Multi-Stage Metallogenesis and Fluid Evolution of the Hongtoushan Cu-Zn Volcanogenic Massive Sulfide Deposit, Liaoning Province, China: Constraints from Sulfur Isotopes, Trace Elements, and Fluid Inclusions

Abstract

The Hongtoushan Cu-Zn volcanogenic massive sulfide (VMS) deposit, located in the Hunbei granite–greenstone terrane of the North China Craton, has undergone a complex, multi-stage metallogenic evolution. The deposit comprises three main types of massive ores: Type-1 ores, characterized by a sulfide matrix enclosing granular quartz and dark mineral aggregates; Type-2 ores, distinguished by large pyrite and pyrrhotite porphyroblasts and a small amount of gangue minerals; and Type-3 ores, mainly distributed in the contact zone between the ore body and gneiss, featuring remobilized chalcopyrite and sphalerite filling the cracks of pyrite. The metallogenic process of the Hongtoushan deposit is divided into three main stages: (1) an early mineralization stage forming Type-1 massive ores; (2) a metamorphic recrystallization stage resulting in Type-2 massive ores with distinct textural features; and (3) a late-stage mineralization event producing Type-3 massive ores enriched in Cu, Zn, and other metals. This study integrates sulfur isotope, trace elements, and fluid inclusion data to constrain the sources of ore-forming materials, fluid evolution and metallogenic processes of the deposit. Sulfur isotope analyses of sulfide samples yield ${\delta }^{34}$S values ranging from −0.7 to 4.2 (mean: 1.8 ± 1.5, 1$\sigma$), suggesting a predominant magmatic sulfur source with possible contributions from Archean seawater. Trace element analyses of pyrite grains from different ore types reveal a depletion of rare earth elements, Cu, and Zn in Type-2 massive ores due to metamorphic recrystallization, and a subsequent re-enrichment of these elements in Type-3 massive ores. Fluid inclusion studies allowed for identifying three types of ore-forming fluids: Type-1 (avg. $T_h$: 222.9; salinity: 6.74 wt.% NaCl eqv.), Type-2 (avg. $T_h$: 185.72; salinity: 16.56 wt.% NaCl eqv.), and Type-3 (avg. $T_h$: 184.81; salinity: 16.22 wt.% NaCl eqv.), representing a complex evolution involving cooling, water–rock interaction and fluid mixing. This multi-disciplinary study reveals the interplay of magmatic, hydrothermal and metamorphic processes in the formation of the Hongtoushan VMS deposit, providing new insights into the fluid evolution and metallogenic mechanisms of similar deposits in ancient granite–greenstone terranes.

1. Introduction

Volcanogenic massive sulfide (VMS) deposits are globally significant sources of both base and precious metals, carrying substantial economic value [1,2,3,4,5]. In addition to Cu and Zn, VMS deposits often contain important precious metals such as Au and Ag, as well as other critical metal elements, including Ni, Co, Pb, and rare earth elements (REEs). These elements have wide applications in modern industries, such as electronics, new energy, aerospace, and other fields, making the study of VMS deposits of great economic and strategic significance [6]. These VMS deposits typically form beneath or directly on the seafloor from the discharge of evolved, high-temperature hydrothermal fluids [7,8]. The metals found in these deposits originate from hydrothermal reaction zones, where they have been leached out by heated seawater that has undergone compositional modifications. This seawater, driven by a steep geothermal gradient, has permeated through the volcanic pile, with or without contributions from magmatic sources for the metals [7,9].

The Hongtoushan Cu-Zn deposit, located in the Hunbei granite–greenstone terrane in the northeastern section of the North China Craton, Liaoning Province, has been extensively studied due to its unique geological features. The terrane itself hosts over a hundred VMS deposits, making it a highly significant mineralization belt for VMS deposits in China. Notably, the Hongtoushan deposit stands out as the only large-scale Archean Cu-Zn VMS deposit identified in China thus far [10,11].

The metallogenesis of the Hongtoushan VMS deposit is quite complex and multistage, as evidenced by the diverse mineralogy, textures, and geochemical characteristics of the ores. Previous studies have made significant progress in understanding the geology and genesis of this deposit. For example, Zhu et al. (2015) [12] obtained a hydrothermal zircon age of 2507 Ma, which is close to the regional metamorphic timing, indicating that the superimposed hydrothermal system is related to metamorphism. Zhang et al. (1984) [10] conducted a systematic study on the geological characteristics of the Hongtoushan deposit, identifying the volcanic–sedimentary sequence that hosts the deposit, suggesting it might be a syngenetic VMS deposit. Gu et al. (2007) [11] further revealed the multistage nature of the mineralization through mineralogical and geochemical investigations, and pointed out that the ore-forming fluids may have undergone an evolution from high to low temperatures. However, there are still significant knowledge gaps regarding the specific mineralization processes and sources of ore-forming elements and fluids in the Hongtoushan deposit. For example, the relative contributions of magmatic fluids versus metamorphic fluids in remobilizing and upgrading the ores remain unclear [13,14]. Additionally, the role of post-mineralization metasomatism in modifying metal distributions and ore grades warrants further investigation using integrated textural and geochemical approaches [14]. Resolving these uncertainties is crucial for refining the genetic model of the Hongtoushan deposit and advancing our understanding of fluid–rock interactions in the broader context of metamorphosed VMS systems. Insights gained from this deposit have important implications for exploration targeting and resource assessments of similar deposits in the region. Therefore, this study aims to provide new constraints on the mineralization processes, fluid sources, and post-mineralization modifications through a comprehensive analysis of the mineral textures, alteration assemblages, and in situ geochemical and isotopic data.

Fluid inclusions, stable isotopes, and analyses of trace elements of minerals, especially pyrite, pyrrhotite and sphalerite, are powerful tools for tracing the sources of ore-forming fluids, reconstructing the hydrothermal evolutionary conditions, and unraveling the origins of the materials involved in the formation of VMS deposits [3,15,16,17,18,19,20,21,22,23,24,25,26,27]. The comprehensive application of these geochemical indicators can provide crucial information for deciphering the complex hydrothermal processes and fluid evolution history in the Hongtoushan deposit.

In this paper, we present a detailed study of sulfide minerals in copper-rich massive ores from the Hongtoushan VMS deposit, focusing on their mineralogy, textures, geochemistry, stable isotopes and fluid inclusions. The main objectives of this study are to: (1) document the mineralogy, mineral assemblages and textures of sulfide minerals; (2) determine the sulfur sources and evolution of sulfur isotopic compositions in sulfide minerals; (3) investigate the rare earth element (REE) patterns and trace element geochemistry of sulfide minerals to understand the nature of the ore-forming fluids; and (4) analyze the fluid inclusions to constrain the physical and chemical conditions of the hydrothermal system. By integrating these datasets, we aim to elucidate the fluid evolution and metal enrichment mechanisms during the multistage mineralization processes in the Hongtoushan VMS deposit.

2. Regional Geology

The Hongtoushan Cu-Zn VMS deposit is located in the Qingyuan granite–greenstone belt, which is part of the North China Craton (NCC). The NCC consists of a metamorphic basement that ranges from the Archean to Paleoproterozoic ages, and is overlain by unmetamorphosed cover rocks that range from the Mesoproterozoic to Cenozoic. The metamorphic basement rocks have ages ranging from 3.8 to 3.2 Ga [28,29,30]. The Neoarchean metamorphic rocks are mainly composed of tonalite–trondhjemite–granodiorite (TTG) gneisses with ages of 2.6~2.5 Ga, syntectonic granites at ~2.5 Ga, and various supracrustal rocks that underwent greenschist to granulite facies regional metamorphism and polyphase deformation around 2.5 Ga [31,32,33]. The NCC is divided into two major Archean to Paleoproterozoic blocks, the Eastern and Western Blocks, separated by the Paleoproterozoic Trans-North China orogenic belt [34]. The Qingyuan greenstone belt, comprising 60~70% Neoarchean plutons and 30~40% supracrustal rocks, is situated at the northern margin of the Eastern Block [35,36] (Figure 1).

