Ore Genesis of the Huanggang Iron-Tin-Polymetallic Deposit, Inner Mongolia: Constraints from Fluid Inclusions, H–O–C Isotopes, and U-Pb Dating of Garnet and Zircon

Abstract

The Huanggang iron-tin deposit, located in the southern Greater Khingan Range, is one of the largest Fe-Sn deposits in Northern China (NE China). Iron-tin mineralization occurs mainly in the contact zone between granitoid intrusions and the marble of the Huanggang and Dashizhai formations. Six mineralization stages are identified: (I) anhydrous skarn, (II) hydrous skarn, (III) cassiterite-quartz-calcite, (IV) pyrite-arsenopyrite-quartz-fluorite, (V) polymetallic sulfides-quartz, and (VI) carbonate ones. Fluid inclusions (FIs) analysis reveals that Stage I garnet and Stage II–III quartz host liquid-rich (VL-type), vapor-rich two-phase (LV-type), and halite-bearing three-phase (SL-type) inclusions. Stage IV quartz and fluorite, along with Stage V quartz, are dominated by VL- and LV-type inclusions, while Stage VI calcite contains exclusively VL-type inclusions. The FIs in Stages I to VI homogenized at 392–513, 317–429, 272–418, 224–347, 201–281, and 163–213 °C, with corresponding salinities of 3.05–56.44, 2.56–47.77, 2.89–45.85, 1.39–12.42, 0.87–10.62, and 4.48–8.54 wt% NaCl equiv., respectively. The H–O–C isotopes data imply that fluids of the anhydrous skarn stage (δD = −101.2 to −91.4‰, δ¹⁸O_H2O = 5.0 to 6.0‰) were of magmatic origin, the fluids of hydrous skarn and oxide stages (δD = −106.3 to −104.7‰, δ¹⁸O_H2O = 4.3 to 4.9‰) were characterized by fluid mixing with minor meteoric water, while the fluids of sulfide stages (δD = −117.4 to −108.6‰, δ¹⁸O_H2O = −3.4 to 0.3‰, δ¹³C_V-PDB= −12.2 to −10.9‰, and δ¹⁸O_V-SMOW = −2.2 to −0.7‰) were characterized by mixing of significant amount of meteoric water. The ore-forming fluids evolved from a high-temperature, high-salinity NaCl−H₂O boiling system to a low-temperature, low-salinity NaCl−H₂O mixing system. The garnet U-Pb dating constrains the formation of skarn to 132.1 ± 4.7 Ma (MSWD = 0.64), which aligns, within analytical uncertainty, with the weighted-mean U−Pb age of zircon grains in ore-related K-feldspar granite (132.6 ± 0.9 Ma; MSWD = 1.5). On the basis of these findings, the Huanggang deposit, formed in the Early Cretaceous, is a typical skarn-type system, in which ore precipitation was principally controlled by fluid boiling and mixing.

1. Introduction

Skarn-type deposits, as products of interactions between postmagmatic hydrothermal fluids and carbonate rocks [1,2], are critical global sources of base metals (tungsten, copper, iron, molybdenum, tin, lead, and zinc) as well as precious metals (gold and silver), which has attracted worldwide attention [2,3,4,5,6]. Fe and Sn skarn systems are widely distributed and economically significant. To date, there are still debates on the sources and migration-enrichment mechanism for Fe and Sn during magmatic-hydrothermal processes [7,8]. There are many factors that affect the precipitation of tin and iron, including temperature, pH value, oxygen fugacity, and volatile matter [9,10,11,12]. A thorough understanding of fluid evolution and metal precipitation mechanisms in skarn-type iron-tin polymetallic systems is fundamental to elucidating the mineralization processes of such deposits.

The southern Greater Khingan Range, situated in the eastern section of the Central Asian Orogenic Belt (CAOB; Figure 1a), has undergone multistage magmatic and tectonic activities [13,14,15,16,17,18], making it a major metallogenic zone for polymetallic deposits in NE China. The Huanggangliang–Ganzhuermiao metallogenic belt is a significant polymetallic metallogenic zone in the CAOB [19,20], in which there are a series of renowned hydrothermal and skarn-type deposits (e.g., Baiyinnuoer, Hongling, Dajing, Shuangjianzishan et al. [21,22,23,24,25,26]). Sn mineralization has been widely confirmed to exist in this metallogenic belt: in addition to the Huanggang deposit, other iron-tin skarn systems such as the Hongling deposit have been discovered in this mineralization zone recently. Research on the Huanggang deposit provides a theoretical foundation for future iron-tin prospecting in this area.

The Huanggang deposit is renowned and has received attention from many scholars for its scale and geological and geochemical characteristics [29]. Current studies on it mainly focus on geochemistry and geochronology; research on fluid origin and evolution remains controversial and lacks detailed insights into fluid characteristics and evolution processes during the stages of skarn → oxide → sulfide. The precipitation of Fe and Sn remains to be further clarified [29,30,31,32,33]. The evidence for the timing of formation of skarn is relatively limited and requires additional garnet geochronological data to better indicate the skarn-type mineralization age, and a comprehensive mineralization model of the Huanggang Fe-Sn deposit is crucial for guiding further exploration and refining the hydrothermal mineralization framework on a regional scale.

In this study, on the basis of detailed field geological investigations combined with new constrains on fluid inclusion (FI) petrological observation, microthermometric data, stable isotopes (H–O, C–O), geochronological data (zircon and garnet U-Pb age determination), we elucidate the sources and evolution of the ore fluids, provide new constrains on the diagenetic and mineralization age values, and establish a mineralization model for the Huanggang deposit.

