Geological, Mineralogical, Geochemical, and Petrogenetic Characteristics of Plutonic Rocks in Çiftehan (Ulukışla-Niğde) Area, South-Central Türkiye: Implication for Genetic Link with Fe-Zn Skarn Mineralization

Abstract

Globally, most skarn deposits show a direct relationship with magmatic activity, indicating a genetic link between the geochemical composition of causative plutons and the metal content of associated skarns. Therefore, this study investigated the Early–Middle Eocene plutonic rocks and their relationship with Fe-Zn skarn deposits in the Esendemirtepe-Koçak and Horoz areas of south-central Türkiye. Despite the regional significance, previous studies have not adequately addressed the petrogenetic evolution of these intrusions and the geochemical characteristics of the related skarns. In particular, the fluid-aided mobility of elements at the contact between the causative plutons and the volcano-sedimentary country rocks remains poorly understood. Therefore, in this study, field studies, petrographic and mineralogical analysis, and whole-rock geochemical analysis were conducted to investigate the genetic link between the plutonic rocks and the skarn deposits. Field studies reveal that the skarn zones are within volcano-sedimentary sequences and marble-schist units intruded by four distinct plutonic bodies: (1) Esendemirtepe diorite, (2) Koçak diorite, (3) Horoz granodiorite, and (4) Çifteköy monzogabbro. These rocks exhibit calc-alkaline, I-type, and metaluminous signatures, except for the Çifteköy monzogabbro, which shows I-type, tholeiitic, and alkaline characteristics. All the plutonic rocks associated with the skarn formation display steep LREE-enriched REE patterns with minor positive Eu anomalies (Eu/Eu* = 0.98–1.35), suggesting a subduction-related volcanic arc setting similar to other granitoids in the Ulukışla Basin. The Horoz skarn exhibits both endoskarn and exoskarn features, while the Esendemirtepe-Koçak deposit is characterized by typical exoskarn features. Dominant ore minerals in both skarn deposits include magnetite, hematite, sphalerite, chalcopyrite, and pyrite, with minor arsenopyrite, galena, and cobaltite. The mineral composition of the skarn also shows the dominance of Na-rich and Mg-rich minerals in both locations. The geochemical compositions of the I-type, metaluminous Esendemirtepe-Koçak, and Horoz plutonic rocks are compatible with Fe-Zn skarn type deposits based on the moderate MgO (0.36–4.44 wt.%) and K₂O (1.38–7.99 wt.%), and Rb/Zr and Sr/Zr ratios. They also show typical volcanic arc features, and the variation in various trace element concentrations shows similarity with Fe-Zn skarn type granitoids. These findings support a strong genetic relationship between the mineralization and the geochemical and mineralogical characteristics of the associated plutonic rocks.

1. Introduction

Skarn deposits are an important class of mineral deposits in Türkiye that formed between the Late Cretaceous and Oligo-Miocene period during the collision between the Afro-Arabian and Eurasian plates [1]. These deposits in Türkiye are mostly calcic exoskarns but sometimes occur as magnesian skarns with economic mineralization mainly restricted to the calcic exoskarns and some Fe-skarns hosted by endoskarns. There are many known and operating Fe-Pb-Zn deposits in the Taurus Mountains. In the Central Taurus Mountains, the main Fe-Zn skarn deposits are the Horoz, Esendemirtepe (Çiftehan, Niğde Province) and Karamadazi (Yahyali, Kayseri Province) deposits. These deposits have attracted the attention of several researchers in recent years, and many different ideas have been put forward regarding the origin of the deposits. Some of the researchers mentioned that the deposits are generally hosted in different stratigraphic units at different ages and are mostly epigenetic and sedimentary in origin [2]. Also, other authors working in the Central Taurus Mountains have proposed that the deposits in the region have a hydrothermal metasomatic origin and occur in a fault-controlled form and fill into karstic cavities (e.g., [3,4]).

It is estimated that the total reserve of the iron in the Fe-Zn skarn deposits in the Central Taurus Mountains is about 200,000 tonnes [5]. The endoskarn and exoskarn zones where the Fe-Zn skarn ore developed are in limestone/marble-granodiorite/volcanite contact, wherein the intrusions intersect the metacarbonate lithologies. The Fe-Zn skarns associated with granitoids in the Central Taurus Mountains, especially in the Niğde area, are the adakitic Horoz granitoids [6]. The exact age of the Horoz granitoids is still a controversy in the scientific discourse, though most authors have proposed an Early-Middle Eocene age (e.g., [6,7,8]). For instance, Ref. [7] reported a zircon U–Pb radiogenic age of 55 Ma for the Horoz granitoids, which is similar to the zircon U–Pb age of 54.74 ± 0.37 Ma reported by [9].

The granitoid plutons in the Niğde Province were emplaced in the eastern part of the Bolkar Mountains and in the valley where the Çiftehan village is located in Ulukışla District (Figure 1). In this area, there are several large- and small-scale mines but only a small number of them are currently under operation [10,11]. The geological [12,13,14,15], petrological [6,7,8,9,16,17], geodynamic and tectonic features [18,19,20,21,22,23,24,25], sediment and ore slag geochemical characteristics [26,27], and detailed studies on the mineral potential of the region [5,28,29,30,31] date back to the 1940s and still continue into the present day. However, the relationship between the Fe-Zn skarns and the associated granitoid plutons, and the possibility of identifying different styles of mineralization related to the granitoid emplacement, are not yet documented in the literature.

Studies on the petrogenetic characteristics of the intrusions in the Çiftehan (Ulukışla-Niğde) area and the skarn formation are lacking in the literature. In this regard, the fluid-aided mobility of elements at the contact between the intrusions and the volcano-sedimentary country rocks is not well understood. In addition, the relationship between the plutons and the associated Fe-Zn skarns, and the possibility of identifying different styles of mineralization related to the granitoid emplacement are not yet documented in the literature. Correlations between granitoid compositions and metal contents of the associated skarns have been reported elsewhere to provide vital constraints on the genetic relation between both. For instance, Ref. [32] analyzed the composition of the Evciler granitoid in Çanakkale, northwestern Türkiye, and found that its geochemical characteristics closely resemble those of metasomatic zones in which the skarn deposits formed. This might indicate that both the emplacement of the granitoids and the skarn formation were contemporaneous from a common source. Similar observations were made in India [33], Mexico [34], Spain [35], Romania [36], Canada [37], Australia [38], Japan [39], and the USA [40]. Therefore, this study is aimed at investigating the geology, petrography, and whole-rock geochemistry of the skarns and associated granitoids in the Çiftehan (Ulukışla-Niğde) area, south-central Türkiye. The main objective is to constrain the petrogenesis of the granitoids and to establish the relationship between the metal contents in the skarns and the whole-rock geochemistry, degree of crystallization, and tectonic settings of the plutonic rocks. Overall, the present study will be the first detailed report of comparisons between the skarns and associated plutonic rocks in the Çiftehan (Ulukışla-Niğde) area, south-central Türkiye.

2. Geological and Geodynamic Settings

The evolution of the Anatolian plate is related to tectonic regimes during the opening and closure of the Tethyan Ocean, which led to the development of paleo basins with different characteristics [18]. The Ulukışla Basin is one of the most important Late Cretaceous-Cenozoic basins in Central Anatolia. It is located in the southern fringe of Central Anatolia (Figure 1) and is geographically bordered to the north by the Niğde Massif, to the south by the Bolkar Mountains and the İvriz Detachment Fault, and to the east by the Aladağ Mountains and the Ecemiş Fault Zone [25,41].

A variety of rock types crop out in the area with age values mostly from Paleozoic to Quaternary. The basement rocks are composed of Late Permian to Late Cretaceous marble and schist belonging to the Bolkardagi Unit (Figure 1), which are then tectonically overlain by Late Cretaceous Alihoca ophiolite. The Alihoca ophiolite is adjacent to ophiolitic mélange made up of peridotite, gabbro, diabase, and diorite [42]. The Early-Middle Eocene adakitic Horoz granitoids intruded on the basement rocks near the Horoz stream [6]. The Cretaceous Kalkankaya Formation, which consists of limestone, sandstone, and shale, overlies the ophiolitic mélange and the basement rocks [26]. Quaternary alluvial deposits cover the Kalkankaya Formation and several units in the southwestern portion of the area (Figure 1). The volcano-sedimentary rocks in the region are approximately 5 km thick and are determined to be formed from the Late Cretaceous to Early Oligocene period [43], while volcanic rocks have a thickness of around 2 km [20]. Most of the volcano-sedimentary units formed in the Late Paleocene to Middle Eocene Ulukışla Formation; however, only sedimentary rocks are observed in the Middle Eocene Hasangazi Formation [26].

