Phosphorus, though comprising only 0.09 wt.% of Earth’s crust [1] , is the tenth most abundant element and plays a critical role in both geological and biological processes. Among minerals, phosphates are notably complex and diverse, with around 460 identified species [2] . Phosphorus is essential for the growth and development of all living organisms and serves as a limiting nutrient for marine biological productivity over geological timescales [3] ]. While it forms part of certain proteins, its primary biological role lies in forming the structural basis of vertebrate bones and teeth, as well as the shells and chitinous exoskeletons of various invertebrates [4] . Marine sedimentary phosphates represent a major sink for phosphorus (P), a vital element for all life forms [5] [6] . Phosphates existing at the interface of biological and geological systems play a key role in the global phosphorus cycle [7] .

Rock phosphate, also known as phosphorite, is a marine biochemical sedimentary rock enriched in phosphorus, typically containing at least 18 wt.% P₂O₅ and in some cases up to 40 wt.%. It is commonly formed in coastal upwelling zones [8] - [12] when specific physical conditions align with biological activity during early diagenesis [7] [13] . Phosphate ( ${\text{PO}}_4^{3-}$) crystallizes into francolite—also known as carbonate-fluorapatite (CFA)—as authigenic cryptocrystalline masses. These phosphatic particles typically occur as concretions, including pellets, nodules, and crusts. The saturation index for fluorapatite [Ca₅(PO₄)₃F] during phosphogenesis ranges from 5 × 10⁻⁸ to 2.5 × 10⁻⁷ g/L, under pH conditions of 7.1 to 7.8, Eh values between −0.1 and 3.0 (indicative of reducing environments), and temperatures from 5˚C to 20˚C [7] [11] [14] . These conditions are typically driven by microbial degradation of organic matter within sediments [9] [15] . Francolite formation is generally associated with suboxic to anoxic sediments, where phosphorus and carbon are released during microbial decomposition. The sedimentation rate also plays a critical role in controlling phosphogenesis [15] .

Geological interest in phosphatic sediments has increased in recent decades due to their economic importance, particularly in the fertilizer industry, which consumes about 90% of global phosphate production [16] [17] . As a critical non-renewable resource for agriculture, phosphorites are essential for sustaining global food demand, with the production of synthetic fertilizers requiring around 150 million tons of phosphate annually [4] . Phosphates are also considered as a potential source for the rare earth elements [12] .

Marine sediments have been known to contain phosphate nodules throughout geological time [18] [19] . In regions such as South Africa, Peru-Chile, and Baja California, organic-rich, diatomaceous sediments containing phosphate deposits are found [19] - [21] . These regions are characterized by exceptionally high levels of global bioproductivity [22] . Nonetheless, phosphates can also be found in low-productivity oceans, where the absorption of phosphorus was likely controlled by bacterial activity and organic matter [23] . Recent studies highlight the significance of bacterial sulfate reduction and sulfide oxidation as the most efficient microbial processes for concentrating and promoting apatite precipitation [3] [24] - [26] . According to [27] and [28] , the late Proterozoic, Early Cambrian, Ordovician, Permian, Late Cretaceous-Eocene, and Miocene were the most optimal times for phosphogenesis, while least favourable conditions characterized the Silurian, Devonian, Carboniferous, and Triassic. However, economically significant Devonian phosphates are rarely found, as indicated by examples in Iran [28] [29] .

The Carboniferous-Permian marks a major phase of global phosphogenesis, with widespread deposits documented globally. The Permian is recognized as one of the key Phanerozoic periods of extensive phosphate accumulation [27] ( Figure 1 ). The Phosphoria Formation in North America is a well-known example of Permian phosphorite development [30] [31] . Other notable Permian phosphorite deposits include Novaya Zemlya in Russia [32] and the Gufeng Formation on the Yangtze Platform in South China [33] .

Despite a century of rigorous research, the mechanisms underlying their formation and the origin of their granular constituents remain subjects of ongoing debate. By accreting and bioerosioning phosphorite grains, microbial mediation activities in phosphogenic regions can contribute to phosphogenesis in either a beneficial or detrimental way [25] [34] - [42] . Marine phosphorites, which document a broad spectrum of depositional and diagenetic changes, are conventionally classified into two main types: granular and non-granular phosphorites [41] [43] . A range of phosphatic particles, such as phosphatised skeletal (for example, as fish bones, shark teeth, and bioclasts) and non-skeletal (for example, as nodules, coprolites, peloids, ooids, oncoids, aggregates, and lithoclasts) grains, make up granular phosphorites [44] . Due to our reliance on phosphate for fertilizers and various agricultural and chemical applications, these deposits are significant not only scientifically but also economically [7] .

