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 δ 56 Fe 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 δ 56 Fe 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 δ 56 Fe 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 δ 56 Fe 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 δ 56 Fe for Mt and Ilm along the profile and the uniform bulk δ 56 Fe 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.