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Introduction

Mississippi Valley-type (MVT) deposits are among the most significant sedimentary lead-zinc deposits globally, contributing approximately 25 % of the world’s lead-zinc resources (Sośnicka and Lüders, 2019

Sośnicka, M., Lüders, V. (2019) Super-deep, TSR-controlled Phanerozoic MVT type Zn-Pb deposits hosted by Zechstein-2 gas reservoir carbonate (Ca2), Lower Saxony Basin, Germany. Chemical Geology 508, 62–77. https://doi.org/10.1016/j.chemgeo.2018.04.025

). Three metallogenic models are proposed for MVT deposits (Sverjensky, 1981

Sverjensky, D.A. (1981) The Origin of a Mississippi Valley-type Deposit in the Viburnum Trend, Southeast Missouri. Economic Geology 76, 1848–1872. https://doi.org/10.2113/gsecongeo.76.7.1848

; Leach et al., 2005

Leach, D.L., Sangster, D.F., Kelley, K.D., Large, R.R., Garven, G., Allen, C.R., Gutzmer, J., Walters, S. (2005) Sediment-hosted lead-zinc deposit: a global perspective. In: Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., Richards, J.P. (Eds.) Economic Geology 100th Anniversary Volume, Society of Economic Geologists Inc., USA, 561–607

): the fluid mixing model, the sulfate reduction model and the reduced sulfur model. The former two models are frequently reported to explain the precipitation of galena and sphalerite (Barrie et al., 2009

Barrie, C.D., Boyce, A.J., Boyle, A.P., Williams, P.J., Blake, K., Wilkinson, J.J., Lowther, M., McDermott, P., Prior, D.J. (2009) On the growth of colloform textures: A case study of sphalerite from the galmoy ore body, Ireland. Journal of The Geological Society 166, 563–582. https://doi.org/10.1144/0016-76492008-080

; Hurtig et al., 2018

Hurtig, N.C., Hanley, J.J., Gysi, A.P. (2018) The role of hydrocarbons in ore formation at the Pillara Mississippi Valley-type Zn-Pb deposit, Canning Basin, Western Australia. Ore Geology Reviews 102, 875–893. https://doi.org/10.1016/j.oregeorev.2018.09.012

; Szmihelsky et al., 2021

Szmihelsky, M., Steele-MacInnis, M., Bain, W.M., Falck, H., Adair, R., Campbell, B., Dufrane, S.A., Went, A., Corlett, H.J. (2021) Mixing of brine with oil triggered sphalerite deposition at Pine Point, Northwest Territories, Canada. Geology 49, 488–492. https://doi.org/10.1130/G48259.1

), while the reduced sulfur model is rarely considered (Sverjensky, 1986

Sverjensky, D.A. (1986) Genesis of Mississippi Valley-type lead-zinc deposits. Annual Review of Earth and Planetary Sciences 14, 177–199. https://doi.org/10.1146/annurev.ea.14.050186.001141

; Leach et al., 2005

Leach, D.L., Sangster, D.F., Kelley, K.D., Large, R.R., Garven, G., Allen, C.R., Gutzmer, J., Walters, S. (2005) Sediment-hosted lead-zinc deposit: a global perspective. In: Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., Richards, J.P. (Eds.) Economic Geology 100th Anniversary Volume, Society of Economic Geologists Inc., USA, 561–607

). The fluid mixing model is particularly effective for rapid formation of MVT deposits (e.g., Barrie et al., 2009

Barrie, C.D., Boyce, A.J., Boyle, A.P., Williams, P.J., Blake, K., Wilkinson, J.J., Lowther, M., McDermott, P., Prior, D.J. (2009) On the growth of colloform textures: A case study of sphalerite from the galmoy ore body, Ireland. Journal of The Geological Society 166, 563–582. https://doi.org/10.1144/0016-76492008-080

); but requires substantial amounts of H₂S. Therefore, the formation of H₂S-rich reservoirs is crucial for controlling the efficiency and scale of MVT mineralisation. Previous studies indicate that at elevated temperatures (e.g., T > 120 °C), thermochemical sulfate reduction (TSR) is the predominant mechanism for producing H₂S within the deeper sections of sedimentary basins (Worden et al., 1995

Worden, R.H., Smalley, P.C., Oxtoby, N.H. (1995) Gas souring by thermochemical sulfate reduction at 140 °C. AAPG Bulletin 79, 854–863.

