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Article

Fluid Mixing, Organic Matter, and the Origin of Permian Carbonate-Hosted Pb-Zn Deposits in SW China: New Insights from the Fuli Deposit

by
Xingyu Liang
1,
Bo Li
1,*,
Xinyue Zhang
2,
Huaikun Qin
3 and
Gao Li
3
1
Faculty of Land Resources Engineering, Kunming University of Science and Technology, Kunming 650093, China
2
Kunming Geological Prospecting Institute, China Metallurgical Geological Bureau, Kunming 650024, China
3
Fuli Pb-Zn Mine Co., Ltd., Fuyuan 655504, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(3), 312; https://doi.org/10.3390/min14030312
Submission received: 1 December 2023 / Revised: 21 February 2024 / Accepted: 13 March 2024 / Published: 15 March 2024
(This article belongs to the Section Mineral Deposits)
Browse Figures
Versions Notes

Abstract

:
The Fuli Pb-Zn deposit is situated at the southwestern margin of the Yangtze Block in Yunnan. The deposit, which is hosted in the Permian Yangxin Formation dolomite, is a recent discovery. Our study indicates a significant presence of fluid inclusions in sphalerite from the Fuli Pb-Zn deposit, with fewer inclusions observed in dolomite and calcite. We conducted comprehensive petrographic, microthermometric, and laser Raman analyses on the inclusions within sphalerite and dolomite. Additionally, six samples of dolomite from the mineralization period were selected for H-O isotope analysis. The results of our study reveal the characteristics of ore-forming fluids and explore the mechanisms of ore formation. The study results indicate that the Fuli Pb-Zn deposit is a low- to medium-temperature hydrothermal deposit with fluid inclusions mainly composed of two-phase gas-liquid inclusions. Salinity and homogenization temperature analyses affirmed that there are two types of fluids present, one with low salinity and the other with high salinity. Laser Raman tests demonstrated the presence of CH4, N2, and CO2 in the gas phase of the inclusions. Microthermometric analyses indicated that the sphalerite ore-forming fluids consist of a multicomponent system of Mg2+ and Ca2+ enriched fluids. The features of the ore-forming fluids in the Fuli deposit arise from a blend of high-temperature, low-salinity metamorphic fluids and low-temperature, high-salinity basin brines. The basin brines in question have the potential to emanate from the Youjiang Basin. The formation of the ore is ascribed to the TSR and the mixing of fluids. The combination of these processes provided the requisite materials (SO42−), catalysts (Mg2+), and reducing agents (organic matter, CH4, and H2S) required to initiate the thermochemical sulfate reduction (TSR). As the TSR proceeded, it caused a shift in the pH of the fluids, thus promoting the precipitation of metal sulfides.

1. Introduction

The southwestern edge of the Yangtze block is a renowned Pb-Zn mineralization zone in China (Figure 1), covering an area of around 170,000 square kilometers. Over 400 Pb-Zn deposits have been discovered within the triangular area made up of southwestern Sichuan, northeastern Yunnan, and northwestern Guizhou [1,2,3,4], making it an essential low-temperature hydrothermal metallogenic region of southern China. The Fuli Pb-Zn deposit is situated in the southeastern part of the southwestern margin of the Yangtze block (Figure 1). The deposit’s ore bodies are layered and appear in the fractured zone of the Permian Yangxin Formation limestone. Prior reports and studies on the deposit focused mainly on its geological features, rock geochemistry, and isotope geochemistry. However, this investigation discovered significant hydrothermal mineralization in the deposit, which is also abundant in organic matter. The investigation of fluid inclusions in minerals is a vital aspect of research on deposits. Various metallic deposits exhibit distinct fluid characteristics that aid in ore formation [5,6]. Nonetheless, there is a scarcity of research on the fluid inclusions of this deposit. The correlation between organic-rich fluids and mineralization has always been a topic of great interest among scholars in the field [7,8,9]. Numerous research studies have demonstrated the close association between metallic deposits and organic-rich fluids, often found in basins or basin margins. These deposits are genetically linked with organic matter, which plays a vital role in activating, extracting, migrating, and depositing metallic elements throughout the various stages of mineralization. Therefore, this study aimed to conduct systematic petrographic and microthermometric analyses on fluid inclusions, with a focus on the Fuli deposit. Laser Raman testing and isotopic analyses of H and O were utilized to summarize the characteristics of the ore-forming fluids in the Pb-Zn deposits and investigate the correlations between the organic matter, mineralization, and ore-forming fluids in the study region.

