Structural and alteration history of the Buffalo gold deposit, Red Lake, Ontario

Abstract

Detailed structural, petrographic, and geochemical analysis of the Buffalo gold deposit in the Archean Red Lake greenstone belt, northwestern Ontario, suggests that gold was localized during D3 mylonite-style deformation and associated retrograde metamorphism. The deposit is therefore younger than much larger deposits elsewhere in the belt. The deposit is hosted by the -7km-diameter, 2718 Ma Dome stock granodiorite. Gold is hosted within two sets of quartz + tourmaline+ pyrite + calcite veins in conjugate orientation. The first set of veins is small (- 3 cm or less wide), with a strike of 239° and dipping 73° north (northeast veins). The second set is larger (- 10 cm wide) with a strike of 119° dipping 76° south (northwest veins). Compilation of structural fabrics in the Buffalo deposit and the surrounding area suggests that the two sets of quartz + tourmaline + pyrite+ calcite veins were formed as a result of the intersection of two D3 mylonitic deformation zones. The Dome stock was originally a white and black granodiorite which has been extensively altered to pink. This was followed by a greenish-grey alteration event, associated with mylonitic deformation during the M3. During a later phase of M3, a massive tourmalinization event replaced the much of the quartz in the northwest veins at the Buffalo deposit. This tourmalinization was associated with bleached pink metasomatic halos (-10 cm), which developed around tourmaline rich portions of the northwest veins Gold mineralization at the Buffalo deposit is divided into two major phases: (1) During metasomatism surrounding the northwest veins, gold and sulphide minerals were enriched with respect to the surrounding granodiorite. Microprobe analysis indicates that gold is associated with calcite and albite alteration of the granodiorite; (2) During late brittle fracturing of the quartz+ tourmaline + pyrite + calcite veins. Gold in this phase is associated with bismuth tellurides as fracture and cavity fillings in quartz and especially late fracture-filling pyrite.