The volcanic rocks in the region consist of alkaline and potassic volcanic units. Alkaline volcanic rocks are present as pillow lavas and massive lavas, whereas the potassic volcanic rocks occur as dykes and massive lavas [43]. Most of the alkaline volcanic rocks are intercalated with sedimentary rocks and are intersected by the Upper Cretaceous Elmalı syenite (Figure 1).

Various ore deposits are found in the Ulukışla Basin within the Tauride belt in Türkiye. The primary mineral deposits in the area include Fe-Zn skarns at the Horoz granitoids and metacarbonate rocks interface, MVT- and CRD-type Pb-Zn and non-sulfide Zn deposits in carbonate rocks, podiform chromite in ophiolites, vein-type Sb-W-Hg-Au in the Niğde Massif, bauxite, Al and Fe-rich laterite, and Mn deposits [1,26,44,45,46]. Small-scale thrust faults greatly influence the rheology of the geological units and aid in ore settlement.

The Niğde Massif at the northern part of the Ulukışla Basin is predominantly composed of high-grade metamorphic rocks (quartzite, marble, and paragneiss) and ophiolites that are intruded on by slightly metamorphosed granitoids [47,48]. It is believed that the metamorphic rocks in the Niğde Massif, which are members of the Central Anatolian Crystalline Complex (CACC), were initially exposed in the late Cretaceous period and were exhumed prior to the end of the Maastrichtian period (e.g., [49,50]). Previous research works in the Niğde Massif have proposed that after the closure of the Neotethys Ocean during the late Cretaceous period, the massif was subjected to a different geodynamic evolutionary history when compared to the other parts of the CACC [19]. Moreover, Ref. [19] indicated that in the Oligocene-late Miocene, the Niğde Massif was exhumed under an extensional regime as a core complex with a low-angle normal fault trending from the southern to the northwestern fringes of the massif. This proposition that the Niğde Massif was exhumed as a core complex has largely been accepted but there are a lot of disagreements on the timing of the exhumation and the exact position of the low-angle normal fault that triggered the exhumation of the massif. For example, it has been proposed that the low-angle normal fault extended from the top to the northeastern part, which resulted in the exhumation of the massif in the Campanian-early Paleocene period [22,47]. Also, other studies have suggested poly-burial exhumation episodes for the massif (e.g., [50,51,52,53]). Paleontological and geochronological data revealed that the first episode of exhumation occurred before the Eocene period [22] and in the Oligocene period, and reburial occurred at the eastern domain of the massif [53]. The second episode postdates the Oligocene period and may have occurred in the Miocene period [54].

In the southern fringe of the Ulukışla Basin, a fault developed along the northern margin of the Bolkar Mountains and is termed the “Bolkar Thrust Fault” [55]. It is along this fault that the rocks belonging to the Bolkar group were thrusted to fill up the Ulukışla Basin [18]. However, other studies have proposed that the fault in the northern margin of the Bolkar Mountains is actually a normal fault known as the “Bolkar Front Fault” [56]. Dilek et al. [56] mentioned that the Bolkar Front Fault is a high-angle normal fault that strongly contributed to the evolution of the Ulukışla Basin. This is consistent with the findings of [20,21], who further suggested that the normal fault facilitated the emplacement of ophiolites to the north of the Bolkar Mountains. However, more recent studies confirmed the earlier thrust fault hypothesis (e.g., [7,23,57]). Moreover, refs. [25,41] stated that the fault is rather a detachment type of fault that was active in the late Cretaceous-Eocene period and is termed the İvriz Detachment Fault, which is trending ENE–WSW and extending between the Bolkar Mountains and the Ulukışla Basin. This has since brought more understanding on the geological evolution of basins in Central Anatolia from a previous perception of collision-related basins to supradetachment basins.

The eastern fringe of the Ulukışla Basin features the left-lateral Ecemiş Fault Zone, one of the important active structures controlling the evolution of basins in Central Anatolia [58]. The Ecemiş Fault Zone was initiated in the late Eocene with geological offsets in the range of 60 ± 5 to 80 ± 10 km [59,60]. The fault kinematics suggest a transition from strike-slip during the middle-late Miocene period to transtensional and extensional in Pliocene-Quaternary period [24,61].

3. Materials and Methods

Field work for this study involved geological mapping, delineating the igneous plutons, the skarn, and contact with the carbonate rocks. The sampling traverses were carried out across the skarn and the host volcano-sedimentary rocks. During the field studies, more than 200 samples were collected from the various rocks in the Esendemirtepe and Horoz areas. Representative samples (n = 30) were selected for thin section preparation to help in understanding the mineral paragenetic sequence. A total of eleven (11) samples were selected and pulverized in an electric agate mortar for whole-rock geochemical analysis. The pulverized samples were analyzed for major elements and selected trace elements by XRF technique at the Central Laboratory of Niğde Ömer Halisdemir University, Türkiye. Some of the powders were analyzed for additional trace and rare earth element (REE) compositions by ICP-MS, following fusion with lithium metaborate/tetraborate, at the Department of Geological Engineering, Istanbul Technical University, Türkiye.

The mineralogical composition of the skarns was determined from twenty-two samples by a D8 ADVANCE BRUKER X-ray diffractometer (XRD) (Billerica, MA, USA) in the Central Research Laboratory of Niğde Ömer Halisdemir University. The XRD instrumentation comprised a theta with a copper sealed tube X-ray source producing Cu Kα radiation at a wavelength of 1.5406 Å from a generator operating at 40 kV and 40 mA. The X-ray energy range of operation was 3–30 keV with a detection limit of ~1% by volume. The XRD data were collected over a 2θ range of 2° to 70°, and the resulting diffraction patterns were identified and indexed with the aid of X’Pert HighScore Plus v. 3.0 software for mineral identification and semi-quantitative mineral content analysis.

4. Results

4.1. Geological, Stratigraphic, and Field Relations

The study area is part of the Late Cretaceous-Tertiary Ulukışla Basin, which developed in different tectonic units (Figure 2). The area is observed in three sections (southern, middle, and northern) that are distinct from stratigraphical and lithological points of view (Figure 2). Whilst the southern section consists of sedimentary rocks that are intruded on by some granitoids and dykes, which developed on the Bolkar Mountain Group, the northern section consists of volcano-sedimentary and plutonic rocks, which developed above the Niğde Group (Figure 2). Also, the middle section developed on the Alihoca ophiolite complex in the form of volcano-sedimentary rocks (Figure 2). The plutons related to the skarn mineralization were emplaced in the southern and middle sections. They are located at the eastern fringe of the Bolkar Mountains in the Niğde Province of south-central Türkiye. The Bolkar Mountains are contiguous to the Inner Taurides Suture Zone sandwiched between the Taurides and the CACC [8]. Details of the southern and middle sections, where the skarn mineralization occurs, are given in the following Sections.

4.1.1. Southern Section

The oldest lithological units in the area form the heights of the Bolkar Mountain with an anticlinal architecture. These units are composed of metamorphosed Permian-Triassic platform carbonates with intercalations of siliciclastic rocks, which were later intruded by the ENE–WSW-trending sill-like pluton known as the Horoz granitoid in the Horoz village with the associated economic skarn mineralization (Figure 2). Locally, the Horoz granitoid is a shallow-seated intrusion that crops out in the Horoz Valley towards the Horoz village. It is proposed that the Horoz granitoid was emplaced in the Early Eocene period (56.1 Ma) [7]. The Horoz granitoid appears as a light-colored medium-fine-grained intrusion with some chilled margins with the country rocks [8]. It contains mafic microgranular enclaves, indicative of mingling during the evolution of the felsic magma. Previous studies suggest that the granitoid can be classified into two different groups, granodiorite and felsic granite, which have a gradational contact [8]. Locally, the felsic granite is relatively fine-grained, fractured, and altered with fewer enclaves than the granodiorite.

