J. Cent. South Univ. (2019) 26: 3516-3533

DOI: https://doi.org/10.1007/s11771-019-4270-9

Genesis of Qujiashan manganese deposit, Shaanxi Province:constraints from geological, geochemical, and carbon and oxygen isotopic evidences

WANG Zi-yong(王子勇)¹, HAN Run-sheng(韩润生)¹, REN Tao(任涛)¹,WU Yong-tao(吴永涛)¹, LI Hu-jie(李虎杰)²

1. Faculty of Land Resource Engineering, Kunming University of Science and Technology,Kunming 650093, China;

2. College of Environment and Resource, Southwest University of Science and Technology,Mianyang 621010, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: The Qujiashan manganese deposit is located in the Longmen-Daba fold belt along the northern margin of the Yangtze Block. The layered ore bodies are distributed within the purple-red calcareous shale. Qujiashan is a high-grade w(MnO)=8.92% to 18.76%) manganese deposit with low-phosphorus w(P₂O₅)=0.08% to 0.16%) content. It also has a low total REEs contents (with an average of 101.3×10^-6), and has inconspicuous Ce (0.81 to 1.29) and Eu (1.00 to 1.25) anomalies. lg(Ce/Ce^*) values are from -0.02 to 0.11. The ores have high SiO₂/Al₂O₃ and Al/(Al + Fe + Mn) ratios. In figures of Fe–Mn–[(Ni+Cu+Co)×10] and lgU–lgTh, all samples show that hydrothermal exhalative fluids played an important role during mineralisation. The δ¹³C_PDB and δ¹⁸O_SMOW values of eight ore samples are from -20.7‰ to -8.2‰ (with an average of -12.4‰) and from 14.3‰ to 18.7‰ (with an average of 17.0‰), respectively. These carbon and oxygen isotopic features indicate that hydrothermal fluids derived from deep earth are participation in the metallogenic process, which is also supported by high paleo-seawater temperatures varying from 47.08 to 73.98 °C. Therefore, the geological and geochemical evidences show that the Qujiashan deposit formed from submarine exhalative hydrothermal sedimentation.

Key words: manganese deposit; element geochemistry; carbon and oxygen isotopes; genesis; Qujiashan manganese deposit

Cite this article as: WANG Zi-yong, HAN Run-sheng, REN Tao, WU Yong-tao, LI Hu-jie. Genesis of Qujiashan manganese deposit, Shaanxi Province: constraints from geological, geochemical, and carbon and oxygen isotopic evidences [J]. Journal of Central South University, 2019, 26(12): 3516-3533. DOI: https://doi.org/10.1007/s11771-019- 4270-9.

1 Introduction

Following the Wuling and Jinning tectonic movements, a crystalline basement formed in the Yangtze Block during the late Mesoproterozoic to the Neoproterozoic. Sedimentary rocks predominantly widely distributed in the north- northeast (NNE) intercontinental extensional rift of Yangtze Block. Many of the manganese deposits have been explored, including the Songtao in Guizhou Province, Xiushan in Chongqing city, Changyang in western Hubei Province and Ziyang in Shaanxi Province.

The Qujiashan manganese deposit is located at the boundary of the northern and southern Daba Mountains (Figures 1(a) and (b)). Layered ore bodies host in the manganese-bearing rocks. The rocks consist of argillaceous volcanic rock, argillite, carbonate rock and siliceous rock. The deposit is a high grade manganese deposit with low phosphorus and iron contents. Although the deposit was discovered in the 1950s, previous studies have mainly focused on geological characteristics, mineral composition and lithofacies paleogeography [1], while many scientific issues, such as source of ore-forming material, metallogenic environment and ore genesis, have not been studied. In this study, the results of systematic geological survey and ore texture observation, major and trace element analysis of ore and wall rock, and carbon and oxygen isotope analysis of ore are used to discuss the source of the metal substance, ore-forming setting and genesis of the Qujiashan deposit.

