Hydrochemical characteristics and genesis of geothermal water in the Zunyi area,north Guizhou
The Zunyi area in north Guizhou is located on the slope section transiting from the Yunnan-Guizhou Plateau to the Sichuan Basin.This area is geotectonically situated on the southwestern margin of the Yangzi plate,where Neoproterozoic strata to Cenozoic strata are exposed with the absence of Devonian and Cretaceous strata.Tectonically,the"Jurassic folds",developed and distributed in the NE or NNE directions,consist of alternating anticlinoria and synclinoria.Fracture structures are mostly distributed in the core of anticlines,spreading in the NE-SW direction or near SN direction.The study area is at normal geothermal temperature gradient and within the regional heat flow value.This area is rich in geothermal resources at low-medium temperature,and its hot springs(wells)are distributed near the fracture zones of anticlines.The thermal reservoirs in this area are mainly composed of dolomite of Cambrian Loushanguan Group and Sinian Dengying Formation.Generally,the insufficient research and unknown causal mechanisms have restricted the rational development and utilization of geothermal resources in this area.This study takes three hot springs and three geothermal wells distributed in the Zunyi area as research objects.Combining the regional geothermal geological data with the analysis of hydrochemical components and H-O isotope compositions of geothermal water,we explore the geothermal water source,the water-rock reaction process,and the elevation and temperature of recharge,calculate the thermal storage temperature,hot and cold water mixing ratio and thermal cycle depth,and summarize the genesis model of geothermal water,so as to provide a basis for the exploration,development and utilization of geothermal water resources in this area.The results show that the temperature of geothermal water is 29-48 ℃;the pH value is 7.30-7.95;the TDS is 187.95-2,322.74 mg·L-1,the δD and δ18O are-72.1 ‰ to-50.0 ‰ and-11.03 ‰ to-8.23 ‰,respectively.Hydrochemically,geothermal water falls into SO4-Ca·Mg,Cl·SO4-Na·Ca,HCO3-Ca·Mg,HCO3-Na·Ca and HCO3-Ca·Na·Mg types,with main beneficial elements of Sr,Li,H2SiO3 and F.Ca2+,Mg2+and HCO3-in geothermal water mainly come from the dissolution of dolomite and calcite.Ca2+in the geothermal water of Yanjinqiao and Tanchang may be added from the dissolution of gypsum in gypsum-rich dolomite or gypsum-salt layers,in addition to Ca2+from dissolution of dolomite and calcite.SO4-ions mainly come from the dissolution of gypsum.Injections of ancient brines in the saline basin of Sichuan led to the enrichment of Na+,K+and Cl-in the geothermal water of Yanjinqiao.The H-O stable isotope signature of the water samples indicates that the origin of geothermal water in the study area is atmospheric precipitation recharge.Based on the elevation effect and temperature effect,the elevations of the recharge areas of geothermal water are estimated to be 1,310.0-1,391.2 m,and the average annual temperatures range from 4.4 ℃to 8.3 ℃.Comprehensive analyses indicate the recharge area is located in Dalou mountain in the central part of the study area.Na-K-Mg triangular diagram shows that the geothermal water in the study area is the unmature water.The reservoir temperatures are 53-95 ℃ by the silica temperature scale,and the reservoir depths are 1,372-2,633 m.The percentages of cold water mixing with geothermal water estimated by the silica-enthalpy model are 76％-92％.Groundwater in the Dalou mountain area is replenished by atmospheric precipitation infiltration,and seeps into the thermal reservoir of Cambrian and Sinian dolomite along the exposed area of Permian-Triassic carbonate bedrock or tectonic fracture zones.Then,under the influence of geothermal gradients,groundwater absorbs heat and increases temperature,forming geothermal water.While becoming warming by increasing temperature,groundwater carrying CO2 reacts with dolomite(mainly composed of dolomite with a small amount of calcite,gypsum,and other minerals)within the thermal reservoir to form geothermal water rich in Ca2+,Mg2+,HCO3,and SO42-.In the Renhuai area,groundwater reacts with gypsum-salt beds of the Dengying Formation and gypsum-rich dolomite rocks of the Loushanguan Group to form geothermal water with high and moderate concentrations of Ca2+and SO2-within the respective thermal reservoirs.Under the action of head pressure,geothermal water is transported along the karst pores,caves or tectonic fissures in the thermal reservoir.Some of the geothermal water is channeled to the surface by NE-SW oriented water-blocking fractures,and is exposed to form natural hot springs at low temperature.Due to the encirclement of the upper overburden(clastic rock),part of the geothermal water gathers in the deep carbonate thermal reservoir to form a pressurized geothermal water reservoir(anticlinal core),which is drilled by human,and geothermal wells come into being.
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