Vol. 3 No. 2

CHIN. J. OCEANOL. LLMNOL.

1985

T H E C O M P O N E N T S OF T H E SALINE L A K E IN X I Z A N G A N D AN A P P R O A C H T O T H E I R ORIGIN* Yang Shaoxiu (Guilin College of Geology, China)

and Zheng Xiyu (Institute of Saline Lakes, Academia Sinica) Abstract

Component data of the saline lakes in Xizang were obtained from field observations in recent" years (1976, 1978). Laboratory studies show that there are nearly 37 chemical components in 63 lakes' brine and 27 evaporative minerals in nearly 40 saline lakes that reach their depositional stage. Their formative conditions, distributive properties, assemblage properties of some salt minerals, and mechanisms affecting the components of the saline lakes are discussed. A sedimentary model of the early Holocene epoch saline lake is suggested. This work is an aid not only to the understanding of the formation of the saline lakes in the said area, but also to the use of their mineral resources.

The saline lakes in Xizang spread out like stars. Their components are complicated, and are especially enriched in lithium and boron, elements seldom seen elsewhere in the worldE1,2,7,s~. The significance of the investigation of these lakes' components and the probe into their origin, is that an understanding of the basic laws of China's saline lakes will lead to a better utilization of their salt resources. This investigation also enables us t o study plateau uplift and geological changes in the natural environment.

I.

T H E DISTRIBUTION OF T H E SALINE LAKES

The saline lakes in Xizang lie in a district north of Xizang Plateau, approximately between 78--92~ and 30--36~ (south of the Kunlun mountains and north of the Gang~ disi-Nianqingtanggula mountains). However, in southern Xizang, there are only a few saline lakes 1) (Fig. 1). According to statistics ~), there are more than 220 saline lakes in the district, with a total area of more than 6000 km ~, about 22.22 % of the total area of the lakes in these regions. Of these saline lakes, 39 are greater than 50 km 2 in area, totalling 4100 km 2, or 68 ~o of the total lake area in Xizang. The average water surface level of these saline lakes is more than 4500 m above sea level. Some are more than 5000 m above sea level. Anglaren Cuo * This paper was published in Chinese in Ocean. Limn. Sinica, 1983, 14 (4): 342--352. 1) Data from Nanjing Institute of Geography, Academia Sinica. Cuochuolong Lake is a saline lake, whose mineralized extent is 154.099 g/l. This paper was published in Chinese in Oceanologia et Limnologia Sinica 14 (4): 342--351, 1983. 2) From the Table of Lake Types in Xizang Plateau by Institute of Saline Lakes, Academia Siniea, 1980

252

CHINESE JOURNAL OF OCEANOLOGY AND LIMNOLOGY

Vol. 3

Lake is the biggest saline lake. It has an area of more than 560 km z, and a height of more than 4689 m above sea level. Qingche Lake with an area of 57 km ~, is the highest saline lake, being 5104 m above sea level. At present on the spot investigation, East-Cuoni Lake is the deepest saline lake with a water depth of 58.7 m ~), an area of 66.5kin z, and a height of 4902 m above sea level.

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Fig. 1 The distribution of the saline lakes in Xizang 1--saline lake, 2 freshwaterlake. II.

THE COMPONENTS OF BRINE

The brine of the saline lakes in Xizang is divided into two types, surface brine and inter~erystal brine. Among these saline lakes, the amounts of brine reserves are very different. F o r instance, in Gaerkunsha Lake there is no surface brine and the intercrystal brine is scarce. O n the other hand, the Cuoni Lakes (both the eastern lake and the western one), the deepest known saline lakes, are full of surface brine. However, in the majority of saline lakes, surface ;brine is shallow, as the water depth is 1 m. Some saline lakes are even always partially dried up. The above-mentioned brine is colourless and odourless, almost transparent, salty or salty-bitter to the taste, alkaline, p H 7---9.3, gravity 1.030---1.329, mineralized extent 50--350 g/l, maximum 365 g/1. According to analysist~, s] the brine contains 37 chemical components without hydrogen and oxygen (Table 1). The content of these components in the brine is very distinct. For instance, the sodium content is 61607 mg/1, but the tin content is only 0.0044 mg/1. The difference in value is a multiple of 14 X l0 s. In Table 1, the content of cation (Na +, K +, Mg +2 and Ca +z) andanion 1) Data taken from an on the spot investigation in 1976 by Nanjing Institute of Geography, Academia $inica.

