Zoning in the Kirka Borate Deposit, Western Turkey: Primary Evaporitic Fractionation or Diagenetic Modifications?

The Kırka borate deposit (Miocene), in western Turkey, the most important B2O3 producer at present in the world, exhibits a symmetrical zonation in a lateral sense; it is comprised of: a central body of Na borate (borax), an intermediate zone of Na–Ca borate (ulexite), and a marginal zone of Ca borate (colemanite). This mineral zonation is also developed in a vertical sense, although it is somewhat asymmetrical because of the presence of a discontinuous Mg borate horizon overlying the central body of borax. The genesis of such a mineral zonation in the Tertiary lacustrine borate deposits of the world has been attributed to a number of diagenetic processes. In contrast, we postulate that the Kırka sequence is merely depositional and represents an evaporitic sequence in a lake basin. The existence of a lateral gradient of salinity in the Kırka lacustrine system would have conditioned the concentric pattern of the facies. The various borax lithofacies (chemical, clastic, mixed) present in the central body reflect precipitation in a lake under evolving conditions. These conditions oscillate from a predominant subaqueous setting at variable depths (perennial lake stage), to an interstitial setting (playa-lake stage). The evaporative concentration of the boratiferous solution in the lake, together with the periodic changes in temperature of the water mass, are considered to be the main controls on the crystallization of borax. No petrographic evidence was found for an inyoite-to-colemanite transformation, as previously proposed. Furthermore, the post-depositional burial of the Kırka deposit is considered to be only moderate, and insufficient for such a generalized transformation. The Mg borates represent the ultimate evaporitic precipitates from the fractionation of the initial boratiferous solution, instead of the reaction products between pre-existing borates and groundwaters. The mineral zonation in Kırka is primary, not only for borax and ulexite, but also for both the colemanite forming the marginal zone and the Mg borates overlying the central body of borax.