Mineralogical and geochemical characteristics of alteration minerals related to fossil geothermal activities in the Kızıldere geothermal field, Western Turkey

Abstract

The Kızıldere geothermal field, located at the eastern part of the Büyük Menderes graben in Western Turkey, is the most important geothermal reservoir suitable for electricity generation. Fossil fumarole fields and alteration or mineralization zones are directly related to the tectonic zones influenced by N-S directional extension. Associated to fossil geothermal activities, calcite, dolomite, strontianite, quartz, gypsum, anhydrite, celestine, kaolinite, smectite, boehmite and goethite/limonite were occurred in the form of void or crack fill within the Paleozoic metamorphic and Miocene-Quaternary clastic and carbonate host rocks. The current mineralogical composition indicates temperature conditions of 100–250 °C which close to current reservoir temperatures. Some of minerals with fossil geothermal origin, i.e. calcite, anhydrite, dolomite, celestine, amorphous silica and quartz, are compatible for mineral precipitations estimated from mineral equilibrium modeling, and scale mineralogy of wells, as well. The formation order of the most common geothermal minerals is determined as calcite → gypsum → anhydrite → quartz direction indicating that alkaline conditions were followed by acidic conditions. The blade-like/prismatic rhombohedral calcites replaced by quartz occurrences in siliceous‑carbonate veins indicate the boiling was occurred in the field. The geothermal mineral zoning determined from drilling samples is anhydrite-dolomite-calcite from shallow to deeper parts. The lateral and vertical distribution of mineral zoning is related to the fact that geothermal waters are mainly affected by host rock compositions, i.e. dissolution from the host rocks and precipitation along the cracks/fractures and bedding planes. Mineralizations in the Kızıldere geothermal field mainly represent the direct precipitations from hot geothermal waters rather than transformations of minerals in the host rocks. Si, Al, Mg, K and Na concentrations in carbonate and sulfate minerals show a positive correlation relationship and are derived from metapelites. Whereas Ca is negatively related to these elements and it is derived from metacarbonate and/or carbonate host rocks. According to the current geothermal water composition, Ca enriches in the deeper parts, while Mg and B enrichment in shallow depths near the basin edge that indicates the different composition of the host rocks where minerals precipitated. The relatively high boron contents at shallow depth indicate that it is retained by the minerals precipitated this level and causes less release to the surface.