Evolution_of_uranium_fractionation_throu.pptx

This presentation covers Uranium fractionation through time. It's decay through its daughter product is elaborated to explain fractionation.

1. Evolution of uranium fractionation through time – Secular variation of Uranium deposit types GOURAV MUKHERJEE DEPARTMENT OF GEOLOGY UNIVERSITY OF CALCUTTA.

2. Introduction : URANIUM ore deposits present the most extreme diversity of concentration process for a metallic element. Uranium deposits vary greatly from meso-archaean to present. Uranium resources are unevenly distributed through geologic time.

3. Geochemistry of uranium: Uranium is a member of actinide group in the periodic table. May exist in four valence state: U +3 ,U +4 ,U +5 ,U +6 but naturally it occurs as U +4 and U +6 . The ionic radius of uranium is 0.93 A . Average abundance of uranium in Earth crust is 2ppm. In nature , uranium is found as uranium-238 (99.2739–99.2752%), uranium-235 (0.7198–0.7202%), and a very small amount of uranium 234 (0.0050–0.0059%)

4. Diversity of U deposit types QPC type. Vein type. Unconformity type. Sandstone type.

5. Major global drivers for various deposit types : (1) Change of geotectonic conditions during the Late Archean . (2) Strong increase of atmospheric oxygen from (2.4 to 2.2 Ga ). (3) Land plant evolution in the Silurian.

6. earth’s uranium fractionation mechanisms evolved through four major periods – First period , from  4.55 and 3.2 Ga  . Second period , from about 3.1 to 2.2Ga. Third period , from 2.2 to 0.45Ga . Fourth period , from 0.45Ga to present.

7. Uranium fractionation of early earth (4.55 – 3.2Ga) : Partial melting of mantle followed by crystal fractionation. Formation of thin mafic crust (dominantly komatiitic and tholeiitic basalt ). The most felsic rock Tonalite-Trondhjemite - Granodiorite (TTG) suite. U concentration in these early felsic rock less than a few ppm . U concentrated as refractory accessory minerals ( e.g zircon , titanite , apatite etc. ). oxygen absent . Thus , no U deposits in pre- mesoarchen time.

8. Uranium fractionation during archean to paleoprotearozoic (3.2 – 2.2Ga) : 1 During this period Uranium undergoes fractiontion and the First Uranium rich granite crystallize uraninite . 2 Possible source of granite to crystallize uraninite : a)highly fractionated high K metaluminous granite b) highly fractionated peraluminous granite c) weakly peraluminous pegmatoids . 3 Proposed hypotheses for highly potassic granitic melt - a. Melt generated from mantle wedge. b. Partial melting of TTG crust and metasedimentary rocks. But these two hypothesis failed to explain the genesis of granitic melt. 4. The prefered model involves d ehydration of subducted crustal material to enrich the subcontinental lithospheric mantle incompatible elements, which are then extracted by partial melting and fractional crystallization .

9. Uranium fractionation during archean to paleoprotearozoic (3.2 – 2.2Ga) : 5 . Low level of oxygen before about 2.2 Ga is supported by lack of red terrestrial strata and abundant detrital Th - and REE-rich uraninite,siderite , and pyrite in QPC, which are only known in significant abundance in strata older than 2.4 Ga. Thus the first economic uanium deposits on earth the Quartz pebble conglomerate (QPC) containing uraninite and variable amounts of gold , are those hosted by the Dominion Reef Group and the Witwatersrand Supergroup which were initially deposited at 3,083 ± 3 Ma.

10. Uranium fractionation during the early paleoproterozoic to paleozoic (2.2 – 0.45Ga) : 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 . Uranium deposits formed through redox processes. Deposit types are vein type, unconformity type.

11. Uranium deposition after ordovician (>0.45Ga) : Major change in environment – land plant occurrence. Deeply affected the metallogeny of Uranium. Siliciclastic sedimentation no longer formed homogenously oxidised succession. Alternate sedimentary sucession of oxidised and reduced layers forms. Formation of the largest economic uranium resources on the Earth hosted by sandstones .

12. THE THREE BOX MODEL EXPLAINING RISE OF ATMOSPHERIC OXYGEN THROUGH GEOLOGIC TIME :

13. References : Michel Cuney (2010) Evolution of Uranium Fractionation Processes through Time : Driving the Secular Variation of Uranium Deposit Types. Understanding Mineral Deposits : Kula C. Misra .