The West Kunlun-Karakoram orogen is divided into the North Kunlun (NKT), South Kunlun (SKT) and Tianshuihai tectonic units (Xiao et al., 2005; Wang et al., 2014; Wang et al., 2017) (Figure 1A). The Luobugaizi Pb-Zn deposit occurs in a rift basin within the Mingtiegai landmass (second-order tectonic unit), which lies on the northern margin of the ancient Tethys tectonic domain, the junction of the South Pamir and the Karakoram Mountains. Stratigraphically, the region belongs to the Qiangbei-Changdu-Simao zone, specifically the Mingtiegai subzone. The exposed strata mainly comprise the Paleoproterozoic Bulunkole Group (Pt₁ B) medium-to high-grade metamorphic rocks, the Lower Silurian Wenquangou Formation (S₁ w) fine-grained clastic rocks and limestone, the Carboniferous Qatir Group (C₂ Q) carbonate, the Jurassic Longshan Formation (J_1-2 l) marble and pyroclastic rocks, and Quaternary (Q) sediments (Figure 1B). The Palaeozoic strata are widespread in the Mingtiegai area, while the Mesozoic and Cenozoic strata occur over limited areas. Besides the Luobugaizi Pb-Zn deposit, the region hosts other deposits, such as the Tuokemansu tungsten deposit, peripheral Pb-Zn and copper occurrences around Luobugaizi, the Qunsagiriya copper and Pb-Zn, the Dasdarnan Pb-Zn, the Athiyayile copper and the Dabalama hematite deposits.

The region experienced intense regional tectonic activity. Major regional faults include the NW-NNW-trending Kangxiwa fault (F1, Figure 1A), the NW-trending Taaxi fault (F2), the Tashkurgan fault (F3) and the SE-trending Dabudaer reverse fault (F4). Third-level tectonic faults include the NW-EW-trending Qunsaragiriya Fault (F5), the NW- trending Talisike fault (F6), the NW-trending Kizilekemu fault (F7) and the NW- trending Athiyayile fault (F8, Figure 1B).

Magmatic rocks are widely distributed across the region. Major regional magmatic rocks include Carboniferous granitoids, Early Paleozoic volcanic rocks (530–450 Ma; Quek, 2018), Early Mesozoic granite (Triassic, 240–200 Ma; Liu et al., 2020b), Late Yanshanian intermediate-felsic intrusive rocks and Cenozoic alkaline mafic rocks. In the vicinity of the study area and within it, Late Yanshanian intermediate-felsic intrusive rocks are predominantly distributed (Figure 1B), which may be closely related to Pb-Zn mineralization.

The exposed strata at the Luobugaizi Pb-Zn deposit are predominantly (95%) composed of the Lower Silurian Wenquangou Formation (S₁ w), a shallowly metamorphosed sandstone-shale sequence. This Formation is divided into five lithologic members, with the third member (S₁ w ³, predominantly slate) serving as the primary ore-hosting layer. Quaternary deposits cover the remaining ∼5% of the surface (Figure 2).

The deposit is structurally controlled by the NW- to EW- trending Qunsaragiriya Fault (F5, Figure 1B), which separates the Yanshanian granodiorite to the south from Jurassic strata to the north. Yanshanian intermediate-felsic intrusive rocks are distributed approximately 2.5 km east of the deposit and also occur within its southern sector (Figures 1, 2A). These intrusions occur predominately as stocks and consist mainly of granodiorite, quartz diorite and biotite granite.

The deposit comprises nine individual orebodies, with total resources exceeding one million tonnes (Wang et al., 2021). The orebodies occur primarily in slate interbeds of the Wenquangou Formation and mainly in stratiform, sub-stratiform and lenticular shapes. While some orebodies conform to the attitude of the surrounding strata, other Pb-Zn veins crosscut or displace the original bedding planes (Figure 2B). Weighted average grades are 2.30% Pb and 3.21% Zn, with a combined Pb + Zn grade of 5.51%, meeting industrial standards. Furthermore, the deposit contains associated elements such as Ag, Cd, and Ga.

The ore deposit has a relatively simple mineral assemblage. The major ore minerals are sphalerite and galena, accompanied by pyrite, chalcopyrite, siderite, cerussite, tetrahedrite, covellite and limonite. Gangue minerals include quartz, sericite, clay minerals, calcite, plagioclase, chlorite, graphite and organic carbon. Ore structures include massive, blocky, angular breccia, and veinlet-like types (Figures 3A–F). The ores mainly show euhedral to subhedral granular, replacement, metasomatic, intergranular, heteromorphic, and exsolution textures (Figures 4A–F). The mineral assemblage was predominantly formed during the hydrothermal mineralization period, with siderite and cerussite being minor products of subsequent supergene alteration.

