PARAGENESIS AND FLUID HISTORY OF THE KARKU UNCONFORMITY-RELATED URANIUM DEPOSIT, PASHA-LADOGA BASIN, WESTERN RUSSIA

The Karku unconformity-related U deposit is located near St. Petersburg, Russia, on the margin of the Pasha-Ladoga intracratonic rift basin. Compared to typical basins hosting unconformity-related U deposits, the Pasha-Ladoga Basin deviates in two key aspects: it formed in the Mesoproterozoic, whereas other basins formed in the Paleoproterozoic; and the sedimentary units in the area of Karku are lithic subarkose to arkose sandstones and conglomerates, which contrast with the quartz arenites in other basins.

In the host Priozersk Formation, early diagenetic cementation created an effective paleoaquitard around the margins of the Pasha-Ladoga Basin, preventing wide-scale circulation of basinal fluids. Diagenetic alteration consists of calcite cement, mixed-layer illite-smectite, kaolinite, and minor chlorite in the matrix. Previously published calcite fluid inclusion data and the presence of swelling clays indicate a maximum diagenetic temperature of 130°C. The stable isotopic composition of the swelling clays is consistent with pore fluids evolved from low-latitude meteoric water.

Barren samples close to mineralization contain Fe-chlorite, kaolinite and calcite cement in the matrix. Stable isotopic compositions of chlorite-forming fluids have near-zero δD values, indicating a modified seawater source that was heated to 210°C, determined by chlorite geothermometry. U mineralization occurred at 1467±39 Ma based on U/Pb dating of uraninite. This syn-ore, U-bearing fluid was forced along the unconformity due to a change in hydraulic gradient, depositing U where it interacted with reduced basement fluids moving through graphitic units. Whole-rock Pb isotope ratios indicate minimal radiogenic Pb from U decay was carried out of the deposit by later fluid events, with most of the Pb remaining in the deposit as galena inclusions in uraninite.

The Karku deposit shows that unconformity-related U deposits may form in Mesoproterozoic basins and in units that appear to be unprospective due to lack of widespread high-temperature alteration and typical geochemical signatures, provided there was sufficient space somewhere in the basin for circulating pore fluids to leach U, and there are hydrologic pathways to suitable basement structures for U deposition.