Summary:
Deposit Types
The MacLellan and Gordon deposits can be considered as belonging to the class of gold deposits referred to as orogenic. The orogenic gold deposit model (Groves, Goldfarb, Gebre-Mariam, Hagemann, & Robert, 1998) characterizes structurally controlled gold occurrences formed during orogenesis by either metamorphic or deeply sourced magmatic fluids.
Geological Setting and Mineralization
The Lynn Lake Greenstone Belt (LLGB) is located within the Paleoproterzoic Churchill Structural Province of the Canadian Shield. The LLGB, is divided into the North and South Belts, both of which are part of a larger litho-structural unit that extends in a north-easterly direction from the La Ronge Greenstone Belt in Saskatchewan. Rocks of the LLGP have undergone upper greenschist to upper amphibolite metamorphism.
Both the North and South Belts are comprised of steeply north-dipping mafic/ultramafic to felsic volcanic rocks, clastic sediments, oxide facies banded iron formation, and mafic to felsic plutonic rocks. The belts are separated and bounded to the north and south by large granitic batholiths.
The MacLellan property is located on the western portion of the North Belt. The MacLellan deposit is in the western portion of the North Belt and consists of multiple high grade ore zones contained within a package of northeast trending, hydrothermally altered, ultramafic flows, basalts and volcaniclastic rocks. The Gordon property is in the eastern portion of the North Belt. The Gordon deposit is characterized by a series of sulphidized, auriferous, moderating dipping veins hosted primarily by iron formation. The hydrothermally altered haloes adjacent to the veins in the iron formation are also mineralized. A second steeping deeply mineralization domain is present along the diorite intrusion to the south. The diorite intrusion itself, is host to discontinuous, mineralized quartz-carbonate veins.
The ore bodies typically form ore shoots. An ore body can vary between 0.5–50 m wide, 100’s of meters long, and consists typically of a vein network, an en echelon vein swarm, or just of one single large vein. The depth extent of an ore body may well be much larger than its extent along strike.
All deposits are characterized by an alteration halo consisting of proximal to distal carbonatization and proximal potassic alteration. Elements enriched typically include arsenic, gold, potassium, rubidium, sulphur, antimony, tellurium, and tungsten; in some cases, also silver, boron, bismuth, cobalt, copper, and selenium are enriched.
MacLellan
Gold and silver mineralization at the MacLellan deposit are synchronous with, and spatially associated with significant D2 shear zones (Hastie, Gagnon, & Samson, 2018). The highest concentrations of Au-Ag within the MacLellan deposit are where east-west trending dextral D2 shear zones are intersected by NNE-SSW trending D4 brittle faults.
Two stages of gold mineralization at the MacLellan deposit were identified by Hastie (2018):
• Au-Ag mineralization associated with pyrrhotite and arsenopyrite replacement of iron carbonate veins parallel to D2 ductile shearing. Biotite plus silica alteration are associated with this mineralization event; and
• Spatial association with the intersection of D2 shear zones and D4 faults. The gold mineralization is characterized by fracture-filling native gold, aurostibite, and Au-rich rims and fractures in arsenopyrite.
Gordon
At the Gordon deposit, a set of extensional and shear veins developed during D2 deformation. The moderately south dipping (25°-45°) iron carbonate-quartz extension veins are preferentially developed in the iron formation, although they are observed in most lithological units. The shear veins are S2 foliation parallel. Both vein orientations show pyrite and pyrrhotite replacement of iron carbonate within the veins and the host iron formation has sulphidized replacement halos peripheral to the veins. Gold is associated with the sulphides in both the veins and sulphidized iron formation.
Weston (2014), conducted a detail investigation of high-grade intersections throughout the Gordon Deposit, concluding that:
• High grade mineralization is associated with shallow-dipping, sulphidized quartzcarbonate veins with an average wide of 9 cm.
• High-grade mineralization is also present within strongly sulphidized iron formation. The sulphide replacement zones average 2.4 m in width however range from 15 cm to 8.3 m wide and often display sharp visual contacts with weakly to non-mineralized iron formation.