Understanding Hydrothermal Activity and Organic Matter Enrichment with the Geochemical Characteristics of Black Shales in Lower Cambrian, Northwestern Hunan, South China

Black shales, which are deposited in marine environment with many features such as fine grain sedimentary rock, organic matter enrichment, and some special geochemical characteristics, have been research hotspot for a long time [1–3]. The black shales in lower Cambrian are mainly distributed in Yangtze and Tarim Block of China and many other places in the world and are a global isochronous stratum and can be compared generally [1, 4, 5]. The black shales widely developed in northwestern Hunan were formed in the Ediacaran-Cambrian transition, which is an important turning interval, and some important events happened during this time [6]. There are five obvious characteristics on the black shales of lower Cambrian in northwestern Hunan from our and other studies. First, the total organic carbon (TOC) content of the black shales is generally high. Many scholars find that the value of TOC is generally no less than 1% and can be up to 30% [7, 8], which means that the organic matter was highly enriched. Second, the black shales have special features and are easy to be identified. The rock color is generally black or gray and black. Based on the cores and outcrops, the lithology set of the black shales of lower Cambrian in northwestern Hunan mainly is mud shale, carbonaceous shale, siliceous rock, and siliceous mudstone and sometimes may have a little of siltstone, carbonatite, marl, phosphorite, or barite. Thirdly, the black shales are famous with multielement enrichment, and there are lots of large or super large metallic deposits in the world [9, 10]. In northwestern Hunan, V, Ni, and Mo are most noticeable for the enrichment [11]. There also is a very large strata-bound barite in Xinhuang of northwestern Hunan and Tianzhu of Guizhou province nearby [12]. The next key feature is Cambrian Explosion, which almost happened in the early Cambrian. There are numerous types and amount of creatures, especially some of them lived in seafloor hydrothermal systems and can be found as fossils in the black shales of northwestern Hunan [13]. Finally, the black shales were deposited in the marine environment with a strong reduction in the study area, which can correspond to universal transgression and oceanic anoxic event in Tommotian [14].

The black shales, forming with hydrothermal origin, have been proved by many studies in the study area. There are many evidences such as mineralogy, petrology, sedimentary, geochemistry, and polymetallic deposits [15, 16]. However, in the early Cambrian, it cannot conclude that hydrothermal activity happened where the black shales developed. Otherwise, the hydrothermal activity would happen nearly everywhere in northwestern Hunan. We have no idea about the influence from the hydrothermal activity although it brought huge heat and mass from the deep until now. Hydrothermal activity can help to a certain extent for organic matter enrichment mainly through increasing productivity and enhancing reducibility in the ocean [17, 18]. But in northwestern Hunan, we do not know clearly about how to cause so much of organic matter enrichment. We also do not know what the effects from the hydrothermal activity in this process. In this paper, mainly based on the element, mineralogy, and TOC analysis from three exploratory wells in lower Cambrian, we try to find out the characteristics of the geological background, sedimentary environment, and evolution. Furthermore, we can learn more about the hydrothermal activity, the organic matter enrichment, and the relationship between them. That will help us to understand the distribution laws of high-quality source rocks and provide evidences on the shale gas exploration.

The study area of near $3.2×104$ km² is located in the northwest of Hunan province, a part of the Yangtze Block (Figure 1). The black shales were formed in the marine environment by large-scale transgression and generally distributed in almost the whole study area [19]. There are two important basement fault belts, F1 fault crossing from Baojing to Cili and F10 fault crossing from Fenghuang to Zhangjiajie. The two faults meet at Zhangjiajie and disappeared when crossing Cili. Since the late Mesoproterozoic, the strata on both sides of the F1 fault have big differences in lithology, lithofacies, thickness, etc. [6, 20]. F1 fault with large scale, long time, multistage activity, and so on is believed to relate to many hydrothermal activities in the early Cambrian of northwestern Hunan [1, 13]. The depth of seawater is increasing basically from northwest to southeast, and the sedimentary facies of northwestern Hunan are mainly deep shelf, slope, and deep basin [19]. The slope is mainly distributed around the area of F1 and F10 fault belts, but the boundary between the slope and the deep basin is not for sure. Many studies identified the slope as transitional facies or marginal zone, and the hydrothermal activity was also occurred in this area [6, 7].

