The Neoarchean Great Bear Gold Deposit: Evidence for a Second Major Mineralization Event in the Red Lake Greenstone Belt (Superior Craton, Canada)?

The Great Bear deposit is a new world-class gold deposit (~6.6 Moz Au) in the Red Lake greenstone belt of the Archean Superior craton in Canada. The deposit is defined by three key zones of mineralization that are located adjacent to an interpreted upper crustal fault (known as the Lithoprobe fault): (1) quartz veining in mafic volcanic rocks in the hinge zone; (2) quartz veining in sulfide-rich argillite horizons in the limb zone; and (3) disseminated and vein-hosted gold within a high-strain zone in the hanging wall of the Lithoprobe fault. The stratigraphy in the hanging wall of the Lithoprobe fault, known colloquially as the Felsic domain, consists of sedimentary rocks (MS1, MS2, and MS3), felsic volcaniclastic rocks (F1), and felsic porphyries (E31 and E32), which were metamorphosed to lower amphibolite facies conditions. We interpret the felsic porphyries, which host a significant fraction of the established gold mineralization, as synsedimentary, emplaced as subvolcanic intrusions and lava domes within an interpreted structural basin. Uranium-lead dating by chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) of magmatic zircons from the synsedimentary quartz-feldspar (QF) porphyry (E31) constrains the age of magmatism to 2710.2 ± 1.2 Ma, which is inconsistent with existing interpretations of the stratigraphy as part of the 2748 to 2742 Ma McNeely sequence of the Confederation assemblage. The E31 porphyry is instead temporally equivalent with a suite of younger syntectonic porphyries and tonalite-trondhjemite-granodiorite intrusions in the Red Lake gold camp. This age also constrains the maximum age of mineralization to <2710 Ma, which is younger than other established gold deposits in the Red Lake greenstone belt, including the giant 2723 to 2712 Ma Campbell-Red Lake deposit, suggesting a second major mineralization event succeeded regional D2 shortening and mineralization in the Red Lake greenstone belt. High-strain zones and structural basins associated with major faults, somewhat akin to the fault-related orogenic gold deposits of the Abitibi subprovince, may therefore represent important first-order targets for exploration in the Red Lake greenstone belt and the adjacent greenstone belts of the Uchi subprovince.