2. Regional Geological Characteristics

The southern Great Khingan Range is tectonically situated in the eastern segment of the CAOB (Figure 1a), bordered by the Xilamulun, Hegenshan-Heihe, and Nenjiang faults (Figure 1c; [15,16,31,34,35]). The lithostratigraphic units in this region mainly comprise Late Paleozoic and Mesozoic strata, while Proterozoic, Early Paleozoic, and Quaternary units are sparsely distributed. The Proterozoic Baoyintu Group is the oldest crystalline basement in the region. The Paleozoic and Mesozoic strata consist of the Silurian Xingshuwa Formation, the Carboniferous Benbatu Formation, the Permian Dashizhai, Huanggangliang, Zhesi and Linxi Formations, the Jurassic Xinmin, Manitu and Manketouebo Formations, and the Cretaceous Baiyingaolao Formation. Quaternary sediments are widespread in this region [23,24,36,37].

The region has undergone multiple complex tectonic activities, forming structures of varying stages, directions, and scales, which were of great significance to both magmatism and mineralization processes (Figure 1c; [38]). The most representative ones are NE-trending structures, consisting of parallel anticlinoriums, synclinoriums, and faults, such as the Xilinhot-Bayanhua anticlinorium, Hanwula synclinorium, Huanggangliang-Ganzhuermiao anticlinorium, Lindong synclinorium, Tianshan anticlinorium, Hegenshan-Heihe fault, Huanggangliang-Ganzhuermiao fault, and Nenjiang fault (Figure 1c; [23]). Additionally, the EW- and NW-trending faults are mostly products of inherited activities of deep faults, which are more localized and smaller in scale.

Roughly, three phases of igneous rock formation occurred in this region: Hercynian, Indosinian, and Yanshanian. The Hercynian phase is represented by Carboniferous-Permian granitoid and dioritoid, mostly occurring in the form of batholiths. The Indosinian magmatic activity is relatively weak, and the Yanshanian magmatic rocks are multistage and compositionally diverse, predominantly consisting of intermediate-acid granitoids, mostly formed between the Late Jurassic and the Early Cretaceous, and are distributed northeastward across the central-eastern part of the southern Great Khingan Range [23,33,36].

3. Ore Deposit Geological Characteristics

The strata at Huanggang comprise the Lower Permian Qingfengshan, Dashizha, Huanggangliang Formations, Upper Permian Linxi Formation, and Middle to Upper Jurassic units [19,24,39,40]. Consistent with the regional structure, the overall stratigraphic inclination is northwestward, with dip angles of 50° to 82° (Figure 2; [41]). The Dashizhai Formation, mostly distributed in the southeastern part of the Huanggang area, consists mainly of andesite, basalt, spilite, hornblende, acidic tuff, sandstone, and slate interbedded with limestone. The lower Huanggangliang Formation, mostly distributed in the northwestern part of the Huanggang area, contains limestone, sandstone, marble, and intermediate acidic volcanic rocks [29].

Structures at Huanggang include NE-, NW-, and EW-trending faults and the Huanggang anticlinorium. The deposit lies at the northwestern wing of the Huanggang anticlinorium [40]. The NE-trending compressive faults and Huanggang anticlinorium are the dominant structures in this area. In addition, there are relatively small faults such as NW-trending tensile faults and EW-trending faults in the ore district (Figure 2a), which are mainly destructive structures of the post-mineralization period [38].

The Yanshanian magmatism was multi-phase and intense, leading to the emplacement of intrusions, which comprise the K-feldspar granite, granite porphyry, and a small amount of diorite. The Huanggang area hosts multiple intrusions, including the named Luotuochangliang intrusion, the 204 intrusion, and several smaller, unnamed intrusions [31,39]. The Luotuochangliang intrusion, situated in the southwestern part of the area, covers an area of 4.6 km², which controls Ore Zones I, II, VI, and VII. The 204 intrusion is situated in the central-northeastern part of the area, covering an outcrop area of 2.0 km², which controls Ore Zones III and IV. The ore-related K-feldspar granite is massive and porphyroid in texture with a pale red color and mainly contains alkali feldspar (35~40%), quartz (25~30%), plagioclase (20%), and biotite (10%). Alkali feldspar mainly includes perthite, orthoclase, and microcline.

One hundred and eighty-five ore bodies were discovered, divided into seven ore zones. The lengths of ore bodies are 10–1475 m, with thicknesses of 2–118 m and plunge depths of up to 500 m. They generally dip to the north at 40–80°. Skarn alteration is dominant, which is predominantly developed along the contact zone between the granitoid intrusions and marble. Alteration and skarn in this area have the characteristics of zoning, there are silicified wall rocks, hornfels, green skarns, ore-barren skarns, and ore-bearing skarns (from the wall rock to skarns), with additional fluoritization, carbonatization, and silicification [24].

The ore minerals comprise oxides (e.g., magnetite, cassiterite, hematite), sulfides (e.g., pyrite, arsenopyrite, pyrrhotite, chalcopyrite, galena, sphalerite, molybdenite, tetrahedrite), and tungstate (e.g., scheelite). Gangue minerals comprise garnet, diopside, idocrase, hornblende, actinolite, epidote, chlorite, quartz, calcite, and fluorite (Figure 3). The mineralization sequence is broadly divided into skarn and quartz-sulfide periods, which can be further subdivided into six distinct stages on the basis of minerals paragenesis and crosscutting relationships of veins (Figure 4), including: (I) anhydrous skarn, (II) hydrous skarn, (III) cassiterite-quartz-calcite, (IV) pyrite-arsenopyrite-quartz-fluorite, (V) polymetallic sulfides-quartz, and (VI) carbonate stages.

