Extraction and application of quantitative indexes for mineralization-alteration in non-magmatic epigenetic hydrothermal deposits--a case of the super-large germanium-rich lead-zinc deposit in Huize, Southwest China

The Northeast Yunnan, which is an integral part of the Sichuan-Yunnan-Guizhou, contains approximately 20 million tons of metal reserves. This area provides an ideal location to evaluate relationship between alteration of the surrounding rocks and the ore body and most wall-rock are carbonates. Wall-rock alteration is a distinct feature of this type of deposit. It is the most direct and accurate indicator for prospecting blind ore bodies; however, it is limited by quantification and therefore cannot be applied. In this study, we examined the Huize super-large lead-zinc deposit in the Northeast Yunnan as an example and proposed quantitative indicators for alteration in the mining area. Based on quantitative alteration methods, including the alteration index (AI), carbonate-pyrite index (CPI), composite index (CI), and alteration box plots, five distinct alteration zones have been delineated within the Huize lead-zinc deposit. The above indices allowed us to determine the patterns of change in elements within the alteration zones and extract the characteristic indexes of these zones. The comprehensive analysis demonstrated that 1) the AI consists of AI_MgO and AI_CaO, which are effective indicators for distinguishing between dolomitization and calcitization; 2) pyrite mineralization is closely related to various types of mineralization elements, and the CPI can be used to assess the strength of pyrite mineralization in different zones; and 3) the CI, comprising the AI and CPI, can effectively reflect the characteristics of alteration zones. The quantitative indexes of each alteration zone are as follows: distal end zone V (CI < 50), zone Ⅳ (51 < CI < 80), intermediate zone Ⅲ (81 < CI < 95), proximal zone Ⅱ (96 < CI < 98), and ore body Ⅰ (99 < CI) zones. In addition, this study utilized the CI to predict the hidden ore body in the deep middle section, which provides a theoretical basis for enriching the quantitative indexes of lead-zinc deposits in the Northeast Yunnan Ore Concentration and realizes the rapid delineation of the target area of similar deposits.

1 Introduction

A hydrothermal deposit is an ore deposit formed by mineralized elements bearing hot water solutions under specific physical and chemical conditions in favorable ore-forming sections through processes such as filling, leaching, and deposition among other ore-forming methods (Zhai et al., 2011). Most metals and nonmetals can form hydrothermal deposits, except for Cr and several elements associated with the platinum group element (Li et al., 2007). Wall-rock alteration is a common feature of hydrothermal deposits that records extensive information about the formation process of deposits and serves as an important indicator in the search for minerals.

Northeast Yunnan, China, is a typical non-magmatic epigenetic hydrothermal deposit mining concentration area, with a predicted Pb-Zn polymetallic reserve of >20 million tons, containing several super-large and large-scale Pb-Zn deposits (Huize, Maoping, MaoZu), as well as numerous small- and medium-sized Pb-Zn deposits (Han et al., 2012). Wall-rock alteration is a typical characteristic of these deposits (Han et al., 2007; Runsheng et al., 2023), however, previous research has mostly been based on qualitative analysis (Ai-Ying et al., 2012; Wen et al., 2014; Wu and Han, 2017; Zhang, 2017) and only semi-quantitative analyses have been conducted for individual deposits (Chen et al., 2016; Zhang et al., 2022). A large number of previous studies has been conducted on wall-rock alteration of different hydrothermal mineral deposits, and recent research has focused on semi-quantitative and quantitative analyses of element migration in alteration zones. For example, Hokke et al. (2024) utilized the mineralization alteration index (AI) for geochemical exploration, defined the scope and shape of alteration zones, determined the alteration types, and established new exploration indicators for the Metsamonttu mining area (Hokka et al., 2024). Similarly, Zeng et al. (2023) developed a vertical zonation model for the Wengkongba copper-polymetallic deposit in southwestern Yunnan Province, combining mineralization and alteration minerals with quantitative analysis using a hydrothermal AI (Zeng et al., 2023); Li et al. (2023) studied the amphibolite porphyry body of the porphyry copper-molybdenum polymetallic deposit, analyzed the interrelationships between the elemental migration of the deposit and alteration of the enclosing rocks, and proposed a new search for mineralization based on this model (Shoukui et al., 2011). In addition, the use of short-wave infrared spectroscopy to rapidly extract alteration mineral features, construct alteration mineral zoning models (Zhai et al., 2022; Tang et al., 2021; SHAO et al., 2021), and construct regional mineralization alteration zoning models based on the interpretation of remote sensing information constitutes key research directions (Ngerezi et al., 2024; Zhang et al., 2025; Xin et al., 2025). Mineralized alteration zoning is a direct and accurate indicator for locating ores in blind ore bodies; however, its promotion and application are hindered by the challenge of establishing quantitative indexes. Failure to establish quantitative indexes for alteration zones restricts the exploration of deposits and is not conducive to the development of deep mineral prospecting.

To address these issues, this study considers the Huize super-large Pb-Zn deposit as the research object. The formation of this deposit occurred during the late Indo-Chinese epoch, spanning 200–230 Ma, and was predominantly influenced by tectonics (Feng hao et al., 2024). Carbonate rocks constitute the primary composition of the ore-bearing host rocks, demonstrating a notable alteration zoning phenomenon. Based on the field mapping of the alteration zones and the AI CPI CI and alteration box plots to quantitatively analyze the different types of alterations in the mining area and their corresponding data indexes. We then extracted quantitative evaluation indexes for various alteration zones of the mining area to provide a basis for deepening the deposit model and predicting the search for minerals.

