Natural nuclear fission reactors: time constraints for occurrence, and their relation to uranium and manganese deposits and to the evolution of the atmosphere

Abstract

Knowledge of the formation conditions of Francevillian uranium and manganese ore deposits as well as natural fission reactors sheds light on the early evolution of the atmosphere between 1950 and 2150 Ma ago. The model explaining the formation of the Oklo uranium deposits suggests that at the time of sediment deposition in the Franceville basin 2150 million years ago, the oxygen deficient atmosphere would have inhibited uranium dissolution. Dissolution of uranium was only possible during later diagenesis, approximately 1950 Ma. Reduction reactions in the presence of hydrocarbons allowed precipitation of dissolved uranium to U⁴⁺, forming deposits with high enough uranium contents to trigger subsequent nuclear fission reactions. Such a model is in agreement with earlier suggestions that oxygen contents in atmosphere increased during a ‘transition phase’ some 2450–2100 Ma ago. The manganese deposits were formed before the uranium deposits, during the deposition of the black shales and very early diagenesis, and thus at a time when oxygen content in atmosphere was very low. Carbon isotopes data of organic matter show decrease of δ¹³C upward in the Francevillian series (−20 to −46% PDB) reflecting the high CH₄ and low O₂ contents in the atmosphere during sediment deposition. This favoured anoxic conditions during deposition of the basinal FB black shales and likewise the migration of Mn over long distances. The manganese precipitated first as Mn-oxides at the shallow edges of the Franceville basin, in photic zones, where photosynthetic organisms flourished. Mn-oxides were then reduced in the black shales forming Mn-carbonates when conditions became more reducing during transgression episodes and/or the first stages of burial. In the black shales, reducing conditions prevailed until recent weathering, allowing the good preservation of organic matter and the Mn deposits. The present-day alteration is responsible for the dissolution of Mn-carbonates and precipitation of Mn-oxides at the water table to form the high grade Mn ore (45–50% Mn). Development of photosynthesizing organisms, a volcanic source of the Mn, and favourable palaeogeography of the Francevillian basins are all important parameters for the formation of the Mn deposits. For the occurrence of the natural nuclear reactors, the age of 2.0 Ga is the main parameter that controls the abundance of fissile ²³⁵U and the critical mass. Before 2.0 Ga the ²³⁵U/²³⁸U ratio was sufficiently high for fission reactions to occur but conditions favourable for forming high grade uranium ores were not achieved. Then, after 2.0 Ga the increase of oxygen in the atmosphere commonly led to the formation of high grade uranium ores in which the ²³⁵U/²³⁸U ratio was too low to support criticality.

Introduction

The natural nuclear fission reactors in Gabon are unique. The possibility that nuclear fission reactions might have occurred in the past was first suggested by Kuroda 1956 but we had to wait until 1972 to discover that such reactions did indeed occur 2.0 Ga ago in the Oklo uranium deposit of the Francevillian Series, Gabon (Neuilly et al., 1972). Other uranium deposits of Proterozoic age in the world (Maas and McCulloch, 1990) do not manifest the characteristic U and REE isotopic anomalies of natural nuclear fission reactions, suggesting that these only occurred in the Paleoproterozoic uranium ore deposits of Gabon. This observation raises the question whether this is fortuitous or due to special geological conditions. This paper points out the main physical and chemical conditions that are necessary to start and to sustain fission reactions in a natural environment and tries to show that most of these conditions are related to the age of the uranium deposits.

Beside uranium deposits, the Franceville basin contains one of the largest Mn-deposits in the world, namely the Moanda deposit. Reserves of the Moanda deposits are estimated to be 200 Mt of Mn at concentrations up to 48%. This deposit has been formed by a ‘lateritic’ alteration process of the Francevillian black shales which contain Mn-bearing carbonates.

The early Proterozoic is considered as one of the main periods for Mn deposit formation. In the early Archean it is presumed by Roy (2000) that ‘Fe²⁺ and Mn²⁺ in solution were separated by preferential precipitation of the former and the retention of the latter in dissolved state’. Precipitation of Mn first happened at a very large scale 2.3–2.1 Ga ago in the Paleoproterozoic Kalahari manganese deposit (Transvaal Supergroup, South Africa). Whether this dissociation is related to the variation of dissolved oxygen in the marine environments during transgression–regression episodes or is due to hydrothermal exhalations is still controversial (Glasby, 1997, Roy, 1992, Roy, 2000, Klemm, 2000). The study of the Moanda deposit allows a better understanding of the relations between the geochemistry of Fe and Mn, the evolution of the composition of atmosphere and the paleogeography of the basins at that time.

The relation between Proterozoic Mn deposits and black shales has been recognized by various authors e.g. in the Azul (Bernardelli and Beisiegel, 1978, Beauvais, 1984) and Amapa deposits of Brazil, and the Tangganshan deposit of China (Roy, 1992). The period between 1950 and 2100 Ma was generally favourable for the deposition of organic-rich sediments and Condie et al. (2001) have inventoried ten basins worldwide where the thickness of black shales sediments range between 150 and 2000 m. This period of black shale deposition is interpreted as recording the break-up of supercontinents resulting in increased numbers of partially closed marine basins with consequent disruption of ocean currents and increased ocean ridge activity, collectively leading to widespread anoxia (Condie et al., 2001). It will be shown here that the differences in carbon isotopic compositions of organic matter of the Francevillian black shales reflect oxic and anoxic environments and provide information on the oxygen concentration of the atmosphere during their deposition.

The present study tries attempts to connect the above observations in order to show that the formation of the Mn and U deposits and the occurrence of nuclear fission reactions are related to the evolution of the composition of the atmosphere between the time of deposition of the Francevillian sediments and their diagenesis, that is between circa 2150 and 1950 Ma ago. For this, we will refer to several models that have been proposed to explain the formation of the manganese and uranium deposits and of the fission reactors which are already published. However, we shall present only the major points of the arguments that sustain these models. The focus is to show the relationships between the evolution of the Proterozoic atmosphere and the geochemistry of U and Mn.