N0.2

THECOMPONENTSOFTHESAL1NELAKE

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254

CHINESE JOURNAL OF OCEANOLOGY AND LIMNOLOGY

Vol. 3

(CI-, SOu 2, HCO7 and CO~ ~) are abundant, accounting for 98 ~o of the brine ionic component. These 8 components are the main brine ionic components of the saline lakes and they are the primary factors by which the type of the saline lake is determined. They also affect lake salification. According to the research of ValyashkoreJ, the roles of these essential ionic components in natural water with various salt contents are different. When the mineralized extent of saline lake increases, the earliest to come into action is CO~ a (including HCO~); then SO~ 2 comes into play. When the mineralized quantity is very high, CI- comes into play. Valyash]ko concludes that the stable sequence in the dissolving of the main anion, in their order, is C

"-S

~CI

The main cation, Ca § plays a role in natural water at first. During the increase of the -mineralized quantity, Na § and K § in succession occupy the chief position, but the relative ,changes have not as yet taken place in Mg +~. In Table 1 there are nearly 30 rare and dispersed elements which appear in addition to the main components. But these 30 represent only 2 ~o of the brine ionic components. Among these, some elements are rare in the earth's crust; others appear in great quantity, but in geochemical distribution, they are widely scattered. Two examples are K § and Rb § (Table 2). Nevertheless, these elements are of great significance as they reflect the properties of the saline lakes. By comparing the saline lakes' brine with seawater, it can be seen that, except for Ca +z and Br-, the content of all elements in the saline lakes is from some dozen times to one thousand times that in sea water. These elements, such as Li +, B +s, K +, Rb +, Cs +, U +6, Th +4, etc., have formed a geochemical concentration. The low content of Ca +~ is the result of this ion's action on the brine-saturated CO~ ~ (HCOy) and SOu ~, thus bringing into being CaCO3 or CaSO4 which easily deposit salt; yet their content of Br- is less than that in seawater, this being a mark for continental origin. Table 2

The average content for some elements in the earth's crust (after Vinogoradof, A. P., 1962)

Element Weight (%)10.00321

2.5

] 0.015[3.7• 0.001211.7•

29

I 0.06612.1•

Owing to the different geological and geographic environments of the region, the elemental content of surface brine of each lake varies. For example, the saline lake with the highest content of boron is Zhabuye Chaka Lake, whose surface brine contains 2g boron a litre. The next highest boron content appears in Chalaka Lake and Nie'er Cuo Lake. They have a boron ,content of more than 1 g/l, a higher content of lithium, with an average of more than 500 mg/l, a n d a maximum of 1200 mg/l. Some lakes with a higher content of potassium are Zhabuye Chaka Lake, Nie'er Cuo Lake, Pengyan Cuo Lake and Zhacang Chaka Lake, all with more than 16 g/1. It can be deduced that the minerals containing potassium may be separated as a result. Nie'er Cuo Lake has a higher content in rubidium and cesium, 22.96 mg/l and 22.72 mgfl respectively. Chalaka Lake and Zhacang Chaka Lake are rich in iodine, containing 0.6 mag/1 and 0.54 mg/l respectively. In addition, the uranium of Bange Lake (1.5 mg/l), tho-

