Why Proterozoic Basins have so much Uranium worldwide?

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The Proterozoic basins around the world shows higher uranium concentration and many deposits are also established in these basins. For example, Proterozoic Athabasca Basin and Thelon Basin; Canada, Paleo- to Mesoproterozoic McArthur Basin; Australia, Paleoproterozoic Roraima Basin; Guyana, Paleo- to Mesoproterozoic Cuddapah Basin and Neoproterozoic Bhima Basin; India etc.

The physicochemical conditions are extensively studied to know the evolution of life and early geological events during Archean and Proterozoic. Geochemistry of uranium is also studied widely due to its vast nature of mineralisation. Based on geochemistry of uranium and physicochemical conditions of Proterozoic; following reasons could be responsible for anomalous enrichment of uranium in these basins.

  1. Development of Organism – During the Paleoproterozoic, blue-green algae, simple bacteria and simple eukaryotic green algae in the ocean produced organic matter. For unconformity related uranium deposits, some of these organic matter accumulations were decomposed and metamorphosed into graphite in the metasedimentary basement rock. The graphitic metasedimentary rocks are favourable basement rock for unconformity related uranium deposits.
  2. Absence of land plants – The Proterozoic era was devoid of land plants and resulting lack of stable soil would have rendered the alluvial plain environment without cohesive riverbanks. As a result, very little mud is found in Paleoproterozoic sedimentary succession deposited in fluvial environment. Such sedimentary deposits serve as the aquifer for later fluid events related to ore formation processes.
  3. Change in oxygen level – During the Archean and early Paleoproterozoic uranium was deposited in placers deposits as Quartz Pebble Conglomerate (QPC) like Elliot lake district, Canada and Walkunji-Yellakki, Karnataka, India.  In late Paleoproterozoic and Mesoproterozoic, formation of uranium mineralisation shifted to sub-surface of intracratonic basins. After the Great Oxidation Event (GOE) at ~ 2.2 Ga, oxidized ground waters and basinal brines mobilized and transported in the form of soluble uranyl (U6+O2)2+. Later it precipitated by effective reductants in unconformity and vein type uranium deposits.