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Textural and compositional evolution of iron oxides at Mina Justa (Peru): Implications for mushketovite and formation of IOCG deposits

  • Xia Hu , Huayong Chen , Georges Beaudoin and Yu Zhang
Published/Copyright: March 1, 2020
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American Mineralogist
From the journal American Mineralogist Volume 105 Issue 3

Abstract

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.

Keywords: Iron oxides; mushketovite; texture; mineral chemistry; hydrothermal fluids; IOCG deposit

Acknowledgments

We thank Xiangping Gu (Central South University) and Changming Xing (Guangzhou Institute of Geochemistry, Chinese Academy of Sciences) for their help in the XRD and EMPA analyses. Discussion with Xiaoliang Liang and Wei Tan (Guangzhou Institute of Geochemistry, Chinese Academy of Sciences) improved the understanding of the structure of magnetite and hematite. We also acknowledge constructive comments and suggestions from Irene del Real Contreras and an anonymous reviewer, and editorial handling by M. Darby Dyar.

  1. Funding

    This study was funded by the National Natural Science Foundation of China (41572059 and U1603244) and the China Scholarship Council Fund (201804910485).

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Received: 2019-02-27
Accepted: 2019-11-15
Published Online: 2020-03-01
Published in Print: 2020-03-26

© 2020 Walter de Gruyter GmbH, Berlin/Boston

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  2. The distribution and abundance of halogens in eclogites: An in situ SIMS perspective of the Raspas Complex (Ecuador)
  3. Pressure dependence of Si diffusion in γ-Fe
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  11. Textural and compositional evolution of iron oxides at Mina Justa (Peru): Implications for mushketovite and formation of IOCG deposits
  12. Siwaqaite, Ca6Al2(CrO4)3(OH)12·26H2O, a new mineral of the ettringite group from the pyrometamorphic Daba-Siwaqa complex, Jordan
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https://doi.org/10.2138/am-2020-7024
Keywords for this article
Iron oxides; mushketovite; texture; mineral chemistry; hydrothermal fluids; IOCG deposit

Articles in the same Issue

  1. Heavy halogen geochemistry of martian shergottite meteorites and implications for the halogen composition of the depleted shergottite mantle source
  2. The distribution and abundance of halogens in eclogites: An in situ SIMS perspective of the Raspas Complex (Ecuador)
  3. Pressure dependence of Si diffusion in γ-Fe
  4. Seismic detectability of carbonates in the deep Earth: A nuclear inelastic scattering study
  5. Equations of state, phase relations, and oxygen fugacity of the Ru-RuO2 buffer at high pressures and temperatures
  6. Experimental determination of the phase relations of Pt and Pd antimonides and bismuthinides in the Fe-Ni-Cu sulfide systems between 1100 and 700 °C
  7. Layer stacking disorder in Mg-Fe chlorites based on powder X-ray diffraction data
  8. Elasticity of single-crystal Fe-enriched diopside at high-pressure conditions: Implications for the origin of upper mantle low-velocity zones
  9. XANES spectroscopy of sulfides stable under reducing conditions
  10. Zircon and apatite geochemical constraints on the formation of the Huojihe porphyry Mo deposit in the Lesser Xing’an Range, NE China
  11. Textural and compositional evolution of iron oxides at Mina Justa (Peru): Implications for mushketovite and formation of IOCG deposits
  12. Siwaqaite, Ca6Al2(CrO4)3(OH)12·26H2O, a new mineral of the ettringite group from the pyrometamorphic Daba-Siwaqa complex, Jordan
  13. Negevite, the pyrite-type NiP2, a new terrestrial phosphide
  14. Transjordanite, Ni2P, a new terrestrial and meteoritic phosphide, and natural solid solutions barringerite-transjordanite (hexagonal Fe2P–Ni2P)
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