0375-6505/87 $3.00 + 0.00 Pergamon Journals Ltd. ~) 1987 CNR. Geothermics, Vol. 16, No. 2, pp. 181-195, 1987. Printed in Great Britain. SURFACE H Y D R O T H E R M A L MINERALS AND THEIR DISTRIBUTION IN THE TENGCHONG GEOTHERMAL AREA, CHINA Z H U M E I X I A N G and T O N G W E I Department of Geology, Peking University, Beijing, China (Received October 1985; acceptedfor publication October 1986) Abstract--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. INTRODUCTION D u r i n g a g e o t h e r m a l survey of the surface manifestations, various sinters (travertine, siliceous sinter, efflorescences and sulfur flowers) have been f o u n d in most of the 58 active h y d r o t h e r m a l areas within T e n g c h o n g C o u n t y , while the alteration products of r e p l a c e m e n t are not well developed. H y d r o t h e r m a l rock alteration has been studied in the H o t Sea and the Ruidian g e o t h e r m a l areas only. X-ray p o w d e r diffraction and classical optical m e t h o d s have been used to identify 150 samples, which include 55 air-dried oriented, u n o r i e n t e d and glycolated samples f r o m each active h y d r o t h e r m a l area. T h e X-ray data were o b t a i n e d on a Hitachi X-ray diffractometer 2038 at 40 kV, C u K a and scanning speed 2-4°/min. I n f r a r e d spectra, electron m i c r o p r o b e , electron microscope analyses and differential thermal analysis were also used to identify a part of the above samples. H y d r o t h e r m a l mineral types, mineral association features, and distribution were d e t e r m i n e d for all 58 h y d r o t h e r m a l areas. T h e characteristics of surface h y d r o t h e r m a l alteration and metallization of uranium in sinter of the H o t Sea were described in detail. F r o m the point of view of h y d r o t h e r m a l alteration in the T e n g c h o n g area, the H o t Sea and the Ruidian h y d r o t h e r m a l systems m a y be of potential g e o t h e r m a l interest. GEOLOGIC SETTING Since the late Paleozoic the T e n g c h o n g area (Figs 1, 2) has been a microcontinent b e t w e e n the G o n d w a n a l a n d and Eurasian. U p to the C e n o z o i c geologic activity was very intense. R e c e n t 181 Zhu Meixiang and Tong We• 182 -, ~ .),. ~oo/~ t, /~Coun<~ ~.) "%~IV'~.~'~ \~ll ~ ~li Te ngc hangt~ll~u~m_mg Jifll ", ~ II ~../ •" .~,w- ~ II . . . . Bali Big Bot.h .t. Yingjiarig _" / i //Mengbeng l / / ~" ..,,*", l I" ;i.~ to "~ ~ ~" ( .- "- ~L "~ / ~ f (iDongji%zhai-¥ujiozhai Sp, L ~--~JStone-wolt( F. N " ~ I Yong-an Bath Pool ~ I /~ Cl ; ~ I ~ .anoo~Hot Pool oox ~~ -°? Of ~i • .~ • ...7 ~ . , / )~..~<,ao ( ~) Hot PooL. I County ~" " ." • / • • \/ ! v ~,k~ "=\Ponzhihua BoiLing Springs / \ .~Bo.~'.~..--'~"4\ ,'--'J ~ _ ~ Long-~n-gioo / . _,," , • • LionThe J" ~Dozhuyuon Sp. \ ~" %. • • / e~) ~ .I C o u n ty = / / ) ( _..,' : }2 e< ; "~ ~ BaosLien r- ~. ?= 1 '~ .,J,.%o .~h~ k • • • ~ Zongfu Sp. . Dacun HOt Pool ~ _ ~ .L • ~. - ~ O, ~CuonLong Bath • {,e,~,y• Tengchong (~ ~ Hat PooL ~ ~ ~" • Oimucum Blxth • •Hueonapo k~ ( • gpo Worm " ~ "~% .# { / 1,/ Shanghoudian Hot , ) .._..~Lionghe ~ j~'~e'~f / f - (.,, ~ I o'zT~,'-o ,o n O. O a toO(lingSps eBeidong Bath /• //~/i I ~ 4 . . Ouingkou MoLishon Sps l(~lili~ ..'/~ Tengchong -'7 ///~ County "dlt ~ )-~ "% rJ ~ Ill. ( Datong Sps Ruidion leaf ietd Xiooshuibo Sp. \',~,~ •~ / ,(: County t " / • . "• Shobo Xiootong Sp i 7:°ox:o:,so \"~ \' • ," Xiootonggou Sp I st•,?