Geochemistry and Ore Genesis of the Huoyanshan Cu-Zn Polymetallic Deposit, North Qilian Orogenic Belt, China: Constraints from Trace Element Compositions and Sulfur Isotopes

Abstract

The Huoyanshan Cu-Zn volcanogenic massive sulfide (VMS) deposit, located in the North Qilian Orogenic Belt, China, is of significant economic importance. This study provides new constraints on the ore-forming processes through high-resolution in situ trace element and sulfur isotope analyses of pyrite and sphalerite using LA-(MC)-ICP-MS. Petrographic and geochemical investigations identified three distinct generations of pyrite (Py l to Py III). Early-stage Py I and Py II are characterized by high trace element contents (Au, As, Bi, Cu, Pb), elevated Co/Ni ratios (>1–10), and enriched δ³⁴S values (+4.98‰ to +7.47‰). These signatures indicate precipitation from high-temperature, reduced magmatic–hydrothermal fluids influenced by thermochemical sulfate reduction (TSR). Late-stage Py IIl exhibits markedly lower Co/Ni ratios (<0.1) and lighter δ³⁴S values (+3.72‰ to 3.89‰). This geochemical shift reflects a transition toward a cooler, more oxidized environment driven by the incursion and mixing of ambient seawater as the hydrothermal system waned. Trace element geochemistry of sphalerite reveals an average crystallization temperature of 265.8 °C (derived from the “GGIMFis” geothermometer), consistent with fluid inclusion data and representing a thermal “snapshot” of the waning hydrothermal stage. Systematic discriminant analysis using Ga/In, Ge/In, and Co-Ni-As systematics further confirms a strong magmatic–hydrothermal affiliation.

1. Introduction

Volcanogenic massive sulfide (VMS) deposits are a significant source of base metals (e.g., Cu, Zn, Pb) and precious metals (e.g., Au, Ag), primarily originating from hydrothermal processes linked to submarine volcanic activity. In addition, some critical metals (e.g., Sn, Co, Ni, In, Ga, Re, and Te) may occur in the VMS deposits [1,2,3]. These deposits are commonly stratiform and are found in various tectonic settings, such as modern mid-ocean ridges, ancient oceanic arc-back, arc-systems, and continental rift environments [4,5,6]. Their formation entails the movement of metal-enriched fluids propelled by magmatic heat, leading to the precipitation of sulfide minerals upon interaction with seawater [7,8,9,10]. Understanding the genesis of VMS deposits is crucial not only for mineral exploration but also for elucidating Earth’s geodynamic history and the interaction between hydrothermal systems and marine ecosystems.

The North Qilian orogenic belt in Western China is a significant VMS metallogenic. The orogenic belt links the Alxa Block with the Qaidam Block, while being separated from the Tarim Basin to the west and the northern China Craton to the east (Figure 1a). The northwestern sector of the Qilian Orogenic Belt is offset by the Altyn Tagh Fault, the largest left-lateral strike-slip fault in western China (Figure 1b). This orogenic belt hosts numerous polymetallic deposits genetically linked to Paleozoic oceanic subduction and subsequent continental collision [11,12,13,14]. The Huoyanshan Cu-Zn deposit is a large volcanic-hosted massive sulfide (VMS) deposit within the Baiyinchang ore field. The Cu, Zn, Pb and Ag resources are 0.251 Mt, 0.013 Mt, 0.001 Mt and 5 t respectively [15]. Numerous studies have investigated its chronology [16,17], fluid inclusions [18,19], and Geochemistry of volcanic rocks [20,21]. However, the Huoyanshan deposit remains poorly constrained at the micro-scale, and key aspects of its ore-forming fluid evolution and metal precipitation mechanisms are still not fully resolved. Specifically, three critical questions require further investigation: (1) the specific enrichment mechanisms of economically critical metals; (2) the precise sources of ore-forming materials—whether they were derived primarily from magmatic fluids or involved significant contributions from seawater and/or wall–rock interactions; and (3) the genetic classification of the deposit. The application and development of in situ microzoning techniques has made mineral geochemistry of sulfides a proxy for a more detailed understanding of the nature and evolution of mineralized systems [10,22]. Pyrite, chalcopyrite and sphalerite are the most important sulfides in each metallogenic stage of Huoyanshan deposit, which can be used to reveal important physicochemical information before and after mineralization.

In recent years, the in situ elemental and isotopic characteristics of sulfides have become key means to define the physicochemical conditions of ore-forming fluids, and thus to reconstruct the original ore-forming hydrothermal system [9,10,18]. Pyrite and sphalerite are ubiquitous sulfide minerals in volcanogenic massive sulfide (VMS) deposits. Although their ideal chemical formulas are FeS₂ and ZnS, respectively, natural specimens typically host a wide range of trace elements [22,23]. Pyrite commonly incorporates Co, Ni, Cu, Zn, As, Au, Ag, Sb, Se, Te, Pb, and Bi, whereas sphalerite often contains Fe, Cu, Cd, Hg, Mn, As, In, Se, Ga, Ge, and Ag, among others [3,22,24,25,26,27,28]. These trace elements can be incorporated into the crystal structure via atomic substitution or occur as nano- to micro-scale inclusions [29,30]. Certain elements such as Pb, Bi, and Cu may exist in both forms within sphalerite [22,23]. The trace element composition of pyrite and sphalerite is strongly influenced by hydrothermal conditions, including temperature, pressure, pH, and oxygen fugacity [25]. Consequently, trace element concentrations can reveal temporal changes in fluid composition and help elucidate metal precipitation and enrichment mechanisms during ore formation. When combined with in situ sulfur isotope analysis of mineral assemblages, trace element data provide a powerful tool for reconstructing the evolution of VMS hydrothermal systems [10,31,32,33]. Such integrated approaches offer critical insights into mineralization processes and contribute significantly to genetic models of VMS deposits.

2. Geological Setting and Ore Geology

2.1. Geological Setting

The North Qilian Orogenic Belt, situated on the northeastern margin of the Qinghai–Xizang Plateau, represents a typical Caledonian orogenic belt. It is bounded by the Alxa Block to the north, the Qaidam Block to the south, and the Tarim Block to the west, separated by the Altun Fault (Figure 1a,b) [34,35,36,37]. This belt extends in an NW–SE direction for approximately 1200 km, with a width of about 300 km [38]. It comprises, from south to north, an Early Paleozoic mid-ocean ridge ophiolite belt, a central island arc volcanic belt, and a northern SSZ-type ophiolite belt (Figure 1b) [13,39]. The structural architecture of the North Qilian Orogenic Belt is complex and diverse, characterized mainly by two phases of faulting: an early NW striking system and a later NE to NNE striking system. A well-developed trench–arc–basin system is present, with various tectonic units—including island arcs, subduction complexes, and oceanic crustal fragments—distributed systematically from north to south [21]. The belt records the northward subduction and eventual closure of the North Qilian Ocean lithosphere between 520 and 440 Ma [13], and has been identified as a Paleozoic oceanic suture zone related to a branch of the North Qilian Ocean and the Proto-Tethys Ocean [40]. During the middle–late Cambrian, extensive basaltic volcanism in an intracontinental rift setting supplied essential metals for Cu-polymetallic mineralization. Subsequently, intense Middle Ordovician submarine volcanism associated with subduction–collision processes generated abundant ore-forming hydrothermal fluids and coeval fault systems, creating favorable geological conditions for volcanogenic massive sulfide (VMS) deposits [41,42].

