Geochemical and Petrographic Analysis of Hydrothermal Mineralization in the Chitral Region, Northern Pakistan: Implications for Tectonic and Diagenetic Processes

Introduction

The Chitral region, situated in Northern Pakistan, is characterized by a complex geological history marked by tectonic activity, particularly related to the ongoing collision between the Indian and Eurasian plates. This tectonic interaction not only shaped the topography of the region, but also played a crucial role in the formation of various mineral deposits, including those associated with hydrothermal processes. This area is notable for its diverse lithologies encompassing limestones, quartz veins, and limonitic zones, all of which exhibit evidence of significant tectonic deformation, chemical weathering, and hydrothermal alteration. The investigation of hydrothermal mineralization in the Chitral area (Figure 1) is of particular significance because of its implications for understanding the broader tectonic and geochemical processes in the region. Hydrothermal processes driven by the heat generated during tectonic collisions and magmatic intrusions are known to concentrate economically important minerals, including galena, antimony, pyrite, and malachite. These minerals typically precipitate from mineral-laden fluids that migrate through fractures and faults within Earth’s crust, rendering the study of such deposits crucial for both academic research and economic exploration. Previous investigations in analogous tectonic settings, such as the Kohistan- Ladakh arc, have emphasized the importance of structural geology in controlling the localization of mineralization as well as the role of hydrothermal fluids in altering the chemical and mineralogical composition of host rocks.

The significance of studying the Chitral area extends beyond regional geology, offering insights into the processes that govern mineralization in orogenic belts worldwide. This area serves as a natural laboratory for examining the interplay between tectonic activity, fluid migration, and mineral deposition. This is particularly relevant in the context of global mineral exploration, where understanding the genesis and distribution of hydrothermal deposits can facilitate the discovery of new resources. Furthermore, the complex geological setting of the Chitral region provides an opportunity to investigate the effects of post-depositional processes, such as diagenesis and chemical weathering, which can significantly alter the original mineral assemblages and geochemical signatures of hydrothermal deposits. The primary objective of this study was to conduct a comprehensive petrographic and geochemical analysis of rock formations in the Chitral area, with a particular focus on elucidating the hydrothermal mineralization processes and their relationship with the tectonic evolution of the region. The specific objectives include:

a. Characterizing the mineralogical and textural features of limestones, quartz veins, and limonitic zones to identify evidence of hydrothermal alteration;

b. Analyzing the geochemical composition of these rocks to determine the source and evolution of hydrothermal fluids;

c. Assessing the role of tectonic structures in controlling fluid flow and mineral deposition; and

d. Comparing the findings with similar studies in other tectonically active regions to place the results within a broader geological context.

Geological Makeup and Tectonic Evolution of North Pakistan

The geological composition of North Pakistan is highly intricate, comprising a variety of terranes that have undergone significant deformation and metamorphism over approximately 120 million years. This complex geological history is largely attributed to subduction and continental collision between the Indian and Eurasian plates (Figure 2) [1-3]. The formation of the Himalayas, a pivotal event in the region’s geological evolution, commenced during the Cretaceous period as the Indian Plate began its northward drift [4]. During this time, the Kohistan-Ladakh arc formed over a subduction zone, an event that remains a subject of debate among geologists. While some studies suggest a northward-dipping subduction zone beneath the arc [5-7], others provide evidence for southward-dipping subduction [8,9], reflecting the ongoing complexity in understanding the tectonic history of the region.

Regional Geology of North Pakistan

The regional geology of North Pakistan can be categorized into three major geotectonic terranes, each delineated by significant suture zones. These include the Shyok Suture Zone (MKT) to the north and the Indus Suture Zone (MMT) to the south, which collectively define the boundaries of the Kohistan Island Arc. Each of these terranes exhibits distinct geological characteristics that contribute to the overall complexity of the region (Figure 3) [1,10].

The Eurasian (Karakoram) Terrane

The Eurasian, or Karakoram, terrane is situated along the southern margin of the Asian continent, extending southward to the Shyok Suture Zone (Searle, 1991; Tahirkheli, 1979). Despite its remote and inaccessible nature, which has limited detailed geological studies, it has been broadly divided into three distinct belts: the Northern Sedimentary Belt (NSB), Granitic Belt, and the Southern Metamorphic Belt (SMB) (Gaetani, 1997) [11]. The Northern Sedimentary Belt (NSB) features a series of tectonostratigraphic units that include the Baltit group, Guhjal unit, Gircha formation, and Misgar slate, encompassing a range of lithologies from Precambrian to Permian age (Gaetani et al., 1996). The Granitic Belt is further subdivided into the Karakoram Granitic Belt, which spans from the Jurassic to Miocene periods, and the Khunjerab-Tirch Mir Granitic Belt, characterized by porphyritic granite gneiss (Searle et al., 1989) [12]. The Southern Metamorphic Belt (SMB), represented by the Darkot Group, comprises slates, schists, and quartzites, all of which are intruded by granitic rocks [10].

The Kohistan Island Arc

The Kohistan Island Arc occupies a pivotal position between the Indian and Eurasian terranes and is a key feature of the geological landscape of North Pakistan. This arc, which experienced Andean-style magmatism, played a significant role in the tectonic evolution of the region prior to its collision with the Asian landmass that occurred approximately 85–104 million years ago (Bignold et al., 2006) Petterson [3]. The geological structure of the arc is composed of several major units, including Yasin Group sediments, Chalt Volcanic Group, Kohistan Batholith, Chilas Complex, Kamila Amphibolite, and Jijal, Sapat, and Tora-Tiga Complexes (Petterson & Windley, 1991) Searle [9]. Each of these units reflects the complex interplay of volcanic, sedimentary, and metamorphic processes that shape the arc. For instance, Yasin Group sediments consist of volcaniclastic and non-volcanic turbidites, while the Chalt Volcanic Group is primarily composed of tholeiitic meta-basalts and rhyolites [8,13]. The Kohistan Batholith includes a diverse assemblage of intrusive rocks such as gabbros, diorites, and granodiorites [9].