Figure 1 (a) Tectonic map of Qinling orogenic belt (modified from Ref. [4]); (b) Regional tectonic map of northern margin of Yangtze Block; (c) Geological map of Qujiashan manganese deposit. NCB—North China Block; NQB—North Qinling Belt; SQB—South Qinling Belt; NDB—Northern Daba Mountains thrust-nappe Belt; SDB—Southern Daba Mountains foreland fold-thrust Belt; YZB—Yangtze Block. 1—Lower Triassic; 2—Cambrian, Shipai Fm; 3—Cambrian, Shilongdong Fm; 4—Cambrian, section I of Dazhaigou Fm; 5—Cambrian, section II of Dazhaigou Fm; 6—Section I-II of Doushantuo Fm; 7—Section III of Doushantuo Fm; 8—Section I of Doushantuo Fm; 9—Section II of Doushantuo Fm; 10—Nantuo Fm; 11—Geoline; 12—Fault; 13—Orebody and number; 14—Line of exploration section; 15—Location and number of cave; 16—Manganese deposit; 17—City; 18—Town

2 Regional geology

The Daba Mountains lie along the northern margin of the Yangtze Block. It formed into the collision between the North China Block and Yangtze Block [2]. The northern margin of the Yangtze Block was an active continental margin in the Neoproterozoic. Carbonaceous shale, siltstone, silicolite, marl, dolomite and submarine volcanic rocks are widely distributed in the region [3]. The northern Qinling Mountains are the southern boundary of the North China Block, whereas the southern Qinling Mountains mark the northern boundary of the Yangtze Block. The Daba Mountains were situated at the northern edge of the Yangtze Block in the early Sinian. From Ordovician to Devonian, the Yangtze Block subducted underneath the North China Block. Both the northern Daba Mountains and the detached area of the northern Yangtze Block formed the Qinling micro-plate [4, 5]. The Qinling micro-plate subducted underneath the North China Block and collided with it in the Late Devonian. The northern and southern Daba Mountains experienced both ocean-continent subduction and continent-continent collision during Indosinian movement [6]. The northern Daba Mountains thrust-nappe belt and the southern Daba Mountains foreland fold-thrust belt formed during these geological movements.

The Bashan manganese metallogenic belt is located in the depression zone of northern margin of the Yangtze Block. This belt is controlled by fractures along both sides of the Daba Mountains (Figures 1(a) and (b)). The Bashan metallogenic belt is further divided into the Shuijingping, Liziya and Qujiashan secondary manganese metallogenic belts.

3 Geology of ore deposit

The study area is located in the northeastern limb of the Songshuba-zihuang anticline (Figure 1(c)), which is a northwest trending overturned anticline. Manganese-bearing rocks have experienced strong deformation due to a series of strong squeezing isoclinal and plunging folds (Figure 2). Sinian to Triassic strata is mainly exposed in the mine (Figure 1(c)). The Sinian Dengying Formation (Zbdn) is subdivided into two sections: 1) the upper section consists of grey massive dolomite with a thickness of more than 100 m; 2) the lower section consists of grey siliceous-bearing limestone, marl and shale, with a thickness of 50-60 m. The lower section has a conformable contact with the underlying strata.

Figure 2 Profile of No.20 exploration line of Qujiashan manganese deposit

Based on rock combination, the Sinian Doushantuo Formation (Zbd) is subdivided into three sections:1) the upper section (Zbd³) is composed of manganese-bearing rock series, including rhodochrosite, Mn-bearing shale and silicolite. Abundant pyrite occurs as fine-grained and disseminated in the Mn-bearing shale. The bottom of the ore body is composed of mainly purple-red, partly reddish or grey-green shale. 2) The middle section consists of grey-green shale, argillaceous sediments and lamellar quartz-bearing fine-grained sandstone. 3) The lower section consists of dark- grey and greyish shale, mudstone and quartz sandstone.

The Sinian Doushantuo Formation is the ore-host stratum. Influenced by tectonic events, both stratum and ore bodies have been folded (Figure 2). The attitudes of the ore bodies are concordant with fold occurrence and present an echelon. The ore bodies mainly consist of dark grey-green shale and siliceous rocks (Figure 3). Reddish-brown rhodochrosite is the main ore mineral. It is occasionally weathered to psilomelane and pyrolusite.

Seven ore bodies have been explored (Figure 1(c)), with a controlled length of 80 to 555 m and a thickness of 0.50 to 6.23 m. Tilt extension depths range from 87 to 350 m. The ore bodies are mainly northwest-trending and dip steeply (approximately 50°-80°). The thickness gradually increases in the order of III-, IV-, V- and VI-ore bodies. Therefore VI-ore body is the largest, with a thickness of 6.23 m. III-, IV- and V-ore bodies have the average grades of 19.24%, 20.48% and 22.91%, respectively.

