Research Paper Mineralogy, diagenesis and hydrochemical evolution in a probertite–glauberite–halite saline lake (Miocene, Emet Basin, Turkey) Javier García-Veigas a, ⁎, Laura Rosell b , Federico Ortí b , İbrahim Gündoğan c , Cahit Helvacı c a Serveis Cientificotècnics, Universitat de Barcelona, 08028, Barcelona, Spain b Departament de Geoquímica, Petrologia i Prospecció Geològica, Universitat de Barcelona, 08028, Barcelona, Spain c Dokuz Eylül Üniversitesi, Jeoloji Mühendisliği Bölümü, Tınaztepe Kampüsü, 35160, Buca-Ízmir, Turkey abstract article info Article history: Received 1 October 2010 Received in revised form 19 November 2010 Accepted 22 November 2010 Available online 29 November 2010 Editor: J. Fein Keywords: Borates Evaporites Biogenic dolomite Biogenic arsenopyrite Sulfate isotopes The Emet basin is one of the Neogene basins in western Turkey containing significant amounts of borate minerals, mainly colemanite. The petrologic study of core samples from two exploratory wells in the Doğanlar sector, under optic and electron microscopy, reveals a complex mineral association in which probertite, glauberite, and halite constitute the major primary phases (without mineral precursors) precipitated in a saline lake placed in a volcano-sedimentary context. Other sulfates (anhydrite, gypsum, thenardite, celestite and kalistrontite), borates (colemanite, ulexite, hydroboracite, tunellite, kaliborite and aristarainite), and sulfides (arsenopyrite, realgar and orpiment) are attributed to early diagenesis. So far, the Doğanlar deposit is the most important deposit of probertite known up to now. Chemical changes in the groundwater inflow led to the precipitation of Ca-bearing borates (colemanite) in the tuff-flat environment surrounding the lake, while Na–Ca sulfates and borates (glauberite and probertite) precipitated in the center of the lake. Fluid inclusion compositions in halite indicate that the advanced brines correspond to the Na-K-Cl-SO 4 type. During restricted stages of the saline lake, the residual brines seeped through the tuff-flat sediments, leading to transformations of previous precipitates that resulted in the formation of K-bearing minerals. The abundance of coccoid-like biogenic dolomite, colloidal arsenopyrite and the isotopic composition of sulfates are indicative of bacterial sulfate reduction. In contrast, arsenic sulfides are attributed to acidophilic micro-organisms in oxidizing conditions. Fluctuations of redox conditions in both free and interstitial brines control the biological influence in some diagenetic transformations. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. 1. Introduction Boron is a non-metallic trace element with an average concentration of 4.5 ppm in seawater, 0.1 ppm in fresh-water, and 10 ppm in the Earth's crust (Garret, 1998). Boron initially accumulates in basaltic magmas, thus mostly all large borate deposits were formed in arid intermontane depressions of active volcanic terrains (Watanable, 1975; Helvacı, 2005; Warren, 2010). In these extrusive environments, boron is leached from massive volcanic and volcaniclastic rocks by hydrothermal waters and transported to endorreic depressions. Subsequently, boron is enriched in ephemeral alkaline-lakes under evaporative conditions, where concentrations reach several hundred or thousand ppm (Harder, 1974) resulting in the precipitation of Ca-, Ca/Na-, Na-, and Mg-bearing borate salts. In this context, the precipitation of borates occurs as: a) salt efflorescences or crusts, b) intrasediment crystals, displacive nodules and cements, and c) subaqueous pavements or beds in a density/ salinity-stratified water column (Inan et al., 1973; Helvacı, 1995; Helvacı and Ortí, 1998; Warren, 2006). The association of borates with other mineral groups (sulfates and chlorides) in evaporite environments has been documented in some modern Quaternary lakes such as Searles Lake (Smith, 1979), the “salares” of the Andean region (Alonso, 1991; Helvacı and Alonso, 1994), and numerous lakes of the Tibet Plateau (Zheng and Liu, 2009). In ancient evaporite formations, however, detailed reports of such complex mineral parageneses are not common. Only a few cases of borate-sulfate associations in lacustrine formations have been described, including the gypsum-probertite Permian deposits of Oklahoma (Ham et al., 1961), the gypsum-hydroboracite Miocene deposits of the Puna region, NW Argentine (Ortí and Alonso, 2000) and the gypsum-priceite Miocene deposits of Sultançayir Basin, Turkey (Ortí et al., 1998). Tertiary evaporites were extensively deposited in various sedi- mentary basins in Turkey. The Miocene boratiferous district of Emet, in western Turkey, is one of the most important producers of Ca- borates, mainly colemanite, for the industry. So far, the recently discovered Doğanlar deposit in the Emet borate district is the largest Chemical Geology 280 (2011) 352–364 ⁎ Corresponding author. E-mail address: garcia_veigas@ub.edu (J. García-Veigas). 0009-2541/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2010.11.023 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo