Gas hydrate stability zone migration occurred in the Qilian mountain permafrost, Qinghai, Northwest China: Evidences from pyrite morphology and pyrite sulfur isotope

• Fracture-filling pyrites occurred in the gas hydrate reservoir system in Qilian mountain permafrost.
• Fracture-filling pyrites mainly consisted of cube crystals and associated with a circular structure.
• The value of δ34SCDT has a more positive excursion than that from authigenic pyrite occurred in marine gas hydrate system.
• The formations of step-like pyrite and circular structure closely related with the process of hydrate decomposition.
• The range of distribution of pyrite recorded the possible largest original gas hydrate stability zone.

Fracture-filling pyrites, which semi-filled or fully filled rock fractures, were commonly found in the cores from all hydrate testing well in the Qilian mountain permafrost. The occurrence of the pyrites is very similar to the “fracture-filling” gas hydrate that occurred in this area, and whose distribution mainly concentrated below the hydrate layer or layer of hydrate associated anomaly. This paper carried out the study in morphology and sulfur isotope for the fracture-filling pyrites. The results show that fracture-filling pyrites consisted of cube form pyrite crystals, directionally spread in a step-like fashion along the fracture surface, and associated with a circular structure; the value of δ34SCDT ranges from 6.761‰ to 41.846‰, and the most positive excursion exists below the deepest layer of hydrate associated anomaly. The characters in pyrite crystal morphology, sulfur isotopic composition and spatial distribution closely related with the secondary change of metastable gas hydrate reservoir. The permafrost degeneration resulting from climate warming is the most direct cause for gas hydrate stability zone (GHSZ) migration that occurred in the Qilian mountain. The zone between the shallowest and the deepest distribution of the fracture-filling pyrite recorded the possible largest original GHSZ. The top and the bottom of GHSZ have moved downward and upward to a certain extent, respectively, further inferring that the depth of permafrost has decreased about 10 m in the boreholes.