Iron isotope fractionation during fluid metasomatism and ore-forming processes in magmatic-hydrothermal systems
Iron isotope fractionation during fluid metasomatism and ore-forming processes in magmatic-hydrothermal systems
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DOI：
10.1016/j.gca.2023.07.001
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发表时间：
2023-07
影响因子：
5
通讯作者：
Wang Liao;Xin-Fu Zhao;L. Zeng;S. Weyer;Chao Zhang;I. Horn;F. Holtz
中科院分区：
文献类型：
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作者：
Wang Liao;Xin-Fu Zhao;L. Zeng;S. Weyer;Chao Zhang;I. Horn;F. Holtz
磁铁矿的铁同位素已经被用来揭示岩浆和矿床的来源和演化过程，但大多数研究都集中在大块岩石样本上，这可能提供混合的信息。磁铁矿在岩浆-热液系统中易受流体诱导的交代作用，如再结晶和耦合溶蚀-再沉淀（CDR）过程，导致磁铁矿的结构和化学改性和再平衡。然而，流体交代过程中铁同位素的行为尚不清楚。氧化铁磷灰石（IOA）矿床中的磁铁矿通常呈现多代交代结构，可能是岩浆熔体/流体到低温热液作用的产物。本研究对中国东部洛河IOA巨型矿床矿石及其寄主粗质山岩中的磁铁矿进行了原位铁同位素和化学分析，以评估流体辅助交代过程对化学和铁同位素组成的影响，并对成矿过程进行了约束。矿石含Mag1、Mag2和Mag3三种类型的磁铁矿，结构均晚于钠长石和透辉石。在高温下形成的原始Mag1晶粒具有钛铁矿析出片层，其Ti （0.61 ~ 1.55 wt%）较低，但Ni/Cr比（主要为> 1）较高（主要为岩浆磁铁矿（MagM））。它们的δ56Fe值（0.12 ~ 0.52‰）低于MagM的δ56Fe值（0.54 ~ 0.65‰），与铁氧化铜金（IOCG）、铁矽卡岩和斑岩型铜矿的高温热液磁铁矿的δ56Fe值重叠，表明它们是由高温高盐流体沉淀而成，早期形成的石榴石、透辉石和同时期的钛矿的流体包裹体记录了它们的δ56Fe值。Mag1的重铁同位素组成解释为高温下流体析出，且同位素轻铁优先于早期透辉石（δ56Fe = - 0.19 ~ - 0.02‰）。在相对较低的温度（~ 480℃）下，通过流体诱导的再结晶过程形成了具有三重结织构的Mag2晶粒。它们的δ56Fe值（0.29 ~ 0.53‰）与Mag1和其他矿床高温热液磁铁矿相似。相比之下，通过CDR在较低温度（<300℃）下形成的Mag3晶粒以边缘形式出现在Mag1晶粒上。它们的δ56Fe值（- 0.15 ~ 0.22‰）显著低于原始Mag1，与已有报道的低温热液磁铁矿的δ56Fe值重叠。从Mag1到Mag3的地层温度下降与微量元素（Ti、Al和V）浓度下降一致。研究表明，IOA矿床是由高温高盐岩浆流体形成的，在流体交代和成矿过程中，磁铁矿的铁同位素发生了明显的变化。因此，结构约束良好的磁铁矿颗粒的微量元素和原位铁同位素分析对于确定岩浆-热液系统的起源和演化具有重要意义。
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 outin situFe 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 andin situFe isotopic analyses on texturally well-constrained magnetite grains are crucial for determining the origin and evolution of magmatic-hydrothermal systems.