DOI: 10.31038/GEMS.2026821

Abstract

The Variscan Ehrenfriedersdorf tin deposit (Germany) is a classic example of extreme enrichment of Sn and associated elements in granite–pegmatite systems. This study investigates the statistical distributions of major and trace elements derived from melt and fluid inclusion data, with particular emphasis on Gaussian, Lorendian, Voigt, and idealized Dirac-like distributions. These distributions provide quantitative insight into the physicochemical processes governing ore formation. A key result is the identification of weakly asymmetric pseudobinary solvus curves in the silicate melt–H₂O (±B₂O₃) system, defined by water concentration versus temperature. Both granite- and pegmatite-related solvus curves exhibit closely similar critical points (≈25–30 % H₂O), despite differences in bulk composition. The region around these critical points coincides with pronounced enrichment of economically important elements, whose concentrations follow characteristic Gaussian, Lorendian, or mixed (Voigt) distributions when plotted against water content of the melt inclusion glass. Gaussian distributions reflect relatively well-mixed systems governed by multiple small-scale processes, whereas Lorendian distributions indicate rare but powerful enrichment events driven by lifetime-limited, interaction-based processes. The frequent occurrence of Lorendian behavior for elements such as Sn, Li, Be, Ta, and W points to episodic supercritical fluid or melt pulses. Extremely high, δ-like “runaway” concentrations are interpreted as the result of trapping stoichiometric daughter minerals during the transition from supercritical to undercritical conditions. These observations demonstrate that solvus geometry, element distribution functions, and critical phenomena are fundamentally linked. The data provide compelling evidence that supercritical, water-rich melts and fluids—likely derived from mantle–crust interaction—play a decisive role in redistributing elements and forming ore in the Ehrenfriedersdorf deposit and comparable Variscan systems.

Introduction

In recently published papers [1,2], in part results from melt and fluid inclusions were used to explain the formation of Sn, Ta, Nb deposits by extraction at the magmatic stages in porphyry deposits, the formation of pegmatites, and the growth of gems from extremely hard fluids (Li, Be, and B). This author and coauthors use a very different model to explain the formation of mineral deposits in general. According to the first author’s research in the last 30 years, especially after the development of a method for the determination of water in glasses and melt inclusions [3], an idea about the extreme element enrichment near the solvus crest of the pseudo binary silicate melt–water system in the form of Gaussian, Lorentzian, and Voigt element distributions was born. The Gaussian, Lorentzian, and Voigt distributions are key to understanding the behavior of silicate melts during ore-forming processes in the famous tin deposit of Ehrenfriedersdorf, Germany. Basic results are in Thomas et al. 2019 [4] and 2022 [5]. Figure 1 shows the generalized pseudobinary solvus curves for the Ehrenfriedersdorf granites and pegmatites. The solvus of the granites (blue curve) of the Ehrenfriedersdorf area is remarkably small due to high concentrations of fluorine, phosphorus, and alkalies (Li, Na, K, Rb, Cs). This curve shows some similarity to the phase diagram of the haplogranite-H₂O system according to Bureau and Keppler (1999) [6], due to the strong deviation of the actual composition from the haplogranite system, resulting in significantly lower pressure (see also Sowerby and Keppler, 2002) [7]. The water content at the critical points (CP) of both curves (850°C, 25 % H₂O and 712°C, 27.7 % (H₂O+B₂O₃) is nearly identical.

Figure 1: Pseudobinary solvus curves of evolved granites (blue) and pegmatites (red) for the Ehrenfriedersdorf tin deposit. CP – critical point. The critical point for the F-rich granites is 850°C and 25% H₂O, and for the pegmatites, the CP is 712°C and 27.7% (H₂O + B₂O₃).

The solvus curves of the granites and pegmatites determine the main processes for the enrichment of ore-forming and rare elements. As we will show below, the region around the critical point is where the crucial processes occur. To discuss the processes, we will first briefly define the relevant element distribution types in our case, including the Diracian distribution used by Vigneresse (2026) [2].

Gaussian, Lorentzian, Voigt, and Diracian distributions (see Linford, 2014 [8], and References in it)

Gaussian Distribution – “Normal Distribution” Natural Variability

The Geochemical meaning of a Gaussian (normal) distribution indicates that many small, random, additive processes produce element variability. That is the most common pattern in geochemistry. This kind of distribution results from measurement noise and multiple small-scale processes adding up (Central Limit Theorem). A Gaussian pattern suggests the system was relatively well-mixed and governed by many equal, independent factors – not dominated by rare or extreme processes. The standard Gaussian frequency distribution (distribution of an element) will not be considered here. We consider, in the following, the frequency distribution relative to another reference of the same sample (melt inclusion), for example, water (H₂O) or (H₂O+B₂O₃), which holds for the Lorentzian distribution (see further below). All points forming the corresponding curves are Gaussian distributed, which results from instrumental noise, sample heterogeneity, and counting statistics. In our case, the peak center corresponds to the sulvus crest, the water concentration at the critical point of the solvus curve. The area under the curve is proportional to the element concentration – giving us, in our case, information on the deviation from the corresponding Clarke concentration (see Rösler and Lange, 1975) [9].

Lorentzian Distribution — Presence of Outliers or Resonant Processes

A Lorentzian distribution has fatter tails than a Gaussian. That indicates occasional large excursions (outliers) and processes dominated by a few strong influences rather than many small ones. Typical causes are supercritical pulses that strongly enrich a trace element, forming a sharp element anomaly. For Lorentzian processes, “lifetime” effects (analogous to Lorentzian broadening in spectroscopy) are characteristic. A Lorentzian pattern implies that rare but powerful events influenced the chemical system. The “heavy tails” correspond to unusually high concentrations due to supercritical pulses. A Lorentzian distribution of an element indicates lifetime-limited or interaction-driven processes. It is important to note that, in our case, the Lorentzian curve is not a classic peak but a curve over another element concentration. Typical causes of the Lorentzian distribution are pressure- or collision-broadening and strong matrix or chemical interactions. The “peak” height becomes unreliable, but the area under the curve remains proportional to concentration.

Voigt Distribution

A Voigt profile is a convolution of Gaussian and Lorentzian effects, meaning both random noise and physical broadening mechanisms are important. The Voigt distribution, as a realistic concentration model, provides information about the instrumental noise (Gaussian component) and the physical or chemical broadening (Lorentzian component).

Dirac (δ) Distribution — Highly Uniform or Idealized Single-Value Concentrations

A Dirac delta distribution represents all values concentrated at one exact number – a perfect spike or an idealized, infinitely sharp line. That is, in our case, not attainable. In reality, this almost never occurs, but it is used conceptually to represent single transition energies or states in analog physical systems (e.g., δ-like spectral lines in the absence of broadening). A Dirac-like pattern suggests a geochemical reservoir with practically no heterogeneity, a mineral phase with stoichiometric composition, and an element controlled by a single dominant process (supercritical process) with negligible variability. An example is the appearance of ideal stoichiometric, however unusual, minerals in melt and fluid inclusions. In natural datasets, a δ distribution rarely occurs; it is more of a reference ideal. We call this distribution here because Vigeresse (2026) [2] used it for his very schematic interpretation.

