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Article

Constraints on the Origin of Sulfur-Related Ore Deposits in NW Tarim Basin, China: Integration of Petrology and C-O-Sr-S Isotopic Geochemistry

by
Shaofeng Dong
1,2,3,*,
Yuhang Luo
2,
Jun Han
4 and
Daizhao Chen
5
1
State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Efficient Development, SINOPEC, Beijing 100083, China
2
School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China
3
Radiogenic Isotope Laboratory, Centre for Microscopy and Microanalysis (CMM), The University of Queensland, Brisbane, QLD 4072, Australia
4
Petroleum Exploration and Production Research Institute, SINOPEC Northwest Oilfield Branch, Urumqi 830011, China
5
State Key Laboratory of Lithospheric and Environmental Coevolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(12), 1265; https://doi.org/10.3390/min15121265 (registering DOI)
Submission received: 24 October 2025 / Revised: 23 November 2025 / Accepted: 25 November 2025 / Published: 29 November 2025
(This article belongs to the Special Issue Formation and Characteristics of Sediment-Hosted Ore Deposits)
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Abstract

Many small-size ore deposits occur in the Lower Paleozoic strata along the ENE-trending imbricate thrust fault in NW Tarim Basin. Based on field investigations and petrographic examinations, sulfur-related deposits mainly occur within the paleo-karst cavities and are composed of elemental sulfur and anhydrite. Elemental sulfur is extensively present, whereas anhydrite is limited to the Topulang area. The over-dispersed δ34S values (−25.2 to +7.4‰ VCDT) suggest that elemental sulfur and anhydrite typically originate from a multi-phase process involving bacterial sulfate reduction (BSR) superimposed stepwise sulfur disproportionation. The source of sulfate most likely derived from the subsurface Cambrian evaporites. The lower δ13C (−6.43 to −3.10‰ VPDB) and δ18O values (−13.49 to −10.30‰ VPDB) and the higher 87Sr/86Sr ratios (>0.7105) further suggest that the calcite cements precipitated from near surface aquifer with significant meteoric water influx and were associated with southeastward propagation since the Cenozoic in response to the remote effects of the India–Eurasia collision. This regional tectonic uplift and meteoric water influx created favorable anoxic environments (“sulfur springs”) for subsequent BSR and sulfur disproportionation along the Kepingtage overthrust fault front, resulting in the mineralization of sulfur-bearing species. This study provides a useful example for understanding the repeated processes of BSR and sulfur disproportionation for deep-buried evaporites associated with tectonic-driven mineralization within the Tarim Basin and elsewhere.

1. Introduction

Sulfur, a vital element for all creatures on Earth, mainly occurs in the form of sulfide and/or sulfate. Elemental sulfur could be formed in various marine and non-marine settings through abiotic and/or biological processes [1,2,3,4,5,6]. However, limited numbers of elemental sulfur deposits are reported except in certain environments [7,8]. The circum-Mediterranean region hosts the world’s largest strata-bound elemental sulfur deposits [8,9,10], while several small commercial native sulfur deposits related to caprock are present in the Texas–Louisiana Coastal salt basin [11]. All these deposits are related to biological processes, regardless of being biosyngenetic or bioepigenetic in origin.
Many abandoned open pits of exploited mines such as fluorite, lead–zinc and sulfur-related ore deposits are present within a 220 km long, NW-SE-trending overthrust fault zone in NW Tarim Basin (Figure 1A). Although many studies have been carried out on the hydrothermal dolomite and associated ore deposits [12,13,14,15], only a few works put the emphasis on the origin of sulfur-related ore [16,17]. Different mechanisms, either organic thermal decomposition [16] or magmatic hydrothermal alteration [17], have been proposed to explain the enrichment of these ore deposits. However, the origin and sources of sulfur remain a matter of debate. In addition, the relationships between the ore mineralization and regional tectonic events were poorly constrained in general.
This study aims to elucidate the mechanism and timing of sulfur-related ore deposits, to determine the source of sulfur-bearing fluids, as well as their links to tectonic deformation. To reach these goals, detailed field investigations and petrographic and geochemical studies were carried out on the exploited and abandoned small galleries in the Keping area, NW Tarim Basin.

