柴达木盆地小柴旦湖硼酸盐的形成

ORIGIN OF BORATES IN XIAO—CHAIDAN LAKE, CHAIDAM BASIN, CHINA

本文探讨了小柴旦湖第四纪沉积物中的钠硼解石和柱硼镁石的特征和成因,并进行了盐湖卤水的模拟实验,认为硼酸盐矿物在卤水浓缩的早期到晚期均可形成。

Xiaochaidan lake is situated in the north of Chaidam basin with the elevation of 3 171 m a. s. 1. , and covers an area of 40 square kilometers. Its surface brine is 340 g/L in salinity and contains 1.25 g/L of B_2O_3. It is a Na_2SO_(4-) subtype saline-lake rich in boron. The borates are mainly distributed in the bottom sediments of the southern lake, and occurs as a veinform, lentiform laminaform or a honeycomb. They mostly consists of ulexite and pinnoite. The former is often associated with mirabilite, halite and gypsum; the latter with dolomite and aragonite, and occurs within the carbonate concretions. Such a set of sediments is over 10 m thickness, and it belongs to the last epoch of Late-Pleistocene to Holocene epoch chronologically. According to their occurences, structures and textures, as well as ^(14)C dating of the sediments, it illustrates that the borates are secondary in genesis and are formed after the overlyint sediments were buried: In the light of the salt minerals paragenerated with berates and the boron isotopic composition, the formative processes of borates can be divided into 2 stages: 1 )the formative stage of Ca-Mg carbonate-pinnoite and 2)the formative stage of mirabilite-haliteulexite. However, the formation of the borates is closely related to the activity of underground water of B-rich in this area. The auther carried out the freezing experiment under low-temperature with the mine water. The result not only first found out the separating condition of ulexite, but also proved the formative process of borate at the second stage.