Seabridge Gold’s FAT (felsic ash tuff) deposit in the Courageous Lake Greenstone Belt (CLGB) is a rare example of a well-preserved epithermal gold system in Archean rocks. CLGB is one of several Archean greenstone belts in the Slave Province. This greenstone belt is part of the Yellowknife Supergroup of the Central Slave Province, NWT, that contains large gold deposits like the past producing Giant, Con, Lupin and Salmita mines. The CLGB is made up of cyclical volcanic eruptions that formed an 1800m thick succession of mafic flows that gave way to a discrete rhyolite tuff series and is host to the FAT deposit. This volcanic stratigraphy is overlain by a thick turbidite sedimentary package. Multiple metamorphic events have affected the CLGB, tilting the strata to near vertical and producing a mid-greenschist metamorphic mineral assemblage.
Felsic volcanic textures of the rocks within the FAT deposit are well preserved, generally representing sub-aerial lapilli tuff and ash tuff laid down during eruptions. Placed between the eruptive events are aqueously reworked volcanoclastic beds. There are eight recognized eruptions forming the felsic volcanic pile that is host to the FAT deposit. At the top of several discrete eruptions are well developed, stratabound chert beds that contain most of the gold-arsenopyrite mineralization. Mineralized volcanic units stretch north-south for about 2,000m. Overall thickness of the volcanic zone is 920m with individual mineral zones between 30m and 65m wide. Drilling has evaluated the mineral system vertically to about 650m. Narrow shear structures cut the stratigraphy at an acute angle, remobilizing and redepositing the gold-arsenopyrite. The FAT deposit is refractory with most of the gold found as small inclusions within the arsenopyrite.