The Qingyuan Group, which is older than 3.0 Ga, experienced upper amphibolite facies metamorphism between 2.9 and 2.8 Ga [38,39]. The medium- to coarse-grained Neoarchean Precambrian basement in Qingyuan mainly comprises TTG gneiss, and is divided into the Hunbei and Hunnan terranes by the Hunhe fault [39,40]. The supracrustal rocks are further subdivided into the Shipengzi, Hongtoushan, and Nantianmen formations, which underwent amphibolite to granulite facies metamorphism [38]. Over 100 occurrences of mineralization have been found within the Hunbei greenstone belt, with the Hongtoushan deposit being the largest VMS deposit in the area. Impermeable volcanic layers control the size and location of the Cu-Zn mineralization in the region. All VMS deposits in the area are confined to a 100 m thick stratigraphic interval within the upper part of the Hongtoushan Formation. This unit, termed the “rhythmical member” by local geologists, is primarily composed of alternating layers of biotite plagioclase gneiss and hornblende plagioclase gneiss [41].

Regional tectonic evolution played a crucial role in controlling the formation of the Hongtoushan deposit. Previous studies have shown that the regional extensional tectonics during the late Paleoproterozoic to early Neoproterozoic led to basic magmatism, creating favorable volcanic environments and hydrothermal systems [35]. The regional metamorphism and granitic intrusions in the late Neoarchean may have promoted the activation and enrichment of ore-forming elements [42,43].

3. Ore Deposit Geology

The Hongtoushan deposit is a notable Archean VMS Cu-Zn deposit located in the Hunbei district. The local residents began surface mining in the early 1930s, and large-scale underground operations commenced in the 1960s after systematic drilling. The deposit holds a metal reserve of 0.5 Mt Cu with grades of 1.5–1.8%, 0.7 Mt Zn with grades of 2.0–2.5% and 20 t Au with grades of 0.5–0.8 g/t [11]. The mining operations have reached a depth of 1337 m [44].

The primary ore deposits in the region are confined within the “rhythmic member” of the upper section of the Hongtoushan formation, which comprises seven layers. These layers include: (1) a 12 m thick layer of biotite gneiss and quartz-feldspar gneiss with intercalations of plagioclase hornblende gneiss; (2) a 25 m thick layer of garnet quartz plagioclase gneiss intercalated with granulite; (3) a 20 m thick layer of plagioclase amphibole gneiss and biotite plagioclase gneiss intercalations; (4) a 20 m thick layer of plagioclase hornblende gneiss intercalated with thin biotite plagioclase gneiss; (5) a 20 m thick layer of biotite plagioclase gneiss with plagioclase hornblende gneiss; (6) a 10 m thick layer of plagioclase hornblende gneiss and biotite plagioclase gneiss; and (7) interbedded plagioclase hornblende gneiss (Figure 2A).

The surrounding rocks comprise mainly amphibolite gneiss, sillimanite–biotite gneiss, and biotite–plagioclase gneiss. The protoliths are believed to be basic-intermediate acid volcanic rocks and sedimentary rocks within volcanic formations [45]. The Archean Qingyuan greenstone belt has undergone multiple episodes of deformation, metamorphism, intrusion by granites, and migmatization, resulting in the Hongtoushan VMS deposit having a highly intricate structure. Above a depth of −467 m, the ore bodies exhibit a Y-shaped geometry, with two eastward-extending branches merging westward into a single ore zone (Figure 2B). According to previous research results [37], three periods of tectonic deformation have been identified in the Hongtoushan VMS deposit: (1) In the initial phase, the north–south tight fold was generated in response to the east–west extrusion force. (2) Following this, the north–south extrusion force led to the formation of the east–west plunging vertical fold in the subsequent stage. Under tectonic compression, the original ore body migrated from the limbs towards the hinge zone, resulting in the thickening and upgrading of the ore body and the formation of the “ore pillar”. (3) Lastly, driven by east–west extrusion and north–south torsion, the third stage culminated in the development of the north–south open fold. Fold deformation not only transforms the morphology of the ore bodies but also plays a crucial role in controlling and hosting the mineralization, leading to the remobilization and enrichment of the ore-forming elements.

The ores in the Hongtoushan deposit have been generally classified by previous researchers into two main types: massive ore and disseminated ore. These two ore types exhibit distinct differences in their mineralogical composition and textural characteristics [13]. The primary ore minerals include pyrite, pyrrhotite, sphalerite and chalcopyrite, accompanied by minor occurrences of magnetite, galena, molybdenite, argentite and electrum. The gangue mineral assemblage comprises quartz, plagioclase, biotite, phlogopite, sericite, garnet, gahnite and sillimanite. The ore bodies have undergone extensive wall rock alteration, which is manifested by tremolitation, phlogopitization, sericitization, silicification, chloritization and carbonation. The pervasive high-grade metamorphism and deformation have significantly obliterated the primary sedimentary structures, posing challenges in deciphering the original depositional environment. To better understand the mineralogical and textural variations within the deposit, we have categorized the massive ore into three distinct types (classified as: Type-1 massive ores, Type-2 massive ores and Type-3 massive ores) based on their characteristic mineral assemblages and textural features. These ore types will be elaborated upon in the forthcoming sections.

4. Sampling and Analytical Methods

4.1. Sampling

Sampling focuses on the copper-rich massive ores of the Hongtoushan deposit, where the three types of massive ores are collected from the Hongtoushan underground mine. Individual minerals (mainly pyrite and pyrrhotite) are extracted from the massive ores and are lightly crushed to a grain size of approximately 40–60 mesh using a carefully pre-cleaned agate mortar and pestle. They are washed and then handpicked to a purity of more than 99% under a binocular microscope. Summarily, 53 thin sections are studied by transmitted- and reflected-light optical microscopy red (OLYMPUS BX51, Olympus Co., Ltd., Tokyo, Japan), and 14 metal sulfides samples are selected for trace (rare earth) element analysis; additionally, 51 samples are selected for microthermometry and Laser Raman microspectroscopy. Additionally, eight pyrite samples are selected for Pb isotope analysis, while 21 sulfides (pyrite, pyrrhotite and sphalerite) are selected for S isotope analysis.

4.2. Geochemistry of Sulfides

The trace (rare earth) element analysis were performed at the Analytical Laboratory Beijing Research Institute of Uranium Geology (ALBRIUG), with Elan DRC-e inductively coupled plasma mass spectrometer red (ICP-MS, PerkinElmer SCIEX, Waltham, MA, USA)and based on the GB/T14506.30 standard [46]. ICP-MS is a highly sensitive and precise technique for trace element analysis, capable of detecting elements at very low concentrations (ppb to ppt levels) with high accuracy and precision. Compared to other techniques, such as neutron activation analysis (NAA) and X-ray fluorescence (XRF), ICP-MS offers lower detection limits, wider dynamic range, and the ability to analyze a larger number of elements simultaneously [47,48]. Analytical uncertainties are 1–3% for major elements. For trace element and rare earth element analyses, rock powders (50 mg) were dissolved using mixed acids (HF + HClO₄) in capped Savillex Teflon breakers at 120 °C for 6 days, and subsequently dried to wet salt and re-dissolved in 0.5 mL HClO₄. The solutions were then evaporated to wet salt at 140 °C and redissolved in 1 mL HNO₃ and 3 mL water for 24 h at 120 °C. The solutions were diluted in 2% HNO₃ for analysis. The uncertainties based on the replicate analyses of internal standards are ±5% for REE and ±5–10% for trace elements. The data of trace elements (including REE) of sulfide are listed in

Table 1. Trace element (ppm) and rare earth element compositions of metal sulfide of Hongtoushan Deposit.

