Tianna Groeneveld MSc thesis abstract

Thesis Title: 
Geochemistry and mineralization of the Archean Titan (Roaring River) intrusion, Thunder Bay, Ontario
The 2690 ± 3.2 Ma Titan intrusion is an approximately 7 by 3 km, roughly ovoid, mafic-ultramafic intrusion, located just north of the inferred boundary between the Winnipeg River and Marmion terranes.  Outcrop in the area is relatively sparse, largely due to cover from pervasive glacial till and Proterozoic diabase sills associated with the Midcontinent Rift.  Titan consists of a variety of lithologies, including  eucogabbros, melagabbros, gabbros, and pyroxenites, which are distributed throughout the intrusion.  Both mafic and felsic dikes are observed in outcrop, as are felsic breccias.  Titan consists of a single magma body, with one pulse of magma which has subsequently undergone fractional crystallization within a closed system.  This is supported by smooth linear trends in the major element bivariate plots, with moderate amounts of scatter, and consistent, tight primitive mantle normalized REE trends.  Titan samples have a range of (La/Sm)N from 0.7 to 3.8, a range of (Gd/Yb)N from 2.3 to 7.4, and a range of Nb/Nb* values from 0.02 to 0.47.
Titan likely formed in a supra subduction zone setting, as evidenced by the negative HFSE anomalies.  This is consistent with the regional context of the Winnipeg River and Marmion terranes during this time period (~2.74-2.69 Ga).  Small amounts of crustal material appear to have been incorporated into Titan, as evidenced by εNd values of 0.70 to 1.82, compared to an estimated depleted mantle at 2.7 Ga which would have a εNd value of +3.  The exact source of the contamination can only be speculated due to the similarity in geochemical and εNd values for the surrounding Roaring River complex, and lack of data from the basement rocks in that area.
Sulphides are found ubiquitously throughout the intrusion, though generally at low abundances (~3%).  The most common sulphide is pyrite, often found as very finegrained blebs with a rim of magnetite, whereas larger aggregates of pyrrhotite, chalcopyrite, and pentlandite are much rarer.  Pyrite is considered to be a hydrothermal phase, likely as secondary precipitation. Sulphide isotopes were gathered from pyrite and a smaller amount of chalcopyrite grains, providing δ34S values with a range of -10.02 to +5.41‰ and Δ33S values with a range of -0.26 to +0.1‰.  The sulphur isotope values are consistent with an initial magmatic sulphur phase, responsible for the large aggregates of pyrrhotite, pentlandite, and chalcopyrite, and a later low temperature (< 400 °C) hydrothermal system.  The hydrothermal system is also likely to have oxidizing conditions, causing the preferential mobility of Se over S.
Titan is compared to the nearby Lac des Iles suite, a collection of mafic to ultramafic intrusions within the Marmion terrane, which includes the Lac des Iles Complex and Tib Lake intrusion.  There are broad similarities between Titan and the Lac des Iles suite, particularly in the regional context of the intrusions, general lithology, age, and in tectonic setting.  However, there are key differences.  Titan consists of one
magmatic body, while the Lac des Iles complex consists of several intrusions and Tib Lake has multiple magmatic pulses.  The  environment around the Lac des Iles suite is also more dynamic, with roughly coeval felsic and mafic magmatism.  The Lac des Iles
complex and Tib Lake are interpreted to have assimilated country rock and felsic magmas associated with surrounding tonalite.  In contrast, Titan is intruded into the middle of a felsic complex, but the magmatism is not coeval.  In general, Titan appears to be a much
simpler intrusion, when compared to intrusions of similar size in the Lac des Iles suite.