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Abstract: Weathering has always been a concerned around the world, as the first and most important step in the global cycle of elements, which leads to the fractionation of isotopes on the scale of geological age. The Middle Ordovician Majiagou Formation in Daniudi area of the Ordos Basin had experienced weathering for >130 Myr. Through thin section observation, major and trace element analysis, carbon, oxygen, and magnesium isotopes composition analysis, the dolomitization modes and weathering of ancient dolomite in Daniudi area were analyzed in detail. The results showed that the Sabkha and brine-reflux dolomitization modes had developed, and the Mg isotopes in different layers of the karst crust were fractionated by various factors. The vertical vadose zone was affected by weathering, the Mg isotope of dolomite (δ26Mgdol) showed a downward decreasing trend; the horizontal underflow zone was controlled by diagenesis and formation fluid, δ26Mgdol showed a vertical invariance and negative; the main reason for Mg isotope fractionation in the deep slow-flow zone was the brine-reflux dolomitization mode during early burial period, which showed a vertical downward increase. Finally, the Mg isotope characteristic data of the ancient weathering crust were provided and the process of Mg isotope fractionation in the karst crust was explained.