Jizerka Gemstone Placer—Possible Links to the Timing of Cenozoic Alkali Basalt Volcanism in Jizera Mountains, Czech Republic

Abstract

The Jizerka Quaternary alluvial placer in the Czech Republic has been a well-known source of gemstones since the 16th century, and the only one in Europe that has yielded a significant amount of jewel-quality sapphire. Besides Mg-rich ilmenite (“iserine”), which is the most common heavy mineral at the locality, some other minerals have been mined for jewellery purposes. These are corundum (sapphire and ruby varieties), zircon (“hyacinth” gemstone variety) and spinel. Here, we present a detailed petrological and geochronological investigation of the enigmatic relationship between the sapphires and their supposed host rocks, supporting their xenogenetic link. Our hypothesis is based on thermal resetting of the U–Pb isotopic age of the zircon inclusion found inside Jizerka blue sapphire to the estimated time of the anticipated host alkaline basalt intrusion. The host rocks of the gemstones (sapphire and zircon) and Mg-rich ilmenite are not yet known, but could be related to the Cenozoic volcanism located near the Jizerka gem placer (Bukovec diatreme volcano, Pytlácká jáma Pit diatreme and Hruškovy skály basalt pipe). The transport of sapphire, zircon and Mg-rich ilmenite to the surface was connected with serial volcanic events, likely the fast ascent of alkali basalts and formation of multi-explosive diatreme maar structures with later deposition of volcanoclastic material in eluvial and alluvial sediments in nearby areas. All mineral xenocrysts usually show traces of magmatic corrosion textures, indicating disequilibrium with the transporting alkali basalt magma. In order to constrain the provenance and age of the Jizerka placer heavy mineral assemblage, zircon inclusion and associated phases (niobian rutile, baddeleyite and silicate melts) in the blue sapphire have been studied using LA–ICP–MS (laser ablation–inductively coupled plasma–mass spectrometry) geochemistry and U–Pb in situ dating. Modification of the zircon inclusion into baddeleyite by exposure to temperature above 1400 °C in a basaltic melt is accompanied by zircon U–Pb age resetting. A zircon inclusion in a Jizerka sapphire was dated at 31.2 ± 0.4 Ma, and its baddeleyite rim at 31 ± 16 Ma. The composition of the melt inclusions in sapphire and incorporated niobian rutile suggests that the parental rock of the sapphire was alkali syenite. The Eocene to late Miocene (Messinian) ages of Jizerka zircon are new findings within the Eger Graben structure, as well as among the other sapphire–zircon occurrences within the European Variscides. Jizerka blue sapphire mineral inclusions indicate a provenience of this gemstone mineral assemblage from different parental rocks of unknown age and unknown levels of the upper crust or lithospheric mantle.

1. Introduction

Zircon is one of the most widely used minerals for obtaining information on the genesis and history of rocks. A zircon and sapphire gemstone assemblage is frequently associated with alkaline basalts [1]. Some authors suggest that large zircon and corundum crystals represent xenocrysts that are transported by alkaline basaltic magmas and do not crystallize primarily from them. Sutherland et al. [2] has recognized two main suites of corundum xenocrysts based on their crystallographic and chemical features: the “magmatic” suite and subsidiary vari-coloured “metamorphic” suites. The “magmatic” (or “BGY”, for blue–green–yellow) suite includes dark green, blue–green, and yellow barrel-shaped sapphires, often with sharp colour zonation. It is the most common suite present in nearly every basalt-related deposit. Inclusions in these sapphires are mostly zircon, K-feldspar, sodic plagioclase, Ti–Nb–Ta oxides, and U and Th minerals.

Gem-quality zircons and sapphires are found in deposits associated with Cenozoic volcanic rocks in various locations, including France, Australia, Vietnam, Cambodia, Laos, Thailand, China, Myanmar, Sri Lanka, and Tanzania [1,3,4,5,6]. Most of their occurrences are located in alluvial or eluvial deposits, making it difficult to study the enigmatic relationship with their host rocks. Very similar sapphire–zircon deposits of Cenozoic age are also known in Central Europe along the Rhine and Eger Neovolcanic Rifts [7,8,9,10].

We performed a careful mineralogical, geochemical and petrological study of the mineral and magmatic melt inclusions enclosed within a blue sapphire isolated from the Quaternary gravel in the Jizerka placer (Figure 1). We focus on the chemical characteristics and U–Pb dating of the enclosed zircon crystal and composition of the melt inclusions and niobian rutile incorporated in the sapphire.

Zircon as a traditional geochronometer is combined here with trace element composition and phase transition (thermal decomposition) into baddeleyite. These data are used to constrain the history and characteristics of the unknown parent rock in which the zircon xenocryst originally crystallized [11,12].

