Formation of giant iron oxide-copper-gold deposits by superimposed, episodic hydrothermal pulses

Iron oxide-copper-gold deposits are a globally important source of copper, gold and critical commodities. However, they possess a range of characteristics related to a variety of tectono-magmatic settings that make development of a general genetic model challenging. 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. We identify at least two mineralization stages comprising an early 675–800 °C iron oxide-apatite type mineralization overprinted by a later copper-rich fluid at around 550–700 °C. We propose that these distinct stages were caused by episodic pulses of injection of magmatic-hydrothermal fluids from crystallizing magmas at depth. 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.