Phreatomagmatic Pipes of the Tunguska basin (Siberia): Aerosols Flux into End-Permian atmosphere

The Tunguska Basin sedimentary sequence consists of Precambrian and Paleozoic evaporites, carbonates and terrigenous rocks including Late Paleozoic coal-bearing strata. Precambrian and early Paleozoic oil source rocks contain numerous high potential oil and gas fields. Paleozoic evaporites contain rock and potassium salts deposits of commercial grade. Tunguska Basin evaporites are considered as a regional seal for the mineralizing brines. Permian-Triassic volcaniclastic rocks overlie this sequence and intrusive rocks have the numerous evidences of magma-sediment interaction. The Siberian Traps dolerites have intruded the Tunguska Basin sequence at different levels of emplacement and magma-sediment-brine interaction result in basalt pipes formation. Intrusions are surrounded by contact aureoles of thicknesses dependent upon sediment composition, dolerite thickness, depth of emplacement and the number of intrusive bodies. The basalt pipes cutting all intrusive bodies are filled with breccias of magmatic, volcaniclastic and sedimentary rocks altered to varying extents. The age of the pipes is geologically well constrained and related to Siberian Traps intrusion emplacement. A dolerite sill intruded into rock salt sequences of the Nepa potash deposit has a U-Pb zircon age of 252 × 0.4 Ma. Basalt pipe cutting all known dolerite intrusions, revealed here during prospecting survey. The phreatomagmatic origin of the basalt pipes is supported by ubiquitous occurrence of altered sedimentary clasts and volcaniclastic lapilli, corroded by brine during initial stages of magma-evaporite (brine) interaction. Corroded lapilli have rims of diopside, chlorine-bearing hornblende, apatite and magnetite. Iron rich garnet is present on the tuff and glass fragments. Our recent study of magnetite-rich coarse lapilli tuffs revealed the specific garnet lapilli rimmed with magnetite cemented by altered clay groundmass. This result corroborates our hypothesis about magnetite formation during initial stage of magma-sediment-brine interaction. We suggest that such conclusion requires a re-examination of lapilli tuff size and composition from basalt pipes of the Tunguska Basin. We suppose that this study could shed light on our understanding of aerosols flux from the Siberian phreatomagmatic pipes and its influence on End-Permian atmosphere.