The Montecristo mining district, northern Chile: the relationship between vein-like magnetite-(apatite) and iron oxide-copper-gold deposits

Our plain-English summary of The Montecristo mining district, northern Chile: the relationship between vein-like magnetite-(apatite) and iron oxide-copper-gold deposits, by L. Mateo, F. Tornos, J.M. Hanchar, et al (Mineralium Deposita, 2023).

The study

• Authors: L. Mateo, F. Tornos, J.M. Hanchar, I.M. Villa, H.J. Stein, A. Delgado
• Journal: Mineralium Deposita
• Published: 2023;58(6):1023-1049
• Read it: PubMed · Publisher · Free full text

Abstract

Unlabelled: The Montecristo district, northern Chile, is one of the few places worldwide where there is a direct relationship between magnetite-(apatite) (MtAp) mineralization and iron oxide-copper-gold (IOCG) mineralization. The MtAp mineralization includes Ti-poor magnetite, fluorapatite, and actinolite and is crosscut and partially replaced by a younger IOCG mineralization that includes a second generation of actinolite and magnetite with quartz, chalcopyrite, pyrite, and molybdenite. The MtAp stage at Montecristo is interpreted as the crystallized iron-rich melts that used the pre-existing structures of the Atacama Fault System as conduits. These rocks later acted as a trap for hydrothermal IOCG mineralization. Geochronology data at Montecristo indicate that the host diorite (U-Pb zircon 153.3 ± 1.8 Ma, 2-sigma), MtAp mineralization (40Ar-39Ar in actinolite, 154 ± 2 Ma and 153 ± 4 Ma, 2-sigma), and the IOCG event (Re-Os on molybdenite, 151.8 ± 0.6 Ma, 2-sigma) are coeval within error and took place in a time span of less than 3.4 Ma. The εHfi and εNdi values of the host diorite are + 8.0 to + 9.8 and + 4.3 to + 5.4, respectively. The whole-rock 87Sr/86Sri values of the IOCG mineralization (0.70425 to 0.70442) are in the lower end of those of the MtAp mineralization (0.70426-0.70629). In contrast, εNdi values for the IOCG mineralization (+ 5.4 and + 5.7) fall between those of the MtAp rocks (+ 6.6 to + 7.2) and the host diorite, which suggests that the IOCG event was related to fluids having a more crustal Nd (εNdi < + 5.7) composition than the MtAp mineralization. This likely reflects the mixing of Nd from the MtAp protolith and a deep magmatic-hydrothermal source, very likely an unexposed intrusion equivalent to the host diorite. Sulfur isotope compositions (δ34S, + 0.3 to + 3.4‰) are consistent with a magmatic source.

Supplementary Information: The online version contains supplementary material available at 10.1007/s00126-023-01172-0.

© The Author(s) 2023.

In plain English

This is a geology study, not an asbestos or health study. The authors examined two veins in the Montecristo mining district, northern Chile: the mined Abundancia vein, with both magnetite-(apatite) (MtAp) and iron oxide-copper-gold (IOCG) mineralization, and the unmined San Juan vein, with MtAp only. The authors analyzed mineral chemistry, including actinolite, and dated the rocks (uncertainties at 2-sigma). Zircon from the host diorite gave 153.3 plus or minus 1.8 million years. MtAp actinolite gave 154 plus or minus 2 and 153 plus or minus 4 million years. Two IOCG molybdenite samples gave 151.8 and 162.4 plus or minus 0.6 million years, and the authors called the older age geologically unrealistic. They judged the events coeval within error, taking place in less than 3.4 million years. Two of three IOCG samples had lower neodymium isotope values (+5.4, +5.7) than the MtAp rocks, suggesting more crustal fluids. The authors interpreted the third value (+7.0) as inherited from the MtAp rock. Sulfur isotopes were consistent with a magmatic source. The authors concluded that their results suggest the two events were genetically independent, though the wider relationship remains debatable.

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 field, mineralogical, geochemical and geochronological (rock-dating) study of ore veins; no human or animal subjects
• Who was studied: Rock and mineral samples from two ore veins in the Montecristo mining district, northern Chile: the currently mined Abundancia vein (MtAp and IOCG mineralization) and the unmined San Juan vein (MtAp only)
• Exposure: Not applicable: no human or animal exposure was studied. The minerals analyzed included actinolite, magnetite, apatite, zircon, molybdenite and copper and iron sulfides, as parts of ore deposits.
• Main results:
  • Zircon from the host diorite (12 grains) gave a Concordia age of 153.3 plus or minus 1.8 million years (2-sigma)
  • Actinolite from the MtAp stage gave plateau ages of 154 plus or minus 2 million years in the Abundancia vein and 153 plus or minus 4 million years in the San Juan vein (both 2-sigma)
  • Two molybdenite samples from the IOCG stage gave Re-Os ages of 162.4 plus or minus 0.6 and 151.8 plus or minus 0.6 million years (2-sigma). The authors called the older age geologically unrealistic and judged redistribution of Re and Os within the molybdenite the most plausible explanation.
  • The authors judged the diorite, MtAp and IOCG events to be coeval within error, taking place in less than 3.4 million years, a span they said cannot be temporally resolved with geochronology
  • Three whole-rock IOCG samples gave initial epsilon-Nd values of +5.7, +5.4 and +7.0 and initial strontium ratios of 0.70425 to 0.70442. The authors read the lower values as reflecting fluids with a more crustal Nd composition than the MtAp mineralization, and they interpreted the +7.0 value as inherited from the MtAp stage.
  • Sulfur isotopes in fifteen chalcopyrite and pyrite aliquots from the IOCG event ranged from +0.3 to +3.4 per mil, with a mean of +1.8 per mil
  • IOCG-stage magnetite (magnetite II) averaged 406 ppm copper, compared with 50 to 64 ppm in MtAp-stage magnetite (magnetite I)
• The authors' conclusion: The authors concluded that, although the genetic relationship between MtAp and IOCG deposits remains debatable, their results suggest the IOCG and MtAp mineralization events in the Montecristo district are genetically independent of each other
• Limitations the authors state:
  • The whole-rock strontium and neodymium samples from the IOCG stage might contain minor contamination from the MtAp stage
  • Zircon uranium contents were very high and the analyses showed discordance, so only 12 grains with less radiation damage were kept for the age calculation
  • The authors acknowledged that their data might lead to different interpretations
  • Erosion has removed any mineralization that originally lay above the veins

Topics

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

Related studies

• Formation of giant iron oxide-copper-gold deposits by superimposed episodic hydrothermal pulses — I. Del Real, M. Reich, et al. (2023)
• Excess mortality from stomach cancer, lung cancer, and asbestosis and/or mesothelioma in crocidolite mining districts in South Africa — J.L. Botha, L.M. Irwig, et al. (1986)
• Re: "Excess mortality from stomach cancer, lung cancer, and asbestosis and/or mesothelioma in crocidolite mining districts in South Africa" — H.P. Hart (1987)
• A review of mortality associated with elongate mineral particle (EMP) exposure in occupational epidemiology studies of gold, talc, and taconite mining — J.H. Mandel, B.H. Alexander, et al. (2016)
• Participation of nitric oxide and iron in the oxidation of DNA in asbestos-treated human lung epithelial cells — C.C. Chao, S.H. Park, et al. (1996)
• Regulation of nitric oxide synthase induction by iron and glutathione in asbestos-treated human lung epithelial cells — S.H. Park, A.E. Aust (1998)

Browse return to medical research.