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Oxygen isotopes of diatom silica and petrographical characterisation of diatomaceous laminated sediments of Lago Chungará (northern Chilean Altiplano) have allowed us to establish its palaeohydrological evolution during the Lateglacial-early Holocene (ca. 12 000-9400 cal. yr BP). These laminated sediments are composed of light and dark pluriannual couplets of diatomaceous ooze formed by different processes. Light sediment laminae accumulated during short-term diatom blooms whereas dark sediment laminae represent the baseline limnological conditions during several years of deposition. Oxygen isotope analysis of the dark diatom laminae show a general 18O enrichment trend during the studied period. Comparison of these 18Odiatom values with the previously published lake-level evolution suggests a correlation between 18Odiatom and the precipitation:evaporation ratio, but also with the evolution of other local hydrological factors as changes in the groundwater outflow as well as shifts in the surface:volume ratio of Lago Chungará. The lake expanded (probably increasing this ratio) during the rising lake-level trend due to changes in its morphology, enhancing evaporation. Furthermore, the lake's hydrology was probably modified as the groundwater outflow became sealed by sediments, increasing lake water residence time and potential evaporation. Both factors could cause isotope enrichment

Chile's boron resources are all located in the north, mainly in the evaporitic basins known as salares or salt flats. They are genetically related to Upper Tertiary and Quaternary volcanic rocks. The most important known deposits are at Surire, Ascotan, Aguas Calientes, Quisquiro, Maricunga and Pedernales; other basins in the Altiplano may also have resources. The borates are present mainly in horizons known as barras, forming lenses interfingered with evaporitic and clastic sediments. In other instances boron minerals form nodules up to decimeters in diameter. The only economic mineral is ulexite, a Ca-Na borate. Boron is also present in the Li-rich brines of the Salar de Atacama, where 0.84 g/l of polyborates can be recovered as by-products. Far less important are borate-rich horizons associated with palaeosols in the Pampa del Tamarugal, while minor boron is also found with geothermal brines and nitrates., Chile's boron resources are all located in the north, mainly in the evaporitic basins known as salares or salt flats. They are genetically related to Upper Tertiary and Quaternary volcanic rocks. The most important known deposits are at Surire, Ascotan, Aguas Calientes, Quisquiro, Maricunga and Pedernales; other basins in the Altiplano may also have resources. The borates are present mainly in horizons known as barras, forming lenses interfingered with evaporitic and clastic sediments. In other instances boron minerals form nodules up to decimeters in diameter. The only economic mineral is ulexite, a Ca-Na borate. Boron is also present in the Li-rich brines of the Salar de Atacama, where 0.84 g/l of polyborates can be recovered as by-products. Far less important are borate-rich horizons associated with palaeosols in the Pampa del Tamarugal, while minor boron is also found with geothermal brines and nitrates.

The sequence of complex parageneses filling the pores and spaces in different host rocks reveals the evolution of the parent brines. A sequence of progressively more soluble minerals was precipitated: silicates (zeolites); calcite; Ca, Na, (K) and Mg sulphates; Na and K-Mg nitrate sulphates; nitratine; and small amounts of iodates, Na and K-Mg iodate sulphates, chromates, borates and perchlorates. Two specific trends have been distinguished: a Na trend characterised by the association of glauberite, darapskite and hectorfloresite, and a K-Mg-Na trend related to bloedite, polyhalite, humberstonite, nitre and fuenzalidaite. Regional data indicate an Oligocene-Miocene age of formation older than 6 Ma, when the main physiographic features were similar to recent ones but the climate was wetter. Brine genesis included thermal processes and leaching of salts previously formed in the arid Atacama desert. Evaporation was also involved, but the vein-infill nitrates cannot be considered as true evaporites in the absence of displacive growths and other typical structures., The sequence of complex parageneses filling the pores and spaces in different host rocks reveals the evolution of the parent brines. A sequence of progressively more soluble minerals was precipitated: silicates (zeolites); calcite; Ca, Na, (K) and Mg sulphates; Na and K-Mg nitrate sulphates; nitratine; and small amounts of iodates, Na and K-Mg iodate sulphates, chromates, borates and perchlorates. Two specific trends have been distinguished: a Na trend characterised by the association of glauberite, darapskite and hectorfloresite, and a K-Mg-Na trend related to bloedite, polyhalite, humberstonite, nitre and fuenzalidaite. Regional data indicate an Oligocene-Miocene age of formation older than 6 Ma, when the main physiographic features were similar to recent ones but the climate was wetter. Brine genesis included thermal processes and leaching of salts previously formed in the arid Atacama desert. Evaporation was also involved, but the vein-infill nitrates cannot be considered as true evaporites in the absence of displacive growths and other typical structures.