Jizerská Louka Meadow sapphire–zircon Quaternary placer is located 45 km east of Liberec district city in the Czech Republic. Today, it is a rare natural area within the Central Europen realm that deserves study and the highest possible protection. The largest part of the placer deposit is in the national nature reserve Jizerka Peat Bogs, where any removal of natural resources is strictly prohibited. The area of the Jizerka Rivulet and Jizera River catchment near the state border between the Czech Republic and Poland is defined as a nature reserve of international importance. Therefore, the investigation was mostly carried out on material from old collections, with very limited access to the Jizerka placer.

Historically, the Jizerská Louka Meadow in northern Bohemia is known as one of the first European alluvial deposits of sapphire and gem-quality zircon (“hyacinth”) to be discovered. According data from early literature and archives, Venetian gemstone prospectors especially sought blue sapphire there. The alluvial placer of the Jizerka Rivulet and its small tributaries, particularly in confluence with Sapphire Creek, have been more or less intensively panned at least since the 16th century. Quaternary sediments rest directly on granitic eluvium. They are mostly between 0.5 and 2 m thick and are largely overlain by a 0.2–3 m thick layer of peat. The heavy minerals concentrate especially at the bedrock of sandy gravel, often containing cobbles of rocks of local origin (granite, aplite, pegmatite and vein quartz). The dominant tracker of sapphires and zircons (hyacinth) is found there: “iserine”, a Mg−Fe³⁺-rich variety of ilmenite, which is the most common heavy mineral in the Jizerka Rivulet Quaternary alluvial placer. The spherical cavities on the surface of some “iserine” grains resemble imprints of the fluid bubbles of magmatic origin [13]. A weak abrade of the bubble imprints into “iserine” nodules indicates a short distance of the “iserine” mineral association from the volcanic source to the Jizerka Rivulet [14].

Sapphire itself was never observed as an inclusion mineral. Its structure suggests crystallization from a highly differentiated alkaline syenite melt [15]. All sapphire macrocrysts usually show magmatic corrosion textures, indicating disequilibrium with the transporting magma, most probably defined as alkali–basalt. The primary host rock of the Jizerka gemstones and “iserine” is not known, but it could be represented by the recently identified volcanic diatreme structure of the “Pytlácká jáma” Pit located ca. 2 km from the Jizerka stream [14].

2. The Study Area

The Jizerka gemstone placer is a well-known mineralogical locality in the Krkonoše-Jizera Granite Massif domain of the Saxothuringian zone [16] of the Bohemian Massif located on the outskirts of the European Variscides (Figure 1b). The study area belongs to the Late Eocene to Oligocene Lusatian Volcanic Field of the Eger Rift in the Czech part of the Bohemian Massif (Figure 1a). Within this volcanic field, there are more than 1000 volcanic structures in association with ca. 500 extinct volcanoes. Remnants of cinder cones, lava lakes, lava flows, and maar diatremes are found in situ near the original syn-volcanic surface. In deeper eroded structures, relics of volcanics are exposed as snouts or supply channels and diatremes, whereas their volcanoclastic equivalents are rare. Such pyroclastics are known from eastern Saxony (Seufzergründel near Hinterhermsdorf and Hofenberg near Lema) and contain abundant porphyroclasts of coloured zircons very similar to those found in the Jizera Meadow placer. The ages of the volcanites from this part of the Lusatian volcanic field range from 35 to 27 Ma (⁴⁰Ar/³⁹Ar) and fall mainly in the Oligocene period [17].

The Jizerská louka Meadow area (Figure 2) is located within the Upper Carboniferous Krkonoše-Jizera Granite Massif with dominant porphyritic biotite granite [18,19]. Quaternary and Cenozoic maar–diatreme volcanoes exploded both within and outside the Eger Rift Graben during syn-rift (42–16 Ma) and late-rift (16–0.3 Ma) stages [20]. Volcanic bodies with NW–SE linear directions dominate the period of 30–26 Ma [21]. The Pytlácká jáma Pit has been mentioned by Ulrych and Uher [22], and was recently studied by Rous et al. [14], as one of these unusual volcanic structures in the Czech part of the Lusatian Volcanic Field [23]. A large depression, 1000 × 600 m in area, with a depth of about 40 m, is located about 2 km north of the Jizerka village (Figure 2). The diatreme infill of the Pytlácká jáma Pit is buried under a thick layer of peat bog within the Jizerské hory Mts. National Nature Reserve and is thus inaccessible for direct sampling and study. The nearby area is known as one of the first discovered European alluvial placers of gemstone-quality sapphire and zircon [14,24]. Quaternary alluvial gravels contain ilmenite, magnetite, zircon, pleonast and corundum as the main heavy minerals and, less commonly, ruby, Fe-rich spinel, Nb-Rb, fergusonite, ferrcolumbite and REE-phosphate minerals [15,22,24,25,26]. There are 10,000 grains of ilmenite (local historic name “iserine”) to every 10 zircons (hyacinths), and one blue sapphire (one tonne of bedrock gravel contains ca. 4 kg of ilmenite and 40 sapphire grains). The Jizerka natural sapphires are blue (80%) or yellow and green (20%), with rare pink sapphires. Their most frequent size is 4 to 6 mm in diameter.

