Mineralogy of Gold, Tellurides and Sulfides from Lianzigou Gold Deposits in the Xiaoqinling Region, Central China: Implications for Ore-Forming Conditions and Processes

Abstract

The Lianzigou deposit, which has an Au–Te paragenetic association, is hosted in plagioclase gneiss of the Qincanggou Formation in the Taihua Group in the Xiaoqinling region, central China. This quartz vein-type Au deposit comprises native Au and a variety of tellurides. The latter include calaverite (AuTe₂), krennerite (Au₃AgTe₈), petzite (Au₃AgTe₂), hessite (Ag₂Te), melonite (NiTe₂), and altaite (PbTe). Four stages have been recognized in this deposit: stage I consists of K-feldspar and quartz; stage II is of milky quartz veins accompanied by coarse-grained disseminated and lumps of pyrite with weak Au mineralization; stage III is composed mainly of Au, tellurides, and sulfides; and stage IV is characterized by abundant carbonate and quartz. Based on mineral assemblage and thermodynamic data, we estimated the physicochemical conditions of the main metallogenic stages. Based on thermodynamic modelling, the physicochemical conditions of Au–Ag–Te mineral associations were estimated. The Au–Ag–Te minerals from stage III formed mainly under conditions of logƒO₂ = −43.15 to −33.31, logƒH₂S = ~−9.29, pH < 7, logfTe₂ = −10.6 to −9.8 and logαAu⁺/αAg⁺ = −7.2 to −6.5. In contrast, the physicochemical conditions of stage II were higher, specifically pH (8.3–8.5) and logƒO₂ (−34.90−31.96). In the ore-forming fluids of the Lianzigou deposit, the dominant Au species was Au(HS)₂⁻ while the dominant Te species were HTe⁻(aq) and Te₂²⁻(aq). Moreover, the Au–Ag–Te metal associations in the Lianzigou Au deposit were derived from mantle materials related to lithospheric thinning of the eastern North China craton in the Early Cretaceous under an extensional tectonic system.

1. Introduction

The Xiaoqinling region, as the second largest Au metallogenic province in China, hosts more than 1200 auriferous quartz veins with over 800 tons (t) of proven Au reserve [1,2]. The Au deposits from the Xiaoqinling region have distinct characteristic: native Au coexists with tellurides of Au, Ag, Pb and Ni, such as Dahu, Jinqu, Yangzhaiyu, Chen’er and Linghu deposits [3,4,5,6,7]. The occurrence of numerous tellurides of Au and Ag was related to magmatic–hydrothermal fluids [8,9,10,11].

The Lianzigou deposit hosted in the plagioclase gneiss of the Taihua Group is one of the typical quartz vein Au−Te deposits in the Xiaoqinling region, where ore minerals include multiple sulfides with several tellurides associated with native Au [12]. Previous studies of this deposit have focused mostly on its mineralogy, geochemistry, stable isotope geochemistry (S, H, O), and fluid inclusions [12,13,14,15,16]. However, issues related to its phase relationships and physicochemical conditions have been less explored.

Tellurides and related sulfides can effectively reflect a mineral deposit’s physicochemical variations during its formation, particularly temperature and fugacity of O, S, and Te [17,18]. Studying them is of great importance for Au mineralization due to the close genetic link between telluride and native Au [18]. Thus, an in-depth study of gold–telluride–sulfide association in the Lianzigou deposit will not only provide critical information governing ore-forming processes and physicochemical conditions, but will also enhance our understanding of plausible sources and transport forms of Au and Te.

Here, we discuss in detail the gold–telluride–sulfide association in the Lianzigou deposit. Based on mineral assemblages and thermodynamic data, we precisely identified the variation of physicochemical conditions during the deposit’s metallogenic process, providing insights into the reasons for the precipitation of native Au and tellurides. This study also boosts our understanding of the relationship between the genesis of Au–Te deposits and tectonics in the Xiaoqinling region.

2. Regional Geology

The Xiaoqinling region, spanning the Henan and Shanxi provinces, is located in the southern margin of the North China craton (Figure 1a,b). As a part of the WNW-trending Qinling–Dabie orogen, it is bounded by the Taiyao fault to the north and the Xiaohe fault to the south (Figure 1c) [3,7]. Between these two faults lies a complex system of nearly E–W-trending and W-plunging folds comprising, from north to south, the Wulicun anticline, the Qishuping syncline, the Laoyacha anticline, the Miaogou syncline, and the Shangyangzhai anticline [6]. Gold deposits occur in these plunging complex folds and their related secondary faults.

