Abstract

The Bigadic borates are the largest colemanite and ulexite deposits in the world and the high-grade colemanite and ulexite ores (30 and 29% B 2 O 3 , respectively) should supply a substantial proportion of the world's needs for many years. The Bigadic deposits formed within Neogene perennial saline lakes sediments located in a northeast-southwest-trending basin. The volcano-sedimentary sequence in the deposits consists of (from bottom to top) basement volcanics, lower limestone, lower tuff, lower borate zone, upper tuff, upper borate zone, and olivine basalt. The borate deposits formed under arid conditions in perennial saline lakes fed by hydrothermal springs associated with local volcanic activity. The deposits are interbedded with tuffs, clay, and limestones. Borate minerals formed in two zones separated by thick tuff beds that have been transformed to montmorillonite, chlorite, and zeolites (mainly heulandite) during diagenesis. Colemanite and ulexite predominate in both borate zones, but other borates, including howlite, probertite, and hydroboracite are present in the lower borate zone; whereas inyoite, meyerhofferite, pandermite, tertschite (?), hydroboracite, howlite, tunellite, and rivadavite are found in the upper borate zone. Calcite, anydrite, gypsum, celestite, K feldspar, analcime, heulandite, clinoptilolite, quartz, opal-CT, montmorillonite, chlorite, and illite are also found in the deposit. Colemanite nodules in both borate zones probably formed directly from solution, within unconsolidated sediments just below the sediment-water interface, and continued to grow as the sediments were compacted. It is unlikely that the colemanite formed by dehydration of inyoite and/or by replacement of ulexite after burial. Later generations of colemanite and ulexite are found in vugs and veins and as fibrous margins of early formed nodules. Other diagenetic changes include the partial replacement of colemanite by howlite and hydroboracite and ulexite by tunellite. Nodular-shaped colemanite and ulexite minerals predominate in both borate zones. Colemanite and ulexite show alternating horizons, and the transformation of one mineral to another has not been observed and the...

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