2 Geological background

2.1 Geological characteristics of the deposit

The northeastern Yunnan polymetallic mineralization is an important part of the Sichuan-Yunnan-Guizhou polymetallic mineralization domain (Han et al., 2012), spreading within the “triangle” enclosed by the SN-oriented Xiaojiang, NE-oriented Mile-Shizong, and NW-oriented Ziyun-Yadu deep Fractures (Zhang, 2008; Zhang et al., 2017; Yan et al., 2023) (Figure 1).

Figure 1

Figure 1. Geological sketch map of the Huize lead-zinc mining area (Han et al., 2006). 1- Upper Permian Emeishan Basalt Formation; 2- Permian Qixia-Maokou Formation tuffs, dolomitic tuffs interbedded with dolomite, and carbonaceous shales and quartz sandstones of the Liangshan Formation; 3- Carboniferous Maping Formation brecciated tuffs, Weining Formation oolitic tuffs, Dengying Formation coarse-crystalline dolomite and dolomitic tuff, and cryptocrystalline tuffs and oolitic tuffs of the Datang Formation; 4- Devonian Zaige Formation tuffs, silicic dolomite and dolomite, and Haikou Formation siltstones and mudstone shale; 5 - Cambrian crippled shale and sandy mudstone of the Qiongzhusi Formation. 6 - Siliceous dolomite of the Dengying Formation; 7 - Fractures; 8 - Stratigraphic boundaries; 9 - Germanium-rich lead-zinc deposits.

The stratigraphy of the Huize Pb-Zn mine area comprises a Precambrian basement (The basement rock is dominantly carbonate rock, chiefly limestone.) and sedimentary cover dating from the Late Ordovician, with an angular unconformity contact between the two. The Upper Paleozoic was fully exposed in the mining area, whereas the Lower Paleozoic Cambrian was only exposed Qiongzhusi Formation. To date, Pb and Zn ore bodies have primarily been deposited in the coarse crystalline dolomite of the Lower Carboniferous Baizuo Formation and the siliceous dolomite of the Upper Sinian Dengying Formation. The upper part of the Baizuo Formation developed shales from the Middle Permian Liangshan Formation, and the upper part of the Dengying Formation developed muddy shales from the Lower Cambrian Qiongzhusi Formation. These rocks are dense and homogeneous, with low porosity and permeability, and thus form an impermeable layer with strong plasticity; This impedes the penetration and escape of mineralizing fluids, thereby creating favorable conditions for these fluids to fill, accumulate, and enrich mineralization in the desirable parts of the ore-bearing layer (Wang et al., 2022; Wang et al., 2023).

The deposit is tectonically controlled and the NE tectonic band constitutes a metallogenic tectonic system. Three NE-directed Transpressional fractures, the Qilingchang, Kuangshanchang, and Yinchangpo Fractures, form imbricated tectonics that control the deposits of Qilingchang, Kuangshanchang, and Yinchangpo, respectively (Han et al., 2007). The ore bodies occur in stratiform-like, pocket-shaped, and lenticular forms, exhibiting sharp contacts with the country rocks. Their emplacement is prominently controlled by interlayer slip zones. The ore bodies predominantly strike NE and dip steeply toward SE, with dip angles ranging from approximately 50°–76. Systematic cross-section interpretations reveal that the lenticular mineralized bodies tend to pinch out toward the top and display a plunging extension along the SW direction, reflecting significant structural controls on ore localization. The orebody extends for approximately 400 m along strike and exceeds 1,500 m down-dip.

Ore bodies occur in the form of layers, capsules, and lenses. The primary ore minerals include sphalerite, galena, and pyrite, with occasional chalcopyrite, bismuth-silver sulfide, and native antimony. The main gangue minerals are dolomite and calcite, with quartz, barite, and chlorite. The ore structures are predominantly dense and massive, featuring dips, bands, and brecciations. These characteristics may be caused by the following patterns: the epigenetic hydrothermal mineralization system utilized the widely distributed primary bedding for initial pervasive infiltration and weak alteration, and was ultimately controlled by higher-order secondary fracture systems, within which it filled and replaced to form economically valuable vein-type ore bodies. Therefore, the emplacement of the ore bodies is not random but is strictly controlled by the pre-existing structural framework of the host rock. Additionally, ore textures include authigenic and heterogenic granular forms, with the occurrence of co-rimming, gap-filling, and other textures.

2.2 Wall-rock alteration type

Under certain physicochemical conditions during the metallogenic period, the following alteration rock types were formed in the Huize Pb-Zn deposit: (1) dolomitization; (2) calcitization; (3) pyrite mineralization; (4) silicification; and (5) chloritization (Figure 2).

Figure 2

Figure 2. Photographs of typical mineralized alteration features within the mine area (a,b) agglomerated coarse-crystalline dolomite; (c) banded dolomite is distributed around the ore body; (d) the massive dolomite shows a sharp contact with the ore body; (e) coarse vein-like calcite cemented to periclinal breccia in the fracture zones; (f) pyrite is encapsulated in reticulated calcite; (g) massive calcite in the ore body; (h) sphalerite overgrows the calcite vein.; (i) sedimentary-age strawberry-like pyrite; (j) pentagonal dodecahedral pyrite; (k) finely veined dipping pyrite produced along fissures; (l) dipping pyrite produced with lead-zinc ore bodies (m–o) quartz veinlets developed next to calcite veinlets; (p) limonitized occurrences of limonite seen next to quartz veinlets; (q) clumped chlorite (r) chlorite cuts through early calcite (s) pyrite seen next to clumped chlorite (t) finely veined xanthite is encapsulated by chlorite; Cal, calcite; Dom, dolomite; Lim, limonite; Py, pyrite; Gn, galena; Sp, sphalerite; Qtz, quartz; Chl, chlorite.

