Hydrogeochemical Characteristics And Genesis Of The Springs In The Simao Basin Of Yunnan

The red beds of low permeability widely occur in the Simao Basin in southwest China.Nevertheless,several types of springs?including hot springs,brackish springs,saline springs and salty springs?exist in the basin,some of which have a large flow?>10 L/s?and others deposit travertines.In this study,hydrochemical and isotopic analyses and models are used to examine the sources of recharge,heat and solutes of 19 water samples of 13 springs and favorable conditions for travertine formation to better understand the subsurface processes and formation mechanisms of different springs,which is of important significance for illuminating the mass transport and regional hydrological circulation in a sedimentary basin.The thick Mesozoic and Cenozoic red beds occur in the depression areas in the basin.Due to the later denudation,the Mesozoic and Paleozoic carbonate rocks outcrop in the central block uplifts and the marginal belt uplifts in the basin.The 13 springs are basically distributed along the three branch faults of the Wuliangshan fault.Temperatures of the spring waters in the Simao Basin range from 26.2 to 65.8oC.The hot spring water receives heat from geothermal heating.The geothermal reservoir temperatures calculated with the quartz geothermometer range from51 to 120oC and the circulation depths of the spring waters vary from 1123 to 3443 m.The geothermal waters are mixed with shallow cold waters in their ascending processes,and the mixing ratios of cold water range from 56%to 94%.Total dissolved solids?TDS?of the water samples in the Simao Basin range from 0.35 to334.68 g/L,including fresh water springs?TDS<1 g/L?,brackish springs?1?TDS<5 g/L?,and saline spring?TDS?5 g/L?and salty spring?TDS?35 g/L?.Incongruent dissolution of evaporite,carbonate rocks and silicate rocks is the dominant process affecting the chemical compositions of the spring waters.The main ions Ca,Mg and HCO₃ of fresh water springs are derived from the dissolution of carbonate rocks.The higher concentrations of Na and Cl in brackish,saline and salty springs are mainly from halite dissolution.The concentrations of Ca,Mg,HCO₃ and SO₄ mainly come from the dissolution of calcite,gypsum and anhydrite.The concentration K originates mainly from the dissolution of sylvite existing in halite.The total concentrations of rare earth elements?REEs?in the spring waters in the Simao Basin range from 0.055 to 6.682?g/L.There are coherent negative correlations of REE contents of the spring waters with p H and Eh.Speciation calculation demonstrates that carbonate complexes are dominated among the dissolved forms of REEs in the spring waters.There are also sulfate and chloride forms in the brackish,saline and salty springs.It is a rare phenomenon that travertine is depositing near the Cl-Na type spring with TDS of as high as about 10 g/L,the highest TDS as far as the authors are aware.The high partial pressure of CO₂(P_CO2)?much higher than that in air?and the common-ion effect are favorable conditions for the travertine deposition near the Mangka saline hot spring.Travertine is only depositing near vent YS2-2A.The reason for the absence of travertine deposits near vents YS2-1 and YS2-3 is that the hydrodynamic conditions to form a thinner flow surface are not conducive to CO₂ outgassing.The occurrence of the spring waters in the Simao Basin is mainly controlled by faults.The springs are formed by precipitation that is recharged in the mountain areas.The groundwater flows in carbonate formations,circulates deeply and is heated by geothermal heat,emerges in the form of hot springs along the permeable fracture or fault zones.The groundwater flows through Paleogene,Cretaceous,and Jurassic red beds,undergoes deep circulation and is heated by heat flow and incongruently dissolves the subsurface minerals at the same time,and emerges in the form of brackish,saline and salty hot springs.The groundwater flows in the red beds,dissolves a large amount of halite,eventually emerges in the form of salty springs.