The primary host rocks of the gemstones (sapphire and zircon) and abundant Mg-ilmenite (“iserine”) in the Jizerka alluvial placer are not known, but could be related to the cluster of alkaline basalt extrusions in the vicinity of Jizerská louka Meadow [14,22]. They are represented by Bukovec hill nephelinite, Pytlácká jáma Pit buried volcanic diatreme and Hruškovy skály basalt pipe (Figure 2).

A special feature of the Jizerka placer in the Jizerské hory Mts. is the occurrence of zircon megacrysts (up to ca. 1 cm in size), with U–Pb ages between ca. 5.6 ± 0.03 and 70.9 ± Ma. The major group of zircons from Jizerka placer with ages between ca. 27 and 35 Ma corresponds to the age of Neovolcanic activity in the Lusatian Volcanic Field [23,27]. Because none of the minerals typical of the gem placer has been found within the volcanic rocks cropping out in the near vicinity of the Jizerská louka Meadow, the buried Pytlácká jáma diatreme is considered the most probable source of the sapphire mineral deposits in the Jizerka Rivulet [14,22]. Similar mineral association of sapphire and zircon megacrysts has been described from Lava Plains gem fields in NE Australia [28], where large zircon is found as megacrystas over 1 cm in size and few smaller (a few mm) zircon crystals are found as inclusions enclosed within sapphire. This contrasts with some sapphire placer deposits from basaltic volcanic sources (e.g., Naryn-Gol in Russia [29]), where zircon megacrysts are absent but zircon is present in the form of rather small inclusions (up to 100 micron) in the sapphire crystals.

3. Samples

Our research was performed on one of the five sapphires with an anhedral to subhedral crystal shape and sizes of 5–20 mm obtained by panning Quaternary sediments of the Jizerka Rivulet at its confluence with the Sapphire Creek in Jizerka village (geographic coordinates 50°49′22.84″/15°20′6.2″ Figure 2).

4. Methods

4.1. LA–ICP–MS U–Pb Dating

A Thermo Scientific Element 2 sector field ICP–MS (inductively coupled plasma–mass spectrometry) coupled to a 193 nm ArF excimer laser (Teledyne Cetac Analyte Excite laser by Teledyne Cetac Technologies, Omaha, NE, USA) at the Institute of Geology of the Czech Academy of Sciences, Prague, Czech Republic, was used to measure the Pb/U and Pb isotopic ratios in zircons. The laser was fired at a repetition rate of 5 Hz and fluence of 3.4 J/cm² with 40 micron (zircon) and 25 micron (baddeleyite) spot size. The carrier gas was flushed through a two-volume ablation cell at a flow rate of 0.9 L/min and mixed with 0.68 L/min Ar and 0.005 L/min N prior to introduction into the ICP. An in-house glass signal homogenizer (with the design of Tunheng and Hirata [30]) was used for mixing all the gases and aerosol, resulting in smooth, spike-free signal. The signal was tuned for maximum sensitivity of Pb and U, with a Th/U ratio close to unity and low oxide level, commonly below 0.2%. Typical acquisitions consisted of 15 s of blank measurement followed by measurement of U, Th, and Pb signals from the ablated zircon and baddeleyite for another 35 s. The total of 420 mass scan data were acquired in time resolved peak jumping pulse counting/analogue mode, with 1 point measured per peak for masses ²⁰⁴Pb + Hg, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th, ²³⁵U and ²³⁸U. Due to a non-linear transition between the counting and analogue acquisition modes of the ICP instrument, the raw data were pre-processed using a purpose-made Excel macro. As a result, the intensities of ²³⁸U were left unchanged if measured in counting mode and recalculated from ²³⁵U intensities if the ²³⁸U was acquired in analogue mode. Data reduction was then carried out off-line using the Iolite data reduction package version 3.4 with VizualAge utility [31]. Full details of the data reduction methodology can be found in Paton et al., [32]. The data reduction included correction for gas blank, laser-induced elemental fractionation of Pb and U and the instrumental mass bias. For the data presented here, blank intensities and instrumental bias were interpolated using an automatic spline function while down-hole inter-element fractionation was corrected using an exponential function. No common Pb correction was applied to the data due to the high Hg contamination of the commercially available He carrier gas, which precludes accurate correction of the interfering ²⁰⁴Hg on the very small signal of ²⁰⁴Pb (common lead).

Residual elemental fractionation and instrumental mass bias were corrected by normalization to the natural zircon reference material Plešovice [33] and baddeleyite Phalaborwa [34]. The U–Pb ages are presented as concordia plots generated with the ISOPLOT program v. 4.16 [35].

4.2. LA–ICP–MS Analysis of Major and Trace Elements

The same instrumental setup was used to acquire the major and trace element data of the mineral and magmatic melt inclusions in the sapphire as well as of the youngest (5.6 Ma) single zircons found in the gem placer (

Table 1. REE and trace element composition (in ppm; quoted as a range and average values) of the zircon inclusion in the Jizerka blue sapphire and of the single megacrysts of Jizerka 5.6 Ma zircons (LA-ICP-MS). n = number of analyses.