Dolomitization: Dolomitization was predominantly observed in the main ore-endowed layers of the Baizuo and Dengying Formations. The dolomite appears irregular (Figure 2a) and clumped (Figure 2b). Most hydrothermal dolomites developed dissolution holes that tended to be filled with post-mineralization calcite clusters (Figure 2c). Remnants of limestone alteration were observed in some dolomite rocks (limestone breccias wrapped in altered dolomite) (Figure 2d). Dolomitization became more intense with an increase in the ore body depth, thickness of the ore body, size of the ore body, and grade, suggesting a close genetic relationship between dolomitization and mineralization (Han et al., 2006).

Calcitization: Calcite is widely developed within the Baizuo, Weining, and Zaige Formations, predominantly appearing in veins, vein networks, and mass distributions within the ore body and surrounding rocks. Based on its formation, it can be divided into two categories: (1) filling fissures or fractures, often found in vein networks, masses, veins, and of hydrothermal origin, and its distribution is mainly controlled by the fracture. Calcitization was typically stronger near fracture zones and gradually weakened away from the fracture zones (Figures 2e–g); (2) fillings in the surrounding rocks, mostly produced in crystal clusters and distributed in the medium-coarse crystal porous dolomite (Figure 2h). Vein calcite was formed in the main and late stages of mineralization, and its formation mechanism was mainly due to the precipitation of sulfides, leading to an increase in the pH of the hydrothermal fluids, which in turn led to the precipitation of vein calcite (Zhang et al., 2016; Zhang, 2016; Cui et al., 2023).

Pyrite mineralization: Pyrite mineralization, which commonly develops in mining areas, predominantly takes the form of veins and dipping structures. Frequently observed in the Baizuo and Weining Formations, pyrite mineralization, characterized by its multiphase nature, can be divided into two phases: (1) sedimentary pyrite of cubic authomorphic form, which can be locally observed in the strawberry structure (Figure 3a); sedimentary formation of pyrite by the hydrothermal mineralization period of recrystallization. The formation of pentagonal dodecahedral authomorphic granular pyrite is distributed in the ore body or leaching produced in the top plate of the ore body of the dolomite (Figures 2.i,j); and (2) metallogenic period pyrite, which can be divided into two stages (Figures 3b,c). The early stage of pyrite is authomorphic-semi-authomorphic grains (maximum 2–5 mm), and the crystal type is pentagonal dodecahedral and cubic polygonal, dominated by massive, dipping, and banded assemblage output (Figure 2l). The late stage often occurs along the galena and sphalerite cleavage in the form of veins, vein-like outputs, or fine-grained assemblages distributed in crushed zones near the ore body (Figure 2k).

Figure 3

Figure 3. Photographs characterizing the morphological changes of pyrite. (a) Strawberry pyrite; (b) Cubic pyrite; (c) Pentagonal dodecahedral pyrite; Py, pyrite; Cal, calcite.

Silicification: Silicification is relatively pronounced in the Dengying Formation and can be categorized into two types based on its formation: (1) It is produced in calcite vein bodies and calcite clusters in the form of fine veins, and the width of the veins is generally smaller than that of calcite veins, which is only 1–2 mm; most of them are oriented and distributed in the same direction as the extension direction of the calcite vein bodies (Figures 2m–o). (2) The veins are produced around limonite (Figure 2p).

Chloritization: Mostly found in fracture zones as a product of tectonic fluid action, often with a seemingly stratified output (Figures 2q,r), the distribution of fine-grained pyrite can be observed in the schistose chloritization near the fracture surface (Figures 2s,t).

2.3 Characterization of alteration zones

Based on the above wall-rock alteration types, we conduct field mapping of typical sections of the mine and choose a typical middle section of 924 in the area was selected to characterize the alteration zoning.

The altered petrographic variations are characterized as follows: the section mainly exposes gray-black fine-microcrystalline limestone of the Datang Formation and gray-white-carnal red mottled coarse-crystalline dolomite of the Baizuo Formation, from SW to NE, showing gray-black fine-crystalline limestone (Zone Ⅴ, indicating weak calcite mineralization), gray-white fine-medium-crystalline dolomite (Zone Ⅳ, indicating the occurrence of dolomitization, calcite mineralization, and remnants of uneroded dolomitic limestone), reddish-pink medium-coarse crystalline dolomite (Zone Ⅲ, strong dolomitization, calcite, and weak pyrite mineralization), beige-colored porous coarse-crystalline dolomite (Zone Ⅱ, indicating weak Pb-Zn mineralization), and Pb-Zn mineralization zones (Zone Ⅰ, Gn-Sp-Py ore bodies, with Gn as the main ore body, Sp as secondary, and Py as the least, with mineralization roughly following the layer’s output; the mineralization was vein-like, mottled, and abundant). Overall, the central part of the alteration in the 924 zones exhibited a symmetrical distribution pattern from the center of the ore body to the sides (Figure 4).

Figure 4

Figure 4. Measured profile of the alteration zones in the section of 924.

Macroscopically, dolomitization was most strongly altered in Zone Ⅱ, calcitization was concentrated in Zone Ⅲ, and pyritization was progressively enhanced from Zones Ⅳ to Ⅰ. The characteristic mineral assemblages in each alteration zone are listed in Table 1.

Table 1

Table 1. Characterized mineral assemblages within each alteration zone.