The Indus Suture Zone

The Indus Suture Zone marks the southern boundary of the Kohistan Arc and represents a crucial tectonic feature formed during the subduction of oceanic lithosphere as the Neo-Tethyan ocean closed beneath the arc [14,15] (Anczkiewicz et al., 1998). This zone is characterized by a variety of geological formations, including tectonic melanges, blueschists, greenschists, and ophiolitic rocks. These formations reflect the complex geological history of the region, including the convergence and eventual collision of the Indian and Eurasian plates [10]. The ophiolitic rocks within the Indus Suture Zone are prominently exposed in multiple locations, including Bajaur-Utmankhel, Skhakot-Qila, and Mingora-Shangla, and are indicative of ancient oceanic crust and mid-ocean ridge environments that once existed in the region (Jan & Jabeen, 1990) [16].

Geological Features of the Indian Terrane

The Indian terrane, south of the Indus Suture Zone, is predominantly composed of marbles, schists, and gneisses dating from the late Precambrian to the early Paleozoic era (Treloar, 1989; Anczkiewicz et al., 1998). Following subduction of the Indian plate beneath the Kohistan Arc, significant crustal thickening and metamorphism occurred, particularly in the northern regions of the terrane [9]. This metamorphism decreases gradually in intensity southward from the Main Mantle Thrust (MMT). The rocks of the Indian terrane are organized into six thrust nappes, including Swat, Besham, Hazara, Banna, Lower Kaghan, and Upper Kaghan, each demarcated by late-stage thrusts (Treloar, 1989). In the Lower Swat area, amphibolite and greenschist facies metamorphism is evident, with notable intrusions by Swat granitic gneisses, further contributing to the region’s complex geological fabric (Anczkiewicz et al., 1998).

Geology and Stratigraphy of Chitral

The Chitral region in North Pakistan presents a complex geological setting characterized by a diverse range of sedimentary, meta- sedimentary, and igneous formations. The geological makeup of this area can be divided into two primary domains: the Karakoram Plate and Kohistan Island Arc (Gaetani et al., 1995). These domains encompass various formations that reveal the intricate tectonic and metamorphic histories of the region.

Sedimentary and Meta-Sedimentary Entities

The sedimentary and meta-sedimentary formations within the Chitral region are categorized into distinct units within the Karakoram Plate and Kohistan Island Arc. Each of these Formations exhibits unique lithological characteristics and reflects different geological processes and periods (Figure 4) (Hayden, 1915; Buchroithner & Gamerith, 1986).

Karakoram Plate

Within the Karakoram Plate, the Chitral region exhibits five distinct sedimentary to meta-sedimentary units ranging from the northernmost to the southernmost extent.

Wakhan Formation and Atark Unit

The Wakhan Formation, first identified by Hayden (1915), extends from east of Baroghail through the Tirch Mir area and westward to the Pak-Afghan border. This formation, mapped and compiled extensively by researchers such as Buchroithner and Gamerith (1986), is primarily composed of dark grey homogeneous slate, siltstone, and quartzite, with occasional calcareous schists. The northern extent of the Wakhan Formation transitioned into gneisses, indicating higher metamorphic grades. The formation is faulted against a carbonate body known as the Atark Formation, which runs parallel to the contact zone of the Wakhan Formation and is considered part of it. The Wakhan Formation is attributed to the Early Triassic, with some evidence suggesting potential extension to the Late Permian (Kafarskiy, 1976) [17].

Awireth Series/Series of Owir

The Awireth Series, or Series of Owir, is a sequence from the Upper Paleozoic era, comprising middle- to dark-grey slates, siltstones, fine-grained quartzites, and argillaceous-arenaceous-calcareous schists with interspersed lenses of limestone and dolomite. The series also includes green meta-volcanic and volcano-sedimentary rocks, highlighting the diverse lithological composition of this stratigraphic unit (Hayden, 1915).

Sarikol Shale

The Sarikol Shale, introduced by Hayden (1915), refers to a primarily slate sequence that extends from Chinese Turkistan to Chitral. This formation, named after the Sarikol Mountain Range, consists predominantly of slates with occasional quartzitic elements and limestone beds. The Sarikol Shale is notably fossiliferous, with nautiloids indicating a Devonian age (Tipper, 1924). The formation is prominently exposed near the Shogram village in Chitral, where it is primarily composed of black shale and slate with minor limestone beds.

Shogram Formation

Desio (1966) characterized the Shogram Formation as one of the most significant Devonian sections in Pakistan. This formation consists of limestone and quartzitic sandstone, and is further subdivided into three units: a lower unit of massive dolomite, a middle unit of well-bedded dolomite and limestone, and an upper unit of quartzite. The Shogram Formation is Devonian and has been correlated with the Nowshehra Formation (Stauffer 1975).

Shoghore Limestone/Reshun Formation/Reshun Conglomerate

This sequence, ranging from Cretaceous to Tertiary, is characterized by a variety of lithologies, including white to light-grey limestones, dolomites, sandstones, conglomerates, and red calcareous schists. These formations are predominantly found in the southwestern region of Chitral and demonstrate significant lithological variability (Calkins 1981; Desio 1959, 1963).

Chitral Slates

The Chitral Slates primarily consist of fine-grained black slates with subordinate dark gray phyllite, quartzite, and layers of limestone or marble. The limestone layers vary in thickness and exhibit a dark gray to black, medium-grained, recrystallized texture. The Chitral Slates are Paleozoic in age, likely encompassing the Late Paleozoic period, and have been correlated with the Darkot group in the Gilgit Agency (Stauffer, 1975).

Kohistan Island Arc

Within the Chitral region, several units of the Kohistan Island Arc are distinguishable, each reflecting the complex volcanic and sedimentary processes associated with this tectonic domain (Khan et al., 1998).

Drosh Formation

The Drosh Formation is characterized by thick-bedded porphyritic andesite with phenocrysts of plagioclase, hornblende, and pyroxene. This formation is prominently exposed along the road south of Drosh and is interbedded with red shales. The Drosh Formation typically overlies the Purit Formation (Tahirkheli et al., 2012).

Purit Formation

The Purit Formation is comprised of medium-bedded sandstone with sedimentary structures indicative of a fluvial origin. This formation also includes interbedded shales, mudstone, and polymictic conglomerates that extend uniformly along the regional extent of formation [6].

