Search

Potash deposits are common in the Mesozoic-Cenozoic Tethyan domain, which features the giant potash deposits in the Late Jurassic Central Asian Basin and the Late Cretaceous Khorat Basin of Thailand-Laos. Widespread evaporitic sequences from these intervals have been found in some marine and transitional basins within the Chinese continental blocks, such as the Yangtze, Simao and Tarim blocks, in which, only one small-scale potash deposit had been found in the Late Cretaceous Simao Basin. These evaporites are commonly characterized by depositional cycles that are more frequent than those of other potash basins around the world. It remains debated whether large or giant potash deposits could be formed in Chinese blocks of Tethyan domain, and if so, what mechanism was responsible for the potash formation. To address these issues, we comprehensively review the Meso-Cenozoic drift history and collision dynamics of these Chinese blocks and summarize the formation mechanism, depositional characteristics, and paleoclimate of the evaporite basins in these blocks. These blocks drifted from the Southern Hemisphere and collided and welded together in the beginning of the late Permian in the Northern Hemisphere. The collisions often commenced in the east and gradually spread to the west of these blocks, resulting in: (1) westward seawater retreat; (2) differential topographic uplift with high topography in the east and low topography in the west and (3) increasingly closed depositional environments in these basins. In addition, these collisions often exerted nearly N-S-oriented principal compressive stress, which resulted in nearly E-W-oriented extension. These forces induced a series of faulted-bounded subsidence belts with N-S-striking extensional sub-basins arranged in an E-W-oriented chain in these blocks. In this chain-like sub-basins system, as sea level rose and seawater flowed from west to east, the western sub-basins proximal to the open sea would have precipitated calcium carbonates and calcium sulfates, resulting in a high degree of concentration by the time the brines reached the distal end, i.e. eastern sub-basins. Ultimately, potash minerals were commonly deposited in the easternmost sub-basins. Based on the above model of potash formation, we predict that potash deposits may be formed in certain sub-basins in the eastern part of these basins in Chinese Tethyan blocks, which maybe lay a theoretical foundation for China’s future exploration of potash deposits.

The Mengyejing potash deposit (MPD) is the first significant ancient potash deposit ever found in China, and it was considered to have formed during the middle to late Cretaceous. The 87 Sr/ 86 Sr ratios of potash bulk samples suggest that both bedded and veined potash orebodies of the MPD have been affected by igneous activity resulting from decompression melting of a metasomatically altered, depleted mantle from 16 Ma to present. The Rb-Sr isotopic systematics of the potash bulk samples define several apparent “isochrons” ranging from 0.609 ± 0.026 to 14.23 ± 0.63 Ma, with the maximum age being consistent with previous K-Ar ages of the K-bearing minerals from the MPD. Such isotopic signatures imply that the MPD may have been affected by multiple-stage fluid flow events since the mid-Miocene, consisting of hydrothermal fluids, continental fresh water, and/or hot spring water. It is very likely that the MPD is still affected by fluid flow-events in modern times. The mineral assemblage of halite + sylvite with minor amounts of carnallite was formed due to the recrystallization and diagenetic modification by hydrothermal fluids. Some characteristics of halite photomicrographs and inclusions and the lack of original halite chevron patterns are also supportive of the hypothesis that the deposit has undergone a diagenetic modification and dissolution–recrystallization processes.