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ABSTRACT

The Namurian black shales studied have a good palaeontological control. Pyrite, illite and organic matter are higher in the marine shales, and siderite and kaolinite are higher in the non-marine shales. The distribution of the diagenetic iron minerals is attributed to anionic activities controlled by salinity. The same control could account for the observed clay-mineral distribution by differential flocculation. The major element geochemistry closely reflects the above mineral variation. Mn and Zr are higher in the non-marine shales due to siderite and zircon respectively. Co, Cr, Rb, Y and Ba are not significantly different and a detrital source, mainly clay minerals, is suggested. In the marine shales the elements Pb, Cu, V, Ni, Sr and Zn are significantly higher. Using a discriminant function analysis the palaeosalinity groups are separated and the contribution of each element to the separation is calculated. The present element locations are thought, from correlation analysis, to be pyrite and organic matter. By analogy with experimental work and modern environments, the element enrichment is mainly attributed to reactions involving organic matter and oxyhydroxide material in environments in which salinity and slow rates of sedimentation were important factors. The element enrichment cannot be related directly to seawater concentrations, unlike some black shales, and the accumulation rates for Pb and Cu are thought to be unusually high.