Formation of the Zarshuran gold deposit in northwest Iran resulted from the mixing of fertile magmatic fluids with basinal brines: Evidence from sulfide trace elements and sulfur isotopes

Schematic diagram illustrating the evolution of the metamorphic core complex and gold mineralization in the Zarshuran deposit. • Five stages of mineralization (pre-ore stage, ore-stage Ⅰ, ore-stage Ⅱ, ore-stage Ⅲ, and late-ore stage) have been identified in the Zarshuran deposit. • Au mineralization primarily occurred during the Miocene. • The ore-forming fluid was formed by mixing of magmatic fluids with basinal brines during the exhumation of the Iman Khan metamorphic core complex. Zarshuran is the largest gold deposit in the Middle East. However, the sources of ore-forming materials at Zarshuran remain poorly understood. This study presents detailed descriptions of paragenetic sequence, in situ calcite U-Pb data, sulfide trace elements and sulfur isotope data to trace the sources of ore-forming materials and track the ore-forming processes. Calcite U-Pb results suggest the gold mineralization occurred during ca. 21–12.0 Ma. Five stages of mineralization can be divided at Zarshuran. They are characterized by pyrite grains with different textures. LA-ICP-MS analyses of the different types of pyrite demonstrate that Au is enriched in ore-stage Ⅰ, ore-stage Ⅱ, and late-ore stage. LA-ICP-MS analyses of the different types of sphalerite show an increasing trend for In and Sn contents but a decreasing trend for Ge and Tl contents from Sp1a to Sp1c. The S isotope compositions of sulfides from different stages yield positive values, suggesting these sulfides were precipitated by thermochemical sulfate reduction. Most δ 34 S values of sulfide minerals of the ore-stages are in the ranges between magmatic sulfur and redbeds sulfates sulfur, suggesting contributions of magmatic and basinal materials during mineralization. The shift of δ 34 S values during late-ore stage indicates the addition of Miocene seawater sulfates sulfur. We propose the Zarshuran gold mineralization was formed by mixing of magmatic fluids with basinal brines during the exhumation of the Iman Khan metamorphic core complex (MCC). The development of MCC and coeval fertile magmatism and basinal brine flow are important for exploration of Zarshuran-type gold ores.