THE COMPONENTS OF THE SALINE LAKE

lq'o. 2

255

ii|

rium of Chana Cuo Lake (0.072rag/l), arsenic of Nie'er Cuo Lake (13 mg/1), fluorine of Longmu Cuo Lake, etc., are all high in content. As far as a single saline lake is concerned, the elemental distribution is also nonhomogeneous. The area with the highest value of elemental distribution lies opposite the material source of the salt, near the surface brine area, where they are affected by intercrystal brine, and near the deposits of evaporite. For instance, Fig. 2 of Zhacang Chaka Lake, the abscissa (S-N) show the plane distance from the intercrystal brine to the surface brine; the ordinate (S-Y) shows the ionic content. It can be seen that an area with the highest value of ionic content is located at 2 km, nearly at the interface of the intercrystal and surface brine. From here towards the northern part, along with the increase of the depth of the lake water, the ionic content decreases inversely. This shows that the ionic content (except for Ca +~) varies directly as mineralized quantity.

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Fig. 2 The section for the ionic content (g/l) of the surface brine in Zhacang Chaka Lake

The mineralized quantity of intercrystal brine is higher than that of surface brine, because ~he intercrystal brine is affected weakly by the natural factors. The two brines mentioned above have a supplementary relationship with each other. The .content of the trace elements of the intercrystal brine, is as follows: Zhabuye Chaka Lake, boron 2.4 g/I, potassium 24 g/l; Zhacang Chaka Lake, lithium 1.2 g/I, potassium 1.7 g/l, rubidium and cesium, higher than that of the surface brine (Table 3). The ionic content of the saline lake is decided by many factors such as material source, migratory ability, extent of enrichment and so on. Among them the elemental feature and the condition of the water medium are factors that always play a role. The element in brine is mainly the halogenide and alkaline metal whose migratory ability is strong. The intrinsic factor affecting elemental migratory ability is the value of the ionic potential (electrovalent/ ionic radius). Goldschmidt firmly believes that the elements whose ionic potential values are

256

CHINESE J O U R N A L OF OCEANOLOGY AND LIMNOLOGY

Vol. 3

,,

Table 3

Thetypes of brine

I I~ content

The chemical components of the intercrystal brine of the saline lakes in Xizang (rag/l)

Na §

K*

Ca2§

Mg~*

C1- I SO~+ HCOi-

Oceanic water*

salt lakes

10500

380

i Maximum 118529.3] 39634

400

1350

70

70 33534.7

25961

1

9.4

2804.8 3343.2

250

0434.3 17336.7

7282.8

652.1

104.04

9.31

Mean

57464

15892

153.8

3252.5

5.47

41.82

0.38

2.41

Li*

Br-

I-

Oceanic water*

2967

194046 50698.9

550

Be+

I

3957.5

11700

[ contentI~

19000 I

656

Minimum

Multiple in oceanic water Theof brinetypes

COl-

!

I

The brine of

,,,

4.75 Rb*

5.84 Cs§

U 6.

Si4*

Sr~§ 8

4.6

0.17

65

0.06

0.12

0.0005

0.003

3

Maximum 1610.7

1207

259.4

0.27

16.97

6.83

2.29

11.4

<2

Minimum

328.3

61.5

38.4

0.008

2.89

~0.05

0.016

Mean

652.1

424.4

119

0.128

10.69

2.612

0.74

5.16

--

Multiple in oceanic water 141.76 2496.47

1.83

2.13

89.98

5224

46.66

1.72

--

The brine of salt lakes

* Taken from "Journal of the oceanographical society science department in a college annuals, 1971" small (0.61--2.56), such as Li § Na § K § R b § and Cs +, can enter into the water solutio~ because their ionic radii are big and are not affected by the value of p H of the water medium~ so that they migrate towards the lake, but the ions Ca +e, Mg § and so on are easily transported in the acid and neutral medium. The ions with ionic potential values of 9.7 45, such as B +3, As +3 and so on, migrate as a complex ion of oxygen and enter into the water solutionr3L Of course, there are many factors affecting the migration and enrichment of elements. Felsman sums up the comprehensive effects for the intrinsic factors (elemental feature, inner tectonics) and the external factor (natural environment). The intrinsic factor is a geochemical problem; external factors include temperature, pressure, ionic concentration, the value of pH, and so on. It is necessary to stress that the effect of the ionic concentration on the migration and enrichment of an element is very obvious, the greater the ionic concentration, the stronger its migratory ability. Elemental migratory abilityr4a is indicated by Kolnesky M = DC