Stage I contains a great deal of anhydrous silicates, including garnet, diopside, and idocrase (Figure 3b,j); Stage II contains hydrous minerals, with abundant magnetite occurring in its late stage, some of which coexist with quartz. The hydrous minerals include hornblende, actinolite, epidote, and chlorite. These hydrous minerals and magnetite replace the early anhydrous minerals (Figure 3c–e,k); Stage III (the oxide stage) is marked by cassiterite-quartz-calcite veins, accompanied by a little magnetite (Figure 3e,f,l). Stage IV, also called the early sulfide stage, is characterized by pyrite-arsenopyrite-quartz-fluorite veins (Figure 3f,m). Stage V is composed of pyrite, chalcopyrite, galena, pyrrhotite, sphalerite, and abundant quartz (Figure 3g,n,o). Stage VI, also called the carbonate stage, contains abundant calcite and a little quartz and pyrite (Figure 3h,p).

4. Samples and Methods

4.1. FI Analysis

A total of 35 doubly polished thin slices, prepared from drill cores and outcrop samples containing garnet, fluorite, quartz, and calcite, were examined. Petrographic characteristics of FIs were conducted with an Olympus BX-50 microscope (10 × 50), and we used a Linkam THMSG-600 stage (−190–600 °C) at the Analytical Testing Center of Geofluids Laboratory, Jilin University (Changchun, China) to complete the microthermometric measurements. The temperature changing rate was lower than 5 °C/min (0.2 °C/min when approaching phase changes). Analytical accuracies were ±2 °C (above 30 °C) and ±0.2 °C (below 30 °C).

4.2. Isotope Analysis

Stable isotope measurements (H–O–C) were all completed at the Beijing Research Institute of Uranium Geology (BRIUG) (Beijing, China). A mass spectrometer (MAT 253) was used to measure the H-O isotopes of garnet from Stage I and quartz from Stages II to V. Oxygen was liberated from minerals by reacting with BrF₅. Water in FIs was extracted via decrepitation and reduced to H₂ [42]. The results were reported relative to V-SMOW, measurement uncertainties are ±2‰ (δD) and 0.2‰ (δ¹⁸O). The δ¹⁸O_fluid values were calculated through fractionation equations [43,44,45]:

$$1000\ln \left[\left({\delta }^{18}{O}_{V-SMOW}+1000\right)/\left({\delta }^{18}{O}_{fluid}+1000\right)\right]=1.27\times {10}^6/T^2-3.65\left(\mathrm{g}\mathrm{a}\mathrm{r}\mathrm{n}\mathrm{e}\mathrm{t}\right)$$

$$1000\ln \left[\left({\delta }^{18}{O}_{V-SMOW}+1000\right)/\left({\delta }^{18}{O}_{fluid}+1000\right)\right]=3.38\times {10}^6/T^2-3.4\left(\mathrm{q}\mathrm{u}\mathrm{a}\mathrm{r}\mathrm{t}\mathrm{z}\right)$$

The CO₂ was extracted from Stage VI calcite powders (200 mesh) by reacting with H₃PO₄ at 50 °C. A mass spectrometer (MAT 251) was used for measurement, and results were reported in the form of V-PDB. Analytical precision (1σ) is ±0.1‰. The δ¹⁸O values were normalized to V-SMOW through equation [46]:

$${\delta }^{18}{O}_{V-PDB}=\left({\delta }^{18}{O}_{V-SMOW}-30.91\right)/1.0309$$

4.3. Garnet and Zircon LA–ICP–MS U-Pb Dating

The U-Pb isotopic analyses were conducted at the North China Mineral Resources Testing Center of the Ministry of Natural Resources (Tianjin, China). The skarn sample (HG–11) was from the inner contact zone of Ore Block III (Figure 2a) and the garnet in this sample was predominantly andradite. A MC−ICP−MS with a 193 nm (ESI, Hartland, WI, USA) laser was used to complete the age determination of garnet. The areas selected for testing were free of inclusions and fractures. The instrumental setups and analytical parameters were: laser spot size of 30 μm, energy density of 5 J/cm², ablation frequency of 8–10 Hz. The ICP–MS Data–Cal 11.8 software was used to process test results [47,48]. The Tera–Wasserberg inverse isochron U-Pb harmonic plot was completed via the Isoplot 4.15 software [49,50], and the crystallization age of garnet was determined by the lower intercept of the inverse isochron diagram.

Zircon U-Pb analysis was performed on K-feldspar granite collected from the fresh rock samples of 204 intrusion. Zircon crystals handpicking, mounting in epoxy resin, polishing, and CL images acquisition were completed at Beijing GeoAnalysis Technology Co., Ltd. (Beijing, China). The experimental instrument consists of a ICP–MS (Agilent, Santa Clara, CA, USA) and a 193 nm laser. The instrumental setups and analytical parameters were: laser spot size of 32 μm, energy density of 10 J/cm², ablation frequency of 8–10 Hz. Test results were processed utilizing the ICP–MS Data–Cal 11.8 and Glitter program [47,48], with concordia and weighted-mean age values generated via Isoplot 4.15 software [49,50].

5. Results

5.1. FI Types

Primary and secondary FIs were distinguished following the classification criteria in [51]. On the basis of phase states and transition behaviors, the primary FIs were classified into: VL-type (liquid-rich; Figure 5c,d,l,m), LV-type (vapor-rich; Figure 5e,n), and SL-type (halite-bearing three-phase; Figure 5b,k) FIs.

(1) VL-type FIs occur in all minerals of all stages. They have diameters of 3–15 μm with a gas/liquid ratio of 5–25 vol%. Their shapes are mostly elliptical, elongated, and irregular. VL-type FIs generally homogenize into the liquid phase when heated; (2) LV-type FIs occur in Stages I to V. They have diameters of 5–15 μm, with a gas/liquid ratio of >65%. They are oval and elongated in shape. LV-type FIs generally homogenize into a single vapor phase when heated; (3) SL-type FIs occur in Stages I to III, containing one halite of cubic habit, brine liquid, and a bubble. They are elliptical and have diameters of 5–12 μm. SL-type FIs generally homogenize into the liquid phase when heated, and halite in SL-type FIs melts before the disappearance of bubbles.