; Machel et al., 2001

Machel, H.G. (2001) Bacterial and thermochemical sulfate reduction in diagenetic settings—old and new insights. Sedimentary Geology 140, 143–175. https://doi.org/10.1016/S0037-0738(00)00176-7

; Sośnicka and Lüders, 2020

Sośnicka, M., Lüders, V. (2020) Fluid inclusion evidence for low-temperature thermochemical sulfate reduction (TSR) of dry coal gas in Upper Permian carbonate reservoirs (Zechstein, Ca2) in the North German Basin. Chemical Geology 534, 119453. https://doi.org/10.1016/j.chemgeo.2019.119453

). TSR thus provides a vital sulfur source for metal sulfide precipitation (Powell and MacQueen, 1984

Powell, T.G., MacQueen, R.W. (1984) Precipitation of sulfide ores and organic matter: sulfate reactions at Pine Point, Canada. Science 224, 63–66. https://doi.org/10.1126/science.224.4644.63

; Sośnicka and Lüders, 2019

Sośnicka, M., Lüders, V. (2019) Super-deep, TSR-controlled Phanerozoic MVT type Zn-Pb deposits hosted by Zechstein-2 gas reservoir carbonate (Ca2), Lower Saxony Basin, Germany. Chemical Geology 508, 62–77. https://doi.org/10.1016/j.chemgeo.2018.04.025

). However, direct evidence supporting the formation and preservation of H₂S-rich reservoirs is still lacking.

Fluid inclusions (FIs) are important geological records of ore-forming fluids, yet H₂S-rich fluids have been rarely identified in FIs from MVT deposits, with most inclusions composed of CO₂ and CH₄ in the gas phase (Jones and Kesler, 1992

Jones, H.D., Kesler, S.E. (1992) Fluid inclusion gas chemistry in east Tennessee Mississippi Valley-type districts: Evidence for immiscibility and implications for depositional mechanisms. Geochimica et Cosmochimica Acta 56, 137–154. https://doi.org/10.1016/0016-7037(92)90122-Y

; Conliffe et al., 2013

Conliffe, J., Wilton, D.H.C., Blamey, N.J.F., Archibald, S.M. (2013) Paleoproterozoic Mississippi Valley Type Pb–Zn mineralization in the Ramah Group, Northern Labrador: Stable isotope, fluid inclusion and quantitative fluid inclusion gas analyses. Chemical Geology 362, 211–223. https://doi.org/10.1016/j.chemgeo.2013.08.032

). Only a few researchers have reported high density H₂S fluids in sedimentary basins (Worden et al., 1995

Worden, R.H., Smalley, P.C., Oxtoby, N.H. (1995) Gas souring by thermochemical sulfate reduction at 140 °C. AAPG Bulletin 79, 854–863.

; Sośnicka and Lüders, 2020

Sośnicka, M., Lüders, V. (2020) Fluid inclusion evidence for low-temperature thermochemical sulfate reduction (TSR) of dry coal gas in Upper Permian carbonate reservoirs (Zechstein, Ca2) in the North German Basin. Chemical Geology 534, 119453. https://doi.org/10.1016/j.chemgeo.2019.119453

), while systematic experimental validation remains scarce. These fluids might actually be elemental sulfur or dissolved H₂S with a Raman peak at 2590 cm⁻¹ (Frezzotti et al., 2012

Frezzotti, M.L., Tecce, F., Casagli, A. (2012) Raman spectroscopy for fluid inclusion analysis. Journal of Geochemical Exploration 112, 1–20. https://doi.org/10.1016/j.gexplo.2011.09.009

; Schmidt and Seward, 2017

Schmidt, C., Seward, T.M. (2017) Raman spectroscopic quantification of sulfur species in aqueous fluids: Ratios of relative molar scattering factors of raman bands of H₂S, HS⁻, SO₂, HSO₄⁻, SO₄²⁻, S₂O₃²⁻, S₃⁻ and H₂O at ambient conditions and information on changes with pressure and temperature. Chemical Geology 467, 64–75. https://doi.org/10.1016/j.chemgeo.2017.07.022

; Hu et al., 2022

Hu, M. Chou, I-M., Wang, R., Shang, L., Chen, C. (2022) High solubility of gold in H₂S-H₂O ± NaCl fluids at 100–200 MPa and 600–800 °C: A synthetic fluid inclusion study. Geochimica et Cosmochimica Acta 330. https://doi.org/10.1016/j.gca.2022.03.006

). Clearly, more reliable geological evidence is needed to test whether high density H₂S fluids can be formed in sedimentary basins and to evaluate the validity of the proposed mineralising mechanisms for MVT deposits.

