In Situ Carbonate U-Pb Dating of Gold and Mercury Deposits in the Youjiang Metallogenic Province, SW China, and Implications for Multistage Mineralization

Abstract

The Youjiang metallogenic province (YMP) is a famous ore-concentrating area in South China, known for its substantial Carlin-type gold deposits, antimony deposits, and mercury deposits. Previous studies have yielded conflicting views regarding the ages of mineralization in this area, particularly regarding the occurrence of Yanshanian versus Indosinian ore-forming events during the Mesozoic era. To resolve these discrepancies, this study utilized in situ LA-ICP-MS U-Pb dating on carbonate minerals from the Lannigou Carlin-type Au deposit, the Lanmuchang Hg-(Tl) deposit, and the Sixiangchang Hg deposit to accurately determine their mineralization ages. Our results indicate that the three deposits formed at 137 ± 9 Ma, ~97 Ma, and 454 ± 21 Ma, respectively. By integrating previously reported geochronological data, we propose that the low-temperature Au-As-Sb-Hg-Tl deposits in the YMP were formed during two major periods, Late Triassic and Late Jurassic to Cretaceous, with the latter being more prevalent. Additionally, there was a Paleozoic hydrothermal mercury mineralization event at the northeastern edge of this region. These newly acquired data significantly enhance our understanding of multistage, low-temperature mineralization events in the YMP. Our study also demonstrates that in situ carbonate U-Pb dating is an excellent method for investigating the timing of low-temperature mineralization events.

1. Introduction

The Youjiang Basin is located at the tri-junction of the Guizhou, Yunnan, and Guangxi provinces in southwest China. This region represents a major Au-As-Sb-Hg-Tl metallogenic province within the South China low-temperature metallogenic domain, covering an extensive area of 90,000 km² [1]. It hosts a variety of Carlin-type Au deposits, in addition to low-temperature hydrothermal Sb, Hg, and As deposits [2,3,4]. Recent isotopic dating results have indicated that Carlin-type Au mineralization in the Youjiang metallogenic province (YMP) occurred during two distinct periods, estimated at 225 to 195 Ma and 145 to 125 Ma [5,6,7,8,9,10,11]. This has led to considerable debate regarding the age of low-temperature mineralization, with the primary point of contention being whether there were one or two mineralization events during the Mesozoic era. Additionally, along the margins of the Youjiang Basin, older ages ranging from 470 to 370 Ma [12,13,14] have been reported, indicating the potential for a Paleozoic low-temperature mineralization event.

In recent years, there has been rapid advancement in the technology for dating carbonate minerals in thin sections using in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb isotopic analysis. This technique has been successfully applied to the dating of calcite cements in fossils [15], calcite veins [16,17,18], and paleosols [19]. In situ carbonate U-Pb dating presents numerous advantages, including high spatial resolution, which enables the acquisition of data for multiple mineralization events at a microscopic level, as well as efficient and rapid data acquisition with minimal sample consumption. Carbonate minerals are prevalent as ore-forming gangue minerals in low-temperature deposits within the YMP [20]. Despite successful applications of carbonate U-Pb dating in this region, such as the Shuiyindong and the Yata Au deposits [10,11], there is a notable lack of reports on the use of this technique for other types of low-temperature deposits in the area. In this study, the formation sequence and trace element content of carbonate minerals were first determined to confirm their roles as indicators of the hydrothermal ore-forming process. Afterwards, in situ carbonate U-Pb dating was applied to the Lannigou Carlin-type Au deposit, the Lanmuchang Hg-(Tl) deposit, and the Sixiangchang Hg deposit within the YMP. The newly obtained data will advance our understanding of multistage, low-temperature mineralization events in the YMP.

2. Regional Geology

The YMP is situated along the southwestern margin of the Yangtze Block (Figure 1). This triangular region is delineated by the Mile–Shizong Fault to the northwest, the Shuicheng–Ziyun–Bama Fault to the northeast, and the Youjiang Fault to the south. This region is characterized by a Precambrian metamorphic basement overlaid by Devonian-to-Triassic marine sedimentary sequences. The carbonate platform and deep-water basin sedimentary system emerged during the late Early Devonian to the Early Triassic. Lithologically, the platform facies comprise carbonate sediments and breccias, while the basin facies consist of deep-water sediments, such as pelites and silicolites [20,21]. Although magmatic rocks are infrequently exposed, notable occurrences include Late Permian dolerite intrusions (259 Ma; [22]), quartz porphyry dykes (140~130 Ma; [23]), and lamprophyre dykes (88~85 Ma; [24]) in the southeastern and northern sectors of the region. Gravitational and magnetic data suggest that igneous intrusions may be present at depths ranging from 2 to 5 km [25]. This area is abundant in low-temperature, hydrothermal Au-Sb-Hg-As-Tl deposits, with a number of Carlin-type Au deposits, known as the “Dian–Qian–Gui Golden Triangle” [1].

3. Geology of Ore Deposits

3.1. Lannigou Au Deposit

The Lannigou (also called Jinfeng) Au deposit is situated within the terrestrial sediments of the carbonate platform in the central part of the YMP (Figure 1). The exposed strata in the ore district are dominated by Triassic sequences, with the ore-bearing strata mainly consisting of Middle Triassic Xumeng Formation sandstone and mudstone, Niluo Formation claystone interbedded with siltstone, and Bianyang Formation sandstone interbedded with mudstone. The gold orebody is approximately 1500 m in length and 1200 m in width and varies from 10 to 50 m in thickness. It is primarily hosted in fault zones (Figure 2A). The orebody exhibits a simple and regular shape with a relatively intact structure. Within or near fault zones, there are clusters or vein-like occurrences of calcite and thin layers of bitumen, with Au-bearing pyrite distributed in a fine-disseminated form. As one of the largest Carlin-type Au deposits in the region, the Lannigou deposit has an estimated gold resource of over 5.3 million ounces, with an average grade of 4.5 g/t Au [26]. Associated metallic minerals include pyrite, realgar, orpiment, and cinnabar, while gangue minerals comprise quartz, calcite, dolomite, ankerite, and bitumen [27]. The predominant alteration types are pyritization, illitization, dolomitization, and silicification, with the presence of milky quartz and calcite veins [28,29].

3.2. Lanmuchang Hg-(Tl) Deposit

The Lanmuchang Hg-(Tl) deposit lies adjacent to the Shuiyindong Carlin-type Au deposit within the same region, and their geological backgrounds exhibit remarkable similarities. The mineralized layers of the Lanmuchang Hg-(Tl) deposit extend up to 15 layers, primarily distributed in the Permian Longtan Formation and Changxing Formation, followed by the Triassic Yelang Formation (Figure 2B). These orebodies owe their formation and occurrence to a combination of structural and stratigraphic controls, characterized by distinctive layered and lens-shaped features that closely resemble the surrounding sediments [30,32]. Additionally, certain orebodies are associated with fault breccias and zones of alteration. Mineralization predominantly occurs within carbonaceous mudstones, heavily silicified limestones, and their interfacing lithologies. Specifically, independent thallium minerals, primarily composed of lorandite, are predominantly found within the carbonaceous mudstones and sandstones of the Longtan and Changxing Formations [27]. In addition to independent thallium minerals and cinnabar, ore minerals include pyrite enriched in thallium and arsenic. The deposit exhibits diverse alteration types, including silicification, pyritization, argillization, and baritization, with argillization dominated by kaolinization [33,34]. Drilling exploration data reveal that pyritization closely associated with Tl mineralization is well developed in strata ranging from the Longtan Formation to the Yelang Formation [32]. The mercury content in the orebodies typically ranges from 0.08% to 0.3%, with the highest concentration reaching 1.17% and the resource amounting to 4874 tons. Thallium grades generally fall between 0.01% and 0.02%, peaking at 0.035%, with resources exceeding 500 tons [33].

