A Continuum from Iron Oxide Copper-Gold to Iron Oxide-Apatite Deposits: Evidence from Fe and O Stable Isotopes and Trace Element Chemistry of Magnetite

• Published in: Economic Geology
• Main Authors: Rodríguez Mustafa, María A., Simon, Adam C., Real Contreras, Irene del, Thompson, John F.H., Bilenker, Laura D., Barra Pantoja, Fernando, Bindeman, Ilya
• Format: Article in Journal/Newspaper
• Language: English
• Published: Society of Economic Geologists (SEG) 2020
• Subjects: Bear magmatic zone; Cu-Au deposit; Kiruna-type; Hydrothermal alteration; Southeast Missouri; Chemical-analysis; IOCG deposits; IOA deposits; Ore genesis; Oxygen; Candelaria; Kiruna
• Online Access: https://doi.org/10.5382/econgeo.4752; https://repositorio.uchile.cl/handle/2250/178342

Iron oxide copper-gold (IOCG) and iron oxide-apatite (IOA) deposits are major sources of Fe, Cu, and Au. Magnetite is the modally dominant and commodity mineral in IOA deposits, whereas magnetite and hematite are predominant in IOCG deposits, with copper sulfides being the primary commodity minerals. It is generally accepted that IOCG deposits formed by hydrothermal processes, but there is a lack of consensus for the source of the ore fluid(s). There are multiple competing hypotheses for the formation of IOA deposits, with models that range from purely magmatic to purely hydrothermal. In the Chilean iron belt, the spatial and temporal association of IOCG and IOA deposits has led to the hypothesis that IOA and IOCG deposits are genetically connected, where S-Cu-Au-poor magnetite-dominated IOA deposits represent the stratigraphically deeper levels of S-Cu-Au-rich magnetite- and hematite-dominated IOCG deposits. Here we report minor element and Fe and O stable isotope abundances for magnetite and H stable isotope abundances for actinolite from the Candelaria IOCG deposit and Quince IOA prospect in the Chilean iron belt. Backscattered electron imaging reveals textures of igneous and magmatic-hydrothermal affinities and the exsolution of Mn-rich ilmenite from magnetite in Quince and deep levels of Candelaria (>500 m below the bottom of the open pit). Trace element concentrations in magnetite systematically increase with depth in both deposits and decrease from core to rim within magnetite grains in shallow samples from Candelaria. These results are consistent with a cooling trend for magnetite growth from deep to shallow levels in both systems. Iron isotope compositions of magnetite range from delta Fe-56 values of 0.11 +/- 0.07 to 0.16 +/- 0.05 parts per thousand for Quince and between 0.16 +/- 0.03 and 0.42 +/- 0.04 parts per thousand for Candelaria. Oxygen isotope compositions of magnetite range from delta O-18 values of 2.65 +/- 0.07 to 3.33 +/- 0.07 parts per thousand for Quince and between 1.16 +/- 0.07 and ...