- ~ow~y / Covi't.ySps I I IV ,o~,,oa ~ I/ ; J • I I " / ., "~. • • LongLing • County "~. •HubengSp. i/.e • Bozhu Bath )l Fig. 1. Main geothermal manifestations in Tengchong, Western Yunnan. ", • Tengchong Geothermal Area, China 183 Fig. 2. Distribution of alteration zones in the Hot Sea geothermal field. A: alunite zone, K: kaolinite zone, I-S: illite-smectite mixed layer mineral zone, FeS2: pyrite vein, F: fault, (1) boilingfountain, (2) boilingspring, (3) fumarole, (4) hot spring, (5) siliceous sinter, (6) efflorescence, (7) native sulfur. volcanoes and different fault structures are the most conspicuous geologic features. The recent tectonics of Tengchong appear to be characterized by near NS linear features. The N E and N N W fault systems form an "arched" structure. There are 58 hydrothermal areas with different temperatures in Tengchong County. Their distribution is controlled by the active faults in the above-mentioned arched structure. All the high-temperature systems are related to recent volcanism. Most of the low-to-medium temperature hydrothermal areas, with mainly warm springs and hot springs, occur along these fault zones on either side of the eastern and western parts of Tengchong, such as Stonewall Bath, Xiabiaoyuan Xiaotang Spring and Qushi Xiaotangba Springs, etc., along the eastern zone and the Black Mud Pool and Stone-flowery Cavity Springs, etc. along the western zone. Most of the high t e m p e r a t u r e areas, with strong manifestations, are distributed along the m a j o r fault in the central part of Tengchong County. Several high-temperature systems and their manifestations were described in detail by Tong et al. (1986). Based on chemical composition, the thermal waters in Tengchong area can be divided into two main types: alkali chloride waters and bicarbonate sodium waters (Zhang et al., 1987). HYDROTHERMAL ALTERED MINERALS The hydrothermal altered minerals identified within Tengchong area are: 184 Zhu Meixiang and Tong Wei (',l) Sulfates The varieties of sulfates recognized include gypsum, thenardite, alunogen, halotrichite, rozenite, coquimbite and alunite (Table l). Alunite is the direct replacement of feldspar ill primary rocks in the Hot Sea area. Others are usually euhedral, often in hairy and fibrous aggregates. For example, alunogen often occurs in hairy form, and is rarely the crystal of a group of parallel thin flaggy crystals (Fig. 3). Halotrichite occurs in the form of macroflaggy crystals (Fig. 4). Thenardite is found only in Longpu Hot Pool, and is associated with rhodochrositc and manganocalcite (Fig. 5). (b) Carbonates Calcite and trona are the dominant phases; calcite being the main component of travertine, it is determined by main diffraction peaks of 3.04 ,~(100), 3.86 ,~(10), 2.29 A(20) and 2.10 A(18). It is observed as late stage veinule aggregates in altered wall rock in Ruidian hydrothermal area. A crustose aragonite (Table 2) is found as travertine within Xiabiaoyuan Xiaotang Spring. Other carbonates, such as trona, thermonatrite (Table 2), rhodochrosite and manganocalcite, are main components of efflorescences. Trona and thermonatrite are widespread in lowtemperature hydrothermal areas. (c) Silica minerals In decreasing order opal is the major phase, followed by chalcedony and hydrothermal quartz. All are main components of siliceous sinter, and are the secondary minerals of altered wall rocks. The opal X-ray diffractogram refers to the disordered form, with major broad X-ray peaks from 4.06 to 4.12 ,~, similar to the fl-cristobalite described by Keith etal. (1978), and also to the precious opal described by Jones et al. (1964). Infrared spectrogram shows that it is a typical opal (Fig. 6). Although the main X-ray peaks of quartz, 3.34 A(100), 4.26 A(35), 2.45 A(10) and 2.28 .