Zhangjiajie is the concentration region of hydrothermal activity due to its hydrothermal vent, polymetallic Ni-Mo-PGE ore deposit, some special geochemical features, and others in the lower Cambrian [1, 15]. Xinhuang and Tianzhu with hydrothermal sedimentation and very large strata-bound barite can also be considered the concentration area of hydrothermal activity (Figure 1); it also has found some creatures related to a hydrothermal vent [16]. Because of the transformation from extension to subsidence on tectonic, the intensity of fault activity was decreasing in early Cambrian, and the intensity of the hydrothermal activity was also reducing with it [1]. Besides, the polymetallic deposit in Zhangjiajie and large strata-bound barite in Xinhuang and Tianzhu are all developed at the bottom of Niutitang formation. Many scholars believe that it was a normal sedimentary environment in the middle or upper Niutitang formation of lower Cambrian in northwestern Hunan [7, 15].

All cores are from three wells, HY1, XJD1, and XAD1 in northwestern Hunan, and they are all collected at the bottom of Niutitang formation (Figure 2). Dengyi and Liuchapo formation with different depositional facies and lithology is all formed at the same time, late Ediacaran [6]. We can find that the Ediacaran-Cambrian boundary between Niutitang formation and Dengyi or Liuchapo formation is clear in these wells. In effect, the boundary is also very clear in some outcrops in the study area [8].

The samples were first crushed into powder with less than 200 meshes in size, weighed, and then washed with dilute HCl solution more than 2 hours. Then, the samples were dried at 60°C after the pH value of the solution reaches about 7, which make sure that all inorganic carbon was dissolved and removed especially in carbonate. Finally, the samples can be analyzed in the LECO infrared carbon/sulfur analyzer. All operations must follow Chinese national standard GB/T 19145-2003. To get the mineral composition, the samples were also grounded no more than 40 μm and mixed with ethanol. After that, the samples were smeared on glass slides for X-ray diffraction analysis. All operations also must follow Chinese industrial standard SY/T 5163-2010.

On major element analysis, the samples were first grounded into powder with less than 100 μm, roasted, and then melted with solvent lithium tetraborate and lithium nitrate in high temperature furnaces. Then, the samples were cast as glasses for X-ray fluorescence (XRF) spectrometry. All operations must follow Chinese national standard GB/T 21114-2007. For trace element and rare earth element (REE), the samples were crushed until there grain size is less than 74 μm. The samples need to be burned out for removing high content of organic matters. After that, the process needs to be repeated that the samples were digested with a standard multiacid (HNO₃-HClO₄-HF) and then dried. Finally, the samples were made a constant volume with dilute HCl solution and analyzed by inductively coupled plasma-mass spectrometry (ICP-MS). Similarly, all operations must follow Chinese national standard GB/T 14506.30-2010.

Analytical results of TOC and mineral composition are presented in Table 1. The lithology of the samples is mud shale; others are carbonaceous shale, siliceous mudstone, siliceous rock, and marl. The content of TOC is generally high in all wells. Specifically, they are ranging from 0.53% to 10.82% with an average of 4.02% in HY1, ranging from 1.62% to 11.32% with an average of 6.41% in XJD1, and ranging from 1.14% to 14.38% with an average of 6.55% in XAD1. Quartz, feldspar, calcite, dolomite, pyrite, and clay minerals can be found in these samples. Quartz is a majority component in black shales, especially there is an average content 72.36% in XAD1. The content of carbonate minerals is generally low, and most of them are calcite.

Based on XRF and ICP-MS analysis, some important contents of major and trace elements can be seen in Table 2. We can find that SiO₂ is the main component with an average of 65.79% in all wells, which may correspond to high content of quartz and widespread clay minerals. In XAD1, the content of SiO₂ is associated well with quartz due to it having a few clay minerals that series aluminosilicate minerals. Most of the samples have lower content of CaO and MgO, which means the origin of the shales is marine clastic sediments. The element Al has a few dissolved quantities in water and stable chemistry [21]. Al₂O₃ and TiO₂ have a close correlation and can be seen as a signal that inputs terrigenous matter [22]. In our study, the correlation coefficient between Al₂O₃ and TiO₂ is 0.77, and the content of Al₂O₃ is generally low compared with the Clarke value. This phenomenon is consistent with the three wells located away from the continent and a lack of terrigenous matter supply. XAD1 with extremely low content of Al₂O₃ in the deep marine basin, by contrast, is much more far away from the continent. There is a basic trend that the content of Al₂O₃ is increasing from the bottom up in HY1 and XJD1, which means the process of the regression, closing to the continent and increasing inputs of terrigenous material.

