Ta/Nb values in mica: from 0.20 to 0.24 in S-type systems, and from 0.13 to 0.34 at Cínovec as a representative of A-granites.
• In general, no difference was found in the structural or chemical development of pervasive greisen in A- and S-type granites. The formation of pervasive greisen was determined by the subvolcanic depth of the intrusion, which triggered explosive degassing and subsequently enabled intensive circulation of hydrothermal fluids.

Despite the large amount of evaluated data, the results are, due to the low inhomogeneity of the greisenization processes, only indicative. Further research in this direction, including isotopic methods in particular, is highly desirable.