Details
| Original language | English |
|---|---|
| Pages (from-to) | 78-93 |
| Number of pages | 16 |
| Journal | Geochimica et cosmochimica acta |
| Volume | 388 |
| Early online date | 26 Nov 2024 |
| Publication status | E-pub ahead of print - 26 Nov 2024 |
Abstract
The genesis of high-grade Fe-Ti-V oxide ores (up to >90 vol%) in layered intrusions remains highly debated. Here, on example of Hongge layered intrusion in China, we show that hydrothermal dissolution and precipitation of Fe-Ti-V oxides played a critical role in forming high-grade massive ore deposits as demonstrated by textural-compositional evidence and in-situ iron isotope data (δ56Fe), analyzed with femtosecond laser ablation multicollector (LA-MC-) ICP-MS. Hongge is a mafic layered intrusion composed of a Lower olivine clinopyroxenite Zone (LZ), a Middle clinopyroxenite Zone (MZ), where thick massive ore layers (with up to 90 % Fe-Ti-V oxides) formed, and an Upper gabbro Zone (UZ). Magnetite in Hongge exhibits two contrasting generations: 1) Mag1, observed in all lithological zones and formed at the magmatic stage, has extensive ilmenite exsolution lamellae and high Ti and Cr content. The δ56Fe of Mag1 shows considerable variations from −0.23 to 0.63 ‰ and strikingly an offset of ∼0.3 ‰ towards lower values in the massive ore zone compared to the zones below and above; 2) Mag2, concentrated mainly in thick massive ore layers in MZ without exsolution lamellae, is almost pure magnetite (with low Ti, Al content) and has extremely low δ56Fe values (−1.24 to −0.09 ‰), indicating precipitation from Fe-enriched hydrothermal fluids. Similarly, the δ56Fe of ilmenite shows significant variations from −1.08 to −0.27 ‰ and is significantly lower than typical values for igneous ilmenite (−0.4–0 ‰). Ilmenite displays a similar Fe isotope variation pattern to Mag1 along the stratigraphic position, i.e., with significantly lower δ56Fe in the massive ore zone. As magnetite and ilmenite together contain essentially all Fe, the isotopic shift of these minerals in the ore zone translates to a bulk isotopic offset of ∼−0.3 ‰ compared to the zones below and above. This requires a bulk flux of isotopically light Fe resulting in Fe enrichment in this zone to form massive or even monomineralic ores. The very light isotopic values, particularly hydrothermal magnetite (Mag2) and petrologic evidence, strongly indicate that the Fe flux into the massive ore layers occurred during hydrothermal reworking. This scenario is furthermore supported by magnetite-ilmenite elemental and isotopic thermometry, according to which Fe-Ti oxides experienced hydrothermal re-equilibration in a temperature range of 400–300 °C. Iron isotopic mass balance calculations imply that ∼20–30 % of the Fe in the thick massive ore layers may result from secondary enrichment through hydrothermal precipitation, significantly increasing the ore tonnages and grades. Potentially, other layered intrusions experienced similar mechanisms of hydrothermal Fe enrichment, which will have to be proven in future investigations.
Keywords
- Fe-Ti-V oxide deposit, Hydrothermal reworking, In-situ iron isotopes, Layered intrusion, Massive and monomineralic ores
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)
- Geochemistry and Petrology
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In: Geochimica et cosmochimica acta, Vol. 388, 01.01.2025, p. 78-93.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
T1 - Fe-isotopic evidence for hydrothermal reworking as a mechanism to form high-grade Fe-Ti-V oxide ores in layered intrusions
AU - Wang, Dachuan
AU - Hou, Tong
AU - Botcharnikov, Roman
AU - Weyer, Stefan
AU - Haselbach, Sarah Lynn
AU - Zhang, Zhaochong
AU - Wang, Meng
AU - Horn, Ingo
AU - Holtz, Francois
N1 - Publisher Copyright: © 2024 Elsevier Ltd
PY - 2024/11/26
Y1 - 2024/11/26
N2 - The genesis of high-grade Fe-Ti-V oxide ores (up to >90 vol%) in layered intrusions remains highly debated. Here, on example of Hongge layered intrusion in China, we show that hydrothermal dissolution and precipitation of Fe-Ti-V oxides played a critical role in forming high-grade massive ore deposits as demonstrated by textural-compositional evidence and in-situ iron isotope data (δ56Fe), analyzed with femtosecond laser ablation multicollector (LA-MC-) ICP-MS. Hongge is a mafic layered intrusion composed of a Lower olivine clinopyroxenite Zone (LZ), a Middle clinopyroxenite Zone (MZ), where thick massive ore layers (with up to 90 % Fe-Ti-V oxides) formed, and an Upper gabbro Zone (UZ). Magnetite in Hongge exhibits two contrasting generations: 1) Mag1, observed in all lithological zones and formed at the magmatic stage, has extensive ilmenite exsolution lamellae and high Ti and Cr content. The δ56Fe of Mag1 shows considerable variations from −0.23 to 0.63 ‰ and strikingly an offset of ∼0.3 ‰ towards lower values in the massive ore zone compared to the zones below and above; 2) Mag2, concentrated