~(10), are shown on the X-ray diffractogram of sinter from Spectacles Spring at Hot Sea, chalcedony showing aphanitic aggregates is the dominant phase in thin section, whereas quartz appears only as a few druses filling small cracks of chalcedony aggregates (Fig. 7). (d) Clay minerals Kaolinite and I-S mixed layer mineral, which have replaced feldspar and mica in the primary rock, are the major clay minerals. Sometimes kaolinite has been replaced by halloysite and allophane on the surface. The clay minerals of the Hot Sea have been discussed in detail (Zhu, 1986). In the Ruidian several samples determined by X-ray diffraction and electron microscope show that kaolinite is the dominant phase, and, rarely the I-S mixed layer mineral. According to the standard diffraction pattern of Thorez (1976), the X-ray powder diffractogram (Fig. 8) indicates the presence of high order kaolinite in the Ruidian area. The d(001) value invariably increased and the peak split by glycolation (Fig. 9), revealing the presence of expandable layers in the illite. Increments of d(001) show that the minerals in this area are illite-smectite random mixed layer minerals. (e) Uranium minerals Pitchblende and coffinite are found in chalcedony sinter from Spectacles Spring. The pitchblende has a zonal (Fig. 10), crustose or spherulitic structure. Its chemical composition determined by electron microprobe analysis is shown in Table 3. Coffinite is determined by X-ray diffraction (Wei, 1980). 185 Tengchong GeothermaIArea, China (f) Sulfides Pyrite and marcasite usually occur in euhedral crystals (Figs 11, 12). Pyrite is rose-coloured under the reflected light microscope. Both contain high cobalt (Co 0.726% in pyrite, Co 0.677% in marcasite) by electron microprobe analysis. In addition, trace amounts of chabazite and analcime also occur in altered rock of the Hot Sea geothermal field. Native sulfur is well developed in high-temperature hydrothermal areas. Table 1. X-ray powder diffraction data for several sulfate minerals Alunogen d(~l) 1 13.4 Halotrichite Rozenite (2) (3) d(A) I d(A) I Alunite ~) d( I 60 10.4 9.5 7.81 6.73 100 30 20 3.96 3.89 3.62 3.59 20 20 20 10 3.46 3.36 15 20 3.10 3.02 15 15 10 20 5.96 5.23 4.76 4.62 30 10 100 10 4.27 4.09 3.93 3.74 50 30 30 40 3.47 3.36 9.40 8.52 8.26 40 20 90 6.36 5.49 10 80 4.74 4.60 20 40 10 10 4.49 4.37 4.31 (1) (2) (3) (4) (5) Coquimbite Gypsum (4) (5) d(A) I d(A) 1 95 30 3.16 8 2.96 2.87 2.75 2.67 2.61 10 20 30 20 15 7.55 6.89 5.49 5.20 4.77 4.50 40 90 5 10 100 3.99 70 3.62 10 3.41 60 3.29 3.24 10 50 2.99 2.97 2.77 2.71 2.58 2.48 2.44 2.38 2.36 2.27 1.97 40 35 10 10 40 10 30 20 18 30 20 3.64 3.56 3.50 40 20 10 3.36 3.28 60 10 3.11 3.04 30 30 2.76 80 2.53 20 2.34 10 2.28 1.93 1.85 30 10 30 and (6) Sulfur-scooping Hole, Hot Sea. Cucumber Gully, Hot Sea. Pine Gully, Hot Sea. Panzhihua Boiling Springs. Gentlemen Bath, Ruidian. 100 4.27 40 3.78 50 3.06 30 2.86 10 2.68 2.59 10 1.897 1.81 16 10 5.75 10 4.99 30 3.52 70 3.36 15 2.99 100 2.89 10 2.48 5 2.21 1.92 1.90 1.74 1.50 10 10 30 20 30 Zhu Meixiang and Tong Wei 186 Fig. 3. Thin flaggy alunogen, Sulfur Pond. Scale: 501,m. A Fig. 4. Halotrichite in macroflaggy crystals, Cucumber Gully. Scale: 5 #m. Ma RhTh Th ,. I [ I Rh 210 I I I 1251 Th I I I Th I 310 , I I MG 351 t I I I 401 i I I I 451 I I I I 501 I 2e Fig. 5. X-ray diffraction pattern of mixed sample of thenardite (Th), rhodochrosite (Rh) and manganocalcite (Ma Longpu Hot Pool. Tengchong Geothermal Area, China 36 28 20 18 16 14 12 IO 8 4 187 xlO 2 cm / Fig. 6. Infrared spectra of opal, Big Boiling Pan. Fig. 7. Drusy quartz, Spectacles Spring. 188 Zhu Meixiang and Tong Wei Table 2. X-ray powder diffraction data for several carbonate minerals Aragonite Thermonatrite Trona (2) o (11 d(A) I 3.40 3.27 100 60 2.71 40 2.49 40 2.41 2.37 2.34 2O 35 40 2.19 2.11 10 20 1.98 80 1.88 1.82 1.75 35 25 20 t3) d(/k) I 9.82 50 4.91 4.00 40 20 3.21 3.08 2.77 6O 80 30 2.65 2.58 2.51 100 1() 30 2.48 2.45 2.42 5 45 10 2.26 2.18 2.14 2.06 2.04 1.997 1.96 1.88 d(A) 1 5.32 5.24 20 20 2.77 100 2.75 60 2.69 50 2.67 55 2.66 10 2.63 8 2.479 30 2.45 20 50 10 10 20 40 40 20 10 2.37 6(1 2.18 15 2.06 2.01 2.00 20 15 10 (1) Xiabiaoyuan Xiaotang Spring. (2) Stone-wall Bath. (3) Qushi Xiaotangba Springs. O 0 (/3 i i I0 i i i i t 15 , i i J / 20 ~ I i