Disequilibrium of uranium mineralization – a case study of Devri area, Surajpur district, Chhattisgarh, India

Disequilibrium in the uranium series is a vital parameter to ascertain the state of equilibrium between uranium group (parent uranium) and its daughters, i.e. radium group of elements. The ²³⁸U isotope of uranium is the parent of 14 daughter isotopes forming the uranium series. The first five isotopes of ²³⁸U series, i.e. ²³⁸U, 234Th, 234Pa, 234U and 230Th form uranium group and the sixth to last isotopes of ²³⁸U series, i.e. ²²⁶Ra, ²²²Rn, ²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi, ²¹⁴Po, ²¹⁰Pb, ²¹⁰Bi, ²¹⁰Po and ²⁰⁶Pb form radium group. Isotopes in the uranium and radium group usually remain in equilibrium within the group, but disequilibrium commonly exists between the two groups. In an open system, the mobility of radium is significant due to radon gas produced as a daughter element in the radium series. Anomalies with excess radium are commonly found around springs and seepages, whereas uranium anomalies are usually found under reducing environments. A study was carried out in Devri area, Surajpur district, Chhattisgarh, India, to decipher the state of disequilibrium in the area by analysing the drill core samples (n = 4470) from 43 boreholes. This study has revealed a strong disequilibrium (65%) in favour of the parent uranium. This disequilibrium is due to cycles of dissolution and deposition of parent uranium; aided by the movement of groundwater and diverse geochemical behaviour of different radionuclides. In addition, loss of radon from the system might have also contributed to the disequilibrium in favour of the parent uranium. Linear regression coefficient between U₃O₈ (parent uranium) and RaEq.(U₃O₈) was calculated to be 0.82. Strong disequilibrium in favour of the parent uranium will enhance the uranium ore reserve in Devri area.