The influence of meteoric water on skarn formation and late-stage hydrothermal alteration at the Evciler skarn occurrences, Kazdağ, NW Turkey

Abstract

Skarn ore deposits are generally characterized by two distinctly different alteration styles: an early prograde stage with anhydrous minerals, such as garnet (grossular/andradite) and pyroxene (diopside/hedenbergite), which forms in the presence of relatively high-temperature fluid and; a later retrograde stage with hydrous minerals, such as epidote, amphibole (actinolite), and chlorite (from most to least abundant) plus sulfides, which forms in the presence of the lower-temperature fluid. These two alteration stages commonly have been thought to reflect a dominance of magmatic and meteoric water, respectively, with relevance to the source of ore metals. We report data from the Evciler Au–Cu skarn deposit (Kazdağ, NW Anatolia). The aim of the analyses was to determine temperatures of formation, the extent of equilibration among the mineral phases and/or of possible alteration through fluid–mineral isotopic exchange. Stable isotope compositions of anhydrous and hydrous alteration minerals indicate that garnet (grossular/andradite)–pyroxene (diopside/hedenbergite) skarn (prograde stage) was produced by predominantly magmatic fluids during initial skarn-forming metasomatism in the study area and actinolite–epidote rich skarn (retrograde stage) was formed by magmatic water mixed with meteoric water. The δ 18O values of garnet from the Evciler district range from 4.6 to 8.2‰ (mean 6.7‰), δ 18O values of pyroxene range from 6.2 to 10.3‰ (mean 8.2‰), δ 18O values of amphibole range from 6.3 to 8.2‰ (mean 7.4‰), and the δ 18O values of epidote range from − 3.3 to 5.1‰ (mean 0.9‰). However, the δD values of retrograde alteration minerals such as amphibole and epidote indicate both magmatic and lighter values for Evciler skarn deposit that could be explained by mixing with meteoric water. We conclude that the isotopic evolution of the hydrothermal fluid can be accounted for by circulation of meteoric water through a convection system heated by the Evciler granitoid, causing exchange of oxygen isotopes with the granitoid and country rock, and possibly involving some admixture of magmatic water.