Location and geological setting

The Heinitang hot spring is located some 6 km to the northwest of the Houqiao town near Tengchong in western Yunnan. Tengchong is situated at the border area of southwestern China (Fig. 1).
Fig. 1. The location of Yunnan and Tengchong (Adapted from National Geomatics center of China)
The Tengchong area lies in the Tengchong Block (Fig. 2a), where the magmatism and volcanic activities occurred frequently and many hot springs exist.
The field site for this study lies in the northwest of Tengchong in Yunnan, which is underlain by Paleogene, Triassic, Permian, Devonian and Proterozoic rocks. Lithologically, the Paleogene rocks are composed of moyites. The Triassic rocks appear mainly in the west of the area and consist of monzonitic granite. The Permian rocks consist of slates, sandstones and quart arenites. The Devonian rocks consist of argillic and carbonaceous slates and high permeability limestones. The Proterozoic rocks consist of mica schist, plagiogneiss, quartzites and granulites of low permeability (Fig. 2b).
The Heinitang hot spring is located at the western edge of the Permian rocks and it is structurally controlled by a NW-SE fault. The Heinitang hot spring includes of 7 vents (S1, S2, S3, S4, S5, S6 and S7) (Fig. 3). The temperature of the hot water ranges from 50.9 ℃ to 66.6 ℃. In addition, there are several fossil travertine mounds and cones near the hot spring vents. The travertine mounds and cones were weathered to some degree. The travertine cones range in height from 0.3 m to 5.5 m (Fig. 3). Apparent change in morphologies from travertine mounds to travertine cones indicate that the discharge of the Heinitang hot springs decreased with time in the past. In recent decades, the hot water was used for bathing by constructing pools near vents S2, S3 and S4. New travertine deposits near these pools as a result of outflow of hot water from the pools. The C8 travertine cone is relatively well preserved (Fig. 4a). Travertine deposited near vent S2 in 2018 (Fig. 4c) are obviously larger than that in 2013 (Fig. 4b). Small scales of travertine are depositing near vent S1, but in 2013 no travertine was found to deposit near this vent.
Fig 2. (a) Simplified tectonic map of the western Yunnan (Adapted from Bureau of Geology and Mineral Resources of Yunnan Province) and (b) the location of the Heinitang hot spring (Adapted from Bureau of Geology and Mineral Resources of Yunnan Province, 1990) 1 Paleogene Moyites; 2 Triassic Monzonitic granit; 3 Sandstones of the Permian; 4 Devonian Argillic and carbonaceous slates; 5 Proterozoic Quartzites.
Fig. 3. Plan diagram and profile of the Heinitang hot springs with 2 travertine mounds and 14 cones. (The number is the height of the inactive travertine cone (m)).
Fig. 4. Field photographs of travertines: (a) The inactive travertine cone (C8); (b) Travetines near vent S2 in 2013 and (c) Travertines near S2 in 2018.