The Chamba basin in Himachal Pradesh, which represents the Tethyan domain in the northwest Himalaya, has been investigated since 1882, when McMahon, who was the first, surveyed the region and also established the earliest geological map [45] [46] . The Chamba region is part of a NW-SE trending longitudinal valley, located between the Zanskar and Dhauladhar ranges in the northeast and southwest, respectively. The Chamba region is part of a longitudinal valley with axial trace orienting NW-SE with gently southeastward-plunging axis that stretches over 100 km from Bhalesh to Bharmour via Kalhel with a width of approximately 20 km between its two limbs [47] [48] . In later years, contributions to the area’s stratigraphy include [49] - [55] .

The Salooni Formation of the Chamba basin is characterized by the presence of phosphorites, which occur as phosphate nodules. The Geological Survey of India was the first to find the phosphate nodules at the Chikoli bridge (32˚45'05'' N, 75˚59'40'' E)—Bhatiyund (32˚43' N, 76˚04' E) area of Chamba district, Himachal Pradesh ( Figure 2 ). Despite extensive investigation into the area, a thorough

mineralogical assessment of the phosphate nodules in these deposits and a detailed explanation remains missing. The present investigation addresses the mineralogical features, distribution patterns, textures, and optical properties of phosphate-bearing minerals and associated gangue. It involves thin-section analysis, scanning electron microscopy (SEM), and X-ray diffraction (XRD) of phosphate nodules of the Chakoli bridge-Bhatiyund area of Chamba district, Himachal Pradesh to learn about their formation processes and the depositional environment of the basin.

Since no petrographic studies have previously been conducted in the study area, this paper focuses on the petrographic characteristics of phosphate nodules from the Chamba basin to better understand their genesis and depositional environment.

Chamba nappe is located between the Chenab Normal Fault (CNF) to the NW and the Chail Thrust (also referred to as Panjal Thrust— [56] [57] ) to the SW while having biostratigraphic similarities with the Tethys Himalaya Zone [58] . The Chamba sequence thrusts over the Lesser Himalayan succession to the southwest along the Chail Thrust (CT) and Higher Himalaya Crystallines (high-grade metamorphic rocks) to the northeast side along the CNF and is classified as the Chamba nappe (CN) [47] [59] [60] . The structure of the nappe represents two regional-scale synclinoriums named Chamba syncline and Tandi syncline, both exhibiting vergence in opposing directions. Vergence of the Chamba syncline is towards SW along the Chail Thrust [58] [61] [62] and the Tandi syncline is towards NE along the CNF [58] [63] [64] . Both the synclines possess several synforms and antiforms and were created when the succession of the nappe was tectonically folded into an overturned fold during the first phase of deformation [52] [65] [66] . This longitudinal valley (Chamba syncline) of axial trace orienting NW-SE with a gently southeastward-plunging axis stretches over 100 km from Bhalesh to Bharmour via Kalhel with a width of approximately 20 km between its two limbs and both limbs of which dip on the northeasterly side by an average of 30˚ [47] [48] . In the Churah area, the northwestern closure of the Chamba syncline suggests the plunging of the syncline is towards the NW side [47] .

Chamba basin comprises roughly 8 to 10 kilometers of low-grade metasedimentary rocks arranged in a folded sequence, covering an area of nearly 140 km in length by 70 km average width [47] [58] . The study area is situated within the Chamba Basin of northwestern Himachal Pradesh, approximately 48 km north of Chamba city and about 90 km northeast of Dalhousie town. The rock formations of the Chamba syncline encompass very diverse lithologies in age ranging from the late Proterozoic era to the Triassic period [50] [54] [67] . The basement of the Chamba nappe sequence is the Salkhala group of rock formations which is predominantly exposed in the anticline’s core on the northwest side of Chamba area as well as along the Panjal Thrust side limb of the Chamba syncline as a long narrow band that lies in the southwestern part [47] [53] [58] .