Gawuch Formation

The Gawuch Formation is composed mainly of metavolcanics, carbonate sediments, and diorite. The formation exhibits a basal part of phyllite or greenschist facies metavolcanics, while the upper half consists of metavolcanic layers interspersed with metasedimentary rocks, such as quartzites and marble. The Gawuch Formation is correlated with the Chalt-Yasin Group in the eastern part of the Kohistan [6].

Igneous Bodies

The sedimentary units of the Karakoram Plate in the Chitral are intruded by granitic and granodioritic plutons, including the Buni- Zom, Kafristan, Tirch Mir, and Garam Chashma plutons.

Tirch Mir Pluton

The Tirch Mir Pluton, dated at 115 ± 4 Ma (Rb–Sr on biotite), exhibits a foliated and augen-textured structure. The pluton is composed of quartz, plagioclase, K-feldspar, biotite, and muscovite with accessory minerals such as apatite, zircon, and opaques. Its formation is attributed to subduction of the Paleo-Tethys oceanic lithosphere beneath the Hindu Kush-Karakoram plate margin [18,19].

Kafristan Pluton

The Kafristan Pluton consists of strongly foliated porphyric gra nodiorites and granites with a mineralogical composition including quartz, plagioclase, K-feldspar, biotite, and muscovite. The pluton was assigned a whole-rock Rb–Sr age of 483 Ma and is linked to the extensional tectonic regime associated with the breakup of Gondwana during the Paleozoic [20] (Faisal et al., 2018).

Garam Chashma Pluton

The Garam Chashma Pluton, identified as a two-mica leucogranite, has intruded staurolite-grade schist and migmatites. The U–Pb ages indicate crustal melting around 24 Ma, concurrent with the Baltoro melting event [12]. The pluton is composed of quartz, alkali feldspar, plagioclase, muscovite, and biotite, with accessory minerals, including tourmaline and zircon [19].

Kesu-Buni Zom Pluton

The Kesu-Buni Zom Pluton comprises foliated medium-grained diorites and granodiorites, with some granite pegmatite sheets. The formation of the pluton is attributed to subduction of the Paleo- Tethys oceanic lithosphere beneath the Hindu Kush-Karakoram plate margin [19,21].

Methodology

Field Method

A comprehensive geological field expedition was conducted over a two-week period in the Awireth, Siwakht, and Krinj regions of Chitral District. The primary objective was to systematically collect samples from hydrothermal veins enriched with metallic minerals and their host rocks. A total of 26 rock samples were collected during the fieldwork (Figure 5). Of these, 16 representative samples were selected for detailed petrographic analysis and X-ray fluorescence (XRF) analysis. The initial field observations revealed several significant mineralogical features.

Thin Section Preparation

For petrographic examinations, thin sections were prepared using a systematic process. Initially, rock samples were cut into slabs measuring 4 × 2 cm using a precision rock cutting machine. One side of each slab was polished with abrasives to achieve a smooth surface suitable for microscopic analysis. Subsequently, the polished surface of the slab was mounted onto the etched surface of a glass slide using an epoxy resin. The thick portion of the slab was then trimmed using a small cutting machine, and the remaining section was thinned using a grinding machine. Final buffing was performed with silicon carbide abrasives of varying grain sizes to achieve the requisite thickness for petrographic investigation. The thin sections were analyzed at the Department of Geology, University of Peshawar, using a polarizing microscope. Photomicrographs were acquired at the National Center of Excellence in Geology to document the mineralogical and textural characteristics of the samples.

Analytical Method

The elemental compositions of the ore samples were determined using X-ray fluorescence (XRF) analysis conducted in the laboratory of the Chinese Mineral and Mines. An energy-dispersive portable XRF (pXRF) analyzer, specifically the Bruker S1 Titan, was employed for this purpose. The analytical procedure commenced with uniform pulverization of the rock samples. A representative portion of each pulverized sample was obtained using quartering and coning techniques to ensure homogeneity. The prepared rock powders were subsequently packed into sealed pack tubes, and each sample was placed in a pXRF chamber. The detection time was set to 60 seconds, operating under the Geo Mining and Geo Exploration mode, which is optimized for geological sample analysis. This method provides expeditious and precise quantification of the major oxide components present in the samples.

In addition to XRF analysis, Energy Dispersive X-ray (EDX) analysis was conducted to further elucidate the elemental composition of the ore samples. EDX analysis complements XRF by providing additional data on the elemental distribution within mineral phases. The combination of XRF and EDX analyses ensured a comprehensive characterization of the major and trace elements in the samples, facilitating a detailed interpretation of the geological processes involved in the formation of hydrothermal veins and associated mineralization.

Results

Petrographic studies provide a comprehensive and precise description of the modal mineralogy and textural relationships within the rocks. These investigations encompass both microscopic analysis and field observations, with a focus on variations in lithology, color, and texture. Detailed microscopic examination of thin sections is essential for elucidating the origin of rocks and is frequently complemented by geochemical analysis. In the Chitral region, specifically in the Awireth and Siwakht areas, the Reshun thrust fault intersects rock formations, resulting in significant deformation including fracturing, folding, and shearing. The primary lithologies under investigation in this area are the quartz veins of the Awireth and limestone of the Shoghore carbonates.

Petrography

The petrographic analysis of rock samples from the Chitral area, along with their modal mineralogy, are summarized in Table 1. The subsequent sections provide a detailed examination of the petrography of limestone and quartz veins observed in the study area.

Limestone

Based on comprehensive field observations and petrographic analyses, the limestone of the Shoghore carbonates has been classified into several distinct subtypes.

Limestone Containing Malachite Vein

The sample (Sh) from the limestone of the Shoghore carbonates exhibited a malachite vein approximately 100 μm thick embedded within the limestone matrix. The limestone displays a fine-grained texture and features a complex network of calcite veins, as illustrated in Figure 6. The presence of malachite indicates secondary mineralization within the carbonate matrix.

Table 1: Major and trace element concentrations in limestone, limonite, and quartz veins as determined through X-ray fluorescence (XRF) analysis.

