Advances in Distribution Pattern and Enrichment Mechanism of Associated Cobalt Resources in Skarn-Type Deposits, China

Abstract

Although skarn-type deposits represent significant hosts for Co resources, the distribution patterns and enrichment mechanisms of associated Co resources within these deposits have not been systematically investigated. This study summarizes relevant data on Co resources from representative skarn-type deposits in China to comparatively reveal the grade and reserve characteristics, spatiotemporal distribution patterns, and coupled enrichment mechanisms of Co across three principal skarn mineralization subtypes: iron-, copper-, and lead–zinc polymetallic-dominated deposits. Studies demonstrate that Fe-dominated skarn-type cobalt deposits exhibit widespread distribution, high Co grades (100–2000 ppm), and abundant Co reserves (4000–32,000 t), demonstrating significantly superior Co resource potential compared to Cu-dominated (Co grades: 20–200 ppm, Co reserves: 3000–10,000 t) and Pb-Zn polymetallic-dominated (Co grades: 140–853 ppm, Co reserves: approximately 3000 t) subtypes. In these skarn-type cobalt deposits, cobalt is mainly hosted in sulfide minerals. Influenced by tectonic settings, magmatic activity, and hydrothermal fluid evolution, associated Co resources in these skarn-type deposits exhibit both regional zonation and stage-specific differential enrichment patterns. In the formation of skarn-type cobalt deposits, mantle-derived magmas play a critical role in the pre-enrichment of Co. The injection of mafic magmas, assimilation of evaporite sequences, and the dissolution–reprecipitation mechanism of hydrothermal fluids collectively promote the re-enrichment of Co during magmatic evolution. These findings provide a theoretical foundation for targeted exploration, sustainable development, and comprehensive utilization of associated Co resources in skarn-type deposits.

1. Introduction

Cobalt (Co), an emerging critical metal resource, is widely utilized in lithium-ion batteries, superalloys, and the aviation industry due to its ferromagnetism, low thermal conductivity, and electrical conductivity, and its importance in modern industrial systems has grown substantially [1,2]. Global demand for Co, significantly driven by the transition towards low-carbon energy infrastructures, increased from less than 1 × 10⁵ t in 2015 to 1.87 × 10⁵ t in 2022, with a Compound Annual Growth Rate (CAGR) of approximately 9.6% [3]. Consequently, cobalt has been classified as a critical mineral resource by numerous nations, including the European Union, the United States, Japan, and China [4,5,6]. Against this global backdrop, China, the largest Co consumer globally, experiences a severe imbalance between resource scarcity and industrial demand. In 2022, domestic Co production in China was reported at only 2200 t, contrasting sharply with consumption of 1.41 × 10⁵ t, resulting in an import dependency rate of 98% [3]. Furthermore, Chinese Co deposits are characterized by being small-scale and low-grade and having a dispersed distribution, further exacerbating the existing supply–demand imbalance [7]. To mitigate this critical bottleneck, intensifying exploration and development efforts targeting potential Co resources is imperative for enhancing future supply security.

Globally, principal Co deposit types comprise stratiform sediment-hosted Cu-Co deposits (41%), lateritic Ni-Co deposits (36%), magmatic Ni-Cu-Co deposits (15%), and hydrothermal–volcanic polymetallic Co deposits (8%) [6]. Conversely, Co deposits in China are primarily of magmatic and hydrothermal origin [8,9]. Hydrothermal Co deposits in China, which are widely distributed and cover diverse subtypes including skarn, hydrothermal metasomatic, and porphyry, are increasingly recognized as a focus for the exploration and development of Co resources [10,11]. Historically, research on skarn-type deposits has primarily focused on the metallogenic mechanisms of major elements such as Fe and Cu [12,13]. However, understanding of the distribution patterns and enrichment mechanisms of associated Co remains inadequate, reflecting a persistent tendency toward prioritizing major elements over associated elements. Given the high dependence on Co resources and surging demand from emerging industrial sectors, enhancing the understanding of Co metallogenic mechanisms in skarn-type deposits is considered essential and urgent [3].

Skarn-type deposits represent significant sources of Co in China, accounting for 28.2% of the national Co reserves, with Co mainly hosted in Fe-, Cu-, Pb-, and Zn-bearing minerals [14,15,16,17,18,19,20]. Furthermore, significant Co enrichment has recently been identified in skarn-type deposits within major metallogenic belts of China, including the Handan–Xingtai Mining Area, the Middle-Lower Yangtze River Metallogenic Belt, and the Gangdese Metallogenic Belt, underscoring the considerable Co metallogenic potential of such deposits [21,22,23]. Consequently, this study focuses on investigating the grade, reserves, distribution patterns, occurrence states, migration, precipitation, and enrichment mechanisms of associated Co resources in skarn-type deposits to comprehensively understand the key geological controls and processes of cobalt enrichment, thereby advancing the understanding of Co metallogenic mechanisms.

2. Grade and Reserves of Associated Cobalt Resources in Skarn-Type Deposits

Although skarn-type deposits represent significant hosts for Co resources [24,25], notable variations in Co grade and reserves are observed among different deposits. According to the Comprehensive Exploration and Evaluation Specification for Mineral Resources (GB/T 25283-2023) [26], the boundary grade for the comprehensive utilization of associated cobalt is 100 ppm. This study summarizes relevant data on associated Co resources from 42 skarn-type deposits and classifies these deposits into three subtypes for comprehensive assessment: Fe-dominated, Cu-dominated, and Pb-Zn polymetallic deposits (Figure 1 and Figure 2).

2.1. Fe-Dominated Skarn-Type Cobalt Deposits

Fe-dominated skarn-type cobalt deposits are globally distributed, with significant occurrences documented in the Sayan Mountains of Russia, Tuva–Mongolia region of Russia, Yamato mining area of Japan, Sesia–Lanzo metallogenic belt in the Western Alps, and Banatic Belt in Eastern Europe (Romania) [27,28,29,30,31]. In China, a significant concentration of such deposits primarily occurs in the Middle-Lower Yangtze River Metallogenic Belt (MLYB), Handan–Xingtai Mining Area (HXMA), and Shandong Mining Area of the eastern regions (SDMA), with minor occurrences in the Gangdese Metallogenic Belt (GMB) and East Kunlun Metallogenic Belt (EKOB) of the western regions (Figure 2a). These deposits are characterized by high Co grades, ranging from 100 ppm to 2000 ppm. Co reserves commonly exceed 4000 t, and several deposits exceed 10,000 t, indicating substantial potential for economic exploitation (Figure 2b). Consequently, it is proposed that, given their widespread distribution, high grades, and abundant reserves, Fe-dominated skarn-type cobalt deposits represent critical targets for Co resource exploration.

The Zhongguan deposit, a representative “Hanxing-type” skarn Fe deposit, is dominated by primary magnetite ores with total reserves of 94.89 Mt [32]. Associated Co reserves are estimated at 9489 t, with an average Co grade of 180 ppm [32,33]. Notably, pyrite in the deposit is characterized by a generally elevated Co content, which facilitates effective Co resource extraction through optimized beneficiation to improve recovery rates [34]. Similarly, the Baijian Fe deposit, also located in the Handan–Xingtai Mining Area, has been identified as the largest recently discovered Fe deposit in the region [21]. This deposit hosts massive Fe ore reserves of 112 Mt, accompanied by associated Co reserves of 17,186 t, with an average Co grade of 150 ppm and localized high-grade zones reaching 320 ppm [35].

The Longqiao Fe deposit, located in the Luzong Basin of the Middle-Lower Yangtze River Metallogenic Belt, is characterized by a principal ore body extending 1000 m in length [19]. This ore body has been recognized as the largest single Fe ore body among the skarn-type deposits in eastern China [19]. It contains Fe ore resources estimated at 101 Mt, accompanied by Co reserves of 10,810 t with an average Co grade of 110 ppm [19,22]. Notably, sulfur–cobalt concentrates from this deposit exhibit an average Co content ranging from 0.16% to 0.20%, meeting the minimum grade requirement for Grade VI sulfur–cobalt fine powder, which demonstrates potential for Co resource utilization [36]. The Zhangjiawa Fe deposit, located in the Laiwu area of Shandong Province, represents the largest-scale deposit within the regional ore field and contains abundant reserves of open-hearth-furnace-rich ore [37]. Hosted by the inclined termination of the mineralized arc-shaped anticline, it comprises three distinct mining areas: Zhangjiawa I, II, and III. Total proven Fe ore resources are estimated at 290 Mt, accompanied by Co reserves of 48,548 t, which represents 63.66% of the total Co reserves within the ore field [38]. Within the Zhangjiawa I Mining Area, the associated Co reserves are estimated at 6180 t, with an average Co grade of 150 ppm. The Zhangjiawa II Mining Area (also known as Xiaoguanzhuang) contains Co reserves of 13,611.5 t, exhibiting an average Co grade of 150 ppm and localized zones reaching 410 ppm [38]. Meanwhile, Zhangjiawa III Mining Area (also known as Gangli) contains Co reserves of 28,757 t, with an average Co grade of 190 ppm and localized zones reaching up to 2600 ppm [38].

Among global Fe-dominated skarn-type cobalt deposits, the Cornwall deposit (Pennsylvania, USA) is recognized for its substantial Fe resources and high-grade associated critical metals, including Co, Au, and Ag [39]. The deposit exhibits an average Co grade of 300 ppm, making it a significant source of Co in the United States and serving as a representative example for associated Co recovery from skarn-type deposits [40,41]. In Russia, the Goroblagodat Fe deposit exhibits an average Co grade of 200 ppm, while the Magnitogorsk Fe deposit exhibits an average Co grade of 180 ppm [15,42].

2.2. Cu-Dominated Skarn-Type Cobalt Deposits

Compared to Fe-dominated skarn-type cobalt deposits, Cu-dominated skarn-type cobalt deposits are less abundant and primarily concentrated in the Middle-Lower Yangtze River Metallogenic Belt (MLYB). These deposits are typically characterized by low Co grades, ranging from 20 ppm to 200 ppm (Figure 2a). Despite the relatively low average Co grade, total Co reserves are substantial, mostly ranging from 3000 t to 10,000 t (Figure 2b). Consequently, it is proposed that, given their concentrated distribution, low grades, but substantial reserves, Cu-dominated skarn-type cobalt deposits represent an important supplementary source for Co exploitation.

