J. Cent. South Univ. (2020) 27: 1828-1845

DOI: https://doi.org/10.1007/s11771-020-4411-1

Provenance and salt structures of gypsum formations in Pb-Zn ore-bearing Lanping basin, Southwest China

ZHANG Zhi-bo(张治波)^{1, 2}, ZHU Zhi-jun(朱志军)¹, LI Huan(李欢)³, JIANG Wei-cheng(蒋维诚)⁴,

WANG Wen-feng(王文峰)¹, XU Ying(徐颖)¹, LI Li-rong(李丽荣)¹

1. College of Earth Sciences, East China University of Technology, Nanchang 330013, China;

2. School of Resources and Geosciences, China University of Mining and Technology,Xuzhou 221116, China;

3. Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment

Monitoring, Ministry of Education; School of Geosciences and Info-Physics, Central South University,Changsha 410083, China;

4. School of Earth Resources, China University of Geosciences, Wuhan 430074, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: Large-scale gypsum rocks associated with world-class Pb-Zn ore formations are widely distributed in the Lanping Basin, Sowthwest China. Geochemical studies alongside field investigations were conducted in this study to determine the source and evolutionary processes of the gypsum rocks in this area. The gypsum sequences in the Lanping Basin developed in two formations: the Triassic Sanhedong Formation and the Paleogene Yunlong Formation. The gypsum hosted in the former displays a primary thick-banded structure with δ³⁴S_V-CDT values in the range of 14.5‰-14.8‰. Combined with the ⁸⁷Sr/⁸⁶Sr values (0.707737-0.707783) of limestone, it can be suggested that the Sanhedong Formation is of marine origin. In contrast, the gypsum from the Paleogene Yunlong Formation is characterized by the dome, bead and diapiric salt structures, wider range of both ⁸⁷Sr/⁸⁶Sr (0.707695-0.708629) and δ³⁴S_V-CDT values (9.6‰-17‰), thus indicating a marine source but with the input of continental materials. The initial layered salt formations were formed by chemical deposition in a basin and were later intensely deformed by collisional orogeny during the Himalaya period. As a result, variable salt structures were formed. We hereby propose an evolutionary model to elucidate the genesis of the gypsum formations in the Lanping Basin.

Key words: Lanping basin; gypsum; marine sediment; Sr–S isotopes; salt structure

Cite this article as: ZHANG Zhi-bo, ZHU Zhi-jun, LI Huan, JIANG Wei-cheng, WANG Wen-feng, XU Ying, LI Li-rong. Provenance and salt structures of gypsum formations in Pb-Zn ore-bearing Lanping basin, Southwest China [J]. Journal of Central South University, 2020, 27(6): 1828-1845. DOI: https://doi.org/10.1007/s11771-020-4411-1.

