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Article

Geochemical and Textural Features of Apatites from Propylitic to Advanced Argillic Hydrothermal Alteration Zones in the Sharlo Dere Area, Chelopech Cu-Au Deposit, Bulgaria

by
Radoslav Kalchev
1,*,
Irena Peytcheva
1,
David Chew
2,
Atanas Hikov
1 and
Elitsa Stefanova
1
1
Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
2
Department of Geology, School of Natural Sciences, Trinity College Dublin, D02 PN40 Dublin, Ireland
*
Author to whom correspondence should be addressed.
Minerals 2026, 16(2), 150; https://doi.org/10.3390/min16020150
Submission received: 19 December 2025 / Revised: 21 January 2026 / Accepted: 26 January 2026 / Published: 29 January 2026
(This article belongs to the Special Issue Critical Mineral Exploration: Innovations, Challenges and Future Directions)
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Abstract

Apatite is a widespread accessory mineral, which can provide information on the geochemical characteristics of magma and the conditions of hydrothermal alteration of the rocks in magmatic–hydrothermal deposits. This study aims to understand the relationships between the geochemical and textural features of apatites from diorite porphyries that have undergone different degrees of hydrothermal alteration in the Sharlo Dere area, Chelopech epithermal Cu-Au deposit, Bulgaria. The apatites were characterized by laser ablation–inductively coupled plasma mass spectrometry, scanning electron microscopy with energy-dispersive X-ray spectroscopy, electron probe microanalysis with wave-dispersive spectroscopy, optical cathodoluminescence and multi-element mapping. Magmatic apatites from “hematitic”, propylitic and propylitic-sericitic zones of alteration are distinguished by euhedral crystals with oscillatory zoning and brown luminescence in CL images. In quartz-sericitic alteration zones, apatite has a yellow CL response. Hydrothermally altered apatites in the diorite porphyries overprinted by advanced argillic alteration have corroded, irregular forms and pink-green luminescence. Apatite crystals of magmatic origin reveal high contents of chlorine, strontium, light rare earth elements (LREE), negative Eu anomalies and high LaN/SmN and CeN/YbN ratios. Hydrothermally altered or hydrothermal apatites are distinguished by their higher contents of Na2O, F, SO3, Y and middle rare earth elements (MREEs) and their low LaN/SmN and CeN/YbN ratios. The intensity of hydrothermal alteration affects the luminescence and major and trace element contents, including the rare earth element patterns in the apatites, implying apatite can be used as a geochemical indicator to study magmatic–hydrothermal ore deposits.

1. Introduction

Apatite is a calcium phosphate mineral with the general chemical formula of Ca5(PO4)3(F,Cl,OH) and is one of the most common accessory minerals in igneous rocks [1,2]. The calcium (Ca) site can be isomorphically occupied by elements with large cations such as Na+, K+, Rb+, Mg2+, Mn2+, Fe2+, Sr2+, Ba2+, Pb2+, Y3+, REE3+, Th4+ and U4+ [3,4,5], while the phosphorus (P) site is favorable for substitution by highly charged cations such as Si4+, S6+, V5+ or As5+ [3,4,5]; finally, the halogen site apatite hosts F, Cl or OH and can sometimes incorporate Br and I [3].
Common trace elements in magmatic apatites are the rare earth elements (REEs), Fe, Mn, Sr, Y, S, Ba, Pb, Th and U. They can serve as key indicators of magma composition and physicochemical changes during magmatic–hydrothermal evolution [2,6,7,8]. The temperatures for the saturation and crystallization of apatite in silicate magmatic melts are influenced by the contents of SiO2 and P2O5 and are lower for low abundances of both SiO2 and P2O5 [9]. Higher contents of fluorine relative to chlorine and hydroxyl (OH) also indicate lower temperatures [10]. The crystallization temperature of apatites is suggested to be an important parameter controlling the incorporation of REEs, as the average distribution coefficient for REE3+ approximately doubles with a decrease in temperature from 1080 to 950 °C [11].
The geochemical trace element characteristics of apatite can also be used to determine the potential ore-bearing capacity/fertility [12] and the redox potential of magmas (e.g., Eu and Ce anomalies [7]), which expands its potential use as an indicator and pathfinder mineral for ore exploration.
Hydrothermal and hydrothermally altered apatites have often been shown to be characterized by distinct textural and geochemical features. CL imaging of hydrothermal apatite commonly reveals alteration zones on the rims of magmatic apatite crystals, often with a yellow-to-green CL response and with corroded textures or dendritic forms [13,14,15,16,17]. Hydrothermal apatites can be distinguished from magmatic apatite by higher contents of Y, F, Na and MREEs [12,13,15,16,18], positive Eu anomalies [15,19] and lower Sr, Cl, Mn, Mg, Fe and light rare earth element (LREE) abundances [8,12,13,15,18,19].
Optical cathodoluminescence (cold-CL) is a rapid and effective method for visualizing microtextures in mineral phases. The resultant colored image reflects the presence of elements which act as either CL “activators” or “quenchers” [20]. In the past few years, this technique has been widely applied in Earth sciences, especially in mineralogy, ore geology, geochronology and sedimentology. CL images combined with the trace element composition of apatites can be integrated and used as an indicator for determining the type of hydrothermal alteration in magmatic–hydrothermal systems [8,13].
The aim of this study is to establish the geochemical and textural features of apatites from a rock suite that has undergone different degrees of hydrothermal alteration, to understand and distinguish the primary (magmatic) features of apatites from those related to imposed alteration. The case study was conducted in the Sharlo Dere area of the Chelopech high-sulfidation epithermal Cu-Au deposit, Bulgaria. The copper–gold mineralization is deposited in a diorite porphyry system that has undergone variable hydrothermal alteration. Apatites from five hydrothermal alteration zones were studied using a suite of geochemical, mineralogical–geochemical and optical methods, namely LA-ICP-MS—laser ablation–inductively coupled plasma mass spectrometry; scanning electron microscope with energy-dispersive X-ray spectroscopy—SEM-EDS; electron probe microanalysis with wave-dispersive spectroscopy—EPMA-WDS; petrographic observations on polished sections; and optical cathodoluminescence (cold-CL). The data enable the delineation of distinctive characteristic features of the apatites and contribute to a better understanding of the conditions of magma generation and subsequent hydrothermal alteration of the rocks. The most diagnostic trace elements and/or textural relationships in apatite are then proposed as potential geochemical indicators for the exploration and mining of magmatic–hydrothermal deposits.

