The LaRonde Penna Au-Rich Volcanogenic Massive Sulfide Deposit

Abstract

Abstract The 2698 Ma LaRonde Penna deposit, with over 71 Mt of ore at 3.9 g/t Au (280 t Au or ~9 Moz Au), is the second largest Au-rich volcanogenic massive sulfide (VMS) deposit in the world. It is part of the Doyon-Bousquet-LaRonde mining camp in the eastern part of the Blake River Group. The deposits of the Doyon-Bousquet-LaRonde mining camp are hosted by the volcanic rocks of the Hébé-court (base) and Bousquet (top) formations that form a southward-younging homoclinal sequence, with nearly vertical dips due to a north-south compressional event responsible for the development of an E-W–trending, steeply S-dipping, penetrative schistosity under prograde, upper greenschist to lower amphibolite facies meta-morphism. The E-trending, steeply S-dipping schistosity is associated with strong flattening, transposition, and minor folding of the volcanic rocks, alteration zones, and sulfide lenses. The ore lenses at LaRonde Penna, which are stacked in the upper half of the Bousquet Formation, are characterized by semimassive to massive sulfides or narrow intervals of transposed sulfide veins and veinlets. The synvolcanic hydrothermal alteration at LaRonde Penna now corresponds to mappable upper greenschist-lower amphibolites-grade metamorphic assemblages. In the upper part of the deposit, the 20 North lens comprises a transposed pyrite-chalcopyrite (Au-Cu) stockwork (20N Au zone) overlain by a pyrite-sphalerite-galena-chalcopyrite-pyrrhotite (Zn-Ag-Pb) massive sulfide lens (20N Zn zone). The 20 North lens (20N Au and 20N Zn zones) is underlain by a large, semiconformable alteration zone that comprises a proximal quartz-Mn-garnet-biotite-muscovite alteration assemblage. The 20N Zn zone tapers with depth in the deposit and gives way to the 20N Au zone. At depth in the deposit, the 20N Au zone consists of semimassive sulfides (Au-rich pyrite and chalcopyrite) enclosed by a large aluminous alteration assemblage interpreted to be the metamorphic equivalent of an advanced argillic alteration zone. At LaRonde Penna, the presence of sulfide lenses characterized by Au-rich portions and base metal-rich portions demonstrates that a VMS system can generate mineralization styles that gradually evolve, both in space and time, from neutral (Au-Cu-Zn-Ag-Pb ore), to transitional, to acidic (advanced argillic alteration and Au ± Cu-rich ore) in response to the evolving local geologic setting.