The Horoz granitoid pluton intrudes marbles belonging to the Bolkar Mountain units, which are also intruded by tonalitic and dioritic dykes and some schist-like units (Figure 2). According to [8], aplite veins crosscut the unit. The entire unit is overlain by the Upper Cretaceous Alihoca Ophiolites (Figure 2), which have been thrusted to form an ophiolitic mélange. An erosional episode postdates the thrusting event with the Cretaceous Kalkankaya Formation unconformably overlying the ophiolitic unit. This formation generally consists of limestone and conglomerate (Figure 3). It was first named by [13] according to Kalkankaya Hill southwest of Alihoca village. Later, Refs. [55,62] also adopted the same name. The conglomerate level at the base is red and grain supported. The ophiolitic rocks cover the recrystallized limestone pebbles. The pebbles are generally medium sized and not very rounded. The limestone levels of the formation are light gray, medium-thick bedded (Figure 4a), and contain abundant Loftusia and rudist crustal limestone. The formation unconformably overlies the Bolkardağı Group, which contains marble-schist-like rocks interbedded with the Horoz granitoids (Figure 4b–d). There is also an aplite dyke intruding the Bolkardağı Group towards the Kalkankaya units (Figure 4e). Overlapping units known as the Madenköy ophiolitic mélange also underlie the formation after an erosion zone. However, the Madenköy ophiolitic mélange is mainly confined to the eastern boundary of the Ulukışla Basin and is therefore not part of the study area. The Yağbağ Formation conglomerates between Kevenniboyun and Sivri Tepe on the road to Horoz village also belong to this formation. The Kalkankaya Formation is about 300 m thick. Minor Luftusia and Omphalocyclus sp. obtained from the limestone levels indicate that the age of the unit is Late Maastrichtian [15]. However, the fauna of Miliolidae, Microcodium, and Mississipina sp. indicate Late Maastrichtian-Early Paleocene age [15]. On this basis, the formation may have been deposited under shallow shelf conditions.

The Yağbağ Formation consists of limestone, including sandy varieties (Figure 3). Yağbağ, located in the southwestern part of Alihoca village, is named after the Yağbağ stream [15]. The formation is mainly represented by black, thin-medium bedded, sometimes sandy, bituminous limestone. It occasionally includes black clayey limestone interbeds and brecciated dolomite (Figure 4f). The formation unconformably overlies the Madenköy ophiolitic mélange and is transitional with Saçkayası limestone on top. Its maximum thickness is 50 m. Globorotallia sp. and Globigerina sp. fauna obtained from the clayey limestone levels indicate Paleocene age [15]. It is likely that the formation was deposited beneath these features.

The Geyik Pinar conglomeratic unit is the conjugate unit of the Yağbağ Formation and consists entirely of conglomerate (Figure 3). It also contains small amounts of ophiolitic rock pebbles in a calc-silicate unit (Figure 4g). The pebbles are sub-angular and poorly sorted. The cracks in the calc-silicate unit are filled with sphalerite ore (Figure 4h). Previously, Ref. [55] included the Geyik Pinar conglomeratic unit in the Halkpinar Formation, but [15] separated it as a member of the Yağbağ Formation. Its thickness is a maximum of 100 m. It is transitional with the Kirkpinar Formation and was probably deposited under riverine conditions.

The Saçkayası limestone unit (Figure 3) is named after Saçkaya Hill in the north of Madenköy. It is also a member of the Yağbağ Formation. However, it appears to be a later unit. It consists of thin-medium-bedded claystone and alternating sandstone. The middle level consists of off-white, thick-bedded sandstone and abundant fossils. The upper level is represented by massive limestone. The unit is about 20 m thick and was probably deposited in a shallow shelf environment.

The Kırkpınar Formation consists of conglomerate, limestone and sandstone–cla–stone succession (Figure 3). It is named after the Kırkpınar Spring southwest of Madenköy. Demirtaşlı et al. [55] considered the unit as a part of the Halkpinar Formation. The formation consists of limestone at the lower levels and sandstone-shale succession at the upper levels. The limestone level is generally thin to medium-bedded. The upper-level units are laminated and contain sandstone lenses showing channel geometry. Within the sandstones, ophiolitic rock fragments and claystone forming fault breccia within the formation are observed (Figure 4h). The Kırkpınar Formation, which is transitional with the underlying Yağbag Formation, is 350–400 m thick. According to the fauna found in it, the formation is Lutesian in age and was deposited in a wave base environment [15].

4.1.2. Middle Section

The middle section of the Esendemirtepe-Koçak area predominantly consists of volcano-sedimentary formations intruded on by igneous rocks, dated from the Late Cretaceous to Middle Eocene (Figure 5). The basement begins with the Alihoca Ophiolitic Complex, including quartz porphyry and dioritic intrusions associated with skarn mineralization. These units are overlain by the Kırkgeçit Formation, which comprises limestone, sandstone, conglomerate, shale, and basalt [15,55].

The Kırkgeçit Formation, named after the Kırkgeçit Stream, includes limestone, sandstone–shale alternations, and basaltic units. Discontinuous limestone sections dominate from the northeastern border to the southwest (Figure 6a). The Karakaya basaltic unit, composed of pillow lavas and flow breccia, overlies the Alihoca Ophiolitic Complex and transitions into the Çiftehan Unit. Fossil evidence suggests a Maastrichtian age [15,55].

The Tabaklı Formation, named after the Tabaklı village, contains sandstone and limestone intercalations with planktonic foraminifera such as Abathomphalus mayaroensis and Morozovella uncinata. This formation, deposited under wave-base conditions, is Early-Middle Paleocene in age and transitions into the Hasangazi Formation [15].

The Hasangazi Formation features sandstone–shale alternations interspersed with mudstones and conglomerates. Missing Bouma sequences and fossil records indicate a Middle-Early Eocene (Lutetian) age. The Bozbel Tepe andesitic unit, part of this formation, contains red micritic limestone fragments and continues with the Ardıçlı Formation [15,55].

The Ardıçlı Formation, characterized by andesite, trachyandesite, and pillow lavas, reflects Early-Middle Paleocene to Middle-Early Eocene seafloor development. Fossil micritic limestone fragments suggest a marine setting [15].

The Zeyve Evaporite, deposited after erosion, consists of gypsum, sandstone–mudstone, and anhydrite alternations. It shows angular unconformity with the underlying Hasangazi Formation and dates back to the Oligocene. Quaternary alluvium caps the sequence in this section of the study area [14,63].

4.2. Petrographic Characteristics of Plutonic Rocks

4.2.1. Monzogabbro

Monzogabbroic rocks are widespread in the Çifteköy village. They exhibit a granular (holocrystalline) texture containing biotite phenocrysts and some undifferentiated opaque minerals (Figure 7a,b). The minerals are partially altered so that the feldspars cannot be recognized. Plagioclase, orthoclase, and biotite minerals are highly altered due to alkaline metasomatism. Therefore, calcitization, epidotization, and argillization are very intense, indicating that calcic alteration developed over sodic alteration (Figure 7a,b).

4.2.2. Granodiorite

Granodiorites are found only in the Horoz region in the study area. They exhibit granular (holocrystalline) textures containing quartz, plagioclase, orthoclase, microcline, amphibole, and biotite phenocrysts (Figure 7c). They also show microcrystalline porphyritic textures, but locally glomeroporphyric textures are common. They contain phenocrysts of quartz, plagioclase, amphibole, biotite, and muscovite (Figure 7d,e). The minerals are partially altered towards the skarn zone, such that the feldspar minerals show intense epidotization. Wavy extinction is common in quartz minerals (Figure 7c–e). Lattice twinning is evident in microcline (Figure 7c), and zoning is observed in plagioclase (Figure 7d).

Other granodiorites in the Horoz area exhibit hypidiomorphic granular and myrmekitic textures with quartz, orthoclase, biotite, and plagioclase as the dominant phenocrysts (Figure 7f). Plagioclase typically shows polysynthetic twinning and zoning, while quartz shows undulose extinction (Figure 7f), indicating stages of deformation. Plagioclase feldspar is partially altered around the skarn zone with calcitization and silicification as common types of alteration associated with the granodiorite.

