Abstract

The breccia-hosted epithermal Ag-Au deposit of Chah Zard is located within a high K, calc-alkaline andesitic to rhyolitic volcanic complex in central part of Urumieh-Dokhtar Magmatic Arc (UDMA), Iran. U–Pb zircon geochronology yields ages between 6.29±0.29 and 6.19±0.24 Ma, and a mean age of 6.24±0.16 Ma for magmatic activities at Chah Zard. Systematic geochemical investigations on the volcanic rocks show that they 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 data indicate magmatic arc affinity for the volcanic rocks. Hornblende fractionation was an important controlling factor on the evolution of mineralized subvolcanic rocks. Breccias and veins formed during and after the waning stages of strong explosive eruption of magmatic-hydrothermal and phreatomagmatic brecciation events due to shallow emplacement of the rhyolite porphyry. Detailed systematic mapping leads to the recognition of three distinct breccia bodies: the volcaniclastic breccia, the polymict breccia, and the mono-polymict breccia. The polymictic breccias generated bulk-mineable ore, whereas the volcaniclastic breccia is relatively impermeable and largely barren. Hydrothermal alteration assemblages, covering an area about 9 km², are zoned around the breccias and veins, consisting of secondary quartz, illite, pyrite, adularia, chlorite, various carbonate minerals, and minor K-feldspar. Iron oxides-hydroxides, jarosite, gypsum, kaolinite, halloysite and rare alunite are the supergene alteration minerals. Precious metals occur with sulfide and sulfosalt minerals as disseminations, as well as in the veins and breccia cement. There is a progression from pyrite-dominated (stage 1) to pyrite-base metal sulfide and sulfosalt-dominated (stages 2 and 3) to base metal sulfide-dominated (stage 4) breccias and veins. Deposition of gangue minerals progressed from illite-quartz to quartz-adularia, carbonate and finally gypsum-dominated assemblages. Free gold occurs in stages 2 and 4, principally as intergrown with pyrite, quartz, chalcopyrite, galena, sphalerite, and Ag-rich tennantite-tetrahedrite; and also as inclusions in pyrite. The U-Pb zircon age of 6.19±0.24 Ma for emplacement of the rhyolitic magmas represents the maximum age of mineralization at Chah Zard. It may indicate that there was a previously unrecognized mineralization event in UDMA at this time.