The predominant strata in the Baiyinchang orefield comprise the Cambrian Heicigou Formation, Xiangmaoshan Formation, and Ordovician Baiyin Group [43]. This region exhibits a sequence of Cambrian–Ordovician submarine volcanic eruptive sedimentary rocks, encompassing bimodal volcanic and sedimentary rock formations. Predominantly Middle Cambrian volcanic rocks shape a substantial volcanic dome within the ore field, primarily consisting of medium acid quartz keratophyre and keratophyre, with medium basic volcanic rocks surrounding it. Intrusive rocks in the region can be categorized into three types: (1) quartz diorite porphyry intrusions as rock stocks and dikes within acidic volcanic rocks; (2) diorite situated south of the Zheyaoshan deposit and sporadically at the Xiaotieshan deposit; and (3) back-arc granite mainly found at the Xiaotieshan deposit, interspersed with ore bodies [15]. The sedimentary strata in the area are relatively simple, with tuffaceous phyllite from the second to third cycles being the predominant lithology. Acting as the ore-bearing horizon of Baiyinchang type deposits, intermediate-acid volcanic rocks host five significant Cu polymetallic industrial deposits at various structural positions of the Baiyinchang volcanic dome. Notably, the NEE F1 fault, a larger fault, serves as a crucial structure controlling both rocks and ore in the area (Figure 2) [43].

2.2. Ore Geology

Huoyanshan deposit is one of the large-scale VMS deposits in North Qilian orogenic belt, with average ore grades of 1.3% Cu, 1.4% Zn, 0.5% Pb, 0.79 g/t Au and 9.28 g/t Ag [15]. The ore-bearing strata of the Huoyanshan deposit are quartz keratophyre tuff (Figure 1b). The ore belt spans approximately 1400 m in length from east to west and 300 m in width. The general orientation is NWW in the east, transitioning to NNW in the west, with a steep dip towards the SW [44]. There are 221 ore bodies, Cu, Pb and Zn reserves of about 265,000 tons. The grade of ore bodies is generally rich in the east and poor in the west. The eastern orebodies are thick, concentrated and steep, and are mostly characterized by lenticular aggregates. The western ore bodies are thinner in horizon, multi-branched and complex in shape, and are mostly vein-like [44]. Generally, the orebodies are thicker in the upper and middle parts, and have a tendency for branching and pinching out to the deep upper and lower plates.

The ore textures of the Huoyanshan deposit are dominated by massive, disseminated, and vein-type structures. The metallic mineral assemblage principally consists of pyrite, chalcopyrite, sphalerite, and galena, with associated minerals such as pyrrhotite and bornite also present (Figure 3). Gangue minerals are predominantly quartz, sericite, chlorite, and calcite. Petrographic observations reveal that the ore exhibits a variety of textures, including euhedral to subhedral granular texture (Figure 3a), common rim texture (Figure 3b), brecciated texture (Figure 3c), inclusion texture (Figure 3c,d), and intergrowth texture (Figure 3g–i). Although chemical etching was not performed, distinct generations of minerals were rigorously recognized. Based on detailed petrographic observations (including cross-cutting relationships, overgrowths, and co-crystallizing mineral assemblages), and corroborated by systematic trace element variations observed during LA-ICP-MS profiling, pyrite can be categorized into three distinct types: (1) euhedral granular pyrite (Py I; Figure 3a,e); (2) pyrite coexisting with chalcopyrite (Py II; Figure 3b,c,h,i); and (3) pyrite associated with sphalerite and galena (Py III; Figure 3d,f,g).

These microtextural relationships collectively define a two-stage temporal sequence for the principal sulfide precipitation. The early stage is characterized by high-temperature mineral assemblages, predominantly euhedral Py I, followed by the co-crystallization of Py II and early chalcopyrite (Ccp I) (Figure 3a–c). The late stage records a cooler environment influenced by seawater mixing, characterized by the precipitation of sphalerite, galena, late-stage Py III, and late chalcopyrite (Ccp II), which frequently cross-cut or replace the earlier sulfides (Figure 3d,f–i).

3. Samples and Analytical Methods

3.1. Samples

Samples were collected from the underground tunnel (elevation 1655~1741 m) in Huoyanshan deposit, including typical massive sulfide ore, quartz-sulfide vein ore, chalcopyrite-rich sulfide ore, altered wall rock and mineralized wall rock. These samples were collected from different locations, aiming to collect samples at different stages and different spatial locations as much as possible.

3.2. LA-ICP-MS Trace Element Analysis

Trace element analysis of sulfides, specifically pyrite and sphalerite, was carried out on thin sections using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at Nanjing FocuMS Technology Co., Ltd., Nanjing, China. The instrumentation consisted of a Teledyne Cetac Technologies Analyte Excite laser ablation system coupled with an Agilent Technologies 7700x quadrupole ICP-MS (Santa Clara, CA, USA). A 193 nm ArF excimer laser, homogenized through a series of beam delivery optics, was directed onto the sulfide surfaces with a fluence set at 3.0 J/cm². Each analytical run included a 15-s background measurement, followed by 40 s of ablation using a 40 μm spot diameter at a repetition rate of 5 Hz. To avoid polyatomic mass interferences, the following specific isotopes were monitored during the analysis: ³⁴S, ⁵⁵Mn, ⁵⁷Fe, ⁵⁹Co, ⁶⁰Ni, ⁶⁵Cu, ⁶⁶Zn, ⁷⁵As, ⁷⁷Se, ¹⁰⁹Ag, ¹¹⁵In, ¹¹⁸Sn, ¹²¹Sb, ¹²⁵Te, ¹⁹⁷Au, ²⁰⁸Pb, and ²⁰⁹Bi. Helium was employed as the carrier gas at a flow rate of 370 mL/min to effectively transport the ablated material from the cell, subsequently mixed with argon (approximately 1.15 L/min) via a T-connector prior to introduction into the ICP torch. Calibration was performed externally using the USGS pressed polymetal sulfide pellet MASS-1 and the synthetic basaltic glass GSE-1G. Off-line data processing was carried out using ICPMSDataCal (Ver. 9.5) software, applying a 100% normalization strategy without internal standard correction [45].

3.3. LA-MC-ICP-MS Sulfur Isotopes Analysis

Sulfur isotope analysis of sulfide minerals (pyrite, sphalerite, and chalcopyrite) was carried out in situ using a Neptune Plus multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) manufactured by Thermo Fisher Scientific, Bremen, Germany. This instrument was fitted with a Geolas HD excimer ArF laser ablation system from Coherent, Göttingen, Germany. The analyses were performed at Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. Helium was employed as the carrier gas within the ablation cell, subsequently mixed with argon (as makeup gas) downstream of the cell. A relatively large spot diameter of 44 μm and a slow pulse frequency of 2 Hz were selected in order to mitigate the down-hole fractionation effect [46]. The laser energy density was maintained consistently at approximately 5 J/cm².

