Search

Aluminum phosphate-sulfate (APS) minerals formed around the Athabasca unconformity-type deposits and those from their Australian counterparts are chemically very similar showing the same continuum between the diagenetic Sr-rich APS minerals of the barren sandstones and the LREE-rich composition of the APS minerals in the hydrothermally altered sandstone. The P- and LREE-rich compositions were controlled by the transport and the redistribution of P and LREE elements released from the dissolution of phosphate minerals (principally monazite) in the basement rocks and in the basin during the syn-ore alteration processes. The S/Sr ratio measured in the APS minerals from unaltered sandstone away from the unconformity and any mineralization is preserved during the syn-ore alteration processes suggesting that the fluids involved in both the deep burial diagenetic processes and the syn-ore alteration system were derived from a similar diagenetic reservoir in both the Athabasca and Kombolgie regions. The trioctahedral chlorite host-rock alteration around the Australian basement-hosted U deposits, as compared to the illite and sudoite associated with the Athabasca basement-hosted, along with the more LREE-rich APS compositions in the Australian deposits, suggests that the pH and oxygen fugacity ( f O 2 ) of the syn-ore fluids differed in the alteration systems of the two regions at the time of the U deposition.

The anomalously high Th/La ratio (∼1.14) of the Early Proterozoic silicified sandstones of the Franceville basin (Gabon), compared to Archean and Proterozoic metasedimentary rocks (Th/La ∼0.27), results from extreme light rare earth element (REE) migration during diagenesis. Monazite, which represents the main light REE-bearing phase in the sandstones, was altered by diagenetic brines at 140 °C and 1 kbar. The alteration phase is a microcrystalline Th-silicate phase, indicating low Th solubility at these conditions. Light REEs are simultaneously leached out together with P and U. The increase in Th/La from detrital monazite to residual Th-silicate phase indicates that about 76% of the light REEs were leached out, corresponding to a global amount of 2.01 × 109 metric tons at the scale of the FA Formation in the Franceville basin. Uranium was also leached during monazite alteration and may have contributed significantly to the genesis of the high-grade uranium deposits of the Franceville basin that host the natural nuclear reaction zones.