The Xiaoqinling metamorphic core complex is composed mainly of the Late Archean Taihua Group, which is a suite of amphibolite to granulite facies metamorphic rocks including amphibolite, felsic gneiss, migmatite, and metamorphosed supracrustal rocks [19]. The zircon U−Pb geochronological data of Taihua Group metamorphic rocks indicate that they formed during the Neoarchean to Paleoproterozoic (2.8–2.3 Ga) [20,21,22]. These rocks underwent amphibolite-facies metamorphism during the Paleoproterozoic. Mesoproterozoic Guandaokou Group carbonate rocks and clastic rocks are distributed in the south of the Xiaoqinling region [23].

Multiple and widespread intrusive events occurred during the Paleo- to Mesoproterozoic and Mesozoic eras. The Guijiayu biotite hornblende granite and the Xiaohe biotite granite feature zircon U–Pb ages of 1.8 and 1.6 Ga, respectively [24,25]. Additionally, three Mesozoic granitoid intrusions are distributed successively from west to east: the Huashan, Wenyu, and Niangniangshan plutons. Zircon U–Pb dating indicates that they were emplaced between 146 and 138 Ma [3,26,27]. The Taihua Group likely underwent partial melting, resulting in the formation of the Early Cretaceous plutons [28]. There exist also mafic dykes, such as diabase, gabbro, and lamprophyre, which intruded in two separate episodes: 1850–1810 Ma and 140–125 Ma [27]. Diabase and lamprophyre dikes are widely distributed in most Au deposits, existing at pre-, syn-, and post-ore stages.

Quartz vein-type Au deposits are developed mainly in the Xiaoqinling ore concentration area. At present, more than 50 Au deposits and more than 1200 auriferous Au veins have been found (Figure 1c). Gold mineralization occurs mainly in the metamorphic basement of the Taihua Group. In addition, molybdenum mineralization of industrial value is also developed in the quartz vein-type Au mineralization system in the northern ore belt of the ore concentration area, such as the Dahu Au (Mo) deposit, which has a reserve of 30,000 tons of Mo [4]. Previous studies have shown two peaks in the metallogenic age of the deposits in the Xiaoqinling area, namely in the Late Triassic and Early Cretaceous [2].

Figure 1. (a) Sketch maps of eastern China and the North China Craton. (b) Geological map of the Qinling Orogen showing the location of the Xiaoqinling region (adapted from [29]). (c) Geological map of the Xiaoqinling Au district (adapted from [11]).

Figure 1. (a) Sketch maps of eastern China and the North China Craton. (b) Geological map of the Qinling Orogen showing the location of the Xiaoqinling region (adapted from [29]). (c) Geological map of the Xiaoqinling Au district (adapted from [11]).

Many intrusive events took place in the Xiaoqinling area from the Late Triassic to the Late Jurassic–Early Cretaceous. For example, the Wengyu amphibole monzogranite has a zircon LA–ICP–MS U–Pb age of ~205 Ma [30] while the adakitic and shoshonitic intrusions in the Laoniushan granitic complex were formed between 228 and 215 Ma [31]. Late Jurassic plutons such as the Laoniushan granodiorite, quartz diorite, and biotite have zircon LA–ICP–MS U–Pb ages of 153–146 Ma [32] and the Huashanyu biotite monzogranite has a zircon SHRIMP U–Pb age of ~146 [26]. Early Cretaceous plutons include the Wenyu biotite monzogranite [33], the Niangniangshan biotite monzogranite [28], and the Fangshanyu biotite monzogranite [30].

3. Deposit Geology

The Lianzigou deposit in the western part of the Xiaoqinling region (Figure 1c) is located in Luonan City of Shaanxi Province (110°09′15″ to 110°11′00″ E, 34°21′45″ to 34°23′15″ N). The outcropping strata in this region include mainly plagioclase gneiss of Qincanggou Formation of the Neoarchean Taihua Group and quartz sandstone interbedded with argillite-sandy slate of the Mesoproterozoic Gaoshanhe Formation [13]. The former is the main ore-bearing strata, while the latter covers these strata with angular unconformity. The orebodies are hosted in the alteration fracture zones formed by the NE-striking fault. The scale of individual mineralization is directly proportional to an alteration zone (Figure 2a). There are nine alteration zones with Au mineralization, including seven economic Au orebodies. Orebodies I-2, II-2, and III-1 are the main economic ones with average grades of 1.48 g/t, 2.22 g/t, and 2.32 g/t, respectively [15,34]. The orebodies strike 30–80° and dip SE with dip angles of 30°–50° (Figure 2b). Most of the orebodies are vein-type and lenticular with thicknesses of 0.50–15.00 m and lengths of 55–550 m. Based on the characteristics of Au mineralization, the ore type is divided into: quartz-vein type with 2.0–6.0 g/t Au and altered-rock type with 0.8–3.0 g/t Au. Spatially, it is often observed that quartz vein-type Au ore is superimposed upon altered-rock type Au ore and distributed at the center of an orebody.

