Lehmann et al. (2007) reported trace, rare earth element (REE), and Mo-isotope data from the Ni-Mo sulfide layer and host black shale of the Lower Cambrian Niutitang Formation in southern China. They argue that the metals in the sulfidic rocks originated purely from Early Cambrian seawater. Here we would like to point out that the underpinning assumption of their model (i.e., the stratigraphic correlation of a variety of geochemically anomalous sedimentary beds) is debatable. Furthermore, their geochemical and isotopic evidence is far from conclusive.
Lehmann et al. claim that “the metal-rich sulfidic sediments are stratigraphically equivalent to a spectrum of sediments, including sapropelic alginite,…, large stratified barite and phosphorite deposits” (p. 403). It has been well established in numerous studies that these deposits do not occur at the same stratigraphic level. The major phosphorite and barite deposits in the lower Cambrian black shale sequence occur far below the Ni-Mo sulfide layer (Zhu et al., 2003). In contrast to a single “euxinic basin” as suggested by Lehmann et al., recent geochemical studies suggest that the depositional environment from the phosphorite and barite deposits to the Ni-Mo sulfide layer changed from oxic/dysoxic to anoxic/euxinic (e.g., Goldberg et al., 2007).
The reference by Lehmann et al. to the footwall of the rocks in question as being “Late Proterozoic dolomite of the Doushantuo Formation” (p. 403) is incorrect. The underlying sedimentary rocks are those of the Dengying Formation dolomite, which is more than 50 m.y. younger than the Doushantuo Formation.
TRACE AND RARE EARTH ELEMENT EVIDENCE
Lehmann et al. claim that the seawater-like trace element patterns (their Fig. 2) provide strong support for their seawater hypothesis for the Ni-Mo sulfide formation. However, modern seafloor hydrothermal sediments (Cave et al., 2003; Pašava et al., 2004; Dias and Barriga, 2006; Pašava et al., 2007) show similar patterns as those from the Niutitang Formation. Consequently, a hydrothermal contribution to the geochemical signature of the mineralized black shales cannot be excluded. In addition, the pronounced positive Y anomaly, cited by Lehmann et al. as representative of Early Cambrian seawater, has also been observed in many seafloor massive sulfide deposits (Bau and Dulski, 1999) and thus it is not conclusive.
Lehmann et al.'s interpretations of the Mo-isotopic data are problematic. They assumed that the small scatter of the δ98/95Mo values (−1.24‰ ± 0.10‰) for the Ni-Mo sulfides record that of the Early Cambrian seawater. If this is the case, then some of the shale samples have significantly heavier Mo than the proposed seawater value. This is not known in the current oceans where seawater and euxinic sediments have the heaviest Mo isotope signature. Here we present an alternative interpretation for the Mo data. We suggest that the large scatter in δ98/95Mo values (−1.82‰ to −0.40‰) for the shale samples may represent a heavier authigenic euxinic-sourced Mo (seawater Mo isotopes) mixed with a lighter Mo source of terrigenous materials in the shales (Poulson et al., 2006), whereas the Ni-Mo sulfides are not derived from euxinic seawater but from a third component (hydrothermal fluids). In examining the Mo content and Mo isotope data of the shale samples, it seems that those with heavier Mo isotopic compositions also show higher Mo contents, which can be readily explained by two end-member mixing, although more data are needed to confirm a good correlation between them. The Ni-Mo sulfides and two shale samples with high-Mo content (>150 ppm) (see Table DR2 in the GSA Data Repository 2007097 of Lehmann et al., 2007) plot away from the trend for the low-Mo shale samples, but show uniform δ98/95Mo values, suggesting a dominant additional Mo source with a unique Mo isotope ratio most likely from seafloor hydrothermal solutions. This is consistent with Mo-isotopic data from sediment-hosted modern hydrothermal systems that display high Mo concentration and uniform Mo-isotopic compositions (δ98/95Mo = −1.5‰) (McManus et al., 2002).
In light of the above comments, the model of an exclusive seawater origin of the metal anomalies in the Niutitang Formation shales is still questionable, and a hydrothermal component to the mineralizing fluid mix remains a viable alternative.
This study is financially supported by the National Science Foundation of China program (40221301).