New Re-Os Isotopic Constrains on the Formation of the Metalliferous Deposits of the Lower Cambrian Niutitang Formation

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1 Journal of Earth Science, Vol. 27, No. 2, p , April 2016 ISSN X Printed in China DOI: /s New Re-Os Isotopic Constrains on the Formation of the Metalliferous Deposits of the Lower Cambrian Niutitang Formation Yong Fu 1, 2, Lin Dong* 3, Chao Li 4, Wenjun Qu 4, Haoxiang Pei 2, Wenlang Qiao 5, Bing Shen 3 1. College of Resource and Environmental Engineering, Guizhou University, Guiyang , China 2. Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing , China 3. Key Laboratory of Orogenic Belts and Crustal Evolution, MOE; School of Earth and Space Science, Peking University, Beijing , China 4. Re-Os lab, National Research Center for Geoanalysis, Chinese Academy of Geological Sciences, Beijing , China 5. Institute of Guizhou Geological Survey, Guiyang , China ABSTRACT: The Terreneuvian Epoch ( Ma)is also an important period for metallogenesis in South China, as was represented by the widespread occurrences of Ni-Mo polymetallic layers on the antecedent shallow platform margin and the V-enriched black shales in deeper slope-basin settings. In this study, we have measured Re-Os isochron ages of Ni-Mo polymetallic layers (Songlin, Niuchang, Sancha, Chuanpengwan), V-rich black shales (Bahuang), and non-metalliferous black shales (Shuidong) in the basal Niutitang Formation in Guizhou and Hunan province, South China. The Ni-Mo polymetallic layers and V-enriched black shales have similar Re-Os isochron ages, suggesting concurrent deposition of these two types of metalliferous ores. This suggestion is consistent with the traditional stratigraphic correlation by using the nodular phosphorite bed directly underlying these metalliferous layers as a stratigraphic marker. Furthermore, the metalliferous ores and non-metalliferous black shales have similar initial 187 Os/ 188 Os ratios of , arguing for a dominant seawater origin with minor contribution of hydrothermal activity. Furthermore, Re-Os isotopic data also imply that Ni-Mo and V ore might have derived from the same source. We suggest that the spatial distribution of metalliferous ores can be explained by the development of non-sulfidic anoxic-suboxic wedge (NSASW) in the slope-basin and sulfidic wedge in the previous platform margin. Upwelling of deep water first transects the mildly reduced, organic rich NSASW, in which V (V) is reduced to V (IV), and is preferentially removed from seawater by organometallic complex formation. As a result, V-rich black shale deposits in the slope-basin of Yangtze Platform. Further movement of deep water into the sulfidic platform margin results in Ni-Mo polymetallic layer formation. KEY WORDS: Niutitang Formation, South China, Ni-Mo polymetallic layer, V-rich black shale, Re-Os isochron ages. 0 INTRODUCTION The Ediacaran-Cambrian boundary is marked by the first appearance of trace fossil Treptichnus pedum at ~541 Ma (Cohen et al., 2013, Brasier et al., 1994, Narbonne and Myrow, 1988). However, the establishment of the Phanerozoic Earth system, which was marked by the Cambrian great bioradiation of metazoans (Cambrian explosion) (Shu, 2008; Marshall, 2006; Conway Morris, 1989; Gould, 1989), occurred ~20 million years after the beginning of the Cambrian Period. The *Corresponding author: lin.dong@pku.edu.cn China University of Geosciences and Springer-Verlag Berlin Heidelberg 2015 Manuscript received December 01, Manuscript accepted May 06, Phanerozoic earth system was characterized by substantially oxygenated atmosphere and ocean, and diversified biosphere (Wang et al., 2015; Och et al., 2013; Wang J et al., 2012; Maloof et al., 2010; Zhou and Jiang, 2009; Shu, 2008). Thus, the first 20 million years of Cambrian ( Ma), now officially defined as the Terreneuvian Epoch in the geological time scale (Cohen et al., 2013), is critical for understanding the evolution of Phanerozoic Earth system. In the Yangtze Platform, the deep-water Cambrian succession equivalent to the Terreneuvian series is represented by a set of chert, black shale, and phosphorite deposits. In Guizhou and Hunan provinces, they are typically called the Liuchapo and Niutitang formations in ascending order (Wang J et al., 2012; Wang X et al., 2012; Yang et al., 2009). The basal Niutitang black shale enriches in redox-sensitive trace metals, such as Ni, Mo, V, Cr, PGE, Au, and U, representing one of the most Fu, Y., Dong, L., Li, C., et al., New Re-Os Isotopic Constrains on the Formation of the Metalliferous Deposits of the Lower Cambrian Niutitang Formation. Journal of Earth Science, 27(2): doi: /s

