Zircon U-Pb Age, Geochemical Characteristics and Geological Significance of Diabase in the Yanlinsi Gold Deposit, Northeastern Hunan Province

Abstract

The Yanlinsi gold deposit, located in the middle section of the Jiangnan Orogenic Belt, is one of the typical gold deposits in northeastern Hunan Province. Diabase dikes are exposed by underground workings and drill holes in the mining area. The dikes strike NW and cut the NE-trending gold ore body. To investigate the petrogenetic age, characteristics of the magmatic source area, and tectonic setting of the diabase dikes in the Yanlinsi gold mining area, northeastern Hunan, and to determine the mineralization age of the deposit, in this paper, diabase dike LA-ICP-MS zircon U-Pb dating, whole-rock geochemistry, and gold-bearing quartz vein LA-ICP-MS zircon U-Pb dating were studied. The results of LA-ICP-MS zircon U-Pb dating indicate that the diabase was emplaced at an age of 219.5 Ma, belonging to the late Indosinian. The investigated diabase dikes are characterized by low SiO₂ (43.68%–46.55%), high MgO (7.78%–9.84%), and high Mg# (65.0–68.7) values, belonging to the alkaline basalt series with high potassium. The chondrite-normalized REEs patterns show highly fractionated LREEs and HREEs ((La/Yb)_N = 11.21–14.82), and the primitive mantle-normalized spider patterns show enrichment in large ion lithophile elements (e.g., Rb, Ba, K and Sr) and relative depletion in high field strength elements (e.g., Nb, Ta, and P), similar to those of ocean island-like basalt (OIB). Rock geochemical characteristics indicate that the magma of the Yanlinsi diabase was formed by partial melting of the enriched mantle (EM II), with the source region being spinel-garnet lherzolite. The degree of partial melting was approximately 10%–15%, and the assimilation and contamination with continental crustal materials were weak. Meanwhile, weak fractional crystallization of olivine, clinopyroxene, and apatite occurred during the magma evolution process. On the basis of a synthesis of previous research results, it is concluded that the Yanlinsi diabase formed in an extensional tectonic setting after intracontinental collisional orogeny. The LA-ICP-MS U-Pb age of hydrothermal zircons from quartz veins in the main mineralization stage of the Yanlinsi gold deposit is 421.9 ± 1.5 Ma. Combined with the cross-cutting relationships between mafic dikes and gold veins (ore bodies), it is determined that the main mineralization stage of the deposit formed during the Caledonian Period.

1. Introduction

The northeastern Hunan region is located in the middle segment of the Jiangnan orogenic belt, which lies in the junction zone between the Yangtze block and the Cathaysia block (Figure 1a). The multi-stage and multi-phase tectonic evolution of this region [1,2,3,4,5,6,7,8,9] enables it to play an important role in explaining the amalgamation and evolutionary process of the Yangtze and Cathaysia blocks, as well as the lithospheric structure of south China and regional metallogenic regularity [10,11,12,13,14,15,16]. Intense magmatic activity has occurred in northeastern Hunan, forming intermediate-acid intrusions (dikes) and a small number of mafic dikes. Previous studies have focused relatively more on intermediate-acid magmatic rocks [8,17,18,19,20,21,22,23,24], while relatively less attention has been paid to mafic magmatic activity in the region [25,26,27]. In particular, regarding research on Indosinian-period mafic dikes in the region, only Zhou Chao et al. [27] have conducted studies on the chronology (zircon U-Pb age of 225 Ma), whole-rock geochemistry, isotope geochemistry, and petrogenetic tectonic setting of the diabase dikes in the Tuanshanbei gold mining area, northeastern Hunan. Meanwhile, there are numerous gold deposits in the Liuyang-Liling Guanzhuang area of northeastern Hunan, and previous researchers have carried out extensive studies on the characteristics, genesis, and chronological background of these gold deposits [28,29,30,31,32,33,34,35,36,37]. Among these topics, the mineralization age of the gold deposits in the region remains highly controversial, with different viewpoints such as Indosinian-period mineralization and Yanshanian-period mineralization [11,29,36,38]. In recent years, reports have indicated that there is also Caledonian-period mineralization in this area [36,39,40].

The Yanlinsi gold deposit is a typical gold deposit in the Liuyang-Liling Guanzhuang area of northeastern Hunan Province. Previous studies have extensively investigated various aspects of this deposit, including its geological features, ore-forming material sources, ore-forming fluid sources, enrichment mechanisms, mineralization age, and genetic type [29,31,35,36,41,42]. However, due to limitations in suitable testing objects and methods, there remains significant controversy regarding the mineralization age of the Yanlinsi gold deposit. The primary differing viewpoints focus on mineralization occurring during the Indosinian Period [29,36] and the Yanshanian Period [29], which has hindered a comprehensive understanding of the deposit’s metallogenic regularity. In recent years, during the exploration of the Yanlinsi gold deposit in northeastern Hunan, the authors discovered northwest-trending diabase dikes in both drill holes and mine workings, which cut through the northeast-trending gold veins. Therefore, on the basis of detailed field investigations, this study systematically conducted petrological, elemental geochemical, and zircon U-Pb chronological analyses on the diabase in the Yanlinsi mining area to reveal the nature of its magmatic source and the tectonic setting of its formation. Meanwhile, LA-ICP-MS U-Pb dating was performed on hydrothermal zircon grains from auriferous quartz veins. Combined with the cross-cutting relationship between diabase dikes and auriferous quartz veins, the mineralization age of the deposit was determined. This study holds significant importance for in-depth understanding of the geochemical properties of the deep mantle source and deep lithospheric information in northeastern Hunan Province.

Figure 1. (a). Sketch map showing tectonic units of the Jiangnan orogenic belt, Modified after [43]. (b). Simplified geological map of the Yanlinsi Au ore prospect. Abbreviations: LTPT—Longtoupu Town; FXS—Fengxi Street; NYQV—Nanyangqiao Village; XGT—Xiangeng Town; MZV—Mingzhao Village; WLDV—Wulidun Village; BGT—Baiguan Town; HXQ—Hongxiaqiao Rock Mass; XJV—Xianjin Village; ZT—Zhentou; GQT—Guanqiao Town; GQ—Guanqiao; DGC—Daguanchong; BHL—Baohualing; GZT—Guanzhuang Town; CL—Changlian; BSP—Banshanpu Rock Mass; YJBV—Yaojiaba Village; XXT—Xianxia Town; HLZT—Huanglanzui Town; BSV—Banshan Village; LSR—Lushui River; JJ—Jinji; XHJ—Xiaohengjiang; PJT—Puji Town; YHR—Yanghe River; CCT—Chengchong Town; HJC—Hengjiangchong; LSP—Lishupo; TSB—Tuanshanbei; ZC—zhengchong; YLS—Yanlinsi; BBS—Banbianshan; XHT—Xianghetian; DYC—Dayuanchong; QC—Qincao; SJAF—Shuangjiangao Fracture; YHV—Yanghua Village; DYZ—Dayao Town; JGT—jingang Town; NQT—Nanqiao Town; FLT—Fuli Town; BTTT—Baitutan Town; PKT—Pukou Town; WX—Wangxian; DBV—Dongbao Village; HD-LLF—Hengdong-Liling Fracture; WFT—Wangfang Town. 1. Cretaceous–Quaternary; 2. Upper Triassic–Upper Jurassic; 3. Middle Devonian–Lower Triassic; 4. Banxi group; 5. Dayaogu Formation; 6. Xiaomuping Formation; 7. Huanghudong Formation; 8. Leishenmiao Formation; 9. Panjiachong Formation; 10. Yijiaqiao Formation; 11. Silurian tonalite; 12. Silurian granodiorite; 13. Silurian monzogranite; 14. Granitic porphyry dikes (plutons); 15. Diabase dikes; 16. Hornfelsed alteration zone; 17. Geological boundary; 18. Unconformity geological boundary; 19. Thrust fault and its attitude; 20. Faults with uncertain nature, inferred faults; 21. Inferred concealed rock mass; 22. Lode gold deposits; 23. Placer gold deposits.

