Formation mechanisms of paleokarst and karst collapse columns of the Middle Cambrian-Lower Ordovician carbonates in Huainan coalfield, Northern China

Abstract

Karst water in the Middle Cambrian-Lower Ordovician carbonates is the ecological resources and domestic water in the Huainan coalfield (Anhui Province, China), but it also directly threatens the safe production of coal mines due to the development of paleokarst and karst collapse columns (KCCs). Most of the Middle Cambrian-Lower Ordovician carbonates in Northern China experienced multistage tectonic movements and were affected by multi-type corrosive fluids, but very few studies focused on the effect of multistage fluid-rock reaction on the formation of paleokarst and KCCs. To investigate the formation mechanisms and characteristics of paleokarst and KCCs, this study integrated petrographic studies, isotope geochemistry (C and O) and minor elements (Ba, Mn, and Sr), and clarified the sources and types of corrosive fluids. Through this study, meteoric water, formation water, hydrothermal fluids, and mixing fluids were determined as the four main types of corrosive fluids that formed pores, vugs, fractures, caves, and KCCs in the Middle Cambrian-Lower Ordovician carbonates. The meteoric dissolution is controlled by carbonic acid solution recharge conditions which affect the karst development in the Cambrian and Ordovician paleoweathering crusts and local carbonate outcrops. Hydrothermal fluids with high-temperature, high-pressure and high-corrosivity can develop a strong hydrothermal pore-fracture system in the Cambrian carbonate, which is the reason that KCCs can develop in the Cambrian strata in the Huainan coalfield. The mixing dissolution is controlled by sulfuric acid dissolution and usually occurs in the water tables and the fault and fracture zones, which are conducive to the development of caves and KCCs. In addition to the above four corrosive fluids, the development of paleokarst and KCCs in the Huainan area is also controlled by stratigraphic lithology and geological structures, especially for faults and fractures, which are the main migration channels of corrosive fluids. An evolution model of paleokarst and KCCs was established, providing a plausible interpretation for better understanding of the spatial distribution of paleokarst and KCCs. In practice, this study provides critical references for predicting the spatial distribution of paleokarst and KCCs in Northern China coalfields, as well as the exploration and development of karst water around the world.