2. Geological Setting

Since the Early Paleozoic, a thick carbonate sequence (>2000 m) had accumulated on a stable shallow marine platform along the northwest Tarim Basin, where the study area is located [18]. This depositional pattern persisted until the end of the Ordovician, after which it shifted to siliciclastic deposition due to the continuous subsidence. During the Early Permian, a large igneous province (LIP) was formed [19,20,21] in response to the convergent subduction of the Middle Tienshan arc to the north [22]. This event resulted in rapid uplift, exhumation and erosion, particularly along the northwestern flank of Tarim Basin.
Throughout the Mesozoic, intense collision between the Qiangtang Terrane and the Eurasian plate induced continuous uplift and erosion in this region, thereby resulting in the widespread absence of the Mesozoic successions, which are only locally present. During the Cenozoic, A-type subduction between the South Tienshan and the northwestern Tarim block occurred due to the Indo-Asia collision [23,24,25,26]. Since the Oligocene, a series of arcuate, emergent imbricate thrust belts have been formed and propagated southeastward along the northwestern flank of Tarim block [27,28,29]. In front of these thrust sheets, many north-dipping overthrust high-angle faults (50–70°), such as the Shajingzi–Kepingtage fault and Yimugantawu fault, crop out and parallel to the strike of the successions (Figure 1A) [30,31]. All the imbricate thrust faults are rooted in the underlying Cambrian evaporite decollement horizon, forming a thin-skinned structure [27,32]. Conversely, the basin interior underwent flexural subsidence, descending to greater depths and forming the present tectonic configuration of the Tarim Basin (Figure 1).
Post-Cambrian strata crop out along the hanging wall of the thrust faults (Figure 1). A summary of stratigraphic units is illustrated in Figure 1B, and detailed information can also be found in previous studies [13,33]. This study focuses on the Yijianfang Formation, Middle Ordovician (Figure 1B). The Yijianfang Formation (~60 m thick) is dominated by gray, thick-bedded to massive bioclastic limestones deposited in marginal reef or shoal settings and conformably overlies the gray grainy limestones of Yingshan Formation. An unconformity surface is present between the carbonate and overlying red argillaceous bioclastic rocks.
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Figure 1. (A) Simplified geological map of the Keping fold-and-thrust belt along the northwestern flank of Tarim block (modified from Huang et al., 2019 [34]). (B) Stratigraphic and lithologic succession of the Cambrian–Ordovician strata in the studied area (modified from Zhou et al., 2001 [33] and Dong et al., 2013 [13]).
Figure 1. (A) Simplified geological map of the Keping fold-and-thrust belt along the northwestern flank of Tarim block (modified from Huang et al., 2019 [34]). (B) Stratigraphic and lithologic succession of the Cambrian–Ordovician strata in the studied area (modified from Zhou et al., 2001 [33] and Dong et al., 2013 [13]).
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3. Analytical Methods

Field observation and sampling were carried out upon the selected outcrop near the Keping county (Figure 1). A typical wildcat well (BT5) with thick evaporites in the interior of the Tarim Basin was also investigated for comparison. Detailed petrographic studies were performed on hand specimens and thin sections that were stained with Alizarin Red-S and potassium ferricyanide [35]. Afterward, the carbon, oxygen and strontium isotope compositions of the host limestone and calcite cement were determined following standard procedures described in Dong et al. [13,36].
For S isotope analysis, anhydrite powder was dissolved in 0.15 M HCl and subsequently reacted with 0.25 M BaCl2 solution to precipitate BaSO4 [37]. The BaSO4 was thoroughly recovered by filtration and then dried. Purified BaSO4 and elemental sulfur were converted to SO2 by combustion. The extracted SO2 gases were analyzed for δ34S with a Finnigan MAT 252 mass spectrometer (Finnigan Mat Ltd., Bremen, Germany) [38]. Calibration was conducted using IAEA-S3 and NBS-127 international standards. δ34S signatures are reported in per mil (‰) relative to the Vienna Canyon Diablo Troilite (VCDT) standard. Precision was better than ±0.3‰. All the experiments were performed at the Laboratory for Stable Isotope Geochemistry, Institute of Geology and Geophysics, Chinese Academy of Sciences.

4. Petrology of Sulfur-Related Minerals

Sulfur-associated ore deposits, now exhausted (Figure 2A), generally occur in the Middle Ordovician thick-bedded carbonates along the front of the Kepingtage thrust belt. The host carbonates were commonly subjected to intense fracturing and brecciation, forming numerous fractures/fissures (Figure 2B). Limestones are dominated by abundant peloids with minor bioclasts cemented by early marine sparry calcite (Figure 2C). Some grains are truncated by calcite-filled tectonic fractures. The high-amplitude stylolites are present preferentially in more argillaceous parts and often charged by bitumen/organic materials (Figure 2D).
The dome-shaped ore bodies (Figure 2A and Figure 3A) are mainly trapped in the paleo-karst cavities crosscut by later high-angle-to-subvertical fractures/faults filled with encrusting calcite cements (Figure 3B). White anhydrite grains exclusively occur in the Topulang (TPL) area and exhibit a radial-to-acicular texture (Figure 3C). A beige-brown powdery texture was also observed when anhydrite had suffered weathering (Figure 3D,E). Elemental sulfur occurs as veinlet and/or clump aggregation in the TPL area, forming irregular-to-anastomosed geometry (Figure 3D). In contrast, only elemental sulfur is observed in the Liuhuanggou (LHG) area and disperses over the antecedent travertine, while laminar and/or radial cement fills the paleo-karst cavities and subvertical fractures (Figure 3F,G). Some of these fractures even extend into the overlying Silurian yellowish-greenish mudstone (Figure 3H).
Thick evaporite layers are extensively present and composed of halite and anhydrite in the Early-to-Middle-Cambrian core interval of the BT5 wildcat well (Figure 4). Halite is colorless to red in hand specimens and is translucent to transparent with minor mudstone breccias. Anhydrite, commonly milky-white in hand specimens, typically displays a nodule or lump texture intercalating with dark-gray laminar to stripped argillaceous dolomicrite (Figure 4A,Β). Rarely, anhydrite also occurs in the form of cement completely occluding the former dissolved ooidal grainstone (Figure 4C,D).

