| The research area is located in the Nimu-Nagchu geothermal belt of south-central Tibet and mainly controlled by NE-trending active fault belt of Damxung-Yangbajing-Duoqingcuo,which holds one of the most concentrated hydrothermal activity manifestations.From north to south,the research area could be divided into 5 hydrothermal activity areas,and there are massive sinter platforms deposited in Qialagai and Bujiemu areas.In this work,we carry out fieldwork and collect sinter samples in Yangyi geothermal field,gather geochemical data of geothermal fluids and magmatic rocks.On the basis of discussions about the age,mineral composition,petrofabric and geochemical characteristic of sinters,we explore the material source of sinter,build and discuss the spatial-temporal pattern and genetic mechanism of hydrothermal activity in the research area,and obtained the following understandings:Geothermal waters in the research area belongs to theNa-HCO3·Cl type and are characterized with high Li,F,B and SiO2 concentrations,which might imply the interfusion of deep-seated magmatic water.We could define the reasonable reservoir temperatures in the research area by combining the calculated results of chemical geothermometers,silica-enthalpy mixing model and multicomponent chemical equilibrium analyses of geothermal fluid with drilling data of geothermal wells we collected so far.There are two geothermal reservoirs(intermediate and high temperature)in the Yangyi geothermal fields with temperature ranges of 163?172? and 192?200?,respectively.In the enthalpy vs.chloride plot,the deep parent fluid of Yangyi is estimated to have a enthalpy of 1020J/g(water temperature of 242?).The deep geothermal fluids in Bujiemu area may experience complicated ascent cooling process,react with shallow surrounding rock,and emerged as unequilibrated waters.by combining the observation of hand specimens,microscopic and scanning electron microscopic samples with X-ray diffraction analysis,we describe mineral composition and petrofabric characteristic of sinters in detail.Travertine is mainly composed of calcite and aragonite,while siliceous sinter is mainly composed of opal-A or opal-CT.The difference of microbefiber diameters in travertine and siliceous sinter indicate that these two type sinters are deposited from middle-high and low temperature waters,respectively.by comparing the trace element and rare earth element distribution pattern of travertine,siliceous sinter,typical volcanic rocks,basement granite and Linzizong volcanic rocks,we find sinters and volcanic rocks of Yangyi geothermal field have the homologous ingredient characteristic,which are both the results of crust melting and magmatic differentiation,and are related to the structure environment of continental collision and crust thickening.Deep geothermal fluids in the research area leach and inherit the trace element and rare earth element distribution pattern of surrounding rocks of geothermal reservoir,then ascend to the surface and formed the sinters.In addition,the hydrothermal sedimentation and hypergene process lead to the enrichment(Cs,Sr)and depletion(K)of some trace element in the sinters,and the complicated ascent cooling process of geohermal waters bring about positive Eu anomaly of travertine from Bujiemu area.We expound the forming stage and scale of hydrothermal activity in the research area by combing the age data with field distribution characteristics of sinter.The hydrothermal activity shows a trend of sorthward movement,and influenced by Tibetan plateau uplift and neotectonic movement since middle-late pleistocene.We also explore and analyze the spatial distribution characteristics and the intensity differences of hydrothermal activity in the research area.Finally,by combining the characteristics of heat source,geothermal reservoir,caprock and thermal chamber,the conceptual model of geothermal system is put forward,and the genetic mechanism of hydrothermal activity is analyzed. |
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