2 272 Yong Fu, Lin Dong, Chao Li, Wenjun Qu, Haoxiang Pei, Wenlang Qiao and Bing Shen important metallogenetic periods in the Yangtze Platform. Such polymetallic enrichment is represented by the deposition of Ni-Mo polymetallic layer (Ni+Mo>10 wt.%) in the antecedent carbonate platform marginal zones and V-rich black shale (V 2 O 5 >1 wt.%) in slope-basin settings (Pi et al., 2013; Guo et al., 2007; Jiang et al., 2007a; Lehmann et al., 2007; Jiang et al., 2003; Bao et al., 2002) (Fig. 1). Both types of metalliferous ores overlie a thin (1-2 m thick) nodular phosphorite marker bed (Jiang et al., 2007b). Traditional stratigraphic correlation indicates that the Ni-Mo polymetallic and V-enriched layers might be correlatable (Zhu et al., 2003). The Ni-Mo polymetallic layers are restricted in the previous shallow carbonate platform margins in the Yangtze Platform (Fig. 1, 2). In contrast, the equivalent V-rich black shales are deposited in deeper slope-basin environments (Figs. 1, 2). Because of greater economic importance, previous studies mainly focused on the Ni-Mo polymetallic layers (Guo et al., 2007; Jiang et al., 2007a; Jiang et al., 2007; Lehmann et al., 2007; Orberger et al., 2007), but little attention has been paid to the V-rich black shales (Su et al., 2012; Bao et al., 2002). As such, current metallogenetic models only consider the origin of Ni-Mo polymetallic layers deposited on the platform margin (Guo et al., 2007; Jiang et al., 2007a; Lehmann et al., 2007), but do not appreciate their possible linkage with the V-rich black shale deposition in the slope-basin settings. Earlier Re-Os studies of the Ni-Mo polymetallic layers were deposited near the Ediacaran-Cambrian boundary (Jiang et al., 2007; Li et al., 2002; Mao et al., 2002), but recent Re-Os isochron and U-Pb ages imply much younger ages (Chen et al., 2015; Wang X et al., 2012; Xu et al., 2011; Jiang et al., 2009; Zhou et al., 2008). As another focus of the Ni-Mo polymetallic layer formation, the metal source has been extensively studied, and both hydrothermal and seawater origin of Ni-Mo polymetallic layers have been proposed (Pi et al., 2013; Xu et al., 2013; Jiang et al., 2007a, c; Lehmann et al., 2007). In this study, we present new Re-Os isochron ages of the Ni-Mo polymetallic layers, non-metalliferous black shales, and V-enriched black shales of the basal Niutitang Formation. These new Re-Os ages confirm the deposition of Ni-Mo polymetallic layers ~20 million years posterior to the Ediacaran-Cambrian transition, and concomitant precipitation of the Ni-Mo polymetallic layers and V-rich black shales as inferred by the persistent underlying nodular phosphorite marker bed. Furthermore, the similar initial 187 Os/ 188 Os ratios of all measured samples imply that both metalliferous horizons derived dominantly from seawater with minor contributions of hydrothermal fluids. 1 SAMPLE LOCALITIES AND GEOLOGICAL SET- TINGS Samples were collected from 6 sections in Guizhou and Hunan provinces, South China (Fig. 1). Brief descriptions for the stratigraphy and sampling information are given below. 1.1 Shuidong Section (Fig. 2) In the Shuidong Section, the Dengying dolostone is unconformably overlain by a 2-m-thick dark grey, thin-bedded, phosphorus dolostone intercalated with black shales of the basal Niutitang Formation, followed by a thick sequence of black shales. The Ni-Mo polymetallic layer occurs about 5 m above the base of the black shale (Fig. 3a). The Ni-Mo-enriched layer is about 5~100 cm thick and is underlain by a 10-cm thick nodular phosphorite marker bed. Six black shale samples were collected from the black shale sequence underneath the nodular phosphorite marker bed. 1.2 Songlin Section (Fig. 2) The Niutitang Formation in the Songlin Section unconformably overlies the dolostone of the Dengying Formation. Figure 1. (a) Geographic location of the Yangtze Platform; (b) paleogeographic map of the Yangtze Platform in Early Cambrian (modified from Wang et al., 2012a). Black and white circles show the vanadium ore deposits, whereas squares indicate the major localities of the Ni-Mo polymetallic layers in the basal Niutitang Formation. The sampling localities are represented by red circles.

3 New Re-Os Isotopic Constrains on the Formation of the Metalliferous Deposits 273 Figure 2. Stratigraphic columns of the Late Ediacaran-Early Cambrian successions in the Yangtze platform. (1) Shuidong section, (2) Songlin Section, (3) Wenquan Section, (4) Niuchang Section, (5) Sancha Section, (6) Chuanpengwan Section, (7) Zhenyuan Section. The section localities are illustrated in Fig. 1. Figure 3. Field photograph of the Ni-Mo polymetallic layer and V-rich black shale in the Lower Niutitang Formation, Guizhou and Hunan provinces. (a): Ni-Mo polymetallic layer (Ni-Mo) in the Shuidong Section; (b) the Ediacaran-Cambrian boundary in the Songlin Section is marked by the unconformable contact between the Dengying (dolomite) and the Niutitang formations (black shale series). The distance between the Ni-Mo polymetallic layer (Ni-Mo, marked by yellow dash lines) and this boundary is about 80 cm; (c) Ni-Mo polymetallic layer in the Sancha Section (marked by yellow dash lines); (d) V-rich black shale in the Bahuang Section; the major V-ore horizons marked by yellow dash lines.

4 274 Yong Fu, Lin Dong, Chao Li, Wenjun Qu, Haoxiang Pei, Wenlang Qiao and Bing Shen A sharp erosional surface at the top of the Ediacaran Dengying dolostone implies a long-lasting subaerial exposure of carbonate platform prior to deposition of Niutitang Formation (Fig. 3b). The Niutitang Formation begins with a 3-m thick dolostone horizon that is overlain by a 1.6-m thick phosphorite. The overlying 1.4-m thick bedded chert is separated from the underlying phosphorite by a volcanic ash bed, which was dated to 530±0.7 Ma (U-Pb) (Jiang et al., 2009). The nodular phosphorite marker bed occurs 2.4 meters above the base of black shale, above which is a 12-cm thick Ni-Mo polymetallic layer. Six samples were collected from the Ni-Mo polymetallic layer. 1.3 Niuchang Section (Fig. 2) The Niutitang Formation unconformably overlies the dolostone of the Dengying Formation in the Niuchang Section. The Niutitang Formation begins with a 1-m thick bedded chert, followed by a thick black shale sequence. The 15-cm thick nodular phosphorite marker bed associated with a 10-cm thick Ni-Mo polymetallic layer occurs 3 m above the base of black shale. Four samples were collected from the Ni-Mo polymetallic layer. 1.4 Sancha Section (Fig. 2) In the Sancha Section, the thick sequence of black shale in the Niutitang Formation is directly in contact with the underlying Dengying dolostone by an unconformity surface. A Ni-Mo polymetallic layer associated with a nodular phosphorite marker bed lies 0.7~1 m above the base of the Niutitang Formation (Fig. 3c). Five samples were collected from the Ni-Mo polymetallic layer. 1.5 Chuanpengwan Section (Fig. 2) The Niutitang Formation conformably overlies the Dengying Formation in the Chuanpengwan Section. A Ni-Mo polymetallic layer along with the underlying nodular phosphorite marker bed occurs ~3 m above the base of black shale sequence. The Ni-Mo polymetallic layer and the nodular phosphorite marker bed are 3 and 12 cm thick, respectively. Eight samples were collected from the Ni-Mo polymetallic layer. 1.6 Bahuang Section (Fig. 2) The Late Ediacaran-Early Cambrian thin-bedded chert of the Liuchapo Formation conformably overlies the Ediacaran Doushantuo Formation in the Bahuang Section. The 15-m thick Liuchapo Formation is composed of thin-bedded chert at the base and bedded chert intercalated with black shale at the top. The boundary between the Liuchapo and Niutitang formations is marked by the disappearance of thin-bedded chert. The nodular phosphorite marker bed is located at 0.4 m above the base of the Niutitang formations. Unlike other sections described above, a V-rich horizon, rather a Ni-Mo polymetallic layer presents above this marker bed (Fig. 3d). Six samples were collected from the V-rich black shale above the nodular phosphorite marker bed. 2 METHODS Re-Os isotope analyses were conduct in the Re-Os laboratory, National Research Center for Geoanalysis, Chinese Academy of Geological Sciences. The chemical separation follows the procedure described by Du et al., (1994), Qu et al., (2003) and Li et al., (2010, 2009). About 100 g of fresh rock sample was baked in an oven at 60 ºC for 12 hours. Samples were first crushed to 5 10 mesh size, and then powdered to 200 mesh in agate mortar. About 100 mg of sample powder was loaded into a Carius tube, and was dissolved by a mixture of 3 ml of 10 M HCl and 5 ml of 16 M HNO Os and 185 Re spike solutions (Oak Ridge National Laboratory) were also added into the Carius tube. The bottom of the Carius tube was frozen at -50 to -80 ºC in an ethanol-liquid nitrogen slush. The Carius tube was sealed with an oxygen-propane torch, and placed in a stainless steel jacket. After being heated at 200 ºC for 24 hours, Os was converted into OsO 4. Os was distilled by heating at ºC for 50 minutes, and collected in 10 ml deionlized (DI) water. Re is retained in the residual solution. The residual solution is transferred into a Teflon beaker and dried up in a hotplate. Residual is re-dissolved by 1 ml of DI water, and dried up in hotplate. This procedure was repeated twice, 10 ml of 5M NaOH solution was added into the Teflon beaker to dissolve the residual. Re is collected by 10 ml acetone in a Teflon separation funnel. The Re-bearing acetone was washed with 2 ml of 5M NaOH for 2 minutes. The solution was dried at 50 ºC to remove all acetone, and then was dissolved in 2% HNO 3. Os isotope ratios and Re contents were determined by a high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS, Element 2). 3 RESULTS Re-Os abundance and isotope data are tabulated in Table 1. Re and Os concentrations in the Ni-Mo polymetallic layers range from 11.2 to ng/g, and 0.53 to ng/g, respectively. The 187 Re/ 188 Os and 187 Os/ 188 Os ratios vary between 86.6 and ng/g, and 1.61 and ng/g, respectively. The non-metalliferous black shales underlying the Ni-Mo polymetallic layer in the Shuidong Section have lower Re and Os contents, varying from 36.8 to ng/g, and 1.15 to ng/g, respectively. Regression of Re-Os isotope data for the Ni-Mo polymetallic layers yields similar Re-Os age of 528±170 Ma (2σ, n = 6, MSWD = 30, initial 187 Os/ 188 Os ratio = 1.00±2.10) at Songlin, 528.1±6.3 Ma (2σ, n=4, MSWD=0.89, initial 187 Os/ 188 Os ratio=0.84±0.03) at Niuchang, 522±23 Ma (2σ, n=5, MSWD=7.4, initial 187 Os/ 188 Os ratio=0.94±0.23) at Sancha, 519±38 Ma (2σ, n=7, MSWD=7.0, initial 187 Os/ 188 Os ratio=0.89±0.04) at Chuanpengwan, respectively (Fig. 4). Black shale samples underneath the nodular phosphorite marker bed in the Shuidong section yield similar Re-Os age of 532 ± 14 (2σ, n=5, MSWD=1.7, initial 187 Os/ 188 Os ratio=0.85±0.07). Samples of V-enriched black shales collected from the Bahuang Section contain lower Re and Os contents, having ng/g and ng/g, respectively. The 187 Re/ 188 Os and 187 Os/ 188 Os ratios vary between 15 and 744, and between 1 and 7.3, respectively. The Re/Os isochron age by regression of V-rich black shale at the Bahuang section is 520.3±9.1 (2σ, n=6, MSWD=7.6, initial 187 Os/ 188 Os ratio=0.86±0.05) (Fig. 4). The combined isochron age estimated from all Re-Os data of Ni-Mo polymetallic layers and V-enriched black shale is 525.9±8.9 Ma (2σ, n=28, MSWD=16, initial 187 Os/ 188 Os ratio=0.90±0.09) (Fig. 5).