Ore-Type	Type-1 Massive Ores
Sample No.	Mineral	Li	Be	Sc	V	Cr	Co	Ni	Cu	Zn	Ga	Rb	Sr	Y	Mo	Cd	In	Sb
1	Pyrrhotite	0.477	0.047	0.88	8.8	2.31	0.002	4.71	1557	1769	4108	127	0.888	2.06	0.462	0.444	-	23.5
2	Pyrrhotite	0.436	0.043	1.81	36.6	24.7	1.39	2.42	240	2265	60.6	1.15	4.25	2.29	0.707	9.48	0.758	0.786
3	Pyrrhotite	0.216	0.079	1.67	22.1	8.01	2.43	18.2	5889	3067	216	0.532	3.08	16.9	1.89	10.7	0.743	1.76
avg	Pyrrhotite	0.376	0.056	1.453	22.500	11.673	1.274	8.443	2562.000	2367.000	1461.533	42.894	2.739	7.083	1.020	6.875	0.751	8.682
4	Pyrite	0.043	0.037	0.567	0.431	30.1	276	7.32	454	587	0.19	0.258	0.372	0.046	0.626	1.78	0.068	0.236
5	Pyrite	0.022	0.006	0.128	0.548	30.6	322	6.77	218	138	0.155	0.074	0.311	0.02	0.17	0.367	0.047	0.134
6	Pyrite	0.028	0.024	0.316	1.21	0.064	395	5.77	677	729	0.269	0.08	0.351	0.039	0.296	2.17	0.25	0.19
7	Pyrite	0.029	0.033	2.7	1.49	34	402	8.62	1034	301	0.207	0.127	0.34	0.051	0.685	0.95	0.128	0.305
avg	Pyrite	0.031	0.025	0.928	0.920	23.691	348.750	7.120	595.750	438.750	0.205	0.135	0.344	0.039	0.444	1.317	0.123	0.216
Sample No.	Mineral	Cs	Ba	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	W
1	Pyrrhotite	0.037	0.131	1.84	0.627	1.47	0.829	0.779	0.192	0.081	0.238	0.034	0.243	0.054	0.222	0.037	0.245	0.027
2	Pyrrhotite	0.491	9.49	1.77	3.57	0.421	1.63	0.356	0.156	0.337	0.066	0.389	0.082	0.232	0.043	0.294	0.043	0.556
3	Pyrrhotite	0.305	5.28	11.7	25.5	3.2	13.7	3.01	0.59	2.8	0.552	2.88	0.548	1.53	0.234	1.38	0.187	0.926
avg	Pyrrhotite	0.278	4.967	5.103	9.899	1.697	5.386	1.382	0.313	1.073	0.285	1.101	0.291	0.605	0.166	0.570	0.158	0.503
4	Pyrite	0.022	1.44	0.053	0.101	0.011	0.043	0.01	-	0.006	-	0.005	-	0.004	-	0.003	-	0.349
5	Pyrite	0.005	2.35	0.014	0.025	0.002	0.052	0.004	-	-	-	0.01	-	-	-	0.002	-	0.024
6	Pyrite	0.011	0.624	0.02	0.033	0.004	0.016	0.004	-	0.002	-	0.003	-	0.003	-	0.004	-	0.182
7	Pyrite	0.009	0.811	0.035	0.066	0.007	0.027	0.009	-	0.005	-	0.004	-	-	-	0.005	-	0.073
avg	Pyrite	0.012	1.306	0.031	0.056	0.006	0.035	0.007	-	0.004	-	0.006	-	0.004	-	0.004	-	0.157
Sample No.	Mineral	Re	Tl	Pb	Bi	Th	U	Nb	Ta	Zr	Hf	$\mathrm{\Sigma }$REE	$\mathrm{\Sigma }$LREE	$\mathrm{\Sigma }$HREE	LREE:HREE	$\mathbf{\delta }$Eu	$\mathbf{\delta }$Ce	(La:Yb)N
1	Pyrrhotite	0.016	0.208	52.9	51.1	1.41	0.179	0.156	0.005	1.38	0.034	6.891	5.737	1.154	4.971	1.213	0.086	33.527
2	Pyrrhotite	0.016	0.407	82.3	2.15	0.178	0.042	0.287	-	0.18	0.008	9.389	7.903	1.486	5.318	1.358	0.965	4.059
3	Pyrrhotite	0.016	0.783	122	4.07	1.21	0.122	0.948	0.019	0.482	0.035	67.811	57.700	10.111	5.707	0.612	0.987	5.716
avg	Pyrrhotite	0.016	0.466	85.733	19.107	0.933	0.114	0.464	0.012	0.681	0.026	28.030	23.780	4.250	5.332	1.061	0.679	14.434
4	Pyrite	0.017	0.041	30	0.9	0.024	0.017	0.023	-	0.061	-	2.347	0.808	1.539	0.525	-	0.957	11.911
5	Pyrite	0.016	0.033	4.18	0.504	0.015	0.021	0.014	-	0.086	-	2.230	0.687	1.543	0.445	-	1.005	4.719
6	Pyrite	0.016	0.071	13.1	0.914	0.005	0.005	0.032	-	0.035	-	2.200	0.667	1.533	0.435	-	0.839	3.371
7	Pyrite	0.017	0.075	174	4.64	0.026	0.012	0.05	0.03	0.079	0.002	2.272	0.734	1.538	0.477	-	0.959	4.719
avg	Pyrite	0.0165	0.0550	55.3200	1.7395	0.0175	0.0138	0.0298	0.0300	0.0653	0.0020	2.262	0.724	1.538	0.471	-	0.940	6.180
Sample No.	Mineral	Li	Be	Sc	V	Cr	Co	Ni	Cu	Zn	Ga	Rb	Sr	Y	Mo	Cd	In	Sb
1	Pyrrhotite	0.397	0.046	0.674	26	39.1	0.791	22.6	633	2817	1.83	1.04	1.19	5.12	2.11	9.8	1.19	0.714
2	Pyrrhotite	0.3	0.07	1.41	5.27	3.06	1.85	17.9	3285	1410	5.89	0.432	3.37	3.94	0.878	5.24	0.334	0.455
3	Pyrrhotite	0.533	0.054	3.74	19.2	39.1	6.79	9.52	12869	1504	15.2	0.416	5.03	14	7.93	5.07	1.37	3.81
avg	Pyrrhotite	0.41	0.057	1.941	16.823	27.087	3.144	16.673	5595.667	1910.333	7.640	0.629	3.197	7.687	3.639	6.703	0.965	1.660
4	Pyrite	0.01	0.022	9.07	0.739	3.24	135	3.49	186	1603	0.183	0.086	0.72	0.068	0.619	6.57	0.355	0.464
5	Pyrite	0.048	0.003	0.715	24	1.78	366	7.67	407	197	0.222	0.086	0.579	0.421	0.304	0.647	0.14	0.677
6	Pyrite	0.042	0.027	0.218	0.852	4.19	443	16.1	222	97.3	0.124	0.074	0.364	0.023	0.189	0.273	0.014	0.136
avg	Pyrite	0.033	0.017	3.334	8.530	3.070	314.667	9.087	271.667	632.433	0.176	0.082	0.554	0.171	0.371	2.497	0.170	0.426
Sample No.	Mineral	Cs	Ba	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	W
1	Pyrrhotite	0.089	5.03	3.95	7.98	1.04	4.96	1.32	0.151	1.18	0.215	1.01	0.166	0.395	0.057	0.327	0.043	0.341
2	Pyrrhotite	0.062	4.72	2.38	5.22	0.633	2.67	0.612	0.154	0.592	0.127	0.668	0.137	0.361	0.06	0.376	0.053	0.448
3	Pyrrhotite	0.127	4.08	11.9	26.3	3.35	14.3	3.04	0.765	2.66	0.516	2.71	0.522	1.4	0.22	1.36	0.182	1.03
avg	Pyrrhotite	0.093	4.610	6.077	13.167	1.674	7.310	1.657	0.357	1.477	0.286	1.463	0.275	0.719	0.112	0.688	0.093	0.606
4	Pyrite	0.008	0.682	0.097	0.176	0.017	0.057	0.009	0.004	0.007	-	0.005	-	0.004	-	0.01	-	0.098
5	Pyrite	0.01	2.33	0.062	0.15	0.022	0.104	0.051	0.003	0.046	0.012	0.067	0.012	0.038	0.005	0.028	0.005	0.948
6	Pyrite	0.012	1.05	0.018	0.036	0.004	0.007	-	-	-	-	-	-	-	-	-	-	0.022
avg	Pyrite	0.010	1.354	0.059	0.121	0.014	0.056	0.030	0.004	0.027	0.012	0.036	0.012	0.021	0.005	0.019	0.005	0.356
Sample No.	Mineral	Re	Tl	Pb	Bi	Th	U	Nb	Ta	Zr	Hf	$\mathrm{\Sigma }$REE	$\mathrm{\Sigma }$LREE	$\mathrm{\Sigma }$HREE	LREE:HREE	$\mathbf{\delta }$Eu	$\mathbf{\delta }$Ce	(La:Yb)N
1	Pyrrhotite	0.017	0.364	75.2	2.24	0.122	0.035	0.46	0.018	0.344	0.008	22.794	19.401	3.393	5.718	0.363	0.929	8.144
2	Pyrrhotite	0.016	0.286	60.5	2.4	0.274	0.057	0.139	-	0.149	0.008	14.043	11.669	2.374	4.915	0.773	1.004	4.268
3	Pyrrhotite	0.016	1.42	137	4.95	1.54	0.347	0.627	0.055	3.63	0.148	69.225	59.655	9.570	6.234	0.805	0.989	5.899
avg	Pyrrhotite	0.016	0.690	90.900	3.197	0.645	0.146	0.409	0.037	1.374	0.055	35.354	30.242	5.112	5.622	0.647	0.974	6.104
4	Pyrite	0.016	0.09	47.8	2.36	0.014	0.005	0.02	-	0.065	-	1.826	0.360	1.466	0.246	1.487	0.963	6.540
5	Pyrite	0.017	0.107	216	6.92	0.038	0.036	0.175	0.002	0.082	-	0.605	0.392	0.213	1.840	0.186	0.976	1.493
6	Pyrite	0.015	0.039	29.2	1.02	0.004	0.007	0.012	-	0.044	-	0.332	0.119	0.213	0.559	0.186	0.981	0.433
avg	Pyrite	0.016	0.079	97.667	3.433	0.019	0.016	0.069	0.002	0.064	-	0.921	0.290	0.631	0.882	0.620	0.973	2.822
Sample No.	Mineral	Li	Be	Sc	V	Cr	Co	Ni	Cu	Zn	Ga	Rb	Sr	Y	Mo	Cd	In	Sb
1	Pyrrhotite	0.199	0.038	0.021	1.44	4.01	2.22	45.8	971	953	0.353	0.206	1.15	0.276	1.51	20.4	1.29	0.035
2	Pyrrhotite	0.933	0.031	1.14	1.03	0.523	223	105	872	4586	0.788	0.146	0.712	1.5	1.44	1.7	0.036	0.011
avg	Pyrrhotite	0.566	0.0345	0.5805	1.235	2.2665	112.61	75.4	921.5	2769.5	0.5705	0.176	0.931	0.888	1.475	11.05	0.663	0.023
3	Pyrite	0.239	0.021	0.35	1.42	0.113	120	245	41303	3844	0.308	0.454	0.526	0.322	4.66	38.9	1.52	0.038
4	Pyrite	0.004	0.013	0.53	0.571	0.303	104	46.7	1732	473	0.135	0.049	0.434	0.281	3.68	0.801	0.089	0.053
avg	Pyrite	0.122	0.017	0.440	0.996	0.208	112.000	145.850	21517.500	2158.500	0.222	0.252	0.480	0.302	4.170	19.851	0.805	0.046
Sample No.	Mineral	Cs	Ba	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	W
1	Pyrrhotite	0.078	2.83	0.285	0.618	0.074	0.285	0.069	0.011	0.054	0.009	0.049	0.01	0.039	0.006	0.026	0.004	0.088
2	Pyrrhotite	0.065	5.78	8.44	18.3	2.23	9.26	1.89	0.201	1.4	0.208	0.726	0.07	0.126	0.008	0.056	0.005	0.06
avg	Pyrrhotite	0.0715	4.305	4.3625	9.459	1.152	4.7725	0.9795	0.106	0.727	0.1085	0.3875	0.04	0.0825	0.007	0.041	0.0045	0.074
3	Pyrite	0.014	2.41	1.16	2.47	0.306	1.23	0.261	0.042	0.198	0.026	0.104	0.013	0.029	0.003	0.021	0.003	0.158
4	Pyrite	0.017	1.38	0.267	0.552	0.069	0.243	0.054	0.006	0.056	0.008	0.046	0.009	0.021	0.003	0.019	0.002	0.235
avg	Pyrite	0.016	1.895	0.714	1.511	0.188	0.737	0.158	0.024	0.127	0.017	0.075	0.011	0.025	0.003	0.020	0.003	0.197
Sample No.	Mineral	Re	Tl	Pb	Bi	Th	U	Nb	Ta	Zr	Hf	$\mathrm{\Sigma }$REE	$\mathrm{\Sigma }$LREE	$\mathrm{\Sigma }$HREE	LREE:HREE	$\mathbf{\delta }$Eu	$\mathbf{\delta }$Ce	(La:Yb)N
1	Pyrrhotite	0.016	0.033	6.85	4.53	0.097	0.072	0.037	0.003	0.182	0.005	1.539	1.342	0.197	6.812	0.532	1.002	7.390
2	Pyrrhotite	0.018	0.04	9.71	1.79	4.38	0.875	0.07	0.008	0.207	0.01	42.920	40.321	2.599	15.514	0.362	0.995	101.611
avg	Pyrrhotite	0.017	0.0365	8.28	3.16	2.2385	0.4735	0.0535	0.0055	0.1945	0.0075	22.230	20.832	1.398	11.163	0.447	0.999	54.500
3	Pyrite	0.021	0.065	24.9	11.6	0.424	0.158	0.071	0.004	0.208	0.005	5.866	5.469	0.397	13.776	0.543	0.978	37.241
4	Pyrite	0.016	0.103	306	26.4	0.009	0.02	0.02	-	0.061	-	1.355	1.191	0.164	7.262	0.331	0.958	9.474
avg	Pyrite	0.019	0.084	165.450	19.000	0.217	0.089	0.046	0.004	0.135	0.005	3.611	3.330	0.281	10.519	0.437	0.968	23.358