The Ediacaran Doushantuo (Z₁d) and Dengying (Z₂dn) formations are important host rocks for MVT deposits in the Yangtze Block, South China (Zhang et al., 2022

Zhang, W.-D., Li, B., Lu, A-H., Elatikpo, S.M., Chen, H., Chen, X.-D. (2022) Origin of the Early Cambrian Huayuan carbonate-hosted Zn-Pb orefield, South China: Constraints from sulfide trace elements and sulfur isotopes. Ore Geology Reviews 148, 105044. https://doi.org/10.1016/j.oregeorev.2022.105044

). Here we report the discovery of H₂S-rich FIs from the Z₁d dolomite at the Caojunba section, South China. This study includes: (1) thorough petrographic observations, in situ Raman spectroscopy and microthermometry, of high purity (∼100 %) and high density (0.68–0.76 g/cm³) H₂S FIs are discovered for the first time, (2) the temperature-pressure (T-P) conditions of entrapment (134–158 °C; 50–106 MPa) of the high density H₂S fluids are reconstructed, affirming an origin from TSR, and (3) by incorporating previous carbon-oxygen isotope analyses, we propose that calcite precipitation consumes CO₂, which in turn purifies H₂S within the fluid. In summary, this study presents a new mechanism for the formation of high density, high purity H₂S fluids in sedimentary basins, and provides new insights for the precipitation of metal sulfides in MVT deposits.

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Samples and Methods

The Ediacaran strata cover the entire Yangtze Block in South China, including Z₁d and Z₂dn. The nodular pore-filling quartz-calcite cements are common in the upper Z₁d and the lower to middle Z₂dn dolomites hosting MVT deposits (e.g., Yangjiaping, Zhongling, Caojunba, Nanbeizhen, and Zouma sections; Shi et al., 2022

Shi, H., Ouyang, Q., Zhou, C., Xiao, S., Chen, Z., Guan, C. (2022) Integrated study of the Doushantuo Formation in northwestern Hunan Province: Implications for Ediacaran chemostratigraphy and biostratigraphy in South China. Precambrian Research 377, 106699. https://doi.org/10.1016/j.precamres.2022.106699

; Cui et al., 2022

Cui, H., Kaufman, A.J., Xiao, S., Zhou, C., Zhu, M., Cao, M., Loyd, S., Crockford, P., Liu, X.-M., Goderis, S., Wang, W., Guan, C. (2022) Dynamic interplay of biogeochemical C, S and Ba cycles in response to the Shuram oxygenation event. Journal of The Geological Society 179, jgs2021-81. https://doi.org/10.1144/jgs2021-081

). The studied dolomite and nodular cement samples are collected from the Z₁d strata at the Caojunba section (Northwestern Hunan Province, South China), where quartz and calcite nodules commonly occur along the bedding (Shi et al., 2022

Shi, H., Ouyang, Q., Zhou, C., Xiao, S., Chen, Z., Guan, C. (2022) Integrated study of the Doushantuo Formation in northwestern Hunan Province: Implications for Ediacaran chemostratigraphy and biostratigraphy in South China. Precambrian Research 377, 106699. https://doi.org/10.1016/j.precamres.2022.106699

; Fig. S-1).

The collected rock samples are prepared as doubly polished thin sections for petrographic observations. Compositions of selected FIs are characterised by using Raman spectrometer. Microthermometry and thermodynamic modelling are conducted to reconstruct the FIs trapping T-P conditions. Detailed descriptions of geological settings and methods can be found in the Supplementary Information.

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Results

Petrographic characteristics. There is no obvious cross-cutting relationship between the quartz and calcite cements within the nodules (Fig. 1a,b), indicating that they likely formed contemporaneously. Due to the small size of the FIs in the calcite cement (<5 μm), this study primarily focuses on the petrology, microthermometry, and Raman spectroscopy of quartz cement-hosted FIs. Only a limited number of two phase aqueous inclusions in the calcite cement are subjected to microthermometric analyses. At room temperature, the FIs are classified into three types based on the phase states of their contents: gas-liquid-solid three phase inclusions, gas-liquid two phase inclusions, and solid-liquid two phase inclusions (Fig. 1c,d). Both three phase and solid-liquid two phase inclusions exhibit irregular shapes, with long axes ranging from 5 to 15 μm. The gas-to-liquid ratio in the three phase inclusions varies from 1 to 5 %. These FIs typically occur in clusters or linear arrays. Gas-liquid two phase inclusions also show irregular shapes, with a gas-to-liquid ratio of ∼2 % and a long axis of 3 to 12 μm. They often occur with three phase inclusions (Fig. 1c).