(D spread coefficient 1), C ionic concentration). I f the ionic concentration is known, the migratory ability of an element can be calculated from this formula. Because the effect of p H on the element whose concentration is less than 0.1---0.001 mg/1 is very small or even nullE4~, the lake water with various values of p H may contain traces, 1) When the temperature is 20~

the value of D is 0.25---2.5g day and night.

No. 2

THE COMPONENTS OF THE SALINE LAKE

257

of rare and dispersed elements. When the ionic concentration is big, and especially during the formation of salt deposits, the value of pH plays a role. The brine component in Xizang is related to the special geological tectonic and geographic environments in Qinghai-Xizang Plateautl'~"sL The saline lakes which are rich in Li +, B § Rb § Cs + and so on, are mainly located near magmatite, volcanic rock, and a gross fault with a violent hot-water movement. Hot-springs and majestic sinters of carbonate can, always be seen around a saline lake. At present there are some hot-springs that flow directly into saline lakes such as Yibu Chaka Lake, Caiduo Chaka Lake, Bange Lake, Zhacang Chaka Lake and Dong Cuo Lake. According to investigation, there is an obvious. abnormal phenomena (Fig. 3) of water temperature in the vertical section for East-Cuoni Lake 1'2) and Qiangma Cuo Lake s), which indicates that there is violent hydrothermal movement on the bottoms or the coasts of the lakes. It appears that a violentnaturaI hydrothermal circulation is the basic reason for the concentrations of Li +, B +3 and so on. This conclusion has been confirmed from the investigation of terrestrial heat a).

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Fig. 3 A vertical section for East-Cuoni Lake (1978, 5)

III.

T H E M I N E R A L COMPONENTS

(1) The minerals of the saline lakes According to the investigations of many years (from 1958 to 1961; in 1976 and 1978) ir~ the saline lakes in Xizang, the discovered evaporative minerals are of 27 kinds (Table 4). Among them is a kind of chloride (Halite), 8 kinds of sulphate (gypsum, syngenite, glaserite, tychite, hydroglauberite*, epsomite*, thenardite and mirabilite); 7 kinds of borate (ulexite, inyoite, tincalconite, borax, pinnoite, inderite and kurnakovite), and 11 kinds of carbonate (calcite, aragonite, hydromagnesite, magnesite, trona, natron, thermonatrite, nesquehonite, northupite, naheolite and caylussite). 1) Data taken from an on-the-spot investigation in 1976 by Nanjlng Institute of Geography, Academiat Sinica. 2) Data taken from an on-the-spot investigation in 1978 by Institute of Saline Lakes, Academia Sinica. 3) After the data by Zhu Meixiang et al. in 1980. * First discovered mineral in saline lake in Xizang.

258

CHINESE JOURNAL

OF OCEANOLOGY

AND LIMNOLOGY

Vol, 3

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NO. 2

THE COMPONENTS

.

