STRATIGRAPHY, MINERALOGY, AND GENESIS OF THE BIGADIC BORATE DEPOSITS,WESTERN TURKEY

The Bigadic berates are the largest colemanite and ulexite deposits in the world and the high-grade colemanite and ulexite ores (30 and 29% B2O3, respectively) should supply a substantial proportion of the worl d's needs for many years. The Bigadic deposits formed within Neogene p erennial saline lakes sediments located in a northeast-southwest-trend ing basin. The volcano-sedimentary sequence in the deposits consists o f (from bottom to top) basement volcanics, lower limestone, lower tuff , lower berate zone, upper tuff, upper berate zone, and olivine basalt . The berate deposits formed under arid conditions in perennial saline lakes fed by hyrdrothermal springs associated with local volcanic act ivity. The deposits are interbedded with tuffs, clays, and limestones. Borate minerals formed in two zones separated by thick tuff beds that have been transformed to montmorillonite, chlorite, and zeolites (mai nly heulandite) during diagenesis. Colemanite and ulexite predominate in both berate zones, but other berates, including howlite, probertite , and hydroboracite are present in the lower berate zone; whereas inyo ite, meyerhofferite, pandermite, tertschite (?), hydroboracite, howlit e, tunellite, and rivadavite are found in the upper berate zone. Calci te, anhydrite, gypsum, celestite, K feldspar, analcime, heulandite, cl inoptilolite, quartz, opal-CT, montmorillonite, chlorite, and illite a re also found in the deposit. Colemanite nodules in both berate zones probably formed directly from solution, within unconsolidated sediment s just below the sediment-water interface, and continued to grow as th e sediments were compacted. It is unlikely that the colemanite formed by dehydration of inyoite and/or by replacement of ulexite after buria l. Later generations of colemanite and ulexite are found in vugs and v eins and as fibrous margins of early formed nodules. Other diagenetic changes include the partial replacement of colemanite by howlite and h ydroboracite and ulexite by tunellite. Nodular-shaped colemanite and u lexite minerals predominate in both berate zones. Colemanite and ulexi te show alternating horizons, and the transformation of one mineral to another has not been observed and the boundary between them is always sharp. Because these minerals are readily dissolved, secondary pure a nd transparent colemanite and ulexie are often encountered in cavities of nodules and cracks. Some colemanite and ulexite is weathered and c ompletely replaced by calcite. Probertite bands are found in some ulex ite horizons, especially in the lower berate zone. It forms in the sam e chemical environment as ulexite and indicates a period of more extre me desiccation and possibly subaerial exposure within the lakes. Euhed ral tunellite formed during dissolution and recrystallization of some Sr-rich ulexite horizons. In the Bigadic deposits, hydroboracite forme d by replacement of colemanite, with Mg2+ ions supplied from adjacent tuffs and clays by ion exchange. Howlite grew in clays alternating wit h thin colemanite bands and coincided with periods of relatively high Si concentrations. Diagenetic processes also produced small howlite no dules embedded in unconsolidated colemanite nodules. The initial solut ions that formed the alkaline perennial saline lake(s) were low in Cl- and SQ(4)(2-) and high in boron and Ca2+, with subordinate Na+.