Figure 5

Figure 5. Photographs of typical characteristics of alteration minerals in different alteration zones (a) calcitization, calcite is veinlike; (b) coarse crystalline granular dolomite with a haze-centred bright-edged structure; (c) calcitised coarse crystalline dolomite, see stellate pyrite; (d,e) pyrite in the form of fine to medium-crystalline semiautomorphic grains that fill along the cleavages; (f) dolomitic galena is in the form of a harbor-like structure; (g) sedimentary pyrite, euhedral texture; (h) pyrite precipitate along grain boundaries; (i) Pyrite-accounted pyrite in galena, see black triangular hole formed by cleavage within galena; Py, pyrite; Gn, galena; Sp, sphalerite; Dol, dolomite; Cal, calcite.

3 Sample test methods and results

3.1 Sample collection and test methods

Regarding the elemental migration pattern of the Huize Pb-Zn deposit during hydrothermal alteration, a typical middle alteration section was selected and representative samples from each alteration zone were collected for analysis (36 samples). The samples were tested at the Northwest Nonferrous Geological Research Institute Testing Center. The samples were decomposed using nitric, hydrochloric, hydrofluoric, and perchloric acids. Subsequently, the perchloric acid was evaporated, and the residue was dissolved in hydrochloric acid (1 + 1). The resulting solution was transferred to a 10 mL plastic cuvette and allowed to stabilize. The characteristic spectral intensities of the investigated components were measured at specific wavelengths using an inductively coupled plasma atomic emission spectrometer. Matrix effects were corrected to calculate the quantity of the components of interest in the specimen. To assess horizontal sample precision, four to five standard substances with varying mass fractions were selected. Each horizontal sample was measured in triplicate and the raw data were subjected to statistical analysis to ensure compliance with the precision requirements (precision standard reference DZ/T0279.2–2016).

For rare earth elements such as La, the test material was dissolved with hydrofluoric and nitric acids in a closed dissolver under high temperature and pressure, evaporated, re-dissolved with nitric acid, and shaken well to form a solution to be measured, which was introduced into the RF plasma by pneumatic atomization, evaporated, atomized, ionized, and separated according to the ion-mass ratio with a quadruple rod mass spectrometer and detected with a detector. The amount of elements to be measured was analyzed by quantitative analysis of the calibration curve method. The elemental amount, sample substrate caused by the instrument response inhibition or enhancement effect, and instrument drift can be compensated using the internal standard, in accordance with GB/T6379.2-2004. A different mass fraction range of five standard substances was selected, each level of the sample was measured three times, the raw data were statistically analyzed, and the analytical method of precision refers to the document DZ/T 0279.32-2016.

3.2 Analysis of results

The variations in the whole-rock major trace elements across each alteration zone for the middle sections 924 and 1,404 are detailed in Schedules 1, 2. Within each alteration zone, CaO and MgO concentrations were generally >45%. The SiO₂ concentration was higher in Zone V, whereas the Na₂O, K₂O, MnO, and P₂O₅ contents were lower in all the zones. The CaO/MgO ratios in all zones, except Zone V, ranged from 1.55 to 2.20, with a mean value of 1.73. These ratios were slightly higher than the theoretical values for pure dolomite (CaO/MgO = 1.39).

Schedule 1

Schedule 1. Major element data for different alteration zones.

Schedule 2

Schedule 2. Trace element data for different alteration zones.

4 Results

4.1 Mineralization-alteration index (AI)

The Alteration Index (AI) was used to reflect the strength of alteration development within a mining area (Ishikawa et al., 1976); The patterns of element migration during the alteration process are the key factors determining this index. The key reactions involved in calcite and dolomite alteration of the Huize mine include:

1. ${\text{Ca}}^{2+}+{\text{Mg}}^{2+}+2{\text{OH}}^{-}+2{\text{CO}}_2=\text{CaMg}{\left({CO}_3\right)}_2\downarrow +{\mathrm{H}}_2\mathrm{O}$

2. ${\text{Ca}}^{2+}+2{\text{OH}}^{-}+{\text{CO}}_2={\text{CaCO}}_3\downarrow +{\mathrm{H}}_2\mathrm{O}$

Reaction (1) shows the early stage of mineralization when carbonate rocks react with acidic fluids to form dolomitization (Yan et al., 2023), in which a large amount of Mg²⁺ migration occurs. Reaction (2) indicates the precipitation process of calcite in a Mg²⁺-deficient environment, which embodies a large amount of Ca²⁺ migration. In order to determine the migration pattern of elements during the alteration process, this is based on the Akella and Gresens hypothesis that one or more components are inactive during the opening of a geological system (Akella and Winkler, 1966; Gresens, 1967). Based on previous studies, we used TiO₂ as the inactive component (Guo et al., 2009), performed mass-balance calculations as proposed by Zhang Kecheng (Equation 1), and the migration of major elements into and out of the alteration zones was calculated (Zhang and Yang, 2002). Figure 6 shows the elemental changes in each zone.

${T_i=C}_i^A·\left(\frac{M_i^O}{M_i^A}\right){-C}_i^O(1)$

where $T_i$ is the relative value of migration in and out of each element, $C_i^A$ is the mass fraction of component i in the altered rock, $C_i^O$ is the mass fraction of component i in the unaltered rock, $M_i^A$ is the mass fraction of inactive component i in the altered rock, and $M_i^O$ is the mass fraction of inactive component i in the unaltered rock.

Figure 6

Figure 6. The migration pattern of elements in and out of each alteration zone. 1-SiO₂; 2-Al₂O₃; 3-TFe₂O₃; 4-FeO; 5-MgO; 6-CaO; 7-Na₂O; 8-K₂O; ; 9-MnO; 10-P₂O₅; ①-Ⅰ→Ⅱ; ②-Ⅱ→Ⅲ; ③-Ⅲ→Ⅳ; ④-Ⅳ→Ⅴ.

