Abstract:

There are a mass of white sandstone in the top of Yanan Formation in northeastern Ordos Basin which formed the local large kaolin deposit, and its bearing horizons were similar and adjacent to Zhiluo Formation in DongshengHangjinqi Area. Studies have shown that the whitening was closely related to uranium enrichment and reservation, green alteration of the orecontrolling sandstone, carbonation and so forth. These phenomena were related to the flowrock effect which was caused by the upper Paleozoic natural gas migrating northward. This paper mainly talks about the formation process and geochemical genesis mechanism of the white sandstone through the stable isotope tracing. The paper also indicates that there existed a natural gas dissipation dynamics background in this area, and that the hydrogen and oxygen isotopes of the kaolinite revealed that the formation of kaolin deposit was based on low temperature hydrothermal event, with the same properties and events as Dongsheng uranium deposit. The silicon isotopes of the kaolinite were similar to the uranium silicon isotope data, but far different from the clay silicon isotope data, which further evidenced that the white sandstone and uranium mineralization were the products of the same low temperature hydrothermal events. Carbon and oxygen isotopes showed that the white sandstone formation fluid was associated with organic matter source. Combined with organic geochemistry of Dongsheng sandstone type of uranium deposits and local geological background events of natural gas dissipation, it is suggested that the organic matters came from the natural gas dissipation northward in the Upper Palaeozoic. The characteristics of hydrogen and oxygen isotope of calcite inclusion indicated that the water of the fluid was atmospheric precipitation. In summary, the forming processes of white sandstone in the study area were as follows: the microthermal natural gas in the Upper Palaeozoic migrated from the central part northward to the shallow YananZhiluo Formations; this microthermal natural gas along with the underground water made up the microthermal mixed hydrothermal “natural gaswater”; the reduction effect of hydrocarbons in the microthermal liquid also led to the alterations, causing the formation of white sandstone in the top of Yanan Group and the Dongsheng uranium deposit in Zhiluo Formation, causing the greenization of the sandstone, the carbonatization and so on. These alterations formed the Dongsheng super large uranium deposit, super large kaolin deposit and hydrocarbon alteration. These all brought about the situation that multienergy sources and ores coexisted in the same basin with common prosperity.

Key words: white sandstone, stable isotope, tracing, the natural gas dissipation, reduction of the hydrocarbons, sandstone type uranium deposit, epithermal