Iron Isotope Systematics of the Panzhihua Mafic Layered Intrusion Associated With Giant Fe‐Ti Oxide Deposit in the Emeishan Large Igneous Province, SW China

Abstract

In order to investigate how Fe was enriched from parental high‐Ti basaltic magma to form the stratigraphically thick Fe‐Ti oxide ore at the bottom layers, we present a systematic study for Fe isotopic compositions of whole rocks and mineral separates (clinopyroxene, magnetite, and ilmenite) throughout the Panzhihua intrusion. Whole rock δ⁵⁶Fe ranges from 0 ± 0.02‰ to 0.15 ± 0.04‰, consistent with the range of clinopyroxene (0.01 ± 0.02‰ to 0.16 ± 0.05‰). On the contrary, magnetite (Mt) separates have δ⁵⁶Fe ranging from 0.17 ± 0.05‰ to 0.62 ± 0.02‰, showing a strikingly complementary trend with coexisting ilmenite (Ilm) separates (−0.52 ± 0.03‰ to −0.09 ± 0.02‰) along the profile. The calculated bulk δ⁵⁶Fe of Fe‐Ti oxides (Mt + Ilm), however, has a small range from 0.01‰ to 0.16‰, identical to those for clinopyroxene separates and whole rocks. The uniform δ⁵⁶Fe of clinopyroxene may have resulted from the small Fe isotope fractionation between clinopyroxene and parental magma in early‐stage magma differentiation before substantial crystallization of Fe‐Ti oxides. The complementary trends of δ⁵⁶Fe for Mt and Ilm along the profile and the uniform bulk δ⁵⁶Fe of Fe‐Ti oxides are better interpreted as in situ crystallization of Fe‐Ti oxides from the interstitial liquid. Our Fe isotopic data and petrographic observations indicate that the thick Fe‐Ti oxide ore layers in the lower zone of the Panzhihua intrusion may be attributed to in situ crystallization of Mt and Ilm from the interstitial, immiscible Fe‐rich melt in the lower part of the magma chamber.