1 Introduction

Gypsum can be well preserved in the evaporite environment and is one of the most common and predominant minerals hosted in sedimentary outcrop [1, 2]. In addition, the Gypsum (formation or layer) is also found to be associated with Pb-Zn ores in some basins recently [3-8]. It is well-known that gypsum has multiple genesis and sources due to its wider distribution and various speciation, e.g., anhydrite, satin spar, alabastrine gypsum and selenite [9-12]. For example, in Spain, well- developed, Permian-Jurassic gypsum formations were controlled by the warming climate [2], but instead, young gypsums were more affected by regional factors (e.g., tectonism, volcanism and saline diapirism) during Cretaceous and Neogene [2, 11]. These sulfate rocks (gypsum and anhydrite) were precipitated from natural brines, which are either of continental (dissolution of previous sulfate layers), marine (episodic transgression), transitional origin, or even hydrothermal origin [2, 11, 13]. OGAWA et al [14] suggested that the anhydrite in Kuroko sulfide deposits, Japan, was produced by hybrid brines which are composed of preheated seawater and hydrothermal fluids. LAURENT et al [12] proposed that the diagenetic gypsum deposit from the Namibe Basin, Angola, is most continental in origin, whereas the formation of vein-type gypsums results from the telogenetic process. Fascinating gypsum megacrystals uniquely found in Naica, Mexico, were interpreted to be generated by the chemical interaction between the shallow high-salinity fluid and some sort of deep brine. The interaction occurred under a self-sufficient mechanism driven by a solution-mediated, gypsum- anhydrite phase transition during the late hydrothermal event [9, 15, 16]. The sulfur in desert gypsum was concluded to have originated from atmospherically transported marine aerosols, marine salt, evaporated river water and weathering of igneous or metamorphic rocks in basin [1]. Also gypsum crystallized mainly from the mixing of meteoric water and connate water in the Newark rift basin, North America [17]. Although sources of gypsum differ in different regions, it is generally accepted that it is mainly sourced from continental and marine salt lakes. The gypsum formed from these two sources is highly different in composition, size and genetic model; for example, the scale of latter is bigger than the former. There are multiple sources of marine gypsum, one of which may be deep hot brine [18, 19]. By comparison, continental gypsum is sourced from water bodies of the earth’s surface and of the sea’s regression and transgression. The concentration of these water bodies can be elevated in the evaporative environment, resulting in the precipitation of water into gypsum [20]. Additionally, the continental salt lake is produced by surface runoff and deep brine that are controlled by fault, climate, and topography, whereas the source of marine salt lake is seawater, which is dominated by tectonic setting and sea level [21]. In terms of the salt structure, its classification varies based on different benchmarks [22-25]: 1) three categories can be classified, i.e., shallow, medium, and deep, based on the buried depth of the salt body; 2) according to the external geometry of the salt structure, it can be grouped into salt anticline, salt pillow, salt dome, salt roll, salt mat, salt awning and salt glacier; 3) it can be divided into the primary, secondary and residual salt structure in the light of their origins; 4) compared with the salt structure and its contact relation with the wallrock, the salt structure can be divided into integrated type, transitional type, piercing type, and gushing type. Previous workers tend to focus more on the well-known Jinding Pb-Zn deposit and the relationship that exists between the salt structure and Pb-Zn mineralization by employing geochemical approaches, thus neglecting the source and genesis of salt rocks (e.g., gypsum), which may be closely associated with the regional mineralization. Also, well-developed salt structures including their occurrence, characteristics and isotopic compositions are likewise ignored.

Gypsum formations are widely distributed in the Lanping Basin of western Yunnan (Sowthwest China), spaciously associated with some world-class Pb-Zn deposits such as Jinding and Baiyangping [7, 26–28]. Previous studies focused more on the relationship between gypsum formations and the Pb-Zn mineralization in this basin, suggesting that the gypsum hosed in the Triassic marine strata provided the ore-forming elements. These ores formed within a dome structure as a result of the diapiric migration of evaporates, during the Paleogene deformation and subsequent thrusting [29, 30]. However, few studies related to the source and salt structure of gypsums have been carried out in this area. Sr and S isotope can be combined to better constrain the material sources, whereas the characteristics of salt structures are reflective of tectonic evolution for salt rocks (e.g., gypsum). Therefore, given the weakness on local gypsum study, on the one hand, this contribution integrated the varied salt structures and Sr, S isotopic geochemistry to improve the understanding of the aspects and formation environment of gypsum from different strata within the Lanping Basin. On the other hand, we aim to illuminate how these various kinds of gypsums formed and where they came from, and eventually, establishing a two-stage evolutionary model not only for the genesis of the gypsum but also guiding the prospecting of metallic ores in this region.

2 Regional geology

The Lanping Basin is located in the northern part of the Simao block that belongs to the Sanjiang Tethys tectonic domain [31-34] (Figure 1). The tectonic lines of this basin are NS- or NNW-trending, mainly developing three faults: the Lancang River fault, the Jinsha River-Ailao Mountain fault and the Lanping-Simao fault. Since the Mesozoic, the tectonic setting of the Lanping Basin has transformed from post-collision fault depression to intracontinental depression [35].