5.2. FI Microthermometric Data

Microthermometric data of FIs are shown in

Table A1. Microthermometric data of FIs from the Huanggang deposit.

Stage	Mineral	Fis Type	Frequency	Size (μm)	Tm-ice (°C)	Tm-NaCl (°C)	Th-tot (°C)	Salinity (wt% NaCl Equiv.)
I	Garnet	LV	40	5 to 15	−5.4 to −1.8		411 to 513	3.05 to 8.40
		VL	39	3 to 15	−13.2 to −9.2		392 to 497	13.33 to 17.19
		SL	16	5 to 12		351 to 475	401 to 501	42.50 to 56.44
II	Quartz	LV	37	5 to 15	−4.6 to −1.5		336 to 429	2.56 to 7.30
		VL	39	3 to 15	−13.6 to −10.0		317 to 426	13.99 to 17.56
		SL	23	5 to 12		314 to 403	332 to 420	39.27 to 47.77
III	Quartz	LV	29	5 to 15	−4.0 to −1.7		292 to 418	2.89 to 6.43
		VL	37	3 to 15	−11.1 to −8.7		286 to 411	12.54 to 15.15
		SL	12	5 to 12		301 to 385	272 to 396	38.24 to 45.85
IV	Quartz	LV	24	5 to 12	−3.5 to −0.8		231 to 327	1.39 to 5.70
		VL	19	3 to 12	−8.6 to −7.1		224 to 318	10.62 to 12.42
	Fluorite	LV	23	5 to 12	−3.8 to −0.9		255 to 347	1.56 to 6.14
		VL	15	3 to 12	−8.4 to −7.3		243 to 331	10.87 to 12.19
V	Quartz	LV	27	5 to 12	−1.5 to −0.5		223 to 274	0.87 to 2.56
		VL	34	3 to 12	−7.1 to −4.9		201 to 281	7.72 to 10.62
VI	Calcite	VL	38	3 to 10	−5.5 to −2.7		169 to 213	4.48 to 8.54

, Figure 6 and Figure 7. There are VL-, LV-, and SL-type FIs occurring in Stage I garnet (Figure 5a–f). For VL-type, the ice melting temperatures (T_m-ice) are −13.2 to −9.2 °C, with fluid salinities of 13.33–17.19 wt% NaCl equiv. and final homogenization temperatures (T_h-tot) of 392–497 °C. LV-type FIs yield T_m-ice of −5.4 to −1.8 °C, fluid salinities of 3.05–8.40 wt% NaCl equiv., and T_h-tot of 411–513 °C. SL-type FIs have halite melting temperatures (T_m-NaCl) of 351–475 °C with salinities of 42.50–56.44 wt% NaCl equiv. and T_h-tot of 401–501 °C (Figure 6a,g).

Stage II quartz contains all types of FIs (Figure 5g,h). For VL-type FIs, T_m-ice are −13.6 to −10.0 °C, with fluid salinities of 13.99–17.56 wt% NaCl equiv. and T_h-tot of 317–426 °C. LV-type FIs yield T_m-ice of −4.6 to −1.5 °C, salinities of 2.56–7.30 wt% NaCl equiv. and T_h-tot of 336–429 °C. SL-type FIs yield T_m-NaCl of 314–403 °C with salinities of 39.27–47.77 wt% NaCl equiv. and T_h-tot of 322–420 °C (Figure 6b,h).

There are VL-, LV-, and SL-type FIs occurring in quartz crystals of Stage III (Figure 5i). For VL-type FIs, T_m-ice are −11.1 to −8.7 °C, with fluid salinities of 12.54–15.15 wt% NaCl equiv. and T_h-tot of 286–411 °C. LV-type FIs have T_m-ice values of −4.0 to −1.7 °C, salinities of 2.89–6.43 wt% NaCl equiv., and T_h-tot of 292–418 °C. SL-type FIs have T_m-NaCl of 301–385 °C, salinities of 38.24–45.85 wt% NaCl equiv., and T_h-tot of 272–396 °C (Figure 6c,i).

There are VL- and LV-types occurring in fluorite and quartz of Stage IV (Figure 5j,o). VL-type FIs in quartz have T_m-ice of −8.6 to −7.1 °C, salinities of 10.62–12.42 wt% NaCl equiv., and T_h-tot of 224–318 °C. LV-type FIs in quartz yield T_m-ice of −3.5 to −0.8 °C, salinities of 1.39–5.70 wt% NaCl equiv., and T_h-tot of 231 to 327 °C. VL-type FIs in fluorite yield T_m-ice of −8.4 to −7.3 °C, calculated salinities of 10.87–12.19 wt% NaCl equiv. and T_h-tot of 243–331 °C. LV-type FIs in fluorite yield T_m-ice of −3.8 to −0.9 °C, salinities of 1.56–6.14 wt% NaCl equiv., and T_h-tot of 255–347 °C (Figure 6d,j).

Stage V quartz contains VL- and LV-types FIs (Figure 5p,q). VL-type FIs yield T_m-ice of −7.1 to −4.9 °C, salinities of 7.72–10.62 wt% NaCl equiv., and T_h-tot of 201–281 °C. LV-type FIs yield T_m-ice of −1.5 to −0.5 °C, salinities of 0.87–2.56 wt% NaCl equiv. and T_h-tot of 223–274 °C (Figure 6e,k).

Stage VI calcite contains VL-types FIs (Figure 5r). VL-type FIs yield T_m-ice of −5.5 to −2.7 °C, fluid salinities of 4.48–8.54 wt% NaCl equiv., and T_h-tot of 169–213 °C (Figure 6f,l).

