IOCG deposits in this district are spatially and temporally associated with a north-northwest sinistral fault system and are interpreted to be coeval with a northwest transpressive deformation 6. The Candelaria-Punta del Cobre district is located south of the city of Copiapó in northern Chile and comprises more than nine active IOCG mines, all interpreted to be part of the same hydrothermal system 6 (Fig. Geology and mineralization in the Candelaria-Punta del Cobre district

This study provides the empirical evidence indicating a key role for successive magmatic fluid injections in the formation of IOCG deposits and provides insight on how iron oxide-apatite (IOA) and IOCG deposits genetically relate. In this contribution, we examine micro-textural and compositional variations of actinolite recovered along a ~1 km-long drill core to determine the thermal evolution of the Candelaria IOCG deposit. In the case of actinolite, its composition has been successfully used for estimating temperature of formation in IOA and Cu-Ni-PGE deposits 16, 23, 32, 33, 34, and preliminary data using actinolite in IOCG deposits suggests that it could be similarly useful 16, 35. Compositional variations in minerals have been used previously for temperature estimations, e.g., titanium content in zircons 24 or Mg-Fe silicate minerals in porphyry Cu-Au-Mo systems 25, 26, 27, 28, 29, 30, 31. Experimental data demonstrate that the thermal stability of actinolite depends on its Fe and Mg content, which decrease linearly over a wide P-T range, and is applicable as a proxy for estimating temperature of formation 23. One of the most common minerals found in both (early) Fe-rich and (later) Cu-rich mineralization stages in IOCG deposits is actinolite, (Ca 2)(Mg 4.5–2.2Fe 0.5–2.5)(Si 8O 22)(OH) 2. Understanding the temporal and spatial relationship between these two stages is critical to development of a universally applicable genetic model. In particular, the thermal and spatial evolution constraints of early Cu-poor and Fe-rich alteration and the main Cu(-Au) mineralization stages have not been addressed. Despite this progress, the thermal evolution of IOCG systems in space and time remains poorly constrained as IOCG deposits lack substantial quartz veining hindering the use of fluid inclusion data 18, 19, 20, 21, 22. Geochemical proxies, particularly the chemical composition and Fe-O-H-S isotopic signatures of selected silicates, magnetite, and pyrite, have provided some insights into the nature of the hydrothermal fluids 8, 9, 10, 11, 12, 13, 14, 15, 16, 17. Intrusions are inferred at depth but the magmatic events leading to mineralization are not well defined. Even though there is abundant evidence for magmatic signatures in IOCG deposits-geochemical and isotopic 1, 3, 7-most of these deposits show no apparent genetic relationship with exposed coeval igneous intrusions 1, 2, 3, 7.

Previous studies on important IOCG deposits, such as Olympic Dam (Australia), Candelaria (Chile), and Salobo and Sossego (Brazil), have shown that they are closely related to basement penetrating fault systems 1, 2, and have undergone an early magnetite-actinolite event followed by main-stage mineralization of copper and other elements 4, 5, 6. By-product strategic elements including Co, U and REE add to their attractiveness. Iron oxide-copper-gold (IOCG) systems are among the world’s richest mineral deposits, making for highly profitable mining operations 1, 2, 3. We suggest that the mineralisation stages we identify were the result of temperature gradients attributable to changes in the magmatic source, rather than variations in formation depth, and that actinolite chemistry can be used as a proxy for formation temperature in iron oxide-copper-gold systems.

We propose that these distinct stages were caused by episodic pulses of injection of magmatic-hydrothermal fluids from crystallizing magmas at depth. We identify at least two mineralization stages comprising an early 675–800 ☌ iron oxide-apatite type mineralization overprinted by a later copper-rich fluid at around 550–700 ☌. Here we investigate micro-textural and compositional variations in actinolite, to constrain the thermal evolution of the Candelaria iron oxide-copper-gold deposit in Chile. However, they possess a range of characteristics related to a variety of tectono-magmatic settings that make development of a general genetic model challenging.

Iron oxide-copper-gold deposits are a globally important source of copper, gold and critical commodities.

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