Petrogenesis of Fe-Ti-P mineral deposits associated with Proterozoic anorthosite massifs in the Grenville Province: insights from oxide and apatite trace-element geochemistry at Lac à l’Orignal, Quebec, Canada

Proterozoic anorthosite massifs can host significant amounts of critical and strategic metals such as Ti, V, and P, associated with magmatic Fe-Ti oxides and apatite. Yet their petrogenesis is much less understood than Fe-Ti-V-P deposits hosted in layered intrusions within large igneous provinces. Several mineralized lenses of Fe-Ti-P outcrop near the border of the 1080 (±2) Ma Vanel and the 1016 (±2) Ma Mattawa Anorthosite Massifs, in the Central Grenville Province, Quebec, Canada. For example, the Lac à l’Orignal Fe-Ti-P deposit, hosted in the Vanel Anorthosite near the northern border of the Mattawa Anorthosite, comprises a lenticular structure of oxide apatite norite (OAN) with thin layers of apatite-bearing anorthosite and minor amounts of nelsonite (massive Fe-Ti oxides and apatite), indicating accumulation by density differences. Oxide settling generated the melanocratic OAN cumulates and nelsonite. Plagioclase flotation generated the leucocratic apatite-bearing anorthosite layers. The mineralization is dominated by hemo-ilmenite, accompanied by apatite and a minor amount of magnetite at the borders, whereas the core is dominated by ilmenite, magnetite, and apatite. In-situ U-Pb dating of magmatic zircon indicates that the Lac à l’Orignal deposit is a multistage intrusion with two different crystallization ages between the younger core (993 ± 13 Ma) and the older upper border (1069 ± 12 Ma) of the intrusion. These ages are similar to those of nearby anorthosite-massifs (Mattawa and Vanel Anorthosites, respectively). In-situ trace element analysis of plagioclase, apatite and oxides, by laser ablation ICP-MS, reveals subtle variations in certain trace elements (e.g., Cr, Ni, V) related to differentiation under relatively high-fO₂ conditions (FMQ = +0.9 to +1.7). Calculated melt compositions from apatite indicate a similar parental magma for both the border and core that matches the composition of high-Fe-Ti-P ferrodiorite dykes at Lac à l’Original. This high-Ti-P ferrodiorite magma was probably residual after anorthosite formation. Sub-solidus inter-oxide equilibration modified the original composition of the different cumulates in the intrusion. The absence of extensive massive oxide cumulates and the presence of higher amounts of cumulus magnetite and apatite, supported by mineral chemistry, denotes a more evolved character for the Lac à l’Orignal deposit compared with other Fe-Ti-(P) deposits in the Grenville Province (e.g., Lac Tio Fe-Ti and Grader intrusion Fe-Ti-P deposits in the Havre St. Pierre Anorthosite, eastern Quebec). Petrogenetically, the Lac à l’Orignal Fe-Ti-P deposit corresponds to an evolved part of a low-Ti/Fe system in the Grenville Province in the late stages of differentiation of ferrodiorite/jotunite magmas.