N and O isotopes and the ore-forming mechanism of nitrate deposits in the Turpan-Hami Basin, Xinjiang, China

The recently discovered nitrate ore field in the Turpan-Hami Basin of western China represents an estimated resource of 2.5 billion tons, and is comparable in scale to the Atacama Desert super-scale nitrate deposit in Chile. The research on this area is rarely carried out, and the origin of the deposits remains uncertain. In this study, new methods were used to systematically analyze N and O isotopes in nitrate minerals collected from the Kumutage, Xiaocaohu, Wuzongbulak, Dawadi, Tuyugou, and Shaer ore deposits in the Turpan-Hami Basin. The data showed that the delta N-15(Air) value ranges from 0.7aEuro degrees to 27.6aEuro degrees, but mostly between 2aEuro degrees and 6aEuro degrees, which was similar to atmospheric NO (3) (-) . The O-18 was highly enriched with delta O-18(V-SMOW) varying from 30.2aEuro degrees to 46.7aEuro degrees. This differs from levels in deposits derived from microbial nitrogen fixation, but is similar to those in atmospheric nitrates. N and O isotopes data indicated that nitrate deposits in Turpan-Hami Basin must be the result of deposition of atmospheric nitrate particles. Although atmospheric nitrate particles are common, the nitrate deposits could form only under the condition of long-term extreme drought climate and very limited biological activity. This paper summarized the ore-forming mechanism of different types nitrate deposits based on their geological setting.