Genesis of High-Grade Gold Mineralization at the Guocheng Deposit, Jiaodong Peninsula: Constraints from Magnetite Geochemistry

Abstract

The processes responsible for high-grade disseminated gold mineralization remain poorly constrained, hindering effective exploration. This study integrates petrography, BPMA, and LA-ICP-MS analysis of magnetite from marble- and granite-hosted ores with contrasting gold grades, to constrain wall-rock-induced changes in the thermodynamic environment. BPMA results show distinct mineral assemblages: granite-hosted ores are characterized by quartz (52.31%)-K-feldspar (19.65%)-sericite (9.56%)-pyrite (8.36%), whereas marble-hosted ores feature pyrrhotite (33.90%)-chlorite (27.50%)-pyrite (15.22%)-magnetite (1.94%). The closed intergrowths of magnetite with gold and sulfides, along with the magnetite Ga-V (Grant-Vaughan) discrimination diagram, indicate a hydrothermal origin for magnetite formed during the mineralization stage. Geochemical data show that marble-hosted magnetite has lower V and chalcophile element (Co, Ni, Sn, Zn) concentrations than granite-hosted magnetite. Considering the partitioning behavior of these elements in magnetite, these differences indicate magnetite crystallization under increasing oxygen fugacity (fO₂) and decreasing sulfur fugacity (fS₂). Thermodynamic modeling results demonstrate that these changes in fO₂ and fS₂ destabilized gold-sulfur complexes in the ore-forming fluid, significantly enhancing gold precipitation efficiency and ultimately leading to the formation of high-grade ores in marble.

1. Introduction

High-grade disseminated gold ores constitute an economically crucial component of many gold deposits, as their formation directly determines the overall ore grade and mining potential of a deposit [1,2,3,4]. The formation of disseminated gold ores is widely recognized to be controlled by fluid–rock interactions, through which ore-forming fluids exchange components with wall rocks, leading to thermodynamic variations such as changes in redox state, pH, and sulfur activity [5,6,7,8]. These variations can significantly influence the solubility and transportation of gold, thereby controlling the efficiency of gold precipitation [9,10,11]. However, because different deposits mostly form under distinct temperature, pressure, and fluid-composition conditions, it remains challenging to systematically compare gold-precipitation efficiency among disseminated gold systems [5,12,13]. Consequently, the influence of different fluid–rock interaction mechanisms on gold-precipitation efficiency and ore grade is still unclear [14,15,16]. This uncertainty limits our understanding of gold precipitation processes in disseminated system and impedes the advancement of exploration efforts.

The Jiaodong gold province, located on the southeastern margin of the North China Craton, is the largest gold-producing region in China, with total gold resources exceeding 5000 tonnes [17,18]. The majority of gold deposits in this region display highly consistent mineral assemblages, alteration zonation, mineralization stages, isotopic signatures and ages [19,20,21,22,23]. Their ore-forming fluids are widely regarded as having similar source and properties, characterized by medium-to-low temperatures (~200–400 °C), low salinities (2–10 wt% NaCl eqv.) and near-neutral to weakly acidic pH [19,24]. The Guocheng gold deposit is situated within this province, shares these common features. Previous studies on the Guocheng gold deposit indicate that mineralization occurred at 119 ± 2 Ma [25], and the ore-forming fluids were of medium–low temperature, low salinity, and weakly acidic to near-neutral properties [26,27,28], consistent with the mineralization peak (120 ± 2 Ma) and fluid characteristics of the Jiaodong gold deposits [29,30]. These similarities suggest that the Guocheng deposit shares the same ore-forming fluid system with other deposits in the province, and that it was formed by the fluid which ascended along the Guocheng fault. However, field investigations at Guocheng reveal a marked difference in gold grade between ores hosted in its two main wall rock (marble and granite): marble-hosted ores often display high-grade gold mineralization (gold grade typically 5–10 g/t Au), whereas granite-hosted ores generally show lower gold grades (~1.4 g/t Au) [31,32]. The significant gold-grade disparity between the two types of disseminated ores within the same metallogenic system at Guocheng gold deposit, provides ideal conditions to investigate how different fluid–rock interactions control gold-precipitation efficiency and the formation of high-grade disseminated gold ores. Nevertheless, the processes controlling the distinct contrast in gold grades between the two types of disseminated ores within the same system have not yet been convincingly explained.

In recent years, the trace element composition of magnetite has been demonstrated as an effective tracer of the thermodynamic environment during its formation [33,34,35]. For example, by comparing two types of magnetite in the Weizigou gold deposit, previous researchers found that the V content of early-generation magnetite (Mag-II-B) was significantly higher than that of late-generation magnetite (Mag-II-A), revealing that the ore-forming fluid evolved from relatively high oxygen fugacity in the early stage to low oxygen fugacity in the late stage [15]. At the Guocheng gold deposit, magnetite is widely developed in the gold ores and can serve as a reliable indicator for reflecting the physicochemical conditions of mineralization. To better understand the mineralogical characteristics associated with gold mineralization, we employed the BGRIMM Process Mineralogy Analyzer (BPMA)—the first automated mineralogy system independently developed in China by the Beijing General Research Institute of Mining & Metallurgy (BGRIMM Technology Group). This SEM–EDS–based technology enables rapid and quantitative characterization of mineral composition [36,37,38], providing an effective tool for investigating ore-forming processes [39]. In this study, we conducted an integrated investigation combining microscopic petrography, automated mineralogy (BPMA), and LA-ICP-MS trace-element analysis of magnetite from representative disseminated marble-hosted and granite-hosted ores at Guocheng gold deposit. By systematically comparing mineral assemblages and magnetite geochemical characteristics between the two ore types, we constrained the variations in thermodynamic conditions during gold precipitation, and revealed how different fluid–rock interactions controlled gold precipitation efficiency and the formation of high-grade disseminated ores at Guocheng gold deposit. The study can advance the understanding of the formation mechanisms of high-grade disseminated gold ores and provide a theoretical basis for their exploration and targeting.

