August 2024
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Mineralium Deposita
Iron-titanium-vanadium (Fe-Ti-V) oxide mineralisation is commonly associated with Proterozoic massif-type anorthosites, but the conditions required for their formation remain poorly understood. The Etoile Suite Mafic Intrusion (1149 ± 11 Ma), in the Grenville Province, Quebec (Canada), comprises a layered mafic intrusion that is coeval with nearby massif-type anorthosites. The mafic intrusion consists of troctolite and olivine gabbro cumulates, where magnetite and ilmenite are intercumulus at the base (Zone A) and top (Zone C) but cumulus (<30 modal %) in the centre (Zone B). Towards the base of Zone B, vanadium mineralisation occurs in a 1-km-thick oxide-rich wehrlite horizon, where V-rich titanomagnetite (<1.85 wt% V2O5) and ilmenite form semi-massive oxide layers. From the base to the top of Zone B there is an overall progressive decrease in Anpl, Fool, and Mg#cpx, and in Cr and Ni concentrations of magnetite, albeit with several reversals to more primitive compositions, including one near the base of Zone C. This indicates fractional crystallisation in an open magma chamber. The intrusion crystallised at moderate fO2 (~FMQ 1.1 ± 0.3), resulting in the late crystallisation of V-rich magnetite from a relatively evolved magma. The parental magma was likely a high-Al basalt derived from a depleted mantle source, recording minimal crustal contamination, in contrast to coeval massif-type anorthosites that commonly contain orthopyroxene reflecting higher degrees of crustal contamination. As a result, V mineralisation in noritic anorthosites formed at higher fO2, with early crystallisation of relatively V-poor magnetite, whereas magnetite in troctolitic-olivine gabbroic intrusions crystallised later with higher V contents, due to lower fO2.