Geochemical variation of oxide-apatite mineralization associated with Proterozoic massif-type anorthosites in the Central Grenville Province, Quebec, Canada

A variety of magmatic oxide-apatite mineralization is spatially and temporally associated with Proterozoic AMCG (Anorthosite–Mangerite–Charnockite–Granite) suites, being of both economic and scientific interest, hosting significant amounts of critical and strategic metals, such as Ti, V and P. However, the origin and genetic relationship of both AMCG host and oxide-apatite mineralization are still highly debated, as well as the exact tectonic setting. Oxide-apatite mineralization is commonly interpreted as crystallizing from residual Fe-Ti-V-P rich melts (ferrodiorite/jotunite composition), after extensive polybaric crystallization of plagioclase and mafic silicates during the formation of anorthosite massifs. The two younger AMCG suites in the Central Grenville Province (1082-1045 Ma Pipmuacan and 1020-1008 Ma Valin) have a complex petrogenetic evolution, presenting a geochemical variation of oxide-apatite mineralization (from Ti-magnetite-dominated to hemo-ilmenite-dominated mineralization) according to the host anorthosite composition (labradorite- or andesine-type) and its age. Andesine-type anorthosite massifs that are younger than 1100 Ma are orthopyroxene-bearing and host hemo-ilmenite (± apatite) mineralization, whereas labradorite-type anorthosite massifs which are older than 1100 Ma, are olivine-bearing and host Ti-magnetite (± apatite) mineralization. The reason for this mineralogical and geochemical variation of oxide-apatite mineralization with time is not understood but could be due change in parental melts and/or degrees of crustal contamination or fractional crystallization. This thesis aims to better characterize this geochemical variation in both ore-deposit and regional scales in the Central Grenville Province, understanding the processes that control their compositional variation (e.g., source of magma, crustal contamination, evolution of residual ferrodiorite/jotuntite magmas) through LA-ICP-MS whole rock and mineral chemistry of plagioclase, ilmenite, magnetite and apatite, as well U-Pb-Hf isotopes in zircon. This methodology was applied to Fe-Ti-(P) mineralization (hemo-ilmenite dominated) at three different scales to better characterize the geochemical variation within: I) a single lenticular oxide-apatite-norite deposit (Lac à l’Orignal); a larger scale through II) several mineralized lenses composed by different Fe-Ti-P cumulates (e.g., massive oxides, nelsonite and oxide-apatite-norite at Lac Mirepoix); a regional scale, III) comparing the different locations of Fe-Ti-P mineralization hosted in the anorthosites and mangerites of the Pipmuacan (Lac de l’Abbondance, L’Étang, Lac Périgny) and Valin (Lac Brûlé, Mattawa and La Hache) AMCG suites with mineralization at Lac à l’Orignal and Lac Mirepoix (Vanel Anorthosite), as well as published data from Fe-Ti-P deposits from the Grenville Province (e.g, Lac Tio, Grader Intrusion, Saint Urbain). The Lac à l’Orignal Fe-Ti-P deposit, hosted in the 1080 (±2) Ma Vanel Anorthosite near the northern border of the 1016 (±2) Ma 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). 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 from the mineralization indicates that the Lac à l’Orignal deposit is a multi-injection 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 reveals subtle variations in Cr, Ni and V related to differentiation under relatively high-fO2 conditions (FMQ = +0.9 to +1.7). Calculated melt compositions from apatite indicates 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. 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, Grader intrusion). 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. The nearby Lac Mirepoix Fe-Ti-P mineralization is also (hemo)-ilmenite-dominated, accompanied by magnetite and apatite. This ‘layered intrusion’ 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 (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 (10-25m), 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 reveals cryptic variations related to magma differentiation and multiple injections of ferrodiorite parental magmas of similar composition from which the OAN and nelsonite crysatllized from. Similar to Lac à l’Orignal, in-situ U-Pb dating of zircon from the OAN mineralization itself indicates two different crystallization ages between zone III (1048 ± 8Ma) and zone I (964 ± 9 Ma), favoring a model of multi-injections rather than in-situ crystallization of a single layered 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. Lac à l’Orignal Fe-Ti-P mineralization). 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. The regional study of several Fe-Ti-P mineralization in the two younger Pipmuacan and Valin AMCG suites demonstrates a similar geochemical evolution related to fractional crystallization of parental ferrodiorite magma, with massive oxides (hemo-ilmenite ± magnetite) crystallizing first from high Ti/Fe residual melts followed by apatite-bearing nelsonites ((hemo)-ilmenite + magnetite + apatite), oxide apatite norite ((hemo)-ilmenite + magnetite + apatite + silicates) crystallizing from low-Ti/Fe melts. The most evolved melts (jotunite composition) formed mineralized mangerites (ilmenite + magnetite + apatite + oligoclase + K-feldspar + quartz). Most of the Fe-Ti-P mineralization in this study is within the range of previous ages for the Pimuacan and Valin AMCG suites (1080 – 950Ma). However, U-Pb dating of the mineralized cumulates in this study resulted in quite different ages (>50Ma) from previously publications on each respective host-AMCG. We propose a complex 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. In-situ U-Pb-Hf isotopes in zircon from Fe-Ti-P mineralization in this study have mainly chondritic to slightly positive and negative εHf values, in which mineralization in the Labrieville and Mattawa anorthosites present more suprachondritic Hf signature (εHf: +3.0 to +7.0; U-Pb dating: 1065 (± 5Ma) to 990 (± 7Ma)), possibly reflecting contamination from juvenile crust, similar to mineralization in the Saint Urbain Anorthosite. Mineralization associated with the Vanel Anorthosite and the mangerites of La Hache and Poulin de Courval present a more enriched Hf signature (εHf: -5.0 to +7.4; U-Pb dating: 964 (± 9Ma) to 1115 (± 11Ma)), either being relatively more contaminated, and/or associated with more enriched mantle source(s) in relation to the mineralization at Labrieville and Mattawa. The trace element patterns of apatite from all of the Fe-Ti-P mineralization have LREE enriched patterns very similar to apatite from mafic layered intrusions, associated with mantle plumes, with negligible crustal contamination. This indicates the involvement of an asthenospheric enriched mantle source in forming the Fe-Ti-P rich melts for the different Fe-Ti-P mineralization in this study, rather than a depleted mantle source or enriched subcontinental lithospheric mantle as commonly proposed in previous tectonic models for the Grenville AMCG magmatism. Moreover, the Lu-Hf data reveal that Fe-Ti-P mineralization in Vanel Anorthosite and the mangerites of La Hache and Poulin de Courval probably underwent low- to moderate lower crustal contamination. Therefore, we show for the first time that apatite and zircon geochemistry provide important constraints on the mantle source and role of contamination in the petrogenesis of AMCG suites in the Grenville Province, which should be taken into account for the tecto

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