The geochemical and organic petrological characteristics of kolm (upper Cambrian, Sweden): Implications for genesis

Kolm refers to uraniferous (1280–7100 ppm) and organic-rich (25–71 wt%) lenses occurring exclusively within the Furongian part of the Alum Shale Formation in south-central Sweden. It typically less than 7 cm thick and forms thin discontinuous layers. This study investigates the geochemical and organic petrological characteristics of kolm, and it is shown that the organic matter likely represents secondarily formed solid bitumen rather than a primary organic-rich component. The high uranium content is concentrated in specific uranium‑yttrium‑zinc-rich (U-Y-Zn-rich) particles. A model for kolm formation is presented, suggesting that during sedimentation, initial uranium-enriched particles were formed and then became concentrated, probably by winnowing at the sea floor under euxinic conditions. This lag deposit rich in uranium particle subsequently formed the radioactive nuclei (U-Y-Zn-rich particles) for the kolm nodules that grew during the early diagenesis. Initial kolm was apparently formed by in-situ accumulation of diagenetically formed solid bitumen (Ro < 0.5 %) onto these strongly radioactive U-Y-Zn-rich particles. The more abundant development of kolm in the Billingen area of Västergötland, compared to other regions in south-central Sweden where kolm occurs, is likely due to increased generation of solid bitumen associated with localized heating from Permo-Carboniferous intrusions.