Rare metal sequestration and mobility in mineralized black shales from the Zunyi region, South China

The micromineralogical and chemical study of two black mineralized shales from the Zunyi region show that they experimenced a low diagenetic degree still preverving microbial phosphate and sulfides and poorly crystallized phases composed of metals or Al and Si. The two shales differ in diagenetic vein mineralogy. CH11 is carbonate-rich whereas CH25 is sulfate-rich. As CH11 is twice enriched in the redox sensitive elements Ni, Se, As, Mo, Sb, U, Zn, Pb, Cd, a remobilization by CO2 (may be derived from organic matter oxidation) rich fluids is suggested. Seawater characteristic REE element patterns, but Se/S-ratio typical for a volcanogenic environment, favor a fluid mixture during early diagenesis.