REE Geochemistry of the Uranium Phases in Syn-Magmatic and Hydrothermal-Type U-Mineralisation: Two Case Histories from India

Abstract

Rare Earth Elements (REE) geochemistry of uranium-bearing phases from two genetically different types of uranium mineralisation is presented here. These include the syn-magmatic type, hosted by part of the Suryamalai or Sankari granite in the Kullampatti area, Tamil Nadu and hydrothermal vein type, hosted by both basement granitoid and its overlying Shahabad Limestone in the Bhima basin at Gogi, Karnataka. The study is based on the Electron Microprobe (EMP) analysis of selected REEs from different U-bearing phases, viz., uraninite, its pseudomorphic altered product of gummite, pitchblende and coffinite. Those hosted by granite, compared to that in limestone, have higher contents of XREE. Those in syn-magmatic type contain HREE>LREE and vice-versa in hydrothermal type. The former pattern is reflected in the co-existing Y- and HREE-bearing multiple oxides of fergusonite, samarskite and betafite. Furthermore, U-bearing organic matter and smectite-illite clay at Gogi contain higher contents of LREE. Pitchblende in both granite and limestone shows perceptible positive Ce-anomaly whereas the coffinite in limestone shows negative Ce-anomaly.

In the plot of EREE vs. chondrite-normalised LREE/HREE, except for those in the granite-hosted syn-magmatic type, the rest of U-phases as well as uraninite from other types (taken from literature) define a trend of positive correlation. The minimum-end of the ZREE is represented by the unconformity-type of Collins Bay, Canada and Pine Creek deposits, Australia, and the maximum-end of SREE by hydrothermal vein type in the granite at Gogi. The syn-magmatic granite-hosted uraninite from Kullampatti, Rossing and Faraday plot close to each other, but away from the main positive trend toward a higher EREE.

In simple oxides of uranium, LREE, MREE and HREE, appear to co-exist or substitute, respectively, for Ca, Th and U, as indicated by the corresponding increasing trends. During post-magmatic alteration of uraninite to gummite and samarskite to betafite, there is a notable depletion of especially the HREE. During coffinitisation of pitchblende or replacement of coffinite by pitchblende, there is a similar depletion, especially the LREE in the former and the MREE in the latter.