Note: “-” means below detection limit.

.

4.3. Microthermometric and Laser Raman Spectroscopy

The fluid inclusion and Laser Raman spectroscopic analyses were performed at The State Key Laboratory for Mineral Deposits Research at Nanjing University. The study primarily focused on microthermometric and Raman analysis investigations on quartz of ores.

The LinkamTHMS600 heating–freezing stage, with a temperature range of −196 to +600 °C, was used for the fluid inclusion microthermometry. Calibration of the stage involved measuring the melting points of pure water inclusions (0 °C), pure CO₂ inclusions (−56.6 °C), and potassium bichromate (398 °C). The temperature measurements had an accuracy of approximately ±0.2 °C during cooling and slightly deviated to approximately ±2 °C between 100 and 600 °C. The salinities of the NaCl-H₂O fluid inclusions were calculated using the final ice melting temperatures, following the revised method of Bodnar (1993) [49]. The model of Sterner et al. (1988) [50] and Bodnar (1983) [51] were used to estimate the salinity and density of halite-bearing inclusions. The ore-forming pressure was calculated using the empirical equation proposed by Roedder (1984) [52].

A RM2000 Raman microprobe red (Renishaw, New Mills, UK)was used to specify the compositions of single fluid inclusions. The Ar ion laser, with a surface power of 5 mW exciting the radiation (514.5 nm), the detector charge-coupled device (CCD) area of 20 $\mathsf{\mu }$m², and the spectra scanning range of 1000–4000 cm⁻¹ established the analysis parameters. Each scan had an accumulation time of 30 s.

4.4. Sulfur Isotopic Analysis

Sulfur isotope analysis was carried out by the Analytical Laboratory Beijing Research Institute of Uranium Geology (ALBRIUG) and based on the GB/T0184.14 standard [53]. The sulfur isotopic compositions of sulfide and rock samples were determined using MAT251 and isotope ratio mass spectrometers. The analytical methods have been described in detail by Giesemann et al. (1994) [54]. The sulfur isotope results are reported relative to the Canyon Diablo Troilite (CDT) standard, and expressed as ${\delta }^{34}$S values, with an analytical precision of ±0.3‰. The analyzed ${\delta }^{34}$S data is listed in

Table 2. ${\delta }^{34}$S(‰) data of metal sulfide from Hongtoushan Deposit.

No.	Ore Type	Mineral	σ34S (‰)	No.	Ore Type	Mineral	σ34S (‰)	No.	Ore Type	Mineral	σ34S (‰)
1	Type-3	Pyrite	−0.1	8	Type-2	Pyrite	0.6	15	Type-1	Pyrite	0.4
2	Type-3	Pyrite	0.2	9	Type-2	pyrrhotite	2.5	16	Type-1	Pyrite	0.3
3	Type-3	Pyrite	−0.7	10	Type-2	pyrrhotite	−0.1	17	Type-1	pyrrhotite	4.2
4	Type-3	pyrrhotite	−0.2	11	Type-2	pyrrhotite	0.1	18	Type-1	pyrrhotite	1.7
5	Type-3	pyrrhotite	2	12	Type-2	sphalerite	0.4	19	Type-1	pyrrhotite	0.1
6	Type-2	Pyrite	0.3	13	Type-1	Pyrite	0.8	20	Type-1	sphalerite	0.2
7	Type-2	Pyrite	0.6	14	Type-1	Pyrite	0.2	21	Type-1	sphalerite	−0.2

.

5. Results

5.1. Ore Mineralogy

5.1.1. Type-1 Massive Ores

Type-1 massive ore is characterized by a sulfide matrix enclosing granular quartz and dark mineral aggregates. The sulfide matrix, which accounts for 70–80% of the ore, consists primarily of pyrite (35%), pyrrhotite (25–65%), and minor amounts of chalcopyrite and sphalerite (5–10%) (Figure 3A). The granular to euhedral mineral inclusions comprise 20–30% of the ore, and include granular quartz (approximately 10%, 3–20 mm in size, Figure 3D,E) and euhedral columnar dark mineral aggregates (approximately 5%, 3–25 mm in size) composed of hornblende and quartz. Some of the granular quartz exhibits fracturing, dynamic recrystallization and undulatory extinction. Additionally, biotite is present within the pyrite matrix.

5.1.2. Type-2 Massive Ores

Type-2 massive ore is distinguished by large pyrite and pyrrhotite porphyroblasts (90%) and a small amount of gangue minerals (10%). The sulfide minerals are dominated by pyrrhotite (50%) and pyrite (40%), with minor amounts of magnetite, chalcopyrite, and sphalerite (10%) (Figure 3B). The main gangue minerals are quartz and biotite. Pyrite and pyrrhotite porphyroblasts often form triple-junction recrystallization annealing structures with 120° contact angles. Some pyrite grains have been replaced by pyrrhotite. A small amount of chalcopyrite and sphalerite (5–10%) is distributed along the boundaries of the pyrite porphyroblasts (Figure 3F).

5.1.3. Type-3 Massive Ores

Type-3 massive ores are mainly distributed in the contact zone between the ore body and gneiss, and some occur in the host rock adjacent to the ore body (Figure 3C). The main metal minerals are pyrite, pyrrhotite, chalcopyrite and a small amount of sphalerite. Crushed pyrite and sphalerite are distributed in the chalcopyrite matrix (Figure 3H). It can be also observed that the remobilized chalcopyrite and sphalerite filled in the cracks of pyrite. Chalcopyrite is observed to occur within sphalerite in the form of emulsion droplets or bands (Figure 3I). This occurrence has been referred to by some scholars as the “chalcopyrite disease structure” [55,56], elucidating the product of the iron–sphalerite reaction with copper ions in the hydrothermal solution [57].

5.2. Geochemistry and Sulfur Isotope of Sulfide

The analysis of pyrite, pyrrhotite, and a small amount of sphalerite from the three types of massive ore deposits included a broad spectrum of elements, ranging from base metals, such as Cu and Zn, to rare earth elements (REEs), including La and Ce (Table 1). Additionally, this study examined the ${\delta }^{34}$S isotopic composition of these sulfide minerals (Table 2). The specific analytical results are presented as follows. It is worth noting that a small number of sphalerite samples were also tested for sulfur isotopes, with ${\delta }^{34}$S values ranging from −0.2‰ to 0.4‰, and all sulfide minerals, with the exception of a few samples, did not exhibit remarkable negative Ce anomalies.

5.2.1. Pyrite

In Type-1 massive ores, pyrite typically exhibits a significantly higher Co content (average Co: 348.750 ppm) than the pyrite found in Type-2 and Type-3 massive ores, and is depleted in rare earth elements (REEs), with an average $\mathrm{\Sigma }$REE of 2.262 ppm and a (La:Yb)_N ratio of 6.180 (Figure 4A). The average contents of Ni, Cu, Zn, and Pb in pyrite are 7.120 ppm, 595.750 ppm, 438.750 ppm, and 55.320 ppm, respectively. In these Type-1 massive ores, pyrite has ${\delta }^{34}$S values ranging from 0.2‰ to 0.8‰.

In Type-2 massive ores, pyrite has an average content of Co = 314.667 ppm, Cu = 271.667 ppm, and Zn = 632.433 ppm, and is more depleted in REEs compared to the pyrite found in Type-1 and Type-3 massive ores, with an average $\mathrm{\Sigma }$REE of 0.921 ppm and a (La:Yb)_N ratio of 2.822 (Figure 4A). The average contents of Ni and Pb are 9.087 ppm and 97.667 ppm, respectively. In these Type-2 massive ores, pyrite has ${\delta }^{34}$S values ranging from 0.3‰ to 0.6‰.

In Type-3 massive ores, pyrite typically has higher contents of Cu and Zn (averaging Cu = 21,517.5 ppm and Zn = 2158.5 ppm) than the pyrite found in the two other types of massive ores, and is depleted in REEs, with an average $\mathrm{\Sigma }$REE of 3.611 ppm and a (La:Yb)_N ratio of 23.358 (Figure 4A). The average contents of Co, Ni, and Pb are 112.000 ppm, 145.850 ppm, and 165.450 ppm, respectively. In these Type-3 massive ores, pyrite has ${\delta }^{34}$S values ranging from −0.7‰ to 0.2‰.

5.2.2. Pyrrhotite

In Type-1 massive ores, pyrrhotite has an average content of Ni = 8.443 ppm, Zn = 2367 ppm, and Cu = 2562 ppm, and is enriched in rare earth elements (REEs), with an average $\mathrm{\Sigma }$REE of 28.030 ppm and a (La:Yb)_N ratio of 14.434 (Figure 4B). The average contents of Co and Pb in pyrrhotite are 1.274 ppm and 85.733 ppm, respectively. In these Type-1 massive ores, pyrrhotite has a wider range of ${\delta }^{34}$S values, from 0.1‰ to 4.2‰.

The pyrrhotite in Type-2 massive ores has a significantly high content of Cu (averaging Cu = 5595.667 ppm), and is also enriched in REEs, with an average $\mathrm{\Sigma }$REE of 35.354 ppm and a (La:Yb)_N ratio of 6.104 (Figure 4B). The average contents of Co, Ni, Zn and Pb in pyrrhotite are 3.144 ppm, 16.673 ppm, 1910.333 ppm and 90.900 ppm, respectively. In these Type-2 massive ores, pyrrhotite has a wider range of ${\delta }^{34}$S values, from −0.1‰ to 2.5‰.

