This paper deals with sedimentologic and diagenetic aspects of the evaporitic facies of the Sijes Formation (Miocene, central Andes, NW Argentina), which contains the largest known hydroboracite reserves in the world. In outcrop, the sulfate minerals are secondary gypsum and minor anhydrite, and the borate minerals are hydroboracite with subordinate inyoite and colemanite, and some ulexite. In the Monte Amarillo Member of the Sijes Formation it is possible to distinguish two coeval, shallow lacustrine subbasins, in which the gypsum accumulated in the margins and the hydroboracite in the centers, the intermediate zones being characterized by mixed gypsum-hydroboracite layers. In the depositional sequence, primary gypsum (gypsarenite) and syndepositional anhydrite, in association with limited amounts of calcium borates (colemanite, inyoite) precipitated first, followed by hydroboracite (calcium/magnesium borate). Alternations of gypsum and hydroboracite layers also formed. Hydroboracite is mainly a primary mineral, although it replaced some gypsum under synsedimentary conditions. The formation of colemanite, which occurred during early diagenesis, is linked to the precipitation of calcium sulfates (gypsum and anhydrite), whereas inyoite coexists with both calcium sulfates and magnesium-bearing borates. Transformations among the various borate minerals during burial diagenesis were not detected. Primary gypsum was transformed into anhydrite from early diagenesis to moderate burial diagenesis. The boron source of these deposits seems to be related to the volcanic/hydrothermal activity in the central Andes during the Miocene.

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