Qujiashan deposit has simple ore mineral assemblages that consist of psilomelane, pyrolusite, braunite, rhodochrosite and manganocalcite, and gangue minerals include calcite, quartz, feldspar, chlorite, antigorite and barite. The large variety of ore textures present in the Qujiashan deposit. The most important textures include the following features. Banded texture consists of rhythmically banded ore containing distinct generations of rhodochrosite or gangue mineral of calcite, braunite, hydromuscovite, kaolinite and chert. The widths of bandeds range from a few millimeters to a dozen centimeters (Figure 4(a)). Rhodochrosite occurs as densely disseminated in argillaceous minerals (Figure 4(e)). Irregularly banded and lenticular texture consists of discontinuous or lentoid rhodochrosite vein. It mainly appears at the edge of the ore bodies or near the upper and lower wall rock (Figure 4(b)). Volcanic rock is often observed in the contact area between rhodochrosite and surrounding rock (Figure 4(f)). Massive texture formed when width of the rhodochrosite banded is larger than 20 cm. The Mn grade of ore is generally higher than 25% (Figure 4(c)). And Mn-bearing calcite vein filled in fissure of the manganese- bearing rocks (Figure 4(d)).

4 Methods

The major elements were analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Leeman) at ALS Minerals (Guangzhou), with uncertainties of analytical results of 2%-8%. 30 mg powdered sample was weighed into a graphite crucible and 130 mg lithium metaborate was added. The mixture was then placed in a temperature-controlled furnace (at 1000 °C) for 15 min to facilitate the dissolution. After the sample was cooled to room temperature, 5% aqua regia was added until the sample was completely dissolved by ultrasonic extraction. The liquid phase was then placed into 25 mL colorimetric tubes and 25 μL of cadmium standard solution (1 μg/mL) was added. The solution was then diluted with 5% aqua regia.

Trace elements were analyzed by inductively coupled plasma-mass spectrometry at the State Key Laboratory of Ore Deposit Geochemistry of the Institute of Geochemistry, Chinese Academy of Sciences. The precision and accuracy of the data for the trace elements were determined using international standards GBPG-1, OU-6 and the Chinese National standard GSR-1, and were generally better than 5%. 50mg powder sample was accurately weighed and fully digested using 1 mL HF in a Ploytetrafluoroethylene (PTFE) crucible. The sample was then heated on an electric hot plate to remove most of the SiO₂and the residue was dissolved in a solution of 1 mL HF and 0.5 mL HNO₃ at 200 °C for 48 h. Once the sample was cooled to room temperature, 1 mL HNO₃ was added. The dissolution process was repeated and the second step was performed once more. After the liquid phase was evaporated to dryness, 2 mL HNO₃ and 5 mL distilled water were added to redissolve the residue. The solution was heated at 130 °C for 3 h. Once the solution was cooled to room temperature, 1 mL of 1 μg/mL Rh solution was added. The insoluble solid residue portions were separated by centrifugation, and the clear solution was diluted with 30 mL water to a total volume of 50 mL. The analytical methods and procedures are described in Ref. [7].

Figure 3 Representative sketch maps of Qujiashan manganese deposit

Figure 4 Photographs and micrographs of representative ores from Qujiashan deposit

Rhodochrosite samples were crushed to 75 μm powders and reacted with concentrated phosphoric acid at 25 °C. The CO₂ was extracted from rhodochrosite with pure phosphoric acid (H₃PO₄) at 50 °C. Evolved CO₂ was then purified and its carbon and oxygen isotope compositions were measured by a Finnigan MAT 251 at the State Key Laboratory of Environmental Geochemistry, Chinese Academy of Sciences. The precision was better than 0.1‰ for oxygen and carbon isotopic compositions. The results are reported relative to Pee Dee Belemnite (PDB), and expressed as δ¹⁸O_SMOW =1.03086δ¹⁸O_PDB+30.86.

