Formation of the Mantoverde iron oxide-copper-gold (IOCG) deposit, Chile: insights from Fe and O stable isotopes and comparisons with iron oxide-apatite (IOA) deposits

Abstract

The Mantoverde iron oxide-copper-gold (IOCG) deposit, Chile, contains hundreds of millions of tonnes (Mt) of mineable iron oxide and copper sulfide ore. While there is an agreement that mineralization at Mantoverde was caused by hydrothermal fluid(s), there is a lack of consensus for the role(s) that non-magmatic vs. magmatic fluid(s) played during the evolution of the mineralized system. In order to overcome the hydrothermal overprint at Mantoverde, which is known to disturb most conventional stable isotope systems (e.g., oxygen), we report the first delta Fe-56 and delta O-18 pairs for early-stage magnetite and late-stage hematite that provide information on the source reservoir of the hydrothermal fluids. Magnetite delta Fe-56 values range from 0.46 +/- 0.04 to 0.58 +/- 0.02 parts per thousand and average 0.51 +/- 0.16 parts per thousand (n= 10; 2 sigma). Three hematite delta Fe-56 values were measured to be 0.34 +/- 0.10, 0.42 +/- 0.09, and 0.46 +/- 0.06. Magnetite delta O-18 values range from 0.69 +/- 0.04 to 4.61 +/- 0.05 parts per thousand and average 2.99 +/- 2.70 parts per thousand (n= 9; 2 sigma). Hematite delta O-18 values range from - 1.36 +/- 0.05 to 5.57 +/- 0.05 parts per thousand and average 0.10 +/- 5.38 parts per thousand (n= 6; 2 sigma). These new delta Fe-56 and delta O-18 values fingerprint a magmatic-hydrothermal fluid as the predominant ore-forming fluid responsible for mineralization in the Mantoverde system.