THE ORIGINS OF BORATES IN SALINE LAKES, QINGHAI-XIZAIG PLATEAU

This paper delt with the origins of borates in the saline lakes of Qinghai-Xizang plateau mainly on the basis of the characteristics of the borate distribution, the paragenesis mineral group and some results of our experiments. 1. The distribution and paragenesis mineral group of borates in the saline lakes of Qinghai-Xizang Plateau Borates are mainly distributed in a lot of saline lakes from the southern side of the Qilian mountain to the north side of the Gangdisi-Tanggula mountains. As to the age, most of them exist in the Holocene sediments ( up to now, borates have been precipitating in some salt lakes yet) ,but only a few of them in the Late Pleistocene. However, they are mostly concentrated at the side of lake-basin supplied with hotspring water. In the salt lakes of this area,the discovered borates are as follows: tincalconite, ulexite, pinnoite, kurnakovite, inderite, hungchaoite, hydroboracite, trigonomagne-borite and carboborite etc. Among them borax and tincalconite ( secondary ) are mainly found in the salt lakes of carbonate-type, and the pinnoite, kurnakovite and other Mg-borates in those of sulphate type. Borates are always in paragenesis with clay and various saltminerals. Borax and tincalconite ( secondary ) are always associated with mirabilite, thenardite, hy-dromagnesite, calcite, northupite, thermonatrite,and soda. Ulexite with carbonate-clay and mirabilite. Pinnoite is always associated with ulexite and may also be associated with carbonate, mirabilite, or bloedite-epsomite. Kurnakovite is always associated with inderite and the both letters can form a single ore-layer respectively. Borates can be formed in the different concentrating-stages of saline lakes. 2. The origins of borates in the saline lakes of Qinghai-Xizang Plateau In the Qinghai-Xizang Plateau, the geothermal activity is very strong, and the geothermal water ( hot spring water ) contains a lot of boron ( B₂O₃ being 84mg/l on the average, the maximum 1561mg/l ) . The acid rocks ( neutral rocks and basic rocks) are well developed there. Their B-content is higher ( the maximum coateat of B₂O₃ up to 2000 p. p. m. ) , so that it can provide abundant substance resources for the boron concentration and borate formation in the saline lakes of this area. Besides, since the Late Pleistocene the Qinghai-Xizang Plateau has been rapidly rising up, the climate gradually turning to cold and dry. It is helpful to provide advantageous conditions for the formation of saline lakes and their borates. In order to find out the forming conditions of borates, we have simulated the natural condition to carry out the experiments of diluting, freezing evaporation, freezing and unmoved- setting using the brines, spring water and hot spring water taken from Dachaidan Salt Lake ( Mg₂S0₄-subtype ) , Zhacang-Caka Salt Lake ( Na₂SO₄-subtype ) and Jilin Lake ( carbonate type) and the distilled water. Borates have been obtained from over 4o samples. Inderite is predominant and there are also kurnakovite, as well as other Mg-borates. They appeared at the different stages of the brine concentration. ( 1 ) Mixing action: The borates associated with clay carbonate, gypsum and mirabilite would be formed when they were distributed in the belt supplied with external water and the lake water was at the diluting stage. ( 2 ) The condition of low-temperature: It may impel and speed up the formation of borates iundern the low-temperature condition. Generally, the borates in paragenesis with low-temperature minerals of mirabilite etc. should be formed under this condition. ( 3 ) Stable condition: Generally, the fomation of all the borates requires a rather stable environment (i. e. stable equilibrium condition) except the extreme high-content of boron in brines. For example: after having bu riedthe salt sediments the borates were separated from intercrystal brine, pore water and interstitial water in the salt sediments. ( 4 ) The type and composition of B-bearing brine and the correlation between boron and other components in the B-bearing brine decide to a certain extent thespecies and paragenesis mineral group of forming borates， Especially. the Mg-bo-ates might be formed in the sulphate-type water-bodies of the different concentrat-ing stages and compositions. The difference between the borate species obtained from the experiments andthe nature mainly results from the difference of brine composition and the latteralteration of brine composition，as well as the latter secondary change of minerals.