4.2.3. Diorite

Among the intrusive rocks surfacing in the study area, diorites are mostly located in the Esendemirtepe-Koçak region. They exhibit microcrystalline and porphyritic textures where quartz, orthoclase, plagioclase, biotite, epidote, chlorite, and undifferentiated opaque minerals are the dominant minerals (Figure 7g,h). Plagioclase feldspar minerals are transformed into epidote (Figure 7g) during the skarnization process. Zonation is observed in places in plagioclase (Figure 7h). Sericitization and calcitization are also common (Figure 7h).

Overall, the microscopic images of the plutons in the region show that porphyritic textures are common and thus, it can be said that the plutons crystallized in a shallow environment.

4.3. Whole-Rock Geochemical Constraints

4.3.1. Major Element Variation and Rock Classification

The different types of plutons were distinguished by field observations and petrographic studies as monzogabbro, granodiorite, and diorite. Results of the whole-rock geochemical analysis of the plutonic rocks are presented in

Table 1. Whole-rock geochemistry of plutonic rocks in the Çiftehan (Ulukışla-Niğde) area, south-central Türkiye (ND = Not Detected).

Sample No.	C1	C3	H3B	H16	H19	KE1	EM13	EM10	E4B	ED2A	ED2C
Rock Name	Çifteköy Monzogabbro		Horoz Granodiorite			Koçak Diorite	Esendemirtepe Diorite
Major elements (wt.%)
SiO2	40.5	40.0	71.0	67.6	71.2	56.4	59.9	56.4	60.5	57.8	55.6
Al2O3	18.9	18.8	15.7	15.2	13.8	17.8	16.7	17.2	17.2	18.5	18.5
Fe2O3	13.9	14.9	1.63	4.83	2.15	8.69	7.12	8.04	5.08	6.13	6.19
CaO	10.2	11.3	2.93	3.33	1.85	4.27	1.95	3.33	2.11	2.73	3.13
MgO	4.49	4.14	0.42	0.60	0.36	2.80	0.95	2.19	1.44	2.22	1.86
Na2O	1.71	1.50	4.05	3.42	3.02	4.61	5.24	4.95	4.71	4.16	3.66
K2O	2.75	1.96	2.68	3.11	4.85	2.13	3.96	1.38	5.18	4.58	6.66
MnO	0.07	0.07	0.02	0.02	0.01	0.05	0.06	0.02	0.04	0.09	0.09
TiO2	1.84	2.16	0.33	0.29	0.18	0.66	0.50	0.76	0.90	0.77	0.93
P2O5	1.15	1.26	0.18	0.19	0.11	0.19	0.13	0.20	0.30	0.36	0.42
Cr2O3	0.01	0.00	0.01	0.00	0.01	0.01	0.00	0.00	0.01	0.02	0.01
LOI	3.50	2.29	0.84	1.15	2.30	2.03	2.95	3.82	1.83	1.90	1.95
Total	99.0	98.4	99.8	99.7	99.8	99.7	99.4	98.3	99.3	99.2	99.1
Trace elements (ppm)
Cr	68.4	26.0	34.2	27.4	54.8	34.2	27.4	27.4	34.2	130	82.1
Ni	39.3	29.8	23.6	47.1	7.86	15.7	15.7	7.86	15.7	62.8	23.6
Pb	27.9	11.8	10.2	9.29	ND	ND	27.9	9.29	9.29	9.29	9.29
Cu	7.99	8.79	7.99	7.99	24.0	104	31.9	39.9	31.9	16.0	7.99
Mn	531	568	157	164	106	393	475	147	280	702	678
V	448	482	44.8	28.0	39.2	168	33.6	140	84.0	140	118
Ga	13.1	13.0	10.1	11.8	7.50	13.5	11.8	13.0	12.8	11.1	12.9
Rb	99.1	79.3	60.0	71.8	113	39.5	102	48.3	104	135	154
Pd	0.02	0.07	0.08	0.01	0.01	0.02	0.02	0.06	0.02	0.02	0.02
Rh	0.24	0.24	0.01	0.01	ND	0.02	ND	0.01	ND	0.04	0.01
In	0.00	0.00	0.00	0.00	0.00	0.02	0.02	0.08	0.00	0.00	0.05
Cs	3.17	4.30	0.53	0.56	3.02	0.68	0.84	3.21	0.49	2.68	0.86
Tl	0.47	0.42	0.07	0.22	0.26	0.16	0.69	0.23	0.45	0.78	0.74
Zr	133	112	111	118	59.2	81.4	355	81.4	311	185	185
Y	19.4	26.4	17.3	13.3	10.9	12.1	19.0	18.7	21.0	17.0	21.0
Th	8.26	12.0	13.7	9.72	14.9	3.14	35.0	2.12	33.7	19.6	23.3
U	1.39	2.16	3.66	3.35	3.10	0.80	6.32	0.31	3.62	1.86	2.65
Hf	3.05	15.9	0.79	1.73	0.56	0.21	5.61	0.17	3.28	2.06	2.53
Nb	8.50	4.30	7.70	10.5	6.60	10.9	5.80	4.86	8.30	5.95	8.70
Rare earth elements (ppm)
La	63.1	68.4	19.9	113	20.7	11.9	88.0	11.4	123	87.2	109
Ce	138	148	44.0	160	39.8	24.5	157	24.9	232	158	209
Pr	17.1	18.5	5.25	5.18	4.22	2.93	16.0	3.24	25.0	17.2	22.8
Nd	65.3	72.0	18.6	19.4	15.2	11.9	50.1	14.2	87.8	61.4	75.7
Sm	12.6	12.6	3.54	3.30	2.69	2.81	7.57	3.38	14.15	9.88	13.6
Eu	3.29	4.11	1.01	0.95	0.80	1.01	2.11	1.29	4.22	3.55	4.58
Gd	8.36	9.99	2.80	2.65	2.01	2.57	5.17	3.70	8.52	6.56	8.42
Tb	1.09	1.33	0.46	0.44	0.37	0.35	0.73	0.57	1.07	0.89	1.13
Dy	4.85	6.12	2.91	2.21	2.31	1.97	3.97	3.50	4.88	3.92	5.18
Ho	0.82	0.91	0.52	0.44	0.34	0.41	0.74	0.68	0.79	0.68	0.75
Er	2.03	2.75	1.65	1.61	1.09	1.30	2.34	2.17	2.49	1.95	2.39
Tm	0.16	0.21	0.18	0.13	0.10	0.15	0.27	0.22	0.24	0.11	0.25
Yb	1.42	1.94	1.60	1.51	1.50	1.32	2.12	1.90	1.81	1.44	1.65
Lu	0.20	0.31	0.18	0.23	0.19	0.15	0.32	0.27	0.28	0.17	0.22
∑REE	318	347	103	311	91	63	336	71	506	353	455
Eu/Eu*	0.98	1.12	0.98	0.98	1.05	1.15	1.03	1.12	1.17	1.35	1.31
(La/Yb)N	27.4	21.7	7.66	46.1	8.50	5.55	25.6	3.70	41.9	37.3	40.7
(Gd/Yb)N	4.32	3.78	1.28	1.29	0.98	1.43	1.79	1.43	3.45	3.34	3.74
(Ce/Yb)N	23.2	18.2	6.55	25.2	6.32	4.42	17.6	3.12	30.5	26.1	30.2

