Summary:
The Paleoproterozoic Lynn Lake greenstone belt (LLGB) is located within the Churchill Structural Province of the Canadian Shield. The LLGB comprises Paleoproterozoic arc-related ultramafic to felsic rock assemblages and represents a portion of the internal Reindeer zone of the TransHudson Orogen. The LLGB is correlative with the La Ronge greenstone belt in Saskatchewan to the southeast. To the north, the LLGB is bound by the South Indian Domain, and to the south, it is separated from the Flin Flon greenstone belt by the Kisseynew Domain (Gilbert, Syme, & Zwanzig, 1980; Beaumont-Smith & Böhm, 2004).
Rocks of the LLGB have undergone upper-greenschist to upper-amphibolite-facies metamorphism (Gilbert, Syme, & Zwanzig, 1980; Glendennings, Gagnon, & Polat, 2015).
The LLGB has been subdivided into the following three sub-domains based on isotopic, geochemical and age differences (Beaumont-Smith & Bohm, 2003):
• North Belt;
• South Belt; and
• Fox Belt.
The North and South Belt are east-trending, steeply dipping belts comprised of supracrustal rocks of the Wasekwan Group and younger sedimentary rocks of the Sickle Group. Thought to occupy the upright limb of a north-facing antiform, the North Belt is comprised of a steeply north-dipping, subaqueous equence of volcanic rocks containing basalt, andesite, dacite, and rhyolite overlain by volcaniclastic rocks and epiclastic sediments, intruded by mafic to felsic plutonic rocks (Fedikow & Gale, 1982). A relatively narrow, stratigraphically and structurally distinct zone consisting of ultramafic flows, banded iron formations and associated exhalative and epiclastic rocks is spatially associated with both the MacLellan and Gordon deposits in the North Belt (Yang & Beaumont-Smith, 2016). Historically, this unit was termed the Agassiz metallotect in the MacLellan deposit area and the Nickel Lake shear zone in the Gordon deposit area (Fedikow & Gale, 1982).
The South Belt consists largely of tholeiitic to calcalkalic volcanic and volcaniclastic rocks. A major belt wide deformation zone termed the Johnson Shear trends east-west across the property and is spatially associated with several gold deposits and occurrences including the past-producing Burnt Timber mine.
Burnt Timber deposit geology
The Burnt Timber deposit is one of several gold deposits along the Johnson Shear Zone, a major east-trending fault zone on the southern margin of the Lynn Lake greenstone belt in northern Manitoba. The deposit is hosted by mafic tectonite of the Wasekwan Group. Gold is associated with silica, pyrite and carbonate enrichment that occurred during dextral shearing along the shear zone.
The Johnson Shear Zone has a strong east-trending shear foliation. This foliation is deformed by chevron folds, which are, in turn, sheared along the shear foliation, forming isolated fold packets in the shear zone. These structures are overprinted by a crenulation cleavage and kinks that formed during later dextral shearing. East-trending cataclasite units and pseudotachylite veins cut across the earlier ductile structures, and are displaced by late, north-trending, sinistral faults.
The deposit is hosted by strongly sheared mafic tectonite, which underwent strong silicification, carbonatization and pyritic alteration over a width of approximately 100 m within the 250 m wide Johnson Shear Zone (JSZ). The JSZ is a regional dextral shear zone that predates peak regional metamorphism but cuts across regional F1 folds of the Lynn Lake belt (Beaumont-Smith and Rogge, 1999). A late brittle fault, the T1 fault, is subparallel to the JSZ. The T1 fault is a north-dipping reverse fault (Peck et al., 1998). The deposit is in the hanging wall of the fault and extends approximately 40 m from the fault.
The Burnt Timber deposit is hosted by mafic volcanic rocks of the Wasekwan Group (Bateman, 1945). The rocks are fine- grained, dark green, mafic tectonite, in which all primary structures have been erased by the strong deformation in the shear zone. Near the deposit, the mafic tectonite has alternating dark grey and light grey bands that weather pink and green, respectively, on outcrop surface. The mafic tectonite is locally strongly magnetic due to the presence of magnetite crystals up to 2 mm in size.
To the north and south of the JSZ, the mafic volcanic rocks are fine-grained massive flows with few preserved quartz- filled amygdules. The mafic flows are dark green on fresh surface and light green on outcrop surface. Minor units of mafic volcaniclastic rock and felsic tuff are interlayered with the mafic flows. The mafic volcaniclastic rocks are exposed for more than 100 m across strike. They are finely laminated with alternating, light grey and dark grey layers less than 0.5 cm wide. Younging indicators are rare, but graded bedding on two outcrops suggests that the beds young to the north. To the north of the JSZ, felsic tuff occurs as thin layered units, which are typically less then 4 m thick. The tuff is white on outcrop surface, light grey on fresh surface, and finely laminated to thickly bedded (10 cm maximum thickness). The felsic tuff is cut by a cleavage that refracts toward the bedding as it passes from the more massive thicker beds to the thinner, presumably less competent beds.
South of the deposit, the mafic metavolcanic rocks are cut by a coarse-grained granodiorite of the Pool Lake suite (Gilbert et al., 1980). The intrusion comprises 25% quartz, 60% feldspar and 15% biotite. Biotite is altered to chlorite in the margins of late fractures cutting through the intrusion. North of the Burnt Timber deposit, a poorly exposed, dark grey gabbro contains large amphibole grains up to 2 cm in size. The granodiorite and gabbro are not in contact, so the relative age of the two intrusions is not known.
Burnt Timber deposit mineralization
Quartz-carbonate alteration of the mafic tectonite occurs across the JSZ. Quartz and iron carbonate veins are folded by F2 chevron folds and overprinted by the sericitic S2 shear foliation. In high-grade zones of the orebody, the mafic tectonite underwent strong silicification, pyritization and carbonatization. Sericite along S2 foliation planes is replaced by chlorite. This supports the conclusions of a geochemical study by Peck and Eastwood (1996), who reported an inverse correlation between gold and potassium values in the orebody.
A spatial relationship between felsic dykes and gold mineralization is observed in several drillholes through the deposit. Gold assay values increase where felsic dykes are present. This suggests a possible genetic association between the dykes and the gold mineralization.