5. Isotopic Geochemistry

Anhydrite (n = 11) sampled from the BT5 well has the highest δ34S values, ranging from +24.4 to +35.4‰ (aver. +32.6‰) (Figure 5; Table 1), which lie within the sulfur isotope range of Early Paleozoic seawater (δ34S: +24~+34.5‰) [39,40]. In contrast, anhydrite samples (n = 5) from the TPL area yield significantly lower δ34S values, varying from −20.0 to −5.5‰ (Table 1). These data, however, apparently cluster on two groups: group one, clustering between −20.0 and −18.1‰, and group two, clustering between −6.4 and −5.5‰. The paragenetic mineral, elemental sulfur, exhibits the lowest δ34S values, varying between −25.2 and −18.0‰ (aver. −22.3‰). Three elemental sulfur samples from the LHG area have medium δ34S values ranging from −6.2 to +7.4‰ (aver. +1.6‰), exhibiting relatively higher values than those of elemental sulfur in the TPL area (Table 1).
Table 1. Summary of the δ34S isotopic signatures of anhydrite and elemental sulfur in the Keping area, Tarim Basin, NW China.
Table 1. Summary of the δ34S isotopic signatures of anhydrite and elemental sulfur in the Keping area, Tarim Basin, NW China.
Sample No.Lithologyδ34SVCDT (‰)Error (2σ)
14TPL-06Aanhydrite−20.00 0.016
14TPL-09Aanhydrite−19.82 0.025
14TPL-04Banhydrite−18.05 0.029
14TPL-09Banhydrite−6.36 0.035
14TPL-10Banhydrite−5.49 0.058
T081223-1 **anhydrite−10.85 —
T081223-2 **anhydrite−7.56 —
TP25-3 **anhydrite+2.92 —
TP26-1 **anhydrite−6.93 —
TP26-2 **anhydrite−23.64 —
TP26-3 **anhydrite−23.56 —
14TPL-01Aelemental sulfur−21.13 0.015
14TPL-02Aelemental sulfur−24.44 0.008
14TPL-04Aelemental sulfur−22.81 0.018
14TPL-05Aelemental sulfur−25.15 0.031
14TPL-08Aelemental sulfur−18.00 0.013
14PTL-01Belemental sulfur−21.44 0.016
14PTL-04Belemental sulfur−23.71 0.011
14PTL-05Belemental sulfur−21.95 0.022
14PTL-06Belemental sulfur−21.84 0.012
14LHG-01Aelemental sulfur+3.46 0.012
14LHG-03Aelemental sulfur+7.39 0.016
14LHG-04Aelemental sulfur−6.18 0.009
13BT5-27-Є2aanhydrite+34.93 0.004
12BT5-02-Є2aanhydrite+24.43 0.018
12BT5-07-Є2aanhydrite+29.76 0.013
12BT5-08-Є2aanhydrite+35.44 0.009
13BT5-32-Є2aanhydrite+34.23 0.020
13BT5-37-Є1wanhydrite+31.30 0.015
13BT5-40-Є1wanhydrite+33.10 0.012
13BT5-42-Є1wanhydrite+35.33 0.009
13BT5-46-Є1wanhydrite+33.46 0.018
13BT5-50-Є1wanhydrite+34.47 0.008
13BT5-51-Є1wanhydrite+31.68 0.016
Note: **—data from Pan et al., 2012 [16].
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Figure 5. δ34S values of anhydrite and elemental sulfur from field outcrop and BT5 drill core in NW Tarim Basin. The shaded area represents the sulfur isotopic values of Early Paleozoic seawater (+24 to +34.5‰) [39,40].
Figure 5. δ34S values of anhydrite and elemental sulfur from field outcrop and BT5 drill core in NW Tarim Basin. The shaded area represents the sulfur isotopic values of Early Paleozoic seawater (+24 to +34.5‰) [39,40].
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Host limestones (n = 4) sampled from the TPL area yield δ13C values from −2.30 to −0.50‰ VPDB (aver. −1.41‰) and δ18O values from −7.50 to −6.20‰ VPDB (aver. −6.77‰) (Figure 6; Table 2), which lie within their ranges for carbonates in equilibrium with Early Paleozoic seawater (δ18O: −10 to −6‰, δ13C: −2.5 to +1.5‰) [41,42]. Two host limestones yield a very narrow range of 87Sr/86Sr ratios between 0.7086 and 0.7087 (Figure 7), developing overlap with those of Ordovician seawater [42,43].
In contrast, the δ13C and δ18O values of laminar calcite cements of the LHG area (Figure 6; Table 2) are distinctly lower than those of host limestones and vary from −6.43 to −3.10‰ VPDB (aver. −5.68‰) and from −13.49 to −10.30‰ VPDB (aver. −12.25‰), respectively. Accordingly, their 87Sr/86Sr ratios are more radiogenic than that of the Early Paleozoic seawater, varying from 0.7108 to 0.7105 (Figure 7).
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Figure 6. Cross-plot of δ13C and δ18O (VPDB) values for the sampled host limestone and radial travertine in the NW Tarim Basin. The shaded area represents the stable isotopic composition of Early Paleozoic marine calcite [41,42]. †—data from Zhang and Munnecke (2016) [44]; §—data from Jiang et al. (2015) [14]; #—data from Dong et al. (2013) [13]; *—data from Zhong et al. (2012) [17].