i i , , 25 i i i 30 28 J i i i , 35 . . . . ~ 40 , Tengchong Geothermal Area, China 189 Table 3. Chemical composition (%) of a zonal pitchblende Light zone Darkzone U Ca Fe Pb 80 68.2 0.3 0.5 0.9 0.6 0.8 0.9 Th - Si - - -- - -- ID 05 AI tO A g ~o O3 u) 03 ed 0"I tO. i /3' I< ,¢ OLO C' I _ ~ m r6 r6 ~ N Y -'g D' ~ ~ 0 N v l'to D m ~ tO I'-: v_ ~ .,,e. 1.0 od to I',,- I D !o , Lo ,2., i-,:,. ,.,.,.,.,.1:,. ,.,.,., 4 6 8 I0 [~ 14 1611 2 0 2 2 ~ 1 2 6 L ~ 3 0 4 6 8 1012 141618 L~02224262830 28* 28 ° 1.1.1. i~ I~ItlLIAI t I , I . I . L . ~ Fig. 9. X-ray diffractograms of clay minerals in the Hot Sea geothermal field. Samples A to D are air-dried, and A' to D' are glycolated. I-S: illite-smectite mixed layer mineral, K: kaolinite, Q: quartz; sample AA' from Sulfur Pond; BB' from Pine Gully, CC' from Dabaiyan, DD' from Bath Stream. Fig. 8. (Opposite) X-ray diffraction of unoriented kaolinite. K: kaolinite, I-S: illite-smectite mixed layer mineral, O: quartz. Ruidian geothermal field. 190 Zhu Meixiang and Tong Wei Fig. 10. Zonal pitchblende, electron image (left), X-ray image of uranium (right), x30(t. Spectacles Spring. Fig. I I. Euhedral crystals of pyrite, x 160. Pine Gully. Fig. 12. Spear-like marcasite, ×65. Pine Gully. Tengchong Geothermal Area, China 191 PARAGENESIS AND DISTRIBUTION OF HYDROTHERMAL MINERALS The above-mentioned sulfates, carbonates and silica minerals are the dominant phases in various sinters, while clay minerals and sulphides are found only in altered wall rock of high-temperature hydrothermal areas. The travertine mainly composed of calcite is distributed widely in Tengchong active hydrothermal area along the eastern and western fault zones. Large-scale travertine bodies exist in some low-temperature areas, such as the travertine terrace in Black Mud Pool, the travertine mushroom in Stone-wall Bath, the travertine hill in Stone-flowery Cavity Springs. The crustose aragonites only occur on the surface of the travertine fan in Xiabiaoyuan Xiaotang Spring. The efflorescences consisting of trona and thermonatrite occur in the low-temperature areas. Trona efflorescences are widespread on travertine bodies in several hydrothermal areas, for instance in Black Mud Pool and Stone-wall Bath. Thermonatrite is only found on laminated travertine in Qushi Xiaotangba Springs and is associated with trona, and, rarely, halite. The distribution of sihceous sinter, etflorescences of complex sulfate with AI, Fe and carbonate with Mn, sulfur flowers, as well as hydrothermal minerals in altered wall rock are concentrated in several high-temperature areas along the fault zone in the central part of Tengchong. Siliceous sinters with small bodies seem to be restricted to the Hot Sea area. Alunogens are well developed on steaming grounds, around fumaroles and boiling springs in the Hot Sea and the Black Stone Stream Spring, where it is associated with native sulfur. Efflorescences consisting of sulfate with AI, Fe have a restricted distribution; for example, coquimbite is only developed in the fractures of biotite-gneiss and is closely associated with native sulfur in Panzhihua Boiling Spring. The efflorescence consisted of carbonate with Mn, which is only found near a boiling pond in Langpu Hot Pool. Hydrothermal minerals of wall rock alteration have been studied in the Hot Sea and Ruidian. The primary rock in Ruidian is quartz-monzonite, which has been altered by the geothermal fluid near the fault zone. The primary feldspars are commonly replaced by kaolinites, and, rarely, by I-S minerals. Many calcite aggregates as veinlets penetrated the veins of kaolinite. The zonation of the hydrotherreal minerals in Ruidian has not yet been studied in detail. The hydrothermal alteration of the Hot Sea thermal field is described below. HYDROTHERMAL ALTERATION OF THE HOT SEA GEOTHERMAL FIELD The basement