Geochemistry and paragenesis of magnetite bearing gabbros from the Mine Block Intrusion at Lac des Iles Mine, Northern Ontario

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. [...]