The Anqing Cu-Fe deposit (also known as Xima’anshan), located in the Middle-Lower Yangtze River Metallogenic Belt, is characterized by Cu and Fe as the principal ore types. Cu ore reserves are estimated at 23.28 Mt, accompanied by Co reserves of 4905 t, with an average Co grade of 166 ppm [25,43]. Through comparative analysis of porphyry-skarn deposits in the Middle-Lower Yangtze River Metallogenic Belt, Zhong et al. revealed that the average Co grade in Cu ores of the Anqing deposit is 3 to 8 times higher than that of the Cu ores in the Dongguashan, Xinqiao, and Wushan deposits (average Co grade: 20 ppm to 50 ppm) and approximately 33 times higher than that of the Cu ores in the Shaxi deposit (average Co grade: 5.2 ppm) [43,44]. This anomaly is attributed to the unique cobalt-hosting monoclinic pyrrhotite in the deposit, characterized by numerous crystallographic vacancies and strong magnetism that significantly enhance its Co enrichment ability, resulting in significantly higher Co grades relative to regional analogous deposits [43,45,46].

Significant spatial variations in Co resource distribution are observed between the Wushan and Chengmenshan Cu deposits, both located in the Jiurui Mining Area of the Middle-Lower Yangtze River Metallogenic Belt [22]. The Wushan deposit is divided into northern and southern ore zones, which exhibit distinct differences in Co grades and reserves. In the northern zone, associated Co reserves are estimated at 7603 t, with an average Co grade of 56 ppm. In contrast, the southern zone contains lower associated Co reserves of 1995 t, with a lower average Co grade of 22 ppm [44]. The Chengmenshan Cu deposit exhibits an average Co grade of 85 ppm, and its abundant ore reserves of 220 Mt result in substantial associated Co reserves of 18,382 t, demonstrating obvious low-grade but high-reserve characteristics [22,47].

The Mount Elliott deposit (Queensland, Australia) serves as a representative example of a global Cu-dominated skarn-type cobalt deposit. It exhibits an average Co grade of 1000 ppm [48], substantially higher than that in analogous deposits in China (Figure 2a), indicating significant Co metallogenic potential.

2.3. Pb-Zn Polymetallic-Dominated Skarn-Type Cobalt Deposits

Compared to Fe-dominated and Cu-dominated skarn-type cobalt deposits, Pb-Zn polymetallic-dominated skarn-type cobalt deposits are less numerous, with primary occurrences concentrated in the Gangdese and East Kunlun Metallogenic Belts of western China. Significant variations in Co grades are documented for this deposit type, ranging from 140 ppm to 853 ppm (Figure 2a). The total Co reserves are estimated at approximately 3000 t (Figure 2b). Consequently, it is proposed that Pb-Zn polymetallic-dominated skarn-type cobalt deposits are characterized by a spatially constrained distribution, moderate grades, and relatively low reserves and thus require further exploration and development.

The Galinge deposit, located in the Qimantage area of the East Kunlun Metallogenic Belt, is characterized by a polymetallic Fe-Cu-Pb-Zn mineralization assemblage [49]. Geological exploration data indicate that total Co resources (proven, controlled, and inferred) are estimated at 4.34 Mt of ore, accompanied by Co reserves of 3538 t. The average Co grade for the deposit is reported as 600 ppm, with the No. II ore body exhibiting a significantly higher grade of 1230 ppm, reflecting localized high Co enrichment [49]. The Pusangguo deposit, located in the Gangdese Metallogenic Belt, was formed primarily through late-stage mineralization and hydrothermal replacement [50]. Total proven metal resources of Cu, Pb, Zn, and Co in this deposit exceed 300,000 t, including Co reserves exceeding 280 t. An average Co grade of 140 ppm is recorded for this deposit [20,51].

The Yaojialing deposit, located in the Tongling Mining Area of the Middle-Lower Yangtze River Metallogenic Belt, is recognized as the sole large-scale Cu-Pb-Zn polymetallic deposit [52]. Associated Co reserves in this deposit are estimated at 2920 t, with an average Co grade of 450 ppm [53]. Mineralization primarily comprises four ore body types: skarn, stratiform, vein-type, and stratabound-type. Significant Co enrichment is observed in skarn-type and stratiform ore bodies formed during the early high-temperature stage, indicating economic recovery potential [53].

Based on the preceding analysis, it is proposed that Fe-dominated skarn-type cobalt deposits are characterized by their abundance and wide distribution, with significantly higher Co grades than Cu-dominated and Pb-Zn polymetallic-dominated cobalt deposits, coupled with relatively abundant Co reserves. In contrast, Cu-dominated skarn-type cobalt deposits generally exhibit lower Co grades, although certain deposits exhibit relatively high grades along with considerable reserves. Pb-Zn polymetallic-dominated skarn-type cobalt deposits demonstrate higher Co grades than Cu-dominated deposits but are constrained by relatively limited Co reserves. Nevertheless, Liang et al. (2023) conducted a comparative analysis of Co grade and reserve characteristics in the Fe- and Cu-dominated skarn-type cobalt deposits in the Middle-Lower Yangtze River Metallogenic Belt and concluded that no obvious correlation exists between Co mineralization and specific deposit type [54]. Thus, further research is required to investigate the correlation between Co metallogenic potential and skarn-type deposit types.

Table 1. The grade and reserves of associated cobalt resources in skarn-type deposits.

No.	Name	Location	Type	Average Co Grade (ppm)	Co Reserves (t)	Reference
1	Henghui	Xingtai, Hebei Province, China	Fe	100	3000	[55]
2	Baijian	Xingtai, Hebei Province, China	Fe	150	17,186	[35]
3	Yushiwa	Handan, Hebei Province, China	Fe	150	2800	[33]
4	Zhongguan	Xingtai, Hebei Province, China	Fe	180	9489	[32,33]
5	Xishangzhuang	Laiwu, Shandong Province, China	Fe	150	6600	[33]
6	Gujiatai	Laiwu, Shandong Province, China	Fe	160	5000	[33]
7	Chuiyang	Laiwu, Shandong Province, China	Fe	110	——	[38]
8	Zhangjiawa I Mining Area	Laiwu, Shandong Province, China	Fe	150	6180	[38]
9	Zhangjiawa II Mining Area	Laiwu, Shandong Province, China	Fe	150	13,612	[38]
10	Zhangjiawa III Mining Area	Laiwu, Shandong Province, China	Fe	190	28,757	[38]
11	Xinzhuang	Zibo, Shandong Province, China	Fe	157	——	[56]
12	Jinzhao	Zibo, Shandong Province, China	Fe	193	4100	[33]
13	Houjiazhuang	Zibo, Shandong Province, China	Fe	113	2100	[33]
14	Tonglvshan Fe Ore	Daye, Hubei Province, China	Fe	35	——	[57]
15	Wushan South Ore Zones	Ruichang, Jiangxi Province, China	Cu	22	1995	[44]
16	Jinshandian	Daye, Hubei Province, China	Fe	67	7178	[58]
17	Wushan North Ore Zones	Ruichang, Jiangxi Province, China	Cu	56	7603	[44]
18	Chengchao	Ezhou, Hubei Province, China	Fe	170	32,717	[59]
19	Tieshan	Huangshan, Hubei Province, China	Fe	170	25,818	[57]
20	Fengshandong	Huangshan, Hubei Province, China	Cu	79	7384	[60]
21	Chengmenshan	Jiujiang, Jiangxi Province, China	Cu	85	18,382	[22,47]
22	Xujiazu	Daye, Hubei Province, China	Cu	160	——	[57]
23	Daguangshan	Daye, Hubei Province, China	Fe	200	4200	[33]
24	Anqing Cu Ore	Anqing, Anhui Province, China	Cu	166	4905	[43]
25	Zhangsizhu	Daye, Hubei Province, China	Fe	500	——	[57]
26	Tonglvshan Cu Ore	Daye, Hubei Province, China	Cu	50	——	[57]
27	Zhuchong	Anqing, Anhui Province, China	Fe	190	10,045	[61]
28	Baixiangshan	Maan, Anhui Province, China	Fe	80	10,584	[62]
29	Dongguashan	Tongling, Anhui Province, China	Cu	56	9093	[63]
30	Tongshankou	Daye, Hubei Province, China	Cu	59	3099	[64,65]
31	Longqiao	Hefei, Anhui Province, China	Fe	110	10,810	[19]
32	Xinqiao	Tongling, Anhui Province, China	Fe	113	14,129	[66,67]
33	Anqing Fe Ore	Anqing, Anhui Province, China	Fe	126	3864	[43]
34	Yaojialing	Wuhu, Anhui Province, China	Pb-Zn polymetallic	450	2920	[53]
35	Chunzhe	Rikeze, Xizang Province, China	Fe	898	——	[68]
36	Pusangguo	Rikeze, Xizang Province, China	Pb-Zn polymetallic	140	——	[20]
37	Bangpu	Lasa, Xizang Province, China	Pb-Zn polymetallic	363	——	[68]
38	Jiama	Lasa, Xizang Province, China	Pb-Zn polymetallic	853	——	[68]
39	Zhibula	Lasa, Xizang Province, China	Pb-Zn polymetallic	410	——	[17]
40	Galinge	Geermu, Qinghai Province, China	Pb-Zn polymetallic	600	3538	[49]
41	Haisi	Dulan, Qinghai Province, China	Fe	1800	——	[69]
42	Niukutou	Geermu, Qinghai Province, China	Pb-Zn polymetallic	120	——	[70]
43	Zhanbuzhale IV Mining Area	Dulan, Qinghai Province, China	Fe	240	——	[71]
44	Mount Elliott	Queensland, Australia	Cu	1000	——	[48]
45	Goroblagodat	Middle Urals, Russia	Fe	200	——	[15]
46	Magnitogorsk	Baimak-Buribai, Russia	Fe	180	——	[42]
47	Cornwall	Pennsylvania, America	Fe	300	——	[40]

Table 1. The grade and reserves of associated cobalt resources in skarn-type deposits.

