Geochemistry of the Enjerd Skarn and Its Association with Copper Mineralization, Northwestern Iran

The Enjerd skarn is located 28 km northwest of Ahar in East Azarbaijan Province, northwestern Iran. Based on mineral composition, paragenesis, and mineral relationships, two stages of hydrothermal alteration and related mineralization have been identified. Stage I was produced by early pulses of Fe-rich, magmatically derived fluids coincident with potassic alteration of the main stock. This event caused the formation of oscillatory zoned garnets, followed by pyroxene (diopside-heden-bergite) + wollastonite + magnetite and rare MoS at high temperatures (∼445°C). Molybdenite was the only sulfide mineral deposited at this stage. Stage II occurred at a lower temperature (∼360°C), and is characterized by tremolite-actinolite, epidote, quartz, calcite, apatite, titanite, and hematite crystallization and by copper deposition. Copper mineralization was due to decreasing fO2 and increasing pH as temperatures fell below 400°C. Fluctuations in the Al+3/Fe+3 ratio of hydrothermal fluids at the Enjerd skarn resulted in complex oscillatory zoning in the garnets from andradite100 to andradite76-grossular24. Individual zones are typically composed either of near-end-member andradite or andradite containing approximately 20 mole % grossular. Introduction of fresh batches of Fe-rich fluid caused crystallization of near-end-member andradite garnet because the system was buffered by the fluid. The Al in the system was derived from Cretaceous marls and limestone sequences, because the Al available from granitoid magmatic solutions was very low and almost constant.