Geochemical, fluid inclusion and O–H–S isotope studies of the Milajerd Au-polymetallic prospect, Central Iran: Implications for ore genesis

Abstract

The Milajerd Pb-Zn ± Au polymetallic prospect in Naein-Qom Magmatic Zone is a favorable target for exploration. Geological and geochemical characteristics such as setting, alteration assemblages, fluid inclusion microthermometry and stable isotopes are consistent with the Milajerd Pb-Zn ± Au polymetallic prospect being a typical low-sulfidation (LS) epithermal deposits. The Au-polymetallic mineralization occurs mainly as structurally controlled veins, and brecciated dikes, hosted by calc-alkaline volcanic rocks. Mineralization at the Milajerd prospect is controlled by an annular fault system. The sulfide mineralogy of the deposit includes sphalerite, galena, pyrite, arsenopyrite, and marcasite. The principal gangue minerals are quartz, calcite, sericite, and adularia. The alteration assemblages include quartz ± pyrite, quartz-sericite-pyrite and chlorite-calcite-epidote. Fluid inclusions in quartz-sulfide veins have homogenization temperatures (Th) of 158–255 °C and salinities of 4.7–7.7 wt% NaCl equiv., whereas those in barren quartz ± carbonate veins yield Th values of 135–176 °C and salinities of 1.1–4.3 wt% NaCl equiv. The H and O isotopic composition (δDwater = − 120 to − 92‰; inferred δ18Owater = − 6.2 to 3.5‰) of fluid inclusions in quartz veins imply that cooling a mixed magmatic and meteoric waters was the main mechanisms of ore deposition at Milajerd deposit. The δ34S values of ore sulfides are mostly close to zero (avg. − 0.1‰), indicating a magmatic source of sulfur, which could have emanated directly from magmatic activity or have been leached from the surrounding volcanic rocks.