Element in ppm (Range/Mean)	P	Ti	Y	Nb	La	Ce	Pr	Nd	Sm	Eu	Gd
Zircon inclusion in sapphire n = 5	220–422 289	17–10 14	859–2809 1847	24–154 73	0.12–0.22 0.17	8.4–24.3 17	0.09–0.15 0.1	0.5–1.6 1	1.5–4.1 2	1.2–3.4 2	11–34.1 20
Jizerka zircons 5.6 Ma n = 5	80–210 130	4–18 9	654–2546 1879	4–48 26	0.03–0.09 0.1	14–201 98	0.06–0.9 0.5	1–13 8	3–22 14	2–15 9	14–97 62
Element in ppm (range/mean)	Tb	Dy	Ho	Er	Tm	Yb	Lu	Hf	Ta	Th	U
Zircon inclusion in sapphire n = 5	5.3–15.7 10	67–237 143	27–95 58	129–433 271	97–31 62	332–906 599	48–126 84	12,031–17,186 15,265	8.6–47.6 22	373–1334 737	23–367 101
Jizerka zircons 5.6 Ma n = 5	5–29 20	62–296 208	23–90 66	106–361 256	21–71 50	204–579 401	40–98 66	5616–6221 6221	1.4–13 7	21–1112 429	39–638 307

and

Table 2. Chemical composition of the mineral and melt inclusions in the Jizerka blue sapphire.

wt.%	Cl	Al2O3	Na2O	SiO2	K2O	CaO	FeO	TiO2	Nb2O5	Ta2O5	ZrO2	HfO2	MgO	* Sum wt.% **
Niobian rutile		6.53		0.35			2.56	87.02	6.00					102.46
Niobian rutile		1.17		0.44			2.79	85.67	7.12					97.19
Niobian rutile		1.91					11.01	45.83	34.93	2.47			0.28	96.43
Baddeleyite		0.60					1.42	4.62			83.22	3.29		93.15
Zircon				26.33							61.54	2.75		90.62
3 melt *	0.34	18.65	3.26	42.53	8.50	1.69	1.06	1.47	0.68					74.92
2 melt *	0.63	19.61	6.06	37.01	5.79	1.79	0.48	2.54	1.38					75.29

* Melt inclusions in niobian rutile. ** Differences from 100 wt.% oxides sum represent mostly volatile (H₂O + CO₂) content or mixed analysis in the baddeleyite symplectitic rim. The low totals of some analyses might be due to the content of elements that are below the detection limit of the analysis (e.g., REE in zircon).

). The laser was fired at a repetition rate of 10 Hz and fluence of 3.4 J/cm² with 30 micron laser spot size. The TE data were collected at the low mass resolution mode (m/Δm = 300) and the measurement sequence consisted of the repeated blocks of two analyses of NIST SRM612, one analysis of BCR-2 standard and ten unknowns (zircon). Trace element data were calibrated against the NIST SRM612 glass using the Si as an internal standard/reference. The minimum detection limit values were calculated by multiplying the variance of the background by 3.25 for individual major and trace elements. The time-resolved signal data were processed using the Glitter software [36]. The precision of the analyses (1 RSD) ranges between 5 and 15% for most elements. The accuracy was monitored by a homogenized basalt reference material BCR-2 (USGS).

5. Results

5.1. Jizerka Blue Sapphire

A set of polished sections of several blue, blue–green, and green and yellow barrel-shaped sapphire megacrysts (Figure 3) from the Jizerka placer were investigated for the presence of mineral inclusions. A small group of isolated anhedral grains of zircon, and tentatively identified niobian rutile, were detected inside one dark blue sapphire showing well-rounded edges (Figure 4). Several globular inclusions of the magmatic melts with diameters of several to tens of microns were also found inside the sapphire megacryst, as well as in the niobian rutile inclusions. The largest inclusion of the magmatic melt, 46 µm in diameter, was located in the niobian rutile (Figure 4d). Such syngenetic inclusions demonstrate that the sapphire crystallised in an environment enriched with incompatible trace elements and volatiles [3,27,37,38].

5.2. Zircon Inclusion in the Jizerka Blue Sapphire

The zircon in the blue sapphire is a single triangular-shaped light brown xenocryst about 500 × 200 µm² in size (at the section level), which is mantled by a continuous irregular rim of syngenetic symplectite of baddeleyite and quartz. These mineral phases result from solid solution breakdown of the zircon inclusion along the contact with the surrounding sapphire. The zircon inclusion is characterised by CL sector zoning with some sectors having individual planar oscillatory growing zones; there is no inherited core in the zircon but there is obvious disturbance of the CL pattern in the central part of the grain in the deeper level of the zircon (Figure 5). Baddeleyite (monoclinic ZrO₂) and quartz symplectitic mixtures document quick thermal cooling (Figure 6). According to Melnik et al. [39], desilification rims of zircon xenoliths represented by baddeleyite record the timing of kimberlite emplacement [40]. It indicates that such baddeleyite reaction rims can also be used for constraining the timing of emplacement of the alkaline-basalt-hosting gemstone minerals (zircon and sapphire).