2. Geological Setting

The Chelopech high-sulfidation epithermal deposit is one of the largest Cu-Au deposits in Europe. It is part of the “Apuseni–Banat–Timok–Srednogorie” (ABTS) magmatic and metallogenic belt (MMB; [21,22,23]; Figure 1), which was formed during the north-vergent Late Cretaceous subduction of the Neotethys Ocean beneath the European continent [24,25,26].
The Chelopech deposit is part of the Elatsite–Chelopech ore field, which is located in the northernmost parts of the Central Srednogorie zone in Bulgaria, termed the Panagyurishte ore region [21]. It is hosted by Late Cretaceous explosive and effusive products of the Chelopech stratovolcano and is intruded by subvolcanic andesitic dykes and bodies. This sequence lies transgressively or tectonically on a basement of Meso-Neoproterozoic high-grade and Paleozoic low-grade metamorphic rocks [21,27,28,29,30]. The volcanic rocks are subdivided into three units [31]: (1) volcanic domes composed of andesites and trachydacites with an age of 92.22 ± 0.30 Ma (U-Pb zircon dating); (2) lava flows with latitic-to-dacite composition; (3) volcanics and volcanic breccias with andesitic to latitic composition, with the rocks of units (2) and (3) yielding a magmatic crystallization age of 91.3 ± 0.3 Ma (U-Pb zircon dating). Chambefort et al. [32] regarded the ore bodies in the deposit to be hosted in shallow-intrusive bodies of andesitic composition and provided an age of 91.45 ± 0.15 Ma (U-Pb zircon ID-TIMS technique), which reflects the maximum age of the deposit. The volcanic activity in the area took place in a submarine environment, as evidenced by the presence of accretionary lapilli and hyaloclastic and pillow breccias [33].
The initial model of the deposit was of a stratovolcano edifice characterized by explosive and effusive products of andesitic composition (Chelopech volcano) [27,34,35]. In the recent years, new genetic models for the formation of the deposit have been developed. Marton et al. [36] associated ore deposition with a maar-diatremic system in an intrusive setting that hosts several types of mineralization: (1) Cu-Mo-Au porphyry mineralization in the western part (in the footwall of the Petrovden fault); (2) polymetallic sulfide veins (in the Vozdol and Petrovden areas); (3) high-sulfidation Cu-Au mineralization in the central and western parts of the Chelopech deposit, where the Sharlo Dere exploration area is situated (Figure 2). The formation of the Chelopech deposit is associated with a magmatic–hydrothermal system, with a gradual transition from a deeper subvolcanic/porphyritic to a shallower high-sulfidation epithermal environment observed. The ore mineralization is mainly hosted in subvertical phreatomagmatic breccias and subhorizontal hydromagmatic pyroclastic flows [37].
The Sharlo Dere area is located 1 km northeast of the Chelopech mine (Figure 2). The host rocks were initially described as volcanic breccias and andesites, which are hydrothermally affected by propylitic, quartz-sericitic and advanced argillic type alteration [38]. According to new data for the deposit, the host rocks in this area are diorite porphyries and phreatomagmatic breccias [37] overprinted by three types of hydrothermal alteration, which from the center to the periphery are zoned as follows: advanced argillic, quartz-sericitic and propylitic alteration [39,40].

3. Materials and Methods

Seven rock samples from two types of igneous rocks were studied—diorite porphyry (five samples) and diorite-porphyritic clasts in phreatomagmatic breccia (two samples)—which represent several types of hydrothermal alteration: relatively weak alteration with imposed hematitization (called “hematitic”), propylitic, transitional propylitic-sericitic, quartz-sericitic and advanced argillic alteration. The samples were selected from drill cores EX_SDW_04 and EX_SD_51 in the Sharlo Dere area, Chelopech (Figure 2). The samples RK23_3, RK23_4, RK23_10, RK23_17 and RK23_22 are from drill core EX_SDW_04 and are at depths of 382, 410, 754, 980 and 1080 m, respectively. The samples RK23_39 and RK23_40 are from drill core EX_SD_51 and are at depths of 246 and 247 m, respectively.
Microscopic observations of the apatites and their relationships with other minerals in the rocks were studied in polished sections with a Leica DM750P (Leica Microsystem, Wetzlar, Germany) polarization microscope at the Geological Institute of the Bulgarian Academy of Sciences GI-BAS. To reveal the internal structure (e.g., oscillatory zoning) of the apatites, CL studies were performed using a Cathodyne optical cathodoluminescence system (NewTec Scientific, Caveirac, France) at GI-BAS. This is a cold cathodoluminescence system coupled to a Leica DM2700 M (Leica Microsystem, Wetzlar, Germany) optical microscope, equipped with 5×, 10× and 20× objectives. The images were acquired under the following operating conditions: gun current (70 mA) and voltage (7000 V). Controlled sample positioning was applied.
The major element composition of the apatites was determined by two methods: scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) and an electron probe micro analyzer (EPMA) using wave-dispersive spectroscopy (SEM-WDS). The SEM-EDS analysis at the Department of Geology, Trinity College, Dublin, Ireland, followed the protocol described in the article by [41]. The only difference in the setup was the use of a beam current of 3 nA (EPMA) versus 300 pA (SEM-EDS). The EPMA analyses were performed using a JEOL JXA 8530F (JEOL Ltd., Tokyo, Japan) field-emission electron microprobe in wavelength-dispersive mode (WDS) at the Earth Science Institute of the Slovak Academy of Sciences in Banská Bystrica. The following analytical conditions were applied: probe current 10–20 nA; acceleration voltage 15 kV; beam diameter 3–10 µm; counting time 10–30 s for the peak and 5–15 s for the background. The standards used for EPMA via by SEM-WDS are as follows: Diopside (Ca, Mg), Tugtupite (Cl), Barite (S), Apatite (P), Fluorite (F), Albite (Si, Al, Na) and Rhodonite (Mn).
Trace elements were measured by LA-ICP-MS (laser ablation–inductively coupled plasma mass spectrometry) at the (GI-BAS). The system includes a New Wave Research 193 nm Excimer UP-193FX laser (ATLEX-LR, ATL Lasertechnik GmbH, Kriftel, Germany) and a Perkin-Elmer ELAN DRC-e quadrupole-inductively coupled mass spectrometer (Perkin Elmer, Woodbridge, ON, Canada). Calcium, phosphorous and a total of 31 trace elements were analyzed: 45Sc, 49Ti, 51V, 55Mn, 57Fe, 66Zn, 71Ga, 88Sr, 89Y, 91Zr, 93Nb, 139La, 140Ce, 141Pr, 146Nd, 147Sm, 151Eu, 157Gd, 159Tb, 163Dy, 165Ho, 167Er, 169Tm, 173Yb, 175Lu, 178Hf, 181Ta, 184W, 208Pb, 232 Th and 238U. The laser spot diameter was 20 µm with a repetition rate of 5 Hz for epoxy-mounted apatite separates from the quartz-sericitic alteration zone, to 25–35 µm and 6 Hz for other samples. NIST SRM 610 and NIST SRM 612 were used for external calibration of the apatite, and calcium was used as the internal standard. Calcium concentrations for all samples assumed a stoichiometry of 393,300 ppm. The results were processed using the Sills program [42].
Multi-element mapping of apatite crystals was performed using a Teledyne Photon Machines (Eugene, OR, USA) Iridia 193 nm ArF Excimer laser ablation system coupled to an Agilent 7900 ICP-MS (Santa Clara, CA, USA) at the Department of Geology, Trinity College, Dublin, Ireland. Trace element mapping was undertaken using a repetition rate of 50 Hz, a square spot of 8 μm width, a laser scan speed of 5 μm/sec and a fluence of 2.5 J/cm2. NIST612 glass [43] was analyzed as the primary reference material. Mapping was undertaken by defining a selected rectangular area composed of successive line scans (rasters) [44]. During the mapping session, 13 major and trace elements were acquired: 44Ca (2.5), 89Y (5), 139La (10), 140Ce (5), 141Pr (10), 146Nd (50), 172Yb (10), 202Hg (2.5), 204Pb+Hg (2.5), 206Pb (20), 207Pb (40), 208Pb (2.5), 232Th (10) and 238U (10) (dwell times listed in brackets) with a total sweep cycle time of 206 ms. The NIST612 standard was measured at the start and end of every mapping sequence. The time-resolved laser ablation ICP-MS data were reduced using the Trace_Elements data reduction scheme (DRS) in the Iolite Version 4 data reduction software [45]. Maps were reduced employing semi-quantitative normalization relative to the reference material NIST 612 glass.

