U-Pb geochronology and Pb isotope characteristics of Chah Zard Ag-Au epithermal deposit, West-Central Iran

The Chah Zard Ag-Au epithermal deposit is located within an andesitic to rhyolitic volcanic complex in central part of Urumieh-Dokhtar Magmatic Arc (UDMA), west- central Iran. The deposit is formed in breccia bodies dominantly hosted by rhyolite porphyries. Geological field relationships show that epithermal mineralization occurred in close association with a phreatomagmatic fracturing event, being localized in breccia cements, and formed as disseminated sulfides in the matrix of the more permeable breccias, and in veins cutting breccia bodies. No volcanic and/or subvolcanic flow and domes have been observed to cross-cut zones of alteration, phreatomagmatic breccias or veining. The volcanic rocks are high K, calc-alkaline, and show rare earth and trace elements features characteristic of arc magmatism. Zircons extracted from intermediate and felsic volcanic rocks of the Chah Zard deposit yield concordant U-Pb ages of 6.29±0.29, 6.25±0.29 and 6.19±0.24 Ma, and a mean age of 6.24±0.16 Ma. This age is interpreted to represent the maximum age of mineralization of the Chah Zard deposit, and may indicate that there was a previously unrecognized mineralization event in UDMA. Galena mineral separates from the breccia cements and veins have uniform lead isotope ratios of ²⁰⁶Pb/²⁰⁴Pb=18.71-18.77, ²⁰⁷Pb/²⁰⁴Pb=15.61-15.73, and ²⁰⁸Pb/²⁰⁴Pb=38.95-39.01, implying derivation of Pb from a homogeneous upper crustal source.