5.3. Stable Isotopes

5.3.1. H–O Isotopes

The H–O isotopes data of garnet and quartz (n = 10) are shown in

Table A2. Oxygen-hydrogen isotope data of garnet and quartz from the Huanggang deposit.

Sample	Stage	Mineral	δ18OV-SMOW (‰)	δD (‰)	Th (°C)	δ18OH2O (‰)
HG-1-1	I	Garnet	3.8	−92.3	453	5.0
HG-1-2	I	Garnet	3.8	−101.2	453	5.0
HG-2-1	I	Garnet	4.8	−91.4	453	6.0
HG-3-1	I	Garnet	4.2	−96.6	453	5.4
HG-5-1	II	Quartz	9.5	−106.3	376	4.9
HG-5-2	II	Quartz	8.9	−104.7	376	4.3
HG-7-1	III	Quartz	9.2	−106.2	353	4.0
HG-8-1	IV	Quartz	7.6	−108.6	288	0.3
HG-9-2	IV	Quartz	6.1	−117.4	288	−1.2
HG-10-1	V	Quartz	5.9	−114.3	243	−3.4

Note: T_h (°C) is the average T_h-tot for each stage.

and Figure 8. The calculated δ¹⁸O_H2O values of garnet in Stage I are 5.0‰ to 6.0‰ (mean = 5.4‰), and the δD values are −101.2‰ to −91.4‰ (mean = −95.4‰). The δ¹⁸O_H2O values of quartz in Stage II are 4.3‰ to 4.9‰ (mean = 4.6‰), with δD values of −106.3‰ to −104.7‰ (mean = 105.5‰). The δ¹⁸O_H2O value of quartz in Stage III is 4.0‰, and the δD value is −106.2‰. The δ¹⁸O_H2O values of quartz in Stage IV are −1.2‰ to 0.3‰ (mean = −0.4‰), and the δD values are −117.4‰ to −108.6‰ (mean = −113.0‰). Quartz from Stage V yields δ¹⁸O_H2O values of −3.4‰, with δD values of −114.3‰.

5.3.2. C–O Isotopes

The C–O isotope data of calcite (n = 4) are presented in

Table A3. Oxygen-carbon isotope data of calcite from the Huanggang deposit.

Sample	Mineralization Stages	Mineral	δ13CV-PDB (‰)	δ18OV-PDB (‰)	δ18OV-SMOW (‰)
HG-7	VI	Calcite	−11.1	−30.9	−0.9
HG-12-1	VI	Calcite	−11.5	−31.9	−1.9
HG-7-1	VI	Calcite	−10.9	−30.7	−0.7
HG-13-1	VI	Calcite	−12.2	−32.1	−2.2

and Figure 9. The δ¹³C_V-PDB values are −12.2‰ to −10.9‰ (mean = −11.4‰), δ¹⁸O_V-PDB values are −32.1‰ to −30.7‰ (mean = −31.4‰), and the calculated δ¹⁸O_V-SMOW values are −2.2‰ to −0.7‰ (mean = −1.4‰).

5.4. Geochronological Data

5.4.1. Garnet U-Pb Age Data

The garnet U-Pb testing results are provided in

Table A4. LA–ICP–MS U-Pb dating results of garnet from the Huanggang deposit.

Samples	Th	U	Th/U	Isotopic Ratios
	ppm	ppm		206Pb/238U	1σ	207Pb/235U	1σ	207Pb/206Pb	1σ
HG-1	0.87	2.01	0.43	0.0630	0.0016	6.054	0.1976	0.6244	0.0383
HG-2	1.23	2.63	0.47	0.1184	0.0026	13.014	0.3314	0.7973	0.0257
HG-3	0.33	0.95	0.35	0.1885	0.0041	21.678	0.5510	0.8341	0.0275
HG-4	0.49	2.14	0.23	0.2858	0.0083	34.925	1.0865	0.8863	0.0453
HG-5	0.31	0.74	0.42	0.0212	0.0005	2.395	0.0508	0.0835	0.0055
HG-6	0.85	1.73	0.49	0.0299	0.0008	3.382	0.1745	0.3093	0.0135
HG-7	1.77	2.21	0.80	0.0329	0.0011	3.747	0.1138	0.3651	0.0136
HG-8	0.63	1.58	0.40	0.1112	0.0029	12.085	0.4862	0.7884	0.0260
HG-9	0.55	2.57	0.21	0.2242	0.0062	26.412	1.6145	0.8543	0.0334
HG-10	0.31	0.88	0.35	0.2683	0.0133	32.980	1.9301	0.8916	0.0596
HG-11	0.74	2.19	0.34	0.1246	0.0034	12.843	0.4170	0.7476	0.0223
HG-12	0.71	2.97	0.24	0.1158	0.0028	12.046	0.3725	0.7546	0.0215
HG-13	1.41	3.51	0.40	0.1518	0.0050	17.385	0.7369	0.8305	0.0531
HG-14	0.76	1.23	0.62	0.1999	0.0041	23.362	0.3878	0.8478	0.0159
HG-15	0.98	2.26	0.43	0.0642	0.0013	7.467	0.1917	0.6437	0.0169
HG-16	0.33	0.96	0.34	0.1445	0.0063	16.862	0.8729	0.8463	0.0706
HG-17	0.79	3.21	0.25	0.1045	0.0041	10.495	0.5164	0.7286	0.0263
HG-18	0.28	1.44	0.19	0.1720	0.0043	20.857	0.6284	0.8794	0.0387
HG-19	1.01	2.31	0.44	0.1252	0.0040	13.224	0.5320	0.7661	0.0412
HG-20	0.42	0.89	0.47	0.1340	0.0032	14.325	0.4402	0.7751	0.0262
HG-21	0.64	1.02	0.63	0.1386	0.0042	14.478	0.5420	0.7574	0.0231
HG-22	0.75	2.63	0.29	0.0865	0.0026	8.819	0.3127	0.7390	0.0296
HG-23	1.23	2.37	0.52	0.3161	0.0085	38.561	1.3332	0.8848	0.0284
HG-24	1.35	2.86	0.47	0.0946	0.0031	9.786	0.4235	0.7503	0.0510
HG-25	0.73	2.4	0.30	0.0923	0.0030	8.969	0.3640	0.7050	0.0541

. A total of 35 test points from sample HG-1 were analyzed, but due to the possible influence of inclusions or cracks during the testing process, some data signals were unstable or poor. Therefore, 25 reasonable and effective data points were ultimately selected. Th (0.28–1.77 ppm) and U (0.74–3.51 ppm) contents yield Th/U = 0.19–0.80. The Tera-Wasserburg diagram yielded a lower-intercept age of 132.1 ± 4.7 Ma (MSWD = 0.64; Figure 10a).

