Geochemical, geochronological, and textural relationships between magnetite-(apatite) and iron oxide-copper-gold mineralization, Montecristo district, northern Chile

Abstract

The genetic relationship, if one exists, between magnetite-(apatite) (MtAp) and iron oxidecopper- gold (IOCG) deposits has been debated for many years, and to this day, few representative places have been found where both types of mineralization occur. The Jurassic age Montecristo district in northern Chile is one of a few places in the world where both MtAp and IOCG mineralization occur with vertical and crosscutting relationships. Consequently, this is one of the best sites to better understand the possible relationship between these two types of mineralization and their formation, and is the main objective of this thesis project. For this project, the samples studied were selected from: (1) a MtAp vein (the San Juan vein), comprising magnetite, and pegmatite-like fluorapatite, and actinolite; (2) a vein with MtAp mineralization in the deeper parts of the system, and an overprinting IOCG assemblage of chalcopyrite, pyrite, magnetite, actinolite, quartz, gold, and molybdenite, in the shallower parts of the system (the Abundancia vein); and (3) the host rock that contains both types of mineralization. The results of this study reveal that the host diorite, the MtAp mineralization in the Abundancia and San Juan veins, and the IOCG mineralization, are all coeval within error, and that their genetic relationship cannot be discriminated by geochronology alone. However, intermediate εNdi values of the IOCG event suggest a mixing of Nd from the MtAp rocks and a deep magmatic-hydrothermal source. Hf, Sr–Nd, and sulfur isotopes support a model in which the IOCG mineralization was derived from magmatic-hydrothermal fluids. We propose that an ascending, immiscible iron-rich melt utilized structures of the Atacama fault system as pathways for the deposition of the MtAp mineralization, followed by precipitation of the IOCG sulfidemagnetite- actinolite-quartz-gold assemblage from magmatic-hydrothermal fluids that used the earlier MtAp rocks as a trap.