of the Hot Sea consists of Yanshanian granite. In late Tertiary age it may have been a graben-like basin but it is now filled with Miocene granitic clastic sedimentary ro~k, which acts as a shallow thermal reservoir within the basin. Coarse- to medium-grained sandstone is the dominant lithologic type, and conglomerate is also relatively abundant. Breccia is sporadically and irregularly distributed within the clastic rock. The breccia comprises angular detrital matter, including rock fragments (abundant granite and scarce quartzite) and clastic minerals (abundant quartz and scarce feldspar) and a siliceous or argillaceous matrix. The rock fragments are characterized by angular and variable size (generally 20-30 cm - 1 - 2 cm in diameter). It remains to be seen whether the rock fragments originated from a hydrothermal eruption. There are a series of faults of north-south trend used as efficient channels for the ascent of geothermal fluid within the Hot Sea. In addition, some pyrite and quartz veins with a near N-S strike exist in the clastic sedimentary rocks (Fig. 2). The intense surface manifestations occur in the Hot Sea, whose host rocks were extensively affected by hydrothermal activity. High-temperature surface manifestations, such as hot pools, boiling springs, fumaroles and steaming grounds, are concentrated along the Sulfur Pond- 192 Zhu Meixiang and Tong Wei Cucumber Gully fault zone, while hot springs or warm springs, as well as a boiling spring near Dadijiao fault, are distributed along fault zones on either side of the eastern and western parts of the H o t Sea area. D I S T R I B U T I O N A N D Z O N A T I O N OF H Y D R O T H E R M A L ALTERED PRODUCTS The distribution of various sinters are concentrated along the Sulfur P o n d - C u c u m b e r Gully fault. In the eastern and western parts of this area there are no sinters except a few efflorescences. Alunogens are widespread on the surface of the steaming grounds on the wall of a solfataric fumarole and in the cavities or cracks of the host rock within the steaming ground, and are associated with native sulfur, and, rarely, gypsum. Halotrichite and rozenite were found on the surface of all pyrite veins. All three siliceous sinters found in Tengchong are located in the H o t Sea area. There is a fossil sinter terrace of 1.5 m height nearby Past Boiling Pan. It is a white laminated opal sinter, containing abundant casts of plant stems, roots and leaves. Recently an opal sinter rim was still precipitating around the margins of Big Boiling Pan. A chalcedony sinter with developed lamination can be observed around Spectacles Spring. It has the form of a shallow d o m e 3 m in diameter at the bottom. It contains disseminated pitchblende, coffinite, pyrite, marcasite and hematite. The ribboned texture of the sinter resulted from the orientation of these mineral aggregates along its lamination (Fig. 13). Fig. 13. Ribboned sinter, x 1.5; Spectacles Spring. The Tertiary clastic rocks were altered by the geothermal fluid. The alteration of the wall rock and paragenesis of the hydrothermal minerals are closely related to the fissures, the permeability of the primary rocks, and the temperature and nature of the geothermal fluid. Two altered centres have developed (Fig. 2). In Sulfur P o n d - C u c u m b e r Gully fault, this centre develops outwards as alunite-kaolinite z o n e - k a o l i n i t e - I - S zone. In Dadijiao fault the inner zone is I-S and its outer zone is I-S kaolinite. In both zones the intensity of alteration is closely related to fissures. The nearer the zone is to the centre, the stronger is alteration. All altered rocks consisting of pure kaolinite, pure alunite and pure I-S appeared as lenticular bodies distributed Tengchong Geothermal Area, China 193 in or near the faults. This distribution is similar to