Numerous sulfides and tellurides have been identified in the Lianzigou deposit. Pyrite, native Au, galena, and chalcopyrite are the major ore minerals associated with the deposit, while the minor ones are sphalerite, tetrahedrite, krennerite, calaverite, petzite, hessite, altaite, melonite, tellurium, magnetite, hematite, bornite, digenite, and electrum (Figure 3). The associated gangue minerals are mainly quartz, followed by K-feldspar, calcite, and sericite. The ore structures of the Lianzigou deposit are lumps, veins, networks, dissemination, speckled, and banded. The ores show the following textures: euhedral–subhedral granular, anhedral granular, emulsion, and inclusion textures. Wall rock alteration occurs mainly on both sides of an orebody, thus exhibiting zoning characteristics. The inner wall rock alteration zone is characterized mainly by K-feldspar and silicification, with minor carbonatization. Sericitization developed mainly in the transition zone between the inner and outer wall rock alteration zones. Thus, the main types of alteration in the external wall rock are chloritization, epidotization, and minor carbonatization.

Based on the cross-cutting relationships of veins and on the characteristics of mineral assemblages, two main intervals can be distinguished in the mineralization processes of the Lianzigou deposit: hyogenesis and supergenesis. In the former, a four-stage paragenesis has been recognized (Figure 4), namely, K-feldspar-quartz stage, pyrite–quartz stage, gold–polymetallic sulfides–tellurides stage, and carbonate–quartz stage.

The K-feldspar–quartz stage (Stage I) consists mainly of K-feldspar and quartz (Figure 3a). Additionally, medium- to fine-grained disseminated pyrite is distributed in the inner wall rock alteration zone.

In the pyrite–quartz stage (Stage II), the hydrothermal fluid migrated and filled along the fault zone. There are milky quartz veins accompanied by coarse-grained disseminated pyrite and lumpy pyrite at this stage (Figure 3b,c). In addition, this stage contains small amounts of magnetite.

The gold–polymetallic sulfides–tellurides stage (Stage III) is characterized by gray quartz, which crosses or superimposes milky quartz veins, and abundant polymetallic sulfides including chalcopyrite, sphalerite, galena, and tetrahedrite (Figure 3d–h). The important characteristics of this stage are the presence of native Au and tellurides, including krennerite, calaverite, petzite, hessite, altaite, and melonite (Figure 3d–i). The tellurides are closely related to polymetallic sulfides such as sphalerite and chalcopyrite and commonly occur in early pyrite fractures (Figure 3e–h). This stage is the main metallogenic stage with intense Au mineralization.

The carbonate–quartz stage (Stage IV) is characterized by abundant carbonate and quartz with weak Au mineralization.

4. Sample and Analytical Methods

All samples were collected from II-2 and III-1 orebodies in the Lianzigou deposit. Systematic studies of mineralogy were carried out in polished thin sections of ore samples using reflected-light microscopy (Leica DM 4500P) (Leica Camera AG, Wetzlar, Germany) at the China University of Geosciences (Beijing). The major composition of minerals was determined using a JEOL JXA 8230 electron probe microanalyzer equipped with an energy-dispersive X-ray Spectrometer (EDS) (JEOL, Tokyo, Japan) and an AsB detector (Backscatter Electron, BSE) (Monterey, CA, USA) at the Institute of Mineral Resource, Chinese Academy of Geological Sciences. The conditions of EMPA involved an acceleration voltage of 20 kV, a current of 20 nA, and an electron beam of 1–5 μm diameter. The detection limit was 0.0081%. ZAF corrections were applied to the processing of quantitative analysis. The standards minerals and metals used for calibration included: Au–Au–Ag alloy (Au–Mα), AgSbS₂ (Ag–Lα), PbTe (Te–Lα), CuFeS₂ (Cu–Kα), FeS₂ (Fe–Kα, S–Kα), FeAsS (As–Lα), Sb₂S₃ (Sb–Lα), PbS (Pb–Mα), and ZnS (Zn–Lα).

To reveal compositional differences, line scanning analysis of the grayish-with-blue and grayish-with-yellow domains of krennerite was carried out at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Beijing) using a Zeiss SUPRA 55 Field Emission Scanning (FESEM) (Göttingen, Germany) equipped with an Oxford energy-dispersive X-ray spectrometer (EDS) (Oxford Instruments, Abingdon, UK) and an AsB detector (Backscatter Electron, BSE, image) (Monterey, CA, USA). It operated under the following conditions: acceleration voltage of 15 kV, specimen current of 200 nA, with a working distance of 15 mm.