Instrumental mass fractionation was corrected using the standard-sample bracketing (SSB) method. To account for potential matrix effects, matrix-matched reference materials were utilized: pyrite standard PPP-1 and chalcopyrite standard GBW07268 [46] were applied for correcting isotope analyses of natural pyrite and chalcopyrite, respectively [46]. The accuracy of this calibration approach was verified through repeated analyses of in-house reference materials, including pyrrhotite SP-Po-01 (δ³⁴Sv-CDT = 1.4 ± 0.4‰) and chalcopyrite SP-CP-01 (δ³⁴Sv-CDT = 5.45 ± 0.3‰), which were treated as unknown samples during the measurement sessions. All data reduction for sulfur isotope ratios obtained by MC-ICP-MS was performed using the “Iso-Compass” (Ver. 1.0) software package [47].

4. Results

4.1. Trace Element Compositions

The in situ trace element compositions of pyrite and sphalerite exhibit systematic variations. To ensure analytical accuracy, inclusion-free grains were selectively targeted during analysis. Representative time-resolved depth profiles, which illustrate the distribution and homogeneity of the analyzed elements, are provided in Figure 4.

4.1.1. Pyrite Trace Element Compositions

In situ trace element compositions of pyrite, as determined by laser ablation–inductively coupled plasma-mass spectrometry (LA-ICP-MS), are presented in

Table 1. LA-ICP-MS trace element analysis results of pyrite from the Huoyanshan deposit (Element Concentration Unit: ppm).

Sample/Element	Ti	V	Mn	Co	Ni	Cu	Zn	Ga	Ge	As	Se	Ag	Au	Cd	In	Sn	Sb	Tl	Pb	Bi
Pyrite I (n = 10)
23HB-15-Q1-1	35.20	0.11	46.94	68.04	5.09	54.08	2.99	0.06	1.14	666.28	2.27	0.80	0.23	0.04	-	0.14	11.30	0.20	78.67	3.76
23HB-15-Q1-2	34.39	0.11	3.66	52.68	7.34	96.11	0.90	0.01	1.19	1821.18	13.52	0.66	0.40	0.05	0.01	-	2.38	0.04	44.36	9.40
23HB-25-2-Q4-1	95.08	0.28	4.11	14.51	11.64	33.23	0.82	0.28	1.84	2407.70	3.35	2.39	0.87	0.03	-	-	4.12	0.01	183.73	67.52
23HB-25-2-Q4-2	96.11	0.29	4.29	63.17	21.74	30.30	0.80	0.07	1.77	1161.64	17.71	1.55	0.21	0.02	0.01	0.10	1.35	-	73.01	89.90
23HB-25-2-Q4-3	81.15	0.24	3.84	105.12	24.20	32.65	0.88	0.08	1.84	956.32	26.61	0.96	0.05	-	-	0.04	1.06	-	57.98	70.97
23HB-33-1-Q5-1	203.40	0.60	1.50	7.88	2.32	56.07	0.76	0.04	1.58	2.76	9.52	0.01	0.01	-	-	0.17	-	-	3.75	3.33
23HB-33-1-Q5-2	74.30	0.16	0.99	84.92	1.48	0.98	0.65	0.01	1.60	325.26	12.34	0.10	-	0.01	-	-	1.01	-	16.58	4.47
23HB-33-1-Q5-3	73.70	0.14	0.81	205.87	78.54	0.25	0.55	0.02	1.43	0.85	254.41	-	0.02	0.05	-	0.03	-	-	0.03	0.21
23HB-33-1-Q5-4	65.80	0.78	3.32	52.34	7.25	48.04	0.58	0.14	1.49	14.99	10.93	3.35	0.02	-	0.07	18.36	327.18	1.33	837.32	55.53
23HB-33-1-Q5-5	64.66	0.20	5.07	494.75	36.03	36.87	0.98	0.02	1.44	570.72	27.66	0.50	0.05	-	-	0.09	1.00	0.06	32.11	11.08
Min.	34.39	0.11	0.81	7.88	1.48	0.25	0.55	0.01	1.14	0.85	2.27	0.01	0.01	0.01	0.01	0.03	1.00	0.01	0.03	0.21
Max.	203.40	0.78	46.94	494.75	78.54	96.11	2.99	0.28	1.84	2407.70	254.41	3.35	0.87	0.05	0.07	18.36	327.18	1.33	837.72	89.90
Avg.	82.38	0.29	7.45	114.93	19.56	38.86	0.99	0.07	1.53	792.77	37.83	1.03	0.19	0.02	0.01	1.89	34.94	0.16	132.75	31.62
Pyrite II (n = 13)
23HB-6-Q5-1	35.15	0.11	0.97	336.73	39.75	5.47	1.03	-	1.35	710.84	255.19	0.01	-	0.01	0.01	0.10	0.04	-	0.19	2.74
23HB-6-Q5-2	36.23	0.14	1.42	5.45	0.36	264.34	10.44	0.22	1.62	32.61	106.49	0.34	-	0.09	4.26	25.05	0.01	0.02	0.44	20.99
23HB-6-Q5-3	33.42	0.12	2.05	652.29	15.20	50.47	1.43	0.06	1.17	1223.63	57.94	0.21	0.47	0.04	-	0.05	0.36	0.01	80.87	8.23
23HB-15-Q5-3	30.95	0.10	2.79	8.67	3.85	1255.77	623.12	0.36	0.98	615.10	75.38	2.40	0.08	2.58	12.57	16.34	2.41	0.04	210.68	71.95
23HB-15-Q5-4	29.65	0.12	0.77	6.28	4.88	0.03	0.68	0.02	0.86	53.55	14.28	-	0.01	0.01	-	0.03	0.06	0.01	0.01	0.02
23HB-12-3-Q2-1	33.46	0.07	0.92	0.90	0.57	45.82	0.75	0.01	1.48	2209.34	16.96	0.39	1.19	0.01	0.01	-	56.98	-	321.92	3.28
23HB-12-3-Q2-3	34.02	0.10	0.96	173.97	51.73	-	1.10	-	1.32	370.58	44.87	0.01	0.01	-	-	0.02	0.01	-	0.06	0.14
23HB-12-3-Q7-2	36.12	0.04	0.69	764.59	8.83	6.83	0.99	0.03	1.36	118.60	335.63	0.13	0.01	-	-	-	0.05	0.02	5.00	23.47
23HB-12-3-Q7-5	35.89	0.04	0.93	320.77	19.94	4.25	0.91	0.01	1.14	123.04	28.71	0.40	0.06	0.07	-	-	0.84	0.03	4.30	2.77
23HB-12-3-Q7-6	32.02	0.12	0.75	116.68	19.61	0.14	0.66	0.01	1.13	198.24	33.36	0.01	-	0.04	-	-	0.01	-	0.06	0.14
23HB-54-4-Q5-4	56.27	0.16	1.02	1164.98	0.90	1256.29	17.71	0.03	1.56	562.96	93.40	0.50	0.02	0.14	4.31	2.80	0.21	-	4.05	32.46
23HB-54-4-Q5-5	52.72	0.13	0.77	1192.33	29.63	31.06	6.51	0.52	1.65	11.53	8.02	0.03	0.01	0.10	0.51	32.59	0.23	-	1.35	2.91
23HB-54-4-Q7-3	50.88	0.02	0.78	1905.31	1.30	317.70	9.34	0.02	1.18	4264.06	114.65	0.61	0.80	0.05	0.03	0.24	0.17	-	77.84	29.50
Min.	29.65	0.02	0.69	0.90	0.36	-	0.66	-	0.86	11.53	8.02	0.01	0.01	0.01	0.01	0.02	0.01	0.01	0.01	0.02
Max.	56.27	0.16	2.79	1905.31	51.73	1256.29	623.12	0.52	1.65	4264.06	335.63	2.40	1.19	2.58	12.57	32.59	56.89	0.04	321.92	71.95
Avg.	38.21	0.10	1.14	511.46	15.12	249.09	51.90	0.10	1.29	807.24	91.14	0.39	0.20	0.24	1.67	5.94	4.72	0.01	54.37	15.28
Pyrite III (n = 4)
23HB-18-4-Q1-4	31.72	0.09	0.84	10.57	496.30	-	1.11	0.01	0.94	61.99	12.81	-	-	0.05	-	-	-	0.01	0.72	-
23HB-18-4-Q1-5	33.97	0.08	0.98	54.99	105.42	0.86	0.82	0.03	0.88	108.43	9.54	0.01	0.01	0.02	-	-	0.01	0.01	1.29	-
23HB-47-1-Q5-1	68.66	0.29	2.24	0.08	10.70	52.15	26.90	1.20	1.97	164.25	0.52	5.33	0.07	0.02	0.01	0.26	3.43	0.06	105.80	0.01
23HB-47-1-Q5-4	63.41	0.10	1.03	0.10	7.18	1.60	1.14	0.04	1.45	70.75	-	0.19	0.02	0.02	-	-	0.20	0.03	79.35	0.01
Min.	31.72	0.08	0.84	0.08	7.18	0.86	1.11	0.01	0.88	61.99	0.52	0.01	0.01	0.02	0.01	0.26	0.01	0.01	0.72	0.01
Max.	68.66	0.29	2.24	54.99	496.30	52.15	26.90	1.20	1.97	164.25	12.81	5.33	0.07	0.05	0.01	0.26	3.43	0.06	105.80	0.01
Avg.	49.44	0.14	1.27	16.43	154.90	13.65	7.49	0.32	1.31	101.36	5.72	1.38	0.02	0.03	0.01	0.07	0.91	0.03	46.79	0.01