Based on the above indices and the internal alteration development of the Huize lead-zinc mining area, we proposed new indices to assess the strength of dolomitization and calcite alteration in the mining area. the AI was defined based on the main migration patterns of elements in and out during the reaction process, following (Ishikawa et al., 1976).

$AI=\frac{100*{\displaystyle {\sum }_{i=CaO, MgO}}\omega \left(i\right)}{{\displaystyle {\sum }_{i=CaO, MgO, {Na}_{2}O, K_{2}O}}\omega \left(i\right)}(2)$

$AI\left(CaO\right)=\frac{100*CaO}{{\displaystyle {\sum }_{i=CaO, MgO, {Na}_{2}O, K_{2}O}}\omega \left(i\right)}(3)$

$AI\left(MgO\right)=\frac{100*MgO}{{\displaystyle {\sum }_{i=CaO, MgO, {Na}_{2}O, K_{2}O}}\omega \left(i\right)}(4)$

where the unit of the oxides in the calculation of AI is mass percent (wt.%) (Equation 2).

The results of the AI calculations for each alteration zone are presented in Table 2, indicating the alteration intensities of calcite (Equation 3) and dolomite (Equation 4).

Table 2

Table 2. Changes in AI and CPI indices of each alteration zone.

The alteration rules for the AI in each zone are shown in Figure 7. In general, from Zone V to I, AI_MgO first increased and then decreased, whereas AI_CaO exhibited the opposite trend.

Figure 7

Figure 7. Change pattern of alteration index and carbonate-pyrite mineralization index in the middle section of 924.

4.2 Carbonate-pyrite index (CPI)

A single AI is insufficient to address the overall development of alterations in a mine area (Chmielowski et al., 2016; Zheng et al., 2015). CPI can effectively address the alteration intensity of petrified carbonate and pyritization in the mine area; therefore, the introduction of the CPI complements the AI (Equation 5):

$CPI=\frac{100*{\displaystyle {\sum }_{i=MgO, {TFe}^2{O}^3}}\omega \left(i\right)}{{\displaystyle {\sum }_{i={TFe}^2{O}^3, MgO, {Na}_{2}O, K_{2}O}}\omega \left(i\right)}(5)$

${CPI}_{MgO}=\frac{100*MgO}{{\displaystyle {\sum }_{i={TFe}^2{O}^3, MgO, {Na}_{2}O, K_{2}O}}\omega \left(i\right)}(6)$

$CPI{TFe}_2{O}_3=\frac{100*{TFe}_2{O}_3}{{\displaystyle {\sum }_{i={TFe}_2{O}_3, MgO, {Na}_{2}O, K_{2}O}}\omega \left(i\right)}(7)$

where the unit of the oxides in the calculation of the CPI is mass percent (wt.%).

To visualize the changes in carbonatization and pyrite mineralization across each alteration zone (Equations 6, 7), CPI_TFe2O3 and CPI_MgO were calculated separately, and the results are presented in Table 2.

The alteration patterns of the CPI for each zone are shown in Figure 7. Overall, CPI_TFe2O3 exhibits an increasing trend across the alteration zones, whereas CPI_MgO exhibits an initial increase followed by a decrease.

5 Discussion

5.1 Indications of alteration index

The AI can be used to gauge the intensity of alteration development, as previously verified (Zhang et al., 2022; Zeng et al., 2023). From Zone V to Ⅰ, AI_MgO showed a trend of increasing and then decreasing, whereas AI_CaO exhibited the opposite trend (Figure 8). AI_MgO and AI_CaO were essentially symmetrically distributed.

Figure 8

Figure 8. Characteristics of the distribution of calcite and dolomitization alteration indices in each zone. (a) AI_MgO, AI_CaO statistical histograms for each zone (b) AI_MgO violin plot for each zone (c) AI_CaO violin plot for each zone.

The greater the extent of dolomitization and calcitization, the higher the corresponding AI values, indicating a positive correlation with alteration intensity. Different alteration indexes were analyzed using histograms (Figure 8a), revealing distinct corresponding values for each alteration type. The variation range of AI_MgO in each zone was concentrated between 30 and 45, whereas the AI_CaO was concentrated between 50 and 70, indicating that the AI is a good indicator for distinguishing between dolomitization and calcitization in Zones Ⅱ and Ⅲ (Figures 8b,c). In comparison, Zone II had low AI_CaO and high AI_MgO, whereas Zone III has the opposite. This indicates that Zone II underwent greater dolomitization, which is consistent with the fluid-rock reaction process between the fluid and surrounding rock. As the fluid migrated into Zone I (the ore-bearing zone), it had sufficient time to react with the surrounding rock. The acidic fluid dissolves dolomitic tectonic rocks in the interlayer fault zone, raises the pH, and mixes with the reducing sulfur fluid to form an orebody with obvious mineral assemblage zoning (Zhang et al., 2016). The residual fluid, having unloaded the mineralized material, diffused outward, centered on Zone I and dissolved the surrounding rocks in each zone (at this time, different alteration zones were formed with a spatially symmetrical distribution). The fluid from Zone I, which dissolved a large number of surrounding rocks in Zone II, was enriched in Ca²⁺ and Mg²⁺, supersaturated, and precipitated iron dolomite (beige) (Zone II has the characteristics of low AI_CaO and high AI_MgO content). In Zone III, Mg²⁺ is more depleted, and iron calcite (flesh red) begins to precipitate, resulting in an alteration phenomenon in which strong dolomitization develops in near-mineral Zone II, whereas strong calcitization dominates in Zone III.

