The Mechanism of Fluid Exsolution and Sn–W Precipitation: Example from the Hongling Pb–Zn Polymetallic Deposit and the Surrounding Area in Northern China

Abstract

Metal migration and precipitation in hydrothermal fluids are important topics in economic geology. The Hongling polymetallic deposit comprises one of the most important parts of the Huanggangliang–Ganzhuermiao polymetallic metallogenic belt, which is in eastern Inner Mongolia. Except for lead–zinc skarn, minor cassiterite in the skarn and disseminated W–Sn mineralization in granitic rocks have also been found. The dominant Sn–W mineralization is in the northern part of the deposit, occurring as disseminated wolframite and cassiterite in aplite hosted in Mesozoic granite porphyry. The aplite together with pegmatite K-feldspar–quartz comprises vein dikes hosted in the granite porphyry, providing evidence for the transition from melt to fluid. The veins, dikes, and Sn–W mineralization in the aplite provide an opportunity to investigate fluid exsolution and the mechanics of metal precipitation. Based on field observations, the micrographic and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) results of the vein dikes, chronology, and the whole-rock geochemistry of the host rock, together with the fluid inclusion results, this paper discusses the characteristics of the causative magma, the mechanics of fluid exsolution and W–Sn precipitation. Our results show that the causative magma is of highly fractionated A-type granite affinity and has an intrusive age of late Mesozoic (133.3 ± 0.86 Ma). The magmatic evolution during shallow emplacement led to immiscibility between highly volatile, high-silica, and W- and Sn-enriched melts from the parent magma, followed by fluid exsolution from the water-rich melt. The alkaline-rich fluid exsolution led to a change in the redox state of the magma and the chilling of the melt. Fluid boiling occurred soon after the fluid exsolution and was accompanied by the degassing of CO₂. The boiling and escape of CO₂ from the fluid led to changes in fluid redox and W and Sn precipitation; thus, the W and Sn mineralization are mostly hosted in causative intrusions or peripheral wall rocks, which can be used as indicators for Sn–W exploration in the area.

1. Introduction

Metal migration and precipitation in hydrothermal fluids have important implications for the formation of mineral deposits and have long been important topics in economic geology. During magma upwelling and shallow emplacement, large volumes of aqueous fluids can exsolve from the magma [1] because of decompression (first boiling) or fractional crystallization (second boiling). Fluid exsolution and evolution not only affect the redox state of the magma but also lead to metal enrichment, which is essential to the formation of magmatic–hydrothermal mineral deposits [2,3]. For many of the hydrothermal mineral deposits, the causative magmatic rock is usually vague because of their distal mineralization. Magmatic–hydrothermal transition textures provide evidence for the causative magma and have been widely documented for porphyry Cu–Mo–Au [4,5,6,7,8,9,10,11], granite-related Sn–W [12], and barren granitic systems [13]. Unidirectional solidification textures (USTs), miarolitic cavities, and vein dikes in felsic intrusions are typical magmatic–hydrothermal transition textures. Fluid inclusions hosted in quartz in vein dikes or miarolitic cavities provide the best samples of the magmatic fluid that directly exsolved from magma and provide important information on metal migration and precipitation.

The Hongling (Haobugao) Pb–Zn polymetallic deposit is in the Huanggang–Ganzhuermiao tin and silver polymetallic metallogenic belt in the eastern part of the Central Asian Orogenic Belt (CAOB; Figure 1a,b), which formed by the collision between the North China Craton and Siberia Craton during the late Paleozoic to early Mesozoic [14,15,16,17,18,19]. During the Mesozoic, the eastern CAOB was affected by the subduction of the Pacific plate underneath the North China Craton (NCC) [20,21,22,23,24], forming a NE-trending Mesozoic tectonic–magmatic belt and the Great Xing’an Range belt. The Hongling deposit is situated on the southwest slope of the Great Xing’an Range. A series of Sn, Ag, and Pb–Zn deposits related to Mesozoic granite are reported in the belt, such as the Huanggang Fe–Sn skarn [25], Weilasituo Sn-polymetallic epithermal deposit [26], Dajing Sn-polymetallic deposit [27], Baiyinnuoer Pb–Zn-polymetallic skarn [28], Meng’entaolegai hydrothermal Ag–Pb deposit [29], and Baiyinchagan Sn-polymetallic deposit [30]. Hongling Pb–Zn skarn comprises an important part of this belt, with a total resource of 0.29 million metric tons (Mt) Zn at an average grade of 4.24%, 0.15 Mt Pb at an average grade of 2.25%, and 2.91 Mt Fe at an average grade of 28.7%, accompanied by less important Sn, Cu, Bi, and Au [31,32]. Previous studies have focused on the Pb–Zn skarn-related geology, chronology, and fluid inclusions [32,33,34,35,36], with minor focus on Sn mineralization in the skarns [37]. Except for Fe skarn, Fe–Zn skarn, and Pb–Zn–(Cu) skarn, minor Sn and W mineralization is also reported, not only in the skarns, but also in the granite porphyry of the surrounding area. To the north of the deposit, the Sn and W mineralization occurs as disseminated cassiterite and wolframite in aplite or the host granite porphyry. The aplite comprises part of the vein dikes together with pegmatitic quartz and K-feldspar hosted in the granite porphyry, providing evidence for the genetic link between the granite porphyry and Sn–W mineralization. The discovery of Sn–W mineralized veins and dikes hosted in the granite porphyry provide an opportunity to investigate both the characteristics of causative magma and the fluid exsolution from the magma, which is responsible for the Sn–W mineralization. Based on field observations, micrographic and SEM/EDS results of the vein dikes, zircon U–Pb chronology, and whole-rock geochemistry of the host granite porphyry, together with fluid inclusion results, this paper discusses the characteristics of the causative magma, the fluid exsolution process, and the mechanics of Sn–W precipitation. It is proposed that during the shallow emplacement of the parent magma, a highly volatile and alkaline-enriched melt immiscible from the parent magma led to Sn and W enrichment in the volatile-rich melt. The following CO₂-rich fluid exsolution led to the chilling of the magma and the formation of the aplite together with pegmatitic K-feldspar and quartz. The coexistence of aqueous-rich, vapor-rich, and high salinity fluid inclusion hosted in pegmatitic quartz implies a boiling process soon after the fluid exsolution. CO₂ escape during boiling and alkaline fluxing by alkaline alteration led to a change in the redox state of the fluid and W and Sn precipitation. The causative granite porphyry and the peripheral wall rock may be the best target for Sn–W exploration in the area.