4. Results

4.1. Textures of Apatites

Apatites in the slightly altered diorite porphyries with “hematite” alteration are euhedral, prismatic and hexagonal, with a crystal size from 50 to 200 µm. They occur as inclusions in amphibole phenocrysts or in the groundmass (Figure 3A,B). Zircon inclusions are observed in some apatites (Figure 3B,C). Backscattered electron (BSE) images show that the apatite crystals are relatively homogeneous, with lighter to darker zones and dispersed microcracks (Figure 3B). Cathodoluminescence (CL) images reveal light to dark brown colors with well-developed magmatic oscillatory zoning, with yellow-brown coloration observed along the microcracks (Figure 3C).
In the propylitic-altered rocks, apatite crystals have prismatic and hexagonal shapes with sizes from 50 to 150 µm. They are located near plagioclase and pyrite crystals in the groundmass (Figure 3D). Backscattered electron images show homogeneity and multiple cracks in the crystals (Figure 3E). In CL images, apatite crystals are brown in color, and near the cracks they are yellow-green (Figure 3E).
In diorite porphyries with propylitic-sericitic alteration, the apatites have prismatic crystal forms, 50 to 100 µm size, and are associated with sericite, pyrite and carbonates (Figure 3G). In BSE images, the same crystals show relative homogeneity with lighter and darker zones and microcracks, with slight corrosion of the crystals observed at the peripheries (Figure 3H). Cathodoluminescence images demonstrate zoning with dark to light brown colors in the central parts, while the peripheral zones are brighter and distinguished by a green hue (Figure 3I).
In the quartz-sericitic-altered rocks, apatite crystals have prismatic and hexagonal forms and a crystal size of 50 to 100 µm. They are associated with quartz, sericite, pyrite and anhydrite (Figure 3J). Backscattered electron images show the apatites are BSE-homogenous with some cracks, with some resorption on the periphery of crystals (Figure 3K). In CL images, the crystals are light brown in the central parts, and variable zones with yellow coloration are observed on the periphery of the crystals (Figure 3L).
Apatites from diorite-porphyry clasts in a phreatomagmatic breccia with advanced argillic alteration have irregular, corroded-like shapes and a crystal size from 25 to 80 µm. They are associated with quartz and dickite (Figure 3M). On BSE images, the crystals show homogeneity, microcracks and corroded peripheries (Figure 3H). Cathodoluminescence images show a pink-green color of the crystals (Figure 3O).

4.2. Apatite Major Element Contents

The results for major element analyses in the apatites from the various alteration zones in diorite porphyries and diorite-porphyry clasts in phreatomagmatic breccia obtained from SEM-EDS and EPMA analyses are presented in Table 1 and Table 2, respectively.
The apatite CaO contents from the variably altered rocks range from 52.15 to 54.83 wt% (4.71 to 4.86 apfu) and for P2O5 from 38.72 to 42.92 wt% (2.76 to 2.99 apfu) (Figure 4A).
The F content in the apatites varies widely from 0.18 to 4.39 wt% (0.049 to 0.92 apfu), in “hematitic”, propylitic and propylitic-sericitic-altered rocks. In the quartz-sericitic and advanced argillic-altered rocks, F varies from 3.06 to 5.0 wt% (0.81 to 0.99 apfu) (Figure 4B). Chlorine (Cl) contents are in the range 0.01–1.24 wt% (0.001 to 0.18 apfu), with the highest values in the “hematitic” alteration zone and the lowest in advanced argillic alteration (Figure 4C). A significant portion of the apatites from the hydrothermally altered rocks are determined as fluorapatites, while some are hydroxylapatites (Figure 5), using the classification diagram of [46]. The OH concentration in apatite was calculated based on the measured concentration of F and Cl, assuming the halogen site is fully occupied (XF-ap + XCl-ap + XOH-ap = 1) [4].
The SiO2 content varies from 0.03 to 1.02 wt% with variations between the distinct alteration zones, being highest in the advanced argillic alteration zone (Figure 4D). The apatite SO3 content increases with the intensity of hydrothermal alteration, with variations in the “hematitic” alteration rocks ranging from 0.01 to 0.18 wt% and in the advanced argillic alteration reaching 0.48–2.94 wt% (Figure 4E). Apatite crystals from all altered rocks show low values of K2O (≤0.17 wt%) and MnO (≤0.22 wt%). Sodium contents (estimated as Na2O) in apatites from the “hematitic” to quartz-sericitic alteration zones range from 0.01 to 0.58 wt%., while in the advanced argillic-altered rocks they reach 0.70 wt%.

4.3. Trace Elements in Apatites

Apatite trace element contents determined by LA-ICP-MS analyses are presented in Table 1 *, Table 3 # and Table 4 # (* crystal mounts; # polished sections). The average trace element contents in apatites from the alteration zones of the Sharlo Dere area Chelopech deposit are summarized in Table 5. The strontium content in the apatites from the slightly altered zones (847–935 ppm) and those in the propylitic zones (808–887 ppm) is similarly high, decreasing slightly in the apatites from the propylitic-sericitic (700–826 ppm) and quartz-sericitic zones (725–855 ppm). In the latter, it strongly decreases in the alteration rims with yellow coloration from CL images, where the values are in the range of 290 to 441 ppm, similar to the contents in apatites from the advanced argillic zones (339–459 ppm) (Figure 4E). The apatite Y distribution shows the opposite behavior. The yttrium content is lowest in apatites from rocks with “hematite” (178–257 ppm), propylitic (188–253 ppm), propylitic-sericitic (178–286 ppm) and quartz-sericitic alteration (182–261 ppm), but increases from 310 to 816 ppm in the outer rims in the latter. In apatites from the advanced argillic zone, the Y content is 515–1040 ppm (Figure 4G).
In all analyzed apatites from different hydrothermally altered rocks, the trace elements Sc, Ti, Zn, Zr, Nb and Hf are below the limit of detection. The V contents in the apatites from different alteration zones vary slightly from 8.53–14.1 ppm to 2.21–10.5 ppm, as seen in Table 4, but in advanced argillic rocks it decreases, being below the limit of detection in one of the analyses.
The Mn and Fe contents remain relatively constant in most apatites, while in the altered zones (yellow coloration in the CL images) in the quartz-sericitic rocks a significant increase in Mn content is observed, although the Fe content is generally below the limit of detection. In the advanced argillic rocks, the Mn and Fe values in the apatites are below the limit of detection.
The Ga contents in the apatites from the most altered rocks are below 40 ppm, while in the advanced argillic rocks the contents are below the limit of detection. No significant difference is observed in the U content of the apatites from most alteration zones, but in the advanced argillic zone its content drops below 2 ppm or below the limit of detection (below 0.96 ppm). No significant differences are observed in the Th contents. The Pb content is relatively constant in most altered apatites (2–6 ppm), but in the most altered zones (the quartz-sericitic zone and the advanced argillic rocks), Pb increases to 30 ppm and 285 ppm, respectively.
The contents of rare earth elements and Y in the apatites are shown in Table 3 and Table 4 and Figure 6 and Figure 7. On a chondrite-normalized REE plot, the apatites from the “hematitic”, propylitic, propylitic-sericitic and unaltered zones from quartz-sericitic-altered rocks are characterized by enrichment in LREE, negative Eu anomalies (Eu/Eu*), positive Gd anomaly (Gd/Gd*) and a smooth slope towards the heavy rare earth elements (HREEs) (Figure 6A). In the alteration zones of apatite from the quartz-sericite rocks, a trend of depletion of La, Ce and Pr, moderately negative Eu/Eu*, moderately positive Gd/Gd* and relative enrichment in MREE–HREE is observed (Figure 6B). Apatites from advanced argillic rocks are characterized by moderately positive Eu/Eu* and Gd/Gd* and variable but depleted light REEs, significant enrichment in MREEs (Sm-Dy) and depletion of HREEs (Figure 6B).
The box plots (Figure 4H,I) present the contents of La and Ca in ppm, which both decrease with an increasing degree of hydrothermal alteration of the rocks. Multi-element LA-ICP-MS mapping of selected apatite crystals from alteration zones (with yellow coloration from CL images) from quartz-sericite rocks demonstrates enrichment of Y and depletion of La and Ce (Figure 7B–E,G,H).
The CeN/YbN ratio was used to assess the distribution of REEs and the enrichment or depletion of LREEs and HREEs. It is highest in apatites from weakly altered rocks (22–26) and lowest in the advanced argillic rocks (0–2). In the alteration zones of quartz-sericite rocks, the values are in the range of 5–11.

