Paragenesis and fluid history of the Karku unconformity-uranium deposit, Pasha-Ladoga Basin, Western Russia
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The Karku unconformity-type uranium deposit is located in the Proterozoic Pasha-Ladoga Basin in western Russia. The deposit is located in basinal sandstones near a reverse fault that offsets the unconformable contact with underlying graphite-bearing biotite schist basement rocks. Meteoric water-sourced diagenetic fluids altered sandstones outside mineralized zones, producing clay assemblages of interstratified illite-smectite with minor dickite and Fe-rich chlorite, and fluid isotopic compositions of d18O = 3.3‰ to 6.0‰ and dD = -55‰ to -44‰ at 130°C. Altered sandstones within zones of mineralization include a clay mineral assemblage of Fe-rich chlorite with minor illite, and fluid isotopic compositions of d18O = 6.0‰ to 12.7‰ and dD = -8‰ to -4‰ at 210°C, caused by seawater-derived fluids related to ore formation. In addition, a Mg-chlorite occurring with uraninite in the mineralized lenses was identified. Dating of Karku uraninite using U/Pb isotope systematics from LA-MC-ICP-MS, LA-HR-ICP-MS, and SIMS gave upper-intercept ages of 1467 ± 39 Ma, 1459 ± 74 Ma, and 1302 ± 150 Ma respectively. The lower-intercept age of all methods was around 300 Ma. A formation model for the Karku uranium deposit is proposed based on the results of this study and incorporating previously published. Early, low-temperature diagenesis resulted in cementation of distal, basin-marginal sandstones by mixed-layer illite-smectite clays, preventing later, higher-temperature diagenetic alteration in this unit. Sandstones towards the centre of the basin allowed the circulation of higher-temperature basinal fluids that later flowed along the unconformity to the graphitic schist unit. A fault intersecting this unit facilitated the mixing of basement and basinal fluids, causing uraninite to precipitate at the unconformity.