2. Geological Setting of the Region and Deposit

2.1. Geological Setting of the Region

The southwestern margin of the Yangtze block borders the southwestern Sanjiang orogenic belt (SB), northwestern Songpan–Ganzi orogenic belt (SGB), northern Qinling–Dabie orogenic belt (QDB), and southern Huaxia block (CB). It is limited by the northeast-trending Mile–Shizong–Anshun (MSA) fault, northwest-trending Kangding–Ziyun–Yadu (KYZ) fault, and north-directed Anning River–Luzhijiang (AL) fault. This region has experienced numerous tectonic events, magmatic occurrences, and metamorphic deformations. The stratigraphy primarily comprises metamorphic basement and sedimentary cover layers. The sedimentary cover layers mainly comprise Paleozoic, Mesozoic, and Cenozoic sedimentary rocks, predominantly marine carbonate rocks. The junction of the sedimentary blanket layers and the underlying basement exhibits an angular unconformity, whereas the connections between distinct stratigraphic units display either conformable or parallel unconformities.

2.2. Deposit Geology Setting

2.2.1. Tectonic Features

The Fuli Pb-Zn deposit is located between the Qujing Fault and the Mile–Shizong Fault (Figure 1). The basement in the study area is represented by the Middle to Late Proterozoic Kunyang Group, which is exposed only in the southwestern part of the study area [13]. The sedimentary record is missing from the Sinian to the Silurian period above it. The Middle Devonian Haikou Formation directly contacts the Kunyang Group with an angular unconformity. The Paleozoic tectonic events mainly had an east–west orientation. After several movements, including the Jinning Movement, Caledonian Movement, and Hercynian Movement, the regional tectonics changed to a north–south orientation as the main trend, followed by a northeast trend. After the tectonic movements of the Yanshan and Himalayan orogeny periods, the study area developed a north–south structural pattern as the dominant trend, with a secondary northeast trend, as well as east–west and north–west structures [14]. The sedimentary rocks in the study area include Proterozoic and Paleozoic carbonate and clastic rocks with a total thickness of more than 10,000 m, indicating long-term basin sedimentation. The development of the stratigraphy from Cambrian to Permian is relatively extensive but intermittent. During the Late Hercynian Movement, large-scale magmatism occurred in the region, consisting only of eruptions of basalt lava. The structural framework is characterized by parallel and nearly equidistant north–south trending faults. The northeast-trending structures are also well defined, large in scale, and have a long history. The north–south and northeast-trending structures play an important controlling role in the formation, distribution, and enrichment of zinc-lead mineralization in the area [12].
The northeast-trending Mile–Shizong Fault is the major fault in the area. Several gentle anticlines, synclines, and steep reverse fault structures are distributed throughout the study area. The distribution of regional stratigraphy, secondary structures, and mineralization is controlled by faults and folds. Together, the faults and folds control the distribution of regional stratigraphy, as well as the distribution of secondary structures and mineralization in the area [14].

2.2.2. Stratigraphy and Lithology

The study area’s exposed stratigraphy consists of the Lower Permian Liangshan Formation, Middle Permian Yangxin Formation, Upper Permian Emeishan Formation, Lower Triassic Yongningzhen Formation, and Feixianguan Formation (Figure 2A). The ore-bearing formation in the deposit is the Permian Yangxin Formation (P2y), which is mainly made up of interbedded dolomite and limestone and can be divided into three units (Figure 2B). The lower unit (P2y1) comprises light gray limestone. The middle unit (P2y2) contains interbedded light gray limestone and dolomite, with siliceous dolomite being localized and serving as the primary ore-bearing unit at the deposit. The upper unit (P2y3) is made up of gray or dark gray thinly bedded dolomite, interbedded with gray or off-white dense limestone. The Emeishan Formation is an assemblage of dense massive basalt and tuffaceous basaltic breccia. The Yongningzhen Formation and Feixianguan Formation in the Lower Triassic consist of alternating layers of mudstone, limestone, and shale.

2.2.3. Ore Bodies

The Fuli Pb-Zn deposit comprises two ore bodies situated within dolomitic limestone (Figure 2b), having a Pb+Zn grade of over 25%. The largest ore body is around 320 m long, 210 m wide, and has a thickness of roughly 1.2 m. It dips at a 15° angle towards the southeast and is thicker in the middle while being thinner at both ends. The ore body at the shallow part of the Fuli Pb-Zn deposit is relatively thin and has a lower content of metals. As depth increases, the ore body gradually becomes thicker, and the content of Zn and Pb increases.