Banded Limestone

Sample (SM-1) is characterized by fine-grained and recrystallized calcite particles, exhibiting alternating stratification of light and dark color bands, as illustrated in Figure 7. The sample contained opaque minerals, such as pyrite, lead, and antimony, with particle sizes of approximately 200 μm dispersed between the bands. Furthermore, recrystallized calcite particles (approximately 2300 μm) were observed and were encapsulated by opaque mineralization. Quartz grains measuring approximately 300 μm were also present adjacent to coarse-grained calcite. Stylolites, ranging from 100 to 600 μm in thickness, were evident and predominantly mineralized with Sb and Pb.

Limestone Hosting Disseminated Pyrite and Ore Minerals

Sample (SM-1B) comprised recrystallized calcite (approximately 1400 μm) and quartz grains (approximately 1000 μm). These grains contained euhedral to subhedral pyrite grains and other opaque minerals, as illustrated in Figure 8. Pyrite grains with quartz inclusions are particularly prominent. In certain regions, fine-grained calcite also serves as a matrix for euhedral pyrite and metallic mineralization, with quartz grains measuring approximately 100 μm. Sample (SM-1C) exhibited recrystallized calcite containing euhedral to subhedral pyrite grains with dimensions ranging from 200 μm to 2600 μm, as illustrated in Figure 9. The pyrite grains incorporated quartz inclusions (~ 50 μm). Certain calcite grains display a cloudy appearance due to alteration, and quartz, measuring approximately 250 μm, is also present and occasionally observed as inclusions within the opaque mineralization.

Limestone with Stylolite Features

Sample (SM-3) exhibited pressure dissolution features, specifically stylolites filled with opaque minerals with a width of approximately 250 μm. Euhedral pyrite was observed in the metallic mineralization, as illustrated in Figure 10. Certain sections revealed unfilled fractures, potentially post-mineralization, with iron leaching observed in some fractures. The unfilled fractures, approximately 100 μm wide, were oriented perpendicular to the stylolites.

Altered Limestone

Sample (SM-2) exhibits antimony (Sb) alteration, predominantly characterized by a light to dark brown coloration, as illustrated in Figure 11. Unaltered Sb minerals, appearing dark owing to the absence of light transmission, were observed in specific regions. Finegrained calcite was also present in this sample, which is indicative of a complex alteration history.

Quartz Vein Containing Metallic Mineralization

Sample (AW), obtained from the Awireth Gol area, was characterized by a quartz vein containing euhedral to subhedral pyrite. The quartz matrix also encompasses an opaque mineralization vein approximately 100 μm in width, as illustrated in Figure 12. In certain regions, calcite grains with quartz inclusions were observed. Specific portions of the sample exhibit carbonate grains with widths of approximately 500 μm, which function as hosts for metallic mineralization veins. This mineralization suggests substantial hydrothermal activity within quartz veins.

Major Elements Geochemistry

Geochemical analysis of rock samples from the Chitral region revealed significant variations in the concentrations of major elements, reflecting the diverse lithological characteristics of the area (Table 1). The SiO₂ content in these samples exhibited a wide range from 0.53% to 48.03%. Limestone samples demonstrated the lowest SiO₂ values, while the highest values were observed in limonitic samples and quartz veins at 48.03% and 41.84%, respectively, presumably because of their elevated quartz content. The Al₂O₃ content varied considerably across the samples. The lowest Al₂O₃ concentration was recorded in limestone at 0.19%, whereas the limonitic samples and quartz veins displayed higher values, reaching 13.07% and 11.2%, respectively. This increase in Al₂O₃ content is indicative of the presence of aluminosilicate minerals in these rock types. Fe₂O₃ concentrations ranged from 3.38% to 25.76%, with limestone exhibiting the highest values due to the presence of pyrite. In contrast, limestone samples without pyrite demonstrated the lowest Fe₂O₃ concentrations.

The MnO content was the highest in limestone (4.98%) and lowest in quartz veins, suggesting that manganese-bearing minerals are more prevalent in carbonate rocks. The SO₂ content was notably high in limestone rocks, with values reaching 11.48%, indicating an elevated sulfide mineral content. In contrast, limonitic samples, representing barren zones, exhibited the lowest SO₂ values. The MgO content ranged from 0.6% to 7.54%, with limestone displaying the highest values, which is likely due to the presence of dolomite. The lowest values were also observed for limestone, indicating lower dolomite content. The CaO content was particularly high in limestone samples, with values reaching up to 37.84%, corresponding to a high calcite content. Limonitic samples exhibited lower CaO values (3.74%), while quartz veins showed CaO concentrations ranging from 5.94% to 7.75%. K₂O values ranged from 0.01% to 1.82%, with the highest values recorded in limonite and the lowest in limestone. P₂O₅ content was highest in limestone and limonite samples, reaching 1.99% and 1.81% respectively, while the lowest P₂O₅ values were observed in quartz samples. This distribution suggests that phosphorus-bearing minerals were more concentrated in carbonate- and iron-rich lithologies.

Table 2: Analysis of economic metals concentration in limestone and quartz utilizing Energy-dispersive X-ray spectroscopy (EDX).

Minor and Trace Element Geochemistry

The minor and trace element compositions of the rock samples provide additional insights into the geochemical characteristics of the Chitral region (Table 1). Antimony (Sb) concentrations were substantial, reaching up to 112,500 ppm. The highest Sb concentrations were observed in limestone and quartz vein samples, whereas limonitic samples exhibited the lowest Sb levels.

a. Zinc (Zn) concentrations ranged from 100 to 296,400 ppm, with the highest levels observed in limestone and the lowest in limonite. Lead (Pb) concentrations also demonstrated significant variation, ranging from 100 ppm to 128,300 ppm, with limestone exhibiting the highest values and limonite the lowest.

b. Arsenic (As) concentrations ranged from 100 ppm to 5,400 ppm, with the highest levels detected in quartz vein samples, while the lowest concentrations were observed in both limestone and limonite. Strontium (Sr) concentrations ranged from 100 to 600 ppm, with the highest values in limestone and the lowest in quartz veins and limonitic samples.

c. Copper (Cu) concentrations reached 900 ppm, with the highest values recorded in limestone and the lowest in limonite. Chromium (Cr) concentrations peaked at 1,900 ppm in limestone, with lower values observed in the quartz vein samples. Other trace elements include tin (Sn), present in varying amounts across different lithologies, and tungsten (W), which is primarily observed in limestone samples.

