Temperature Characteristics of Hydrothermal Alteration Minerals In Yangbajing Geothermal Field, Xizang（tibet）

Yangbajing Geothermal Field in Xizang（Tibet）is controlled by the active fault zone at the southeast foot of Nianqing Tanggula. Its hydrothermal alteration is strongly developed. Hydrothermal alteration is an alteration phenomenon after water rock reaction in a certain temperature environment. The types of hydrothermal alteration are different under different temperature conditions. Through the study of hydrothermal alteration types and combinations, the temperature of paleogeothermal fluid can be infered, and then the temperature range of the thermal reservoir can be judged. Based on the field geological survey, microscopic observation, electron microprobe analysis, X-ray diffraction clay whole rock analysis and clay quantitative analysis of Yangbajing Geothermal Field, this paper summarizes the main alteration types, subdivides four hydrothermal alteration zones: yellowish brown pyrite silicification alteration center zone, gray white medium strong silicification clay alteration zone, gray white medium clay alteration zone and light gray white weak clay alteration zone, The characteristic minerals of the alteration zone are pyrite, secondary microcrystalline quartz and clay (illite). The results show that pyrite is pentagonal dodecahedral euhedral crystal, and its formation temperature is more than 200℃; Secondary microcrystalline quartz is strongly developed, and its formation temperature is higher than 150℃; The formation temperature of illitization is higher than 120℃. The temperature characteristics are consistent with the temperature measurement results of secondary fluid inclusions in rocks in strong alteration zone (145~318℃), with the zonal distribution characteristics of hydrothermal alteration minerals, and with the temperature spatial distribution characteristics of hydrothermal activity from fracture channel to surrounding rock temperature reduction on both sides. The spatial distribution characteristics of the three minerals in the north and south areas are consistent with the temperature distribution characteristics obtained from geothermal exploration. Euhedral pyrite, secondary microcrystalline quartz (silicification, tufa, quartz vein) and illite widely developed on the surface are the indicators of altered minerals in medium and high temperature thermal reservoirs. The deep part of the NE trending fault structure controlling the spatial distribution of hydrothermal alteration in the north area is one of the important directions of geothermal exploration.