The former stage occurred during the middle Triassic to early Jurassic, following the “lancang movement”. Between middle Permian to early Triassic, the basin experienced uplift and sediments therein gradually lost due to strong collision and compression. In the Early to Middle Triassic, fault depressions were formed in the eastern and western margins of the basin, with the development of neritic to littoral marine facies clastic sedimentary formation. Subsequently, the extremely thick acidic volcanic rock formation was deposited ascribing to the rapid rise of the deeper level of the basin along faults, also the central part of the basin. In the Early to Late Triassic, the basin was further faulted and the marine transgression was widely developed. The basin was uplifted again in response to the opening of the Tethys ocean in the middle part of the Bangong Lake to Nujiang, during the Late Triassic. The rift valley basin entered the atrophic stage, depositing the sedimentary sequence of the she-delta facies clastic rocks in the gulf-sea-flat facies and the marine regression sequence of the lacustrine clastic rocks in the Early Jurassic. At the end of the Early Jurassic, the basin was completely uplifted and regressed, forming a parallel unconformity contact between the Middle and Early Jurassic strata. The intracontinental depression occurred over the period from Middle Jurassic to Early Cretaceous. In the outset, the Yarlung Zangbo River expanded rapidly and thus led to the gradual closure of the Tethys ocean in the Nujiang. By Late Jurassic, the Nujiang Ocean was completely closed and the basin re-uplifted, resulting in the erosion of the Late Jurassic to Early Cretaceous strata as well as the formation of the parallel unconformity contact surface. By Early Cretaceous, continental clastic sediments began to form, with minor lithological variation and basin shrinkage. At the end of the Late Cretaceous, and with the closure of the Yarlung Zangbo River, the front edge of the Indian plate collided with the Eurasian plate. In the meantime, the basin kept rising and being denuded, giving rise to the formation of parallel unconformity contact between the Late Cretaceous and the Paleocene. Therefore, the varied appearance of the Lanping Basin is a consequence of long-term complex tectonic movement and sedimentary evolution.

Figure 1 Simplified geological map and tectonic sketch of Lanping basin

The stratigraphic successions in this basin also varied for the period of the Mesozoic tectonism. The Lower Triassic strata are absent in the basin; the Middle Triassic Shanglan formation is composed of a set of deep-water basin facies rocks, and the Upper Triassic strata consist of the Maichunjing and Sanhedong Formations. The Maichunjing Formation is a tidal delta coal-bearing clastic formation, whilst the Sanhedong Formation is a suite of sediments of carbonate facies, which is composed of thin, medium and thick layered dolomitic, crystalline, and bioclastic limestones and siliceous rocks. Most of them are interbedded with gypsum rocks. The lithological evolution of Triassic strata is the response to the closure of the ancient Tethys Ocean [26, 33]. Red clastic rock is the dominant lithology of the Jurassic strata and can be subdivided into three formations as the Lower Jurassic Yangjiang, the Middle Jurassic Huakaizuo, and the Upper Jurassic Bazhulu Formations [36-38]. This reflects a sedimentary characteristic of an intracontinental single-fractured dustpan-like basin [38]. There is a bottom-top vertical zonation in the Cretaceous strata, i.e., the Nanxin, the Hutousi, and the Hemankuan Formations, indicating a sedimentary change from early lacustrine facies fan-delta to late fluvial and lacustrine deposits [39-41]. The gypsum-bearing layers in the basin are seen distributed in the Palaeogene Yunlong Formation, which unconformably lies on either the underlying Hutousi Formation or the Hemankuan Formation. The Yunlong Formation contains grey-purple calcareous siltstones, interbedded with micritic dolomites in the lower part and evaporative salt lacustrine facies mudstone, silty mudstone and marl (locally gypsum) in the upper part.

3 Salt structures and gypsum textures

Unlike the counterparts in the Junggar Basin and Ordos Basin [42, 43], the gypsum rocks in the Lanping Basin are characterized by variable salt structures in the different formations. In the Jinding mining area, the gypsum developed in the form of salt walls, salt beads, and salt pillows seen in the fuchsia sandstone of the Yunlong Formation (Figures 2(a) and (b)). The gypsum located in the Minjiang River area is however characterized by grey-green salt walls (Figure 2(c)). In the same vein, the salt bodies around Yunlong area display a similar grey-green gypsum mud underlying the fuchsia sandstone of the Yunlong Formation (Figure 2(d)). However, salt bodies seen in the Baiyangchang mining area occurred as pillow-like or strip shapes, interspersing the aubergine mudstone of the Yunlong Formation (Figure 2(e)). The gypsum rocks within the Hexi and Cijiao mining area overthrust on the fuchsia sandstones of the Yunlong Formation as salt nappes (Figures 2(f) and (g)). Banded layers structures are peculiar to gypsum of the Sanhedong Formation (Figure 2(h)).On the scale of hand specimens, some gypsum rocks show a grey to white laminar crumple texture (Figure 3(a)). Under the microscope, these gypsum rocks also display a corrugation-deformation characteristic (Figure 3(b)). Some gypsum aggregates intergrown with limestone in sedimentary formations, where the brecciated limestone is cemented by gypsum (Figure 3(c)). Also observed under microscopic study were some anhydrites intergrown with carbonate minerals (Figure 3(d)). In addition, some gypsum aggregates show a lamellar texture, interlayered with lenticular limestone (Figure 3(e)) and are characterized by a directional flow texture in which the anhydrite particles are extremely fine (Figure 3(f)). The original sedimentary gypsum is greyish and white with well-developed thin layers (Figure 3(g)), and prismatic and strip-shaped with obvious mineral boundaries (Figure 3(h)).