2. Geological Background

The Hongling Pb–Zn multimetal skarn is located 70 km north of the Chifeng city in eastern Inner Mongolia, northern China. Tectonically, it is situated in the eastern Central Asia Orogenic Belt (CAOB, Figure 1a), which was formed by accretion of island arcs, ophiolites, oceanic islands, seamounts, accretionary wedges, oceanic plateaus, and micro-continents during Paleozoic to early Mesozoic [42]. The CAOB has a long and complex tectonic evolution from at least ca. 1.0 Ga to late Paleozoic–early Mesozoic [43,44], but the tectonic setting of the eastern part of CAOB might be linked to the NNW subduction of the Paleo-Pacific plate (Izanagi) during Late Mesozoic, which affected most of eastern China [45]. The formation of the great Xing’an Range NE-trending Mesozoic tectonic-magmatism belt and accompanied Sn, W, Ag, and Pb-Zn polymetallic mineralization (Figure 1b), such as the Huanggang Fe-Sn skarn, are the results of the Paleo-Pacific plate subduction-related magmatism [45]. The Hongling deposit is situated on the southwest slope of the Great Xing’an Range.

The lithologies of the deposit and the surrounding area are mainly Permian sedimentary–volcanic rocks, Jurassic volcanic rocks, and Mesozoic granitoids. The Permian marine sedimentary and volcanic rocks, including black shale, limestone, siltstone, tuff, and alkaline to felsic–intermediate volcanic rocks (Dashizhai Formation), comprise the main wall rock of the ore body [46]. Jurassic Manketouebo formation rhyolite and volcanic pyroclastic rock unconformably overlie the Permian [41,46,47]. Intrusive rocks in the Hongling area are mainly Mesozoic granite (with zircon U-Pb ages of 144.8 to 139.8 Ma, [33]) and granite porphyry (with zircon U-Pb ages of 136.7 Ma, [33]). The NE- and NW-trending biotite syenite porphyry and lamprophyre dikes intruded into Permian and granitic rocks [48]. Permian sedimentary–volcanic rocks were strongly deformed during the late Permian–Early Triassic tectonic event and resulted in NE-trending folds [48]. The major faults are NE-trending and NW-dipping faults that control the occurrence of skarn and alteration. Both the NE-trending faults and folds are crosscut by NW-trending faults or related fracture zones (Figure 2).

Garnet-, diopside-, and actinolite-skarn host most of the ore bodies in the Hongling deposit, including Fe, Fe–Zn–(Cu), and Pb–Zn–Ag–(Cu) ore bodies. Minor veinlet, stockworks, or disseminated Cu–Ag and Pb–Zn–Ag mineralization are also confirmed in the hanging and footwall of the skarn. The ore minerals of the Hongling deposit include mainly magnetite, sphalerite, galena, and chalcopyrite, with less important cassiterite, wolframite, argentite, tetrahedrite, bismuthinite, and stibnite. Despite the dominant skarn Fe, Fe–Zn, and Pb–Zn (Ag) mineralization of the deposit, minor Sn mineralization is reported in the Pb-Zn skarn, as well. The discovery of Sn–W mineralization in granite porphyry outcrops to the north of the deposit implies important potential for W–Sn exploration in the area.

3. Sampling and Analytical Methods

The samples for this study were collected from outcrops to the north of the mining area, including granite porphyry (HL-5087 and HL-5086), aplite, and associated pegmatitic K-feldspar and quartz. Pegmatitic K-feldspar and quartz were also collected from granite porphyry host rocks.