The black shales, named metalliferous shales, are easily identified with their enrichment in some metallic elements [3, 23]. Because the content of Al₂O₃ is generally low, we will make mix information when using the formula with Al normalized (⁠$XEF=X/Alsample/X/AlNASC$⁠), where $X$ is the element of interest and NASC is the North American Shale Composite standard value [24]. So, we can use the samples directly compared with NASC. The enrichment factor (EF) is used to describe the degree of each element enrichment by normalizing NASC. The EF distribution of some important elements is shown in Table 3. We can easily find that these elements mentioned above have similar enrichment phenomena especially in Ag, Ba, Mo, U, and V. The EF of element Th, by contrast, is generally low in all wells when compared with NASC.

REE have special and stable geochemical properties and can reflect many geological information [5]. The content of each REE can be obtained free of charge, and some indicators are shown in Table 4. The total content of REE (∑REE) is all the contents from element La to Lu. All light REE (LREE) include from element La to Eu, and all heavy REE (HREE) include from element Gd to Lu. Sample XAD1-6 has extremely high content of quartz or SiO₂ but only has 25.39 ppm of all REE content, which is a little low when compared with other samples. Due to XAD1 being far away from the continent and the increasing of ∑REE depending on the terrigenous material input, ∑REE of XAD1 show a variation from 25.39 ppm to 157.58 ppm and 88.38 ppm on average. In contrast, ∑REE of XJD1 is ranging from 45.54 ppm to 177.55 ppm with an average of 131.50 ppm; ∑REE of the HY1 well is ranging from 107.99 ppm to 214.50 ppm with an average of 165.72 ppm. Cerium anomaly (Ce/Ce$∗$⁠) can be calculated using the formula (⁠$Ce/Ce∗=CeN/LaN×PrN1/2$⁠), and Eu anomaly (Eu/Eu$∗$⁠) can be got by the formula (⁠$Eu/Eu∗=EuN/SmN×GdN1/2$⁠) according to Shields and Stille [25]. The subscripts $N$ refer to normalization of concentrations against the standard value of chondrite. These indicators are also listed in Table 4.

There is a significant difference between hydrothermal and normal sediments on many geochemical features [13]. The siliceous rocks formed by hydrothermal sediments only have an $Al/Al+Fe+Mn$ value of 0.01, and formation by biogenic sediments in offshore is 0.60 [26]. In the study area, samples HY1-7, HY1-8, XJD1-7, XJD1-8, XAD1-6, XAD1-7, and XAD1-8 are siliceous mudstone or siliceous rock with very high content of quartz; they are also at the bottom of Niutitang formation. Their value of $Al/Al+Fe+Mn$⁠, respectively, is 0.66, 0.78, 0.61, 0.46, 0.08, 0.51, and 0.51. Except XAD1-6, the others are generally high and indicate there is a normal siliceous deposition. There are also normal sediments in the Al-Fe-Mn triangle chart of determining origin. Their value of Fe/Ti, respectively, is 7.94, 9.78, 14.13, 22.75, 77.01, 19.30, and 15.71, which XJD1-8 and XAD1-6 are more than 20 and the others also present normal siliceous deposition. We believe that XAD1-6 is an abnormal sample and has no useful information on most indicators. About XJD1-8, the seawater in early Cambrian generally is in the anoxic and iron-rich environment, and Fe/Ti become larger and may bring a little mistake on the judgment [27]. More clearly, Eu/Eu$∗$ with a positive anomaly (>1) is the main feature in hydrothermal sediments. The degree of positive anomaly can even reflect the intensity of hydrothermal activity participation [28]. Eu/Eu$∗$ with negative anomaly (<1) can refer to the normal marine sedimentary rock [29]. We can find that Eu/Eu$∗$ are 0.62 on average, which are all less than 1 in Table 4 and close to 0.65 of the NASC standard value [30]. All of these mean it is a normal marine sedimentary.