5. Discussion

5.1. Relationship Between Geochemical Characteristics and CL Response of Apatites

The apatites show a number of textural features similar to apatites from hydrothermally altered rocks in copper–porphyry deposits in other ore belts, including Cu-Au deposits in the Khionkan and Shigadze regions in China, British Columbia (Canada), and the Olympic Dam Cu–U–Au–Ag deposit in South Australia [13,14,15,17]. Based on CL images, the apatites from variably hydrothermally altered rocks in the Sharlo Dere area of the Chelopech epithermal Cu-Au deposit show clear differences in their macro- and microelement compositions. They can be used as criteria for distinguishing primary magmatic apatites from those with subsequent hydrothermal alteration.
Apatite crystals from the “hematitic”, propylitic, propylitic-sericitic and quartz-sericitic alteration have typical magmatic oscillatory zoning (Figure 3B,E,I and Figure 7A,D) characteristic of magmatic apatites [13]. Altered zones with yellow coloration observed in CL images of quartz-sericite rocks (Figure 3L and Figure 7A,D) probably reflect hydrothermal alteration of primary magmatic apatites under the influence of hydrothermal fluids [13,14,15,17]. Apatites from the advanced argillic rocks show significant changes in their texture (Figure 3H,M,O) and can be considered as hydrothermal apatites [15].
Apatites associated with “hematitic”, propylite and propylite-sericite-altered rocks show weak variations in Na2O content, while those from quartz-sericite and advanced argillic-altered rocks exhibit higher Na2O concentrations. This is also confirmed by CL images. A possible reason for the higher values is that Na can isomorphically replace Ca in the apatite crystal lattice [48].
The chlorine content is the highest in magmatic apatites and the lowest in some apatites from quartz-sericite and advanced argillic-altered rocks (Figure 4B). According to Palma et al. [49], higher Cl contents are typical of hydrothermal apatites in IOCG (iron oxide copper–gold) deposits. However, Wei et al. [19] in Carlin-type gold deposits and Li et al. [8] in granodiorite of the Songnen-Zhangguangcai Range Massif reported an inverse/opposite relationship of lower Cl contents in hydrothermally altered apatites. This difference is likely due to the different evolution of the fluids after their separation from the magmatic source [19]. The F content is lower in apatites from rocks with a weaker hydrothermal overprint (“hematite”, propylite and propylite-sericite zones) and increases in quartz-sericite and advanced argillic rocks (Figure 4B). This F enrichment may be due to an increase in the intensity of hydrothermal alteration [16].
The increase in SO3 content in apatites from the quartz-sericite and advanced argillic alteration zones is likely associated with increased oxygen fugacity (fO2) or oxidized conditions [50,51]. A moderate negative correlation was observed between SO3 and P2O5 and SiO2 and P2O5 in apatites from the quartz-sericite (−0.9, −0.8) alteration zone and −0.5 and −0.8 in the advanced argillic alteration zone, respectively. This may be the result of isomorphic substitution: S6+ + Si4+ = 2P5+ [48].

5.2. The Role of Sr, Y and REE as Geochemical Indicators in Magmatic and Hydro-Thermal Apatites

On a Sr-Y biplot (Figure 8A), the strontium content in magmatic apatites from the Chelopech area is >700 ppm, while in apatites from the quartz-sericite and advanced argillic alteration zones it is <500 ppm. The Y content in magmatic apatites is 170–300 ppm, while in apatites from the quartz-sericite and advanced argillic alteration zones it is 310–816 ppm and 515–1040 ppm, respectively. In the box plots (Figure 4E,G), a decrease in Sr and an increase in Y are observed with an increase in the degree of hydrothermal alteration of the rocks. The increase in Y is well demonstrated in the multi-element image maps (Figure 7B–E,G,H). The depletion of Sr is considered to be characteristic of hydrothermal apatites [13], which according to Tan et al. [18] is also combined with Y enrichment. From a stoichiometric point of view, Y3+ can isomorphically enter the apatite structure, replacing Ca2+ [52], and possibly its substituting element Sr.
The normalized LaN/SmN–CeN/YbN ratios (Figure 8B) show distinct trends in the transition from magmatic apatites from weakly hydrothermally altered zones to strong hydrothermally altered apatites. The magmatic apatites from the “hematite”, propylitic, propylitic-sericitic and quartz-sericitic alteration show a trend of decreasing LaN/SmN–CeN/YbN ratios with increasing hydrothermal alteration intensity in the rocks. Apatites from the quartz-sericitic and advanced argillic alteration zones have the lowest values of these ratios. The lower LaN/SmN values are probably related to the relative depletion of light rare earth elements with increasing alteration intensity, combined with an increase in MREEs [12], as seen in the chondrite-normalized REE plots (Figure 6B–D). Such REE behavior in apatite has been described in other deposits with hydrothermally altered apatites [15,19] and hydrothermal apatites.

5.3. Apatite Geochemistry as an Indicator of the Ore-Bearing Potential of Magmas

Previous studies of apatite geochemistry have demonstrated its potential to distinguish between fertile and unfertile Cu–(Au) magmatic–hydrothermal systems [12,52]. The predictive diagram (DP1-1/DP1-2) of ([52], explanation of axes titles on Figure 8) shows that apatites from all altered rocks of the Sharlo Dere area of the Chelopech deposit fall into the “ore deposit” field (Figure 8B). An exception is apatite from the advanced argillic rocks which lack Mn and cannot be plotted. This feature highlights the specific geochemical characteristics of hydrothermal apatites for which application on discriminant diagrams for magma fertility should be used with caution or avoided.