Pyrrhotite in Type-3 massive ores has a higher Zn content (average Zn = 2769.5 ppm) than the pyrrhotite found in the two other types of massive ores, and is enriched in REEs, with an average $\mathrm{\Sigma }$REE of 22.230 ppm and a (La:Yb)_N ratio of 54.500 (Figure 4B). The average contents of Co, Ni, Cu, and Pb in pyrrhotite are 1.12 ppm, 610.000 ppm, 75.400 ppm, and 921.500 ppm, respectively. In these Type-3 massive ores, pyrrhotite has a wider range of ${\delta }^{34}$S values, from −0.2‰ to 2.0‰.

5.3. Fluid Inclusions

Fluid inclusion studies were conducted on the quartz coexisting with sulfides in three types of ore samples (Type-1, Type-2, and Type-3) from the Hongtoushan Cu-Zn VMS deposit. This quartz typically interpreted to have formed during the main stage of sulfide mineralization. For simplicity, the fluid inclusions in the quartz from three types of massive ores (Q-T1MO to Q-T3MO in table); we also directly refer to them as Type-1 to Type-3 fluid inclusions. A total of 51 fluid inclusions were measured, including 13 from Type-1 ore, 11 from Type-2 ore, and 27 from Type-3 ore. All fluid inclusions are two-phase (V_H2O + L_H2O), except for two inclusions from the Type-2 ore that contain halite daughter minerals (V_H2O + L_H2O + S_NaCl) (Figure 5F). The primary fluid inclusions in the three types of massive ores are randomly distributed, and occur individually within the quartz crystals. These fluid inclusions are typically 4~12 $\mathsf{\mu }$m in size, mainly exhibit polygonal shapes, and have vapor bubbles occupying ~10% (

Table 3. Fluid inclusion petrography and micromometric data from the Hongtoushan deposit.

Mineral	Num.	Size($\mathsf{\mu }$m)	Vol.%	Types	Tice (°C)	Th (°C)	Salinity (wt%Nacl eqv)	$\mathit{\rho }$	P (MPa)
Q-T1MO	H2	6×8	10	VH2O + LH2O	−0.3	200	0.53	0.87	12.29
Q-T1MO	H3	4 × 12	10	VH2O + LH2O	−2.8	142.7	4.65	0.96	11.67
Q-T1MO	H5	10 × 8	10	VH2O + LH2O	−0.1	291	0.18	0.72	17.33
Q-T1MO	H8	2 × 10	10	VH2O + LH2O	−7.8	193.5	11.46	0.96	20.96
Q-T1MO	H9	4 × 6	15	VH2O + LH2O	−6.8	279.3	10.24	0.85	29.06
Q-T1MO	H10	4 × 2	10	VH2O + LH2O	−3.1	253.9	5.11	0.83	21.28
Q-T1MO	H12	4 × 4	10	VH2O + LH2O	−8.4	210.8	12.16	0.95	23.33
Q-T1MO	H18	8 × 10	10	VH2O + LH2O	−8.6	217.3	12.39	0.94	24.21
Q-T1MO	H19	4 × 10	10	VH2O + LH2O	−5.8	220.1	8.95	0.91	21.87
Q-T1MO	H33	4 × 6	10	VH2O + LH2O	−4.9	222.3	7.73	0.9	21.05
Q-T1MO	H34	4 × 6	10	VH2O + LH2O	−2	223.7	3.39	0.86	16.99
Q-T1MO	H35	3 × 8	10	VH2O + LH2O	−2.5	228.7	4.18	0.86	18.22
Q-T1MO	H36	4 × 8	10	VH2O + LH2O	−4.1	214.8	6.59	0.9	19.36
Q-T2MO	H13	4 × 8	10	VH2O + LH2O	−10.2	195.5	14.15	0.98	22.87
Q-T2MO	H15	4 × 12	10	VH2O + LH2O	−10.4	181.7	14.36	1.00	21.37
Q-T2MO	H17	8 × 10	10	VH2O + LH2O	−9.4	189.7	13.29	0.98	21.68
Q-T2MO	H20	10 × 10	10	VH2O + LH2O	−7.3	177.4	10.86	0.97	18.85
Q-T2MO	H25	10 × 6	10	VH2O + LH2O	−15.1	199	18.72	1.02	25.85
Q-T2MO	H26	6 × 6	10	VH2O + LH2O	−16.1	172.8	19.53	1.05	22.8
Q-T2MO	H27	4 × 8	10	VH2O + LH2O	−19.6	198.9	22.1	1.05	27.49
Q-T2MO	H28	6 × 8	10	VH2O + LH2O	−16.4	179.6	19.76	1.05	23.81
Q-T2MO	H29	4 × 6	10	VH2O + LH2O	−12.3	176.9	16.24	1.02	21.78
Q-T2MO	H95	10 × 10	10	VH2O + LH2O + SNaCl		165.00TNaCl	30.27	1.14	5.00
Q-T2MO	H100	10 × 14	10	VH2O + LH2O + SNaCl		183.60TNaCl	31.09	1.13	8.00
Q-T3MO	H39	10 × 8	15	VH2O + LH2O	−13.4	127.4	17.26	1.07	16.05
Q-T3MO	H40	4 × 12	10	VH2O + LH2O	−9	158.8	12.85	1.00	17.92
Q-T3MO	H42	4 × 8	15	VH2O + LH2O	−20.7	177.5	22.85	1.08	24.84
Q-T3MO	H43	10 × 16	20	VH2O + LH2O	−19.2	186.5	21.82	1.06	25.66
Q-T3MO	H45	6 × 6	10	VH2O + LH2O	−11.5	170.8	15.47	1.02	20.65
Q-T3MO	H47	8 × 10	15	VH2O + LH2O	−12.3	190	16.24	1.01	23.4
Q-T3MO	H48	10 × 6	10	VH2O + LH2O	−14	180.7	17.79	1.03	23.02
Q-T3MO	H49	6 × 12	10	VH2O + LH2O	−15.9	254.9	19.37	0.97	33.53
Q-T3MO	H51	8 × 6	15	VH2O + LH2O	−15	159.1	18.63	1.05	20.63
Q-T3MO	H52	8 × 6	10	VH2O + LH2O	−13.5	209	17.34	1.00	26.38
Q-T3MO	H53	8 × 6	10	VH2O + LH2O	−13.5	208.5	17.34	1.00	26.32
Q-T3MO	H54	8 × 6	10	VH2O + LH2O	−14.4	171.5	18.13	1.04	22.01
Q-T3MO	H55	6 × 4	10	VH2O + LH2O	−12.8	198.1	16.71	1.00	24.66
Q-T3MO	H56	6 × 4	10	VH2O + LH2O	−14.2	210	17.96	1.00	26.85
Q-T3MO	H57	6 × 4	10	VH2O + LH2O	−13.7	204.3	17.52	1.00	25.88
Q-T3MO	H58	6 × 4	10	VH2O + LH2O	−12.6	147	16.53	1.04	18.22
Q-T3MO	H59	3 × 12	10	VH2O + LH2O	−14.4	214.1	18.13	1.00	27.48
Q-T3MO	H63	6 × 12	15	VH2O + LH2O	−11.3	169.5	15.27	1.02	20.4
Q-T3MO	H65	4 × 8	10	VH2O + LH2O	−11.8	190.1	15.76	1.00	23.15
Q-T3MO	H67	8 × 6	10	VH2O + LH2O	−8.7	205.9	12.51	0.96	23.02
Q-T3MO	H69	6 × 4	10	VH2O + LH2O	−15.8	197.6	19.29	1.03	25.95
Q-T3MO	H75	6 × 10	15	VH2O + LH2O	−6.1	164.1	9.34	0.97	16.54
Q-T3MO	H78	8 × 6	10	VH2O + LH2O	−10.9	143.4	14.87	1.03	17.09
Q-T3MO	H82	6 × 8	10	VH2O + LH2O	−3.8	197.2	6.16	0.92	17.41
Q-T3MO	H91	4 × 10	10	VH2O + LH2O	−12.3	179.5	16.24	1.01	22.1
Q-T3MO	H93	4 × 6	10	VH2O + LH2O	−11.5	197.9	15.47	0.99	23.93
Q-T3MO	H94	6 × 10	10	VH2O + LH2O	−7.5	176.5	11.1	0.97	18.9

Abbreviations: T_ice: final melting temperatures of ice; T_h (°C): homogenization temperature; T_NaCl: temperature of halite crystal vanish; S_NaCl: halite crystal; V_H2O: H₂O vapor; L_H2O: H₂O liquid; vol.%: the volume proportion of the vapor phase; Q-T1MO = quartz in Type-1 massive ores; Q-T2MO = quartz in Type-2 massive ores; Q-T3MO = quartz in Type-3 massive ores.

