CHARACTERISTICS AND ORIGINS OF ANALCIMES OF THE PERMIAN PINGDIQUAN FORMATION IN THE NORTHEAST OF JUNGGAR BASIN, CHINA

Two types of analcimes are found in sedimentary rocks of Permian Pingdiquan Formation in the northeast of Junggar Basin, China. During Pingdiquan period, the study area was an intracontinental rift basin based on the ocean basin formed earlier, where magmatic and hydrothermal activities were intensive. This formation consists of sandstone, siltstone, mudstone, shale with minor intervals of marlstone, dolomicrite and tuff.

The result of X-ray diffraction based on 55 samples containing analcime shows that the average content of analcime is 10.53% (varying from 1% to 70.7%) while clay is just 4.9%. Analcime is the only zeolite which can be detected. Two different types of analcime can be distinguished by lithological, mineralogical and chemical features. The first type was found in tuff, siltstone, oolitic dolomite and mudstone. They are shaped in multiple structures such as equigranular texture in lamina, cryptocrystal distributing in fine-grained sediments， coarsed-grain filling in stringers and pores. It is associated mainly with albite, K-feldspar, quartz, dolomite, calcite and pyrite. The other type of analcime is red on hand sample. The whole rock contains 70.7 % of analcime. This analcime is subhedral, coarsed, bad sorting and surrounded with orange colored matrix. Only quartz (18.2 %) and feldspar (11.1 %) can be observed besides of analcime. On the basis of EMPA, the mean Si/Al ratio of type one is 2.56, while type two is 2.81. Type two analcime contains larger amounts of Si (35.96 per unit cell) and less Al（12.8 per unit cell）and Na（7.01 per unit cell）than the first type of analcime（35.02 Si, 13.68 Al, 13.85 Na per unit cell）.

The origin of the first type of analcime is preliminary interpreted as a precipitation from mixed fluid by pyroclastic material and Na-rich interseptal water. The second type may have been formed directly by the reaction of the saline and alkaline-lake water with volcanic glass.