2. Regional Geologic Setting

The Jiaodong Peninsula lies along the southeastern margin of the North China Craton, bounded by the Tan–Lu strike-slip fault zone to the west, the Yellow Sea to the east, and surrounded on the remaining three sides by the Bohai Sea (Figure 1A). The region is tectonically divided by the N–NE-trending Wulian–Yantai Fault Zone into the Sulu Terrane to the southeast and the Jiaobei Terrane to the northwest, the latter of which is further subdivided into the Jiaobei Uplift and the Jiaolai Basin (Figure 1B) [19]. The Sulu Terrane is mainly composed of metamorphic basement rocks overlain by Jurassic–Cretaceous volcanic–sedimentary sequences and intruded by Cretaceous magmatic bodies [17]. In contrast, the Jiaobei Terrane, occupying the northwestern to central-northern part of the peninsula, exposes extensive Neoarchean–Paleoproterozoic basement rocks of the North China Craton [20], consisting of Precambrian metamorphic sequences, Mesozoic granites, and volcanic–sedimentary assemblages. The metamorphic basement includes the Neoarchean Jiaodong Group dominated by tonalite–trondhjemite–granodiorite (TTG) gneisses [29], the Paleoproterozoic Jingshan and Fenzishan Groups characterized by gneiss, marble, calc-silicate, graphitic schist assemblages [40], and the Neoproterozoic Penglai Group composed mainly of marble, slate, and marl [41]. Spatially, the Jingshan Group is predominantly exposed east of the Wulian–Qingdao–Yantai Fault Zone. The Sulu Terrane, on the other hand, exhibits a distinct dual structural framework, with an ultrahigh-pressure metamorphic belt distributed linearly along the Wulian–Qingdao–Yantai Fault Zone and forming continuous outcrops in its eastern part [42].

Mesozoic magmatism in Jiaodong was driven by the subduction of the Paleo-Pacific Plate. It comprises two main phases: (1) emplacement of the Linglong biotite granite (160–145 Ma) and (2) emplacement of the Guojialing porphyritic granodiorite (130–122 Ma) [40,43]. During the Triassic to Jurassic, flat-slab subduction induced a compressional collisional stress regime in the North China Craton lithospheric mantle [43]. Magmatism was minimal in Jiaodong during this period, represented mainly by the Linglong granites [44]. A fundamental tectonic shift occurred in the Cretaceous with the onset of slab rollback of the Paleo-Pacific Plate. This triggered intense NW-SE lithospheric extension and large-scale thinning beneath the North China Craton [44,45]. Concurrently, widespread magmatic activity peaked around 125 Ma, producing abundant intermediate-acid intrusive rocks (notably the Guojialing suite) and volcanic assemblages [46,47]. Critically, the major gold mineralization events in Jiaodong are both temporally and genetically associated with this Early Cretaceous (ca. 130–120 Ma) magmatic–hydrothermal activity, particularly with the Guojialing intrusions and related fluid systems, wherein gold deposits are primarily hosted within the Linglong and Guojialing granites [29,43].

The Jiaodong region is predominantly governed by E-W and NNE-NE trending tectonic systems (Figure 1B). E-W trending structures mainly manifest as ancient basement folds, whereas the spatial distribution of gold deposits is strictly controlled by NNE-NE trending regional fault zones and their subsidiary structures (Figure 1B) [48]. First-order lithospheric-scale faults include the Wulian-Yantai Fault Zone and the Tan-Lu Fault Zone, with NNE-NE trending second- and third-order fault networks constituting their branch systems [49]. Within the region, five major fault zones develop from west to east: Sanshandao-Cangshang, Jiaojia, Zhaoyuan-Pingdu, Penglai-Qixia, and Muping-Rusha [49]. These fault systems fundamentally control gold deposit localization (Figure 1B). Among these, three major ore belts develop along the Penglai-Qixia Fault Zone in western Jiaodong [19,48], while the Muping-Rushan metallogenic belt dominates the east. The NE-NNE trending fault systems control over 90% of the region’s gold resources, concentrated particularly within the three major ore belts in the west [50].

3. Deposit Geology

The Guocheng gold deposit is situated in the northeastern part of the Jiaolai Basin, occupying a key tectonic position at the junction of the Sulu ultrahigh-pressure metamorphic belt, the Jiaolai Basin, and the Jiaobei Terrane [19,51]. The exposed strata are mainly composed of Archean Jingshan Group metamorphic rocks and Mesozoic Laiyang Group sedimentary rocks (Figure 2). The Jingshan Group is dominated by marbles, which are metamorphosed carbonate rocks formed during the Paleoproterozoic regional metamorphic event associated with the Jingshan orogeny [52]. Intrusive rocks are widely distributed in the southern and eastern parts of the area, covering about half of the total surface. The dominant intrusive unit is the Late Proterozoic Muniushan adamellite pluton, which intrudes the Archean Jingshan Group with a tongue-like morphology. Mesozoic mafic to felsic dikes, including dolerite, monzonite porphyry and lamprophyre, are also common. These bodies are mainly of Late Yanshanian age and cut through both the Jingshan Group and the Muniushan pluton.