that of the Otake field (Hayashi, 1973). The zonal patterns of two centres also exist in the Los Azufres field in Mexico. Cathelineau et al. (1985) showed that two zones resulted from a main aquifer at depth, which discharged through two circulation zones, which probably originate from two main fracture systems. Because of limited drillings in the Hot Sea the situation at depth has not been studied. Pyrite veins in primary rock were also affected by hydrothermal activity. Primary coarse pyrite was repeatedly replaced by hydrothermal pyrite and marcasite (five or six times) (Zhu et al., 1985). The temperature of primary pyrite and hydrothermal pyrite is 300°C and 180°C respectively (Jiang Li-min, oral comm.), which suggests that the former is of magmatic origin and the latter of geothermal origin. ZONAL CAUSES OF HYDROTHERMAL ALTERATION The chemical composition of the geothermal fluid is evidently an important factor controlling altered zonation. Judging from the mineral associations the first centre has an acid environment, and the second has a neutral-weak alkaline one. During ascent of deep geothermal fluid part of the HzS and CO2 rich solution ascends to or near the surface along the Sulfur Pond-Cucumber Gully fault. The solution becomes acid due to oxidation of hydrogen sulfide. On the one hand, the association of kaolinite with alunite and/or SiO 2 minerals have occurred as a result of interaction between the reservoir rock and the acidic fluid. Zoning from the centre outwards is consistent with acid solutions moving outwards from channels and becoming more alkaline by reaction with the reservoir rocks (Browne, 1978). At the same time, the original pyrite vein in the clastic rock is replaced by hydrothermal pyrite and marcasite under the action of the hydrothermal fluid. The close spatial relationship between Fe-bearing sulfate and pyrite-marcasite veins indicate that pyrite is the source of iron. Another part of the deep fluid might rise directly to or near the surface along the Dadijiao fault. The fluid represented basically the property of a deep fluid (Zhang Zhifei, oral comm.) which reacts with the wall rock and results in I-S mixed layer minerals. URANIUM MINERALS IN SILICEOUS SINTER The basement rocks in and near the Hot Sea area have a relatively high radioactive background; the granite is 67 y-ray equivalents and the clastic rocks 61 ~' (Dai etal., 1984). There are some small post-leaching uranium deposits in the clastic rock in an area near the Hot Sea.* Radioactive anomalies in these deposits prior to geothermal activity reached 1000-2000 V, and the highest 3000 y. Uranium mineralization extends to a depth of 0-400 m. Geothermal activity facilitates uranium leaching, transportation and concentration from reservoir rocks. Barbier (1974) indicated that much of the uranium in granitic rocks is leachable even during surface weathering. Thus, strong geothermal activity and the good permeability of the clastic rocks provide more favourable conditions for uranium leaching. Robert (1982) studied the mobility of uranium during alteration of rhyolite ash to montmorillonite and showed that the mobility of uranium is related to dissolubility of SIO2. It is inferred that intense alteration and the high-temperature fluid rich in SiO2 favoured leaching and transportation of uranium. The alkaline environment facilitated the formation of siliceous sinter (Walter, 1978), and in the alkaline solution UO2(CO3)~- and UO2(CO3)4- are probably the major species of uranium; *GeologicalBrigade209, GeologicBureau, Ministryof NuclearIndustry,1979,Internalreport. 