5.3. Chemistry of Minerals and Melt Inclusions in the Sapphire

The zircon inclusion in the Jizerka sapphire contains about 1269 ppm of REE, about 1847 ppm of Yttrium and a very high Hafnium content of about 1.5%. LA–ICP–MS analysis indicates that the zircon inclusion in the Jizerka blue sapphire has similar concentrations of trace elements and REE as the youngest, ca. 5.6 Ma old, Jizerka placer zircons (REE: 1258 ppm; Yttrium: 1879 ppm; Table 1 and Figure 7). However, their Hafnium content of ca. 0.6 wt.% is much lower than that in the zircon inclusion in the blue sapphire (Table 1). Khamloet et al. [41], report a similar difference between zircons and zircon inclusions in sapphires from basalt-related deposits in Thailand.

Analyses of the dated zircon inclusion and 5.6 Ma old zircon megacrysts from the Jizerka placer (Table 1) revealed contrasting geochemistry. The zircon inclusion was relatively rich in trace elements (av. in ppm: Hf 15 265, Th 737, Ta 22, Nb 73, P 289) compared to the 5.6 Ma old Jizerka zircon megacrysts (Hf 6 221, Th 429, Ta 7, Nb 26, P 130). In the zircon inclusion, the elements with low-to-medium REE (av. ppm) values include low Pr (0.1), Ce (17), Nd (1), Sm (2), Eu (2) and Gd (20). Heavy REE values were observed only for Yb (599) and Lu (84).

Chondrite-normalized REE patterns of the zircon inclusion interior in the Jizerka blue sapphire have a steep slope from light REE (LREE) to heavy REE (HREE), with pronounced positive Ce-, but with a lack of a Eu anomaly (Figure 7). Almost identical chondrite-normalised REE plots are known from zircon from syenite xenoliths in Loch Roag alkalic basalts (Scotland) and zircon from nepheline syenite xenoliths from Bizac basaltic tephra in France [3].

5.4. The U–Pb Ages of the Zircon Inclusion and Its Baddeleyite Rim in the Jizerka Sapphire

Both the zircon core and the baddeleyite rim of the zircon inclusion in the Jizerka blue sapphire were dated using the U–Pb in situ LA–ICP–MS method (Figure 8). The zircon inclusion core is visually older than its baddeleyite rim. In addition, the presence of zircon inclusions in the sapphire indicates their similar provenance and formation from the same parental rock. However, U–Pb dating conducted on the zircon core and baddeleyite rim of the inclusion in the Jizerka blue sapphire provided ages that are identical within their analytical uncertainties. The age of the zircon was found to be 31.2 ± 0.4 Ma (Figure 9), while a highly imprecise age of 31 ± 16 Ma was estimated for the baddeleyite rim (Figure 9). The high uncertainty of the baddeleyite U–Pb age is attributed to the low content of radiogenic Pb, low volume of the material available for dating and an excess of common lead due to intergrowth with other phases in the symplectitic mixture [42].

5.5. Salic Glass Inclusions in Jizerka Sapphire

According to many authors, silicate inclusions in igneous minerals may have compositions corresponding to the chemistry of their parent rocks [42]. The Jizerka sapphire and its niobian rutile inclusion also contain scarce primary melt inclusions in the form of small droplets of salic glass that could have been trapped in these minerals during their growth in magma (Figure 5). These melt inclusions provide an unambiguous method to directly determine the composition of melts from which both minerals crystallized. The chemical compositions of the melt inclusions analysed in this study are reported in Table 2. The glasses of inclusions are similar in chemistry to silica-undersaturated feldspathoids (37–42.5 wt.% SiO₂, total Na₂O + K₂O 6.52–11.85 wt.%) corresponding to foidolites or the quartz nepheline syenite field in the R₁–R₂ diagram [43] (Figure 10). There are also high contents of TiO₂ (~2.5 wt.%), Nb₂O₅ (1.68 wt.%), total Fe as FeO (2.5 wt.%) and CaO (0.8 wt.%) (Table 2). The small size of the analysed inclusions indicates that the analysis might have been compromised by the addition of elements from the host mineral during an LA–ICP–MS analysis. Volatile contents of the glasses (up to 25 wt.%) are measured as the difference between the sum of the measured oxides and 100 wt.% total (Table 2).