5. Results

Natural elements in the Lianzigou deposit include native Au, electrum, and native Te. The tellurides in the deposit consist mainly of calaverite, krennerite, petzite, hessite, melonite, and altaite. Figure 5 illustrates a summary of the telluride minerals and native Au assemblages in an Au–Ag–Te–Pb–Ni diagram.

5.1. Natural Elements

Native Au, which is the prevalent form of Au present, occurs in different modes, namely fissure Au, associated Au, intergranular Au, and encapsulated Au. Of these, fissure Au is the most common, with irregular granular or veinlet native Au filling in pyrite’s fissure (Figure 3e). The native Au is irregular with particle sizes of 5–30 μm. Associated Au coexists with krennerite, altaite, melonite, and chalcopyrite in fissures of pyrite (Figure 3e,h). Intergranular Au occurs in between pyrite or quartz grains. Encapsulated Au occurs in pyrite or quartz. The data of EMPA indicated that encapsulated Au was composed of Au and Ag with mass fractions of 90.07–91.85 wt% and 5.96–6.45 wt%, respectively, and there were small amounts of Hg and Se (Table 1); the values of Au/(Au + Ag) ranged 0.884–0.894. The Au and Ag in electrum were 89.24 wt% and 10.59 wt%, respectively; the Au/(Au + Ag) was 0.819.

Native Te, which is rare, coexists with chalcopyrite as mineral inclusions in pyrite. The results of EMPA show that native Te contained Te concentrations of 98.76–99.47% wt%.

5.2. Tellurides

Calaverite (AuTe₂) was commonly observed with quartz (Figure 3i). It is white with light yellow with sizes of ~50 μm. The EMPA data indicated that it was composed of Au, Ag and Te with mass fractions of 36.82–38.65 wt% (average = 37.73 wt%), 2.51–3.57 wt% (average = 3.04 wt%), and 56.52–57.90 wt% (average = 57.21 wt%), respectively (Table 1). The calculated average formula of calaverite was Au_0.85Ag_0.13Te_2.00.

Krennerite (Au₃AgTe₈) exists in granular sizes of 50–200 μm. It is reflective grayish with yellow tints exhibiting reflectivity lower than that of the native Au. Krennerite is common in the Lianzigou deposit and it occurs mainly in native gold−altaite−melonite−chalcopyrite assemblages (Figure 3 and Figure 6), which occur in fissures of pyrite or between pyrite grains. Altaite–melonite formed around the rim of krennerite. Krennerite contains 30.13–38.50 wt% Au (average = 34.99 wt%), 55.54–58.28 wt% Te (average = 36.84 wt%), and 4.50–9.90 wt% Ag (average = 6.67 wt%) (Table 1). The calculated average formula based on EPMA data was Au_3.20Ag_1.11Te_8.00. According to elements line scanning analysis of EDS, the domain of Ag-rich exhibited grayish-with-yellow whereas the domain of Au-rich exhibited grayish-with-blue. The EPMA mapping-scan and EDS line-scan images show that the element distribution is inhomogeneous in krennerite (Figure 6 and Figure 7). Previous studies demonstrated that the Au–Ag–Te system was stable at higher temperatures in the early stage of the deposit, and the deposition of calaverite and krennerite is related to breakdown products of the Au–Ag–Te system (γ- and χ-phases) due to quick cooling [35,36]. Therefore, inhomogeneous element distribution in the krennerite can be caused by quick cooling, resulting in the Au–Ag–Te system.

Petzite (Ag₃AuTe₂) shows light gray reflection color without bi-reflectance and polychromatism. Petzite coexisting with hessite, chalcopyrite, bornite, and sphalerite occurs in fissures of pyrite (Figure 3h). Petzite contains 22.57–24.25 wt% Au (average = 23.64 wt%), 30.94–33.43 wt% Te (average = 32.53 wt%), and 41.60–45.92 wt% Ag (average = 43.23 wt%) (Table 1). Its chemical formula can be calculated as Ag_3.14Au_0.94Te_2.00. Hessite (Ag₂Te) shows blue-gray reflection color without bi-reflectance and polychromatism. The calculated average formula of hessite was Ag_1.88Te.