EDX Analysis of Ore Samples

Sample SM6

Energy Dispersive X-ray (EDX) analysis of Sample SM6 revealed a heterogeneous elemental composition comprising Zn, Ca, S, C, Mg, Fe, Na, and O. The EDX spectrum exhibits prominent peaks at energy levels of 0.8 and 8.4 keV, corresponding to the presence of Zn, as presented in Table 2. Furthermore, a distinct narrow peak observed at approximately 3.5 keV was attributed to calcium, indicating its substantial presence in the sample.

Sample SMA

The EDX analysis of the SMA sample revealed the presence of Sb, S, and O. The EDX spectrum exhibits a peak at 0.4 keV corresponding to Oxygen, and distinct peaks at approximately 2.4 keV and 3.5 keV corresponding to Sulfur and Antimony, respectively.

Sample K1

EDX analysis of Sample K1 revealed the presence of Sb, S, Si, and O. The peak observed at 0.5 keV is attributed to Oxygen, while distinct peaks at 1.6 keV, 2.2 keV, and 3.5 keV correspond to Silicon, Sulfur, and Antimony respectively.

Sample AW

Energy-dispersive X-ray spectroscopy (EDX) analysis of the AW sample revealed the presence of Pb, S, Si, Ca, Fe, and O. The observed peaks at 0.2 keV, 0.4 keV, and 1.5 keV are attributed to Calcium, Oxygen, and a combination of Lead and Silicon, respectively. The distinct peak at 2.2 keV corresponds to the presence of sulfur.

Sample SM1

The Energy-dispersive X-ray spectroscopy (EDX) analysis of Sample SM1 revealed the presence of Oxygen (O), Sulfur (S), Silicon (Si), Calcium (Ca), Iron (Fe), Aluminum (Al), and Carbon (C). Peaks observed at 0.2 keV correspond to Calcium and Carbon, while peaks at 0.4 keV, 1.6 keV, and 2.2 keV correspond to Oxygen, Silicon, and Sulfur respectively.

Discussions

Mineralogy of Different Rock Types in the Chitral Area

Limestone

The limestone samples from the Chitral area exhibited a variety of mineralogical characteristics, including the presence of calcite, malachite, and various opaque minerals such as pyrite, lead, and antimony. The fine-grained texture of limestone, along with the presence of malachite veins, indicates secondary mineralization processes. The recrystallization of calcite, as observed in samples SM-1B and SM-1C, suggests diagenetic alterations that have enhanced the grain size and possibly the purity of calcite. The presence of stylolites, particularly in sample SM-3, indicates pressure dissolution processes that are common in tectonically active regions.

Quartz Veins

The quartz veins, particularly sample AW, showed significant mineralization with metallic minerals, including pyrite, galena, and antimony. The presence of euhedral to subhedral pyrite grains within the quartz matrix indicates a well-developed crystallization environment, which is likely influenced by hydrothermal processes. The occurrence of carbonate grains hosting metallic mineralization within the quartz veins further supports the hypothesis of hydrothermal activity.

Limonite

The limonite samples from the Chitral area, as indicated by the EDX analysis, showed high concentrations of iron, with lower concentrations of other major elements, such as SiO₂ and CaO. This is typical for limonitic zones, which are generally characterized by the presence of goethite, hematite, and other iron oxides. The relatively high Fe₂O₃ content in these samples suggests an environment of intense weathering and oxidation, which is consistent with the findings of Ullah et al. [22] who documented similar mineralogical trends in weathered iron formations in northern Pakistan. The mineralogical characteristics observed in the Chitral area are consistent with the regional geology of northern Pakistan, which is influenced by the tectonic setting of the Himalayas and the associated subduction-related processes. The presence of sulfide minerals in both limestone and quartz veins indicates a significant contribution from hydrothermal fluids, a common feature in tectonically active regions. The consistency of these findings with recent studies suggests that the Chitral area is part of a broader metallogenic province with a complex history of mineralization linked to tectonic and hydrothermal processes.

Depositional Environment of Different Rock Types in the Chitral Area

Limestone

Understanding the depositional environment of sedimentary rocks is fundamental for reconstructing their geological history. Geochemical proxies such as SiO₂ vs. Al₂O₃/SiO₂, CaO vs. MgO, and Fe₂O₃ vs. MnO ratios are widely used to infer environmental conditions during deposition.

a. SiO₂ vs. Al₂O₃/SiO₂ Ratio

The SiO₂ vs. Al₂O₃/SiO₂ ratio is a critical proxy for distinguishing between clastic inputs and carbonate-dominated environments. A higher SiO₂/Al₂O₃ ratio, along with low Al₂O₃ content, typically indicates a marine setting where siliceous material dominates and clay input is minimal. In this study, Sample SM5 exhibited these characteristics, indicating deposition in a low-clay, high-siliceous marine environment. This interpretation is consistent with established geological models of marine settings, where reduced terrestrial influence results in clearer water conditions that are conducive to carbonate formation [23]. Transitional environments, characterized by moderate SiO₂/Al₂O₃ values, suggest sedimentation in settings such as lagoons or estuaries. Samples SM1, SM2, SM4, SM6, and SM8 displayed intermediate values, indicating mixed sediment input from both marine and terrestrial sources. These transitional settings likely experienced fluctuations in energy and salinity, which are characteristic of environments with periodic tidal and seasonal influences. In contrast, Sample SM3, with high Al₂O₃ content and a lower SiO₂/Al₂O₃ ratio, reflects greater clay content, indicative of a terrestrial depositional environment [24]. Such environments are typically closer to landmasses or fluvial systems where higher clastic inputs are prevalent.