. The monzogabbro displays major element variation in SiO₂ (40.0–40.5 wt.%), Al₂O₃ (18.8–18.9 wt.%), Fe₂O₃ (13.9–14.9 wt.%), CaO (10.2–11.3 wt.%), MgO (4.14–4.49 wt.%), Na₂O (1.50–1.71 wt.%), K₂O (1.96–2.75 wt.%), MnO (0.07–0.07 wt.%), TiO₂ (1.84–2.16 wt.%), P₂O₅ (1.15–1.26 wt.%), and Cr₂O₃ (0.00–0.01 wt.%) (Table 1). The granodiorite also displays major element variations in SiO₂ (67.6–71.2 wt.%), Al₂O₃ (13.8–15.7 wt.%), Fe₂O₃ (2.15–4.83 wt.%), CaO (1.85–3.33 wt.%), MgO (0.36–0.60 wt.%), Na₂O (3.02–4.05 wt.%), K₂O (2.68–4.85 wt.%), MnO (0.01–0.02 wt.%), TiO₂ (0.18–0.33 wt.%), P₂O₅ (0.11–0.19 wt.%), and Cr₂O₃ (0.00–0.01 wt.%) (Table 1). Similarly, the diorite displays major element variations in SiO₂ (55.6–60.5 wt.%), Al₂O₃ (16.7–18.5 wt.%), Fe₂O₃ (5.08–8.69 wt.%), CaO (1.95–4.27 wt.%), MgO (0.95–2.80 wt.%), Na₂O (3.66–5.24 wt.%), K₂O (1.38–6.66 wt.%), MnO (0.02–0.09 wt.%), TiO₂ (0.50–0.93 wt.%), P₂O₅ (0.13–0.42 wt.%), and Cr₂O₃ (0.00–0.02 wt.%) (Table 1). This indicates that the plutons are fairly enriched in both alkali elements and ferromagnesian oxides, which is consistent with their mineralogy. The granodiorite and diorite are more abundant in alkali elements than the monzogabbro, whereas the monzogabbro contains more ferromagnesian oxides (Table 1). Also, the diorite is slightly more enriched in alkali elements than the granodiorite (Table 1). The Na₂O contents correlate positively with increasing silica (Figure 8a,b), whereas the K₂O, Al₂O₃, Fe₂O₃, CaO, MgO, TiO₂, and P₂O₅ contents correlate negatively with increasing silica content (Figure 8c–h). The major element variation diagrams also reveal that the Al₂O₃, Fe₂O₃, CaO, MgO, TiO₂, and P₂O₅ values appear to be more depleted in the granodiorite and diorite than in the monzogabbro (Figure 8a–h).

The major element compositions of most plutons associated with the skarn deposits indicate calc-alkaline affinities but the monozogabbro shows tholeiitic affinity in the [64] classification diagram (Figure 9a). All the studied plutons plot as two distinct groups in the AFM diagram [64]. The monzogabbro plots in the high Fe₂O₃ (Figure 9a) and low SiO₂ fields (Figure 9b), whereas the granodiorite and diorite plot in the moderate-to-high total alkali field (Figure 9a) and high SiO₂ fields (Figure 9b). This trend is similar to granitoids from the Ulukışla-Niğde area (Figure 9b) [7,16,17,65]. Also, on the Al-saturation index diagram of [66], the studied plutonic rocks all have metaluminous compositions except for the Koçak diorite and one sample from the Horoz granodiorite, which plots in the peraluminous field (Figure 9c).

In this study, granite discrimination diagrams were used to interpret the geochemical characteristics of monzogabbro, diorite, and granodiorite, despite their compositional differences, in order to provide a comparative framework for understanding their petrogenetic relationships. While granite diagrams are traditionally designed for felsic rocks and may not fully capture the geochemical behavior of mafic rocks like monzogabbro, their use here is justified by the need to evaluate all three rock types within a unified geochemical context. This approach allows for a clearer visualization of magmatic differentiation trends and potential genetic links among the rocks. The authors acknowledge the limitations of applying felsic-based diagrams to mafic compositions; however, by normalizing the data and interpreting the results cautiously, these diagrams offer valuable comparative insights, especially when the rocks are part of the same intrusive suite or magmatic system. Therefore, in the classification diagram of Winchester and Floyd [67], the plutonic rocks in the study area form a continuous spectrum from the field of felsic rocks (granodiorite) to the field of mafic-ultramafic plutonic rocks (Figure 9d). The Horoz granodiorite, Esendemirtepe diorite, and Koçak diorite all plot in the field of felsic rocks, whereas the Çifteköy monzogabbroic rocks characteristically plot in the field of mafic-ultramafic plutonic rocks due to their high TiO₂ and low Zr contents (Figure 9d).

Figure 9. Rock classification showing (a) calc-alkaline affinity on the AFM diagram [64], (b) alkaline to subalkaline affinity, (c) the Al-saturation index [68], and (d) the continuous spectrum from the field of felsic rocks (granodiorite) to the field of mafic-ultramafic plutonic rocks [67]. Legend: [7,16,17,65].

Figure 9. Rock classification showing (a) calc-alkaline affinity on the AFM diagram [64], (b) alkaline to subalkaline affinity, (c) the Al-saturation index [68], and (d) the continuous spectrum from the field of felsic rocks (granodiorite) to the field of mafic-ultramafic plutonic rocks [67]. Legend: [7,16,17,65].

4.3.2. Trace and Rare Earth Element (REE) Variations

Trace and REEs have been proven to be one of the best tools in petrogenetic studies since these elements can indicate the source of a melt as well as when a melt is differentiated or contaminated by crustal materials during its ascent [69]. The petrogenetic characteristics of the studied plutonic rocks show contrasting differences among the monzogabbro, granodiorite, and diorite. The emplacement of the monzogabbroic rocks likely played a significant role in the formation of the skarn deposits in the area and they display the lowest Rb (79.3–99.1 ppm) and Th (8.26–12.0 ppm) contents (Table 1; Figure 10). However, the diorite displays the highest Rb (39.5–154 ppm) and Th (2.12–34.95 ppm) contents (Table 1; Figure 9). The Rb (60.0–113 ppm) and Th (9.72–14.9 ppm) contents of the granodiorite are also more enriched than that of the monzogabbro (Table 1; Figure 10). The Y content of monzogabbro (19.4–26.4 ppm) is higher than that of the granodiorite (10.9–17.3 ppm) and the diorite (12.1–21.0 ppm) (Table 1; Figure 10). The monzogabbro is also fairly enriched in Zr (112–133 ppm), whereas the granodiorite (59.0–118 ppm) and diorite (81.4–355 ppm) show moderate to high enrichment in Zr, respectively (Table 1; Figure 10). The Cr content of the monzogabbro is low (26.0–68.4 ppm) compared to that of the granodiorite (27.4–54.8 ppm) and diorite (27.4–130 ppm) (Table 1; Figure 10). However, the V content of the monzogabbro (448–482 ppm) is higher than that of the granodiorite (28.0–44.8 ppm) and diorite (33.6–168 ppm) (Table 1). The other trace elements like Cr, Ni, Ga, Cd, In, Cs, Tl, and U show low concentrations in all the plutonic rocks (Table 1).

The diorite shows considerable enrichment in REEs compared to monzogabbro and the granodiorite (Table 1). The total REE abundance in the monzogabbro is in the range of 318–347 ppm, whereas those of the granodiorite and diorite are in the range of 91–311 ppm and 63–506 ppm, respectively (Table 1). The Eu/Eu* values for the monzogabbro vary from 0.98 to 1.12 while the Eu/Eu* values for the granodiorite and diorite vary from 0.98 to 1.05 and 1.03–1.35, respectively (Table 1). The monzogabbro exhibits (La/Yb)_n ratios in the narrow range of 27.4–21.7 and (Ce/Yb)_n ratios of 18.2–23.2. The (Gd/Yb)_n ratios (3.78–4.32) of the monzogabbro are also elevated (Table 1). Moreover, the granodiorite shows a broader range of REE fractionation, with (La/Yb)_n ratios in the range of 7.66–46.1, (Ce/Yb)_n ratios in the range of 6.32–25.2, and (Gd/Yb)_n ratios in the range of 0.98–1.29 (Table 1). The diorite also shows a wide range of REE fractionation characterized by (La/Yb)_n ratios (3.70–40.7), (Ce/Yb)_n ratios (3.12–30.5), and (Gd/Yb)_n ratios (1.43–3.74) (Table 1).

4.4. Skarn Occurrences

4.4.1. Esendemirtepe-Koçak Skarn

The skarn mineralization in the Esendemirtepe-Koçak area occurs as an exoskarn characterized by various calc-silicate minerals along the skarn zone. It is classified as an exoskarn because these kinds of skarns are identified by the overlap in both location and timing of an intrusive body with sedimentary wall rocks [70]. However, in the field, there was no clear contact between the Esendemirtepe-Koçak diorites and the skarn mineralization and thus the endoskarn zone could not be determined. Moreover, mineralization was mostly determined within the exoskarn towards the outer zone, mostly in the hornfelsic zone. Therefore, alteration in the Esendemirtepe-Koçak area is predominantly affecting the sedimentary wall rocks with retrograde metamorphism evidenced by intense epidotization and transformation of pyroxene minerals into actinolite.