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Raman spectroscopic characteristics. In three phase inclusions, the typical Raman peaks for the gas and liquid phases appear at ∼2610 cm⁻¹ and ∼2580 cm⁻¹, respectively, with water peaks being nearly undetectable (Fig. 2a). The solid phase exhibits major Raman peaks at ∼221 cm⁻¹, ∼473 cm⁻¹, ∼823 cm⁻¹, ∼873 cm⁻¹, ∼2500 cm⁻¹, and ∼2567 cm⁻¹ (Fig. 2a). In the solid-liquid two phase inclusions, the solid and liquid phases exhibit similar Raman peaks with those observed in the three phase inclusions (Fig. 2a). Gas-liquid two phase inclusions can be further divided into two types based on the liquid phase composition, although they exhibit no obvious difference under the microscope. One type (i) has a liquid phase composed of aqueous solution (3000–3700 cm⁻¹) and dissolved H₂S (∼2590 cm⁻¹) and the gas phase exhibits Raman spectral peaks at ∼2610 cm⁻¹ (Fig. 2b). H₂S is not identified in the other type (ii) of gas-liquid two phase inclusions (Fig. 2b) which are simple two phase aqueous inclusions (Fig. 2b).

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Microthermometric data. Figure 3a-i presents the phase transition of three phase inclusions during heating and cooling. Upon heating, the volume of the gas bubble gradually decreases, achieving gas-liquid homogenisation (partial homogenisation; L + V > L) at 33 to 57 °C (Fig. 3a-c, Table S-1). As heating continues, the solid components progressively dissolve, resulting in L + V + S > L which produces total homogenisation at temperatures between 113 and 156 °C (Fig. 3d,e, Table S-1). After cooling to room temperature (Fig. 3f), the FIs are cooled to −120 °C, at which point the fluids are completely frozen (Fig. 3g). The freezing of the liquid into a solid can deform the original gas bubble, forming multiple bubbles within the inclusion. Figure 3g shows the formation of a large bubble in the lower right corner and a smaller bubble in the upper left corner of the inclusion. Upon heating, the solid phase begins melting into the liquid phase. For instance, Figure 3h shows that the small bubble in the upper left corner disappears at −86 °C, whereas the larger bubble in the lower right corner remains an irregular shape, indicating incomplete melting of the solid phase. The point at which the bubble transforms from an irregular to a regular spherical shape marks the complete melting of the solid phase (Fig. 3i). This temperature is recorded as the triple point of the fluid within the inclusion (−85.1 to −83.2 °C; Table S-1).

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For the two phase aqueous inclusions, accurately determining the ice melting temperature is challenging due to their small size. As a result, only the Aq + V > Aq homogenisation temperatures are obtained (134–158 °C; Table S-1). By fitting the O-H stretching band of water in the aqueous phase (Wang et al., 2013

Wang, X., Hu, W., Chou, I.-M. (2013) Raman spectroscopic characterization on the OH stretching bands in NaCl–Na₂CO₃–Na₂SO₄–CO₂–H₂O systems: Implications for the measurement of chloride concentrations in fluid inclusions. Journal of Geochemical Exploration 132, 111–119. https://doi.org/10.1016/j.gexplo.2013.06.006

), the chloride concentration is obtained, which ranges from 4.3 to 12.8 wt. % NaCl equiv. Additionally, homogenisation temperatures for a few two phase aqueous inclusions in calcite cement are measured between 138 and 160 °C (Table S-1), which closely align with the homogenisation temperatures of the aqueous inclusions hosted in quartz cement.

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Discussion

Identification of high density H₂S inclusions. The 2580 cm⁻¹ and 2610 cm⁻¹ peaks in the three phase inclusions are typical ν₁(H₂S) vibrational peaks, corresponding to H₂S in the liquid and gas phases, respectively (Burke, 2001

Burke, E.A.J. (2001) Raman microspectrometry of fluid inclusions. Lithos 55, 139–158. https://doi.org/10.1016/S0024-4937(00)00043-8

; Frezzotti et al., 2012

Frezzotti, M.L., Tecce, F., Casagli, A. (2012) Raman spectroscopy for fluid inclusion analysis. Journal of Geochemical Exploration 112, 1–20. https://doi.org/10.1016/j.gexplo.2011.09.009

; Yu et al., 2021

Yu, Y., Hu, W., Chou, I.-M., Jiang, L., Wan, Y., Li, Y., Xin, Y., Wang, X. (2021) Species of Sulfur in Sour Gas Reservoir: Insights from In Situ Raman Spectroscopy of S–H₂S–CH₄–H₂O System and Its Subsystems from 20 to 250°C. Geofluids 2021, 1–14. https://doi.org/10.1155/2021/6658711

; Hu et al., 2022

Hu, M. Chou, I-M., Wang, R., Shang, L., Chen, C. (2022) High solubility of gold in H₂S-H₂O ± NaCl fluids at 100–200 MPa and 600–800 °C: A synthetic fluid inclusion study. Geochimica et Cosmochimica Acta 330. https://doi.org/10.1016/j.gca.2022.03.006