5 New Re-Os Isotopic Constrains on the Formation of the Metalliferous Deposits 275 Table 1 Re-Os abundance and isotope data for samples collected from 6 sections in Guizhou and Hunan provinces, South China Section Lithology No. Re (ppb) 2SD Os (ppb) 2SD 187 Re/ 188 Os 2SD 187 Os/ 188 Os 2SD Shuidong Black shale Xj Black shale Xj Black shale Xj Black shale Xj Black shale Xj Black shale Xj Songlin Ni-Mo ZL Ni-Mo ZL Ni-Mo ZL Ni-Mo ZL Ni-Mo ZL Ni-Mo ZL Niuchang Ni-Mo ZHS Ni-Mo ZHS Ni-Mo ZHS Ni-Mo ZHS Bahuang V-shale DT V-shale DT V-shale DT V-shale DT V-shale DT V-shale DT Chuanpengwan Ni-Mo CPW Ni-Mo CPW Ni-Mo CPW Ni-Mo CPW Ni-Mo CPW Ni-Mo CPW Ni-Mo CPW Sancha Ni-Mo SC Ni-Mo SC Ni-Mo SC Ni-Mo SC Ni-Mo Sc DISCUSSION 4.1 Age of Metalliferous Ore Deposition Previous Re-Os isochron ages of the Ni-Mo polymetallic layers in the basal Niutitang Formation, dated at 542±11 Ma (Li et al., 2002), 541±16 Ma (Mao et al., 2002), and 535±11 Ma (Jiang et al., 2007), imply that these metalliferous layers were deposited close to the Ediacaran-Cambrian boundary age. But much younger ages have been obtained from recent studies, including the Re-Os age of 521±5 Ma (Xu et al., 2011) and SHRIMP U-Pb zircon ages of 532.3±0.7 Ma (Jiang et al., 2009), 522.7±4.9 Ma (Wang X et al., 2012), and 518±5 Ma (Zhou et al., 2008). Our new Re-Os isochron ages confirm that the deposition of the Ni-Mo polymetallic layers was ~20 Ma posterior to the onset of Cambrian (Figs. 4, 5). The four Ni-Mo polymetallic layers have yielded Re-Os isochron ages overlapped with large uncertainties (Fig. 4). Furthermore, a Re-Os isochron age of 520.3±9.1 Ma is obtained from the V-rich black shales collected from the Bahuang Section in northeastern

6 276 Yong Fu, Lin Dong, Chao Li, Wenjun Qu, Haoxiang Pei, Wenlang Qiao and Bing Shen Figure 4. Re-Os isochron diagrams. (a) Re-Os isochron diagram of the non-metalliferous black shale beneath the nodular phosphorite maker bed in the Shuidong Section; (b) Re-Os isochron diagram of the Ni-Mo polymetallic layer from the Songlin Section; (c) Re-Os isochron diagram of the Ni-Mo polymetallic layer from the Niuchang Section; (d) Re-Os isochron diagram of the Ni-Mo polymetallic layer from the Sancha Section; (e) Re-Os isochron diagram of the Ni-Mo polymetallic layer from the Chuanpengwan Section; (f) Re-Os isochron diagram of the V-rich black shale from the Bahuang Section. Guizhou province (Figs. 1, 4), indistinguishable from the Re-Os ages of the Ni-Mo polymetallic layers (Fig. 4) and recent U-Pb zircon ages of the Niutitang base (Chen et al., 2015; Wang X et al., 2012). These new age data imply that the deposition of Ni-Mo polymetallic layers and V-rich black shales may be concomitant, agreeing with the traditional stratigraphic correlation marked by the underlying nodular phosphorite bed over entire Yangtze Platform (Jiang et al., 2007; Yang et al., 2004; Chen et al., 2003; Zhu et al., 2003). A combined Re-Os age of 525.9±8.9 Ma obtained from all Ni-Mo sulfide samples collected from the four studied sections and V-rich black shale samples in the Bahuang section is also indistinguishable (within the analytical error) from recent U-Pb zircon ages of ± 4.9 Ma at Taoying section (Wang X et al., 2012), 522.3±3.7 Ma at Bahuang (Chen et al., 2015) and 524.2±5.1 Ma at Panmen from the base of Niutitang Formation (generally 1 2 m below the metalliferous layers) in northeastern Guizhou Province (Wang X et al., 2012) (Fig. 5). 4.2 Source of Polymetallic Ore Deposits Similar Re-Os ages of the Ni-Mo polymetallic layers and V-rich black shales in the basal Niutitang Formation may indicate possible synchronous deposition of these two types of polymetallic ores. To further constrain the source of ore-forming metals, the initial 187 Os/ 188 Os ratio is used. To avoid interference of 187 Re, which decays to 187 Os, the

7 New Re-Os Isotopic Constrains on the Formation of the Metalliferous Deposits 277 Figure 5. Combined isochron diagram of all measured Re-Os data of Ni-Mo polymetallic layers from four studied sections and V-rich black shale from the Bahuang Section. Re corrected initial 187 Os/ 188 Os ratio has been widely used as a proxy for seawater Os isotopic compositions. Seawater Os isotopic composition is mainly controlled by the relative contributions of riverine and hydrothermal influxes (Peucker-Ehrenbrink and Ravizza, 2000; Sharma et al., 1999; Sharma et al., 1997; Sharma and Wasserburg, 1997). Although extraterrestrial contribution from meteorite and cosmic dust is the third largest Os source for seawater, the absolute contribution from this source is insignificant as compared with the former two (Peucker-Ehrenbrink and Ravizza, 2000; Sharma et al., 1997). Riverine influx of Os mainly derives from chemical weathering of continental crust, and thus is highly radiogenic ( 187 Os/ 188 Os ratio=1.4) (Sharma et al., 1997; Sharma and Wasserburg, 1997). In contrast, hydrothermal Os influx is produced by hydrothermal alteration of juvenile oceanic crust, and has the mantle-like unradiogenic signature ( 187 Os/ 188 Os ratio=0.127) (Sharma et al., 2000). In the modern ocean, seawater has a homogeneous 187 Os/ 188 Os ratio of 1.06 (Woodhouse et al., 1999), so riverine influx accounts for ~75% of modern marine Os budget. Seawater Os isotopic composition shows secular variation in Earth s history (Du Vivier et al., 2014; Zhu et al., 2013; Rooney et al., 2011; Kendall et al., 2009; Turgeon and Creaser, 2008; Kendall et al., 2006; Oxburgh, 2001; Cohen et al., 1999; Sharma et al., 1999; Pegram et al., 1994). The long term fluctuation in seawater 187 Os/ 188 Os ratio is mainly controlled by chemical weathering of continents. For example, a gradual increase in seawater 187 Os/ 188 Os ratio in the last 40 million years is suggested to be related to the enhanced chemical weathering after the rise of Himalayas (Sharma et al., 1999). In contrast, multiple short-lived negative spikes in seawater 187 Os/ 188 Os ratio are best interpreted as bursts of hydrothermal influx. A decrease of seawater 187 Os/ 188 Os ratio from 0.8 to 0.2 during the Mid-Cretaceous oceanic anoxic event (AOE2) is attributed to the emplacement of Columbia Flood Basalt during the Cenomanian-Turonian boundary at ~93.5 Ma (Du Vivier et al., 2014, Turgeon and Creaser, 2008). Similar negative excursions in seawater 187 Os/ 188 Os ratio have also been reported from the Cretaceous-Tertiary and Triassic-Jurassic boundaries (Cohen et al., 1999; Pegram et al., 1994). Thus, seawater 187 Os/ 188 Os ratio is sensitive to short-term hydrothermal activities. In this study, all Ni-Mo polymetallic layers and V-rich black shales have similar initial 187 Os/ 188 Os ratios ranging from 0.8 to 0.9 (Fig. 4), generally consistent with previous studies (Xu et al., 2011; Jiang et al., 2007a; Mao et al., 2002). These values are also similar to the initial 187 Os/ 188 Os ratio (0.85±0.07) of the non-metalliferous black shale underlying the nodular phosphorite marker bed in the Shuidong Section (Fig. 4). In previous studies, the hydrothermal origin of Ni-Mo polymetallic layers of the basal Niutitang Formation was proposed, in terms of their lower initial 187 Os/ 188 Os ratio than that of the Early Cambrian black shale ( ) from Lesser Himalaya (Jiang et al., 2007; Singh et al., 1999). However, the only slightly lower 187 Os/ 188 Os ratios of these metalliferous and non-metalliferous layers imply a major contribution of normal seawater, likely with minor inputs of hydrothermal fluids through which the initial 187 Os/ 188 Os ratio was drew down slightly in comparison with modern seawater. This scenario is also supported by recent Mo isotope, trace elements, and REE data (Xu et al., 2013). Therefore, we suggest that normally seawater might be the most likely metal source for the polymetallic ores deposits. 