5 Results

5.1 Major elements

The major element compositions of 27 samples are presented in Table 1. The CaO and MgO contents in the ores range from 1.89% to 5.99% and from 1.92% to 5.58%, respectively, and the loss on ignition (LOI) ranges from 11.39% to 21.67%. The Na₂O, TiO₂, P₂O₅ and SO₃ contents are lower than 1%, with values of 0.12% to 0.85%, 0.10% to 0.35%, 0.08% to 0.16% and 0.02% to 0.20%, respectively. The MnO and Fe₂O₃contents range from 8.92% to 18.76% and from 1.42% to 3.18%, respectively, with Fe/Mn molar ratios of 0.07 to 0.27. The ores have high contents of SiO₂ (16.28%-31.06%) and Al₂O₃ (2.04%- 6.36%).

The surrounding rocks are mainly shale, limestone or calcareous shale. They have CaO and MgO contents range from 0.53% to 20.30%, and 4.03% to 17.80%, respectively. Their LOI values range from 3.94% to 28.22%. Compared to the manganese ores, they have higher contents of K₂O (0.18% to 3.21%), Na₂O (0.12% to 1.77%), TiO₂ (0.20% to 0.82%), P₂O₅ (0.10% to 0.30%), SO₃ (0% to 0.68%), Al₂O₃ (3.83% to 13.56%), Fe₂O₃ (1.54% to 6.37%) and SiO₂ (28.07% to 63.89%),but lower MnO contents (0.06% to 8.01%).

Table 1 Major element compositions of ores and wall rocks in Qujiashan manganese deposit (mass fraction, %)

5.2 Trace elements

The results of the rare earth elements (REEs) analysis of eighteen ore and wall rock samples from the Qujiashan deposit are shown in Table 2, which can be summarised as follows: 1) The total REEs contents of limestone and marl are relatively low (30.92×10^-6 to 96.94×10^-6); 2) The shale has the highest total REEs contents in the mine (53.82×10^-6 to 155.15×10^-6, with a mean of 117.5×10^-6). However, these values are lower than the average total REEs content of shale in South China Block with an average of 200×10^-6 to 300×10^-6 [8]; 3) The total REEs contents of the ores range from 50.88×10^-6 to 108.99×10^-6, with a mean of 92.91× 10^-6, which is considerably higher than marine carbonate of 19×10^-6 [9].

Table 2 Rare-earth element compositions of ores and wall rocks in Qujiashan manganese deposit

The PAAS-normalized REEs patterns of ores are flat (Figure 5(a)), with the La/Yb ratios ranging from 0.95 to 1.39 (with a mean of 1.16). REEs fractionation is not obvious and has different characteristics compared with modern seawater (Figure 6(a)) [14]. Most samples have weakly positive Eu and Ce anomalies, except for one sample. The Eu and Ce negative anomalies for one sample may be influenced by hydrothermal alteration and calcite vein. Wall rocks have slightly negative Eu anomalies.

Representative trace element results of eight ore samples are shown in Table 3. The contents of V range from 56×10^-6 to 79×10^-6, Cr from 20×10^-6 to 40×10^-6, Co from 11.4×10^-6 to 22.3×10^-6, Ni from 32.5×10^-6 to 77.2×10^-6, Sr from 127.5×10^-6 to 260×10^-6, Mo from 6.98×10^-6 to 15.7×10^-6, Th from 1.0×10^-6 to 2.6×10^-6 and U from 0.6×10^-6 to 1.0×10^-6. The Th/U ratios range from 0.17 to 3.71, V/Cr from 1.87 to 3.90, Ba/Sr from 5.94 to 31.80 and V/(V+Ni) from 0.43 to 0.71.

The V contents of wall rocks range from 23×10^-6 to 78×10^-6, Cr from 30×10^-6 to 90×10^-6, Co from 10.0×10^-6 to 16.7×10^-6, Ni from 39.4×10^-6 to 56.3×10^-6, Sr from 103.5×10^-6 to 205×10^-6, Mo from 0.19×10^-6 to 5.84×10^-6, Th from 3.3×10^-6 to 7.3×10^-6 and U from 0.5×10^-6 to 1.0×10^-6.

6 Discussion

6.1 Sources of manganese

Al₂O₃ is obviously positive correlation with SiO₂, TiO₂, K₂O, TFe₂O₃, P₂O₅ (Figure 7), respectively, but is negative correlation with MnO and S in the ores from the Qujiashan deposit. This geochemical feature suggests that MnO and S derived from submarine exhalative hydrothermal fluids. The SiO₂/Al₂O₃ and Al/(Al+Fe+Mn) ratios range from 4.8 to 9.6 (>3.6) and from 0.1 to 0.27, respectively, also suggesting that the submarine exhalative fluids played an important role in the mineralization [16]. Furthermore, the low Al₂O₃+ TiO₂ content indicates that the ore contains a small amount of terrigenous components [17, 18].

