Mineralogy and sedimentology of the Miocene Gocenoluk borate deposit, Kirka district, western Anatolia, Turkey

摘要

The Miocene boratiferous district of Kirka, in western Anatolia (Turkey), is the most important Na-borate (borax) resource in the world. Two separate deposits in the Kirka district are located near the villages of Sankaya and Gocenoluk (Eskisehir Province). Borax is intensively exploited in open-pit mines in the Sankaya deposit while only small quarries of colemanite are known in the Gocenoluk deposit. Recent exploratory drilling in the Gocenoluk area intersected a thick succession of dolostones, tuffs and three borate-bearing units (Lower, Intermediate and Upper Borate Units). In them, the most abundant borate mineral is ulexite (Ca-Na-borate) passing at depth to probertite. Borax (Na-borate) is only present in the Intermediate Borate Unit. Minor amounts of colemanite (Ca-borate) and hydroboracite (Ca-Mg-borate) occur at the base, and/ or top, of each mineralized unit. Pyroclastic layers within the borate units show intense alteration by alkaline, boron-bearing waters and formation of diagenetic clay minerals (smectites), zeolites (analcime) and borosil-icates (searlesite). The Gocenoluk succession is interpreted as a shallow, ephemeral, alkaline lake deposit in which carbonates formed as stromatolites and travertines. Borate precipitation in the Gocenoluk area took place interstitially within muddy and carbonate sediments in a lateral progression from marginal Ca-borates towards Na-Ca-borates and rarely to Na-borates in the center of the lake. Authigenic silicate mineral distribution shows parallel changes toward the center of the lake that reflect increasing pH gradient.