The Guocheng gold deposit contains five main mines: Tuidui, Shawan, Dongliujia, Houkuang, and Longkou, which are distributed along subsidiary NE–SW-trending faults related to the Guocheng Fault (Figure 2). The gold orebodies are predominantly fault-controlled, occurring as NE–SW–striking veins and lenses within the marble of the Jingshan Group and the granite of the Muniushan pluton (Figure 2) [53]. The ore exhibits disseminated mineralization (Figure 3), characterized by a pronounced contrast in gold grade between the two main wall-rock types, with altered granites hosting relatively low-grade ores (~1.4 g/t Au) and altered marbles containing significantly higher-grade ores, typically ranging from 5 to 10 g/t Au [27]. The BGRIMM Process Mineralogy Analyzing System (BPMA) indicates that the dominant ore minerals in the Guocheng deposit include pyrite, pyrrhotite, and magnetite, accompanied by minor chalcopyrite, sphalerite, and galena (Figure 4 and Figure 5). The gangue minerals are mainly composed of quartz, dolomite, calcite, chlorite, and sericite (Figure 5). Distinct contrasts in mineral assemblages are observed both among different ore types and among their corresponding wall rocks. The marble-hosted ores comprise pyrrhotite (33.90%), chlorite (27.5%), pyrite (15.22%), actinolite (5.82%), pyroxene (4.64%), sericite (4.47%), calcite (1.99%), magnetite (1.94%), dolomite (1.35%), and accessory minerals (1.33%), with magnetite commonly intergrown with pyrrhotite and pyrite (Figure 4A–G, Figure 5A–C and Figure 6C,D). In contrast, the barren marble is dominated by dolomite (72.46%) and calcite (18.85%), with minor actinolite (3.28%), chlorite (1.24%), pyrrhotite (0.33%), pyrite (0.21%), and magnetite (0.63%) (Figure 6A,B). The granite-hosted ores contain quartz (52.31%), K-feldspar (19.65%), sericite (9.56%), and pyrite (8.36%), with trace pyrrhotite (0.14%) and magnetite (<0.01%) (Figure 4H,I, Figure 5D–F and Figure 6G,H). The barren granite consists mainly of quartz (47.84%) and plagioclase (41.81%), with muscovite (4.24%) after feldspar and biotite alteration (Figure 6E,F).

4. Samples and Analysis Methods

4.1. Samples

Systematic sampling was conducted at the Guocheng gold deposit to collect both marble-hosted and granite-hosted ores, along with their respective barren wall rocks (Figure 3). This sampling strategy aimed to compare and evaluate mineralization differences between ore units hosted in distinct barren lithologies.

4.2. BGRIMM Process Mineralogy Analyzing System (BPMA)

To mitigate analyst-induced bias, representative samples of marble-hosted ore, granite-hosted ore, and their wall rocks—selected based on detailed fieldwork—were subjected to automated mineral identification testing. Analyses were performed at the State Key Laboratory of Mineral Processing (BGRIMM Technology Group, Beijing, China), using a TESCAN VEGA scanning electron microscope coupled with a Bruker energy dispersive spectrometer and the BPMA software (version 2.0). Operational parameters included high-vacuum mode, a 15 mm working distance, 20 kV accelerating voltage, and a 2 nA beam current, with backscattered electron detection. A whole-particle measurement mode was employed to automatically identify metallic sulfides. Quantitative parameters (e.g., sulfide content, particle size distribution, and dissemination complexity) were acquired under high magnification. Detailed analytical procedures follow references [38].

4.3. LA-ICP-MS Analysis of Magnetite

In situ trace element analyses of magnetite were conducted via laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) at the National Research Center for Geoanalysis (CAGS, Beijing, China). An Agilent 8900 ICP-MS/MS Triple Quad coupled with an NWR 193™ laser (Elemental Scientific Lasers, Bozeman, MT, USA) was utilized. Helium carrier gas transported ablated aerosols from the chamber, where they were mixed with argon and nitrogen (as a diatomic gas, N₂) to enhance sensitivity. Laser ablation employed a 6 Hz repetition rate at 5 J/cm² energy density with 30 μm spot diameters. A baffle-type smoothing device installed upstream of the ICP-MS minimized signal fluctuations induced by laser pulses, improving data quality. Each analysis comprised 10 s of gas blank (background) acquisition followed by 30 s of sample data acquisition. Calibration standards NIST SRM 610 and MASS-1 were analyzed after every ten samples to correct for time-dependent instrumental drift and mass bias. Data reduction utilized ICPMSDataCal 11.8 software employing specific protocols and calibration methods following [54]. A total of 59 magnetite analyses were performed, of which 56 were retained after applying strict quality control criteria, including rejection of analyses with low signal intensity, unstable baselines, or >15% relative standard deviation for key elements (e.g., Ti, V, Fe, Mn, Mg). The detection: limits for Ti, V, Ga, Mn, Co and Ni are <0.1 ppm. For Sn and Zn, the detection limits are <1 ppm; for Mg and Al, it is approximately 3 ppm. The precision and accuracy are better than 10% for most analyzed elements.

