THE MONTECRISTO IRON OXIDE-COPPER-GOLD (IOCG) DISTRICT IN NORTHERN CHILE: NEW EVIDENCE ABOUT THE GENETIC RELATIONSHIP BETWEEN IOCG AND MAGNETITE-(APATITE) DEPOSITS

It has been debated for several decades if there is a genetic relationship between iron oxide-copper-gold (IOCG) and magnetite-apatite (MtAp) deposits, and if these deposits are of magmatic or hydrothermal origin or a combination of these two processes. The lack of occurrences globally showing both IOCG and MtAp events in the same deposit has made it difficult to understand this possible link between both types of mineralization, leading to conclusions that are usually based only in the local coexistence of mineralization assumed to belong to these types. The Montecristo district in northern Chile is an ideal natural laboratory to resolve this debate, where a vertical and lateral transition occurs between both styles of mineralization. The Abundancia IOCG deposit in the Montecristo district contains both mineralization types, often with crosscutting relationships, whereas the San Juan vein is a MtAp dyke and thought to be temporally and genetically related to the IOCG veins in the district. The identification of chronological and geochemical similarities between the IOCG and MtAp mineralization events in the Abundancia and San Juan deposits contributes to our understanding of the formation conditions and determine the existence of any genetic relationship between them. Textural, chronological, and crosscutting relationships suggest an early emplacement of the dioritic host rock, followed by a MtAp event in the San Juan and Abundancia deposits, and a later IOCG event in the Abundancia deposit. Electron Probe Micro Analyses on actinolite, magnetite, and apatite, from both types of mineralization in the Abundancia and San Juan deposits show similar mineral compositions, indicating that the formation of each mineral occurred under similar physico-chemical conditions. Sr-Nd radiogenic isotopes determine that the IOCG event has more crustal contribution compared to the MtAp event, being more similar to the host rocks, this suggesting a different origin for the MtAp and IOCG mineralization. Furthermore, sulfur isotopes from the IOCG event indicate mainly mantle derived sulfur (inherited sulfur) with some influence of non-magmatic sulfur (e.g., sedimentary rocks), or sulfur leached from the host rock, also consistent with a cortical contribution.