2.2.4. Petrography and Mineralogy

Based on field observations, mineral assemblages, and preliminary lithological studies, the formation of the deposit can be classified into three stages: early mineralization, main mineralization, and post mineralization. Dolomite is the primary gangue mineral in the deposit, and all were formed by hydrothermal activity (Figure 3 and Figure 4).
(1)
Early-Mineralization Stage: The primary characteristic of the ore in the early-mineralization stage includes the presence of two different types of hydrothermal dolomitization: D1 and D2. D1 dolomite is gray-black with irregular, fine-grained particles that are typically cemented by mineralizing gray dolomite. Conversely, D2 dolomite is gray-white with coarse-grained particles, featuring a white core and a dark gray-to-gray-white edge (Figure 3a). Blocky or impregnated sphalerite and galena have been discovered as replacements within the dolomite (Figure 3b).
(2)
Main Mineralization Stage: This stage entails minerals forming within interlayer fractured zones, following the stratigraphic trend. Primarily, the main minerals are vein-like, impregnated, or blocky sphalerite, whereas galena appears in cubic crystals. Overall, sulfides display granular structures (Figure 3e,f) and replacement structures (Figure 3c,d), some of which form blocky or conglomerate-like structures, with impregnated structures present as well (Figure 3b). At this stage, sphalerite can be divided into two mineralization phases: Phase I sphalerite appears predominantly reddish brown to dark-brown in transmitted light, with mainly euhedral-to-subhedral crystals, while Phase II sphalerite appears predominantly brownish yellow to white in transmitted light, with mainly subhedral-to-anhedral crystals. The HD3 dolomite is a grey-white, meso-coarse-grained semi-autotype dolomite with a grain size ranging from 0.1 to 0.8 mm; it is distributed along the edges of sphalerite and galena and directly contacts or replaces wall rock.
(3)
Post-Mineralization Stage: The HD4 dolomite often appears as saddle-like, euhedral coarse crystals that are white-milky white in color. It fills open spaces or cement fissures and is mostly in contact with the grey dolomite, and it was formed later than the grey dolomite. Coarse, vein-like calcite (C5) forms during the post-mineralization stage and envelops the sphalerite and galena from the main mineralization stage (Figure 3b,f).

3. Materials and Methods

3.1. Fluid Inclusion Microthermometry

All fluid inclusion microthermometric measurements were conducted at Nanjing University’s State Key Laboratory for Mineral Deposits Research in China using the Linkam THMS600 heating–cooling stage installed on an Olympus BX-51 microscope (Tokyo, Japan). The Linkam THMS600 stage was calibrated by measuring the melting points of pure CO2 (−56.6 °C) and pure water (0 °C), as well as the critical point of pure water (374.1 °C). The cooling and heating stages were calibrated over a temperature range of −195 °C to 600 °C, with uncertainties of around ±0.2 °C and ±2 °C, respectively [15].
The salinities of fluid inclusions lacking or having relatively low amounts of CO2 were determined by the final melting of ice. All inclusions were cooled to below −120 °C to ensure unobstructed observations of the phase transition temperatures. Density calculations were conducted using the MS Excel 2010 (Redmond, WA, USA) spreadsheet “HOKIEFLINCS_H2O-NACL” [16] and Excel VBA macros to calculate the compositions of the H2O-NaCl-CaCl2 fluid inclusions [17]. FIA (Fluid Inclusion Assembly) averages with 1σ uncertainties were used to report the temperatures, salinities, and densities.

3.2. H-O Isotope Analysis

Six samples of dolomite were selected for H-O isotope analysis at the analytical laboratory of the China National Nuclear Corporation Beijing Research Institute. The H-O isotope composition was analyzed using a Finnigan MAT253-EM mass spectrometer. The oxygen and hydrogen isotopic compositions were determined using the traditional BrF5 method and obtained from the FIs in dolomite, respectively. The details of this process were described by the authors of [18]. The H-O isotope values were standardized against the Vienna Standard Mean Ocean Water (V-SMOW). The δ18O analytical precision exceeded 0.2‰, and the δD analytical precision was better than 2‰. δ18Ofluid was calculated using the formula in [19].

3.3. Laser Raman

Laser Raman spectroscopy was conducted at the State Key Laboratory for Mineral Deposits Research, Nanjing University, China, using a Renishaw RM-2000 laser Raman spectrometer. The experiment utilized a 514.5 nm argon ion laser as a light source. The laser was equipped with a slit width of 25 μm and operated with an exposure time of 30 s. The scanning range extended from 1000 to 4100 cm−1.