6. Discussion

Zircon (ZrSiO₄) is known as a common inclusion in sapphires, and may undergo solid-state thermal dissociation to zirconia and silica at extreme conditions such as high temperature and/or high pressure as follows: ZrSiO₄ (zircon) = ZrO₂ (baddeleyite) + SiO₂ [6]. Borisova et al. [44] tested thermobarometric effects of hot basaltic melt on zircon xenoliths. Their data suggest high solubility of zircon in basaltic magma and very fast congruent dissolution of zircon in basaltic melt at pressures of 0.2 to 0.7 GPa. They calculated timescales of zircon survival in tholeiitic melts. For example, spherical zircon grains about 1 cm in diameter will be completely dissolved at 1300 °C and 0.5 GPa in a tholeiitic melt within ~11 years. A 100-micron zircon will dissolve in 9.7 h, a zircon sphere of 50 microns in 2.6 h and a 10-micron zircon in 0.2 h [44]. Paquette and Margoil-Daniel [3] also report such dissolution reactions of zircons during their residence in basaltic magma. The nearby Bukovec hill nephelinite (Figure 2) contains elevated Zr values of ca. 415 ppm, which would be released from over 830 ppm of dissolved zircon. According to Morales et al. [45], zircon derived from crustal rocks can completely survive dissolution in basaltic melt only if they are shielded as an inclusion phase in early formed minerals (e.g., sapphires) in non-disaggregated xenoliths. Our data indicate that we are observing the very same process in the case of alkaline basalts cropping out in the studied area of Jizerka placer.

According to Váczi et al. [46], the high-temperature breakdown of unaltered zircon below the generally accepted thermodynamical dissociation temperature (1665–1676 °C) is a surface corrosion represented by formation of crystalline ZrO₂ and amorphous SiO₂. Silica is inferred to evaporate below 1400 °C. According to annealing experiments, the temperature of thermal dissociation of ZrSiO₄ was assessed at 1673 ± 10 °C. ZrSiO₄ decomposes by a solid-state reaction, and released SiO₂ in the form of discrete metastable intermediate phases with super stoichiometric Si content. The eutectic temperature in the ZrO₂–SiO₂ system was set to 1687 ± 10 °C [47].

Wang et al. [48] investigated the effects of heat treatment on the gemmological and spectroscopic features of zircon inclusions in sapphires. Progressive decomposition of zircon and chemical reactions between zircon and the host sapphire occurred at temperatures between 1400 °C and 1850 °C. Sub-solidus reactions (i.e., the decomposition of zircon into its component oxides without melting) of some zircon inclusions started at temperatures as low as 1400 °C, as evidenced by the formation of baddeleyite (ZrO₂) and a SiO₂-rich phase.

A desilicification rim was found in the baddeleyite around the zircon inclusion in the blue sapphire from the Jizerka gemstone placer (Figure 5). Clusters of dendritic baddeleyite and a SiO₂-rich phase intergrown with some discrete baddeleyite crystals in rounded centres (Figure 5) intermittently line the zircon margins. The zircon inclusion is apparently older than its baddeleyite rims. Baddeleyite symplectitic textures document quick cooling of the basaltic lava during volcanic eruption. This baddeleyite rim was formed by the interaction with host basalt magma before or soon after emplacement into the crust.

A large number of sapphire-bearing gem fields all over the world are time-related to the host alkali basaltic volcanism. Guo et al. [49], suggested that the relatively high temperature of basaltic magmas might reset the U–Pb isotopic system in zircons due to enhanced Pb diffusion, giving the zircons the same age as the carrier magmas.

Coenraads et al. [50] document a genetic link of the 35 Ma age of the zircon inclusions in the sapphires from East Australia to the alkali basalt volcanism of almost the same age. Paquette and Mergoil-Daniel [3] described a similar genetic relationship between syenite xenoliths containing 1 Ma old zircon and host lava of the same age at Bizac in France. Likewise, Zeug et al. [51] report a similar time coincidence between 4.3 and 0.8 Ma ages of alkaline basaltic rocks for zircon inclusions in sapphires with a U–Pb age of 0.93 Ma from Ratanakiri Province, Cambodia [52]. The zircon xenocrysts from the same province give U–Pb ages of 0.88–1.56 Ma, which predates some of the alkali basalts (~0.7 Ma), indicating very short mantle residence times before entrainment in the erupting magma and an earlier beginning of basaltic volcanism. According to Piilonen et al. [1], the similarity of He and U–Pb ages points to an extremely fast cooling of the zircons from magmatic temperatures through the ~180 °C isotherm, and suggests that the dated zircons erupted to the surface approximately at the same time between 1.02 and 0.86 Ma. This fast cooling process during the transport of sapphires with zircon inclusions to the surface facilitated by the alkali basalts is also documented by the symplectitic mixture of the baddeleyite–quartz rim around the zircon inclusion in the Jizerka sapphire. Similar symplectitic mixtures of baddeleyite–quartz were also found in rims and interiors of about 25% of 83 zircons from the Jizerka placer with an age ranging from 5.6 to 70 Ma (Figure 11).

According to Melnik et al. [39], the desilicification rims of zircon xenoliths represented by baddeleyite form in the crust shortly before or soon after the emplacement of kimberlites and thus record the timing of kimberlite emplacement. This indicates that the baddeleyite reaction rim in the studied material is a good target for deciphering the emplacement age of the alkali basalt hosting gemstone minerals (zircon and sapphire) in the Jizerka Quaternary placer.