5.4. Eu and Ce Anomalies in the Apatite—Indicators of the Redox Potential of Magmas

The magmatic apatites from our study show negative Eu anomalies. This is characteristic of magmatic apatites and is typically associated with the fractional crystallization of plagioclase [6,52]. Apatites from the quartz-sericitic alteration zone have a moderately negative Eu anomaly, while apatites from advanced argillic altered rocks have a positive Eu anomaly, which is characteristic of hydrothermal apatites [15,52]. The ratio between Eu/Eu* and Ce/Ce* anomalies can serve as an indicator for establishing the redox potential of magmas [7,12]. Magmatic apatites fall in the zone of moderately oxidizing to strongly oxidizing conditions of the magma, while apatites from the quartz-sericitic and advanced argillic alteration zones show strongly oxidizing conditions (Figure 8D).

6. Conclusions

Apatites from diorite porphyries and diorite-porphyry clasts in phreatomagmatic breccias that have undergone variable degrees of hydrothermal alteration in the Sharlo Dere area of the Chelopech high-sulfidation epithermal Cu-Au deposit, Bulgaria, allow the following conclusions to be drawn:
(1)
The apatites are primary magmatic, magmatic with hydrothermally altered zones, and completely hydrothermally altered/hydrothermal.
(2)
The combination of CL images, major and trace element composition and REE distribution patterns of apatites serves as a powerful tool for tracking the degree of hydrothermal alteration in the rocks.
(3)
Magmatic apatites from “hematite”-, propylite-, propylite-sericite- and quartz-sericite-altered rocks are characterized by high Cl and Sr contents, negative Eu anomalies, high LaN/SmN and CeN/YbN ratios and enrichment of LREEs.
(4)
Apatites from the quartz-sericite alteration zones and hydrothermal apatites from the advanced argillic altered rocks have increased Na2O, F, SO3, Y contents, low LaN/SmN and CeN/YbN ratios and enrichment of MREEs.
(5)
Hydrothermal apatites from the advanced argillic rocks are characterized by a positive Eu anomaly.
(6)
The geochemical behavior of trace elements in apatite can be used to constrain redox conditions (e.g., Eu and Ce anomalies), which is important for the exploration and exploitation of magmatic–hydrothermal deposits.
(7)
In assessing the ore-fertility of magmas, it is recommended to use magmatic apatites as the geochemistry of hydrothermally apatites results from overprinting processes of fluid–rock interaction and may compromise conclusions about the primary magma characteristics.

Author Contributions

Conceptualization, R.K., I.P. and A.H.; methodology, R.K., I.P., D.C., A.H. and E.S.; software, R.K., I.P., D.C. and E.S.; validation, R.K., I.P., D.C., A.H. and E.S.; formal analysis, R.K., I.P., D.C., A.H. and E.S.; investigation, R.K., I.P., D.C., A.H. and E.S.; resources, R.K., I.P., D.C., A.H. and E.S.; data curation, R.K., I.P., D.C., A.H. and E.S.; writing—original draft preparation, R.K., I.P., D.C., A.H. and E.S.; writing—review and editing, R.K., I.P., D.C., A.H. and E.S.; visualization, R.K., I.P., D.C., A.H. and E.S.; supervision, R.K., I.P., D.C., A.H. and E.S.; project administration, R.K., I.P., D.C., A.H. and E.S.; funding acquisition, R.K., I.P., A.H. and E.S. All authors have read and agreed to the published version of the manuscript.

