The Finlayson and Lumby Lake Greenstone Belts are located approximately 200km west of Thunder Bay, Ontario, north of Atikokan, Ontario.
Within both the Finlayson Lake and Lumby Lake Greenstone belts two distinct sequences of sedimentary rocks are present. Each of the belts contains an upper and a lower sedimentary rock sequence which differ in age and chemical composition.
The lower sequence of the Finlayson Lake Greenstone Belt is represented by the Little Falls Lake metasedimentary rocks and the laterally equivalent lower Finlayson Lake metasedimentary rocks. These rocks consist of coarse-grained sandstones, conglomerates, and lesser interbedded mafic detritus-rich metasedimentary rocks and are laterally continuous with felsic volcanic rocks to the south. Deposition of these sedimentary rocks was by high-density turbidity current processes. Their chemical composition is distinct from that of the upper Finlayson Lake metasedimentary rocks and suggest a single felsic volcanic source with composition similar to that of the Steep Rock Upper Felsic unit and the Old Tonalite unit. U-Pb geochronology again supports a single source rock, with an age of 2996 ± 0.8 Ma.
The upper band of Finlayson Lake sedimentary rocks are distinct from the sedimentary rocks present in the Little Falls Lake area and lower Finlayson Lake areas. Their chemical composition suggests that the upper Finlayson Lake sedimentary rocks are similar and were continuous with, although fault offset from, the upper Lumby Lake sedimentary rocks (Fenwick, 1976; Stone and Pufahl, 1995). U-Pb data from conglomerate in the southern Finlayson Lake area yield zircon ages ranging from 2997 ± 2.5 to 3002 ± 0.9 Ma. Sm-Nd data suggest that the basin received detritus derived from tonalitic intrusions as well as a slightly older mafic volcanic component. These data agree with geochemical data which suggest that the composition of the upper Finlayson Lake sedimentary rocks lies on a mixing line between the Old Tonalite and the Steep Rock Upper Mafic unit or the Finlayson Lake mafic volcanic rocks.
A well developed coarsening upward sequence is preserved within the upper Finlayson Lake sedimentary rocks. The sequence consists of iron formation and chemical sedimentary rocks at the base, overlain by DE turbidites, which coarsen to pebbly sandstones and conglomerates near the top of the sequence. There is also some lateral facies variation with coarsest-grained metasedimentary rocks exposed in the southern part of Finlayson Lake. These rocks are consistent with deposition from both high- and low-density turbidity currents and were likely deposited by a prograding delta system that was centered south of the area.
As in the Finlayson Lake Belt, the Lumby Lake Belt also contains two stratigraphically distinct sedimentary units. The lower sedimentary unit is represented by the sedimentary rocks present within the Hock Lake area, whereas the upper sedimentary unit is represented by the sedimentary rocks near Norway Lake and west to the Keewatin-Hematite Lakes area.
The lower Lumby Lake sediments are laterally continuous with 2999 Ma old felsic volcanic rocks to the east (Jackson, 1985) and are the resedimented equivalent of them. Their chemical composition is similar to that of the Little Falls Lake sedimentary rocks, and a chemically similar source is suggested.
The upper Lumby Lake sedimentary rock sequence is similar to the upper Finlayson Lake sequence and is the fault offset equivalent (Fenwick, 1976; Stone and Pufhal, 1995). The chemical composition of the sedimentary rocks suggests that the upper Lumby Lake sedimentary rocks had source rocks of the same composition as the source of the upper Finlayson Lake sediments. The upper Lumby Lake sequence is dominated by iron formations and chemical precipitates, with lesser fine-grained clastic sedimentary rocks. There are lateral facies variations from a clastic dominance in the west to chemical precipitate dominance in the east. The predominance of chemical precipitates is evidence of widespread hydrothermal activity throughout the area. It is possible that the upper Lumby Lake clastic sedimentary rocks represent the distal equivalent of the turbidite system developed in the Finlayson Lake area. Alternatively, the upper Lumby Lake portion of the basin may have been fed by a localized source centred in the Norway Lake area. Evidence of this includes a dominance of clastic sedimentary rocks and the presence of debris flow conglomerates in this area.
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