The Riverdale Sill is located within the southern city limits ofThunder Bay. Although the sill consists mainly of gabbronorite, olivine gabbro can be found in a number of localities. The intrusive contact between the sill and the Paleoproterozoic Rove Shale is sharp and the chilled margins are only a few centimetres thick. The sill is exposed over an area approximately 6 km long and 2 km wide but the true thickness is unknown because the upper contact is not visible.
In comparison to other Midcontinent Rift-related intrusions, the Riverdale Sill is geochemically similar to ultramafic intrusions such as Hele and Disraeli (Hollings et al. 2007). Although it is found south ofThunder Bayit most closely resemble the Jackfish Sill of the Nipigon Embayment rather than the Logan and Nipigon Sills which surround it.
The Riverdale Sill crops out in a quarry on West Riverdale Road where it has an exposed thickness of 10 m. Detailed sampling was carried out every metre up through the sill to investigate geochemical variations within it. There is no geochemical evidence for fractionation within the sill which is supported by the absence of cumulate textures.
On a primitive mantle-normalised plot, a number of samples of the Riverdale Sill broadly resemble Ocean Island Basalts. However, some samples within the sill have negative niobium anomalies though geochemical evidence for contamination of the sill is localized to within less than 1m of the contact with Rove Shale. Therefore, this contamination is best interpreted to be the result of crustal contamination at depth. The less-contaminated samples are typically found towards the center of the sill. The geochemistry of the center of the sill varies slightly from that near the sill margins. Samples of gabbronorite taken adjacent to a shale xenolith within the sill do not display a negative Nb anomaly. The lack of this anomaly, combined with the lack of contamination above the contacts in the sill at the quarry, supports a model in which contamination is occurring at depth rather than during emplacement.
Geochemical variations within the sill can be interpreted as the result of two pulses of magma, with the first more-contaminated pulse intruded by the second, less-contaminated magma shortly after the emplacement within the shale. The less-contaminated magma may have pushed the contaminated magma to the edges of the sill, leaving a less-contaminated core.