Under microscopic observation, pyrite is commonly paragenetic with quartz and is enclosed by sphalerite (Figure 4D). Both in hand specimens and under the microscope, sphalerite veins can be clearly observed to be crosscut by galena-quartz veins (Figures 3E, 4B,F), whereas the quartz-carbonate veins, representing the latest mineralization stage, crosscut either sphalerite veins or galena veins (Figures 3A,D). Based on these observations, the hydrothermal mineralization of this deposit can be divided into four stages (Figure 5). Stage 1 is the quartz-pyrite stage characterized by quartz–sericite assemblage with intergrown pyrite. Stage 2 is the quartz-sphalerite-carbonate stage. It is dominated by sphalerite precipitation, with minor pyrite-chalcopyrite-galena. Sphalerite is yellowish-brown in hand specimens, dark to brownish-grey in reflected light, and sometimes shows “chalcopyrite disease” (Figure 4A). Stage 3 is the quartz-galena-carbonate stage. Ore minerals include galena, chalcopyrite and trace sphalerite. Galena veins are observed to crosscut sphalerite (Figure 4E). Stage 4 is the quartz-carbonate stage. It represents the latest hydrothermal phase, characterized by the formation of quartz and calcite, interspersed with minerals from earlier stages.

Alteration is dominated by silicification and sericitization, with subordinate dolomitization, carbonatization and chloritization. Silicification and sericite alteration are well developed and most evident in the surrounding rock of Pb-Zn orebodies. Supergene alteration produces cerussite and smithsonite, which are vivid red or orange-yellow, making them key exploration indicators.

In this study, ten representative samples were collected from the initial mining area (eastern mining zone) of the Luobugaizi Pb-Zn deposit. These samples were prepared as polished thin sections for optical microscopic examination to characterize their mineralogy and paragenetic relationships. To guide precise spot selection for in situ analysis, micrographs at multiple scales were first acquired for all samples. These images ensured that measurement points on sphalerite grains were accurately targeted for subsequent in situ trace element (LA-ICP-MS) and S isotope (LA-MC-ICP-MS) analysis.

Separately, individual sphalerite and galena grains were hand-picked from eight ore samples for Pb isotope analysis.

The trace element compositions of sphalerite from the Luobugaizi Pb-Zn deposit are summarized in Table 1.

Sphalerite contains low to moderate concentrations of Fe, ranging from 0.55% to 4.25%. Cadmium concentrations are high and relatively uniform (905–1925 ppm, mean 1,261 ppm), whereas Co concentrations are lower lower but also consistent (14.0–87.6 ppm, mean 44.9 ppm). Lead and Cu show the greatest concentration ranges (Pb: from below the detection limit up to 25,800 ppm, mean 1,388 ppm; Cu: 2.89–2,780 ppm, mean 143 ppm). Elements including Ge, Ga, In, Sb, and Ag demonstrate–moderate concentrations that vary over one to two orders of magnitude (0–50.2 ppm, 0.64–390 ppm, 0–237 ppm, 0.70–2,672 ppm, and 0.58–181 ppm, respectively). Several elements are consistently present in low concentrations. Arsenic concentrations are generally low (1.95–68.6 ppm, mean 9.80 ppm), while elements like Ni, Se, and Sn are the least concentrated, with mean values all below 5 ppm.

The–Sulfur isotope compositions of sphalerite are reported in Table 2 and Figure 6. The δ³⁴S values for sphalerite range from −1.2‰ to +3.3‰ (mean + 1.3‰), exhibiting a relatively small variation compared to most sediment-hosted deposits. The values display a distinct “tower-like” distribution pattern, with the majority concentrated within a narrow peak between +1‰ and +2‰.

The Pb isotopic compositions of sulfides from the Luobugaizi deposit are summarized in Table 3. The sulfides from the deposit have ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb values of 18.41–18.44 (mean 18.43), 15.70 to 15.71 (mean 15.70), and 38.70 to 38.76 (mean 38.74), respectively.

The distribution of trace element concentrations in sulfides is controlled by their modes of occurrence. Numerous studies have shown that trace elements are incorporated into sulfides via various mechanisms, including homovalent substitution and as micro-inclusions of discrete minerals (Benedetto et al., 2005; Ciobanu et al., 2012; Reich et al., 2013; George et al., 2015; Yang et al., 2022). The investigation into the occurrence modes and substitution mechanisms of trace elements in sulfides is a prerequisite for using these elements to reconstruct the physicochemical conditions of deposit formation.

