Textural and geochemical investigation into uraninite–ilmenite symplectite from the Singhbhum Shear Zone, India: Possible mechanism of formation and timing.

The Singhbhum Shear Zone, eastern India is known for its large uranium and copper resources. Uraninite, the most abundant U-ore mineral, occurs as inclusions in magnetite, ilmenite, apatite, tourmaline, biotite, chlorite and allanite. Shared grain boundaries of uraninite with magnetite and ilmenite are common. Uraninite also shows complex symplectic intergrowth with ilmenite. In some places, the symplectite shares a straight grain boundary with ilmenite and magnetite and is surrounded and enveloped by magnetite. The symplectite could be formed by the reaction of separate U-bearing and Fe–Ti-bearing minerals or by the breakdown of a single U–Ti–Fe-bearing mineral under changed physicochemical conditions. Using the textural settings, geochemistry, and known occurrence of brannerite (UTi2O6) in the shear zone, we propose that the uraninite–ilmenite symplectite is most likely formed by the breakdown of brannerite by interaction with a Fe-bearing hydrothermal fluid. Based on in-situ EPMA chemical dating of uraninite in the symplectite and the known ages of uraninite formation/recrystallization in the shear zone, we further suggest that the symplectite formed during 950±50 Ma, an age which represents a pervasive tectonothermal event in the shear zone. [ABSTRACT FROM AUTHOR]