4.3 A Working Model of Ni-Mo and V Ore Formation Similar Re-Os ages and seawater-like initial 187 Os/ 188 Os ratios of the Ni-Mo polymetallic layers and V-rich black shale suggest that the Ni-Mo and V ores probably both originate from normal seawater. The Ni-Mo polymetallic layers normally contain a few weight percent of Ni and Mo and several hundred ppm of V (Pi et al., 2013; Lehmann et al., 2007). In contrast, the V-rich black shale has wt.1% 2% of V 2 O 5, but a few hundred ppm of Ni and Mo (Su et al., 2012; Bao et al., 2002). These observations imply the separation of V from Ni and Mo during metal ore deposition. V, Ni, and Mo are redox-sensitive trace elements in seawater. V and Mo have strong euxinic affinity, i.e., they are highly enriched in sulfidic conditions, whereas Ni has weak euxinic affinity (Algeo and Maynard, 2004). Therefore, the enrichment of Mo and Ni without concomitant accumulation of V in the Ni-Mo polymetallic layers implies the scavenging of seawater V before Ni-Mo ore deposits, otherwise V would have co-precipitated with Ni and Mo as polymetallic sulfide (Algeo and Maynard, 2004). In the Yangtze Platform, Ni-Mo polymetallic layers are restricted in the platform margin, whereas the V-rich black shales are widely distributed in the slope-basin environments (Fig. 1). As such, to separate V from Ni and Mo, one possibility is to remove seawater V by black shale deposition during the upwelling of more distal deep-basin water into the slope-basin of the Yangtze platform (Fig. 6). This interpretation also implies that the distal deep-basin water mass might be the major source of the metallic ores. Below, we propose a working model for the formation of metal ores deposition in the basal Niutitang Formation. In this model, depositions of polymetallic layers in platform margins and V-rich black shale in slope-basin environments are attributed to the redox stratification in Early Cambrian oceans. Our model is rooted from the stratified redox model proposed by Feng et al. in But different from the stratified redox model (Feng et al., 2014), our model suggest that the distal deep-basin water

8 278 Yong Fu, Lin Dong, Chao Li, Wenjun Qu, Haoxiang Pei, Wenlang Qiao and Bing Shen was oxic or suboxic. The distal deep-basin water could not be sulfidic, because V, Ni, and Mo would be scavenged from seawater by sulfide precipitation (Adelson et al., 2001; Morse and Luther Iii, 1999; Huerta-Diaz and Morse, 1992). Neither could it be organic rich (e.g., containing high concentration of dissolved organic carbon), because all these metals also have strong affinity with organic matter, resulting in organometallic complex formation (Achterberg and Van Den Berg, 1997; Achterberg et al., 1997; Wehrli and Stumm, 1989; Templeton III and Chasteen, 1980). Thus, the deep water might be oxic or slightly anoxic and organic-poor. An oxic/sub-oxic deep water is plausible, because organic productivity might be extremely low in the distal oceans (due to insufficient nutrient supply from continents), which in term might have facilitated O 2 penetration into deeper water column. It is unclear whether the metal contents in the deep water could be higher than the base-level, however, additional enrichment of metals in seawater might not be required for the metallic ore deposition as estimated in previous study (Lehmann et al., 2007). Separation of V from Ni and Mo in the upwelling deep water might take place in the slope-basin of Yangtze Platform, where a non-sulfidic anoxic-suboxic wedge (NSASW) might have been developed. Similar to the formation of oxygen minimum zone in the modern ocean, NSASW could be dynamically sustained by continuous supply of particulate organic matter from the overlying euphotic zone, thus physical oceanic stratification is not needed. We speculate that NSASW might be organic rich due to excessive supply of organic matter, but should not be sulfidic, since sulfate might have been depleted in the platform margin, where a sulfidic wedge was developed. When the upwelled deep water transects the mildly reduced, organic-rich NSASW, redox sensitive elements compete for organic ligands. Under non-euxinic, anoxic-suboxic redox conditions, V has higher enrichment factor than Ni and Mo (Algeo and Maynard, 2004), thus V is preferentially scavenged from seawater by chelating with organic ligands. The distinct behavior between V and Ni/Mo under suboxic to anoxic conditions may be attributed to the step-wise reduction of V. In oxic seawater, V remains V(V) in vanadate ion (e.g., HVO 2-4 and H 2 VO - 4) (Wehrli and Stumm, 1989). Under mildly reducing conditions with the presence of humic and/or fulvic acid, vanadate is reduced to species with the valence state of V(IV) (such as VO 2+, VO(OH) -, VO(OH) 2 ) (Templeton III and Chasteen, 1980). The V(IV) species tend to chelate with organic matter, forming organometallic complex (Templeton III and Chasteen, 1980; Wilson and Weber, 1979), followed by absorption onto clay minerals. Further reduction of V(IV) to V(III) occurs under euxinic conditions, within which V is precipitated as solid (V 2 O 3 or V(OH) 3 ) (Wanty and Goldhaber, 1992; Breit and Wanty, 1991). After depletion of V in the upwelling seawater in the NSASW, the deep water migrates upward along the continental shelf into the shallow interior of Yangtze Platform. In the platform margin, where sulfidic wedge transect, Ni and Mo are rapidly scavenged by H 2 S, forming the Ni-Mo polymetallic layer. The sulfidic wedge might be sustained in the platform margin by high organic productivity in the surface oceans and terrestrial supply of sulfate (Fig. 6). In this model, we suggest that the developments of NSASW in the slope-basin and sulfidic wedge in the continental margin are responsible for the separation of V from Ni-Mo during metallic ores formation. Thus, the ocean chemistry might have placed the key control on the metallogenesis in the basal Niutitang Formation in the Yangtze Platform. 5 CONCLUSIONS In this study, we report new Re-Os isochron ages of Ni-Mo polymetallic layers, V-rich black shale, and non-metalliferous black shale in the basal Niutitang Formation Figure 6. Model showing the formation of metalliferous deposits in the basal Niutitang Formation in the Yangtze platform.

