Helvaci, Cahit
(1977)
Geology, mineralogy and geochemistry of the borate deposits and associated rocks at the Emet valley, Turkey.
PhD thesis, University of Nottingham.
Abstract
The Emet borate deposits are shown to be older and mineralogically more complex than previously thought. They are considered to have formed within the muds of separate or interconnected playa lakes, in areas of volcanic and seismic activity, fed partly by thermal springs and partly by streams draining the catchment areas, under arid or semi-arid climatic conditions during the Middle Oligocene.
The borate minerals formed in two geochemically distinct sedimentary basins and are interbedded with limestone, marls, volcanic tuffs and clays, much of which appear to have been derived mostly from a volcanic terrain. The borates show mineralogical and geochemical features in the two basins which are sufficiently different to suggest that the chemical compositions of ground and surface waters in the two basins differed at least from time to time. Basement metamorphic rocks and Tertiary limestone might also have been exposed and erosion of these may have contributed Ca and Sr to the lake waters.
Sediments in the borate lakes are cyclical with both lateral and vertical facies changes. The clastic sediments in both basins are similar but are very much thicker in the northern basin. The total Tertiary sedimentary thickness exceeds 750 metres. The borate zone varies in thickness between 0-100 metres and reaches its maximum thickness in the northern deposits.
The Middle Oligocene sedimentary formations, including the borate zone, are far more faulted than previously thought. Thermal springs, which at present deposit travertine and sulphur, are active and widespread west of the Emet River.
Colemanite predominates and occurs in many different forms. Other borates include meyerhofferite, ulexite, tunellite, teruggite, cahnite, hydroboracite and either veatchite or p-veatchite. Cahnite has not hitherto been identified from borate deposits. Calcite, gypsum, celestite, native sulphur, realgar and orpiment are the principal associated minerals. Montmorillonite and illite are the only clay mineral groups identified.
Volcanic rocks, closely related to the Middle Oligocene lacustrine sediments, range from rhyolites to basalts and are very rich in potash relative to soda. A complete transition exists between tuffs and clays in the Emet borate zone sediments and is reflected in their geochemistry. All tuffs, clays and limestones are characterized by a relatively high concentration of B, As, S and Sr, and high Fe203:FeO ratios which indicate strongly oxidising conditions of sedimentation, with a very low rate of leaching.
The deposits are characterized by high Ca borate (colemanite), very low Na, poor Cl and relatively high Mg, Sr, As and S concentrations compared with other borate deposits. Like the sediments, the borates are also characterized by a relatively high Fe203:FeO ratio, suggesting strongly oxidising conditions of precipitation.
The geochemical investigations suggest that the most likely sources of B, As and S were from the Tertiary volcanic rocks and associated thermal springs. The major sources of Ca, Mg and Sr are considered to be due to leaching of underlying basement rocks and Tertiary limestone by thermal springs.
Investigation of the genesis of colemanite reveals no field evidence to unambiguously support its formation by replacement of ulexite or dehydration of inyoite after burial. The early colemanite nodules were probably formed directly from brines, penecontemporaneously within the unconsolidated sediments below the sediment/water interface, and continued to grow as the sediments were compacted. Possibly the brines were never sufficiently concentrated to allow borate precipitation until the lakes were partially or wholly dried up.
Later generations of colemanite occur in vughs, veins and as fibrous margins to colemanite nodules. Other diagenetic changes include the partial replacement of colemanite by hydroboracite, calcite, cahnite and a veatchite mineral. When weathered, colemanite is often almost completely replaced by calcite. Realgar, celestite and sulphur appear to have been deposited both during and after the formation of the borates.
Quantitatively known reserves in the Emet district are more adequate but their quality is less assured. The presence of arsenic in both the sulphide and borate phases can present problems during mineral processing. However, the Emet borates are high-grade colemanite deposits and should supply a substantial quantity of the world's needs for many years.
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