Petrography, geochemistry and genesis of dolomites in the upper Cambrian Sanshanzi Formation of the western Ordos Basin, northern China

Abstract

The Cambrian Sanshanzi Formation is an important dolomite horizon in the Ordos Basin of northern China, and is well exposed in the western part of that basin. This paper focuses on the petrography and geochemistry of dolomites in this formation with an aim to provide a better understanding of their genesis. Reflux seepage, burial, and hydrothermal dolomites are identified in the Sa nshanzi Formation. Reflux seepage dolomites include dolomitic grainstone and powder crystal dolomite, and have high Sr and low Mn contents, a high CaO/MgO ratio and low δ13 C values, and the reflux seepage dolomitization was formed by high-salinity and Mg2+-enriched fluid penetrates downward into the limestone. Burial dolomites are the main type of dolomite distributed through the Sanshanzi Formation. Burial dolomites are composed of fine- to coarse-crystal dolomite and have low Sr and high Mn contents, a low CaO/MgO ratio, and high δ13C values. The features of high salinity, strong reducibility and negative δ18O values indicate the burial dolomites formed under high-temperature and high-pressure conditions, the burial dolomitization was due to the limestone deeply buried during the later diagenetic stage. Hydrothermal dolomites are mainly characterized by saddle dolomite features, a positive δEu anomaly, and low δ18O values, and the hydrothermal dolomitization occurred during Mg2+-rich hydrothermal fluid invaded. The reflux seepage and hydrothermal dolomite developed in local areas, with the hydrothermal dolomite affected by the distribution of faults.