However, some other indicators have different results. U/Th is a general indicator for redox conditions and hydrothermal activity [8]. There are many samples having a quite high value of U/Th radio, such as 20.98 in HY1-5 and 17.33 in XJD1-8. These values are much higher than 1, and it only can be explained that they are affected by hydrothermal activity. The value of Co/Zn radio is very low in hydrothermal sediments (0.15 on average) and is generally high in submarine sediments (2.5 on average in Fe-Mn nodules) [9]. The values of all samples are ranging from near 0 to 0.45 with 0.10 on average, indicating that most of the samples are hydrothermal sediments. Compared with normal marine sediments, hydrothermal sediments have lower ∑REE and obviously are rich in LREE. The ∑REE value of NASC is 173.20 ppm, and the continental upper crustal is 146.40 ppm on average, but ∑REE in seawater and hydrothermal sediments is very low [24]. $LaN/CeNandGdN/YbN$ all with higher value, respectively, are 2.40 and 2.57 on average in typical hydrothermal sediments [31]. Based on our datum in Table 4, ∑REE of XAD1 is generally low and different with the other two wells, which may be due to XAD1 being also far away from continental margin and lacking terrigenous clastic with high ∑REE supply. But the obvious decrease in ∑REE is also controlled by hydrothermal sediments in all three wells. Most values of $LaN/CeNandGdN/YbN$ are more than 1 but generally lower than hydrothermal sediments, which mean the intensity of enrichment in LREE is not very strong in HY1, XJD1, and XAD1. So, based on the features of REE, most samples are normal marine sediments, but some of them may be hydrothermal sediments.

Summarizing all these indicators, we can find that there are so confused results on the characteristics of the sedimentary rocks. In simple terms, we conclude that there are normal marine sediments based on $Al/Al+Fe+Mn$⁠, Fe/Ti, and especially Eu/Eu$∗$⁠. Some indicators including U/Th and REE features mean that there are normal marine sediments, and some of them are hydrothermal sediments. But there may be all hydrothermal sediments according to Co/Zn. In Figure 3(a), many samples are located in the shared area of sedimentary rock and tholeiite according to La/Yb versus ∑REE, where oceanic tholeiite is formed in the midocean ridge with the matters from the deep [32]. That means some of the sediments may be hydrothermal sediments.

Furthermore, we can identify how much of the influence of hydrothermal activity is based on Y/Ho, Eu/Sm, and Sm/Yb. The value of Y/Ho radio in seawater is much higher (44-74) than in chondrite and upper crustal (26-28) [28]. A little contribution from hydrothermal fluids can produce an observable high Eu/Sm ratio in the chart [33]. Our samples are very different from hydrogenetic Fe-Mn crusts in the seabed and seawater, but the Eu/Sm radios are a little low when compared with the Pongola formation of South Africa which confirmed that there is a little hydrothermal fluid input (Figure 3(b)). Mixing calculations indicate that a 1-5% contribution of hydrothermal fluids may account for the Y/Ho and Sm/Yb observed within Pongola formation [33], and our samples are similar with Pongola formation in Figure 3(c). We can conclude easily that there is also a little hydrothermal fluid when using Sm/Yb and Eu/Sm (Figure 3(d)). Our wells are much close to the black shales from Xiamaling formation of North China [34], and we can also conclude that there are a few hydrothermal sediments in the sedimentary process.

These wells are all far away from the concentration region of hydrothermal activity (Figure 1). The hydrothermal sediments can be spread from the center of hydrothermal activity to all around, and the matters also can be brought to the continental shelf by submarine currents or others [17]. Therefore, we believe that these wells may have no or little hydrothermal activity and have also limited effects by the hydrothermal activity. However, the hydrothermal sediments can come to our wells during the sedimentary period of Niutitang formation. The content of trace elements may be varied and bring confusing results when using the element indicator analysis. Although they may be hardly affected by the hydrothermal activity, HY1 and XJD1 basically show some diminishing trend of hydrothermal activity intensity. That is accord with the geological background which the hydrothermal activity is reducing in the study area, but it is not very clear in XAD1 (Figure 4). A reasonable explanation is that XAD1 is much more far away from the concentration region and located in the oceanic basin. Hydrothermal sediments need more time spread to XAD1 and are hard to show some trend of hydrothermal activity intensity.

