Abstract

Chah Zard in the central part of the Urumieh-Dokhtar Magmatic Arc (UDMA) of Iran is a breccia-hosted low- to intermediate-sulfidation epithermal deposit (~2.5 Mt @1.7 g/t Au and 12.7 g/t Ag). The deposit is hosted in a 6.2±0.2 Ma volcanic complex in the UDMA and was formed by relatively low temperature, dominantly magmatic fluids at shallow epithermal environment. Fluid inclusion data reveal the presence of two fluids with different salinities during the breccia and vein paragenesis. The saline fluid was present only during pre-ore stage, whereas the other moderately saline liquid is thought to be the main ore-transporting media during ore stages. Mineralization at Chah Zard occurred in two stages. Fluids in the early stage Au mineralization were relatively hotter (280–345 °C) with a slightly higher salinity (12.7 wt. % NaCl equiv.), relative to fluids in the late stage Ag-bearing base metal mineralization (262–311 °C; 10.9 wt. % NaCl equiv.). These conditions are typical of LS to IS epithermal environment, with salinity decreasing from early to late stage. Boiling has occurred at Chah Zard, which may have been an important mineralization mechanism for Au and Ag. Direct evidence for boiling is the coexistence of primary and pseudosecondary LV and VL fluid inclusions in ore-stage quartz and sphalerite, as well as the presence of adularia and bladed calcite in breccia and veins. Extensive hydrothermal brecciation in the ore-zones may have been caused by explosive phase separation, or flashing. Mixing between magmatic fluids and meteoric water has also been important to ore deposition at Chah Zard. Penetration of meteoric water toward deeper levels via breccia and faults may have caused temperature decrease and pH increase of the metal-bearing hydrothermal fluids. Such changes may have contributed to the precipitation of base metal sulfides.