On the Munyeijin potassium salt deposits in Yunnan Province, People's Republic of China

In Langping-Simao basins of Southwestern Yunnan Province, a number of salt deposits and salt springs are distributed (Fig. 1). The salt-bearing formations are sandstones, silt and mudstones of the Cretaceous to Paleogene ages, although the ages of the potassium-salts deposits seem to be younger. The Munyeijin potassium deposit was found on the eastern side of the Simao basin in early nineteen sixties. Although the salt deposit has been strongly deformed by tectonic movements (Fig. 2), the deposit has salt-dome structure and three depositional stages have been identified in each cycle of depositional sequence. They are, from the bottom to the top, red salts, black and white salts and carnallite-bearing clayey rocks (see Fig. 3). The red colour of the bottom zone is due to globular debris of silt in the red salts, whereas the top clayey rocks are high in organic materials, being characteristic of the residue of the last stage evaporation of a salt lake. Potassium salt is found in all the three zones, filling up the grain gaps of sodium chloride crystals or running through salt beds in veins and veinlets, although the black-white salts are the most fertile in potassium and have been mined for commercial use. Small amounts of carbonates and sulfates (gypsum and anhydrite) also exist as fine, dispersed grains. Thin layers of gypsum are not uncommon in dark portions of the red and black-white salts. Tachhydrite (CaCl(2)・2MgCl(2)・6H(2)O) is also observed in the top clayey deposits. From the field observation, geological, mineralogical, and geochemical considerations, the paleohydrogeological environments which led to the formation of potassium-rich salt deposits at Munyeijin are reconstructed as follows: 1) The depositional stage of the carnallite-bearing sediments: This is the last stage of evaporation of a salt lake. The sediments would have had up to 80 vol. % interstitial waters saturated with carnallite and sylvite. 2) Diagenetic stage: The carnallite-clay deposits were covered by younger salt deposits and the pore water was gradually squeezed out by compression. The pore water penetrated into the underlying sodium chloride deposits, where the pore water precipitated sylvite as the sodium salts were essentially devoid of potassium. Because the pore water contained organic materials (Table 2), the sylvite precipitates were dark in colour. 3) Tectonic stage: The Langping and Simao basins are lined along the Sanjian tectonic belts. The tectonic activity which presumably started in Eocene must be responsible for the intense deformation of the salt beds. Enrichment of potassium along the axis of folding and the anhydrite formation with the axis of crystallization tilted to the plane of salt beds are some of the important paleohydrogeological results of such movements. Several lines of evidence strongly suggest that hydrothermal activity took place widely in Munyejin basin during this stage. The origin of the potassium-bearing brines has been debated in China since the discovery of the deposits. Many lines of evidence suggest it be of marine origin. However, the presence of tachhydrite in the carnallite-bearing clayey deposits requires some additional source(s) of calcium in addition to seawater. The highly saline groundwaters in Triassic through Jurassic formations of Sichuan Province often are rich in Ca(2+) as well as Mg(2+) and K(+) as some examples are shown in Table 3. If such saline ground waters flew into a salt lake and was subjected to evaporation, calcium-bearing salts such as tachhydrite may form at the last stage deposit of the lake. The origin of such groundwaters is an interesting problem to be studied in future