O F T H E SALINE L A K E

259

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260

CHINESE JOURNAL OF OCEANOLOGY AND LIMNOLOGY

vol. :5

Clay mineral is mainly illite, next is chlorite, montmorillonite is last1), and coUyrite only exists in the individual lake. (2) Primary and secondary borate mineral Based on their genetic features, salt minerals are always divided by scholars here and abroad into two types, the primary (syngenetic) and the secondary (epigenetic). In actual work, it is of significance to divide them for an understanding of both their mineral origin and their prospects of salification. Now let us discuss the following actual examples. Kurnakovite: The kurnakovite in Zhacang Chaka Lake is idiomorphic and semi-idiomorphic granular in the cyrstal shape, grain size is usually 0.3---0.5 mm, aggregate is a "granulated sugar-form", and stratoid in form, has certain stratified position, whose strata from top to bottom and interlayer are composed of sand, gravel and clay, as well as carbonate with tiny stratification. These primary marks indicate it is a chemical sediment of the lake's facies and that it is primary mineral. Pinnoite: The pinnoite is widely distributed and combines with clay of carbonate to form "'hard ernst". Microscopic examination shows replacement of kurnakovite by pinnoite, indicating pinnoite is a secondary salt mineral. Ulexite: There is still controversy on how ulexite is formed. Evanoff considers the tiny spherical granular ulexite combined with bloedite is formed by "the action of atmospheric water or coagulative water on dispersive boronous mud". The substitution of kurnakovite and pinnoite by ulexite can be seen on the sample taken from Zhacang Chaka Lake. Under the microscope (Fig. 4), it is obvious that the ulexite is secondary. In 1978 crystallization of ulexite in spherical granular aggregate (grain size 4 mm) form could be seen directly on the surface of the sand layer on the coast of Laguo Cuo Lake (the water depth for this lake about 8 cm, gravity 1.065, pH 8.5, content of B2Oa 2303 mg/1). Even if the above-mentioned mark is primary, we still have not been able up to now to find experimental proof concerning boron enriched brine crystallized on ulexite.

~ !

0

U

|

0.2 mm

Fig. 4 Sketch under microscope pinnoite (P) replaced kurnakovite (K), both are also replaced by ulexite (U) (3) The characteristic mineral assemblage Saline lakes of the carbonate type have a characteristic sodic mineral assemblage, such as trona, natron, thermonatrite, northupite, naheolite as well as caylussite. If the saline lake is enriched with boron, its boronate is a sodie one mainly borax. The sulphate type saline lake has a characteristic calcic mineral assemblage, such as by1) After the data by Xu Chang in 1980.

No. 2

THE COMPONENTS OF THE SALINE LAKE

261

droglauberite, syngenite and gypsum. If the saline lake is enriched with boron, there must be a typically magnesic borate-kurnakovite, inderite and pinnoite. The minerals of the Xizang saline lakes are similar to those of the hot-springs. Reports that the various minerals crystallized from hot-spring (salt sinter) always can be seen on evaporite in Xizangrs'a~, indicate that the components of the saline lakes and the hot-springs have a common source. Isotope analysist51 of sulfur in areas of hot-water movement provides proof that hotspring, volcano and magmatic source are closely related. But as far as mineral components are concerned, there is a genetic relationship between the saline lakes (especially those enriched with boron), terrestrial heat and volcanism, as they are direct or indirect reflections of magmatism of different forms on the shallow portion or the surface of the earth crust. (4) A sedimentary model (taking Zhacang Chaka Lake as example) In the Pleistocene epoch about 20,000 years ago, the lake basin in the early Quaternary Period was formed on the basis of the subsidence of the rift massif (Fig. 5). Later, hot-springs (or thermal springs, boiling springs) developed along the lake coast. Great numbers of sinters stood out like beauties of the woods. The hot-springs, carrying the elements of Lithium and boron etc., flowed into the lake. Salification began and sediment of carbonate formed in the lake water. From the late Pleistocene epoch to the early Holocene epoch, the basin of Zhacang Chaka Lake had a varied topography. There was a sunken area and a shoal (Fig. 6). The mineralized extent of the lake water had reached such a stage as to effect salification, or make it a saline lake. The sedimentary model at this stage is that of the sedimentary facies for the clay of carbonate and mirabilite in the sunken area of the lake basin (which is formed mainly by mechanical sedimentation) and the kurnakovite's sedimentary facies on the lake coastal shoal (which is formed mainly by chemical sedimentation). Both existed in common (Fig. 7).

