Sulfide-associated hydrous fluids drive Fe isotope disequilibrium in magmatic Ni-Cu sulfide systems: Insights from coexisting silicates and sulfides in the Hongqiling deposit

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ID: 328037
2026
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Abstract
Abstract Iron isotopes are increasingly recognized as a key tracer for magmatic evolution and sulfide mineralization in magmatic Ni-Cu sulfide deposits. While these deposits form primarily from high-temperature melts, exsolved magmatic volatile bubbles can spontaneously attach to segregated sulfide droplets. However, whether and how these sulfide-associated volatile-rich fluids influence the Fe isotopic compositions of coexisting silicate minerals during subsequent cooling remain poorly understood. Here, we present high-precision Fe isotopic data of various minerals (silicates, oxides, and sulfides) from the Hongqiling Ni-Cu sulfide deposit in the Central Asian Orogenic Belt. Systematic Fe isotopic fractionation is observed among silicates, oxides and sulfides, with increasing δ56Fe values from olivine (-0.19 to 0.11‰), orthopyroxene (-0.07 to 0.09‰), clinopyroxene (0.05 to 0.20‰), and hornblende (0.03 to 0.29‰) to Cr-spinel (0.08 to 0.37‰), and from pyrrhotite (-1.19 to -0.44‰) and pentlandite (-1.09 to 0.95‰) to chalcopyrite (0.17 to 0.72‰). Notably, quantitative isotope thermometry for coexisting chalcopyrite and pyrrhotite indicates extensive sub-solidus re-equilibration at temperatures predominantly below 200 °C. Silicates and oxides in barren rocks display narrow inter-mineral Δ56Fe values consistent with high-temperature equilibrium, whereas those in mineralized rocks exhibit significant inter-mineral Fe isotopic disequilibrium, characterized by markedly lower δ56Fe values in olivine and higher δ56Fe signatures in hornblende. Positive correlations between δ56Fe and Mg# in silicate minerals, combined with selective alteration patterns and mantle-like sulfur isotopic signatures, indicate that Fe isotopic disequilibrium in mineralized rocks is not driven by fractional crystallization, crustal contamination, or hydrothermal overprinting, but rather by sub-solidus elemental exchange mediated by sulfide-associated hydrous fluids. Upon their release from sulfide melts during crystallization, these fluids induced the observed disequilibrium by facilitating Fe mobility, driving the preferential incorporation of light Fe isotopes into olivine through kinetic diffusion, while simultaneously enriching the remaining fluid in heavy isotopes to form hornblende with elevated δ56Fe values. Our findings highlight that silicate minerals in mineralized systems are modified by sulfide-associated fluids, emphasizing the need to carefully assess such fluid-mediated processes when using these minerals as petrogenetic proxies.
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Authors Mengmeng Cui, Ben-Xun Su, Jing Wang, Yang Bai, Shou-Qian Zhao
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Journal journal of petrology
Year 2026
DOI
10.1093/petrology/egag074
URL
https://doi.org/10.1093/petrology/egag074
Keywords Keywords not found

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