Note: “-” represents below minimum detection limits.

and illustrated in Figure 5. A representative LA-ICP-MS time-resolved ablation profile is provided in Figure 4. The analytical results reveal systematic variations in trace element concentrations among three generations of pyrite (Py I, Py II, and Py III). Ore-forming elements including Au, As, Bi, Cu, Pb, Sb, Se, and Te are generally present at low concentrations in Py I, become significantly enriched in Py II, and are relatively depleted in Py III. In contrast, elements such as Ag, Cr, Ga, and Ge exhibit a progressive depletion from Py I to Py III. Conversely, Co, Zn, and Mn display lower abundances in Py I and Py II but are markedly elevated in Py III. Ni is low in Py I and shows a gradual increase from Py II to Py III. These compositional variations likely reflect evolving physicochemical conditions during hydrothermal mineralization, suggesting changes in fluid source, precipitation mechanisms, or redox environment over the paragenetic sequence.

4.1.2. Sphalerite Trace Element Compositions

In situ trace element compositions of sphalerite, as determined by laser ablation–inductively coupled plasma-mass spectrometry (LA-ICP-MS), are presented in

Table 2. LA-ICP-MS trace element analysis results of sphalerite from the Huoyanshan deposit (Element Concentration Unit: ppm).

Sample/Element	Zn	Fe	Mn	Co	Ni	Cu	Ga	Ge	As	Se	Ag	Cd	In	Sn	Sb	Pb	Bi
23HB-18-4-6	670,532	1633.69	3.28	0.19	-	25.13	0.91	0.03	1.53	24.55	11.65	2561.66	20.90	0.33	1.59	13.15	0.02
23HB-18-4-6	671,453	1537.74	1.49	2.95	-	32.46	1.12	0.06	1.00	17.85	7.62	2503.49	21.48	0.48	0.77	7.12	0.17
23HB-44-1-2-1	706,186	13,632.27	313.45	0.01	-	90.19	21.78	0.35	1.76	5.13	21.16	2563.44	3.28	2.78	7.33	10.28	-
23HB-44-1-2-2	704,905	13,842.51	310.30	-	-	72.88	21.82	0.32	1.99	8.85	19.29	2556.15	3.27	2.83	9.03	11.79	-
23HB-44-1-2-3	703,920	13,837.50	304.25	-	0.03	89.22	22.31	0.43	1.53	-	17.72	2465.25	3.17	3.54	11.74	10.62	0.01
23HB-44-1-6-1	704,172	11,592.13	342.51	-	0.03	427.66	20.41	0.02	1.73	13.63	12.28	2611.56	9.08	1.74	3.41	20.88	0.01
23HB-44-1-6-2	703,302	12,056.19	342.41	0.01	0.01	97.63	19.43	0.98	0.72	-	18.12	2459.89	8.82	1.52	3.44	23.67	0.01
23HB-44-1-6-3	700,973	9816.43	329.83	-	0.03	296.72	20.75	0.24	2.59	-	27.62	2624.28	9.07	1.59	3.43	636.21	-
23HB-44-1-6-4	704,424	9501.14	323.26	-	0.07	257.71	22.43	1.05	2.81	10.55	25.58	2471.50	9.00	3.21	6.73	22.97	0.01
23HB-44-1-6-5	704,705	9032.18	329.23	-	0.02	1206.19	21.88	2.12	2.85	5.06	17.56	2666.38	9.17	12.20	6.79	27.03	-
23HB-47-1-3	654,169	14,809.09	100.47	0.01	0.01	1087.72	17.42	0.23	1.10	3.59	15.85	2057.11	10.66	1.30	5.62	9.00	0.01
23HB-51-2-3-1	686,164	18,122.58	59.94	-	0.07	102.45	5.36	-	5.15	4.21	22.42	2703.33	82.79	1.94	4.24	16.43	-
23HB-51-2-3-2	709,397	7432.29	50.93	0.01	-	84.91	5.95	0.23	2.39	-	18.72	2744.21	86.28	1.30	1.16	9.98	0.01
23HB-51-2-3-3	706,335	6947.94	56.35	-	-	81.69	6.27	0.22	1.10	1.86	17.57	2647.16	84.03	0.85	0.14	4.46	-
23HB-51-2-4-1	688,103	18,481.04	20.84	0.01	0.24	387.19	3.64	0.55	19.50	-	26.04	2715.71	78.17	32.11	10.56	27.12	-
23HB-51-2-4-2	690,792	17,478.29	19.09	0.02	0.15	756.21	2.37	0.24	6.47	-	46.46	2578.36	70.04	9.83	8.97	51.15	0.01
23HB-51-2-5-1	706,637	5636.99	13.76	-	0.02	1299.25	2.35	0.25	34.98	-	104.39	2696.33	71.04	7.49	22.75	149.96	0.02
23HB-51-2-5-2	554,263	90,863.08	17.86	0.16	1.17	97.40	2.14	0.90	10.74	-	28.98	2169.36	47.32	4.94	5.94	24.63	-
23HB-51-2-5-3	651,769	37,374.75	20.21	0.04	0.32	164.10	2.14	0.52	12.17	5.97	19.83	2819.31	58.47	3.32	6.39	26.47	-
23HB-51-2-5-4	702,594	7692.77	28.13	0.01	-	106.41	3.46	0.29	3.56	-	23.88	2740.60	64.30	2.61	5.09	23.09	-
23HB-51-2-5-5	700,982	7586.24	27.33	-	-	76.18	3.43	0.24	1.60	-	21.37	2723.31	67.29	0.85	0.60	9.09	-
23HB-54-4-3	647,027	18,889.14	1.14	3.33	0.07	2332.44	0.89	0.02	0.58	98.66	9.45	2141.86	111.62	3.59	0.89	6.30	5.14
Min.	554,263	1537.74	1.14	0.01	0.01	25.13	0.89	0.02	0.58	1.86	7.62	2057.11	3.17	0.33	0.14	4.46	0.01
Max.	709,397	90,863.08	342.51	3.33	1.17	2332.44	22.43	2.12	34.98	98.66	104.39	2819.31	111.62	32.11	22.75	636.21	5.14
Avg.	685,128	15,808.91	137.09	0.31	0.10	416.90	10.38	0.42	5.36	9.09	24.25	2555.47	42.24	4.56	5.76	51.88	0.25

