A new Tertiary borax deposit in the Andes

Abstract

The Loma Blanca borate deposit was formed in the muds of playa-lake environments during the Late Miocene and is the fourth Tertiary commercial borax deposit discovered within the borate districts of the world. It is the only South American deposit known to contain any of the minerals colemanite, inyoite, ulexite, borax, tincalconite and teruggite with a unique and characteristic mineral sequence among the other Argentinian borate deposits. The Loma Blanca deposit is characterized by abundant Ca, Na and B, very low Cl and relatively high As, S and Mg concentrations compared with other borate deposits. Thermal springs and hydrothermal solutions associated with local volcanic activity are thought to be the source of the borates. The early colemanite, inyoite, ulexite, borax and teruggite nodules and crystals appear to have been formed directly from brines penecontemporaneously within the unconsolidated sediments, and they continued to grow as the sediments were compacted. Later generations of borate minerals occur in vughs, veins and as thin layers. Diagenetic alterations include the partial replacement of borax by ulexite and tincalconite; when weathered, borates are often almost completely replaced by calcite.