The Physical Setting of the Chilean Borate Deposits

Abstract

North of the Tropic of Capricorn, the Andes of Chile and Bolivia stand forth as two massive chains with a lofty plateau between. The two great ranges of Andes are known as the Cordillera Occidental and the Cordillera Oriental, or Cordillera Real. The lofty tableland between passes under the name of Altaplanicie, or Great Central Plateau of Bolivia. It is in some respects one of the most remarkable topographic features of South America. Throughout nearly ten degrees of latitude its surface maintains an elevation of approximately I2,000-13,000 feet above sea-level. But apart from its greater elevation this broad inter-Andine tableland shows a resemblance to portions of the great basin region in the western United States. This plateau is boxed in on the east by the great wall-like ridge of the Eastern Cordillera which culminates in the giant peaks of Illimani (21,200 feet) and Illampu (21,490 feet). On the west the plateau is bounded by a remarkable string of volcanoes which constitute the Western Cordillera. The two systems of mountains are totally unlike. While the eastern range, or Cordillera Real, is composed of folded Paleozoics, with the still older granites and gneisses exposed in the axis of the range, the Western Cordillera is conspicuous chiefly as a chain of volcanic cones perched upon the western edge of the great plateau. These volcanic cones rest upon folded Mesozoic strata, chiefly of Jurassic and Cretaceous age. The body of the plateau itself is composed principally of beds ranging in age from the Devonian to the Cretaceous. Throughout the extent of the plateau, in Chile, Bolivia, and Peru, are numerous lakes, saline marshes, and beds of former lakes, most of which possess no outward drainage. The large bodies of water which still persist, like Lake Titicaca and Lake