Note: “-” represents below minimum detection limits.

and illustrated in Figure 6. A representative LA-ICP-MS time-resolved ablation profile is provided in Figure 4. Compositional analysis of Huoyanshan deposit sphalerite reveals Zn and S contents ranging from 55.43–70.94 wt.% (avg. 68.51) and 27.67–35.20 wt.% (avg. 29.5), respectively. The mineral is notably enriched in several trace elements, including Fe (0.15–9.09 wt.%, avg. 1.58 wt.%), Cd (2057.11–2819.31 ppm, avg. 2555.47 ppm), Cu (25.13–2332.44 ppm, avg. 416.90 ppm), Mn (1.14–342.51 ppm, avg. 137.09 ppm), In (3.17–111.62 ppm, avg. 42.24 ppm), and Ag (7.62–104.39 ppm, avg. 24.25 ppm). Moderately elevated concentrations are also observed for Ga (0.89–22.43 ppm, avg. 10.38 ppm), Sb (0.14–22.75 ppm, avg. 5.76 ppm), and Sn (0.33–32.11 ppm, avg. 4.56 ppm). In contrast, elements such as Co, Ni, and Bi occur in relatively low abundances.

4.2. S Isotopic Compositions

The sulfur isotopic compositions (δ³⁴S) of sulfide minerals from the Huoyanshan deposit, including pyrite (n = 18), chalcopyrite (n = 10), and sphalerite (n = 5), were systematically analyzed. The results are summarized in

Table 3. S isotope composition of sulfides in Huoyanshan deposit.

Sample	Mineral	δ34S (‰)	2SE
23HB-33-1-Py1	Pyrite I	7.47	0.08
23HB-33-1-Py2	Pyrite I	4.98	0.13
23HB-33-1-Py3	Pyrite I	6.68	0.09
23HB-6-2-Py1	Pyrite II	4.49	0.08
23HB-6-2-Py2	Pyrite II	4.54	0.08
23HB-6-2-Py3	Pyrite II	6.75	0.1
23HB-12-3-Py2	Pyrite II	6.07	0.08
23HB-12-3-Py3	Pyrite II	5.78	0.07
23HB-12-3-Py4	Pyrite II	4.83	0.07
23HB-12-3-Py3	Pyrite II	4.41	0.08
23HB-12-3-Py4	Pyrite II	4.48	0.09
23HB-54-4-Py1	Pyrite II	3.87	0.05
23HB-54-4-Py2	Pyrite II	4.22	0.06
23HB-54-4-Py1	Pyrite II	3.75	0.1
23HB-54-4-Py3	Pyrite II	5.03	0.07
23HB-47-1-Py1	Pyrite III	3.89	0.09
23HB-47-1-Py2	Pyrite III	3.83	0.1
23HB-47-1-Py3	Pyrite III	3.72	0.08
23HB-6-2-Ccp4	Chalcopyrite	4.13	0.12
23HB-6-2-Ccp5	Chalcopyrite	4.50	0.10
23HB-12-3-Ccp1	Chalcopyrite	4.89	0.12
23HB-12-3-Ccp5	Chalcopyrite	4.14	0.11
23HB-12-3-Ccp1	Chalcopyrite	3.71	0.11
23HB-12-3-Ccp2	Chalcopyrite	3.60	0.10
23HB-54-4-Ccp3	Chalcopyrite	4.49	0.11
23HB-54-4-Ccp4	Chalcopyrite	4.61	0.1
23HB-54-4-Ccp2	Chalcopyrite	4.82	0.11
23HB-54-4-Ccp4	Chalcopyrite	5.09	0.08
23HB-47-1-Sp1	Sphalerite	3.12	0.11
23HB-47-1-Sp2	Sphalerite	3.18	0.09
23HB-47-1-Sp4	Sphalerite	2.98	0.11
23HB-47-1-Sp5	Sphalerite	3.28	0.10
23HB-47-1-Sp6	Sphalerite	3.17	0.10

and illustrated in Figure 7. The δ³⁴S_V-CDT values range from 3.72 to 7.47‰ (avg. 4.93‰) for pyrite, 3.60 to 5.09‰ (avg. 4.40‰) for chalcopyrite, and 2.98 to 3.28‰ (avg. 3.15‰) for sphalerite. Notably, the average δ³⁴S value of chalcopyrite (4.40‰) is heavier than that of sphalerite (3.15‰). In a hydrothermal system at isotopic equilibrium, sphalerite should be enriched in δ³⁴S relative to chalcopyrite. This reversed isotopic fractionation indicates that the coexisting sulfides did not attain isotopic equilibrium. This disequilibrium was likely caused by rapid mineral precipitation or the rapid mixing of magmatic–hydrothermal fluids with cooler ambient seawater.

5. Discussion

5.1. Occurrence and Substitution Mechanisms of Trace Elements in Pyrite and Sphalerite

In pyrite studies, trace element geochemistry is commonly employed to decipher the formation environments and origins of pyrite [48]. As shown in the LA-ICP-MS depth profiles of three representative pyrite samples, elements such as Co, Ni, and As display stable and flat signals (Figure 4a–c), reflecting their homogeneous distribution within the pyrite structure [49]. These elements are incorporated into the pyrite lattice primarily through isomorphic substitution [50]. In particular, Ni and Co replace Fe²⁺ according to the coupled substitution mechanism: 2Fe²⁺ ↔ Co²⁺ + Ni²⁺.

A pronounced negative correlation between As and S²⁻ is observed, in contrast to a weak correlation between As and Fe²⁺ (Figure 8a,b), suggesting that As replaces S in the crystal lattice via an isomorphic mechanism [51,52]. This geochemical behavior implies that pyrite crystallized under reducing conditions [25,53]. Therefore, Co, Ni, and As likely occur in pyrite in isomorphic form, although the presence of nanoscale mineral inclusions hosting these elements cannot be entirely ruled out [53].

Due to its large ionic radius, Pb²⁺ is unlikely to be incorporated into the pyrite lattice via isomorphic substitution. Thermodynamic studies further indicate that Pb has a stronger tendency to precipitate as discrete sulfide minerals in aqueous solutions compared to iron [52,54,55,56]. LA-ICP-MS trace element analysis reveals characteristic spectral peaks associated with Pb–Bi mineral inclusions (Figure 4b). It should be noted that while the transient peaks for Pb are highly prominent and easily identifiable in representative time-resolved profiles (e.g., Figure 4b), the corresponding Bi signals are often significantly weaker due to their low absolute concentrations, occasionally falling near the background detection limits. Consequently, Bi is not explicitly labeled in the representative visual profile to avoid misinterpreting near-background noise. Nevertheless, the quantitative data (Table 1) and the robust positive correlation between Pb and Bi globally across the dataset (Figure 8f) consistently support their co-occurrence in these micro-inclusions. In addition, Pb exhibits strong positive correlations with Sb, Cu, Ag, and Bi (Figure 8c–f), which—combined with petrographic observations (Figure 3d,f,g)—collectively demonstrate that lead predominantly occurs in pyrite as nanoscale mineral inclusions [57,58]. Similarly, Au shows positive correlations with Ag, Cu, and Pb (Figure 8g–i), indicating that gold is also mainly present in the form of mineral inclusions within pyrite.

In the Huoyanshan deposit, LA-ICP-MS time-resolved spectra reveal that elements such as Fe, Mn, Cd, In, and Ga in sphalerite display profiles approximately parallel to that of Zn. This suggests their incorporation into the mineral structure either through isomorphic substitution or as homogeneously distributed nanoparticles. In contrast, elements including Cu, Ge, Sn, and Pb exhibit anomalous spectral curves (e.g., peaks or troughs; Figure 4d,e), indicating a dual mode of occurrence: partial isomorphism alongside micro-mineral inclusions. Previous studies on trace element substitution in sphalerite have demonstrated that divalent cations (e.g., Fe, Mn, Cd), due to their similar ionic radii to Zn²⁺, can directly replace Zn in the crystal lattice [3,22,23,29,56,59]. This mechanism is consistent with our data: the combination of a significant negative correlation between the (Fe + Mn + Cd) sum and Zn (Figure 9a) and the smooth, flat time-resolved profiles of these elements supports the occurrence of a direct substitution mechanism (Fe²⁺, Mn²⁺, Cd²⁺) ↔ Zn²⁺ in the deposit.

Furthermore, the extent of iron incorporation into the sphalerite lattice is known to be influenced by temperature, sulfur and/or oxygen fugacity during mineral formation [60,61,62]. Consequently, the high Fe content observed in some sphalerite samples from Huoyanshan suggests mineralization likely occurred under conditions of relatively low sulfur and/or oxygen fugacity, or at elevated temperatures [63,64].

Sphalerite from the Huoyanshan deposit exhibits a notably high Cu content. LA-ICPMS time-resolved spectra reveal two distinct ablation patterns for Cu (Figure 4): (1) a relatively smooth profile, indicative of Cu in solid solution within the sphalerite lattice; and (2) profiles featuring sharp, intermittent peaks, signaling the presence of micron-scale chalcopyrite inclusions—a texture commonly referred to as “chalcopyrite disease” [65]. While such physical interference can introduce statistical ‘noise’ in geochemical interpretations, these effects were rigorously addressed during our data processing. The significantly higher concentration of Cu compared to Ag in our samples suggests that Cu⁺ serves as the primary monovalent cation facilitating coupled substitutions. The positive correlations of Cu with Sb³⁺ and Sn⁴⁺ (Figure 9d,e) indicate that Cu enters the sphalerite lattice via the substitution mechanisms: 2Zn²⁺ ↔ Cu⁺ + Sb³⁺ and 3Zn²⁺ ↔ 2Cu⁺ + Sn⁴⁺. Furthermore, a significant positive correlation between Cu and In (Figure 9f) confirms the well-established 2Zn²⁺ ↔ Cu⁺ + In³⁺ substitution [66]. The correlation observed between In and Sn (Figure 9c) further suggests the potential operation of more complex mechanisms, such as 3Zn²⁺ ↔ In³⁺ + Sn²⁺ + (Cu, Ag) ⁺ or 4Zn²⁺ ↔ In³⁺ + Sn⁴⁺ + (Cu, Ag) ⁺ + □ [22,23].

To ensure the accuracy of the trace element data and the subsequent geothermometry, all LA-ICP-MS time-resolved spectra were carefully inspected. Analytical segments exhibiting sharp, intermittent Cu and Fe spikes—characteristic of chalcopyrite inclusions—were strictly excluded during off-line data reduction. Only the stable plateau signals representing the elements held within the sphalerite crystal lattice were integrated. Consequently, the high Fe and Cu concentrations discussed in the following sections reflect the true lattice-bound composition of the sphalerite, rather than contamination from mineral inclusions.

5.2. Mineralization Temperature

Trace element concentrations in sphalerite (e.g., Fe, Mn, Ga, Ge, In) are highly sensitive to formation temperature, making it an effective geothermometer [23,29,61,67]. While conventional methods relying solely on FeS content have certain limitations [68,69], recent multivariate approaches, such as the trace element-based “GGIMFis” geothermometer developed by Frenzel et al. [23], provide a robust alternative for constraining mineralization temperatures.

