Micron-scale distribution of metals in Cambrian metalliferous shales, South China: Insights into local biologically driven redox disequilibrium

Early Cambrian black shales of the Niutitang Formation are found across the Yangtze Platform in a 1600-km belt extending across south China. A thin organic matter-rich layer in the lowermost part of this formation contains exceptional concentrations in Mo, Ni, Se, Re, Os, As, Hg, Sb, Ag, Au, Pt, Pd, and Ag. Due to their extreme metal enrichment, these black shales provide a rare opportunity to study the interface between semi-metals, metals and biogenic material. We report the first detailed μm-scale investigation of metal distributions and associations in samples from two sites, Zunyi and Sancha, located hundreds of km apart, using a combination of analytical techniques including X-ray fluorescence (XRF) mapping, scanning electron microscopy (SEM), Synchrotron-based XRF mapping, electron probe micro-analyser (EPMA), particle induced x-ray emission (PIXE) probe and x-ray absorption near-edge structures (XANES) imaging. Strong μm-scale variations in metal and semi-metal distributions were highlighted by Synchrotron-based XRF mapping of samples from both sites. At both locations, U is present within phosphorite nodules. Arsenic, Mo and Se are particularly abundant in the organic-rich matrix, showing strong associations with organic matter. Nickel, however, shows different distributions between the two sites. It is mainly present in association with the organic matter at the Sancha site while it is found in abundance in millerite at the Zunyi site, suggesting slightly different local conditions at Zunyi favouring Ni-S associations over Ni-organic matter. At both sites, biogenic structures were re-mineralised with laminations dominated by different metals, indicating a likely control of organic matter over metal distributions. In addition, the XANES imaging highlighted different redox states of As over μm-scale areas. While As−1 appears to be mainly present in pyrite, As+3 was mainly detected in association within the carbon and MoS2 mixed layer phase (MoSC). Overall, the present results emphasize the complexity of metal associations in this mineralised layer, the significant role of organic matter in the accumulation and precipitation of metals and semi-metals in these metalliferous shales and highlight how biogenic activity can induce μm-scale variations in redox conditions in sediments.