Geology, mineralogy and depositional setting of the Beypazarı Trona (Natural Soda) Deposit (Ankara, Türkiye)
Authors: Helvacı Cahit 
Published: November 2024

The Beypazarı district is a large area of volcano-sedimentary rocks in the interior of central Anatolia, situated ~100 km northwest of Ankara. Trona, lignite, and bituminous shale occur in the lower part, and Na-sulfate and gypsum occur in the upper part of the sedimentary sequence of the Miocene Beypazarı basin. The Neogene Beypazarı Basin extends in an east-west direction from Beypazarı to Nallıhan and consists of a sedimentary sequence of up to 1200 m total thickness. The pre-Neogene basement rock assemblages of the Pontides limits the basin to the north. The Central Sakarya Region consisting of ultrabasic, granitic and metamorphic rocks bounds the basin to the south.
The trona deposit, located 250–300 m below the surface, was discovered incidentally in the summer of 1982 by the General Directorate of Mineral Research and Exploration (MTA) while carrying out a drilling project on lignite deposits. An extensive exploratory drilling program was undertaken by MTA during the period 1983–1985 on behalf of Etibank. Proven trona reserves are 210 million metric tons, and total reserves are estimated to be 240 million metric tons. The Beypazarı trona deposit is the world’s second largest trona deposit after the Green River deposit, Wyoming, USA. In addition, there are ~400 million metric tons of lignite, 340 million metric tons of bituminous shale, and 1 million metric tons of Na-sulfate in the Beypazarı Basin.
The trona deposit located north of Zaviye village is associated with shale in the lower part of the Hırka Formation and alternates with bituminous shale and claystones. Based on borehole data, it is estimated that the areal extent of the trona deposit is ~8 km2. The trona beds were deposited as two lens-like bodies within a 70–100 m thick zone in the lower part of the shale unit. A total of 33 trona beds are known: 16 in the lower trona lens and 17 in the upper lens. The total thickness of the lower trona horizon ranges from 40 to 60 m, and the total thickness of the upper trona horizon is ~40 m. The interval between the lower and the upper trona horizons varies from 30 m to 35 m. The central part of the trona deposit is generally thicker than the marginal parts, and the trona beds grade laterally into dolomitic mudstones and claystones toward the edges of the basin. The total thickness of the trona beds in both lenses varies between 21 and 34 m in the central parts, and 2.5 and 12 m in the marginal parts of the ore bodies. The common thickness of the individual trona beds in both trona horizons ranges from 0.4 m to 2 m. The isopach contours of both trona horizons are restricted by the Zaviye fault.
The principal sodium-carbonate minerals are trona and minor nahcolite occurring in the marginal parts of the trona deposits, and trace amounts of pirssonite and thermonatrite occur locally. Trona and dolomite are associated throughout the trona zone. Calcite, zeolites, feldspars, and clays are the most common minerals within the rocks associated with the trona deposit. Trona crystals, generally white and occasionally grayish due to the presence of impurities, formed as massive crystals and as disseminated crystals in the claystone and shales. The products of zeolitization, dolomitization, and chloritization are rather widespread within the rock units associated with trona.
The Beypazarı Basin was affected by an extensional tectonic regime during the Middle-Late Miocene period. This extensional regime converted to a unidirectional compressional regime during the Late Miocene–Early Pliocene period. The sediments associated with the trona, lignite, and bituminous-shale deposits formed in fluvial, lacustrine, and playa-lake (perennial and ephemeral) environments. The Beypazarı Basin is mainly filled by clastic materials and the penecontemporaneous products of adjacent volcanic activity, centered northeast of the basin. The most likely sources of Na for the formation of trona and other sodium-carbonate salts were thermal springs, tuffs interbedded with the sediments, and extensive Neogene volcanic rocks interfingering with sedimentary rocks in the northeastern part of the basin.