Figure 2 Field observation on salt structures of gypsum bodies in the Lanping basin:

4 Samples and analytical methods

Representative gypsum rocks and related carbonate rocks were collected from Sanhedong and Yunlong Formations in the Guolang, Baiyangchang, Hexi, Cijiao, Weixi and Jiayashan areas. The sampling locations are shown in Figures 1 and 4, and the detailed sample information is listed in Table 1.

S isotopes were analyzed at the Key Laboratory of Nuclear Resources and Environment, Ministry of Education, East China Institute of Technology. The detailed analytical procedures were introduced [44]. Thirty-three gypsum rock samples were chosen for analysis. The 400 μg powder (>74 μm size) from each sample was weighed, and wrapped in aluminum foil, and then placed in EA of EA-MS connection measuring device, where the powder will be oxidized by cuprous oxide at 1020 °C to produce SO₂ gas. The pure SO₂ gas was separated by gas chromatography column, and the S isotope ratio of SO₂ gas was determined by a Flash-EA elemental analyzer and a MAT253 gas isotope mass spectrometer. All S isotope ratio data were based on the V-CDT standard, and the analytical error was better than 0.2‰ (1σ).

Figure 3 Hand specimens and microscopic features of gypsum rocks:

Figure 4 Lithologic histogram and sampling horizon

Table 1 Sr and S isotopic data of gypsum rocks and related carbonate rocks in the Lanping basin

Sr isotope analysis was conducted at the Beijing Institute of Geology of the Nuclear Industry, China. Totally 11 carbonate rocks and 7 gypsum rocks are selected for analysis. The samples were pulverized to 74 μm powder in an agate mortar. 0.1-0.2 g powder from each sample was put into a low pressure sealed solution sample tank with the addition of a hydrazine diluent and mixed acid for 24 h, completely dissolving the powder. After evaporate drying, 6 mL hydrochloric acid was added in order to convert the solution into chloride, followed by the supplement of 0.5 mL hydrochloric acid for re-dissolving the sample. After that, the clear solution was separated from the mixture using centrifuge separation and added into a cation exchange column (φ0.5 cm×15 cm, AG50W×8 (H+) 74-149 μm). The Rb and Sr were rinsed with a 1.75 mL and 2.5 mL hydrochloric acid solution, respectively, and evaporated to dryness for mass spectrometry. The isotope analysis was performed using an ISOPROBE-T thermal ionization mass spectrometer, with a single band, M+, adjustable multi Faraday receiver. In the course of the experiment, the relative humidity was 21% and the temperature was 19°C. Mass fractionation was corrected by ⁸⁶Sr/⁸⁸Sr=0.1194. Reproducibility and accuracy of Sr isotopic analyses were periodically checked by running standards NBS 987, with a mean ⁸⁷Sr/⁸⁶Sr value of 0.710250±0.000007 (certified value: 0.710340±0.000260, 2σ). The analytical blank was <2×10^-10 g for Rb and Sr. Isotope ratio error was 2σ. The details of analytical procedures were similarly described in Ref. [45].

5 Results

The Sr and S isotope analytical results are shown in Table 1. For the Sr isotopes, seven gypsum samples and nine limestone samples from the Yunlong Formation show characteristic and variable ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.707695 to 0.708629 (average=0.708030) and from 0.707732 to 0.710189 (average=0.708293), respectively. Two limestone samples from the Sanhedong Formation yielded ⁸⁷Sr/⁸⁶Sr ratios of 0.707737-0.707783 (average=0.707760). The S isotopes of 28 gypsum samples analysed from the Yunlong Formation show variable δ34SV-CDT contents in the range of 9.6‰-17‰ (average=13.7‰). Other three gypsum samples from the Sanhedong Formation are characterized by consistent δ34SV-CDT contents in the range of 14.5‰-14.8‰ (average=14.7‰).