U–Pb ages of the zircon inclusion and its baddeleyite rim in the Jizerka blue sapphire indicate similar timing (zircon 31.2 ± 0.4 Ma and baddeleyite 31 ± 16 Ma). However, the low accuracy of the baddeleyite age analysis does not allow for better interpretation of the timing of baddeleyite formation with respect to the age of the host rock. This is because of the open behaviour of the U–Pb isotopic system during zircon growth and storage at high-temperature conditions within the Jizerka sapphire. According to Kober [53,54], the zircon whole-grain evaporation utilises the decomposition of zircon into oxides (ZrO₂ + SiO₂) at elevated temperatures, where Pb is removed from zircon through the volatilisation of the silica component. This partial alteration of zircon to Pb-free zircon and the formation of Pb-free baddeleyite within the altered zircon generate a resetting of the U–Pb geochronometers [12].

This information is in agreement with U–Pb ages of zircon and baddeleyite from the Jizerka blue sapphire. Although there is a difference between their ages with respect to the relative scale of their respective experimental errors, they may be combined to a mean age of ca. 31 Ma for both related mineral phases. This timing corresponds to 30 ± 1 Ma K-Ar whole-rock dating of the limburgite stock, next to Hašler Chalet (13 km to the west from the Jizerka placer [55]). According to this time coincidence, we assume that the thermobarometric effects of the supposed hot host basaltic magma may be responsible for both the zircon inclusion age resetting and the concomitant formation of the baddeleyite rim. Interestingly, a very similar case was found at Hofeberg Hill near Leuba in the Lausitz Volcanic Field in Saxony (E. Germany), which is part of the same system of the Ohře rift as the studied Jizerka locality with its volcanics. In the Hofeberg, which is located ca. 36 km from Jizerka, the local basanitic rocks locally host very abundant zircon megacrysts with a red colour (variety “hyacinth”) and a size of up to 15 mm [9,10]. The zircon from Hofeberg is affected by magmatic corrosion and is more or less mantled by baddeleyite reaction rims. The Hofeberg zircon U–Pb age of ca 30.5 Ma corresponds with the Ar/Ar data of the host basanite, which was determined as being between ca 30 and 31 Ma [9].

The high similarity of REE and trace element composition of the most frequent population of 5.6 Ma old Jizerka zircon xenocrysts [27] and the zircon inclusion in blue sapphire (Table 1) may indicate their similar lithospheric source within magmatic activity of the Jizerka area. Small or absent Eu anomalies may characterize mantle-derived 5.6 Ma old zircons and zircon inclusion in Jizerka sapphire [56].

Wark and Miller (1993) [57] showed that the Hf contents in zircon can roughly reflect the degree of differentiation of the parental melt. The most frequent 5.6 Ma Jizerka zircons are anomalously Hf-poor, with about 0.6 wt.% Hf (Table 1). Such zircons are characteristic for mantle, basic and mainly alkaline igneous rocks, syenites, nepheline syenites and their pegmatites [3,37,58,59]. On the other hand, Hafnium content in the zircon inclusion in the blue Jizerka sapphire is about twice as high (ca. 1.5 wt.% Hf). Zircon from common felsic crustal magmatic rocks, especially granites, usually contains >1 wt.% Hf [56,60,61].

High alkaline content of the salic glass inclusions in niobian rutile inclusions in the Jizerka sapphire may indicate a relationship with the assumed parental alkali syenite (Figure 4 and Figure 6, and Table 2). High alkaline felsic melt is proposed as the crystallization environment for the formation of these sapphires, probably in the lower crust [41,62]. Upton et al. [63] proposed the relevance of the salic melt fractions in the ultrasalic xenoliths of the alkali basalts in Scotland from the shallow mantle.

The high similarity of the chondrite-normalised REE composition of the zircon inclusion in the Jizerka blue sapphire to zircons from syenite xenoliths Loch Roag [3] and 5.6 Ma old zircons from Jizerka placer with zircon from nepheline syenite xenoliths from Bizac basalt [37] indicate their genetic link to alkaline igneous rocks (syenites and nepheline syenites) (Figure 7). However, according to Hoskin and Ireland [64], REE chemistry of zircons from a wide range of crustal rock types and tectonic settings exhibit similar characteristics and therefore zircon chemistry is not generally useful as an unequivocal indicator of provenance.

The mineral xenocrysts found in the Jizerka gemstone placer were most likely transported to the surface by repeated explosions of alkali basalt magmas between ca 30 and 5.6 Ma [27]. This transport may be preferentially linked to the buried Pytlácká jáma Pit and the Bukovec volcanic diatremes near the Jizerka placer (Figure 2). Timing of the volcanic activity in the Jizerka area is indirectly documented by the relative frequency of the 170 zircon U–Pb dates of the Jizerka gemstone placer (Figure 11). The age of zircon megacrysts ranging between ca 70 and 5.6 Ma indicates either older undiscovered volcanic activity in the nearby units or survival of older zircon xenocrysts during very short transport by younger volcanic rocks.