3.1. Petrography and SEM/EDS Analyses

Microscopy and SEM/EDS were completed at the School of Civil and Resource Engineering, University of Science and Technology Beijing (USTB) using an Olympus BX53M microscope and a Phenom XL SEM equipped with an EDS system, respectively. The EDS analysis was performed under high-vacuum conditions with an accelerating voltage of 15 kV.

3.2. Whole-Rock Geochemical Analyses

Whole-rock major and trace element determination was conducted at the Analytical Laboratory of Beijing Research Institute of Uranium Geology (BRIUG) using X-ray fluorescence (XRF) spectrometry and inductively coupled plasma–mass spectrometry (ICP–MS), respectively. For the major elements, the standard sample was AB104L, with uncertainties of less than 5% [51]. The analytical precision for the trace elements was less than 3% RSD (relative standard deviation). The G2 and GSR-1 standards were used as reference materials.

3.3. LA–ICP–MS Zircon U–Pb Dating

The zircon U–Pb analyses were conducted on the NWR 193 UC laser ablation–ICP–MS (LA–ICP–MS) instrument at the Laboratory of Mineral/Inclusion Microanalysis, Institute of Geology, Chinese Academy of Geological Sciences (CAGS). The mass spectrometer that was used was an Agilent 7900 inductively coupled plasma mass spectrometer (Agilent, Palo Alto, CA, USA). The spot size was 32 μm with a 5 Hz laser and 2 J/cm² energy density. The Isoplot 3.0 program was used for age calculation and diagraming [52]. The Iolite program was used for data processing [53], with zircon 91,500 and GJ-1as the standard. The standards were run between every 10–12 samples. The analytical procedures were described in detail by Yu et al. (2019) [54].

3.4. Fluid Inclusion Microthermometry and Laser Raman Microprobe (LRM) Analyses

The petrography and microthermometry of the fluid inclusions was completed in the Fluid Inclusion Laboratory of USTB. The instrument used for microthermometry was a Linkam THMS 600 cooling and heating stage with a temperature range of −196~+600 °C. The measurement accuracies for freezing and heating were ±0.1 °C and ±2 °C, respectively. An LRM analysis of the fluid inclusions was carried out at the Analytical Laboratory of BRIUG. The instrument that was used was a LabRAM HR800 Evolution high-resolution laser Raman spectrometer produced by the Horiba Jobin-Yvon Company of Longjumeau, France.

4. Results

4.1. Petrology and Mineralogy of Vein Dikes and Host Granite Porphyry

Based on field observations, the granite porphyry occurs as small stocks that intruded the granite (with zircon U-Pb ages of 144.8 to 139.8 Ma, [33]). The vein dikes hosted in the granite porphyry comprise aplite (Figure 3a,b) and pegmatitic to coarse-grained K-feldspar–quartz (Figure 3c). Vein dikes usually occur as veins (Figure 3a), irregular globes (Figure 3c), or dikes (Figure 3b) in granite porphyry. The pegmatitic quartz–(K-feldspar) comprises the center of the vein dike (Figure 3a) or occurs as veins or miarolitic cavities (Figure 3d) in the granite porphyry.

The granite porphyry is pink in color and has a massif structure and porphyritic texture (Figure 4a,b). The phenocrysts include K-feldspar and quartz, and the matrix is quartz, K-feldspar, plagioclase, and minor biotite (Figure 4a). The accessory minerals include allanite (Figure 4c), ilmenite, monazite, apatite, zircon, and rutile.

The vein dikes hosted in the granite porphyry comprise aplite and pegmatitic or coarse-grained K-feldspar–quartz. The aplite shows a fine-grained texture (Figure 4d) with a mineral assemblage of K-feldspar and quartz, with minor plagioclase, biotite, and hornblende. The petrographic and SEM/EDS observations confirmed that cassiterite (Figure 5a,b) and wolframite (Figure 5c,d) occur as disseminated crystals in aplite or in pegmatitic quartz (Figure 5c) and K-feldspar.

4.2. Zircon U–Pb LA–ICP–MS Results of Granite Porphyry

The cathodoluminescence (CL) images show that the zircons in the granite porphyry are prismatic-shaped with clear oscillatory zonation (Figure 6a). The LA–ICP–MS U–Pb analysis results show that the Th/U ratios of these zircons are 0.27–0.59 (Table 1), indicating a magmatic origin [55,56]. Eight of them show good correlation and give a concordant age of 133.3 ± 0.86 Ma (n = 8, mean standard weighted deviation (MSWD) = 1.6, Figure 6b).

