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Wensheng Ge, China University of Geosciences (China)
Keqin Cai, China University of Geosciences (China)
Shaojie Sang, No.1 Geological Team, Xinjiang Geological Exploration Bureau (China)
Bin Qiu, No.1 Geological Team, Xinjiang Geological Exploration Bureau (China)
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The Kumtag large nitrate deposit is located in the Turfan-Hami Basin, Xinjiang, China. Its reserves are estimated at present to be 170 million tons.
The nitrate deposit is distributed in the Pleistocene stratum of unconsolidated gritrock. High-grade nitrate ores are found on lower slopes of hills and in alluvium-filled depressions. The nitrate ores occur as stratiform seams 0.3-3.65m thick, present at depths of 0.2-0.5m below the gobi surface. A vertical zoning of ores exists in the nitrate deposit. The ore is a 0.4- to 0.7-m-thick layer of thenardite at lower level and grades to a 0.8- to 1.4-m-thick upper zone of halite associated with nitratite and thenardite, capped by a 0.4- to 2.0-m-thick zone of nitratite associated with halite and thenardite.
Two types of nitrate ore are distinguished as halitic gritrock ore and halitic breccia ore, corresponding to two principal lithologic units of the alluvial fan discriminated as gritrock unit and breccia unit. The mineral assemblage of ore is made up of halite, darapskite, nitratite and thenardite. The content of NaNO3 in typical nitrate ore ranges from 2% to 27.98%, with an average of over 10%. The saline minerals occur as fine-grained cements of gritrock and breccia or as disseminated and veined infill in pores and fractures of bedrock.
Abundant saline constituents supply, hyper-arid climate and suitable structure and topography for transportation and concentration of nitrate are favorable for the mineralization of the nitrate deposit. The distribution range of the nitrate is roughly corresponding to that of the Jurassic coaly strata in the Basin. The compositional analyses of the Jurassic coal strata combined with N and O isotopic data of nitrate ores support derivation of saline materials in the Turpan-Hami Basin from diverse sources. It is suggested based on the analyses and data that nitrate might be derived mainly from the Jurassic coal strata, and partly from Carboniferous volcanic products, and other saline materials might come from the Cretaceous and Tertiary saline strata through weathering and leaching. The hyper-arid climate of the Turpan-Hami basin, with annual average rainfall and evaporation of less than 25mm and over 2700mm, respectively, is important for formation and preservation of the nitrate deposit. Occurrence of "high mountain surrounding deep basin" structure pattern and the Aidin lake of 154m below sea level formed by the Himalayan movement is favorable for transportation and accumulation of saline materials.
The data mentioned above are consistent with formation of the nitrate deposit by a multiple-phases process of direct crystallization of saline minerals by capillary evaporation of groundwater, followed by leaching of the saline constituents by rainwater, and remigration, reprecipitation and recrystallization of ground brine.
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