4. Results

4.1. Characteristics of Fluid Inclusions

Fluid inclusions were prominently observed in sphalerite samples and more rarely in dolomite and calcite (Figure 5a–f). The primary fluid inclusions showcased lengths ranging from 4 to 25 µm and manifested in various shapes, like negative crystal, elliptical, circular, rectangular, and irregular shapes. The secondary fluid inclusions, on the other hand, were smaller in size (1–6 µm) and had mainly circular or irregular shapes, as shown in Figure 5b. Furthermore, primary FIAs were discovered throughout the sphalerite’s growth zonation (Figure 5a,b). The inclusions in dolomite and calcite, ranging in size from 2 to 8 µm, are in negative crystal, elliptical, and rectangular shapes. The inclusions’ phase types at room temperature (20 °C) were used to divide the fluid inclusions into three groups:
  • Liquid-rich fluid inclusions: This group included inclusions consisting of a liquid and vapor (L+V-type) (Figure 5a–h): These inclusions included more than 50% liquid phase and contained both gas and liquid phases. The fluid inclusions displayed diverse shapes, such as negative crystal, rectangular, and irregular shapes, with occasional elliptical and circular forms. Approximately 95% of all inclusions in this group were measured within 1–25 μm. The gas phase content of these inclusions ranged between 5% and 40%. Upon heating, the inclusions homogenized into a liquid phase, primarily consisting of H2O.
  • Pure gas inclusions (V type) (Figure 5f): The inclusions observed were clusters of single-phase vapor, ranging in size from 1 to 5 μm, and commonly coexisted with the L+V-type inclusions.
  • Daughter mineral-bearing fluid inclusions: These rare multiphase inclusions contained gas, liquid, and solid phases and showed near-elliptical or irregular shapes (Figure 5c). Their sizes ranged from approximately 5 to 20 μm. The daughter minerals frequently exhibit a cubic crystal morphology, which may suggest the existence of NaCl minerals and indicate the presence of high-salinity elements in the ore-forming fluids of the Fuli deposit.

4.2. Microthermometry Results of Fluid Inclusions

By using fluid inclusion microthermometry, temperature and salinity results for the fluid inclusions of the Fuli Pb-Zn deposit were obtained (Table 1), The complete dataset is included in Table S1. The predominant type of fluid inclusions in this deposit is L+V-type inclusions. The temperature measurements indicate that the eutectic temperatures of the fluid inclusions in sphalerite range from −59.1 to −35.5 °C, suggesting the presence of a certain amount of Ca2+ and Mg2+ in the fluids during this stage [20]. The salinity and homogenization temperatures of the fluid inclusions have a wide range of values, ranging from 182 to 237 °C (mean value: 203 °C) for dolomite, from 117 to 197 °C (mean value: 162 °C) for Sp-1, and from 112 to 171 °C (mean value: 142 °C) for Sp-2. The salinity of the fluid inclusions in dolomite ranges from 4.3 to 9.3 wt.% NaCl Eqv, while for Sp-1, it ranges from 14.3 to 22.8 wt.% NaCl Eqv, and for Sp-2, it ranges from 2.3 to 9.2 wt.% NaCl Eqv. Based on these observations, it can be concluded that the Fuli Pb-Zn deposit is a typical meso-low-temperature hydrothermal deposit that possibly contains the presence of two types of fluids characterized by low and high salinity (Figure 6).

4.3. Laser Raman Characteristics

The fluid inclusions found in the Fuli Pb-Zn deposit primarily consist of two-phase inclusions (L+V-type inclusions). Laser Raman spectroscopy findings have, however, revealed the diverse compositions of the fluid inclusions and subsequently led to the classification of two subtypes, namely Type I and Type II. Type I comprises two-phase gas–liquid inclusions characterized by definite bubble boundaries and an overall transparent colorless appearance (Figure 5a,d). Laser Raman analysis indicates that these inclusions are primarily composed of H2O, and this type of inclusion is mainly distributed in dolomite. In contrast, Type II two-phase gas–liquid inclusions exhibit black bubbles with rough black margins (Figure 5e). In addition to H2O, the inclusions also contain CH4, CO2, and N2 (Figure 7); most of the inclusions in sphalerite are Type II inclusions.

4.4. H-O Isotopic Analysis

The hydrogen and oxygen isotope compositions obtained from dolomite are listed in Table 2. The δDV-SMOW values range from −49.8 to −96.7‰, with an average value of −65.3‰. The δ18O values range from +12.1 to +20.8‰, with an average value of +18.9‰.

5. Discussion

5.1. Fluid Characteristics

Different fluid systems possess distinct eutectic temperatures. When a fluid keeps a constant composition, the eutectic temperature does not alter with varied concentrations. Thus, it can work as a criterion for identifying the fluid system [20]. The eutectic temperatures for the NaCl-H2O, CaCl2-H2O, and MgCl2-H2O binary systems are −21.2 °C, −49.8 °C, and −33.6 °C, respectively. The eutectic temperatures for the NaCl-CaCl2-H2O, NaCl-MgCl2-H2O, and CaCl2-MgCl2-H2O ternary systems are −52.0 °C, −35.0 °C, and −55.0 °C, respectively [20]. The fluid inclusions discovered in early-stage minerals like calcite indicate a NaCl-H2O system, as evidenced by their eutectic temperatures of −21.8 °C and −22.6 °C. Furthermore, the fluid inclusions in sphalerite have an eutectic temperature range of −55.4 °C to −35.8 °C, which suggests a NaCl-CaCl2-H2O fluid system.
During the initial stage of mineralization, carbonate mineral precipitation leads to a decrease in ion concentration due to the formation of Mg2+ and Ca2+, resulting in the dominance of the NaCl-H2O fluid composition. In the primary mineralization stage, water/rock reactions keep carbonate rocks in a cycle of dissolution and reprecipitation [12], with the main mineralization fluids containing ions like Mg2+, Ca2+, and SO42−. This discovery aligns with previous studies on the liquid-phase compositions of neighboring deposits including Daliangzi, Tianbaoshan, Jinding [21,22]. Our laser Raman analysis of the gas phase identified CH4, CO2, and N2, which are often linked to reducing conditions, implying that the mineralizing fluids have reducing properties.