In this study, the crystallization temperatures of sphalerite were determined using the “GGIMFis” geothermometer developed by Frenzel et al. [23]. This trace-element-based thermometer calculates temperature (T) using the principal component (PC 1′), which is derived from the concentrations of Ga, Ge, Fe, Mn, and In. The specific equations are as follows:

$$\mathrm{T}\left(°\mathrm{C}\right)=-\left(54.4\pm 7.3\right)·\mathrm{PC} 1^{\prime } + (208\pm 10)$$

(1)

$$\mathrm{PC} 1^{\prime }=\ln \left({\displaystyle \frac{{\mathrm{C}}_{\mathrm{Ga}}^{0.22}·{\mathrm{C}}_{\mathrm{Ge}}^{0.22}}{{\mathrm{C}}_{\mathrm{Fe}}^{0.37}·{\mathrm{C}}_{\mathrm{Mn}}^{0.20}·{\mathrm{C}}_{\mathrm{In}}^{0.11}}}\right)$$

(2)

where C_Ga, C_Ge, C_Mn and C_In are trace element concentrations in ppm, and C_Fe is the iron concentration in wt.%. Applying these equations to the trace element data presented in Table 2, the calculated temperatures range from 225 °C to 301.6 °C (avg. 265.8 °C). These values are in close agreement with fluid inclusion homogenization temperatures (ca. 265 °C) previously reported for the same deposit by the Gansu Nonferrous Geological Survey Bureau [43], effectively constraining the thermodynamic regime of the mineralization system during its waning stage. Since the analyzed sphalerite is intimately intergrown with Py III and galena (Figure 3d,f,g), this average temperature specifically represents a thermal “snapshot” of the waning hydrothermal system. The medium-to-low temperature regime captures the cooling trajectory from the initial high-temperature core (>300 °C) associated with early-stage mineralizing fluids. The transition from high-temperature Py I and II to the 265.8 °C environment of sphalerite and Py III reflects the typical spatial and temporal evolution of VMS hydrothermal upflow zones.

5.3. Origin of Sulfur

Volcanic massive sulfide (VMS) deposits originate from prolonged and complex interactions between seawater-derived hydrothermal fluids and the crust [4,8,70,71,72,73,74,75,76]. Extensive research indicates that the sulfur in these hydrothermal mineralization systems is primarily derived from three principal sources: (1) leaching of sulfur-bearing components from the surrounding coeval felsic volcanism [77]; (2) magmatic sulfur contributions [78,79,80]; and (3) reduction of coeval seawater sulfate via thermochemical sulfate reduction (TSR) or bacterial sulfate reduction (BSR) [7,9,81].

A minor presence of barite and gypsum, along with other silicate minerals, is observed in the Huoyanshan deposit. This suggests the probable involvement of seawater sulfate during the early stages of mineralization. However, the δ³⁴S values of sulfides analyzed in this study exhibit a high degree of homogeneity and are characterized by a tightly clustered distribution (Figure 7). According to the model established by Ohmoto [82], under conditions where large-scale sulfate precipitation is absent and the fluid maintains a low oxygen fugacity (fO₂), the δ³⁴S values of resultant sulfides can closely approximate the δ³⁴S composition of the total sulfur in the hydrothermal fluid.

During the early-to-middle stages of mineralization, pyrite (Py I and Py II) exhibits relatively enriched δ³⁴S values ranging from +4.98‰ to +7.47‰. These values are consistent with the δ³⁴S ranges documented for many Paleozoic volcanic-hosted massive sulfide (VMS) deposits [7]. Although sulfides precipitated from pristine magmatic–hydrothermal fluids are commonly characterized by lighter S isotopes (approaching 0 ± 3‰) [83], the enriched δ³⁴S values of Py I and Py II (+4.98‰ to +7.47‰) observed in the Huoyanshan deposit represent a significant positive isotopic shift. This enrichment is interpreted to reflect the dominant influence of thermochemical sulfate reduction (TSR) [7,77]. High-temperature magmatic fluids, which are supported by the elevated Co/Ni ratios, likely interacted with infiltrated Paleozoic seawater, promoting the reduction of isotopically heavy marine sulfate to H₂S [82,84]. The incorporation of this reduced, heavy sulfur into the hydrothermal system overprinted the original magmatic signature, thereby reconciling the enriched sulfur isotopes with the magmatic-hydrothermal origin of the deposit.

The late-stage pyrite (Py III) records a marked geochemical shift, characterized by a decrease in δ³⁴S values to approximately +3.72‰ to +3.89‰ and a pronounced drop in the Co/Ni ratio to below 0.1. This synchronous isotopic and elemental shift reflects an increase in oxygen fugacity (fO₂) and cooling of the fluid, driven by the collapse of the hydrothermal plume and the incursion of oxygenated seawater [82,85]. Although the dataset for Py III is limited, the consistent clustering of these values near the host rocks composition (+3.68‰; Figure 10) implies that, while oxidation can drive fractionation towards lighter values [82], the sulfur budget became increasingly dominated by the recycling of sulfur from the volcanic wall-rock sequence as the direct magmatic input waned [51,86]. Furthermore, as noted in Section 4.2, the sulfur isotopic disequilibrium observed between chalcopyrite (δ³⁴S avg. 4.40‰) and sphalerite (δ³⁴S avg. 3.15‰) strongly corroborates the multi-stage nature of the Huoyanshan deposit. Chalcopyrite, primarily associated with the Py II stage, reflects the earlier, isotopically enriched magmatic–hydrothermal fluid modified by TSR. In contrast, sphalerite co-precipitated with Py III during the waning stage, capturing the isotopic shift towards lighter values caused by the progressive incursion and mixing of oxidized seawater.

5.4. Ore Genesis

The trace element composition of sulfide minerals acts as a sensitive archive of mineralization conditions, capturing the complex interplay between crystallization temperature, fluid source, and precipitation mechanism. Therefore, systematic geochemical analysis of sulfides, particularly pyrite, provides a powerful diagnostic tool for discriminating between distinct genetic deposit types and for reconstructing the temporal evolution of hydrothermal systems [28].

Within the pyrite lattice, cobalt and nickel are incorporated primarily via isomorphous substitution for iron. The partitioning of these elements is strongly temperature-dependent: Co is preferentially incorporated into pyrite at elevated temperatures, whereas Ni exhibits limited temperature sensitivity or may even be slightly enriched in lower-temperature fluids [84]. Consequently, the Co/Ni ratio in pyrite is widely employed as a sensitive discriminant for deposit genesis [29,91]. Typically, pyrite formed from high-temperature hydrothermal fluids in volcanogenic massive sulfide (VMS) systems displays Co/Ni ratios between 5 and 100, in contrast to sedimentary or diagenetic pyrite, which generally yields ratios below 1 [53].

Based on integrated in situ trace element and sulfur isotope data, together with the multivariate geochemical discrimination presented in Figure 11, the Huoyanshan deposit is interpreted to have formed in a dynamic magmatic–hydrothermal system that underwent two distinct evolutionary stages. During the early high-temperature core stage (>300 °C), metal-rich, reduced fluids driven by magmatic heat ascended and precipitated pyrite (Py I and Py II). This stage is represented by the precipitation of Py I and Py II, which exhibit euhedral to subhedral textures and smooth mutual boundaries with chalcopyrite, suggesting primary crystallization from the ore-forming fluid rather than later metamorphic recrystallization (Figure 3a–c). These pyrites plot above the 1:1 line in Figure 11b and exhibit elevated Co/Ni ratios, and consistently fall within the magmatic-hydrothermal fields in Figure 11c,d, supporting a magmatic fluid source [5,84]. Their relatively heavy δ³⁴S values (+4.98‰ to +7.47‰) reflect thermochemical sulfate reduction (TSR) of entrained Paleozoic seawater sulfate by high-temperature magmatic fluids, introducing isotopically heavy sulfur into the system [7,77]. As the hydrothermal system waned, the late stage was characterized by progressive seawater ingress [92]. Cool, oxidized seawater infiltrated the fluid conduits, leading to mixing and increased oxygen fugacity (fO₂) [51,82]. Py III displays a pronounced shift toward the Ni-enriched domain in Figure 11b,d, with Co/Ni ratios (<0.1) approaching those of sedimentary pyrite. This coupled isotopic and elemental shift records fluid cooling, oxidation, and seawater mixing during the final stage of mineralization. It should be noted, however, that the trace element dataset for Py III is currently limited (n = 4). While the observed geochemical shift is consistent with late-stage seawater incursion and cooling, these interpretations regarding the final mineralization phase should be treated with caution. Further analytical work with a larger sample size of Py III is required to fully validate this evolutionary model.

