A Preliminary Study on the Aghbolaq (Fe, Cu) Skarn Deposit, Oshnavieh, NW Iran: Constraints on Metasomatic Fluid Evolution

Abstract

AbstractThe Aghbolaq skarn deposit is located in the Urumieh‐Golpayegan plutonic belt, NW Iran. The garnetite skarn (stage I) has been intensely cross‐cut by the magnetite‐garnet skarn (stage II) which were, in turn, cut and offset by the ore‐hosting quartz veins/veinlets (stage III). The predominance of andradite (Adr82.5–89.1) and its high Fe3+/Al ratio (up to 1685) apparently supports the high fO2, salinity and prevalence of magmatic/hydrothermal fluids involved, rather than meteoric waters, during the magnetite‐garnet skarn formation. Two major groups of fluid inclusions, namely aqueous (LV, LVS) and aqueous–carbonic (LVC, LLCVC), were recognized in garnet and quartz veins that, especially in growth zones and along intra‐granular trails, better display fluid inclusion assemblages (FIAs) than those in clusters. The prograde magnetite‐garnet skarn was formed by the metasomatic fluid at relatively high Th (209–374°C), under a lithostatic pressure of ∼200 bars. The retrograde mineralized quartz veins were formed at temperatures ranging from 124°C to 256°C, by dilute and less saline (2.57–11.93 wt% NaCl eq.) hydrothermal fluids under a hydrostatic pressure of ∼80 bars. The fluid evolution of the Aghbolaq skarn began with an earlier simple cooling of metasomatic fluid during the prograde stage, followed by the later influx of low salinity meteoric fluids during the retrograde stage.