Figure 5 PAAS-normalized REEs patterns of ores from (a) Qujiashan (modified from Ref. [10]), (b) Xiangtan [11],(c) Xixibao [12] and (d) Gaozeng [13] deposits

Figure 6 Pacific seawater at different depth (a) (modified from Ref. [14]) and Co-rich crusts in Pacific seafloor (b) (modified from Ref. [15]) PAAS-normalized REEs distribution patterns

Table 3 Trace element compositions of ores and wall rocks in Qujiashan manganese deposit

However, the SiO₂/Al₂O₃ and Al/(Al+Fe+Mn) ratios in surrounding rocks show a normal marine sedimentary.

Fe-Mn-(Ni+Cu+Co)×10 and lgU-lgTh discrimination diagrams are often used to study the genesis and source of wall rock and ore [19]. All of the results plotted within or near the Palaeo- hydrothermal sedimentation field and near the Fe–Mn end-member (Figure 8(a)), as well as lgU–lgTh diagram (Figure 8(b)). These values are similar to those of the Taojiang hydrothermal manganese deposit [20, 21], indicating that the genesis of Qujiashan deposit may be related to hydrothermal sedimentation.

The trace element data also support this speculate. Researchers have proposed that the ratio of Ba/Sr is an indicator of the formation setting of deposit. The value of Ba/Sr less than 1 represents a normal sedimentary environment; and the value of Ba/Sr is greater than 1, which represents a submarine hydrothermal environment [22, 23]. The Ba/Sr ratio in the Qujiashan deposit varies from 5.94 to 31.80 indicating the deposit formed in a hydrothermal environment. Th/Sc ratios of ores and wall rocks range from 0.16 to 0.56 and from 0.42 to 1.70, respectively. Both of the ratios are less than the average value of the upper crust of 1.0 [24], indicating the participation of the mantle material in the metallogenic process [16].

Figure 7 Correlograms of some major elements in Qujiashan deposit:

Figure 8 lgU-lgTh diagram (modified from Ref. [19]) (a) and Fe-Mn-(Ni+Cu+Co)×10 diagram (modified from Ref. [35]) (b) of Qujiashan deposit. HD–hydrothermal sediment; HN-hydrolith; CR-Fe-Mn crust; ED-hydrothermal metal deposit of Eastern Pacific; I-Normal ocean sediments; II-Deposited Pacific Rise; III-Palaeo-hydrothermal sedimentation

The carbon and oxygen isotopic values of ores are shown in Table 4. The δ¹³C_PDB values range from -20.7‰ to -8.2‰, which are significantly depleted compared with marine carbonates (-1‰ to 2‰) [25-27] and freshwater carbonates (-4.9‰) [26, 27]. They are also distinctly lower than those of MORE and IAB (-2‰ to -11‰) [28-30]. This isotopic feature suggests that the carbon likely derived from inorganic carbon [31]. The δ¹⁸O_SMOW values range from 14.3‰ to 18.7‰ (with a mean of 17.0‰), which are lower than the oxygen isotope values of modern marine carbonates (28‰ to 30‰) [26]. The Qujiashan deposit has significantly different carbon and oxygen isotopic compositions compared with manganese deposits distribute at the margin of the Yangtze Block (Figure 9). Previous studies have suggested that diagenesis, metamorphism and hydrothermal alteration could decrease the carbon and oxygen isotope values of carbonate rocks. However, such large isotopic fractionation is not resulted by above stated mechanism [32-34]. In the δ¹³C_PDB-δ¹⁸O_SMOW diagram, most of the samples plotted in the mixing field of the marine carbonate, sedimentary organic matter and mantle source (Figure 9). This especial isotopic feature suggests that Qujiashan deposit has genetic relationship with submarine exhalative hydrothermal fluids.