). The three phase inclusions contain minimal to no water, with solid phase Raman peaks at ∼221 cm⁻¹, ∼473 cm⁻¹, ∼823 cm⁻¹, and ∼873 cm⁻¹. These peaks can be attributed to elemental sulfur (i.e. S; Yu et al., 2021

Yu, Y., Hu, W., Chou, I.-M., Jiang, L., Wan, Y., Li, Y., Xin, Y., Wang, X. (2021) Species of Sulfur in Sour Gas Reservoir: Insights from In Situ Raman Spectroscopy of S–H₂S–CH₄–H₂O System and Its Subsystems from 20 to 250°C. Geofluids 2021, 1–14. https://doi.org/10.1155/2021/6658711

; Hu et al., 2022

Hu, M. Chou, I-M., Wang, R., Shang, L., Chen, C. (2022) High solubility of gold in H₂S-H₂O ± NaCl fluids at 100–200 MPa and 600–800 °C: A synthetic fluid inclusion study. Geochimica et Cosmochimica Acta 330. https://doi.org/10.1016/j.gca.2022.03.006

). The peak at ∼2500 cm⁻¹ is likely ascribed to polysulfane (H₂S_n; Hurai et al., 2019

Hurai, V., Černušák, I., Randive, K. (2019) Raman spectroscopic study of polysulfanes (H₂S_n) in natural fluid inclusions. Chemical Geology 508, 15–29. https://doi.org/10.1016/j.chemgeo.2018.04.007

), while the ∼2567 cm⁻¹ peak has been hypothesised by Hurai et al. (2019)

Hurai, V., Černušák, I., Randive, K. (2019) Raman spectroscopic study of polysulfanes (H₂S_n) in natural fluid inclusions. Chemical Geology 508, 15–29. https://doi.org/10.1016/j.chemgeo.2018.04.007

to be H₂S bound to crown-shaped cycloocta-sulfur. In two phase inclusions, the Raman peak at ∼2590 cm⁻¹ is assigned to dissolved H₂S (Frezzotti et al., 2012

Frezzotti, M.L., Tecce, F., Casagli, A. (2012) Raman spectroscopy for fluid inclusion analysis. Journal of Geochemical Exploration 112, 1–20. https://doi.org/10.1016/j.gexplo.2011.09.009

; Schmidt and Seward, 2017

Schmidt, C., Seward, T.M. (2017) Raman spectroscopic quantification of sulfur species in aqueous fluids: Ratios of relative molar scattering factors of raman bands of H₂S, HS⁻, SO₂, HSO₄⁻, SO₄²⁻, S₂O₃²⁻, S₃⁻ and H₂O at ambient conditions and information on changes with pressure and temperature. Chemical Geology 467, 64–75. https://doi.org/10.1016/j.chemgeo.2017.07.022

; Hu et al., 2022

Hu, M. Chou, I-M., Wang, R., Shang, L., Chen, C. (2022) High solubility of gold in H₂S-H₂O ± NaCl fluids at 100–200 MPa and 600–800 °C: A synthetic fluid inclusion study. Geochimica et Cosmochimica Acta 330. https://doi.org/10.1016/j.gca.2022.03.006

).

In situ Raman spectra of the solid and liquid phases of three phase inclusions are collected during heating (20 °C, 50 °C, 100 °C, and 150 °C; Fig. 3k,j). Previous studies suggest that polysulfane is thermally (e.g., >70 °C) and photochemically sensitive and tends to decompose into hydrogen sulfide and elemental sulfur (H₂S_n→^(n-1)/₈S₈ + H₂S; Hurai et al., 2019

Hurai, V., Černušák, I., Randive, K. (2019) Raman spectroscopic study of polysulfanes (H₂S_n) in natural fluid inclusions. Chemical Geology 508, 15–29. https://doi.org/10.1016/j.chemgeo.2018.04.007

; Hu et al., 2022

Hu, M. Chou, I-M., Wang, R., Shang, L., Chen, C. (2022) High solubility of gold in H₂S-H₂O ± NaCl fluids at 100–200 MPa and 600–800 °C: A synthetic fluid inclusion study. Geochimica et Cosmochimica Acta 330. https://doi.org/10.1016/j.gca.2022.03.006

). This is confirmed by our in situ Raman spectroscopic characterisations. At 100 °C, the polysulfane signal disappears, but elemental sulfur remains detectable in the solid phase (Fig. 3k). With increasing temperature, the Raman peaks of elemental sulfur (∼221 cm⁻¹ and ∼473 cm⁻¹) in the supercritical H₂S phase become visible and intensify (Fig. 3j), indicating the progressive dissolution of S in H₂S.