No.	Name	Location	Type	Average Co Grade (ppm)	Co Reserves (t)	Reference
1	Henghui	Xingtai, Hebei Province, China	Fe	100	3000	[55]
2	Baijian	Xingtai, Hebei Province, China	Fe	150	17,186	[35]
3	Yushiwa	Handan, Hebei Province, China	Fe	150	2800	[33]
4	Zhongguan	Xingtai, Hebei Province, China	Fe	180	9489	[32,33]
5	Xishangzhuang	Laiwu, Shandong Province, China	Fe	150	6600	[33]
6	Gujiatai	Laiwu, Shandong Province, China	Fe	160	5000	[33]
7	Chuiyang	Laiwu, Shandong Province, China	Fe	110	——	[38]
8	Zhangjiawa I Mining Area	Laiwu, Shandong Province, China	Fe	150	6180	[38]
9	Zhangjiawa II Mining Area	Laiwu, Shandong Province, China	Fe	150	13,612	[38]
10	Zhangjiawa III Mining Area	Laiwu, Shandong Province, China	Fe	190	28,757	[38]
11	Xinzhuang	Zibo, Shandong Province, China	Fe	157	——	[56]
12	Jinzhao	Zibo, Shandong Province, China	Fe	193	4100	[33]
13	Houjiazhuang	Zibo, Shandong Province, China	Fe	113	2100	[33]
14	Tonglvshan Fe Ore	Daye, Hubei Province, China	Fe	35	——	[57]
15	Wushan South Ore Zones	Ruichang, Jiangxi Province, China	Cu	22	1995	[44]
16	Jinshandian	Daye, Hubei Province, China	Fe	67	7178	[58]
17	Wushan North Ore Zones	Ruichang, Jiangxi Province, China	Cu	56	7603	[44]
18	Chengchao	Ezhou, Hubei Province, China	Fe	170	32,717	[59]
19	Tieshan	Huangshan, Hubei Province, China	Fe	170	25,818	[57]
20	Fengshandong	Huangshan, Hubei Province, China	Cu	79	7384	[60]
21	Chengmenshan	Jiujiang, Jiangxi Province, China	Cu	85	18,382	[22,47]
22	Xujiazu	Daye, Hubei Province, China	Cu	160	——	[57]
23	Daguangshan	Daye, Hubei Province, China	Fe	200	4200	[33]
24	Anqing Cu Ore	Anqing, Anhui Province, China	Cu	166	4905	[43]
25	Zhangsizhu	Daye, Hubei Province, China	Fe	500	——	[57]
26	Tonglvshan Cu Ore	Daye, Hubei Province, China	Cu	50	——	[57]
27	Zhuchong	Anqing, Anhui Province, China	Fe	190	10,045	[61]
28	Baixiangshan	Maan, Anhui Province, China	Fe	80	10,584	[62]
29	Dongguashan	Tongling, Anhui Province, China	Cu	56	9093	[63]
30	Tongshankou	Daye, Hubei Province, China	Cu	59	3099	[64,65]
31	Longqiao	Hefei, Anhui Province, China	Fe	110	10,810	[19]
32	Xinqiao	Tongling, Anhui Province, China	Fe	113	14,129	[66,67]
33	Anqing Fe Ore	Anqing, Anhui Province, China	Fe	126	3864	[43]
34	Yaojialing	Wuhu, Anhui Province, China	Pb-Zn polymetallic	450	2920	[53]
35	Chunzhe	Rikeze, Xizang Province, China	Fe	898	——	[68]
36	Pusangguo	Rikeze, Xizang Province, China	Pb-Zn polymetallic	140	——	[20]
37	Bangpu	Lasa, Xizang Province, China	Pb-Zn polymetallic	363	——	[68]
38	Jiama	Lasa, Xizang Province, China	Pb-Zn polymetallic	853	——	[68]
39	Zhibula	Lasa, Xizang Province, China	Pb-Zn polymetallic	410	——	[17]
40	Galinge	Geermu, Qinghai Province, China	Pb-Zn polymetallic	600	3538	[49]
41	Haisi	Dulan, Qinghai Province, China	Fe	1800	——	[69]
42	Niukutou	Geermu, Qinghai Province, China	Pb-Zn polymetallic	120	——	[70]
43	Zhanbuzhale IV Mining Area	Dulan, Qinghai Province, China	Fe	240	——	[71]
44	Mount Elliott	Queensland, Australia	Cu	1000	——	[48]
45	Goroblagodat	Middle Urals, Russia	Fe	200	——	[15]
46	Magnitogorsk	Baimak-Buribai, Russia	Fe	180	——	[42]
47	Cornwall	Pennsylvania, America	Fe	300	——	[40]

3. Distribution Patterns of Associated Cobalt Resources in Skarn-Type Deposits

Distinct distribution patterns are observed for associated Co resources in skarn-type deposits. With respect to host minerals, Co enrichment is mainly associated with sulfide minerals, with Co occurrence states varying systematically across deposit types. Regionally, Co resources exhibit zonal distribution patterns controlled by tectonic settings and magmatic activity. During the mineralization process, stage-specific variations in Co enrichment are governed by the evolving composition of hydrothermal fluids and the physicochemical conditions.

3.1. Distribution Patterns of Cobalt in Host Minerals

In skarn-type deposits, Co occurs not only as independent cobalt minerals but is also mainly hosted in sulfides, arsenides, and oxides through isomorphic substitution [54,72,73,74]. To further elucidate its mineralogical distribution, available data on cobalt-hosting minerals and average Co grades in major cobalt-bearing minerals from skarn-type deposits are summarized (Table 2).

Early on, Xu et al. (1979) pointed out that in skarn-type Fe-Cu deposits, pyrite is the primary Co-bearing mineral, with its proportion of Co reaching as high as 50%–90% [75]. Based on this conclusion, we propose that Co in Fe-dominated skarn-type cobalt deposits is mainly hosted in pyrite and magnetite through isomorphic substitution for Fe, with a minor proportion occurring in pyrrhotite (Table 2). Additionally, Co occurs as independent Co minerals in specific deposits, such as cobaltite (CoAsS) in the Baijian deposit [35], grimmitte (NiCo₂S₄) in the Zhuchong deposit [54], and glaucodot [(Co,Fe)AsS] in the Longqiao deposit [76]. During hydrothermal mineralization, the complexity of fluid interactions leads to a wide variation in the average Co content of pyrite [17], ranging from 220 ppm to 13,000 ppm, and with most values exceeding 1000 ppm (Table 2). In contrast, magnetite, as the other major cobalt-bearing mineral, exhibits a narrower range of average Co content, from 17 ppm to 320 ppm, with most values below 100 ppm (Table 2). This distribution pattern is exemplified by the Baijian Fe deposit, where Co is mainly hosted in pyrite and magnetite through isomorphic substitution, with a minor proportion occurring as an independent cobalt mineral, cobaltite (CoAsS) [35]. Specifically, pyrite exhibits a significant Co content variation, ranging from 0.187 ppm to 33,331 ppm, while magnetite ranges from 15.2 ppm to 107 ppm [35].

In Cu-dominated skarn-type cobalt deposits, Co is mainly hosted in pyrite and chalcopyrite through isomorphic substitution, with a minor proportion occurring in bornite (Table 2). Additionally, Co occurs as independent cobalt minerals in specific deposits, such as cobaltite, carrollite, and glaucodot in the Dongguashan Cu deposit [63] and siegenite [(Ni,Co)₃S₄] in the Anqing Cu-Fe deposit [43]. In this subtype of deposits, pyrite exhibits an average Co content ranging from 626 ppm to 1365 ppm, values typically lower than that in pyrite from Fe-dominated skarn-type cobalt deposits (Table 2). Chalcopyrite (253–500 ppm) exhibits a lower average Co content than pyrite (Table 2). This distribution pattern is exemplified by the Anqing Cu-Fe deposit, where pyrite and chalcopyrite serve as primary cobalt-bearing minerals through isomorphic substitution, with an average content of 1358 ppm and 260 ppm, respectively, with a minor proportion occurring as an independent cobalt mineral, siegenite [43,77,78].

In Pb-Zn polymetallic-dominated skarn-type cobalt deposits, Co is mainly hosted in pyrite, arsenopyrite, and sphalerite through isomorphic substitution, with minor incorporation into chalcopyrite and pyrrhotite (Table 2). Distinct from skarn-type Fe and Cu deposits, these systems commonly develop abundant independent cobalt minerals, including cobaltite and glaucodot [(Co,Fe)AsS] in the Niukutou deposit [70], linnaeite in the Pusangguo deposit [20], and skutterudite (CoAs₃) in the Galinge deposit [73]. Notably, pyrite exhibits a highly variable average Co content, ranging from 42.5 ppm to 27,400 ppm, with most values exceeding 100 ppm (Table 2). Sphalerite, a distinctive cobalt-bearing mineral in this deposit type, exhibits an average Co content from 203.7 ppm to 8100 ppm (Table 2). This distribution pattern is exemplified by the Galinge deposit, where Co occurs in two principal modes: One is as independent cobalt minerals, including skutterudite and cobaltite [73]. The other is through isomorphic substitution in arsenopyrite, sphalerite, pyrite, pyrrhotite, and chalcopyrite, with average Co contents of 34,077 ppm, 471 ppm, 194 ppm, 145 ppm, and 2 ppm, respectively [73]. It is evident that the Co content in arsenopyrite is significantly higher than that in pyrite. This further indicates that when both arsenopyrite and pyrite are present in a deposit, Co primarily occurs in arsenopyrite, with only a minor proportion hosted in pyrite [75]. This phenomenon is attributed to the fact that sulfarsenides such as arsenopyrite have a crystal structure more conducive to accommodating cobalt compared to pyrite [70,79].

Based on the preceding analysis, pyrite, chalcopyrite, sphalerite, and arsenopyrite are identified as the primary cobalt-bearing minerals across the three types of the skarn-type deposits (Table 2). These minerals exhibit elevated Co contents, confirming sulfide minerals as the dominant hosts for Co enrichment [23,43,80]. Notably, the development of independent cobalt minerals is restricted to specific deposits.

