Characterization of a saline hot spring depositing travertine in the red beds in the Simao Basin of China

The Mangka saline hot springs, in the Simao Basin in southwest China, are unique in having relatively large discharge (>10 L/s) from the red beds (relatively poor permeability) and travertine deposition from Cl-Na type hot water with high content of total dissolved solids (10 g/L). The stable hydrogen and oxygen isotopes indicate that the Mangka spring waters are meteoric in origin and the average altitude of the recharge area is estimated as 1,750 m above sea level. The high temperature of the spring water is derived from geothermal heating. The estimated geothermal reservoir temperature range is 86–91 °C and the circulation depths of the spring waters are 2,318–2,454 m. Incongruent dissolution of salt-bearing formations contributes to the mineralization of the Mangka saline hot springs, including dissolution of halite, gypsum, carbonate and sylvite. Some suitable hydrochemical factors provide beneficial conditions for travertine deposition such as a higher CO₂ partial pressure of 10^–1.94–10^–0.9, γCa/γHCO₃ (the milliequivalent ratio of Ca and HCO₃) ranging from 5.76 to 6.67, and saturation indices for both calcite and aragonite over or equal to zero. There is also a favorable hydrodynamic condition for the spring water flowing down steep slopes and forming fast-flowing shallow channels with a relatively large surface area. The formation of the Mangka saline hot springs is summarized as follows: the groundwater receives recharge of infiltration from precipitation, undergoes deep circulation in the salt-bearing strata, receives heat from the geothermal heat flow, and ascends to the land surface along the permeable fault.