Some elements including Ag, As, Ba, Cu, Mo, Ni, Se, U, V, and Zn are enriched in black shales; Au, P, PGE, and some REE also may be enriched, and some kinds of them have in connection with the hydrothermal activity [1, 35]. Different wells have similar kinds and strengths of the element enrichment (Table 3). That means they have similar sources and deposits in a similar environment when considering the same conditions and changes after the sedimentation. The enrichment characteristic of U, V, Mo, Ni, and Cu enriched at the same time and generally has high TOC in the samples, which reveals that they are formed in anoxic or near euxinic sedimentary environment [18]. This environment formed in early Cambrian is with transgression and may be strengthened by hydrothermal activity. Some metal sulfides can be enriched in the hydrothermal sediments. The content of some metal elements can easily be enriched as sulfide or in the pyrite [35]. We can often find many different metal sulfide grains in the Ni-Mo ore-hosting horizon, and the sedimentary environment with hydrothermal H₂S can be confirmed in Zhangjiajie [36]. Pyrite is also generally high in the black shales of our wells (Table 1). Therefore, in the enrichment elements, Ag, As, Cu, Se, and Zn are chalcophile elements and easily enriched in this sedimentary environment. In addition, organic matter may make a significant role in metal transport and accumulation. They can react with metal and sulfur species through the redox process [10]. Meanwhile, some metal elements as nutrients take part in the biological activity and become part of organic matters such as Ba, Cu, and Zn [37].

As mentioned earlier, Ni-Mo polymetallic deposits are mainly in Zhangjiajie; barite deposits are mainly in Xinhuang and Tianzhu in the study area. There is a normal value of Mo/TOC (<100) and Ni/TOC (<50) in normal euxinic sediments including the black shales, and in most situation, the value is far below the boundary [16, 38]. In the study area, most samples are less than 100; only XJD1-6 has 116.22 of the Mo/TOC ratio. At the same time, many samples are less than 50 of Ni/TOC radio but HY1-2, HY1-6, HY1-7, and HY1-8 with high value, and some others are close to the boundary. The EF distribution of Mo and Ni is also very different in Table 3. All these phenomena indicate that the enrichment mechanism of Mo and Ni and their primary source is different, which is consistent with other studies [39]. Mo can be easily from seawater and biological activity and then easily deposited in an anoxic environment [5]. But Ni was probably derived from the leaching of underlying old basement rocks by the hydrothermal fluid [40]. That may lead to the EF of Mo being extremely high, and the EF of Ni is relatively low.

Mo, Ni, U, and V are all redox-sensitive elements, and they can be enriched in anoxic sedimentary environment [3]. But the correlation coefficients between these elements and TOC are very low except U. U has 0.61 in HY1 well and 0.43 in all wells, which can be explained that U has strong correlation with organic matter [41]. The distribution of V concentration is very uneven in these wells. Some samples are several thousand ppm, and the maximum can be up to 6920 ppm, but many of them are several hundred ppm. V is easily enriched by biological activity, and it can scavenge efficiency from seawater much higher than Mo and U under anoxic/euxinic conditions [39]. V can also be from the hydrothermal fluids, but the distribution enrichment of V is very dispersed in the study area [14], which indicate V which may be enriched mainly from seawater.

Considering underlying Ediacaran series with high content of Ba [12] and Ba can be derived from the reaction of hydrothermal fluid with sediments [42], Ba was easily enriched in the lower Cambrian black shales by the hydrothermal activity. Although Ba can be from seawater and play an important role in biological activity and productivity [43], generally high content in the study area and very high content of barium sulfate in the thick ore horizon of Xinhuang-Tianzhu indicate there are different sources of Ba.

