Origin of the ore-forming fluids of the Zefreh porphyry Cu–Mo prospect, central Iran: Constraints from fluid inclusions and sulfur isotopes

The Zefreh porphyry Cu–Mo prospect is located in the central part of the NW-trending Cenozoic Urumieh–Dokhtar Metallogenic Belt, 75 km NE of Isfahan city, central Iran. Chalcopyrite and molybdenite mainly occur as disseminated sulfides in the host rocks and, to a lesser extent, in the surrounding volcanic rocks. Four types of fluid inclusions were identified in quartz-rich veins: i.e., two-phase liquid-rich, two-phase vapor-rich, simple brine, and multi-phase brine inclusions. Liquid-rich inclusions in ore-bearing, A-type quartz veins homogenized between 350° and 370 °C, suggesting that fluids trapped in these inclusions were derived from magmatic fluids at high temperatures. Simple and multi-phase brine inclusions exhibited homogenization or partial homogenization over a wide range from 350° to 1209 °C. The very high homogenization temperatures and the presence of up to six solid phases in multi-phase brine inclusions indicate heterogeneous trapping of melt, fluid, and vapor. The Zefreh porphyry prospect is thought to have formed from hypersaline liquids and vapors that exsolved from a magmatic intrusion assembled in the shallow crust. The δ³⁴S values of sulfides from the Zefreh porphyry prospect range from +0.8‰ to +10.7‰. This indicates that the early, ore-bearing quartz veins dominated by high-δ³⁴S pyrites may have interacted with later hydrothermal fluids, or alternatively, the pyrites are paragenetically late products of propylitic and K-feldspar-dominated potassic alteration assemblages. Another plausible explanation for significantly enriched δ³⁴S values of sulfides may be mixing of hydrothermal fluids of magmatic origin with the Cenozoic seawater (δ³⁴S ≈ +22‰). It is suggested that the Zefreh prospect likely would be a poor target for further exploration.