Formation Conditions And Hydrogeochemical Characteristicsof The Geothermal Water In Typical Coastal Geothermal Field With Deep Faults,Guangdong Province

Guangdong province belongs a famous volcanic geothermal area located in the hinterland of Catyaysia Block along the SE margin of the South China Block,where multi-stage magma and repeated activities of deep faults play a control function on the formation of geothermal resource.A typical coastal geothermal field with deep faults in Guangdong Province,the Xinzhou geothermal field,was selected to discuss the formation conditions of the geothermal water and the genetic model.The research canenrich geothermal water deep cycle theory and provide a scientific basic for the developmentand utilization of geothermal resources in a reasonable way.The paper firstly analysed the thermal structure of the crustal zone and thecharacteristics of deep geothermal field in the study area.The hydrogeothermal anomalyzones was characterized using thespatial distributions of surface temperature anomalies,Bouguer gravity field and aeromagnetic anomaliesas discriminant factors,by means of geophysical prospecting,thermal infrared remote sensing,and boretemperature logging.It is found that the distribution of geothermalwater anomalies is mainly controlled by the following three factors:(1)The distribution of hydrogeothermal anomaly zone is controlled by NW deep faults obviously;(2)the exposure of hot spingsare closely related to the magmatic intrusion activities,and most of them can be exposed in the edge of magmatic rock,later intrusionsor in outer contact zone between rock mass and strata;(3)The Mesozoic-Cenozoic sedimentary basinsof large thickness often hold massive untapped reserves ofthe sedimentary-basin type geothermal systems.On the basis of fully summing upa combination modelof the “four elements”for the formation conditions ofgeothermal water,various geochemical geothermometers,multi-mineral balance simulations,and Fix Al method were used to estimate theunderground temperatures and the depth ofgeothermal water circulation in geothermal systems.The deep hydrodynamic conditions and response of geothermal water were also determined to establish a conceptual hydrogeochemical-hydrogeological model in Xinzhou.The deep faults played a key role inthe formation of geothermal water.On the one hand,the groundwater flow in the area was forced to run deep into a regional flow system and being heated under the background ofhigh regional heat flow.On the other hand,the intersection of the NE-and NW-trending faults produces aheat pipe effect which can act as preferential channels for convective circulation of hydrothermal fluids and exert primary control on geothermal activity.Based on the above-mentioned concept model,a series of geochemicalinformations were extracted to analyze the hydrogeochemical evolution process and runoff characteristics of geothermal water.The geothermal water has high Na+?Cl-,TDS concentrations and lightest ?18O and ?D values,derived from a regional flow system with large circulation depth and long residence time.The hydrochemical characteristics of the thermal waters were examinedand characterized as Na-Cl and Ca-Na-Cl types,which are very similar to that of seawater.The hydrochemical evolution is revealedby analyzing the relationship between the chemical constituentsand temperature,ion ratios and their relative changes for different water samples,reflecting differentextents of water-rock interactions and clear mixing trends with seawaters.Nevertheless,isotopic dataindicate that thermal watersare all of themeteoric origins.It was recharged by preciptitation in the high lands of northern piedmont of Mt.Ziluo,abount 20-11.5 ka BP,presumably under a colder climate in the last glacial period.Isotopic data also allowed determination of different runoffconditions andpresentation of differentmixing proportions of seawater with thermal waters.This study has conducted daily and annual variationmonitoring of geothermal flows and numerical simulationsto investigate the physical states of the geothermal water inside the fault zone.Flow variations reflect responses of the deep geothermal regionto tidal waves.The study result show that the influence of the solid tide on the conductivity of deep falut depends on the degree between significant of solid tide deformation and deep fault directions.The results also detail the concept of a thermal reservoir interms of its formation and heat distribution.Owing todriving buoyancy deep geothermal water in states of high pressure and high temperaturecan effectively flow to the shallow partsthroughthe deep fault.The invasive seawater caninvade deep underground under certain favorable hydrodynamicconditions,and could seep further into the region where the hotspring geothermal water cycle occurs.The results also show the temperaturedistribution is strongly controlled by the fault fracture system and a high temperature cylinder in the NE-trending fault formed by convection of hot water heats top surface of bedrock and thus formsthe center high temperature area.