Metal enrichment in lithologically complex black shales: a case study from the Tremadocian of NE Estonia

Significantly elevated U, Mo, Zn and Pb contents characterize the Early Ordovician black shales in the Sillamäe area, NE Estonia. The presence of silty interlayers with sulphidic mineralization and authigenic carbonates suggests unique physicochemical conditions for metal enrichment in this location. We investigated metallogenesis of these shallow-water black shales based on high-resolution mapping of element distribution in the Sõtke drill core and nearby Päite outcrop using X-ray fluorescence spectroscopy, inductively coupled plasma mass spectrometry and organic elemental analysis, complemented by optical microscopy and scanning electron microscopy of selected samples. Enriched metals in the black shales of the study area show dissimilar distribution trends with sharp vertical concentration gradients. The recorded variance ranges were 88–275 ppm for U, 70–2467 ppm for Mo, 85–1600 ppm for V, 21–17 283 ppm for Zn and 95–26 549 ppm for Pb, while total organic carbon varied from 0.5 to 13 wt%. In most cases, the metals showed no clear covariance with organic matter or other major compounds such as S or P. The development of a sulphate reduction zone near the sediment–water interface with a sharp decrease in Eh, production of H2S, elevated alkalinity and pH, and (re)distribution of phosphorus probably controlled the syngenetic capture of Mo and U. Enhanced transfer of fluids and solutes in coarse-grained permeable beds facilitated the accumulation of metals, while the deposit probably acted as a semi-open geochemical system throughout its geological evolution. Local Zn and Pb enrichment developed due to the intrusion of late diagenetic metal-bearing fluids and the entrapment of metals in beds that contained authigenic carbonates.