In western Himachal Pradesh, the Salkhala group has been categorized into two units, namely the Bhalai Formation and the Chamba Formation. A set of concordant granitoid intrusions has been emplaced within the Salkhala Group of rocks, locally referred to as the Dhauladhar Granite, Dalhousie Granite, Mandi Granite, and Kersog Granite [68] . Figure 3 and Table 1 provide the geology and stratigraphy of the study area in Chamba region is described as follows: The Chamba Formation which forms the basal unit of the study area is succeeded by Manjir Formation, Katarigali Formation (Lower Salooni Formation/LSF), Saho Volcanics, Upper Salooni Formation and Kalhel Limestone—which occupies the core of the Chamba syncline.

The Chamba Formation consists of a thick succession comprising thinly bedded dark grey carbonaceous slates with limonitic encrustations, puckered lustrous phyllites, metapelitic schist, metasiltstones, metagreywacke and laminated to cross-bedded quartzites displaying strong flyschoid characteristics. The Manjir Formation unconformably overlies the Chamba formation, consists of grey and purple slates. Both the normal and inverted limbs of the formation have characteristic diamictite-bearing lithology and localized pebbly horizons. It comprises quartzite bands, pebble beds, shales, and interbedded limestone bands. The formation is sharply overlain by the Katarigali Formation. The Katarigali Formation (LSF) comprises dark grey to black carbonaceous pyritous slates (hard, compact, non-laminated, with poorly developed slaty cleavage) and thick, pyrite-disseminated phyllites, interbedded with quartzite, limestone, and graphite quartz-schist.

Within the phyllites pyrite commonly occurs as disseminations, stringers, and veins, while quartz as veins and pockets.

Phosphate nodules are confined only to the black carbonaceous slate unit of the Lower Salooni Formation (LSF) ( Figures 4(a)-(c) ) and occur parallel to both the cleavage and rock bedding. These nodules are dark grey to black, sub-rounded to oval in shape, and vary in size from 3 × 4 cm to 15 × 35 cm across. Phosphate nodules generally lack internal structures, although concentric layering may occasionally be observed. These nodules, sporadically distributed, exhibit conchoidal fracture and are often flattened due to tectonic compression ( Figures 5(a)-(c) ). Additionally, the nodules contain disseminations, streaks, and specks of pyrite. These nodules having P₂O₅ concentration ranging from 20% to 30% indicate their economic importance [69] .

Flows ranging from composition of basaltic to andesitic in the form of sills, and dykes (10 - 50 m thick) intrude the Salooni Formation (Lower and Upper Salooni) aligning with NW-SE foliation trends constitute the Saho Volcanics. These intrusive vary from light grey to dark greenish-grey in color, with cryptocrystalline to holocrystalline textures, and in amygdaloidal nature appear to be doleritic composition. The Upper Salooni Formation comprises interbedded grey slate, carbonaceous slate, calcareous phyllite, quartzite, and limestone and the presence of fossils including Modiola lidarensis, Spiriferalla rajah, and Productus sp. further corroborate its Permian age [51] . The Kalhel Formation (Triassic), the youngest stratigraphic unit in the area, comprises 400 m of interbedded crystalline limestone (variegated grey, bluish-grey, and pink) and dolomitic limestone (including crinoidal and cherty varieties), alongside calcareous phyllite, orthoquartzite, and shaly slate [70] .

Sampling was carried out from important locations of the area named as (1) near the Chikoli bridge, which is 8.5 km northwest of Salooni tehsil, and (2) at the Bhatiyund locality, which is about 13 km west of Salooni tehsil. Using a geological map and toposheet.map (Nos. 52 D/2 and 43P/13) of the examined area, 35 representative phosphate nodule samples were taken from the Chikoli bridge (32˚45'05'' N, 75˚59'40'' E) and the Bhatiyund outcrop (32˚43' N, 76˚04' E). Samples from several stratigraphic levels and horizons were systematically taken at regular intervals to minimize any sampling bias within the bed, if present, and to achieve an optimal range of nodule variation. Thin sections were prepared and examined with a polarizing microscope at various magnifications to determine their optical properties of mineral assemblages.

The samples were ground to a size of about 200 mesh to get distinct mineral diffraction patterns. Using a Shimadzu LabX XRD-6100 with a Cu-Kα radiation source (λ = 1.54), the mineralogy of the sample was analyzed by powder X-ray diffraction (XRD) to determine the type and distribution of crystalline phases present in powder samples with a scanning rate of 8 deg/min over a 2θ range of 10-65˚ at room temperature. The X-rays were generated at a voltage of 40 kV and a current of 30 mA. The JCPDS cards’ X-ray identification charts and the PANalytical X’pert High Score Plus® software were utilized for data analysis.

The freshly broken surfaces of the samples were affixed to SEM stubs applying black carbon tape. The samples were then gold coated via a sputter coater and were subsequently analyzed using a scanning electron microscope (SEM) (JOEL JSM-6510, Japan) that included an EDX (energy dispersive X-ray) microanalyzer at the University Sophisticated Instruments Facility, AMU, Aligarh, to analyze the crystal morphology, shape, size, and characteristics of the mineral grains. The SEM images shown in this study were taken at various magnifications.

The methodology employed for the petrographic analysis of phosphate nodules is illustrated in a stepwise workflow diagram summarizing each procedural step ( Figure 6 ).

Microscopic examination reveals that phosphorites consist of phosphatic (principal), non-phosphatic grains and organic matter in smaller amounts. The phosphatic constituents occur as pellets, nodules, and apatite-rich matrix in the groundmass, predominantly composed of cryptocrystalline carbonate-fluorapatite (CFA), the primary phosphate mineral within the nodules. In contrast, microquartz, pyrite, dolomite and siliceous material represent the non-phosphatic constituents. In plane-polarized light, CFA appears dark brown, while under crossed nicols it exhibits a black, pseudoisotropic appearance, attributed to the aggregate polarization caused by its cryptocrystalline texture. The groundmass of structureless cryptocrystalline CFA contains well-rounded phosphate peloids dispersed throughout ( Figures 7(a)-(c) ). The presence of phosphatic pellets within the nodules suggests an early diagenetic origin, formed shortly below the sediment-water interface and is supported by the stable-isotopic study [9] [71] - [73] . Organic matter present in the phosphate nodules imparts a brown to brownish-black coloration. In areas where the organic content is higher, the color appears notably more intense ( Figure 7(b) ). Organic matter is broken down by microbes, which is usually associated with the reduction of nitrate (NO3⁻) and sulphate (SO4²⁻). This process releases labile phosphorus, and free sulphide which drives the precipitation of pyrite [74] [75] . It observed that in certain pellets within the nodules and phosphate material has been partially replaced by siliceous matrix ( Figures 7(a)-(c) ) with relict patches of the original phosphatic composition preserved in certain places ( Figure 7(c) ). Siliceous material has replaced the initial phosphate matrix of the nodules, forming a network-like structure ( Figure 7(d) and Figure 7(e) ). Replacement of phosphates by silica and pyrite shows a succession of post-depositional diagenetic changes [74] [76] . A few peloids appear as “phosphate envelopes”—grains with a phosphatic rim similar in composition to typical peloids, but with calcite cement filling the core. These uncommon structures are interpreted as the result of internal dissolution and subsequent calcite cementation ( Figure 7(f) ). The present work has been corroborated with the other workers like [2] [77] - [79] .

CFA (also Francolite) and hydroxylapatite are the main constituents of the phosphate nodules while the latter is in major proportion were identified by sharp and well-defined diffraction peaks revealed in the X-ray diffraction (XRD) results. The other minor (also non-phosphatic) mineral phases contain quartz and kaolinite or chlorite ( Figures 8(a)-(c) ). Comparable results have been documented in phosphate nodules from Guizhou Province, South China [80] .