3.3. Sixiangchang Hg Deposit

The Sixiangchang Hg deposit is situated at the northeastern corner outside the YMP. Mercury serves as the primary economic element in this deposit, accompanied by gold as an auxiliary element, with an average gold grade of 7.19 g/t [31]. The ore-bearing stratum is the Upper Cambrian Yangjiawan Formation, which consists of muddy siltstone, thick-bedded detrital limestone interbedded with laminated muddy limestone, limestone, and dolostone (Figure 2C). The principal ore minerals include cinnabar, pyrite, native mercury, and stibnite, while the main gangue minerals are quartz, calcite, dolomite, and ankerite. The deposit exhibits various types of alteration, with silicification, pyritization, calcitization, and dolomitization being predominant [35]. Structural activity within the mining area is intense, characterized by complex and variable faults and folds. Notably, no igneous rock exposures have been observed in the mining area.

4. Sample Description

In the low-temperature deposits of the YMP, multiple stages of carbonate mineral crystallization are observed. The selection of carbonate minerals formed during the metallogenic stage is crucial for the precise determination of the mineralization age. Therefore, we prepared 100 μm thin sections of the samples and performed observations using polarized optical microscopy and cathodoluminescence microscopy. We identified calcite and dolomite, intimately associated with ore minerals in the thin sections, as suitable candidates for U-Pb dating. Most natural calcite and dolomite contain uranium concentrations below 100 ppb, which approaches the resolution limit of LA-ICP-MS technology. Consequently, we conducted preliminary trace element analyses on the calcite and dolomite within the thin sections [33] and selected samples with elevated uranium content (>0.1 ppm) for U-Pb dating.

The dating sample from the Lannigou Au deposit is a sandstone specimen (lng-8) with a notable gold grade of 13.65 g/t. The sample is characterized by the presence of multiple calcite veins (Figure 3A). Notably, the fine calcite veins within the sandstone matrix harbor several realgar particles. Furthermore, the sandstone is interspersed with copious amounts of Au-bearing pyrite (Figure 3B), indicating that these calcite veins crystallized contemporaneously with the ore-forming process. Under cathodoluminescence observation, a close association between quartz and calcite is evident, with the latter exhibiting bright luminescence, in contrast to the subdued appearance of quartz (Figure 3C). Additionally, the fine calcite veins are observed to encapsulate numerous quartz and pyrite particles of varying sizes (Figure 3D).

In the Lanmuchang Hg-(Tl) deposit, dating analyses were conducted on two limestone specimens, each featuring calcite veins, which were extracted from drill hole ZK901 (Figure 4). The first sample, lmc-2, originated from the Permian Longtan Formation at a depth of 67 m. The second, lmc-29, was obtained from a depth of 310 m within a structural alteration zone, also referred to as the SBT. The SBT is a product of regional detachment structures and hydrothermal activities occurring at the interface of the Permian Maokou and Longtan Formations. Lithologically, it includes highly silicified limestone, brecciated silicified limestone, silicified cataclasite, and brecciated claystone, constituting the principal ore-bearing stratum in the YMP [36]. An examination of the microscopic images of these samples shows that calcite is present alongside significant quantities of pyrite and arsenopyrite, suggesting that the calcite deposition coincided with the metallogenic stage (Figure 4C,F).

Regarding the Sixiangchang Hg deposit, the selected sample for dating is a metasedimentary sandstone specimen (sxc-29), derived from the Cambrian strata within the mining district. This sample is rich in dolomite and cinnabar particles (Figure 5A). Upon fresh fracturing, the specimen discloses numerous tiny droplets of silver-white liquid mercury. Cathodoluminescence analysis demonstrates that the dolomite exhibits more intense luminescence compared to cinnabar, and visible cinnabar grows on the dolomite matrix (Figure 5B). This observation is consistent with the microstructures observed under both transmitted light and reflected light (Figure 5C,D), implying a near-simultaneous formation of dolomite and cinnabar. Consequently, the age of the dolomite provides a reliable estimate for the timing of mercury mineralization.