b. CaO vs. MgO Ratio

The CaO vs. MgO plot is commonly used to distinguish between limestone and dolostone and assess the extent of dolomitization. Marine limestones generally exhibit low MgO content, suggesting minimal post-deposition dolomitization. In this study, Samples SM1, SM3, SM4, SM5, and SM8 all displayed low MgO contents, aligned with the characteristics of marine limestone formations. These results suggest deposition in a stable marine setting with little alteration by Mg-rich fluids. Such environments typically maintain open marine conditions with favorable carbonate preservation [23]. In contrast, transitional environments are often marked by a moderate MgO content, indicating partial dolomitization due to varying salinity and water chemistry. Samples SM2 and SM6, which exhibited moderate MgO values, are indicative of such settings. This partial dolomitization may be attributed to the interaction of carbonate sediments with magnesium-enriched waters, likely in restricted marine or lagoonal environments, where fluctuating conditions are common [24]. Sample SM9, with its significantly higher MgO content, indicates substantial dolomitization, a characteristic feature of terrestrial environments where arid conditions often result in a concentration of magnesium-rich brines.

c. Fe₂O₃ vs. MnO Ratio

The Fe₂O₃ vs. MnO plot is a valuable tool for interpreting the redox conditions in depositional environments. A low MnO content is typically indicative of well-oxygenated marine environments, where oxidizing conditions dominate. Sample SM4, which exhibited low MnO content, reflects deposition under stable marine redox conditions. Such environments are typical of offshore marine settings, where oxygen levels remain high, limiting the accumulation of manganese [23]. Transitional environments, represented by moderate MnO content, reflect intermediate redox conditions often found in lagoonal or estuarine settings. Samples SM1, SM2, SM3, SM6, and SM8, all of which displayed moderate MnO values, suggest deposition in environments where fluctuating oxygen levels and periodic anoxia may have influenced the geochemical signature. These environments likely experienced episodic oxygen depletion, which contributed to the moderate MnO content observed [24]. In contrast, terrestrial environments, characterized by high MnO content, indicate reducing conditions that are more prevalent in settings with limited water circulation and organic matter accumulation. Samples SM5 and SM9, both of which exhibited higher MnO contents, suggest deposition under reducing conditions typical of terrestrial environments. These findings align with the idea that terrestrial settings, particularly under low-oxygen conditions, foster Mn enrichment due to the reduction of manganese oxides.

Comparison with Recent Research

The geochemical findings in this study align closely with recent studies by Garzanti et al. and Zhang et al. [23,24], who employed similar proxies to differentiate depositional environments. Garzanti et al. demonstrated the efficacy of Al₂O₃/SiO₂ ratios in distinguishing between marine, transitional, and terrestrial settings, which supports the current study’s interpretation of Sample SM5 as marine and Sample SM3 as terrestrial. Similarly, Zhang et al.’s work on carbonate platforms emphasized the role of CaO vs. MgO ratios in identifying dolomitization, which is consistent with the identification of significant dolomitization in Sample SM9. Additionally, the use of Fe₂O₃ vs. MnO ratios to infer redox conditions is well supported by both studies, further validating the interpretation of Sample SM4 as deposited under oxidizing marine conditions and Sample SM9 as representative of reducing terrestrial environments. The consistency between these findings and previously published research underscores the robustness of the geochemical methods employed in this study, enhancing our understanding of the depositional settings of the limestone samples analyzed.

Quartz Veins

The quartz veins in the Chitral area, particularly in sample AW, suggest a depositional environment linked to hydrothermal processes. The mineralization patterns, including the presence of pyrite, lead, and antimony, indicate that these veins were formed from mineral-laden fluids that infiltrated fractures and faults within the host rocks. The association of quartz veins with hydrothermal activity is typical in tectonically active regions, where deep-seated fluids are driven towards the surface through crustal pathways.

Limonite

The limonite samples, characterized by a high iron oxide content, are indicative of a weathering environment where intense oxidation processes dominate. Limonite typically forms in oxidizing environments such as lateritic soils or weathered profiles of sulfide- rich deposits. The presence of such minerals suggests that the region has experienced prolonged periods of exposure to weathering processes, likely in a tropical to subtropical climate, where chemical weathering is intense. The depositional environments inferred for the rock types in the Chitral area are consistent with the region’s broader geological setting. The shallow marine carbonates, hydrothermal quartz veins, and weathering-related limonites all indicate a complex interplay of depositional and post-depositional processes driven by the tectonic evolution of the Himalayas.

Paleotemperature of Different Rock Types in the Chitral Area

Limestone

Limestone samples from the Chitral area provide insights into the paleotemperature conditions during their formation and diagenesis. The presence of recrystallized calcite and the development of stylolites are indicative of burial diagenesis, which often occurs under increased temperature and pressure conditions. Stylolites typically form at temperatures ranging from 60 to 150°C, depending on the burial depth and geothermal gradient. The secondary mineralization of malachite within limestone also suggests a range of paleotemperatures, as malachite formation is generally associated with low to moderate temperatures, typically between 25°C and 75°C, often in the presence of oxidizing fluids. These temperatures reflect conditions consistent with shallow burial or diagenesis occurring near the surface or at relatively shallow depths.

Quartz Veins

The quartz veins in the Chitral area, particularly those containing metallic minerals, such as pyrite and antimony, suggest a higher paleotemperature environment associated with hydrothermal activity. Quartz veins typically form at temperatures ranging from 200°C to 400°C, depending on the depth and source of hydrothermal fluids. The mineral assemblages within these veins, especially the presence of pyrite, indicate that the fluids were likely of moderate to high temperatures, consistent with deep-seated orogenic processes or magmatic activity. The paleotemperature conditions inferred for the different rock types in the Chitral area highlight the diverse thermal histories of the region. The limestones reflect moderate burial temperatures associated with diagenesis, whereas the quartz veins indicate higher temperatures linked to hydrothermal processes. However, the limonitic zones suggest low-temperature weathering environments typical of surface conditions.

