Microthermometry and noble gas isotope analysis of magmatic fluid inclusions in the Kerman porphyry Cu deposits, Iran: constraints on the source of ore-forming fluids

Abstract

This paper reports microthermometric and noble gas isotope data for fluid inclusion assemblages (FIAs) with evidence of phase separation, i.e. coexisting vapor-rich and halite-saturated inclusions, hosted in the early-formed quartz stockwork veins and post-magmatic quartz eye crystals in two economic porphyry Cu deposits (PCDs; Sar Cheshmeh and Miduk) and two sub-economic prospects (Sar Kuh and Abdar) from the Kerman porphyry copper belt (KPCB), Iran. The multiphase halite-saturated inclusions (i.e., Type I) in all studied PCDs and prospects had the highest homogenization temperature (Th = 525–594 °C) and salinities (63–73 wt% NaClequiv), whereas vapor-rich inclusions (Type II) had lower Th (362–460 °C). Fluid inclusion data show that like economic PCDs, the sub-economic prospects were formed in a fertile hydrothermal system and benefited from a mineralizing fluid, which evolved from a primary hot (mostly > 400 °C), metal-rich and oxidized fluid (as evidenced by the presence of opaque- and hematite-bearing fluid inclusions) of unknown salinity, which underwent a phase separation process to form both brine and vapor phases in the early stage of mineralization. The helium abundance and its isotopic composition document a mantle-derived magmatic source for the primary ore fluid in the formation of the studied PCDs and prospects (3He/4He ratios ranging from 0.46 to 2.8 Ra, corresponding to a mantle He contribution in ore fluids between ~ 7 and 45%). However, subsequent hydrothermal processes, i.e., vapor–brine phase separation, fluid-rock interaction with crustal rocks, and mixing with meteoric pore water containing dissolved atmospheric (e.g., Ne and Xe) and some crustal noble gases (e.g., Ar), changed the initial noble gas composition of the magmatic ore fluid to predominantly atmospheric- and crustal-like compositions. A significant proportion of mantle-derived He (up to 45%) in high-temperature (513–594 °C) and high-salinity (61.5–73 wt% NaClequiv) FIAs may indicate the existence of buried, economic, porphyry Cu mineralization in the Abdar prospect; therefore, it is suggested to be a possible target for further exploration. Comparing the He and Ar noble gas isotope composition in porphyry copper systems of different size and economic importance in this study showed that the ore-forming fluids of the outsized PCD (i.e., Sar Cheshmeh) have higher contributions of crustal-derived fluids characterized by predominantly radiogenic noble gas signatures (4He and 40Ar) than the smaller PCDs. This could have been achieved by a prolonged hydrothermal circulation in a large volume of crustal rocks containing radiogenic noble gases under a long-lived heat regime resulting from a deeply emplaced and slowly cooled composite intrusive body.