5. Analytical Methods

In situ carbonate LA-ICP-MS U-Pb dating was performed at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, using an Agilent 7700× plasma mass spectrometer equipped with a GeoLasPro 193 nm ArF excimer laser system (Agilent Technologies, Santa Clara, CA, USA). The parameters for ablation were as follows: spot size, 120 μm; repetition rate, 10 Hz; and energy density, 8 J/cm². Prior to experimentation, it is essential to optimize the performance of ICP-MS using standard reference material Nist 610 in order to achieve optimal sensitivity and ionization efficiency (U/Th ≈ 1) while also minimizing oxide yield (ThO/Th < 0.3%) and background values. The signal acquisition of a single sample consists of approximately 20 s of baseline signal, followed by a sampling period lasting 30 s, and concluding with an additional 40-s decay phase until the signal returns to the background level. Nist 610 was used to correct the ²⁰⁷Pb/²⁰⁶Pb ratios, and a matrix-matched carbonate WC-1 was used for the fractionation correction of the ²³⁸U/²⁰⁶Pb ratios [37]. The data were processed using ICPMSDataCal [38] to select sample and blank signals, correct for instrument sensitivity drift, and calculate U-Pb isotope ratios. The Tera–Wasserburg diagram was employed in ISOPLOT [39] to derive the lower intersection point age (2σ). The experimental results for WC-1 are listed in Supplementary Table S1. After eliminating any abnormal values, an age of 254 ± 16 Ma was calculated for WC-1 (Supplementary Figure S1), which corresponds to the established standard value [37].

6. Results

The concentrations and isotopic ratios of U, Th, and Pb in the analyzed samples and WC-1 are detailed in Supplementary Table S1. The majority of the analyzed spots exhibit U concentrations above 0.1 ppm, alongside consistent U/Pb ratios and minimal average standard weighted deviations. The ²⁰⁷Pb/²⁰⁶Pb and ²³⁸U/²⁰⁶Pb values demonstrate a pronounced linear correlation, affirming the reliability of the experimental results. Dating based on the U-Pb isotopic ratios resulted in an age of 137 ± 9 Ma (MSWD = 3.0) for calcite within the Lannigou Au deposit (Figure 6A). Two calcite samples from the Lanmuchang Hg-(Tl) deposit yielded ages of 96 ± 2 Ma (MSWD = 3.71) and 97 ± 9 Ma (MSWD = 1.09), respectively (Figure 6B,C). Dolomite from the Sixiangchang Hg deposit was dated at 454 ± 21 Ma (MSWD = 1.17) (Figure 6D). These ages correspond to the formation times of the respective carbonate minerals within each deposit.

7. Discussion

7.1. Interpretation of the Carbonate U-Pb Ages

The U-Pb age of calcite from the Lannigou Au deposit obtained in this study is 137 ± 9 Ma, which is younger than the 148 ± 6 Ma hydrothermal apatite U-Th-Pb age previously reported for this deposit [40]. It is widely accepted that the initial mineralization stage of Carlin-type Au deposits in the YMP is characterized by decarbonation, followed by significant calcite deposition during the late mineralization stage [3,25]. The calcite veins analyzed herein intersect with sandstone ore rich in Au-bearing pyrite (Figure 3B), indicating that the formation of the calcite veins postdates the precipitation of Au-bearing pyrite. Consequently, the calcite veins are indicative of the late mineralization stage. Thus, the U-Pb age of the calcite indicates a minimum age constraint for the mineralization of the Lannigou Au deposit, suggesting that the large-scale precipitation of Au in the deposit did not occur later than 135 Ma. Combining this with the hydrothermal apatite dating results [40], it is inferred that the Lannigou Au deposit formed during the Yanshanian mineralization period.