6 Discussion

Provenance or tracing the source of rocks using Sr and S isotopes as fingerprint has been widely used in recent time due to some degree of its reliability. Sr isotopes have four stable isotopes (⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr and ⁸⁸Sr), among which only ⁸⁷Sr is a radioactive product that formed by beta decay of ⁸⁷Rb. It often occurs in minerals containing calcium and potassium in a dispersed state. Shell-derived silica-alumina rocks have a higher ⁸⁷Sr/⁸⁶Sr value (0.720±0.005 on average), but with a lower concentration of Sr. The ⁸⁷Sr/⁸⁶Sr ratios in mafic rocks (mean=0.704±0.002) and marine carbonates and sulfates (mean=0.708±0.001) are generally lower but show higher Sr contents. Although whole-rock Sr isotopic features can be altered by some geological processes (such as evaporation), the ⁸⁷Sr/⁸⁶Sr value of water in the same area can be preserved for a longer time so that contrasting Sr isotopic characteristics between marine and terrestrial can be recognized [46]. Different geological bodies have characteristic S isotopic ratio that can be distinguished. There are four stable isotopes (δ³²S, δ³³S, δ³⁴S, δ³⁶S) of S, among which δ³⁴S serves as a better fingerprint of rock tracers. Three sources of sulfur were found in this study: 1) mantle sulfur; showing δ³⁴S close to 0 with a range of 3‰, 2) sea water sulfur; δ³⁴S is approximately 20‰, 3) reduced sulfur or called biological sulfur; characterized by negative δ³⁴S which is primarily controlled by two mechanisms, one is thermodynamic equilibrium fractionation and another is kinetic fractionation [46-49]. The thermodynamic equilibrium fractionation relys more on temperature and occurs in hot aqueous system with magma degasification process and temperature change. As for the kinetic fractionation control, it refers to isotopic composition variation caused by the inconsistent reaction rate of isotopic atoms or groups such as oxidation reaction, bacterial reduction, thermal decomposition of organic matter, organic reduction, high-temperature inorganic reduction process, and disproportionation [47]. Biological bacterial reduction and pyrolysis of organic matter were recognized as the main cause to increase the S isotopic fractionation by means of the S isotope studies of Pb-Zn deposits in the Lanping Basin. Moreover, the indispensable sulfur for local mineralization derived from this gypsum is also a shred of convincing evidence that the high sulfur from gypsum is a result of the above two mechanisms [48]. Similarly, the increase of sulfur from the marine gypsum hosted in Sanhedong Formation is also because of the two effects. In contrast, marine sulfur in conjunction with external water sulfur participates in the salification for gypsum in Yunlong Formation. In spite of the development of a regionally deep-seated fault, no evidence of magmatism has been reported during either salification or mineralization that could bring sulfur of mantle origin to this shallow part of the Lanping Basin. Thus, mantle sulfur cannot be said to play a role in the formation of gypsum as well as metal sulfides in this case.

6.1 Sedimentary environment of gypsum: Sr isotopic implications

Depositional environment of sedimentary formations can be determined using the ⁸⁷Sr/⁸⁶Sr ratios among other numerous techniques [48, 49]. Previous studies have found that the residual time of Sr in seawater (2.5 Ma) is much longer than the mixing time of seawater (1.6 Ka). On a million-year time scale, the distribution of Sr in the ocean can be considered to be uniform, thus the ⁸⁷Sr/⁸⁶Sr ratio of an oceanic reservoir in a specified time span is unique, which is completely independent of location or water depth [50-53]. Seawater occupies most part of the earth’s surface, so most of the geological effects will affect the composition of seawater, accordingly changing the Sr isotopic compositions. In general, Sr in seawater is derived from three sources: 1) crust-sourced Sr resulting from weathering of aluminosilicate rocks; 2) mantle-sourced Sr produced by seafloor heat flow; and 3) sea-sourced Sr generated by re-dissolution of ocean carbonate and sulfate [50-52]. The crust-sourced Sr has a high ⁸⁷Sr/⁸⁶Sr value [54], whereas the mantle-sourced Sr possesses a relatively low ⁸⁷Sr/⁸⁶Sr. In contrast, the ⁸⁷Sr/⁸⁶Sr ratio of sea-sourced Sr is between that of crust-sourced and mantle-sourced [55]. The average ⁸⁷Sr/⁸⁶Sr value of modern seawater is ~0.709 [56], Cretaceous seawater is 0.707-0.708 [55], and Triassic marine carbonate rock is 0.70695-0.70845 [57-60].