In general, changes in AI values were consistent with the law of macroscopic alteration zoning. In Zone I, the surrounding rock underwent a strong hydrothermal action. The acidic fluid with low pH reacted with the surrounding rock, dissolving large amounts of Ca and Mg, which then migrated out. At this time, the AI value showed the characteristics of low AI_MgO and high AI_CaO, and only a small amount of iron dolomite and part of the dolomite were deposited. The remaining dissolved substances were transported to Zone II in conjunction with the fluid, and substantial quantities of Ca and Mg migrated into Zone II. Under suitable physical and chemical conditions, large amounts of dolomite and a portion of calcite were precipitated. The AI value, characterized by high AI_MgO values, indicated that at the end of dolomite precipitation, Ca migrated to Zone III, where calcite precipitation commenced, resulting in strong calcitization in Zone III with high AI_CaO. As the dissolution decreased, when the ore-forming fluid migrated to Zone IV, the pH of the fluid gradually increased to moderate alkalinity, and the fluid-rock reaction could no longer occur. In summary, the AI can be visualized to reflect the change rule of calcite, dolomitization strength, and weakness experienced in each zone.

5.2 Indications of the carbonate-pyrite index

As a complement to the AI, the CPI focuses on the analysis of pyrite and carbonate petrogenesis occurring during the alteration process (Zheng et al., 2013; Mathieu, 2018). The main ore-bearing host rock of the Huize Pb-Zn deposit is dolomite, and such rocks have undergone strong carbonate petrogenesis; therefore, the CPI_MgO value is larger in each zone except Zone Ⅴ, which is far from the orebody. From Zone I to V, CPI shows a decreasing and then an increasing trend; CPI_TFe2O3 is the largest in Zone I and the smallest in Zone IV, implying that pyrite mineralization is most developed in Zone I.

The CPI _MgO was higher in Zone I and showed a decreasing trend in Zones II and III (Figure 8). This indicates that Zone I underwent significant dolomitization. During dolomitization, the rock porosity increases by approximately 12% owing to the molar substitution between calcite and dolomite (Weyl, 1960), On the one hand, the stronger dolomitization in Zone I provides a channel for mineralizing hydrothermal fluids (producing a large number of dissolution holes); on the other hand, it also increases the contact area for hydrothermal fluids to react with the surrounding rocks, which makes Zone I a favorable area for mineralization. Although the CPI values in Zones II and III are close to those in Zone I, Mg and Ca ions show migratory enrichment in Zones II and III, and oversaturated Ca and Mg fluids continue to be supplied after the dolomitization process. Overdolomitization is likely to occur, which leads to a decrease in porosity and affects the unloading of mineralized materials (Saller and Henderson, 1998; Saller and Henderson, 2001), which is not conducive to the metallogenic process.

5.3 Correlation analysis of two types of indices with Pb and Zn elements

Based on the correlation analysis of 14 samples in the middle section of 924, the correlation data were first processed using the interquartile range method, and the data were fitted by removing an outlier (HZK-12), and the fitting results are shown in Figures 9, 10. There was a significant positive correlation between the two types of indexes (AI and CPI) and Pb and Zn, and the correlation coefficients (R²) were 0.873 and 0.891, respectively; the two are not simple linear correlations, but quadratic exponential correlations. In the correlation diagram of the AI with Pb and Zn (Figure 10), excluding the distal point of X = 95.89, 90% of the points are located inside the confidence interval, and when X > 100, the bandwidth of the confidence interval is rapidly expanded, and the extrapolation risk is significantly increased, which indicates that the correlation is only applicable to the case of AI<100. However, the formula for calculating the corrosion index (Equation 2) shows that the ratio of the numerator to the denominator cannot be > 1, i.e., the value of AI cannot be > 100; therefore, the correlation is reliable.

Figure 9

Figure 9. Correlation of alteration index with Pb + Zn content.

Figure 10

Figure 10. Correlation of alteration carbonate-pyrite index with Pb + Zn content.

For the correlation between the CPI and elements Pb and Zn (Figure 10), the two also show an obvious exponential correlation, and the correlation has no obvious extrapolation boundaries; 80% of the points fall within the confidence interval, indicating that the correlation is also reliable.

As mentioned before, the sizes of AI and CPI react to the intensity of the development of different alteration types, and they show good correlation with Pb and Zn, implying that the area of strong wall-rock alteration is also a mineralization-rich area, which is also consistent with the pattern revealed by the field macro-catalog. In the near-mineral end (alteration zones Ⅱ and Ⅰ), the carbonation is strong, and the calcite and dolomite are mostly produced in the form of agglomerates and bands, with coarse grains and often accompanied by sparse dipping pyrite, indicating that it has been subjected to more intense hydrothermal alteration and the originally dense massive dolomite has become loose and porous due to the hydrothermal action, which provides good physical conditions for mineralization. Therefore, it shows the characteristics of high CPI and AI in alteration zones Ⅰ and Ⅱ. As the distance from the ore body increased, the alteration gradually weakened, the calcite and dolomite production gradually transitioned to fine veins, and the surrounding rocks showed obvious changes in grain size and porosity, corresponding to the reduction of the AI and CPI values and weakening of the mineralization. In summary, the comprehensive use of the AI and CPI to predict mineralization is an effective means of quantification.

Although the correlation within this specific high-grade zone is strong (R² = 0.873 R² = 0.891, n = 14), the limited sample size introduces uncertainty; future sampling within similar zones is encouraged to refine this relationship.