Typical Examples of Distributions Obtained from Melt Inclusions in Quartz of Pegmatite and Pegmatite-Like Rocks from the Ehrenfriedersdorf Sn Deposit

It is important here that we correlate the distribution of trace and major elements with the water content of the melt inclusions, determined by micro-Raman spectroscopy [3]. The trace and major elements were determined using different analytical methods (microprobe, SIMS, LA-ICP-QMS, synchrotron radiation XRF, and Raman spectroscopy) – see Borisova et al. (2012) [10] and Thomas et al. (2019, 2022) [4,5]. First, we show the Rb vs. H₂O distributions for a pegmatite from the Sauberg mine near Ehrenfriedersdorf, Central Erzgebirge, Germany. We see a distribution of Rb vs. H₂O in melt and fluid inclusions, forming a solvus-like curve in the silicate-water range (up to about 50% H₂O), and a distribution of Rb in the fluid phase at high water concentration (filled triangles).

A similar relationship holds for the B₂O₃-H₂O and F-H₂O systems of the Ehrenfriedersdorf pegmatite [11]; Thomas and Rericha, 2023). That means at least that the solvus curve obtained for the pegmatites related to the Variscan granites of the Ehrenfriedersdorf region is determined primarily by H₂O, B₂O₃, Rb as well as by F. We will see that at least all elements forming Gaussian and Lorentzian curves over the water concentration take part in the formation of the characteristic solvus curves, because this refrains from the region around the critical point, the fringes fit well with the solvus curves. A similar plot result for antimony (Sb) vs. water. The Sb data for this plot were obtained using the Synchrotron radiation XRF technique with Monte Carlo-based quantification [12,13]. This plot clearly shows that trace elements like Sb also follow the solvus crest of the pegmatite-H₂O system. In the fluid part of the system, even 700-900 ppm Sb is possible [10], obviously related to chlorine complexes.

We will later see that, around the critical point of the solvus, Gaussian and Lorentzian element distribution curves are sitting. That means the solvus, the Gaussian, and the Lorentzian curves are not independent. Figures 2 and 3 clearly show that, generally, on the right side of the solvus curves, a fluid phase coexists with the corresponding curve [14]. That means the coexistence of two melt inclusion types (A- and B-type MI) with fluid phases containing different daughter phases, often Al- and Si-bearing (topaz, muscovite). Now we will show a Gaussian distribution curve. As an example of such a Gaussian element distribution, see Figure 4: the distribution of Be versus water content in the measured melt inclusions.

Figure 2: Plot of the Rb concentration in melt and fluid inclusions versus the water content. Black points represent the so-called A-type melt inclusions, and half-filled points represent the water-rich B-type melt inclusions, and the black triangles stand for fluid inclusions. The isotherms are drawn in for both melt inclusion types.

Figure 3: Distribution of antimony (Sb) in melt (red) and fluid (grey line) inclusions.

Figure 4: Gaussian distribution of Be versus the water content of melt inclusions in pegmatite quartz from Ehrenfriedersdorf (beryl-quartz vein in the Sauberg mine). The center is at 26.4% H₂O, the half-maximum distance is at 9.5% water, the maximum of the Gaussian curve is at 12075 ppm Be, and the offset corresponds to 214 ppm Be. The offset represents the regional enrichment of Be for the given mineralization.

Gaussian curves are rare in Ehrenfriedersdorf Sn deposits. Most of the studied elements follow a Lorentzian distribution, as shown in Figure 5. The relationship between the solvus curve of pegmatite-like mineralizations is presented by Thomas and Rericha (2024) [15]. In this paper, the authors also establish a clear relationship between the solvus curve and the Lorentzian distribution of Sn, as well as a generalization of the Lorentzian distribution using normalized element concentrations C_A/C_A-crit versus the normalized water concentration of the solvus H₂O/H₂O-crit. This correlation enables the estimation of the Lorentzian distribution of any element from only a couple of measurements (Table 1).

Figure 5: The figure shows the Lorentzian distribution of Sn vs. H₂O, and in Table 1 are the resulting fitting data summarized. The center (25.7 % H₂O) is the position of the peak’s center, which corresponds to the critical point of the solvus curve and the maximum (height) of the Sn concentration (here, 16400 ppm Sn). Width is the half-width at half-maximum (HWHM). The offset refers to the displacement of the Lorentzian curve from its original position along the x-axis (H₂O concentration) corresponding to 644 ppm.

Table 1: Lorentzian fit of Sn, determined in silicate melt inclusions from the pegmatite system of the Sauberg mine near Ehrenfriedersdorf (46 measuring points). Each point is the mean of 5 to 10 single measurements. The values in the second data row are calculated (see Thomas 2025a).

Thomas (2025a) [16] also discusses the Lorentzian distribution in more detail. In this contribution, the Lorentzian data for the 10 elements Be, B, P, Cl, Zn, As, Cs, Sn, Ta, and W are tabulated. More elements of other mineralizations are in Thomas et al. 2019, 2022. Here, we will include the results of another element, which is of great economic significance at the time: Li.

Table 2 summarizes the Lorentzian data for Li shown in Figure 6. According to Rösler and Lange (1975) [9], the mean of granitic rocks is 40 ppm.

Table 2: Lorentzian fit of Li, determined in silicate melt inclusions from the pegmatite system of the Sauberg mine near Ehrenfriedersdorf.

Figure 6: Lorentzian distribution of Li versus the water content in melt inclusions in pegmatite quartz from the Sauberg mine, Ehrenfriedersdorf/Germany.

In contrast to Sn, the width of Li is almost three times as large, and the value at the center (water content at the critical point) is 666.5 times that of the normal granite. The large width of the Li curve is a hint for the participation of Li in the formation of the solvus curve. Sometimes we also observe overlapping Lorentzian curves for a single element, as shown here for Be (Figure 7 and Table 3) [5].

Figure 7: Distribution of Be in some melt inclusions in pegmatite quartz from Ehrenfriedersdorf (sample Qu8). The sum curve (grey) results from the overlapping of two Lorentzian components caused by different Be species in the melt inclusions. Peak 1 is representative of beryllonite [NaBePO₄], and peak 2 (blue) is for hambergite [Be₂BO₃(OH,F)] as a daughter mineral.

Table 3: Lorentzian fit parameters for both Be curves.

In a single-melt inclusion, we found up to 71500 ppm Be as a large daughter crystal of beryllonite. That would, after the classic idea, be a (however real!) runaway value. This observation is typical for Lorentzian element distribution curves: peak height becomes unreliable. We see that the positions of the second Lorentzian Be peaks shift to higher H₂O values with increasing bulk volatile concentration. This observation confirms the term “critical range.”

The fourth distribution, the Diracian distribution, is characterized by a signal that represents an idealized, infinitely sharp line. We often found extremely high concentrations at the centers of the Lorentzian distributions (mostly at the solvus crest). Such behavior is very typical for the Ehrenfriedersdorf case and does not approach the pure Lorentzian distribution. The distribution of tin (Figure 5) already suggests such a distribution. Also, the high Be value of 71500 ppm Be in the case of the first peak form, at least a so-called runaway value, which cannot be ignored. In the past, we often encountered so-called runaway data during our analytical work, which irritated us and others because it was mostly uncorrelated with other elements. See the extended discussion of our data by London and Evensen (2002) [17]. The first such example we found during the development of the first SYXRF-microprobe spectrometer at DESY/Hamburg in 1995, Thomas et al. (1995) [18], on a melt inclusion in quartz from the Sauberg mine near Ehrenfridersdorf. In the first studied melt inclusion, we found high Rb and Cs concentrations and extremely high Sn values, which could be traced to a cassiterite daughter mineral phase, as indicated by microscopic studies.