In sphalerite, the substitution of Zn²⁺ by minor/trace elements mainly occurs via simple or coupled substitution. Cations like Fe²⁺, Cu²⁺, Mn²⁺, Cd²⁺, and Co²⁺, which have similar ionic radius and charge to Zn²⁺, can directly substitute (M²⁺ ↔ Zn²⁺) into the crystal lattice (Cook et al., 2009; Lockington et al., 2014; Belissont et al., 2016; Babedi et al., 2019). In contrast, monovalent (Cu⁺, Ag⁺), trivalent (As³⁺, Ga³⁺, Sb³⁺, and In³⁺) and tetravalent (Sn⁴⁺, Ge⁴⁺) cations with larger ionic radius are primarily incorporated via coupled substitution mechanisms (Cook et al., 2009; 2012; Ye et al., 2011; Bonnet et al., 2016; Li et al., 2020; Luo et al., 2022; Yang et al., 2022).

Binary plots show a generally negative correlation between Fe and Zn contents in sphalerite (Figure 7A), indicating a possible substitution of Zn²⁺ by Fe²⁺. A weak positive correlation between Co and Fe (Figure 7B) implies that Fe²⁺ and Co²⁺ may jointly replace Zn²⁺. In many Pb-Zn deposits, Ge and Ga in sphalerite show a correlation with Cu (Ye et al., 2016; Li et al., 2020; Zhang J. K. et al., 2022). Here, the strong positive correlation between Ga and Cu (Figure 7C) and the weak positive correlation between Ge and Cu (Figure 7D) indicate the following coupled substitutions: Ga³⁺ + Cu⁺ ↔ 2Zn²⁺, Ge⁴⁺ + 2Cu⁺ ↔ 3Zn²⁺.

Trivalent cations Sb³⁺, Sn³⁺, and In³⁺ are more likely to substitute Zn²⁺ via coupled substitution with monovalent cations Cu⁺ and Ag⁺ (Makovicky and Topa, 2015; Torró et al., 2022; Xiao et al., 2023). In the present study, the strong positive correlations observed between Sb and Cu (Figure 7E) and between Sb and Ag (Figure 7F) support the mechanisms: 2Zn²⁺ ↔ Cu⁺ + Sb³⁺ and 2Zn²⁺ ↔ Ag⁺ + Sb³⁺. Similarly, the correlation between In and (Cu, Ag) (Figure 7H) can be explained by the substitution: In³⁺ + (Cu⁺, Ag⁺) ↔ 2Zn²⁺.

Arsenic in sphalerite is typically present as As³⁺ (Yang et al., 2022; Li et al., 2024). The weak positive correlation between As and Cu (Figure 7G) suggests a possible coupled substitution of 2Zn²⁺ ↔ As³⁺ + Cu⁺. The strong positive correlation between Cu + Ag (monovalent cations) and Ga + As + Sb + Ge (tri- and tetravalent cations) (Figure 7I) implies that another substitution mechanism may exist: (Cu, Ag)++ (Ga, As, Sb) ³⁺ + Ge⁴⁺ ↔ 4Zn²⁺.

The concentrations of key trace elements in sphalerite (e.g., Fe, Mn, Co, Cd, In, Ga, Ge) are widely recognized as sensitive indicators of mineralization temperature (Möller, 1985; Frenzel et al., 2016; Frenzel et al., 2022; Bauer et al., 2019b; Xing et al., 2021). Sphalerite associated with high-temperature or magmatic-hydrothermal fluids generally has high Fe, In, Mn, and Se concentrations but low Ga, Ge, and Ti concentrations, exhibiting low Ga/In and Ge/In ratios (Frenzel et al., 2016; Bauer et al., 2019a; Li et al., 2022; Liu et al., 2025). In contrast, medium-temperature sphalerite commonly shows high Cd and In concentrations, with Ga/In ratios of 0.01–5 and Cd/Fe ratios of 0.02–1 (Li et al., 2022; Sun et al., 2023; Liu et al., 2025). Low-temperature sphalerite is typically lighter in colour and has relatively high Ga, Ge and Cd concentrations, with a Ga/In ratio ranging from 1 to 100 and elevated Ge/In values (Ye et al., 2011; Cook et al., 2012; Zhuang et al., 2019; Hu et al., 2021).

Sphalerite from Luobugaizi is typically light in colour and contains 1.71–4.25 wt% Fe, which is below the typical threshold for high-temperature sphalerite (Fe > 10%). The Ge content in sphalerite systematically increases with declining mineralization temperature. High-temperature sphalerite generally contains <5 ppm Ge, medium-temperature sphalerite contains 5–50 ppm Ge, and low-temperature sphalerite usually contains >50 ppm Ge (Cugerone et al., 2018; Wei et al., 2021). In this study, Ge contents in sphalerite range from 0.30 to 50.2 ppm, predominantly falling within the medium-temperature range. The In/Ge ratios vary from 0.002 to 545 (average 67.4), which are elevated relative to low-temperature deposits but notably lower than those reported for high-temperature deposits (e.g., Goutouling mine in Furong tin ore field, In/Ge = 2091–16923; Ye et al., 2012). Ga/In ratios primarily range from 0.26 to 60, with occasional values exceeding 100, consistent with low-to medium-temperature sphalerite.

