Geochemistry, petrography, geochronology, and radiogenic isotopes of the weakly mineralized intrusions in Thunder Bay North Igneous Complex

Abstract

The Thunder Bay North Igneous Complex (TBNIC) is located approximately 50 km northeast of Thunder Bay, Ontario, Canada. It comprises two Cu–Ni–PGE-mineralized mafic–ultramafic intrusions, the Escape and Current Intrusions, and five smaller mafic–ultramafic intrusions whose mineralization potential has yet to be fully assessed, namely the Lone Island Lake, Greenwich, Southeast Anomaly (SEA), and 025 intrusions, and the East–West Connector (EWC). The five gabbroic to peridotitic intrusions are the focus of this study. They are characterized by high magnetic anomalies and crosscut the Archean granitoids and metasedimentary rocks of the Quetico Subprovince. U-Pb dating yielded an age of 1107.6 ± 0.9 Ma for the Escape intrusion, which is the oldest date among TBN intrusions, and an age of 1105 ± 0.9 Ma for the Greenwich intrusion. Field observations and drill core logging of the five intrusions indicates that they are gabbroic to peridotitic in composition with approximately 6-9% sulfides. The sulphides are dominantly pyrite, with lesser amounts of chalcopyrite and pyrrhotite. Petrographic analysis of the five intrusions show them to comprise gabbronorite with lesser leuco-gabbro, gabbro, and websterites for the Lone Island intrusion, gabbronorite to websterite and lherzolite for 025, gabbro to websterite for SEA, and highly altered gabbro in the EWC and Greenwich intrusions. [...]