Characteristics and Genesis of Dolomite in Majiagou Formation of Ordovician,South of Ordos Basin

There developed four kinds of dolomitization in genesis in Majiagou Formation of southern Ordos basin:penecontemporaneous dolomitization,circumfluence and mixing water dolomitization,burial dolomitization and hydrothermal dolomitization,based on the study of macro-analysis,micro-analysis and geochemical characteristics.Penecontemporaneous dolostone developed in 1st,3rd and 5th members of Majiagou Formation,and its genesis is evaporative pumping dolomitization.Circumfluence and mixing water dolostone is produced by two kinds of effects: one is salt water rich in magnesium,another is the mixture of salt water rich in magnesium and fresh water,which finally cause metasomatism of limestone,and produce circumfluence and mixing water dolostone.Burial dolostone results from metasomatism of limestone by salt water rich in silicon and aluminum and seawater or salt lake water along the pore space.Hydrothermal fluid dolostone is mostly produced by hydrothermal fluid which is supersaturated in pore space of coarse-crystaline dolostone and structural fracture in Yanshan era.