6.2 Metallogenic environment

The solubility of redox-sensitive elements, such as U, V and Mo, can be used to determine the redox state of ancient oceanic sediments or sedimentary rocks [36-40]. Soluble U⁶⁺ represents an oxidizing environment while insoluble U⁴⁺ represents a reducing environment [41, 42]. V and Mo have similar geochemical behavior to U. Weakly oxidizing-reducing condition is beneficial for the precipitation of Mn⁴⁺ as oxides or hydroxides. Mn-oxyhydroxide can oxidize Ce³⁺ to Ce⁴⁺ and adsorb Mo, which can lead to positive Ce and Mo anomalies in sediments [43]. The ores have high Mo contents (6.98×10^-6 to 15.70×10^-6), which are 5 to 10 times higher than upper crust (1.5×10^-6) [16]. Based on the above analyses, we suggest that Qujiashan deposit formed in a weakly oxidizing- reducing transitional environment.

The ratios of Th/U, V/(V+Ni) and V/Cr are important parameters to identify the depositional environment [41, 44-46]. And they are widely used to constrain paleoceanographic, diagenetic and metallogenic conditions [41, 47, 48]. The ratios of Th/U are from 0 to 2, which indicating anoxic environment; and the ratios are above 3.8 indicating oxic environment. V/Cr ratios are below 2 indicating oxygen-enriched environment [45, 46], and the ratios are from 2 to 4.25 indicating intermediate oxygen-enriched environment and above 4.25 indicating hypoxic environment [44]. In our study, the ratios of Th/U, V/(V+Ni) and V/Cr in the ores range from 0.17 to 3.71 (with an average of 2.18), 0.37 to 0.71 (with an average of 0.526) and 1.87 to 3.90 (with an average of 2.43), respectively. In addition, the Fe/Mn ratios can reflect the redox state [49, 50]. When the sedimentary environment is highly oxidizing or reducing, Fe and Mn tend to deposit synchronously. While when the sedimentary setting is moderately oxidizing, Fe and Mn may separate from each other [51]. The Fe/Mn ratios of the Qujiashan deposit are from 0.07 to 0.27 (with an average of 0.17), indicating that rhodochrosite formed in a weakly oxidizing to reducing transitional environment.

Table 4 Summary of carbon and oxygen isotopic compositions of ores from representative manganese deposits in Yangtze Block

Figure 9 δ¹³C–δ¹⁸O diagram of ores from Qujiashan deposit (The data come from Table 4)

Redox-change may lead to Eu and Ce exhibiting more than one form (Ce³⁺, Ce⁴⁺, Eu³⁺, Eu²⁺) [52]. Under high temperature (>250 °C) and highly reductive conditions, Eu occurs as Eu²⁺, and in the PAAS-normalized REEs patterns it shows negative Eu anomaly [53]. In contrast, under low temperature and relatively oxidizing condition, Eu presents as Eu³⁺, and in the PAAS-normalized REEs patterns it shows positive Eu anomaly [54-56]. The soluble Ce³⁺ will be oxidized to insoluble Ce⁴⁺ under oxidizing and alkaline condition. And it will adsorb to the surface of Fe and Mn oxides or oxyhydroxides [57-60]. This change leads to negative Ce anomaly in seawater [61] and positive Ce anomaly in ferromanganese sediment(Figure 6). In addition, a formula, (Ce/Ce^*)_SN= 2Ce_SN/(La_SN+Pr_SN), is used to calculate Ce anomaly. And previous studies show that the (Ce/Ce^*)_SN- (Pr/Pr^*)_SN diagram can judge whether Ce anomaly affects by La, where (Pr/Pr^*)_SN calculates from the following equation: (Pr/Pr^*)_SN=2Pr_SN/(Ce_SN+Nd_SN). Ce anomaly does not affect by La when (Pr/Pr^*)_SN> 1 or (Pr/Pr^*)_SN<1, but affection happens when (Pr/Pr^*)_SN=1 [62, 63]. The (La/Pr)_SN values vary from 0.82 to 1.04 (with an average of 0.94) in Qujiashan deposit. In the (Ce/Ce^*)_SN-(Pr/Pr^*) _SN diagram (Figure 10), all samples plotted in the field of IIIa, indicating that the Ce anomalies do not affect by La. WRIGHT et al [64] suggested that value of -0.1 for lg(Ce/Ce^*) is a redox index. When lg(Ce/Ce^*) value is higher than -0.1, it representes reductive environment; and when the value is lower than 0.1 it representes oxidation environment. Furthermore, there is a positive correlation between Al₂O₃ and δCe, but a negative correlation between MnO and δCe (Figure 7), which suggests that the δCe anomaly is exaggerated by the terrigenous detritus. The range of δEu and lg(Ce/Ce^*) values in the ores are from 1.00 to 1.25 (with an average of 1.12) and from -0.02 to 0.11 (with an average of 0.08), respectively, indicating that Qujiashan deposit formed in a weakly reducing to weakly oxidizing hydrothermal environment.

