Mineralogical, petrografical and geochemical investigation of the Pırnakapan evaporites (Aşkale, Erzurum)

In this study, the mineralogy, petrography and geochemistry of the Lower Miocene evaporites from Adilcevaz Formation exposed around Pırnakapan (Aşkale) area were investigated and thepaleoclimatic environments during their formation and source-origin relationships were revealed. The evaporitic units composed of gypsum, anhydrite, clayey-gypsum bearing limestone and clay-rich levels were determined as lenses and interbeds. In the evaporitic unit, secondary evaporitic lithofacies are described as laminated, banded, nodular, nodular-banded, chicken-wire and entrolithic. Additionally, alabastrine, porphyroblastic and satin spar textured secondary gypsum formed via hydration of anhydrite are identified. The gypsum nodules, banded or laminated structures show syn- and post-sedimentary deformational structures such as microfaults, slumps and undulations. Gypsum nodules up to 30 cm in diameter have composite structure with numerous small nodules. Based on the optical-, scanning electron- microscopy and X-ray diffraction studies, the main mineral is secondary gypsum accompanied by minor anhydrite, celestite, calcite, dolomite, lutecite llite/smectite and smectite.The whole-rock and isotope geochemistry of the evaporites are used as an important criteria to distinguish ancient marine types from non-marine types. The studied gypsum and anhydrite samples from different lithofacies display enrichment in LREE (LaN/LuN, 4.243-0.964) and differences in Ce/Ce* anomaly (1.283-0.859), Ni, Cu, Co, Fe and Mg contents implying continental input. The Sr contents of the samples are from 806 to 6464 ppm in gypsum, 1023-1197 ppm in anhydrite and 631-15150 ppm in claystone-mudstone-limestone suggesting importance of marine-derived Sr, but particularly high Sr contents are related to epigenetic celestite formation. Mineralogic, petrographic and geochemical evidences indicate Ca-sulphate precipitation in a shallow marine environment.Key Words: Pırnakapan, evaporite, secondary gypsum lithofacies, gypsum, anhydrite, celestite, geochemistry