Trace-element thermometers for sphalerite provide a quantitative tool for estimating its mineralization temperature. Keith et al. (2014) proposed a geothermometer based on the Fe and Zn contents of sphalerite (Equation 1), which yields crystallization temperatures of 233 °C–276 °C (mean = 257 °C) for the Luobugaizi deposit. Temperature calculation using the GGIMF thermometer (Frenzel et al., 2016) yields a range from 140 °C to 283 °C, with a mean of 195 °C. Fe / Zn   s p h a l e r i t e = 0.0013 T − 0.2953 (1)

Recently, machine learning approaches have been applied to trace element data for sphalerite geothermometry. Meng et al. (2024), Zhao et al. (2024) developed a Sphalerite Random Forest Thermometer (SPRFT) software. Applying this software to Luobugaizi trace element data yields temperatures of 135 °C–279 °C, with a mean of 220 °C ± 26 °C. All the calculated temperatures are listed in the Supplementary Material.

The average values calculated by three different methods for the formation temperature of sphalerite range from 195 °C to 257 °C, indicating a low-to medium-temperature mineralization condition for Luobugaizi.

The mineralization of Luobugaizi Pb-Zn deposit is characterized by an assemblage of sphalerite, galena, pyrite, and minor chalcopyrite, accompanied by hydrothermal alteration such as silicification, sericitization, and chloritization. Together, this mineral assemblage and alteration features indicate a low-to medium-temperature hydrothermal origin for the deposit.

Trace element contents and ratios in sulfides of Pb-Zn deposits can indirectly indicate the deposit’s genetic type (Cook et al., 2009; Belissont et al., 2014; Yuan B. et al., 2018). For instance, in magmatic-hydrothermal deposits, the Cd/Fe ratio is generally stable and below 0.1, In/Ge ratio typically exceeds 0.1, commonly approaching 50. MVT deposits show considerable variation of Cd/Fe ratios ranging from 0.08 to 10, with In/Ge typically less than 0.1, while sandstone-hosted deposits exhibit the widest Cd/Fe ratio range of 0.15–100, In/Ge less than 0.03 (Cook et al., 2009; Ye et al., 2011; Gao et al., 2024). At Luobugaizi, sphalerite yields Cd/Fe ratios of 0.02–0.08 (mean 0.03) (Figure 10A) and In/Ge predominantly exceeds 0.1 (mean 50.1) (Figure 10B), indicating a clear genetic link to magmatic activity.

Isotope geochemistry further supports this interpretation. Both the δ³⁴S values (showing a narrow, near-zero, and–tower-shaped distribution) and the Pb isotopic composition (similar to Cretaceous granites) suggest a common origin from a magmatic-related hydrothermal fluid.

The Luobugaizi deposit exhibits a close spatial and temporal association with Late Yanshanian magmatism. In the southern part of the deposit, there are outcrops of Late Yanshanian biotite granite (Figure 1B) and to the east, Late Yanshanian monzonitic granite aligns along the Qunsaragiriya Fault (F5). The granite exposed to the east of the deposit has yielded a weighted mean age of 107–102 Ma (Liu et al., 2020a). Wang et al. (2021) obtained a monazite U-Pb age of ca. 99 Ma from light-colored veins closely related to Pb-Zn mineralization, which they interpreted as the mineralization age of the Luobugaizi deposit. This mineralization age overlaps, within analytical uncertainty, with the emplacement age of the granite, underscoring their temporal connection.

Previous studies on regional plate tectonic evolution indicate that during the Early Cretaceous, the Neo-Tethyan oceanic lithosphere likely underwent low-angle to flat subduction beneath South Pamir-Karakoram (Liu et al., 2020a; Zhang C. L. et al., 2022; Yang F. et al., 2024). Consequently, a series of granitoids were interpreted in previous studies as products of partial melting of the Precambrian lower crust with mantle-derived inputs during ca. 107–102 Ma (Liu et al., 2020a; Yang F. et al., 2024). Subsequent differentiation and fluid exsolution from this crust-mantle interaction derived magmas provided a potential source for the ore-forming materials (Figure 11).

The host rock of Luobugaizi deposit is dominated by relatively soft and deformable slate. Regional tectonic deformation induced tensile fracturing within these rocks, creating pathways for hydrothermal Pb-Zn-bearing fluids to migrate along faults and fractures and ultimately precipitate ore minerals within the host sequence.

In conclusion, the mineralogical, geochemical, and geological characteristics of Luobugaizi Pb-Zn deposit are consistent with a magmatic-hydrothermal origin.