4.3. Whole-Rock Geochemistry of Granite Porphyry

4.3.1. Major Elements

The major element results of the granite porphyry are listed in Table 2. The results show high SiO₂ (76.99%–77.52%), Al₂O₃ (12.31%–12.56%), and K₂O+Na₂O (7.95%–8.21%, and K₂O/Na₂O ratios of 1.16–1.21), but low MgO (0.10%–0.11%), FeO (0.68%–0.82%), and CaO (0.35%–0.38%) contents. The loss on ignition (LOI) values (0.51%–0.61%) are lower than most of the granite rocks. The total Fe₂O₃ values of the granite porphyry vary from 0.98 to 1.16, and they have higher Fe²⁺ than Fe³⁺ values (Fe₂O₃/FeO = 0.21–0.65). It plots in the granite field in the TAS diagram (Figure 7a). In the w(SiO₂)–w(K₂O) diagram, the granite porphyry plots in the high-potassium calc-alkaline series (Figure 7b). In the 10,000 Ga/Al–Nb diagram (Figure 8a), the granite porphyry plots in the A-type granite area, with 10,000 Ga/Al values of 3.02–3.32 and Nb values of 9.82–22.2. In the (Zr+Nb+Ce+Y)–(K₂O+Na₂O)/CaO diagram, they plot inside the fractionated granite field and close to the A-type granite area (Figure 8b).

4.3.2. Trace and Rare Earth Elements

The trace element results are listed in Table 3. The results show that the total rare earth element (REE) contents of the rock are 109.70 × 10⁻⁶–179.39 × 10⁻⁶, with ∑light REE/∑heavy REE (∑LREE/∑HREE) = 2.19–2.77 and (La/Yb)N = 4.93–7.59 (average of 6.28). The chondrite-normalized REE pattern shows LREE enrichment and a flat HREE model, with strong negative Eu anomalies (δEu = 0.08–0.11) and slight Ce anomalies (δCe = 0.53–1.06) (Figure 9a). The trace element results show Rb-, Th-, U-, K-, and LREE-enrichment and Sr, P, and Ti depletion (Figure 9b).

4.4. Fluid Inclusion Results

4.4.1. Micrography

Various types of fluid inclusions have been observed in pegmatitic quartz under a microscope. Based on the phase association at room temperature (~25 °C), the fluid inclusions can be grouped into four types, including C-type, L-type, V-type, and S-type inclusions.

The L-type fluid inclusions comprise a vapor bubble and an aqueous phase with sizes of 5–20 μm. The vapor bubbles are 25%–30% (with minor bubbles up to ca. 50%) of the inclusion volume, and while heating, the inclusion homogenizes into the aqueous phase. The inclusions occur as round, elliptical, negative-crystal, elongated, or irregular shapes (Figure 10a).

The V-type fluid inclusions comprise a vapor bubble with or without a liquid phase, with vapor volumes up to 50%–100%, and while heating, the inclusions homogenize to the vapor phase. The sizes of the inclusions are approximately 10–20 μm in the long axes, with round, negative-crystal, or irregular shapes (Figure 10b,c).

The S-type fluid inclusions are mostly elliptical, negative crystals and are elongated in shape, with a vapor bubble, an aqueous phase, and one or two daughter phases. The daughter phases in the S-type inclusions include halite (Figure 10b,c), carbonate minerals, and some unknown minerals. Normally, the bubble phase takes 25%–40% of the inclusion volume, and while heating, the inclusion homogenizes into the aqueous phase. In total, the S-type fluid inclusions have sizes of 10–15 μm.

The C-type fluid inclusions contain vapor or vapor–liquid CO₂ bubbles together with the aqueous phase. They usually show negative crystals and polygon to elongated shapes. The CO₂ phase accounts about for 20%–50% of the inclusion volume (Figure 10d). During heating, the fluid inclusions homogenize to the aqueous phase. Some of the C-type fluid inclusions have one or two non-salt daughter phases.

4.4.2. Microthermometric Results

During cooling, C-type fluid inclusions usually form three phases: liquid CO₂, vapor CO₂, and aqueous phases. With increasing temperature, the CO₂ vapor bubble becomes increasingly larger and then homogenizes to the vapor phase at 22.8 °C–30.8 °C. The melting temperatures of solid CO₂ are approximately −59.2 °C–61.2 °C, with CO₂ clathrate melting temperatures of 9.4 °C–10.2 °C. While heating is maintained, the fluid inclusions finally homogenize to the aqueous phase at 356 °C–388 °C. Based on the CO₂ clathrate melting temperature, the calculated salinity of the fluid host in the C-type fluid inclusion is 0.00–1.23 wt.% NaCl eq., according to Hall et al. (1988) [61].

During heating, the L-type inclusions homogenize to the liquid phase at 312 °C–411 °C and have an ice-melting temperature range of −6.8 °C~−2.0 °C. The V-type fluid inclusions homogenize to the vapor phase at 321 °C–442 °C and have ice-melting temperatures of −6.8 °C–2.0 °C. Based on the ice-melting temperature and Roedder et al.’s (1984) [62] formula, the calculated salinities of the fluid host in the L- and V-type fluid inclusions are 3.4–10.2 and 3.1–9.5 wt.% NaCl eq., respectively.