The paleo-seawater temperatures can be calculated by the isotope external temperature method on the basis of knowing the oxygen isotopic composition of paleo-seawater and synsedimentary minerals. The ore bodies of the Qujiashan deposit are hosted in the third lithologic section of the Sinian Doushantuo Formation, and the stratum is conformable contact with the Dengying Formation. Therefore the deposit formed during the Ediacaran- Cambrian (ca. 635 to 520 Ma). Previous studies on the Ediacara Formation in worldwide shown that the δ¹⁸O_PDB values of the Ediacara Doushantuo Formation vary from -8‰ to -4‰ [65-69]. These values also represent the oxygen isotopic compositions of paleo-seawater. The paleo- seawater temperatures are calculated according to Eqs. (1)-(4) [70, 71]:

t₁=14.8-5.41×(δ¹⁸O_C-δ¹⁸O_W)                (1)

t₂=1.62×10⁴/(56.75+(δ¹⁸O_C-δ¹⁸O_W)) -273     (2)

t₃=16.9-4.38×(δ¹⁸O_C-δ¹⁸O_W)+0.1×(δ¹⁸O_C-δ¹⁸O_W)² (3)

t₄=16-5.17×(δ¹⁸O_C-δ¹⁸O_W)+0.092×(δ¹⁸O_C-δ¹⁸O_W)² (4)

In the above formulas δ¹⁸O_C represents the δ¹⁸O_PDB value of rhodochrosite, which is analytical data for rhodochrosite in this paper and δ¹⁸O_W represents the δ¹⁸O_PDB value of paleo-seawater, which refer to previous studies in worldwide. The calculated paleo-seawater temperatures range from 47.08 to 73.98 °C (Table 5). These calculated temperatures are obvious higher than normal sea water temperature indicating hydrothermal fluids played an important role in manganese mineralization.

Figure 10 (Ce/Ce^*)_SN -(Pr/Pr^*)_SN diagram of Qujiashan deposit. Field I—neither Ce nor La anomaly; Field IIa—positive La anomaly and no Ce anomaly; Field IIb—negative La anomaly and no Ce anomaly; Field IIIa—real positive Ce anomaly; Field IIIb—real negative Ce anomaly

Table 5 Estimated formation temperatures of rhodochrosites in Qujiashan manganese deposit (°C)

6.3 Genesis of Qujiashan manganese deposit

Based on mineralogical and geochemical characteristics, sedimentary manganese deposit can be divided into three genetic mechanism types: hydrogen-containing compounds type, sedimentary- diagenetic type and hydrothermal type [72, 73]. Hydrogen-containing manganese deposit is represented by ferromanganese crusts. It is slow deposition from seawater, and ratio of Mn/Fe is close to 1. It is enrichment in trace elements such as Ni and Cu, and has high total REEs contents and positive Ce anomaly (Figure 6(b)) [74, 75]. Sedimentary-diagenetic manganese deposit precipitated directly from seawater inducing by redox condition change [76]. Hydrothermal manganese deposit formed in a hydrothermal environment, which has high Mn/Fe ratio, low trace element and REEs contents, and negative Ce anomaly [72, 77]. Ores from Qujiashan deposit have high Mn/Fe ratios and low total REEs contents (93.67×10^-6 to 108.99×10^-6). It is lower than modern seawater. Moreover, in the PAAS- normalized REEs patterns, all samples show flat shape, with unconspicuous Eu and Ce anomalies. This REEs patterns are obviously different from the “back-raise-shaped” REEs patterns of seawater at different depths in the Pacific and the “sawtooth-like” REEs patterns of Co-, Fe- and Mn-rich crusts in the Pacific seafloor (Figure 6). However, they are similar to those of hydrothermal manganese deposits around the Yangtze Block, such as Xiangtan, Xixibao and Gaozeng (Figure 5). Thus the Qujiashan manganese deposit is likely relate to submarine exhalative hydrothermal sedimentation.