The so-called “Runaway-Points” can be explained very simply: At the transition of a supercritical fluid to the undercritical state, insoluble cassiterite microcrystalls form, are suspended in the melt, and are trapped by the change. A similar process can also be observed in the case of Be distribution, where different large beryllonite daughter crystals are trapped quite by chance. A completely different case is the sporadic occurrence of diamond crystals in the typical greisen rock of the Ehrenfriedersdorf tin deposits. The appearance of spherical diamonds (Raman peak at 1326 ± 9.8 cm^-1, n = 13 different crystals) in such a typical crustal rock is clear proof of interaction with supercritical fluids coming from the Earth’s mantle. Another or additional explanation for the presence of diamonds in crustal rocks is the formation of extreme shock-like pressure at the transition of a supercritical fluid or melt into the undercritical state (see the whisker-like diamond needles in quartz crystals from Zinnwald, Thomas (2025b) [19]. Another surprising discovery is the presence of graphite and diamond aggregates or crystals (see Figure 8) in quartz, characterized by numerous melt and fluid inclusions, as the second explanation underline (Table 4).

Figure 8: Raman spectrum of diamond (D)-containing graphite (Gr) in pegmatite quartz (Qu8) from the Sauberg mine near Ehrenfriedersdorf. The leaf-like graphite (right at the top) is 80 µm deep from the sample surface. The FWHM is 77 cm^-1 (FWHM is Full Width at Half Maximum).

Table 4: Results of the Raman spectroscopic determination of diamond in pegmatite quartz (Qu8) from the Saubach mine near Ehrenfriedersdorf, Central-Erzgebirge/ Germany.

1s: Standard Deviation, FWHM: Full Width at Half Maximum, n: Measured Points (at different points of the leaf; see Figure 8).

Discussion

In this contribution, we show, using the well-studied example from Ehrenfriedersdorf/Germany, that melt inclusions provide important information about element distribution. During the determination of water by Raman spectroscopy [3], a natural example of a solvus curve (water versus temperature) is identified for the first time. Further studies on the same example (Ehrenfriedersdorf) brought more information about the behavior of a couple of elements. Important ore-deposit-forming elements exhibit characteristic distributions that are Gaussian, Lorentzian, or mixed, such as the Voigt distribution. Very important points are the critical ones. It is remarkable that this point ± coincidence with all solvus, Gaussian, Lorentzian, and Voigt curves. Why is that so?. It is well known that, above the critical point, the matter is in the supercritical state. The presence of a Gaussian and/ or Lorentzian distribution indicates that we have not only a sharp peak but also a relatively large temperature range, from about 850 to 600°C (see Figure 1). Unusual physicochemical properties characterize this range: extremely low viscosity and extremely high diffusivity (see Thomas et al., 2019 [4], 2022 [5], 2023 [20]). These conditions enable the extreme enrichment of normally rare elements. By the transition of the supercritical fluid or extremely water-rich melt from the mantle region into under-critical conditions at the crustal level, a first crystallization of ore-forming minerals occurs – the trapping of such minerals (e.g., cassiterite, beryllonite, hambergite, and others with stochiometric composition) follows the Lorentzian distribution. A single dominant process with negligible variability controls such minerals. Similar processes also occur during a high-temperature hydrothermal stage (see Borisova et al. 2012) [10]: the trapping of Zn-rich minerals (88, 121, and 46,049 ppm) in two fluid inclusions, only some micrometers apart, in pegmatite quartz. The trapped mineral phase, according to Raman (room temperature), is a daughter crystal of K₂[ZnCl₄] (flinteite) in both fluid inclusions in pegmatite quartz (Qu8) from the Ehrenfriedersdorf tin deposit, with high Rb, Cs, and Br concentrations in the flinteite formula.

The finding of co-trapped high-pressure minerals, including diamond, moissanite, coesite, lonsdaleite, kumdykolite, reidite, as well as high-pressure orthorhombic cassiterite, in nearly all parts of the Ehrenfriedersdorf deposit (diamonds and BN also in the greisen part), underscores the significance of the supercritical state and the decisive role of supercritical fluids or extreme water-rich melts (see e.g. Thomas and Trinkler, 2024) [21]. First hints of such a scenario were discussed by Schütze et al. 1983 [22]. These authors, based on their studies, conclude that the inducing processes are subduction- or subfluence-related and involve older ocean crust. Our studies provide evidence that supercritical, water-rich melts and/or fluids have a significant influence on the entire Variscan ore mineralization in the Central Erzgebirge and elsewhere. The often-cited evidence that, for example, a large part of the tin is transported as orthorhombic cassiterite underscores the active interaction between mantle and crust. By the existence of orthorhombic cassiterite crystals in the supercritical fluid, it is plausible that this fluid is also saturated in Sn. That is compatible with the new discussion that a lot of water is concentrated in the deep Earth (see, for example, Mohn et al., 2025) [23-25] and may move episodically from the mantle deeps into the crust (see also Thomas et al., 2025a) [15].

Acknowledgments

We thank Jean-Louis Vigneresse for initiating this contribution, which will address some differences in his viewing and our understanding of the meaning of melt inclusions for ore-forming processes.

References

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DOI: 10.31038/GEMS.2026852

Abstract

Barite–galena mineralization constitutes an important component of Nigeria’s industrial mineral and base metal resource potential. This study presents an integrated geophysical investigation of structurally controlled mineralization within the Precambrian basement terrain of Iyamitet, Obubra Local Government Area, south-eastern Nigeria. This deployment of ground magnetic and Very Low Frequency Electromagnetic (VLF-EM) methods produced robust interpretation of concise data for delineating Barite–Galena mineralisation within the Iyamitet area. Residual Magnetic Intensity, Analytical Signal, 3-D Euler Solution, and Q-Factor maps were generated to identify conductive sulphide-bearing structures, lithological contacts, hydrothermal alteration zones, and fracture-controlled mineralized pathways. The magnetic maps revealed pronounced low magnetic anomalies associated with hydrothermal alteration and sulphide mineralisation, while the Analytical Signal and Euler Solution maps delineated dominant NE–SW and NW–SE structural trends interpreted as faults, fractures, and vein systems favourable for hydrothermal fluid migration and ore emplacement. The Q-Factor map derived from VLF-EM filtering revealed conductive linear anomalies corresponding to sulphide-rich fracture zones spatially associated with magnetic lows and clustered Euler depth solutions. The mineralisation potential zonation further identified moderate to high prospective zones characterized by structurally controlled conductive and resistive signatures indicative of hydrothermal Pb–Zn–Barite mineralisation. The continuity and spatial distribution of these anomalies suggest significant economic potential for Barite–Galena deposit development within the area.

The integrated interpretation confirms that the Iyamitet area is highly prospective for structurally controlled hydrothermal Barite–Galena mineralisation. The results demonstrate the effectiveness of integrated geophysical techniques for mineral exploration in structurally complex basement terrains and provide a scientific framework for targeted drilling and sustainable mineral resource development in southeastern Nigeria.

Keywords

Barite–galena, Basement mineralization, Magnetic survey, VLF-EM, Structural controls, Nigeria

Introduction

Barite (BaSO₄) is a high-density industrial mineral widely used in petroleum drilling, chemical industries, and radiation shielding applications, while galena (PbS) remains the principal ore of lead globally. In Nigeria, barite–galena mineralization is commonly associated with hydrothermal vein systems controlled by tectonic structures within basement and sedimentary terrains. Oladapo et al. (2011) [1], Ezekwesili et al. (2012) [2] and Obaje et al. (2009) [3], the aforementioned carried out related studies on Galena and Barite deposit in different location within Nigeria. The workdone by Akinde et al. 2019 [4] recommended the use of multiple geophysical techniques to uncover the presence of Barite – Galena Mineralisation since the results of Vertical Electrical Sounding alone could not properly delineate the presence of Barite-Galena mineralisation. Recently, Akinde et al 2026b [5] adopted magnetotelluric to delineate structural traps hosting groundwater bodies, the work focussed on the important roles played by structural traps which is also relevant in hydrothermal mineral flows. Despite early documentation of barite occurrences, many deposits remain poorly characterized geophysically, limiting effective exploration and resource development. In the Iyamitet area, artisanal mining has revealed occurrences of barite–galena assemblages within quartz-schist host rocks. This study thus integrates the deployment of ground magnetic and Very Low Frequency VLF geophysical methods to characterize structural controls, delineate mineralized zones, and evaluate Iyamitet’s Barite-Galena mineralistion deposit (Figures 1 and 2).

