Genesis of the Longmendian Ag–Pb–Zn Deposit in Henan (Central China): Constraints from Fluid Inclusions and H–C–O–S–Pb Isotopes

School of Resources and Safety Engineering, Central South University, Changsha 410083, China Key Laboratory of Metallogenic Prediction of Non-Ferrous Metals and Geological Environment Monitoring (Central South University), Ministry of Education, Changsha 410083, China School of Geosciences and Info-Physics, Central South University, Changsha 410083, China Faculty of Science, Universiti Brunei Darussalam, Gadong BE1410, Brunei Darussalam


Introduction
The Longmendian Ag-Pb-Zn deposit is located in the Shagou (Xiayu) orefield of the Xiong'er region (western Henan Province) in central China. Tectonically, the deposit is located at the southern margin of the North China Craton (NCC), bordered with the eastern Qinling-Dabie Orogen. The NCC is one of the world's oldest craton, with a history dating back to >3.8 Ga. It is also an important mineral province, hosting a variety of ore deposits [1,2]. The Mesozoic granites and hydrothermal mineralization systems in the Xiong'er region (southern NCC margin) have been widely studied [3][4][5][6][7][8][9][10][11]. The regional Au and Mo mineralization was coeval with the first Mesozoic magmatic phase there (late Jurassic-early Cretaceous, ca. 158-136 Ma), which was related to the Paleo-Pacific plate subduction beneath Eastern China [1]. Meanwhile, Ag-Pb-Zn mineralization was mostly coeval with the second magmatic phase (early Cretaceous, ca. 134-108 Ma) [12][13][14], which is related to lithospheric thinning-asthenospheric upwelling caused by the subductiondirection change of the Paleo-Pacific Plate [1,2]. In addition, Neoproterozoic Mo mineralization was also reported locally, as represented by the Zhaiao and Longmendian deposits [15,16].
The Longmendian Mo mineralization is spatially related to the migmatites but distal from intrusions or faults [17]. Four types of fluid inclusions (FIs) have been recognized in the Mo-bearing quartz and calcite grains, i.e., CO 2 ± CH 4 -bearing FIs (PC-type), CO 2 -H 2 O-bearing FIs (C-type), daughter mineral-bearing FIs (S-type) and H 2 O-NaCl FIs (W-type). The ore fluid temperature and pressure were estimated to be around 225 to 390°C and 114 to 265 MPa, respectively [17]. Geological and FI features indicate that the Mo mineralization, which was molybdenite Re-Os dated to be 1875 Ma, was caused by the high temperature migmatitic fluid/melt [15][16][17]. The Paleoproterozoic mineralization event was interpreted to have linked to the collision between the western and eastern NCC along the Trans North China Orogen at 1.85 Ga [15,16].
Meanwhile, Longmendian Ag-Pb-Zn mineralization is of a much larger scale than the Mo mineralization. The relationship between the Ag-Pb-Zn and Mo mineralization, as well as their respective ore material/fluid source and metallogenesis, remains unclear. In this study, therefore, we addressed these issues via FI microthermometric and H-O-C-S-Pb isotope analyses.

Regional Geology
The Xiong'er terrane is an important part of the Kunlun-Qinling-Dabie Orogen. The terrane, encompassing an area of 80 km long (E-W) and 15 km wide (N-S), is bound by the EW-trending Machaoying fault in the south and the Luoning fault in the north (Figure 1). Prolonged and multiphase intensive orogenic events have controlled the formation of the structural framework, magmatism, and sedimentation of the region, with consequently influenced the Xiong'er metallogeny. The regional tectonic events include the formation of the continental nucleus and then the cratonic basement in the Paleoproterozoic, Mesoproterozoic rifting, Neoproterozoic-to-Paleozoic sedimentation, and Mesozoic tectonic reactivation. The study area is located in the western Xiong'er ( Figure 1). Local stratigraphy is characterized by crystalline basement with metavolcanic-sedimentary cover. The crystalline basement is primarily composed of the Neoarchean Taihua Group (Gp) metamorphosed terrane, while the metavolcanic-sedimentary cover is composed of Mesoproterozoic Xiong'er Group (Gp.) low-grade metavolcanic rocks. The Taihua Group in the study area strikes ENE and is dominated by biotite/amphibole plagioclase interlayered with (plagioclase) amphibolite, granulite, and leptite. Regionally, the Xiong'er Gp. volcanic rocks overlie the Taihua Group along an unconformity, which was formed by the 1.85 Ga collision between the eastern and western NCC. The rock types of the Xiong'er Gp. consist of mainly (pyroxene) andesite porphyry with local siltstonemudstone/shale interbeds.