At the contact between the volcano-sedimentary rocks and the skarn, several centimeter-scale irregularly distributed hematite assemblages are accompanied by garnet, actinolite, calcite, and quartz (Figure 11a). At this contact, a highly fractured structure dominated by magnetite with calcite and chalcopyrite filling in the fractures was observed, indicating that magnetite formed before calcite (Figure 11b). Epidote and chalcopyrite inclusions are observed around fracture-filling-type magnetite (Figure 11c). A few meters away from the skarn zone, epidote continues to develop within the volcanic breccia (Figure 11d). There are quartz veins cross-cutting the epidote grains (Figure 11e). The Esendemirtepe-Koçak skarn zone is characterized by irregular garnet aggregates coexisting with calcite, quartz, and hematite (Figure 11f). Such garnet aggregates are observed close to the ore body and range from centimeters to decimeters. Although there is no distinct zoning as one moves towards the wall rocks from the ore, epidotization and chloritization become more common as one moves away. Hematite is observed in rock fractures, and the width of the zone exceeds 200 m in places.

4.4.2. Horoz Skarn

The skarn mineralization in the Horoz area shows both endoskarn and exoskarn features considering the close contact between the Horoz granitoids and the calc-silicate-bearing sedimentary rocks in the area with alteration affecting both the sedimentary wall rocks and the intrusive bodies (Figure 4). In this area, garnets are coarse-crystalline and massive (Figure 12a). Sporadic nodular sphalerite and irregular quartz are common in the granitoids (Figure 12b). Banded crystallization of calcite, quartz, and sphalerite occurs towards the ore zone, which is at the contact of the granitoids and the carbonate unit (Figure 12c). The mineralization occurs as vein-type mineralization due to hydrothermal fluid ingress. Sphalerite formation between the contact of calcite (Figure 12d) indicates fracture-filling-type retrograde sphalerite. Other macroscopic endoskarn features are characterized by massive (Figure 12e) and large garnet crystals surrounded by smaller fine-grained garnet crystals (Figure 12f). These garnets may have formed simultaneously closer to the skarn zone suggesting reactive infiltration, which caused zoning [70,71,72,73,74].

4.5. Mineralogical Characteristics of the Skarn Deposits

XRD analyses were conducted to identify the various primary and secondary minerals in the skarn deposits. However, some analyses involving Electron Probe Microanalysis (EPMA) of the skarn deposits are reported in an upcoming related work. A summary of all minerals identified in skarn-related rocks is given in Table S1.

From the XRD analyses, skarns in the Esendemirtepe-Koçak area essentially consist of Na-rich minerals with calc-silicate minerals (Figure 13a). The volcano-sedimentary rocks in the Esendemirtepe-Koçak area also contain Na-rich clinopyroxene and orthopyroxene (Figure 13b). Therefore, in the calcic skarn zone, calc-silicate minerals such as grandite garnet (grossular-andradite), clinopyroxene (diopside-hedenbergite-johansenite), epidote, and actinolite, among others, are commonly observed. Hydrothermal alteration products in the Esendemirtepe-Koçak skarns include epidote, calcite, dolomite, and clinochlore (Figure 13c). The hydrothermal alteration also resulted in the formation of quartz, magnetite, hematite, and subordinate spinel in the skarns (Figure 13d).

A summary of all minerals identified in skarn-related rocks in the Horoz region is given in Table S1. On the basis of the XRD analyses, skarns in the Horoz area contain abundant calc-silicate minerals and Na-K-rich minerals (Figure 14a). The garnet crystals observed in the Horoz area are actually andraditic to grossularite in composition in view of the XRD patterns. Pyroxene end members (diopside) and augite alongside a sodium-rich member of the amphibole group of silicate minerals (riebeckite) also developed with the garnet end member andradite (Figure 14b). Alteration may have resulted in the formation of quartz, calcite, piemontite, actinolite, grossular, and vermiculite (Figure 14c) and continued with the formation of dolomite, muscovite, and kaolinite (Figure 14d).

5. Discussion

5.1. Petrogenetic Evolution of the Çiftehan Plutonic Rocks

The major element variations, especially the SiO₂ and Na₂O contents, K₂O/Na₂O ratios, molecular A/CNK ratios, and mineralogical composition, all suggest that the plutons in the Çiftehan (Ulukışla-Niğde) area show I-type characteristics when compared with the classification scheme of [75]. These characteristics also indicate that the monzogabbro, granodiorite, and diorite have a similar origin, probably derived from partial melting and crystallization from meta-igneous rocks, but due to magmatic differentiation and influence from crustal materials, the granodiorite and diorite became late-stage products of the same magma [32,76].

The observed geochemical trends in the studied samples revealing positive correlations of SiO₂ with Na₂O and negative correlations with K₂O, Al₂O₃, Fe₂O₃, MgO, and TiO₂ (Figure 8) are indicative of advanced fractional crystallization processes and evolving magma chemistry. As magmas differentiate, silica and alkali contents typically increase due to the progressive removal of early-crystallizing mafic minerals such as olivine, pyroxene, and amphibole, which preferentially incorporate Fe, Mg, and Ti. The depletion of Al₂O₃ may reflect the fractionation of plagioclase, which is a major Al-bearing phase, especially in intermediate to felsic magmas [77]. The positive trend with Na₂O suggests the increasing role of alkali feldspars and sodic plagioclase (e.g., albite) in the residual melt, consistent with I-type granitoid evolution [78]. The equilibrium between plagioclase and amphibole is particularly significant in controlling the trace element and major oxide evolution of the melt. Amphibole can incorporate significant amounts of Fe, Mg, and Ti, and its crystallization alongside plagioclase can drive the melt toward more felsic compositions. Moreover, the presence of amphibole suggests water-rich conditions, which can suppress plagioclase crystallization at early stages, delaying its appearance until lower temperatures, thereby influencing the observed Al₂O₃ depletion [79]. These mineralogical controls, combined with the redox-sensitive behavior of elements like Ti and Fe, further support a scenario of oxidizing to mildly reducing conditions during magma evolution.

The trace element concentrations of the studied plutonic rocks were normalized to the Chondrite, Ocean Ridge Granite (ORG), Upper Crust, and Lower Crust values of [80] to further confirm their source characteristics. The Chondrite, ORG, Upper Crust, and Lower Crust-normalized patterns of the plutonic rocks indicate enrichment in large ion lithophile elements (LILEs) like Rb and Th and depletion in Y (Figure 15a–d), implying interaction with crustal materials for all the plutonic rocks. However, the higher Rb and Th values for the granodiorite and the diorite than the monzogabbro is interpreted to be because of extensive crustal input from a subduction-related origin since Rb and Th are enriched in crustal sources [81,82].

According to Rudnick and Gao [81], plutonic rocks with a Zr content higher than 132 pm and a low Y content may point to crustal mixing with their magmas. Therefore, the low Zr and Y content in the monzogabbro imply limited crustal interaction, but most of the granodioritic and dioritic rocks, especially the Esendemirtepe diorite, show extremely high Zr and Y contents, implying extensive input from crustal materials during their emplacement. The low Cr content of the monzogabbro compared with that of the granodiorite and diorite further indicates more input from crustal materials to the magma of the granodiorite and diorite. The Çifteköy monzogabbro and the Esendemirtepe diorite are exceptionally rich in Cs when compared with the Horoz granodiorite and the Koçak diorite (Figure 15a–d). Such enrichment might be due to the presence of accessory minerals like apatite [83]. On the Upper Crust and Lower Crust-normalized diagrams (Figure 15c,d), the studied rocks are fairly enriched in LILEs compared to the Upper Crust and are more enriched than the Lower Crust, consistent with patterns of magma that have undergone significant crustal mixing. When compared with geochemical data of granitoids in the Ulukışla-Niğde area [7,16,17,65], the studied rocks show overlapping patterns, indicating that they may be coeval members.