9 New Re-Os Isotopic Constrains on the Formation of the Metalliferous Deposits 279 in South China. All samples have a similar Re-Os isochron age, suggesting concurrent deposition of these two types of metals ores in different depositional settings. Furthermore, the metalliferous ores and non-metalliferous black shales have similar initial 187 Os/ 188 Os ratios of , suggesting that the metalliferous ores were mainly precipitated from seawater with minor contribution of hydrothermal activity. The concomitant deposition of these two types of ores in different depositional settings implies the earlier scavenging of V from seawater prior to the precipitation of Ni-Mo polymetallic layer, otherwise V and Mo may have been co-precipitated as sulfide due to their similar strong euxinic affinity. We propose the following the model to interpret the concomitant precipitation of V-rich black shales in the slope-basin and Ni-Mo polymetallic layers in the platform margin of the Yangtze platform. The distal deep water is the major source of metals. When upwelling first transects the non-sulfidic anoxic-suboxic wedge that enriches inorganic matter, V(V) is reduced to V(IV), resulting in the V-rich black shale deposition in the slope-basin environments. Further upward migration of V-depleted deep water into sulfidic platform margin subsequently causes rapid precipitation of Ni-Mo polymetallic layers. We suggest that the spatial distribution of metal ores in the basal Niutitang Formation is mainly controlled by the redox condition of paleoceans. ACKNOWLEDGMENTS This work is supported by the National Natural science Foundation of China (Nos , , , ), the Chinese Geological Survey Program (Nos , , , , J(2010)KP010705), Laboratory of Paleontology and Stratigraphy Open-lab grant (133103), the Science and Technology Foundation of Guizhou (No. QKHJZ(2012)2163) and the Talents Introduction Foundation of Guizhou University (No. GDRJHZ(2011)17). We thank Prof. Chen Daizhao and Prof. Chu Xuelei (Institute of Geology and Geophysics, Chinese Academy of Sciences) and Prof. Li Chao and Prof. Jiang Shaoyong (China University of Geosciences, Wuhan) for their constructive comments on the manuscript, and Prof. Yang Ruidong (Guizhou University) for providing the funds for field sampling. REFERENCES CITED Achterberg, E. P., Van Den Berg, C. M. G., Chemical Speciation of Chromium and Nickel in the Western Mediterranean. Deep Sea Research Part II: Topical Studies in Oceanography, 44(3 4): Achterberg, E. P., van den Berg, C. M. G., Boussemart, M., et al., Speciation and Cycling of Trace Metals in Esthwaite Water: A Productive English Lake with Seasonal Deep-Water Anoxia. Geochimica et Cosmochimica Acta, 61(24): Adelson, J. M., Helz, G. R., Miller, C. V., Reconstructing the Rise of Recent Coastal Anoxia, Molybdenum in Chesapeake Bay Sediments. Geochimica et Cosmochimica Acta, 65(2): Algeo, T. J., Maynard, J. B., Trace-Element Behavior and Redox Facies in Core Shales of Upper Pennsylvanian Kansas-Type Cyclothems. Chemical Geology, 206(3 4): Bao, Z. X., Wan, R. J., Bao, J. M., Vanadium Deposits of Black Shale in Upper Yangtze Platform. Yunnan Geology, 21: Brasier, M., Cowie, J., Taylor, M., Decision on the Precambrian-Cambrian Boundary Stratotype. Episodes, 17(1 2): Breit, G. N., Wanty, R. B., Vanadium Accumulation in Carbonaceous Rocks: A Review of Geochemical Controls During Deposition and Diagenesis. Chemical Geology, 91(2): Chen, D., Zhou, X., Fu, Y., et al., New U Pb Zircon Ages of the Ediacaran-Cambrian Boundary Strata in South China. Terra Nova, 27(1): Chen, Y., Jiang, S., Ling, H., et al., Pb Pb Isotope Dating of Black Shales from the Lower Cambrian Niutitang Formation, Guizhou Province, South China. Progress in Natural Science, 13: Cohen, A. S., Coe, A. L., Bartlett, J. M., et al., Precise Re Os Ages of Organic-Rich Mudrocks and the Os Isotope Composition of Jurassic Seawater. Earth and Planetary Science Letters, 167(3 4): Cohen, K. M., Finney, S. C., Gibbard, P. L., et al., The ICS International Chronostratigraphic Chart. Episodes, 36: Conway Morris, S., Burgess Shale Faunas and the Cambrian Explosion. Science, 246: Du, A., He, H., Yin, W., et al., A Study on the Rhenium-Osmium Geochronometry of Molybdenites. Acta Geologica Sinica (English Edition), 68: Du Vivier, A. D. C., Selby, D., Sageman, B. B., et al., Marine 187 Os/ 188 Os Isotope Stratigraphy Reveals the Interaction of Volcanism and Ocean Circulation during Oceanic Anoxic Event 2. Earth and Planetary Science Letters, 389(0): Feng L., Li C., Huang J., et al., A Sulfate Control on Marine Mid-Depth Euxinia on the Early Cambrian (ca Ma) Yangtze Platform, South China. Precambrian Research, 246: Gould, S. J., Wonderful Life: The Burgess Shale