[en] Full text: The most important borate deposits in South America are concentrated in the Central Andes. The Neogene deposits are located in the Puna Plateau of N W Argentina. These continental deposits are stratiform in the tectonically deformed Tertiary rocks. The largest borate accumulations Tincalayu, Sijes and Loma Blanca are part of the Late Miocene Sijes Formation, composed by different evaporitic and clastic units. In the main borate units of each location different phases of borates dominate. In Tincalayu the mayor mineral is borax with minor amounts of kernite and other rare borate minerals (ameginite, rivadavite, etc.). The principal minerals in Loma Blanca are borax with minor ulexite and inyoite. In the two main units of Sijes hydroboracite and colemanite are the major minerals; inyoite and ulexite appear subordinately. The deposition of the borates is due to a strong evaporation in playa lakes, which were fed by boron bearing thermal fluids (Alonso and Viramonte 1990). From Loma Blanca we determined δ¹¹B values of ulexite (- 6.3 %0), inyoite (-12.7 %0) and terrugite (-16.2 %0); and from Tincalayu the δ¹¹B values of borax (-10.5 %0), tincal (-12.2 %0) kernite (-11.7 %0) and inderite (-15.4 %0). The borates of Sijes are hydroboracite (-16.8 %0 to -17.2 %0), ulexite (-22.4 %0) and inyoite (-28.5 %0 to -29.6 %0). In order to get information about the δ¹¹B values and pH of a boron solution we analysed the thermal spring of Antuco. It has a δ¹¹B of -12.5%0 at a pH of 7.9. The presently forming ulexite deposit has a δ¹¹B of -22.4%0. Borates within one depositional unit show a decreasing δ¹¹B value sequence from the Na-Borates to the Ca-Borates related to the boron coordination of the minerals (Oi et al. 1989). The difference in the δ¹¹B values excludes the precipitation in equilibrium from solutions with constant pH. According to results from previous work on Neogene borates (Turkey, USA) we interpret the borate succession due to precipitation at variable pH values (Palmer and Helvaci 1995, Oi et al. 1989). Because of the identical δ¹¹B values of each borate sequence from Sijes, similarly composed thermal brines are likely. Taking the Antuco data into account we calculate an original δ¹¹B of -12%0 for the Sijes brines. The difference to the value (-16%0), calculated according to Oi et al. (1989), is small and can be explained by lower temperature and pH as well different chemical composition in the required fluid. In Tincalayu we measured the boron isotopes from different minerals with the same boron atomic coordination. The δ¹¹B values are similar for the borates, which agrees with the work from Oi et al. (1989). The calculated δ¹¹B value for the boron rich brines is around -10%0. In a first approximation, the isotopic composition of the boron source must be similar for Tincalayu and Sijes. A different situation is found in Loma Blanca. There a δ¹¹B value between 0.0%0 and +4%0 for the fluid is calculated, which is clearly due to different source material. (author)