During heating, the halite daughter phase in the S-type fluid inclusions dissolve first, and the vapor bubbles disappear after halite dissolution. The temperatures of halite dissolution are 227.7 °C–243.7 °C, and the fluid inclusions finally homogenize to the aqueous phase at 293 °C–310 °C. The calculated salinity values of the fluid host in the S-type fluid inclusions are 33.48–34.68 wt.% NaCl eq, according to Hall et al. (1988) [61]. The coexistence of V- and high salinity fluid inclusions and similar homogenization temperatures suggests a boiling fluid inclusion association. The higher homogenization temperatures for the V-type fluid inclusions may be a result of the inhomogeneous trapping.

4.4.3. Raman Results

The Raman analysis results show that the CO₂ phase of the C-type inclusions is mainly CO₂ and less CH₄ (Figure 11a). The vapor phase of the L- and V-type inclusions also contain CO₂, but the peaks at 1284 cm⁻¹ and 1386 cm⁻¹ are much weaker. The aqueous phase in all kinds of fluid inclusions is H₂O. The daughter mineral in the S-type fluid inclusions is halite and carbonate mineral, whereas the daughter phases in the C- and V-type fluid inclusions are carbonate minerals and albite (Figure 11b,c).

5. Discussion

5.1. Causative Magma and Mineralization Age

Aplite or aplite together with pegmatite has been widely reported in Sn mineralized granite, such as the Central and West African rare-metal granitic pegmatites [63], the Nong Sua Sn–W deposit of Thailand [64], and the Limu Sn–W district in southern China [65]. The transition between pegmatites and aplites indicate that they are part of the same differentiation process [63]. The aplite together with inward growth pegmatitic quartz and K-feldspar has now been widely accepted as vein dikes that record the melt–fluid transition [66]. In the Hongling area, cassiterite- and wolframite-bearing vein dikes occur as irregularly shaped veins, dikes, or rounded globes hosted in granite porphyry that intruded into granite (with zircon U-Pb age of 144.8–139.7 Ma, [33]). Except for pegmatitic quartz and K-feldspar, which host abundant fluid inclusions, vein dikes also contain aplite, which shows typical magmatic mineralogical and textural characteristics. The sharp contact of the aplite (Figure 3a,b) with the host granite porphyry and the irregular shapes, especially the rounded globes (Figure 3c) of the vein dikes in the host rock, imply an immiscible origin from the parent granitic magma; thus, the granite porphyry comprises the parent magma of W and Sn mineralization in the area.

The zircon U-Pb results of the vein dikes host granite porphyry show a Late Cretaceous intrusive age (133.3 ± 0.86 Ma, Figure 6b), which is similar, but hardly younger than the data (136.70 ± 0.85 Ma) reported by Li et al. (2016) [33] in the area. Approximately 136.70 ± 0.85 Ma to 133.3 ± 0.86 Ma may record the duration of the Sn–W mineralization age in the Hongling area, as the Sn–W precipitated from the hydrothermal fluid derived from late-stage water-rich melt.

Mesozoic, especially Cretaceous, is the most important metallogenic age for tungsten and tin mineralization in China [67]. The late Mesozoic Sn–W deposits occur mostly in southeastern China, with a few present in northeastern China, such as in the Huanggang Fe–Sn skarn [45], Dajing Sn–multimetal deposit [27], and Weilasituo Sn–polymetallic epithermal deposit [26] in the Great Xing’an Range. The eastern part of the CAOB is characterized by the widespread occurrence of late Mesozoic volcanic and intrusive rocks, mainly comprising I- and A-type granitoids [68,69]. There is still controversary about the causative magma for Sn–multimetal mineralization in northeastern China and the proposed opinions include A-type [68,70] and highly fractionated I-type granite, which is partly caused by the difficulties in distinguishing highly differentiated I-type granite and A-type granitoids [71,72]. Highly fractionated granite usually has high silica and alkalinity, is enriched in W, Sn, Li, F, and rare metals [73], and is genetically related to Sn and W mineralization. Fractional crystallization of initially Sn-rich felsic melts under reduced conditions can further tin enrichment and produces Sn-bearing granites [74]. Our whole-rock geochemical results show that the granite porphyry in the Hongling area has high SiO₂, Al₂O₃, and Na₂O+K₂O contents, and in the Zr+Nb+Ce+Y versus (K₂O+Na₂O)/CaO diagram (Figure 8b), they plot in the fractionated granite field. Magmatic evolution may have played an important role in the enrichment of Sn and W [75,76,77]. The lack of abundant water-bearing minerals, such as mica, biotite, and hornblende, and low LOI values imply dry magma. The Ga/Al value and the trace element (incl. REE) partition pattern (Figure 9a,b) show similarity with the A-type granite reported in northeastern China [68]. Wu et al. (2002) proposed that the formation of Cretaceous A-type granite in northeastern China is possibly related to extension following lithospheric delamination in eastern China associated with subduction of the Paleo-Pacific Plate. The variation of subduction direction of the Paleo-Pacific Ocean likely triggered a change in the stress regime at ca. 136 Ma and likely promoted the lithospheric delamination beneath the southern Great Xing’an range, resulting in intense magmatism; thus, the late Mesozoic Sn–W mineralization in eastern China, in both the south [78,79] and the north [33], were likely formed in an extensional tectonic setting, which is linked with the Paleo-Pacific Ocean subduction.