Besides tellurides of Au–Ag, altaite and melonite were also identified in the Lianzigou deposit. Under reflected light, melonite (NiTe₂) is pale-pink with non-bi-reflectance and is homogeneous. It occurs mainly in native gold−altaite−melonite−chalcopyrite assemblages. Melonite coexists with native Au, altaite, and chalcopyrite in fissures of pyrite, or it appears in chalcopyrite–sphalerite–melonite inclusions in altaite. Major element compositions of melonite show that Ni ranged 15.26–18.36 wt% with an average of 17.35 wt%, and Te ranged 76.73–81.04 wt% with an average of 80.01 wt% (Table 1). The average calculated formula of melonite was Ni_0.94Te_2.00.

Altaite (PbTe) is grayish with sizes of 5–20 μm. It is associated with krennerite, calaverite, melonite, and native Au (Figure 3). The content of Pb in altaite was 59.29–60.99 wt% and that of Te was 37.69–38.59 wt% (Table 1), and its formula was Pb_0.97Te_1.00.

Table 1. EPMA analyses (wt%) of native elements and tellurides in the Lianzigou.

Minerals	Native Au	Electrum	Native Te	Krennerite	Calaverite	Petzite	Hessite	Altaite	Melonite
Au	90.07–91.85	87.24	0.00–0.02	30.13–38.50	36.82–38.65	22.57–24.25	0.23–0.29	0.00–0.29	0.00–0.46
Ag	5.96–6.45	10.59		3.70–9.90	2.51–3.57	41.60–45.92	59.25–60.25	0.00–0.08	0.00–0.03
Hg	0.53–1.54	0.01	0.00–0.02	0.06–0.54	0.34–0.48	0.00–0.16	0.00–0.03		0.00–0.06
Sb		0.00–0.18
Pb								59.29–60.99	0.00–0.10
Bi	0.36–0.50	0.45	0.00–0.01	0.11–0.29	0.26–0.32	0.00–0.19	0.00–0.01		0.00–0.07
Co	0.00–0.01	0.04
Ni	0.00–0.11	0.08		0.00–0.03		0.00–0.03	0.00–0.01	0.00–0.04	15.26–18.36
Cr	0.00–0.48	0.40
As	0.00–0.02
Fe	0.17–0.90	0.10	0.40–0.48	0.05–0.18	0.05–0.13	0.54–0.94	1.20–1.73	0.19–1.18	0.09–1.74
Cu	0.24–0.57		0.00–0.04	0.07–1.36	0.46–0.92	0.02–0.32	0.22–0.34	0.00–2.03	0.00–0.43
Zn			0.06–0.11	0.00–0.01	0.00–0.02			0.02–0.10	0.00–0.14
Ge		0.03
Ga	0.10–0.18	0.13
Se	0.00–4.56		0.12–0.18	0.04–0.22	0.07–0.08	0.04–0.06	0.05	0.00–0.02	0.07–0.18
Te	0.00–0.11	0.13	98.76–99.47	55.54–58.28	56.52–57.90	30.94–33.47	38.20–38.48	37.69–38.59	76.73–81.04
S	0.05–0.12	0.14	0.10–0.12	0.00–0.03	0.00–0.04	0.16–0.21	0.26–0.48	0.00–0.07	0.03–1.73

Table 1. EPMA analyses (wt%) of native elements and tellurides in the Lianzigou.

Minerals	Native Au	Electrum	Native Te	Krennerite	Calaverite	Petzite	Hessite	Altaite	Melonite
Au	90.07–91.85	87.24	0.00–0.02	30.13–38.50	36.82–38.65	22.57–24.25	0.23–0.29	0.00–0.29	0.00–0.46
Ag	5.96–6.45	10.59		3.70–9.90	2.51–3.57	41.60–45.92	59.25–60.25	0.00–0.08	0.00–0.03
Hg	0.53–1.54	0.01	0.00–0.02	0.06–0.54	0.34–0.48	0.00–0.16	0.00–0.03		0.00–0.06
Sb		0.00–0.18
Pb								59.29–60.99	0.00–0.10
Bi	0.36–0.50	0.45	0.00–0.01	0.11–0.29	0.26–0.32	0.00–0.19	0.00–0.01		0.00–0.07
Co	0.00–0.01	0.04
Ni	0.00–0.11	0.08		0.00–0.03		0.00–0.03	0.00–0.01	0.00–0.04	15.26–18.36
Cr	0.00–0.48	0.40
As	0.00–0.02
Fe	0.17–0.90	0.10	0.40–0.48	0.05–0.18	0.05–0.13	0.54–0.94	1.20–1.73	0.19–1.18	0.09–1.74
Cu	0.24–0.57		0.00–0.04	0.07–1.36	0.46–0.92	0.02–0.32	0.22–0.34	0.00–2.03	0.00–0.43
Zn			0.06–0.11	0.00–0.01	0.00–0.02			0.02–0.10	0.00–0.14
Ge		0.03
Ga	0.10–0.18	0.13
Se	0.00–4.56		0.12–0.18	0.04–0.22	0.07–0.08	0.04–0.06	0.05	0.00–0.02	0.07–0.18
Te	0.00–0.11	0.13	98.76–99.47	55.54–58.28	56.52–57.90	30.94–33.47	38.20–38.48	37.69–38.59	76.73–81.04
S	0.05–0.12	0.14	0.10–0.12	0.00–0.03	0.00–0.04	0.16–0.21	0.26–0.48	0.00–0.07	0.03–1.73

