EVIDENCE FOR HYDROTHERMAL VENTING OF SULFUR-RICH FLUIDS MOBILIZED BY SIBERIAN TRAP INTRUSIVES AT THE END-PERMIAN

The Siberian Trap flood basalts erupted into and through the Siberian craton around 252 million years ago, coincident with the end-Permian mass extinction, and emplaced sill intrusions up to hundreds of meters thick into the sediments of the Tunguska Basin in Eastern Siberia. The emplacement of these high temperature intrusives led to the development of hydrothermal circulation cells that mobilized and transported fluids from the sedimentary basin to the paleosurface. Evidence of such fluid transport and venting are found in the hundreds of ore-rich breccia pipes identified throughout the Tunguska Basin, rooted in Siberian Trap sills. Previous work has proposed that contact metamorphic carbon release through these hydrothermal vent structures may represent a large component of the carbon degassing budget for the entire Siberian Traps province. Questions remain, however, regarding the mode, composition, and volume of other released volatiles. We have conducted a detailed investigation of the petrography and isotope composition of materials in a Siberian Trap breccia pipe and crater to assess the composition and source of the hydrothermal fluids once active in this system. We focus, in particular, on the extensive sulfur and strontium observed in the diatreme and crater sediments. Strontium isotope (⁸⁷Sr/⁸⁶Sr) values in the crater sediments of 0.708497 to 0.708563 are most consistent with those in the Tunguska Basin anhydrite. Pyritic sulfur isotopic values range from -7‰ to 24‰ δ³⁴S, reflecting both a sedimentary origin for the sulfur and the involvement of H₂S. We propose that the contact metamorphism in the presence of saline fluids and reducing organic material, led to anhydrite mobilization and transport of sulfurous brines from deep in the Tunguska Basin to the surface biosphere at the end-Permian. This hydrothermal venting may have been a mechanism and source of volcanism-induced sulfur degassing contributing to the end-Permian environmental crisis.