Alteration of the Damiao anorthosite complex in the northern North China Craton: Implications for high-grade iron mineralization

摘要

The Damiao type iron deposit is hosted in a typical Proterozoic anorthosite complex in the northern North China Craton. The types of ores in Damiao mainly comprise massive Fe ores, massive Fe-P ores, and disseminated Fe and Fe-P ores. The disseminated Fe and Fe-P ores formed by fractional crystallization are generally hosted in oxide-apatite gabbronorite and account for 70% of the proven reserve of the Damiao type iron ore. The massive Fe and Fe-P ores account for 30% of the proven reserve of the Damiao type deposit iron ore and generally occur as irregular dykes or veins filling vertical fractures of the previously consolidated anorthosite, showing typical features of hydrothermal mineralization. The contact between the massive orebodies and wall rods is sharp and straight. The anorthosite comprises white and dark varieties, with the former resulted by the alteration of the latter that occurs as relicts. Petrographic observation and electron microprobe analyses show abundant Fe-Ti oxide inclusions in plagioclase which impart the dark color to the rock. The similar spider diagram patterns between fresh and altered plagioclase and between dark- and white-colored anorthosite imply a genetic relationship between the dark and white types. During the alteration of anorthosite, CaO and MgO were slightly decreased, the SiO_2, Al_2O_3 and Na_2O were significantly increased, and the TFe_2O_3 and TiO_2 were significantly decreased. The TFe_2O_3 and TiO_2 in the dark-colored anorthosite have a range of 4.86-12.18 wt.% and 0.37-1.65 wt%, respectively. However, The TFe_2O_3 and TiO_2 in the white-colored anorthosite have a range of 1.67-3.1 wt.% and 0.14-0.31 wt.%, respectively. These features suggest that the alteration of the anorthosite led the Fe element by leaching from the dark-colored anorthosite at highly oxidized condition, and then precipitated within the fractures of the anorthosite, thus forming the massive Fe and Fe-P orebodies. Because the estimated amount of transported Fe is much more abundant than the proven ore reserve, we infer that there should be huge potential for prospecting Damiao type iron ores.