The content of some elements is shown in Figure 5, and we can find the overall trend in some kinds of element in the wells. Specifically, the content of Ag in HY1 and XJD1 is basically decreased from the bottom up, although near the bottom is a little low. As is the same chalcophile and low temperature ore-forming element like Ag and has almost the same trend, Ni also has a similar trend in HY1 and XJD1, and Ba and V may only have some similar trend in XJD1 but are not very clear in HY1. At the same time, the trend of Mo and U in HY1 and XJD1 is not for sure, and XAD1 basically has no trend in all kinds of the elements. All these trends are in accordance with the hydrothermal activity trend in Figure 4. The EF of most elements in XJD1 are generally higher than in HY1 (Table 4), and the hydrothermal activity trend in XJD1 is also clearer than that in HY1 (Figure 4), which seems that the influence of the hydrothermal activity is stronger in XJD1 than in HY1. That may be because XJD1 is closer to Zhangjiajie and easier to sediment the clastics from the concentration area of hydrothermal activity than HY1 according to paleogeographic environment. The basement fault F10 is also closer to XJD1, which means there may also have some hydrothermal activity near XJD1. Finally, comparing the EF in XJD1 with more influence by the hydrothermal activity and XAD1 probably with little influence, we can find that Ag and V are generally high in XAD1 than XJD1, but As, Ba, Cu, Mo, Ni, and Zn are generally low. That means As, Ni, and part of Ba may be more related to the hydrothermal activity when considering the trend in XJD1 and HY1. The content of Ag and V is high in XAD1 and may have more different sources in the deep sea.

In summary, the enrichment of the elements is a result of the combination of effects by hydrothermal activity, anoxic/euxinic conditions, biological forces, deep-sea sedimentary environment, the chemical property of the element, and so on. Different elements have clear and different enrichment mechanisms, and some elements can have multifactors to be highly enriched such as Ba, Mo, U, and V. However, it needs more evidences and investigation to confirm that there is much contribution from the hydrothermal activity or others. It is possible for this element enrichment with high EF from seawater mainly because of the very low sedimentation rate, precipitation-scavenging, and fresh seawater replenishment in the stagnant oceanic basin [11, 14]. The influence of the hydrothermal activity existed but may be not very much because of the wells being far away from the concentration region of hydrothermal activity, which is consistent with the study in Section 5.1.

TOC and some indexes which can reflect the features of the sedimentary environment have been seen in Figure 6. Basically, there is no vertical trend of TOC in all wells. The EF of Cu and P is the same. Although Cu and P are related to paleoproductivity [44], the correlation coefficient between the EF of the elements and TOC is not high. It may be because the connection is very complex and easily affected [37]. However, the indicators of redox condition show good relationship among them although Ni/Co is a little low (Table 5). It is a reductive sedimentary environment when $Ni/Co>7$⁠, $V/V+Ni>0.54,V/Cr>4.25,U/Th>1.25,orδU>1$ (Figure 6). These indicators are increased with enhanced reducibility [16]. So, they can show well about the redox conditions and vertical trend. We can easily find that XJD1 has a clear trend where reducibility is reducing from bottom to up. The redox conditions in the bottom of HY1 are inconsistent, but only Ni/Co show some similar trend like XJD1, but XAD1 also has no trend.

Negative Ce anomaly can indicate the depth of seawater. In some ways, the smaller the value of Ce/Ce$∗$⁠, the deeper the seawater and the more lack of oxygen in the ocean [45]. HY1 and XJD1 show well about becoming shallow with the transgression, while XAD1 also has no trend because it is in the deep sea all the time. $La/YbN$ represent the differentiation of REE, and the absolute value of $La/YbN−1$ can represent the sedimentary rate [22]. Only XJD1 show a totally slower rate of sedimentation from bottom to up. Maybe it is also related to the hydrothermal activity, due to the sedimentary rate of the hydrothermal activity being much higher than normal sedimentation. Co/Zn and ∑REE that represent the intensity of the hydrothermal activity well are also shown in Figure 6.

All these phenomena reveal that there is a logical but not high corelationship between these indicators (Table 5). The cross-correlation in XJD1 which has more influence by the hydrothermal activity and clear trend is shown in Table 6. Compared with Table 5, it is surprising that the cross-correlation of redox conditions is higher, and the reductive conditions have a clearer positive correlation with the depth in XJD1. However, there is a big difference between all wells and XJD1 on the influence by the hydrothermal activity. We can find that neither Co/Zn nor ∑REE has inconsistent correlation with other indicators, but they have good correlations in XJD1. The relationship between the influence by the hydrothermal activity and the other conditions is credible in XJD1 based on the above study. Co/Zn and ∑REE have a good positive correlation with Ce/Ce$∗$ and have a good negative correlation with the other indicators except the EF of P. Therefore, the hydrothermal activity may provide excess Cu or improve the paleoproductivity, enhance the reducibility in the sedimentary environment, and reduce with the seawater depth becoming shallow [46]. The hydrothermal activity promoted organic matter enrichment especially on redox conditions based on our examples. According to the correlation coefficient in XJD1 and all wells in Tables 5 and 6, TOC has a positive correlation with paleoproductivity, clear positive correlation with reducibility, positive correlation with the depth of seawater, and some negative correlation with the sedimentary rate. Reducibility is one of the major factors that affect organic matter enrichment. But some conditions like paleoproductivity lack representative indexes and need further investigation.

