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Orthorhombic Orthorhombic refers to a rectangular crystal with three axes of different lengths and all at right angles to each other. Oxide Oxide refers to a group of minerals where oxygen joined with a metal is a major constituent.
Oxidized zone Oxidized zone refers to the portion of an ore body that has been altered by downward percolating groundwater and which contains dissolved oxygen and carbon dioxide.
Although none of the pristine, unaltered uraninites that formed during the above mentioned events contain significant intrinsic minor or rare earth elements, the basement uraninites are consistently much enriched in thorium compared to those from the supracrustal.
Based on the compositions, we propose that the basement uraninites formed from a high temperature magmatic/metamorphic fluid, whereas those in the supracrustal rocks crystallized from a low temperature, presumably oxidized fluid.
However, we do not entirely reject the possibility of neo-mineralization.
The discrete events deciphered from uraninite in the Samarkiya area can also be broadly linked to some major magmatic-metamorphic events, identified from other independent studies, in the ADFB.The ∼1.24–1.20 Ga event appears to be somewhat enigmatic in the context of well-known geological events in the area.However, based on some very recently published data, we interpret this to be a post-peak metamorphic (∼1.37–1.35 Ga) hydrothermal event or even a new metamorphic event, hitherto unknown.Back-scattered electron images, X-ray elemental mapping of selected elements and EPMA spot analysis of large uraninite grains (both from the basement and the supracrustals) collectively demonstrate that subsequent to the major mineralizing event at ∼1.24–1.20 Ga, the mineralized rocks were subjected to fluid-mediated alteration, which resulted in ∑REE Y- and Si (Ca)-enrichment of existing ∼1.24–1.20 Ga uraninites in the basement and supracrustal rocks, respectively.We cannot constrain the exact timing of this alteration event.The results obtained have been compared with the available isotopic ages of the studied minerals; the compared data are satisfactorily consistent.