Table 2. Summary table of host minerals of cobalt and the average content in typical skarn-type cobalt deposits.

Name	Type	Cobalt-Bearing Minerals	Main Cobalt-Bearing Minerals and the Average Content (ppm)	Independent Cobalt Minerals	Reference
Zhongguan	Fe	Pyrite, Magnetite, Pyrrhotite, Hematite	Pyrite (1750), Magnetite (42.8)		[34]
Baijian	Fe	Pyrite, Magnetite	Pyrite (4251), Magnetite (47.6)	Cobaltite	[35]
Henghui	Fe	Pyrite	Pyrite (5233)		[55]
Yushiwa	Fe	Pyrite, Magnetite	Pyrite (2800), Magnetite (320)		[75,81]
Zhangjiawa	Fe	Pyrite	Pyrite (6300)		[38]
Jinshandian	Fe	Pyrite, Magnetite	Pyrite (590), Magnetite (52)		[58]
Baixiangshan	Fe	Pyrite, Magnetite	Pyrite (785), Magnetite (17)		[62]
Chengchao	Fe	Pyrite, Magnetite	Pyrite (1550)		[57,59]
Tieshan	Fe	Pyrite, Magnetite	Pyrite (1580), Magnetite (1046)		[57]
Zhuchong	Fe	Pyrite, Magnetite, Pyrrhotite, Chalcopyrite	Pyrite (2138), Magnetite (72.1)	Grimmite	[43]
Longqiao	Fe	Pyrite, Magnetite	Pyrite (1115), Magnetite (30)	Cobaltite, Carrollite, Glaucodot	[19,82]
Goroblagodat	Fe	Pyrite	Pyrite (220)		[15]
Cornwall	Fe	Pyrite	Pyrite (13,000)		[40]
Peña Colorada	Fe	Pyrite	Pyrite (1200)		[83]
Dongguashan	Cu	Pyrite, Magnetite, Chalcopyrite, Pyrrhotite	Pyrite (626)	Carrollite, Cobaltite, Glaucodot	[63]
Tongshankou	Cu	Pyrite, Chalcopyrite	Pyrite (629), Chalcopyrite (253)		[64,65]
Fengshandong	Cu	Pyrite, Chalcopyrite	Pyrite (689)		[60]
Anqing	Cu	Pyrite, Magnetite, Chalcopyrite, Pyrrhotite	Pyrite (1358), Chalcopyrite (260)	Siegenite	[43,77,78]
Chengmenshan	Cu	Pyrite, Chalcopyrite	Pyrite (790)		[47]
Tonglvshan	Cu	Pyrite, Magnetite, Chalcopyrite, Bornite	Pyrite (1365), Chalcopyrite (500)	Carrollite, Cobaltite, Safflorite	[84]
Pusangguo	Pb-Zn polymetallic	Pyrite, Sphalerite, Chalcopyrite, Galena	Pyrite (490.9), Sphalerite (1102)	Cobaltite, Carrollite, Linnaeite	[20,85]
Niukutou	Pb-Zn polymetallic	Pyrite, Arsenopyrite, Pyrrhotite, Sphalerite, Chalcopyrite,	Pyrite (42.5), Sphalerite (230.7), Arsenopyrite (3755.9)	Cobaltite, Glaucodot	[70]
Yaojialing	Pb-Zn polymetallic	Pyrite, Sphalerite, Chalcopyrite	Pyrite (193.2), Sphalerite (733.9)	Carrollite	[53]
Lalingzaohuo	Pb-Zn polymetallic	Pyrite, Chalcopyrite, Pyrrhotite, Sphalerite	Pyrite (27,400), Sphalerite (8100)	Cobaltite	[18]
Zhibule	Pb-Zn polymetallic	Pyrite, Magnetite, Galena, Sphalerite	Pyrite (2454), Magnetite (228)	Siegenite	[17]
Galinge	Pb-Zn polymetallic	Pyrite, Arsenopyrite, Sphalerite, Pyrrhotite, Chalcopyrite	Pyrite (194), Sphalerite (471), Arsenopyrite (34,077)	Skutterudite, Cobaltite	[73]

Table 2. Summary table of host minerals of cobalt and the average content in typical skarn-type cobalt deposits.

Name	Type	Cobalt-Bearing Minerals	Main Cobalt-Bearing Minerals and the Average Content (ppm)	Independent Cobalt Minerals	Reference
Zhongguan	Fe	Pyrite, Magnetite, Pyrrhotite, Hematite	Pyrite (1750), Magnetite (42.8)		[34]
Baijian	Fe	Pyrite, Magnetite	Pyrite (4251), Magnetite (47.6)	Cobaltite	[35]
Henghui	Fe	Pyrite	Pyrite (5233)		[55]
Yushiwa	Fe	Pyrite, Magnetite	Pyrite (2800), Magnetite (320)		[75,81]
Zhangjiawa	Fe	Pyrite	Pyrite (6300)		[38]
Jinshandian	Fe	Pyrite, Magnetite	Pyrite (590), Magnetite (52)		[58]
Baixiangshan	Fe	Pyrite, Magnetite	Pyrite (785), Magnetite (17)		[62]
Chengchao	Fe	Pyrite, Magnetite	Pyrite (1550)		[57,59]
Tieshan	Fe	Pyrite, Magnetite	Pyrite (1580), Magnetite (1046)		[57]
Zhuchong	Fe	Pyrite, Magnetite, Pyrrhotite, Chalcopyrite	Pyrite (2138), Magnetite (72.1)	Grimmite	[43]
Longqiao	Fe	Pyrite, Magnetite	Pyrite (1115), Magnetite (30)	Cobaltite, Carrollite, Glaucodot	[19,82]
Goroblagodat	Fe	Pyrite	Pyrite (220)		[15]
Cornwall	Fe	Pyrite	Pyrite (13,000)		[40]
Peña Colorada	Fe	Pyrite	Pyrite (1200)		[83]
Dongguashan	Cu	Pyrite, Magnetite, Chalcopyrite, Pyrrhotite	Pyrite (626)	Carrollite, Cobaltite, Glaucodot	[63]
Tongshankou	Cu	Pyrite, Chalcopyrite	Pyrite (629), Chalcopyrite (253)		[64,65]
Fengshandong	Cu	Pyrite, Chalcopyrite	Pyrite (689)		[60]
Anqing	Cu	Pyrite, Magnetite, Chalcopyrite, Pyrrhotite	Pyrite (1358), Chalcopyrite (260)	Siegenite	[43,77,78]
Chengmenshan	Cu	Pyrite, Chalcopyrite	Pyrite (790)		[47]
Tonglvshan	Cu	Pyrite, Magnetite, Chalcopyrite, Bornite	Pyrite (1365), Chalcopyrite (500)	Carrollite, Cobaltite, Safflorite	[84]
Pusangguo	Pb-Zn polymetallic	Pyrite, Sphalerite, Chalcopyrite, Galena	Pyrite (490.9), Sphalerite (1102)	Cobaltite, Carrollite, Linnaeite	[20,85]
Niukutou	Pb-Zn polymetallic	Pyrite, Arsenopyrite, Pyrrhotite, Sphalerite, Chalcopyrite,	Pyrite (42.5), Sphalerite (230.7), Arsenopyrite (3755.9)	Cobaltite, Glaucodot	[70]
Yaojialing	Pb-Zn polymetallic	Pyrite, Sphalerite, Chalcopyrite	Pyrite (193.2), Sphalerite (733.9)	Carrollite	[53]
Lalingzaohuo	Pb-Zn polymetallic	Pyrite, Chalcopyrite, Pyrrhotite, Sphalerite	Pyrite (27,400), Sphalerite (8100)	Cobaltite	[18]
Zhibule	Pb-Zn polymetallic	Pyrite, Magnetite, Galena, Sphalerite	Pyrite (2454), Magnetite (228)	Siegenite	[17]
Galinge	Pb-Zn polymetallic	Pyrite, Arsenopyrite, Sphalerite, Pyrrhotite, Chalcopyrite	Pyrite (194), Sphalerite (471), Arsenopyrite (34,077)	Skutterudite, Cobaltite	[73]

3.2. Distribution Patterns of Cobalt in Metallogenic Regions

It is proposed that the distribution of Co resources in skarn-type deposits exhibits regional zonation patterns, as revealed by the preceding analysis. The Handan–Xingtai and Shandong Mining Areas are dominated by skarn-type Fe deposits, while the East Kunlun and Gangdese Metallogenic Belts mainly host skarn-type Pb-Zn polymetallic deposits. Furthermore, the Middle-Lower Yangtze River Metallogenic Belt hosts multiple types of skarn-type deposits, including Fe, Cu, and Pb-Zn polymetallic deposits (Figure 1). These regional distribution patterns are fundamentally controlled by regional tectonic settings and associated magmatic activities (Table 3).

The Handan–Xingtai Mineral Area (HXMA) is recognized as a prominent skarn-type Fe metallogenic province in China, hosting over 100 Fe deposits of various sizes, with total reserves exceeding 1000 Mt [12,86]. Notably, significant Co mineralization occurs at the Baijian, Zhongguan, and Henghui Fe deposits in the region, with Co grades consistently ranging from 100 ppm to 200 ppm (Figure 2a). This area is located in the southern segment of the Taihang Mountain Uplift Belt in the central North China Craton [12]. During the Indosinian period, a series of EW-NNE-trending concealed basement fault zones were developed through subduction interactions between the Paleo-Siberian Plate and the Yangtze Block, coupled with the Pacific Plate [21,87]. The metallogenic age of the skarn-type cobalt deposits in this region is primarily concentrated between 135 and 125 Ma [21], with rock types mainly comprising diorite, monzonite, and monzodiorite [21,34,35,55]. The mining area experienced three stages of magmatic activity consisting of eight intrusive episodes [12]. During this process, repeated injection of mafic magma into felsic magma chambers significantly increased the Co content in the hydrothermal fluids, while well-developed fault structures provided migration pathways for cobalt-bearing fluids from deep to shallow crustal levels [12,21,88]. Host strata are dominated by Ordovician carbonate sequences, primarily composed of dolomite, quartz sandstone, and gypsum breccia, which provide prerequisites for Co mobilization and enrichment [21].

The Gangdese Metallogenic Belt (GMB), which hosts numerous skarn-type Pb-Zn polymetallic deposits, is recognized as another Co-enriched belt comparable to the Middle-Lower Yangtze River Metallogenic Belt [23]. This belt exhibits distinct east–west contrasts in Co distribution patterns and enrichment mechanisms [23,68]. In the eastern segment of this belt, deposits such as Jiama, Zhibula, and Chunzhe exhibit an average Co content exceeding 400 ppm in ores, whereas ores from the Bangbule and Chagele deposits in the western segment generally exhibit values of less than 3 ppm [17,68]. This belt is located in the southern Lhasa Terrane, between the Indus-Yarlung Tsangpo Suture Zone and the Milashan–Luobadui Fault [89,90,91,92]. It was influenced by the Jurassic–Cretaceous subduction of the Neo-Tethyan oceanic crust, which resulted in the development of a nearly EW-trending tectonic–magmatic zone [93,94]. During the Paleogene, the region experienced the main collision (65–41 Ma) and late collision (40–26 Ma) between the Indian and Asian continents, entering the Miocene post-collisional tectonic stage (25–0 Ma) [17]. The mineralization ages of skarn-type cobalt deposits in the area are primarily concentrated in 65–41 Ma and 33–13 Ma [23], corresponding to four major stages of magmatic activities [95,96,97,98,99,100]. The rock types are predominantly granodiorite, quartz monzonite, and monzogranite [23]. Western cobalt-depleted deposits were formed in Paleocene continental collision settings, with mineralization ages concentrated around 61 Ma [23,101,102]. These deposits are associated with limited magmatic activity, lacking juvenile lower crust development or mantle-derived magma injection [23,103]. In contrast, eastern Co-enriched deposits were formed in Miocene post-collisional extensional or Oligocene late-collisional settings, with mineralization ages concentrated around 15 Ma [17,104,105,106,107,108,109,110]. This eastern mineralization phase demonstrates intense magmatic activity, featuring juvenile lower crustal additions and mantle-derived magma injection [23,103]. Furthermore, the differential Co enrichment between segments is further enhanced by adakitic characteristics in eastern intrusions compared to inherited arc magma signatures in western intrusions [23,103].

Extensive Co geochemical anomalies identified at a 1:50,000 scale in the East Kunlun Metallogenic Belt (EKOB) are spatially associated with the Galinge, Haisi, and Niukutou Pb-Zn polymetallic deposits, indicating significant Co metallogenic potential [49]. Considerable variations in Co grades are observed in these deposits, ranging from 120 ppm to 1800 ppm (Figure 2a). The belt is located in the western Central Orogenic Belt, where its formation is closely related to the collision of the Bayan Har and Qiangtang terranes with the Qaidam Basin after the Paleo-Tethys closure [111,112,113,114,115]. The belt is tectonically subdivided by major crustal-scale fault zones, including the nearly EW-NWW-trending North Kunlun, Central Kunlun, and South Kunlun Faults [49]. The Paleozoic–Triassic development of a trench–island arc–backarc system facilitated Co deposit formation through continuous northward subduction–accretion along the Kunlun suture zone, as exemplified by the Haisi and Xiarihamu deposits [69,116,117]. The mineralization ages of skarn-type cobalt deposits in this belt are mainly concentrated between 237 and 212 Ma, with rock types mainly comprising granodiorite and monzogranite [49,69,70]. The mineralizing magmas were primarily derived from partial melting of the lower crust induced by upwelling asthenosphere, with additional mantle-derived contributions, representing typical crust–mantle mixed sources [118]. Intense magmatic activities promoted Co migration from the mantle to the crust, thus providing a material basis for the development of cobalt-bearing skarn-type deposits [18,119].

The Middle-Lower Yangtze River Metallogenic Belt (MYLB) hosts significant Co resources, totaling approximately 2.79 × 10⁵ t, classified into three types based on the relationship between Co grades in deposits and the boundary grade for the comprehensive utilization of associated cobalt (100 ppm) [22]. Approximately 1.08 × 10⁵ t is classified as favorably recoverable (reaching 100 ppm), while 4.6 × 10⁴ t exhibits moderate recovery potential (approaching 100 ppm), and the remaining 1.25 × 10⁵ t represents potential future resources (below 100 ppm) [22]. The belt is dominated by skarn-type Fe and Cu deposits, where Fe deposits exhibit average Co grades ranging from 35 ppm to 500 ppm, while Cu deposits range from 22.1 ppm to 166 ppm (Figure 2a). This belt is located in the northern margin of the Yangtze Craton, and it was formed through subduction and collision between the Yangtze and North China blocks along the Xiangfan–Guangji and the Tan–Lu Faults [120,121]. The mineralization ages of skarn-type cobalt deposits in this belt are mainly concentrated between 145 and 127 Ma [54], with rock types mainly comprising granodiorite, diorite, and quartz diorite [19,43,54,122]. The deep material sources for the mineralization system include mantle upwelling, partial melting of the lower crust, and crust–mantle mixing [123]. These magmatic activities were controlled by the Mesozoic tectonic regime transformation and are closely associated with large-scale Mesozoic mineralization events in eastern China [124]. However, systematic investigations into Co enrichment mechanisms in skarn-type deposits of this belt remain insufficient, hindering a comprehensive understanding of metallogenic processes.

Table 3. Summary table of characteristics and controls on cobalt resources in skarn-type cobalt deposits across metallogenic regions.

Metallogenic Region	Geotectonic Settings	Main Mineralization Ages of Deposits	Main Rock Types of Deposits	Magmatic Source	Main Types of Deposits	Typical Deposits	Distribution Patterns of Co Resources	Influencing Factors	Reference
Han–Xing Mining Area in China	Subduction of the Paleo-Siberian, Yangtze, and Pacific Plates	135–125 Ma	Diorite, Monzonite, Monzodiorite	Predominantly mantle-derived with minor crustal or mixed crust-mantle sources	Fe	Baijian, Henghui, Zhongguan	High Co grade and abundant Co reserves	Multi-stage magmatic activities, relatively developed fault structures, ordovician carbonate strata	[12,21,34,35,55,88]
Gangdese Metallogenic Belt in China	Subduction of the Neo-Tethyan oceanic crust	65–41 Ma and 33–13 Ma	Granodiorite, Quartz Mozonite, Monzogranite	Predominantly mantle-derived with minor mixed crust-mantle sources	Pb-Zn polymetallic	Jiama, Zhibula	East segment Co-enriched, west segment Co-poor	Eastern Co-enriched deposits with adakitic ore-forming intrusions, young mineralization age, and intense magmatic activity	[23,68,93,94,103]
East Kunlun Metallogenic Belt in China	Collision between the Bayan Har or Qiangtang terrane and the Qaidam Basin	237–212 Ma	Granodiorite, Monzogranite	Mantle-derived, mixed crust-mantle sources	Pb-Zn polymetallic	Galinge, Niukutou	Significant variation in Co grade	“Trench–island arc–back arc basin” tectonic system, intense magmatic activity	[49,69,70,111,112,113,114,115,118,119]
Middle-Lower Yangtze Metallogenic Belt in China	Subduction and collision of the Yangtze and North China blocks	145–127 Ma	Granodiorite, Diorite, Quartz diorite	Mantle-derived, mixed crust-mantle sources	Fe, Cu	Zhuchong, Anqing	Extremely abundant Co reserves	————	[19,22,43,54,120,121,122,123]
Cornwall mining district in America	Variscan orogeny	295–270 Ma	Granite	————	Fe	Cornwall	Relatively high Co grade	————	[39,41,125]
Cloncurry mining district (Mount Isa Eastern Fold Belt) in Australia	Late tectonism	1510 Ma	Granite	————	Cu	Mount Elliott	Extremely high Co grade	————	[48]
Magnitogorsk zone in Russia	Island arc	Late-Devonian to Early Carboniferous	Gabbroid	Mantle-derived	Fe	Magnitogorsk	Relatively high Co grade	————	[126,127,128]

Table 3. Summary table of characteristics and controls on cobalt resources in skarn-type cobalt deposits across metallogenic regions.

Metallogenic Region	Geotectonic Settings	Main Mineralization Ages of Deposits	Main Rock Types of Deposits	Magmatic Source	Main Types of Deposits	Typical Deposits	Distribution Patterns of Co Resources	Influencing Factors	Reference
Han–Xing Mining Area in China	Subduction of the Paleo-Siberian, Yangtze, and Pacific Plates	135–125 Ma	Diorite, Monzonite, Monzodiorite	Predominantly mantle-derived with minor crustal or mixed crust-mantle sources	Fe	Baijian, Henghui, Zhongguan	High Co grade and abundant Co reserves	Multi-stage magmatic activities, relatively developed fault structures, ordovician carbonate strata	[12,21,34,35,55,88]
Gangdese Metallogenic Belt in China	Subduction of the Neo-Tethyan oceanic crust	65–41 Ma and 33–13 Ma	Granodiorite, Quartz Mozonite, Monzogranite	Predominantly mantle-derived with minor mixed crust-mantle sources	Pb-Zn polymetallic	Jiama, Zhibula	East segment Co-enriched, west segment Co-poor	Eastern Co-enriched deposits with adakitic ore-forming intrusions, young mineralization age, and intense magmatic activity	[23,68,93,94,103]
East Kunlun Metallogenic Belt in China	Collision between the Bayan Har or Qiangtang terrane and the Qaidam Basin	237–212 Ma	Granodiorite, Monzogranite	Mantle-derived, mixed crust-mantle sources	Pb-Zn polymetallic	Galinge, Niukutou	Significant variation in Co grade	“Trench–island arc–back arc basin” tectonic system, intense magmatic activity	[49,69,70,111,112,113,114,115,118,119]
Middle-Lower Yangtze Metallogenic Belt in China	Subduction and collision of the Yangtze and North China blocks	145–127 Ma	Granodiorite, Diorite, Quartz diorite	Mantle-derived, mixed crust-mantle sources	Fe, Cu	Zhuchong, Anqing	Extremely abundant Co reserves	————	[19,22,43,54,120,121,122,123]
Cornwall mining district in America	Variscan orogeny	295–270 Ma	Granite	————	Fe	Cornwall	Relatively high Co grade	————	[39,41,125]
Cloncurry mining district (Mount Isa Eastern Fold Belt) in Australia	Late tectonism	1510 Ma	Granite	————	Cu	Mount Elliott	Extremely high Co grade	————	[48]
Magnitogorsk zone in Russia	Island arc	Late-Devonian to Early Carboniferous	Gabbroid	Mantle-derived	Fe	Magnitogorsk	Relatively high Co grade	————	[126,127,128]

Based on the preceding analysis, significant Co enrichment occurs in skarn-type deposits across multiple major metallogenic regions. The spatially heterogeneous enrichment of Co is systematically controlled by regional geotectonic settings, rock types, and magmatic activities, which collectively govern the migration pathways, enrichment zones, and transport efficiency of ore-forming fluids, thereby influencing the effective enrichment of Co [12,18,21,23,88,103,119]. These factors result in a distinct zonal distribution pattern of associated Co resources in these regions (Figure 1).

3.3. Distribution Patterns of Cobalt in Metallogenic Process

The metallogenic process of skarn-type deposits is typically divided into distinct stages, including the dry skarn, wet skarn, oxide, sulfide, and carbonate stages [129]. During mineralization, the geochemical behavior of Co varies significantly with the evolving composition of hydrothermal fluids, resulting in stage-specific enrichment patterns [130,131]. Therefore, the distribution patterns of Co and its controlling factors are summarized for 10 skarn-type deposits (

Table 4. Summary table of stage distribution patterns and controls on cobalt enrichment in typical skarn-type cobalt deposits.

Name	Main Metallogenic Stage	Enrichment Degree of Co Element	Influencing Factors	Reference
Zhongguan	Wet skarn	Depleted	Continuous high-temperature oxidation conditions prevent the decomposition of cobalt complexes	[34]
	Oxide	Enriched	Decreased fluid temperature, elevated oxygen fugacity
Henghui	Early sulfide	Enriched	Magnetite crystallization and multi-stage mafic magmatism causing decreased fluid temperature and salinity	[55]
	Late sulfide	Depleted	Hydrothermal alterations release Co into fluid
	Carbonate	Re-enriched	Meteoric water injection causes fluid cooling and dilution
Anqing	Early quartz–sulfide	Enriched	Meteoric water injection causes fluid temperature and chloride complex concentration to decrease	[43]
	Late quartz–sulfide	Depleted	Continuous meteoric water injection causes further fluid cooling and dilution
Zhuchong	Sulfide	Enriched	Magnetite crystallization and wall-rock Mg-Ca strata cause fluid temperature and oxygen fugacity to decrease	[54]
Wushan	Quartz–sulfide I	Depleted	Formation of limited pyrite amount	[44]
	Quartz–sulfide II	Enriched	Elevated fluid temperature, the massive formation of pyrite
Xinqiao	Quartz–sulfide	Enriched	Elevated fluid temperature	[122]
Galinge	Early sulfide I	Enriched	————	[73]
	Early sulfide II	Re-enriched	Elevated fluid temperature and pH, fluid–rock interaction
	Late sulfide	Depleted	Decreased fluid temperature
Zhibula	Quartz–sulfide	Enriched	————	[17]
Yaojialing	Early sulfide	Enriched	————	[53]
	Late sulfide	Depleted	Decreased fluid temperature
Niukutou	Pyrrhotite	Depleted	————	[70]
	Chalcopyrite	Enriched	Decreased fluid temperature and oxygen fugacity
	Sphalerite–Galena	Depleted	————

).

In the Henghui deposit, Co is mainly precipitated and enriched during the sulfide and carbonate mineralization stages [55]. During the early sulfide stage, the highest average Co content of 9122 ppm is recorded in coarse-grained disseminated pyrite (Py_2a), resulting from the combined effects of decreasing hydrothermal fluid temperature and salinity, extensive magnetite crystallization, and multi-stage mafic magma injection [55,88,132,133,134,135,136]. During the late sulfide stage, shearing fragmented Py_2a into coarse-grained cataclastic pyrite (Py_2b) [55]. Hydrothermal fluids infiltrating fractures generated abundant secondary pores, promoting extensive hydrothermal alteration that mobilized Co into the hydrothermal system and reduced the average Co content of Py_2b to 848 ppm [55,137]. During the carbonate stage, the injection of meteoric water caused cooling and dilution of the hydrothermal fluids, triggering the reprecipitation and enrichment of Co and elevating the average Co content in Py₃ to 6870 ppm [55,138,139].

In the Anqing Cu-Fe deposit, Co is mainly precipitated and enriched during the quartz–sulfide stage [43]. Although skarn minerals and magnetite crystallize during the skarn stage, Co enrichment remains limited due to low fluid–mineral partition coefficients in these phases [43]. During the early quartz–sulfide stage, the injection of meteoric water reduces the hydrothermal fluid temperature and the concentration of chloride complex [43]. These variations promote Co incorporation into pyrite and pyrrhotite lattices, resulting in precipitation and concentration, with their average Co contents reaching 4871 ppm and 753 ppm, respectively [43,131,138]. Progressive meteoric water injection during the late quartz–sulfide stage further cooled and diluted the hydrothermal fluids. These physicochemical changes decrease Co solubility, leading to inhibited significant precipitation and enrichment in pyrite and a decrease in the average Co content to 95.4 ppm [43,131,138].

In the Galinge Pb-Zn deposit, Co is mainly enriched during the sulfide stage [73]. In the early sulfide stage I, Co occurs primarily in arsenopyrite, with minor proportions occurring in pyrrhotite and pyrite, with an average content of 13,018 ppm, 67.7 ppm, and 192.6 ppm, respectively [73]. As the system progresses to early sulfide stage II, elevated As/S ratios in arsenopyrite indicate rising fluid temperatures, which increases the average Co content of arsenopyrite to 47,703 ppm and that of pyrrhotite to 359 ppm [73,140,141]. Concurrently, an increase in fluid pH promotes the reaction CoCl₄²⁻ + 3H₃AsO₃⁰ → CoAs₃ + 4Cl⁻ + 2H⁺ + 3.5H₂O + 2.75O₂, facilitating the precipitation of abundant skutterudite (CoAs₃). Continuous fluid–rock interactions deplete the As concentrations, leading to decreased As/S ratios and the transformation of skutterudite into cobaltite (CoAsS) [73,142]. The formation of these cobalt independent minerals contributes to the highest Co concentrations observed in minerals [73]. During the late sulfide stage, fluid inclusion evidence indicates decreasing temperature, which reduces the average Co content in pyrrhotite to 69 ppm and in pyrite to 95 ppm, respectively [73,143].

Based on the preceding analysis, the stage-specific distribution patterns of Co reflect the cumulative effects of multi-stage mafic magma injection, fluid–rock interaction, and meteoric water injection. These processes collectively drive changes in physicochemical conditions of the hydrothermal fluids, including temperature, oxygen fugacity, and salinity, which govern differential Co enrichment across mineralization stages [43,55,73,88,131,132,133,134,135,136,138,139,140,141,142,143]. Precipitation and enrichment of Co mainly occur during sulfide stages, although notable variations exist between different deposits (Table 4).

4. Enrichment Mechanism of Associated Cobalt Resources in Skarn-Type Deposits

Cobalt, as a typical element with a pronounced mantle affinity, is enriched in skarn-type deposits via processes intrinsically linked to deep-seated magmatic activities [14,144]. Within the high-temperature, high-pressure, and reduced deep-seated magma chambers, Co mainly forms hydride or carbonyl–hydride complexes. These compounds migrate upward through magmatic–hydrothermal conduits along fractures [144,145,146,147]. The decomposition of cobalt-bearing compounds occurs in response to evolving physicochemical conditions in the hydrothermal system, enabling migration and culminating in the effective enrichment under favorable metallogenic constraints [34]. During this process, the material contribution from mantle-derived magmas provides the primary condition for Co enrichment, while the injection of mafic magmas, the assimilation of evaporite sequences, and the dissolution–reprecipitation mechanism of hydrothermal fluids during the magmatic evolution further promote the re-enrichment of Co [21,23,35,54,55,110]. These four factors synergistically drive the effective enrichment of Co in the magmatic–hydrothermal systems (Table 5).

4.1. Magmatic Source

Cobalt, a transition metal, is widely distributed throughout the Earth’s layers with significant abundance variations, decreasing systematically from the core (420 ppm) through the lower mantle (200 ppm) and the upper mantle (160 ppm) to the crust (25 ppm) [148]. The mantle serves as a significant source reservoir due to its high Co abundance, which provides the material foundation for mantle-derived Co deposits such as Volcanogenic Massive Sulfide (VMS) deposits [149,150]. VMS deposits are volcanic in origin, and skarn-type deposits are of a hydrothermal–metasomatic origin [129]. Given their classification as hydrothermal deposits, Co enrichment in skarn-type deposits is inferred to be closely associated with mantle-derived magmas. Known mantle-derived magmatic skarn-type deposits are mainly restricted to the Handan–Xingtai Mining Area and the Gangdese Metallogenic Belt [23,151,152,153,154].

Table 5. Controlling factors and specific mechanisms of associated cobalt resources in skarn-type deposits.

Controlling Factors	Specific Mechanisms	Representative Examples	Reference
Mantle-derived magma sources	High Co content in mantle-derived magmas provides the additional Co source	Handan–Xingtai Ming Area, Gangdese Metallogenic Belt	[23,151,152,153,154]
Mafic magma injection	Inherited high Co content from mantle-derived magmas and mixed with felsic magmas	Baijian deposit, Henghui deposit	[21,55,110]
Assimilation of evaporite sequences	Extensive assimilation of evaporite sequences with magmatic–hydrothermal fluids alter fluid composition and metallogenic environment	Baijian deposit, Zhuchong deposit	[35,54]
Dissolution and re-precipitation mechanism of hydrothermal fluids	Enhanced activity of late-stage hydrothermal fluids drives the reprecipitation and enrichment of Co	Henghui deposit, Zhongguan deposit, Baijian deposit	[35,55]

Table 5. Controlling factors and specific mechanisms of associated cobalt resources in skarn-type deposits.

Controlling Factors	Specific Mechanisms	Representative Examples	Reference
Mantle-derived magma sources	High Co content in mantle-derived magmas provides the additional Co source	Handan–Xingtai Ming Area, Gangdese Metallogenic Belt	[23,151,152,153,154]
Mafic magma injection	Inherited high Co content from mantle-derived magmas and mixed with felsic magmas	Baijian deposit, Henghui deposit	[21,55,110]
Assimilation of evaporite sequences	Extensive assimilation of evaporite sequences with magmatic–hydrothermal fluids alter fluid composition and metallogenic environment	Baijian deposit, Zhuchong deposit	[35,54]
Dissolution and re-precipitation mechanism of hydrothermal fluids	Enhanced activity of late-stage hydrothermal fluids drives the reprecipitation and enrichment of Co	Henghui deposit, Zhongguan deposit, Baijian deposit	[35,55]

Recent investigations into the attributes of magma sources in the Handan–Xingtai Mining Area have been conducted by many scholars [151,152,153,154]. Amphiboles from the Ziquan, Guzhen, Fushan, Wuan, and Qicun intrusions were analyzed by electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) [151,152,153,154]. Integrated with TiO₂-Al₂O₃ discrimination diagrams, these data reveal that the magma source of these intrusions is mainly mantle-derived or crust–mantle-mixed, with minor crust-derived contributions [151,152,153,154]. Our research group performed Sr isotope analyses of apatite in diorite intrusions from the Zhongguan deposit using LA-ICP-MS. These analyses yield initial ⁸⁷Sr/⁸⁶Sr(i) ratios ranging from 0.7053 to 0.7076. Integrated with Sm-La-Th-Y relationships in apatite magma source discrimination diagrams, these isotopic signatures indicate a dominantly mantle-derived magma source with minor crustal-derived components (unpublished data). These results collectively demonstrate that the magma source in the Handan–Xingtai Mining Area is primarily mantle-derived. We propose that this mantle-dominated magma source characteristic is closely linked to the regional development of numerous skarn-type Fe deposits with significant Co enrichment.

For the Gangdese Metallogenic Belt, Wang et al. (2025) demonstrated that mantle-derived magmas or involvement of mafic juvenile lower crust provide the essential material basis for the significant enrichment of Co through a multi-isotope system (Pb-Hf-Os) [23]. The Pusangguo (18.555, 15.716, 39.044), Jiama (18.608, 15.618, 38.990), and Zhibula (18.437, 15.574, 38.536) deposits exhibit relatively low ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios [155,156,157,158,159,160], whereas the Jiama (6.0), Nuri (6.4), and Zhibula (7.2) deposits exhibit higher ε_Hf (t) values [161,162,163]. Furthermore, the initial ¹⁸⁷Os/¹⁸⁸Os ratios of the Zhibula (0.49), Leqingla (~0.1), Chengba (~0.1), and Jiama (~0.05) deposits approach or fall below the mantle value (~0.13) [17,109,164,165,166]. These isotopic signatures, combined with widespread occurrences of mafic gabbro enclaves, collectively provide compelling evidence for contributions from mantle-derived magma sources or juvenile mafic lower crust [23]. The involvement of these components is consequently considered to provide additional sources of Co for the formation of cobalt-enriched skarn-type deposits throughout the belt, thereby promoting effective Co enrichment [23].

In summary, despite contrasting tectonic settings between the Handan–Xingtai Mining Area and the Gangdese Metallogenic Belt (Table 3), both regions exhibit significant Co enrichment in their skarn-type deposits that are primarily derived from mantle-derived magmas. Thus, we propose that mantle-derived magmas represent a key controlling factor for Co enrichment.

4.2. Injection of Mafic Magmas

Fe-dominated skarn-type cobalt deposits associated with intermediate-mafic magmas generally exhibit higher average Co grades than Cu-dominated deposits associated with intermediate-felsic magmas (Figure 2a). This contrast demonstrates the critical control of magma composition on Co enrichment. Regarding material sources, as products of mantle partial melting, mafic magmas provide abundant Co to metallogenic systems through inherited high Co content [167], while the mafic magmas’ low viscosity and high mobility enable efficient Co migration and enrichment [168]. During the metallogenic processes, mafic magma injection modifies mineralization environments through mixing with magmatic–hydrothermal fluids and enables Co migration and precipitation [21,55]. This dual control mechanism, which encompasses both material source supplementation and modification of the metallogenic environment, drives significant Co enrichment in skarn-type deposits, as exemplified by the Baijian, Henghui, Zhongguan, and Pusangguo deposits [21,55,110].

The Baijian Fe deposit exhibits exceptional Co enrichment characterized by multi-stage precipitation at low temperatures, which contrasts with conventional hydrothermal mineralization theory that typically posits Co precipitation during high-temperature hydrothermal stages [21,130]. Notably, late-stage low-temperature veinlet pyrite exhibits an anomalously high Co content that mostly exceeds 10,000 ppm [21]. This phenomenon is genetically related to multiple injections of mafic magma into a felsic magmatic chamber, as evidenced by mineral chemical data of the Qicun intrusion [88]. Specifically, the Mg^# values of amphibole phenocrysts are positively correlated with temperature and pressure but negatively correlated with oxygen fugacity [88]. Based on this evidence, Qin (2022) proposed that the injection of mafic magma not only provides a supplementary Co source but also modifies physicochemical conditions of temperature, oxygen fugacity, and pressure in the metallogenic system through felsic magma mixing [21]. This process establishes favorable conditions for Co migration and precipitation, ultimately leading to significant Co enrichment in the late low-temperature stage [21].

In the Henghui deposit, the sulfide-stage pyrite exhibits a high Co content of 9122 ppm, attributed to the combined effects of decreasing temperature and salinity of hydrothermal fluids, among other factors [55]. Furthermore, mantle-derived mafic magma ascent and subsequent mixing with crust-derived felsic magma are demonstrated by plagioclase zoning data from the Wuan intrusion, amphibole thermobarometer calculations (yielding temperatures of 697–754 °C and pressures of 39–65 MPa), and shallow-level metasomatic characteristics [169]. Based on this evidence, Dong et al. (2024) propose that Co enrichment in the pyrite is primarily controlled by multi-stage mafic magma injection and associated magma mixing processes [55]. Similarly, Cao et al. (2025) demonstrate that oscillatory zoning of Co in pyrite from the Jinchang porphyry deposit records multiple injections of mafic magma, proposing that such events significantly elevate the Co content in the ore-forming fluids and promote Co enrichment in the porphyry deposits [170]. Additional evidence for mafic magma injection is provided by the discovery of gabbroic inclusions in the Pusangguo deposit and mafic inclusions in the Zhongguan deposit (Figure 3), although their specific roles in Co enrichment remain underexplored [110] (unpublished data).

In summary, the enrichment of Co in skarn-type deposits is significantly facilitated by multi-stage mafic magma injections and associated mixing processes. The Baijian, Henghui, Zhongguan, and Pusangguo deposits, which are primarily concentrated in the Handan–Xingtai Mining Area and Gangdese Metallogenic Belt, are associated with mantle-derived magmas, and this geological association provides both the material basis for Co enrichment and favorable conditions for mafic magma injections.

4.3. Assimilation of Evaporite Sequences

Evaporite sequences, which are evaporative sedimentary units primarily composed of gypsum, anhydrite, and sulfate minerals, contain abundant saline components including SO₄²⁻, Cl⁻, CO₃²⁻, Na⁺, and K⁺. These constituents control metal enrichment and mineralization through diverse geochemical mechanisms [171,172,173,174,175,176,177,178,179,180,181]. SO₄²⁻ serves as an oxidizing agent that transforms low-valence Co into higher oxidation states. This process inhibits the premature removal of Co through sulfide precipitation during early mineralization stages, thereby preserving Co for subsequent enrichment in later stages [181]. On the other hand, other saline components including Cl⁻, CO₃²⁻, Na⁺, and K⁺ act as complexing agents that enhance the dissolution and migration of metallic elements through complexation reactions [181]. Furthermore, the low mechanical strength of evaporite sequences provides favorable conditions for magma emplacement and hydrothermal fluid migration [182]. While existing studies mainly focus on the controlling mechanisms of evaporite sequences on Fe mineralization in skarn-type deposits, systematic investigations into their influence on Co enrichment remain inadequate [181]. In this paper, the Baijian, Zhongguan, and Zhuchong deposits are examined to systematically investigate this mechanism [35,54].

The Baijian deposit is located in the contact zone between Yanshanian intermediate diorite and Middle Ordovician wall rocks dominated by a gypsum-bearing limestone formation [183,184,185]. The extensive development of evaporite sequences provides the sulfur source for mineralization and effectively promotes Co enrichment through extensive assimilation by magmatic–hydrothermal fluids [35]. Sulfur isotopic analyses reveal that δ³⁴S values of pyrite in magnetite ores (18.3%) are closely comparable to those of stratigraphic pyrite (18.9%), indicating a dominant evaporite-derived sulfur source for the ore body, with 45%–79% of the stratigraphic sulfur (mainly from gypsum–salt layers) participating in mineralization [35,186,187]. Meanwhile, substantial amounts of SO₄²⁻, Cl⁻, CO₃²⁻, and other saline components were released through the assimilation of these layers [35]. Specifically, SO₄²⁻ oxidized Co²⁺ to Co³⁺, facilitating Co incorporation through isomorphic substitution for Fe³⁺ and leading to significant Co enrichment [35]. Co mobility in hydrothermal fluids was substantially enhanced by chlorine complexation (e.g., [CoCl₄]²⁻), a process driven by Cl⁻ [21,188,189]. Additionally, these saline components facilitated fluid-mediated Co extraction from wall rocks, providing an essential material source for subsequent enrichment [21,175,190]. In the Zhongguan deposit, we performed chemical analyses of major and trace elements in amphibole and apatite in diorite using EPMA and LA-ICP-MS. A distinct core–rim structure was observed in apatite, with rims showing enrichment in S, Na, and Cl, while cores were enriched in SiO₂, MnO, and Ce. Simultaneously, amphibole matrices were found enriched in Cl and Ca relative to phenocrysts. These results collectively demonstrate a similar extensive assimilation process involving evaporite sequences within the deposit. This process significantly altered hydrothermal fluid composition and magmatic–hydrothermal physicochemical conditions, creating favorable conditions for Co enrichment (unpublished data).

