Sergio Bautista MSc Thesis Abstract
The Mine Block Intrusion (MBI) is a 3 km by 1.5 km tear-shaped mafic complex that was emplaced along with other mafic-ultramafic plutonic rocks of the South Lac des Iles Complex (LDIC) between 2,699 and 2,686 Ma in the Marmion terrane of the Superior Province. The MBI consists of gabbronorite and magnetite-rich gabbronorite, and a later series of noritic rocks and their metamorphosed altered equivalents.
Major element data of the MBI indicate that the rocks have a broadly gabbroic composition with protoliths dominated by plagioclase, a predominance of orthopyroxenes over clinopyroxene, and titano-magnetite, which is consistent with the petrographic observations. Magnetite gabbronorite rocks in the MBI were derived from a metasomatized mantle, consistent with the arc setting proposed for the Lac des Iles Complex as a whole.
Plagioclase compositions decrease up the analyzed section, with no sharp change, suggesting a normal fractionation process with no magma injection events. Plagioclase identified within the magnetite gabbronorite of the MBI is labradorite-bytownite (An-number ranging from 0.58 to 0.75), which is less evolved than observed in layered intrusions globally suggesting that saturation of Fe-Ti oxides occurred at an early stage of differentiation compared with other mafic intrusions.
Coarse primary ilmenite grains with hematite lamellae in the MBI indicate early ilmenite crystallization, which could suggest a high TiO2 concentration in the magma. However, comparison with rocks of deposits dominated by hemo-ilmenite (high Ti content) and titanomagnetite (low Ti content) suggest that the primary ilmenite crystallized from a low-Ti magma.
When comparing the analyzed samples of magnetite gabbronorite from the MBI with Fe-oxides of well-characterized rocks from the lowermost layers of Fe-Ti-V deposits (primitive signature) and the uppermost layers of Fe-Ti-P deposits (evolved signature), the pattern of compatible elements such as V, Ni, and Cr in magnetite show a distribution from primitive to evolved magnetite. This suggests that magnetite at the MBI crystallized from a magma of primitive composition associated with Fe-Ti-V deposits rather than from a more evolved magma typically linked to Fe-Ti-P mineralisation. This is consistent with the absence of apatite and the low P contents observed in whole rock geochemistry in the MBI. The upward decrease of elements compatible with magnetite, such as Mg, Ni, V, and Cr, and the upward increase of magnetite from elements incompatible with magnetite, such as Mo, Zn, and Ti in the magnetite from the MBI, indicate fractionation of the silicate melt from a less evolved melt to a more evolved one. It is likely that during fractionation plagioclase would have accumulated at the top of the magma chamber whereas the denser iron oxides would sink, resulting in the cumulate textures observed in the magnetite gabbronorite in the MBI.
The magnetite in the MBI gabbronorite at Lac des Iles can be considered to belong to the Fe-Ti-V deposit group hosted in the lower parts of a layered intrusion, characterized by titanomagnetite as the dominant oxide mineral occurring in layers and oxide mineralization associated with gabbronoritic cumulates. Magnetite chemistry also suggests a magmatic Fe-Ti-V deposit with a potential for V mineralization at Lac des Iles since several samples have concentrations (up to 1.3 wt. % V2O5) above those of the Panzhihua V mine (0.3 wt. % V2O5) or the Rhovan (Bushveld Complex) mine (0.5 wt. % V2O5).