Surface Hydrothermal Minerals and Their Distribution in the Tengchong Geothermal Area, China

-In the active hydrothermal areas of Tengchong there is widespread evidence that hydrothermal minerals are deposited directly from the geothermal fluid or from water-rock interactions. X-ray powder diffraction, electron microprobe analyses and classical optical methods were used to identify these hydrothermal minerals. Sulfates (gypsum, alunite, alunogen, halotrichite, etc.), carbonates (calcite, trona, thermonatrite, etc.), clay minerals (kaolinite, illite-smectite mixed layer mineral, etc.) and silica minerals (opal, chalcedony, etc.) are the dominant phases. Native sulfur, pyrite, marcasite and aragonite are next in order of abundance. Some chabazite, analcime, pitchblende, coffinite, hematite, thenardite, rozenite, coquimbite, manganocalcite and rhodochrosite is also present. Although travertine and efflorescences, along with carbonates and simple sulfates, are widespread in the low-temperature hydrothermal areas, siliceous sinters and hydrothermal altered minerals, such as clay minerals, zeolites and efflorescences with complex sulfates containing Fe, AI, are only found in a few high-temperature hydrothermal areas, such as in the Hot Sea and the Ruidian hydrothermal areas. Most of the wall rock was intensely altered by geothermal fluid in the Hot Sea and Ruidian, zoning in the characteristic feature of the altered minerals within the Hot Sea. Pitchblende, cofflnite, pyrite, marcasite and hematite, which are all of hydrothermal genesis, as well as the sulfate with AI and Fe, seem to be the result of water-rock interaction. I N T R O D U C T I O N Dur ing a geothermal survey of the surface manifestat ions, various sinters ( travert ine, siliceous sinter, efflorescences and sulfur flowers) have been found in most of the 58 active hydro thermal areas within Tengchong County , while the al terat ion products of rep lacement are not well developed. Hydro the rma l rock al terat ion has been studied in the Ho t Sea and the Ruidian geothermal areas only. X-ray powder diffraction and classical optical methods have been used to identify 150 samples, which include 55 air-dried or iented, unor ien ted and glycolated samples f rom each active hydro thermal area. The X-ray data were ob ta ined on a Hitachi X-ray di f f ractometer 2038 at 40 kV, CuKa and scanning speed 2-4°/min. Inf rared spectra, e lectron microprobe , electron microscope analyses and differential thermal analysis were also used to identify a par t of the above samples. Hydro the rma l mineral types, mineral association features, and distribution were de termined for all 58 hydro thermal areas. The characteristics of surface hydro thermal al terat ion and metall ization of uranium in sinter of the Hot Sea were described in detail. F rom the point of view of hydro thermal al terat ion in the Tengchong area, the Hot Sea and the Ruidian hydro thermal systems may be of potent ial geothermal interest. G E O L O G I C S E T T I N G Since the late Paleozoic the Tengchong area (Figs 1, 2) has been a microcont inent be tween the G o n d w a n a l a n d and Eurasian. U p to the Cenozoic geologic activity was very intense. Recen t