A genetic model for Na-carbonate mineral precipitation in the Miocene Beypazari trona deposit, Ankara province, Turkey

The Miocene Beypazari trona deposit in Central Anatolia (Turkey), with estimated reserves of 250 million of tons of soda ash (sodium carbonate), is the second largest Na-carbonate deposit in the world. Petrographic observations of sodium-carbonate evaporites from basin-center cores show that the evaporites and associated minerals underwent significant early diagenetic mineral transformations in the Beypazan Basin. Trona, pirssonite and nahcolite are the major evaporite minerals, with subordinate bradleyite, shortite and thenardite. Pirssonite occurs in organic-rich muds (oil shales). Pirssonite is not a primary mineral but rather appears to have formed diagenetically from displacive gaylussite. Trona precipitated subaqueously (in lake waters) and as interstitial phase in playa muds. Nahcolite is a later diagenetic mineral replacing both trona and pirssonite beds. Evaporite minerals are closely related to pyrite in oil shales and are also associated with diagenetic silicates (Mg-rich smectites, zeolites, K-feldspar, searlesite, and idiomorphic quartz) formed by reaction of alkaline brines with pyroclastic deposits. The proposed sedimentary model involves an alkaline lake in which water depth fluctuated from deep, perennial, meromictic (stratified) stages to shallow, ephemeral, playa lake stages. Lake margin clastic deposits are absent in the center of the basin wherein Na-carbonate minerals were formed, suggesting the prevalence of a groundwater regime during drawdown events. (C) 2013 Elsevier B.V. All rights reserved.