Figure 1. (a). Sketch map showing tectonic units of the Jiangnan orogenic belt, Modified after [43]. (b). Simplified geological map of the Yanlinsi Au ore prospect. Abbreviations: LTPT—Longtoupu Town; FXS—Fengxi Street; NYQV—Nanyangqiao Village; XGT—Xiangeng Town; MZV—Mingzhao Village; WLDV—Wulidun Village; BGT—Baiguan Town; HXQ—Hongxiaqiao Rock Mass; XJV—Xianjin Village; ZT—Zhentou; GQT—Guanqiao Town; GQ—Guanqiao; DGC—Daguanchong; BHL—Baohualing; GZT—Guanzhuang Town; CL—Changlian; BSP—Banshanpu Rock Mass; YJBV—Yaojiaba Village; XXT—Xianxia Town; HLZT—Huanglanzui Town; BSV—Banshan Village; LSR—Lushui River; JJ—Jinji; XHJ—Xiaohengjiang; PJT—Puji Town; YHR—Yanghe River; CCT—Chengchong Town; HJC—Hengjiangchong; LSP—Lishupo; TSB—Tuanshanbei; ZC—zhengchong; YLS—Yanlinsi; BBS—Banbianshan; XHT—Xianghetian; DYC—Dayuanchong; QC—Qincao; SJAF—Shuangjiangao Fracture; YHV—Yanghua Village; DYZ—Dayao Town; JGT—jingang Town; NQT—Nanqiao Town; FLT—Fuli Town; BTTT—Baitutan Town; PKT—Pukou Town; WX—Wangxian; DBV—Dongbao Village; HD-LLF—Hengdong-Liling Fracture; WFT—Wangfang Town. 1. Cretaceous–Quaternary; 2. Upper Triassic–Upper Jurassic; 3. Middle Devonian–Lower Triassic; 4. Banxi group; 5. Dayaogu Formation; 6. Xiaomuping Formation; 7. Huanghudong Formation; 8. Leishenmiao Formation; 9. Panjiachong Formation; 10. Yijiaqiao Formation; 11. Silurian tonalite; 12. Silurian granodiorite; 13. Silurian monzogranite; 14. Granitic porphyry dikes (plutons); 15. Diabase dikes; 16. Hornfelsed alteration zone; 17. Geological boundary; 18. Unconformity geological boundary; 19. Thrust fault and its attitude; 20. Faults with uncertain nature, inferred faults; 21. Inferred concealed rock mass; 22. Lode gold deposits; 23. Placer gold deposits.

2. Regional Geology and Ore Deposit Geological Features

The Yanlinsi gold deposit is located in the Liuyang-Liling Guanzhuang area in the southern part of northeastern Hunan Province (Figure 1b). The strata exposed in the region are dominated by the neoproterozoic Lengjiaxi group, which accounts for nearly 70% of the total area of the region, followed by the neoproterozoic Banxi group, Devonian system, Carboniferous system, Permian system, Triassic system, Cretaceous system, and Quaternary system. Among them, the Lengjiaxi group consists of a set of shallowly metamorphosed flysch turbidite deposits—abyssal facies clastic rocks intercalated with pyroclastic rock deposits. It constitutes the regional folded basement and serves as the main ore-hosting horizon for gold deposits in the region, with an Au content several to dozens of times the average value of the upper continental crust [44,45]. The neoproterozoic Banxi group is composed of a set of regionally shallowly metamorphosed greywackes and sandy-argillaceous rock deposits; the strata from the late Paleozoic to Mesozoic are mainly composed of terrestrial, marine, and marine-continental transitional facies sedimentary rocks such as shales, sandstones, siltstones, limestones, and dolomites.

The regional structure takes the Jiangnan orogenic belt as its basic framework, with the main structural trend being northeast-north-northeast. It is dominated by a series of composite anticlines (with cores composed of neoproterozoic Lengjiaxi group epimetamorphic rocks) and multiple northeast-north-northeast trending faults. In local sections, northwest-trending faults are superimposed on the northeast-north-northeast structural framework [46,47,48].

Magmatic activity in the region is frequent and intense, encompassing not only large-scale Banashipu and Hongxiaqiao granitoid intrusions in the southern part but also numerous basic-acid dikes, granitic porphyry intrusions (the Wangxian intrusion), and granodiorite stocks (the Tuanshanbei intrusion). The rock types of the Banashipu and Hongxiaqiao intrusions are mainly monzogranite, granodiorite, and tonalite, with diagenetic age concentrated in the range of 434–421 Ma. These intrusions formed during the extensional relaxation stage in the late phase of Early Paleozoic intracontinental orogeny in South China [18,19].

The sedimentary strata exposed in the Yanlinsi gold mining area are mainly the Huangxudong formation of the Neoproterozoic Lengjiaxi group. On the basis of lithological associations and sedimentary cycles, it is divided into three lithological members from bottom to top: Lower member (Pt₃h¹): The main rocks are grayish-green to grayish-black tuffaceous sandstones, sandy slates or slates, which constitute the core of the Yanshan thrust overturned anticline. Middle member (Pt₃h²): The main rocks are light-colored shallowly metamorphosed sandstones, silty slates, or slates, distributed on both sides of the overturned anticline core. Upper member (Pt₃h³): The main rocks are dark grayish-green to grayish-black shallowly metamorphosed siltstones, silty slates, slates, and phyllites, distributed in the northwest and southeast of the mining area. All three lithological members serve as ore-hosting strata. The main ore vein zone I is hosted in the Lower and Middle members; vein zones II and III are hosted in the Lower member of the northwest limb of the overturned anticline; and vein zone IV is hosted in the Middle and Upper members of the southeast limb of the overturned anticline (Figure 2).

The main structures in the mining area are the Yanshan thrust overturned anticline and the northeast-trending ductile shear zone. The Yanshan thrust overturned anticline is a secondary structure of the regional Jianganshan overturned composite anticline, with a length exceeding 2000 m, an axial direction of north-northeast-northeast, and an axial plane dipping northwest (290°–320°) at an angle of 30°–62°. Its southeast limb is the overturned limb, while the northwest limb is the normal limb. Cleavage zones parallel to the fold axial plane and interlayer fracture zones nearly parallel to the strata are developed in both limbs; these cleavage zones and interlayer fracture zones serve as the ore-hosting structures for gold orebodies in the mining area (Figure 2). Northeast-trending ductile shear zone: ductile deformation is superimposed on the early brittle structures of the mining area, forming an inherited fault with multiple activity features. It consists of a series of small-scale northeast-trending brittle-ductile shear zones that constitute a large-scale shear zone, and the gold orebodies themselves exhibit ductile shear deformation features. The shear zone extends over 1 km along its strike, with an attitude of 310°–330° ∠ 47°–61°. A large number of macroscopic indicators of ductile deformation are developed within the shear zone, forming kink structures of quartz veins, composite vein folds, strong compressive schistosity zones, and ptygmatic structures; locally, horsetail structures, lenticular structures, and asymmetric folds formed by ductile deformation can be observed. The slates within the shear zone undergo extensive deformation and intense rock alteration, showing (proto-) mylonitization features. In addition, northwest-trending faults are developed in the mining area and belong to the second phase of faults in the area. These faults are small in scale, have short strike extensions, and dip northeast or southwest. Diabase dikes and quartz veins are often developed in the northwest-trending fault zones and cut through the northeast-trending gold veins (orebodies).

No obvious magmatic activity is observed on the surface of the mining area; however, geological information revealed by drill holes and adits indicates the presence of concealed basic dikes in the area, with diabase as the main rock. According to underground investigations, the diabase occurs in a dike-like form with branching and merging phenomena. The dike width varies from 0.5 to 10 m, with an overall strike of approximately 310°, dipping northeast, and a dip angle generally ranging from 65° to 75° (nearly vertical in local sections), and the strike extension exceeds 200 m. Observations in drill holes and mines show that the diabase dikes intrude into the Lengjiaxi Group and cut through the northeast-trending gold veins (Figure 3). The diabase is generally grayish-black in color, with a massive structure and a diabasic texture (Figure 4e,f). It consists of plagioclase, pyroxene, carbonate, and a small amount of magnetite, with occasional minor pyrite and chalcopyrite (Figure 4g,j–l). Plagioclase mostly occurs as subhedral to euhedral prisms; twinning is mostly indiscernible, with only local polysynthetic twins visible, and most of it has undergone carbonation and chloritization alteration. Pyroxene mainly occurs as anhedral grains, appearing dark brown under plane-polarized light, and fills the triangular interstices formed by relatively euhedral plagioclase, presenting a diabasic texture (Figure 4g,j).

The wall-rock alteration of the deposit is extensively developed; centered on the gold orebody, a distinct alteration zone is generally formed within a range of 0.5–3.0 m on either side. All-rock alteration of the deposit is well-developed: centered on the gold orebody, a distinct alteration zone is formed within a range of generally 0.5–3.0 m on both sides. The main types of wall-rock alteration include sericitization, carbonatization, pyritization, and arsenopyritization; no obvious superimposed or secondary alteration phenomena are observed. Pyrite and arsenopyrite in the altered wall rocks generally have larger grain sizes and well-developed crystal forms, which is different from the fine-grained, xenomorphic-hypidiomorphic pyrite and arsenopyrite in the gold orebody.

The gold orebodies in the mining area are mainly concentrated in the core and northwest limb (normal limb) of the Yanshan thrust overturned anticline. A total of 75 gold orebodies have been discovered; among these, three of the main ore bodies have relatively simple morphologies, and their attitudes are basically consistent with those of the country rocks. Most of them are stratabound orebodies, presenting stratoid or layered shapes; some orebodies have a dip extension larger than the strike extension, showing a podiform shape. Within the mining area, the ore (mineralized) bodies can be divided into four groups of vein zones in plane view, among which vein zone I is the main vein zone (Figure 2); it strikes 35°–55°, dips northwest with a dip angle of 30°–60°, generally has a length ranging from several tens of meters to over 900 m, a thickness of 0.67–6.68 m, and a grade of 0.52–51.62 g/t. In addition, a very small number of orebodies (V4, V26) occur in northwest-trending fractures (Figure 2). According to the actual mining data of the area, the northwest-trending ore veins have short extensions and large grade variations, and basically have no mining value.

Ore types can be divided into two categories—banded quartz vein type and altered rock type—on the basis of differences in ore texture—structural features, mineral assemblages, and ore-hosting rocks (Figure 4a–c). The banded quartz veins vary in thickness from 0.02 to 0.30 m, with relatively regular shapes and clear boundaries with the surrounding rocks. The quartz veins are white in color, and dark gray bands are well-developed inside them, mainly composed of minerals such as sericite, fine-grained pyrite, arsenopyrite, subordinate amounts of chalcopyrite, galena, sphalerite, and native gold (Figure 4g–i). The altered rock type occurs on both sides of the banded quartz veins, with a thickness of approximately 0.4 m on a single side. It is characterized by “bleaching” alterations such as sericitization, carbonation, and silicification. The metallic minerals are mainly pyrite and arsenopyrite, distributed in a sparse disseminated manner, with a gold grade generally ranging from 0.63 to 1.90 g/t.

3. Materials and Methods

3.1. Materials

The diabase samples in this study were collected from the drift of the 115th middle level in the mining area. To exclude and minimize the impact of mineralization and alteration on the sample analysis results, all samples in this study were collected from diabase dikes with large widths, and the sampling sites were far from the edges of the diabase dikes.

Samples for hydrothermal zircon research were collected from the main orebody V11 in the 115th middle level of the mining area. The collected samples are quartz vein-type ores, mainly composed of quartz, pyrite, and arsenopyrite. Quartz accounts for more than 90% of the total composition, with a large grain size variation ranging from 0.01 to 2 mm. The ores also contain small amounts of sericite, galena, sphalerite, and native gold.

3.2. Methods

On the basis of detailed petrological studies, fresh samples without late-stage metasomatic vein intrusion were selected, cleaned, and air-dried. Samples for zircon separation were crushed to 80–120 mesh; zircon grains were separated via electromagnetic method and finally purified under a binocular microscope. For major element analysis, samples were comminuted to 200 mesh in a vibratory crusher use an agate mortar tray, while samples for trace element analysis were crushed to below 200 mesh.

The separation of single zircon minerals, mounting of samples, and cathodoluminescence (CL) imaging for the study samples were completed at Guangzhou Tuoyan Testing Technology Co., Ltd. in Guangzhou, China. LA-ICP-MS zircon U-Pb dating was carried out at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences. Zircon age determination adopted the laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analytical technique, which coupled a laser ablation system (Geolas2005) with an inductively coupled plasma-mass spectrometer (Agilent7500a). The Geolas2005 equipment was manufactured by Coherent Lambda Physik GmbH (formerly Lambda Physik AG) in Göttingen, Germany, and the country of purchase is Germany. The Agilent 7500a equipment was manufactured by Agilent Technologies Inc. in Tokyo, Japan, and the country of purchase is Japan. The laser ablation parameters were as follows: spot size of 32 μm, ablation depth of 20–40 μm, pulse frequency of 10 Hz, and laser energy of 110 mJ. The zircon standard 91500 was used as an external standard for isotopic fractionation correction, and detailed instrument operating conditions and testing procedures referred to relevant detailed descriptions [49,50]. Offline processing of the analytical data was performed using ICPMSDataCal software, Version 11.8. The error of individual dating isotopic age was 1σ, and the weighted mean age had a confidence level of 95%.

The major oxides and trace elements analyses of diabase were conducted at Guangzhou ALS Analytical Testing Co., Ltd in Guangzhou, China. The instrument for major element analysis of rocks was the PANalytical PW2424, The PANalytical PW2424 equipment was manufactured by PANalytical B.V. in Almelo, The Netherlands, and the country of purchase is The Netherlands. adopting the ME-XRF26 X-ray fluorescence spectrometry fusion method. Two samples were weighed: one sample (0.66 g) was mixed thoroughly with a flux of 4% lithium tetraborate-lithium metaborate (66:34), followed by high-temperature fusion. The molten product was poured into a platinum mold to form a flat glass slide, which was then analyzed by X-ray fluorescence spectrometry. Meanwhile, the other sample (1.0 g) was placed in a muffle furnace and calcined at 1000 °C. After cooling, it was weighed, and the weight difference of the sample before and after heating was the loss on ignition (LOI). The sum of the LOI result and the oxide results of elements determined by XRF is the “total” sum of this method, with the detailed determination process referring to the national standard GB/T 14506.31-2019 [51]. Ferrous oxide (FeO) was tested by the Fe-VOL05 titration method: the sample was decomposed with hydrofluoric acid-sulfuric acid, the remaining fluorine in the solution was complexed with boric acid, and sodium diphenylamine sulfonate was used as an indicator to titrate the content of ferrous iron (Fe²⁺) with a standard potassium dichromate solution, followed by calculating the amount of FeO. The instrumentation for trace and rare earth element analyses comprised the Agilent 7900, The Agilent 7900 equipment was manufactured by Agilent Technologies Inc. in Santa Clara, CA, USA, and the country of purchase is Singapore, adopting the ME-MS61r four-acid digestion inductively coupled plasma mass spectrometry (ICP-MS) and ME-MS81g fusion inductively coupled plasma mass spectrometry (ICP-MS), respectively. The detailed determination process refers to the national standard GB/T 14506.30-2010 [52]. For the ME-MS61r method: The sample was digested with perchloric acid, nitric acid, hydrofluoric acid, and hydrochloric acid, volumetrically adjusted with dilute hydrochloric acid, and then analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES); if the contents of Bi/Hg/Mo/Ag/W are high, appropriate dilution is required before analysis by ICP-MS. For the ME-MS81g method: Lithium borate (LiBO₂/Li₂B₄O₇) flux was added to the sample, mixed uniformly, and fused in a furnace at 1025 °C. After the molten liquid cooled, it was digested with nitric acid, hydrochloric acid, and hydrofluoric acid, volumetrically adjusted, and then analyzed by ICP-MS. The relative deviation (RD) for analytical precision control is <10% for ME-MS61r, and <5% for both XRF and LOI in ME-XRF26.

4. Results

4.1. Zircon U-Pb Age of Diabase

In this study, 31 zircon grains selected from sample 115CM3-1 were tested, and the ages of 24 zircon grains with high concordance were analyzed. Zircon grains from the Yanlinsi diabase dike are long prismatic, short prismatic, or irregular in shape, with particle sizes ranging from 40 to 160 μm and length–width ratios of 1:1 to 3:1. In cathodoluminescence (CL) images, most zircon grains appear dark gray; most exhibit obvious rhythmic compositional zoning, while some show uniform streaky absorption (Figure 5). The Th/U ratios of the zircon grains range from 0.15 to 0.78, with an average of 0.43 (

Table 1. LA-ICP-MS zircon U-Pb data for the diabase dike from the Yanlinsi gold deposit.

Number	Content (×10−6)		Th/U	Isotope Ratio								Age (Ma)
	Th	U		207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ	208Pb/232Th	1σ	207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ	208Pb/232Th	1σ
115CM3-1-1	48	243	0.20	0.1520	0.0021	8.8929	0.1656	0.4233	0.0060	0.1274	0.0065	2368.8	23.0	2327.1	17.0	2275.2	27.0	2424.3	117.4
115CM3-1-2	70	115	0.61	0.1012	0.0016	3.6921	0.0588	0.2643	0.0031	0.0804	0.0039	1646.6	29.5	1569.7	12.7	1511.8	15.7	1563.9	72.2
115CM3-1-4	823	3217	0.26	0.0516	0.0007	0.2463	0.0036	0.0346	0.0005	0.0118	0.0005	333.4	29.6	223.6	2.9	219.2	3.0	236.4	10.4
115CM3-1-5	1001	3781	0.26	0.0521	0.0010	0.2457	0.0086	0.0350	0.0022	0.0115	0.0006	300.1	44.4	223.1	7.0	221.5	14.0	231.6	12.5
115CM3-1-6	396	1017	0.39	0.0505	0.0010	0.2401	0.0044	0.0345	0.0004	0.0108	0.0005	216.7	44.4	218.5	3.6	218.7	2.6	216.8	10.1
115CM3-1-7	279	1037	0.27	0.1334	0.0798	0.2645	0.0126	0.0346	0.0013	0.0139	0.0030	2142.9	1288.9	238.3	10.1	219.0	8.3	278.9	59.6
115CM3-1-8	467	599	0.78	0.0525	0.0012	0.2537	0.0059	0.0350	0.0005	0.0100	0.0005	309.3	53.7	229.6	4.8	222.0	2.9	201.3	10.8
115CM3-1-9	271	522	0.52	0.0489	0.0013	0.2327	0.0056	0.0346	0.0005	0.0110	0.0006	142.7	65.7	212.5	4.6	219.3	3.4	222.0	12.9
115CM3-1-10	988	2743	0.36	0.0513	0.0008	0.2440	0.0034	0.0345	0.0004	0.0124	0.0006	257.5	39.8	221.7	2.8	218.4	2.3	249.7	12.1
115CM3-1-11	194	447	0.44	0.0531	0.0015	0.2535	0.0059	0.0347	0.0004	0.0113	0.0006	344.5	30.6	229.4	4.8	219.9	2.3	227.1	12.1
115CM3-1-12	698	1919	0.36	0.0504	0.0008	0.2377	0.0039	0.0342	0.0005	0.0108	0.0005	213.0	41.7	216.5	3.2	217.0	2.9	218.0	10.4
115CM3-1-13	595	3095	0.19	0.0510	0.0008	0.2448	0.0059	0.0348	0.0007	0.0120	0.0006	239.0	32.4	222.3	4.8	220.5	4.5	241.3	12.1
115CM3-1-14	1563	3420	0.46	0.0507	0.0011	0.2429	0.0077	0.0359	0.0026	0.0108	0.0006	227.8	50.0	220.8	6.3	227.3	16.5	216.2	11.1
115CM3-1-15	489	1576	0.31	0.0516	0.0012	0.2473	0.0052	0.0348	0.0007	0.0105	0.0006	333.4	51.8	224.4	4.2	220.8	4.4	211.1	13.0
115CM3-1-16	562	762	0.74	0.0558	0.0011	0.2653	0.0057	0.0345	0.0004	0.0108	0.0007	442.6	44.4	238.9	4.5	218.5	2.5	216.3	13.2
115CM3-1-17	207	553	0.37	0.0517	0.0016	0.2384	0.0069	0.0336	0.0005	0.0107	0.0008	333.4	70.4	217.1	5.7	212.7	3.3	214.5	15.8
115CM3-1-19	217	1472	0.15	0.0558	0.0015	0.2667	0.0071	0.0346	0.0004	0.0119	0.0007	442.6	59.3	240.1	5.7	219.5	2.3	238.3	13.3
115CM3-1-21	341	494	0.69	0.0519	0.0016	0.2493	0.0077	0.0349	0.0005	0.0107	0.0006	279.7	72.2	226.0	6.3	220.9	2.9	216.1	11.1
115CM3-1-22	100	242	0.41	0.0519	0.0025	0.2469	0.0112	0.0346	0.0005	0.0105	0.0006	279.7	112.9	224.0	9.1	219.3	3.3	210.8	12.8
115CM3-1-24	437	1166	0.37	0.0963	0.0020	2.5499	0.1426	0.1883	0.0089	0.0581	0.0043	1553.7	38.3	1286.4	40.8	1112.1	48.2	1141.6	82.4
115CM3-1-26	1014	1322	0.77	0.0531	0.0010	0.2536	0.0048	0.0346	0.0005	0.0104	0.0005	331.5	36.1	229.5	3.9	219.2	2.8	209.2	10.1
115CM3-1-27	489	670	0.73	0.0942	0.0045	2.4685	0.2146	0.1793	0.0079	0.0463	0.0025	1522.2	289.8	1262.8	62.9	1062.9	43.1	914.7	48.1
115CM3-1-29	464	1075	0.43	0.0539	0.0010	0.2580	0.0053	0.0346	0.0004	0.0106	0.0005	368.6	42.6	233.0	4.3	219.5	2.7	212.7	10.7
115CM3-1-31	512	1075	0.48	0.0554	0.0016	0.2546	0.0068	0.0333	0.0004	0.0096	0.0006	431.5	64.8	230.3	5.5	211.4	2.3	193.0	11.8
YLS-CM1-4-02	530	667	0.79	0.0688	0.002	1.1931	0.0453	0.1235	0.0016	0.0378	0.0010	891.7	70.4	797.4	21.0	750.8	9.4	749.1	19.1
YLS-CM1-4-03	895	2789	0.32	0.0515	0.001	0.3381	0.0090	0.0470	0.0005	0.0136	0.0004	264.9	58.3	295.7	6.9	296.0	3.0	272.1	7.1
YLS-CM1-4-4	316	1101	0.29	0.0576	0.0019	0.5377	0.0177	0.0671	0.0008	0.0201	0.0006	516.7	72.2	436.9	11.7	418.7	4.8	401.3	12.5
YLS-CM1-4-05	460	456	1.01	0.0710	0.003	1.4064	0.0538	0.1426	0.0020	0.0468	0.0012	966.7	77.5	891.6	22.7	859.1	11.1	924.5	23.3
YLS-CM1-4-7	1116	1979	0.56	0.0588	0.0019	0.5570	0.0172	0.0679	0.0007	0.0220	0.0006	566.7	70.4	449.6	11.2	423.7	4.3	439.5	11.3
YLS-CM1-4-08	3209	23348	0.14	0.0522	0.001	0.2341	0.0050	0.0320	0.0003	0.0106	0.0003	294.5	45.4	213.6	4.1	203.1	1.6	213.8	5.4
YLS-CM1-4-09	242	349	0.70	0.1169	0.003	5.5237	0.1409	0.3377	0.0036	0.0938	0.0021	1909.6	44.9	1904.3	22.0	1875.6	17.2	1812.5	39.5
YLS-CM1-4-10	884	1706	0.52	0.0499	0.002	0.3280	0.0109	0.0473	0.0006	0.0149	0.0004	190.8	77.8	288.0	8.3	297.7	3.6	299.4	7.7
YLS-CM1-4-11	754	1513	0.50	0.0537	0.0017	0.5242	0.0172	0.0699	0.0009	0.0216	0.0006	366.7	72.2	428.0	11.5	435.7	5.3	432.7	11.5
YLS-CM1-4-13	29	67	0.43	0.1271	0.006	4.7259	0.2176	0.2732	0.0063	0.1124	0.0046	2058.3	78.7	1771.8	38.6	1556.8	31.8	2152.9	83.0
YLS-CM1-4-14	213	351	0.61	0.1372	0.003	7.8591	0.1880	0.4103	0.0046	0.1188	0.0029	2191.7	41.1	2215.0	21.6	2216.1	20.8	2269.0	52.9
YLS-CM1-4-15	836	1240	0.67	0.0592	0.0018	0.5321	0.0152	0.0648	0.0007	0.0204	0.0005	576.0	66.7	433.2	10.1	404.6	4.1	407.3	9.4
YLS-CM1-4-16	812	686	1.18	0.0567	0.0028	0.4956	0.0234	0.0635	0.0008	0.0194	0.0005	479.7	109.2	408.7	15.9	397.1	4.7	388.6	9.1
YLS-CM1-4-17	832	1148	0.72	0.0567	0.0020	0.5526	0.0185	0.0704	0.0008	0.0211	0.0005	479.7	77.8	446.7	12.1	438.4	4.6	422.0	10.0
YLS-CM1-4-19	1460	2550	0.57	0.0549	0.002	0.3431	0.0115	0.0453	0.0005	0.0143	0.0004	409.3	86.1	299.5	8.7	285.8	2.9	287.7	7.1
YLS-CM1-4-21	573	1272	0.45	0.0698	0.0026	0.6374	0.0213	0.0666	0.0008	0.0282	0.0009	924.1	70.2	500.7	13.2	415.5	4.6	562.3	18.0
YLS-CM1-4-22	1513	1834	0.82	0.0540	0.0016	0.5097	0.0158	0.0678	0.0008	0.0208	0.0005	372.3	68.5	418.2	10.6	423.0	4.7	416.0	10.0
YLS-CM1-4-25	794	5675	0.14	0.0657	0.002	1.2208	0.0295	0.1334	0.0013	0.0412	0.0010	794.4	55.7	810.1	13.5	807.0	7.5	815.8	18.7
YLS-CM1-4-26	523	1579	0.33	0.0794	0.003	1.3558	0.0428	0.1227	0.0010	0.0451	0.0013	1183.3	69.4	870.1	18.5	745.9	6.0	892.1	24.5
YLS-CM1-4-27	959	1046	0.92	0.0598	0.0020	0.5815	0.0185	0.0701	0.0007	0.0217	0.0005	596.0	70.4	465.4	11.9	436.9	4.1	434.4	10.2
YLS-CM1-4-29	600	1272	0.47	0.0589	0.0020	0.5593	0.0198	0.0682	0.0008	0.0216	0.0006	561.1	74.1	451.1	12.9	425.0	5.0	432.6	11.3

), indicating that they are magmatic zircon grains or captured zircon grains of magmatic origin [53]. The zircon chronological data results for the sample show that among the 24 analysis spots, 20 spots yield a ²⁰⁶Pb/²³⁸U age that lies on or near the concordia line in the zircon U-Pb concordia diagram. The calculated concordia age is 219.5 ± 1.7 Ma (MSWD = 1.10, n = 20), which is basically consistent with the weighted mean age of 218.3 ± 1.5 Ma (MSWD = 1.00, n = 20). This consistency indicates that the isotopic system of the zircon grains has remained essentially closed since their formation, representing the crystallization age of the magma (Figure 6). The remaining 4 analysis spots have significantly older and scattered ages, with ²⁰⁷Pb/²³⁵U age ranging from 2327 to 1263 Ma, and are inferred to be xenocrystic zircon grains derived from the basement.

4.2. U-Pb Age of Hydrothermal Zircon Grains in Quartz Veins

In this study, 30 zircon grains separated from sample YLS-CM1-4 were tested, and the ages of 21 zircon grains with high concordance were obtained. Among these, the analytical spots No. 3, 8, 10, and 19 deviated significantly from the concordia line and discordia line of zircon grains, so they were not included in the zircon age calculation. The ²⁰⁷Pb/²³⁵U ages of spots No. 2, 5, 25, and 26 ranged from 797 to 892 Ma, basically consistent with the peak age of zircon grains in the Lengjiaxi Group clastic rocks in the study area; these zircon grains are inferred to be captured detrital zircon grains. The ²⁰⁷Pb/²³⁵U ages of spots No. 9, 13, and 14, which are presumably xenocrystic zircon grains derived from the basement, ranged from 1557 to 2216 Ma. The remaining 10 zircon grains are mostly short prismatic, granular, or irregular in shape, with particle sizes ranging from 30 to 80 μm and length–width ratios of 1:1 to 2:1 (Figure 7). In cathodoluminescence (CL) images, the zircon grains are mostly grayish-white to dark gray; the CL images show oscillatory zoning of whole grains or weak zoning in their interiors, and some have irregular luminescent areas and white altered rims, exhibiting typical hydrothermal zircon features [54,55,56]. Zircon chronological data results for the sample show that the ²⁰⁶Pb/²³⁸U ages of 10 analytical spots all lie on or near the concordia line in the zircon U-Pb concordia diagram, with apparent ²⁰⁶Pb/²³⁸U ages ranging from 397 to 438 Ma (Table 1). The calculated weighted mean age was 421.9 ± 1.5 Ma (MSWD = 8.4, n = 10), indicating that the isotopic system of the zircon grains has remained essentially closed since their formation, representing the formation age of the hydrothermal zircon grains (Figure 8).

4.3. Whole-Rock Geochemistry

The analysis results for major, trace, and rare earth elements in 12 diabase samples show that the SiO₂ content of the diabase samples ranges from 43.68% to 46.55%, the Al₂O₃ content is in the range of 13.88% to 15.16%, the MgO content varies between 7.78% and 9.84%, and the Mg# values are relatively concentrated, ranging from 65.0 to 68.7%. The total alkali (Na₂O + K₂O) content ranges from 3.42% to 4.15% (

Table 2. Major oxide (wt%) and trace oxide (×10⁻⁶) element data for the Yanlinsi diabase samples.

