Wieland Boehme, PhD

The Great Bear deposit is a new world-class gold deposit, in the Red Lake greenstone belt of the Mesoarchean to Neoarchean Uchi sub-province, Superior Craton (measured + indicated resource of 2.7 Moz at 2.81 g/t Au). Gold mineralization principally occurs in the LP Zone, a strongly foliated, up to 400 m wide zone at the contact between a northeastern felsic domain and a southwestern mafic domain. Stratigraphic and petrographic analyses of mineralized intervals were conducted to characterize the host lithology, ore assemblages and textures in the LP Zone and to constrain gold mineralization and remobilization processes. 

The volcano-sedimentary host rocks in the LP Zone consists mainly of strongly foliated metasedimentary and felsic metavolcanic rocks. Massive coherent volcanic flows or intrusions dominate the stratigraphy and consist of a porphyritic unit containing coarse, rounded quartz and blocky plagioclase phenocrysts (<15 %) in a dark grey, fine-grained quartz-plagioclase-biotite matrix and an altered porphyritic unit with a very fine-grained to aphanitic quartz-plagioclase-biotite matrix. The metasedimentary lithologies are dominated by a dark grey, very fine-graine, massive to thinly bedded, biotite-rich groundmass with locally occurring garnet and staurolite porphyroblasts. Gold mineralization is associated with quartz ± carbonate veins that crosscut and are aligned parallel to the host strata. Mineralized intervals also correlate with varying degrees of silica, albite, and sericite alteration as well as moderate carbonate and minor chlorite alteration. 

The ore assemblage consists predominantly of disseminated pyrite, arsenopyrite, and pyrrhotite, with minor chalcopyrite, sphalerite, galena, and magnetite, as well as trace Bi±Ag-Te minerals, scheelite, and gold. A local enrichment in gold occurs in the central and southeastern LP Zone and correlates with higher sulfide abundance, stronger alteration and elevated strain. The gold enrichment is characterized by pyrite-arsenopyrite (central LP Zone) and pyrite-pyrrhotite (southeastern LP Zone) dominated ore assemblages. 

Native gold occurs primarily as round, disseminated blebs within and close to quartz ± carbonate veins. Gold occurs as interstitial grains along coarse recrystallized grain boundaries within the veins and is locally intergrown with altered, foliation-parallel biotite within the host strata. Further, gold shows close spatial and textural relationships with Bi±Ag±Pb tellurides, galena, and coarse, deformed pyrite. These preliminary observations suggest a pre- to syndeformational emplacement of gold and a potential syndeformational remobilization accompanied by pyrite recrystallization.