5.2. Fluid Exsolution from the Melt and Characteristics of the Primary Magmatic Fluid

The vein dikes hosted in the granite porphyry comprise hydrothermal quartz, K-feldspar, and aplite, implying the water-rich characteristics of the melt, and record the fluid exsolution process. Liquid/liquid immiscibility can better explain the genetic link between the vein dikes and the host granite porphyry. No fluorite has been found in the vein dikes, implying a low F content in the parent magma. Experimental results show that granitic melts with high F content could extract and enrich Sn and W in the melt [80,81], and the partition coefficient of tin between aqueous and silicate melt would increase significantly when the F content in the melt were below 1 wt.% [80]; therefore, the decrease in F and increase in Cl content is favorable to the partition of Sn and W into aqueous fluid [80,81]. High salinity fluid inclusion hosted in hydrothermal quartz indicates a higher Cl in the water-rich melt; thus, the immiscibility between low F, water-rich, and water-poor silicate melts may lead to Sn and W enrichment in the water-rich melt. The LA-ICP-QMS microanalysis of the pegmatite–leucogranite melt and fluid inclusions confirmed that tin and the associated metal would enrich the Cl⁻ rich brine during water-melt immiscibility [82]. The inward growth direction of pegmatitic quartz and K-feldspar in the aplite, together with the dominant fluid inclusions hosted in pegmatitic quartz imply fluid exsolution from the water-rich melt. The fine-grained texture of aplite and pegmatitic quartz-K-feldspar implies a chilled character. During the upwelling of the magma, the depressurization of the magma may have caused the oversaturation of water and CO₂ in the melt and rapid fluid exsolution, which may be responsible for the chilling character. The fluid inclusions hosted in pegmatitic quartz may record the primary fluid derived from this late-stage melt.

The fluid inclusion results show that the primary magmatic fluid had a high to medium temperature, low to medium salinity, and was CO₂-rich. The homogenization temperatures of the fluid inclusions range from 312 °C to 442 °C, which are much lower than those of the porphyry Cu system [83]. The low solidus felsic melt has been widely reported both from experimental results and nature samples [83,84,85,86]. High alkalinity (such as K and Na), which can be evidenced by the dominant K-feldspar in the vein dikes and the halite, the albite daughter phase in the fluid inclusions, and high volatiles in the melt, such as water and CO₂, may have largely decreased the solidus of the magma and led to the delay of fluid exsolution [87], as well as the much lower temperature of the exsolution fluid.

5.3. Sn–W Precipitation Mechanics and Implications for Sn–W Exploration in the Area

Disseminated cassiterite and wolframite mainly occur in aplite, which has also been reported in the Cornwall Sn deposit of the UK [88]. Cassiterite and wolframite can also be found in the quartz of the miarolitic cavity or the quartz–K-feldspar veins in the granite porphyry, implying a hydrothermal fluid product. Many different cassiterite precipitation mechanics have been reported, such as fluid colling [89,90] and fluid mixing [79,91]. The coexistence of V- and S-type fluid inclusions hosted in quartz and their similar homogenization temperatures suggest a fluid boiling process that occurs shortly after fluid exsolution. The bubble phase of the V- and C-type fluid inclusions hosted in pegmatitic quartz contains dominant CO₂ and shows a much lower salinity than the S- and L-type fluid inclusions, indicating that CO₂ escapes together with vapor during fluid boiling. Tin is a metal that exhibits both hard acid (as Sn⁴⁺) and borderline divalent (Sn²⁺) traits. In an oxide condition, the Sn and W are more likely complex, with a hard base, such as OH⁻, HCO₃⁻, or CO₃ ²⁻; thus, the boiling and escape of CO₂ from the fluid may have led to the enrichment of Sn and W in the fluid and the decreasing of CO₃²⁻ in the fluid, which may be responsible for W and Sn precipitation. The W and Sn mineralization are mostly hosted in causative intrusions or peripheral wall rocks. Based on the intrusive sequence of the granitic rocks in the area, the spatial relationships between the vein dikes and host granite porphyry, together with our understanding of the mechanics of Sn–W precipitation, the deposit model is proposed as Figure 12. Well-developed vein dikes can be used as indicators for Sn–W exploration both in the study area and in the other areas of the world.

6. Conclusions

• The Sn–W mineralization in the area is related to late Mesozoic granite porphyry, which shows highly fractionated A-type granite affinity. The U–Pb age (133.3 ± 0.86 Ma) of the granite porphyry may represent the Sn–W mineralization age in the area.

• Sn–W mineralization mainly occurs in vein dikes hosted in the granite porphyry. The rapid upwelling of the parent magma and depressurization led to melt immiscibility between water-rich and water-poor melts. The melt immiscibility may have played an important role in the Sn–W enrichment in the late-stage water-rich melt.

