Being the most important geothermal area with active hydrothermal manifestations in China,Tengchong is world-famous for its geothermal resources. Tectonically, Tengchong is located close to the junction zone of the Eurasian plate and the Indian plate. The residual magma gradually cooled down and became an ideal heat source for its overlying hot spring system. It is inferred from the thermometry of geyserite and adarce inclusions that underground temperatures for the hot spring system in Tengchong are 157—270℃. Surface hydrothermal alterations are well developed in the Tengchong active hot spring system. Tengchong has a great variety of special surface geothermal manifestations, such as warm springs, hot springs, boiling springs, sinters (geyserite, travertine, efflorescenceetc.), hydrothermal explosive breccias, and hydrothermal altered rocks, which are surface manifestations of geothermal activity in the interior of the earth’s crust. In this paper, mineral composition and chemical composition of geyserite in the Tengchong active hot spring system of Yunnan Province were studied by means of analytical electron microscopy (AEM) including transmission electron microscopy (TEM) and energy dispersive X-ray spectra (EDS),scanning electron microscopy (SEM), X-ray diffraction (XRD), infrared absorption spectra (IR), chemical analysis and optical examination. The results indicate that geyserite consists mainly of amorphous silica (silica gel), opal, chalcedony and quartz, and that silica gel is composed of micron-sized (0.2—2μm) SiO2spherulites. Silica gel has played an important role in the enrichment of gold. Geyserite is only present in a few high-temperature hot water areas, such as the Hot Sea and Ruidia. It is found that the hot spring system contains abundant gold mineralization. Quartz veins and geyserite have remarkably high gold abundance: 0.198 ppm for quartz veins and 0.155 ppm for geyserite (the highest value being 0.39 g/t). The enrichment of gold in geyserite provides an important inspiration for the prospecting of hot spring gold deposits. Hence, Tengchong is a very promising area for ore prospecting. Geyserite may serve as an excellent ore-prospecting indicator and has important geological implications.