5.2. Fluid Mixing

The Fuli deposit consists of three groups of fluid inclusions with varying homogenization temperatures and salinities, exhibiting a broad range of temperature (112–237 °C) and salinity (2.3–22.8 wt.% NaCl eqv.) variations (Figure 8). These variations are often attributed to fluid mixing during the ore deposition processes [23,24,25,26]. The inclusions in the HD3 dolomite demonstrate the properties of metamorphic fluids in terms of both temperature and salinity. The inclusions found in Sp-1 are within the range of basin brines and display a decrease in temperature but a considerable increase in salinity, thus indicating the mixing of new external basin brines with metamorphic fluids.
Employing H and O isotopes to trace the origin of ore-forming fluids is a well-known and efficient method in ore deposit research, as fluids deriving from various sources have hydrogen and oxygen isotopes with unique compositions [27,28,29,30,31,32]. Most of the isotopic data from dolomite fluids in the δD versus δ18Ofluid diagram (Figure 9) lie within the range of metamorphic water, with one data point falling within the range of organic water. A trend towards atmospheric precipitation or basin brines is generally observed in the isotopic data of dolomite fluids, suggesting the mixing of these two fluids.
Although the fluids in the initial mineralization stage are characterized by metamorphic fluids (Figure 8), their high temperatures can also be associated with mantle fluids (or magmatic fluids), particularly given the presence of Emeishan basalt coverage in the study area (Figure 2) [26,33].
Minerals 14 00312 g009
Figure 9. Diagram of δD vs. δ18Ofluid for dolomite from Fuli deposit and some Pb-Zn deposits in SYGMB (modified from Kesleret al. [28], Taylor [34], and Wang et al. [31], respectively). Data from Wang et al. [26], Zhou et al. [35], Zhao et al. [36], Hai et al. [37], Ding et al. [38], Wen et al. [39], Wu et al. [40], Yuan et al. [41], Yuan et al. [42], and Yang et al. [43], The complete dataset is included in Table S2.
Figure 9. Diagram of δD vs. δ18Ofluid for dolomite from Fuli deposit and some Pb-Zn deposits in SYGMB (modified from Kesleret al. [28], Taylor [34], and Wang et al. [31], respectively). Data from Wang et al. [26], Zhou et al. [35], Zhao et al. [36], Hai et al. [37], Ding et al. [38], Wen et al. [39], Wu et al. [40], Yuan et al. [41], Yuan et al. [42], and Yang et al. [43], The complete dataset is included in Table S2.
Minerals 14 00312 g009

5.3. Organic Matter and TSR Interaction

Organic matter is crucial to the formation and evolution of petroleum and natural gas, as well as to the complexation, transportation, unloading, and accumulation of certain metallic elements [44]. Certain geological conditions can lead to the creation of a biotic–organic matter ore-forming fluid system through geological processes. During the formation of the Fuli Pb-Zn deposit, organic fluids were present as significant components of ore-forming fluids during the hydrothermal mineralization stage. The study of the Pb-Zn deposits near the Fuli deposit also indicate the possibility of organic fluid activity in the study area, as evidenced by the discovery of organic fluids containing asphalt and methane-bearing inclusions (unpublished data).
The association between Pb-Zn minerals and organic matter has been a central focus of research. Fluid inclusions, crucial constituents of ore-forming fluids, provide crucial proof of organic matter’s involvement in the mineralization process. Methane inside fluid inclusions can be produced in multiple ways, such as microbial sulfate reduction (BSR) and thermochemical sulfate reduction (TSR). BSR reactions take place at temperatures below 80 °C [45]. Based on the temperature results of the fluid inclusions in this deposit, it can be concluded that BSR reactions are not the primary source of H2S. Hence, the hypothesis that BSR reactions are the main reason behind CH4 can be discarded.
If the TSR occurs and provides reduced sulfur for mineralization, sulfur isotope fractionation can be observed. The difference in δ34S between sulfate minerals and sulfide minerals (Δδ34S) is significant, and temperature also influences this difference. For instance, during the TSR, fractionation takes place between seawater sulfate and reduced sulfur. At temperatures of 100 °C, Δδ34S is 20‰; at 150 °C, Δδ34S is 15‰; and at 200 °C, Δδ34S is 10‰ [12,41,45,46]. During sulfide precipitation, there is no noticeable sulfur isotope fractionation between metal sulfides and reduced sulfur. In metallic deposits, the δ34S values of metal sulfides depend on the reduced S from the TSR and are typically 10% to 20% lower than the δ34S values of seawater sulfate in the surrounding rocks [2,42,47,48,49]. At the Fuli Pb-Zn deposit, sulfide minerals including sphalerite, galena, and minor pyrite are present, but no sulfate minerals have been detected. Consequently, the sulfides mean δ34S value is suitable for representing the δ34SΣS of the hydrothermal fluids. Fluid inclusion temperature results demonstrate that mineralization temperatures range from 117 °C to 237 °C. If the TSR occurs in the Fuli deposit, the sulfide δ34S values in the ore should be 10‰ to 20‰ below the seawater sulfate δ34S values in the surrounding rocks, resulting in a Δδ34S of 10‰ to 20‰. Sulfur isotope testing was carried out on sphalerite samples from the Fuli deposit, revealing δ34S values ranging from +15.57‰ to +16.91‰, with an average of +16.26‰ [50]. The deposit’s sedimentary strata house numerous evaporite gypsum formations, with marine sulfates δ34S values being concentrated between +20.0‰ and +38.7‰, averaging at +29‰ [51,52,53,54]. Compared to the δ34S values of sulfides found in Fuli ores, a difference in the Δδ34S value of approximately 13‰ implies the presence of the TSR in the Fuli Pb-Zn deposit. This discovery suggests that the TSR played a role in acting as a source of reduced sulfur for mineralization.

5.4. Ore-Forming Mechanism

There is a significant correlation between orogenic belts and sedimentary basins as they depend on each other in terms of spatial distribution and material conversion [55]. The Youjiang Basin, situated on the western edge of the Yangtze Block, forms a composite “basin-mountain system” with the surrounding orogenic belts. This system plays a crucial role in controlling the metallogenic and mineralization processes within the basin [10,11,33]. The Fuli deposit is located on the southwestern margin of the Youjiang Basin, which underwent significant Indosinian orogeny from the Late Triassic to the Early Jurassic. This resulted from the intense compression of the southwestern part of the Yangtze Block [10,56,57,58,59]. Due to uplift of the Youjiang Foreland Basin on the Youjiang fold belt, basin brines originating from the Youjiang Basin migrated northwestward towards the southwestern part of the Yangtze Block due to the effects of gravity and/or tectonic forces [4,60]. During the migration of brines from the basin, elements responsible for ore-forming, such as Pb and Zn, were extracted consistently from both cover or basement rocks. This led to the creation of ore fluids presenting higher concentrations of Zn2+, Pb2+, Mg2+, Ca2+, and Cl− with medium temperature (117–237 °C) and high salinity (14–22.8 wt.% NaCl Eqv). Subsequently, methane (CH4) from coal-bearing strata widely distributed in the eastern and southeastern portions of the Fuli mining region acted as a reducing agent. This conversion process transformed the SO42− obtained from the evaporite gypsum layers in the geological formations of the mining area into H2S through the thermochemical sulfate reduction (TSR). The mixing of low-temperature, high-salinity basin brines with high-temperature, low-salinity metamorphic fluids in the Middle Permian Yangxin Formation resulted in mineralization.
The sulfur isotope signatures indicate that the sulfur present in the Fuli deposit originates from seawater sulfate [3,12,50]. The reduction of SO42− in seawater can produce the reduced sulfur required for the precipitation of metal sulfides through a series of processes [61]. The high-salinity ore fluids at medium temperature (117–237 °C) provide the SO42− required for the TSR. Mg2+ serves as a catalyst, accelerating the initiation of the TSR [62,63]. The ore-forming fluids present in the Fuli deposit contain Mg2+ (refer to Section 5.1), reaching the threshold temperature of 100–140 °C [45], which is required for the TSR to take place. The addition of CH4 and H2S from the coal-bearing strata in the mining region promotes the smooth progression of the TSR [45,64].
When the thermochemical sulfate reduction (TSR) takes place, organic matter serves as the reducing agent that transforms SO42− into reduced sulfur [8,45,65,66,67]. The following equations, Equations (1)–(3) [45,68], represent the possible reactions:
4CH2 + 3SO42− + 2H2O → 4CO2 + 3H2S + 6OH−
Large amounts of H2S are produced in the process [69]:
CH4 + SO42− → CO32- + H2S + 2H2O
Organic matter + SO42− → Hydrocarbons + bitumen + CO32− + H2S + CO2 + H2O
Mg2+ + Ca2+ + 2CO2 + 2H2O → CaMg(CO3)2↓ + 4H+
Zn2+ + H2S → ZnS↓ + 2H+
2Fe3+ + 3H2S → FeS2↓ + FeS↓ + 6H+
Pb2+ + H2S → PbS↓ + 2H+
Reactions (4)–(7) occur with a large amount of H+, which changes the pH of the ore-forming fluid, leading to a decrease in the solubility of metal ions, which further promotes the precipitation of minerals.

6. Conclusions

(1)
The formation process of the Fuli Pb-Zn deposit can be divided into three stages: (I) the precipitation of hydrothermal dolomite, (II) the precipitation of sphalerite + galena, and (III) the precipitation of calcite. The first stage occurs at a medium temperature (182–237 °C) and in fluids with low salinity (4.3–9.3 wt.% NaCl eqv.). In the early part of the second stage, low to medium temperatures (117–197 °C) and high-salinity (14.3–22.8 wt.% NaCl eqv.) fluids were observed, while the late part of the second stage is characterized by low to medium temperatures (112–171 °C) and low salinity (2.3–9.2 wt.% NaCl eqv.).
(2)
Petrographic studies show that the inclusions are two-phase gas–liquid inclusions with complex compositions. Laser Raman spectroscopy showed the presence of components other than H2O, such as CH4, N2, and CO2. The initial melting temperature of the inclusions indicates that the fluids in the hydrothermal calcite stage consist mainly of a NaCl-H2O fluid system, while the sphalerite + galena stage contains multiple ions, such as Mg2+ and Ca2+, forming a multicomponent fluid system.
(3)
The Fuli deposit was formed by the mixture of low-temperature, medium–high salinity basin brine and high-temperature, low-salinity metamorphic fluids, and mineralization occurred in the Middle Permian Yangxin Formation. At an appropriate temperature range (120–260 °C), the sulfate (SO42−) carried by the ore-bearing fluid and the catalyst (Mg2+) jointly promote the initiation of TSR, leading to the precipitation of metal sulfides. The change in fluid pH during the reduction of sulfate by organic matter and CH4 further facilitated the precipitation of metal sulfides. The presence of asphalt, CH4, N2, CO2, and sulfur isotope fractionation characteristics in the ore-forming fluids constitute evidence of the occurrence of the TSR.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min14030312/s1, Table S1: Results of fluid inclusion analysis from the Fuli Pb-Zn deposit; Table S2: Comparison of the H-O istopic data for carbonate-hosted Pb-Zn deposits in the SYGMB.

Author Contributions

Formal analysis, X.Z.; Investigation, H.Q. and G.L.; Writing—original draft, X.L.; Writing—review & editing, B.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

All data are contained within the article and Supplementary Materials.

Conflicts of Interest

Author Huaikun Qin and Gao Li was employed by the company Fuli Pb-Zn Mine Co., Ltd., Author Xinyue Zhang was employed by the company Kunming Geological Prospecting Institute, China Metallurgical Geological Bureau. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. (a) Simplified geological framework of SW China showing the SYG province. (b) A sketched geological map of the SYG metallogenic province showing the distribution of Pb-Zn deposits and Emeishan basalt and the location of the Fuli deposit (modified from Han et al. [10,11], Zhou et al. [12], and Ren et al. [3]).
Figure 1. (a) Simplified geological framework of SW China showing the SYG province. (b) A sketched geological map of the SYG metallogenic province showing the distribution of Pb-Zn deposits and Emeishan basalt and the location of the Fuli deposit (modified from Han et al. [10,11], Zhou et al. [12], and Ren et al. [3]).
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Figure 2. (a) Geological sketch map of the Fuli Pb−Zn deposit (modified from Zhou et al. and Ren et al. [2,3]). (b) A-B cross-section of the Fuli deposit.
Figure 2. (a) Geological sketch map of the Fuli Pb−Zn deposit (modified from Zhou et al. and Ren et al. [2,3]). (b) A-B cross-section of the Fuli deposit.
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Figure 3. Photographs of the field and specimen studied and its textures from the Fuli Pb-Zn deposit. (a) Massive Sp are cemented by Cal veins. (b) Sphalerite veins filled in dolomite fissures. (c) Galena metasomatic pyrite and tetrahedrite. (d) Metasomatic relict pyrite and xeno-morphic sphalerite in an anhedral sphalerite fracture. (e,f) Sphalerite and dolomite with a granular structure.
Figure 3. Photographs of the field and specimen studied and its textures from the Fuli Pb-Zn deposit. (a) Massive Sp are cemented by Cal veins. (b) Sphalerite veins filled in dolomite fissures. (c) Galena metasomatic pyrite and tetrahedrite. (d) Metasomatic relict pyrite and xeno-morphic sphalerite in an anhedral sphalerite fracture. (e,f) Sphalerite and dolomite with a granular structure.
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Figure 4. Mineral paragenesis in the Fuli Pb-Zn deposit.
Figure 4. Mineral paragenesis in the Fuli Pb-Zn deposit.
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Figure 5. Characteristics of the fluid inclusions in the sphalerite and dolomite of the Fuli Pb-Zn deposits. (a,b) L + V-type fluid inclusions in Sp-1. (c) S + L + V-type fluid inclusion in Sp-1. (d,e) L+V-type fluid inclusions in Sp-2. (f) L+V- and V-type fluid inclusions in Sp-1. (g,h) L+V-type fluid inclusions in dolomite. Abbreviations: L = liquid; V = vapor; S = solid.
Figure 5. Characteristics of the fluid inclusions in the sphalerite and dolomite of the Fuli Pb-Zn deposits. (a,b) L + V-type fluid inclusions in Sp-1. (c) S + L + V-type fluid inclusion in Sp-1. (d,e) L+V-type fluid inclusions in Sp-2. (f) L+V- and V-type fluid inclusions in Sp-1. (g,h) L+V-type fluid inclusions in dolomite. Abbreviations: L = liquid; V = vapor; S = solid.
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Figure 6. Stacked histograms of fluid inclusion homogenization temperatures (a) and salinity (b) in the Fuli deposit.
Figure 6. Stacked histograms of fluid inclusion homogenization temperatures (a) and salinity (b) in the Fuli deposit.
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Figure 7. Laser Raman spectra for the fluid inclusions in the sphalerite and dolomite of the Fuli Pb-Zn deposits. (a) L + V-type fluid inclusions in dolomite; (b,c) L + V-type fluid inclusions in Sp-2; and (d) L + V-type fluid inclusions in Sp-1.
Figure 7. Laser Raman spectra for the fluid inclusions in the sphalerite and dolomite of the Fuli Pb-Zn deposits. (a) L + V-type fluid inclusions in dolomite; (b,c) L + V-type fluid inclusions in Sp-2; and (d) L + V-type fluid inclusions in Sp-1.
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Figure 8. Diagram of homogenization temperatures and salinities of fluid inclusions in different stages of Fuli deposit. I HD3 dolomite, II Sphalerite with high salinity, III Sphalerite with low salinity.
Figure 8. Diagram of homogenization temperatures and salinities of fluid inclusions in different stages of Fuli deposit. I HD3 dolomite, II Sphalerite with high salinity, III Sphalerite with low salinity.
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Table 1. Summary of microthermometric results of fluid inclusions in dolomite and sphalerite in the Fuli deposit.
Table 1. Summary of microthermometric results of fluid inclusions in dolomite and sphalerite in the Fuli deposit.
MineralStageNTm (°C)Mean ValueTh (°C)Mean ValueSalinity (wt.% NaCl Eqv.)Mean Value
dolomiteHD321−2.6 to −6.1−4.5182 to 2372034.3 to 9.37.1
sphaleriteI31−10.3 to −20.6−15.3117 to 19716214.3 to 22.818.8
sphaleriteII17−1.3 to −6−3.4112 to 1711422.2 to 9.25.5
Table 2. Compositions of hydrogen and oxygen isotopes in dolomite from the Fuli Pb-Zn deposit.
Table 2. Compositions of hydrogen and oxygen isotopes in dolomite from the Fuli Pb-Zn deposit.
Sample No.HostδDV-SMOW/‰δ18OV-SMOW/‰T/ °Cδ18Ofluid/‰
FL8-1dolomite−62.120.320210.4
FL8-2dolomite−52.420.820210.9
FL14dolomite−52.819.62029.7
FL15dolomite−60.319.22029.3
FL10dolomite−96.712.12022.2
FL13dolomite−49.818.92029
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Liang, X.; Li, B.; Zhang, X.; Qin, H.; Li, G. Fluid Mixing, Organic Matter, and the Origin of Permian Carbonate-Hosted Pb-Zn Deposits in SW China: New Insights from the Fuli Deposit. Minerals 2024, 14, 312. https://doi.org/10.3390/min14030312

AMA Style

Liang X, Li B, Zhang X, Qin H, Li G. Fluid Mixing, Organic Matter, and the Origin of Permian Carbonate-Hosted Pb-Zn Deposits in SW China: New Insights from the Fuli Deposit. Minerals. 2024; 14(3):312. https://doi.org/10.3390/min14030312

Chicago/Turabian Style

Liang, Xingyu, Bo Li, Xinyue Zhang, Huaikun Qin, and Gao Li. 2024. "Fluid Mixing, Organic Matter, and the Origin of Permian Carbonate-Hosted Pb-Zn Deposits in SW China: New Insights from the Fuli Deposit" Minerals 14, no. 3: 312. https://doi.org/10.3390/min14030312

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