Figure 6. Cross-plot of δ13C and δ18O (VPDB) values for the sampled host limestone and radial travertine in the NW Tarim Basin. The shaded area represents the stable isotopic composition of Early Paleozoic marine calcite [41,42]. †—data from Zhang and Munnecke (2016) [44]; §—data from Jiang et al. (2015) [14]; #—data from Dong et al. (2013) [13]; *—data from Zhong et al. (2012) [17].
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Table 2. Isotopic data (δ13C, δ18O and 87Sr/86Sr) of host limestones and calcite cements in the Keping area, Tarim Basin, NW China.
Table 2. Isotopic data (δ13C, δ18O and 87Sr/86Sr) of host limestones and calcite cements in the Keping area, Tarim Basin, NW China.
Sample No.Lithologyδ13CVPDB (‰)δ18OVPDB (‰)87Sr/86Sr±2σ
14TPL-11AHost limestone−0.67 −6.54 0.708717 0.000006
14TPL-02BHost limestone−2.15 −6.83 0.708570 0.000009
14TPL-12AHost limestone−0.50 −6.20 ——
14TPL-03BHost limestone−2.30 −7.50 ——
YJK14-1 †Limestone−0.46 −7.19 ——
YJK14-2 †Limestone−0.46 −7.33 ——
YJK14-3 †Limestone−0.33 −7.18 ——
YJK14-4 †Limestone−0.22 −6.90 ——
YJK14-5 †Limestone−0.53 −6.79 ——
YJK14-6 †Limestone−0.63 −6.69 ——
YJK14-7 †Limestone−0.58 −6.72 ——
YJK14-8 †Limestone−0.39 −7.02 ——
YJK14-9 †Limestone−0.21 −6.83 ——
YJK14-10 †Limestone−0.13 −6.81 ——
YJK14-11 †Limestone−0.17 −6.04 ——
YJK14-12 †Limestone−0.47 −5.97 ——
YJK14-13 †Limestone−0.04 −6.14 ——
YJK14-14 †Limestone−0.08 −6.14 ——
YJK14-15 †Limestone+0.02 −6.23 ——
YJK14-16 †Limestone−0.01 −6.08 ——
YJK14-17 †Limestone+0.01 −6.24 ——
YJK14-18 †Limestone−0.17 −6.31 ——
14LHG-01A’-2Laminar calcite−6.20 −12.70 ——
14LHG-02A-1Laminar calcite−5.90 −12.70 ——
14LHG-02A-2Laminar calcite−6.40 −12.90 ——
14LHG-05B-1Laminar calcite−6.10 −10.30 ——
14LHG-05B-2Laminar calcite−3.10 −11.00 ——
14LHG-01A’Laminar calcite−6.43 −13.49 0.710769 0.000004
14LHG-02ALaminar calcite−5.66 −12.69 0.710546 0.000015
09SC-82 #Vein calcite 1−2.78 −13.36 0.709238 0.000013
09SC-84 #Vein calcite 1−2.42 −11.07 ——
09SC-87 #Vein calcite 1−2.69 −9.26 ——
09SC-90 #Vein calcite 1−3.02 −10.53 0.709142 0.000013
09SC-91 #Vein calcite 1−3.13 −11.45 0.709132 0.000011
09SC-96 #Vein calcite 1−2.75 −13.49 0.709277 0.000013
09SC-98 #Vein calcite 1−2.38 −10.99 0.709407 0.000015
09SC-110 #Vein calcite 1−2.40 −11.62 0.709440 0.000012
09SC-112 #Vein calcite 1−3.75 −11.07 0.709001 0.000013
09SC-112 #Vein calcite 2−2.85 −13.01 0.709481 0.000013
09SC-110 #Vein calcite 2−1.64 −12.76 0.709630 0.000010
TPL-1 §Vug calcite−1.66 −16.13 0.710313 —
TPL-2 §Vug calcite−2.32 −17.39 0.710445 —
TPL-3 §Vug calcite−2.72 −14.99 0.709537 —
TPL-4 §Vug calcite−2.66 −15.50 0.709625 —
TPL-5 §Vug calcite−4.98 −13.30 0.709905 —
17-5 *Travertine−7.8−14.20 ——
17-11 *Travertine−4.6−14.70 ——
18-2 *Travertine−5.9−14.10 ——
TB001 *Travertine−5.6−13.50 ——
TB079 *Travertine−5.9−14.70 ——
TB001 *Travertine−5.2−14.00 ——
TB023 *Travertine−5.1−14.70 ——
TB006 *Travertine−5.2−12.90 ——
TB013 *Travertine−4.9−12.70 ——
TB022 *Travertine−4.6−11.60 ——
TB013 *Travertine−6.1−13.50 ——
TB016 *Travertine−6.4−13.10 ——
TB024 *Travertine−5−14.70 ——
TB080 *Travertine−5−14.20 ——
TB082 *Travertine−5.2−14.70 ——
TB099 *Travertine−5.4−15.50 ——
Note: †—data from Zhang and Munnecke, 2016 [44]; §—data from Jiang et al., 2015 [14]; #—data from Dong et al., 2013 [13]; *—data from Zhong et al., 2012 [17].
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Figure 7. Cross-plot of δ18O (VPDB) values and 87Sr/86Sr ratios of host limestone and laminar calcite samples. The shaded area represents the Sr isotopic values of the Early Paleozoic seawater [42,43,45,46,47]. §—data from Jiang et al., 2015 [14]; #—data from Dong et al., 2013 [13].
Figure 7. Cross-plot of δ18O (VPDB) values and 87Sr/86Sr ratios of host limestone and laminar calcite samples. The shaded area represents the Sr isotopic values of the Early Paleozoic seawater [42,43,45,46,47]. §—data from Jiang et al., 2015 [14]; #—data from Dong et al., 2013 [13].
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6. Discussion

6.1. Origin and Sources of Sulfur for Anhydrite and Elemental Sulfur Ore Deposits

Different mechanisms have been proposed to explain the enrichment of ore deposits in NW Tarim Basin: (1) magmatic hydrothermal alteration [17] and (2) thermal decomposition of organic matter [16]. The first scenario can be readily ruled out in view of the depleted δ34S values (−25.2‰) that could not be reached solely by a magmatic sulfur source through fluid–rock interaction (δ34S = 0‰ ± 3‰) [48,49]. Thermal decomposition of organic matter mainly inherits the δ34S value of precursor marine kerogen and oil, which is significantly higher than the obtained values of sulfur-bearing minerals (Table 1). Furthermore, organic thermal decomposition generally occurs at a temperature excess of 175 °C [50], which exceeds the maximum burial temperature of the Ordovician strata in NW Tarim Basin [13]. Thus, the possibility of organic thermal decomposition could also be discounted.
The dome-like geometry of sulfur-related minerals along the front of Kepingtage NW Tarim Basin (Figure 3A,B) implies that they are of post-depositional diagenetic origin. The extremely negative δ34S values (Table 1), which are far below the signatures of sulfur-bearing species resulting from thermochemical sulfate reduction (TSR) [51,52], indicate that microbial/bacterial sulfate reduction (BSR) is the most plausible scenario for the enrichment of sulfur-bearing deposits. The extensive presence of oil seepages [17,53,54] and bitumen along high-amplitude stylolite (Figure 2D) provides the prerequisite for BSR. In addition, the underlying Cambrian evaporites are probably the most likely sources of required sulfate (Figure 3). The maximum δ34S discrepancies between elemental sulfur and the Cambrian anhydrite (δ34S: +24.4~+35.4‰) and the Cambrian seawater (δ34S: +24~+34.5‰) [39,40] are up to 60.6‰ and 59.7‰, respectively, which approach the maximum sulfur isotope fractionation [55,56,57], indicating that intense BSR had taken place in the TPL area [55,56,57]. The presence of heterogeneity for individual elemental sulfur (Figure 5; Table 1) is presumably driven by either variations in reaction kinetics or the extent of involvement of intermediates. Anhydrites with equivalent δ34S compositions probably resulted from subsequent re-oxidation of isotopically light sulfur, in view of negligible isotopic fractionation during oxidation [55]. However, the presence of slightly heavier δ34S anhydrites (Figure 5; Table 1) implies that not all intermediates were reduced back to H2S. These anhydrites could serve as a representative of relict reactants that have suffered moderate BSR. Otherwise, a uniform and strongly negative δ34S signal would be expected across all anhydrite phases. These scattered δ34S data in the TPL area thus well record the different extent of isotope fractionation by stepwise bacterial reduction in an open system.
Although only three δ34S measurements of elemental sulfur are available in the LHG area (Figure 5; Table 1), they still offer invaluable insights into the evolution of BSR. One of the measured δ34S values is negative and overlaps with those of anhydrites in the TPL area, suggesting that approximately one third of elemental sulfur in the LHG area may have formed through moderate BSR. In theory, TSR can reach a maximum of ~20‰ sulfur isotopic fractionation [58], but abundant TSR case studies testified that such large isotopic fractionation induced by TSR seems impossible [59,60,61]. Most importantly, TSR generally occurs in closed systems, contradicting the fact that sulfur-bearing deposits formed in open systems. Therefore, elemental sulfur in the LHG area is also plausibly attributed to BSR. The positive δ34S values may be caused by either (1) the early phase of BSR; (2) elevated ambient temperatures; or (3) limited sulfate availability. The first scenario appears to be plausible. Minor isotopic fractionation is common during the initial phase of BSR. Elemental sulfur via re-oxidation of this-stage H2S yields accordingly heavy δ34S value. As the reaction proceeds, intermediate elemental sulfur can be either further reduced to H2S or re-oxidized back to sulfate that suffered repeated redox, thereby promoting an increasingly negative δ34S signal. The sulfur-bearing ore deposits, particularly along the front of the Kepingtage overthrust nappe in Tarim Basin, thus probably formed through successive BSR and sulfur disproportionation.