5.4 Significance of alteration box plots for indicating alteration zones

The alteration box plot comprehensively reflects different alteration types and intensities using two indicators: the mineralization AI and the CPI (Zheng et al., 2015; Large et al., 2001; Chen et al., 2013; Sharma, 2014). As illustrated in Figure 11, the alteration system of the Huize Pb-Zn deposit is predominantly hydrothermal in nature, with a relatively limited influence of diagenesis and metamorphism. The alteration system comprises three principal types of alteration: (1) pyritization, which is situated within the Pb-Zn mineralized zone (Zone I), the near-mineralized zone (Zone II), and the transition zone (Zone III) (Region a); (2) dolomitization/iron dolomitization (Region b), located in close proximity to the ore body and within the near-mineral zones (Zones I and II); and (3) calcitization (Region c), situated within the transition zone (Zone III). Most samples in Zone I are located in Region a in Figure 11, and the characteristics of high CPI and high AI in Region a demonstrate that the wall-rock alteration of Zone I is primarily characterized by dolomitization and pyrite alteration. In contrast, samples from Zone II were predominantly distributed across Regions a and b, suggesting that, compared to the other zones, Zone II exhibited stronger dolomitization accompanied by relatively weaker pyritization (as indicated by the lower CPI). The drop points in Zone III were concentrated in Regions b and c, indicating that the dominant alteration process in Zone III was calcitization accompanied by pyritization. The drop point of Zone IV is situated in the region designated as d, indicating that calcitization and dolomitization were relatively weak in Zone IV and that pyritization was pronounced, which, in conjunction with microscopic observations, suggests that pyrite formed during the sedimentary period. Zone V was located in Region e, indicating that it did not undergo significant hydrothermal alteration.

Figure 11

Figure 11. Three-dimensional alteration box plot [base image according to Ross Large (Large et al., 2001)].

Although pyrite mineralization developed in Zones Ⅰ, Ⅱ, and Ⅲ, the CPI values of each zone decreased sequentially from Zone Ⅰ to Zone Ⅲ, implying a gradual weakening of pyrite mineralization. Pyrite, identified as a penetrating mineral in the Pb-Zn mineralization process (Sheng-Ping et al., 2024), exhibits alteration strength that, to a certain extent, corresponds to the intensity of the hydrological reactions between the ore-forming fluids and the surrounding rocks in various alteration zones. In the alteration box plot, the five zones reflect the sequence of alteration between the fluid and the surrounding rock during outward transportation: When the ore-forming fluid rises to a favorable ore-forming section owing to tectonic drive, the magnesium-rich hydrothermal fluid has a high temperature, and the dolomite in the original strata is transformed by the hydrothermal fluid to produce high-Ca²⁺/Mg²⁺ dolomite (Zhao et al., 2025), which is unstable. Some of the high Ca²⁺/Mg²⁺ dolomite undergoes de-dolomitization and generates calcite (Wang et al., 2009), while the lower pH of the deep-source mineralizing fluids promotes the dissolution of the dolomite (Zhang, 2016). Therefore, the characteristic alteration combination type of strong pyrite mineralization + dolomitization + weak calcite mineralization within Zone I was formed, which is reflected in the alteration box plot as 10<AI_MgO<45, 95<CPI<100. When the precipitation of the main metal minerals ended, the fluids continued to be transported outward. During this process, part of the pyrite continued to precipitate because the previous stage of dedolomitization consumed part of the Ca²⁺ and Mg²⁺, the Ca²⁺/Mg²⁺ in the fluid tended to equilibrate, and no further dedolomitization occurred. Dolomite was produced in large quantities in Zone II, forming a combination of strong dolomitization and pyritization in Zone II (10<AI_MgO<70, 80<CPI<100). In the first two stages, owing to the precipitation of dolomite, the Mg²⁺ content in the fluid decreases, the Ca²⁺/Mg²⁺ ratio increases, the precipitation of dolomite becomes less favorable, and calcite begins to precipitate in large quantities. Therefore, there was a combination of strong calcite and weak pyritization alteration in Zone III (70<AI<100, 60<CPI<97). With the end of the large amount of calcite precipitation, the alteration In Zone IV was already weak, with essentially no generation of hydrothermal calcite and dolomite. Therefore, the combination of alteration types included weak calcite and weak dolomitization. In the alteration box plot, the process of I → II → III → IV reflects the strength and sequence of the metallogenic fluids interacting with the wall rock in different stages of the Huize Pb-Zn mining area.

5.5 Extraction of quantitative indicators for each alteration zone

To comprehensively extract the effects of CPI and AI on the entire alteration process, the CPI and AI of each zone were processed using the CI:

$CI=\left[\log \left(AI*CPI\right)-3.9\right]*1000(8)$

After performing CI calculations on 12 original dolomite samples from the mining area, the outliers appeared in the second decimal place. To amplify the outliers, 3.9 was selected as the background value for the following reasons:

The truncation test results showed that when truncated to the first decimal place, all 12 original rock samples yielded 3.9 (100% coverage rate). A single-sample t-test confirmed no deviation from 3.9 (t = 0, p = 1.0). Based on this, a residual analysis was conducted: after subtracting 3.9, the residuals showed extremely small variance (s = 0.000897), confirming that the outliers only appeared in higher decimal places. Additionally, we performed a confidence interval estimation: the 95% confidence interval for the proportion of samples with a background value of 3.9 is [0.59, 1.0].

A contour map of the deep middle part of the mine was plotted (Figure 12), which yielded the quantified indexes of the alteration zones. Zone Ⅴ: CI < 50; Zone Ⅳ: 51 < CI < 80; Zone Ⅲ: 81 < CI < 95; Zone Ⅱ: 96 < CI < 98; Zone Ⅰ: CI > 99.

Figure 12

Figure 12. Kuangshanchang deep-section composite index (CI) contour map.

