The geobiology of the extremely enriched polymetallic sulfides in the black shale of the lower Cambrian Niutitang formation, Southwestern China

Abstract

The Precambrian-Cambrian transition is a period with enormous geological and biological changes. There is a wide distribution of black shale sequence in the Late Sinian and Early Cambrian strata along the passive southern margin of the Yangtze Platform in South China. The remarkable polymetallic sulfide extremely enriched ore layer is embedded at the bottom of the Lower Cambrian Niutitang Formation, but its genesis remains highly disputable. Known mechanisms can hardly explain the extreme enrichment and paragenesis of multimetal sulfides with regard to their highly variant properties.

Here, a case study is performed about the polymetallic sulfide enriched ore layer and related strata from the black shale-dominated Niutitang Formation in Zunyi, Guizhou Province and Zhangjiajie, Hunan Province. A phosphorite-rich layer is situated near the bottom of the Niutitang Formation. A few meters above the phosphorite is the polymetallic sulfide enriched ore layer embedded in the carbonaceous black shale wall-rock. A Combination of different methodologies were used to examine the polymetallic enriched ore, the black shale wall-rock and the adjacent phosphorite samples, including optical microscopy, electron microscopy, Mössbauer spectroscopy, Raman spectroscopy, electron microprobe analysis, X-ray absorption spectroscopy, etc.

Submicron-scale organic vesicles resembling green algae were noted in the polymetal sulfide enriched layer, indicating the key role of microbial activities during the mineralization of the polymetallic sulfide ores. Larger biogenic structures with possible hydrozoan and anthozoan (or algal) affinities were discovered from the polymetal sulfide ores and phosphorites, respectively, suggesting the participation of metazoan during the mineralization process. The ability of biomineralized molybdenite and apatite to preserve pseudomorphs is attributed to their fine crystal sizes, even if they were precipitated under drastically different geological conditions. The geochemical cycling of phosphorus and other nutrients probably involves multiple marine life-forms. Nickel and iron sulfides, on the contrary, were suggested to be incapable of preserving fine fossil structures because of high-degree recrystallization. In the polymetallic ores, pyrite was proved to be the predominant form of iron, and the uranium minerals were recognized as mainly coffinite and a small portion of uraninite. The uranium radioactive decay-caused carbonization effects were scrutinized in the micro-environment, suggesting the authigenecity of these uranium minerals.

With these novel mineralogical, paleontological and geochemical evidence at the Precambrian-Cambrian boundary, new perspectives on the geobiology of the polymetallic sulfide ores are presented as follows. Both benthic and planktonic organisms might have actively participated in the early Cambrian metallogenesis. Polymetal biosorption by live and dead biomass, especially those from algal blooms, is hypothesized to be a major cause of the unusual polymetallic sulfide ore layer, followed by microbial reduction and immobilization in a stratified water column. This unique ore formation reflects the complicated mutual relationships between Cambrian biota and its paleoenvironment. These results may provide a better understanding of the role of biological activities in the problematic metallogenesis of the polymetal sulfide enriched ores and open a new window to the cognition of the hypothetic Cambrian Explosion.