Overall, the trace element patterns of the studied plutonic rocks suggest that they may have been derived from a hybrid source, characterized by crustal and mantle components. This interpretation is supported by the Chondrite, ORG, Upper Crust, and Lower Crust-normalized trace element patterns of the studied rocks as discussed above.

The studied plutonic rocks were also normalized to the Chondrite and Continental Crust REE values of [80]. A steep negative REE slope (La > Yb) with considerable enrichment in LREEs against the MREEs and HREEs, smaller positive Eu anomalies (Eu/Eu* = 0.98–1.35), and a flat normalized pattern for the HREEs are observed in all the plutonic rocks (Figure 16a,b). This trend is also comparable to that of granitoids in the Ulukışla-Niğde area [7,16,17,65], which implies contribution from crustal sources to the melt composition and accumulation of plagioclase feldspars during partial melting [84,85].

The Eu/Eu* ratios ranging from 0.98 to 1.35 suggest limited to moderate plagioclase accumulation or retention in the source magma and relatively reducing conditions that allowed for the partial stabilization of Eu²⁺. Studies have shown that such ratios are typical of calc-alkaline granitoids where plagioclase crystallization plays a dominant role in early magmatic differentiation [86]. The presence of positive Eu anomalies in the monzogabbro or less-evolved rocks, and their gradual decline in more-evolved granodioritic and dioritic rocks, further supports the role of plagioclase fractionation in controlling Eu behavior [87]. The samples also show negative Tm anomalies, indicating the absence of Tm-bearing minerals [88]. The Continental Crust-normalized patterns of the Horoz granodiorite and Koçak diorite are fairly similar and thus, their magmas may have been derived from mantle sources and later mixed with continental crustal sources (Figure 16b).

The (La/Yb)_n, (Ce/Yb)_n, and Gd/Yb)_n ratios of the monzogabbro indicate strong LREE and MREE enrichment relative to HREEs. These patterns are characteristic of magmas derived from a deep-seated, garnet-bearing source, where garnet retains HREEs during partial melting [89]. The elevated MREE/HREE ratios further imply that amphibole may also be present in the residue, contributing to the fractionation pattern [90]. Tectonically, such REE signatures are typical of continental arc settings, where subduction-related fluids and melts metasomatize the overlying mantle wedge, enriching it in LREEs. The high-pressure melting conditions inferred from the garnet signature suggest that these magmas were generated in a thickened crustal environment, possibly during the early stages of arc development or post-collisional crustal reworking [91].

Nevertheless, the variability in the (La/Yb)_n, (Ce/Yb)_n, and Gd/Yb)_n ratios of the granodiorite and diorite suggest heterogeneous source compositions or variable degrees of partial melting and fractional crystallization. The samples with lower (La/Yb)_n and (Ce/Yb)_n ratios (e.g., EM10 and EM13—Esendemirtepe diorite) likely reflect shallower melting conditions, where garnet is absent and clinopyroxene or plagioclase dominates the residue. In contrast, samples with higher ratios (e.g., samples H16, E4B, ED2A, and ED2C—Horoz granodiorite and Esendemirtepe diorite) resemble the monzogabbro, indicating deeper melting with garnet stability. This geochemical diversity implies a complex magmatic evolution, possibly involving magma mixing, crustal assimilation, or polybaric melting. The tectonic setting remains consistent with a continental arc to post-collisional regime, but the broader range of REE patterns suggests progressive crustal thickening, slab rollback, or delamination processes influencing magma genesis [92,93].

5.2. Tectonic Environments

Trace element data are widely used in discriminating the tectonic environments associated with the formation of different magma types (e.g., [66,94]). In the Rb versus Y + Nb (Figure 17a) and Rb/Zr versus SiO₂ (Figure 17b) diagrams, the plutonic rocks from the study area all plot in the Volcanic arc granite (VAG) and Syn-collisional granite (Syn-COLG) + VAG fields just as samples from previous studies in some localities within the Ulukışla-Niğde area. The tectonic discrimination of the rocks as volcanic arc granites and compositionally as calc-alkaline granites implies subduction-related settings for their emplacement [76,94]. It might also indicate regional metamorphism around active continental margins. Furthermore, on the [95] diagram (Figure 17c), the samples spread across the Late-orogenic to Syn-Collision granite zones that are consistent with previous studies [7,16,17,65] and this is a typical geochemical signature of late orogenic plutonic rocks. Late orogenic granites are known to be calc-alkaline rocks characterized by more than one tectonic environment [96], which reflects the hybrid composition of their source magmas like a depleted/enriched mantle and crust. The thick nature of the crust and extensive interaction between mantle-derived magma and the crust makes most late orogenic granites show evidence of crustal assimilation [96].

5.3. Genetic Link Between Plutonic Rocks and Skarn-Type Formation

Some studies have documented the relationship between the composition of plutonic rocks and skarn-type deposits (e.g., [32,70,97]). For example, Ref. [97] reported that plutonic rocks associated with the formation of Fe, Zn, Au, and Cu skarns contain moderate levels of MgO and K₂O. In contrast, plutonic rocks associated with Sn and Mo skarns have less MgO and more K₂O [97]. Furthermore, Ref. [70] revealed that most of the plutons associated with skarns show calc-alkaline trends and aluminum saturations that are mostly metaluminous.

In this study, with the exception of the Çifteköy monzogabbro, all plutonic rock samples from the Esendemirtepe-Koçak and Horoz areas were in the calc-alkaline field in the AFM diagram (Figure 18a). In addition, the aluminum saturation index values of the plutonic rocks ranged from 0.52 to 1.08 and therefore, they plot in the metaluminous field, except for some Horoz granodiorite and Koçak diorite samples, which plot in the peraluminous field (Figure 18b). In this context, the Esendemirtepe-Koçak and Horoz granitoids have the potential to produce skarns and their geochemical compositions are similar to all skarn-forming plutonic rock types except the Sn-type. Similar findings were reported by Demir and Bayraktar [98] in the Düzköy Fe-Cu skarn deposit of Gümüşhane in northeastern Türkiye. In addition, the geochemical composition of the Esendemirtepe-Koçak and Horoz plutonic rocks is similar to Fe-, Au-, Cu-, and Zn-type skarn granitoids in SiO₂ versus K₂O (Figure 18c) and SiO₂ versus MgO (Figure 18d) diagrams. However, the Esendemirtepe-Koçak and Horoz plutonic rocks differ from the geochemical composition of W, Sn, and Mo skarn-type granitoids.

Accordingly, Fe-, Cu-, Zn-, and Au-type skarn granitoids occur as volcanic arc granitoids (VAGs) and are therefore usually located in the VAG area in tectonic discrimination diagrams such as Rb versus Y + Nb and Nb versus Y [70]. Such granitoids typically have lower Rb and Zr contents compared to W, Mo, and Sn skarn-related granitoids. The Esendemirtepe-Koçak and Horoz plutonic rocks have a similar composition to volcanic arc granitoids and a close relationship with Fe and Zn-type skarn granitoids and partly with Cu-type skarn granitoids in the Rb versus Y + Nb diagram (Figure 19a). In addition, the studied samples are similar to Fe-, Cu-, and Zn-producing granitoids in Rb/Zr and Sr/Zr diagrams (Figure 19b,c).

On the basis of the above explanations, both major and trace elements of the Esendemirtepe-Koçak and Horoz plutonic rocks show similar features and a close relationship with Fe- and Zn-type skarn granitoids with a slight overlap with Cu-type skarn granitoids. Similar findings have been reported in the skarn deposits of Düzköy (Gümüşhane), Sivrikaya (Rize), Dağbaşı (Trabzon), and Eğrikar (Gümüşhane) and a strong relationship between skarn mineralogy and the geochemical properties of skarn-related granitoids has been demonstrated [74,98,99,100].