6 Application examples

6.1 Prospecting model

As a typical representative of germanium-rich Pb-Zn deposits in the Northeast Yunnan Ore Concentration, the formation process of the Huize Pb-Zn deposits can be summarized into three main stages: (1) the formation of the strike-slip fault zone and the large-scale transport of the fluid; (2) the stage of the formation of the fluid-mixing, and ore-rich fluids; and (3) the stage of the mineralization by mineral precipitation and tectonic-fluid coupling (Han et al., 2012; Runsheng et al., 2023). As a product of tectonic-fluid coupling mineralization, the strength of alteration in each alteration zone has an obvious correlation with mineralization; during the evolution process of the diffusion of mineralizing fluids into the surrounding rocks, carbonatization and metal mineralization gradually weakened as they moved farther away from the ore body. The alteration types of the surrounding rocks in this area are mainly dolomitization, calcite, pyrite, and silicification, among which dolomitization and calcite are more widely distributed, and the fading phenomenon is obvious in the mining area; dolomitization and calcitization are gradually enhanced from the far mine to the near mine; thus, the characteristic alteration assemblage of strong dolomitization, calcitization, and strong pyrite (evidenced by flesh-red and beige coarse crystalline dolomite zones) serves as an important predictor for identifying hidden ore bodies and can be used as important searching markers for predicting hidden ore bodies. As mentioned previously, the alteration box plot can indicate the sequence and intensity of water-rock reactions within the ore zone, and the characteristic indexes of each zone extracted from the alteration box plot provide quantitative indexes for the delineation of alteration zones. The AI and CPI characteristics in each zone are listed in Table 3. The results of the macro-alteration zoning and analysis of the AI and CPI of each alteration zone were combined, and comprehensive evaluation indexes were extracted to construct a representative alteration model map of the zone. (Figure 13).

Table 3

Table 3. CPI and AI index division of each alteration zone.

Figure 13

Figure 13. Alteration zoning pattern of Huize lead-zinc deposit.

6.2 Examples of applications

Systematic sampling was conducted in the deep middle section of the Qilingchang Fracture according to the alteration zoning pattern map and quantitative analysis results mentioned above. The sampling interval was set to 10 m. However, in some sections where slurry spraying prevented sampling, the interval was increased to 15 m. Fourteen samples were collected, and their CI values were calculated. Contour maps of the profiles were plotted based on the obtained data (Figure 14).

Figure 14

Figure 14. Validation map of the deep middle target area of the Qilingchang.

As shown in Figure 14, the overall contour zoning phenomenon is evident, showing a progression from the outer zone (70–80) to the near-mineral zone (88–94) in the NE→SW direction. Dark-colored sections (>99.5) were considered favorable for metallogenesis. An actual geological compilation and field study of the alteration zoning confirmed that the results from the contour map delineation and field analysis aligned with the drilling project findings. This verification indicates that the discovered ore body was located in the deep part of HZY-11–HZY-13.

While the initial definition of the AI CPI CI and zones was based on the primary dataset (Sections 5.1–5.3), their successful application to independently drill-tested areas (Section 6.2) demonstrates their robustness and potential for guiding exploration within the Huize deposit.

Future research should rigorously test the applicability of the AI, CPI, and CI indices in diverse carbonate-hosted Pb-Zn systems beyond the Huize deposit to evaluate their general exploration utility.

7 Conclusion

1. The AI is an effective indicator for distinguishing between dolomitization and calcitization. Furthermore, changes in AI values reflect the migration of elements into and out of the surrounding rocks within each zone.

2. The CPI is closely related to the mineralizing elements. A change in its value indicates the strength of pyritization in the alteration zones. The analysis results show that AI and CPI have good correlation with Pb and Zn elements.

3. The alteration box plots reflect the type and intensity of alteration of the surrounding rocks in the alteration zones. This indicates alteration by metallogenic hydrothermal fluids during fluid transport. The quantitative indexes of the alteration zones are as follows: Pb-Zn mineralization zone 61<AI_CaO<64, 95<CPI<100, 99 < CI; beige-colored porous coarse-crystalline dolomite zone 51<AI_CaO<84, 80<CPI<100; 96 < CI < 98; reddish-pink medium-coarse crystalline dolomite zone 50<AI_CaO<82, 60<CPI<97, 81 < CI < 95; gray-white fine-medium-crystalline dolomite zone 58<AI_CaO<80, 99<CPI<100, 51 < CI < 80; gray-black fine-crystalline limestone zone 80<AI_CaO, 90<CPI<100, and CI < 50.

4. The use of the CI to delineate the target area represents an efficient, convenient, and accurate methodology.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

Author contributions

JY: Data curation, Investigation, Methodology, Software, Visualization, Writing – original draft, Writing – review and editing, Conceptualization. YZ: Funding acquisition, Methodology, Supervision, Writing – review and editing. RH: Funding acquisition, Methodology, Supervision, Writing – review and editing. PW: Investigation, Writing – review and editing, Software. FL: Data curation, Visualization, Writing – review and editing. HG: Visualization, Writing – review and editing. JW: Conceptualization, Writing – review and editing.

Funding

The authors declare that financial support was received for the research and/or publication of this article. This work was financed jointly by the National Natural Science Foundation of China (42472127, 42172086) the Yunnan Major Science and Technological Projects (202202AG050014), the Yunnan Major Project of Basic Research (202502AB080007-1), Yunnan Mineral Resources Prediction and Evaluation Engineering Research Center (2011), Innovation Team Program of Kunming University of Science and Technology, Yunnan Province.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The authors declare that no Generative AI was used in the creation of this manuscript.

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