6. Discussion

6.1. Ore-Forming Processes and Physicochemical Conditions

Thermodynamic phase diagrams such as logfO₂–logfS₂, logfO₂–logfH₂S, logfO₂–pH, logαAu⁺/αAg⁺–logfTe₂, logfTe₂–logfS₂, or related diagrams can be used to constrain the characteristics of ore-forming fluids and the metallogenic mechanism [35,37,38,39,40]. In addition, because tellurides are very susceptible to variations in environmental conditions, they can be useful markers of metallogenic conditions [35]. Utilizing these characteristics, phase diagrams were used to determine the phase stability relationships of the Lianzigou deposit based on the mineral assemblages. Previous studies have indicated that the homogenization temperatures of 280 °C and 230 °C are typical of stages II and III, respectively [41]. The diagrams were estimated using the HSC Chemistry 6.0 software package and Mcphail [42]. All solid phases and reactions were judged to have occurred under ideal conditions.

For the mineral deposition in stage II at 280 °C, the pyrite−magnetite assemblage reflects that the phase stability domain was situated near the equilibrium boundary of pyrite and magnetite. The calculated logƒO₂ values ranged from −34.90 to −31.96 and the logƒS₂ values were from −11.92 to −9.96 (Figure 8a). The calculated logƒO₂–logƒH₂S phase diagram indicates that the logƒH₂S values ranged from −3.47 to −2.98 (Figure 8b). Additionally, the logƒO₂–pH phase diagram shows that the range of pH was 8.3–8.5 (Figure 8c).

For the main mineralization of Au, polymetallic sulfides, and tellurides at 230 °C, the logƒO₂–logƒS₂ phase diagram for stage III was set up. Based on the existence of pyrite, chalcopyrite, and bornite in stage III, we obtained a calculated logfS2 of ~ −9.29 (Figure 8a), which is constrained by the bornite + pyrite–chalcopyrite equilibrium line. Likewise, the logƒH₂S value was ~ −3.45 (Figure 8b). In addition, the boundary between anglesite (PbSO₄) and galena (PbS) equilibrium indicated the logƒO₂ values were less than −33.31, and the occurrence of barite defined that the logƒO₂ values were higher than −43.15 (Figure 8a). The pH–logfO₂ phase diagram reveals that the stable domain of the calaverite–hessite assemblage in stage III precipitated at pH < 7 (Figure 8d). The range of logƒTe₂ value ranged from −12.54 to −8.69 (Figure 9a), as defined by the stability field of hessite, melonite and altaite, and the equilibrium boundaries between pyrite and pyrrhotite and between Te(s) and Te₂(g). Moreover, based on the calaverite–petzite–native gold assemblage equilibrium, the estimated logαAu⁺/αAg⁺ values for stage III tellurides were between −7.2 and −6.5 (Figure 9b). Figure 9b further constrains the range of logƒTe₂ values from −10.6 to −9.8.

The calculated physicochemical conditions revealed decreases in pH and oxygen fugacities, while the fugacities of H₂S and S₂ exhibited smaller fluctuations during the shift from stage II to stage III. The phenomenon of telluride mineralization occurring under low pH and low oxygen fugacity conditions is consistent with multiple Au–Ag deposits in the Xiaoqinling region, such as the Dahu Au–Mo deposit and the Jinqu Au deposit. Similarly, the Dongping Au–Te deposit in the North China craton also experienced a lowering of temperature and oxygen fugacity from the early to the late stages of mineralization. Therefore, the main reason for Au–Te mineralization in the Lianzigou deposit may be similar to those for other Au–Te deposits, including fluid immiscibility, fluid mixing and water–rock interaction. For example, cooling caused by mixing with low-temperature meteoric water and by water–rock reaction resulted in the mineralization of the Dabaiyang Au deposit [37]. The metallogenic mechanisms of Jinqu Au deposit could also be fluid immiscibility, fluid mixing, and water–rock reaction [3]. The precipitation of tellurides in Au–Te deposits is mainly influenced by physicochemical conditions, with lower oxygen fugacity and lower temperature playing crucial roles, whereas sulfur fugacity exerts a relatively limited influence on this process.