Figure 1: Geological Exploration Base Map of the Iyamitet Prospect Area Showing Hydrographic Network, Exploration Access Routes, Vegetated Terrain and Dominant NE-SW Trending Mineralised Structural Lineament Interpreted to Control Subsurface Mineralisation.

Figure 2: Elevation and 3-D Elevation Map of the Study Area.

Geological Setting

The study area lies within the southeastern Nigerian Precambrian basement complex, comprising schists, phyllites, gneisses, and granitic intrusions. Regionally, the Oban Massif forms part of a tectonically reworked crustal block influenced by Pan-African orogeny. Hydrothermal mineralization is structurally controlled and typically occurs along fracture-filling systems within metamorphic lithologies. Oden et al. (2012) [6] carried out comparative analysis of fracture lineaments in Oban and Obudu areas, SE Nigeria.

Materials and Methods

Magnetic Survey

Ground magnetic data were collected along 21 traverses with 20 m station spacing. Data enhancement techniques included:

• Reduction to magnetic equator (RTE).
• Upward continuation filtering.
• Pseudo-gravity transformation.
• Analytical signal analysis.
• Spectral depth estimation.
• 3-D Euler deconvolution.

Electromagnetic Survey

VLF-EM data were acquired using ABEM WADI equipment along established grid lines. Filtered real (Q-factor) anomalies were used to map conductive structures interpreted as fracture systems associated with mineralization (Figures 3.1-3.3).

Figure 3.1: Magnetic Intensity Map of Iyamitet Obtained from Ground Magnetic Data.

Figure 3.2: Ground Magnetic Intensity Map of Iyamitet (Upward Continued to 20 m.

Figure 3.3: Reduction to the Equator Map of Iyamitet

Results

1. Integrated geophysical interpretation revealed: A dominant NE–SW-trending structural corridor controlling mineralization.
2. Low magnetic intensity zones coinciding with high pseudo-gravity anomalies.
3. Conductive zones delineated by VLF-EM corresponding to fracture systems.
4. Depth estimates indicating shallow (<10 m) to intermediate (~50 m) mineralized bodies.
5. Analytical signal and Euler solutions confirmed structural controls on mineral emplacement (Figures 4.1-4.7).

Figure 4.1: Residual Ground Magnetic Intensity Map of Iyamitet.

Figure 4.2: Analytical Signal Map of the Study Area.

Figure 4.3: 3-D Euler Solution Map of Iyamitet (SI = 0).

Figure 4.4: 3-D Euler Solution Map of Iyamitet (SI = 0.5).

Figure 4.5: Radially Averaged Spectrum Map of Iyamitet.

Figure 4.6: Raw Real Component Map of Iyamitet.

Figure 4.7: Q-Factor Map of Iyamitet Obtained from Filtering of the Raw Real Data.

Discussion

The VLF-EM and ground magnetic maps of the Iyamitet area reveal significant subsurface structural and lithological characteristics that strongly favour the occurrence of structurally controlled barite–galena mineralisation. The integration of the Residual Magnetic Intensity Map, Analytical Signal Map, Euler Depth Solutions, and Q-Factor filtering maps provides convincing evidence for fracture-controlled hydrothermal mineral deposition within the study area. The Residual Ground Magnetic Intensity Map (Figure 4.1) shows pronounced magnetic contrasts characterized by alternating zones of high and low magnetic intensities. The dominant low magnetic closures (blue to cyan colours) observed particularly within the central and northeastern portions of the study area are interpreted as zones of hydrothermal alteration, demagnetization, and structural weaknesses. These low magnetic signatures are typical of sulphide-bearing mineralized zones because galena (PbS) and barite (BaSO₄) are essentially non-magnetic minerals and are commonly associated with altered host rocks that have undergone destruction of magnetic minerals such as magnetite during hydrothermal fluid circulation. Conversely, the moderate to high magnetic anomalies (yellow, red, and purple zones) surrounding the low magnetic corridors likely represent relatively fresh basement rocks or ferruginized lithologies which acted as competent host rocks and structural traps for mineralizing fluids. The sharp magnetic gradients observed between magnetic highs and lows indicate lithological contacts and faulted boundaries that served as migration pathways for hydrothermal solutions responsible for barite–galena emplacement. The Analytical Signal Map (Figure 3.5) further enhances the delineation of structural contacts and subsurface discontinuities independent of magnetization direction. The strong analytical signal amplitudes trending predominantly NE–SW and NW–SE suggest the presence of deep-seated fracture systems and fault intersections. These structural trends are highly significant because barite–galena mineralisation within the Benue Trough and adjoining basement terrains of Nigeria is commonly controlled by fault systems, shear zones, and fracture networks. The analytical signal closures around the central part of the map indicate concentrated subsurface structural disturbances which may represent mineralized veins or hydrothermal conduits. The clustering of anomalies around these lineaments strongly suggests structurally localized sulphide mineralisation. The intersection zones of the lineaments are especially important because they usually provide enhanced permeability for ascending hydrothermal fluids and consequently form favourable sites for barite and galena deposition.

The 3-D Euler Solution Maps (Figures 4.3 and 4.4) provide depth estimations and geometric characterization of the causative bodies. The Euler solutions with Structural Index (SI = 0) are indicative of contact-type geological structures such as faults, lithological boundaries, and vein systems. The clustering of Euler depth solutions along elongated trends suggests the presence of continuous structural corridors. These corridors are interpreted as fracture-controlled mineralized zones favourable for epigenetic barite–galena deposition. Similarly, the Euler solutions with SI = 0.5 indicate dyke-like or vein-like bodies occurring at shallow to intermediate depths. The concentration of these solutions within the central and southern parts of the study area strongly supports the existence of vein-controlled mineralisation. Barite and galena deposits in Nigeria are commonly emplaced as hydrothermal veins along fault planes and fractures; therefore, the observed Euler signatures strongly favour this style of mineral occurrence. The depth estimates ranging approximately from near-surface to moderate depths further increase the economic significance of the mineralization because shallow hydrothermal sulphide systems are generally more accessible for exploration and possible exploitation. The Q-Factor Map (Figure 4.7), derived from filtering of the VLF-EM real component data, provides one of the strongest evidences for conductive mineralized structures within the area. The map reveals several conductive zones represented by anomalous closures and linear conductive trends. These conductive anomalies are interpreted as sulphide-rich fracture zones because galena exhibits relatively high electrical conductivity compared to surrounding country rocks. The conductive linear features identified on the Q-Factor map correspond spatially with the structural trends mapped from the magnetic interpretation. This correlation strongly validates the interpretation of interconnected fracture-controlled mineralized systems. The conductive anomalies labelled around the central and western portions of the study area are particularly significant because they coincide with zones of magnetic lows and Euler structural clustering.

Such coincidence between:

• conductive VLF-EM anomalies,
• magnetic discontinuities,
• analytical signal peaks,
• And a clustered Euler depth solution is a classical geophysical signature of structurally controlled hydrothermal sulphide mineralisation.

