Magnetite is a common mineral in many ore deposits and their host rocks. It contains a wide range of trace elements that can be used to fingerprint deposit types and hydrothermal processes. In this study, we present detailed textural and compositional data on magnetite of the Mina Justa deposit in southern Perú to constrain the formation of iron oxides in the iron oxide Cu-Au (IOCG) deposit type.
Two types of magnetite, i.e., mushketovite (TM1) and granular (TM2) magnetite, are identified based on their morphology. Mushketovite shows three different zones (central bright, dark, and outer bright) in backscattered electron (BSE) images. The central bright part (TM1-1), characterized by abundant porosity and inclusions, was intensively replaced by dark magnetite of the median rim (TM1-2). The outer rim (TM1-3) is also bright but lacks porosity and inclusions. Granular magnetite (TM2) is anhedral and shows two different brightness levels (dark and bright) in BSE images. The dark (TM2-1) and bright (TM2-2) domains are intergrown, with irregular boundaries. In general, the dark zones of both magnetite types (TM1-2 and TM2-1) are characterized by higher Si, Ca, Al, and lower Fe contents than the bright zones. Additionally, the lattice parameters of the two types of magnetite are similar and slightly lower than that of pure magnetite, indicating that some cations (e.g., Si4+, Al3+) whose ionic radii are smaller than Fe2+ or Fe3+ may have entered into the magnetite structure by simple or coupled substitutions.
Our study shows that oxygen fugacity and temperature change are the dominant mechanisms leading to the formation of different types of magnetite at Mina Justa. Primary hematite, identified by Raman spectroscopy, was transformed into magnetite (TM1-1) due to a sharp decline of fO2 and then replaced by TM1-2 magnetite during temperature increase, followed by the formation of TM1-3 due to decreasing temperature, eventually forming the mushketovite with different zones. The granular magnetite may have originally precipitated from hydrothermal fluid that crystallized TM2-1 and also TM1-2 magnetite and was then modified by changing temperature and fO2 to form TM2-2. Even though the iron oxides in IOCG deposits may have formed in the same alteration stage, they could undergo a very complicate evolution process. Therefore, it is important to combine texture and mineral chemistry to investigate the origin and evolution history of iron oxides.