6.2. Possible Sources and Transport Forms of Au and Te

As a dispersed element, Te concentration in the mantle (22 ppb) is significantly greater than in the crust (2–3 ppb) [43]. For telluride-rich Au deposits, the hydrothermal fluid source is thought to be related to magmatic activity, and Te is mostly of magmatic origin [44,45]. The presence of tellurides in the deposit may be an indication that their ore-forming minerals originated from deep sources. Condensation of volatiles from Te-bearing magmas was an effective mechanism for the deposition of Te-rich deposits, with Te transported mostly in the gaseous phase [42,46]. At the Lianzigou deposit, H–O–He–Ar isotopic data of ore and alteration minerals demonstrated that the ore-forming fluids mostly came from deep mantle-derived fluids, underwent strong water–rock interaction and mixing with crustal fluids during upwelling, and were influenced by atmospheric precipitation in the shallow crust, which are harmonious with those of other typical Au deposits in the Xiaoqinling area [15]. The Pb–S isotopic data revealed that the source of ore-forming material was deep-seated, which may be related to magmatic activity or mantle devolatilization under the Mesozoic extensional background [14,15]. Additionally, some ore-forming materials from crustal sources may also have been present in the ore-forming materials [15]. In summary, the occurrence of tellurides and the geochemical characteristics indicate that ore-forming materials in the Lianzigou deposit originated from mantle materials. In addition, water–rock interaction led to the addition of crustal materials into the ore-forming system [15].

Gold migrated as a bisulfide complex in medium- and low-temperature hydrothermal deposits [47,48], with AuHS⁰ being the dominant species at acidic conditions and Au(HS)²⁻ being the dominant species at alkaline to weakly acidic conditions [5]. The abundant precipitation of pyrite in stage II of the Lianzigou deposit led to an increasing H⁺ and a decrease in HS⁻ and H₂S activity. The solubility of Au as Au(HS)²⁻ exhibited a slight decrease with decrease in pH from alkaline to weak acidity. The solubility of Au as Au(HS)²⁻ in H₂S_(aq)-dominated fluids was determined by the following series of reactions:

Fe²⁺_(aq) + H₂S = FeS_2(s) + 2H⁺_(aq)

(1)

Fe²⁺_(aq) + HS⁻ = FeS_2(s) + H⁺_(aq)

(2)

Au(HS)₂⁻_(aq) + 0.5H₂O(l) = Au_(s) + H₂S_(aq) + HS⁻_(aq) + 0.25O_2(g)

(3)

Au(HS)₂⁻_(aq) + H⁺_(aq) +0.5 H_2(g) = Au_(s) + 2H₂S_(aq)

(4)

Au(HS)₂⁻_(aq) + 0.5H_2(g) = Au_(s) + H₂S_(aq)

(5)

The above process led to the consumption of HS⁻ and H₂S while increasing the ƒTe₂/ƒS₂ ratio, whereby tellurides begin to precipitate. The thermodynamic parameters of Te provided by Mcphail [42] indicate that the most probable dominant Te species in the ore-forming fluids were HTe⁻_(aq) and Te₂²⁻_(aq) (Figure 8d). The solubility of Au as Au(HS)²⁻ and the precipitation of tellurides were controlled by:

Au(HS)₂⁻_(aq) + 2HTe⁻_(aq) + 3H⁺_(aq) = AuTe_2(s) + 2H₂S_(aq) + 1.5H_2(g)

(6)

Au(HS)₂⁻_(aq) + Te₂²⁻_(aq) + 3H⁺_(aq) = AuTe_2(s) + 2H₂S_(aq) + 0.5H_2(g)

(7)

2Ag(HS)₂⁻_(aq) + HTe⁻_(aq) + 2H⁺_(aq) + 0.5H_2(g) = Ag₂Te_(s) + 4H₂S_(aq)

(8)

4Ag(HS)₂⁻_(aq) + Te₂²⁻_(aq) + 6H⁺_(aq) + H_2(g) = 2Ag₂Te_(s) + 8H₂S_(aq)

(9)

Au(HS)₂⁻_(aq) + 3Ag(HS)₂⁻_(aq) + 2HTe⁻_(aq) + 6H⁺_(aq) = Ag₃AuTe_2(s) + 8H₂S_(aq)

(10)