, (Figure 5B,C,E)). In our observations, we noted the presence of secondary fluid inclusions. These secondary fluid inclusions tend to be small, often measuring less than 5 $\mathsf{\mu }$m. They frequently align along specific fractures or cleavage planes, occurring in lines, arrays, or string shapes (Figure 5A,D). Due to their relatively small size, we did not perform microthermometric studies on them.

5.3.1. Microthermometric Results

Type-1 ore fluid inclusions have homogenization temperatures (T_h) ranging from 142.7 °C to 291 °C, with salinities varying from 0.18 to 12.39 wt.% NaCl eqv. The majority of these inclusions have T_h values between 200 °C and 250 °C, and salinities lower than 10 wt.% NaCl eqv. (Figure 6B,C).

Type-2 ore fluid inclusions exhibit T_h values from 172.8 °C to 199.0 °C, and higher salinities ranging from 10.86 to 22.1 wt.% NaCl eqv. The majority of these inclusions (63%) have T_h values between 150 °C and 200 °C, and salinities between 10 and 20 wt.% NaCl eqv. (Figure 6E,F). Two halite-bearing fluid inclusions have halite dissolution temperatures (T_NaCl) of 165 °C and 183.6 °C with salinities of 30.27 and 31.09 wt.% NaCl eqv.

Type-3 ore fluid inclusions show a wide range of Th values from 127.4 °C to 254.9 °C, with salinities varying from 6.16 to 22.85 wt.% NaCl eqv. The majority of these inclusions have T_h values between 150 °C and 200 °C, and salinities between 10 and 20 wt.% NaCl eqv. (Figure 6H,I).

The densities of the fluid inclusions from all the types of ores range from 0.72 to 1.08 g/cm³, with corresponding trapping pressures from 11.67 to 33.53 MPa (Figure 6A,D,G).

5.3.2. Laser Raman Spectroscopy

The representative fluid inclusions in quartz were measured using RM2000 Raman microprobe red (Renishaw, UK) to constrain their gas compositions. The analyses of both inclusions of quartz from massive and dissminated ores show a broad band with a maximum of around 3450 cm⁻¹ (Figure 7A). It is concluded that H₂O is the dominant component of fluid inclusion from the Hongtoushan deposit ores.

6. Discussion

6.1. Sulfur Isotope Constraints on Metal Sources

The sulfide mineral assemblage of the Hongtoushan Cu-Zn VMS deposit mainly includes pyrite (FeS₂), pyrrhotite (Fe_1−XS), chalcopyrite (CuFeS₂), and sphalerite (ZnS). The degree of sulfur isotope fractionation is influenced by the temperature, with smaller fractionation occurring at higher temperatures and more pronounced fractionation between light and heavy sulfur isotopes at lower temperatures [58]. And, under reducing conditions, pyrite tends to be enriched in light sulfur isotopes, while under oxidizing conditions, it is more likely to be enriched in heavy sulfur isotopes [59].

The presence of pyrite and pyrrhotite porphyroblasts with widespread triple-junction recrystallization annealing textures in the Type-2 massive ores indicates high-temperature conditions. Toulmin and Barton (1964) demonstrated that the pyrite–pyrrhotite assemblage can be used as a geothermometer [60]. Applying this geothermometer to the triple junction textures in the Type-2 massive ores suggests formation temperatures of around 600–800 °C, indicative of peak metamorphic conditions.

At high temperatures, pyrite is unstable, and transforms to pyrrhotite through desulfurization. In this process, the heavy ³⁴S is enriched in the production of pyrrhotite, and the sulfur with a lighter ${\delta }^{34}$S composition is released into the fluid [61]. The wider and positive ${\delta }^{34}$S values of pyrrhotite in the studied Type-1 and Type-2 massive ores might indicate that the pyrrhotite is formed by the desulfurization of pyrite under relatively reducing environments and higher temperature conditions (Figure 8A). At the same time, the pyrite desulfurization would release sulfur with negative S isotope values into the fluid. The pyrite and pyrrhotite in the Type-3 massive ores have ${\delta }^{34}$S values of −0.7 to 0.2‰ and −0.2 to 2.0‰ (Figure 8A), which are significantly lower than the other sulfides in the deposit. This suggests that the Type-3 massive ores may have formed under relatively oxidizing environments and lower temperature conditions.

The observed ${\delta }^{34}$S values of most sulfides from the ores in the Hongtoushan VMS deposit are mostly distributed in a narrow range close to 0‰ (−0.7 to 4.2‰), which indicated a magmatic (mantle-derived) source [62,63]. The slightly higher ${\delta }^{34}$S values of the sulfides suggest that the sulfur may have undergone a hydrothermal fractionation processes. Based on the evidence presented, we propose that the sulfur was more likely derived from igneous rocks through a mobilization processes. However, given the submarine hydrothermal setting of VMS deposits, the involvement of seawater and its potential influence on the sulfur isotope composition cannot be overlooked.

6.2. Trace Element Signatures of Ore-Forming Processes

Sulfide minerals often undergo a “cleansing” process during metamorphic growth or recrystallization [3]. This process leads to a significant decrease in the content of trace elements within the recrystallized minerals. The decreasing trend is attributed to the coarsening of pyrite grains and the loss of inclusions during recrystallization, a phenomenon well-documented in deposits subjected to metamorphism and deformation [64,65].

Most rare earth elements, Cu and Zn in the studied pyrite of Type-2 massive ores are depleted (Figure 4A), while there are no significant changes in the ${\delta }^{34}$S values (Figure 8A), suggesting that the inclusions in Type-2 massive ores are released during recrystallization. These elements dissolve in the fluid, resulting in an enrichment of trace elements within the fluid phase. Meanwhile, the pyrite of the Type-3 massive ores exhibits sufficient enrichment of Cu, Zn, and REEs with negative ${\delta }^{34}$S values. Consequently, the Type-2 massive ores were possibly derived from the recrystallization of Type-1 massive ores, and the trace elements released during the formation of Type-2 massive ores may play an important role in the metal element re-enrichment in Type-3 massive ores.

The Co/Ni ratio in pyrite is an important geochemical indicator that provides insights into the ore-forming environment and metal sources in hydrothermal deposits, particularly volcanogenic massive sulfide (VMS) deposits [66,67,68]. In sedimentary and diagenetic environments, pyrite typically has a low Co/Ni ratio (<1), as Ni is more readily incorporated into the pyrite structure compared to Co under these conditions [67,68]. However, the pyrite formed in high-temperature hydrothermal systems associated with VMS deposits tends to have higher Co/Ni ratios (>1), due to the preferential incorporation of Co into the pyrite lattice at elevated temperatures [69,70]. Furthermore, the Co/Ni ratio in the pyrite can reflect the source of metals in the hydrothermal system. Pyrite with high Co/Ni ratios (>10) is often associated with a magmatic–hydrothermal origin, as magmatic fluids are typically enriched in Co relative to Ni. In contrast, pyrite with lower Co/Ni ratios (1–10) may indicate a mixed source of metals, with contributions from both magmatic fluids and leached host rocks [66,71].