5. Results

5.1. Mineral Size Distribution and Mineral Intergrowth Assemblages

Mineral intergrowth assemblages can reveal spatial relationships and crystallization sequences between minerals, providing key insights into the mineralization process [55]. In the marble-hosted ores, pyrite most frequently intergrows with pyrrhotite (47.84%), chlorite (12.96%), and magnetite (6.42%), while pyrrhotite mainly associates with pyrite (15.97%), iron-rich chlorite (42.95%), and magnetite (4.59%) (Figure 4A–G, Figure 5A,B and Figure 6C,D;

Table 1. Statistical table of the association and combination of metal sulfides.

Mineral	Marble-Hosted Ores		Granite-Hosted Ores
	Pyrrhotite	Pyrite	Pyrite
pyrrhotite	\	47.84	8.06
pyrite	15.97	\	\
magnetite	4.59	6.42	\
sericite	7.31	5.74	15.01
iron chlorite	42.95	12.96	2.80
feldspar	\	\	6.72
quartz	\	\	39.14
other	29.18	27.04	22.27

). In the granite-hosted ores, pyrite primarily intergrows with quartz (39.14%) and sericite (15.01%), along with minor associations with plagioclase (6.72%) and pyrrhotite (8.06%) (Figure 4H,I, Figure 5D–F and Figure 6G,H; Table 1).

Sulfides such as pyrrhotite and pyrite can be classified by grain size into ultrafine (<20 µm), fine (20–100 µm), medium (104–295 µm), coarse (295–417 µm), and ultra-coarse (>417 µm) categories [56]. Grain size differences reflect variations in crystallization rate and the stability of the hydrothermal environment during mineralization [57]. In the marble-hosted ores, pyrite is predominantly medium to coarse grained, with medium grains accounting for 39.64%, coarse grains for 30.80%, ultra-coarse grains for 12.49%, while fine and ultrafine grains make up 9.49% and 7.58%, respectively (Figure 7A). Pyrrhotite shows a more scattered distribution, with medium grains comprising 29.62%, coarse grains 26.19%, ultra-coarse grains 10.62%, fine grains 26.83%, and ultrafine grains 6.74% (Figure 7B). In the granite-hosted ores, pyrite is mainly coarse to ultra-coarse grained, with medium grains accounting for 32.62%, coarse grains for 16.39%, and ultra-coarse grains for 37.28% (Figure 7C), while fine and ultrafine fractions are almost absent.

5.2. LA-ICP-MS Trace Element Analysis of Magnetite

The LA-ICP-MS results of magnetite from the Guocheng gold deposit are summarized in

Table 2. Trace element characteristics of magnetite analyzed by LA-ICP-MS in marble- and granite-hosted gold ores from the Guocheng deposit.

Type	Samples	Ti	V	Al	Ga	Mg	Mn	Co	Ni	Sn	Zn
Magnetite in marble-hosted ores	25GC-M-01	1.55	1.53	92.64	5.14	18,889.51	1178.56	0	0.30	0.80	420.36
	25GC-M-02	2.38	13.18	74.98	4.31	7893.42	1366.66	0	0.99	0.92	47.92
	25GC-M-03	7.48	128.52	160.68	4.73	7095.37	536.03	0.03	0.99	0.80	33.91
	25GC-M-04	9.65	44.02	308.51	4.85	10,008.75	1136.88	0.07	0.01	0.98	70.14
	25GC-M-05	2.68	21.37	125.60	6.83	3969.90	605.63	0.30	0.65	0.69	31.93
	25GC-M-06	14.70	34.86	512.36	5.41	10,867.31	981.48	0.09	0.19	0.94	77.45
	25GC-M-07	20.97	47.04	321.09	2.96	15,027.13	1039.42	0.03	6.06	1.16	48.09
	25GC-M-08	2.96	3.74	293.61	1.59	34,613.43	1334.99	0	0.34	1.29	103.06
	25GC-M-09	4.77	4.69	217.89	1.26	7811.52	1378.32	0.03	0.24	1.05	55.59
	25GC-M-10	0.60	0.53	49.97	1.94	25,943.93	1371.73	0	0.83	5.78	71.40
	25GC-M-11	0.85	1.07	29.27	1.18	12,772.48	1907.82	0	1.25	3.23	70.33
	25GC-M-12	0.70	1.02	27.67	1.44	2195.17	1146.40	0	1.26	0.37	20.75
	25GC-M-13	6.12	16.14	261.17	3.55	3874.90	1198.41	0	0.71	0.91	50.25
	25GC-M-14	2.16	2.42	167.76	2.04	1002.34	626.55	0.10	1.15	1.15	132.13
	25GC-M-15	1.95	2.33	130.12	2.05	1742.14	902.58	0	0.75	1.42	29.70
	25GC-M-16	1.23	1.83	124.46	1.09	465.84	693.22	0	0.83	1.30	25.81
	25GC-M-17	4.62	9.39	482.87	1.78	1822.56	898.60	0	3.20	2.96	22.28
	25GC-M-18	8.01	4.46	454.02	5.15	32,495.20	1669.53	0.02	0.50	2.89	113.97
	25GC-M-19	46.81	254.25	1953.28	19.38	2374.26	203.66	0.13	3.43	2.15	9.89
	25GC-M-20	5.84	144.75	1095.38	11.81	2817.30	174.57	0.04	8.66	1.97	8.68
	25GC-M-21	52.20	13.00	452.32	4.59	18,043.83	832.00	0.42	0.64	0.54	52.54
	25GC-M-22	60.29	12.82	527.37	4.44	17,130.37	2245.09	0.17	1.11	0.65	97.04
	25GC-M-23	6.43	7.60	314.82	2.68	15,909.26	910.37	0.08	0.76	0.69	49.77
	25GC-M-24	5.56	14.83	177.34	0.96	12,459.35	722.42	0	0.23	42.93	46.82
	25GC-M-25	3.05	7.72	242.24	1.42	25,015.49	1235.95	0.07	0.14	0.93	72.68
	25GC-M-26	5.94	5.60	384.56	1.18	14,140.47	430.01	0.02	4.80	0.46	44.40
	25GC-M-27	25.61	126.38	1305.63	3.58	17,423.18	1006.48	0	0.79	16.46	45.08
	25GC-M-28	9.64	7.49	492.82	3.84	20,988.47	848.98	0	0.54	0.54	77.17
	25GC-M-29	5.51	4.32	91.20	3.82	11,728.98	963.50	00	0.82	0.85	0.00
	25GC-M-30	2.73	21.21	564.11	5.96	13,807.69	3027.67	0.02	1.10	1.17	65.55
	25GC-M-31	0.70	20.12	140.46	3.95	7452.43	2585.04	0.04	0.86	0.82	36.35
	25GC-M-32	1.68	4.62	371.26	2.50	8322.82	2331.17	0.01	2.34	1.18	36.33
	25GC-M-33	7.21	13.07	339.63	2.74	18,549.06	2541.47	0	1.02	1.14	63.45
	25GC-M-34	3.04	13.62	406.69	2.93	18,903.83	3907.35	0	1.77	1.22	77.24
	25GC-M-35	5.50	29.81	432.78	5.17	7239.37	471.84	0.25	7.77	0.72	45.16
	25GC-M-36	1.07	24.56	60.22	3.61	8445.28	3839.19	0.06	2.75	0.90	25.51
	25GC-M-37	0.21	9.50	23.63	4.09	8595.96	3865.44	0.00	2.68	1.01	81.98
	25GC-M-38	15.31	303.28	1705.34	4.35	9295.10	1274.92	0.98	3.74	0.85	66.29
Magnetite in granite-hosted ores	25GC-G-01	8.41	40.95	3388.67	8.24	4266.75	338.51	0.20	4.99	0.44	32.59
	25GC-G-02	3.30	66.98	0.00	0.00	0.00	0.00	0.32	1.65	0.72	0.00
	25GC-G-03	7.64	12.52	527.87	8.41	65.03	125.09	0.09	2.98	8.61	47.18
	25GC-G-04	4.02	63.44	288.65	29.14	8178.99	3209.20	2.83	3.16	1.92	261.97
	25GC-G-05	6.73	54.43	983.83	7.68	2024.78	316.57	0.75	5.08	102.85	15.84
	25GC-G-06	8.92	45.70	519.42	7.23	957.40	380.63	0.73	4.54	25.94	11.10
	25GC-G-07	7.57	46.13	13,684.61	24.44	13,563.58	338.38	0.30	8.39	0.63	59.69
	25GC-G-08	18.81	66.40	1571.77	9.23	2748.99	510.74	0.32	6.41	15.18	16.16
	25GC-G-09	2.74	34.76	263.77	5.70	2257.63	911.11	0.53	5.61	0.58	8.10
	25GC-G-10	13.31	51.00	654.10	11.92	2665.75	1952.74	1.24	3.45	112.74	13.35
	25GC-G-11	18.11	52.19	1019.86	15.62	17,227.56	2550.01	0.85	5.62	0.95	32.31
	25GC-G-12	13.99	61.56	154.75	11.49	1064.08	347.78	0.48	5.08	276.48	16.78
	25GC-G-13	6.22	48.94	1082.95	13.49	23,869.25	3654.22	2.26	4.36	0.98	161.14
	25GC-G-14	20.74	41.07	14,453.95	20.46	17,632.71	798.36	0.85	6.53	4.02	163.69
	25GC-G-15	3.69	31.34	359.86	5.49	2052.51	621.53	0.20	2.30	0.48	12.66
	25GC-G-16	4.83	39.22	2511.24	7.83	2435.40	232.85	0.39	3.22	77.15	443.82
	25GC-G-17	8.80	52.20	674.62	6.65	829.39	79.89	0.23	3.75	9.20	22.53
	25GC-G-18	6.71	30.37	212.22	8.09	222.02	289.74	0.11	3.31	36.81	19.59

. A total of 56 analytical spots were conducted, including 38 spots on magnetite in marble-hosted ores and 18 spots on magnetite in granite-hosted ores. Mag in marble-hosted ores exhibits larger and more euhedral crystal forms (Figure 7A–C), whereas Mag in granite-hosted ores commonly contains vugs filled with calcite cementation (Figure 7A–C). For magnetite in marble-hosted ores, the lithophile elements show highly heterogeneous distributions. Ti concentrations are consistently below 20 ppm (average = 9.41 ppm), whereas V contents are irregular but mostly <25 ppm (average = 36.23 ppm). Al and Ga have mean concentrations of 393 ppm and 3.96 ppm, respectively. Mg is notably enriched (average = 12,030 ppm), while Mn averages 1405 ppm. The chalcophile elements in magnetite in marble-hosted ores are generally low: Co is frequently below the detection limit, with an average of 0.12 ppm for detected samples; Ni and Sn are mostly below 3 ppm, averaging 1.74 ppm and 2.78 ppm, respectively; and Zn averages 63.87 ppm.

In magnetite from granite-hosted ores, the lithophile elements remain low but display distinct differences compared with marble-hosted ores. Ti concentrations are similar (<20 ppm), whereas V contents are slightly higher (40–70 ppm, average = 46.21 ppm). Al and Ga are significantly enriched (average = 888 ppm and 11.17 ppm, respectively), while Mg and Mn are depleted (average = 5670 ppm and 925 ppm, respectively). Although the chalcophile elements in Mag in granite-hosted ores also exhibit low concentrations, they are notably higher than those in magnetite in marble-hosted ores: Co concentrations, though low, are detectable, averaging 0.71 ppm; Ni concentrations mostly range between 2 and 8 ppm, averaging 4.47 ppm; Sn concentrations vary widely, averaging 37.54 ppm; and Zn averages 74.36 ppm.

6. Discussion

6.1. Classification and Origins of Magnetite in Marble-Hosted Ores and Granite-Hosted Ores

Magnetite develops in diverse geological environments, and its composition is influenced by physicochemical conditions as well as by the chemistry of both fluids and wall rocks, resulting in a wide compositional range [58,59,60,61]. Consequently, comparisons of magnetite compositions between different regions or deposits are often difficult to establish. However, the different wall rocks but coeval mineralization styles at the Guocheng gold deposit provide an ideal opportunity to compare hydrothermal magnetite compositional variations controlled by the same mineralizing event under comparable physicochemical conditions. Based on its formation process, magnetite can be classified into hydrothermal and magmatic types [33,62]. In this study, petrographic observations reveal that magnetite is closely associated with pyrite, pyrrhotite and gold (Figure 4C and Figure 5A–F), combined with the Ga–V discrimination diagrams (Figure 8A) that classify this magnetite from both ore types into the low-temperature hydrothermal category [63]. This suggests that the magnetite crystallized during the main mineralization stage and can be used to constrain the thermodynamic conditions of gold precipitation (Figure 4C).

In hydrothermal systems, magnetite can form either through in-situ dissolution-recrystallization or through dissolution-migration-recrystallization [59,63]. The former preserves pseudomorphic structures inherited from precursor minerals, while the latter involves direct precipitation of minerals from hydrothermal fluids without the participation of specific precursors [64,65]. Petrographic observations reveal that magnetite exhibits two distinct textural types in both granite-hosted and marble-hosted ores. Most of these magnetite grains exhibit euhedral to subhedral crystal forms and show no evidence of replacing any precursor minerals (Figure 5), indicating that they formed via dissolution–migration–recrystallization [59]. The formation mechanism of this type of magnetite indicates that elements such as Fe were released into the fluid as chloride complexes (for example, Fe-Cl) from iron-magnesium-rich minerals (ankerite, biotite, amphibole) in wall rocks (Reactions (1)–(3)). These components were then transported along permeable pathways and eventually co-precipitated with sulfides (mainly pyrite, pyrrhotite) in favorable locations (Reaction (4)). Additionally, some magnetite replaces pyrite (Figure 5), indicating formation through replacement of precursor pyrite. This reaction process is shown in Equation (5). Notably, the primary textures of both types of magnetite are well preserved and show no visible signs of secondary alteration, such as corrosion, dissolution, or replacement by hematite (Figure 5). This suggests that their chemical compositions have likely retained primary hydrothermal signatures without significant post-mineralization alteration. Although these two types of magnetite formed through different mechanisms, both of them occur in close association with gold and sulfides (Figure 4C and Figure 5A–F) and crystallized from the same ore-forming hydrothermal system. Therefore, they can be regarded as reliable tracers for constraining the physicochemical conditions of gold precipitation of Guocheng gold deposit.

$$\mathrm{C}\mathrm{a}(\mathrm{F}\mathrm{e},\mathrm{M}\mathrm{g})(\mathrm{C}{\mathrm{O}}_3{)}_{2\left(\mathrm{D}\mathrm{o}\mathrm{l}\right)}+4{\mathrm{H}}^{+}=\mathrm{C}{\mathrm{a}}^{2+}+\mathrm{F}{\mathrm{e}}^{2+}+\mathrm{M}{\mathrm{g}}^{2+}+2\mathrm{C}{\mathrm{O}}_2\uparrow +2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{a}\mathrm{q}\right)}$$

(1)

$$2{\mathrm{K}\mathrm{F}\mathrm{e}}_3{\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_{10}(\mathrm{O}\mathrm{H}{)}_{2\left(\mathrm{B}\mathrm{t}\right)}+14{\mathrm{H}}^{+}+6{\mathrm{H}}_2\mathrm{O}=6{\mathrm{F}\mathrm{e}}^{2+}+2{\mathrm{K}}^{+}+2\mathrm{A}\mathrm{l}(\mathrm{O}\mathrm{H}{)}_{3\left(\mathrm{a}\mathrm{q}\right)}+6{\mathrm{H}}_4{\mathrm{S}\mathrm{i}\mathrm{O}}_{4\left(\mathrm{a}\mathrm{q}\right)}$$