In the Lanping Basin, the ⁸⁷Sr/⁸⁶Sr ratios of seven gypsum rocks from the Yunlong Formation range from 0.707695 to 0.708629 (average= 0.70803). This range is consistent with that determined by Ref. [61] and close to the ⁸⁷Sr/⁸⁶Sr value of the Paleogene ancient seawater (0.7078 [59]), but different from those ⁸⁷Sr/⁸⁶Sr values of the crust (about 0.720), the mantle (about 0.704) and other typical gypsum deposits (such as Dongpu and Qaidam) in China (Figure 5).

This indicates that the gypsum rocks in the Yunlong Formation may have formed in a marine sedimentary environment, and the gypsum-forming materials were mainly derived from seawater. In contrast, the nine related carbonate rock samples from the Yunlong Formation show a wider range (0.707732 to 0.710189, average=0.708293) in ⁸⁷Sr/⁸⁶Sr ratios which are obviously higher than those of the gypsum rocks and coeval ancient seawater, probably due to the influence of mixing of continental terrigenous sediments. This is evidenced by one extremely high ⁸⁷Sr/⁸⁶Sr value (0.710189) of the sample B-007 (grey-black limestone) (Table 1), which contains debris components such as terrigenous clay minerals. The two contrasting Sr isotopic features may imply a hybrid source for the Yunlong gypsums probably derived from more terrestrial materials, freshwater and clay. The two latter components are in virtue of the transgression during this period. All of these indicate that the Yunlong Formation gypsum and related carbonate rocks were formed in a marine environment but subjected to the contamination of continental materials.

Figure 5 Strontium isotope ranges from different sources, showing Sr patterns of Paleogene Yunlong Formation and Triassic Sanhedong Formation (Crust and mantle data from Ref. [40]; marine data from Ref. [47]; Qaidam and Dongpu data from Refs. [46] and [54], respectively)

In contrast, the two limestone samples from the Triassic Sanhedong Formation possess the ⁸⁷Sr/⁸⁶Sr values of 0.707737-0.707783 (average= 0.707760) (Table 1), close to the coeval marine ⁸⁷Sr/⁸⁶Sr vales (0.707) [57-60]. This indicates that the limestone of the Triassic Sanhedong Formation was formed in a marine environment and seawater provided materials for the chemical deposition of gypsum with minor addition of continental detrital materials.

6.2 Origin of materials: S isotopic insights

Hydrothermal mineralization is characterized with significant S isotopic fractionation. Both S isotope and ore mineral component can be applied to reveal the physico-chemical conditions under which a material formed, and to trace the source of sulfur and also further to investigate the mineralization environment and ultimately the origin of ore-forming substances. The S isotope fractionation coefficient is extremely low not only between dissolved SO₂^4- and precipitated sulfate at 25 °C, but also during gypsum crystallization in some other basins worldwide [62]. These studies further suggest that S isotopic ratio of sulfate (in evaporites) can represent that of ancient lake or sea water, thereby serving as a good discriminating agent of paleoenvironment. In contrast to marine facies, continental salt lake varies greatly in S isotopic compositions of sulfate [63]. It is generally considered that the best approach to resolving the S isotopic compositions for basin water bodies and sediments is provenances and bacterial reduction [64]. Sulfates originated from old evaporites are likely to enrich heavy S isotopes, whilst those sourced from weathering products of black shale will comparatively be enriched in light S isotopic components [65]. Bacterial reduction, as one of the sulfur circulation processes in nature is the foremost fractionation process with regard to S isotope, resulting in the maximal S isotope fractionation [66].

In the Lanping Basin, the δ³⁴S_V-CDT values of the gypsum from the Paleogene Yunlong Formation range from 9.6‰ to 17‰ (mostly 13‰-15‰, average=13.7‰, n=28). These values fall into but are slightly lower than the δ³⁴S_V-CDT ranges (14‰-21‰) of the seawater during the Late Cretaceous (Figure 6), showing similar source properties with the gypsum of Kuqa Basin, thus indicating contamination of continental freshwater origin of the gypsum in the Lanping Basin.

