Origin of galena and sphalerite mineralization in the Koru Volcanics, Biga Peninsula, Turkey: Insights from MELTS modelling and SUPCRT calculations

Tertiary volcanism (34.3 ± 1.2 Ma and 0.4 ± 0.7 Ma) in NW Turkey produced lead + zinc bearing trachyte, latite, dacite and rhyodacite tuffs, and agglomerates, as well as basaltic andesite, andesite, dacite, and rhyolite lavas. The primary objective of this study is to develop quantitative models for the evolution of magmas that produced these rocks and for the dissolution, transport, and deposition of disseminated ore as sphalerite and galena vein deposits. Our petrogenetic modelling using the MELTS algorithm (Ghiorso, M.S., and Sack, R.O., 1995, Chemical mass transfer in magmatic processes IV: a revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid-solid equilibria in magmatic systems at elevated temperatures and pressures: Contributions to Mineralogy and Petrology, v. 119, p. 197-212) indicates that the ore-bearing volcanic rocks reflect mixing of a basaltic andesite parental magma with a rhyolite magma. In addition, our aqueous geochemistry modelling using the SUPCRT indicates that controlling parameters of the vein deposits were pH = 5.3-9.2, fO2 = 10 ^-55-10^-46 at 150°C and + S = 10^-3 molal. At 250°C, compared with 150°C, the mineral stability field is more oxidized, yet the pH range is not significantly different (pH = 5.8-9.8, fO2 = 10^-34-10^-43).