7 Conclusions

1) The ore bodies in the Qujiashan manganese deposit are hosted in the contact zone of the purple-red calcareous shale in the third lithologic section of the Sinian Doushantuo Formation and siliceous carbonate of the Sinian Dengying Formation. The manganese-bearing rocks are composed of siderite, siliceous rock and other products related to a hydrothermal sedimentary environment. The manganese ores are mainly composed of rhodochrosite and psilomelane.

2) The ores have low total REEs contents with (La/Yb)_N=6.65-13.9, δEu=1.00-1.25, δCe=0.81- 1.29 and lg(Ce/Ce^*)=-0.02-0.11. These elemental characteristics suggest that rhodochrosite in the Qujiashan manganese deposit formed in a weakly reducing to weakly oxidizing hydrothermal environment.

3) The SiO₂/Al₂O₃, Al/(Al+Fe+Mn) and Co/Ni ratios of the ores range from 4.8 to 9.6, 0.1 to 0.27 and 0.08 to 0.38, respectively. In lgU-lgTh diagram, the results plotted in the field of an ancient hydrothermal sedimentary environment. Eight δ¹³C_PDB and δ¹⁸O_SMOW values range from -20.7‰ to -8.2‰ (with an average of -12.4‰) and from 14.3‰ to 18.7‰ (with an average of 17.0‰), respectively. The isotopic feature of the Qujiashan manganese deposit also indicates that the source of Mn is from hydrothermal fluids. And paleo- seawater temperatures which range from 47.08 to 73.98 °C, also support this suppose. All evidences indicate that the metal substance is derived from hydrothermal fluids, and the deposit might be a product of submarine exhalative hydrothermal sedimentation.

Acknowledgments

We are grateful to Mr. XIE Xin-guo and Mr. PU Zheng-wu for helping with field work, and Dr. ZHOU Jia-xi for helping with trace element, carbon and oxygen isotope analyses.

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(Edited by ZHENG Yu-tong)

中文导读

陕西屈家山锰矿床成因研究：来自地质、地球化学和C-O同位素的证据

摘要：屈家山锰矿床位于扬子地块北缘龙门-大巴山褶皱带内，矿体赋存于上震旦统陡山沱组第三岩性段紫红色钙质页岩中，呈层状、似层状产出。赋矿地层中见火山岩和硅质岩等，同时见大量黄铁矿和少量菱铁矿。锰矿石中MnO含量介于8.92%~18.76%，P₂O₅含量介于0.08%~0.16%，为低磷、高品位锰矿床。矿石的稀土总量较低(平均为101.3×10^-6)，轻、重稀土分馏不明显，中稀土略微富集，基本无Eu (平均值为1.12)和Ce(平均值为1.09)异常，lg(Ce/Ce*)值为-0.02~0.11。矿石中S与Al₂O₃呈负相关关系，而与MnO呈正相关关系，表明S可能来源于海底喷流热液，同时矿石具有较高的SiO₂/Al₂O₃比值(4.8~9.6)和较低的Al/(Al+Fe+Mn)比值(0.1~0.27)，在Fe-Mn-(Ni+Cu+Co)×10和lgU-lgTh图解上，所有样品落入古热水沉积区域反映海底喷流热液参与成矿。矿石中TiO₂、SiO₂、TFe₂O₃与Al₂O₃表现为正相关关系，MnO与Al₂O₃、TiO₂、SiO₂表现出负相关关系，表明可能有少量陆源物质的输入。矿石中δ¹³C_PDB和δ¹⁸O_SMOW值分别为-20.7‰~-8.2‰(平均为-12.4‰)和14.3‰~18.7‰(平均为17.0‰)，根据氧同位素外部计温法估算出该矿床形成时古海水温度为47.08~ 73.98°C，表明深部流体参与了成矿。综合矿床地质和地球化学特征，本研究认为该矿床为海底喷流热液成因。

关键词：锰矿床；元素地球化学；C-O同位素；矿床成因；屈家山锰矿

Foundation item: Project(41663006) supported by the National Natural Science Foundation of China; Project(1212011220725) supported by the Geological Survey Project of the China Geological Survey

Received date: 2019-01-30; Accepted date: 2019-03-26

Corresponding author: REN Tao, PhD, Professor; Tel: +86-15198791060; E-mail: rtao1982@126.com; ORCID: 0000-0001-8951-3926