5.4.2. Zircon U-Pb Age Data

The zircon U-Pb testing results are provided in

Table A5. LA–ICP–MS U-Pb dating results of zircon of K-feldspar granite from the Huanggang deposit.

Samples	Content			Isotopic Ratios						Isotopic Age Values (Ma)
	Th ppm	U ppm	Th/U	207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ	207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ
HG-1-1	1347.3	8426.5	0.16	0.05037	0.00094	0.15625	0.00319	0.02085	0.00023	212.2	42.75	147.4	2.8	133.0	1.4
HG-1-2	1898.2	10,293.0	0.18	0.05194	0.00062	0.1582	0.002	0.02069	0.00021	282.8	27.12	149.1	1.8	132.0	1.4
HG-1-3	4042.2	12,729.4	0.32	0.04928	0.00219	0.14391	0.00704	0.02067	0.00029	161.2	100.71	136.5	6.3	131.9	1.8
HG-1-4	544.1	1497.1	0.36	0.04948	0.00188	0.1423	0.00586	0.02112	0.00027	170.9	86.26	135.1	5.2	134.7	1.7
HG-1-5	1035.2	6655.9	0.16	0.04805	0.00143	0.13836	0.00449	0.02097	0.00025	101.6	69.08	131.6	4.0	133.8	1.6
HG-1-6	1106.2	2769.6	0.40	0.04848	0.00145	0.14002	0.00455	0.0207	0.00025	122.9	68.74	133.1	4.1	132.1	1.6
HG-1-7	504.6	4015.5	0.13	0.04929	0.00071	0.15076	0.00234	0.02077	0.00022	198	31	133	1.7	129.5	1.2
HG-1-8	3915.4	6824.9	0.57	0.0549	0.0007	0.1554	0.0034	0.0205	0.0004	406	23	147	2.9	130.6	2.4
HG-1-9	2073.5	9005.6	0.23	0.0525	0.0005	0.1539	0.0017	0.0212	0.0001	309	24	145	1.5	135.2	0.9
HG-1-10	227.8	445.6	0.51	0.05124	0.00141	0.14641	0.00442	0.0208	0.00024	251.6	62.29	138.7	3.9	132.7	1.5
HG-1-11	90.9	1690.4	0.05	0.0528	0.0007	0.1533	0.0026	0.0209	0.0002	320	25	145	2.3	133.5	1.3
HG-1-12	1061.9	7061.0	0.15	0.0491	0.0005	0.1403	0.0017	0.0207	0.0002	150	24	133	1.5	132.0	1.1
HG-1-13	1839.2	10,368.3	0.18	0.05283	0.00059	0.15964	0.00187	0.02064	0.00021	321.4	25.19	150.4	1.6	131.7	1.3
HG-1-14	454.9	6221.5	0.07	0.0506	0.0005	0.1450	0.0018	0.0207	0.0002	220	22	137	1.6	132.3	1.1

. The crystals are euhedral, granular, or subhedral columnar with grain sizes of 80−150 μm, and the length/width ratios are 1:1–1:1.8 (Figure 10b). Th (90.9–4042.2 ppm) and U (445.6–12729.4 ppm) yield Th/U = 0.05–0.57. A total of 14 points were selected, with a concordant age of 133.1 ± 1.2 Ma (MSWD = 0.97; Figure 10c). The weighted-mean age is 132.6 ± 0.9 Ma (MSWD = 1.5; Figure 10d).

6. Discussion

6.1. Source of Ore-Forming Fluids

Fluid mixing of postmagmatic and meteoric fluids is a process commonly used to explain skarn evolution [6]. However, skarns can also form from the evolution of a single magmatic fluid system [59]. Therefore, the integrated discussion on FI and stable isotopes is required to assess the source of fluids.

As illustrated in Figure 8, the δ¹⁸O_H2O and δD values (δ¹⁸O_H2O = 5.0–6.0‰) of garnet in Stage I fall close to the range of primary magmatic water [52], suggesting that fluids of the anhydrous skarn stage were of magmatic origin. The relatively lower hydrogen isotopic compositions (δD = −101.2‰ to −91.4‰) of garnet might record magmatic degassing [60,61,62,63], residual magmatic water could be depleted in δD during open-system magmatic degassing; meanwhile, the effect on δ¹⁸O_H2O is smaller [62,64,65,66,67]. Quartz samples of Stages II and III yield slightly lower δ¹⁸O_H2O and δD values (δ¹⁸O_H2O = 4.0–4.9‰; δD = −106.3‰ to −104.7‰) than Stage I, yet the plotted points remain well away from meteoric water line, suggesting a dominantly magmatic origin with possibly addition of certain amount of meteoric water. Stage IV quartz yields significantly lower δ¹⁸O_H2O and δD values (δ¹⁸O_H2O = −1.2‰ to 0.3‰; δD = −117.4‰ to −108.6‰) than those of Stage III, drifting to the meteoric water line, demonstrating a substantial contribution of meteoric water. Quartz of Stage V has lower δ¹⁸O_H2O and δD values (δ¹⁸O_H2O = −3.4‰; δD = −114.3‰) than those of all stages, indicating that the contribution ratio of meteoric water increased significantly. The previous data also show a similar trend from primary magmatic water to the meteoric water line, which confirms the conclusion of this study.