This evolutionary path—from a magmatically dominated, high-temperature core to a seawater-inundated, cooler margin—is a diagnostic feature of classical VMS systems worldwide [4,8]. From a spatial and exploration perspective, although our current sampling was primarily restricted to specific underground elevations (1655~1741 m), the observed geochemical transition offers potential vectoring tools for future mineral exploration. The high Co/Ni ratios and enriched heavy trace elements in early-stage pyrite (Py I and Py II) can serve as a robust proxy for proximity to the high-temperature upflow zones or vent cores of the VMS system. Conversely, the lower Co/Ni ratios and specific trace element signatures in late-stage sulfides reflect peripheral or cooler environments. Recognizing these micro-scale trace element patterns can thus aid in vectoring towards hidden high-grade ore bodies in the deeper parts or margins of the Baiyinchang ore field. Therefore, based on the integrated evidence from early-stage pyrite and sphalerite geochemistry, the Huoyanshan deposit is classified as a magmatic–hydrothermal-type VMS deposit.

Further genetic constraints can be derived from the ratios of As/Ag and Sb/Bi. According to Augustin et al. [93], diagenetic pyrite is characterized by low As/Ag and medium Sb/Bi, magmatic pyrite by low values in both ratios, and hydrothermal pyrite by high As/Ag and medium Sb/Bi. When plotted on a binary As/Ag–Sb/Bi diagram (Figure 11c) and a Co–Ni–As ternary diagram (Figure 11d), both early (Py I) and late (Py II) generations of Huoyanshan pyrite predominantly fall within the magmatic–hydrothermal field. This consistent signature further corroborates a magmatic–hydrothermal origin for the pyrite mineralization in the deposit.

Figure 11. Discriminant diagrams for the Huoyanshan deposit. (a) Plot of lnGa vs. lnIn in sphalerite (Base map after [94]); (b) Co vs. Ni diagram for pyrite; (c) As/Ag vs. Sb/Bi diagram (Base map after [93]); (d) Co-Ni-As ternary diagram for pyrite (Base map after [95]).

Figure 11. Discriminant diagrams for the Huoyanshan deposit. (a) Plot of lnGa vs. lnIn in sphalerite (Base map after [94]); (b) Co vs. Ni diagram for pyrite; (c) As/Ag vs. Sb/Bi diagram (Base map after [93]); (d) Co-Ni-As ternary diagram for pyrite (Base map after [95]).

The Ga/In and Ge/In ratios in sphalerite serve as sensitive geochemical indicators for discriminating deposit genesis, particularly in identifying magmatic–hydrothermal affiliations. Typically, sphalerite from magmatic–hydrothermal systems is characterized by Ga/In < 1 and Ge/In < 0.1, whereas sphalerite from sedimentary or stratiform deposits commonly exhibits both ratios greater than 10 [29,92]. In the Huoyanshan deposit, sphalerite displays Ga/In ratios ranging from −8 to 7.05 (avg. 1.63) and Ge/In ratios from 0.1 to 0.23 (avg. 0.04). Notably, the average values of both ratios fall within the range indicative of a magmatic–hydrothermal origin. This interpretation is further supported by a sphalerite trace element discrimination diagram (Figure 11a), in which all analytical data plot within the fields for volcanic-associated and magmatic–hydrothermal deposit types. Collectively, these lines of evidence strongly suggest that the Huoyanshan deposit is genetically linked to magmatic–hydrothermal activity.

6. Conclusions

Based on comprehensive fieldwork, petrographic studies, and in situ LA-(MC)-ICP-MS analysis of sulfide minerals from the Huoyanshan Cu-Zn polymetallic deposit, the following conclusions can be drawn:

(1) In pyrite, trace elements such as Co, Ni, and As are hosted primarily by isomorphic substitution within the crystal lattice. Pb and Bi occur predominantly as micro-to-nano-scale mineral inclusions; notably, Bi signals are visually subdued in time-resolved profiles due to their low absolute concentrations. In sphalerite, Fe, Mn, and Cd enter the lattice via direct substitution for Zn, while Cu incorporation is achieved through coupled substitution with elements such as Sb, Sn, and In (e.g., 2Zn²⁺ ↔ Cu⁺ + In³⁺).

(2) Trace element compositions in sphalerite indicate formation in a medium-to-low-temperature environment. Application of the “GGIMFis” geothermometer yields crystallization temperatures ranging from 225 °C to 301.6 °C (avg. 265.8 °C), which is consistent with fluid inclusion homogenization data. This temperature regime specifically captures a thermal “snapshot” of the waning hydrothermal system during the precipitation of late-stage sulfides.

(3) The mineralization evolved through two distinct geochemical stages characterized by a lack of sulfur isotopic equilibrium, strongly supporting the multi-stage nature of the deposit. Early High-Temperature Stage: This stage was dominated by high-temperature (>300 °C), reduced magmatic–hydrothermal fluids. It precipitated Py I and Py II, characterized by high Co/Ni ratios (>1). Their enriched δ³⁴S signatures (+4.98‰ to +7.47‰) reflect the reduction of entrained Paleozoic seawater sulfate via thermochemical sulfate reduction (TSR) by the ascending magmatic fluids, which modified the original magmatic sulfur isotopic signature. Late Seawater-Influenced Stage: Following the fracturing of the primary hydraulic seal, the incursion of cold, oxygenated seawater led to the precipitation of Py III and sphalerite. This stage is marked by sediment-like Co/Ni ratios (<0.1) and lighter sulfur isotopic compositions, reflecting the oxidative mixing of residual hydrothermal fluids with seawater. The observed isotopic reversal (δ³⁴Spy > δ³⁴Sccp > δ³⁴Ssp) confirms this disequilibrium and distinct paragenetic shift.

(4) Integrated trace element diagnostic ratios (Co/Ni in pyrite; Ga/In and Ge/In in sphalerite) and sulfur isotopes definitively classify Huoyanshan as a typical magmatic–hydrothermal VMS deposit genetically linked to Ordovician submarine volcanism. Importantly, the observed micro-scale transition from a high-temperature, Co/Ni-enriched magmatic core to a cooler, seawater-mixed margin provides a robust geochemical vectoring tool. Recognizing these temporal and spatial trace element patterns can significantly aid in vectoring towards hidden high-temperature upflow zones or vent centers in the Baiyinchang ore field.