mainly in thick massive ore layers in MZ without exsolution lamellae, is almost pure magnetite (with low Ti, Al content) and has extremely low δ56Fe values (−1.24 to −0.09 ‰), indicating precipitation from Fe-enriched hydrothermal fluids. Similarly, the δ56Fe of ilmenite shows significant variations from −1.08 to −0.27 ‰ and is significantly lower than typical values for igneous ilmenite (−0.4–0 ‰). Ilmenite displays a similar Fe isotope variation pattern to Mag1 along the stratigraphic position, i.e., with significantly lower δ56Fe in the massive ore zone. As magnetite and ilmenite together contain essentially all Fe, the isotopic shift of these minerals in the ore zone translates to a bulk isotopic offset of ∼−0.3 ‰ compared to the zones below and above. This requires a bulk flux of isotopically light Fe resulting in Fe enrichment in this zone to form massive or even monomineralic ores. The very light isotopic values, particularly hydrothermal magnetite (Mag2) and petrologic evidence, strongly indicate that the Fe flux into the massive ore layers occurred during hydrothermal reworking. This scenario is furthermore supported by magnetite-ilmenite elemental and isotopic thermometry, according to which Fe-Ti oxides experienced hydrothermal re-equilibration in a temperature range of 400–300 °C. Iron isotopic mass balance calculations imply that ∼20–30 % of the Fe in the thick massive ore layers may result from secondary enrichment through hydrothermal precipitation, significantly increasing the ore tonnages and grades. Potentially, other layered intrusions experienced similar mechanisms of hydrothermal Fe enrichment, which will have to be proven in future investigations.
AB - The genesis of high-grade Fe-Ti-V oxide ores (up to >90 vol%) in layered intrusions remains highly debated. Here, on example of Hongge layered intrusion in China, we show that hydrothermal dissolution and precipitation of Fe-Ti-V oxides played a critical role in forming high-grade massive ore deposits as demonstrated by textural-compositional evidence and in-situ iron isotope data (δ56Fe), analyzed with femtosecond laser ablation multicollector (LA-MC-) ICP-MS. Hongge is a mafic layered intrusion composed of a Lower olivine clinopyroxenite Zone (LZ), a Middle clinopyroxenite Zone (MZ), where thick massive ore layers (with up to 90 % Fe-Ti-V oxides) formed, and an Upper gabbro Zone (UZ). Magnetite in Hongge exhibits two contrasting generations: 1) Mag1, observed in all lithological zones and formed at the magmatic stage, has extensive ilmenite exsolution lamellae and high Ti and Cr content. The δ56Fe of Mag1 shows considerable variations from −0.23 to 0.63 ‰ and strikingly an offset of ∼0.3 ‰ towards lower values in the massive ore zone compared to the zones below and above; 2) Mag2, concentrated mainly in thick massive ore layers in MZ without exsolution lamellae, is almost pure magnetite (with low Ti, Al content) and has extremely low δ56Fe values (−1.24 to −0.09 ‰), indicating precipitation from Fe-enriched hydrothermal fluids. Similarly, the δ56Fe of ilmenite shows significant variations from −1.08 to −0.27 ‰ and is significantly lower than typical values for igneous ilmenite (−0.4–0 ‰). Ilmenite displays a similar Fe isotope variation pattern to Mag1 along the stratigraphic position, i.e., with significantly lower δ56Fe in the massive ore zone. As magnetite and ilmenite together contain essentially all Fe, the isotopic shift of these minerals in the ore zone translates to a bulk isotopic offset of ∼−0.3 ‰ compared to the zones below and above. This requires a bulk flux of isotopically light Fe resulting in Fe enrichment in this zone to form massive or even monomineralic ores. The very light isotopic values, particularly hydrothermal magnetite (Mag2) and petrologic evidence, strongly indicate that the Fe flux into the massive ore layers occurred during hydrothermal reworking. This scenario is furthermore supported by magnetite-ilmenite elemental and isotopic thermometry, according to which Fe-Ti oxides experienced hydrothermal re-equilibration in a temperature range of 400–300 °C. Iron isotopic mass balance calculations imply that ∼20–30 % of the Fe in the thick massive ore layers may result from secondary enrichment through hydrothermal precipitation, significantly increasing the ore tonnages and grades. Potentially, other layered intrusions experienced similar mechanisms of hydrothermal Fe enrichment, which will have to be proven in future investigations.
KW - Fe-Ti-V oxide deposit
KW - Hydrothermal reworking
KW - In-situ iron isotopes
KW - Layered intrusion
KW - Massive and monomineralic ores
UR - http://www.scopus.com/inward/record.url?scp=85210126736&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2024.11.017
DO - 10.1016/j.gca.2024.11.017
M3 - Article
AN - SCOPUS:85210126736
VL - 388
SP - 78
EP - 93
JO - Geochimica et cosmochimica acta
JF - Geochimica et cosmochimica acta
SN - 0016-7037
ER -