The VLF-EM response also suggests that the mineralized fractures are likely steeply dipping and laterally extensive. This interpretation is consistent with hydrothermal barite–galena veins commonly associated with tectonic reactivation and fluid migration along regional fractures. Geologically, the integrated geophysical signatures suggest that the Iyamitet area experienced intense tectonic deformation that created interconnected fracture systems which later became conduits for hydrothermal fluids rich in barium, lead, zinc, and associated sulphides. The hydrothermal fluids precipitated barite and galena within structurally weak zones, especially along lithologic contacts, fractures, and fault intersections.

The spatial association of:

• low magnetic anomalies,
• conductive VLF-EM zones,
• fracture-related Euler clusters,
• and analytical signal lineaments therefore strongly favours the presence of structurally controlled barite–galena mineralisation within the Iyamitet area. The integrated interpretation of the VLF-EM and aeromagnetic and ground geophysical datasets confirms that the Iyamitet area possesses favourable subsurface structural architecture and hydrothermal conditions for economically viable barite–galena mineral deposition (Figure 4.8).

Figure 4.8(a-g): Maps Showing Synthesis of Results.

The synthesis maps (Figure 4.8 a–g) show the integrated interpretation of the geophysical datasets within the study area. Variations in colour patterns indicate contrasts in subsurface properties, structural features, and mineralisation potential. High-anomaly zones represented by red, pink, and purple colours suggest possible mineralised or conductive regions, while blue and green zones indicate relatively resistive formations.

The maps reveal structurally controlled anomalies associated with fractures, faults, and alteration zones that may have served as pathways for mineralising fluids. Concentrated and continuous anomalous zones observed in the central and lower sections are interpreted as favourable targets for mineralisation. Generally, the synthesis results indicate significant subsurface heterogeneity and delineate prospective zones suitable for further detailed exploration, trenching, and drilling activities [7-9] (Figure 5).

Figure 5: Mineralization Potential Zonation Map of Iyamitet.

Conclusions

The mineralisation potential zonation map of the Iyamitet area reveals well-defined anomalous zones characterized by structurally controlled conductive and resistive signatures favourable for Barite–Galena mineralisation. The distribution of the anomalous zones suggests hydrothermal fluid emplacement along fractures, faults, and lithological contacts within the subsurface. The moderate to high mineralisation potential zones identified on the map indicate significant concentration of sulphide-bearing minerals, particularly galena associated with barite veins. The continuity and spatial extent of the favourable zones further suggest that the Iyamitet area possesses appreciable economic potential for Barite–Galena deposit development. The integrated interpretation therefore confirms that the area is prospective for hydrothermal Pb–Zn–Barite mineralisation and justifies detailed exploration involving trenching, core drilling, geochemical sampling, and reserve estimation to delineate the ore body and evaluate its commercial viability.

Acknowledgement

The author acknowledges field support and geophysical data acquisition assistance from relevant technical teams, Contribution of late Mr Romanus E.J. to his blessed memory is well appreciated in this research work.

References

1. Oladapo MI, Oladapo-Adeoye OO (2011) Geophysical Investigation of Barite Deposit in Tunga, Northeastern Nigeria. International Journal of the Physical Sciences 6: 4760-4774.
2. Ezekwesili GE, Celestine OO, Chidozie IP (2012) Structural styles and economic potentials of some barite deposits in the Southern Benue Trough, Nigeria. Romanian Journal of Earth Sciences 86: 27-40.
3. Obaje NG (2009) Geology and Mineral Resources of Nigeria, London. Springer Dordrecht. Pp 221.
4. Akinde AS, Dikedi PN, Ogharandukun (2019) Adopting Geo-electric Approach in Mineral Characterisation of Iyamitet Settlement. Journal of Geology and Geophysics 8: 1-7.
5. Akinde AS, Afuwai CG, Gata TB (2026b) Efficacy of Magnetotelluric Method in Delineating Structural Traps Hosting Groundwater Bodies. International Journal of Natural and Practical Sciences 8: 1-3.
6. Oden MI, Okpamu TA, Amah EA (2012) Comparative analysis of fracture lineaments in oban and obudu areas, SE Journal of Geography and Geology. 4. ISSN 19169779. E-ISSN 1916-9787. Published by Canadian Center of Science and Education.
7. Geosoft (2005) Quick start Tutorial and User guide applications (Electronic Version); Oasis Montaj data processing and analysis (DPA) system for Earth Science applications, Euro Technologies, 2007, Version.5.1.4, 242.
8. Oyeniyi TO, Salami AA, Ojo SB (2016) Magnetic Surveying as an Aid to Geological Mapping: A Case Study from Obafemi Awolowo University Campus in Ile-Ife, Southwest Nigeria. Ife Journal of Science 18.
9. Spector A, Grant FS (1970) Statistical Models for interpreting aeromagnetic data. Geophysics 35: 293-302.

DOI: 10.31038/GEMS.2026814

Abstract

In this short contribution, we show the complexity of mineral-forming processes using the example of the Variscan Ehrenfriedersdorf tin deposit and, by way of a further unexpected example, the occurrence of typical alkaline mineralization as an alkaline remnant of vuoriyarvite-K in fluorite in the tin paragenesis.

Keywords

Raman spectroscopy, Vuoriyarvite-K, Strong Alkalinity, Variscan Tin Mineralisation, Ehrenfriedersdorf

Introduction

The author and coauthors have previously shown that exceptional minerals are present in the classic tin deposit of Ehrenfriedersdorf. To these belong nepheline crystals (Figure 1), and the high-pressure minerals diamond, moissanite, orthorhombic cassiterite, and others [1]. In granite quartz, there are melt inclusions of peralkaline granitic composition [2]. That means, at last, that the famous and very rich tin deposits from the Variscan granites around Ehrenfriedersdorf are not the result of a single granite intrusion or a single mineral-forming process. Also, supercritical fluids and melts participate in mineral-forming processes. In Thomas and Rericha (2023) [3], a new microphotograph of nepheline (Figure 4) is presented, and the nepheline composition and that of the surrounding K-feldspar are tabulated. According to melt inclusion studies, there are generally two granite trends: the peraluminous and the peralkaline, as well as an alkaline mineralization, which are detectable to varying degrees and are roughly equal in abundance. In Figure 1, exemplarily shown are the nepheline aggregates in quartz from Ehrenfriedersdorf (Sauberg mine). The figure is from Thomas et al. (2006) [2] and serves as a reminder that nepheline is not typically present or possible in a granite environment; it is nevertheless present!

Furthermore, we will show that remnants of older mineral inclusions in younger ones demonstrate that multi-stage processes are responsible for the exceptional tin-rich deposit. One such example is the occurrence of vuoriyarvite-K [K₂(Nb,Ti)₂(Si₄O₁₂)(O,OH)₂ · 4H₂O]. Simple one-sided geochemical studies cannot solve the complex puzzle of mineral-forming processes.

Figure 1: Back-scattered electron images of nepheline inclusions in feldspar (a) and quartz (b) from the Ehrenfriedersdorf pegmatite. Nepheline shows kalsilite and aegirine exsolution lamellae. Ae – aegirine, Kfs- K-feldspar, Kls – kalisilite, Ne-nepheline, Qtz – quartz.