Stage VI calcite yield δ¹³C_V-PDB of −12.2 to −10.9‰ with δ¹⁸ O_V-SMOW of −2.2 to −0.7‰, as illustrated in Figure 9. The samples show a trend of being influenced by the meteoric water, suggesting fluid mixing with meteoric water during the last stage.

6.2. Fluid Evolution

On the basis of petrographic observations and microthermometry of FIs and stable isotope data, the fluid evolution can be summarized as follows:

Stage I: All three types of FIs coexisting in garnet exhibit overlapping T_h-tot (392–513 °C), yet they show marked divergence in salinities (3.05–56.44 wt% NaCl equiv.) and homogenization behaviors (into liquid or vapor), indicating fluid boiling [51,68]. The decompression might be the cause of fluid boiling during this stage: when skarn minerals formed, a large amount of free space was created, which led to pressure release and caused fluid boiling [69,70]. As mentioned before, fluids in the anhydrous skarn stage originated magmatically, belonged to a magmatic-derived boiled high-temperature, high-salinity NaCl–H₂O system.

Stage II: All three types of FIs coexisting in quartz exhibit similar T_h-tot (317–429 °C) and contrasting salinities (2.56–47.77 wt% NaCl equiv.) and different phase transition modes, indicating that the fluids continued to boil at this stage [71,72]. Both T_h-tot and salinities decreased, which might be the result of the upward migration of fluids and the gradually decreased influence of magmatic rocks. The decompression might be the cause of fluid boiling: in a boiling system [73,74], the pressure can be estimated through the isochoric diagram [75] for a NaCl–H₂O boiling system, where the estimated pressures decreased from up to 500 bars (Stage I) to 100–350 bars (Stage II) (Figure 7b). The fluid system of the hydrous skarn stage was a boiled high-temperature, high-salinity NaCl–H₂O system.

Stage III: Fluids in this stage show similar characteristics with the hydrous skarn stage and a slight decrease of T_h-tot (272–418 °C) compared with Stage II. While salinities (2.89–45.85 wt% NaCl equiv.) remained relatively high, the NaCl–H₂O fluid system continued to boil at this stage. The estimated trapping pressures are 50–300 bars (Figure 7b). The H–O isotopes results are similar to Stage II. Fluids of this stage were characterized by magmatic-meteoric water mixing, and belonged to medium-to-high temperature, high-salinity NaCl–H₂O boiling hydrothermal system.

Stage IV: The decreases in T_h-tot (224–347 °C), salinities (1.39–12.42 wt% NaCl equiv.), and δD and δ¹⁸O_H2O values indicate the mixing of a great deal of meteoric water in fluids (Figure 8). In this stage, the estimated pressures declined drastically to ~200 bars (Figure 7b). The fluid system turned out to be a medium-temperature, medium-to-low salinity NaCl–H₂O boiling hydrothermal system.

Stage V: Both the T_h-tot (201–281 °C) and fluid salinities (0.87–10.62 wt% NaCl equiv.) continued to decrease, which might be led by the increasing influx of meteoric water. The fluids were characterized as a medium-temperature, low-salinity NaCl–H₂O system.

Stage VI: There are only VL–type FIs developed, characterized by significantly lower T_h-tot (169–213 °C) and salinities (4.48–8.54 wt% NaCl equiv.). In this stage, meteoric water was the dominant component in ore fluids. The fluid system became a medium-to-low-temperature, low-salinity NaCl–H₂O system.

6.3. Timing of Magmatic-Hydrothermal Activity

Previous studies revealed that magmatic activities during the Mesozoic were crucial for mineralization in the Huanggangliang-Ganzhuermiao Polymetallic metallogenetic belt, and mineralization age values were 150–130 Ma [76]. The diagenetic age values of igneous rocks in the Huanggang area were concentrated in the Early Cretaceous (138–132 Ma; [30,38,40,77]), and the lithology was mainly granitoid, including granite porphyry, potassium feldspar granite, etc. Among them, potassium feldspar granite was the main ore-forming rock body [29]. In terms of mineralization age values, previous studies mainly conducted Re–Os isotope analysis on molybdenite, determining that the mineralization formed during 135–133 Ma [29,40,78]. Yao et al. [79] suggested that, in addition to the Yanshanian mineralization, the Huanggang deposit also exhibited SEDEX-type mineralization in the Permian.

In this study, the garnet samples yield a U-Pb age of 132.1 ± 4.7 Ma, which is consistent with previous research [38]. The ore-forming-related K-feldspar granite samples yield the zircon U-Pb age of 132.6 ± 0.9 Ma, which is in agreement with the garnet age within error. So far, previous research and this study have suggested that the formation of skarns predominantly formed at 136–132 Ma, and yet no Permian skarn has been found, indicating that skarn might have mainly formed during the Early Cretaceous. This provides more specific, sufficient evidence for the limitation of the timing of hydrothermal activity and skarn-type mineralization time of the deposit, which better limits the formation time of skarn-type mineralization in the Huanggang deposit in the Early Cretaceous.

6.4. Ore Genesis

The formation of silicate minerals in skarn is closely associated with high-temperature, high-salinity magma-derived fluids, with fluid boiling playing a critical role in metal precipitation in the skarn-type mineralization system [80,81].