and the Nature of History. Norton, New York. 347 Guo, Q., Shields, G. A., Liu, C., et al., Trace Element Chemostratigraphy of Two Ediacaran-Cambrian Successions in South China: Implications for Organosedimentary Metal Enrichment and Silicification in the Early Cambrian. Palaeogeography, Palaeoclimatology, Palaeoecology, 254(1 2): Huerta-Diaz, M. A., Morse, J. W., Pyritization of Trace Metals in Anoxic Marine Sediments. Geochimica et Cosmochimica Acta, 56(7): Jiang, S. Y., Yang, J. H., Ling, H. F., et al., 2007a. Extreme Enrichment of Polymetallic Ni-Mo-PGE-Au in Lower Cambrian Black Shales of South China: An Os Isotope and PGE Geochemical Investigation. Palaeogeography, Palaeoclimatology, Palaeoecology, 254(1 2): Jiang, S. Y., Zhao, H. X., Chen, Y. Q., et al., 2007b. Trace and Rare Earth Element Geochemistry of Phosphate Nodules

10 280 Yong Fu, Lin Dong, Chao Li, Wenjun Qu, Haoxiang Pei, Wenlang Qiao and Bing Shen from the Lower Cambrian Black Shale Sequence in the Mufu Mountain of Nanjing, Jiangsu Province, China. Chemical Geology, 244(3 4): Jiang, S. Y., Zhao, K. D., Li, L., et al., 2007c. Highly Metalliferous Carbonaceous Shale and Early Cambrian Seawater: Comment and Replay: Comment. Geology, 35(1): e158 e159 Jiang, S. Y., Pi, D. H., Heubeck, C., et al., Early Cambrian Ocean Anoxia in South China. Nature, 459(7248): E5 E6 Jiang, S., Yang, J., Ling, H., et al., Re-Os Isotopes and PGE Geochemistry of Black Shales and Intercalated Ni-Mo Polymetallic Sulfide Bed from the Lower Cambrian Niutitang Formation, South China. Progress in Natural Science, 13: Kendall, B., Creaser, R. A., Selby, D., Re-Os Geochronology of Postglacial Black Shales in Australia: Constraints on the Timing of Sturtian Glaciation. Geology, 34: Kendall, B., Creaser, R. A., Gordon, G. W., et al., Re Os and Mo Isotope Systematics of Black Shales from the Middle Proterozoic Velkerri and Wollogorang Formations, McArthur Basin, Northern Australia. Geochim. Cosmochim. Acta, 73: Lehmann, B., Nägler, T. F., Holland, H. D., et al., Highly Metalliferous Carbonaceous Shale and Early Cambrian Seawater. Geology, 35: Li, C., Qu, W.,Du, A., Comprehensive Study on Extraction of Rhenium with Acetone in Re-Os Isotopic Dating. Rock and Mineral Analysis, 28: Li, C., Qu, W., Zhou, L., et al., Rapid Separation of Osmium by Direct Distillation with Carius Tube. Rock and Mineral Analysis, 29: Li, S., Xiao, Q., Shen, J., et al., Re-Os Isotopic Constraints on the Source and Age of Lower Cambrian Platinum Group Element Ores in Hunan and Guizhou. Science in China (Series D), 32(7): (in Chinese) Maloof, A. C., Porter, S. M., Moore, J. L., et al., The Earliest Cambrian Record of Animals and Ocean Geochemical Change. Geological Society of America Bulletin, 122(11 12): Mao, J., Lehmann, B., Du, A., et al., Re-Os Dating of Polymetallic Ni-Mo-PGE-Au Mineralization in Lower Cambrian Black Shales of South China and Its Geologic Significance. Economic Geology, 97(5): Marshall, C. R., Explaining the Cambrian explosion of Animals. Annual Review of Earth and Planetary Sciences, 34: Morse, J. W.,Luther Iii, G. W., Chemical Influences on Trace Metal-Sulfide Interactions in Anoxic Sediments. Geochimica et Cosmochimica Acta, 63(19 20): Narbonne, G. M., Myrow, P., Trace Fossil Biostratigraphy in the the Precambrian-Cambrian Boundary Interval. New York State Museum Bulletin, 463: Och, L. M., Shields-Zhou, G. A., Poulton, S. W., et al., Redox Changes in Early Cambrian Black Shales at Xiaotan Section, Yunnan Province, South China. Precambrian Research, 225: Orberger, B., Vymazalova, A., Wagner, C., et al., Biogenic Origin of Intergrown Mo-Sulphide- and Carbonaceous Matter in Lower Cambrian Black Shales (Zunyi Formation, southern China). Chemical Geology, 238(3 4): Oxburgh, R., Residence Time of Osmium in the Oceans. Geochemistry, Geophysics, Geosystems, 2: 2000GC Pegram, W. J., Esser, B. K., Krishnaswami, S., et al., The Isotopic Composition of Leachable Osmium from River Sediments. Earth and Planetary Science Letters, 128(3 4): Peucker-Ehrenbrink, B., Ravizza, G., The Marine Osmium Isotope Record. Terra Nova, 12: Pi, D. H., Liu, C. Q., Zhou, G. A., et al., Trace and Rare Earth Element Geochemistry of Black Shale and Kerogen in the Early Cambrian Niutitang Formation in Guizhou Province, South China: Constraints for Redox Environments and Origin of Metal Enrichments. Precambrian Research, 225: Qu, W., Du, A., Highly Precise Re-Os Dating of Molybdenite by ICP-MS with Carius Tube Sample Digestion. Rock and Mineral Analysis, 22: Rooney, A. D., Chew, D. M., Selby, D., Re-Os Geochronology of the Neoproterozoic-Cambrian Dalradian Supergroup of Scotland and Ireland: Implications for Neoproterozoic Stratigraphy, Glaciations and Re-Os