The Shrimp Lake intrusion is part of the suite of TTG-like granitoid intrusions found throughout the Superior Province. Although originally classified as a trondhjemite, major element geochemistry of the Shrimp Lake intrusion, specifically SiO2, Al2O3, MgO and Fe2O3 abundances paired with specific Yb and La/Yb values suggest a TTG rock suite.
The Shrimp Lake intrusion is interpreted to be synvolcanic with the intermediate metavolcanics and the Hewitt Lake intrusion which has been dated at 2743±2 Ma and has been taken to be representative of all three synvolcanic intrusions in the North Spirit greenstone belt, and 2735±10 Ma for the Hewitt volcanic assemblage. The three synvolcanic intrusions of the North Spirit Lake greenstone belt show similar compositions of porphyritic granodiorite-tonalite with abundant plagioclase phenocrysts in a fine-grained groundmass of quartz, plagioclase and K-feldspar. The age of the Shrimp Lake intrusion is interpreted to be slightly younger relative to the other two intrusions because of the lack of deformation and alteration observed in the Hewitt Lake and Makataiamik Lake intrusions.
Mineralization within the Shrimp Lake intrusion is found in two outcrops: one containing massive sulphides of pyrite, pyrrhotite chalcopyrite and sphalerite. The other is similar in composition, however alteration is less intense with the plagioclase phenocrysts preserved in the host rock indicating a granodiorite-tonalite host. The host rock to the massive sulphides is interpreted to be the granodiorite-tonalite, however intense alteration and the presence of a mafic xenolith from an adjacent volcanic assemblage make it difficult to confirm this. Geochemical analysis of the rock show a typical granitic trend comparable to the rest of the intrusion.
The magmatic source for North Caribou superterrane TTG suites is typically derived from a depleted mantle with minor contamination from old remelted crust. Formation of these TTG's is commonly associated with oceanic arc-type settings, however, the lack of necessary data for the Shrimp Lake intrusion prevents it being characterized in this way.
Avery is currently working for Fladgate Exploration in Thunder Bay
For more details about this thesis contact Dr. Peter Hollings