Local structure is controlled by a series of NE-trending detachment faults along/around a metamorphic core complex, the most important of which lies on the unconformity between the Xiong'er Group and Taihua Group ( Figure 2). The main detachment fault and the NE-trending shears are the major Ag-Pb-Zn ore-hosting structures [20]. Local folds include mainly the EW-trending recumbent-overturned folds and the NS-trending anticlines and synclines. The Machaoying fault is the major regional fault and is located in the southern Xiong'er. Faults/fractures are well developed in the study area and are mainly NE-trending and minor NW-trending and NS-trending. NE-trending structures are the main ore host.
In the study area, the widely exposed magmatic rocks were mainly formed in the Archean, Mesoproterozoic, and Mesozoic: Archean magmatic rocks include mafic to felsic volcanic rocks and late-stage ultramafic intrusions (e.g., dykes). The rocks are metamorphosed into gneiss, plagioclase amphibolite, and migmatite; Mesoproterozoic magmatic rocks comprise extensive intermediate-mafic volcanic rocks (ca. 1780-1320 Ma [21]) and the Xiong'er diorite in Songxian County (zircon U-Pb age: 1440 Ma [22]); Mesozoic magmatic rocks are widespread and multiphase. Their formation was primarily restricted largely to the Yanshanian (Jurassic-Cretaceous), although Indosinian (Triassic) one is also reported [23]. The Yanshanian magmatic rocks are mostly granite batholiths and minor granite stocks and medium-felsic dykes. Furthermore, numerous outcrops of small-sized explosive-breccias are observed and generally regarded to be closely ore-related [23]. The granite batholiths are distributed mainly in the eastern Xiong'er, e.g., the Huashan, Jinshanmiao, Wuzhangshan, and Heyu intrusions. Previous works indicated that Yanshanian magmatism may have facilitated extensive migration of metallogenic materials along/across the sequences and that the postmagmatic hydrothermal fluids constitute an important component of the ore fluids [24].

Deposit Geology
The Longmendian deposit is located to the west of the Xiong'er metamorphic core complex ( Figure 3). The exposed metamorphosed sequences comprise primarily the Neoarchean Taihua Group and the Mesoproterozoic Xiong'er Group. The Taihua Group is distributed to the north of Longmendian and comprises dominantly biotite plagioclase gneiss and migmatized amphibole-plagioclase gneiss. The sequence is underlain by numerous amphibolite bodies. The Xiong'er Group is distributed to the southeast of Longmendian and comprises mainly andesite, particularly pyrite-bearing amygdaloidal and porphyritic ones. Andesitic rocks at Longmendian are generally altered, with the plagioclase strongly sericitized and the groundmass chlorite-and epidote-altered.
At Longmendian, detachment faults and secondary NE faults are the dominating structures, whereas folds are not well developed. The detachment faults strike NE with dip angles of 20-35°. The unconformity interface between the Taihua Group and Xiong'er Group is overprinted by slickensides of the detachment faults. The NE-trending (NW-dipping) faults, tectonic breccia, and mylonites are commonly altered. Magmatic rocks include mainly diabase and few ultramafic rocks. The diabase intruded into the Taihua Gp. sequence in the form of near-vertical dykes. The major minerals include pyroxene and plagioclase. The plagioclase phenocrysts (5-10 vol%) are commonly sericite- 2 Geofluids and epidote-altered, while the pyroxenes are generally altered to actinolite. Accessory minerals include ilmenite, magnetite, apatite, zircon, and ilmenite. Ultramafic dykes (100 m long and 10 m wide) are locally exposed to the north of Longmendian. The orebodies are mainly of quartz vein and altered-rock types ( Figure 4). Metallic minerals include primarily galena and sphalerite, followed by pyrite, chalcopyrite, Ag-bearing tetrahedrite, pyrargyrite, argyrite, polybasite, native Ag, tetrahedrite, and bornite. Nonmetallic minerals include primarily quartz, calcite, siderite, ankerite, fuchsite, and sericite, followed by feldspar, chlorite, amphibole, biotite, and apatite ( Figure 4). Ore structures include mainly massive, vein/stockwork, banded, brecciated, and disseminated, while ore textures include (hyp)idiomorphic, xenomorphic granular, skeletal, rimmed, and zoned ( Figure 4). Alteration types include silicic, sericite, chlorite, carbonate, and pyrite. Ore proximal wallrock alterations are dominated by silicic and sericite, and locally weak potassic feldspar, while the ore distal ones include mainly chlorite and carbonate.