Chemical Weathering and Diagenesis of Different Rock Types in the Chitral Area

Limestone

Limestone samples from the Chitral area exhibit features indicative of both chemical weathering and diagenesis. The presence of recrystallized calcite and stylolites suggests that these rocks underwent significant diagenetic alteration. The recrystallization of calcite is typically a diagenetic process that occurs during burial, where the original carbonate grains are restructured under pressure and temperature conditions that facilitate the growth of larger, more stable calcite crystals. The formation of stylolites, which are pressure dissolution features, further supports the occurrence of chemical compaction during diagenesis, indicating that the limestone experienced considerable burial and tectonic stress. Chemical weathering in limestones is indicated by the presence of secondary mineralization, such as malachite, which forms as a result of the oxidation of primary copper minerals under surface or near-surface conditions. This suggests that the limestone was exposed to surface weathering processes after diagenesis, leading to the alteration of the original mineral content.

Quartz Veins

Quartz veins in the Chitral area, particularly those containing significant metallic mineralization, such as pyrite and antimony, suggest a history of chemical alteration associated with hydrothermal activity. The formation of these veins involves the precipitation of quartz and associated minerals from hydrothermal fluids that are typically enriched in dissolved silica and various metals. The chemical composition of hydrothermal fluids and their interaction with the surrounding rock can lead to alteration of the host rock and the development of mineralized zones. During diagenesis, quartz veins may also undergo chemical weathering, particularly when exposed to surface conditions. Oxidation of sulfide minerals, such as pyrite, within the veins can lead to the formation of iron oxides, such as limonite or goethite, indicating a transition from reducing to oxidizing conditions. This process reflects the chemical weathering of the vein material under atmospheric conditions.

Limonite

The formation of limonite in the Chitral area is primarily the result of intense chemical weathering. Limonite, composed mainly of iron oxides, such as goethite and hematite, is formed through the oxidation of iron-bearing minerals in a highly oxidizing environment. This process occurs under surface conditions where oxygen is abundant, leading to the breakdown of primary minerals and concentration of iron oxides. The development of limonite is indicative of advanced chemical weathering, often associated with tropical or subtropical climates, where high temperatures and humidity promote the rapid breakdown of silicate minerals and leaching of more soluble components. This leaves iron oxide as a residual product of weathering. Chemical weathering and diagenetic processes observed in the Chitral area reflect the complex interplay between tectonic activity, burial history, and surface exposure. The limestones showed clear evidence of diagenesis under burial conditions, followed by chemical weathering after uplift and exposure. Quartz veins formed from hydrothermal fluids also exhibit signs of chemical alteration, particularly through the oxidation of sulfide minerals. Limonitic zones represent the end products of intense chemical weathering in an oxidizing environment. These processes are consistent with the geological evolution of the region, which has been shaped by the ongoing India-Eurasia collision, leading to significant tectonic uplift and subsequent exposure of rocks to weathering processes.

Tectonic Environment of Different Rock Types in the Chitral Area

Limestone

The geochemical analysis of the limestone samples using PAAS-normalized concentrations of Al₂O₃, Fe₂O₃, and MnO revealed key insights into their depositional environments. These trends suggest a mixed influence from inland continental sources and tectonically active regions, with evidence pointing to volcanic and hydrothermal activity. We discuss our findings in detail below.

Inland or Continental Influence

Certain samples, notably SM9 with a normalized Al₂O₃ value of 0.696, aligned with geochemical signatures typical of inland or continental settings. Such elevated Al₂O₃ concentrations indicate significant input from continental sources, likely the result of detrital material being carried into the depositional environment through riverine or aeolian processes. The presence of aluminum-rich clays in the sediments is consistent with the weathering of nearby landmasses and the subsequent transportation of these materials into the basin. This supports the idea that limestone in these settings was influenced by continental weathering processes, contributing to the overall geochemical makeup of the deposits.

Tectonically Active Environments

The high Fe₂O₃ concentrations observed in samples SM1 and SM9 (up to 3.57) strongly indicate deposition in a tectonically active setting. These elevated iron levels are consistent with environments associated with volcanic arcs such as forearc or back-arc basins. In such settings, volcanic eruptions, hydrothermal vents, and other magmatic processes introduce iron-rich materials into surrounding sediments. The high Fe₂O₃ levels in these samples suggest that the limestones were deposited in proximity to volcanic activity and received volcanic ash and other detrital materials. This supports the interpretation of a forearc or back-arc tectonic setting, where volcanic inputs are significant contributors to sediment composition.

Unusual MnO Enrichment

One of the most striking features of the dataset was the exceptionally high MnO concentration, ranging from 8.0 to 40.64. In most marine and inland settings, MnO concentrations are typically low (generally less than 0.1, normalized values). The unusually high MnO concentrations in these samples suggest the involvement of post-depositional processes such as hydrothermal alteration. In tectonically active regions, hydrothermal systems can mobilize Mn, which then precipitates in the sediments under oxidizing conditions. This MnO enrichment further supports the hypothesis that a forearc or back-arc basin setting is influenced by hydrothermal processes, likely in the vicinity of a volcanic arc system.

Mixed Marine and Tectonic Inputs

The variations in Al₂O₃ and Fe₂O₃ concentrations across the samples indicate a combination of marine and tectonic inputs. Samples such as SM1, with low Al₂O₃ concentrations (approximately 0.033), are more typical of marine environments where the input of continental detrital material is minimal. Despite the low Al₂O₃ values, the elevated Fe₂O₃ levels suggest the influence of volcanic input, indicating a depositional environment where marine conditions coexisted with volcanic activity from a nearby tectonic setting. This dual influence highlights the complexity of the depositional environment, which likely occurs in a marine basin near volcanic arcs or active tectonic zones.

Implications

The geochemical trends observed in the limestone samples, particularly the elevated concentrations of Fe₂O₃, Al₂O₃, and MnO, provide strong evidence of a tectonically active setting with significant contributions from both volcanic and continental sources. Specifically, these trends support the following interpretations.

Forearc or Backarc Basin Setting

The high Fe₂O₃ concentrations (up to 3.57) are characteristic of forearc or back-arc basins, which are regions typically associated with volcanic arcs. In these settings, Fe is often enriched because of volcanic activity or hydrothermal processes. Both the forearc and backarc basins receive volcanic ash and other detrital materials from nearby volcanic arcs, which contribute to the sedimentary record. The high Fe₂O₃ values in your samples suggest proximity to an active volcanic arc system, likely influencing the geochemistry of the deposited limestones.