In contrast, the δ³⁴S_V-CDT values of the gypsum from the Triassic Sanhedong Formation show insignificant variation, i.e. 14.5‰-14.8‰ (average=14.7‰, n=3), which is consistent with that of the Triassic seawater (~15‰,) [67] and those of the barite from the Shuixie (12.3‰-19.0‰) and Bailongchang (13.1‰) deposits in the Lanping Basin [68]. With the formation age progressively getting younger, the value of the δ³⁴S_V-CDT of gypsum in Kuqa Basin decreased from 20‰ to 9‰-10‰ and then rose to 12.7‰, showing the transition from marine facies to terrestrial facies and then back to marine facies. By comparison between the two basins, we can infer that materials that produced the gypsum in Sanhedong Formation within the Lanping Basin are mostly provided by marine facies without obvious continental input, while gypsum hosted in Kuqa Basin is characterized by multi-sources [69, 70].

Figure 6 Sulfur isotope range from different sources, showing the S patterns of the gypsum from Paleogene Yunlong Formation and the Triassic Sanhedong Formation [59]

6.3 Evolutionary model of gypsum formations

Previous studies suggested that anhydrite is distributed at the margin or deeper part of the Pb-Zn orebody in the Jinding deposit [71], in which a mineralization zonation can be identified from the inner part to the outer part as Pb-Zn, celestite, and anhydrite orebody. WANG et al [72] further divided the ore-bearing breccia into non- mineralized tectonic-gypsum breccia and diapiric- emplacement breccia and combined Pn-Zn, pyrite, celestite and anhydrite mineralization with the superposition relationship between the sandstone- type orebody and limestone breccia-type orebody, proposing that the gypsum (or anhydrite) and celestite orebodies provided the space for mineralization [72]. These authors further built a multivariate metallogenic model that is composed of and interrelated by tectonic nappe, salt dome, regional extension, oil and gas accumulation, diapir of flow sand, fluid drainage and metallic precipitation [73]. In addition, the mineralization in the Jinding area was believed to be relevant to the salt diapir structure caused by thrust nappe, and diapiric breccia or post-diapiric salt-soluble breccias are important parts of mineralization [74]. DONG [75] and LI et al [76] demonstrated that there are numerous salt dome structures developed in the Jinding orefield, whilst mineralization and alteration zoning are reckoned to be affected by the diapirism of salt structures based on the study regarding the metallogenic situation around the salt dome [75, 76]. As mentioned earlier, distinct salt structures are widely developed in the gypsum formations in different areas of the Lanping Basin. In the Jinding mining area, we propose that the salt structures such as salt anticline, salt bead and salt pillow in the Sanhedong Formation were formed by early compression and differential compaction and these salt bodies have undergone thrusting- extrusion and diapirism (Figure 7(a)). Such salt structures are distributed in the interlaminar detachment fractures near regional faults, which provide spaces for the salt bodies as well as Pb-Zn ores. Additionally, based on the Sr and S isotope studies, the source and formation environment of gypsum salts can be revealed, which supplies essentials (e.g., sulfur) not only for the sedimentation and salification of gypsum but also for the Pb-Zn mineralization.

Figure 7 Outline diagram of salt structures in Lanping basin:

The salt structures in the Baiyangchang deposit are characterized by diapiric layers of the Yunlong Formation, with gypsum mud veins intruding along the fracture zones of the overlying purple-red sandstone and mudstone (Figures 7(b) and (c)). The development of these salt structures is due to the structural movement and differential compaction of the overlying rock mass. The gypsum mud migrated along the minimum stress direction in faults and fractured zones. In addition, a large number of limestone breccia can be observed in the Yunlong Formation. These breccias may have been carried out by gypsum mud during the deformation process, due to its good plasticity served as lubricant and cementing agent.

In the Cijiao area, two sets of gypsum rock formations were determined (Figure 7(d)). The upper salt layer is lenticular and lamellar with black gypsum hosted in the Yunlong Formation purple-red sandstones. The salt bodies intruded into the Yunlong Formation as mushroom shapes.

