The only known case of natural fission reactors is hosted by high-grade uranium (U) deposits at Oklo-Okelobondo and Bangombé in sandstones of the ~2.1 Ga Francevillian Group, Gabon. However, the geochemical influence of the depositional environment on this unique natural nuclear phenomenon has not been clearly established. Localized, unusually high vanadium (V) enrichments are thought to have prevented such natural nuclear fission reactors from occurring in other Francevillian U deposits (e.g., Mounana, Boyindzi, and Mikouloungou). However, while U-bearing detrital monazite derived from Archean rocks surrounding the Francevillian basin is viewed as the main source of U, the source of V remains poorly constrained. Here, we combine petrographic and whole-rock geochemical data for the Francevillian Group sedimentary rocks, coupled with previously documented geochemical data for the Archean basement. These data suggest that, although ultramafic to mafic igneous rocks of the Mesoarchean Bélinga Group, and to some extent Archean granitoids, were likely important sources of V to the Francevillian U deposits, they were not the only source of the abnormally high V concentrations in the U deposits that did not produce natural nuclear reactors. Instead, hydrocarbon migration from V-rich black shales of the Upper Francevillian Group, deposited during widespread and protracted deoxygenation of the Paleoproterozoic ocean at the end of the Lomagundi Carbon Isotope Excursion (LE) at ~2.11–2.06 Ga, resulted in a redox front that precipitated the U deposits. These migrated V-rich hydrocarbons likely account for the high V concentrations, which ultimately prevented natural fission reactions from occurring in these U deposits. Similarities with other pyrobitumen-bearing Paleoproterozoic U deposits worldwide suggest that organic-rich source rocks, which deposited in open-marine settings under widespread hyper-euxinic conditions in the aftermath of the LE, played a key role in preventing the Oklo natural nuclear reactor phenomenon from reaching a larger extent.

天然裂变反应堆的唯一已知案例是在加蓬〜2.1 Ga Francevillian Group的砂岩中的Oklo-Okelobondo和Bangombé的高品位铀（U）矿床。但是，沉积环境对这种独特的自然核现象的地球化学影响尚未明确。人们认为，局部富集异常高的钒（V）阻止了此类自然核裂变反应堆在其他Francevillian U矿床（例如Mounana，Boyindzi和Mikouloungou）中发生。然而，尽管来自弗朗西斯维利亚盆地周围太古代岩石的含U碎屑独居石被视为U的主要来源，但V的来源仍然受限制。在这里，我们结合了Francevillian Group沉积岩的岩石学和全岩地球化学数据，以及先前记录的太古代地下室的地球化学数据。这些数据表明，尽管MesoarcheanBélinga组的超镁铁质至镁铁质火成岩，以及一定程度的太古宙花岗岩，可能是Francevillian U矿床中重要的V来源，但它们并不是V异常高V浓度的唯一来源。没有产生天然核反应堆的U矿床。取而代之的是，上法兰西维利亚群富V黑色页岩中的碳氢化合物迁移是在Lomagundi碳同位素偏移（LE）末期〜2.11-2.06 Ga末古元古代海洋的广泛而持久的脱氧过程中沉积的，从而导致了氧化还原锋。沉淀了U沉积物。这些迁移的富V的碳氢化合物可能是高V浓度的原因，最终阻止了这些铀矿床中发生自然裂变反应。与全球范围内其他含焦土的古元古代U矿床的相似性表明，在LE发生后，在广泛的高油化条件下沉积在开放海洋环境中的富含有机物的烃源岩在防止奥克洛天然核反应堆现象方面发挥了关键作用从更大的范围。