The average Co/Ni ratios of pyrite in Type-1 and Type-2 massive ores are 49 and 53, respectively, which are significantly higher than the average Co/Ni ratio of 0.96 in Type-3 massive ores (Figure 8B). The high Co/Ni ratios in Type-1 and Type-2 massive ores suggest a dominant magmatic–hydrothermal input during the early stages of mineralization, while the low Co/Ni ratio in Type-3 massive ores points to a later stage of mineralization characterized by increased fluid–rock interactions [66,72,73,74], and also by a possible mixing of magmatic and circulating seawater-derived fluids, according to the sulfur isotope characters discussed above.

6.3. Nature and Evolution of Ore-Forming Fluids

Fluid inclusions provide direct evidence for identifying the composition, pressure, temperature, and redox conditions of the mineralization environment [75,76]. Although the post-mineralization deformation and metamorphism can obliterate most of the evidence of the original ore-forming fluids [77], the primary fluid inclusions in the quartz from all types of massive ore in the studied deposit are well-preserved.

The fluid inclusions in the quartz from the three types of massive ores exhibit distinct homogenization temperatures and salinity characteristics, which may represent different stages of ore-forming fluid evolution and correspond to the formation of the different types of massive ores. Type-1 fluids, characterized by higher homogenization temperatures (avg. 222.9 °C) and lower salinities (avg. 6.74 wt.% NaCl eqv.), likely represent the early stage high-temperature fluids responsible for the formation of Type-1 massive ores. The Type-2 and Type-3 fluid inclusions have relatively low homogenization temperatures (avg. 185.7 °C and 184.81 °C, respectively) and similarly high salinities (avg. 16.56 and 16.22 wt.% NaCl eqv., respectively), indicating that they share most characteristics. However, the Type-2 fluid inclusions are distinguished by the presence of a small number of halite-bearing high-salinity fluid inclusions (salinity range from 30.27 to 31.09 wt.% NaCl eqv.), which are absent in the Type-3 fluid inclusions. We infer that the high salinity of the Type-2 fluids, particularly the presence of halite-bearing inclusions, is more likely caused by the mixing of magmatic fluids, which is consistent with the magmatic–hydrothermal origin of the Type-1 and Type-2 massive ores, as indicated by the sulfur isotope and trace element characteristics. However, the relatively low homogenization temperatures of Type-2 fluid inclusions suggest that they were possibly trapped during a later stage of the hydrothermal system, after the fluids had undergone cooling from the peak metamorphic conditions (600–800 °C) associated with the formation of the Type-2 massive ores. In contrast, the absence of halite-bearing inclusions in the Type-3 fluid inclusions, combined with their sulfur isotope and trace element characteristics, points to a greater contribution from seawater in the formation of the Type-3 massive ores. The generally low Th/La and Nb/La ratios (<1) in pyrite and pyrrhotite across all of the ore types indicate that the hydrothermal fluids were enriched in Cl, facilitating the complexation and transportation of REE and HFSE. These observations suggest a trend of cooling and dilution from Type-1 to Type-2 and Type-3 fluids, with an increasing involvement of seawater in the hydrothermal system.

6.4. Implications for Metallogenic Models

The integrated evidence from sulfur isotopes, trace elements, and fluid inclusions suggests a complex, multi-stage metallogenic process for the Hongtoushan deposit:

(1) Early mineralization stage: This stage mainly formed Type-1 and Type-2 ores. The seawater penetrated deep into the crust along fracture systems, mixing with magmatic hydrothermal fluids, and continuously circulated. This process leached the metallic elements from the basement rocks, forming Cu- and Zn-rich hydrothermal fluids. As the fluids ascended along the faults and fractures, they underwent cooling and decompression, leading to the precipitation of sulfides and the formation of the Type-1 massive sulfide ores. The high formation temperatures are evidenced by the high Co/Ni ratios in the pyrite and the characteristics of the Type-1 fluid inclusions.

(2) Metamorphic recrystallization stage: During the peak metamorphic conditions, the Type-1 ores underwent recrystallization and coarsening, forming the Type-2 massive ores. This process is supported by the widespread triple-junction textures in the pyrite and pyrrhotite porphyroblasts. The recrystallization led to the release of trace elements from the pyrite into the hydrothermal fluids, as indicated by the depletion of the rare earth elements, Cu, and Zn in Type-2 ores. The characteristics of the Type-2 fluid inclusions, with lower homogenization temperatures but higher salinities, suggest that water–rock interaction played a significant role during this stage.

(3) Late-stage mineralization: The trace-element-enriched fluids generated from the metamorphic recrystallization of the Type-2 ores migrated and cooled significantly. The decrease in temperature resulted in the precipitation of Cu, Zn, and other elements from the hydrothermal fluids, forming the Type-3 massive ores. The lower Co/Ni ratios in pyrite and the characteristics of the Type-3 fluid inclusions support a lower formation temperature for these ores. The negative ${\delta }^{34}$S values of the pyrite in the Type-3 ores indicate that the sulfur was likely derived from the desulfurization of pre-existing sulfides in the Type-1 and Type-2 ores.

The Hongtoushan deposit is a product of multi-stage hydrothermal activity in a submarine volcanic setting, with mineralization influenced by both magmatic and seawater sources. The early stage involved high-temperature hydrothermal fluids leaching metals from the basement rocks and precipitating massive sulfides. Subsequent metamorphic recrystallization led to the release of trace elements, which were re-enriched in the late-stage ores. This complex interplay of magmatic, hydrothermal, and metamorphic processes highlights the dynamic nature of the VMS deposit formation.

Further research on the regional tectonic setting, the timing of mineralization events, and the relationship between the Hongtoushan deposit and the other VMS deposits in the region will provide a more comprehensive understanding of the metallogenic system. Detailed geochemical and isotopic studies on the host rocks and the alteration zones may offer additional insights into the source of metals and the extent of the fluid–rock interaction. Investigating the potential role of biological activity in the precipitation of sulfides could also shed light on the complex processes involved in the formation of the Hongtoushan deposit.

7. Conclusions

Based on the comprehensive discussion of sulfur isotopes, trace elements, and fluid inclusions, the following conclusions can be drawn:

(1) Sulfur isotope analysis indicates that the sulfur in the Hongtoushan Cu-Zn VMS deposit is primarily derived from magmatic sources. Most sulfides have ${\delta }^{34}$S values distributed within a narrow range close to 0‰ (−0.7 to 4.2‰), suggesting a magmatic origin, while slightly higher ${\delta }^{34}$S values may be attributed to hydrothermal fractionation processes.

(2) Trace element studies reveal that the pyrite in Type-2 massive ores underwent a “cleansing” process during metamorphic recrystallization, resulting in a significant decrease in the trace element contents. The elements released during this process dissolved in the hydrothermal fluids, and may have played a crucial role in the re-enrichment of metal elements in Type-3 massive ores.

(3) Fluid inclusion studies identified three types of fluid inclusions (Type-1 to Type-3) with distinct homogenization temperatures and salinity characteristics, representing different stages of ore-forming fluid evolution and corresponding to the formation of different types of massive ores. Type-1 fluids represent early high-temperature fluids, and Type-2 and Type-3 fluids were influenced by water–rock interactions and experienced further cooling and dilution.

(4) Integrating the evidence from sulfur isotopes, trace elements and fluid inclusions, it is proposed that the Hongtoushan deposit underwent a complex, multi-stage metallogenic process: the early mineralization stage formed Type-1 and Type-2 ores; during peak metamorphic conditions, the Type-1 ores underwent recrystallization and coarsening, forming the Type-2 massive ores; in the late-stage mineralization, trace element-enriched fluids generated from the metamorphic recrystallization of the Type-2 ores migrated and cooled significantly, forming the Type-3 massive ores.

(5) The Hongtoushan deposit is a product of multi-stage hydrothermal activity in a submarine volcanic setting, with the mineralization influenced by both magmatic and seawater sources. The early stage involved high-temperature hydrothermal fluids leaching metals from the basement rocks and precipitating massive sulfides. Subsequent metamorphic recrystallization led to the release of trace elements, which were re-enriched in the late-stage ores.