Geochemical Characteristics of Dolomite from 5th Member of the Ordovician Majiagou Formation in Sulige Area,Ordos Basin

The dolomite in the 5th Member of the Ordovician Majiagou Formation is a significant reservoir in Sulige area.The origin of the Ordovician Majiagou Formation has been a hot topic for many years.The trace element and carbon,oxygen,strontium isotope compositions of dolomite developed in this deposit were determined in this study.The results show that the content of manganese,sodium and strontium is low.Meanwhile, the values of isotopes ofδ 13 C andδ 18 O in secondary void-filling dolomite are much similar to those of the dolomite and dolomitic breccia.Theδ1 8O values of the dolomite are in the range of 0.81‰~5.68‰(averagely 3.442‰),lower than those of the Ordovician seawater.The 87 Sr/ 86 Sr ratio is higher than that of the Ordovician seawater.An analysis shows that the fluids that generated dolomite and void-filling dolomite might have come from the same fluid source.The negative values ofδ1 8O of dolomite result probably from temperature effect of the burial process.The high values of 87 Sr/ 86 Sr may be explained by the hypothesis that metasomatic fluids came from or flew though aluminosilicate basal rock or siliciclastic rock,which had rich radioactive strontium.Those characteristics of dolomite from 5th Member of Majiagou Formation were probably formed in a burial environment.