(2)

$${\mathrm{C}\mathrm{a}}_2{\mathrm{F}\mathrm{e}}_5{\mathrm{S}\mathrm{i}}_8{\mathrm{O}}_{22}(\mathrm{O}\mathrm{H}{)}_{2\left(\mathrm{H}\mathrm{b}\mathrm{l}\right)}+14{\mathrm{H}}^{+}=2{\mathrm{C}\mathrm{a}}^{2+}+5{\mathrm{F}\mathrm{e}}^{2+}+8{\mathrm{H}}_4{\mathrm{S}\mathrm{i}\mathrm{O}}_{4\left(\mathrm{a}\mathrm{q}\right)}$$

(3)

$$3{\mathrm{F}\mathrm{e}\mathrm{C}\mathrm{l}}^{+}+4{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{a}\mathrm{q}\right)}={\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_{4\left(\mathrm{M}\mathrm{a}\mathrm{g}\right)}+{\mathrm{H}}_{2\left(\mathrm{a}\mathrm{q}\right)}+6{\mathrm{H}}^{+}+3{\mathrm{C}\mathrm{l}}^{-}$$

(4)

$$3\mathrm{F}\mathrm{e}{\mathrm{S}}_{2\left(\mathrm{P}\mathrm{y}\right)}+4{\mathrm{H}}_2\mathrm{O}=\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_{4\left(\mathrm{M}\mathrm{a}\mathrm{g}\right)}+6{\mathrm{H}}_2{\mathrm{S}}_{\left(\mathrm{a}\mathrm{q}\right)}$$

(5)

6.2. Thermodynamic Conditions of High-Grade Disseminated Gold Ore Formation Constrained by Magnetite Geochemistry

Disseminated mineralization is widely attributed to fluid–rock interactions [5,6,7,8]. Both the marble-hosted and granite-hosted ores at Guocheng exhibit typical disseminated mineralization (Figure 3A,C), with gold grades strictly controlled by the lithology of the surrounding rock (marble vs. granite). These observations suggest that the significant difference in ore grade is closely related to distinct fluid–rock interactions with the different wall rocks. In this section, we compare the ore-forming environments of marble-hosted and granite-hosted ores by contrasting magnetite geochemistry and mineral assemblages, and discuss variations in (1) oxygen fugacity and (2) sulfur fugacity, as well as their roles in controlling gold precipitation and ore grade. Furthermore, magnetite from both ore types exhibits similarly low Ti concentrations (mostly <20 ppm, Table 2). Given that Ti incorporation into magnetite is highly sensitive and positive correlated with temperature [63,66], this consistent low-Ti signature confirms that both two types of magnetite crystallized under similar low-temperature conditions.

(1)

Oxygen fugacity (fO₂)

Magnetite from marble-hosted ores contains significantly lower vanadium (V) concentrations (mostly <25 ppm) than that from granite-hosted ores, where V contents are typically concentrated between 40 and 70 ppm (Figure 8B). Because V is highly sensitive to the redox state of hydrothermal fluids, this compositional contrast directly reflects differences in the oxygen fugacity during magnetite formation [67,68,69]. In fluids, V mainly exists as V³⁺ and V⁴⁺, among which V³⁺ can replace Fe³⁺ in magnetite through isomorphic substitution via exchange reactions [70,71]. As oxygen fugacity increases, V³⁺ in the fluid is oxidized to V⁴⁺, resulting in less V entering the magnetite lattice [67]. Therefore, the lower V concentrations in marble-hosted magnetite indicate crystallization under more oxidizing conditions than granite-hosted magnetite. Thermodynamic models have demonstrated that fluid oxidation can occur due to different degrees of fluid-rock interaction, primarily through carbonation reactions, without requiring externally derived oxidizing fluids [72]. The elevated fO₂ in the marble-hosted system likely resulted from strong fluid–rock interactions with the Fe-rich marble rocks of the Jingshan Group, which formed during the Paleoproterozoic Lomagundi-Jatuli oxidation event [52]. Consequently, the elevated fO₂ in the marble-hosted system likely resulted from strong fluid–rock interactions with the Fe-rich marble rocks of the Jingshan Group. During carbonatization, Fe²⁺ released from these wall rocks was oxidized and precipitated as ankerite and magnetite, thereby increasing the oxygen fugacity of the fluid [73,74]. This interpretation is supported by the widespread ankerite and magnetite assemblages and pervasive carbonatization observed in marble-hosted ores (Figure 6C,D). The ore-forming fluids of the Guocheng deposit belong to the medium- to low-temperature reduced H₂O-CO₂-NaCl ± CH₄ system [26,27], consistent with other Jiaodong gold deposits [19]. In these deposits, gold is mainly transported as hydrosulfide complexes (AuHS(aq) and Au(HS)₂⁻) [75,76]. Thermodynamic modeling results demonstrate that elevated oxygen fugacity in the ore-forming fluid would lead to the destabilization of Au–S complexes, reduce gold solubility, and consequently enhance gold precipitation efficiency [24,77]. Consequently, the elevated oxygen fugacity induced by carbonate–fluid interaction is a primary control on the formation of high-grade disseminated gold ores in marble-hosted zones.

