Characteristics And Their Indicating Significance Of Minerals Associated With Gas Hydrate In Sanlutian Of Muli In Qilian Mountains

It is a common phenomenon that gas hydrate occurs with carbonate and pyrite in Sanlutian of Muli in Qilian Mountains. In order to discuss the association between carbonate, pyrite formation and gas hydrate, this paper do research on 31 carbonate samples and 25 pyrite samples and the mineralogy test, trace elements test, rare earth minerals test, carbonate carbon and oxygen isotope test and sulfur isotope test of pyrite were carried out. According to the field observation and microscopic identification of associated carbonate and pyrite, carbonate can be divided into four types:(1) thin layered carbonate, with a thickness of 2-4 cm, mainly distributed in the mudstone of gas hydrate layer;(2) thin shell-like carbonate, with a thickness of less than 1mm, generally distributed in fracture of the mudstone of gas hydrate layer;(3) diamond drusy carbonate, with good crystal, mainly distributed in the mudstone, silty sandstone and fine sandstone of gas hydrate layer;(4) micro disseminated carbonate, mainly distributed in the black mudstone and carbonaceous mudstone. Pyrite is divided into three types:(1) Thin-film pyrite, with a thickness of less than 1mm, generally distributed in the fracture of the silty sandstone and fine sandstone of gas hydrate layer;(2) banded pyrite, 3-5 cm in length, mainly distributed in grayish black mudstone;(3) Nodular pyrite, with a diameter of less than 1 cm, mainly distributed in the black mudstone. The results showed that the mineralogical and geochemical characteristics of different types of carbonate and pyrite were obviously different. Diamond drusy carbonate with clear crystal, have a low content of the trace element and high content of rare earth element, Ni/Co>7. Compared to the other three types of carbonate that diamond drusy carbonate formed in a relatively reducing environment. And carbon isotopes of different types of carbonates are also obviously different. The δ13CVPDB of thin layered and thin shell-like carbonate is relative positively skewed, with an average of 2.5‰ and 3.5‰. The average δ13CVPDB of disseminated carbonate is-1.3‰, and the δ13CVPDB of diamond drusy carbonate is relatively negative, with a minimum of-6.4‰, and the average is-2.41‰. It is believed that the formation of the diamond drusy carbonate may be related to the decomposition of the gas hydrate, and the carbon source of diamond drusy carbonate is partly derived from the release of methane from the gas hydrate. A new crystal form, truncated octahedron, is observed in the thin-film pyrite. The banded pyrite and nodular pyrite in the microscope can not be observed good crystal form, and showed other mineral filled, reflects the older formation time. Compared with the other two types of pyrite, the thin-film pyrite showed relatively low sulfur isotope values, its δ34SV-CDT is-20.9‰ to 17%, and the average value is 1.25‰. The form of thin-film pyrite is similar to flaky gas hydrate, and the thin-film pyrite is usually symbiotic with gas hydrate or distributed near the location of the gas hydrate, which may is the result of reduction reaction between sulfate and methane which may comes from gas hydrate decomposition. Obtained in the study, the diamond drusy carbonate and film-like pyrite formation may be related to gas hydrate decomposition, these two kinds of authigenic minerals may record the once top and bottom boundaries of gas hydrate layer, they can be used as trace mineral in the future gas hydrate exploration in Qilian Mountain permafrost and other permafrost.