Mixed Continental and Marine Influence

The Al₂O₃ concentrations show evidence of both continental and marine input. Higher Al₂O₃ concentrations (e.g., 0.696 in SM9) indicate detrital input from nearby landmasses, possibly through weathering or fluvial transport. Conversely, samples with lower Al₂O₃ values (such as SM1) suggest a more marine-dominated environment with a limited continental input. This mixed signature suggests that the limestone was deposited in a marine basin close to a volcanic arc but was also influenced by continental sediment sources.

Hydrothermal Alteration and Diagenesis

The unusually high MnO concentrations indicate the likely influence of hydrothermal processes. Such high MnO values suggest that these limestones were exposed to hydrothermal alteration, which is possibly related to tectonic activity in the surrounding area. Hydrothermal systems commonly associated with volcanic arcs can mobilize and deposit Mn in sediments, resulting in the significant MnO enrichment observed in these samples.

Overall Tectonic Setting

The combined evidence points to a depositional environment most likely located in a forearc or back-arc basin associated with an active volcanic arc. The geochemical signature, including elevated Fe₂O₃, MnO, and variable Al₂O₃, reflects contributions from volcanic activity, hydrothermal alteration, and continental sediment input, suggesting a dynamic and complex tectonic environment.

Quartz Veins

The quartz veins observed in the Chitral area, particularly those containing significant metallic mineralization, such as pyrite and antimony, are indicative of a tectonic environment characterized by active faulting and fluid migration. These veins likely formed in a tectonically active setting where crustal movements facilitated the circulation of hydrothermal fluids through fractures and faults. The presence of these veins suggests a post-collisional extensional regime, in which tectonic forces have created space for fluids to infiltrate and deposit minerals.

Limonite

The formation of limonite in the Chitral area suggests a tectonic environment that has been exposed to prolonged weathering and oxidation, possibly linked to uplift and subsequent exposure of the region during the Himalayan orogeny. The high iron content and weathered nature of these rocks indicate that they have been subjected to surface processes for an extended period, which is consistent with the uplift and exhumation phases of the mountain buildings. The tectonic environments inferred for the different rock types in the Chitral area are indicative of a region that has been heavily influenced by the ongoing India-Eurasia collision. The combination of compressional forces, extensional faulting, and uplift has created a diverse tectonic landscape where a variety of depositional and post-depositional processes have occurred. This complex tectonic setting is reflected in the deformation features, mineralization patterns, and weathering profiles observed in the rocks of the region.

Hydrothermal Implications and Mineralization Processes in the Chitral Area

The geological setting of the Chitral area, coupled with its complex tectonic history, has played a significant role in the hydrothermal processes and subsequent mineralization observed in the region. The presence of quartz veins and metallic minerals such as pyrite, antimony, and secondary mineralization in limestones and limonitic zones provides crucial insights into the hydrothermal activity that has shaped the mineral deposits in this area.

Hydrothermal Processes

Hydrothermal processes in the Chitral area are primarily linked to the region’s tectonic activity, particularly the ongoing collision between the Indian and Eurasian Plates. This tectonic environment has facilitated the circulation of hydrothermal fluids, which have played a pivotal role in the formation of mineralized quartz veins and the alteration of surrounding rocks.

a. Source of Hydrothermal Fluids:

The hydrothermal fluids responsible for mineralization are likely derived from magmatic sources, as indicated by the high-temperature conditions inferred from quartz veins (200–400°C). These fluids are typically rich in silica and various metals, which are mobilized and transported through fractures and faults created by tectonic stresses.

b. Fluid Pathways:

The structural geology of the region, characterized by numerous faults and fractures, has provided pathways for these fluids to ascend from deeper crustal levels. The interaction of these fluids with the surrounding rocks, particularly carbonate rocks and existing sulfide minerals, has led to the deposition of a variety of mineral species.

c. Temperature and Pressure Conditions:

The formation of quartz veins at elevated temperatures (200– 400°C) indicates that these fluids were capable of carrying and depositing metals such as antimony and pyrite. The presence of sulfide minerals suggests that hydrothermal fluids were reduced, facilitating the precipitation of metal sulfides.

Conclusion

This study offers a detailed geochemical and petrographic analysis of hydrothermal mineralization in the Chitral region of Northern Pakistan, underscoring the critical role of tectonic structures, fluid migration, and mineral deposition within an orogenic setting. These findings confirm that tectonic deformation facilitated the flow of mineral-rich hydrothermal fluids, leading to the deposition of economically important metals such as Pb, Zn, and Sb. Major element analysis revealed significant concentrations of Fe₂O₃, CaO, and MnO in limestones, while quartz veins exhibited high SiO₂ content, indicative of silica-rich compositions. Trace element analysis further highlighted elevated Pb, Zn, and Sb levels in the limestones and quartz veins, confirming their hydrothermal origin. Additionally, EDX analysis reinforced these conclusions by identifying key elements such as Zn, Pb, Sb, and Fe in specific rock samples.

The observed mineralization patterns, characterized by structure- controlled deposition, presence of sulfides, and low-temperature conditions, suggest that the Chitral region experienced MVT -Type mineralization. This conclusion not only enhances our understanding of the region’s tectonic evolution and hydrothermal processes but also points to its significant potential as a source of critical metals, particularly Pb, Zn, and Sb, which are crucial for industrial applications. Considering these findings, further efforts should be undertaken to fully assess the commercial viability of mineral deposits. Detailed geophysical surveys and targeted drilling programs should be conducted to better define the extent of mineralized zones and estimate resource potential. Advanced geochemical modeling and isotopic studies can also provide deeper insights into the source and evolution of hydrothermal fluids, offering more clarity on the mineralization processes in the study area.

Statements and Declarations

a. Ethical Approval: This manuscript has not been published elsewhere in part or in entirety and is not under consideration by another journal. We have read and understood your journal’s policies, and we believe that neither the manuscript nor the study violates any of these. We have not submitted our manuscript to a preprint server before submitting it here.

b. Consent to Participate: Not applicable.

c. Consent to Publish: Not applicable.

Availability of data and materials: Not applicable.

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