Due to the exceptionally fine-grained character of CFA e.g. fabric, crystal dimensions, and crystal form, individual crystals in sedimentary phosphate rocks cannot be observed with a standard petrographic microscope. Scanning electron microscopy (SEM) analysis of phosphate nodules was conducted on freshly fractured surfaces to examine their internal texture. SEM observations reveal that the apatite crystals in semi-euhedral form with little erosion seen on the edges of the crystals indicate the recrystallization of CFA crystal ( Figure 9 ). Crystalline apatite identified in the samples ( Figure 9 ) may support to the claims of [81] and [82] that phosphates first precipitated as non-crystalline or slightly crystalline metastable phases before changing into crystalline apatite based on the level of fluorapatite supersaturation and the present findings support conclusions of [83] . In the photomicrograph successive stages of growth and crystallization within an apatite crystal exhibit ( Figure 10 ) and these findings align with the findings published by [83] . A highly porous texture is formed by the aggregation of fine-grained phosphate crystals of micrometre scale, exhibiting predominantly equant granular and short-prismatic eucrystalline as seen in microstructure of CFA and the elemental composition of phosphate particles is further confirmed by EDX analysis ( Figure 11(a) and Figure 11(b) ). Basal sections of CFA appear as hexagonal prisms, forming a polygonal ultrastructure composed of laminated or randomly arranged hexagonal apatite crystals (0.9 - 1.5 μm wide), which are either welded or retain their separate crystal morphology. [2] [84] - [86] came to similar conclusions, which also corroborate the current findings. In the examined samples, two modes of occurrence were identified: diagenetic replacement and direct precipitation. Diagenetic francolites display fibrous crystals with radial arrangements relative to the grain surface ( Figure 12 ).

By contrast, authigenic francolites—formed via direct precipitation—are distinguished by large, well-developed euhedral crystals within micropores ( Figure 9 ). The variation in crystallinity may be attributed to differences in algal assemblages, organic matter content, and pore-water chemistry, and is possibly more directly influenced by the rate of crystal growth or the presence of impurities. This explanation aligns with the work of [87] [88] . Quartz is present at the center which occurs on the surface of micro-structured globular phosphatized particles, accompanied by well-crystallized CFA particles and the elemental composition of quartz is further confirmed by EDX analysis ( Figure 13(a) and Figure 13(b) ). Platy crystals appear as loosely arranged, euhedral to subhedral outlines, aligned face-to-face along their elongation, and is observed filling a microfracture between two CFA grains that may be either kaolinite or chlorite as kaolinite (7 Å mineral), cannot be distinguished from chlorite using XRD analysis when both minerals are present ( Figure 14 ) and has been supported by other researchers including as [89] - [91] .

The phosphate nodules of the Chikoli-Bhatiyund region in Chamba district are primarily hosted within the black carbonaceous slate of the Lower Salooni Formation. It is concluded that carbonate fluorapatite (CFA) is the chief phosphate mineral, with quartz as the major gangue mineral, pyrite, carbonate, and clay as minor gangue constituents. Analytical methods including X-ray diffraction, thin-section petrography, and scanning electron microscopy along with EDX collectively reveal apatite crystals that are euhedral to subhedral and exhibit equant granular to prismatic forms, accompanied by a porous matrix and siliceous replacement textures occurring both within individual phosphate peloids and throughout the phosphate matrix as a network of silica veins. Disseminated pyrite and organic matter observed in the nodules suggest localized reducing conditions during diagenesis. Evidence of microbial filaments seen in SEM images supports the involvement of microorganisms which indicate the early diagenetic processes in phosphogenesis. Microfracture fillings by kaolinite/chlorite suggest that the phosphate-bearing sequences formed under conditions of intense chemical weathering of the continental landmass.

The mineral assemblages and replacement textures imply initial precipitation under shallow marine suboxic to anoxic conditions. The mutual replacement of phosphate by silica indicates the last stage of diagenesis and varying chemical conditions during burial. While these nodules are currently limited in distribution, their high phosphorus content underscores their potential as an alternative resource for fertilizer production.

The present study adds new insight into the genesis and mineral composition of phosphatic nodules within the Chamba Basin.

The first and third authors are thankful to the Chairperson, Department of Geology, and University Sophisticated Instruments Facility, Aligarh Muslim University, Aligarh, for providing the necessary facilities. Shamim A. Dar acknowledges the support of the HOD, Department of Geology, B.H.U., Varanasi, for providing the required facilities. Shamim A. Dar is highly thankful to the Anusandhan National Research Foundation (ANRF), Science and Engineering Research Board (SERB), Department of Science & Technology, and Government of India for a start-up research grant (M-14/0599; Sanction order no. SRG/2022/001478) and Seed Grant under Institutions of Eminence (IoE), Banaras Hindu University (BHU) (Dev. Scheme No. 6031) for financial assistance.