~~

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Fig. 5 The changes for the lake basin in Zhacang Chaka Lake The boundary line of the lake basin for the early Quaternary period; The boundary line of the lake basin at present; Salt sediment; Lake water.

262

CHINESE JOURNAL OF OCEANOLOGY AND LIMNOLOGY

1

2

3

Vol. 3

4

5

I

0

4

8kin

" r

.N

S

I

k

"-..

~

)

.-

~

/

I

Fig. 6 The sedimentary model of salt from the late Pleistocene epoch to the early Holocene epoch in Zhacang Chaka Lake a. A plane figure b. Section 1. The sinter of hot-spring; 2. The boundary line of the lake; 3. The shoal on the lake; 4. The direction for supplying of spring water; 5. The direction for supplying of surface water.

F r o m late Holocene epoch to the present, the lake water has had strong concentrations o f salt sediments, mainly mirabilite and halite. These salt sediments form deposits of the saline lake facies at present (Fig. 8).

No. 2

THE COMPONENTS O F THE SALINE LAKE

263

] - - I ] Lal/e r, dge CK7611

CK7821

(secUon)

9." """

CK762t

CK7831

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8 I0

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Fr'q r-~ r':-1[53 ~ t rz'q tr:~ f--q r:~

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2

3

4

5

6

7

8

9

Fig. 7 A correlation for salt deposits in Zhacang Chaka Lake 1. Halite; 2. Mirabilite; 3. Gypsum; 4. Kurnakovite; 5. Pinnoite; 6. Carbonate; 7. Clay; 8. Sand; 9. Gravel A 5600-4-150 y; B 80004-130 y; C 109004-200 y; D 90604-120 y; E 156004-600 y; F 70004-110 y; G 14004-690 y; H 47804-180 y; I 134004-160 y; J 154004-160 y; K 200004-350 y.

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%~

_

_

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1. 5. 9.

2

4

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Fig. 8 The distribution of the salt deposits in Zhacang Chaka Lake Kurnakovite; 2. Pinnoite; 3. The position of drill hole; 4. Halite; Mirabilite; 6. Clay; 7. Spring and river; 8. Sinter terrace; Road; 10. Lake water.

IV. C O N C L U S I O N 1. There are 37 kinds o f chemical components in the brine o f saline lakes in Xizang~ whose content (except for Ca +e, Br-) is higher than in seawater. They f o r m a geochemical area enriched in the elements o f B +s, Li +, R b +, Cs + et al. 2. The evaporative minerals (carbonate, borate, sulphate and chloride) are o f 27 kinds~

264

CHINESE J O U R N A L OF OCEANOLOGY AND LIMNOLOGY

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in this area. They have formed two characteristic mineral assemblages of the hydrochemical type. 3. The boron enriched saline lakes are distributed in three zones of the regional tectonics. Terrestrial heat movement and volcanism have proved to be contributing factors for the presence of the elements of B § Li § Rb § Cs § et al. of the boron enriched saline lakes. 4. The sedimentary model from the late Pleistocene epoch to the early Holocene epoch in Zhacang Chaka Lake is that of the sedimentary facies for the clay of carbonate and mirabilite in the sunken area of the lake basin (formed mainly by mechanical sedimentation) and the sedimentary facies of the kurnakovite on the lake coastal shoal (which is formed mainly b y chemical sedimentation). Both existed in common. This paper reports on the results of an integrated scientific survey in the Qinghai-Xizang Plateau. Unless otherwise acknowledged as coming from other sources, all experimental data are from the Institute of Saline Lakes, Academia Sinica. The author is thankful to Yang Wenbo, Hu Jinquan, Liu Jiahua and Wei Xiangtai, for their help. References

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