The Huanggang deposit, characterized by classic geological, fluid, and isotopic features of skarn mineralization, is a typical skarn-type deposit. In comparison to skarn-type deposits in the southern Greater Khingan Range (e.g., Hongling lead-zinc, Chaobuleng iron-zinc, and Baiyinnuoer lead-zinc deposits; [22,23,24,82,83]), the Huanggang deposit exhibits similar fluid sources, fluid evolution paths, and hydrothermal activity. These systems initially involved high-temperature, high-salinity boiling systems in earlier stages, and later evolved into medium-to-low-temperature, low-salinity fluid systems. Fluid boiling was always the mechanism of precipitation of oxide minerals, while fluid mixing was commonly associated with deposition of sulfides. Specifically, the genesis of the Huanggang deposit is as follows:

During the Early Cretaceous, the post-orogenic extension of the Mongolia-Okhotsk Ocean tectonic domain and the subduction of the Paleo-Pacific plate jointly triggered the upwelling of asthenosphere material, inducing lithospheric delamination and thinning [84,85,86,87]. The newly formed lower crustal material partially melted and eventually formed K-feldspar granite magma under the extensional tectonic background [29]. The magma intruded upwards and migrated along deep faults to shallow positions. The ore-forming material continuously differentiated and enriched in the ore-bearing hydrothermal fluids. At the contact zone with marble of Huanggangliang and Dashizhai Formations, the hydrothermal fluids underwent intense metasomatism with the surrounding rocks, resulting in the skarn-type mineralization (Figure 11).

The hydrothermal fluids derived from magma during the earlier stage were rich in ore-forming materials and volatile components, with high temperature and oxygen fugacity. These fluids underwent strong metasomatism reactions with carbonate rocks, forming anhydrous minerals such as garnet, diopside, idocrase, and so on [59,88]:

$${3CaCO}_3+{Fe}_2{O}_3+{3SiO}_2⟶{Ca}_3{Fe}_2{Si}_3{O}_{12}\left(andradite\right)+{3CO}_2$$

(1)

$${CaCO}_3+{MgCO}_3+{2SiO}_2⟶CaMg{Si}_2{O}_6\left(diopside\right)+{2CO}_2$$

(2)

As the fluids migrated, the hydrous skarn stage commenced. During this stage, the influence of the granite intrusions diminished, and the ore-forming environment began to change. The ore-forming pressure decreased significantly, from approximately 400 bars in the anhydrous skarn stage to around 200 bars. The fluid boiling was obvious, and the temperature was slightly lower than anhydrous skarn stage, a great deal of magnetite precipitated in this situation [89]:

$${4CaCO}_3+{2FeCl}_3+{FeCl}_2+{4H}_2\mathrm{O}⟶{Fe}_3{O}_4\downarrow \left(magnetite\right)+{4CaCl}_2+{4H}_2{CO}_3$$

(3)

During the mineralization stage of cassiterite (oxide stage), the fluid temperature decreased, which might be caused by the involvement of minor meteoric water or the upward migration of the fluids along the channel. In magmatic fluids, tin mainly migrates as divalent (Sn²⁺) chlorine-bearing complexes in magmatic fluids [90,91,92], whereas Sn in cassiterite occurs as tetravalent (Sn⁴⁺) valent. Therefore, the redox reaction mechanism exerts a strong influence on Sn precipitation [11]. As the fluids underwent metasomatism with carbonate rocks during the skarn stage, a large number of acidic substances were consumed, and the fluid exhibited relatively alkaline conditions. Under the combined effects of fluid mixing caused by meteoric water and fluid boiling induced by pressure drop, tin precipitated to form cassiterite:

$${Sn}^{4+}+{4OH}^{-}⟶{SnO}_2\downarrow \left(cassiterite\right)+{2H}_{2}O$$

(4)

At the beginning of the early sulfide stage, substantial meteoric water added into the hydrothermal system, resulting in a sharp reduction in fluid temperature and salinity, reducing the solubility of sulfide minerals and promoting their precipitation. Furthermore, the mixing of meteoric water also likely elevated the pH of the hydrothermal fluid, drove the equilibria of sulfide-forming reactions toward mineral precipitation:

$${Zn}^{2+}+{HS}^{-}⟶ZnS\downarrow \left(sphalerite\right)+H^{+}$$

(5)

$${Pb}^{2+}+{HS}^{-}⟶PbS\downarrow \left(galena\right)+H^{+}$$

(6)

$${Fe}^{2+}+{Cu}^{2+}+{2HS}^{-}⟶{CuFeS}_2\downarrow \left(chalcopyrite\right)+{2H}^{+}$$

(7)

$${Fe}^{2+}+{2HS}^{-}⟶{FeS}_2\downarrow \left(pyrite\right)+{2H}^{+}$$

(8)

In the final stage, considerable meteoric water mixed within the hydrothermal fluids, causing the formation of abundant calcite, minor pyrite and quartz, forming Stage VI veins. The widespread development of carbonate veins signifies the termination of hydrothermal mineralization.

7. Conclusions

• The Huanggang skarn-type iron-tin deposit comprises seven stages of mineralization: (I) anhydrous skarn, (II) hydrous skarn, (III) cassiterite-quartz-calcite, (IV) pyrite-arsenopyrite-quartz-fluorite, (V) polymetallic sulfides-quartz, and (VI) carbonate stages.
• Fluid inclusion and C–H–O isotopic evidence suggests that the ore-forming fluids were initially a high-temperature high-salinity magmatic-derived system with intense boiling and changed to a medium-to-low-temperature, low-salinity NaCl–H₂O system in the later stages with meteoric water mixing.
• The precipitation of magnetite and cassiterite was closely related to the joint effects of fluid boiling caused by decompression and fluid mixing induced by the adding of a certain amount of meteoric water.
• U-Pb dating of garnet and zircon yields a lower-intercept age of 132.1 ± 4.7 Ma and a weighted mean age of 132.6 ± 0.9 Ma, respectively, corresponding to the timing of skarn formation and crystallization of K-feldspar granite.