194 Z h u Meixiang and Tong Wei CO2 may be important for uranium transport (Rich et al., 1977). Since the geothermal fluid in the area is rich in C O 2, uranium could be transported as a uranic complex with carbonic acid. Because of convective heat loss, direct concentration and cooling, the highly supersaturated silica precipitated from the fluid rising to the surface. CO2 loss from ore solution during boiling played a major role in the precipitation of pitchblende (Rich et al., 1977). Part of the uranium formed pitchblende, and another part combined with SiO2 formed coffinite. Since the geothermal fluid is rich in SiO2, coffinite content should be higher, but this has not been observed. Bei Yuming (oral comm.) showed that coffinite is a p s e u d o m o r p h of pitchblende under the reflecting microscope. Hostether et al. (1962) showed that coffinite rapidly altered under oxidizing conditions. Judging from the development of the drusy quartz, after the sinter formed it may have developed from opal to chalcedony. This transformation was also observed in Steamboat Spring (White, 1964). Thus, the chalcedony sinter in Spectacles Spring is considered to be an older sinter. During transformation the coffinite was altered by oxidation or replaced by pitchblende. CONCLUSIONS H y d r o t h e r m a l mineral associations are controlled by intensity of geothermal activity in Tengchong area. Large-scale travertine bodies can be seen in the low-temperature areas, whereas siliceous sinters only occur in the high-temperature areas. Efflorescences made up of trona and thermonatrite in the low-temperature areas are the direct precipitations of ascending fluid, while efflorescences of sulfate with AI, Fe and carbonates with Mn in the high-temperature areas are the products of w a t e r - r o c k interaction. Alunogen and halotrichite are often found in the hot springs in some active volcanic areas, such as Mount St Helens (Keith et al., 1983) and Latera caldera (De Rita et al., 1983). In Tengchong area they occur in the high-temperature areas with steam grounds and fumaroles and are closely associated with native sulfur. This seems to suggest that geothermal activity is closely related to young volcanic activity. Although there have been many discussions about the relationship between metallization and active hydrothermal systems (White, 1955; Weissberg, 1979; Sims et al., 1981; Ellis, 1969: Dickson et al., 1971; Browne, 1971) uranium mineralization in recent hydrothermal systems has never been reported in the literature as far as we know. This paper suggests that the precipitation of pitchblende and coffinite in sinter are related to hydrothermal activity in the Hot Sea. The uranium disseminated in the clastic rocks was enriched and carried up to or near the surface by geothermal fluids rich in SiO2, and then deposited together with silica, as a result of water-rock interaction. The replacement of original pyrite by post-depositional pyrite and marcasite is also the result of water-rock interaction. This research on hydrothermal alteration in the Tengchong area indicates that a few intensive active hydrothermal areas, such as the Hot Sea and the Ruidian geothermal area, may be potential exploration regions. Acknowledgements--We would like to thank Peng Zhizhong (Beijing Graduate School, Wuhan College of Geology), Ren Leifu, Cao Zhengmin and Jiang Shaoying (Geological Company of State Bureau of Building Materials Industry) for their advice and help in identificationof minerals. We are also grateful to Liao Zhijie, Liu Shibin and Zhou Changjin (Commission on Multidisciplinary Exploration of Natural Resources, Academia Sinica, Beijing, China) for collecting part of the samples. We are also greatly indebted to Sheng Minzi for useful advice on the manuscript. REFERENCES Barbier, J. (1974) Continental weathering as a possible origin of vein-type uranium deposits. Mitt. Dep. 9,271-288. Browne, P. R. L. (1971) Mineralization in the Broadlands Geothermal Field, Taupo Volcanic Zone. Soc. 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