Summary:
The Great Bear deposit is an Archean mesothermal gold deposit. The prolific Red Lake greenstone belt in Canada hosts numerous high-grade Archean mesothermal gold deposits that have produced more than 28 Moz of gold (Armstrong et al., 2018).
The Property area lies within a regional northwest-southeast trending belt of metavolcanic and metasedimentary rocks which are bounded by intrusive batholiths. The regional tectonostratigraphic assemblages of the East Uchi Subprovince (SanbornBarrie et al., 2004a) have recently been subdivided by Great Bear into new lithologies based on visual core logging, geochemistry, and petrology. The division between mafic and intermediate-felsic domain still exists.
The southwestern portion of the Property is within the mafic domain and consists of mafic volcanic flows (high Fe-tholeiites and high Mg-tholeiites) intercalated with argillite, siltstone, iron formation, and minor local felsic volcanics. The association of these rocks is interpreted to be the sequence formed in a marine setting, in proximity to active venting in pre-existing anoxic basins. The strong magnetic response associated within this sequence is related to horizons of iron formation and argillites.
The younger sequence of intermediate to mafic volcanic and volcanic derived sedimentary rocks is located at the centre of the Property and has a similar stratigraphy to the western and eastern portions of the Property. However, these areas have a much higher proportion of felsic pyroclastic rocks in the strata. These are also interpreted to have been submarine flows. Basin development is characterized by relatively thinbedded, silty argillite and common iron formation. The fine-grained volcanic facies suggest quiescent depositional conditions with subdued modification of the sedimentary sequences caused by volcanism.
The felsic domain dominates the northeastern portion of the Property. It consists of porphyritic felsic flows (dacites) and volcaniclastics intercalated with sedimentary rocks. The sequence is interpreted as a deformed felsic flow-dome complex.
The mafic domain is in contact with a largely felsic/sedimentary domain in the northeast portion of the Property. The contact between the two domains is best described as gradational from mafic to sedimentary and felsic rocks and, where drilled, is marked by a highly strained sedimentary sequence.
Topping directions, determined from graded bedding using oriented core data, are generally fining towards the northeast.
Mafic volcanic dykes and sills are common throughout the Property, ranging from lamprophyre to gabbro/diorite (i.e., high level apophyses injected into and disrupting the stratigraphy). Intermediate felsic intrusive rocks are also noted throughout the region. Small intrusive bodies are mapped and have been intersected in both the historic and present drill campaigns.
Gold mineralization styles on the Property include:
- Silica-sulphide replacement of meta-sediments along the limb of D2 folds (Limb Zone);
- Quartz veining in mafic volcanics oriented axial planar to D2 folds (Hinge Zone);
- Shear hosted; disseminated gold within high strain zones (LP Zone).
Silica Sulphide Replacement – Limb Zone
The silica sulphide replacement zones have been the focus of exploration at the Limb Zone (historically the 88-4 Zone target). This zone is associated with the rheological and geochemical contact between pillow basalt (Fe-tholeiites) and massive basalt (Mgtholeiites). The contact is often marked by argillite/siltstones. Mineralization occurs as replacement of sediments, if present, or as silica flooding and quartz-calcite veining in the absence of sediments. Pyrrhotite is the dominant sulphide, with sulphides ranging in concentrations from 2% to 40% pyrrhotite, 2% to 15% pyrite, 1% to 4% arsenopyrite, 2% chalcopyrite, minor less than 2% sphalerite, and trace magnetite. Visible gold is not uncommon and, where observed, is associated with strong pyrrhotite and weaker arsenopyrite-pyrite mineralization. Higher-grade and more intense visible gold correlates well with a thinning or absence of sedimentary host rocks at the contact. An increase in silica flooding at the high Fe-tholeiite basalt and high Mg-tholeiite basalt contact is observed where sediments are thin or not deposited. Petrographic work by Ross (2004) identified the presence of gold-silver and lead-tellurides locally encapsulated within arsenopyrite. All native gold in the polished thin sections occurred as free gold crystals up to 50 microns in size.
Alteration and mineralization are strongly correlated with the sulphidized sedimentary layer, both commonly exhibiting very sharp contacts with unmineralized or unaltered host rock. A strong shear component is present within and adjacent to the mineralized zone. The zone is approximately 800 m long and has been drilled to a vertical depth exceeding 400 m. Mineralization plunges steeply northwest in a fold limb host dipping steeply to subvertically northeast. It is generally considered that the Limb Zone lies on the north limb of a property scale F2 fold.
Quartz Veining – Hinge Zone
Quartz veining has been observed throughout the Property and has been the main focus of exploration at the Hinge Zone. The quartz veining is hosted by multiple lithologies including massive basalt (high Mg-tholeiite), argillite, and pillow basalt (high-Fe tholeiite). Individual veins are variable in width ranging from 1 cm to 5 m and can create zones of up to 40 m. They are generally mineralized with fine-grained disseminated sulphides consisting of 1% to 3% pyrrhotite, 1% to 2% pyrite, 1% to 2% chalcopyrite, less than 1% arsenopyrite, and trace sphalerite. Visible gold is very common ranging from trace to 5% as pin pricks, centimetre scale clusters, and fracture fill. These veins have a weak to strong, patchy to pervasive biotite and carbonate alteration halo ranging from several centimetres to approximately 2 m in width.
The Hinge Zone vein system is comprised of several sub-parallel anastomosing veins formed along the axial trace of a property wide D2 fold. The intersection between the fold and stratigraphy marks a plunge control on the higher grades within the veins.
Disseminated Gold in High Strain Corridor – LP Zone
The LP Zone exhibits a style of mineralization which is not observed in other parts of the Red Lake greenstone belt. The zone is associated with a high degree of deformation, widespread alteration, and transposition of primary textures, as well as complete flattening of stratigraphy.
The LP Zone mineralization occurs within a wide zone of high strain and increased metamorphic grade. Up to 500 m wide, the strain zone is very continuous for over 4 km and is slightly oblique to stratigraphy, intersecting multiple lithologies including the porphyritic felsic volcanic, metasediment 2, felsic volcanic 2, and metasediment 3. The higher-grade gold mineralization appears to be controlled by the intersection of this strain zone and the metasediment 2 unit. Ongoing LP Zone drilling has demonstrated that most of the greater than 5.0 g/t Au intercepts and nearly all of the greater than 10 g/t Au intercepts drilled along the LP Zone to date occur within 50 m to 100 m of the metasedimentary/felsic volcanic contact.
Gangue mineralization is variable across the zone and locally ranges from 0% to any amount of the following: 1% to 15% disseminated pyrite, 1% to 10% arsenopyrite (blebby and matted), 1% to 5% red and yellow sphalerite, 1% to 5% pyrrhotite, 1% to 5% chalcopyrite, 1% to 5% galena, and 1% to 3% scheelite. The LP Zone has been further sub-divided into six subzones named, from northwest to southeast, Discovery, Bruma, Yuma, Yauro, Auro, and Viggo.