Sample #	115CM3-1-1	115CM3-1-2	115CM3-1-3	115CM3-1-4	ZK002-2A	ZK002-2B	ZK002-2C	ZK002-2D	ZK701-1	ZK701-2	ZK701-3	ZK701-4
SiO2	45.88	45.87	46.55	46.19	43.68	43.95	45.08	44.12	45.65	45.63	46.29	46.34
TiO2	1.58	1.56	1.57	1.59	1.84	1.66	1.68	1.65	1.52	1.55	1.58	1.58
Al2O3	14.76	14.64	14.73	14.88	15.16	13.88	14.27	13.96	14.14	14.36	14.44	14.51
Fe2O3T	11.36	11.27	11.26	11.45	9.78	10.26	10.39	9.86	10.79	11.05	11.22	11.20
MnO	0.14	0.14	0.17	0.14	0.13	0.14	0.14	0.15	0.14	0.14	0.15	0.15
MgO	9.84	9.84	9.80	9.80	7.78	9.24	9.78	8.90	9.62	9.56	9.56	9.58
CaO	7.00	7.42	7.27	6.48	6.97	7.82	7.21	7.91	7.68	7.56	7.41	7.35
Na2O	2.24	2.23	2.32	2.24	2.27	2.24	2.30	2.22	2.42	2.48	2.66	2.54
K2O	1.70	1.74	1.83	1.74	1.15	1.43	1.65	1.61	1.35	1.42	1.42	1.42
P2O5	0.30	0.30	0.30	0.30	0.39	0.35	0.35	0.35	0.29	0.29	0.30	0.30
LOI	5.03	4.92	4.54	5.47	9.85	8.63	7.05	8.51	5.44	5.61	5.28	4.87
Total	99.83	99.93	100.34	100.28	99.00	99.60	99.90	99.24	99.04	99.65	100.31	99.84
Mg#	66.9	67.0	67.0	66.6	65.0	67.7	68.7	67.8	67.5	66.8	66.5	66.6
Na2O + K2O	3.94	3.97	4.15	3.98	3.42	3.67	3.95	3.83	3.77	3.90	4.08	3.96
Rb	54.6	56.0	60.4	52.2	38.8	42.4	50.5	50.4	45.5	44.7	47.5	43.0
Ba	641	627	635	647	525	679	740	710	540	550	568	640
Th	4.53	4.66	4.51	4.59	5.46	4.88	4.91	5.27	4.42	4.52	4.37	4.55
U	0.78	0.76	0.78	0.80	1.09	0.89	0.94	0.93	0.73	0.76	0.76	0.76
Nb	32.8	32.1	33.1	33.1	40.6	36.6	37.9	36.0	30.4	30.4	29.8	29.1
Ta	1.75	1.75	1.78	1.78	2.5	2.2	2.2	2.1	1.7	1.6	1.7	1.7
La	31.0	31.2	31.2	30.8	41.0	37.5	37.8	36.3	32.0	31.4	31.8	28.9
Ce	56.3	56.4	57.5	55.4	76.6	71.7	72.6	76.2	61.4	61.2	61.6	61.6
Pr	6.31	6.39	6.54	6.21	8.00	7.33	7.52	7.79	6.38	6.30	6.31	6.27
Sr	505	501	491	517	475	607	594	617	493	499	470	497
Nd	22.8	23.2	23.8	22.5	27.8	25.1	25.6	27.2	22.4	21.7	21.6	23.0
Sm	4.13	4.20	4.27	3.97	4.60	4.18	4.14	4.31	3.74	3.77	3.74	3.79
Zr	140	137	140	140	174	153	151	154	135	137	142	137
Hf	3.3	3.4	3.3	3.4	3.8	3.3	3.6	3.4	3.1	3.0	3.1	3.1
Eu	1.34	1.34	1.37	1.30	1.52	1.34	1.42	1.51	1.25	1.27	1.27	1.32
Gd	3.88	3.91	4.01	3.70	4.55	3.93	3.98	4.01	3.64	3.78	3.83	3.76
Tb	0.65	0.64	0.62	0.58	0.65	0.57	0.60	0.63	0.61	0.60	0.58	0.62
Dy	3.83	3.84	3.62	3.46	3.90	3.63	3.46	3.71	3.62	3.48	3.55	3.70
Y	18.9	18.9	19.7	18.3	22.0	19.8	19.3	20.1	19.9	19.6	19.8	19.3
Ho	0.76	0.75	0.76	0.74	0.80	0.72	0.70	0.78	0.72	0.72	0.73	0.74
Er	1.99	1.98	2.04	1.96	2.29	2.04	2.10	1.98	2.04	1.95	2.08	2.02
Tm	0.29	0.29	0.30	0.29	0.31	0.28	0.27	0.29	0.29	0.27	0.28	0.29
Yb	1.92	1.85	1.93	1.88	2.00	1.83	1.83	1.81	1.82	1.78	1.92	1.85
Lu	0.30	0.29	0.31	0.30	0.31	0.26	0.28	0.27	0.28	0.28	0.28	0.28
Co	46.7	47.8	48.2	48.6	42.1	47.1	51.6	49.7	53.8	51.6	51.7	53.1
Cr	400	400	400	400	290	350	380	360	370	350	360	350
Ni	244	249	255	245	163.0	222	249	224	257	242	245	252
Rb/Sr	0.11	0.11	0.12	0.10	0.08	0.07	0.09	0.08	0.09	0.09	0.10	0.09
Ba/Rb	11.74	11.20	10.51	12.39	13.53	16.01	14.65	14.09	11.87	12.30	11.96	14.88
Nb/Ta	18.74	18.34	18.60	18.60	16.24	16.64	17.23	17.14	17.88	19.00	17.53	17.12
Zr/Hf	42.42	40.29	42.42	41.18	45.79	46.36	41.94	45.29	43.55	45.67	45.81	44.19
Dy/Yb	1.99	2.08	1.88	1.84	1.95	1.98	1.89	2.05	1.99	1.96	1.85	2.00
ΣREEs	135.50	136.28	138.27	133.09	174.33	160.41	162.3	166.79	140.19	138.5	139.57	138.14
LREEs	121.88	122.73	124.68	120.18	159.52	147.15	149.08	153.31	127.17	125.64	126.32	124.88
HREEs	13.62	13.55	13.59	12.91	14.81	13.26	13.22	13.48	13.02	12.86	13.25	13.26
LREEs/HREEs	8.95	9.06	9.17	9.31	10.77	11.10	11.28	11.37	9.77	9.77	9.53	9.42
(La/Yb)N	11.58	12.10	11.60	11.75	14.70	14.70	14.82	14.39	12.61	12.65	11.88	11.21
δEu	1.02	1.01	1.01	1.04	1.02	1.01	1.07	1.11	1.04	1.03	1.03	1.07
δCe	0.99	0.98	0.99	0.98	1.04	1.06	1.06	1.11	1.05	1.07	1.07	1.12

). The loss on ignition (LOI) of the samples is between 4.54% and 9.85%, with an average of 6.27%, indicating that the rocks have undergone a certain degree of hydrothermal alteration after their formation. Considering that the rocks have a certain degree of alteration, elements that are immobile during metamorphism and alteration can be used to indicate the geochemical features of the diabase samples. In the Nb/Y-Zr/TiO₂ diagram, all sample points fall into the alkaline basalt area (Figure 9a); in the SiO₂-K₂O diagram, all sample points fall near the boundary between the high-K calc-alkaline series and the shoshonitic series (Figure 9b).

In the primitive mantle-normalized trace element spider diagram, all diabase samples show a highly consistent variation trend, characterized by enrichment in large ion lithophile elements (LILEs) such as Rb, Ba, K, and Sr, and relatively weak depletion in high field strength elements (HFSEs) such as Nb, Ta, and P. Overall, these features are similar to those of intraplate ocean island basalts (OIB) (Figure 10a). The total rare earth element (ΣREE) contents of the diabase samples are relatively low, ranging from 133.09 × 10⁻⁶ to 174.33 × 10⁻⁶. The LREE/HREE ratios are 8.95–11.37, and the (La/Yb)_n ratios are 11.21–14.82, indicating significant fractionation between light and heavy rare earth elements. The REE distribution patterns are right-inclined, with LREE enrichment, which is also similar to the features of intraplate OIB (Figure 10b). The δEu values range from 1.01 to 1.11, and the δCe values range from 0.98 to 1.12, showing no obvious anomalies. This suggests that plagioclase fractional crystallization was not significant during magma evolution.

5. Discussion

5.1. Age of Diagenesis

In this study, high-precision laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was used for zircon U-Pb dating of diabase in the Yanlinsi mining area. The ²⁰⁶Pb/²³⁸U ages of 20 analytical spots all lie on or near the concordia line in the zircon U-Pb concordia diagram. The calculated concordia age is 219.5 ± 1.7 Ma (MSWD = 1.10, n = 20) (Figure 6a), which is essentially consistent with the weighted mean age of 218.3 ± 1.5 Ma (MSWD = 1.00, n = 20) (Figure 6b). Cathodoluminescence (CL) images (Figure 5) show that most zircon grains are dark gray, with clearly visible rhythmic compositional zoning, and some exhibit uniform streaky absorption, indicating that these zircon grains crystallized from the diabase magma. The remaining four analytical spots have significantly older and scattered ages, which are interpreted as those of zircon grains captured from the basement during the emplacement of the diabase. Therefore, the age of (219.5 ± 1.7) Ma is determined as the diagenetic age of the diabase dike, corresponding to the Indosinian period. This age is generally consistent with the whole-rock K-Ar age of 238.4 Ma for the diabase dike intruding into the Banashipu intrusion in the region. Previous zircon U-Pb dating results for magmatic activities that mainly formed large acidic intrusions in the Liuyang-Liling Guanzhuang area are concentrated in the range of 430–420 Ma [8,18,19,60]. In recent years, reports on Indosinian magmatic activities in the Liuyang-Liling Guanzhuang area, such as the zircon U-Pb age of 224.7 ± 4.4 Ma for the Wangxian granodiorite porphyry stock [22] and the zircon U-Pb age of 225.2 ± 1.9 Ma for the diabase dike in the Tuanshanbei gold mining area [27], further confirm that in addition to the Caledonian magmatic activities, there were also Indosinian magmatic activities in the region that mainly formed intermediate-basic stocks and dikes.

5.2. Crustal Contamination and Fractional Crystallization

Assessing fractional crystallization and crustal contamination is a critical prerequisite for investigating the features of a rock’s source region. The diabase in the Yanlinsi mining area exhibits high Mg# values (65.0–68.7, average 67.0), high TiO₂ contents (1.52%–1.84%, average 1.61%), and low SiO₂ contents (43.68%–46.55%, average 45.44%), suggesting that the magma was derived from partial melting of the mantle [61,62]. Mantle-derived magmas formed in continental intraplate settings typically pass through the continental crust during ascent, making them susceptible to varying degrees of continental crustal material contamination. Since the continental crust is strongly depleted in Nb, Ta, and Ti [63], magmas that have undergone significant continental crustal contamination during emplacement and ascent typically acquire crustal attributes, exhibiting geochemical features similar to those of island arc magmatic rocks [64]. Trace element analysis of the Yanlinsi diabase shows relatively weak negative anomalies of Nb and Ta (Figure 10a), suggesting that the magma may have experienced minor contamination by continental crustal materials. Additionally, the presence of Proterozoic captured zircon grains (2327–1263 Ma) in the zircon U-Pb dating of the diabase samples further indicates that the magma was contaminated by country rocks during its ascent and emplacement. However, the Nb/Ta ratios of the diabase samples range from 16.24 to 19.00 (average 17.75), Zr/Hf ratios from 40.29 to 46.36 (average 43.74), and Nb/U ratios from 37.3 to 42.44 (average 40.4). These ratios are significantly higher than those of the continental crust (Nb/Ta = 11, Zr/Hf = 33, Nb/U = 6.15), indicating that the influence of crustal contamination on the diabase magma was insignificant [63,65,66,67,68,69,70,71]. Furthermore, compared to mantle-derived materials, the continental crust is characterized by lower Nb, lower Nb/U ratios, enriched LREEs, and higher La/Nb ratios [72]. If the Yanlinsi diabase had undergone significant crustal contamination, positive correlations would be expected in the La/Nb-Ba/Nb and Nb/U-Nb diagrams. However, no obvious correlations are observed in these diagrams (Figure 11a,b), and the sample data points mostly plot within or near the OIB field, with higher Nb contents and Nb/U ratios than the crust and far from crustal data points. This also indicates that contamination by continental crustal materials had little impact on the high field strength elements in the basic dike magma of the Yanlinsi mining area.

The Mg# values for diabase samples from the Yanlinsi gold mining area are relatively high and concentrated, ranging from 65.0 to 68.7 with an average of 67.0, which is comparable to the Mg# values for primitive magma (63–73; [73]). This indicates that the magma did not undergo significant fractional crystallization. Meanwhile, the contents of Cr (290–400 × 10⁻⁶) and Ni (163–257 × 10⁻⁶) in the diabase samples are lower than those in the initial magma (Cr > 400 × 10⁻⁶ [74], Ni = 300 × 10⁻⁶~400 × 10⁻⁶ [75]), suggesting that the diabase may have experienced minor fractional crystallization of ferromagnesian minerals (such as olivine and pyroxene) during its formation. There is an overall positive correlation between Al₂O₃ and SiO₂ (Figure 12a), indicating that there may be fractional crystallization of felsic minerals. The Co content is relatively high and shows a positive correlation with MgO content (Figure 12d). Additionally, SiO₂, Ni, and Cr contents are positively correlated with MgO content (Figure 12b,e,f), implying that fractional crystallization of olivine and clinopyroxene occurred during magma evolution [76,77]. The weak depletion of P in the trace element spider diagram (Figure 10a) suggests that low-degree apatite fractional crystallization may have also occurred during magma evolution [78]. MgO and TiO₂ show a significant negative correlation (Figure 12c), indicating that Ti-bearing minerals (such as rutile, ilmenite, and sphene) were not the main crystallizing phases. Meanwhile, there are no obvious Eu anomalies in the REE distribution patterns (δEu = 1.01–1.11) (Figure 10b), indicating the absence of significant plagioclase fractional crystallization [79].

5.3. Nature of the Source Region

As analyzed above, the diabase in the Yanlinsi mining area is characterized by low silica (average SiO₂ = 45.44%), high magnesium (average MgO = 9.44%), high titanium (average TiO₂ = 1.61%), and high Mg# values (average = 67.0). Meanwhile, both the trace element spider diagram and rare earth element distribution pattern (Figure 10a,b) show that the Yanlinsi diabase has elemental features similar to those of ocean island basalts (OIBs), suggesting that the diabase in the Yanlinsi mining area may have been generated by the melting of an enriched mantle.

Certain trace element ratios remain unaffected by partial melting and fractional crystallization, making them reliable tracers for identifying the nature of magma source regions [80]. The average Nb/Ta and Zr/Hf ratios of the diabase samples are 17.75 and 43.74, respectively, which are very close to the primitive mantle values (17.8 and 37; [81]). This similarity suggests that the magma was derived from the partial melting of mantle peridotite with a composition similar to primitive mantle. Parameters of the Yanlinsi diabase, such as Zr/Nb (4.0–4.8), La/Nb (0.93–1.07), Th/Nb (0.13–0.16), and Th/La (0.13–0.16), all fall within the range of EM II-type OIB [71,82]. In the Nb/Yb–Ta/Yb diagram (Figure 13a) and Nb/Yb–La/Yb diagram (Figure 13b), the diabase samples plot within the mantle array and are concentrated near ocean island basalts (OIBs), further indicating that the diabase magma in the Yanlinsi mining area was generated by the melting of an enriched mantle.

Ratios such as Sm/Yb, Ce/Y, and Zr/Nb are less affected by magma fractional crystallization, making them useful for determining the nature of the source region and the degree of partial melting [84,85]. In the Zr/Nb–Ce/Y diagram (Figure 14a), all diabase samples from the Yanlinsi mining area plot between primitive garnet-facies lherzolite and primitive spinel-facies lherzolite, with a very narrow range of variation. In the Sm/Yb–Sm diagram (Figure 14b), the values of the diabase samples are all close to or slightly higher than the partial melting line of spinel-garnet (1:1) lherzolite, indicating that the mantle source region is spinel-garnet lherzolite, with a partial melting degree of 10%–15%.

On the basis of the above features, this study concludes that the diabase magma in the Yanlinsi mining area was formed by the partial melting of an enriched mantle (EM II). The source region is composed of spinel-garnet (1:1) lherzolite, with a partial melting degree of 10%–15%.

