Constraints on ore formation conditions at the Mazra’eh Shadi epithermal deposit, NE Tabriz, Iran: evidences from geochemistry, sulphur isotope, quartz textures and fluid inclusion studies

Abstract

The Mazra’eh Shadi deposit is one of the most representative gold deposits in the Ahar-Arasbaran Belt. The main minerals are galena, sphalerite, pyrite and chalcopyrite. Concentrations of Au-Ag occur mainly within quartz veins. Five textures (crustiform, comb, microcrystalline, cockade, and mosaic) are distinguished by field reconnaissance and hand specimen observations. The d34S values suggest an increasing role of meteoric water from the deepest levels to the shallow level and surface. Fluid inclusion data show that the mineralisation at the Mazra’eh Shadi deposit can be classified as a volcanic-rock-hosted intermediate-sulphidation epithermal deposit. Fluid inclusions in vein quartz can be distinctly divided into three types according to interpretation of petrographic features: intense boiling, gentle boiling and non-boiling conditions. The presence of intense and gentle boiling among different substages at the same level in the Mazra’eh Shadi deposit indicates that the base of the boiling zone likely shifted upward and downward during vein formation. The concentrations of Au-Ag occur mainly within quartz veins in the shallow level with gentle boiling (max. 813 ppb Au) and with intense boiling (max. 2420 ppb Au), whereas lower Au-Ag concentrations are associated with base metal-rich (Pb-Zn) in the deepest levels with non-boiling fluids (max. 52 ppb Au).