Fe isotopic fractionation during the magmatic–hydrothermal stage of granitic magmatism

Abstract

Felsic intrusive magmatic systems are more complex than volcanic systems as they involve prolonged magmatic processes in addition to potentially multiple phases of magmatic–hydrothermal activity within a single pluton. In particular, the competition between exsolved magmatic fluids and crystallizing minerals for elements and isotopes during the magmatic–hydrothermal stages of evolution of these systems remains unclear despite fluid exsolution being one of the key steps in ore formation. This study focuses on a suite of representative Late Jurassic W–Sn mineralization-related granites within the Nanling Range of southern China and investigates the behavior of Fe isotopes during magmatic differentiation and magmatic–hydrothermal processes recorded within these granites. The granites in the study area have variable δ56Fe values (relative to IRMM-014) ranging from 0.12‰ ± 0.04‰–0.42‰ ± 0.01‰. These values do not correlate with bulk-rock geochemical indicators of fluid exsolution (e.g., Zr/Hf, Nb/Ta, K/Rb, and F/Cl ratios), qualitatively indicating that fluid exsolution has an insignificant effect on Fe isotopic fractionation. We further quantified the relative contributions of fluid exsolution and crystal fractionation on bulk-rock Fe isotopic variability using a Rayleigh fractionation model. This modeling indicates that fluid exsolution can only generate a <0.07‰ increase in bulk-rock δ56Fe values of the Nanling granites even in the most fluid-abundant scenario. This means that Fe isotopic fractionation is controlled mainly by fractional crystallization during the magmatic–hydrothermal stages of the evolution of granitic magmas. This further suggests that Fe isotopes could be used to study the magmatic differentiation of felsic magma chambers, including those within fluid-rich mineralizing magmatic systems.