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Assessing the role of evaporite assimilation for the formation of iron oxide-apatite deposits.

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Title: Assessing the role of evaporite assimilation for the formation of iron oxide-apatite deposits.
Authors: Meng, Xuyang1 (AUTHOR) xmeng1@cugb.edu.cn, Simon, Adam C.2 (AUTHOR), Mao, Jingwen1,3 (AUTHOR), Duan, Chao3 (AUTHOR), Kitajima, Kouki4 (AUTHOR), Neill, Owen K.2 (AUTHOR), Bindeman, Ilya5 (AUTHOR)
Source: Geochimica et Cosmochimica Acta. Jul2026, Vol. 424, p242-256. 15p.
Subjects: Hydrothermal alteration, Magmas, Isotopic analysis, Iron oxides, Geochemistry
Geographic Terms: East Asia, Yangtze River (China)
Abstract: Iron-oxide-apatite (IOA) deposits occur globally and contain tens of millions to several billions of tonnes of iron resources as well as substantial amounts of phosphorus, rare earth elements, cobalt, and vanadium in some deposits. There is geochemical evidence in some IOA deposits of an evaporite component, but it remains poorly constrained whether the evaporite component was necessary for mineralization. It has been hypothesized that magmatic assimilation of evaporites is an essential process that contributes to the Cl-S-rich nature and oxidation of causative magmas for forming IOA deposits. Here, we test this hypothesis by measuring the O isotope composition (δ18O) of apatite and zircon grains, as well as the major and trace element abundances of zircon-hosted apatite inclusions, sampled from IOA-ore-related dioritic rocks in the Ningwu IOA ore district of the Middle-Lower Yangtze River metallogenic belt in Eastern China. The data reveal a concomitant decrease in δ18O values of zircon-hosted apatite inclusions from 7.5‰ to 2‰ with S and Cl concentrations of apatite. Apatite separates from samples with low-δ18O zircon-hosted apatite inclusions yielded comparably low δ18O values of 2.2–3.8‰, suggesting the low δ18O-S-Cl apatite grains record post-crystallization hydrothermal alteration. In contrast, the molar fractions of F, Cl, and OH in the S-Cl-rich zircon-hosted apatite inclusions are consistent with their crystallization from volatile-undersaturated silicate melt. Equilibrium δ18O values for S-Cl-rich apatite inclusions and their zircon hosts range from mantle-like values (δ18O = 5.3 ± 0.6‰) to values requiring crustal contamination (δ18O as high as 7.5‰) and are negatively correlated with published magmatic f O 2 values for spatially and temporally associated igneous rocks. These results are consistent with a metasomatized mantle source for the S-Cl-rich IOA-ore-forming magmas, which were contaminated by relatively reduced crustal materials during ascent to shallow crustal levels in the Ningwu ore district. The previously estimated Cl/H 2 O ratios of 0.11 ± 0.03 based on the high-S-Cl apatite composition are inconsistent with saturation and exsolution of hypersaline liquids directly from the dioritic melts. These findings rule out the assimilation of oxidized evaporites during magma ascent and indicate that an evaporite component detected in some ore samples in some IOA deposits was the product of post-mineralization incursion of non-magmatic hydrothermal fluids. [ABSTRACT FROM AUTHOR]
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Abstract:Iron-oxide-apatite (IOA) deposits occur globally and contain tens of millions to several billions of tonnes of iron resources as well as substantial amounts of phosphorus, rare earth elements, cobalt, and vanadium in some deposits. There is geochemical evidence in some IOA deposits of an evaporite component, but it remains poorly constrained whether the evaporite component was necessary for mineralization. It has been hypothesized that magmatic assimilation of evaporites is an essential process that contributes to the Cl-S-rich nature and oxidation of causative magmas for forming IOA deposits. Here, we test this hypothesis by measuring the O isotope composition (δ18O) of apatite and zircon grains, as well as the major and trace element abundances of zircon-hosted apatite inclusions, sampled from IOA-ore-related dioritic rocks in the Ningwu IOA ore district of the Middle-Lower Yangtze River metallogenic belt in Eastern China. The data reveal a concomitant decrease in δ18O values of zircon-hosted apatite inclusions from 7.5‰ to 2‰ with S and Cl concentrations of apatite. Apatite separates from samples with low-δ18O zircon-hosted apatite inclusions yielded comparably low δ18O values of 2.2–3.8‰, suggesting the low δ18O-S-Cl apatite grains record post-crystallization hydrothermal alteration. In contrast, the molar fractions of F, Cl, and OH in the S-Cl-rich zircon-hosted apatite inclusions are consistent with their crystallization from volatile-undersaturated silicate melt. Equilibrium δ18O values for S-Cl-rich apatite inclusions and their zircon hosts range from mantle-like values (δ18O = 5.3 ± 0.6‰) to values requiring crustal contamination (δ18O as high as 7.5‰) and are negatively correlated with published magmatic f O 2 values for spatially and temporally associated igneous rocks. These results are consistent with a metasomatized mantle source for the S-Cl-rich IOA-ore-forming magmas, which were contaminated by relatively reduced crustal materials during ascent to shallow crustal levels in the Ningwu ore district. The previously estimated Cl/H 2 O ratios of 0.11 ± 0.03 based on the high-S-Cl apatite composition are inconsistent with saturation and exsolution of hypersaline liquids directly from the dioritic melts. These findings rule out the assimilation of oxidized evaporites during magma ascent and indicate that an evaporite component detected in some ore samples in some IOA deposits was the product of post-mineralization incursion of non-magmatic hydrothermal fluids. [ABSTRACT FROM AUTHOR]
ISSN:00167037
DOI:10.1016/j.gca.2026.05.032