According to O’Reilly and Griffin [65], magmas carrying mantle xenoliths must reach the surface within a maximum of about 8 to 60 h of picking up these dense fragments from depths of about 200 to 80 km depth. An average ascent rate through the whole lithosphere (mantle and crust) in the range of 0.2 to 0.5 m s⁻¹ (about 0.5 to 2 km/h). The ascent rates through the shallow crust may be much higher: 20 m s⁻¹ and up to supersonic speeds (300 m s⁻¹) in the uppermost crust. Once magma reaches a 1–2 km depth, eruption is likely in less than 12 h [64]. That equates to this basalt magma only taking several hours to travel from the depth of origin to erupt at the Earth’s surface. Such a rapid ascent of the basalt magma to the Earth’s surface can be similar to the ascent rate determined for kimberlite magmas and documents the thermobarometric effect onto mineralogical and isotopic composition of these zircon xenoliths.

The published data and results of our study indicate specific features of the Jizerka sapphire–zircon formation, which are taken into consideration when interpreting the origin of the Jizerka gemstone placer:

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Time and space relationship of the Jizerka placer with the Cenozoic alkali basalt volcanism in the area;

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Occurrence of Mg-rich ilmenite megacrysts indicating the existence of a deep-seated mafic magma chamber under the Krkonoše-Jizera granite massif;

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Large zircon crystals in the Jizerka gemstone placer record their thermal age resetting, synchronous with several episodes of basalt eruptions known from the Lusatian Volcanic Field in the Eger Neovolanic Rift [17];

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Presence of syngenetic symplectite of baddeleyite forming rims on the Jizerka zircons and indicating their residence in hot magma and fast transport in host rocks to the surface;

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Inclusions of zircon, niobian rutile and droplets of syenitic silicate melt trapped by sapphire crystal during its growth demonstrate their genetic relationship to hypothetical parent rocks of alkaline syenitic composition.

7. Summary and Conclusions

The main conclusions from this study can be summarized as follows:

• Zircon and niobian rutile in the Jizerka blue sapphire represent syngenetic inclusions;
• Zircon inclusion in the Jizerka blue sapphire has been partly mineralogically modified during transport of the sapphire xenocryst in the assumed host alkali basaltic magma. This modification of the zircon inclusion is manifested by development of baddeleyite–quartz rim;
• The width of the baddeleyite rim of the zircon inclusion indicates a short residence time of the blue sapphire in the sub-crustal magma chambers at temperatures higher than closure of the U–Pb system (>900 °C);
• Baddeleyite symplectitic textures around the zircon inclusion in the blue sapphire document quick cooling of the supposed host basalt magma during fast ascent to the earth’s surface;
• The zircon inclusion is apparently older than its baddeleyite rims but the U–Pb dating conducted on the core of the zircon inclusion in the Jizerka blue sapphire provides an age of 31 ± 0.4 Ma indistinguishable from the highly imprecise age of 31 ± 16 Ma of the baddeleyite rim. Such synchronicity suggests their minimal residence in silica-undersaturated magma and rapid ascent to the near-surface temperatures;
• The thermobarometric impact on the zircon inclusion in the Jizerka blue sapphire could have resulted in a U–Pb isotopic age reset at about 31 Ma corresponding to the ages of the alkali basaltic rocks located near the Jizerka gem placer;
• LA–ICP–MS analyses indicate that the zircon inclusion in the Jizerka blue sapphire has similar levels of REE and trace elements as 5.6 Ma old Jizerka zircon xenocrysts. Significant amounts of P, Y, Hf, and Th in zircon inclusion in the Jizerka sapphire imply high-alkali felsic (syenite) source melt for the host sapphire;
• High alkaline content of the magmatic melts in niobian rutile inclusions in the Jizerka blue sapphire represents another indirect evidence for the parental alkali (nepheline) syenite;
• Basaltic magmas can be responsible for violent desegregation of lithospheric rock sequences and participate in upward transport of the corundum-bearing accidental rock xenoliths and mineral megacrysts to the surface during the volatile-rich, explosive eruptions;
• The host rocks of the Jizerka sapphires and zircons are not yet known, but are most probably related to the Cenozoic volcanism located near the Jizerka gem placer.

Eight episodes of zircon U–Pb resets were documented with the U–Pb dating of zircon megacrysts: the ages of 5.6 ± 0.03, 24.2 ± 0.4, 27.9 ± 0.3, 31.9 ± 0.7, 35.4 ± 1.0, 45.9 ± 1.6, 57.7 ± 1.4 and 70.9 ± 1.1 Ma are all linked to the multistage evolution of the Eger Rift basalt magmatism (Figure 11). These variously aged zircons may be related to periodic Cenozoic alkaline basaltic eruptions produced by the deep-seated intermediate magma chamber beneath the Krkonoše-Jizera granite massif. Such continuous eruptions ensured the transport and age resetting of zircons from the parent rocks into erosional cycles continuing into the Quaternary.