Funding

This study is supported by REXPro project, contract BG-RRP-2.011-0040-C01/02, financed by NextGeneration EU, investment C2.I2. “Increasing the innovation capacity of the Bulgarian Academy of Sciences in the field of green and green and digital technologies” of the Recovery and Resilience Mechanism. All responsibility for the content is borne by the authors, and the views expressed herein can in no way be taken to reflect the official opinion of the European Union and the Bulgarian Academy of Sciences.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The logistic help of the DPM Metals exploration team and the support during field work and sampling are highly appreciated. The multi-element mapping was undertaken at Trinity College, Dublin, with the support of the Erasmus+ Programme to R.K. Paul Guyett is thanked for help with the SEM-EDS analyses at Trinity College, Dublin. D.C. acknowledges support from Research Ireland through research grant 13/RC/2092_P2 (iCRAG Research Centre). The three anonymous reviewers are thanked for their constructive comments that helped to improve the manuscript, and to the Minerals editorial team for assistance and handling of the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Geological map of the Apuseni–Banat–Timok–Srednogorie (ABTS) magmatic and metallogenic belt with the location of the Cu-Au deposit Chelopech indicated (after Gallhofer et al. [23]).
Figure 1. Geological map of the Apuseni–Banat–Timok–Srednogorie (ABTS) magmatic and metallogenic belt with the location of the Cu-Au deposit Chelopech indicated (after Gallhofer et al. [23]).
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Figure 2. Geological map of the Chelopech deposit and the Sharlo Dere area with the location of the boreholes EX_SDW_04 and EX_SD_51 marked with black lines (source: DPM Metals).
Figure 2. Geological map of the Chelopech deposit and the Sharlo Dere area with the location of the boreholes EX_SDW_04 and EX_SD_51 marked with black lines (source: DPM Metals).
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Figure 3. Optical microscope, BSE and CL photomicrographs of apatites from diorite porphyries and diorite-porphyry clasts in a phreatomagmatic breccia from the Sharlo Dere area, Chelopech deposit, with various and increasing hydrothermal alteration grade from top to bottom: (A–C) “hematitic”; (D–F) propylitic; (G–I) propylitic-sericitic; (J–L) quartz-sericite; (M–O) advanced argillic alteration. Abbreviations: Anh—anhydrite; Ap—apatite; Cc—calcite; Dic—dickite; Hb—amphibole; Py—pyrite; Pl—plagioclase; Qz—quartz; Ser—sericite; Zrn—zircon. (A,D,M) images in transmitted light; (G,J) cross-polarized light; (B,E,H,K,N) BSE images, and (C,F,I,L,O) CL images.
Figure 3. Optical microscope, BSE and CL photomicrographs of apatites from diorite porphyries and diorite-porphyry clasts in a phreatomagmatic breccia from the Sharlo Dere area, Chelopech deposit, with various and increasing hydrothermal alteration grade from top to bottom: (A–C) “hematitic”; (D–F) propylitic; (G–I) propylitic-sericitic; (J–L) quartz-sericite; (M–O) advanced argillic alteration. Abbreviations: Anh—anhydrite; Ap—apatite; Cc—calcite; Dic—dickite; Hb—amphibole; Py—pyrite; Pl—plagioclase; Qz—quartz; Ser—sericite; Zrn—zircon. (A,D,M) images in transmitted light; (G,J) cross-polarized light; (B,E,H,K,N) BSE images, and (C,F,I,L,O) CL images.
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Figure 4. Distribution of major and trace element contents in apatite crystals from the variably hydrothermally altered rocks: Major elements (wt%) (A–E): (A) P2O5; (B) F; (C) Cl; (D) SiO2; (E) SO3; Trace elements (ppm) (F–I): (F) Sr; (G) Y; (H) La; (I) Ce.
Figure 4. Distribution of major and trace element contents in apatite crystals from the variably hydrothermally altered rocks: Major elements (wt%) (A–E): (A) P2O5; (B) F; (C) Cl; (D) SiO2; (E) SO3; Trace elements (ppm) (F–I): (F) Sr; (G) Y; (H) La; (I) Ce.
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Figure 5. Classification ternary diagram (Cl–F–OH) for chlorapatite, fluorapatite and hydroxylapatite, according to [46].
Figure 5. Classification ternary diagram (Cl–F–OH) for chlorapatite, fluorapatite and hydroxylapatite, according to [46].
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Figure 6. Chondrite-normalized REE distribution patterns (C1 chondrite values according to [47]) in apatites from different alteration zones of diorite porphyries and diorite-porphyry clasts in phreatomagmatic breccias from the Sharlo Dere area, Chelopech deposit: (A) magmatic apatites; (B) hydrothermal altered apatites; (C) hydrothermal apatites; (D) all types of apatites.
Figure 6. Chondrite-normalized REE distribution patterns (C1 chondrite values according to [47]) in apatites from different alteration zones of diorite porphyries and diorite-porphyry clasts in phreatomagmatic breccias from the Sharlo Dere area, Chelopech deposit: (A) magmatic apatites; (B) hydrothermal altered apatites; (C) hydrothermal apatites; (D) all types of apatites.
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Figure 7. Cathodoluminescent images (A,E) and multi-element (Y, La and Ce) mapping (B–D,F–H) of selected apatite crystals from the quartz-sericite alteration zone in the Sharlo Dere area, Chelopech deposit.
Figure 7. Cathodoluminescent images (A,E) and multi-element (Y, La and Ce) mapping (B–D,F–H) of selected apatite crystals from the quartz-sericite alteration zone in the Sharlo Dere area, Chelopech deposit.
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Figure 8. Biplots demonstrating relationships between certain trace and rare earth elements in magmatic and hydrothermally altered apatites: (A) Sr vs. Y; (B) LaN/SmN vs. CeN/YbN; (C) Discriminant plot DP1-1 vs. Dp1-2 of [52] (DP1-1 = −0.06461logMn − 1.56logSr + 2.609logY + 0.3631logLa − 1.766logCe + 0.6243logEu − 3.642logDy + 0.7086logYb − 1.178logPb + 0.4161logTh + 0.963logU + 6.589; DP1-2 = 0.2073logMn − 1.035logSr + 15.1logY + 4.995logLa − 5.804logCe + 0.1741logEu − 8.771logDy − 4.326logYb − 0.6719logTh + 0.02096logU − 10.45); (D) (Eu/Eu* = EuN/[(SmN × GdN)^0.5]) vs. (Ce/Ce* = CeN)/[(LaN × PrN)^0.5]) plot after [7].
Figure 8. Biplots demonstrating relationships between certain trace and rare earth elements in magmatic and hydrothermally altered apatites: (A) Sr vs. Y; (B) LaN/SmN vs. CeN/YbN; (C) Discriminant plot DP1-1 vs. Dp1-2 of [52] (DP1-1 = −0.06461logMn − 1.56logSr + 2.609logY + 0.3631logLa − 1.766logCe + 0.6243logEu − 3.642logDy + 0.7086logYb − 1.178logPb + 0.4161logTh + 0.963logU + 6.589; DP1-2 = 0.2073logMn − 1.035logSr + 15.1logY + 4.995logLa − 5.804logCe + 0.1741logEu − 8.771logDy − 4.326logYb − 0.6719logTh + 0.02096logU − 10.45); (D) (Eu/Eu* = EuN/[(SmN × GdN)^0.5]) vs. (Ce/Ce* = CeN)/[(LaN × PrN)^0.5]) plot after [7].
Minerals 16 00150 g008
