Geochemical Characteristics of Dolomites in Lower Ordovician Majiagou Formation, Ordos Basin

Key words: 
• Majiagou Formation /
• burial dolomite /
• hydrothermal dolomite /
• geochemical characteristics

Abstract: Lower Ordovician Majiagou Formation in Ordos Basin is a shallow marine carbonate sequence with evaporate stratum. The centralwestern part in the basin often contains several dolomite units, which are major reservoir rock. Some researchers considered them as reflux dolomite near surface. However, petrography and geochemistry of dolomite suggest that this dolomite have evidence of burial origin.
〓〓Majiagou dolomite (M5) included dullred cathodoluminescence, trace element of 170 ~ 210 ppm for Mn and 5500 ~ 6200 ppm for Fe, stable isotopic oxygen composition of -7.093~-9.932 PDB, the average is -8.671 PDB, for matrix and vugfilling saddle dolomite and bright dolomite cement, stable isotopic strontium composition of 0.709 766~0.708 65 for 87Sr/86Sr (influenced by silicaclastics materialities, sea water). Dolomite replaced host rock along the fractures in the section. These data about Majiagou dolomite (M5) was formed in the burial condition.
〓〓REE pattern of dolomite are distinctly different from sea water, commonly are short of negative anomaly of Eu and Ce, such distinct REE signatures probably reflected interaction between diagenetic fluid and host rock and siliciclastic material. Homogenization temperature of 160℃～220℃, salinity of 5wt% ~ 25wt%, suggest dolomitization occurred at high temperature and larger pressure, replacing fluids is brines.
〓〓The pervasive dolomite, trace MVT mineral (sphalerites, galenas and iron sulfides), potassium(K)silicate mineral (Kfeldspar), quartz, volcanic tuffs, in these rock, which preserve a regional hydrothermal signatures.
〓〓The petrography and geochemistry of Majiagou dolomite show that this dolomite (M5) has evidence of hydrothermal dolomite (HTD) and central field (gas) are now identified to be of HTD type, typically with a strong structural control on localization. 