• The vein dikes record the transition from melt to fluid. The hydrothermal fluid exsolved from the water-rich melt is of high to medium temperature and low salinity and is CO₂-rich. Boiling occurs shortly after fluid exsolution and leads to Sn and W enrichment and the escape of CO₂ from the fluid, which may play an important role in W–Sn precipitation.

• The W and Sn mineralization is mostly hosted in highly fractionated granite intrusion or peripheral wall rocks. Well-developed vein dikes in the parent intrusion can be used as indicators for Sn–W exploration both in the Hongling area and in other W–Sn areas of the world.

Author Contributions

Data curation, X.S.; funding acquisition, Y.X.; investigation, X.S., C.Y., J.X., Q.Y., J.C. and A.L.; project administration, Y.X.; resources, Y.X.; supervision, Y.X.; writing—original draft, X.S.; writing—review and editing, Y.X. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Acknowledgments

Many thanks to Chao Yu for his help with the zircon U–Pb dating analysis at CAGS, and to the geology team from Chifeng Hongling Nonferrous Mining Co., Ltd. for their help with the fieldwork.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. (a) Tectonic location of the Central Asian Orogenic; modified after Jian et al. (2012) [38] and Sengör et al. (1993) [39]. (b) Simplified geological map of the Great Xing’an Range; modified after Qi et al. (2005) [40] and Zhai et al. (2014b) [41].

Figure 2. (a) Geological map of the Hongling area; modified after Wan et al. (2014) [49]. (b) Geological map of the Hongling deposit; modified after the exploration report of Hongling deposit [50].

Figure 3. Field outcrop photographs of granite porphyry and vein dikes comprising of pegmatite K-feldspar–quartz and aplite: (a) vein dike in granite porphyry; (b) cassiterite-bearing aplite (Sn-aplite) in the granite porphyry; (c) pegmatitic K-feldspar–quartz together with aplite; (d) quartz miarolitic cavity in granite porphyry; Qt—quartz; Kfs—K-feldspar; Sn-aplite—Sn mineralized aplite.

Figure 4. Photomicrographs of granite porphyry and aplite (crossed polarized light): (a,b) porphyritic texture of granite porphyry with quartz and K-feldspar as phenocryst; (c) allanite in granite porphyry; (d) fine-grained texture of aplite; Qt—quartz; Kfs—K-feldspar; Bt—biotite; Aln—allanite.

Figure 5. BSE image and EDS spectrum of cassiterite and wolframite: (a) BSE image of cassiterite in aplite; (b) EDS spectrum of cassiterite; (c) BSE image of wolframite in quartz; (d) EDS spectrum of wolframite; Qt—quartz; Kfs—K-feldspar; Mlc—malachite; Ilm—ilmenite; Wol—wolframite; Cst—cassiterite.

Figure 6. Zircon CL images and LA–ICP–MS U–Pb concordia curve of granite porphyry in the Hongling area: (a) CL images of zircon grains from granite porphyry; (b) LA–ICP–MS U–Pb concordia curve of granite porphyry (the blue circle is calculated result); red circles denote U–Pb analysis spots.

Figure 7. TAS igneous rock classification diagram ((a), based on Maitre et al., 1989 [57]) and K2O versus SiO2 diagram; ((b), based on Peccerillo and Taylor, 1976 [58]) for the granite porphyry at Hongling; Filled blue circles show the Hongling granite porphyry.

Figure 8. Plots of 10,000 Ga/Al versus Nb (a) and Zr+Nb+Ce+Y versus (Ka2O+Na2O)/CaO (b) for granite porphyry based on Whalen et al. (1987) [59]; A—A-type granites; FG—fractionated granites; OGT—unfractionated M-, I-, and S-type granites; Filled blue circles show the Hongling granite porphyry.

Figure 9. Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element patterns (b) for granite porphyry from the Hongling area (normalizing values from Sun and McDonough, 1989 [60]).

Figure 10. Microphotographs of fluid inclusions in quartz: (a) L-type inclusions; (b,c) V-type, S-type, and L-type inclusions; (d) C-type inclusions; LCO2—liquid CO2; VCO2—vapor CO2; LH2O—liquid H2O; VH2O—vapor H2O; H—halite.

Figure 11. Laser Raman spectra of fluid inclusions: (a) vapor CO2 and CH4 in C-type fluid inclusion; albite (b) and natrite (c) daughter mineral in S-type fluid inclusions. + Location of analytical spot.

Figure 12. The genetic model showing the relationship between Sn–W mineralization and the parent magmatic rock. Age data of the granite from Li et al., 2016 [33].

Table 1

LA–ICP–MS zircon U–Pb data for the Hongling granite porphyry.

