Organic clots and their differential accumulation of Ni and Mo within early Cambrian black-shale-hosted polymetallic Ni–Mo deposits, Zunyi, South China

The celebrated early Cambrian Ni–Mo deposits that occur near Zunyi City of Guizhou Province, South China, are hosted in shales. The ore formation has been widely accepted to be related to biotic impact, but direct evidence is still lacking. In the present study of the Huangjiawan ore deposit, organic clots are found to occur widely in the Ni–Mo ore beds, but not in the barren layers. The clots are generally elliptic, with sizes ranging from 200 to 600 μm. The clot morphology is similar to that of modern rhodophyta as reported in the literature. We tentatively interpret the clots as rhodophyte cystocarps. Electron microprobe analysis revealed that the clots have a differential accumulation of Ni and Mo, with Ni in the inner part and Mo in the outer part (of the cystocarp). Our study seems to have provided direct evidence for the biotic impact on the ore mineralization.

► Direct evidence for biotic impacts on early Cambrian shale-hosted Ni–Mo ore formation in Zunyi City, South China.
► The widely occurred organic clots in the ore bed are interpreted as rhodophyte cystocarps.
► Differential accumulation of Ni and Mo in the cystocarps was revealed for the first time.