5.4. Tectonic Setting

Magmatic processes such as fractional crystallization and partial melting have minimal effects on incompatible elements like Zr, Y, Nb, and Ti [88,89]. Therefore, these trace elements can be used to determine the tectonic setting of basaltic magma formation. In the Zr-Zr/Y diagram (Figure 15a), all diabase samples from the Yanlinsi mining area plot within or near the within-plate basalt (WPB) field. In the 2Nb-Zr/4-Y diagram (Figure 15b), the samples project into the within-plate alkaline basalt field. In the Ti/100-Zr-Y×3 diagram (Figure 15c), they all fall within the within-plate basalt field. In the Ta/Hf-Th/Hf diagram (Figure 15d), the samples project into the continental rift alkaline basalt field. Overall, the diabase in the Yanlinsi mining area primarily exhibits a within-plate tectonic setting similar to that of ocean island basalts (OIBs).

Previous studies have shown that during the Indosinian tectonic movement, the Paleo-Tethys Ocean closed and disappeared, leading to intracontinental collision orogeny among multiple blocks around South China [5,92,93,94,95]. Within this tectonic framework, the South China Block experienced various tectonic environments during the Indosinian period, including crustal stacking, thickening, and thermal-stress relaxation. The early stage (251–228 Ma) was characterized by syn-collisional compression, while the late stage (228–199 Ma) saw a local transition to an extensional environment, triggering stratigraphic unconformities, rock metamorphism and deformation, large-scale granitic magmatism, and minor basic magmatism in the South China Block. Indosinian granitic magmatism in the South China Block can be divided into an early stage (243–233 Ma) and a late stage (224–204 Ma) [96]. Specifically, in Hunan Province, early Indosinian magmatism is not prominent, with late Indosinian granites being the main representatives [97], such as the Yajiangqiao intrusion in eastern Hunan (213–212 Ma) [98], the Wangxian intrusion in eastern Hunan (224.7 Ma) [22], the Xitian A-type granite in eastern Hunan (221 Ma) [99], the Ziyunshan intrusion in central Hunan (223.2 Ma) [100], the Weishan intrusion in central Hunan (215.7 Ma) [101], the Miaoershan intrusion in southwestern Hunan (214 Ma) [102], and the Yuechengling intrusion (219–215 Ma) [103,104]. In terms of basic magmatism, apart from the LA-ICP-MS zircon U-Pb age of 219.5 ± 1.7 Ma for the diabase dikes in the Yanlinsi mining area in this study, other basic magmatic rocks have been discovered in Hunan Province, including the alkaline basalts from Baoanwei, Zhongxinpu, and Lizhaixiang in Ningyuan (205.5 Ma and 212.3 Ma) [105], basic rock inclusions from Huziyan in Daoxian (220–233 Ma) [106], the Changchengling diabase in Yizhang (227 Ma) [107], the Jiangshiqiao diabase in Taojiang (229.9 Ma) [108], and the Tuanshanbei diabase in northeastern Hunan (225 Ma) [27]. The occurrence of A-type granites and basic magmatism both indicate that the South China Block entered a local extensional environment in the late Indosinian period. Thus, the diabase in the Yanlinsi mining area should have formed in an extensional tectonic setting after intracontinental collision orogeny in the late Indosinian period, which is consistent with the aforementioned petrogeochemical analysis results.

5.5. Geological Significance

The Guanzhuang gold ore concentration area in Liuyang-Liling, northeastern Hunan, is located in the southern part of the eastern segment of the Jiangnan Uplift metallogenic belt. Within this area, there are a series of small to medium-sized gold deposits (occurrences), including Yanlinsi, Tuanshanbei, Zhengchong, Hongyuan, Lishupo, and Qingcao. On the basis of the types of ore-hosting structures, the gold veins in the area are mainly divided into two groups: northeast-trending and northwest-trending. Northeast-trending veins: The ore-hosting structures are (sub-)bedding ductile shear zones and (interlayer) faults. In some local areas, they may be superimposed by later structures, resulting in a northwest strike (e.g., Jinhong gold deposit). The mineralization is mainly of the types of (banded) quartz veins and altered rocks, like the representative deposits including Yanlinsi and Tuanshanbei. Northwest-trending veins: They are mainly controlled by northwest-trending faults (including ductile shear zones) or fractures, cutting through the northeast-trending veins (ore bodies) or being restricted by northeast-trending ore-hosting faults and developing in their hanging walls. The mineralization type is mainly structural fracture zone type [31,109], with typical deposits including Zhengchong and Qingcao. In recent years, some scholars have studied the metallogenic age of gold deposits in the area. Peng Bo and Liu Xiang [38] obtained an Rb-Sr isochron age of 236 ± 14 Ma for the Qingcao gold deposit, which belongs to the Indosinian period. Han Fengbin et al. [11] obtained an Rb-Sr isochron age of 222.4 ± 9.4 Ma for the Tuanshanbei gold deposit, also belonging to the Indosinian period. Huang Cheng et al. [29] obtained the ESR age of quartz from the Yanlinsi gold deposit as 214.2 Ma (for northeast-trending ore veins) and 177.4–155.0 Ma (for northwest-trending ore veins), suggesting that the deposit formed during the Indosinian and Yanshanian periods. Zhang et al. [36] obtained the sericite Rb-Sr age of 216.9 ± 7.8 Ma for the Yanlinsi gold deposit, as well as the rutile U-Pb age of 415.2 ± 8.3 Ma and sericite Rb-Sr age of 417.6 ± 12.9 Ma for the Tuanshanbei gold deposit. Due to the limitations of various isotopic dating methods, there have been significant controversies regarding the mineralization age of gold deposits in the region, which has hampered an improved understanding on the gold mineralization regularity in the area. As mentioned earlier, the Yanlinsi gold deposit is a representative quartz-vein type gold deposit in the Liuyang-Liling Guanzhuang area. The gold ore veins occur in the Huangxudong Formation of the Lengjiaxi Group, with most orebodies striking 35°–55°, dipping northwest, and having a dip angle of 30°–60°. In this study, the northwest-trending diabase dikes discovered in the Yanlinsi mine were found to cut the northeast-trending banded gold-bearing quartz veins (gold orebodies) (Figure 3), and their zircon U-Pb age is 219.5 ± 1.7 Ma, indicating that the formation of the Yanlinsi gold deposit is no later than 220 Ma. Meanwhile, the authors obtained the LA-ICP-MS U-Pb age of hydrothermal zircon from quartz in the main mineralization stage of the Yanlinsi gold deposit as 421.9 ± 1.5 Ma (MSWD = 8.4, n = 10) (Figure 8), indicating that the main mineralization stage of the deposit formed during the Caledonian period, which is consistent with the mineralization age constrained by the cross-cutting relationship with diabase.

6. Conclusions

(1) The LA-ICP-MS U-Pb zircon age of diabase in the Yanlinsi gold mining area is 219.5 ± 1.7 Ma, indicating that it formed in the late Indosinian period.

(2) Overall, the diabase in the Yanlinsi gold mining area is characterized by low SiO₂, high MgO, and high Mg# values, with trace and rare earth element distribution patterns generally similar to those of ocean island basalt (OIB).

(3) The whole-rock geochemistry characteristics of the diabase in the Yanlinsi gold mining area indicate that its source area is spinel-garnet lherzolite in the enriched mantle (EM II), with a partial melting degree of 10%–15%. It has weak assimilation and contamination with continental crust materials, and weak fractional crystallization of olivine, clinopyroxene, and apatite occurred during magmatic evolution.

(4) A comprehensive analysis of the trace element tectonic discrimination and zircon U-Pb age of the Yanlinsi diabase shows that it formed in an extensional tectonic setting after the intracontinental collision orogeny of the South China Plate in the late Indosinian period.

(5) The LA-ICP-MS U-Pb zircon age of hydrothermal quartz veins from the main metallogenic stage in the Yanlinsi gold mining area is 421.9 ± 1.5 Ma. Combined with the cross-cutting relationship between basic dikes and gold veins (ore bodies), it is determined that the main metallogenic stage of the deposit formed in the Caledonian period.