Minerogenic Theory of the Superlarge Lop Nur Potash Deposit, Xinjiang, China

Abstract  Located in the eastern part of the Tarim basin, Xinjiang, the Lop Nur was an ultimate water catchment area of the Tarim basin during the Quaternary. Through nearly ten years of investigation and research, the authors have found a superlarge brine potash deposit in the Luobei subbasin—a secondary basin of the Lop Nur depression. The deposit has been mined now. On that basis, the authors propose new theories on the genesis of the potash rock deposit. In the tectonic and geomorphologic contexts, the Tarim basin lies in a “high mountain‐deep basin” environment. At the beginning of the Quaternary, influenced by the neotectonic movement, the Lop Nur evolved into a “deep basin” in the Tarim basin. At the end of the middle Pleistocene, neotectonic migration began to take place in the interior of the Lop Nur and a new secondary deep basin—the Luobei subbasin—formed gradually. Despite its small area, it is actually the deepest subbasin in the Lop Nur depression, where brines of the Lop Nur Salt Lake gather and evaporate, thus providing materials for the formation of a superlarge brine potash rock deposit. With respect to the phenomenon of brine concentration and change with deepening of the lake, the authors propose a model of “high mountain‐deep basin” tectonic migration for potash concentration. In the sedimentological context, the honeycomb‐shaped voids developed in glauberite rock in the subbasin are good space for potash‐rich brine accumulation. Study indicates that the deposition of glauberite requires recharge of calcium‐rich water. In the Tarim area the calcium‐rich water might come from deep formation water or oilfield water, and the river water recharging the Lop Nur Salt Lake was rich in sulfate radicals and other components; in addition, the climate in the area was very dry and the brine evaporated steadily, thus resulting in deposition of substantial amount of glauberite, potash accumulation in intercrystal brine and final formation of the potash deposit. Generally, potash formation in a salt lake undergoes a three‐stage process of “carbonates→sulfates (gypsum and glauberite)→chlorides (halite etc.)”, but in the study area there only occurred a two‐stage process of “carbonates→sulfates (gypsum and glauberite)”. The authors call this new geological phenomenon the “two‐stage potash formation” model. In conclusion, the superlarge Lop Nur potash deposit is the result of combined “high mountain‐deep basin” tectonism and “two‐stage potash formation”.