It is a universal phenomenon for gas hydrate associated carbonates in subsurface sediments, but it is rarely reported in the permafrost. Based on microscopic observations and mineral analyses on carbonates associated with gas hydrate, mineral species and occurrence modes of carbonates are determined in the Qilian Mountain permafrost:
white thin-layered carbonate;
smoky rhombic crystal calcite aggregates,
darkish gray thin crust-like carbonate,
sparsely disseminated calcite or carbonate.
Among these carbonates, typed II carbonates is rich in large calcite aggregates, and is abound in a certain amount of aragonite and strawberry-like pyrite. For the typed II carbonates, element concentrations are generally very low, and particularly some element concentrations and elemental ratios are obviously different from other typed carbonates. For example concentrations of Sr, Ba and Eu are abnormal in the typed II carbonates and its Chemical Index of Alteration (CIA) is the lowest among the four types of carbonates. The values of δ13CV-PDB ‰ range from -6~-0.8 ‰ while the values of δ18OV-PDB ‰ range from -17.7~-15.2 ‰ in the typed II carbonates, appearing relatively independent of other three types. Hence the typed II carbonates are indicative of association with gas hydrate decomposition. Namely they possibly form from carbonated reworking after gas hydrate dissociation in the geological history in the Qilian Mountain permafrost.