Methods and Samples

Raman Spectroscopy

Raman spectra were recorded with the EnSpectr Raman microscope RamMics R532 in the spectral range of 0 – 4000 cm^-1 using a 50 mW single-mode 532 nm laser, an entrance aperture of 20 μm, a holographic grating of 1800 g/mm, and a spectral resolution of 4 cm^-1. Depending on the grain size, we used microscope objectives with magnifications between 3.2x and 100x. As 100x objective, we used the long-distance LMPLFLN100x from Olympus. The laser energy on the sample can continuously be adjusted down to 0.02 mW. Generally, we used 30 mW on the sample for the present study. The positions of the Raman bands were controlled before and after each series of Si-band measurements using a single-crystal semiconductor-grade silicon chip. The run-to-run repeatability of the line position (from 20 measurements each) was ± 0.3 cm-1 for Si (520.4 ± 0.3 cm^-1) and 0.5 cm^-1 for diamond (1332.3 ± 0.5 cm^-1 over the range 80 – 2000 cm^-1), respectively. As a diamond reference, we used a water-clear natural diamond crystal (from Brazil). An essential advantage of the Raman spectrometer is the simple zero-point calibration. For the identification of minerals using Raman microspectroscopy, we used the RRUFF database and the Crystal Sleuth program [4].

Reference Sample

As a reference for our study, we used crystals of vuoriyarvite-K [K₂(Nb,Ti)₂(Si₄O₁₂)(O,OH)₂ · 4H₂O] from Pitkäranta (Lupikko shaft) on the northern shore of Lake Ladoga, Karelian SSR/Russia, which were proven with X-ray techniques (sample from Crystal-Treasure, Kassel).

Figure 2 shows an example of the nearby colorless reference vuoriyarvite-K material from Pitkäranta. The spectrum of reference vuoriyarvite-K is given in Figure 3. The RRUFF data file (RRUFF ID R070703 for a 532 nm laser) of vuoriyarvite-K is a little different, but similar. The sample is from another place: Kirov Mine, Kuukisvumchorr Mt. Khininy Massif, Kola Peninsula, Russia.

Figure 2: Reference crystals of vuoriyarvite-K from Pitkäranta/Lake Ladoga, Russia. The spectrum of reference vuoriyarvite-K is given in Figure 3.

Figure 3: Reference Raman spectrum of the vuoriyarvite-K [K₂(Nb,Ti)₂(Si₄O₁₂)(O,OH)₂ 4H₂O] from Pitkäranta (Lupikko shaft) on the northern shore of Lake Ladoga, Karelian SSR/Russia, which was proven with X-ray techniques.

Sample

As a sample, we used a polished mostly green fluorite crystal from a cassiterite paragenesis (Sn-70 from the Prinzler ancillary vein) with a small violet rim at one end, measuring about 2.5 x 2.6 x 0.2 cm. Figure 4 shows the used sample.

Figure 4: Double-sided polished fluorite sample. The green part contains many remnants of vuoriyarvite-K (Vuo). In the fluorite part, the market with arrows contains a high concentration of this mineral.

Results – Key Observations

The Raman spectrum of the fluorite host is given in Figure 5. According to Chukanov and Vigasina (2020) [5], the fluorite is characterized by a single strong Raman band at about 322 cm¹.

Figure 5: Raman spectrum of the fluorite host of vuoriyarvite-K. The lines beside the strong, sharp fluorite line at 320.7 cm^-1  may be fluorescence lines.

The fluorit sample contains many small corroded crystal remnants of vuoriyarvite-K. By the refractive index (fluorite n = 1.44, vuoriyarvite-K = 1.73), the crystals under the microscope are well seen. During the study of the large fluorite crystal, unusual, corroded, and more or less colorless vuoriyarvite-K crystals were observed (Figures 6 and 7).

Figure 6: Vuoriyarvite-K (Vuo) remnant in the fluorite matrix.

Figure 7: A larger vuoriyarvite-K (Vuo) crystal in fluorite (CaF₂) surrounded by dark, undefined material (product of decomposition – maybe oxides of Nb).

Figures 8 and 9 show the Raman spectra of the vuoriyarvite-K-like mineral.

Rastsvetaeva and Chukanov (2002) [6] reported that the labuntsovite group contains more than 20 minerals, with compositions that vary widely. For example, K₂O can vary from 0 to 15%. From most minerals of this group, there are no Raman spectra. Therefore, the interpretation of the labuntsovite-group minerals by Raman and IR spectroscopy is very difficult.

Figure 8: Raman spectrum of vuoriyarvite-K-like mineral in fluorite from the Ehrenfriedersdorf tin deposit, Sauberg mine. The sharp line at 320.5 cm^-1 is from the fluorite host.

Figure 9: Raman spectrum of a vuoriyarvite-K-like mineral. This spectrum corresponds well to the RRUFF ID R070703 (Lafuente et al., 2016). The 320.3 cm^-1 Raman band comes from the fluorite host.

Discussion

The labuntsovite group minerals, with the here described vuoriyarvite-K, are typical of alkaline massifs of the Kola Peninsula and Greenland [6,7]. Here in the Variscan tin deposit of Ehrenfriedersdorf in the Central-Erzgebirge, Germany, are the occurrences of such minerals as nepheline, kalisilite, and vuoriyarvite-K-like minerals, at first sight untypical. However, they clearly demonstrate that the classic tin mineralization is not the result of a single enrichment process from the surrounding Sn-granite. The formation of the tin deposit results from multiple evolutionary stages, including alkaline stages and the interaction of supercritical fluids coming from Earth’s mantle. In such fluids, chromatographic processes of element enrichment and depletion can occur.

References

1. Thomas R, Recknagel U, Rericha A (2023) A Moissanite-diamond-graphite paragenesis in a small beryl-quartz vein related to the Variscan tin-mineralization of the Ehrenfriedersdorf deposit, Geosciences. 13: 1-13.
2. Thomas R, Webster JD, Rhede D, Seifert W, Rickers K, et al. (2006) The transition from peraluminous to peralkaline granitic melts: Evidence from melt inclusions and accessory Lithos 91: 137-149.
3. Thomas R, Rericha A (2023) The function of supercritical fluids for the solvus formation and enrichment of critical Geol Earth Mar Sci. 5(8): 1-4.
4. Lafuente B, Downs RT, Yang H, Stone N (2016) The power of database: The RRUFF In Highlights in Mineralogical Crystallography; Armbruster T, Danisi RM (Eds.). De Gruyter: Berlin, München, Boston. 1-30.
5. Chukanov NV, Vigasina MF (2020) Vibrational (Infrared and Raman) spectra of minerals and related compounds. Springer Mineralogy. Pg: 1376.
6. Rastsvetaeva PK, Chukanov NV (2002) X-ray diffraction and IR spectroscopy study of the labuntsovite-group Crystallography reports. 47: 939-945.
7. Pekov IV (2000) Lovorzero Massif. Moscow. Pg: 480.

DOI: 10.31038/GEMS.2026851

Abstract

This study investigates topaz from Altenberg pycnite rock and the Schneckenstein deposit, with emphasis on fluorine variability and the occurrence of carbon phases. Raman spectroscopy was used to characterize topaz, determine fluorine contents, and identify inclusions. The results show that Altenberg pycnite is compositionally heterogeneous, containing both fluorine-rich and fluorine-poorer domains, whereas Schneckenstein topaz is associated with unusual Ti-rich topaz-rutile inclusions and small spheres of diamond-like carbon and diamond. The discovery of diamond and related carbon phases in both occurrences is interpreted as evidence that supercritical fluids and/or deeper melts participated in the formation of these rocks and minerals.