In contrast to the Baijian deposit, the Zhuchong deposit exhibits relatively limited Co enrichment despite extensive evaporite development. Sulfur isotopic analyses of pyrite reveal δ³⁴S values ranging from 5% to 12% [54]. These values are intermediate between magmatic sulfur (−3%–+3%) and stratigraphic sulfur (+25.4%–+34.4%), indicating a dominantly magmatic sulfur source with minor stratigraphic contributions [54,191,192]. Whole-rock Co analyses reveal a relatively elevated Co content in hydrothermally altered rocks: diopsidized diorite (26.34 ppm), diopside skarn (41.43 ppm), and skarn-proximal siltstone (17.21 ppm) [54]. Notably, high-Co pyrite (2088 ppm) in altered siltstone correlates with lower δ³⁴S values (6.1%–7.8%), whereas low-Co pyrite (94.5 ppm) in gypsum-bearing siltstone correlates with higher δ³⁴S values (12.0%–12.7%) [54]. Furthermore, early high-temperature pyrite spatially associated with magnetite and diopside in magnetite ores is enriched in Co relative to late low-temperature pyrite associated with calcite and anhydrite [54]. Collectively, these results indicate that Co was primarily derived from deep magmatic-hydrothermal fluids, with limited evaporite-derived sulfur involvement and insufficient assimilation, which consequently did not significantly promote cobalt mineralization [54].

In conclusion, the influence of evaporite sequences on Co enrichment in skarn-type deposits varies significantly. It is proposed that sufficient assimilation of evaporite sequences by magmatic–hydrothermal fluids, which effectively modifies fluid composition and physicochemical conditions of the ore-forming system, significantly enhances Co grades. In contrast, insufficient or localized assimilation exerts a negligible influence on Co enrichment.

4.4. Dissolution–Reprecipitation Mechanism of Hydrothermal Fluids

Based on the preceding analysis, it is proposed that sulfide minerals in skarn-type deposits exhibit significant enrichment of Co (Table 2), which is closely associated with a dissolution–reprecipitation mechanism. This mechanism, essentially a dynamic equilibrium process, typically occurs during middle–late mineralization stages, where sulfide minerals thoroughly react with hydrothermal fluids, leading to the activation, mobilization, and reprecipitation of cobalt [193]. Specifically, under high-temperature hydrothermal conditions, increased Co solubility releases ionic Co²⁺ and Co³⁺ [139,194]. These ions subsequently complex with ligands such as Cl⁻ and HS⁻ to form stable migratory complexes [131]. Evolving physicochemical conditions trigger complex decomposition, causing Co to reprecipitate at favorable sites [195]. Throughout this process, porous textures that commonly develop in sulfide minerals such as pyrite enhance fluid–mineral interaction, thereby accelerating re-equilibration kinetics and promoting effective Co enrichment [196,197,198]. This mechanism is exemplified in the Henghui, Zhongguan, and Baijian deposits in the Handan–Xingtai Mining Area [35,55].

The sulfide stage of the Heng Hui Fe deposit was characterized by pervasive hydrothermal fluid infiltration along fracture margins, which induced hydrothermal alteration [55]. Concurrently, porous textures that developed along the margins of Py_2b further enhanced fluid–mineral interaction through increased reactive surface area [55]. This alteration process elevated fluid temperatures and pressures, increasing the solubility of Co [139,194]. Consequently, Co was activated and mobilized from pyrite lattices into the hydrothermal fluids, which decreased the Co content in Py_2b (average: 848 ppm) [55]. During the subsequent carbonate stage, H-O isotope compositions of fluid inclusions from the Fushan deposit (δ¹⁸O: 5.7% to 7.8%, δD_H2O: −78% to −83%) indicate the injection of meteoric water, which caused a further decrease in fluid temperature and salinity [138,139]. This physicochemical shift triggered the reprecipitation and enrichment of Co in Py₃, resulting in a significantly higher Co content (average: 6870 ppm) [55].

Two distinct stages of pyrite formation are recognized in the Baijian deposit. Early-stage pyrite exhibits a relatively low Co content (average: 2000–10,000 ppm), whereas late-stage pyrite exhibits a significantly elevated Co content (average: 10,000–47,340 ppm) [35]. Furthermore, the distribution of Co within individual late-stage pyrite grains is highly heterogeneous, with concentration variations approaching three orders of magnitude in single crystals [35]. Similarly, in the Zhongguan deposit, early-stage pyrite exhibits a relatively lower and uniform average Co content, ranging from 195 ppm to 331 ppm. In contrast, late-stage pyrite exhibits a higher Co content and substantial variability, ranging from 0.35 ppm to 10,000 ppm (unpublished data). Further analysis of Zhongguan late-stage pyrite reveals a distinct rim-enriched and core-depleted zoning pattern for Co, with rim concentrations exceeding core concentrations by approximately four orders of magnitude (Figure 4) (unpublished data). Based on these observations, it is proposed that these differential enrichment patterns are closely related to a dissolution–reprecipitation mechanism. Specifically, enhanced fluid activity during late hydrothermal stages promotes Co remobilization and re-enrichment through leaching of pre-existing cobalt-bearing minerals and intensified fluid–rock interaction.

In conclusion, the differential enrichment of Co in multistage pyrite from skarn-type deposits of the Handan–Xingtai Mining Area is primarily controlled by dissolution–reprecipitation mechanisms. Under this mechanism, enhanced late-stage hydrothermal fluid activity drives Co mobilization and reprecipitation through changes in temperature, pressure, salinity, and other physicochemical conditions [131,139,193,194,195].

5. Exploration Strategy of Associated Cobalt Recourses in Skarn-Type Deposits

Based on a comprehensive analysis of the distribution patterns and enrichment mechanisms of associated cobalt resources in global skarn-type deposits (Figure 1, Table 3), it is recommended that future cobalt exploration focus on the following aspects: (1) deposits with mantle-derived magmatic sources should be prioritized (Table 3); (2) regarding geotectonic settings, attention should be concentrated on subduction or collision zones at convergent plate margins (e.g., the Middle-Lower Yangtze River metallogenic belt in China and the Cornwall mining district in the USA); (3) the mineralization ages of these deposits vary widely, specifically exhibiting older ages in the United States and Australia and relatively young ages in China (mainly Mesozoic to Cenozoic); (4) the associated rock types exhibit a wide range, covering mafic to intermediate-felsic intrusive rocks (e.g., gabbro, diorite, monzonite, and granite), particularly gabbro and diorite.

Based on the above synthesis (Table 3 and Table 5), the metallogenic model for associated cobalt resources in skarn-type deposits can be summarized as follows: (1) The higher Co abundance in the mantle provides the material basis for Co enrichment, making mantle-derived magmas a primary condition for mineralization. For instance, the skarn-type Fe deposits in the Han–Xing Mining Area of China and the Magnitogorsk zone in Russia are dominated by mantle-derived magmas, exhibiting significantly higher Co grades (Table 3). (2) The injection of mafic magmas serves as an additional source of Co and represents one of the important conditions contributing to enrichment. For instance, late-stage mafic magma injection in the Baijian and Henghui Fe deposits in China has resulted in significant Co enrichment (average Co content in pyrite reaching about 10,000 ppm) [21,55]. (3) The contribution of assimilation of evaporite sequences to Co enrichment remains unclear and requires further investigation. (4) The dissolution–reprecipitation mechanism of hydrothermal fluids facilitates Co enrichment and represents another important condition for mineralization. In the Zhongguan deposits in China, this mechanism enhanced Co concentrations in pyrite by approximately three to four orders of magnitude (Figure 4). In addition, deep major fault systems provide favorable pathways for Co migration. The coupling of these factors collectively promotes the enrichment of associated cobalt in skarn-type deposits. Based on this metallogenic model, utilizing artificial intelligence technology to integrate multi-source geological data and exploring the cobalt mineralization patterns can optimize future exploration strategies for cobalt deposits.

6. Conclusions

• Fe-dominated skarn-type cobalt deposits are characterized by a widespread distribution, high Co grades, and abundant reserves, exhibiting significantly superior cobalt resource potential compared to Cu-dominated or Pb-Zn polymetallic-dominated skarn-type deposits.
• In skarn-type cobalt deposits, cobalt mainly occurs in sulfide minerals, particularly pyrite, while independent cobalt minerals occur only in specific deposits.
• Associated cobalt resources in skarn-type deposits are observed to exhibit regionally zonal distribution features.
• Associated cobalt resources in skarn-type deposits are characterized by stage-specific differential enrichment, with effective enrichment particularly exhibited during the sulfide stage.
• Mantle-derived magmas provide additional cobalt sources for enrichment in associated skarn-type cobalt deposits.
• The injection of mafic magmas creates favorable conditions for cobalt enrichment in skarn-type cobalt deposits.
• The assimilation of evaporite sequences alters hydrothermal fluid composition, thereby affecting cobalt enrichment in skarn-type cobalt deposits.
• The dissolution–reprecipitation mechanism of hydrothermal fluids drives the secondary enrichment of cobalt in skarn-type cobalt deposits, significantly contributing to cobalt enrichment.
• The formation of skarn-type cobalt deposits is a complex process. Mantle-derived magmatic sources represent the primary condition for mineralization, while the injection of mafic magmas and the dissolution–reprecipitation mechanism of hydrothermal fluids play crucial roles in promoting cobalt enrichment.
• Cobalt exploration should emphasize mantle-derived magmatic rocks (primarily mafic-intermediate lithologies) in convergent plate tectonic settings such as subduction or collision zones.
• Given the multiple factors influencing cobalt enrichment, multidimensional data analysis under AI-aided frameworks will significantly enhance cobalt exploration efficiency.