Abstract:

Massiftype anorthosite is referred to igneous rocks consisting of over 90％ plagioclase. The massiftype anorthosite bodies formed in the Proterozoic ranging from 2.1 Ga to 0.9 Ga, and commonly host abundant FeTiP oxide ores. FeTiP ore bodies occurred as conformable layers or irregular shapes as lenses, sheets or veins. The ores usually are of massive and disseminated types; the former type contains more than 70％ ore minerals, and the latter one contains 20% to 70％. Ore minerals from some deposits are mainly titanomagnetite with minor ilmenite, whereas the others are mainly hemoilmenite with minor magnetite. Some of ores contain abundant amounts of apatite, which forms nelsonite containing only FeTi oxides and apatite. These FeTiP ores have been commonly interpreted as forming from a FeTirich magma which usually represents residual magma after extensive crystallization of plagioclase in a deep chamber. However, it is controversial on the oreformation mechanisms. They were assumed to have crystallized from immiscible FeTiP liquids separated from silicate magma or represent cumulate rocks that directly crystallized from silicate magmas. The Damiao FeTiP oxide deposit is the unique deposit hosted in massiftype anorthosite in China. Its ore bodies commonly occur as irregular lenses, veins or pods with sharp contact with anorthosite, and consist of different zones of FeTiP ores upwards instead of obvious igneous layering. Based on detailed petrography, mineral proportions, mineral compositions and wholerock geochemistry, we conclude that various ores at Damiao are mixtures of cumulus mineral and trapped liquids, and that they formed from a ferrodioritic magma by fractional crystallization and accumulation. The discordant occurrence of various ores may be related to the dynamic crystallization process and/or subsolidus remobilization.

Key words:  massiftype anorthosite, nelsonite, polybaric crystallization, immiscibility, Damiao FeTiP deposit