Table 1. Representative SEM-EDS and LA-ICP-MS data for major and trace elements in apatites from various hydrothermal alterations of diorite porphyries and diorite-porphyritic clasts in a phreatomagmatic breccia (*) from the Sharlo Dere area, Chelopech deposit. BLD, below limit of detection.
Table 1. Representative SEM-EDS and LA-ICP-MS data for major and trace elements in apatites from various hydrothermal alterations of diorite porphyries and diorite-porphyritic clasts in a phreatomagmatic breccia (*) from the Sharlo Dere area, Chelopech deposit. BLD, below limit of detection.
“Hematitic”PropyliticPropylitic-SericiticQuartz-SericiticAdvanced Argillic *
SampleRK23_6RK23_6RK23_3RK23_3RK23_3RK23_3RK23_10RK23_10RK23_17RK23_17RK23_17RK23_17RK23_39RK23_39RK23_39
Analysis4314341d351365397477513559567568569584585657
Major elements (wt%)
SiO20.230.150.470.520.400.280.170.300.410.340.030.280.890.780.31
TiO20.000.000.000.270.130.300.080.160.000.000.430.080.000.000.00
Al2O30.140.160.190.070.060.110.030.100.130.150.070.160.570.540.21
P2O541.5040.4641.5842.0941.5641.6041.9541.6742.9241.6442.5442.2838.7239.8041.00
SO30.180.000.510.210.260.210.270.660.170.300.000.452.942.821.97
MgO0.000.000.000.240.000.030.030.000.140.000.000.000.000.200.14
FeO0.230.000.000.160.230.140.170.280.260.000.020.040.000.020.00
MnO0.090.000.000.150.000.090.000.080.050.220.100.000.000.000.00
CaO53.2653.6253.2853.5652.5753.3654.3053.4554.1354.0053.8153.8552.1552.5654.83
Na2O0.060.000.070.030.150.290.040.000.000.290.010.220.040.700.42
K2O0.050.000.000.320.080.000.010.100.000.000.090.030.170.040.00
F2.032.464.390.182.751.531.812.003.923.354.273.064.063.483.20
Cl1.191.080.691.090.901.240.550.640.510.780.610.500.020.000.00
Meas98.9697.93101.1898.8999.0999.1899.4199.44102.64101.07101.98100.9599.56100.94102.08
O=F0.861.041.850.081.160.640.760.841.651.411.801.291.711.471.35
O=Cl0.270.240.160.250.200.280.120.140.120.180.140.110.010.000.00
Total97.8496.6599.1898.5797.7398.2698.5298.45100.8799.48100.0499.5597.8599.48100.73
Calculated Formula (9 apfu)
Ca4.9235.0264.7684.9344.7994.8994.9844.8854.7694.8734.7774.8334.7074.6774.863
Mn0.0070.0000.0000.0110.0000.0070.0000.0060.0030.0160.0070.0000.0000.0000.000
Na0.0100.0000.0110.0050.0250.0480.0070.0000.0000.0470.0020.0360.0070.1130.067
K0.0060.0000.0000.0350.0090.0000.0010.0110.0000.0000.0100.0030.0180.0040.000
Fe0.0170.0000.0000.0120.0160.0100.0120.0200.0180.0000.0010.0030.0000.0010.000
Ti0.0000.0000.0000.0170.0080.0190.0050.0100.0000.0000.0270.0050.0000.0000.000
Al0.0140.0160.0190.0070.0060.0110.0030.0100.0130.0150.0070.0160.0570.0530.020
Total4.9775.0424.7985.0214.8634.9945.0124.9424.8034.9514.8314.8964.7894.8484.950
P3.0312.9962.9403.0642.9983.0183.0433.0092.9882.9692.9842.9982.7612.7992.873
S0.0120.0000.0320.0140.0170.0140.0170.0420.0100.0190.0000.0280.1860.1760.122
Si0.0200.0130.0390.0450.0340.0240.0150.0260.0340.0290.0020.0230.0750.0650.026
Total3.0633.0093.0113.1233.0493.0563.0753.0773.0323.0172.9863.0493.0223.0403.021
F0.5540.6810.9220.0490.7410.4150.4900.5400.9350.8890.9290.8110.9970.9140.838
Cl0.1740.1600.0780.1590.1300.1800.0800.0930.0650.1110.0710.0710.0030.0000.000
OH0.2700.1600.0000.7920.1290.4050.4300.3700.0000.0000.0000.1180.0000.0860.162
Total0.9981.0011.0001.0001.0001.0001.0001.0031.0001.0001.0001.0001.0001.0001.000
Trace elements (ppm)
V9.4312.079.1114.868.2310.667.389.87.4910.1113.697.728.84BLD9.37
Mn161116931378154814331652135813551394133312761315BLDBLDBLD
Fe11421485912123310461184933903991648686713939BLDBLD
Ga29.7835.0421.5540.228.4430.2121.9716.9722.9320.618.3620.04BLDBLD3.39
Sr935930807885878849762768775751745736459339341
Y2122252123092002121782062322271991901040515872
La868882576116789889445641752956444446943.132.344.36
Ce167217051184226716541707999984123212751074108688.677.6614.28
Pr15316212423115916510710313413411811312.463.863.23
Nd55359244382157157537937351348840540066.1951.1643.02
Sm80.9180.4559.61116.972.7275.7264.5855.9185.4769.8768.9163.62254245223
Eu15.1412.9412.9620.6413.514.7110.2913.9818.3216.2114.1212.14150109141
Gd51.3563.6157.9597.0957.5858.2242.9839.7671.2463.2848.9347.1668368582
Tb6.667.415.829.756.816.55.375.378.376.975.666.178.6434.7869.33
Dy38.0536.1932.5151.9333.8936.3231.1130.839.0235.1934.2132.75281138254
Ho7.117.086.969.377.116.735.596.857.378.826.155.9731.7816.8529.95
Er20.523.0920.9526.5818.9319.1517.123.8729.8326.5819.9916.6752.5827.4342.08
Tm2.782.32.494.042.072.432.012.283.22.722.622.163.651.52.89
Yb19.6416.7817.720.8613.9616.0814.6819.3324.3519.8815.981612.819.2911.06
Lu2.62.242.413.542.372.562.182.512.83.382.892.890.950.70.95
Pb3.44.353.674.12.373.222.582.465.455.892.652.0813486.4285
Th16.9619.5917.7841.5420.0919.1915.8917.4624.5317.3216.7212.9238.6924.3530.39
U6.556.88.1411.17.517.466.416.698.816.777.467.411.70.68BLD
Notes: “Hematitic”, propylitic-sericitic and advanced argillic are marked in gray; propylitic and quartz-sericitic are marked in black.
Table 2. Representative EPMA-WDS data for major elements in apatites from various hydrothermal alterations of diorite porphyries (diorite-porphyritic clasts in a phreatomagmatic breccia (*)) from the Sharlo Dere area, Chelopech deposit.
Table 2. Representative EPMA-WDS data for major elements in apatites from various hydrothermal alterations of diorite porphyries (diorite-porphyritic clasts in a phreatomagmatic breccia (*)) from the Sharlo Dere area, Chelopech deposit.
Quartz-SericiticAdvanced Argillic *
SampleRK23_22RK23_22RK23_40RK23_40RK23_40RK23_40
AnalysisAp 1Ap 2Ap 1Ap 2Ap 3Ap 4
SiO2, (wt%)0.170.941.020.430.330.35
TiO20.000.000.000.000.000.00
Al2O30.070.660.980.280.100.43
P2O539.5840.3640.1340.7341.6140.51
SO31.931.200.640.860.480.72
FeO0.070.050.050.060.130.06
MnO0.060.100.010.030.000.00
CaO54.2254.0152.5554.0054.3152.78
Na2O0.580.490.130.180.060.16
K2O0.000.000.000.000.000.00
F4.640.003.653.765.004.91
Cl0.010.000.010.000.010.01
Meas101.3397.8199.17100.33102.0399.93
O=F1.950.001.541.582.112.07
O=Cl0.000.000.000.000.000.00
Total99.3797.8197.6398.7599.9297.86
Calculated Formula (9 apfu)
Ca4.8775.0234.8004.8944.8344.788
Mn0.0040.0070.0010.0020.0000.000
Na0.0940.0820.0210.0300.0100.026
K0.0000.0000.0000.0000.0000.000
Fe0.0050.0040.0040.0040.0090.004
Ti0.0000.0000.0000.0000.0000.000
Al0.0070.0680.0980.0280.0100.043
Total4.9875.1844.9244.9584.8634.861
P2.8132.9662.8962.9172.9262.904
S0.1220.0780.0410.0550.0300.046
Si0.0140.0820.0870.0360.0270.030
Total2.9493.1263.0243.0082.9832.980
F0.9990.0000.9841.0000.9990.999
Cl0.0010.0000.0010.0000.0010.001
OH0.0001.0000.0140.0000.0000.000
Total1.0001.0000.9991.0001.0001.000
Notes: Quartz-sericitic is marked in black; advanced argillic * is marked in gray.
Table 3. Trace element composition, determined by LA-ICP-MS of separated apatites from hydrothermal alterations of diorite porphyries from the Sharlo Dere area, Chelopech deposit.
Table 3. Trace element composition, determined by LA-ICP-MS of separated apatites from hydrothermal alterations of diorite porphyries from the Sharlo Dere area, Chelopech deposit.
“Hematitic”PropyliticPropylitic-SericiticQuartz-Sericitic
SampleRK23_4RK23_4RK23_4RK23_4RK23_4RK23_3RK23_3RK23_3RK23_3RK23_3RK23_10RK23_10RK23_10RK23_10RK23_10RK23_17RK23_17RK23_17RK23_17RK23_17
Analysisgr1Rgr7Rgr12Rgr19Rgr29Rgr2Rgr5Rgr9Rgr43Rgr51Rgr1Rgr9Rgr16Rgr18Rgr23Rgr1Rgr4Rgr7Rgr16Rgr17R
Trace elements (ppm)
V14.18.971210.58.5310.36.757.3712.912.79.542.216.669.4310.58.88109.9410.67.82
Mn15401436147514581338139415241508151515071262140013141369128013451331136914001287
Fe129210081083120911789861169119712931271750721690812818864865818844723
Ga1812.815.215.415.117.11213.715.820.812.78.510.915.311.91114.513.515.410.5
Sr871857885896839841808825860887763732815826700795810813855725
Y257178212218218212188208199253200234239286286230234234261182
La10227638548538408627037628231076502302513623433548636570641424
Ce195514071578159915701629129414791546201810717171136138210631165132912301371948
Pr19513715715915216512814815519711685124150125125139133146100
Nd707501574588548584467548545710437359486622528480528524580394
Sm10065.282.077.579.782.373.676.977.010063.071.882.797.487.378.076.682.685.953.5
Eu16.611.414.115.414.513.713.713.814.318.812.411.515.218.116.613.715.014.616.613.6
Gd83.855.264.771.862.968.854.665.863.078.952.464.076.287.082.660.358.066.573.847.1
Tb9.376.027.537.206.626.756.096.676.288.576.298.118.149.798.356.776.907.598.275.48
Dy44.832.139.838.737.139.835.338.73446.534.75146.34653.737.539.345.242.632.2
Ho8.776.217.677.288.037.526.797.257.359.016.909.248.238.8510.27.658.118.188.926.17
Er26.817.223.620.819.719.718.420.620.123.72024.623.926.826.124.52420.923.217.6
Tm3.491.812.622.712.492.762.102.092.363.062.502.863.213.363.592.432.963.193.452.72
Yb21.114.817.116.218.517.216.114.814.518.217.817.617.319.817.919.721.720.221.818.6
Lu2.981.902.572.271.912.402.762.422.443.282.802.662.493.833.443.393.092.953.712.37
Pb2.473.272.243.232.973.064.172.782.973.323.182.662.134.161.862.052.322.434.263.01
Th32.514.219.121.120.320.810.715.61834.2170.911.616.516.72024.317.823.316.3
U9.675.657.147.547.198.115.666.966.9710.87.033.856.507.857.347.978.477.448.696.68
Notes: “Hematitic” and propylitic-sericitic are marked in gray; propylitic and quartz-sericitic are marked in black.
Table 4. Trace element composition (in ppm), determined by LA-ICP-MS of alteration zones of apatites from quartz-sericitic alteration in diorite porphyries from the Sharlo Dere area, Chelopech deposit. BLD, below limit of detection.
Table 4. Trace element composition (in ppm), determined by LA-ICP-MS of alteration zones of apatites from quartz-sericitic alteration in diorite porphyries from the Sharlo Dere area, Chelopech deposit. BLD, below limit of detection.
Quartz-Sericitic (Alteration Zones)
SampleRK23_17RK23_17RK23_17RK23_17RK23_17RK23_17
Analysis1grR 2grR 4grR 12grR 16grR 18grR
Trace elements (ppm)
Mn8141082159514557562068
FeBLDBLDBLD295216281
Ga15.411.523.11624.711.5
Sr331290376346389320
Y602720816590692478
La191250258162274127
Ce633800934580977459
Pr1041311619516380
Nd616650892565971524
Sm210225258232286231
Eu47.449.261.555.556.471
Gd227219292200292238
Tb30.13137.430.839.728.2
Dy164165209153182145
Ho25.724.630.721.528.821
Er5865.576.351.75550.4
Tm5.747.897.093.586.674.84
Yb33.238.527.92822.721.9
Lu7.128.265.691.663.713.84
Pb11.717.830.116.35.310.5
Th3737.740.922.840.660.8
U3.413.442.049.572.2225.7
Table 5. Average of trace elements contents (in ppm) in apatites from alteration zones of diorite porphyries and diorite porphyritic clasts in phreatomagmatic breccia (*) of the Sharlo Dere area, Chelopech deposit; seven to eleven analyses are used to calculate to average trace element contents. BLD, below limit of detection.
Table 5. Average of trace elements contents (in ppm) in apatites from alteration zones of diorite porphyries and diorite porphyritic clasts in phreatomagmatic breccia (*) of the Sharlo Dere area, Chelopech deposit; seven to eleven analyses are used to calculate to average trace element contents. BLD, below limit of detection.
“Hematitic”PropyliticPropylitic-SericiticQuartz-Sericitic(Qz-Ser)(Qz-Ser) (Altered Zones)Advanced Argillic *
MinMaxAverageMinMaxAverageMinMaxAverageMinMaxAverageMinMaxAverageMinMaxAverage
V8.5314.111.36.7514.910.82.2110.56.367.4913.710.6BLDBLDBLD8.849.379.11
Mn143616931565137816521515131414001357127614001338756268512BLDBLDBLD
Fe10081485124791212931103690933812648991820216295256BLD939939
Ga12.83523.91221.616.88.52215.31122.917.08.8624.716.8BLD339339
Sr839935887807887847726800763736855796290441366341459400
Y178258217.71883092491782862321902612264788166475151040778
La102276389257611678723026234634446365401272742012.3443.122.74
Ce14071955168111842018160171713821050948137111604599777187.6688.748.17
Pr13719516612419716185150118100146123801631223.2312.57.85
Nd50170760444382163235962249140058049052497174843.166.254.6
Sm62.510081.359.611788.355.997.476.753.585.969.7210286248223254239
Eu11.416.6141320.616.810.318.114.212.116.614.447.47159.2109150130
Gd51.483.867.654.678.966.839.88763.447.173.860.5200292246368668518
Tb6.029.377.75.828.577.25.399.797.595.668.276.9728.239.73434.878.656.7
Dy32.144.838.532.546.539.530.853.742.334.245.239.7145209177138281210
Ho6.218.777.496.799.017.95.5910.27.905.978.927.452130.725.916.931.824.32
Er17.226.82218.423.721.117.126.82216.724.520.595476.365.227.452.640.01
Tm1.813.492.72.073.062.572.013.592.82.163.452.813.587.895.741.53.652.58
Yb14.821.1181418.216.114.719.817.21621.818.921.938.530.29.2912.811.1
Lu1.92.982.442.373.282.832.183.833.012.083.712.901.668.264.960.70.950.83
Pb2.243.272.762.374.173.272.134.163.152.054.263.165.330.117.786.4285186
Th14.232.523.410.734.222.50.917.59.1812.924.318.622.860.841.824.438.731.5
U5.659.677.665.6611.18.383.857.855.856.778.697.732.0425.713.90.681.71.19
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Kalchev, R.; Peytcheva, I.; Chew, D.; Hikov, A.; Stefanova, E. Geochemical and Textural Features of Apatites from Propylitic to Advanced Argillic Hydrothermal Alteration Zones in the Sharlo Dere Area, Chelopech Cu-Au Deposit, Bulgaria. Minerals 2026, 16, 150. https://doi.org/10.3390/min16020150

AMA Style

Kalchev R, Peytcheva I, Chew D, Hikov A, Stefanova E. Geochemical and Textural Features of Apatites from Propylitic to Advanced Argillic Hydrothermal Alteration Zones in the Sharlo Dere Area, Chelopech Cu-Au Deposit, Bulgaria. Minerals. 2026; 16(2):150. https://doi.org/10.3390/min16020150

Chicago/Turabian Style

Kalchev, Radoslav, Irena Peytcheva, David Chew, Atanas Hikov, and Elitsa Stefanova. 2026. "Geochemical and Textural Features of Apatites from Propylitic to Advanced Argillic Hydrothermal Alteration Zones in the Sharlo Dere Area, Chelopech Cu-Au Deposit, Bulgaria" Minerals 16, no. 2: 150. https://doi.org/10.3390/min16020150

APA Style

Kalchev, R., Peytcheva, I., Chew, D., Hikov, A., & Stefanova, E. (2026). Geochemical and Textural Features of Apatites from Propylitic to Advanced Argillic Hydrothermal Alteration Zones in the Sharlo Dere Area, Chelopech Cu-Au Deposit, Bulgaria. Minerals, 16(2), 150. https://doi.org/10.3390/min16020150

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