Découverte d’une faille dans une zone de cisaillement : levé magnétique et étude de carottes de forage de la propriété du lac Valentine, à Terre-Neuve, Canada
DOI :
https://doi.org/10.4138/atlgeo.2023.008Résumé
La propriété du lac Valentine dans le sud-ouest de Terre-Neuve abrite cinq gîtes aurifères connus sur une distance de 20 km le long d’un axe du nord-est au sud-ouest et on anticipe qu’elle constituera l’emplacement de la plus vaste mine d’or au Canada atlantique. La propriété recèle des filons de quartz-tourmaline-pyrite orogéniques régis par la structure le long ou à proximité de la zone de cisaillement du lac Valentine. La minéralisation est principalement présente sur le flanc nord-ouest de la zone de cisaillement dans des roches intrusives et elle est rare dans le conglomérat sur le flanc méridional. Le secteur renferme de nombreux dykes mafiques, qui sont bien apparents lorsqu’on effectue un levé magnétique en raison de leur teneur magnétique. Ces dykes, à l’instar de la minéralisation, se manifestent surtout dans les roches intrusives et l’axe essentiellement subparallèles à la zone de cisaillement, et ils peuvent par conséquent faciliter l’élucidation de la structure. Pour circonscrire l’emplacement de la zone de cisaillement (et en conséquence la région minéralisée) dans un secteur où l’on soupçonnait un rejet, on a réalisé un levé magnétique détaillé suivant une grille de 200 m sur 300 constituée de 16 lignes. Les données recueillies ont permis la création et le traitement de cartes de l’intensité magnétique. On a alors repéré une région où la configuration des crêtes magnétiques linéaires s’estompe et semble prendre fin près du rejet soupçonné. Les données, conjuguées à l’information obtenue de carottes de forage, nous amènent à interpréter l’endroit comme un rejet vertical de la zone de cisaillement. L’affaiblissement de l’intensité magnétique est possiblement dû à une altération ou un amincissement des dykes mafiques près de la faille.
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