Volatiles other than the H₂S phase (sometimes minor water) at room temperature are not identified in the three phase and solid-liquid two phase inclusions (Fig. 2a), indicating the presence of high purity H₂S. Freezing experiments show that the triple points of the volatiles in the three phase inclusions range from −85.1 to −83.2 °C (Table S-1), which is very close to the triple point of pure H₂S system (−85.5 °C; Sakoda and Uematsu, 2004

Sakoda, N., Uematsu, M. (2004) A thermodynamic property model for fluid phase hydrogen sulfide. International Journal of Thermophysics 25, 709–737. https://doi.org/10.1023/B:IJOT.0000034234.06341.8a

). Obviously, our microthermometric observations further confirm the presence of high purity H₂S. At present, there is no equation of state for the S-H₂S system. In addition, the proportion of solid phase in the solid-liquid two phase inclusions is greater than that in the three phase inclusions (Fig. 1c,d). Consequently, three phase inclusions with a lower solid phase proportion (e.g., Fig. 1c) are approximated as pure H₂S systems. Based on the homogenisation temperature for liquid and vapour H₂S (Lemmon and Span, 2006

Lemmon, E.W., Span, R. (2006) Short fundamental equations of state for 20 industrial fluids. Journal of Chemical and Engineering Data 51, 785–850. https://doi.org/10.1021/je050186n

), and the relationship between the ν₁(H₂S) Raman peak position and H₂S density for the homogeneous H₂S phase (Jiang et al., 2018

Jiang, L., Xin, Y., Chou, I., Chen, Y. (2018) Raman spectroscopic measurements of ν1 band of hydrogen sulfide over a wide range of temperature and density in fused‐silica optical cells. Journal of Raman Spectroscopy 49, 343–350. https://doi.org/10.1002/jrs.5293

), the H₂S densities of three phase inclusions are estimated to be 0.68–0.76 g/cm³. Apparently, the investigated inclusions are characterised by high H₂S density.

The coexistence of three phase inclusions, liquid-solid two phase inclusions, and H₂S-bearing two phase aqueous inclusions suggests that the FIs are formed in H₂S-saturated immiscible fluid systems (Goldstein, 2001

Goldstein, R.H. (2001) Fluid inclusions in sedimentary and diagenetic systems. Lithos 55, 159–193. https://doi.org/10.1016/S0024-4937(00)00044-X

). Under such circumstances, the homogenisation temperature of the two phase aqueous inclusions can be approximated as their trapping temperature (Goldstein, 2001

Goldstein, R.H. (2001) Fluid inclusions in sedimentary and diagenetic systems. Lithos 55, 159–193. https://doi.org/10.1016/S0024-4937(00)00044-X

), which ranges from 134 to 158 °C (Table S-1). By extrapolating along the 0.68–0.76 g/cm³ isochores of the three phase inclusions to 134–158°C yields trapping pressures of 50–106 MPa (Lemmon and Span, 2006

Lemmon, E.W., Span, R. (2006) Short fundamental equations of state for 20 industrial fluids. Journal of Chemical and Engineering Data 51, 785–850. https://doi.org/10.1021/je050186n

).

Generation of high purity H₂S fluid and implications for MVT deposit formation. Previous studies suggest three primary sources of H₂S in sedimentary basins: bacterial sulfate reduction (BSR), thermal decomposition of sulfur-bearing compounds, and TSR (Worden et al., 1995

Worden, R.H., Smalley, P.C., Oxtoby, N.H. (1995) Gas souring by thermochemical sulfate reduction at 140 °C. AAPG Bulletin 79, 854–863.

; Sośnicka and Lüders, 2020

Sośnicka, M., Lüders, V. (2020) Fluid inclusion evidence for low-temperature thermochemical sulfate reduction (TSR) of dry coal gas in Upper Permian carbonate reservoirs (Zechstein, Ca2) in the North German Basin. Chemical Geology 534, 119453. https://doi.org/10.1016/j.chemgeo.2019.119453

). The sulfur content within the organic matter of these formations is typically limited, resulting in H₂S concentrations rarely exceeding 3 % (Machel, 2001

Machel, H.G. (2001) Bacterial and thermochemical sulfate reduction in diagenetic settings—old and new insights. Sedimentary Geology 140, 143–175. https://doi.org/10.1016/S0037-0738(00)00176-7

). Cui et al. (2016

Cui, H., Xiao, S., Zhou, C., Peng, Y., Kaufman, A.J., Plummer, R.E. (2016) Phosphogenesis associated with the Shuram Excursion: Petrographic and geochemical observations from the Ediacaran Doushantuo Formation of South China. Sedimentary Geology 341, 134–146. https://doi.org/10.1016/j.sedgeo.2016.05.008