6.2. Timing of BSR Reaction

BSR generally occurs in a low-temperature diagenetic environment (T < 60–80 °C), while elevated temperature would inhibit bacterial metabolism [56]. Under such conditions, elemental sulfur and associated anhydrite are estimated to be formed at shallow burial depths (<2 km), assuming a normal geothermal gradient (30 °C/km) and a surface temperature of 20~25 °C [62,63]. Although multiple geologic epochs meet the prerequisite of this situation since the deposition of Ordovician successions, the possibility of BSR occurring prior to the Mesozoic could be readily excluded in view of the persistent burial before the Late Permian in the Keping area [13]. Extensive uplift and denudation took place throughout the Mesozoic due to the amalgamation between the Middle Tienshan arc and Tarim block. However, the Paleozoic strata had not been exposed to the surface at that time and the influx of meteoric water was negligible [13]. BSR could not have occurred during the Mesozoic. The Cenozoic is thus the most likely era for the precipitation of these sulfur-related ore deposits.
C-O-Sr systematics of host carbonates also support this hypothesis. In terms of the crosscutting relationships, two episodes of coarse equant to columnar vein-filling calcite cements have been recognized at the underlying Middle Ordovician strata in the Sancha area, ~80 km SW of the study area [13]. These calcites are believed to have been formed from basinal fluids with minor downward infiltration of meteoric waters during the extensive uplift in the Triassic and Late Cretaceous, respectively [13]. Conversely, the calcite cements in the study area exhibit laminar-to-radial geometries (Figure 3), indicating that they are of distinctive origin. The δ13C values of calcite cements in the LHG area (Figure 6; Table 2), which are lower than the isotopic composition of calcites in the Sancha area [13], presumably reflect an increased influence of meteoric water [64] or incorporate significant organic carbon [65]. Moreover, the depleted excursion of the O isotope related to host limestones (Figure 6), together with the radiogenic 87Sr/86Sr isotope signatures (Table 2), excludes the possibility of hydrocarbon involvement and further corroborates an increased influx of CO2 derived from soil processes during precipitation [36,66]. Enhanced subaerial exposure and subsequent denudation mainly occurred in response to the intense overthrust activity during the Cenozoic in view of the reconstructed burial history of the Ordovician carbonate in the Κeping area [13,67]. Therefore, the deposition of elemental sulfur, which postdates the laminar calcite cements (Figure 3F), probably occurred near the surface aquifer in the Cenozoic when the intense thrust propagation with continuous infiltration of meteoric fluids created a favorable environment for the bacterial metabolism.

6.3. Relationship with the Regional Tectonic Evolution

The emplacement of sulfur-bearing deposits is closely linked to the evolution of regional tectonic activity. Since the Cenozoic, the ENE-striking, imbricated overthrust nappe was considered to form due to the remote effects of the India–Eurasia collision [23,24,25,26], which possibly had commenced around 65 Ma [68]. Due to the progressively southeastward propagation, a series of arcuate, emergent imbricate nappes were finally formed, including the Keping–Shajingzi overthrust fault, along the northwestern flank of Tarim block [27,28,29,32,69].
As all the thin-skinned imbricate overthrust faults are rooted in the Cambrian evaporite decollement horizon at depth [27,32], fluids enriched in SO42− squeezed from these underlying evaporites. These SO42−-rich fluids could have been driven to migrate upward along the overthrust base [70,71], forming an array of “sulfur springs” in front of overthrust faults (Figure 8), similar to modern deep-sea hydrothermal vents [72,73]. When these fluids mixed with the downward-infiltrated meteoric water, an anoxic environment suitable for the thriving of anaerobic bacteria would have been created. Under this regime, bacterial metabolism would have extensively taken place through the reduction of SO4 to H2S in the favorable conditions in either the paleo-karst cavities or the fractures. Subsequently, some H2S was re-oxidized, forming intermediates (elemental sulfur), which further underwent disproportionation, eventually resulting in the enrichment of sulfur-related ore deposits (Figure 3).

