Oklo

Abstract: The origins and near-surface distributions of the ~250 known uranium and/or thorium minerals elucidate principles of mineral evolution. This history can be divided into four phases. The first, from ~4.5 to 3.5 Ga, involved successive concentrations of uranium and thorium from their initial uniform trace distribution into magmatic-related fluids from which the first U4+ and Th4+ minerals, uraninite (ideally UO2), thorianite (ThO2), and coffinite (USiO4), precipitated in the crust. The second period, from ~3.5 to 2.2 Ga, saw the formation of large low-grade concentrations of detrital uraninite (containing several wt% Th) in the Witwatersrand-type quartz-pebble conglomerates deposited in a highly anoxic fluvial environment. Abiotic alteration of uraninite and coffinite, including radiolysis and auto-oxidation caused by radioactive decay and the formation of helium from alpha particles, may have resulted in the formation of a limited suite of uranyl oxide-hydroxides. Earth’s third phase of uranium mineral evolution, during which most known U minerals first precipitated from reactions of soluble uranyl (U6+O2)2+ complexes, followed the Great Oxidation Event (GOE) at ~2.2 Ga and thus was mediated indirectly by biologic activity. Most uraninite deposited during this phase was low in Th and precipitated from saline and oxidizing hydrothermal solutions (100 to 300 °C) transporting (UO2)2+-chloride complexes. Examples include the unconformity- and vein-type U deposits (Australia and Canada) and the unique Oklo natural nuclear reactors in Gabon. The onset of hydrothermal transport of (UO2)2+ complexes in the upper crust may reflect the availability of CaSO4- bearing evaporites after the GOE. During this phase, most uranyl minerals would have been able to form in the O2-bearing near-surface environment for the first time through weathering processes. The fourth phase of uranium mineralization began ~400 million years ago, as the rise of land plants led to non-marine organic-rich sediments that promoted new sandstone-type ore deposits. The modes of accumulation and even the compositions of uraninite, as well as the multiple oxidation states of U (4+, 5+, and 6+), are a sensitive indicator of global redox conditions. In contrast, the behavior of thorium, which has only a single oxidation state (4+) that has a very low solubility in the absence of aqueous F-complexes, cannot reflect changing redox conditions. Geochemical concentration of Th relative to U at high temperatures is therefore limited to special magmatic-related environments, where U4+ is preferentially removed by chloride or carbonate complexes, and at low temperatures by mineral surface reactions. The near-surface mineralogy of uranium and thorium provide a measure of a planet’s geotectonic and geobiological history. In the absence of extensive magmatic-related fluid reworking of the crust and upper mantle, uranium and thorium will not become sufficiently concentrated to form their own minerals or ore deposits. Furthermore, in the absence of surface oxidation, all but a handful of the known uranium minerals are unlikely to have formed.

Abstract: Uranium deposit types have evolved considerably from the Archean to the Present. The major global drivers were (1) change of geotectonic conditions during the Late Archean, (2) strong increase of atmospheric oxygen from 2.4 to 2.2 Ga, and (3) development of land plants during the Silurian. Other significant variations of uranium deposit types are related to unique conjunctions of conditions such as those during phosphate sedimentation in the Cretaceous. Earth’s uranium fractionation mechanisms evolved through four major periods. The first, from 4.55 and 3.2 Ga, corresponds to formation of a thin essentially mafic crust in which the most fractionated trondheimite-tonalite-granodiorite (TTG) rocks attained uranium concentrations of at most a few parts per million. Moreover, the uranium being essentially hosted in refractory accessory minerals and free oxygen being absent, no uranium deposit could be expected to have formed during this period. The second period, from about 3.1 to 2.2 Ga, is characterized by several widespread pulses of highly fractionated potassic granite strongly enriched in U, Th, and K. Late in this period peraluminous granite was selectively enriched in U and to a lesser extent K. These were the first granite and pegmatite magmas able to crystallize high-temperature uraninite. The erosion of these granitic suites liberated thorium-rich uraninite which would then be concentrated in placer deposits along with pyrite and other heavy minerals (e.g., zircon, monazite, Fe-Ti oxides) within huge continental basins (e.g., Witwatersrand, South Africa, and Bind River, Canada). The lack of free oxygen at that time prevented oxidation of the uraninite which formed the oldest economic uranium deposit types on Earth, but only during this period. The third period, from 2.2 to 0.45 Ga, records increased oxygen to nearly the present atmospheric level. Tetravalent uranium from uraninite was oxidized to hexavalent uranium, forming highly soluble uranyl ions in water. Uranium was extensively trapped in reduced epicontinental sedimentary successions along with huge quantities of organic matter and phosphates accumulated as a consequence of biological proliferation, especially during the Late Paleoproterozoic. A series of uranium deposits formed through redox processes; the first of these developed at a formational redox boundary at about 2.0 Ga in the Oklo area of Gabon. All known economically significant uranium deposits related to Na metasomatism are about 1.8 Ga in age. The high-grade, large tonnage unconformity-related deposits also formed essentially during the Late Paleoproterozoic to early Mesoproterozoic. The last period (0.45 Ga-Present) coincided with the colonization of continents by plants. The detrital accumulation of plants within continental siliciclastic strata represented intraformational reduced traps for another family of uranium deposits that developed essentially only during this period: basal, roll front, tabular, and tectonolithologic types. However, the increased recognition of hydrocarbon and hydrogen sulfide migration from oil or gas reservoirs during diagenesis suggests potential for sandstone-hosted uranium deposits to be found within permeable sandstone older than the Silurian. Large uranium deposits related to high-level hydrothermal fluid circulation and those related to evapotranspiration (calcretes) are only known during this last period of time, probably because of their formation in near-surface environments with low preservation potential.