, 2017

Cui, H., Kaufman, A. J., Xiao, S., Zhou, C., Liu, X.-M. (2017) Was the Ediacaran Shuram Excursion a globally synchronized early diagenetic event? Insights from methane-derived authigenic carbonates in the uppermost Doushantuo Formation, South China. Chemical Geology 450, 59–80. https://doi.org/10.1016/j.chemgeo.2016.12.010

) propose that anaerobic oxidation of methane (AOM) contributes to the formation of ¹³C-depleted calcite cement (δ¹³C = −5 ∼ −37 ‰; VPDB). However, our FI observations show that the high density, high purity H₂S fluids are generated at 134–158 °C, which exceeds the temperature range conducive to BSR (Machel et al., 2001

Machel, H.G. (2001) Bacterial and thermochemical sulfate reduction in diagenetic settings—old and new insights. Sedimentary Geology 140, 143–175. https://doi.org/10.1016/S0037-0738(00)00176-7

). On the contrary, this temperature range aligns with the TSR active range (Machel et al., 2001

Machel, H.G. (2001) Bacterial and thermochemical sulfate reduction in diagenetic settings—old and new insights. Sedimentary Geology 140, 143–175. https://doi.org/10.1016/S0037-0738(00)00176-7

; Worden et al., 1995

Worden, R.H., Smalley, P.C., Oxtoby, N.H. (1995) Gas souring by thermochemical sulfate reduction at 140 °C. AAPG Bulletin 79, 854–863.

), suggesting that the formation of H₂S fluids is more likely attributed to TSR.

Detailed petrological observations of the upper Z₁d and the lower to middle Z₂dn have been conducted by Cui et al. (2016

Cui, H., Xiao, S., Zhou, C., Peng, Y., Kaufman, A.J., Plummer, R.E. (2016) Phosphogenesis associated with the Shuram Excursion: Petrographic and geochemical observations from the Ediacaran Doushantuo Formation of South China. Sedimentary Geology 341, 134–146. https://doi.org/10.1016/j.sedgeo.2016.05.008

, 2017

Cui, H., Kaufman, A. J., Xiao, S., Zhou, C., Liu, X.-M. (2017) Was the Ediacaran Shuram Excursion a globally synchronized early diagenetic event? Insights from methane-derived authigenic carbonates in the uppermost Doushantuo Formation, South China. Chemical Geology 450, 59–80. https://doi.org/10.1016/j.chemgeo.2016.12.010

, 2022

Cui, H., Kaufman, A.J., Xiao, S., Zhou, C., Zhu, M., Cao, M., Loyd, S., Crockford, P., Liu, X.-M., Goderis, S., Wang, W., Guan, C. (2022) Dynamic interplay of biogeochemical C, S and Ba cycles in response to the Shuram oxygenation event. Journal of The Geological Society 179, jgs2021-81. https://doi.org/10.1144/jgs2021-081

), including outcrops at Zhongling, Yangjiaping and Nanbeizhen (South China), which are only a few kilometres away from the Caojunba section. In the Nanbeizhen section, the nodules are surrounded by finely warping phosphatic dolomite laminations, suggesting that they formed before sediment compaction (Cui et al., 2022

Cui, H., Kaufman, A.J., Xiao, S., Zhou, C., Zhu, M., Cao, M., Loyd, S., Crockford, P., Liu, X.-M., Goderis, S., Wang, W., Guan, C. (2022) Dynamic interplay of biogeochemical C, S and Ba cycles in response to the Shuram oxygenation event. Journal of The Geological Society 179, jgs2021-81. https://doi.org/10.1144/jgs2021-081

). Chicken wire and ghost gypsum textures, indicative of original gypsum precipitation characteristic of evaporative environments, are observed within the dolomite sequences (Cui et al., 2022

Cui, H., Kaufman, A.J., Xiao, S., Zhou, C., Zhu, M., Cao, M., Loyd, S., Crockford, P., Liu, X.-M., Goderis, S., Wang, W., Guan, C. (2022) Dynamic interplay of biogeochemical C, S and Ba cycles in response to the Shuram oxygenation event. Journal of The Geological Society 179, jgs2021-81. https://doi.org/10.1144/jgs2021-081

). Therefore, we propose that the initial minerals in these nodules are likely evaporites (e.g., gypsum), which provide the sulfates necessary for TSR. The extremely negative δ¹³C_carb values (−6.3 to ∼−32.9 ‰, VPDB; Shi et al., 2022

Shi, H., Ouyang, Q., Zhou, C., Xiao, S., Chen, Z., Guan, C. (2022) Integrated study of the Doushantuo Formation in northwestern Hunan Province: Implications for Ediacaran chemostratigraphy and biostratigraphy in South China. Precambrian Research 377, 106699. https://doi.org/10.1016/j.precamres.2022.106699

) indicate that carbon in calcite cement is likely of organic origin, possibly involving methane as an electron donor in the TSR process (Cui et al., 2016

Cui, H., Xiao, S., Zhou, C., Peng, Y., Kaufman, A.J., Plummer, R.E. (2016) Phosphogenesis associated with the Shuram Excursion: Petrographic and geochemical observations from the Ediacaran Doushantuo Formation of South China. Sedimentary Geology 341, 134–146. https://doi.org/10.1016/j.sedgeo.2016.05.008

). Therefore, TSR is invoked to be responsible for the formation of high density H₂S fluids.