Moreover, the mineral composition of the Esendemirtepe-Koçak skarns is characterized by a dominance of Na-rich and calc-silicate minerals, including grandite garnet (grossular-andradite), clinopyroxenes (diopside-hedenbergite-johansenite), epidote, and actinolite. Hydrothermal alteration in this area has produced secondary minerals such as epidote, calcite, dolomite, clinochlore, quartz, magnetite, hematite, and minor spinel. In comparison, skarns from the Horoz area also exhibit a rich assemblage of calc-silicate and alkali-rich minerals, but with a broader diversity. Garnets here range from andraditic to grossularite, and are associated with pyroxenes like diopside and augite, as well as sodium-rich amphiboles such as riebeckite. Alteration in the Horoz skarns has led to the formation of minerals like piemontite, vermiculite, muscovite, and kaolinite, in addition to those found in the Esendemirtepe-Koçak area. Overall, while both areas share common skarn-forming minerals, the Horoz area displays a more complex alteration assemblage and a wider range of alkali-bearing silicates, suggesting variations in fluid composition and metasomatic conditions during skarn formation. Most of these alkali-bearing silicates were observed in the plutonic rocks indicating consanguinity. Similar skarn mineralogy has been reported in the Biga Peninsula in Türkiye by [101]. Therefore, considering the genetic relationship between skarn mineral composition and the geochemical properties of the Esendemirtepe-Koçak and Horoz plutonic rocks, it is thought that this relationship may have occurred on a regional scale.

In Türkiye, recent research on skarn systems offers significant analogs for elucidating the genetic processes seen in the Çiftehan area. For example, Ref. [102] demonstrated that comprehensive mineral chemistry and iron isotope studies of magnetite from the Pertek Fe-skarn deposit can accurately delineate the origins and evolutionary trajectories of hydrothermal fluids. Likewise, the sulfur and carbon-oxygen isotopic compositions, together with fluid inclusion data from the Yolindi Cu-Fe skarn deposit, underscore the intricacy of fluid development and the multistage mineralization processes [101]. Furthermore, the genetic connection between calc-alkaline magmatic activity and skarn mineralization, as shown by [103] in the Biga Peninsula, robustly substantiates the hypothesis that the plutonic rocks associated with skarn formation in the Çiftehan region are probable outcomes of subduction-related magmatism. The findings of the comparison substantiate the perspective that the Çiftehan skarn system emerged via the interaction of developing magmatic-hydrothermal fluids with carbonate host rocks, under tectonic settings shaped by active subduction processes. Moreover, the tectonic and genetic evolution of granitoids associated with skarn formation in the Biga Peninsula, Kazdağ Massif, and the Eastern Taurus Belt of Türkiye also indicate similarity with the plutonic rocks associated with skarn formation in the Çiftehan (Ulukışla-Niğde) area. Their evolution reflects a complex interplay of magmatic, metasomatic, and tectonic processes. For instance, in the Biga Peninsula, the Yolindi Cu-Fe skarn deposit exemplifies the influence of calc-alkaline magmatism in an island-arc setting, where the Şaroluk quartz monzonite intruded Upper Palaeozoic metamorphic rocks, initiating prograde and retrograde metasomatic reactions that formed garnet-pyroxene skarns and later hydrous mineral assemblages [101]. Similarly, in the Kazdağ Massif, skarn alteration and Au-Cu mineralization are linked to Tertiary I-type, magnetite-series, calc-alkaline granitoids. These intrusions generated oxidized skarn systems with hedenbergitic pyroxene and andraditic garnet during early stages, followed by retrograde assemblages of amphibole, epidote, and chlorite, influenced by a transition from magmatic to meteoric fluid sources [104]. In the Eastern Taurus Belt, skarn systems are also genetically tied to post-collisional granitoids, reflecting a tectonic regime shift from subduction to continental collision, which facilitated the emplacement of metaluminous, calc-alkaline intrusions and the associated mineralization. These granitoids share geochemical signatures with global skarn-forming systems, indicating a convergent tectonic heritage and a consistent magmatic-hydrothermal evolution [1].

Figure 18. (a) Comparison of plutonic rock samples with plutonic rocks associated with skarn formation in the AFM diagram of Irvine and Baragar [64]; (b) Aluminum saturation index classification of plutonic rock samples with plutonic rocks associated with skarn formation in the A/CNK versus A/NK diagram [66]; (c,d) SiO₂ versus K₂O and SiO₂ versus MgO variation diagrams and comparison with the composition of plutonic rocks associated with skarn formation [70,105].

Figure 18. (a) Comparison of plutonic rock samples with plutonic rocks associated with skarn formation in the AFM diagram of Irvine and Baragar [64]; (b) Aluminum saturation index classification of plutonic rock samples with plutonic rocks associated with skarn formation in the A/CNK versus A/NK diagram [66]; (c,d) SiO₂ versus K₂O and SiO₂ versus MgO variation diagrams and comparison with the composition of plutonic rocks associated with skarn formation [70,105].

Figure 19. Classification of the plutonic rock samples on the (a) Rb versus Y+Nb tectonic discrimination diagram of [66] and comparison with the plutonic rocks associated with skarn formation (VAG: volcanic arc granitoids; syn-COLG: syn-collision granitoids; WPG: within plate granitoids; ORG: ocean ridge granitoids); (b,c) Rb versus Zr and Sr versus Zr trace element variation diagrams and comparison with the compositions of different types of plutonic rocks associated with skarn formation [97]. Legends: [70,105].

Figure 19. Classification of the plutonic rock samples on the (a) Rb versus Y+Nb tectonic discrimination diagram of [66] and comparison with the plutonic rocks associated with skarn formation (VAG: volcanic arc granitoids; syn-COLG: syn-collision granitoids; WPG: within plate granitoids; ORG: ocean ridge granitoids); (b,c) Rb versus Zr and Sr versus Zr trace element variation diagrams and comparison with the compositions of different types of plutonic rocks associated with skarn formation [97]. Legends: [70,105].

6. Conclusions

The Esendemirtepe-Koçak and Horoz Fe-Zn skarn deposits in the Çiftehan (Ulukışla-Niğde) area in south-central Türkiye formed in the Ulukışla Basin, which is predominantly composed of volcano-sedimentary rocks and various igneous intrusions. The skarn deposits formed due to the intrusion of the Esendemirtepe diorite and Horoz granodiorite into carbonate rocks of the Bolkar Mountain Carbonate Platform. These intrusions show calc-alkaline affinities with I-type and metaluminous characteristics. They are more enriched in LILEs and LREEs than HFSEs, MREEs, and HREEs with smaller positive Eu anomalies (Eu/Eu* = 0.98–1.35) indicating subduction-zone settings for their emplacement. All the plutonic rocks associated with skarn formation are classified as volcanic arc granites similar to other granitoids in the Ulukışla Basin. The tectonic discrimination of the rocks as volcanic arc granites and compositionally as calc-alkaline granitoids confirms their subduction-related settings during emplacement and regional metamorphism around active continental margins.

The endoskarn and exoskarn zones where the Fe-Zn skarn ore developed are located at the limestone-marble-granodiorite-volcanite contact, wherein the intrusions intersect with the metacarbonate lithologies. The plutonic rocks associated with the Fe-Zn skarn formation are Early-Middle Eocene in age. Endoskarn and exoskarn zones are mostly observed in the Horoz area, whereas only the exoskarn zone is located in the Esendemirtepe-Koçak area. The endoskarn zone is characterized by the presence of actinolite and alteration product epidote, which are commonly observed in the calc-silicate rocks. Sporadic nodular-shaped sphalerite and irregular quartz developed along the marble-schist skarn contact. Banded crystallization of calcite, quartz, and sphalerite occurs towards the skarn zone. This zone also shows intensive fracturing and alteration of the granitoids with epidote as the main alteration product. Massive garnet crystals are also common in the endoskarn zone.

The exoskarn zone developed along the contacts of the plutonic rocks and the carbonate rocks due to the infiltration of magmatic hydrothermal fluids. The silicate minerals are richer in calcium than magnesium, thus making them calcic skarns rather than magnesian skarns. However, due to the metasomatic infiltration of the silicate melt into the carbonate lithologies, grossular-andradite pyroxene developed. The common minerals of the exoskarn zone are garnet, pyroxene, quartz, calcite, actinolite, and epidote. The presence of cobalt and nickel minerals is also common in this area. Magnetite and hematite in the skarns formed as massive and banded structures and were characterized by replacement features in both limestone and host volcanites. However, sulfide ore occurs as breccia-filling structures along the brecciated host rocks and sometimes as fracture-filling structures in the limestones and volcanites. The skarn deposits and their causative plutons show overlapping geochemical signatures, indicating their paragenetic occurrence.