Based on the paleogeography and other analyses, a simple sedimentary model during Ediacaran-Cambrian transition, especially having the strong hydrothermal activity, is shown in Figure 7. First, part of the study area and north is a carbonate platform in the late Ediacaran and even has some paleo-weathering crust in Zhangjiajie [6]. Dengying dolomite and Liuchapo chert in the late Ediacaran have been confirmed that they were formed in the same time but different sedimentary environment mainly due to the seawater depth. The wells show this difference between HY1 and XJD1 although they are not far (Figures 1 and 2).

Then, in the Ediacaran-Cambrian transition, the interior of the Yangtze Block experienced a significant sea level rise [39]. The platform southeastern margin began to enter the deep-water marine environment including deep shelf, slope, and deep basin, and the slope area becomes a vast shallow and restricted sea. With the hydrothermal activity strongly happening, the slope becomes more anoxic/euxinic environment with H₂S-rich conditions [5]. There are many mineralized spots in south China, but only Zhangjiajie and Zunyi of Guizhou which also were confirmed with the hydrothermal activity happened to have a certain number of Ni-Mo deposits [15]. In Xinhuang-Tianzhu barite deposits, the content of barium sulfate can be up to 80%. Only enough supply of hydrothermal matters and plenty of space by the synsedimentary faults in the same time can form so large scale and high content of barite deposits. The homogenization temperature of fluid inclusions in the barite of Xinghuang-Tianzhu is low, 105°C-192°C, and the average content of Ba in lower strata of Jiangkou formation and Banxi group in Ediacaran is 17.7 times than the crustal abundance [12]. Therefore, seawater can infiltrate into the hot source in the deep and extract large amount of metal elements through the water-rock reaction in the high temperature environment [1]. After that, the hot water can come out to the seafloor by the fault systems and formed the hydrothermal activity. The major basement faults F1 and F10 are synsedimentary faults and contributed to this situation. Besides, diffusion and submarine current can deliver the hydrothermal sediments too far away. By contrast, XAD1 has a lower influence on hydrothermal activity and is a little low on these element indicators.

• (1)

  Zhangjiajie and Xinghuang-Tianzhu are the concentration region of the hydrothermal activity in the early Cambrian. Our wells especially XAD1 are far away from them and have not much influence on the hydrothermal activity. However, some element indicators can be confused due to the content of trace elements which may be easily varied and show the contradictory results in hydrothermal sediment judgment

• (2)

  XJD1 has a clear diminishing trend in the hydrothermal activity intensity. It also shows the high element enrichment by the hydrothermal activity. But XAD1 show little influence in contrast, and HY1 is between them. What caused this situation is their location when the hydrothermal activity happened in the early Cambrian. XJD1 is close to Zhangjiajie and the basement fault F10, but XAD1 is far away in the deep sea

• (3)

  The enrichment of the element is a result of the combination of effects by hydrothermal activity, anoxic/euxinic conditions, biological forces, and so on. Different elements have clear but different enrichment mechanisms, and some elements can have multifactors to be highly enriched. We can confirm that the enrichment of As, Ni, and part of Ba in XJD1 and HY1 is more related to the hydrothermal activity, but the content of Ag and V is high in XAD1 which may have more different sources

• (4)

  The hydrothermal activity can improve the paleoproductivity and especially enhance the reducibility in the sedimentary environment. TOC has a positive correlation with paleoproductivity and the depth of seawater, clear positive correlation with reducibility, and some negative correlation with the sedimentary rate. Reducibility is one of the major factors affecting organic matter enrichment. Finally, we conclude the sedimentary model in the Ediacaran-Cambrian transition

All data, models, and code generated or used during the study appear in the submitted article.

The authors declare that they have no conflicts of interest.

The corresponding author Yanran Huang also acknowledges the support of the program of China Scholarships Council (No. 201809480006).