Keywords

Topaz, Pycnite, Altenberg, Schneckenstein, Raman spectroscopy, Fluorine, Diamond, Diamond-like carbon, Supercritical fluids, Mineral inclusions

Introduction

Topaz [Al₂SiO₄F_x(OH)_2-x], normally an orthorhombic F-rich Generally, topaz is orthorhombic and has 2V axis angles of 50-68°. Especially the pycnite from Altenberg is a little bit unusual. The c-axis of the topaz crystals makes an angle of about 12° with mineral, is of mineralogical and genetic interest. The F-content is up to 20.65% F, and x=1.4-2.0. If we speak in%, we mean always [%(g/g)], because wt% is not a permitted unit since 1988 [1]. First, we will present some simple mineralogical characteristics. The F-content and the lattice angle 2V characterize mainly the topaz. Table 1 presents topaz data from a selection of crystals [2-4].

Table 1: Results of the X-ray determination of fluorine in topaz according to Thomas (1979 and 1982) [2-3] using the Ribbe and Rosenberg (1971) [5] method. The (200) peak of NaCl is the reference. Δ₍₀₂₁₎=2ΘNaCl₂₀₀ – 2Θ_{Topaz 021}, the optical 2V-determination, and the Raman determination using equation (1) presented by Loges et al. 2025 [6].

Note that in the original table (Thomas, 1979 and 1982) [2,3], the names of the topazes from San Luis Potosí, Mexico, and Ouro Preto, Brazil, are mixed up.
n.b. – means the samples are not present anymore.

Generally, topaz is orthorhombic and has 2V axis angles of 50-68°. Especially the pycnite from Altenberg is a little bit unusual. The c-axis of the topaz crystals makes an angle of about 12° with the a-b plane. Using conoscopic mode on a fine-grained fraction of topaz/pycnite cleavage lamellae reveals optical biaxial symmetry. However, that is not clear proof because many grains show unclear two axes, unlike the Schneckenstein topaz. In the same riddle fraction from Schneckenstein, most crystals show a clear biaxial behavior. According to Gatta et al. (2006) [7], topaz is the stable phase above 12 GPa and 1100°C. The Raman spectroscopic determination of F according to Loges et al. (2025) [6] and Thomas (2026a) [4] showed that the Altenberg topaz contain an relative high portion of topaz with lower fluorine content: F=15.66 ± 2.01% (n =11). Similar lower results were obtained for the black topaz inclusions in normal Topaz from Schneckenstein/W-Erzgebirge: F=16.52 ± 0.76% (n=10). However, we do not want to repeat the results reported in Thomas (2026a) [4] but rather present completely new results. That concerns the topaz and the not-so-rare diamond inclusions. The diamond is, of course, a little bit different from a perfect diamond because, on the long journey from the place of origin to the place of deposition via supercritical fluid (SCF), it undergoes extreme thermal and pressure stress. The spectren look, in a sense, like nanodiamonds.

Methods and Samples

Raman Spectroscopy

We used the Raman spectroscopy to identify the topaz-specific Raman bands and determine the fluorine concentration according to Loges et al. (2025) [6] and Thomas (2026a) [4]. Furthermore, Raman spectroscopy was very helpful in distinguishing the Ti-rich topaz from the wine-yellow Schneckenstein topaz. Another task is to provide clear proof of diamond and DLC (diamond-like carbon) in topaz from Altenberg and Scheckenstein. According to Zaitsev (2001) [8], the more correct term is “diamond-like materials.” The data presented here were acquired using an EnSpectr Raman microscope (RamMics R532). Measurements covered the spectral range from 0 to 4000 cm⁻¹ and were conducted with a single-mode 532 nm laser operating at a maximum output of 50 mW. Analytical settings included a 20 µm entrance aperture, a holographic grating of 1800 g mm⁻¹, and a spectral resolution of approximately 4 cm⁻¹ (generally for the whole range). For most analyses of different topaz crystals, a long-working-distance Olympus LMPlanFL 100× objective was used. Using lower magnifications introduces the problem of excitation of different F-bearing areas within the topaz crystal (lamellas). Laser power at the sample surface was continuously adjustable down to 0.02 mW. Higher powers (up to 50 mW) were applied only for overview measurements. Generally, for F-concentration measurements, we used 0.9 mW on the sample in the range from 75 to 900 cm⁻¹ to prevent local heating. For the measurement of the OH band of topaz, the 0-4000 cm⁻¹ range was used (which is not, however, the object of the present study). For OH-rich topazes transported via supercritical fluids, we have determined an X_OH=0.83 ± 0.08 (at another place). Raman band positions were calibrated before and after each measurement series using the Si band of a semiconductor-grade single-crystal silicon chip. Based on 20 repeated measurements, run-to-run reproducibility was ± 0.2 cm⁻¹ for silicon (520.2 ± 0.2 cm⁻¹) in the measuring range 75 to 900 cm⁻¹.

Fluorine Determination

The Raman method for fluorite determination is described in detail by Thomas (2026a) [4] and is based on the method of Loges et al. (2025) [6].

Samples

A detailed description of the localities of the topaz crystals used is in both books: Topas (1997) [9] and the licensed edition topaz (2011) – [10]. A comprehensive description of the pycnite-bearing rock, including analyses, from Altenberg/E-Erzgebirge, Germany, is provided by Recknagel (1969), Weinhold (2002) [11], and Lausch (2024) [12]. For the preparation of the Altenberg samples, diamond was never used; for polishing, an alumina suspension (50 nm, pH 7-8, 200 g/l) was used. The topaz from the Schneckenstein dates from a visit to the Schneckenstein in October 1960, together with Peter Haupt. In the study, only centimeter-sized natural crystal cleavages perpendicular to the c-axis are generally used (Figure 1). A description of the occurrence of the “Schneckenstein” is given in Lahl (2012) [13].

Figure 1: Topaz crystal cleavage with Ti-rich topaz and DLC inclusions. The scale is 1 cm.

Results

New Results on Topaz (Pycnite) from Altenberg

The pycnite variety of topaz is not uniform in its F content. The main part has a F-concentration of 19.55 ± 0.18% (n=13), whereas in a smaller part the concentration is lower: F=15.66 ± 2.01% (n=11). By the mixture, the cleavage is not uniform. A 12° “inclination of the c-axis against the a–b plane” is a big red flag that what we are seeing is not a change from orthorhombic → monoclinic topaz, but rather an orientation/domain effect (misindexing, twinning, or internal lattice bending/mosaicity). However, as we will see, the high-pressure modifications (monocline system) are not absolutely impossible. However, the pressure should exceed 24 GPa. Another important new finding is the proof of diamond or DLC (diamond-like carbon).

New Results on Topaz from Schneckenstein

Normally, the topaz from Schneckenstein is yellow, water-clear, and contains only a small number of solid mineral inclusions (besides many fluid inclusions, which often contain sassolite daughter phases (see Gilg and Thomas in: Topaz, 2011) [10]. Later, we will see that the fluid inclusions are, as a rule, secondary, and that the F-concentration-temperature diagram (Figure 7 in Thomas 2026a) [4] has only a limited meaning. During the study of samples collected in 1960, we found that, in addition to black spherical topaz-like aggregates (which contain some graphite), topaz also contains small brown-orange spheres, which are topaz with significantly lower F-content. Striking inclusions in topaz (Figure 2) are composed of rutile and topaz; the relative amount of rutile in these inclusions is similar. So, the composition of this inclusion type is ± the same, suggesting a homogeneous formation. That means that this inclusion was at high temperature and high pressure, a single homogeneous mineral phase – in our case, a Ti-rich topaz, in analogy to the Ge- and Ga-rich analog of topaz, the krieselite (Spivak et al., 2021 [14] and Setkova et al., 2024) [15]. A further large surprise was the finding of small spheres (up to 10 µm in diameter) of diamond and DLC in the topaz host, as well as DLC in the old topaz-rutile inclusions in the same host. The finding of diamond spheres in both topaz types (pycnite from Altenberg, topaz from Schneckenstein) demonstrates that during the formation of the rock (pycnite-rock, topaz breccia), supercritical fluids (SCF) and/or supercritical melts (SCM) participated more or less strongly in the formation.