7. Conclusions

Based on detailed field investigations and petrographic and sulfur–carbon–oxygen–strontium isotopic geochemical studies on the well-exposed sulfur-bearing mines in the Keping area, northwest Tarim Basin, the main conclusions are drawn as follows:
1.
Many abandoned ore deposits occur in the Middle Ordovician carbonates and form dome-like geometries. Two types of sulfur-bearing minerals, elemental sulfur and anhydrite, are distinguished in the field. Elemental sulfur is present in both the TPL and LHG areas, whereas anhydrite exclusively occurs in the TPL area.
2.
Elemental sulfur and anhydrite could have been formed through a multi-stage process involving bacterial sulfate reduction (BSR) and sulfur disproportionation in view of the scattered δ34S values. The required sulfate most likely ascended from the underlying Cambrian evaporites due to the progressive propagation of thrust nappes since the Cenozoic.
3.
In the Keping area, extensive emplacement of sulfur-related ore deposits occurred while the infiltration of meteoric fluids was intensified, corresponding to the remote effects of the India–Eurasia collision. Expulsion fluids rich in SO42− migrated upward along the decollement horizon, forming an array of “sulfur springs”. Subsequently, microbial metabolism, re-oxidation and sulfur disproportionation led to the enrichment of elemental sulfur and anhydrite in paleo-karst cavities and fractured zones along the Keping overthrust front.
4.
This study presents a useful example of the emplacement of deep-burial sulfate in a shallow environment controlled by bacterial metabolism in the thrust fault front, which could be very helpful for understanding the comprehensive process of BSR-induced ore deposits.

Author Contributions

Conceptualization, S.D. and D.C.; Methodology, S.D.; Investigation, J.H.; Writing—original draft, S.D.; Writing—review & editing, D.C.; Visualization, Y.L.; Funding acquisition, S.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by [the National Natural Science Foundation of China] grant number [92255302, 42072139].