(2)

Sulfur fugacity (fS₂)

The concentrations of chalcophile elements such as Co, Ni, Sn, and Zn in magnetite are mainly governed by their partitioning behavior between sulfide and oxide phases [33]. In sulfur-bearing hydrothermal systems, these elements are highly compatible with sulfide minerals but incompatible with magnetite [78,79,80]. As sulfides begin to crystallize from the ore-forming fluid, they preferentially incorporate these elements, thereby depleting the coexisting magnetite in chalcophile components [81]. Consequently, the lower concentrations of chalcophile elements in magnetite from marble-hosted ores, compared with those from granite-hosted ores (Table 2, Figure 8C), indicate more extensive sulfide precipitation. Considering that both ore types were derived from the same ore-forming fluid, this trend suggests a decrease in sulfur fugacity (fS₂) during fluid–rock interaction with the marble wall rocks.

This process of a decrease in sulfur fugacity is further supported by the widespread pyrrhotite–magnetite mineral assemblage observed in marble-hosted ores (Figure 4A–C, Figure 5D–F and Figure 6C,D). Magnetite is an oxidizing mineral, whereas pyrrhotite is a reducing mineral, their coexistence commonly indicates that the ore-forming fluids experienced complex physicochemical evolution during gold mineralization [28,77,82]. Thermodynamic modeling shows that a decrease in total sulfur concentration and an increase in fluid pH can effectively expand the upper stability limit of pyrrhotite with respect to logfO₂, while the decline in sulfur fugacity can broaden the stability field of magnetite, allowing pyrrhotite and magnetite to co-precipitate [24,77]. The abundant occurrence of pyrrhotite in marble-hosted ores, in contrast to its scarcity in granite-hosted ores, suggests that during fluid passage through the Jingshan Group metamorphic wall rocks, the sulfur and H⁺ in fluids were continuously consumed (Reactions (2) and (7)), causing sulfide precipitation to shift from pyrite (Reaction (8)) to pyrrhotite (Reaction (9)). This decrease in sulfur fugacity can significantly reduce gold solubility, thereby destabilizing Au–S complexes and promoting rapid precipitation (Figure 4C) [83,84]. This decrease in sulfur fugacity thus represents a key factor controlling the formation of high-grade marble-hosted ores.

$$\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_{3\left(\mathrm{C}\mathrm{a}\mathrm{l}\right)}+2{\mathrm{H}}^{+}=\mathrm{C}{\mathrm{a}}^{2+}+\mathrm{C}{\mathrm{O}}_2\uparrow +{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{a}\mathrm{q}\right)}$$

(6)

$$\mathrm{F}\mathrm{e}\mathrm{C}{\mathrm{l}}^{+}+2{\mathrm{H}}_2{\mathrm{S}}_{\left(\mathrm{a}\mathrm{q}\right)}=\mathrm{F}\mathrm{e}{\mathrm{S}}_{2\left(\mathrm{P}\mathrm{y}\right)}+\mathrm{C}{\mathrm{l}}^{-}+{\mathrm{H}}_{2\left(\mathrm{a}\mathrm{q}\right)}+2{\mathrm{H}}^{+}$$

(7)

$$\mathrm{F}\mathrm{e}\mathrm{C}{\mathrm{l}}^{+}+{\mathrm{H}}_2{\mathrm{S}}_{\left(\mathrm{a}\mathrm{q}\right)}+4{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{a}\mathrm{q}\right)}=\mathrm{F}\mathrm{e}{\mathrm{S}}_{\left(\mathrm{P}\mathrm{y}\mathrm{h}\right)}+2{\mathrm{H}}^{+}+\mathrm{C}{\mathrm{l}}^{-}$$

(8)

In summary, comparative data on magnetite geochemistry and mineral assemblages indicates that marble-hosted ores formed under increasing oxygen fugacity and decreasing sulfur fugacity. This dual evolution drove large-scale co-precipitation of pyrrhotite and magnetite, while a decrease in dissolved sulfur and an increase in fluid oxidation state destabilized Au-S complexes (e.g., AuHS(aq) and Au(HS)₂⁻), reducing gold solubility and promoting efficient gold precipitation [85], which ultimately formed the high-grade disseminated ores in the marble-hosted zones (Figure 9).

7. Conclusions

(1)

Magnetite in the Guocheng gold deposit is of hydrothermal origin and was formed during the main mineralization stage. Petrographic observations indicate that most magnetite crystallized via dissolution–migration–recrystallization, while a minor proportion formed through in situ dissolution–recrystallization. The close association of all magnetite with gold and sulfides, together with their classification into the low-temperature hydrothermal category via Ga–V diagrams, indicates that they reliably record the ore-forming environment of gold precipitation.

(2)

A systematic comparison of magnetite geochemistry between the two ore types reveals distinct mineralization environments. Specifically, the lower V and chalcophile element (Co, Ni, Sn, Zn) concentrations in marble-hosted magnetite, demonstrate that it crystallized under increasing oxygen fugacity (fO₂) and decreasing sulfur fugacity (fS₂) compared to its granite-hosted counterpart.

(3)

The pronounced contrast in gold grades between the two ore types is closely related to the variations in fluid thermodynamic conditions induced by different fluid–rock reactions. In the marble-hosted ores, coupled carbonate dissolution and sulfide precipitation increased fO₂ and decreased fS₂, driving a mineralogical transition from pyrite to pyrrhotite–magnetite co-precipitation. This transition destabilized gold–sulfur complexes, enhanced gold precipitation efficiency, ultimately leading to the formation of high-grade disseminated ores.