Pétrographie, géochimie et potentiel économique en Fe-Ti-P du secteur du Lac à Paul, partie nord de la suite anorthositique de Lac-Saint-Jean, province de Grenville, Québec /

Abstract

Ill légers que les compositions magmatiques parentales calculées pour le secteur du lac à Paul. Cependant, il est possible qu'elles soient génétiquement associées aux roches enrichies en Fe-Ti±P du secteur du lac à Paul.Les péridotites nelsonitiques, avec 10 à 25 % d'apatite, représentent le meilleur potentiel en terme de phosphore (4 à 10 % P2O5). Elles possèdent une bonne distribution à travers le secteur d'étude et elles pourraient également représenter un potentiel économique' secondaire en ilménite et magnetite. Les roches à oxydes de Fe-Ti sont les plus enrichies fer et titane. Toutefois, dans le contexte de cette étude, leur potentiel économique semble limité car elles possèdent une distribution plutôt erratique à travers le secteur d'étude. IV ABSTRACTThis master's project is a study of the geochemistry, the petrography and economic potential of iron-, titanium-, and phosphorus-rich rocks (Fe-Ti±P) of the lac à Paul area, located in the northern part and on the margins of the Lac Saint-Jean anorthositic suite (LSJAS). The LSJAS is part of the Grenville province, which is characterized by numerous proterozoic AMCG-type anorthositic suites. This type of Fe-TüP enrichment is caracteristic of these suites.The Fe-Ti±P enrichment of the different types of rocks in the lac à Paul area reflects their variable enrichment in magnetite, ilmenite and apatite. The sampled rocks that show this type of Fe-Ti±P enrichment were divided into six lithologies: 1) Fe-Ti oxides-and apatite-bearing anorthosites, leucogabbros, leuconorites and leucogabbronorites; 2) Fe-Ti oxides-and apatite-bearing troctolites and leucotroctolites ±; 3) nelsonitic peridotites; 4) Fe-Ti oxides rocks; 5) Fe-Ti oxides-bearing pyroxenites, and; 6) Fe-Ti oxides-bearing dunites. Some ferrodiorites, in the form of crosscutting dykes, were also sampled.The minerals analysed by microprobe (olivine, plagioclase, orthopyroxene, clinopyroxene, ilmenite, magnetite, spinel and apatite) all display evolved compositions. The plagioclase is mostly andesine (An2? à Anjo) and the forsterite content of the olivines varies from F046 à F063. Unlike the other lithologie, which are characterized by ilmenite, the ferrodiorites contain hemo-ilmenite.The evolved nature of the different rock types of the lac à Paul area, illustrated by the pétrographie and geochemical studies, indicates that the parental magma of these rocks is also evolved. Moreover, the ubiquitous distribution of apatite indicates that the parental magma had a composition where apatite was on the liquidus. The REE composition of the parental magma was calculated using the REE composition of separated apatites analyzed by INAA. This (calculated) composition is similar to the composition of ferrodioritic rocks often associated with anorthosite massifs. The formation of the anorthositic and troctolitic rocks is possible by crystal fractionation, from a magma of ferrodioritic composition. The origin of the nelsonitic peridotites remains problematic and liquid immiscibility cannot be excluded. The Fe-Ti oxid...