Systematics. Precambrian Research, 185(3 4): Sharma, M., Papanastassiou, D. A., Wasserburg, G. J., The Concentration and Isotopic Composition of Osmium in the Oceans. Geochimica et Cosmochimica Acta, 61(16): Sharma, M., Wasserburg, G. J., Osmium in the Rivers. Geochimica et Cosmochimica Acta, 61(24): Sharma, M., Wasserburg, G. J., Hofmann, A. W., et al., Himalayan Uplift and Osmium Isotopes in Oceans and Rivers. Geochimica et Cosmochimica Acta, 63(23 24): Sharma, M., Wasserburg, G. J., Hofmann, A. W., et al., Osmium Isotopes in Hydrothermal Fluids from the Juan de Fuca Ridge. Earth and Planetary Science Letters, 179(1): Shu, D., Cambrian Explosion: Birth of Tree of Animals. Gondwana Research, 14(1 2): Singh, S. K., Trivedi, J. R., Krishnaswami, S., Re-Os Isotope Systematics in Black Shales from the Lesser Himalaya: Their Chronology and Role in the 187 Os/ 188 Os Evolution of Seawater. Geochimica et Cosmochimica Acta, 63(16): Su, D. Y., Wu, Z. C., Zhang, M. Q., et al., Geological Characteristics and Metallogenic Prediction of Vanadium Deposit in Northeast Guizhou. Guizhou Geology, 29: Templeton III, G. D., Chasteen, N. D., Vanadium-Fulvic Acid Chemistry: Conformational and Binding Studies by

11 New Re-Os Isotopic Constrains on the Formation of the Metalliferous Deposits 281 Electron Spin Probe Techniques. Geochimica et Cosmochimica Acta, 44(5): Turgeon, S. C., Creaser, R. A., Cretaceous Oceanic Anoxic Event 2 Triggered by a Massive Magmatic Episode. Nature, 454(7202): Wang Y., Wang X. L., Wang Y., Cambrian Ichnofossils from the Zhoujieshan Formation (Quanji Group) Overlying Tillites in the Northern Margin of the Qaidam Basin, NW China.Journal of Earth Science, 26(2): Wang, J., Chen, D., Wang, D. A. N., et al., Petrology and Geochemistry of Chert on the Marginal Zone of Yangtze Platform, Western Hunan, South China, during the Ediacaran-Cambrian Transition. Sedimentology, 59(3): Wang, J., Chen, D., Yan, D., et al., Evolution from an Anoxic to Oxic Deep Ocean during the Ediacaran-Cambrian Transition and Implications for Bioradiation. Chemical Geology, : Wang, X., Shi, X., Jiang, G., et al., New U-Pb Age from the Basal Niutitang Formation in South China: Implications for Diachronous Development and Condensation of Stratigraphic Units across the Yangtze Platform at the Ediacaran-Cambrian Transition. Journal of Asian Earth Sciences, 48(0): 1 8 Wanty, R. B., Goldhaber, M. B., Thermodynamics and Kinetics of Reactions Involving Vanadium in Natural Systems: Accumulation of Vanadium in Sedimentary Rocks. Geochimica et Cosmochimica Acta, 56(4): Wehrli, B., Stumm, W., Vanadyl in Natural Waters: Adsorption and Hydrolysis Promote Oxygenation. Geochimica et Cosmochimica Acta, 53(1): Wilson, S. A., Weber, J. H., An EPR Study of the Reduction of Vanadium(V) to Vanadium(IV) by Fulvic Acid. Chemical Geology, 26(3 4): Woodhouse, O. B., Ravizza, G., Kenison Falkner, K., et al., Osmium in Seawater: Vertical Profiles of Concentration and Isotopic Composition in the Eastern Pacific Ocean. Earth and Planetary Science Letters, 173(3): Xu, L., Lehmann, B., Mao, J., et al., Re-Os Age of Polymetallic Ni-Mo-PGE-Au Mineralization in Early Cambrian Black Shales of South China-A Reassessment. Economic Geology, 106(3): Xu, L., Lehmann, B., Mao, J., Seawater Contribution to Polymetallic Ni-Mo-PGE-Au Mineralization in Early Cambrian Black Shales of South China: Evidence from Mo Isotope, PGE, Trace Element, and REE Geochemistry. Ore Geology Reviews, 52: Yang, J. H., Jiang, S. Y., Ling, H. F., et al., Paleoceangraphic Significance of Redox-Sensitive Metals of Black Shales in the Basal Lower Cambrian Niutitang Formation in Guizhou Province, South China. Progress in Natural Science, 14: Yang, Y. F., Xu, Y. F., Zhou, C. Y., Discussion of the Relation Between Liuchapo Formation and Dengying Formation of Upper Yantze Platform. Guizhou Geology, 4: (in Chinese with English Abstract) Zhou, C., Jiang, S. Y., Palaeoceanographic Redox Environments for the Lower Cambrian Hetang Formation in South China: Evidence from Pyrite Framboids, Redox Sensitive Trace Elements, and Sponge Biota Occurrence. Palaeogeography, Palaeoclimatology, Palaeoecology, 271(3 4): Zhou, M., Luo, T., Li, Z., et al., SHRIMP U-Pb Zircon Age of Tuff at the Bottom of the Lower Cambrian Niutitang Formation, Zunyi, South China. Chinese Science Bulletin, 53(4): Zhu, B., Becker, H., Jiang, S. Y., et al., Re Os Geochronology of Black Shales from the Neoproterozoic Doushantuo Formation, Yangtze Platform, South China. Precambrian Research, 225(0): Zhu, M. Y., Zhang, J. M., Steiner, M., et al., Sinian-Cambrian Stratigraphic Framework for Shallow- to Deep-Water Environments of the Yangtze Platform: an Integrated Approach. Progress in Natural Science, 13: