The Montecristo mining district, northern Chile: the relationship between vein-like magnetite-(apatite) and iron oxide-copper-gold deposits

The Montecristo district, northern Chile, is one of the few places worldwide where there is a direct relationship between magnetite-(apatite) (MtAp) mineralization and iron oxide-copper-gold (IOCG) mineralization. The MtAp mineralization includes Ti-poor magnetite, fluorapatite, and actinolite and is crosscut and partially replaced by a younger IOCG mineralization that includes a second generation of actinolite and magnetite with quartz, chalcopyrite, pyrite, and molybdenite. The MtAp stage at Montecristo is interpreted as the crystallized iron-rich melts that used the pre-existing structures of the Atacama Fault System as conduits. These rocks later acted as a trap for hydrothermal IOCG mineralization. Geochronology data at Montecristo indicate that the host diorite (U-Pb zircon 153.3 +/- 1.8 Ma, 2-sigma), MtAp mineralization (40Ar/39Ar in actinolite, 154 +/- 2 Ma and 153 +/- 4 Ma, 2-sigma), and the IOCG event (Re-Os on molybdenite, 151.8 +/- 0.6 Ma, 2-sigma) are coeval within error and took place in a time span of less than 3.4 Ma. The epsilon Hf-i and epsilon Nd-i values of the host diorite are + 8.0 to + 9.8 and + 4.3 to + 5.4, respectively. The whole-rock Sr-87/Sr-86(i) values of the IOCG mineralization (0.70425 to 0.70442) are in the lower end of those of the MtAp mineralization (0.70426-0.70629). In contrast, epsilon Nd-i values for the IOCG mineralization (+ 5.4 and + 5.7) fall between those of the MtAp rocks (+ 6.6 to + 7.2) and the host diorite, which suggests that the IOCG event was related to fluids having a more crustal Nd (epsilon Nd-i < + 5.7) composition than the MtAp mineralization. This likely reflects the mixing of Nd from the MtAp protolith and a deep magmatic-hydrothermal source, very likely an unexposed intrusion equivalent to the host diorite. Sulfur isotope compositions (delta S-34, + 0.3 to + 3.4 parts per thousand) are consistent with a magmatic source.

Automated summary

The Montecristo district in northern Chile is one of the few places where there is a direct relationship between magnetite-(apatite) mineralization and iron oxide-copper-gold mineralization. The two mineralizations, with different ages, suggest that the initial iron-rich melts crystallized and later acted as a trap for hydrothermal mineralization. The study also indicates a mixing of protolith and a deep magmatic-hydrothermal source in the formation of the iron oxide-copper-gold mineralization.