Abstract

The Chah Zard Ag-Au deposit, a typical breccia-hosted low- to intermediate-sulfidation epithermal system, is located within late Miocene andesitic to rhyolitic volcanic complex in the central part of the Urumieh-Dokhtar magmatic belt. The orebodies are emplaced in breccia bodies dominantly hosted by rhyolite porphyries. Systematic whole-rock geochemical investigations on the volcanic rocks show that both intermediate and felsic rocks are characterized by significant Large Ion Lithophile Elements (LILE) and Light Rare Earth Elements (LREE) enrichment coupled with High Field Strength Elements (HFSE) depletion. These geochemical data indicate subduction-related magmatic arc affinity for the volcanic rocks, and suggest that hornblende fractionation appears to be an important controlling factor on the evolution of mineralized subvolcanic rocks. Although the rhyolite porphyry has relatively high 87Sr/86Sr ratios, the volcanic rocks have similar Sr and Nd isotopic compositions, displaying 87Sr/86Sr range of 0.704910-0.705967 and εNd(i) values of +2.33 to +2.70. These data suggest that the rhyolitic magmas probably represent the final diffetentiates of parental andesitic magmas with minor crustal contamination. The andesitic magmas generated from partial melting of a mixture of an incompatible element depleted anhydrous asthenospheric mantle source and a hydrous LILE and LREE enriched lithospheric mantle source in response to slab-break-off and upwelling of asthenospheric mantle. The rhyolite porphyry is inferred to have supplied heat that drove the convective hydrothermal system at Chah Zard deposit, but also provided some of the fluid sources responsible for the development of the Chah Zard epithermal system.