As shown in Equation 1, TSR not only produces reduced sulfur but also generates CO₂ (as HCO₃⁻ in the fluid; Worden et al., 1995

Worden, R.H., Smalley, P.C., Oxtoby, N.H. (1995) Gas souring by thermochemical sulfate reduction at 140 °C. AAPG Bulletin 79, 854–863.

; Powell and MacQueen, 1984

Powell, T.G., MacQueen, R.W. (1984) Precipitation of sulfide ores and organic matter: sulfate reactions at Pine Point, Canada. Science 224, 63–66. https://doi.org/10.1126/science.224.4644.63

; Sośnicka and Lüders, 2019

Sośnicka, M., Lüders, V. (2019) Super-deep, TSR-controlled Phanerozoic MVT type Zn-Pb deposits hosted by Zechstein-2 gas reservoir carbonate (Ca2), Lower Saxony Basin, Germany. Chemical Geology 508, 62–77. https://doi.org/10.1016/j.chemgeo.2018.04.025

).

However, CO₂ is absent in the investigated inclusions (Fig. 2). Although the exact formation sequence between calcite and quartz cements is difficult to determine, the homogenisation temperatures of the two phase aqueous inclusions within them are similar (134–158 °C vs. 138–160 °C; Table S-1). Consequently, we tend to conclude that calcite and quartz cements formed in a narrow time range. The sulfates (i.e. gypsum) in the nodules are consumed during the TSR, which generates large amounts of H₂S and CO₂ (Fig. 4). The latter is consumed through the precipitation of calcite (Eq. 2), which reduces the pore volume and results in the development of a high density (high pressure) H₂S fluid.

Full size image

Obviously, calcite precipitation acts as a sieve, purifying the fluid (i.e. calcite purification model). This also lowers the pH, promoting the precipitation of authigenic silica. By the time quartz precipitates (Eq. 3), CO₂ in the fluid has been depleted. As a result,

H₂S and elemental sulfur dissolved in the fluid cannot be incorporated into the calcite lattice and, along with a small amount of water, are trapped as FIs within the quartz crystals.

Both fluid mixing and sulfate reduction are crucial for the formation of MVT deposits (Sverjensky, 1981

Sverjensky, D.A. (1981) The Origin of a Mississippi Valley-type Deposit in the Viburnum Trend, Southeast Missouri. Economic Geology 76, 1848–1872. https://doi.org/10.2113/gsecongeo.76.7.1848

). Our findings are of particular importance to the fluid mixing model. Globally, many MVT deposits exhibit colloform textures in ore minerals such as sphalerite and pyrite, which indicate the rapid precipitation of metal sulfides resulting from the mixing of metal-rich fluids with H₂S-rich fluids (Barrie et al., 2009

Barrie, C.D., Boyce, A.J., Boyle, A.P., Williams, P.J., Blake, K., Wilkinson, J.J., Lowther, M., McDermott, P., Prior, D.J. (2009) On the growth of colloform textures: A case study of sphalerite from the galmoy ore body, Ireland. Journal of The Geological Society 166, 563–582. https://doi.org/10.1144/0016-76492008-080

). Therefore, an H₂S-rich reservoir is essential for the formation of these MVT deposits. This study provides geological evidence that TSR can generate high density H₂S fluids and elucidates the important role of TSR in the metallogenic process of MVT deposits, thereby enhancing our understanding of the mineralisation mechanisms of these deposits.

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Conclusions

In the Z₁d dolomite at the Caojunba section (South China), high density, high purity H₂S inclusions are discovered within nodular quartz cement. Combining petrological, in situ Raman spectroscopic, microthermometric, and previous carbon and oxygen isotope analyses, the following conclusions can be drawn:

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Acknowledgments

This work is supported by the National Natural Science Foundation of China (Grant nos. 42192501, 42173038, 42130109). We thank Qing Ouyang and Zhe Chen for assistance in the fieldwork.

Editor: Rául Fonseca

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References

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Supplementary Information

The Supplementary Information includes:

• Geological Setting and Samples
• Methods
• Table S-1
• Figure S-1
• Supplementary Information References

Download the Supplementary Information (PDF)