Spot	U (10−6)	Th (10−6)	Pb (10−6)	207Pb/235U	206Pb/238U	208Pb/232Th	207Pb/235U (Ma)	206Pb/238U (Ma)	208Pb/232Th (Ma)
5086-3	449	1187.5	12.13	0.1393	0.02088	0.0069	132.5 ± 5.5	133.2 ± 2.5	138.9 ± 3.6
5086-13	541.6	230.2	14.42	0.1414	0.02079	0.00676	133.9 ± 5	132.6 ± 2	136.2 ± 3.5
5086-14	447.3	231.8	13.93	0.1365	0.02056	0.00665	129.4 ± 5.9	131.2 ± 2.6	133.9 ± 4.1
5086-15	738	400	25.7	0.1433	0.02106	0.00692	135.7 ± 4.5	134.3 ± 2.2	139.3 ± 2.9
5086-21	355	210	13.49	0.1472	0.02099	0.007	138.8 ± 6.4	133.9 ± 2.6	141 ± 4
5086-22	418	143.8	9.33	0.1465	0.02079	0.00694	138.9 ± 5.4	132.6 ± 2.5	139.8 ± 5
5086-23	194.8	53.2	3.43	0.1484	0.02112	0.00717	139.5 ± 8	134.7 ± 3.1	144.4 ± 6.7
5086-24	312	161.4	10.53	0.1439	0.02143	0.0069	135.7 ± 7.1	136.7 ± 2.7	139 ± 3.9

Table 2

Major element compositions (wt.%) of granite porphyry in the Hongling area.

Sample Number	HL-5087-2	HL-5087-3	HL-5087-4
Rock Type	Granite Porphyry
SiO2	77.52	76.99	76.98
Al2O3	12.31	12.54	12.56
TFe2O3 a	0.98	1.16	1.08
MgO	0.11	0.11	0.10
CaO	0.37	0.35	0.38
Na2O	3.5	3.5	3.75
K2O	4.45	4.66	4.46
MnO	0.02	0.02	0.02
TiO2	0.07	0.07	0.07
P2O5	0.01	0.01	0.01
LOI	0.61	0.51	0.51
Total	99.97	99.97	99.96
FeO b	0.82	0.75	0.68
Fe2O3 b	0.17	0.46	0.44
Na2O+K2O	7.95	8.16	8.21
(K2O+Na2O)/CaO	21.37	23.38	21.66
A/NK	1.55	1.54	1.53
A/CNK	1.48	1.47	1.46
Mg# c	0.09	0.09	0.10
Fe2O3/FeO	0.21	0.61	0.65

^a Total Fe content; ^b Calculated; ^c Mg^# = 100 × Mg/(Mg + Fe²⁺) (in mole).

Table 3

Trace element (incl. REE) compositions of granite porphyry in the Hongling area.

Sample Number	HL-5087-2	HL-5087-3	HL-5087-4
Rock Type	Granite Porphyry
Trace and Rare Earth Elements (ppm)
Li	11.7	8.94	7.8
Be	8.35	7.65	9.34
Sc	0.66	0.887	0.93
V	4.73	9.26	3.46
Cr	1.04	2.04	0.92
Co	0.23	0.23	0.26
Ni	0.58	1.04	0.73
Cu	1.46	1.82	1.38
Zn	35.2	60.5	39.9
Ga	19.7	21.3	22.1
Rb	175	187	186
Sr	17.2	18.3	17.1
Y	23.9	29.9	37.4
Mo	0.67	1.03	0.95
Cd	0.18	0.38	0.19
In	0.02	0.08	0.32
Sb	0.05	0.05	0.54
Cs	3.53	3.98	5.56
Ba	84.8	86	82.2
La	23.7	24.1	45.1
Ce	37.7	52.2	50.5
Pr	6.07	5.62	11.4
Nd	22.5	20.8	41.7
Sm	4.8	4.31	7.79
Eu	0.15	0.14	0.19
Gd	3.79	3.74	6.17
Tb	0.72	0.76	1.09
Dy	3.76	4.17	5.6
Ho	0.77	0.92	1.13
Er	2.24	2.73	3.26
Tm	0.42	0.53	0.63
Yb	2.69	3.51	4.26
Lu	0.38	0.46	0.58
W	1.21	1.42	0.93
Re	0.01	0.01	0.01
Tl	0.95	0.7	0.7
Pb	16.9	18.5	20.1
Bi	0.05	0.12	0.32
Th	23.4	28.4	32.3
U	4.31	4.18	4.4
Nb	9.82	14.3	22.2
Ta	0.99	1.74	2.85
Zr	79.1	105	115
Hf	3.33	4.92	5.94
10,000 Ga/Al	3.02	3.21	3.32
∑REE	109.7	123.98	179.39
∑LREE/∑HREE	2.42	2.19	2.77
δEu	0.11	0.1	0.08
δCe	0.75	1.06	0.53
(La/Yb)N	6.32	4.93	7.59
(La/Lu)N	6.67	5.63	8.38
(La/Sm)N	3.19	3.61	3.74
(Gd/Yb)N	1.17	0.88	1.2
(Gd/Lu)N	1.23	1.01	1.32

Normalizing values from Sun and McDonough (1989) [60].

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