The lamellar gypsum indicates a plastic deformation caused by reginal structural activities, whereas the lenticular gypsum suggests a plastic flow characteristic. In contrast, the lower salt layer is located in the middle of the thick-layered limestone (interbedded with limestone) in the Sanhedong Formation with original banded sedimentary structures, indicating a less deformation and modification for the Sanhedong gypsum.

Based on the analysis of the salt structures in the Lanping Basin, a two-stage evolution model is proposed to explain the gypsum formations. 1) Paleocene-Eocene intralayer compression-depression stage (Figure 8(a)). The extrusion- depression Lanping basin was formed as a result of the collision between the India Plate and the Eurasian Plate. During the extrusion process, salt materials migrated towards to the basin center as plastic bodies in the layers, forming conformable salt structures such as salt anticline and salt pillow in the Sanhedong Formation and weakly modifying their S and Sr isotopes; 2) Oligocene overthrust- detachment stage (Figure 8(b)). The strata of the basin were subjected to the compression of the EW-trending structures, resulting in overthrust dismantling deformation and migration of salt bodies along with overthrust faults and secondary fractures. As a consequence, salt beads and salt walls were developed in the Yunlong Formation. The S and Sr isotopes in these gypsum were largely modified because of the input of largely continental materials, thereby producing the variable S and Sr isotopic ratios seen in the Formations.

Figure 8 Evolutionary model of gypsum formations in the Lanping basin:

7 Conclusions

1) The gypsum rocks in the Paleogene Yunlong Formation are grey-green and breccia-containing, displaying variable salt structures such as salt beads, salt walls and salt pillows. In contrast, the gypsum rocks from the Triassic Sanhedong Formation are grey-black and thick-layered, with original banded sedimentary structures.

2) The gypsum and related carbonate rocks in the Paleogene Yunlong Formation are characterized by variable S (δ³⁴S_V-CDT=9.6‰-17‰) and Sr (⁸⁷Sr/⁸⁶Sr=0.707695-0.710189) isotopic ranges, indicating a marine origin with input of continental materials. In contrast, the gypsum rocks from the Triassic Sanhedong Formation have narrowed S (δ³⁴S_V-CDT=14.5‰-14.8‰) and Sr (⁸⁷Sr/⁸⁶Sr= 0.707737-0.707783) isotopic ratios, implying a single marine provenance.

3) Combined with field evidence and isotopic analyses, a two-stage evolutionary model is hereby proposed to give a clearer understanding of the formation of the gypsum rocks in the Lanping Basin: Paleocene-Eocene intralayer compression- depression stage and Oligocene overthrust- detachment stage.

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(Edited by FANG Jing-hua)

中文导读

中国西南地区含铅锌矿化兰坪盆地中石膏层的盐结构和来源

摘要：兰坪盆地广泛分布着大规模的石膏层和世界级的铅锌矿床。本研究结合野外调查和地球化学分析结果以确定本区石膏的来源和演化过程。兰坪盆地石膏层序发育于三叠系三河洞组和古近系云龙组两个地层。前者为原生厚条带状结构，δ³⁴SV-CDT值在14.5‰～14.8‰。结合灰岩⁸⁷Sr/⁸⁶Sr比值(0.707737～0.707783)，本文认为三河洞组为海相成因。相反，古近系云龙组石膏具有穹隆、珠状和底辟盐构造特征，⁸⁷Sr/⁸⁶Sr(0.707695～0.708629)和δ³⁴SV-CDT值(9.6‰～17‰)的区间较宽，表明其为海相来源，但也有陆相物质的输入。最初的层状盐层是在盆地中以化学沉积的形式形成的，后来的多种形貌不同的盐结构是在喜马拉雅期受到碰撞造山作用的影响，发生强烈变形而形成的。在此基础上，构建了一个解释兰坪盆地石膏建造成因的演化模式。

关键词：兰坪盆地；石膏；海相沉积；Sr–S同位素；盐结构

Foundation item: Project(41362008) supported by the National Natural Science Foundation of China

Received date: 2019-04-25; Accepted date: 2019-12-14

Corresponding author: ZHU Zhi-jun, PhD, Professor; Tel: +86-13979498401; E-mail: zhuzj013@163.com; LI Huan, PhD, Professor; Tel: +86-13627411322; E-mail: lihuan@csu.edu.cn; ORCID: 0000-0001-5211-8324