Three alteration zones were identified according to the alteration minerals and their assemblage ( Figure 5): (1) Chlorite-sericite zone: this zone is developed on the periphery of the fractured zone. Altered feldspars, hydrothermal quartz, carbonate veinlets, and few galena veinlets are observed in this zone, and alteration intensity is commonly low.
(2) Quartz-calcite-dolomite-sericite zone: this zone is developed within the fracture zone and is characterized by varying widths and ore wallrock interlayers. This stage is mostly associated with Ag mineralization.
(3) Silicic zone: this zone is located at the center of the fractured zone and is the mineralization center and the most altered part. The altered rocks are primarily composed of quartz and sulfides. Concentrations of Si and K tend to increase while that of Na decreases from the fresh wallrock to the silicic core, which corresponds to increasing quartz and sericite but decreasing sodic feldspar contents, respectively, by wallrock alteration.

Geofluids
and pyrite show cataclastic texture and are associated with metallic minerals from the late Ag ore period.
(2) Quartz-polymetallic sulfide stage: this stage represents the main Ag-Pb-Zn ore stage and has a wide variety of metallic minerals including galena, sphalerite, pyrite, chalcopyrite, (Ag-bearing)-tetrahedrite, sulfosalts, and native Ag. Galena, sphalerite, and Ag sulfides are associated with sulfosalts and occur within the fracture-infilling veins.
(  Figure 2: Geologic map of the Shagou (Xiayu) Ag-Pb-Zn ore field (modified from [18]). 4 Geofluids gas chromatograph (Varian) and a DX-120 ion chromatograph (Dionen). The analytical error was less than 5%. was used as an oxidizing agent for the sulfide sample to produce SO 2 , which was subsequently frozen and collected for the S isotope analysis with a MAT-251 mass spectrometer. The international standard V CDT was used in this regard, and the accuracy is ±2‰. The Pb isotope analyses were conducted with an IsoProbe-T thermal ionization mass spectrometer. Lead was separated and purified using the conventional cation-exchange technique (with diluted HBr as the eluant). The 208 Pb/ 206 Pb, 207 Pb/ 206 Pb, and 204 Pb/ 206 Pb ratios of the NBS981 Pb standard were 2:1681 ± 0:0008 (2σ), 0:91464 ± 0:00033 (2σ), and 0:059042 ± 0:000037 (2σ), respectively. The analyses were conducted at the BRIUG Analytical Laboratory.        Table 2. The vapor phase comprises primarily H 2 O and minor CO 2 , H 2 , CO, N 2 , and CH 4 . The liquid phase contains Ca 2+ , Na + , K + , SO 4 2− , Cl − , and F − . For the cations, the Ca 2+ content is the highest (followed by Na + ), whereas the Mg + and K + contents are the lowest. For the anions, SO 4 2− content is the highest (followed by Cl − ), and the F − content is the lowest. The ore fluid belongs to a Cl-(SO 4 2− )-Na-K-(Mg) system.    6. Discussion 6.1. Sources of Ore-Forming Material. The δ 18 O H2O values of Stage 1 ore fluids (5.8-7.6‰, avg. 6.7‰) fall inside the range of magmatic water (5.5-9.5‰) defined by Sheppard [27], but different from those of the Taihua Group and pegmatite (δ 18 O H2O = 5:8-6.8‰; [28]). As shown in the δ 18 O H2O vs. δD plot (Figure 9), all Stage 1 data points fall inside/close to the magmatic water field, suggesting a magmatic fluid origin. The Stage 2 δ 18 O Qtz (7.4-11.2‰) and δ 18 O H2O (-4.4 to -0.6‰, avg. -2.7‰) are lower than their Stage 1 counterparts, and the data points fall between the magmatic water and meteoric water fields (Figure 9), indicating probable meteoric water incursion. Stage 3 data points fall close to the meteoric water line, suggesting that the hydrothermal fluid was dominantly meteoric.

H-O Isotope
Most data points from the Longmendian, Shagou, Tieluping, and Haopingou deposits fall within the magmatic water field (Figure 10), pointing to a magmatic fluid source. Furthermore, the Longmendian data points also show well-defined linear trend toward the marine carbonate field (Figure 10),    12 Geofluids suggesting a marine carbonate input for the ore fluids.
Carbonate rocks are abundant in the Longtanggou Formation (Taihua Group) and may represent a potential fluid source for the Ag-Pb-Zn mineralization.
