Geochemical Characteristics of Dolomites in Lower Triassic Feixianguan Formation, Northeast Sichuan, China 

Key words: 
• mixing water dolomite

Abstract: The lower Triassic Feixianguan Formation is a shallow marine carbonate sequence with shale and evaporates. The edge of carbonate platform commonly developed some oolitic banks, which are dolomitized. The oolitic bank dolomites are the most important gas reservoir in Sichuan. Some researchers considered them as the origin of marinefresh water mixing model. However, petrography and chemistry of dolomite are not in consistence with origin of marinefresh water mixing dolomitization. Feixianguan oolitic dolomites included dullred cathodoluminescence, trace elements of 14×10-6~78×10-6 for Mn and 600×10-6~1 000×10-6 for Fe, stable isotopic oxygen composition of 6.73‰ ~ 3.65‰(PDB), the average is 4.89‰（PDB）, for matrix and vugfilling saddle dolomite and bright dolomite cement, stable isotopic strontium composition of 0.707 35 ~ 0.708 00 for 87Sr/86Sr. Dolomite replaced host rock along the fractures in section. These data about oolitic bank dolomite suggest that the dolomitization was formed in the buried condition.