Genesis of hydrothermal stockwork-type magnesite deposits associated with ophiolite complexes in the Kutahya-Eskisehir region, Turkey

Abstract

We report field observations and C and O isotopic compositions of two magnesite deposits within the Lower Cretaceous Dagkuplu ophiolite complex, Eskisehir-Kutahya region, Turkey, to help interpret their genetic significance. The Nemli and Margi deposits are large microcrystalline stockwork-type magnesite systems within the upper part of highly fractured and imbricated ultramafic complexes containing fresh dunite, harzburgite and serpentinite. Magnesite in ultramafic rocks occurs either as stockwork veins or as irregular masses. Centimeter-scale chalcedony bands are observed within magnesite veins. Some serpentinite fragments and platelets are rimmed by fibrous magnesite, indicating significant alteration of the host rock along the WNW-ESE trending Eskisehir fault zone, which is subparallel to the northern edge of Neogene lacustrine sediments. The magnesite deposits are related to tectonic activity along this fracture zone and Middle Miocene calc-alkaline volcanism.

The magnesite samples have δC-13(V-PDB) values varying between -9.1 and -13.7&PTSTHOUSND;, and δO-18(V-SMOW) values between +25.6 and +28.0&PTSTHOUSND;. These data suggest that the mineralized fluids formed from highly evaporated Miocene take water, which migrated through limestone and was heated by nearby hot dikes and volcanic rocks. These fluids then ascended through fractures to the surface. The intensity of magnesite veining can be correlated with the volume of fluid flow of evaporative lake waters, which had passed through fracture systems, and their CO2 contents. The CO2 in MgCO3 was derived from (1) decarboxylation of shale and phyllite enriched in organic matter, and (2) dissolution of limestone at high temperature about 2 kin beneath the surface. Our data suggest that the circulation Of CO2-rich geothermal waters between 65-80° C converted serpentinite to magnesite and opal-CT in the highly fractured veins along the E-W trending main fault zone, which runs subparallel to the Tethyan suture zone.