The δ 34 S values of hydrothermal minerals depend not only on the source δ 34 S values but also on the physicochemical conditions of the S-bearing fluid migration and precipitation. Ohmoto [35] proposed that the hydrothermal mineral   He further hypothesized that the fluid δ 34 S value is characterized by a relatively simple mineral assemblage and (with the absence of sulfates) should be similar to that of total sulfide (i.e., δ 34 S ∑S ≈ δ 34 S sulfide ). No sulfate minerals were found in the Longmendian deposit, and the mineral composition was relatively simple. Therefore, the sulfide δ 34 S could approximate the δ 34 S ∑S of the hydrothermal system. The δ 34 S values of the Longmendian deposit (−1.42 to 2.35‰) are similar to those from magmatic-hydrothermal systems (0 ± 5‰; [36]) and marginally overlap with the Taihua Gp. metamorphic rocks (Figure 11). This indicates that the sulfur was likely derived from magmatic-hydrothermal fluids and Taihua Gp. metamorphic rocks.
Polymetallic sulfide (pyrite, chalcopyrite, and galena) from the Longmendian deposit yields a single-stage model age of 340-566 Ma (Table 6), which is inconsistent with the Neoarchean Taihua Group metamorphosed terrane (>2.2 Ga) and the regional Ag-Pb-Zn vein-type mineralization (145-133 Ma; see Section 6.3), indicating the presence of excess radiogenic Pb in the fluid system, due to either the decay of U and Th or fluid mixing [37][38][39]. In a 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb diagram (Figure 12), the samples plot display a linear correlation. Assuming that the linear distribution of Pb values represents a second isochron (i.e., metallogenesis occurred in multiple phases), we obtain a two-stage model age of 4207-4209 Ma,  14 Geofluids which does not have geological significance (e.g., >3. 8 Ma), indicating that the linear relationship records mixing rather than a second isochron. Therefore, we speculate that the oreforming fluid system at Longmendian was an open system, containing a mixture of external Pb. For a two Pb-source end member mixing system, Pb isotopic data should fall on a line between the two end members [40]. Figure 12 indicates a clear linear relationship for the ore Pb data. Therefore, the ore Pb was likely derived from two end members, one with low radiogenic Pb and the other with high radiogenic Pb contents [40]. On 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb and 208 Pb/ 204 Pb vs. 206 Pb/ 204 Pb diagrams (Figure 12), the samples plot close to the composition of the metamorphic strata of the Taihua Group and Xiong'er Group, indicating that Pb in the Longmendian deposit was likely derived from the metamorphic strata of the Taihua Group and Xiong'er Group during fluid-rock reaction [38,39]. In addition, fluids with more radiogenic Pb were probably derived from concealed granitoids. The ore-forming material of Longmendian deposit mainly derives from magma, and the evidence is as follows: (1) the H-O-C-S-Pb isotope shows that the ore-forming fluid originates from magma. Furthermore, this Na + -Ca 2+ -(K + )-SO 4 2− -Cl − dominated fluids derived from magmatic system also reported in the Jiawula Ag-Pb-Zn deposit [42]. (2) No granite outcrop was found in Longmendian area; only the basic dyke of Paleozoic was found (Figure 3), but the geophysical data also showed that there were concealed intrusions in this area [33]. (3) The wallrock, ore-controlling structure, orebody characteristics, ore minerals, and gangue mineral of the Longmendian deposit were similar to those of the spatially adjacent Haopinggou deposit (Table 7), in which the ore-forming fluid originates from magma [33].
6.2. Ore Fluid Evolution and Ore Precipitation Mechanism 6.2.1. Nature and Evolution of Ore-Forming Fluids. In this study, the ore fluids are determined to be of medium-low temperature and belong to a medium-low salinity H 2 O-NaCl system that contains Na + -Ca 2+ -SO 4 2− -Cl − . From Stage 1 to Stage 3, significant changes occur in the FI types and the ore fluid temperature and salinity (Figures 8 and 13), as described in detail as follows: (1) Stage 1. Magmatic intrusion and its subsequent fractionation may have released magmatic fluid to form hightemperature (198-332°C) Stage 1 ore fluids. The salinityhomogenization temperature plot ( Figure 13) indicates that the salinity of Stage 1 fluids did not change significantly (4.0-13.3 wt% NaCl eqv ), and the major temperature drop could have attributed to cooling.
