Petrogenesis of oxide-apatite mineralization associated with proterozoic anorthosite massifs at Lac Mirepoix, Quebec, Canada: A multi-injection model for Fe-Ti-P mineralization in the Central Grenville Province

The Lac Mirepoix Fe-Ti-P mineralization presents several mineralized lenses that 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. The mineralization is (hemo)-ilmenite-dominated, accompanied by magnetite and apatite. It is subdivided in three different zones due to the appearance of different cumulate phases: zone I comprises mainly massive oxide (>70 % hemo-ilmenite +/- magnetite) layers hosted in anorthosite. Towards the center of the mineralization (zone II), massive oxide layers are less common whereas apatite-bearing cumulates appear, forming massive nelsonite (50-70 % magnetite +/- ilmenite and 25-30 % apatite) and oxide apatite norite (OAN, 15-25 % hemo-ilmenite +/- magnetite and 8-20 % apatite). Finally, zone III is marked by the alternance of OAN layers (10-25 m), richer in magnetite, in addition to (hemo)-ilmenite and apatite, and an absence of massive oxide and nelsonite. In-situ trace element analysis of plagioclase, apatite and oxides by laser ablation ICP-MS, reveals cryptic variations related to magma differentiation and multiple injections of ferrodiorite parental magmas of similar composition. In-situ U-Pb dating of zircon from the OAN mineralization itself indicates two different crystallization ages between zone III (1048 +/- 8 Ma) and zone I (964 +/- 9 Ma), favouring a model of multiple injections of similar Fe-Ti-P-rich melts of similar composition over a period of 80 Mys rather than in-situ crystallization of a single intrusion, which is supported by trace-element geochemistry. The Lac Mirepoix mineralization records the following fractional crystallization sequence of a high-Ti-P magma, residual after the anorthosite formation: first, massive oxides of hemo-ilmenite crystallized (high Ti/Fe) by oxide settling, with primitive compositions, similar to the world-class Lac Tio deposit. The oxide-apatite mineralization (nelsonite and OAN) crystallized from the residual liquid (lower Ti/Fe, evolved compositions) in which magnetite and apatite were more abundant, similar to nearby mineralization in the area (e.g. Lacal'Orignal Fe-Ti-P deposit). The observed decrease in Ti content of the evolving melt is supported by the liquid line of descent of several ferrodiorite dykes within the host-rocks and mineralization. Finally, we propose a model of multiple injections of residual Fe-Ti-P-rich liquids drained from or filter-pressed within diapirs of plagioclase-rich mushes that were emplaced over 80 Mys apart along the same crustal detachment zone.