Geochemistry and iron isotope systematics of coexisting Fe-bearing minerals in magmatic Fe-Ti deposits: A case study of the Damiao titanomagnetite ore deposit, North China Craton

摘要

Geochemical and iron isotopic compositions of magnetite, ilmenite and pyrite separates from the Fe-Ti oxide ores hosted in the Damiao anorthosite-type Fe-Ti ore deposit were analyzed to investigate sub-solidus cooling history of the titanomagnetite. The Fe-Ti oxides form two series of solid solutions, namely, ulvospinel-magnetite (Usp-Mt(ss)) and hematite-ilmenite (Hem-Ilm(ss)) solid solutions. The magnetite separates have 14-27 mol% ulvospinel, while the ilmenite separates have 5-8 mol% hematite. Major element compositions of the mineral separates suggest that the ilmenites were mainly exsolved from the Usp-Mt(ss) by oxidation of ulvospinel in the temperature range of similar to 820-600 degrees C and experienced inter-oxide re-equilibration with the magnetites. Associated with the exsolution is the substantial inter-mineral iron isotope fractionation. The magnetite separates are characterized by high delta Fe-57 (+0.27 - +0.65 parts per thousand), whereas the ilmenite separates have lower delta Fe-57 (-0.65 to -028 parts per thousand). Two types of pyrite are petrographically observed, each of which has a distinctive iron isotope fingerprint. Type I pyrite (pyrite(I)) with higher delta Fe-57 (delta Fe-57 = +0.63 - +0.95 parts per thousand) is consistent with magmatic origin, and type II pyrite (pyrite(II)) with lower delta Fe-57 (delta Fe-57 = -0.90 to -0.11 parts per thousand) was likely to have precipitated from fluids. Iron isotopic fingerprints of the pyrite(I) probably indicate variations of oxygen fugacity, whereas those of the pyrite(II) may result from fluid activities. The iron isotopic fractionation between the magnetite and ilmenite is the net result of sub-solidus processes (induding ulvospinel oxidation and inter-oxide re-equilibration) without needing varying oxygen fugacity albeit its presence. Although varying composition of magnetite-ilmenite pairs reflects variations of oxygen fugacity, inter-oxide iron isotopic fractionation does not. (C) 2019 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.