Fluid evolution in H2O–CO2–NaCl system and metallogenic analysis of the Surian metamorphic complex, Bavanat Cu deposit, Southwest Iran

Abstract

The Bavanat Cu deposit occurs as veins controlled by a NE–trending structure within the Permo–Triassic Surian metamorphic complex (SMC), southwest of Iran. The SMC rocks exposed in the area have undergone greenschist-facies metamorphism. The ore-forming process can be divided into early, middle, and late stages, represented by, respectively, pyrite–quartz, polymetallic sulfide–quartz, and late–stage barren quartz veins. Systematic studies of fluid inclusions (FIs) in the quartz veins found four types: aqueous, mixed aqueous–carbonic, carbonic, and multiphase–bearing inclusions. The FIs of early, middle and late–stages are mainly homogenized at temperatures of 335–417 °C, 230–380 °C, and 190–227 °C, with salinities of 1.1–6.7, 2.9–36.6, and 0.8–2.6 wt.% NaCl equivalent, respectively. The main stage of Cu mineralization is related to the middle–stage, where FIs show evidence of fluid immiscibility. The metal precipitation resulted from a decrease in copper solubility during the fluid immiscibility, cooling, crystallization of multiphase–bearing inclusions, and a small increase in pH. Laser Raman spectroscopy and FIs evidences indicate that the metallogenic system evolved from metamorphic CO2 (+CH4)–rich, relatively high fO2 (10−25 to 10−29 bars) to CO2–poor and relatively low fO2 (10−31 to 10−34 bars). Muscovite from the middle–stage veins yields 40Ar/39Ar plateau age of 195.2 ± 1.0 Ma, suggesting that the Cu mineralization at Bavanat formed in the Early Jurassic coeval with the retrograde metamorphic events during the post–early Cimmerian orogeny.