TY - JOUR

T1 - Iron isotope fractionation during fluid metasomatism and ore-forming processes in magmatic-hydrothermal systems

AU - Liao, Wang

AU - Zhao, Xin Fu

AU - Zeng, Li Ping

AU - Weyer, Stefan

AU - Zhang, Chao

AU - Horn, Ingo

AU - Holtz, Francois

N1 - Funding Information: This study was financially supported by NSFC Projects (41822203; 41972074; 42103064) and a fellowship from the China Postdoctoral Science Foundation (2022M712951). We thank Shiping Sun, Zongjie Zhu and Shixun Chen for their assistance in the field, and Meijun Yang and Xiaolei Nie at Wuhan University of Technology for the EPMA analyses. We are grateful for the thoughtful comments by the Associated Editor, Dr. Adam Simon, and three anonymous reviewers, which greatly improved the manuscript. Prof. Paul T. Robinson is appreciated for polishing English of the manuscript.

PY - 2023/8/15

Y1 - 2023/8/15

N2 - Iron isotopes of magnetite have been used to unravel the sources and evolutionary processes of magma and ore deposits, but most studies have focused on bulk-rock samples, which possibly provide mixed information. Magnetite is susceptible to fluid-induced metasomatism, e.g., recrystallization and coupled dissolution-reprecipitation (CDR) processes in magmatic-hydrothermal systems, resulting in textural and chemical modification and re-equilibration. The behaviors of Fe isotopes during fluid metasomatism, however, are poorly understood. Magnetite from iron oxide-apatite (IOA) deposits commonly shows multi-generation and metasomatic textures, which have been suggested to form from magmatic melts/fluids to low-temperature hydrothermal processes. In this study, we carried out in situ Fe isotopic and chemical analyses on texturally constrained magnetite from ores and their hosting trachyandesite of the giant Luohe IOA deposit, eastern China, to assess the effect of fluid-assisted metasomatic processes on both chemical and Fe isotopic compositions, and to constrain the ore-forming processes. The ores contain three types of magnetite (Mag1, Mag2 and Mag3), all of which are texturally later than albite and diopside. Pristine Mag1 grains, formed at high temperatures (>700 °C), have ilmenite exsolution lamellae, and have lower Ti (0.61–1.55 wt%) but higher Ni/Cr ratios (mostly > 1) than those of magmatic magnetite (MagM) in the hosting trachyandesite. They also have δ56Fe values (0.12–0.52‰) lower than those of MagM (0.54–0.65‰), which overlap with those of high-temperature hydrothermal magnetite from iron-oxide copper–gold (IOCG), iron skarn, and porphyry Cu deposits, indicating that they were precipitated from high-temperature hypersaline fluids, as recorded by fluid inclusions within early-formed garnet, diopside, and coeval titanite. The heavy Fe isotope compositions of Mag1 are interpreted to be due to fluid exsolution under high temperatures and the preference of isotopically light Fe for earlier diopside (δ56Fe = −0.19 to −0.02‰). Mag2 grains, which show triple junction textures, were formed through a fluid-induced recrystallization process at relatively lower temperatures (∼480 °C). They have δ56Fe values (0.29–0.53‰) similar to those of Mag1 and previously reported high-temperature hydrothermal magnetite in other deposits. In contrast, Mag3 grains, which occur as rims on Mag1 grains, were formed via CDR at much lower temperatures (<300 °C). They have δ56Fe values (−0.15 to 0.22‰) significantly lower than those of pristine Mag1 and overlap those of reported low-temperature hydrothermal magnetite. Decreasing formation temperatures from Mag1 to Mag3 are consistent with the decreasing trace element concentrations (Ti, Al, and V). Our study demonstrates that IOA deposits are formed from high-temperature hypersaline magmatic fluids and that the Fe isotopes of magnetite can be significantly modified during fluid metasomatism and ore-forming processes. Therefore, trace element and in situ Fe isotopic analyses on texturally well-constrained magnetite grains are crucial for determining the origin and evolution of magmatic-hydrothermal systems.

AB - Iron isotopes of magnetite have been used to unravel the sources and evolutionary processes of magma and ore deposits, but most studies have focused on bulk-rock samples, which possibly provide mixed information. Magnetite is susceptible to fluid-induced metasomatism, e.g., recrystallization and coupled dissolution-reprecipitation (CDR) processes in magmatic-hydrothermal systems, resulting in textural and chemical modification and re-equilibration. The behaviors of Fe isotopes during fluid metasomatism, however, are poorly understood. Magnetite from iron oxide-apatite (IOA) deposits commonly shows multi-generation and metasomatic textures, which have been suggested to form from magmatic melts/fluids to low-temperature hydrothermal processes. In this study, we carried out in situ Fe isotopic and chemical analyses on texturally constrained magnetite from ores and their hosting trachyandesite of the giant Luohe IOA deposit, eastern China, to assess the effect of fluid-assisted metasomatic processes on both chemical and Fe isotopic compositions, and to constrain the ore-forming processes. The ores contain three types of magnetite (Mag1, Mag2 and Mag3), all of which are texturally later than albite and diopside. Pristine Mag1 grains, formed at high temperatures (>700 °C), have ilmenite exsolution lamellae, and have lower Ti (0.61–1.55 wt%) but higher Ni/Cr ratios (mostly > 1) than those of magmatic magnetite (MagM) in the hosting trachyandesite. They also have δ56Fe values (0.12–0.52‰) lower than those of MagM (0.54–0.65‰), which overlap with those of high-temperature hydrothermal magnetite from iron-oxide copper–gold (IOCG), iron skarn, and porphyry Cu deposits, indicating that they were precipitated from high-temperature hypersaline fluids, as recorded by fluid inclusions within early-formed garnet, diopside, and coeval titanite. The heavy Fe isotope compositions of Mag1 are interpreted to be due to fluid exsolution under high temperatures and the preference of isotopically light Fe for earlier diopside (δ56Fe = −0.19 to −0.02‰). Mag2 grains, which show triple junction textures, were formed through a fluid-induced recrystallization process at relatively lower temperatures (∼480 °C). They have δ56Fe values (0.29–0.53‰) similar to those of Mag1 and previously reported high-temperature hydrothermal magnetite in other deposits. In contrast, Mag3 grains, which occur as rims on Mag1 grains, were formed via CDR at much lower temperatures (<300 °C). They have δ56Fe values (−0.15 to 0.22‰) significantly lower than those of pristine Mag1 and overlap those of reported low-temperature hydrothermal magnetite. Decreasing formation temperatures from Mag1 to Mag3 are consistent with the decreasing trace element concentrations (Ti, Al, and V). Our study demonstrates that IOA deposits are formed from high-temperature hypersaline magmatic fluids and that the Fe isotopes of magnetite can be significantly modified during fluid metasomatism and ore-forming processes. Therefore, trace element and in situ Fe isotopic analyses on texturally well-constrained magnetite grains are crucial for determining the origin and evolution of magmatic-hydrothermal systems.

KW - In situ Fe isotope

KW - IOA deposits

KW - Magnetite

KW - Re-equilibration process

KW - Trace elements

UR - http://www.scopus.com/inward/record.url?scp=85166665680&partnerID=8YFLogxK

U2 - 10.1016/j.gca.2023.07.001

DO - 10.1016/j.gca.2023.07.001

M3 - Article

AN - SCOPUS:85166665680

VL - 355

SP - 161

EP - 172

JO - Geochimica et cosmochimica acta

JF - Geochimica et cosmochimica acta

SN - 0016-7037

ER -