In the Lanmuchang Hg-(Tl) deposit, two samples yield calcite U-Pb ages of 96 ± 2 Ma and 97 ± 9 Ma. Comparing with the neighboring Shuiyindong Au deposit, where calcite U-Pb ages range from 137 to 139 Ma [10], it is evident that the formation of the Lanmuchang Hg-(Tl) deposit occurred at a later stage. Despite their close geographic proximity and similarities in ore-hosting strata and low-temperature ore-forming element association (Au-As-Sb-Hg-Tl), a comparison of the trace element contents in hydrothermal pyrite using LA-ICP-MS reveals distinct differences. The pyrite from the Lanmuchang Hg-(Tl) deposit exhibits enrichment in Tl and W [33], while the pyrite from the Shuiyindong Au deposit is enriched in Au, As, and Cu [41], suggesting variations in ore-forming fluids. The current research proposes that weakly acidic, sulfur-rich, and iron-poor hydrothermal fluids enriched in Au-As-Sb-Hg-Tl-(Cu) interacted with carbonate wall rocks, leading to the decarbonation and subsequent precitation of trace elements (including Au and As) in pyrite [42]. However, the Shuiyindong Au deposit, controlled by the Huaijiapu anticline, lacks well-developed fault structures. Consequently, limited infiltration of rapidly cooling meteoric water and restricted ore-hosting space allowed for residual ore-forming fluids enriched in Tl, W, and Hg to continue migrating along various conduits. Recent studies on the Allchar Au-As-Sb-Tl deposit in North Macedonia, similar to Carlin-type Au deposits, also reveal zonation features from proximal to distal hydrothermal centers, with successive Au, Sb, and As-Tl mineralization [43,44]. Therefore, we posit that both the Lanmuchang and Shuiyindong deposits originate from the same hydrothermal ore-forming system. The mineralization of the Lanmuchang Hg-(Tl) deposit represents the residual enrichment of Hg and Tl fluids after Carlin-type hydrothermal evolution, reacting favorably with geological structures in the surrounding rock. This explanation provides a reasonable account for the delayed occurrence of calcite crystallization in the Lanmuchang Hg-(Tl) deposit compared to the Shuiyindong Au deposit.

The coexistence of dolomite and cinnabar in the samples from the Sixiangchang Hg deposit suggests their simultaneous formation (Figure 4) based on mineral assemblage relationships. In the Sixiangchang mining area, dolomite is widely distributed and serves as the primary gangue mineral associated with hydrothermal activity in this deposit, making its age representative of ore formation. Previous studies have demonstrated that dolomite exhibits higher ablation efficiencies during the laser ablation process compared to calcite and WC-1, and these disparities in ablation rates may result in significant age discrepancies of 4~8% for dolomite [45]. Therefore, despite obtaining a U-Pb age of 454 ± 21 Ma for dolomite in the Sixiangchang Hg deposit, it is important to acknowledge the potential presence of certain systematic errors. Nevertheless, even when accounting for the error range within 50 Ma, the mineralization age of the Sixiangchang Hg deposit remains notably older than the ages of low-temperature deposits in the YMP. Located on the periphery of the YMP, the Sixiangchang deposit is hosted by Cambrian metasedimentary sandstone strata and exhibits distinct differences in terms of ore-forming geological background compared to Carlin-type Au deposits, vein-type Sb deposits, and the Lanmuchang Hg-(Tl) deposit within the YMP. However, the mineralization age of the Sixiangchang deposit aligns with a series of Paleozoic low-temperature Hg-Pb-Zn-Ba-F deposits located in western Hunan province and eastern Guizhou province, southeast of the YMP [14], supporting the formation of the Sixiangchang Hg deposit during the Late Caledonian mineralization period.

7.2. Implications for Dynamic Setting and Multistage Mineralization

Previous studies on the metallogenic chronology of Carlin-type Au deposits in the YMP have demonstrated that there are two distinct stages of Carlin-type Au mineralization [5,6,7,8,9,10,11,40,46]. A comparative analysis of dating results [46] from dissolution-based and in situ data methods reveals that the isotopic ages obtained through single-mineral dissolution samples can be divided into two separate groups, representing Indosinian and Yanshanian mineralization events (Figure 7A). Su et al. further classified Au deposits into two independent metallogenic systems, with those formed during the Yanshanian period located to the north and those formed during the Indosinian period situated to the south along the Youjiang fault [29]. However, based on in situ dating results of hydrothermal accessory minerals, it is evident that Yanshanian mineralization occurred throughout most parts of the region, with only a few deposits (such as the Zhesang deposit) forming during the Indosinian period (Figure 7B). The in situ U-Pb dating of calcite from the Lannigou Au deposit also confirms its formation during the Yanshanian period in this study. Furthermore, previous findings indicate that the mineralization age of Sb deposits in the YMP is attributed to the Yanshanian period, with the Qinglong Sb deposit (the largest Sb deposit in the YMP) was dated to 148~125 Ma [47,48,49]. In addition, regarding the timing of Hg mineralization, the in situ U-Pb dating of calcite from the Lanmuchang Hg-(Tl) deposit, when compared with the adjacent Shuiyindong Au deposit, reveals that the Hg deposits formed later than the Au deposits but still within the Yanshanian mineralization period.

Recent studies have indicated that the W-Sn polymetallic deposits in the eastern part of the South China Block formed during three distinct periods: 230~200 Ma, 160~130 Ma, and 100~80 Ma [50,51]. The occurrence of Carlin-type Au deposits, Sb deposits, and Hg deposits in the YMP aligns closely with the extensive W-Sn polymetallic mineralization during the Indosinian and Yanshanian orogenies, suggesting a potential shared tectonic background. Notably, the Late Triassic mineralization event primarily resulted from intracontinental orogenesis following the collision between the South China Block and multiple surrounding blocks [51,52]. In contrast, the Late Jurassic to Cretaceous mineralization event was mainly controlled by processes such as the subduction–retreat of the ancient Pacific plate [53], the formation of slab windows through the tearing of subducted slabs [50], lithospheric delamination-induced asthenospheric upwelling, and subsequent large-scale lithospheric extension [51,54].

The dating results for the Sixiangchang Hg deposit indicate that it arose from a Paleozoic mineralization event, distinctly different from the mineralization timing observed in the Lanmuchang Hg-(Tl) deposit within the YMP triangle area. Sulfur and mercury isotope compositions of cinnabar from both deposits [33,35] show a significant difference between the Sixiangchang and Lanmuchang deposits (Figure 8), suggesting that they are not products of the same mineralization event.

Considering the timing of mineralization on a larger regional scale, the age of the Sixiangchang Hg deposit is consistent with a series of Hg-Pb-Zn-Ba-F deposits in the western Hunan and eastern Guizhou areas, located in the northeast of the YMP [14]. This similarity indicates that their geodynamic settings may be similar and could have been related to tectono-thermal events during the Late Caledonian orogeny [55]. The Jiangnan Orogenic Belt in South China experienced extensive early Paleozoic granitic magmatism, resulting in a series of gneissic and massive granite intrusions [56]. These granites exhibit zircon U-Pb ages ranging from 480 to 398 Ma, peaking at 456 to 419 Ma, reflecting a transition from an intracontinental orogeny to subsequent crustal extension [57,58]. The movement along NE-SW-trending Caledonian reverse faults led to the inversion of stratigraphic units from the Neoproterozoic sediments to the Early Ordovician sedimentary rocks. The local extensional tectonism during the early Paleozoic orogeny was conducive to the migration of basinal brines along the margins of the Jiangnan Orogenic Belt, leading to the formation of Hg, Pb-Zn, and Ba-F orebodies. The Sixiangchang Hg deposit may represent a distal response to this mineralization event.

8. Conclusions

• The U-Pb age of hydrothermal calcite from the Lannigou Au deposit is 137 ± 9 Ma, indicating that this mineralization event was associated with Yanshanian tectonic movement.
• At the Lanmuchang Hg-(Tl) deposit, two samples yielded hydrothermal calcite U-Pb ages of 96 ± 2 Ma and 97 ± 9 Ma, demonstrating that this mineralization event was also related to Yanshanian tectonic movement. However, its ore-forming time was later than that of Carlin-type Au deposits in the YMP.
• The U-Pb age of hydrothermal dolomite from the Sixiangchang Hg deposit is 454 ± 21 Ma, indicating that this mineralization event was associated with the early Paleozoic Caledonian orogeny.
• Integrating previous chronological data, the low-temperature Au-As-Sb-Hg-Tl deposits in the YMP primarily underwent two phases of mineralization: one in the Late Triassic and the other spanning from the Late Jurassic to the Cretaceous, with the latter being predominant. Additionally, there exists a Paleozoic hydrothermal Hg mineralization event at the northeastern edge of the region.