Khalid Yahia MSc Thesis 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.
The five intrusions are characterized by negative Nb, Zr, Hf, Ti, Y and Sc anomalies, and contain high incompatible element contents, enriched light rare-earth elements (LREE) with La/Smn = 2-3.7, flat to fractionated heavy REE (HREE) with Gd/Ybn = 1-6, and Th/Nbn = 0.09-0.11. The five intrusions also have moderate to strong negative Nb anomalies with Nb/Nb* ranging between 0.90 and 0.05. Generally, the five intrusions show slightly positive Eu anomalies with Eu/Eu*range between 0.53 and 1.14, which is consistent with the elevated plagioclase content of the five intrusions.
Based on major and trace element chemistry, three sample populations were identified. Population A includes Lone Island, EWC, and SEA intrusions, and population B includes part of the 025 intrusion (025-1) and part of the Greenwich intrusion (Greenwich-1). Populations A and B are OIB-like magmas with enriched LREE and steep patterns on primitive mantle-normalized diagrams. They are similar to the Eva Kitto, Jackfish, Seagull, and McIntyre intrusions and relatively close to the Thunder intrusions from the MCR. Population C includes Greenwich-2 and 25-2. This population is less enriched in LREE and has a flatter pattern on primitive mantle-normalized diagrams, and they are similar to Nipigon Sills, Inspiration, Crystal Lake, Coubran Lake and Tamarack intrusions.
Sm–Nd and Rb–Sr isotopes were utilized to assess the role of crustal contamination in the formation of the five intrusions. The intrusions are characterized by generally negative εNd(T) values of -7.4 to +0.14 and Sri of 0.70309 to 0.70587. The most negative εNd(T) values of -7.20 and -7.49 were recorded in samples from the most LREE enriched portions of the Greenwich, whereas the +0.14 value is a hybrid grey (altered grey gabbro) sample from the 025 intrusion. Two country rock samples (an Archean metasedimentary rock and a granitoid) have εNd(1.1 Ga) values of -20.50 and -20.19, Sri values of 0.7636 and 0.7057, and Th/Nbn of 2.33 and 1.50 respectively. The negative εNd(T) values for the intrusions and the country rocks, along with their elevated bulk-rock Th/Nbn ratios, and negative Nb anomalies suggest that the intrusions were contaminated by older crustal material. Three possible sources of contamination were identified, one with less negative εNd(T) and lower Sri that affected Lone Island, SEA, EWC, Greenwich-1, and 025-1. Secondly, one with higher Sri and more negative εNd(T) that affected Greenwich-2. The third type of contamination affected 025-2, and is characterized by a lesser LREE enrichment and lower degree of crustal contamination and it affected 025-2.
The three types of magmas from TBNIC that have been characterized in this study are all plume derived magmas, were generated from distinct melt sources and went through different types and degrees of crustal contamination.