(2) Stage 2. The main-stage mineralization may have contributed by the gradual mixing of magmatic water and meteoric water. The ore fluid temperature and salinity are 132-260°C and 1.1-13.1 wt% NaCl eqv , respectively. The salinityhomogenization temperature plot ( Figure 13) shows bimodal fluid salinity, i.e., high salinity (>6 wt% NaCl eqv ) and low salinity (<5 wt% NaCl eqv ). The coexistence of several FI types (with similar homogenization temperature) further suggests that fluid boiling had taken place (Figures 7(e) and 7(f)).
(3) Stage 3. Fluid inclusions in this stage are dominated by W-type. The further temperature and salinity drop (cf. Stage 1 and 2 FIs) in Stage 3 which could be attributed to the continuous incursion of low-temperature and low-salinity meteoric water (Figures 9 and 13).
Generally, vein-type ore precipitation in hydrothermal deposits can by triggered by fluid boiling, mixing, and/or simply cooling [55][56][57][58]. The physicochemical changes on the fluids by boiling would affect Ag sulfide saturation via H 2 S degassing and/or pH increase [48]. pH increases quite drastically during boiling, especially in Stage 2 because of degassing of CO 2 and to a lesser extent H 2 S. Conductive cooling leads to pH decrease, plus minor effect on aqueous species activities [48]. The fluid cooling and pH increase caused by mixing with meteoric water (seawater or other high-pH fluids) may have led to Zn deposition because of (i) change in the dominating Zn complex (Zn-Cl complexes in high-temperature acidic fluids and Zn(HS) 3 and ZnS(HS) in low-temperature and alkaline fluids), and (ii) Zn solubility drop because of pH increase. Previous experimental studies have shown that temperature drop can effectively dissociate zinc-and lead-chloride complexes (e.g., [59]). As discussed above, fluid inclusion and H-C-O isotope evidence points to fluid boiling, mixing, and cooling at Stage 2 ( Figures 8  and 13), which likely formed the quartz-polymetallic sulfide ore veins.  [15], the existence of molybdenite in the Pb-Zn-Ag-quartz veins has not been confirmed, and thus, the Paleoproterozoic age may reflect an earlier mineralization phase before the Ag-Pb-Zn vein-type mineralization. The ore deposit geology (wallrock, ore-controlling structure) and ore/gangue mineral assemblages of the Longmendian deposit resemble those of the nearby Haopinggou, Shagou, and Tieluping deposits (Table 7), and hence, these broadly coeval (145-133 Ma) deposits may belong to the same Ag-Pb-Zn mineral system. Vein-type Ag-Pb-Zn mineralization at Shagou, Haopinggou, Tieluping, and Longmendian can be attributed to crustal extension-related ( Figure 14;   (157-127 Ma; [24,60,61]) contain abundant mafic microgranular enclaves (MMEs), which are interpreted to represent mixing of mafic and felsic magmas [62,63]. In addition, some of the MME-bearing plutons are intruded by slightly younger mafic dykes [7,[62][63][64]. (2) The presence of Early Cretaceous Xiong'er, Xiaoshan, and Xiaoqinling metamorphic core complexes (Figure 1; [65,66]) demonstrates that large parts of the southern NCC were dominated by extensional tectonics. This extensional event may have linked to lithospheric thinning caused by the west-dipping Paleo-Pacific subduction, which led to partial decratonization of the NCC [7,8,24].
The mineralization was strictly controlled by the NE-NNE-trending fault zones. The Ag-Pb-Zn ore-forming temperature (132-260°C) is low to medium, while the ore fluids progressed from dominantly magmatic-sourced to magmatic-meteoric mixed. The S and Pb isotope compositions indicate that the ore-forming materials were primarily originated from the magma with input from the metamorphosed sequences. Geological and geochemical features suggest that the Longmendian deposit likely belongs to medium-to-low temperature hydrothermal-type related to Cretaceous granitic magmatism (Figure 14).

Conclusions
(1) The H-C-O isotope evidence indicates that the ore fluids were derived from magmatic water with increasing meteoric water input toward the later ore stages. The S-Pb isotope evidence indicates that the ore-forming materials were mainly originated from the granitic magma and the Taihua Group wallrocks (2) Ore-forming fluids were largely of medium-low temperature and medium-low salinity, belonging to a low-density H 2 O-NaCl system that contains Na + -Ca 2+ -SO 4 2− -Cl − . Fluid boiling and cooling, and mixing with meteoric water, may have contributed to the ore precipitation Mineralization at Longmendian shares similar geological characteristics (in lithology, structure, ore/gangue assemblage, and ore fluid temperature and salinity) with typical low-sulfidation to intermediate-sulfidation (LS/IS) epithermal deposits.

Data Availability
The data used to support the findings of this study are included within the article.