Figure 2: Inclusion in topaz (Toz) from Schneckenstein. The red and green minerals are mostly rutile, with a little anatase, and the grey parts are Ti-rich topaz (Ti-Toz). And the black crystals are DLC (diamond-like carbon).

Such quite unusual inclusions, if present, always have the same bulk composition. Of course, that is strictly speaking speculative; however, it looks like so. The substitution of Al by Ti⁴+ ions in the octahedral position is conceivable and, under HT and HP, realistic. The availability of titan is also important. There are well-formed topaz crystals that are very rich in ilmenite. Rutile and anatase are rare minerals and are (in the case of rutile) mostly arranged as a single small crystal in topaz. Anatase is absent in the matrix. The appearance of rutile and anatase clusters within topaz inclusions is remarkable. Figures 3 and 4 show examples of Raman spectra of the Ti-phases.

Figure 3: Raman spectrum of rutile in the inclusion (Figure 2, green crystal).

Figure 4: Raman spectrum of rutile in the inclusion (Figure 2, red crystal).

Figures 3 and 4 show the Raman spectra of rutile in the topaz inclusion in the matrix topaz crystal. Both spectra show remnants of TiO₂-II [16], which means that rutile and TiO₂-II coexisted at HP and HT (Figure 5). The black crystals in Figure 6 are DLC (diamond-like carbon) in the rutile-bearing topaz inclusion in topaz from Schneckenstein.

Figure 5: Raman spectrum of diamond-like carbon (DLC) in Figure 2.

The discovery of diamond-like carbon in the rutile-topaz inclusion in water-clear topaz prompted an intense search for diamonds in both studied topaz types. At first, we found diamond and DLC in pycnite-topaz quartz from Altenberg (Figure 6, and further down in Figure 8b).

Figure 6: DLC and diamond in pycnite-topaz from Altenberg.

Figure 7: Raman spectrum of diamond in pycnite-topaz from Altenberg (see Figure 6).

Figure 8a: Water-clear diamond (D) sphere in topaz (Toz) from Schneckenstein. The Raman spectrum is in Figure 8b.

Figure 8b: Typical first-order Raman spectrum of diamond in topaz from Schneckenstein.

Table 2 presents the Raman results for diamond and DLC in both samples (pycnite quartz, pycnite from Altenberg, and topaz from Schneckenstein.

Table 2: Results of the Raman measurement in the first-order range of diamond.

DLC – Diamond-like carbon, FWHM – Full Width at Half Maximum. n – number of measured crystals. n.d. – not detected.
Note: The intensity of the second and third-order Raman bands (e.g., the 2666 cm⁻¹ and the 3825 cm⁻¹ peaks) of diamond is very weak and has a large FWHM, and at 2664 cm⁻¹, the topaz from Schneckenstein shows a relatively strong Raman background of the (OH)-band in this region.

The discovery of diamond in topaz supports the idea of the formation of Ti-rich diamond, because the interaction of supercritical phases (SCF, SCM) from mantle depths can yield 5 GPa pressures exceeding. At the crustal level, with low HP and HT values, the Ti-topaz phase is clearly unstable. The rutile concentration in the Ti-rich topaz (Figure 2) looks very high. However, if we take the analyses given by Setkova et al. (2024) [15] for Ga- and Ge-rich topaz, we obtain the following relationship.

The pure Ge-substituted topaz analog is krieselite, with the formula Al₂(GeO₄)F₂, and the corresponding Ti-topaz analog is then Al₂(TiO₄)F₂. Besides the remnants of Ti-topaz analog, some topaz crystals contain black, spherical, or rounded crystals that also exhibit the topaz Raman spectrum (Figure 10).

Figure 9: Sum of Ga₂O₃ and GeO₂ versus fluorine in Ga, Ge-rich topaz according to Setkova et al. (2024).

Figure 10a: Black topaz (b-Toz)) in matrix topaz (Toz) from Schneckenstein.

Figure 10b: Raman spectrum of a black topaz inclusion in topaz from Schneckenstein.

Figure 10c: Raman spectrum of black Ti-rich topaz in the first-order carbon range.

The origin of the black color of the topaz is unclear. Possible materials are simple carbon, graphite, and DLC. More studies are necessary.

According to Beny and Piriou (1987) [18], all Raman lines are characteristic of topaz. Using long-time spectra in the narrow range of 50 to 200 and 400 to 900 cm⁻¹, three Raman lines appear, which are characteristic of anatase (145, 514, and 638 cm⁻¹). Obviously, that black Ti-rich topaz is on the verge of disintegrating into anatase and topaz. Additionally, several “anomalous” secondary O-H stretching bands can be seen in the region from 3200 to 3950 cm⁻¹ (see Beny and Piriou, 1987) [18]. The black coloring of the topaz is caused by DLC (see Figure 10c).

Interpretation

After the first proof of spherical diamonds in pycnite-topaz from Altenberg, we have found unusual rutile-(anatase)-rich topaz inclusions in topaz from Schneckenstein. The appearance of this type of inclusion suggests that the inclusion was primarily a homogeneous mineral phase – a very Ti-rich topaz. Because this inclusion is now present in the Schneckenstein topaz in the upper crust, we can assume that the primary homogeneous inclusion originated from greater depth. That is supported by the mostly spherical diamond crystals, which we interpret as evidence of the SCF and/or SCM originating from mantle depths [17,19]. This statement is supportedbynumerous studies over thepastfew years that have provided evidence of HP and HT minerals (cristobalite X-I, coesite, diamond, lonsdaleite, moissanite, orthorhombic cassiterite, stishovite, and others) in Variscan mineralization in the Lusatian Mts, the Saxon Granulite Mts., the Erzgebirge, Slavkovský les, and Thuringia [17, 19, 20-24]. At last, the ascent of SCF and SCM from the mantle into the crust is not a rare, insignificant event. Besides water and other volatiles (F, CO₂, CH₄, higher hydrocarbons), economic important elements like Sn (as orthorhombic cassiterite) and many others, as well as diamond, lonsdaleite, and moissanite, are evidence of the interaction between mantle and crust via SCF and SCM (e.g., Thomas and Rericha, 2025) [25-27]. Such phases from the mantle region must have left their traces not only as minerals but also in significant isotope shifts and trace element ratios.

Conclusions

In summary, Raman spectroscopy shows that topaz from Altenberg pycnite is compositionally heterogeneous and includes domains with distinctly lower fluorine contents, whereas topaz from Schneckenstein contains unusual Ti-rich topaz-rutile inclusions together with diamond-like carbon and diamond. The occurrence of these carbon phases in both localities, combined with evidence for high-pressure Ti-bearing inclusions, supports the interpretation that supercritical fluids and/or melts derived from greater depth contributed to the formation of the pycnite rock and Schneckenstein topaz. These results strengthen the view that mantle-derived components may have played a significant role in the evolution of Variscan mineralization in the Erzgebirge.

Acknowledgment

My interest in topaz began in October 1960 during a visit to the Schneckenstein crag with Peter Haupt, also an apprentice at the Zwickau coal mine, whose father participated in the drilling program by Prof. L. Baumann and S. Gorny.

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