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

Laboratory work was assisted by Lianjun Feng from the Institute of Geology and Geophysics, Chinese Academy of Sciences. Sincere thanks also go to the four referees for the valuable suggestions and comments that greatly improved the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Minerals 15 01265 g002
Figure 2. The characteristics of host limestones in the Keping area, NW Tarim Basin. (A) Field photo showing a typical paleo-karst cavity along the front of the Kepingtage thrust nappe. Note that fractures/faults (red dashed lines) crosscut the thick limestones and paleo-karst cavity. Standing person for scale (1.70 m). (B) Field photo showing intense brecciation of host limestones near the fault. Hammer for scale (35 cm). (C) Photomicrograph showing peloidal grainstone with sparry calcite cements (Cal). Note that later tectonic fracture filled by calcite cements crosscuts peloids (arrow). Plane-polarized light (PPL). (D) Photomicrograph of argillaceous-rich limestone with high-amplitude stylolite (styl) filled by bitumen/organic materials (arrow). Cal: calcite cement.
Figure 2. The characteristics of host limestones in the Keping area, NW Tarim Basin. (A) Field photo showing a typical paleo-karst cavity along the front of the Kepingtage thrust nappe. Note that fractures/faults (red dashed lines) crosscut the thick limestones and paleo-karst cavity. Standing person for scale (1.70 m). (B) Field photo showing intense brecciation of host limestones near the fault. Hammer for scale (35 cm). (C) Photomicrograph showing peloidal grainstone with sparry calcite cements (Cal). Note that later tectonic fracture filled by calcite cements crosscuts peloids (arrow). Plane-polarized light (PPL). (D) Photomicrograph of argillaceous-rich limestone with high-amplitude stylolite (styl) filled by bitumen/organic materials (arrow). Cal: calcite cement.
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Figure 3. The characteristics of sulfur-related minerals in the Ordovician strata in the Keping area, NW Tarim Basin. (A) Field photo showing dome-shaped sulfur-related ore bodies along the front of the Kepingtage thrust nappe. Note a close view of one abandoned small gallery in the lower-right corner. Standing person for scale (1.75 m). (B) Field photo showing the sulfur-related ore body further cut by later calcite-filled fracture (yellow arrow). Hammer for scale (35 cm). (C) Field photo showing anhydrite with radial texture in the TPL area. (D) Field photo showing the co-existence of brecciated limestone (Lim), anhydrite (An) and elemental sulfur (S). Note that the beige-brown anhydrite was subjected to intense weathering. (E) Cross-polarized light photomicrograph of anhydrite showing intense corrosion. (F) Hand sample showing laminar travertine (Tr) with minor elemental sulfur (yellow arrow) occluding the fracture. Lim: limestone. (G) Photomicrograph showing the laminar texture of fracture-filled travertine in the LHG area. (H) Field photo showing travertine-filled fractures (arrows) extending into Silurian yellowish-greenish mudstone (Md). Standing person for scale (1.70 m).
Figure 3. The characteristics of sulfur-related minerals in the Ordovician strata in the Keping area, NW Tarim Basin. (A) Field photo showing dome-shaped sulfur-related ore bodies along the front of the Kepingtage thrust nappe. Note a close view of one abandoned small gallery in the lower-right corner. Standing person for scale (1.75 m). (B) Field photo showing the sulfur-related ore body further cut by later calcite-filled fracture (yellow arrow). Hammer for scale (35 cm). (C) Field photo showing anhydrite with radial texture in the TPL area. (D) Field photo showing the co-existence of brecciated limestone (Lim), anhydrite (An) and elemental sulfur (S). Note that the beige-brown anhydrite was subjected to intense weathering. (E) Cross-polarized light photomicrograph of anhydrite showing intense corrosion. (F) Hand sample showing laminar travertine (Tr) with minor elemental sulfur (yellow arrow) occluding the fracture. Lim: limestone. (G) Photomicrograph showing the laminar texture of fracture-filled travertine in the LHG area. (H) Field photo showing travertine-filled fractures (arrows) extending into Silurian yellowish-greenish mudstone (Md). Standing person for scale (1.70 m).
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Figure 4. The petrographic characteristics of anhydrite of the BT5 well. (A) Core image showing milky-white anhydrite (An) with a lump texture from the Middle Cambrian dolomicrite (Dol). 5221.78 m. (B) Photomicrograph showing stripped anhydrite (An) co-occurring with dark-gray dolomicrite (Dol). 5221.16 m. Cross-polarized light. (C) Core image showing anhydrite completely occluding the dissolved ooidal grainstone. 5218.20 m. Coin for scale (diameter 2 cm). (D) Photomicrograph showing anhydrite (An) completely occluding the dissolved ooidal porosities. Dol: dolomite. 5217.80 m. Cross-polarized light.
Figure 4. The petrographic characteristics of anhydrite of the BT5 well. (A) Core image showing milky-white anhydrite (An) with a lump texture from the Middle Cambrian dolomicrite (Dol). 5221.78 m. (B) Photomicrograph showing stripped anhydrite (An) co-occurring with dark-gray dolomicrite (Dol). 5221.16 m. Cross-polarized light. (C) Core image showing anhydrite completely occluding the dissolved ooidal grainstone. 5218.20 m. Coin for scale (diameter 2 cm). (D) Photomicrograph showing anhydrite (An) completely occluding the dissolved ooidal porosities. Dol: dolomite. 5217.80 m. Cross-polarized light.
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Figure 8. Postulated fluid flow conducive to bacteria sulfate reduction and sulfur disproportionation. Sulfate-rich fluids squeezed from underlying Cambrian evaporites may migrate along the decollement, forming hot “sulfur springs” in front of the thrust belt. This fluid flow (red arrows) may be episodic or transient.
Figure 8. Postulated fluid flow conducive to bacteria sulfate reduction and sulfur disproportionation. Sulfate-rich fluids squeezed from underlying Cambrian evaporites may migrate along the decollement, forming hot “sulfur springs” in front of the thrust belt. This fluid flow (red arrows) may be episodic or transient.
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Dong, S.; Luo, Y.; Han, J.; Chen, D. Constraints on the Origin of Sulfur-Related Ore Deposits in NW Tarim Basin, China: Integration of Petrology and C-O-Sr-S Isotopic Geochemistry. Minerals 2025, 15, 1265. https://doi.org/10.3390/min15121265

AMA Style

Dong S, Luo Y, Han J, Chen D. Constraints on the Origin of Sulfur-Related Ore Deposits in NW Tarim Basin, China: Integration of Petrology and C-O-Sr-S Isotopic Geochemistry. Minerals. 2025; 15(12):1265. https://doi.org/10.3390/min15121265

Chicago/Turabian Style

Dong, Shaofeng, Yuhang Luo, Jun Han, and Daizhao Chen. 2025. "Constraints on the Origin of Sulfur-Related Ore Deposits in NW Tarim Basin, China: Integration of Petrology and C-O-Sr-S Isotopic Geochemistry" Minerals 15, no. 12: 1265. https://doi.org/10.3390/min15121265

APA Style

Dong, S., Luo, Y., Han, J., & Chen, D. (2025). Constraints on the Origin of Sulfur-Related Ore Deposits in NW Tarim Basin, China: Integration of Petrology and C-O-Sr-S Isotopic Geochemistry. Minerals, 15(12), 1265. https://doi.org/10.3390/min15121265

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