Au(HS)₂⁻_(aq) + 3Ag(HS)₂⁻_(aq) + Te₂²⁻_(aq) + 6H⁺_(aq) = Ag₃AuTe_2(s) + 8H₂S_(aq) + H_2(g)

(11)

Au(HS)₂⁻_(aq) + 3Ag(HS)₂⁻_(aq) + 8HTe⁻_(aq) + 12H⁺_(aq) = Ag₃AuTe_8(s) + 8H₂S_(aq) + 6H_2(g)

(12)

Au(HS)₂⁻_(aq) + 3Ag(HS)₂⁻_(aq) + 8Te₂²⁻_(aq) + 12H⁺_(aq) = Ag₃AuTe_8(s) + 8H₂S_(aq) + 2H_2(g)

(13)

Au(HS)₂⁻_(aq) + Pb²⁺_(aq) + 3HTe⁻_(aq) + 2H⁺_(aq) = AuTe_2(s) + PbTe_(s) + 2H₂S_(aq) + 1.5H_2(g)

(14)

Au(HS)₂⁻_(aq) + Pb²⁺_(aq) + 1.5Te₂²⁻_(aq) + 2H⁺_(aq) = AuTe_2(s) + PbTe_(s) + 2H₂S_(aq)

(15)

Au(HS)₂⁻_(aq) + Ni²⁺_(aq) + 4HTe⁻_(aq) + 3H⁺_(aq) = AuTe_2(s) + NiTe_2(s) + 2H₂S_(aq) + 2.5H_2(g)

(16)

Au(HS)₂⁻_(aq) + Ni²⁺_(aq) + 2Te₂²⁻_(aq) + 2H⁺_(aq) = AuTe_2(s) + NiTe_2(s) + 2H₂S_(aq)

(17)

6.3. Genetic Backgrounds

Being the northernmost part of the Qinling–Dabie orogenic belt, the Xiaoqinling region underwent extensive lithospheric thinning at the eastern North China craton during the Early Cretaceous, marking the approximate westernmost limit of lithospheric destruction in the North China craton [1,49,50]. Also, many Au deposits were formed in the Early Cretaceous in the Xiaoqinling region, which were generated dominantly in the transition tectonic settings of the North China craton from compression to extension [2]. The metallogenic age is broadly synchronous with the timing of regional magmatism. In the Xiaoqinling region, there was intense magmatism during the Late Mesozoic, as shown by the major intrusion of the Huashan, Wenyu, and Niangniangshan plutons. The SHRIMP U–Pb age of zircon from the Niangniangshan biotite monzogranite was 141.7 ± 2.5 Ma [28]. The U–Pb ages of zircons in the Wenyu monzogranites ranged from 141.4 to 122.2 Ma [28,51]. Moreover, the age of the monzonitic granite at the Huashan complex was 133.8 ± 1.1 Ma [52].

Because the Re–Os data of molybdenite yielded an age of 128.8 ± 6.5 (MSWD = 15) for the Lianzigou deposit [13], we therefore consider that the metallogenic system of the Lianzigou deposit was related to Au-rich fluids resulting from lithospheric thinning of eastern North China craton in the Early Cretaceous, promoting the rapid release of Au–Te [49,50]. The reactivation of crustal faults, early major faults, and secondary faults under the extensional tectonic system provided sufficient migration channels for the upward migration of ore-forming fluids from deep mantle sources.

7. Conclusions

• The Lianzigou Au deposit developed a variety of tellurides and native Au. The former include calaverite, krennerite, petzite, hessite, melonite, and altaite. Based on the equilibrium boundary of pyrite and magnetite and the metallogenic temperature at stage II, the logƒO₂, logƒS₂, logƒH₂S, and pH were −34.90~−31.96, −11.92~−9.96, −3.47~−2.98, and 8.3~8.5, respectively. Comparably, the calculated physicochemical conditions revealed decreases in pH and oxygen fugacities, while the fugacities of H₂S and S₂ exhibited smaller fluctuations during the transition from stage II to stage III. At stage III, the logƒO₂, logƒS₂, logƒH₂S, and pH were −43.15~−33.31, ~−9.29, ~−3.45, and <7, respectively. Additionally, the logfTe₂ and the logαAu⁺/αAg⁺ values ranged from −10.6 to −9.8 and −7.2 and −6.5, respectively.
• The results of thermodynamic calculations and simulations indicate that the dominant Au species was Au(HS)²⁻ in the ore-forming fluids, while the dominant Te species were HTe⁻_(aq) and Te₂²⁻_(aq). The Au–Ag–Te association in the Lianzigou deposit was derived from mantle materials during the lithospheric thinning of eastern North China craton in the Early Cretaceous under an extensional tectonic system.