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

Our plain-English summary of Formation of giant iron oxide-copper-gold deposits by superimposed episodic hydrothermal pulses, by I. Del Real, M. Reich, A.C. Simon, et al (Scientific Reports, 2023).

The study

• Authors: I. Del Real, M. Reich, A.C. Simon, A. Deditius, F. Barra, M.A. Rodríguez-Mustafa, J.F.H. Thompson, M.P. Roberts
• Journal: Scientific Reports
• Published: 2023;13(1):12041
• Cited by: 5 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Iron oxide-copper-gold (IOCG) deposits are a globally important source of copper, gold and critical commodities. Despite their relevance, IOCG deposits remain an ill-defined clan, with a range of characteristics that has complicated development of the general genetic model. Here we focus on the Candelaria IOCG deposit in Chile and reveal that by using micro-textural and compositional variations in actinolite, a common alteration mineral found in many IOCG deposits, we can constrain the evolution of these systems. We demonstrate that Candelaria formed by the superposition of at least two pulses of mineralization with a late Cu-rich event overprinting and superimposed over an early, and probably higher temperature, iron oxide-apatite (IOA) mineralization event. These distinct pulses were likely caused by episodic injections of magmatic-hydrothermal fluids from crystallizing magmas at depth. Our data provide empirical evidence of grain-to-deposit scale compositional and potentially temperature changes in an IOCG system. The results support the use of actinolite chemistry as a novel approach to understand the formation of IOCG deposits and a potential tool for vectoring in exploration.

© 2023. The Author(s).

In plain English

This was a geology study, not a health study. It examined actinolite, a common mineral in iron oxide-copper-gold (IOCG) ore deposits, at the Candelaria deposit in Chile. The article does not discuss asbestos, fiber shape, or human exposure. The authors measured actinolite chemistry along a 1000 m drill core by electron probe microanalysis and calculated Fe#, the proportion of iron out of iron plus magnesium. Fe# ranged from about 0.19 to about 1. The Fe# of earlier actinolite textures decreased with depth, and the Fe# of later textures showed a broadly positive relationship with copper and gold grades. The compositions formed two distinct clusters. Higher titanium occurred mostly in early, copper-poor samples at depth, and lower titanium in actinolite linked to copper mineralization. The authors interpreted these differences as mainly reflecting temperature. They concluded that the deposit formed from distinct, episodic hydrothermal pulses: an early iron-rich one and a later copper-rich one. They stated that the timing between pulses is yet to be evaluated. A previous version of the article was retracted because of an error in applying an actinolite geothermometer.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Geological and mineralogical study of rock samples from an ore deposit (mineral chemistry and microscopy); not a health study
• Who was studied: Actinolite grains sampled from a 1000 m long drill core (LD1687) through the Candelaria iron oxide-copper-gold deposit, south of Copiapó, northern Chile. The number of samples is not stated in the main text; it is given in supplementary tables that are not included here.
• Exposure: No human or animal exposure was studied. The material analyzed was the mineral actinolite in drill core rock samples, measured by electron probe microanalysis. The article does not describe whether the actinolite was fibrous or asbestiform.
• Main results:
  • Calculated Fe# of actinolite ranged from about 0.19 to about 1. Fe# is the atomic fraction of iron out of iron plus magnesium.
  • The average Fe# of earlier actinolite textures decreased with depth down the drill hole.
  • The Fe# of later actinolite textures showed a broadly positive relationship with copper and gold grades.
  • In the deepest levels, actinolite cores had lower Fe# (0.2 to 0.4) and overgrowths had higher Fe# (0.6 to 0.75).
  • Fe# values from the chemical maps formed two distinct main clusters.
  • Higher titanium in actinolite occurred mostly in grain cores at depth in early, copper-poor samples. Lower titanium was typical of actinolite from the main-stage alteration associated with copper mineralization.
• The authors' conclusion: The authors concluded that world-class IOCG systems such as Candelaria are formed by distinct, episodic pulses of hydrothermal activity: an early, probably higher-temperature iron-rich pulse overprinted by a later copper-rich pulse. They interpreted the actinolite composition differences as predominantly related to temperature.
• Limitations the authors state:
  • Early and later actinolite could not easily be told apart by eye in hand samples from copper-bearing areas.
  • The textures seen in microscope images did not clearly correspond to the early versus copper-stage mineralization.
  • The time separating the two hydrothermal pulses has not yet been properly evaluated.

Topics

Medical terms in this study, explained in our Lexicon: Actinolite.

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